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    • 专利摘要:
    summary patent of invention: “hot rolled steel plate and method to manufacture it”. it is a hot-rolled sheet that includes, by weight%, 0.05 to 0.015% of c, 0 to 0.2% of itself, 0.5 to 3.0% of al, 1.2 to 2.5% of min, not more than 0.1% of p, not more than 0.01% of s, not more than 0.007% of n, 0.03 to 0.10% of ti, 0.008 to 0, 06% of nb, 0 to 0.12% of v, a total of 0 to 2.0% of one or more among cr, cu, ni and mo, 0 to 0.005% of b, a total of 0 to 0, 01% of one or more of ca, mg, la and ce, a total of 0.8x (mn-1)% or more of you and al, and a total of 0.04 to 0.14% of you and nb , the rest being faithful and unavoidable impurities. in the steel structure, the percentage of total surface area of martensite and austenite retained is in the range of 3 to 20%, the percentage of ferrite surface area is in the range of 50 to 96%, and the percentage of area of retained pearlite surface is not more than 3%. in a portion of the surface layer, the thickness of an area in which network oxides are present, in the direction of plate thickness, is less than 0.5 µm. the maximum tensile strength limit is at least 720 mpa.
    公开号:BR112015005020B1
    申请号:R112015005020
    申请日:2013-09-26
    公开日:2020-05-05
    发明作者:Itami Atsushi；Seto Atsushi；Tanahashi Hiroyuki；Maruyama Naoki；Yokoi Tatsuo
    申请人:Nippon Steel & Sumitomo Metal Corp；Nippon Steel Corp；
    IPC主号:
    专利说明:

    Descriptive Report of the Invention Patent for HOT LAMINATED STEEL SHEET AND METHOD FOR MANUFACTURING THE SAME.
    FIELD OF TECHNIQUE [0001] The present invention relates to a hot rolled steel sheet which has a tensile strength limit of 720 MPa or more with excellent corrosion resistance after electroplating coating, fatigue characteristics and curvature capacity , and a method for making hot rolled steel sheet. The present invention relates, in particular, to a hot-rolled steel sheet and a method for making the hot-rolled steel sheet, the hot-rolled steel sheet being favorably used as a frame material, limbs, suspension arms, wheels and the like of automobiles or trucks or, in which the electrodeposition coating is carried out, and as an architectural material and an industrial machinery material.
    BACKGROUND TECHNIQUE [0002] A hot-rolled steel sheet is normally used for a member of an automobile or a truck, such as a chassis or a wheel, and it is required that it has the ability to bend and high durability to fatigue.
    [0003] As a method to increase the forming capacity and fatigue characteristics of a hot rolled steel sheet, the following methods are revealed. As shown in Patent Documents 1 to 3, there are methods for dispersing structures that include rigid martensite into a metal structure that includes mainly flexible ferrite, so that it produces the so-called two-phase steel. In these methods, an alloying element such as Si or Al is added, which has an effect for
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    2/47 promote the formation of ferrite during cooling after final rolling in a hot rolling process.
    [0004] However, in a case where the steel revealed in
    Patent documents 1 to 3 are manufactured in a real line of operation, in some cases, favorable curvature fatigue characteristics are not obtained in a stable manner. In addition, as for a steel to which Si is added, unfortunately, there are cases where the corrosion resistance of the coating obtained after electroplating coating (hereinafter also simply referred to as “resistance to coating corrosion” or “resistance to corrosion after coating) coating ”) is not assured and cases where curvature fatigue characteristics are not obtained, as expected, due to the fact that the roughness of the surface of a steel sheet becomes large.
    [0005] Additionally, it is also required that a steel plate that is used for chassis or for the wheels of cars or trucks has fatigue characteristics of a perforated part. This is due to the fact that the roughness of an end surface formed by drilling with scissors or a hole punch usually has a roughness greater than that of a steel sheet, and the perforated end surface becomes the position where a fatigue crack is preferentially generated.
    [0006] As a method of solving the above problems, for example, Patent Documents 4 and 5 disclose a high strength hot-rolled steel sheet in which damage to the perforated end surface is prevented.
    [0007] Additionally, Patent Documents 6 and 7 disclose steel sheets that have high resistance to indentation fatigue. In steel sheets, the main structure is ferrite and bainite.
    [0008] Patent Document 8 reveals a method for decreasing
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    3/47 widely the roughness of the perforated fracture surface and notably prevent the generation of a fatigue crack from the perforated part, producing a structure in which appropriate amounts of martensite and retained austenite are dispersed, while the strength is increased using precipitation reinforcement of an alloy carbide with the use of a metal structure, as a base, which includes ferrite as a main phase.
    [0009] Additionally, Patent Document 9 discloses a steel sheet that has high adhesion to coating and characteristics of perforation fatigue, while the strength is increased using, as a metal structure that includes a ferrite phase as a main phase, a structure of martensite and bainite.
    BACKGROUND DOCUMENT (S) PATENT DOCUMENT (S) [0010] Patent Document 1 in JP H10-280096A [0011] Patent Document 2 in JP 2007-321201A [0012] Patent Document 3 in JP 2007-9322A [ 0013] Patent Document 4 in JP 2005-298924A [0014] Patent Document 5 in JP 2008-266726A [0015] Patent Document 6 in JP H05-179346A [0016] Patent Document 7 in JP 2002-317246A [0017] Patent Document 8 in JP 2010-159672A [0018] Patent Document 9 in JP 2012-021192A
    SUMMARY OF THE INVENTION PROBLEM (S) TO BE SOLVED BY THE INVENTION [0019] However, in a case of steel sheets revealed in the
    Patent Documents 4 and 5, the improvement in the roughness of the perforated end surface using these methods is not sufficient to improve fatigue characteristics and, in some cases,
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    4/47 the corrosion resistance of the coating is not obtained.
    [0020] Additionally, in a case of steel sheets disclosed in Patent Document 6 and 7, in some cases, characteristics of curvature fatigue and corrosion resistance after coating become inferior. In addition, since the yield ratio is high, the ability to stretch is not always sufficient.
    [0021] Additionally, in a case of the steel plate disclosed in Patent Document 8, in some cases, the limit of curvature fatigue characteristics is low on the original plate and the perforated part. In addition, when electroplating coating is carried out after blasting, favorable corrosion resistance after coating is not always achieved. Additionally, when curvature or a formation process consisting of reduction in area and curvature is performed, in some cases, a break is generated and fatigue characteristics of the limbs are reduced after formation.
    [0022] Additionally, Patent Document 9 has problems in the generation of scale or a break and in fatigue by curvature of a base material, in a case where a heavy process, such as reduction in area, is carried out in the steel plate mainly in order to improve the adhesion to the coating of a black scale material.
    [0023] From the above reasons, it is required that a technique to ensure corrosion resistance after electrodeposition coating and to have excellent curvature fatigue characteristics of a base material and excellent fatigue characteristics of a perforated part is developed for a high-strength hot-rolled steel sheet that has high forming capacity. In particular, a pickled sheet is often curved to be
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    5/47 used, so that the ability to bend is important.
    [0024] The present invention was manufactured in view of the above problems and aims to provide a hot-rolled steel sheet and a method for making the hot-rolled steel sheet, the hot-rolled steel sheet being able to obtain favorable corrosion resistance, fatigue characteristics of a base material, and fatigue characteristics of a perforated part, and also has favorable curvature capability, even in a case where the electrodeposition coating is carried out on a sheet of steel laminated to high strength hotplate that has a maximum tensile strength limit of 720 MPa or more.
    MEANS TO SOLVE THE PROBLEM (S) [0025] First, the focus of the present inventors was a hot-rolled steel sheet that has a maximum tensile strength limit of 720 MPa or more, through which limb weight is expected to be largely reduced, and the study was a method for reducing the roughness of the perforated end surface. Accordingly, the present inventors have concluded that the roughness of the perforated fracture surface is largely reduced and the generation of fatigue cracks in the perforated part is remarkably prevented, producing a structure in which appropriate amounts of martensite and retained austenite are dispersed, while the resistance is increased with the use of precipitation reinforcement of an alloy carbide, typified by TiC and NbC, with the use of, as a base, a metal structure that includes ferrite as a main phase. In addition, the present inventors have confirmed that an excellent stretch forming ability of a double phase steel can be ensured, even during the use of precipitation reinforcement of an alloy carbide.
    [0026] In order to obtain the metal structure above which includes ferrite
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    6/47 as a main phase, it is useful to add appropriate amounts of Si, Al and Mn. However, when the corrosion resistance of the hot rolled steel sheet was evaluated after the electroplating coating, it was concluded that the corrosion resistance has become lower in some cases. Accordingly, the present inventors studied the factor of the same and revealed that an oxide similar to the network containing one or more of Si, Al, Mn and Fe, which are present in a part of the surface layer of a base material, has an influence in corrosion resistance after electroplating coating. In addition, the present inventors have revealed the conditions under which favorable corrosion resistance can be obtained, optimizing the Si, Al and Mn quantity ratio in this case.
    [0027] The present inventors and the like have further observed the metal structure in detail in order to reveal the degradation factor of fatigue characteristics, which can be observed in a two-phase steel containing Si, Al and Mn in some cases. Accordingly, the present inventors have concluded that, in a case where the oxide similar to network 2 containing one or more of Si, Al, Mn and Fe is present immediately under a surface 1a (in the part of the superficial layer) of a material - base 1, as shown in Figure 1, fatigue characteristics are degraded by the oxide similar to network 2, which acts as a starting point for a fatigue fracture. Note that the samples in Figure 1 are a state of a hot-rolled steel sheet with scale, in which black scale (scale) 3 is attached to the surface 1a of the base material 1. In the part of the superficial layer (a region from a predetermined range from the surface 1a of the base material 1 towards the interior of the base material 1) of the base material 1, the network-like oxide 2 containing one or more of Si, Al, Mn and Fe is precipitate
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    7/47 along a limit crystal grain of base material 1. In the present invention, “net-like oxide” refers to oxide 2 which contains one or more of Si, Al, Mn and Fe precipitated along the grain of limit crystal of base material 1 in the surface layer part of base material 1, in this way.
    [0028] Additionally, the present inventors and the like studied the fact that the curvature capacity is degraded with the use of a hot-rolled steel plate from which the scale was removed by stripping. Accordingly, the present inventors have concluded that the ability to bend is degraded when excess network-like oxides containing one or more of Si, Al, Mn and Fe remain in the surface layer part of the steel sheet. Although this factor has not yet been defined, it is considered that this is due to the fact that the presence of an oxide in the grain boundary decreases grain boundary strength, and this part acts as the starting point for the generation of a crack. at the time of a curvature test.
    [0029] The present inventors and the like have intensively studied a method to prevent the formation of this net-like oxide that contains one or more of Si, Al, Mn and Fe. Accordingly, the present inventors concluded that the formation of these oxides can be prevented optimizing the amounts of addition of Mn, Al, Si, Ti, Nb and the like and suppressing remaining water on the surface of a steel plate (water that is present on the surface of the steel plate), rolling conditions and conditions cooling during hot rolling.
    [0030] The intensive studies by the present inventors were based on experimental results. Accordingly, the present inventors concluded that the formation of an oxide that contains one or more of Si, Al, Mn, and Fe present in a format
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    8/47 network in the surface layer part of a base material, can be prevented by adding retained martensite and austenite to a metal structure including mainly precipitation-resistant ferrite and optimizing the amounts of element addition of alloys. As a result, the present inventors have completed a high-strength hot-rolled steel sheet that has a tensile strength limit of 720 MPa or more with excellent fatigue characteristics of a perforated part, stable curvature fatigue characteristics and additionally, excellent corrosion resistance after electroplating coating while favorable curvature forming capacity, favorable curvature capacity for reduction in area and favorable stretching formation capacity are ensured. That is, the summary of the present invention is according to the following:
    [1] [0031] A hot-rolled steel sheet consisting of, in% by mass, [0032] C: 0.05% to 0.15%, [0033] Si: 0% to 0.2%, [0034] Al: 0.5% to 3.0%, [0035] Mn: 1.2% to 2.5%, [0036] P: 0.1% or less, [0037] S: 0.01 % or less, [0038] N: 0.007% or less, [0039] Ti: 0.03% to 0.10%, [0040] Nb: 0.008% to 0.06%, [0041] V: 0% to 0.12%, [0042] one or more among Cr, Cu, Ni and Mo: 0% to 2.0% in total, [0043] B: 0% to 0.005%, [0044] one or more among Ca, Mg, La and Ce: 0% to 0.01% in total,
    Petition 870200007225, of 16/01/2020, p. 11/61
    9/47 [0045] total amount of Si and Al: 0.8x (Mn-1)% or more, [0046] total amount of Ti and Nb: 0.04% to 0.14%, and [0047] balance: Fe and impurities, [0048] where, in a steel structure, a ratio of the total area of martensite and austenite retained is from 3% to 20%, a ratio of ferrite area is from 50% to 96% , and a pearlite area ratio is 3% or less, and [0049] where, in a part of the surface layer, the thickness in a plate-thickness direction of a region in which a network-like oxide is present is less than 0.5 pm, and a maximum tensile strength limit is 720 MPa or more, [2] [0050] Hot-rolled steel sheet, according to [1], [0051] where a average grain diameter of an alloy carbide containing Ti and an alloy carbide containing Nb is 10 nm or less, [3] [0052] The hot-rolled steel plate, according to [1] or [2 ], [0053] where a yield ratio is 0.82 or less, [4] [0054] Hot-rolled steel sheet, according to any uer one of [1] to [4], which consists of, in% by mass, [0055] Si: 0.001% to 0.2%, [5] [0056] The hot-rolled steel sheet, according to any of [1] to [4], which consists of, in% by mass, [0057] V: 0.01% to 0.12%, [6] [0058] Hot-rolled steel plate, of a deal with
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    10/47 any one of [1] to [5], which consists of, in mass%, [0059] one or more of Cr, Cu, Ni and Mo: 0.02% to 2.0% in total, [7] [0060] Hot-rolled steel plate, according to any of [1] to [6], which consists of, in mass%, [0061] B: 0.0003% to 0.005%, [8] [0062] Hot-rolled steel plate, according to any one of [1] to [7], which consists of, in mass%, [0063] one or more of Ca, Mg, La and Ce: 0.0003% to 0.01% in total, [9] [0064] The hot-rolled steel sheet according to any of [1] to [8], [0065] on which a surface is coated or coated and bonded.
    [10] [0066] A method for making a hot-rolled steel sheet, the method including:
    [0067] heat a plate consisting of, in% by mass, [0068] C: 0.05% to 0.15%, [0069] Si: 0% to 0.2%, [0070] Al: 0, 5% to 3.0%, [0071] Mn: 1.2% to 2.5%, [0072] P: 0.1% or less, [0073] S: 0.01% or less, [0074] N: 0.007% or less, [0075] Ti: 0.03% to 0.10%, [0076] Nb: 0.008% to 0.06%, [0077] V: 0% to 0.12%, [0078 ] one or more among Cr, Cu, Ni and Mo: 0% to 2.0% in total,
    Petition 870200007225, of 16/01/2020, p. 13/61
    11/47 [0079] B: 0% to 0.005%, [0080] one or more among Ca, Mg, La and Ce: 0% to 0.01% in total, [0081] total amount of Si and Al: 0 , 8 * (Mn-1)% or more, [0082] total amount of Ti and Nb: 0.04% to 0.14%, and [0083] the balance: Fe and impurities, and subsequently carry out the gross lamination and the final lamination;
    [0084] maintain, from when the stripping is carried out before the final lamination until when the final lamination is finished, for three seconds or more, a state in which water is not present on a steel plate surface, and define that a finishing temperature of the final lamination is 850 ° C or more;
    [0085] perform cooling in which an average cooling speed between the finishing temperature of the final lamination and an Ar3 temperature is 25 ° C / s or more, an average cooling speed between the temperature of Ar3 and 730 ° C is 30 ° C / s or more, an average cooling speed between 730 ° C and 670 ° C is 12 ° C / s or less, and an average cooling speed between 670 ° C and 550 ° C is 20 ° C / s or more; and [0086] laminating the hot rolled steel sheet at 530 ° C or less.
    [11] [0087] A method for making a hot rolled steel sheet, the method comprising:
    [0088] strip off the hot-rolled steel sheet obtained with the method according to [10], and then heat the hot-rolled steel sheet to 800 ° C or less and immerse the hot-rolled steel sheet in a galvanic bath.
    [12] [0089] The method for making a hot rolled steel sheet, according to [11], the method including
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    12/47 additionally:
    [0090] perform treatment of formation of alloy in a coated layer.
    EFFECT (S) OF THE INVENTION [0091] According to the hot-rolled steel sheet of the present invention, by means of the above configuration, it becomes possible to obtain excellent curvature forming capacity, resistance to coating corrosion, curvature fatigue characteristics of a base material and fatigue characteristics of a perforated part. In conventional steel sheets, the component sheet thickness was defined considering the reduction in thickness due to corrosion. In contrast, the hot-rolled steel sheet of the present invention has excellent resistance to coating corrosion and, accordingly, it becomes possible to reduce the sheet thickness of components so as to reduce the weight of cars, trucks or the like. In addition, even when conventional steel sheets are processed to have high strength, the fatigue strength of the perforated part is not substantially improved. In contrast, the hot-rolled steel sheet of the present invention has excellent curvature fatigue characteristics and perforated part fatigue characteristics and, accordingly, is used appropriately to reduce limb weight.
    [0092] Additionally, according to the manufacturing method of the present invention, the addition amounts of alloy element are optimized and the conditions at the moment of hot rolling are controlled and, accordingly, it becomes possible to manufacture a steel sheet hot rolled, which has a maximum tensile strength limit of 720 MPa with excellent bending capacity, corrosion resistance after electrodeposition coating, and fatigue durability.
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    13/47
    BRIEF DESCRIPTION OF THE DRAWING (S) [0093] Figure 1 is a microscope photograph of a periphery of a part of the surface layer of a two-phase steel sheet containing Si, Al and Mn. Note that Figure 1 shows a state of a hot-rolled steel plate with scale, in which black scale (scale) is fixed to a surface of a base material.
    METHOD (S) FOR CARRYING OUT THE INVENTION [0094] Hereinafter, a hot rolled steel sheet embodiment and a method for manufacturing it, in accordance with the present invention, will be described in detail. It should be noted that the present embodiment is described in detail so that the spirit of the hot-rolled steel sheet and the method for making it, according to the present invention, can be understood more clearly. Therefore, the present embodiment should not limit the present invention.
    HOT-LAMINATED STEEL SHEET [0095] The hot-rolled steel sheet, according to one embodiment of the present invention, in percentage by mass consists of, [0096] C: 0.05% to 0.15%, [0097 ] Si: 0% to 0.2%, [0098] Al: 0.5% to 3.0%, [0099] Mn: 1.2% to 2.5%, [00100] P: 0.1% or less, [00101] S: 0.01% or less, [00102] N: 0.007% or less, [00103] Ti: 0.03% to 0.10%, [00104] Nb: 0.008% to 0, 06%, [00105] V: 0% to 0.12%, [00106] one or more among Cr, Cu, Ni and Mo: 0% to 2.0% in total,
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    14/47 [00107] B: 0% to 0.005%, [00108] one or more among Ca, Mg, La, and Ce: 0% to 0.01% in total, [00109] total amount of Si and Al: 0.8 x (Mn-1)% or more, [00110] total amount of Ti and Nb: 0.04% to 0.14%, and [00111] the balance: Fe and impurities, [00112] in a structure steel, the total area ratio of martensite and retained austenite is 3% to 20%, a ferrite area ratio is 50% to 96%, and a pearlite area ratio is 3% or less, and [00113] in a part of the superficial layer, the thickness in the plate thickness direction of a region in which a network-like oxide is present is less than 0.5 pm, and the maximum tensile strength limit is 720 MPa or more.
    [00114] The reasons for limiting the constituents of material steel in accordance with the present modality will be described below.
    STEEL CONSTITUENTS [00115] The steel constituents of the hot rolled steel sheet of the present invention will be described. It should be noted that the representation of% in relation to steel constituents means percent by mass, unless otherwise specified.
    "C: CARBON" 0.05% TO 0.15% [00116] In the present invention, C is used for structure control. When the C content is less than 0.05%, it becomes difficult to ensure the ratio of the total area of martensite and austenite retained to 3% or more. Additionally, when the C content exceeds 0.15%, a pearlite structure appears and fatigue characteristics are reduced in a perforated part. Accordingly, in the present disclosure, the appropriate range of C is limited to a range of 0.05% to 0.15%. Note that the lower limit of the C content is preferably defined as 0.055%, more preferably as 0.06%. In addition, the upper limit of the C content is preferably defined as 0.14%,
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    15/47 more preferably 0.13%.
    "Si: SILÍCIO" 0% TO 0.2% [00117] In the present invention, Si is not necessarily contained; however, by containing Si, the ferrite fraction can be increased. In addition, when the Si content exceeds 0.2%, the net-like oxide is increased in the part of the surface layer and, accordingly, dense zinc phosphate ridges (a chemical conversion coating base film) are not formed in chemical conversion coating in an electrodeposition coating step. As a result, the adhesion between the steel sheet and the coating material becomes unsatisfactory after the electrodeposition coating, and it becomes difficult to maintain corrosion resistance after the electrodeposition coating. In addition, a large amount of Si and Mn oxides are formed in the surface layer during a hot rolling process, which degrades fatigue characteristics and the ability to bend. Because of this, the appropriate range is defined as 0.2% or less. The Si content is preferably defined as 0.18% or less, more preferably 0.15% or less. The lower limit of the Si content is not limited to a particular value, but the content less than 0.001% increases the manufacturing cost, so that 0.001% or more is preferred.
    “Al: ALUMINUM” 0.5% TO 3.0% [00118] Al is an element that increases the ferrite fraction as Si. When the Al content is less than 0.5%, the ferrite fraction cannot be ensured and, accordingly, the strength, forming capacity and fatigue characteristics of the perforated end surface cannot be ensured. On the other hand, when the Al content exceeds 3.0%, a large amount of oxides containing Al and Mn is formed in the part of the surface layer and, accordingly, the fatigue characteristics and the curvature capacity are degraded.
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    16/47
    Because of this, the appropriate range is defined as 0.5% to 3.0%. It is noted that the lower limit of the Al content is preferably defined as 0.6%. In addition, the upper limit of the Al content is preferably defined as 2.0%, more preferably 1.5%.
    “Mn: MAGNESIUM” 1.2% TO 2.5% [00119] Mn is used in order to control the structure and adjust the resistance. When the Mn content is less than 1.2%, it becomes difficult to ensure the ratio of the total area of martensite and austenite retained to 3% or more, and the fatigue characteristics of the perforated part are reduced. On the other hand, when the Mn content exceeds 2.5%, it becomes difficult to ensure a ferrite area ratio of 50% or more, and fatigue characteristics of the perforated part are reduced. In addition, the net-like oxide thickness is increased, so that curvature fatigue characteristics are decreased. Because of this, the appropriate range is defined as 1.2% to 2.5%. Note that the lower limit of the Mn content is preferably defined as 1.3%, more preferably 1.5%. In addition, the upper limit of the Mn content is preferably defined as 2.4%, more preferably 2.3%.
    “P: PHOSPHORUS” 0.1% OR LESS [00120] P can be used to ensure the strength of steel. However, when the P content exceeds 0.1%, the roughness of the perforated end surface is increased and the fatigue characteristics of the perforated part are degraded, so that the appropriate range of P is set to 0.1% or any less. The lower limit of the P content is not limited to a particular value, and can be 0%, but the content less than 0.001% increased the manufacturing cost; accordingly, 0.001% is a substantial lower limit. Note that the P content is preferably defined as 0.05% or less, more preferably 0.03% or less.
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    17/47 “S: SULFUR” 0.01% OR LESS [00121] S is an element that has an influence on the fatigue characteristics of the base material. However, when the S content exceeds 0.01%, the roughness of the perforated fracture surface is increased and favorable fatigue characteristics of the perforated part are not obtained, so that the appropriate range is set to 0.01% or less . In addition, the lower limit of the S content is not limited to a particular value and can be 0%, but the content less than 0.0002% increases the manufacturing cost; accordingly, 0.0002% is a substantial lower limit. Note that the S content is preferably defined as 0.006% or less, more preferably 0.003% or less.
    “N: NITROGEN” 0.007% OR LESS [00122] When the N content exceeds 0.007%, a nitride based on crude Ti-Nb is formed and the formation of Ti and Nb alloy carbides is prevented, so that maximum tensile strength limit of 720 MPa cannot be obtained. Accordingly, the upper limit is defined as 0.007%. Additionally, the lower limit of the N content is not limited to a particular value and can be 0%, but the content less than 0.0003% increases the manufacturing cost; accordingly, 0.0003% is a substantial lower limit. Note that the N content is preferably defined as 0.006% or less, more preferably 0.005% or less.
    “Ti: TITÂNIO” 0.03% TO 0.10% [00123] Ti is used to reinforce steel precipitation. In addition, Ti is effective in preventing the formation of the network-like oxide formed in the surface layer during a hot rolling step. However, when the Ti content is less than 0.03%, it does not prevent the formation of the oxide-like network and, in addition, it becomes difficult to ensure the limit
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    18/47 tensile strength of 720 MPa or more. Additionally, when the Ti content exceeds 0.10%, the saturation effect, the roughness of the perforated part is increased, the fatigue characteristics of the perforated part are decreased, a YR yield ratio is increased and the forming capacity is decreased. Accordingly, the appropriate range is limited to 0.03% to 0.10%. Note that the lower limit of the Ti content is preferably defined as 0.04%, more preferably 0.05%. In addition, the upper limit of the Ti content is preferably defined as 0.09%, more preferably 0.08%.
    “Nb: NIÓBIO” 0.008% TO 0.06% [00124] Nb is used to control the structure and reinforce steel precipitation. In addition, Nb is effective in preventing the formation of the network-like oxide formed in the surface layer during a hot rolling step. However, when the Nb content is less than 0.008%, there is no such effect; when the Nb content exceeds 0.06%, the roughness of the perforated part is increased and the fatigue characteristics of the perforated part are degraded. Accordingly, the appropriate range is limited from 0.008% to 0.6%. Note that the lower limit of the Nb content is preferably defined as 0.009%, more preferably 0.10%. In addition, the upper limit of the Nb content is preferably defined as 0.055%, more preferably 0.05%.
    “TOTAL QUANTITY OF Si AND Al” [00125] Si and Al are elements that increase the fraction of ferrite. By defining the total amount of Si and Al as 0.8x (Mn-1) percentage by mass or more, it becomes possible to ensure a ferrite area ratio of 50%, or more, and to obtain curvature fatigue characteristics perforated part. Additionally, by defining the total amount of Si and Al as an appropriate value,
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    19/47 it is possible to optimize the depth in the direction of sheet thickness in which the network-like oxide formed in the surface layer is present and to improve the curvature fatigue characteristics of the steel sheet. The upper limit of the total amount of Si and Al is not limited to a particular value, but the total amount of Si and Al that exceeds 3.0% decreases the roughness; accordingly, the total amount of Si and Al is preferably defined as 3.0% or less.
    “TOTAL QUANTITY OF Ti AND Nb” [00126] Ti and Nb are used to produce high strength steel by forming an appropriately sized alloy carbide. However, when the total amount of Ti and Nb is less than 0.04%, it becomes difficult to ensure the maximum tensile strength limit of 720 MPa or more. On the other hand, when the total amount of Ti and Nb exceeds 0.14%, the roughness of the perforated part is increased and the fatigue characteristics of the perforated part are degraded. Accordingly, the appropriate range of the total amount of Ti and Nb is limited to 0.04% to 0.14%.
    [00127] In the present modality, as a steel constituent, in addition to the elements above, the following elements can be selectively contained.
    "V: VANADIUM" 0% TO 0.12% [00128] In the present invention, V is not necessarily contained, but can be used to adjust the strength of steel. When the V content is less than 0.01%, there is no such effect. Accordingly, in a case containing V, the V content is desirably defined as 0.01% or more. On the other hand, when the V content exceeds 0.12%, the roughness of the perforated end surface can be increased and the fatigue characteristics of the perforated part can be degraded. Accordingly, the V content is defined as
    Petition 870200007225, of 16/01/2020, p. 22/61
    20/47
    0.12% or less.
    "ONE OR MORE WITHIN Cr, Cu, Ni And Mo: 0% TO 2.0% IN TOTAL".
    [00129] In the present invention, Cr, Cu, Ni and Mo are not necessarily contained, but can be used to control the steel structure. When the total content of one or more of these elements is less than 0.02%, there is no effect accompanied by the addition; accordingly, in a case where one or more of these elements are contained, the total content is desirably defined as 0.02% or more. On the other hand, when the total content of these elements exceeds 2.0%, the resistance to coating corrosion is decreased. Accordingly, the appropriate range of the total content of these elements is defined as 2.0% or less.
    “B: BORO” 0% A 0.005% [00130] In the present invention, B is not necessarily contained, but can be used to control the structure of the steel sheet. When the B content is less than 0.0003%, there is no such effect; accordingly, in a case containing B, the B content is desirably defined as 0.0003% or more. On the other hand, when the B content exceeds 0.005%, it can become difficult to secure 50% or more of ferrite, and the curvature fatigue characteristics can be degraded. Accordingly, the B content is defined as 0.005% or less.
    “ONE OR MORE WITHIN Ca, Mg, La And Ce: 0% TO 0.01% IN TOTAL” [00131] In the present invention, Ca, Mg, La and Ce are not necessarily contained, but can be used for steel deoxidation . When the total amount of one or more of these elements is less than 0.0003%, there is no such effect; accordingly, in a case containing one or more of the following
    Petition 870200007225, of 16/01/2020, p. 23/61
    21/47 these elements, their total content is desirably defined as 0.0003% or more. On the other hand, when their total content exceeds 0.01%, the fatigue characteristics are degraded. Accordingly, the appropriate range of the total content of one or more of these elements is defined as 0.01% or less.
    [00132] In the steel constituents of the hot rolled steel sheet according to the present invention, the balance in addition to the above elements is Fe and impurities. Impurities are included, for example, in raw materials, such as minerals and scraps, or included in a manufacturing process; however, there is no particular limitation. Any element can be contained, as appropriate without impairing the function effect of the present invention.
    STEEL STRUCTURE “REASON FOR TOTAL MARTENSITE AND AUSTENITE RETENTION AREA” [00133] Martensite and retained austenite promote a ductile fracture in a locally deformed region of the perforated part and, accordingly, are effective in smoothing the roughness of the end surface perforated in precipitation-resistant steel. That is, in the present invention whose objective is to increase fatigue characteristics of the perforated part, martensite and retained austenite are significant parameters. Additionally, martensite and retained austenite are effective in increasing the ability to form stretch and ductility.
    [00134] When the ratio of the total area of martensite and austenite retained is less than 3%, there is no such effect. On the other hand, when the ratio of the total area of martensite and austenite retained exceeds%, the roughness of the perforated end surface tends to be increased again. Accordingly, the appropriate range of the ratio of the total area of martensite and austenite retained is limited from 3% to
    Petition 870200007225, of 16/01/2020, p. 24/61
    22/47
    20%. The total area ratio of martensite and austenite retained is preferably defined as 5% or more, more preferably 7% or more. In addition, the total area ratio of martensite and austenite retained is preferably defined as 18% or less, more preferably 15% or less.
    [00135] As for martensite, tempering martensite is also effective in smoothing the perforated end surface. The martensite includes so-called fresh martensite and tempering martensite.
    [00136] In the present context, in a case where martensite is compared to retained austenite, retained austenite has a slightly greater effect in reducing the roughness of the perforated end surface and, accordingly, a certain amount of retained austenite is contained preferably. When the ratio of retained austenite area is less than 1%, the above effect is not shown explicitly; on the other hand, when the retained austenite area ratio exceeds 6%, the fatigue characteristics of the base material are reduced. Accordingly, the retained austenite area ratio is preferably defined as 1% to 6%.
    “FERRITE AREA REASON” [00137] In order to ensure retained martensite or austenite, which contributes to the improvement of the fatigue characteristics of the base material and to the improvement of the roughness of the perforated end surface, the ferrite needs to be contained in an appropriate area ratio. When the ferrite area ratio is less than 50%, it becomes difficult to define the appropriate amount above retained martensite or austenite and the fatigue characteristics of the perforated part are degraded. On the other hand, also in a case where the ferrite area ratio exceeds 96%, the roughness of the perforated end surface is increased and the curvature fatigue characteristics of the
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    23/47 perforated part are degraded. Accordingly, the appropriate range of the ferrite area ratio is limited from 50% to 96%. The lower limit of the ferrite fraction is preferably defined as 70% or more, more preferably 75% or more. In addition, the ferrite area ratio is preferably defined as 93% or less, more preferably 90% or less.
    [00138] Note that the ferrite mentioned in the present context can be any one of polygonal ferrite (ap), pseudopoligonal ferrite (aq), and granular bainitic ferrite (aB) described in “Steel Bainite Photobook -1” The Iron and Steel Institute of Japan (1992) p.4.
    “PEARLITE AREA REASON” [00139] The pearlite increases the roughness of the perforated part. When the area ratio exceeds 3%, the curvature fatigue characteristics of the perforated part tend to be degraded and the tensile strength limit tends to be lowered. Accordingly, the appropriate range is limited to 3% or less. The smaller amount of pearlite is more preferable and the lower limit is 0%.
    [00140] In the present invention, pearlite includes pearlite and pseudo pearlite.
    [00141] It is noted that, in the hot-rolled steel plate of the present invention, the balance of the metal structure can be bainite. In the present context, the bainitic ferrite (a ° B) described in “Steel Bainite Photobook -e1” The Iron and Steel Institute of Japan (1992) p.4, cited above, is classified as bainite.
    [00142] The ratio of areas of ferrite, bainite, pearlite and martensite can be measured using photographs of the structure obtained by photographing with an optical microscope or a scanning electron microscope (SEM) by a scanning method. point counting or by image analysis. The ferrite
    Petition 870200007225, of 16/01/2020, p. 26/61
    24/47 granular bainitic (αΒ) and bainitic ferrite (α ° Β) are determined by observing the structure with a SEM and a transmission electron microscope (TEM) based on Reference Document 1. The fraction of Retained austenite is measured by an X-ray diffraction method.
    NET-LIKE OXIDE IN THE PART OF THE SURFACE LAYER [00143] The net-like oxide that contains one or more of Si, Al, Mn and Fe, formed during a hot rolling step in the surface layer of the degraded steel plate curvature fatigue characteristics and corrosion resistance after coating. When the thickness (depth) in the direction of plate thickness of the region in which this network-like oxide is present is 0.5 pm or more, the curvature capacity, curvature fatigue characteristics and corrosion resistance after coating are degraded. Accordingly, the appropriate range is limited to 0.5 pm or less. It is noted that an oxide that has an influence on curvature capacity and curvature fatigue characteristics is the network-like oxide formed in the boundary crystal grain around the surface of the base material during the hot rolling step and does not include oxides that are formed during the molding and molding stage and are also dispersed in steel. Additionally, as for the internal oxides (oxides precipitated inside a crystal grain), granular oxides are also present in the part of the superficial layer, however they are considered to have an influence on curvature capacity and curvature fatigue characteristics. less than net-like oxides precipitated in the boundary crystal grain of the base material. Accordingly, in the present invention, there is no particular limitation on the granular internal oxides, however there is a limitation on the oxide
    Petition 870200007225, of 16/01/2020, p. 27/61
    25/47 similar to the network formed in the boundary crystal grain in the part of superficial layer.
    AVERAGE GRAIN DIAMETER OF ALLOY CARBONIDE CONTAINING Ti AND ALLOY CARBONIDE CONTAINING Nb [00144] An alloy carbide containing Ti and an alloy carbide containing Nb are precipitated which contribute to reinforcing precipitation. However, when the average grain diameter exceeds 10 nm, it becomes difficult to ensure the maximum tensile strength limit of 720 MPa or more and, accordingly, the appropriate range is desirably limited to 10 nm or less. Note that even when the alloy carbides contain a small amount of N, V and Mo, the precipitation-enhancing effect does not change. The alloy carbide containing Ti can contain N, V and Mo, in addition to Ti and C. Similarly, the alloy carbide containing Nb can contain N, V and Mo in addition to Nb and C. Additionally, N, V and Mo may be contained in addition to both Ti and Nb and C.
    [00145] Note that the grain diameter of the alloy carbide containing Ti and the diameter of the alloy carbide containing Nb are obtained by observing in TEM a precipitate in a sample steel that is tuned by electropolishing or ionic polishing , or by observation in TEM of a residue obtained by electroextraction and calculation, as grain diameter of equivalent circle of 100 or more alloy carbides.
    MAXIMUM STEEL SHEET STRENGTH LIMIT>
    [00146] In the present invention, when the maximum tensile strength limit of a steel plate is less than 720 MPa, the effect to reduce the limb weight becomes small. Accordingly, the range is defined as 720 MPa or more.
    REASON FOR INCOME: 0.82 OR LESS
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    26/47 [00147] For application to members of automobiles or trucks, which need fatigue characteristics, excellent stretch, stretch forming capacity and curvature capacity are required. When a YR yield ratio, defined by YP / TS (YP: yield limit, TS: tensile strength limit), exceeds 0.82, in some cases, a fracture or break is generated during formation and the formation of members becomes unsuccessful. Accordingly, the yield ratio is preferably 0.82 or less.
    METHOD FOR MANUFACTURING HIGH-RESISTANCE HOT-LAMINATED STEEL SHEET [00148] Next, a method for manufacturing the hot-rolled steel sheet of the present invention will be described. The manufacturing method of the present invention is a method that includes: heating a plate consisting of the above constituent composition and sequentially performing the crude and final lamination; maintain, from when stripping is carried out before the final lamination until when the final lamination is finished, for three seconds or more, a state in which water (water on sheet) is not present on a surface of the steel sheet and define a finishing temperature of the final lamination such as 850 ° C or more; perform cooling in which an average cooling speed between the finishing temperature of the final lamination and an temperature of Ar3 is 25 ° C / s or more, an average cooling speed between the temperature of Ar3 and 730 ° C is 30 ° C / s or more, an average cooling speed between 730 ° C and 670 ° C is 12 ° C / s or less and an average cooling speed between 670 ° C and 550 ° C is 20 ° C / s or more; and laminating the hot rolled steel sheet at 530 ° C or less.
    [00149] First, the plate consisting of the above constituent composition is heated and then the crude lamination and the
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    27/47 final lamination are carried out sequentially. At that time, the conditions for heating the plate and the conditions for rough lamination are not limited to particular conditions and any of the conditions that have been used conventionally can be employed. [00150] In the present invention, water (water on sheet) that is present on the surface of the steel sheet from when the stripping is carried out before the final lamination until when the final lamination is finished is a significant factor that has an influence on formation of the net-like oxide in the surface layer of the steel plate. In a final rolling stage, usually high pressure water used for stripping, water used to cool rolling rolls, and water to cool the steel plate between rolling rolls are present on the surface of the steel plate. When the state in which water is not present on the surface of the steel plate for no more than three seconds from when the stripping ends until the final lamination is finished, excess mesh-like oxides remain in the surface layer and curvature fatigue characteristics are degraded. Accordingly, the appropriate range of time in which the state in which water is not present on the surface of the steel sheet is maintained is defined as three seconds or more, preferably four seconds or more.
    [00151] Note that there is no particular limitation in a method to achieve the state in which water is not present on the surface of the steel sheet and to maintain that state. For example, there is a method for removing hydration on the surface of the steel sheet by injecting a gas, such as air in the direction of travel of the steel sheet from an intersecting direction (direction on the side surface side). [00152] An FT finishing temperature of the final lamination is
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    28/47 a significant manufacturing parameter to control oxidation behavior in the part of the superficial layer and a metal structure of the steel plate. When the finishing temperature of the final lamination is less than 850 ° C, as the thickness of the mesh-like oxide in the surface layer part is increased, it becomes more difficult to optimize the metal structure described above. Accordingly, in the present invention, the appropriate finishing temperature range of the final lamination is limited to 850 ° C or more, preferably 870 ° C or more.
    [00153] The cooling speed between the FT finishing temperature of the final rolling mill and the AR3 temperature is a significant manufacturing parameter that has an influence on the microstructure and strength of steel. When the average cooling temperature between these temperatures is less than 25 ° C / s, the ferrite area ratio cannot be optimized. Accordingly, the appropriate range of the average cooling speed between FT and the temperature of AR3 is defined as 25 ° C / s or more, preferably 45 ° C / s or more.
    [00154] Note that the temperature of AR3 is calculated from the following expression (1).
    Ar3 (° C = 910-310xC + 33 (Si + Al) -80xMn-20xCu-15xCr-55xNi-80xMo (1) [00155] In the above expression (1), each atomic symbol denotes the content of each element (percentage in mass) [00156] The cooling speed between the temperature of AR3 and 730 ° C is a significant manufacturing parameter that has an influence on the formation of the network-like oxide in the surface layer. When the average cooling speed between these temperatures is less than 30 ° C / s, the depth at which the network-like oxide is formed from the surface becomes greater.
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    Accordingly, in the present invention, the appropriate range of the average cooling speed between the temperature of AR3 and 730 ° C is defined as 30 ° C / s or more, preferably 35 ° C / s or more.
    [00157] The cooling speed between 730 ° C and 670 ° C is a significant manufacturing parameter to ensure the ferrite area ratio in steel. When the average cooling speed between these temperatures exceeds 12 ° C / s, it is difficult to ensure 50% or more of ferrite. Accordingly, the appropriate range is defined as 12 ° C / s or less, preferably 10 ° C / s or less.
    [00158] The cooling speed between 670 ° C and 550 ° C is a significant manufacturing parameter to optimize the area ratio of martensite and retained austenite. When the average cooling speed between these temperatures is less than 20 ° C / s, pearlite is formed and the roughness of the perforated fracture surface is increased, resulting in degradation of the fatigue characteristics of the perforated part. Accordingly, in the present invention, the appropriate range is defined as 20 ° C / s or more, preferably 25 ° C / s or more.
    [00159] Next, in the manufacturing method of the present invention, the temperature at the moment of rolling of the steel sheet is a significant manufacturing parameter to obtain appropriate amounts of martensite and retained austenite. When the rolling temperature exceeds 530 ° C, appropriate amounts of martensite and retained austenite are not obtained and pearlite is likely to be formed. As a result, the roughness of the perforated fracture surface is increased and the fatigue characteristics of the perforated part are degraded. Accordingly, in the present invention, the appropriate steel sheet rolling temperature range is limited to 530 ° C or less, preferably 510 ° C or less.
    [00160] The finishing lamination or leveling lamination is not
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    30/47 necessary, but it is effective in improving the straightening of shape, aging character and fatigue characteristics, and can be performed after or before blasting, which will be described later. In the case of finishing lamination, the upper limit of lamination reduction is desirably set at 3%. This is due to the fact that the forming capacity of the steel sheet is damaged when the upper limit exceeds 3%.
    [00161] After the hot lamination is finished, stripping is carried out and the black scale (scale) fixed to the surface of the base material is removed. Pickling after finishing hot rolling is effective in removing part of the amount of grain-like oxides of grain contour. However, unless the above manufacturing method is carried out, it is difficult to reduce the thickness of the high mesh-like grain boundary oxide layer only through the stripping step after hot rolling finish.
    [00162] Next, the hot-rolled steel sheet above may be additionally subjected to the surface coating / galvanizing treatment or the surface coating / galvanizing treatment followed by the formation of the coated layer alloy.
    [00163] First, after the hot-rolled steel sheet is stripped, the steel sheet is heated using, for example, continuous galvanizing equipment or continuous galvanizing-annealing equipment. Then, the steel sheet is immersed in a galvanic bath to be subjected to hot immersion, so that a coated layer is formed on the surface of the hot-rolled steel sheet.
    [00164] In this case, when the heating temperature of the
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    31/47 steel sheet exceeds 800 ° C, the metal structure of the steel sheet changed and, in addition, the thickness in the sheet thickness direction of the region containing the network-like oxide in the surface layer part is increased, so that the fatigue characteristics are not ensured. Accordingly, the appropriate range of the heating temperature is limited to 800 ° C or less.
    [00165] Additionally, after hot dipping is carried out, the surface coating / galvanizing treatment followed by alloy formation of the coated layer can be carried out so that an attached hot dip galvanized layer can be formed.
    [00166] Note that there is no particular limitation on the type of surface coating / galvanization. Any type of surface coating / galvanizing can be carried out if the upper limit of the heating temperature is 800 ° C or less.
    [00167] In the following, methods for evaluating the characteristics of the hot rolled steel sheet of the present invention will be described.
    [00168] The curvature fatigue characteristics of the hot rolled steel sheet of the present invention have been evaluated by fatigue limit two million times (stress limit value in which a fatigue fracture does not occur after repeated stress of two million times) obtained by plane curvature fatigue tests under a condition of stress ratio = -1 in accordance with a method described in JIS Z2275, and a fatigue limit ratio was calculated from {fatigue limit / TS (tensile strength limit)}. In the hot rolled steel sheet of the present invention, the fatigue limit ratio of 0.45 or more can be ensured.
    [00169] Additionally, the fatigue characteristics of the perforated part can be briefly evaluated by the following method.
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    32/47 [00170] That is, a curvature test piece that has a hole drilled in a central part is manufactured, and plate curvature fatigue tests are performed to assess the fatigue limit two million times or the fatigue limit ratio (= fatigue limit / TS). In the present context, in a case where the curvature fatigue tests are performed using a test piece that has a width of 30 mm, which is obtained by drilling a drilled hole with a condition of 10% free space using a na 10 mm perforation in the hot rolled steel sheet of the present invention, a fatigue limit ratio of 0.36 or more can be ensured. In addition, a fatigue limit ratio of 0.39 or more is more preferred in the present invention.
    [00171] Additionally, regarding the curvature capacity of the steel plate, the tests were carried out by a method of curvature by pressing, in accordance with a method described in JIS Z2248 with a curved angle of 180 ° and an inner radius of 1, 5t (t is the sheet thickness of the steel sheet). In the hot-rolled steel sheet of the present invention, it is possible to ensure the favorable curvature capacity without a crack or a fracture that is observed in a curvature front.
    [00172] As described above, according to the hot-rolled steel sheet according to the present invention, by the above configuration, excellent bending ability, resistance to coating corrosion and fatigue durability can be obtained. In conventional steel sheets, the component sheet thickness was defined considering the reduction in thickness due to corrosion. In contrast, the hot-rolled steel sheet of the present invention makes it possible to reduce the thickness of the component sheet due to the fact that excellent resistance to coating corrosion can be obtained and enables a reduction in the weight of cars, trucks and
    Petition 870200007225, of 16/01/2020, p. 35/61
    33/47 similar. Additionally, in conventional steel sheets, even when high reinforcement is performed, the fatigue strength of the perforated part has not been substantially improved. In contrast, the hot-rolled steel sheet of the present invention has excellent curvature fatigue characteristics of the base material and excellent fatigue characteristics of the perforated part and also has excellent curvature ability and, accordingly, is excessively suitable for reducing the component weight.
    [00173] Additionally, according to the method for making the hot rolled steel sheet of the present invention, using the conditions and procedures above, it becomes possible to manufacture the hot rolled steel sheet that has the tensile strength limit maximum of 720 MPa or more with excellent bending ability, corrosion resistance after electroplating coating, and fatigue durability.
    EXAMPLES [00174] Examples of the hot rolled steel sheet according to the present invention will be described below and the present invention will be described more specifically. It is noted that the present invention is not limited to the following examples, and can be implemented through the appropriate application of modification without departing from the scope above or below in this document. Such implantation is also included in the technical scope of the present invention.
    [00175] First, plates that have steel constituents A to X shown in Table 1 were melted and then they were heated again in the range of 1050 ° C to 1300 ° C, and the crude lamination was performed. Then, the final rolling, cooling and rolling were carried out under the conditions shown in Table 2, so that hot rolled steel sheets were manufactured. Time periods of the state in which water was not present in the
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    34/47 steel sheet surface from stripping before final lamination to end of final lamination, finishing temperature of final lamination, cooling conditions and lamination temperatures were varied. Then, the pickling treatment was carried out and the steel plates on the surface from which the scale was removed were subjected to evaluation tests.
    [00176] Additionally, as for test number A-12, the hot-rolled steel sheet obtained as test number A-1 was subjected to the choice and, then, the annealing treatment was carried out at 650 ° C and, subsequently , galvanizing treatment was carried out. As for test number A-13, the hot-rolled steel sheet obtained as test number A-1 was subjected to pickling of choice, so the annealing treatment was carried out at 600 ° C, subsequently, the treatment by galvanization and the treatment of alloy formation by galvanization was carried out.
    [00177] Hot-rolled steel sheets of the present examples and hot-rolled steel sheets of comparative examples, which are obtained through the above procedure, were subjected to the evaluation tests described below. Note that alphabetic characters added to the top of “test number”, shown in Table 2, correspond to steel symbols shown in Table 1.
    [00178] Then, the hot-rolled steel sheets of the present examples and the hot-rolled steel sheets of the comparative examples, which were obtained through the above procedure, were subjected to the assessment tests described above.
    [00179] The fatigue characteristics of steel sheets were assessed by fatigue limit two million times obtained by fatigue tests by plane curvature under a condition of stress ratio = -1 in accordance with the method described in JIS Z2275 and fatigue limit ratios were calculated {
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    35/47 fatigue / TS (tensile strength limit)}. It is noted that fatigue limit ratios of 0.45 or more were assessed, as favorable.
    [00180] The fatigue characteristics of the perforated parts were assessed by fatigue limit two million times obtained by fatigue tests by plane curvature, using a curvature test piece that has a hole drilled in a central part, under a condition of a stress ratio = -1 in accordance with the method described in JIS Z2275, and the fatigue limit ratios were calculated from {fatigue limit / TS (tensile strength limit)}. In the present context, the drilling process to provide drilled holes was carried out under a condition of 10% free space using a new φ 10 mm perforation. It is noted that fatigue limit ratios of 0.39 or more were assessed as favorable fatigue characteristics of the perforated parts.
    [00181] The bending capacity of steel sheets was assessed by tests carried out by a method of bending by pressing in accordance with the method described in JIS Z2248 with a curved angle of 180 ° and an internal radius of 1.5t (t is the sheet thickness of the steel sheet) by extracting test pieces in such a way that the longitudinal direction of the test piece becomes perpendicular to the rolling direction. The test pieces in which a crack or fracture was not seen in the frontal curvature were evaluated as A (favorable).
    [00182] The tension characteristics of the steel sheets were assessed by stress tests carried out by extracting a test piece t and JIS No. 5 from each of the steel sheets under a condition in which the tension direction is perpendicular (direction C) the rolling direction.
    [00183] The metal structures along cross-sections of the steel sheets were observed with the SEM and the average values in
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    36/47 three or more regions observed were determined as the thickness of the region in which the net-like oxide is present in the surface layer part of the steel plate.
    [00184] As for the corrosion resistance of the coating, first, pickled hot-rolled steel sheets were degreased and then subjected to zinc phosphate treatment (chemical conversion coating) as a pretreatment and the electrodeposition coating of cation with a thickness of 25 pm was performed and, finally, the cooking treatment was carried out at 170 ° C for twenty minutes. Then, after linear failures were added to the electrodeposition coating surface, salt spray tests (SST tests) were performed for 200 hours, in accordance with a method described in JIS Z2371. After testing, stripped widths of coating films were measured when strip stripping tests were performed. Peeled widths of coating films of 2 mm or less were rated as “A (favorable corrosion resistance)”, and peeled widths of coating films of more than 2 mm were rated “B (unsatisfactory corrosion resistance)”.
    [00185] Table 1 shows the list of steel constituents and Table 2 shows the list of results for assessing the net-like oxide thickness from the surface, the curvature fatigue characteristics, the fatigue characteristics of the perforated parts , the tensile strength limit (TS), the yield ratio and the curvature capacity of the manufactured hot-rolled steel sheets. Note that the indices in Table 2 denote the following items.
    [00186] t: period of time during which water is not present in the steel sheet from stripping until the end of the final rolling mill (seconds)
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    37/47 [00187] FT: final lamination finishing temperature (° C) [00188] CR1: average cooling speed between FT and Ar3 temperature (° C / s) [00189] CR2: average cooling speed between the temperature of Ar3 and 730 ° C (° C / s) [00190] CR3: average cooling speed between 730 ° C and 670 ° C (° C / s) [00191] CR4: average cooling speed between 670 ° C and 550 ° C (° C / s) [00192] CT: rolling temperature (° C) [00193] dMC: average grain diameter of an alloy carbide containing Ti and an alloy carbide containing Nb (nm) [00194] fF: ferrite area ratio (%) [00195] fM: martensite area ratio (%) [00196] f Y : volume fraction of retained austenite (%) [00197] fp: pearlite area ratio (%) [00198] hox: thickness in the direction of plate thickness of the region where the net-like oxide is present in the surface layer part (pm) [00199] EL: total stretching of the steel plate (%) [00200 ] ow / TS: fatigue limit ratio [00201] owp / TS: limit fatigue ratio of the test piece with a drilled hole
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    TABLE 1
    Steel Symbols Steel constituents (percentage by mass) Ar3(° C) Category Ç Si Al Mn P s N You Nb Si + Al Ti + Nb Others THE 0.08 0.1 1.5 2.0 0.010 0.003 0.003 0.08 0.01 1.6 0.09778B 0.13 0.1 2.0 2.4 0.010 0.001 0.003 0.04 0.06 2.1 0.1 V: 0.06 747Ç 0.10 0.1 1.0 2.0 0.010 0.002 0.003 0.05 0.03 1.1 0.08 Mo: 0.2 739D 0.06 0.1 0.8 1.3 0.010 0.002 0.003 0.07 0.01 0.9 0.08 Mg: 0.003 817AND 0.05 0.1 0.6 1.2 0.010 0.002 0.003 0.04 0.05 0.7 0.09 La: 0.0004,Ce: 0.0015 822F 0.13 0.1 1.2 1.8 0.010 0.002 0.003 0.03 0.06 1.3 0.09 V: 0.1, Mo: 0.25,Cr, 0.3 769G 0.06 0.2 0.8 1.3 0.010 0.003 0.003 0.09 0.01 1.0 0.1 B: 0.001,Ca: 0.0015 820H 0.10 0.1 0.7 1.9 0.010 0.003 0.003 0.09 0.03 0.8 0.12 Cu: 0.1, Ni: 0.3 734I 0.04 0.1 1.0 2.0 0.010 0.002 0.003 0.05 0.03 1.1 0.08774J 0.18 0.1 1.0 2.0 0.010 0.002 0.003 0.05 0.03 1.1 0.08731K 0.10 0.3 1.0 2.0 0.010 0.002 0.003 0.05 0.03 1.3 0.08762L 0.10 0.1 0.6 2.0 0.010 0.002 0.003 0.05 0.03 0.7 0.08742
    38/47
    Petition 870200007225, of 16/01/2020, p. 41/61
    M 0.10 0.1 32 2.0 0.010 0.002 0.003 0.05 0.03 3.3 0.08828N 0.10 0.1 1.2 2.6 0.010 0.002 0.003 0.05 0.03 1.3 0.08714O 0.10 0.1 1.0 2.0 0.12 0.002 0.003 0.05 0.03 1.1 0.08755P 0.10 0.1 1.0 2.0 0.010 0.02 0.003 0.05 0.03 1.1 0.08755Q 0.10 0.1 1.0 2.0 0.010 0.002 0.008 0.05 0.03 1.1 0.08755R 0.10 0.1 1.0 2.0 0.010 0.002 0.003 0.12 0.03 1.1 0.15755s 0.10 0.1 1.0 2.0 0.010 0.002 0.003 0.01 0.03 1.1 0.04755T 0.10 0.1 1.0 2.0 0.010 0.002 0.003 0.05 0.07 1.1 0.12755U 0.10 0.1 0.4 1.2 0.010 0.002 0.003 0.05 0.03 0.5 0.08800V 0.10 0.1 1.0 11 0.010 0.002 0.003 0.05 0.03 1.1 0.08827W 0.10 0.1 1.0 2.0 0.010 0.002 0.003 0.05 0.002 1.1 0.05755X 0.10 0.1 1.0 2.0 0.010 0.002 0.003 0.03 0.008 1.1 0.038755
    39/47
    Petition 870200007225, of 16/01/2020, p. 42/61
    TABLE 2
    2- 2 D CH 3 D (D* The D (/) r “Ks FT (° C) CR1 (° C / s) CR2 (° C / s) CR3 (° C / s) CR4 (° C / s) O -1^ 3rd Note dMC (nm) Z ^ p Z ^ p iT + Z ^ p Z ^ p Note(structure) hox (pm) YP (MPa) TS (MPa) < m Z ^ p q $—1 to q $—1 to corrosion after CDQ. CD Category THE -1 5 900 50 50 8 30 4505 80 4 8 00.4 620 800 0.78 22 0.51 0.42 THE THE Present Example THE -2 10 900 50 50 8 30 4505 80 4 8 00.0 620 800 0.78 22 0.52 0.42 THE THE Present Example THE -3 1 900 50 50 8 30 4505 80 4 8 03.0 620 800 0.78 22 0.42 0.33 B B Comparative Example A-4 5 830 50 50 8 30 4505 85 4 9 02.2 600 780 0.77 22 0.44 0.35 B B Comparative Example THE -5 5 900 20 50 20 30 4505 45 2 4 00.4 710 845 0.84 19 0.49 0.35 THE THE Comparative Example THE -6 5 900 50 50 8 4 4505 80 1 2 40.4 655 740 0.89 25 0.49 0.35 THE THE Comparative Example THE -7 5 900 50 50 8 30 58012 97 0 0 00.4 745 825 0.90 20 0.47 0.34 THE THE Comparative Example THE -8 5 900 50 50 8 40 3005 80 3 10 00.4 640 825 0.78 21 0.49 0.40 THE THE Present Example THE -9 5 900 50 50 8 60 205 80 2 15 00.4 655 870 0.75 18 0.49 0.40 THE THE Present Example THE-10 5 900 50 25 8 30 4507 85 2 5 00.6 630 775 0.81 22 0.44 0.35 B B Comparative Example THE-11 5 900 50 8 8 30 45010 90 2 2 20.9 595 720 0.83 23 0.44 0.31 B B Comparative Example
    40/47
    Petition 870200007225, of 16/01/2020, p. 43/61
    THE-12 5 900 50 35 8 30 450 Annealed at 650 ° C and then galvanized 8 80 2 7 0 Martensite is tempering martensite. 0.4 605 790 0.77 21 0.48 0.41 THE THE Present Example THE-13 5 900 50 35 8 30 450 Annealed at 600 ° C and then galvanized 8 80 2 7 0 Martensite is tempering martensite. 0.4 630 805 0.78 18 0.48 0.41 THE THE Present Example B-1 5 930 45 35 8 30 4506 75 3 15 00.3 650 840 0.77 15 0.49 0.40 THE THE Present Example B-2 5 930 45 35 8 60 206 75 2 18 00.3 740 915 0.81 22 0.50 0.42 THE THE Present Example Ç-1 5 880 55 35 8 30 5005 80 3 9 00.3 620 780 0.79 17 0.48 0.41 THE THE Present Example Ç-2 5 880 55 35 8 60 205 80 2 15 00.3 660 850 0.78 23 0.51 0.44 THE THE Present Example D-1 1 880 55 35 8 60 204 90 0 8 02.5 630 785 0.80 23 0.42 0.36 B B Comparative Example D-2 5 880 55 35 8 60 204 90 0 8 00.3 625 780 0.80 24 0.46 0.40 THE THE Present Example D-3 5 880 55 25 8 60 204 92 0 5 10.6 605 760 0.80 27 0.44 0.35 B B Comparative Example D-4 5 880 55 8 8 60 205 93 0 3 20.9 570 715 0.80 19 0.43 0.33 B B Comparative Example E-1 5 900 50 50 8 60 205 90 0 7 00.0 605 760 0.80 21 0.47 0.39 THE THE Present Example
    41/47
    Petition 870200007225, of 16/01/2020, p. 44/61
    F-1 5 900 50 50 8 30 4505 75 4 12 00.3 650 825 0.79 15 0.50 0.40 THE THE Present Example G-1 5 900 50 40 8 60 205 90 1 8 00.0 660 860 0.77 17 0.47 0.41 THE THE Present Example H-1 5 900 50 40 8 30 4505 80 1 6 00.3 630 795 0.79 21 0.48 0.40 THE THE Present Example I-1 5 900 50 40 8 30 4505 80 0 2 00.3 580 700 0.83 26 0.50 0.35 THE THE Comparative Example J-1 5 900 50 40 8 30 4505 80 6 14 50.3 795 900 0.88 15 0.47 0.33 THE THE Comparative Example K-1 5 900 50 40 8 30 4505 85 4 11 02.5 635 805 0.79 23 0.42 0.33 B B Comparative Example L-1 5 900 50 40 8 30 4505 45 2 17 00.0 690 830 0.83 20 0.46 0.35 THE THE Comparative Example M-1 5 900 50 40 8 30 4505 70 3 8 02.8 620 790 0.78 20 0.42 0.35 B B Comparative Example N-1 5 900 50 40 8 30 4505 30 1 15 02.1 610 885 0.69 14 0.43 0.35 B B Comparative Example O-1 5 900 50 40 8 30 4505 80 3 9 00.3 705 840 0.84 17 0.50 0.33 THE THE Comparative Example P-1 5 900 50 40 8 30 4505 75 1 8 00.3 620 780 0.79 21 0.50 0.32 THE THE Comparative Example Q-1 5 900 50 40 8 30 4505 80 3 10 00.3 580 715 0.81 25 0.51 0.38 THE THE Comparative Example R-1 5 900 50 40 8 30 4505 80 2 8 00.0 745 875 0.85 18 0.50 0.33 THE THE Comparative Example S I 5 900 50 40 8 30 4505 85 3 10 42.1 555 715 0.78 20 0.44 0.42 B B Comparative Example
    42/47
    Petition 870200007225, of 16/01/2020, p. 45/61
    T 1 U1
    V-1
    900 50 40 8
    450
    0
    0.0
    630
    795
    0.79
    0.49
    0.35
    900 50 40 8
    450
    0
    0.0
    650
    810
    0.80
    0.48
    0.33
    900 50 40 8
    450
    0.3
    580
    720
    0.81
    0.46
    0.33
    Comparative Example
    Comparative Example
    Comparative Example
    W1
    X-1
    900 50 40 8
    450
    80 3
    600
    780
    0.77
    0.42
    0.33
    900 50 40 8
    450
    80 3
    0.6
    565
    735
    0.77
    0.50
    0.39
    Comparative Example
    Comparative Example
    43/47
    Petition 870200007225, of 16/01/2020, p. 46/61
    44/47 [00202] As shown in Table 2, each of the hot-rolled steel sheets of the present examples, which are within the scope of the present invention, has a fatigue-to-curvature limit ratio of 0.45 or more, a curvature fatigue limit ratio with a drilled hole of 0.39 or more, an A ”corrosion resistance rating after coating, an“ A ”curvature rating and a TS tensile strength limit of the sheet 720 MPa or more steel. Therefore, it is revealed that the hot rolled steel sheet of the present invention has excellent bending ability, resistance to coating corrosion and curvature fatigue characteristics of the steel sheet and the perforated part.
    [00203] In contrast, in each of the hot-rolled steel sheets of the comparative examples, at least one of the above limits in the present invention is beyond the appropriate range. Therefore, at least one of the curvature capacity, resistance to coating corrosion and fatigue characteristics of the perforated part is inferior.
    [00204] As for test numbers A-3 and D-1, since there has been a short period of time t during which water is not present in the steel plate, the region where the net-like oxide is present in the surface layer is thick, the curvature fatigue characteristics of the steel sheet and the perforated part are degraded, and the corrosion resistance after coating is unsatisfactory.
    [00205] As for test number A-4, the FT finishing temperature of the final lamination was less than or equal to the appropriate range, the region where the network-like oxide is present in the surface layer is thick, the fatigue characteristics curvature of the steel sheet and the perforated part are degraded and the
    Petition 870200007225, of 16/01/2020, p. 47/61
    45/47 corrosion resistance after coating is unsatisfactory.
    [00206] As for the test numbers K-1, M-1, N-1, S-1 and W-1, since the steel constituents were not appropriate, an oxide layer inside a surface layer of base iron is thick, the curvature fatigue characteristics of the original plate and the perforated part are degraded, and the corrosion resistance after coating is unsatisfactory.
    [00207] As for test numbers A-10, A-11, D-3 and D-4, since the cooling speed between Ar3 and 730 ° C was slow and the oxide similar to the mesh present in the layer part surface was thick, the curvature ability and fatigue characteristics are degraded.
    [00208] As for test number A-5, since the cooling speed between FT and Ar3 was slow and the cooling speed between 730 ° C and 670 ° C was fast, the ferrite fraction is lower and the characteristics of curvature fatigue of the perforated part are degraded.
    [00209] As for test numbers A-6, A-7, I-1 and V-1, since the area ratio of retained martensite and austenite was slow, the roughness of the perforated fracture surface is increased and the curvature fatigue characteristics of the perforated part are degraded.
    [00210] As for the test numbers J-1, L-1 and U-1, since the steel constituents were not appropriate, any one of the following is satisfied: the ferrite fraction is slow, the area ratio of martensite and retained austenite is beyond the appropriate range and the pearlite area ratio is high. Accordingly, the roughness of the perforated fracture surface is increased and the curvature fatigue characteristics of the perforated part are degraded.
    [00211] As for test numbers I-1, Q-1 and S-1, since
    Petition 870200007225, of 16/01/2020, p. 48/61
    46/47 steel constituents were not appropriate, the maximum tensile strength limit (TS) is beyond the appropriate range.
    [00212] As for the test numbers O-1 and P-1, since the content of P or S was in excess, the roughness of the perforated fracture surface is increased and the curvature fatigue characteristics of the perforated part are degraded .
    [00213] As for test numbers R-1 and T-1, since the Ti or Nb content, or the total amount of Ti and Nb was in excess, the roughness of the perforated fracture surface is increased and the curvature fatigue characteristics of the perforated part are degraded.
    [00214] As for the number of test X-1, although the fatigue characteristics were favorable, since the total amount of Ti and Nb was very small, the limit of maximum tensile strength (TS) is beyond the appropriate range.
    [00215] From the results described above of the examples, according to the hot rolled steel sheet and the method for making the hot rolled steel sheet of the present invention, it is revealed that, even when the electroplating coating is carried out on a high-strength hot-rolled steel sheet that has a maximum tensile strength limit of 720 MPa or more, favorable bending capacity, favorable corrosion resistance and the curvature fatigue characteristics of a material- base and a perforated part are obtained.
    INDUSTRIAL APPLICABILITY [00216] According to the present invention, for example, it becomes possible to provide a high-strength hot-rolled steel sheet with excellent bending ability, resistance to coating corrosion and fatigue characteristics of a base material and part perforated, which can be used properly
    Petition 870200007225, of 16/01/2020, p. 49/61
    47/47 as a material for frames, members, chassis and the like of automobiles or trucks. In this way, by applying the present invention to members, such as frames, members, chassis and the like of automobiles or trucks, the corrosion resistance after coating and the fatigue resistance of a perforated member are increased. In addition, a weight reduction advantage can be obtained sufficiently, so that the industrial effects are extremely high.
    LIST OF BASIC MATERIAL REFERENCE SIGNS
    1st oxide surface similar to the scale
    权利要求:
    Claims (12)
    [1]
    1. Hot-rolled steel sheet, characterized by the fact that it consists of,% by mass:
    C: 0.05% to 0.15%,
    Si: 0% to 0.2%,
    Al: 0.5% to 3.0%,
    Mn: 1.2% to 2.5%,
    P: 0.1% or less,
    S: 0.01% or less,
    N: 0.007% or less,
    Ti: 0.03% to 0.10%,
    Nb: 0.008% to 0.06%,
    V: 0% to 0.12%, one or more among Cr, Cu, Ni and Mo: 0% to 2.0% in total,
    B: 0% to 0.005%, one or more among Ca, Mg, La and Ce: 0% to 0.01% in total, total amount of Si and Al: 0.8 x (Mn-1)% or more, total amount of Ti and Nb: 0.04% to 0.14%, and the balance: Fe and impurities, where, in a steel structure, a ratio of the total area of martensite and austenite retained is 3% to 20%, a ferrite area ratio is 50% to 96% and a pearlite area ratio is 3% or less, and where, in a part of the surface layer, the thickness in a plate thickness direction of a region in which a network-shaped oxide is present and is less than or equal to 0.4 pm, and a maximum tensile strength limit is 720 MPa or more.
    [2]
    2. Hot-rolled steel sheet according to claim 1, characterized by the fact that an average grain diameter of an alloy carbide containing Ti and a carbide of
    Petition 870200007225, of 16/01/2020, p. 51/61
    2/4 alloy containing Nb is 10 nm or less.
    [3]
    3. Hot-rolled steel sheet according to claim 1 or 2, characterized by the fact that the yield ratio is 0.82 or less.
    [4]
    4. Hot-rolled steel sheet according to any one of claims 1 to 3, characterized in that it consists of, in mass%:
    Si: 0.001% to 0.2%.
    [5]
    5. Hot-rolled steel sheet according to any one of claims 1 to 4, characterized in that it consists of, in mass%:
    V: 0.01% to 0.12%.
    [6]
    6. Hot-rolled steel sheet according to any one of claims 1 to 5, characterized in that it consists of, in mass%:
    one or more among Cr, Cu, Ni and Mo: 0.02% to 2.0% in total.
    [7]
    7. Hot-rolled steel sheet according to any one of claims 1 to 6, characterized by the fact that it consists of, in mass%:
    B: 0.0003% to 0.005%.
    [8]
    8. Hot-rolled steel sheet according to any one of claims 1 to 7, characterized by the fact that it consists of, in mass%:
    one or more among Ca, Mg, La and Ce: 0.0003% to 0.01% in total.
    [9]
    Hot-rolled steel sheet according to any one of claims 1 to 8, characterized in that a surface is coated or subjected to the surface coating / galvanizing treatment followed by the formation of the coated layer alloy.
    Petition 870200007225, of 16/01/2020, p. 52/61
    3/4
    [10]
    10. Method for making a hot-rolled steel sheet, characterized by the fact that it comprises:
    heat a plate consisting of, in% by mass,
    C: 0.05% to 0.15%,
    Si: 0% to 0.2%,
    Al: 0.5% to 3.0%,
    Mn: 1.2% to 2.5%,
    P: 0.1% or less,
    S: 0.01% or less,
    N: 0.007% or less,
    Ti: 0.03% to 0.10%,
    Nb: 0.008% to 0.06%,
    V: 0% to 0.12%, one or more among Cr, Cu, Ni and Mo: 0% to 2.0% in total,
    B: 0% to 0.005%, one or more among Ca, Mg, La and Ce: 0% to 0.01% in total, total amount of Si and Al: 0.8x (Mn-1)% or more, amount total Ti and Nb: 0.04% to 0.14%, and the balance: Fe and impurities, and perform crude and final lamination sequentially;
    maintain, from when stripping is carried out before the final lamination until when the final lamination is finished, for three seconds or more, a state in which water is not present on a steel sheet surface, and define a finishing temperature the final lamination is 850 ° C or more;
    perform cooling in which an average cooling speed between the finishing temperature of the final lamination and an Ar3 temperature is 25 ° C / s or more, an average cooling speed between the Ar3 temperature and 730 ° C is 30 ° C / s or more, an average cooling speed between 730 ° C and 670 ° C is 12 ° C / s
    Petition 870200007225, of 16/01/2020, p. 53/61
    4/4 or less, and an average cooling speed between 670 ° C and 550 ° C is 20 ° C / s or more; and laminating the hot rolled steel sheet at 530 ° C or less.
    [11]
    11. Method for making a hot-rolled steel sheet, characterized by the fact that it comprises:
    strip the hot-rolled steel sheet obtained with the method, as defined in claim 10, and then heat the hot-rolled steel sheet to 800 ° C or less and immerse the hot-rolled steel sheet in a galvanic bath .
    [12]
    12. Method for making a hot-rolled steel sheet according to claim 11, characterized by the fact that it comprises:
    perform alloying treatment in a coated layer.
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    JP5574070B1|2014-08-20|
    US20150218708A1|2015-08-06|
    MX2015002759A|2015-05-15|
    TW201425601A|2014-07-01|
    CN104704136A|2015-06-10|
    BR112015005020A2|2017-07-04|
    KR101654492B1|2016-09-05|
    TWI475115B|2015-03-01|
    PL2902520T3|2019-06-28|
    WO2014050954A1|2014-04-03|
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    法律状态:
    2018-11-21| B06F| Objections, documents and/or translations needed after an examination request according [chapter 6.6 patent gazette]|
    2019-04-02| B06A| Patent application procedure suspended [chapter 6.1 patent gazette]|
    2019-08-13| B25D| Requested change of name of applicant approved|Owner name: NIPPON STEEL CORPORATION (JP) |
    2019-12-10| B06A| Patent application procedure suspended [chapter 6.1 patent gazette]|
    2020-03-03| B09A| Decision: intention to grant [chapter 9.1 patent gazette]|
    2020-05-05| 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 26/09/2013, OBSERVADAS AS CONDICOES LEGAIS. |
    2021-08-10| B21F| Lapse acc. art. 78, item iv - on non-payment of the annual fees in time|Free format text: REFERENTE A 8A ANUIDADE. |
    2021-11-30| 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 2640 DE 10-08-2021 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:
    申请号 | 申请日 | 专利标题
    JP2012213728|2012-09-27|
    PCT/JP2013/076027|WO2014050954A1|2012-09-27|2013-09-26|Hot-rolled steel sheet, and production method therefor|
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