cold rolled steel sheet and manufacturing process

专利摘要:
COLD LAMINATED STEEL SHEET AND MANUFACTURING PROCESS. The present invention relates to the amount of C, the amount of Si and the amount of Mn are represented respectively by [C], [Si] and [Mn] in% of mass unit, the cold rolled steel sheet satisfies a ratio of (5x [Si] + [Mn]) / [C]> 10, the metallographic structure contains, by proportion of area, 40% to 90% of a ferrite and 10% to 60% of a martensite, also contains one or more than 10% or less of a perlite by proportion of area, 5% or less of an austenite retained by proportion by volume and 20% or less of a bainite by proportion of area, the hardness of the martensite measured using a nanoindentator satisfies H2O / H101, 10 and (Sigma) HM020 and TSx (Lambda) representing the product of TS which is a tensile strength and (Lambda) which is an orifice expansion ratio is 50000 MPa-% or greater.
公开号:BR112014017042B1
申请号:R112014017042-8
申请日:2013-01-11
公开日:2020-10-27
发明作者:Toshiki Nonaka；Satoshi Kato；Kaoru Kawasaki；Toshimasa Tomokiyo
申请人:Nippon Steel Corporation；
IPC主号:

专利说明:

[Technical Field of the Invention]
[001] The present invention relates to a cold-rolled steel sheet that has excellent plasticity before hot stamping and / or after hot stamping and a manufacturing process thereof. The cold-rolled steel sheet of the present invention includes a cold-rolled steel sheet, a hot-dip galvanized cold-rolled steel sheet, a galvanized and annealed cold-rolled steel sheet, to an electro-galvanization of a sheet cold-rolled steel and cold-rolled steel sheet aluminization.
[002] Priority is claimed in Japanese Patent Application No. 2012-004551, filed on January 13, 2012 and the content of which is incorporated herein by reference. [Related Technique]
[003] Currently, a sheet of steel is needed for a vehicle to be improved for collision safety and to have a reduced weight. Currently, there is a demand for a steel sheet of higher strength class in addition to steel sheets of the 980 MPa class (980 MPa or higher) and steel sheets of the 1180 MPa class (1180 MPa or higher) in terms of tensile strength. For example, there is a demand for a steel sheet that has a tensile strength of more than 1.5 GPa. In the circumstance described above, hot stamping (also called hot pressing, die cooling, press quenching or similar) is drawing attention as a process for obtaining high strength. Hot stamping refers to a forming process in which a steel plate is heated to a temperature of 750 ^ or higher, hot-formed hot-worked (worked worked) in order to improve the plasticity of the steel plate of high resistance and then cooled in order to quickly cool the steel plate, thus obtaining the desired qualities of the material.
[004] A steel plate that has ferrite and martensite, a steel plate that has ferrite and bainite, a steel plate that contains austenite retained in the structure or similar is known as a steel plate that has both compression and high plasticity resistance. Among the steel sheets described above, a multi-phase steel sheet that has martensite dispersed on a ferrite base (steel sheet that includes ferrite and martensite, that is, DP steel sheet) has a low proportion of yield ratio and a high tensile strength and, in addition, excellent elongation characteristics. However, the multiphase steel sheet has a low orifice expandability, as the stress is concentrated at the interface between the ferrite and the martensite, it is likely that if the crack originates from the interface. In addition, a steel plate that has the multifaces described above is not capable of exhibiting a class of 1.5 GPa of tensile strength.
[005] For example, Patent Documents 1 to 3 describe multiphase steel plate as described above. In addition, Patent Documents 4 to 6 describe the relationship between the hardness and plasticity of a high-strength steel sheet.
[006] However, even with the techniques described before the related technique, it is difficult to meet the current requirements for a vehicle such as additional weight reduction, additional increase in strength and more complicated component shapes. [Prior Art Document] [Patent Document]
[007] [Patent Document 1] Unexamined Japanese Patent Application, First Publication No. H6-128688
[008] [Patent Document 2] Unexamined Japanese Patent Application, First Publication No. 2000-319756
[009] [Patent Document 3] Unexamined Japanese Patent Application, First Publication No. 2005-120436
[0010] [Patent Document 4] Unexamined Japanese Patent Application, First Publication No. 2005-256141
[0011] [Patent Document 5] Unexamined Japanese Patent Application, First Publication No. 2001-355044
[0012] [Patent Document 6] Unexamined Japanese Patent Application, First Publication No. H11-189842 [Description of the Invention] [Problems to be solved by the invention]
[0013] The present invention took into account the problem described above. That is, an objective of the present invention is to provide a cold rolled steel sheet that has excellent plasticity and is able to obtain favorable orifice expandability together with strength and a manufacturing process thereof. In addition, another objective of the present invention is to provide a cold-rolled steel sheet capable of guaranteeing a strength of 1.5 GPa or more, preferably 1.8 GPa or more and 2.0 GPa or more after stamping a that forms and obtains a more favorable orifice expandability and a manufacturing process. [Means for Problem Resolution]
[0014] The present inventors carried out intensive studies in relation to a high-strength cold-rolled steel plate that guarantees resistance before hot stamping (before heating in a hot stamping process that includes heating to a temperature in a range from TδO'C to WOO'C, work and cooling) and has excellent plasticity such as orifice expandability. In addition, the inventors carried out intensive studies in relation to cold rolled steel sheet which guarantees a strength of 1.5 GPa or more, preferably 1.8 GPa or more and 2.0 GPa or more after hot stamping (after work and cooling in the hot stamping process) and has excellent plasticity as well as the expandability of the orifice. As a result, it has been discovered that, in cold rolled steel sheet, more favorable plasticity can be guaranteed than ever, that is, the TS tensile strength product and 50000 MPa- À hole expansion ratio (TSXÀ) % or more can be guaranteed by (i), in relation to the steel components, establishing an appropriate relationship between the amounts of Si, Mn and C, (ii) adjusting the ferrite and martensite fractions to predetermined fractions and (iii) adjusting the lamination reduction of the cold rolling to obtain a proportion of hardness (hardness difference) of martensite between the part of the surface of a sheet thickness and the central part of the sheet thickness (central part) steel plate and a distribution of martensite hardness in the central part in a specific range. In addition, it has been found that when the cold rolled steel sheet obtained in the manner described above is used for hot stamping within a certain condition range, a proportion of martensite hardness between the surface part of the sheet thickness and the central part of the cold rolled steel sheet and the destruction of hardness of the martensite in the central part of the sheet thickness are rarely changed even after hot stamping and therefore the cold rolled steel sheet (of hot stamped steel) ) which has great strength and excellent plasticity can be obtained. In addition, it was also made clear that suppression of MnS segregation in the central part of the blade thickness of cold rolled steel sheet is effective in improving the expandability of the orifice in both cold rolled steel sheet before hot stamping and in hot stamping. cold rolled steel plate after hot stamping.
[0015] In addition, it has also been found that, in cold rolling for which a large number of bases are used, adjusting the fraction of the cold rolling rate in each of the highest to the third bases in the total rate of cold rolling (cumulative rolling rate) to a specific range is effective for controlling the martensite hardness. Based on the discovery described above, the inventors have discovered a variety of aspects of the present invention described below. In addition, it was discovered that the effects are not impaired even when submerged hot galvanizing, annealing galvanizing, electro galvanizing and aluminization are performed on cold rolled steel sheet.
[0016] That is, according to a first aspect of the present invention, a cold rolled steel sheet is provided which contains, in mass%, C: more than 0.150% to 0.300%, Si: 0.010% to 1,000 %, Mn: 1.50% to 2.70%, P: 0.001% to 0.060%, S: 0.001% to 0.010%, N: 0.0005% to 0.0100% and Al: 0.010% to 0.050% and optionally containing one or more of B: 0.0005% to 0.0020%, Mo: 0.01% to 0.50%, Cr: 0.01% to 0.50%, V: 0.001% to 0.100% , Ti: 0.001% to 0.100%, Nb: 0.001% to 0.050%, Ni: 0.01% to 1.00%, Cu: 0.01% to 1.00%, Ca: 0.0005% to 0, 0050% and REM: 0.0005% up to 0.0050% and a remainder that includes Fe and unavoidable impurities, where, when an amount of C, an amount of Si and an amount of Mn are represented respectively by [C], [Si] and [Mn] in% of unit by mass, a ratio of the formula 1 below is satisfied, a metallographic structure contains, by proportion of area, 40% up to 90% ferrite and 10% up to 60% martensite, also contains one or more than 10% or less of per lite by area ratio, 5% or less of a retained austenite by volume ratio and 20% or less of a bainite by area ratio, the martensite hardness measured using a nanoindentator satisfies formulas 2a and 3a below and TS * À representing a TS product that is a tensile strength and À that is an orifice expansion ratio is 50000 MPa% or more. (5x [Si] + [Mn]) / [C]> 10 ■■■ (1) H20 / H10 <1.10 ■■■ (2a) oHM0 <20 ■■■ (3a)
[0017] In this case, H10 represents an average hardness of martensite on the surface part of the cold rolled steel sheet, H20 represents an average hardness of martensite in a central part of the thickness of a sheet that occupies a range of ± 100 pm of a center of thickness of the cold rolled steel sheet blade in one direction of the thickness and the oHMO represents the variance of the hardness of the martensite present in the range of ± 100 pm of the central part of the thickness of the plate in the direction of the thickness.
[0018] In cold rolled steel sheet according to (1) above, an area ratio of an MnS that is present in the metallographic structure and has an equivalent circle diameter in the range of 0.1 pm to 10 pm can be 0.01% or less and the following formula 4a can be satisfied. n20 / n10 <1.5 ■■■ (4a)
[0019] In this case, n10 represents an average density in number of MnS per 10,000 pm2 at 1/4 of the blade thickness of cold rolled steel sheet and n20 represents an average density in number of MnS per 10,000 pm2 in central part of the plate thickness.
[0020] In cold rolled steel sheet according to (1) before, additionally, after hot stamping which includes heating to a temperature in the range of 750 ^ up to WOO'C, work and cooling are carried out, the hardness of the martensite measured using a nanoindentator can satisfy formulas 2b and 3b below, the metallographic structure can contain 80% or more of a martensite by proportion of area, optionally also contain one or more than 10% or less of a perlite by proportion of area, 5% or less of a austenite retained by proportion by volume, less than 20% of a ferrite and less than 20% of a bainite by proportion of area and TSXÀ representing the product of TS which is the tensile strength and À which is the expansion ratio of the orifice can be 50,000 MPa% or more. H2 / HK1.10 (2b) oHM <20 ■■■ (3b)
[0021] In this case, H2 represents an average hardness of the martensite in the part of the surface after hot stamping, H2 represents an average hardness of the martensite in the central part of the sheet thickness after hot stamping and oHM represents the variance of the hardness of martensite present in the central part of the sheet thickness after hot stamping.
[0022] In cold rolled steel sheet according to (3) above, a proportion of MnS area that is present in the metallographic structure and has an equivalent circle diameter in the range of 0.1 pm to 10 pm can be 0.01% or less and the following formula 4b can be satisfied. n2 / n1 <1.5 ■■■ (4b)
[0023] In this case, n1 represents an average density in number of MnS per 10,000 pm2 at 1/4 of the thickness of the plate in the cold rolled steel sheet after hot stamping and n2 represents an average density in number of MnS per 10,000 pm2 in the central part of the sheet thickness after hot stamping.
[0024] In the cold-rolled steel sheet according to any one of (1) to (4) above, a hot-dip galvanized layer can also be formed on the surface of the cold-rolled steel sheet.
[0025] In the cold rolled steel sheet according to (5) before, the hot dip galvanized layer may include a galvanized and annealed layer.
[0026] In the cold rolled steel sheet according to any one of (1) to (4) above, a layer of electro galvanizing can also be formed on a surface of the cold rolled steel sheet.
[0027] In the cold-rolled steel sheet according to any of (1) to (4) above, an aluminization layer can also be formed on a surface of the cold-rolled steel sheet.
[0028] According to another aspect of the present invention, in this case a manufacturing process is provided for a cold-rolled steel sheet that includes a molten steel melting casting process that has the chemical components described above (1) and that produce steel; a heating process for heating steel; a hot rolling process to perform hot rolling on steel using a hot rolling installation that has a large number of stands; a winding process for winding the steel after the hot rolling process; a pickling process for pickling steel after the winding process; a cold rolling process for cold rolling steel after the pickling process using a cold rolling cylinder having a large number of bases under conditions where the following formula 5 is satisfied; an annealing process for heating to a temperature in a range of TOO'C to δδO'C and cooling the steel after the cold rolling process and a process of subjecting the steel to hardening rolling to perform the submission of the steel hardening lamination on steel after the annealing process. 1.5 * r1 / r + 1.2xr2 / r + r3 / r> 1.0 ■■■ (5)
[0029] In this case, r1 represents an individual target reduction by cold rolling on the umpteenth (ia) base from the topmost base among a large number of bases in the cold rolling process in% of unit in this case i is 1, 2 or 3 er represents a total reduction by cold rolling in the cold rolling process in% in units.
[0030] In the manufacturing process of obtaining a cold rolled steel sheet according to (9) above, when the winding temperature in the winding process is represented by CT in units of 'C and a quantity of C, a amount of Mn, amount of Si and amount of Mo of the steel are represented respectively by [C], [Mn], [Si] and [Mo]% of mass unit, the formula 6 below can be satisfied. 560-474x [C] -90x [Mn] -20x [Cr] -20x [Mo] <CT <830-27Qx [C] -90x [Mn] - 70x [Cr] -80x [Mo] ■■■ (6 )
[0031] In the manufacturing process of obtaining a cold rolled steel sheet according to (9) or (10) above, when the heating temperature in the heating process is represented by T in unit 13, a period of time po inside the oven is represented by t in minute units and an amount of Mn and an amount of S in the steel are respectively represented by [Mn] and [S] in unit of mass%; formula 7 below can be satisfied. Tx | n (t) / (1.7x [Mn] + [S])> 1500 ■■■ (7)
[0032] In the manufacturing process of obtaining a cold-rolled steel sheet according to any of (9) to (11) above, a hot dip galvanizing process to perform hot dip galvanizing on the steel can also be included between the annealing process and the process of subjecting the steel to hardening lamination.
[0033] In the manufacturing process of obtaining a cold-rolled steel sheet according to any of (9) to (12) above, an alloy treatment process of performing an alloy treatment on can also be included. steel between a hot dip galvanizing process and the process of subjecting steel to hardening lamination.
[0034] In the manufacturing process of obtaining a cold-rolled steel sheet according to any of (9) to (11) above, an electro galvanizing process to perform electro galvanizing on steel can be included afterwards the process of subjecting steel to hardening lamination.
[0035] In the manufacturing process of obtaining a cold-rolled steel sheet according to any of (9) to (11) above, an aluminization process for performing aluminization on steel can also be included among the annealing process and the process of subjecting steel to hardening lamination. [Effects of the Invention]
[0036] According to the aspect of the present invention, as long as an appropriate relationship is established between the amount of C, the amount of Mn and the amount of Si and martensite, an appropriate hardness measured using a nanoindentator is provided, it is possible obtain a cold rolled steel sheet that has a favorable orifice expandability. In addition, it is possible to obtain a cold-rolled steel sheet that has favorable orifice expandability even after hot stamping.
[0037] Meanwhile, cold rolled steel sheet according to (1) to (8) above and hot stamped steel made using cold rolled steel sheet manufactured according to (9) to ( 15) above have excellent plasticity. [Brief Description of the Drawing]
[0038] FIG. 1 is a graph that illustrates a relationship between (5x [Si] + [Mn]) / [C] and TSXÀ.
[0039] FIG. 2A is a graph that illustrates the basis of Formulas 2a, 2b, 3a and 3b and is a graph that illustrates the relationship between H20 / H10 and oHMO of a cold rolled steel sheet before hot stamping and a relationship between H2 / H1 and oHM of a cold rolled steel sheet after hot stamping.
[0040] FIG. 2B is a graph that illustrates the basis of Formulas 3a and 3b and is a graph that illustrates the relationship between oHMO before hot stamping and oHM after hot stamping and TSXÀ.
[0041] FIG. 3 is a graph illustrating the relationship between n20 / n10 of cold rolled steel sheet before hot stamping and n2 / n1 of cold rolled steel sheet after hot stamping and TSXÀ and illustrating the basis of Formulas 4a and 4b.
[0042] FIG. 4 is a graph illustrating the relationship between 1.5xr1 / r + 1.2xr2 / 2 + r3 / r H20 / H10 of the cold rolled steel sheet before hot stamping and H2 / H1 after hot stamping and that illustrates the basis of Formula 5.
[0043] FIG. 5A is a graph that illustrates the relationship between Formula 6 and a fraction of martensite.
[0044] FIG. 5B is a graph that illustrates the relationship between Formula 6 and a fraction of perlite.
[0045] FIG. 6 is a graph that illustrates the relationship between Tx | n (t) / (1,7x [Mn] + [S]) and TSxÀe that illustrates the basis of Formula 7.
[0046] FIG. 7 is a perspective view of a hot stamped steel (cold rolled steel sheet after hot stamping) used in an example.
[0047] FIG. 8 is a flow chart illustrating a manufacturing process for obtaining a cold rolled steel sheet according to an embodiment of the present invention. [Modalities of the Invention]
[0048] As described before, it is important to establish an appropriate relationship between the amounts of Si, Mn and C and, in addition, provide an appropriate hardness to martensite to predetermined parts in the steel plate to improve the expandability of the orifice. So far, in this case there have been no studies on the relationship between the plasticity of cold rolled steel sheet and the hardness of martensite both before and after hot stamping.
[0049] Hereinafter, an embodiment of the present invention will be described in detail.
[0050] First, a cold-rolled steel sheet according to an embodiment of the present invention will be described and the reasons for limiting the chemical components of steel used to manufacture the described cold-rolled steel sheet. Hereinafter, "%" which is the unit of the quantity of each component indicates "% by mass".
[0051] Meanwhile, in the present modality, for convenience, the cold rolled steel sheet that has not been subjected to hot stamping will be called, simply, cold rolled steel sheet, cold rolled steel sheet before stamping a hot or cold rolled steel sheet according to the modality and the cold rolled steel sheet that has been subjected to hot stamping (worked by hot stamping) will be called cold rolled steel sheet after hot stamping or sheet cold rolled steel after hot stamping according to the modality. C: more than 0.150% to 0.300%
[0052] C is an important element to reinforce ferrite and martensite and increase the strength of steel. However, when the amount of C is 0.150% or less, a sufficient amount of martensite cannot be obtained and it is not possible to increase the resistance sufficiently. On the other hand, when the amount of C exceeds 0.300%, the elongation or expandability of the orifice decreases significantly. So far, the C amount range has been adjusted to more than 0.150% and 0.300% or less. Si: 0.010% up to 1,000%
[0053] Si is an important element to suppress the generation of a harmful carbide and to obtain multifaces mainly including ferrite and martensite. However, when the amount of Si exceeds 1,000%, the elongation or expandability of the orifice decreases and the chemical conversion property also decreases. Therefore, the amount of Si is adjusted to 1,000% or less. In addition, Si is added for deoxidation, but the deoxidation effect is not sufficient for an amount of Si less than 0.010%. So far, the amount of Si is adjusted to 0.010% or more. Al: 0.010% to 0.050%
[0054] Al is an important element as a deoxidizing agent. To obtain the deoxidation effect, the amount of Al is adjusted to 0.010% or more. On the other hand, even when Al is added in excess, the effect described above is saturated and on the contrary, the steel becomes brittle and TS * À increases. Therefore, the amount of Al is adjusted in a range of 0.010% to 0.050%. Mn: 1.50% to 2.70%
[0055] Mn is an important element to improve hardenability and reinforce steel. However, when the amount of Mn is less than 1.50%, it is not possible to increase the resistance sufficiently. On the other hand, when the amount of Mn exceeds 2.70%, the temperability becomes excessive and the elongation or expandability of the orifice decreases. Therefore, the amount of Mn is adjusted up to 1.50% to 2.70%. In a case where greater elongation is required, the amount of Mn is desirably adjusted up to 2.00% or less. P: 0.001% to 0.060%
[0056] To a large amount, P secretes at the grain boundaries and deteriorates local elongation and weldability. Therefore, the amount of P is adjusted to 0.060% or less. The amount of P is desirably less, but an extreme decrease in the content of P leads to an increase in the cost of refining and, therefore, the amount of P is desirably adjusted up to 0.001% or greater. S: 0.001% to 0.010%
[0057] S is an element that forms MnS and significantly reduces local elongation or weldability. Therefore, the upper limit of the amount of S is adjusted up to 0.010%. In addition, the amount of S is desirably less; however, due to a refining cost problem, the lower limit on the amount of S is desirably adjusted up to 0.001%. N: 0.0005% to 0.0100%
[0058] N is an important element to precipitate AIN and the like and to miniaturize the crystal grains. However, when the amount of N exceeds 0.0100%, the solid solution with nitrogen remains and decreases the elongation or expandability of the orifice. Therefore, the amount of N is adjusted to 0.0100% or less. Meanwhile, the amount of N is desirably less; however, due to the problem of refining costs, the lower limit of the amount of N is desirably adjusted up to 0.0005%.
[0059] The cold-rolled steel sheet according to the modality has a basic composition that presents the components described above and a remainder of iron and unavoidable impurities, however they can also contain any one or more elements of Nb, Ti, V, Mo, Cr, Ca, REM (rare earth metal), Cu, Ni and B as elements that until now have been used in quantities of the upper or lower limit described below to improve the resistance, the control of the shape of a sulfide or a oxide and the like. The chemical elements described above are not always added to the steel plate and, therefore, their lower limit is 0%.
[0060] Nb, Ti and V are elements that precipitate fine carbonitride and reinforce steel. In addition, Mo and Cr are elements that improve hardenability and reinforce steel. To obtain the effects described above, it is desirable to contain 0.001% or more of Nb, 0.001% or more of Ti, 0.001% or more of V, 0.01% or more of Mo and 0.01% or more of Cr. However, even when more than 0.050% Nb, more than 0.100% Ti, more than 0.100% V, more than 0.50% Mo, and more than 0.50% Cr, the effect of increased resistance is saturated and elongation degradation or expandability of the orifice is caused. Therefore, the upper limits of Nb, Ti, V, Mo and Cr are adjusted up to 0.050%, 0.100%, 0.100%, 0.50% and 0.50%, respectively.
[0061] Steel can also contain Ca in the range of 0.0005% to 0.0050%. Ca controls the shape of a sulfide or an oxide and improves local elongation or orifice expansion. In order to obtain the effect described above, it is desirable to contain 0.0005% or more of Ca. However, when an excessive amount of Ca is contained, the "workability" deteriorates and, therefore, the upper limit of the Ca amount is adjusted to 0.0050%. For the same reason, the lower limit is adjusted up to 0.0005% and the upper limit of the rare earth element (REM) is set to 0.0050%.
[0062] Steel can also contain Cu in a range of 0.01% to 1.00%, Ni in a range of 0.01% to 1.00% and B in a range of 0.0005% to 0, 0020%. The elements described above can also improve the hardenability and increase the strength of the steel. However, to obtain the effect described above, it is desirable to contain 0.01% or more of Cu, 0.01% or more of Ni and 0.0005% or more of B. In the amounts described above or less, the effect reinforcing steel is small. On the other hand, even when more than 1.00% Cu, more than 1.00% Ni and more than 0.0020% B are added, the effect of increased strength is saturated and elongation or the expandability of the orifice decreases. Therefore, the upper limits for the amount of Cu, the amount of Ni and the amount of B are adjusted to 1.00%, 1.00% and 0.0020% respectively.
[0063] In a case where steel B, Mo, Cr, V, Ti, Nb, Ni, Cu, Ca and REM, at least one element is contained. The rest of the steel includes Fe and unavoidable impurities. Steel can also contain elements other than the elements described above (for example, Sn, As and the like), as the unavoidable impurities as well as the characteristics are not impaired. B, Mo, Cr, V, Ti, Nb, Ni, Cu, Ca and REM being contained in amounts less than the lower limits described above are treated as unavoidable impurities.
[0064] Meanwhile, as in this case there is no variation in the chemical components even after hot stamping, the chemical components still satisfy the ranges described above even on a steel plate after hot stamping.
[0065] In addition, in cold rolled steel sheet according to the modality and cold rolled steel sheet after hot stamping according to the modality, when the quantity of C (% by mass), the quantity of Si (% by mass) and the amount of Mn (% by mass) are represented by [C], [Si] and [Mn] respectively, it is important to satisfy the ratio of formula 1 below to obtain sufficient expandability of the orifice as illustrated in FIG. 1. (5x [Si] + [Mn]) / [C]> 10 ■■■ (1)
[0066] When the value of (5x [Si] + [Mn]) / [C] is 10 or less, TSXÀ becomes less than 50,000 MPa% and it is not possible to obtain sufficient expandability of the orifice. This is because, when the C content is high, the hardness of a hard phase becomes too high, the difference in the hardness of a soft phase becomes large and therefore the value of À decreases and, when the Si content or the Mn content is small, TS becomes low. Therefore, it is necessary to control the balance between the quantities of the respective elements also to contain the elements in the ranges described above. The value of (5x [Si] + [Mn]) / [C] does not vary due to lamination or hot stamping. However, even when (5x [Si] + [Mn]) / [C]> 10 is satisfied, in a case where the martensite hardness ratio described below (H20 / H10, H2 / H1) or the dispersion of martensite hardness (oHMO, oHM) does not meet the conditions, sufficient expandability of the hole in cold rolled steel sheet or cold rolled steel sheet cannot be obtained after hot stamping.
[0067] Next, the reason for limiting the metallographic structure of the cold-rolled steel sheet according to the modality and the cold-rolled steel sheet after the hot stamping according to the modality will be described.
[0068] Generally speaking, in cold-rolled steel sheet that has a metallographic structure that includes mainly ferrite and martensite, the dominant factor for plasticity such as orifice expansion is martensite instead of ferrite. The inventors carried out intensive studies regarding the relationship between the martensite's hardness and plasticity such as elongation or expandability of the orifice. As a result, it was found that, as illustrated in FIGS. 2A and 2B, the plasticity such as elongation or expandability of the orifice becomes favorable provided that the proportion of the hardness (difference in hardness) of the martensite between the part of the surface of the sheet thickness and the central part of the sheet thickness and the distribution of the hardness of the martensite in the central part of the sheet thickness are in predetermined states both in cold rolled steel sheet and in cold rolled steel sheet after hot stamping. In addition, it was found that the hardness ratio of martensite and the hardness distribution of martensite on cold rolled steel sheet before hot stamping rarely varied on cold rolled steel sheet after hot stamping obtained by tempering through hot stamping on a cold-rolled steel sheet that has favorable plasticity and consequently, plasticity such as elongation or expandability of the orifice was favorable. This is because the distribution of hardness of the martensite generated in the cold rolled steel sheet before hot stamping still has a significant effect even after hot stamping. Specifically, it is considered to be because the alloy elements concentrated in the central part of the sheet thickness remain in the central part of the sheet thickness in a concentrated state even after hot stamping. That is, in a case where the proportion of the hardness of the martensite between the part of the surface of the sheet thickness and the central part of the sheet thickness is large or a case in which the variance of the hardness of the martensite in the central part of the thickness of the sheet plate is large, the same proportion of hardness and the same variance are obtained even after hot stamping.
[0069] In addition, regarding the martensite hardness measurement measured at a 1000-fold increase using a nanoindentador manufactured by Hysitron Corporation, the inventors found that, in cold rolled steel plate before hot stamping, the plasticity was improved by satisfying Formulas 2a and 3a below. In addition, in relation to the relationships described above, the inventors found that, the cold rolled steel sheet after hot stamping, similarly, the plasticity was improved by satisfying Formulas 2b and 3b below. H20 / H10 <1.10 ■■■ (2a) oHM0 <20 ■■■ (3a) H2 / HK1.10 ■■■ (2b) σHM <20 ■■■ (3b)
[0070] In this case, H10 represents the hardness of the martensite on a part of the sheet thickness of the cold-rolled steel sheet before hot stamping which is 200 pm or less from the outermost layer in the thickness direction. H20 represents the hardness of the martensite in the central part of the blade thickness of the cold-rolled steel sheet before hot stamping, that is, the martensite in a range of ± 100 pm from the center of the sheet thickness in the direction of the thickness. σHMO represents the variance of martensite hardness present in the range of ± 100 pm from the center of the blade thickness of the cold rolled steel sheet before hot stamping in the direction of the thickness.
[0071] In addition, H1 represents the hardness of the martensite on a part of the sheet thickness of the cold-rolled steel sheet after hot stamping which is 200 pm or less from the outermost layer in the direction of the thickness. H2 represents the hardness of the martensite in the central part of the blade thickness of the cold-rolled steel sheet after hot stamping, that is, the martensite in a range of ± 100 pm from the center of the sheet thickness in the direction of the thickness. oHM represents the variance in the hardness of the martensite present in the range of ± 100 pm from the center of the blade thickness of the cold rolled steel sheet after hot stamping in the direction of the thickness.
[0072] Hardness is measured at 300 points for each. The range of ± 100 pm from the center of the sheet thickness in the direction of the thickness refers to a strip that has a center in the center of the sheet thickness and that has a size of 200 pm in the direction of the thickness.
[0073] In addition, the variance of oHMO or oHM hardness is obtained using formula 8 below and indicates the hardness distribution of martensite. Meanwhile, oHM in the formula represents oHMO and is expressed as oHM. [Formula 1]

[0074] Xave represents the average hardness value of the measured martensite and X, represents the hardness of ia. martensite. In the meantime, the formula is still valid even when oHM is replaced by oHMO.
[0075] FIG. 2A illustrates the proportions between the hardness of the martensite on one part of the surface and the hardness of the martensite on the central part of the plate thickness on the cold rolled steel plate before hot stamping and on the cold rolled steel plate after the stamping a hot. In addition, FIG. 2B collectively illustrates the variances in martensite hardness present in the range of ± 100 pm from the center of the sheet thickness towards the thickness of the cold rolled steel sheet before hot stamping and the cold rolled steel sheet after stamping the hot. As illustrated in FIGS. 2A and 2B, the hardness ratio of the cold rolled steel sheet before hot stamping and the hardness ratio of the cold rolled steel sheet after hot stamping are almost the same. Furthermore, the variances in the hardness of the martensite in the central part of the sheet thickness are also almost the same both in cold rolled steel sheet before hot stamping and in cold rolled steel sheet after hot stamping. Therefore, it is found that the plasticity of the cold rolled steel sheet after hot stamping is as excellent as the plasticity of the cold rolled steel sheet before hot stamping.
[0076] The value of H20 / H10 or H2 / H1 which is 1.10 or more indicates that, on cold rolled steel sheet before hot stamping or on cold rolled steel sheet after hot stamping , the hardness of the martensite in the central part of the sheet thickness is 1.10 or more times the hardness of the martensite in a part of the sheet thickness surface. That is, the value that the hardness in the central part of the sheet thickness becomes too high. As illustrated in FIG. 2A, when H20 / H10 is 1.10 or more, oHMO reaches 20 or more and, when H2 / H1 is 1.10 or more, oHM reaches 20 or more. In this case, TSXÀ becomes less than 50,000 MPa%, sufficient plasticity is not obtained both before quenching (that is, before hot stamping) and after quenching (that is, after hot stamping). In addition, theoretically, in this case there is a case in which the lower limits of H20 / H10 and H2 / H1 are the same in the central part of the sheet thickness and in the part of the surface of the sheet thickness provided that no heat treatment is carried out. Special; however, in a real production process that considers productivity, the lower limits are, for example, up to approximately 1.005.
[0077] The variance of oHMO or oHM being 20 or more indicates that, in cold rolled steel sheet before hot stamping and in cold rolled steel sheet after hot stamping, in this case it is a great inequality in hardness of the martensite and in this case are local parts that have an excessively high hardness. In this case, TSXÀ becomes less than 50,000 MPa-% and not enough plasticity is obtained.
[0078] Next, the metallographic structure of the cold rolled steel sheet according to the modality (before hot stamping) and the cold rolled steel sheet after hot stamping according to the modality will be described.
[0079] In the metallographic structure of the cold-rolled steel sheet according to the modality, the proportion of the ferrite area is in a range of 40% to 90%. When the proportion of the ferrite area is less than 40%, the resistance becomes too high even before hot stamping such that in this case there is a case where the shape of the steel plate deteriorates or cutting becomes difficult . Therefore, the proportion of the ferrite area is adjusted up to 40% or more. On the other hand, in cold rolled steel sheets according to the modality, as a large amount of alloying elements are added, it is difficult to adjust the proportion of the ferrite area to more than 90%. The metallographic structure includes not only ferrite, but also martensite and the proportion of the area of the martensite is in the range of 10% to 60%. The sum of the proportion of the ferrite area and the proportion of the martensite area is desirably 60% or more. The metallographic structure can also include one or more of pearlite, bainite and retained austenite. However, when the retained austenite remains in the metallographic structure, the characteristics of secondary work and delayed fracture are likely to degrade and, therefore, it is preferable that the metallographic structure does not substantially include retained austenite. However, inevitably, retained austenite can be included in the volume ratio of 5% or less. As perlite is a hard and brittle structure, the metallographic structure preferably does not include perlite; however, inevitably, perlite can be included in an area proportion of up to 10%. Bainite is a structure that can be generated as a residual structure and is an intermediate structure in terms of strength or plasticity. The absence of bainite makes no difference, but the metallographic structure can include up to 20% bainite by proportion of area. In one modality, in relation to the metallographic structure, ferrite, bainite and perlite were observed through chemical attack with Nital and martensite was observed through chemical attack with Lepera. All structures were observed at 1/4 part of the plate thickness with a 1000-fold magnification using an optical microscope. For retained austenite, the volume fraction was measured using an X-ray diffraction apparatus after polishing a steel plate to a quarter of the depth position in the thickness.
[0080] In the metallographic structure of the cold-rolled steel sheet after hot stamping according to the modality, the proportion of the martensite area is 80% or more. When the proportion of the martensite area is less than 80%, sufficient strength required for a recent hot stamped steel (for example, 1.5 GPa or more) cannot be obtained. Therefore, the proportion of the martensite area is desirably adjusted up to 80% or more. All or the main parts of the cold rolled steel sheet metallographic structure after hot stamping are occupied by martensite, but in this case there is a case where the remaining metallographic structure includes one or more than 10% or less of perlite by proportion area, 5% or less of austenite retained by volume ratio, less than 20% ferrite by area ratio and less than 20% bainite by area ratio. Ferrite is present in a range from 0% to less than 20% depending on the conditions of hot stamping and in this case there is no problem with the strength after hot stamping as long as the ferrite is contained in the range described above. In addition, when the retained austenite remains in a metallographic structure, the fragility characteristics during secondary work and the delayed fracture are prone to degrade. Therefore, it is preferable that the metallographic structure does not substantially include retained austenite; however, inevitably, retained austenite can be included in a volume ratio of 5% or less. As perlite is a hard and brittle structure, the metallographic structure preferably does not include perlite; however, inevitably, pearlite can be included in an area proportion of up to 10%. For the same reason, the metallographic structure can include up to 20% bainite by proportion of area. Similar to the case of cold rolled steel plate before hot stamping, metallographic structures were observed in 1/4 of the part of the plate thickness at a 1000-fold increase using an optical microscope after the chemical attack reaction with Nital for ferrite, bainite and perlite and the chemical attack with Lepera for martensite. For the retained austenite, the volume fraction was measured using an X-ray diffraction apparatus after polishing the steel sheet to a position of a quarter depth in thickness.
[0081] Meanwhile, hot stamping can be carried out according to a conventional method, for example, it can include heating up to a temperature in the range of TδO'C to WOO'C, work and cooling.
[0082] In one embodiment, the hardness of the martensite measured on the cold-rolled steel sheet before hot stamping and on the cold-rolled steel sheet after hot stamping using a nanoindentator with a 1000-fold increase (hardness notched (GPa or N / mm2) or the Vickers hardness value (Hv) converted from notched hardness) is specified. In a common Vickers hardness test, a notch larger than martensite is formed. Therefore, the macroscopic hardness of the martensite and its peripheral structures (of ferrite and the like) can be obtained, but it is not possible to obtain the martensite's own hardness. Since plasticity, such as orifice expansion, is significantly affected by the martensite's own hardness, it is difficult to sufficiently assess plasticity with Vickers' hardness alone. On the contrary, in the modality, as a proportion of hardness and dispersion state measured using a nanoindentator are controlled in an appropriate range, it is possible to obtain an extremely favorable plasticity.
[0083] MnS was observed in the position of a quarter of depth in the thickness (a quarter of the location in the depth of the plate thickness starting from the surface) and in the central part of the blade thickness of the cold-rolled steel plate according to the modality . As a result, it was found that the proportion of the MnS area having an equivalent circle diameter in the range of 0.1 pm to 10 pm was 0.01% or less and, as illustrated in FIG. 3, it is preferable to satisfy formula 4a below to satisfy TS * À> 50000 MPa% favorably and stably. This is considered because when MnS that has an equivalent circle diameter of 0.1 pm is present in an orifice expansion test, the stress is concentrated around MnS and cracking is likely to occur. The reason for not taking into account MnS which has an equivalent circle diameter less than 0.1 pm is that such MnS has a small effect on the concentration of effort. On the other hand, the MnS that is greater than 10 pm is too large and is therefore unsuitable for the job. In addition, when the proportion of the MnS area in a range from 0.1 pm to 10 pm exceeds 0.01%, it becomes easy for fine cracks to be generated due to the spread of stress concentration. Therefore, in this case it is a case in which the expansion of the orifice decreases. n20 / n10 <1.5 ■■■ (4a)
[0084] In this case, n10 represents the number density (grains / 10000 pm2) of MnS that has an equivalent circle diameter in the range of 0.1 pm to 10 pm per unit area (10000 pm2) at 1/4 part of the blade thickness of the cold rolled steel sheet before hot stamping. n20 represents the density in number (mean density in number) of MnS which has an equivalent circle diameter in the range of 0.1 pm to 10 pm per unit area in the central part of the blade thickness of cold rolled steel sheet before hot stamping.
[0085] In addition, the inventors observed MnS at a quarter of the depth position in the thickness (a location at a quarter of the thickness of the sheet in depth starting from the surface) and the center part of the blade thickness of the cold-rolled steel sheet after hot stamping according to the modality. As a result, it was found that, similar to cold rolled steel plate before hot stamping, the proportion of the MnS area that has an equivalent circle diameter in the range of 0.1 pm to 10 pm was 0, 01% or less and, as illustrated in FIG. 3, it is preferable to satisfy the following formula 4b to satisfy TS * À> 50000 MPa% favorably and stably. n2 / n1 <1.5 ■■■ (4b)
[0086] In this case, n1 represents the density of the number of MnS that has an equivalent circle diameter in the range of 0.1 pm to 10 pm per unit area in 1/4 of the part of the blade thickness of the steel plate cold rolled after hot stamping. n2 represents the number density (mean density in number) of MnS which has an equivalent circle diameter in the range of 0.1 pm to 10 pm per unit area in the central part of the blade thickness of cold rolled steel sheet after hot stamping.
[0087] When the proportion of the MnS area that has an equivalent circle diameter in the range of 0.1 pm to 10 pm is more than 0.01%, as described above, plasticity is likely to degrade due to concentration of effort. The lower limit of the proportion of the MnS area is not particularly specified, however 0.0001% or more of MnS may be present due to the limitation of the measurement method, increase and visual field described above, the desulfurization treatment capacity and the amount original Mn or S.
[0088] On the other hand, the value of n20 / n10 or n2 / n1 being 1.5 or more indicates that the density of the number of MnS in the central part of the plate thickness in the cold rolled steel plate before stamping hot or on the laminated steel plate after hot stamping is 1.5 times or more the number density of MnS in 1/4 part of the plate thickness. In this case, the plasticity is likely to degrade due to the segregation of MnS in the central part of the plate thickness.
[0089] In one embodiment, the equivalent diameter of the circle and the density of the number of MnS were measured using a field emission scanning electron microscope (Fe-SEM) manufactured by JEOL Ltd. The increase was 1000 times and the measurement area of the visual field was adjusted to 0.12x0.09 mm2 (= 10800 pm2 = 10000 pm2). Observation was performed at 10 visual fields at the site at a quarter of the thickness of the plate in depth starting from the surface (1/4 part of the plate thickness) and at 10 visual fields at the central part of the plate thickness. The proportion of the MnS area was computed using a computer program for particle analysis. In one embodiment, MnS was observed on cold rolled steel sheet before hot stamping and on cold rolled steel sheet after hot stamping, the MnS shape on cold rolled steel sheet after hot stamping rarely its shape (shape and number) of MnS varied on cold rolled steel plate before hot stamping. FIG. 3 is a view illustrating the relationship between n20 / n10 of the cold rolled steel sheet before hot stamping and n2 / n1 of the cold rolled steel sheet after hot stamping and TS * À. It was found that n20 / n10 of the cold rolled steel sheet before hot stamping and n2 / n1 of the cold rolled steel sheet after hot stamping are almost coincident. This is because of the shape of MnS that does not vary at the standard hot stamping heating temperature.
[0090] The cold rolled steel sheet according to the modality has excellent plasticity. In addition, the cold rolled steel sheet after the hot stamping obtained by performing the hot stamping on the cold rolled steel sheet described above has a tensile strength in the range of 1500 MPa (1.5 GPa) up to 2200 MPa and exhibits excellent plasticity. A significant effect that improves plasticity compared to that of the related technique's cold-rolled steel sheet is obtained particularly and a high strength in a range of approximately 1800 MPa to 2000 MPa.
[0091] It is preferable to perform galvanizing, for example, hot dip galvanizing, galvanizing and annealing, electro galvanizing or aluminizing on the surfaces of the cold-rolled steel sheet according to the modality and of the cold-rolled steel sheet after of hot stamping according to the modality in terms of prevention and rust. The electrodeposition described above does not affect the effects of the modality. The electrodeposition described above can be performed using a well-known method.
[0092] Hereinafter, a process for manufacturing cold rolled steel sheet according to the modality will be described. When the cold-rolled steel sheet is manufactured according to the modality, as a common condition, the molten steel melts so that the chemical components described above are melted continuously after a converter, thereby producing a plate. During continuous casting, when the casting speed is too high, the precipitates of Ti and the like become too fine. On the other hand, when the casting speed is slow, productivity deteriorates and the precipitates described above increase in size and the number of particles decreases such that in this case there is a case where the cold rolled steel sheet gets a shape in other features and thus delayed fractures cannot be controlled. Therefore, the casting speed is desirably adjusted in a range from 1.0 m / minute to 2.5 m / minute.
[095] When the value of Tx | n (t) / (1,7x [Mn] + [S]) is 1500 or less, the proportion of the MnS area becomes large and in this case it is a case in which the difference becomes large between the number of MnS in 1/4 part of the plate thickness and the number of MnS in the central part of the plate thickness.
[096] The plate after melting and casting can be hot rolled as a casting. Alternatively, in a case where the plate has been cooled to a temperature below 1100C, it is possible to reheat the plate in a tunnel oven or similar to a temperature in the range of HOO'C to 1300C and then hot-laminate the plate . When the plate temperature during hot rolling is less than 1100C, it is difficult to guarantee the finishing temperature during hot rolling, which causes elongation degradation. In addition, on a steel plate to which TiNb is added, the precipitates are not sufficiently dissolved during heating and therefore the resistance decreases. On the other hand, when the temperature of the plate is higher than 130013, in this case it is a concern that some scale may be generated and it may be impossible to obtain a favorable quality on the surface of the steel plate.
[0093] In addition, to decrease the proportion of the MnS area, when the amount of Mn (% by mass) and the amount of S (% by mass) of steel are represented by [Mn] and [S], respectively, It is preferable that the temperature T (13) of the heating furnace, the time inside the furnace t (minutes), [Mn] and [S] before hot rolling meet the formula 7 below. Tx | n (t) / (1.7x [Mn] + [S])> 1500 ■■■ (7)
[0094] When the value of Tx | n (t) / (1,7x [Mn] + [S]) is 1500 or less, the proportion of the MnS area becomes large and in this case it is a case in which the difference becomes large between the number of MnS in 1/4 part of the thickness of the MnS plate and the number of MnS in the central part of the thickness of the plate. Meanwhile, the temperature of the heating oven before hot rolling refers to the extraction temperature on the outlet side of the heating oven and the time inside the oven refers to the time that has elapsed since the plate was inserted into the heating oven. for hot rolling up to the extraction of the heating oven plate. Since the MnS does not vary due to lamination or hot stamping as described above, formula 7 is preferably satisfied during the heating of the plate. Meanwhile, the In described above represents a natural logarithm.
[0095] Next, hot rolling is carried out according to a conventional method. On this occasion, it is desirable to perform the hot lamination on the plate with the finishing temperature (the temperature at which the hot lamination ends) adjusted in a temperature range Ar3 to 97013. When the finishing temperature is lower than the temperature Ar3, in this case there is a concern that the lamination can be carried out in a region of two phases of ferrite (a) and austenite (y) and the elongation may degrade. On the other hand, when the finishing temperature is higher than θTO'C, the size of the austenite grain to one hundred and the fraction of ferrite becomes small and, therefore, in this case there is a concern that the elongation may degrade .
[0096] The Ar3 temperature can be obtained by performing a formastor test, which measures the variation in the length of a specimen in response to a temperature variation and which evaluates the temperature at the inflection point.
[0097] After hot rolling, the steel is cooled at a cooling rate measured in a range of 20'C / second to 500 ° C / second and is wound at a predetermined CTC winding temperature. In a case where the cooling rate is less than 20'C / second, it is likely that the perlite will be generated which causes the elongation to degrade, which is not preferable.
[0098] On the other hand, the upper limit of the cooling rate is not particularly specified, however the upper limit of the cooling rate is desirably adjusted up to around δOO'C / second from the point of view of the installation specification, however the upper limit is not limited in this case.
[0099] After cooling, pickling and cold lamination are performed. On this occasion, as illustrated in FIG. 4, cold rolling is carried out under conditions in which formula 5 below is satisfied to obtain a range that satisfies formula 2a described above. When the annealing, cooling and similar conditions described below have also been satisfied after the rolling described above is carried out, the cold rolled steel sheet is obtained in which TSxÀ> 50000 MPa% is satisfied. In addition, the cold rolled steel sheet still satisfies TSXÀ> 50000 MPa% even after hot stamping which includes heating to a temperature in the range of 75CTC to WOO'C, with work and cooling being carried out. Cold rolling is desirably carried out using a series rolling roller in which a steel sheet is rolled continuously in a single direction through a large number of linearly arranged rolling mills, thereby obtaining a predetermined thickness. 1.5xr1 / r + 1.2xr2 / r + r3 / r> 1.0 ■■■ (5)
[00100] In this case, ri (i = 1, 2 or 3) represents the individual target reduction by cold rolling (%) in the ia. (i = 1,2 or 3) base from the base in the upper position in the cold rolling described above and r represents the total reduction by cold rolling reduction (%) in the cold rolling described above. The total reduction by lamination is called the cumulative lamination reduction and is the percentage of the amount of reduction by cumulative lamination in relation to the criterion of the plate thickness at the entrance of the first pass (the difference between the thickness of the plate at the entrance before the first pass and the thickness of the plate at the exit after the final passage).
[00101] When cold rolling is carried out under conditions where formula 5 described above is satisfied, it is possible to divide the perlite sufficiently during cold rolling even when large perlite is present before cold rolling. As a result, it is possible to burn the perlite or suppress the proportion of the perlite area to the minimum extent by annealing performed after cold rolling. Therefore, it is easy to obtain a structure in which Formulas 2 and 3 are satisfied. On the other hand, in a case where formula 5 is not met, reductions by cold rolling at the upper bases of the chain are not sufficient and it is likely that the large pearlite will remain. As a result, it is not possible to generate martensite that has a desired shape in the annealing process.
[00102] In addition, the inventors found that, on cold rolled steel sheet that was subjected to a rolling process that satisfies formula 5, it was possible to maintain the shape of the martensite obtained after annealing (proportion of hardness and variance) almost at same condition even after hot stamping and the cold rolled steel sheet becomes advantageous in terms of elongation or expandability of the orifice even after hot stamping. In a case where the cold rolled steel sheet according to the modality is heated to a region of austenite by means of hot stamping, the hard phase that includes martensite is transformed into an austenite that has a high concentration of C and the ferrite phase becomes austenite, which has a low concentration of C. When the cold-rolled steel sheet is cooled afterwards, austenite becomes a hard phase that includes martensite. That is, when formula 5 is satisfied in order to obtain H20 / H10 described above in a predetermined range, H20 / H10 is maintained even after hot stamping and thus H2 / H1 is obtained in a predetermined range - determined and the cold rolled steel sheet becomes excellent in terms of plasticity after hot stamping.
[00103] In a case in which the hot stamping is performed on the cold-rolled steel plate according to the modality, when heating is carried out at a temperature in the range of 750 * 0 to 1000 * 0, work and cooling of According to a conventional method, excellent plasticity is displayed even after hot stamping. For example, hot stamping is desirably carried out under the following conditions. First, the cold rolled steel sheet is heated to a temperature in the range of 750 * 0 to 1000 * 0 at a rate of temperature increase from 5 * O / second to 500 * 0 / second and is worked (shape da) for one second up to 120 seconds. In order to obtain great resistance, the heating temperature is preferably higher than the point Ac3. The point Ac3 can be obtained by performing a formastor test, which measures the variation in the length of a specimen in response to a temperature variation and evaluating the temperature of the inflection point. After work, the cold rolled steel sheet is preferably cooled to, for example, a temperature in a range from room temperature to 300C at a cooling rate of WC / second to WOOC / second.
[00104] When the heating temperature is below 750C, the martensite fraction is insufficient and in this case there is a concern that it may be impossible to guarantee the resistance. On the other hand, when the heating temperature is above 1000C, the structure becomes too soft and, in a case where the surface of the steel sheet is electrically deposited, in particular, it is electrically deposited with zinc, in this case there is a concern that zinc can be evaporated and burned, which is not preferable. Therefore, the hot stamping heating temperature is preferably in the range of 750C to WOO'C. When the rate of temperature rise is less than 5C / second, control is difficult and productivity is significantly impaired and, therefore, cold rolled steel sheet is preferably heated to a rate of temperature rise of 5C / second or greater. Meanwhile, in this case there is no need to limit the upper limit of the rate of temperature increase; however, when the current heating capacity is taken into account, the upper limit of the temperature rise rate is desirably adjusted to 500C / second. When the cooling rate after work is less than WC / second, speed control is difficult and productivity is significantly impaired. Meanwhile, in this case there is no need to limit the upper limit of the cooling rate; however, when current heating capacity is taken into account, the upper limit of the cooling rate is desirably adjusted to 10OCOCTC / second. The reason for setting a desirable time elapsed until hot stamping after a temperature rise in a range from 1 second to 120 seconds is to avoid evaporation of zinc or the like in a case where the surface of the steel plate galvanized or similar. The reason for a desirable temperature for interrupting cooling in an ambient temperature range up to SOO'C is to guarantee resistance after hot stamping by ensuring a sufficient amount of martensite.
[00105] In one embodiment, r, r1, r2 and r3 represent target reductions by cold rolling. In general, a sheet of steel is cold rolled with a control in order to obtain almost the same value as the real reduction by cold rolling. It is not preferable to perform cold rolling with a real reduction by cold rolling unnecessarily deviated from the target reduction by cold rolling. In a case where there is a big difference between the target reduction by rolling and real reduction by rolling, it is possible to consider that the cold rolled steel sheet is a modality of the present invention as long as the real reduction by rolling meets the formula 5 described above. The actual reduction by cold rolling is preferably converged within a range of ± 10% of the target reduction by cold rolling.
[00106] After cold rolling, annealing is carried out. Annealing causes recrystallization in the steel plate and generates the desired martensite. During annealing, it is preferable, according to a conventional method, to heat the steel sheet to a temperature range from TOO'C to δδO'C and cool the steel sheet to room temperature or at the temperature at which it is carried out. surface treatment such as hot dip galvanizing. When annealing is carried out in the temperature range described above, the predetermined area proportions of ferrite and martensite are obtained and the sum of the proportion of the area of the ferrite and the proportion of the area of the martensite reaches 60% or more and, therefore, TS * Improvement. Conditions other than the annealing temperature are not particularly specified; however, to reliably guarantee a predetermined structure, possibly a predetermined structure, the retention time at a temperature in the range of 700C to 850C is preferably adjusted to 1 second or more, for example, approximately 10 minutes within the scope where productivity is not impaired. The rate of temperature rise is preferably determined as appropriate in a range of 1C / second to the upper limit of the installation's capacity, for example, δOOC / second and the cooling rate is preferably determined as appropriate in a range of 1C / second for the upper limit of the installation's capacity, for example, δOO'C / second.
[0112] After annealing, it is carried out to subject the steel to hardening lamination on the steel. The submission of steel to hardening lamination can be carried out according to a conventional method. The proportion of elongation of the submission of steel to hardening lamination in general is in a range of approximately 0.2% to 5% and a proportion of elongation in which an elasticity limit can be avoided and the shape of the steel sheet can be avoided. being corrected is preferable.
[0113] As an even more preferable condition of the present invention, when the amount of C (% by mass), the amount of Mn (% by mass), the amount of Si (% by mass) and the amount of Mo (% in mass) of steel are represented respectively by [C], [Mn], [Si] and [Mo], the winding temperature CT in the winding process preferably satisfies the following formula 6. 560-474x [C] -90x [Mn] -20x [Cr] -20x [Mo] <CT <830-270x [C] -90x [Mn] - 70x [Cr] -80x [Mo] ■■■ (6 )
[0114] As illustrated in FIG. 5A, when the winding temperature CT is less than 560-474x [C] -90x [Mn] -20x [Cr] - 20x [Mo], that is, CT- (560-474x [C] -90x [Mn ] -20x [Cr] -20x [Mo]) is less than zero, an excessive amount of martensite is generated and the steel sheet becomes too hard such that in this case there is a case where the subsequent cold rolling becomes difficult. On the other hand, as illustrated in FIG. 5B, when the CT winding temperature is greater than 830-270x [C] -90x [Mn] -70x [Cr] -80x [Mo], that is, CT- (830-270x [C] -90x [Mn ] -70x [Cr] -80x [Mo]) is greater than zero, it becomes likely that a strip-like structure that includes ferrite and perlite will be generated. In addition, it is likely that the fraction of perlite in the central part of the plate thickness will become high. Therefore, the uniformity of the distribution of martensite that is generated during the subsequent annealing process is degraded and it becomes difficult to satisfy the formula 2A described above. In addition, in this case there is a case where it is difficult for a sufficient amount of martensite to be generated.
[0115] When formula 6 is satisfied, the distribution of the ferrite and the hard phase becomes ideal on the cold rolled steel plate before the hot stamping as described before. Furthermore, in this case, C and the like diffuse easily evenly after heating and cooling through hot stamping. Therefore, the hardness distribution of the martensite becomes approximately ideal even after cooling is carried out. That is, as long as it is possible to more reliably guarantee the metallographic structure described above by satisfying formula 6, plasticity becomes excellent in both cases before and after hot stamping.
[00107] Furthermore, in order to improve the ability to prevent rust formation, it is preferable to provide a hot dip galvanizing process in which hot dip galvanizing is carried out between the annealing process described above and the process of subjecting the steel to the hardening lamination described above and carrying out a hot dip galvanizing process on the surface of the cold rolled steel sheet. In addition, it is also preferable to provide an alloy formation treatment process in which an alloy formation treatment is carried out between the hot dip galvanizing process and a process of subjecting the steel to hardening lamination to obtain a galvanized plate and annealed by forming an alloy on a hot dip galvanized plate. In a case where an alloying treatment is carried out, a treatment can also be carried out on the surface of the galvanized and annealed plate in which the surface is brought into contact with a substance that oxidizes the plate surface such as steam. 'water, thereby thickening an oxidized film.
[00108] It is also preferable to provide, for example, an electro galvanizing process in which electro galvanizing is carried out on the surface of the cold rolled steel sheet after the process of subjecting the steel to hardening lamination in addition to the dip galvanizing process hot and alloy formation treatment process. In addition, it is also preferable to provide, instead of hot dip galvanizing, an aluminization process in which the aluminization is carried out between the annealing process and a process of subjecting the steel to hardening lamination and performing the aluminization on the plate surface. cold rolled steel. Aluminization is generally and preferably hot-dip aluminum plating with immersion.
[00109] As described before, when the conditions are met, it is possible to manufacture a cold-rolled steel sheet that guarantees the resistance and exhibits a more favorable orifice expandability. In addition, the distribution of hardness or structure is maintained even after hot stamping so that strength is guaranteed and a more favorable orifice expandability is obtained even after hot stamping.
[00110] Meanwhile, FIG. 8 illustrates a gram flow (Processes S1 to S9 and Processes S11 to S14) of an example of the manufacturing process described above. [Example]
[00111] The steel that has the components described in Table 1 was melted continuously at a melting rate in a range of 1.0 m / minute to 2.5 m / minute, a plate was heated in a heating oven under the conditions in Table 2 according to a conventional method such as melted or after cooling the steel once and hot rolling was carried out at a finishing temperature in the range of 910 ° C to θδO'C, thereby producing a sheet of hot rolled steel. Thereafter, the hot rolled steel sheet was wound at a CT winding temperature described in Table 2. After that, the scale on the steel sheet surface was removed by picketing and a sheet thickness in a strip was obtained. from 1.2 mm to 1.4 mm by cold rolling. On this occasion, cold rolling was performed so that the value of formula 5 became the value described in Table 2. After cold rolling, annealing was carried out in a continuous oven for annealing at the annealing temperature described in the Tables 3 and 4. In some of the steel sheets, hot dip galvanizing was carried out in the middle of cooling after immersion in the continuous annealing furnace and then an alloy formation treatment was also carried out on some of the hot galvanized steel sheets. with immersion, thereby galvanizing and annealing. In addition, electro galvanizing or aluminization was performed on some steel sheets. The steel was subjected to hardening lamination at an elongation ratio of 1% according to a conventional method. In this state, a sample was taken to assess the material qualities of the cold rolled steel sheet (before hot stamping) and a test was carried out on the quality of the material or similar. Thereafter, to investigate the characteristics of the cold-rolled steel sheet after hot stamping, hot stamping was carried out in which the cold-rolled steel sheet was heated to a rate of temperature increase in a range of WG / second up to WOG / second at the heat treatment temperature of Tables 5 and 6, maintained for 10 seconds and cooled to 200 * 0 or at a lower temperature at a cooling rate of 1000 / second, thereby obtaining a stamped steel hot runner having a shape as illustrated in FIG. 7. A sample was cut from a location on the hot-stamped steel obtained shown in FIG. 7, a test was performed on the quality of the material and an observation of the structure and the fractions of the respective structures were obtained, the density of number of MnS, the hardness, the tensile strength (TS), the elongation (El), the orifice expansion ratio (À). The results are described in Tables 3 to 8. The expansion ratio of holes A in Tables 3 to 6 were obtained using the formula 11 below. À (%) = {(d’-d) / d} x100 ■■■ (Formula 11)
[00112] d ': diameter of the hole when the cracks penetrate the plate
[00113] d: the initial diameter of the hole
[00114] Considering the types of deposition in Tables 5 and 6, CR represents a non-deposited cold-rolled steel plate. Gl represents a hot-dip galvanized cold-rolled steel sheet, GA represents a galvanized and annealed cold-rolled steel sheet, EG represents an electro-galvanized cold-rolled steel sheet and Al represents an aluminized cold-rolled steel sheet.
[00115] The amount of "0" in Table 1 indicates that the amount is equal to or less than the lower limit of the measure.
[00116] Determinations G and B in Tables 2, 7 and 8 respectively have the following meanings.
[00117] G: the condition of the target formula is satisfied.
[00118] B: the condition of the target formula is not satisfied. [Table 1]



[Table 2]

[Table 3]
[Table 4]
[Table 5]

[Table 6]

[Table 7]
[Table 8]

[00119] It is discovered by Tables 1 to 8 that, when the conditions of the present invention are satisfied, it is possible to obtain a high-strength cold-rolled steel sheet that satisfies TS * À> 50000 MPa-%.
[00120] Furthermore, it is discovered that when hot stamping is carried out under predetermined hot stamping conditions, the cold-rolled steel sheet of the present invention satisfies TSxÀ> 50,000 MPa% even after hot stamping. Industrial Applicability
[00121] According to the present invention, as long as an appropriate relationship is established between the amount of C, the amount of Mn and the amount of Si and the martensite is given an appropriate hardness as it uses a nanoindent, it is possible to provide a cold rolled steel sheet capable of obtaining favorable orifice expandability. [Brief Description of the Reference Symbols] 51: FUSION PROCESS 52: CASTING PROCESS 53: HEATING PROCESS 54: HOT LAMINATION PROCESS 55: COILING PROCESS 56: COLLECTION PROCESS 57: COLD LAMINATION PROCESS 58: PROCESS RECOVERY 59: PROCESS SUBMIT THE STEEL TO CRUISE LAMINATION S11: HOT IMMERSION GALVANIZATION PROCESS S12: TREATMENT PROCESS WITH ALLOY FORMATION S13: ALUMINIZATION PROCESS S14: ELECTRO GALVANIZATION PROCESS

权利要求:
Claims (15)
[0001]
1. Cold-rolled steel sheet characterized by the fact that it consists of, in mass%: C: more than 0.150% to 0.300%; Si: 0.010% to 1,000%; Mn: 1.50% to 2.70%; P: 0.001% to 0.060%; S: 0.001% to 0.010%; N: 0.0005% to 0.0100%; and Al: 0.010% to 0.050%, and optionally one or more of: B: 0.0005% to 0.0020%; Mo: 0.01% to 0.50%; Cr: 0.01% to 0.50%; V: 0.001% to 0.100%; Ti: 0.001% to 0.100%; Nb: 0.001% to 0.050%; Ni: 0.01% to 1.00%; Cu: 0.01% to 1.00%; Ca: 0.0005% to 0.0050%; and REM: 0.0005% to 0.0050%, and a remainder of Fe and unavoidable impurities, where, when an amount of C, an amount of Si and an amount of Mn are represented respectively by [C], [Si ] and [Mn] in% of mass unit, a relation of formula 1 is satisfied, a metallographic structure consists of, by proportion of area, 40% to 90% of a ferrite and 10% to 60% of a martensite , and optionally one or more than 10% or less of a perlite per area ratio, 5% or less of an austenite retained in volume proportion and 20% or less of a bainite per area ratio, a martensite hardness measured using -if a nanoindentator satisfies formulas 2a and 3a below, and TSXÀ, representing a product of TS, which is a tensile strength, and À, which is an orifice expansion ratio, is 50,000 MPa% or more, ( 5x [Si] + [Mn]) / [C]> 10 ■■■ (1) 1.005 <H20 / H10 <1.10 ■■■ (2a) oHM0 <20 ■■■ (3a) in this case, the H10 represents an average hardness of the martensite on the surface part of cold rolled steel sheet, which is 200 pm or less from an outermost layer in a thick direction, H20 represents an average hardness of martensite in a central part of a sheet thickness that occupies a range of ± 100 pm of a center of the plate thickness of the cold rolled steel plate in the thickness direction, and oHMO represents a variance of the hardness of the martensite present in the central part of the plate thickness.
[0002]
2. Cold rolled steel sheet according to claim 1, characterized by the fact that an area ratio of an MnS that is present in the metallographic structure and has an equivalent circle diameter in the range of 0.1 pm to 10 pm is 0.01% or less, and the following formula 4a is satisfied, n20 / n10 <1.5 ■■■ (4a) in this case, n10 represents an average density in number of MnS per 10,000 pm2 at a 1/4 part of the plate thickness of the cold rolled steel plate, and n20 represents an average density in number of MnS per 10000 pm2 in the central part of the plate thickness.
[0003]
3. Cold-rolled steel sheet according to claim 1, characterized in that, additionally, after a hot stamping which includes heating to a temperature in the range of TδO'C to 10OCOCTC, a work and a cooling, the hardness of the martensite measured using a nanoindentador satisfies the formulas 2b and 3b below, the metallographic structure contains 80% or more of a martensite by proportion of area, optionally also contains one or more than 10% or less than one perlite by proportion of area, 5% or less of an austenite retained in proportion by volume, less than 20% of a ferrite and less than 20% of a bainite by proportion of area, and TSXÀ representing the product of TS, which is the tensile strength, and À, which is the expansion ratio of the orifice, is 50000 MPa% or more, H2 / H1 <1.10 ■■■ (2b) σHM <20 ■■■ ( 3b) in this case, H1 represents an average hardness of the martensite on the part of the surface after hot stamping, H2 repr there is an average hardness of the martensite in the central part of the sheet thickness after hot stamping, and oHM represents a variance of the hardness of the martensite present in the central part of the sheet thickness after hot stamping.
[0004]
4. Cold-rolled steel sheet according to claim 3, characterized by the fact that a proportion of MnS area that is present in the metallographic structure and has an equivalent circle diameter in the range of 0.1 pm to 10 pm is 0.01% or less, and the following formula 4b is satisfied, n2 / n1 <1.5 ■■■ (4b) in this case, n1 represents an average density in number of MnS per 10,000 pm2 at 1 / 4 part of the plate thickness on the cold rolled steel plate after hot stamping, and n2 represents an average density in number of MnS per 10,000 pm2 in the central part of the plate thickness after hot stamping.
[0005]
Cold-rolled steel sheet according to any of claims 1 to 4, characterized in that a hot-dip galvanized layer is still formed on a surface of the cold-rolled steel sheet.
[0006]
6. Cold-rolled steel sheet according to claim 5, characterized by the fact that the hot-dip galvanized layer includes a galvanized and annealed layer.
[0007]
7. Cold rolled steel sheet according to any one of claims 1 to 4, characterized by the fact that a layer is still formed by electro galvanizing formed on a surface of the cold rolled steel sheet.
[0008]
Cold rolled steel sheet according to any one of claims 1 to 4, characterized in that an aluminizing layer is still formed on a surface of the cold rolled steel sheet.
[0009]
9. Process of manufacturing a cold rolled steel plate, the process characterized by the fact that it comprises: subjecting the steel to continuous casting, which has the chemical components as defined in claim 1 and obtaining a steel; steel heating; hot rolling of steel using a hot rolling facility that has a large number of bases; steel winding after hot rolling; pickling steel after winding; cold rolling of steel after pickling using a cold rolling cylinder that has a large number of bases under conditions where the following Formula 5 is satisfied; heating the steel to a temperature in a range of TOO'C to δδO'C and cooling the steel after the cold inaction; and subjecting the steel to hardening lamination of the steel after heating and cooling the steel, 1.5xr1 / r + 1.2xr2 / r + r3 / r> 1.0 ■■■ (5) ri represents an individual target reduction by cold rolling in%, in a base ia, where i is 1,2, or 3 counted from the highest base among a large number of bases in cold rolling, er represents a total reduction by cold rolling in cold rolling cold in%.
[0010]
10. Process for the manufacture of a cold rolled steel sheet according to claim 9, characterized by the fact that, when a winding temperature in the winding is represented by CT in ‘C unit; and a quantity of C, a quantity of Mn, a quantity of Cr and a quantity of Mo of the steel are represented respectively by [C], [Mn], [Cr and [Mo] in% of mass unit, Formula 6 then it is satisfied, 560-474x [C] -90x [Mn] -20x [Cr] -20x [Mo] <CT <830-270x [C] - 90x [Mn] -70x [Cr] -80x [Mo] ■■■ (6)
[0011]
Process for the manufacture of a cold rolled steel sheet according to claim 9 or 10, characterized by the fact that, when a heating temperature in the heating is represented by T in unit of 'C; a time inside the oven is represented by t in a minute unit; and an amount of Mn and an amount of S in the steel are respectively represented by [Mn] and [S] in% of unit by mass, the following Formula 7 is satisfied Tx | n (t) / (1.7x [Mn ] + [S])> 1500 ■■■ (7).
[0012]
12. Process for manufacturing a cold-rolled steel sheet according to any of claims 9 to 11, characterized by the fact that it also comprises: hot-dip galvanizing in steel is also provided between annealing and submitting the steel hardening lamination.
[0013]
13. Process for the manufacture of a cold-rolled steel sheet according to claim 12, characterized by the fact that it also comprises: alloy formation with steel between hot-dip galvanizing and the submission of steel to hardening lamination .
[0014]
14. Process for the manufacture of a cold-rolled steel sheet according to any one of claims 9 to 11, characterized by the fact that it also comprises: electro galvanizing of the steel after the submission of the steel to the hardening lamination.
[0015]
15. Process for the manufacture of a cold-rolled steel sheet according to any one of claims 9 to 11, characterized by the fact that it also comprises: aluminization of the steel between annealing and submitting the steel to hardening lamination.

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法律状态:
2018-03-13| B06T| Formal requirements before examination [chapter 6.20 patent gazette]|

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2018-09-11| B07A| Application suspended after technical examination (opinion) [chapter 7.1 patent gazette]|

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2019-08-20| B25D| Requested change of name of applicant approved|Owner name: NIPPON STEEL CORPORATION (JP) |

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2019-12-24| B07A| Application suspended after technical examination (opinion) [chapter 7.1 patent gazette]|

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2020-06-02| B09A| Decision: intention to grant [chapter 9.1 patent gazette]|

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2020-10-27| B16A| Patent or certificate of addition of invention granted [chapter 16.1 patent gazette]|Free format text: PRAZO DE VALIDADE: 20 (VINTE) ANOS CONTADOS A PARTIR DE 11/01/2013, OBSERVADAS AS CONDICOES LEGAIS. |

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2021-11-03| B21F| Lapse acc. art. 78, item iv - on non-payment of the annual fees in time|Free format text: REFERENTE A 9A ANUIDADE. |

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2022-02-22| B24J| Lapse because of non-payment of annual fees (definitively: art 78 iv lpi, resolution 113/2013 art. 12)|Free format text: EM VIRTUDE DA EXTINCAO PUBLICADA NA RPI 2652 DE 03-11-2021 E CONSIDERANDO AUSENCIA DE MANIFESTACAO DENTRO DOS PRAZOS LEGAIS, INFORMO QUE CABE SER MANTIDA A EXTINCAO DA PATENTE E SEUS CERTIFICADOS, CONFORME O DISPOSTO NO ARTIGO 12, DA RESOLUCAO 113/2013. |

优先权:

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申请号 | 申请日 | 专利标题

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JP2012004551|2012-01-13|

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JP2012-004551|2012-01-13|

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PCT/JP2013/050382|WO2013105632A1|2012-01-13|2013-01-11|Cold-rolled steel sheet and method for producing same|

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