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    • 专利摘要:
    It is an object of the invention to provide a thick high strength steel plate of more than 690 MPa with excellent toughness at ultra low temperature (especially toughness at ultra low temperature in the C direction) at -196 ° C or less. and capable of achieving the brittle fracture area ratio at -196 ° C <10% in Ni steel having a Ni content of approximately 5.0 - 7.5%.
    公开号:BE1021357B1
    申请号:E2013/0263
    申请日:2013-04-11
    公开日:2015-11-05
    发明作者:Nako Hidenori；Ibano Akira
    申请人:Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.)；
    IPC主号:
    专利说明:

    "Thick steel plate with excellent toughness at ultra low temperature"
    BACKGROUND OF THE INVENTION 1. Field of the invention
    The present invention relates to a thick steel plate with excellent toughness at ultra-low temperature and more specifically relates to a thick steel plate with excellent toughness at ultra-low temperature equal to or lower than -196 ° (in particular the tenacity in the direction of the width of the plate (direction C)) even when the Ni content is reduced to approximately 5.0 - 7.5%. Below, thick steel plates for liquefied natural gas (LNG) (typically, storage tank, transport vessel, and the like) exposed to the ultra-low temperature described above will be mainly described, but the The thick steel plate of the present invention is not limited thereto and is applicable to thick steel plates generally used for applications exposed to the ultra-low temperature of -196 ° C or less. 2. Description of the Related Art
    In a thick LNG tank steel plate, used for a liquefied natural gas (LNG) storage tank, high toughness that can withstand the ultra-low temperature of -196 ° C or lower is required in addition to high resistance. So far, approximately 9% thick steel plates containing Ni (9% Ni steel) have been used as thick steel plates for this purpose but, as the cost of Ni has increased these In recent years, the development of thick steel plates with excellent toughness at ultra low temperature even with a low Ni content of less than 9% has made progress.
    For example, the non-patent literature 1 (Yano et al., "The influence of ay two-phase coexisting heat treatment region exerted on low temperature toughness of 6% Ni steel", Tetu-To-Hagane (Iron and Steel), 1973 6, pp. 752-763) describes the influence exerted by a heat treatment of a coexistence region of two α-γ phases on the low-temperature steel tenacity at 6% Ni. More specifically, it describes that, by subjecting the coexistence region of two α-γ phases (between α-ASA) (treatment L) to a thermal treatment before exerting a treatment of income, it is possible to confer a tenacity on an ultra-low temperature of -196 ° C equal to or greater than that of 9% Ni steel which has been subjected to tempering and ordinary tempering treatment; this heat treatment also improves the toughness of a direction specimen C (width direction of the plate); these effects result from the presence of residual austenite which is in large quantity, fine and stable even under an impact load at ultra low temperature and the like. But, according to the method, although the ultra-low temperature toughness in the rolling direction (L direction) is excellent, the ultra-low temperature toughness in the direction of the plate width (C direction) tends to be lower than that in the direction L. There is also no description of the ratio of brittle fracture surface.
    Technologies similar to non-patent literature 1 are described in JP-A Nos. S49-135813 and JP-A No. S51-13308. Of these, JP-A No. S49-135813 discloses a process in which 4.0-10% Ni-containing steel with an austenite grain size and the like controlled within a predetermined range is laminated to heat and is then heated between AS1-ASZ, then a cooling treatment (equivalent to the treatment L described in the non-patent literature 1) is repeated once or twice, and the income is then performed at a transformation point temperature Aci or less. JP-A No. S51-13308 also discloses a process in which 4.0-10% Ni-containing steel with AIN size before hot rolling to 1 μm or less is subjected to a similar heat treatment. to that of JP-A No. S49-135813 (treatment of income treatment). The resilience values at -196 ° C (vE - i96) described in these processes are believed to be probably those in the L direction, but the toughness value in the C direction is unclear. In these methods, the resistance is also not taken into consideration and there is no description relating to the brittle fracture surface ratio.
    Non-patent literature 2 (Furuya et al., "Development of 6% Ni Steel for LNG Tank", CAMP-ISIJ, Vol 23 (2010), 1322) also describes the development of 6% Ni steel. for LNG tank that combines L treatment (two-phase region quench treatment) and TMCP. According to the literature, although it is described that the toughness in the rolling direction (L direction) has a high value, there is no description of the toughness value in the direction of the width of the plate ( direction C). JP-A No. 2001-123245 discloses a high tenacity high tenacity steel having excellent toughness in the weld section with 570 MPa or more and containing 0.3% and 0.3% Ni in a predetermined amount. with Mg oxide particles of a predetermined grain size adequately dispersed. JP-A No. 2001-123245 discloses that the heated austenite grain size is refined by controlling the oxide containing Mg and the toughness of the base metal and the heat-affected weld zone (ZAC) improves; and that for this purpose, the amount of O (oxygen) before adding deoxidizing elements and the order of adding Mg and other deoxidizing elements are important, and the molten steel with a quantity of dissolved oxygen of 0.001 - 0.02% is added with Mg, Ti and Al at the same time and is then cast to obtain a billet, or when adding Mg, Ti and Al, Al is added last, and molten steel is then cast to obtain a billet. In an example of JP-A No. 2001-123245, a toughness value in the C direction (breaking surface transition temperature vTrs) is described. Although the property of the 9% Ni steel is excellent (fracture surface transition temperature vTrs <-196 ° C), the property of the Ni steel close to 5% is -140 ° C, and there is still room for improvement.
    As described above, so far, in Ni steel having a Ni content of approximately 5.0 - 7.5%, excellent technologies in terms of ultra-low temperature toughness at -196 ° These have been proposed, but the ultra-low temperature tenacity in the C direction has not been sufficiently studied. In particular, further improvements in ultra-low temperature toughness when base metal strength is high (more specifically, tensile strength TS> 690 MPa, yield strength YS> 590 MPa) (improvement of ultra-low temperature toughness in the C direction) have been in great demand.
    In addition, the ratio of brittle fracture surface has never been studied in the literature described above. The brittle fracture surface ratio is a brittle fracture rate when a load is applied in the Charpy Resiliency Test. In a section where the brittle fracture has occurred, the energy absorbed by the steel over time until the break occurs becomes extremely small and the breakage progresses easily, and therefore, to suppress the break at ultra low In particular, a particularly important requirement is to remove the brittle surface area ratio appearing in the Charpy resiliency test at a low level (10% or less). However, since the strength is higher, the brittle fracture is likely to occur and, therefore, it is generally difficult to obtain a brittle fracture surface ratio <10% with the base metal strength as high as described. above. Therefore, in a thick high strength steel plate whose base metal has a high strength, a technology satisfying both has not yet been proposed.
    SUMMARY OF THE INVENTION
    The present invention has been developed in view of these circumstances and its purpose is to provide a thick, high strength steel plate with excellent ultra low temperature toughness (especially ultra-low temperature toughness in the C direction). ) at -196 ° C and capable of achieving a brittle fracture surface ratio <10% in Ni steel having a Ni content of approximately 5.0 - 7.5%.
    A thick steel plate with excellent ultra low temperature toughness in connection with the present invention that could solve the problems described above is a thick steel plate containing in mass%, C: 0.02-0 , 10%, Si: 0.40% or less (not including 0%), Mn: 0.50 - 2.0%, P: 0.007% or less (not including 0%), S: 0.007% or less ( not including 0%), Al: 0.005 -0.050%, Ni: 5.0 -7.5% and N: 0.010% or less (not including 0%), the balance comprising iron and unavoidable impurities, wherein a volume fraction (V) of a residual austenitic phase present at -196 ° C satisfies 2.0% -12.0%, and when the numerical density of inclusions of more than 1.0 pm of circle equivalent diameter present in the steel is set at Z, Z <200 pieces / mm2, and a value A expressed by an expression (1) below satisfies 11.5 or less. Α = ν2 / 3 + 0.012χπχΖ- (1)
    In one embodiment of the present invention, the thick steel plate satisfies the condition that the residual austenitic phase at -196 ° C is 4.0-12.0% in terms of volume fraction.
    In one embodiment of the present invention, the thick steel plate contains Cu: 1.00% or less (not including 0%).
    In one embodiment of the present invention, the thick steel plate further contains at least one member selected from a group consisting of Cr: 1.20% or less (not including 0%) and Mo: 1.0 % or less (not including 0%).
    In one embodiment of the present invention, the thick steel plate further contains at least one member selected from a group consisting of Ti: 0.025% or less (not including 0%), Nb: 0.100% or less ( not including 0%) and V: 0.50% or less (not including 0%).
    In one embodiment of the present invention, the thick steel plate contains B: 0.0050% or less (not including 0%).
    In one embodiment of the present invention, the thick steel plate further contains at least one member selected from a group consisting of Ca: 0.0030% or less (not including 0%), REM: 0.0050 % or less (not including 0%) and Zr: 0.005% or less (not including 0%).
    According to the present invention, with Ni-low steel in which the Ni content is reduced to approximately 5.0 - 7.5%, it has been possible to provide a thick, high strength steel plate, which has excellent ultra-low temperature toughness at -196 ° C or lower (especially ultra-low temperature toughness in the C direction), more specifically, which satisfies the brittle fracture area ratio at -196 ° C <10 % (preferably at the brittle fracture area ratio at -233 ° C <50%) in the Charpy shock absorption test in the C direction even when the base metal strength is high (more specifically, the resistance tensile strength TS> 690 MPa, yield strength YS> 590 MPa).
    To provide a technology to improve ultra-low temperature toughness satisfying the brittle fracture surface ratio at -196 ° C <10% in the C-direction Charpy resistivity value in a thick high strength steel plate in which Ni content is reduced to 7.5% or less and the tensile strength RS> 690 MPa and YS elastic limit> 590 MPa are satisfied, the present inventors have carried out studies. As a result of these, it was found that the intended purpose is achieved when (a) a volume fraction V of a residual austenitic phase (y residual) present at -196 ° C is controlled at 2.0-12 , 0% (preferably 4.0-12.0% (volume fraction)), and (b) for inclusions of greater than 1.0 μm circle equivalent diameter (may simply be referred to hereinafter as " inclusions ") which promote brittle fracture, the Z numerical density of the inclusions is reduced to 200 units / mm 2 or less, and an A value expressed by an expression (1) below is controlled at 11.5 or less, and the present invention has been completed. Α = ν2 / 3 + 0.012χπ * Ζ ·· (1)
    In particular, a notable feature related to the prior art described above is the latter (b). The manner in which the present invention has been realized will be described below.
    The present inventors have conducted intensive studies to provide a thick steel plate with an excellent low temperature toughness of -196 ° C in Ni steel having a Ni content of 7.5% or less. More specifically, in the present invention, in view of providing a high strength ultra high temperature steel plate with ultra low temperature toughness that satisfies all properties of the brittle fracture surface ratio <10%, the tensile strength TS> 690 MPa and the yield strength YS> 590 MPa in the C direction of the processes taught in the literature described in the prior art were studied.
    In literature, it is taught that it is important to stabilize the residual austenite (residual γ) present at -196 ° C in order to improve the ultra-low temperature toughness of the 5% Ni steel. It is also taught that when the manufacturing process is taken into consideration as a whole, it is recommended a process in which the amount of oxygen in solution before adding deoxidizing elements is controlled in a melting step of the the casting is carried out in such a way that Al is added last in the molten steel, the heat treatment (treatment L) in the region where two α-γ phases coexist (between Aci-Ac3) is carried out, and the treatment The income is then run at a transformation point temperature of Ac 1 or less, and the ultra low temperature toughness is thereby improved. However, according to the results of the studies by the present inventors, it has been found that, although the ultra-low temperature toughness in the L direction has been improved by said process, the ultra-low temperature toughness in the C direction has not been improved. not sufficient, and the target level referred to in the present invention (the ratio of brittle fracture area in the C direction <10%) could not be reached.
    As a result, other studies have been done. As a result, it was found that in order to obtain a thick steel plate with excellent ultra low temperature toughness, it was essential to add other requirements with regard to the thick plate of and manufacturing process for this purpose while basically following the technologies described above. More specifically, it has been found that: (i) in a thick steel plate, it is effective, focusing on inclusions of more than 1.0 pm diameter circle diameter which is proven to be promote the development of brittle fracture, that the numerical density Z of the inclusions be reduced to Z <200 units / mm2, and that a value A expressed by the relational expression (1) between the numerical density Z (units / mm2) inclusions and the volume fraction V (%) of the residual γ phase at -196 ° C is reduced to the value A <11.5 in addition to the fact that the residual γ phase at -196 ° C must be present for the volume fraction V is in a range of V = 2.0% -12.0% and (ii) in order to manufacture such a thick steel plate, it is effective to further control the melting step of the steel in addition to controlling the amount of dissolved oxygen (amount of O free) before adding Al to the melting step of the control, control the heating temperature (T2) of the slab during the hot rolling step, and perform the heat treatment between Aci-AC3 (treatment L) -> treatment of income in a predetermined temperature range after hot rolling, and it is effective to control the cooling time (t2) at 1450-1500 ° C in casting at 300 s or less.
    In addition, the following has been found and the present invention has been completed: (c) by controlling the residual γ phase present at -196 ° C to 4.0-12.0% (volume fraction) in (a) ci above, the ratio of brittle fracture surface can be maintained at an excellent level of 50% or less even at the lower temperature of -233 ° C, and (d) in order to manufacture such a thick steel plate, it is effective that it is maintained for a predetermined time in the heat treatment between AS1-ASZ (treatment L) after hot rolling.
    In this specification, "excellent ultra low temperature toughness" means meeting the brittle fracture area ratio at -196 ° C <10% when the brittle fracture area ratio in the shock absorption test Charpy in the C direction (width direction of the plate) is measured by a method described in a column of an example described below. Although the ratio of brittle fracture surface in the L direction (rolling direction) was not measured in the example described below, it is based on empirical knowledge that the brittle fracture surface ratio in the L direction inevitably becomes 10% or less when the ratio of brittle fracture surface in the C direction is 10% or less.
    In this specification, "thick steel plate" means a steel plate having a thickness of approximately 6 - 50 mm.
    In the present invention, a thick, high-strength steel plate satisfying tensile strength TS> 690 MPa and yield strength YS> 590 MPa is the goal.
    DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The thick steel plate of the present invention will be described in detail below.
    As described above, the thick steel plate of the present invention is a thick steel plate containing, in mass%, C: 0.02-0.10%, Si: 0.40% or less (not including 0%), Mn: 0.50 - 2.0%, P: 0.007% or less (not including 0%), S: 0.007% or less (not including 0%), Al: 0.005-0.050% , Ni: 5.0-7.5% and N: 0.010% or less (not including 0%), the remainder comprising iron and unavoidable impurities, wherein a volume fraction (V) of a residual austenitic phase present at -196 ° C satisfies 2.0% - 12.0%, and when the numerical density of inclusions of more than 1.0 pm of circle equivalent diameter present in the steel plate is fixed at Z, Z <200 units / mm2, and an A value expressed by an expression (1) below satisfies 11.5 or less. Α = ν2 / 3 + 0.012χπ * Ζ ·· (1)
    The composition of the steel will be described first. C: C: 0.02-0.10% C is an essential element for obtaining resistance and residual austenite. In order to perform such an action effectively, the lower limit of the amount of C is set to 0.02% or more. The lower limit of the amount of C is preferably 0.03% or more, more preferably 0.04% or more. However, when C is added in excess, the ultra-low temperature toughness deteriorates due to an excessive increase in resistance and, therefore, the upper limit of C is set to 0.10%. The upper limit of the amount of C is preferably 0.08% or less, more preferably 0.06% or less.
    If: 0.40% or less (not including 0%)
    Si is a useful element as a deoxidizing material. However, when Si is added in excess, the formation of an island-shaped hard martensite phase is favored, the ultra-low temperature toughness deteriorates and, therefore, the upper limit of Si is set to 0.40. % or less. The upper limit of the amount of Si is preferably 0.35% or less, more preferably 0.20% or less.
    Mn: 0.50 - 2.0%
    Mn is a stabilizing element austenite (γ) and is a contributing element to increase the amount of residual γ. In order to exert such action effectively, the lower limit of the amount of Mn is set at 0.50%. The lower limit of the amount of Mn is preferably 0.6% or more, more preferably 0.7% or more. However, when Mn is added in excess, an income-related embrittlement occurs, the desired ultra-low temperature toughness can not be obtained and, therefore, the upper limit of Mn is set at 2.0% or less. The upper limit of the amount of Mn is preferably 1.5% or less, more preferably 1.3% or less. P: P: 0.007% or less (not included 0%) P is an impurity element that becomes a cause of intergranular rupture, and in order to obtain the desired toughness at ultra low temperature, the upper limit of P is set to 0.007% or less. The upper limit of the amount of P is preferably 0.005% or less. Although the amount of P is preferably as small as possible, it is difficult to achieve a 0% P level at the industrial level. S: S: 0.007% or less (not including 0%)
    In a manner similar to P described above, S is also an impurity element which becomes a cause of intergranular rupture and, in order to obtain the desired toughness at ultra low temperature, the upper limit of S is set to 0.007% or less. As shown in an example described below, as the amount of S increases, the ratio of brittle fracture surface increases, and the desired toughness at ultra low temperature (the brittle fracture surface ratio at -196 ° C <10% ) can not be obtained. The upper limit of the amount of S is preferably 0.005% or less. Although the amount of S is preferably as small as possible, it is difficult to arrive at an amount of S of 0% at the industrial level.
    Al: 0.005 - 0.050%
    Al is a deoxidizing element. When the Al content is insufficient, the oxygen content of the steel increases, the numerical density of inclusions of more than 1.0 μm of circle-equivalent diameter increases and, therefore, its lower limit is set to 0.005% or more. The lower limit of the amount of Al is preferably 0.010% or more, more preferably 0.015% or more. However, when Al is added in excess, conglomeration and integration of inclusions are favored, numerical density of inclusions increases, and therefore the upper limit of Al is set to 0.050% or less. The upper limit of the amount of Al is preferably 0.045% or less, more preferably 0.04% or less.
    Ni: 5.0 - 7.5%
    Ni is an indispensable element for fixing residual (residual) austenite which is useful for improving the ultra low temperature toughness. In order to perform such an action effectively, the lower limit of the amount of Ni is set at 5.0% or higher. The lower limit of the amount of Ni is preferably 5.2% or more, more preferably 5.4% or more. However, when Ni is added in excess, the cost of the material increases and, therefore, the upper limit of Ni is set at 7.5% or less. The upper limit of the amount of Ni is preferably 7.0% or less, more preferably 6.5% or less, more preferably 6.0% or less. N: 0.010% or less (not including 0%)
    Since N deteriorates the ultra low temperature toughness by stress aging, the upper limit of N is set to 0.010% or less. The upper limit of the amount of N is preferably 0.006% or less, more preferably 0.004% or less. The thick steel plate of the present invention comprises the compositions described above as base compositions and the balance is iron and unavoidable impurities.
    The present invention may contain the following selective compositions for the purpose of imparting additional characteristics.
    Cu: 1.00% or less (not included 0%)
    Cu is a stabilizing element γ and is a contributing element to increase the amount of residual γ. In order to exert such action effectively, the Cu content should preferably be 0.05% or more. However, when Cu is added in excess, the resistance increases excessively, the desired ultra low temperature toughness can not be obtained and, therefore, the upper limit of Cu is preferably 1.00% or less. More preferably, the upper limit of the amount of Cu is 0.8% or less, more preferably 0.7% or less.
    At least one element selected from a group consisting of Cr: 1.20% or less (not including 0%) and Mo: 1.0% or less (not including 0%)
    Cr and Mo are both elements that improve resistance. These elements can be added alone, and both elements can be used in combination. In order to perform the action effectively, it is preferable to set the amount of Cr to 0.05% or more and the amount of Mo to 0.01% or more. However, when they are added in excess, the resistance increases excessively, the desired toughness at ultra low temperature can not be guaranteed and, therefore, the upper limit of the amount of Cr is preferably 1.20% or less (better still 1.1% or less, better still 0.9% or less, and even better 0.5% or less) and the upper limit of the amount of Mo is preferably 1 0% or less (better still 0.8% or less, even better 0.6% or less).
    At least one element selected from a group consisting of Ti: 0.025% or less (not including 0%), Nb: 0.100% or less (not including 0%) and V: 0.50% or less (not including 0%)
    Ti, Nb and V are all elements precipitating as carbonitride and improving the resistance. These elements can be added alone, and two or more elements can be used in combination. In order to exert the action effectively, it is preferable to set the amount of Ti to 0.005% or more, the amount of Nb to 0.005% or more and the amount of V to 0.005% or more. However, when they are added in excess, the resistance increases excessively, the desired toughness at ultra low temperature can not be guaranteed and, therefore, the upper limit of the amount of Ti is preferably 0.025% or less ( more preferably 0.018% or less, more preferably 0.015% or less), the upper limit of the amount of Nb is preferably 0.100% or less (more preferably 0.05% or less, more preferably 0.02%) % or less) and the upper limit of the amount of V is preferably 0.50% or less (more preferably 0.3% or less, more preferably 0.2% or less). B: B: 0.0050% or less (not included 0%) B is a contributing factor in improving strength by improving quenchability. In order to perform the action effectively, it is preferable to set the amount of B at 0.0005% or higher. However, when B is added in excess, the resistance increases excessively, the desired ultra low temperature toughness can not be guaranteed and, therefore, the upper limit of the amount of B is preferably 0.0050% or less (better still 0.0030% or less, better still 0.0020% or less).
    At least one element selected from a group consisting of Ca: 0.0030% or less (not including 0%), REM (rare earth element): 0.0050% or less (not including 0%) and Zr: 0.005% or less (not including 0%)
    Ca, REM and Zr are all deoxidizing elements. By adding them, the oxygen content of the steel decreases and the numerical density of inclusions of more than 1.0 pm of circle equivalent diameter decreases. These elements can be added alone, and two or more elements can be used in combination. In order to perform the actions effectively, it is preferable to set the amount of Ca to 0.0005% or more, the amount of REM (when REM described hereinafter is contained alone, the quantity is the content alone, and when two or more types are contained, the quantity is the total quantity thereof, and hereinafter also for the amount of REM) at 0.0005% or more and the amount of Zr at 0.0005% or more. However, when they are added in excess, the numerical density of the inclusions on the contrary increases, the ultra-low temperature toughness deteriorates and, therefore, the upper limit of the amount of Ca is preferably 0.0030% or less ( more preferably 0.0025% or less), the upper limit of the amount of REM is preferably 0.0050% or less (more preferably 0.0040% or less) and the upper limit of the amount of Zr is preferably 0.005% or less (more preferably 0.0040% or less).
    In this specification, REM (rare earth element) is a group of lanthanide elements (15 elements of La having the atomic number 57 at Lu having the atomic number 71 in the periodic table) plus Sc (scandium) and Y (yttrium), and they can be used alone or two or more elements can be used in combination. The rare earth elements are preferably Ce and La. The form of REM addition is not particularly limited. REM can be added in the form of a mischmetal containing mainly Ce and La (eg Ce: approximately 70%, La: approximately 20 -30%), or can be added otherwise as a single body of Ce, La and the like.
    The composition of the steel of the present invention has been described above. The thick steel plate of the present invention additionally satisfies 2.0-12.0% (preferably 4.0-12.0%) of the residual phase y present at -196 ° C in terms of fraction. volume.
    It is known that the residual y-phase present at -196 ° C. contributes to improving the ultra-low temperature toughness. In order to exert such action effectively, the volume fraction of the residual phase y relative to the total structure present at -196 ° C is set at 2.0% or higher. However, the residual phase y is comparatively softer than a matrix phase, a predetermined value of YS can not be guaranteed when the residual phase y becomes excessive and, therefore, the upper limit thereof is set to 12. , 0% (see No. 43 of Table 2B below.) For the volume fraction V of the residual y phase, the lower limit is preferably 4.0% or more, more preferably 6.0%. or more, and the upper limit is preferably 11.5% or less, more preferably 11.0% or less.
    By controlling the volume fraction of the residual γ phase relative to the total structure present at -196 ° C to 4.0% or higher, the brittle fracture area ratio can be maintained at an excellent level of 50% or less, even at -233 ° C which is below -196 ° C described above. A more preferable lower limit when such an effect is to be exerted is 6.0% or more, and the preferable upper limit is the same as above.
    In addition, in the thick steel plate of the present invention, the control of the volume fraction V of the residual γ phase is important compared to the structure present at -196 ° C. and the structure other than the residual γ phase. is not limited in any way and may be those usually present in thick steel plates. For example, as a structure other than the residual γ phase, there may be mentioned carbides such as bainite, martensite, cementite and the like.
    In the thick steel plate of the present invention also, with respect to the inclusions of more than 1.0 pm of circle equivalent diameter which are present in the steel plate, the numerical density Z of the inclusions satisfies Z <200 units / mm2, and an A value expressed by the expression (1) below satisfies 11.5 or less. Α = ν2 / 3 + 0.012χπχΖ ·· (1)
    Here, the "circle equivalent diameter" is the diameter obtained as celu. of a supposed circle so that, by observing the size of the inclusion, the areas of the inclusion and the circle become equal to each other.
    Here, we focused on inclusions of greater than 1.0 pm circle equivalent diameter in the present invention because it was clarified that inclusions promoted the development of brittle fracture. That is, to improve the ratio of brittle fracture surface to ultra low temperature while achieving a predetermined high strength, the inclusions that promote brittle fracture should be reduced, but according to the results of the studies by the inventors, it was known that, when the numerical density Z of the inclusions increased, even if the volume fraction V of the residual γ phase at -196 ° C. was controlled in the range described above, the desired toughness at ultra low temperature could not be obtained (see Nos. 33, 36, 47-50 of Table 2B below). The numerical density Z of the inclusions is preferably as low as possible and is preferably 150 units / mm 2 or less, more preferably 120 units / mm 2 or less. In the present invention, the average size of the inclusions having more than 1.0 μm of circle equivalent diameter (average circle diameter) is approximately 2.0 μm or less.
    Inclusions can be measured by a method described in the example below. Here, the type of "inclusions" in inclusions larger than 1.0 pm in circle equivalent diameter is not particularly limited in the present invention. The reason for this is that the occurrence of brittle fracture is more strongly influenced not by the type of inclusions but by the size (average circle diameter) of the inclusions. As regards the type of inclusions, mention may be made in addition to individual particles such as oxides, nitrides, oxynitrides and the like, for example, a complex obtained by combining two or more types of these individual particles, or complex particles. obtained by uniting these individual particles and other elements and the like.
    Moreover, from the sole point of view of inclusion control, similar technology has been disclosed in JP-A No. 2001-123245, but the inclusion control direction is very different from that of the present invention. That is, in JP-A No. 2001-123245, especially Mg is monitored, and the magnification of the austenite grains at high temperature is suppressed and the toughness is improved by dispersing a large number of fine particles of Mg containing oxide having a size of 2.0 μm or less, whereas in the present invention coarse inclusions which become the starting point of brittle fracture or ductile failure and deteriorate toughness are reduced regardless of their type, and both are totally different from each other with respect to the inclusion control method. In addition, according to a preferred manufacturing method of the present invention described below, fine inclusions of 2.0 μm or less of circle equivalent diameter are present at approximately 100-1000 units / mm 2. In addition, when they are limited to Mg-containing oxides among the fine inclusions of 2.0 pm or less of circle equivalent diameter, they are hardly present in the present invention.
    Furthermore, in the present invention, it is necessary not only to control the absolute value of the numerical density Z of the inclusions but also that the value A expressed by the expression (1) above satisfies the value A <11, 5.
    Here, although the value A is calculated from the relation between the numerical density Z of the inclusions and the volume fraction V of the residual austenitic phase (y residual) present at -196 ° C as shown in the expression (1) above, the value A was obtained by experimentally finding the contribution ratio of the two exerted on the ratio of brittle fracture area in the ultra-low temperature region on the basis of a number of basic experiments in view of the In order to reduce the ratio of brittle fracture surface to 10% or less, the shape of the two should be adequately controlled, while the inclusions favoring brittle fracture are reduced and the residual phase, which is suitable for promoting ductile failure, is guaranteed. was included in expression (1) above for the following reason: the empirical formula was deduced from the assumption that the ratio of area (nx radius2) of inclusions became important as a parameter influencing brittle fracture because brittle fracture was considered to be favored when hard inclusions were present in large numbers on the fracture crack development plane in the Charpy test. As shown in examples of the invention below, the ultra-low temperature toughness at -196 ° C, particularly the brittle fracture surface ratio in the Charpy shock absorption test, in the thick The predetermined high-strength steel plate can reach a desired high level only by controlling the A value at 11.5 or lower in addition to controlling the volume fraction V of the residual γ phase and the numerical density Z of the inclusions. On the other hand, when the value A exceeded 11.5, the ratio of brittle fracture area <10% could not be ensured. The value A is preferably as small as possible and is preferably 11.0 or less, more preferably 10.0 or less. Furthermore, although the lower limit of the A value is not particularly limited from the above point of view, it is preferably approximately 2.5 or more, taking into account the equilibrium with the acceptable range of the volume fraction V of the residual γ phase and the numerical density Z of the inclusions.
    A method of manufacturing the thick steel plate of the present invention will now be described.
    The manufacturing method in connection with the present invention is characterized in (A) - (C) below. (A) In the steel melting step, the amount of free oxygen [O] before Al addition is controlled at 100 ppm or less and the cooling time (t2) at 1450-1500 ° C in casting is controlled at 300 s or less. By the method of (A), the numerical density Z of inclusions described above can in particular be reduced to a predetermined range. (B) In the hot rolling step, the pre-rolling heating temperature (T2) is controlled at 1120 ° C or higher. By the method of (B), the numerical density Z of inclusions described above is in particular reduced to 200 units / mm 2 or less. (C) After hot rolling, the steel plate is heated and held in a temperature range of Aci-Ac3 points, is then cooled with water, and then subjected to an income treatment for 10-60 min in a range. temperature of 520 ° C - Ac1 point and is then cooled by air or cooled by water. By the method of (C), in particular, the volume fraction of the residual γ phase at -196 ° is appropriately controlled.
    In addition, the value A can be controlled within the predetermined range by appropriately controlling (A) - (C) above because the value A stipulated in the present invention is a parameter related to the numerical density of inclusions and to the volume fraction of the residual phase y.
    In comparison with the prior art described above, the most distinct characteristic is to control t 2 especially in the process of (A) above.
    The respective steps will be described in detail below. (Melting step)
    In the present invention, particular attention is given to a method for adding Al. This is because, as the inclusions of greater than 1.0 pm diameter circle to be controlled in the present invention are obtained mainly by composite formation secondary inclusions such as oxides, sulfides and the like at the time of cooling from the starting point of the Al-based inclusions formed in the molten metal, the Al-based inclusions are likely to grow by conglomeration and integration, and the numerical density of the inclusions increases.
    Firstly, by adding Al which is a deoxidizing material in the molten steel, the amount of free oxygen (amount of oxygen in solution, can be shortened as amount [O]) before the addition of Al is controlled at 100 ppm or less. This is because, when the amount [O] exceeds 100 ppm, the size of the inclusions formed by adding Al becomes larger, the numerical density of the inclusions of more than 1.0 pm of circle equivalent diameter increases, and the desired toughness at ultra low temperature can not be obtained (see No. 33 of Table 2B below). The amount [O] is preferably as small as possible and is preferably 80 ppm or less, more preferably 50 ppm or less. The lower limit of the quantity [O] is not particularly limited from the point of view of the reduction of the numerical density of the inclusions.
    As a method for controlling the amount [O] as described above, there may be mentioned for example a deoxidation process by adding deoxidizing elements Mn, Si in molten steel. When deoxidizing materials such as Ti, Ca, REM, Zr and the like are added as selective compositions other than the elements described above, the amount [O] can also be controlled by adding them.
    In order to control Al-based inclusions, control of the amount [O] before the addition of Al is important, and the order of addition of Al and other deoxidizing elements is not important. However, when Al is added in a state where the amount [O] is high, the temperature of the molten steel increases due to an oxidation reaction which is dangerous in service and, therefore, it is preferable to add Si and Mn before Al. It is also preferable to add the selective compositions such as Ti and the like in the molten steel after adding Al.
    Then the casting is started. Although the casting temperature range is generally 1650 ° C or lower, according to the present invention, it has been found that it is important to control in particular the cooling time (t2) in the temperature range of 1450.degree. 1500 ° C to 300 s. or less and that the numerical density of the inclusions of more than 1.0 pm circle equivalent diameter was thus appropriately controlled. When t2 exceeds 300 s, the composite formation of the secondary inclusions with the nuclei of the Al-based inclusions is favored, the numerical density of the inclusions of more than 1.0 pm of circle equivalent diameter increases, the value A increases, etc., and the desired ultra-low temperature toughness is not exerted (see Nos. 34, 35 of Table 2B below). From the above point of view, t 2 is preferably as short as possible and is preferably 290 s or less, more preferably 280 s or less. The lower limit of t2 is not particularly limited from the point of view above.
    In the present invention also, the temperature range of 1450-1500 ° C attracts attention particularly outside the casting temperature range because the temperature range is a temperature range where inclusion growth is favored. by the progress of the solidification in casting and the progression of the concentration of the composition towards the molten acid.
    The temperature range of 1450-1500 ° C also means the temperature of the central part of the thickness of the slab. The thickness of the slab is generally 150-250 mm and the surface temperature tends to be lower than the core temperature of approximately 200-1000 ° C. Since the differential variation of the surface temperature is large, the temperature in the central part (near the thickness tx1 / 2) where the variation is small becomes the goal. The temperature of the central portion of the slab thickness can be measured by inserting a thermocouple into a mold.
    Also in the present invention, the cooling time (t 2) in the temperature range of 1450-1500 ° C should only be controlled at 300 s or less, and the method for this purpose is not limited. For example, cooling can be performed at a constant rate in the temperature range at an average cooling rate of approximately 0.17 ° C / sec or less so that the cooling time in the temperature range will be 300 s or less, or the cooling can be run at different rates so that the cooling time in the temperature range will be 300 s or less.
    Also in the present invention, the cooling method for the casting temperature range other than the temperature range described above is in no way limited, and an ordinary method (air cooling or water cooling) can be employee.
    After running the casting as described above, hot rolling is performed, and the steel plate is subjected to a heat treatment. In the hot rolling step, it is preferable to bring the heating temperature before hot rolling (T2) to 1120 ° C or higher. Thus, among the composite secondary inclusions, the sulfides that are comparatively unstable disappear, the size of the inclusions becomes small and, therefore, the numerical density of the inclusions of more than 1.0 pm of circle equivalent diameter decreases. Considering that the effect of the hot rolling temperature on the amount of sulphide formation increases further because the amount of S in the steel in particular is less (ie that the amount of S that contributes to the amount of sulfide formation is less) in the present invention, the heating temperature before hot rolling T2 should be strictly controlled and should be controlled at a temperature above the temperature range general (near approximately 1100 ° C). When T2 is below 1120 ° C, the numerical density of the inclusions to be controlled increases and the desired toughness at ultra low temperature is not exerted (see No. 36 of Table 2B below). From the above point of view, T2 is preferably as high as possible and is preferably 1140 ° C or higher, more preferably 1160 ° C or higher. However, when T2 becomes excessively high, the cost of production increases and, therefore, its upper limit is preferably controlled at approximately 1180 ° C or less.
    The heating time at the preheat heating temperature T2 is preferably approximately in the range of 1-4 hours.
    Steps other than those mentioned above (final rolling, milling rate and the like) are not particularly limited, and commonly used methods can be employed to achieve the predetermined plate thickness.
    After hot rolling, the steel plate is heated to the temperature range of the ASA (TL) points, is maintained and is then cooled with water. These treatments are equivalent to the treatment L described in the prior art discussed above, and the residual phase y stably at -196 ° C can thus be obtained by a predetermined range.
    More specifically, the steel plate is heated up to the temperature of the two-phase region (ferrite (α) -γ) of points Ас1-Асз (TL). By heating the steel plate to the temperature range, alloying elements such as Ni and the like are concentrated at the formed γ phase, and a quasi-stable residual γ phase exhibits almost stably at room temperature. obtained. As a result, below the Aci point or above the Asz point, the residual γ phase at -196 ° C can not be sufficiently ensured (see Nos. 37, 38 of Table 2B below). The preferred heating temperature is approximately 660-710 ° C.
    The heating time (hold time, tL) at the temperature of the two-phase region is preferably in general 10-50 min. When it is less than 10 min, the concentration of the alloy elements of the γ phase does not progress sufficiently, whereas when it is more than 50 min, the a phase is annealed and the resistance deteriorates. The preferred heating time is approximately 15-30 min. The upper limit of the preferential heating time is 30 min.
    Furthermore, by fixing the heating time to 15 minutes or more, the volume fraction of the residual γ-phase at -196 ° C that is obtained is 4.0% or more, and as a result, excellent toughness is obtained. even at the even lower temperature with the brittle fracture area ratio at -233 ° C which is 50% or less. The most preferred lower limit when such an effect is to be exerted is 5.0% or more. The upper limit of the preferential heating time is also the same as mentioned above (30 min or less).
    Then, after cooling with water at room temperature, the income treatment is performed. The tempering treatment is carried out for 10-60 min (t3) in the temperature range 520 ° C - Aci point (T3). Thus, C is concentrated in the quasi-stable residual γ phase and the stability of the quasi-stable residual γ phase increases, and therefore, the residual γ phase stably exhibits even at -196 ° C is obtained. When the tempering temperature T3 is less than 520 ° C., the quasi-stable residual γ phase formed while the two-phase coexistence region is maintained is disintegrated in phase a and cementite phase and the residual γ phase at -196 ° C. can be sufficiently ensured (see No. 41 of Table 2B below). On the other hand, when the temperature of income T3 exceeds the point Aci or the time of income is less than 10 min, the concentration of C in the residual γ phase does not progress sufficiently, and the desired amount of residual γ to - 196 ° C can not be ensured (see No. 42 (the case where T3 is high) and No. 55 (the case where t3 is short) of Table 2B below). In addition, when the recovery time t3 exceeds 60 min, the residual γ phase at -196 ° C is excessively formed and the predetermined resistance can not be ensured (see No. 43 of Table 2B below.
    The preferable income treatment condition is, T3 tempering temperature: 570-620 ° C, t3 revenue time: 15 min or more and 45 min or less (better still 35 min or less, even better 25 min or less).
    After the income treatment has been performed as described above, the cooling is carried out to room temperature. The cooling process is not particularly limited, and either air cooling or water cooling can be employed.
    In this issue, the Ad point and the Aas point are calculated on the basis of the following expressions (from Kouza-Gendai-No Kinzoku-Gaku (Lecture: Contemporary Metallurgy), material part 4, Tekkou-Zairyou (Iran and Steel Material), Japan Institute of Metals).
    Ad point = 723-10.7x [Mn] -16.9x [Ni] + 29.1x [Si] + 16.9x [Cr] + 290x [As] + 6.38x [W]
    Point AC3 = 910-203 x [C] 1 / 2-15,2x [Ni] + 44,7x [Si] + 104x [V] + 31,5x [Mo] + 13,1x [W] where [] means the content (% by weight) of steel alloying elements. In the present invention, in the expressions, the calculation is done with [As] and [W] both 0% because As and W are not included as the composition in the steel.
    [Examples]
    Although the present invention is explained in more detail below by specific reference to examples, the present invention is not limited by the examples below and can also be implemented with modifications added in the scope adaptable to goals described above and below and any of these should be included in the technical scope of the present invention.
    Sample steels of the componential compositions shown in Table 1 (the remainder: iron and unavoidable impurities, the unit is% by weight) were melted under the melting conditions shown in Table 2 using a vacuum melting furnace ( 150 kg, VIF) and were cast and 150 mm x 150 mm x 600 mm ingots were then manufactured by hot forging. In the present example, approximately 50% of the EC-containing mischmetal was used as EM, with approximately 25% being present. In addition, the order of addition of deoxidizing elements was, when the selective compositions were not included, Si, Mn (added simultaneously) -> AI; while, when the selective compositions of Ti, REM, Zr, Ca were included, Si, Mn (added simultaneously) -> AI -> Ti -> REM, Zr, Ca (added simultaneously). In addition, in the present example, the time from the addition of Al to the beginning of the casting (t1) was set at approximately 10 min in all cases (not shown in the tables). In addition, in Table 2, [O] is the amount of dissolved oxygen (ppm) before adding Al, and t2 is the cooling time (s) at 1500-1450 ° C in casting. Cooling at 1500-1450 ° C was performed by air cooling or water cooling and was controlled in such a way that the cooling time was as described above.
    Then, after heating at various temperatures T2 as shown in Table 2, the ingot was rolled to a plate thickness of 75 mm at the temperature of 830 ° C or above temperature, was rolled at 780 ° C. final rolling temperature, was then cooled by water, and a thick steel plate with a thickness of 25 mm was thus obtained. The steel plate thus obtained was heated to the temperature shown in Table 2 (TL in Table 2), was then heated and held for 5-60 min (see Table 2, TL), and was then cooled. by water at room temperature. Then, after the income treatment (T3 = temperature of income, t3 = time of income) was performed as shown in Table 2, the air cooling or water cooling was performed up to room temperature.
    With regard to the thick steel plate thus obtained, the numerical density Z (units / mm 2) of the inclusions of more than 1.0 μm of circle equivalent diameter, the volume fraction (%) of the residual γ phase -196 ° C, tensile properties (tensile strength TS, yield strength YS), and ultra-low temperature toughness (the ratio of brittle fracture surface in the C direction to -196 ° C or -233 ° C). ° C) were evaluated as described below. (1) Measurement of the numerical density Z of inclusions of more than 1.0 pm of circle equivalent diameter
    The t / 4 (t: plate thickness) position of the steel plate was mirror-polished and four fields of view were photographed at 400x magnification using an optical microscope. The area of view was 0.04 mm2 and the total area of the four fields of view was 0.15 mm2. The inclusions observed in these four fields of view were analyzed by "Image-Pro Plus" produced by Media Cybernetics, Inc., the numerical density Z (units / mm2) inclusions greater than 1.0 μm in circle equivalent diameter ( diameter) was calculated, and its average value was calculated.
    (2) Measurement of the volume fraction of the residual γ phase at -196 ° C
    A 10 mm x 10 mm x 55 mm specimen was taken from the t / 4 position of each steel plate, held for 5 min at the liquid nitrogen temperature (-196 ° C) and then X-ray diffraction measurement on a small two-dimensional portion by an X-ray diffraction apparatus (RINT-RAPID II) made by Rigaku Corporation. Then, with respect to respective lattice plane peaks of (110), (200), (211), (220) of the ferritic phase and respective lattice plane (111), (200), (220) peaks. , (311) of the residual γ phase, the volume fractions of (111), (200), (220), (311) of the residual γ phase were respectively calculated on the basis of the integrated intensity ratio of the respective peaks. , and their average value was obtained, the average value of which was made the "volume fraction (%) of the residual γ phase". (3) Measurement of tensile properties (tensile strength TS, yield strength YS)
    Specimen No. 4 of JIS Z 2241 was taken parallel to the C direction of the t / 4 position of each steel plate, the tensile test was performed by a method described in JIS Z 2241, and the resistance TS tensile strength and YS yield strength were measured. In the present example, those with TS> 690 MPa and YS> 590 MPa were rated as excellent in terms of base metal strength. (4) Ultra low temperature toughness measurement (Fracture failure ratio in the C direction)
    Three pieces of Charpy resilience test specimens (V-notch specimen from JIS Z 2242) were taken parallel to the C direction of position t / 4 (t: plate thickness) and position W / 4 (W: plate width) as well as position t / 4 and position W / 2 of each steel plate, the ratio of brittle fracture area (%) to -196 ° C was measured by the method described in JIS Z 2242, and the average value of each was calculated. Of the two values thus calculated, the lower average value of the property (which is large in the brittle fracture area ratio) was used, and one with 10% or less of this value was rated as excellent in terms of toughness at ultra low temperature in the present example.
    These results have been shown side by side in Table 2. For reference, the Aci point and the Asso point have also been shown in Table 1 and Table 2.
    [Table 1 A] S 263
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    The following study is possible from Table 2.
    First, Nos. 1-32 of Table 2A are examples which satisfy all the requirements of the present invention and the thick steel plate with excellent ultra low temperature toughness (more specifically, the average value of the ratio of brittle fracture surface in the C <10% direction) at -196 ° C even when the strength of the base metal was high could be provided. On the other hand, Nos. 33-43, 55 of Table 2B are the comparative examples which do not meet the requirements of the present invention because at least one of the preferred manufacturing conditions of the present invention was not satisfied, and the desired properties could not be assured.
    More specifically, No. 33 is an example in which the steel composition of No. 33 of Table 1B was used which met the requirements of the present invention, but the numerical density Z of the inclusions increased because the amount [O] which was the amount of dissolved oxygen before adding Al was high. As a result, the brittle fracture area ratio has increased and the desired toughness at ultra low temperature has not been achieved.
    No. 34 is an example in which, although the steel composition of No. 34 of Table 1B which met the requirements of the present invention was used, the value A exceeded the predetermined range because the cooling (t2) of 1500-1450 ° C in casting was long. As a result, the brittle fracture area ratio has increased and the desired toughness at ultra low temperature has not been achieved.
    No. 35 is an example in which the steel composition of No. 35 of Table 1B whose amount P was high was used and the cooling time (t 2) of 1500-1450 ° C in casting was long. and, therefore, the numerical density Z of the inclusions has increased. As a result, the brittle fracture area ratio has increased and the desired toughness at ultra low temperature has not been achieved.
    No. 36 is an example in which the steel composition of No. 36 of Table 1B was used, the amount of C of which was high, the heating temperature (T2) before hot rolling was low, therefore the numerical density Z of the inclusions has increased and the value A has exceeded the predetermined range. As a result, the brittle fracture area ratio has increased and the desired toughness at ultra low temperature has not been achieved.
    No. 37 is an example in which the composition of the steel of No. 37 of Table 1B satisfying the requirements of the present invention was used, but the residual amount γ was insufficient because the heating was carried out at a temperature less than the temperature of the two-phase region (TL). As a result, the brittle fracture area ratio has increased and the desired toughness at ultra low temperature has not been achieved.
    No. 38 is an example in which the steel composition of No. 38 of Table 1B was used, the amount of Si of which was high and the heating was carried out at a temperature above the temperature of the region of two. phases (TL) and therefore the amount of residual γ was insufficient. As a result, the brittle fracture area ratio has increased and the desired toughness at ultra low temperature has not been achieved.
    No. 39 is an example in which the composition of the steel of No. 39 of Table 1B satisfying the requirements of the present invention was used, but the amount of residual γ was insufficient because the holding time of heating (tL) at the temperature of the two-phase region (TL) was short. As a result, the brittle fracture area ratio has also increased and the desired ultra low temperature toughness has not been achieved.
    No. 40 is an example in which the steel composition of No. 40 of Table 1B satisfying the requirements of the present invention is used, but the amount of residual γ has increased because the holding time of heating ( tL) at the temperature of the two-phase region (TL) was long. As a result, the yield strength YS and the tensile strength TS deteriorated and the desired strength of the base metal could not be obtained.
    No. 41 is an example in which the composition of the steel of No. 41 of Table 1B satisfying the requirements of the present invention was used, but the amount of residual γ was insufficient because the tempering temperature (T3 ) was low. As a result, the brittle fracture area ratio has increased and the desired toughness at ultra low temperature has not been achieved.
    No. 42 is an example in which the steel composition of No. 42 of Table 1B was used, the amount of Mn of which was high and the tempering temperature (T2) was high and, consequently, the amount of residual γ was insufficient. As a result, the brittle fracture area ratio has increased and the desired toughness at ultra low temperature has not been achieved.
    No. 43 is an example in which the steel composition of No. 43 of Table 1B satisfying the requirements of the present invention has been used, but the amount of residual γ has increased because the tempering time (T3) has been long. As a result, the yield strength YS deteriorated and the desired toughness at ultra low temperature could not be obtained.
    No. 55 is an example in which the steel composition of No. 55 of Table 1B satisfying the requirements of the present invention was used, but the amount of residual γ was insufficient because the time of income (t3 ) was short. As a result, the brittle fracture area ratio has increased and the desired toughness at ultra low temperature has not been achieved.
    Nos. 44-54 are comparative examples made by the process of the present invention using one in which only the steel composition differed.
    More specifically, No. 44 is an example in which the amount of residual γ was insufficient because the steel composition of No. 44 of Table 1B, whose amount of Mn was less, was used. As a result, the brittle fracture area ratio has increased and the desired toughness at ultra low temperature has not been achieved.
    No. 45 is an example in which the composition of the steel of No. 45 of Table 1B whose amount of S was high was used. As a result, the brittle fracture area ratio increased and the desired ultra low temperature toughness could not be obtained.
    No. 46 is an example in which the steel composition of No. 46 of Table 1B was used, the amount of C was lower, the amount of Al was high and the amount of Ni was less and therefore , the numerical density Z of the inclusions increased and the quantity of residual γ was insufficient. As a result, the brittle fracture area ratio has increased and the desired toughness at ultra low temperature has not been achieved. In addition, TS has also deteriorated.
    No. 47 is an example in which the steel composition of No. 47 of Table 1B was used, the amount of Al of which was lower and the amount of N was high, therefore the numerical density Z of the inclusions was increased and the value A has exceeded the predetermined range. As a result, the brittle fracture area ratio has increased and the desired toughness at ultra low temperature has not been achieved.
    No. 48 is an example in which the steel composition of No. 48 of Table 1B was used, the amounts of Cu and Ca, which were the selective compositions, were high and, consequently, the numerical density Z inclusions increased and the value A exceeded the predetermined range. As a result, the brittle fracture area ratio has increased and the desired toughness at ultra low temperature has not been achieved.
    No. 49 is an example in which the steel composition of No. 49 of Table 1B was used, the amounts of Cr and Zr, which were the selective compositions, were high and, consequently, the numerical density Z inclusions increased. As a result, the brittle fracture area ratio has increased and the desired toughness at ultra low temperature has not been achieved.
    No. 50 is an example in which the steel composition of No. 50 of Table 1B was used, the amounts of Nb and REM, which were the selective compositions, were high and, therefore, the numerical density Z inclusions increased and the value A exceeded the predetermined range. As a result, the brittle fracture area ratio has increased and the desired toughness at ultra low temperature has not been achieved.
    In No. 51, the steel composition of No. 51 of Table 1B was used, the amount of Mo, which was the selective composition, was high, so the ratio of brittle fracture area was increased and the The desired toughness at ultra low temperature could not be achieved.
    In No. 52, the steel composition of No. 52 of Table 1B was used. As the Ti-entity, which was the selective composition, was high, the brittle fracture area ratio consequently increased and the desired ultra-low temperature toughness could not be obtained.
    In No. 53, the steel composition of No. 53 of Table 1B was used, the amount of V, which was the selective composition, was high, the ratio of brittle fracture surface therefore increased and the The desired toughness at ultra low temperature could not be achieved.
    In No. 54, the steel composition of No. 54 of Table 1B was used, the amount of B, which was the selective composition, was high, the ratio of brittle fracture surface therefore increased and the The desired toughness at ultra low temperature could not be achieved.
    Example 2
    In the present example, taking into account some of the data used in Example 1 (all of which are examples of the present invention), the brittle fracture area ratio at -233 ° C was evaluated.
    More specifically, for the purposes described in Table 3 (No. in Table 3 corresponds to No. in Table 1 and Table 2), three pieces of specimen were taken from position t / 4 and position W / 4, the Charpy resilience test at -233 ° C was performed by a method described below, and the average value of the brittle fracture area ratio was evaluated. In the present example, one in which the brittle fracture surface ratio <50% has been rated excellent for the brittle fracture area ratio at -233 ° C. "Kouatsu-Gasu" (High Pressure Gas) , flight. 24, p. 181, "Ultra low temperature impact test of austenite-based cast stainless steel"
    These results are shown in Table 3.
    [Table 3]
    All 3, 4, 6, 15, 19, 24 of Table 3 are examples in which the heating time (tL) at the temperature of the two-phase region was monitored at 15 min or more (see table 2A) and the residual γ phase could be ensured by 4.0% or more. As a result, not only was the brittle fracture area ratio at -196 ° C but also the same ratio at -233 ° C that was lower than -196 ° C was excellent and very excellent ultra low temperature toughness was achieved. could be obtained.
    权利要求:
    Claims (2)
    [1]
    1. Thick steel plate with excellent toughness at ultra low temperature containing in% by mass: C: 0.02-0.10%; If: 0.40% or less (not including 0%); Mn: 0.50 - 2.0%; P: 0.007% or less (not including 0%); S: 0.007% or less (not including 0%); Al: 0.005 - 0.050%; Ni: 5.0 - 7.5%; and N: 0.010% or less (not including 0%); the remainder comprising iron and inevitable impurities, in which a volume fraction (V) of a residual austenitic phase at -196 ° C satisfies 2.0% -12.0%, and when the numerical density of inclusions having more than 1.0 pm diameter circle equivalent present in the steel plate is set at Z, Z <200 units / mm2, and a value A expressed by an expression (1) below satisfies 11.5 or less. Α = ν2 / 3 + 0.012χπ * Ζ - (1)
    [2]
    2. The thick steel plate of claim 1 further containing, as other elements, at least one of (a) - (e) below: (a) Cu: 1.00% or less (not included 0 %); (b) at least one element selected from a group consisting of Cr: 1.20% or less (not including 0%) and Mo: 1.0% or less (not including 0%); (c) at least one member selected from a group consisting of Ti: 0.025% or less (not including 0%), Nb: 0.100% or less (not including 0%) and V: 0.50% or less (not included 0%); (d) B: 0.0050% or less (not including 0%); (e) at least one member selected from a group consisting of Ca: 0.0030% or less (not including 0%), REM: 0.0050% or less (not including 0%) and Zr: 0.005% or less ( not including 0%).
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    法律状态:
    优先权:
    申请号 | 申请日 | 专利标题
    JP2012092203|2012-04-13|
    JP2012092203|2012-04-13|
    JP2012172063A|JP6018454B2|2012-04-13|2012-08-02|High strength thick steel plate with excellent cryogenic toughness|
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