WO2021149676A1 - 鋼板およびその製造方法 - Google Patents

鋼板およびその製造方法 Download PDF

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WO2021149676A1
WO2021149676A1 PCT/JP2021/001658 JP2021001658W WO2021149676A1 WO 2021149676 A1 WO2021149676 A1 WO 2021149676A1 JP 2021001658 W JP2021001658 W JP 2021001658W WO 2021149676 A1 WO2021149676 A1 WO 2021149676A1
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steel sheet
point
martensite
temperature range
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PCT/JP2021/001658
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English (en)
French (fr)
Japanese (ja)
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丸山　直紀
匹田　和夫
進一郎 田畑
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日本製鉄株式会社
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Priority to KR1020227028020A priority Critical patent/KR20220127894A/ko
Priority to MX2022008976A priority patent/MX2022008976A/es
Priority to EP21745006.3A priority patent/EP4095272A4/en
Priority to JP2021572738A priority patent/JP7364942B2/ja
Priority to CN202180010399.XA priority patent/CN115003839A/zh
Priority to US17/794,442 priority patent/US20230065607A1/en
Publication of WO2021149676A1 publication Critical patent/WO2021149676A1/ja

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    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the present invention relates to a steel sheet and a method for manufacturing the same.
  • the application of high-strength steel sheets is aimed at steel sheets for automobiles.
  • Automotive members include skeletal members such as pillars, sills, or members, in addition to reinforcing members such as bumpers or door guard bars.
  • High-strength steel plates applied to these members are required to have collision-resistant characteristics that can ensure the safety of passengers in the event of a collision (for example, Patent Documents 1 to 3).
  • the collision resistance characteristic is a characteristic that has a high reaction force characteristic and can absorb energy at the time of collision deformation without brittle fracture even if the member is greatly deformed at the time of collision deformation.
  • Examples of the steel sheet having excellent energy absorption performance include a DP steel sheet having a two-phase structure of ferrite and martensite (for example, Patent Document 4), or a TRIP steel sheet having a residual ⁇ structure in addition to ferrite and bainite (transformation induction). (Plastic steel sheet) is used (for example, Patent Document 5). Further, a steel plate and a member having a high yield stress composed of a structure mainly composed of martensite are disclosed (for example, Patent Documents 6 to 8).
  • Japanese Unexamined Patent Publication No. 2009-185355 Japanese Unexamined Patent Publication No. 2011-111672 Japanese Unexamined Patent Publication No. 2012-251239 JP-A-11-080878 Japanese Unexamined Patent Publication No. 11-080879 Japanese Unexamined Patent Publication No. 2010-174280 Japanese Unexamined Patent Publication No. 2013-117068 JP-A-2015-175050
  • An object of the present invention is to provide a steel sheet having a yield stress of 1000 MPa or more and a method for producing the same.
  • the present inventors diligently studied a method for solving the above problems, and as a result, obtained the following findings.
  • the present invention has been made based on the above findings, and the gist of the following steel sheet and its manufacturing method is.
  • the chemical composition is mass%. C: 0.14 to 0.60%, Si: More than 0% and less than 3.00%, Al: More than 0% and less than 3.00%, Mn: 5.00% or less, P: 0.030% or less, S: 0.0050% or less, N: 0.015% or less, B: 0 to 0.0050%, Ni: 0 to 5.00%, Cu: 0 to 5.00%, Cr: 0 to 5.00%, Mo: 0 to 1.00%, W: 0 to 1.00%, Ti: 0 to 0.20%, Zr: 0 to 0.20%, Hf: 0 to 0.20%, V: 0 to 0.20%, Nb: 0 to 0.20%, Ta: 0 to 0.20%, Sc: 0 to 0.20%, Y: 0 to 0.20%, Sn: 0 to 0.020%, As: 0 to 0.020%, Sb: 0 to 0.020%, Bi: 0 to 0.020%, Mg: 0 to 0.005%
  • Yield stress is 1000 MPa or more, Steel plate.
  • Ms 546 x exp (-1.362 x C) -11 x Si-30 x Mn-18 x Ni-20 x Cu-12 x Cr-8 (Mo + W) ... (vi)
  • the element symbol in the above formula represents the content (mass%) of each element in the steel sheet, and if it is not contained, 0 is substituted.
  • the average particle size of the iron carbide contained in the metal structure is 0.005 to 0.20 ⁇ m.
  • the method for manufacturing a steel sheet according to any one of (1) to (3) above is subjected to a hot rolling step, a cold rolling step, an annealing step and a heat treatment step in this order.
  • the hot rolling step the average cooling rate from the rolling end temperature to 650 ° C. is set to 8 ° C./s or more, and the mixture is cooled to room temperature.
  • the annealing step the temperature range of Ac 3 points to (Ac 3 points + 100) ° C. was maintained for 3 to 90 s, and The average cooling rate between 700 ° C. and (Ms point-50) ° C. is 10 ° C./s or higher.
  • the residence time in the temperature range of (Ms point +50) to 250 ° C. is set to 100 to 10000 s. If the Ms point is less than 250 ° C, The residence time in the temperature range of (Ms point +80) to 100 ° C. is 100 to 50,000 s. Steel sheet manufacturing method.
  • the above Ms point (° C.) and Ac 3 point (° C.) are represented by the following formulas, and the element symbol in the formula represents the content (mass%) of each element in the steel sheet, and when it is not contained. Suppose to substitute 0.
  • the method for manufacturing a steel sheet according to any one of (1) to (3) above is subjected to a hot rolling step, an annealing step and a heat treatment step in this order.
  • the hot rolling step the average cooling rate from the rolling end temperature to 650 ° C. is set to 8 ° C./s or more, and the mixture is cooled to room temperature.
  • the annealing step the temperature range of Ac 3 to (Ac 3 + 100) ° C. was maintained for 3 to 90 s, and The average cooling rate between 700 ° C. and (Ms-50) ° C. is 10 ° C./s or higher.
  • the residence time in the temperature range of (Ms + 50) to 250 ° C. is set to 100 to 10000 s. If the Ms point is less than 250 ° C, The residence time in the temperature range of (Ms + 80) to 100 ° C. is 100 to 50,000 s. Steel sheet manufacturing method.
  • the above Ms point (° C.) and Ac 3 point (° C.) are represented by the following formulas, and the element symbol in the formula represents the content (mass%) of each element in the steel sheet, and when it is not contained. Suppose to substitute 0.
  • the method for manufacturing a steel sheet according to any one of (1) to (3) above The slab having the chemical composition described in (1) above is subjected to a hot rolling step and a heat treatment step in this order.
  • the rolling end temperature is set to Ar 3 points or more, and The average cooling rate from the rolling end temperature to (Ms-50) ° C. is set to 10 ° C./s or higher.
  • the heat treatment step When the Ms point is 250 ° C or higher, The residence time in the temperature range of (Ms + 50) to 250 ° C. is set to 100 to 10000 s. If the Ms point is less than 250 ° C, The residence time in the temperature range of (Ms + 80) to 100 ° C. is 100 to 50,000 s.
  • Ms point (° C.) and Ar 3 point (° C.) are represented by the following formulas, and the element symbol in the formula represents the content (mass%) of each element in the steel sheet, and when it is not contained.
  • Ms 546 x exp (-1.362 x C) -11 x Si-30 x Mn-18 x Ni-20 x Cu-12 x Cr-8 (Mo + W) ...
  • Ar 3 910-310 x C + 33 x Si-80 x Mn-55 x Ni-20 x Cu-15 x Cr-80 x Mo ... (viii)
  • C 0.14 to 0.60%
  • C is an element that is effective in improving the strength and making the block particle size finer.
  • the C content is set to 0.14% or more.
  • the C content exceeds 0.60%, the Ms point decreases, and the average axial ratio described later tends to increase. As a result, brittle fracture occurs in the stress concentration portion at the time of collision deformation, and the impact energy absorption capacity is reduced. Therefore, the C content is set to 0.14 to 0.60%.
  • the C content is preferably 0.15% or more, more preferably 0.18% or more, and preferably 0.50% or less.
  • Si and Al are elements effective for deoxidizing steel, but in the present invention, the effect of increasing the average axial ratio of martensite, the effect of suppressing the formation of iron carbide, and the block particle size of martensite are determined. It has the effect of making it smaller, thereby suppressing cracking during collision deformation of the member and improving the energy absorption capacity.
  • Si and Al are each contained in an amount of more than 0%. It is preferable that Si and Al are each contained in an amount of 0.01% or more.
  • the total content of Si and Al is set to 3.00% or less.
  • the total content is preferably 2.50% or less.
  • the lower limit of the total content is not particularly limited, but it is preferably 0.10% or more in order to surely obtain the effect of reducing the block particle size.
  • Mn 5.00% or less Mn has the effect of suppressing the formation of ferrite and improving the yield stress, and is an element useful for controlling the average axial ratio.
  • Mn content exceeds 5.00%, the Ms point decreases, and the average axial ratio described later tends to increase. As a result, brittle fracture occurs in the stress concentration portion at the time of collision deformation, and the impact energy absorption capacity is reduced. Therefore, the Mn content is set to 5.00% or less.
  • the Mn content is preferably 4.00% or less, 3.00% or less, or 2.00% or less. In order to surely obtain the above effect, it is preferable to contain 0.01% or more.
  • the product of the contents of C and Mn is a parameter that correlates with brittle fracture at the stress concentration portion during collision deformation. If the value of C ⁇ Mn exceeds 0.80, the brittle fracture tendency becomes strong, so the value is set to 0.80 or less. This value is preferably 0.60 or less, and more preferably 0.40 or less.
  • P 0.030% or less
  • P is an element that contributes to the improvement of strength. However, if the P content exceeds 0.030%, the tendency of grain boundary fracture increases during collision deformation, and the impact energy absorption capacity decreases. Therefore, the P content is 0.030% or less. From the viewpoint of resistance weldability, the P content is preferably 0.020% or less.
  • the lower limit is not particularly limited, but reducing it to less than 0.001% leads to an increase in manufacturing cost, so that 0.001% is practically the lower limit.
  • S 0.0050% or less
  • S is an impurity element, and if its content exceeds 0.0050%, fracture occurs from the punched portion or the bent portion at the time of collision. Therefore, the S content is set to 0.0050% or less.
  • the S content is preferably 0.0040% or less, or 0.0030% or less.
  • the lower limit is not particularly limited, but reducing it to less than 0.0002% leads to an increase in manufacturing cost, so that 0.0002% is practically the lower limit.
  • N 0.015% or less
  • N is an element that can be used to control the average axial ratio.
  • the N content is set to 0.015% or less.
  • the N content is preferably 0.010% or less, or 0.005% or less.
  • the lower limit is not particularly limited, but reducing it to less than 0.001% leads to an increase in manufacturing cost, so that 0.001% is practically the lower limit.
  • B 0 to 0.0050% Since B is an element having an effect of enhancing the hardenability of the steel sheet, it may be contained as necessary. However, if the B content exceeds 0.0050%, cracks may occur during collision deformation. Therefore, the B content is set to 0.0050% or less.
  • the B content is preferably 0.0040% or less, or 0.0030% or less.
  • the lower limit is not particularly limited and may be 0%, but when the above effect is desired, the B content is preferably 0.0003% or more.
  • Mo 0 to 1.00%
  • W 0 to 1.00%
  • the lvalue of the above equation (iii) is preferably 1.00 or more.
  • the upper limit is not particularly limited, but if it exceeds 4.00, the Ms point decreases and the average axial ratio described later tends to increase. As a result, brittle fracture may occur in the stress concentration portion during collision deformation, and the impact energy absorption capacity may decrease. Therefore, the lvalue of the above equation (iii) is preferably 4.00 or less.
  • Ni and Cu are preferably 4.00% or less, more preferably 3.00% or less, and further preferably 1.00% or less, respectively.
  • the Cr content is preferably 3.00% or less, more preferably 1.00% or less.
  • the contents of Mo and W are preferably 0.80% or less, and more preferably 0.60% or less, respectively.
  • the total content exceeds 0.20%, a large amount of alloy precipitates are precipitated and cracks are likely to occur at the time of collision deformation, so the total content is set to 0.20% or less.
  • the total content is preferably 0.010% or more.
  • Sn 0 to 0.020%, As: 0 to 0.020%, Sb: 0 to 0.020%, and Bi: 0 to 0.020%, and Sn + As + Sb + Bi ⁇ 0.020 ⁇ ⁇ ⁇ (v)
  • Sn, As, Sb and Bi are elements used to obtain a predetermined metal structure, one or more selected from these elements may be contained as necessary. However, if the total content of these exceeds 0.020%, the tendency of grain boundary fracture increases during collision deformation, so the upper limit is set to 0.020%.
  • the lower limit is not particularly limited, but reducing it to less than 0.00005% leads to an increase in manufacturing cost, so that 0.00005% is practically the lower limit.
  • Mg 0 to 0.005%, Ca: 0 to 0.005%, and REM: 0 to 0.005% Since Mg, Ca and REM are elements having an action of controlling the morphology of oxides and sulfides, one or more selected from these elements may be contained as required. However, if the content of any of the elements exceeds 0.005%, the addition effect is saturated and the energy absorption capacity at the time of collision deformation is lowered, so the content is set to 0.005% or less.
  • the contents of Mg, Ca and REM are all preferably 0.003% or less. When the above effect is desired, it is preferable to contain one or more selected from Mg: 0.001% or more, Ca: 0.001% or more, and REM: 0.001% or more.
  • REM refers to 15 elements of lanthanoids, and the content of the REM means the total content of lanthanoids.
  • Lanthanoids are industrially added in the form of misch metal.
  • the balance is Fe and impurities.
  • impurity is a component mixed with raw materials such as ore and scrap, and various factors in the manufacturing process when the steel sheet is industrially manufactured, and is allowed as long as it does not adversely affect the present invention. Means something.
  • Martensite 85% or more Having martensite as the main structure is indispensable for ensuring a yield stress of 1000 MPa or more. If the volume fraction of martensite is less than 85%, it becomes difficult to secure a yield stress of 1000 MPa or more. Therefore, the volume fraction of martensite is set to 85% or more. In order to stably secure the yield stress, the volume fraction of martensite is preferably 90% or more.
  • martensite shall contain tempered martensite, that is, martensite in which carbides are formed. The form of martensite may be any of lath, butterfly, twin, thin plate and the like.
  • Residual austenite 15% or less Residual austenite is a metal structure effective for improving molding processability and impact energy absorption characteristics. However, if the volume fraction exceeds 15%, the yield stress tends to decrease and brittle cracks tend to occur during collision deformation. Therefore, the volume fraction of retained austenite is set to 15% or less.
  • the volume fraction of retained austenite is preferably 12% or less.
  • the lower limit is not particularly limited, but is preferably 0.1% or more.
  • the remaining organization other than the above organization is bainite.
  • the bainite includes a lower bainite and an upper bainite, and the bainitic ferrite ( ⁇ ° B) described in Non-Patent Document 1 is further classified into bainite.
  • the tempered martensite may be difficult to separate from bainite even according to Reference 1.
  • the tissue fraction is calculated by regarding it as martensite. It is not necessary to set an upper limit on the area ratio of the remaining bainite, but it is substantially 15% or less, preferably 10% or less.
  • the volume fraction of the metal structure is determined by the following procedure. First, a 1/4 thickness portion of a surface parallel to the rolling direction and the thickness direction of the steel sheet is mirror-polished, and then nital corrosion is performed. Then, the surface is observed with a scanning electron microscope (SEM) or a transmission electron microscope (TEM), and the area ratios of martensite and bainite are obtained by a point counting method or image analysis using the photographed tissue photograph. , Let this be the volume ratio.
  • the volume fraction of retained austenite is determined by an X-ray diffraction method. The area to be observed is 1000 ⁇ m 2 or more in the case of SEM and 100 ⁇ m 2 or more in the case of using TEM.
  • the average block particle size and the average axial ratio of martensite and bainite are also defined as follows.
  • Average block particle size of martensite and bainite 3.0 ⁇ m or less
  • the block particle size of martensite affects the occurrence and propagation of brittle fracture during collision deformation, and the smaller the value, the better the impact characteristics. If the average block particle size exceeds 3.0 ⁇ m, plate breakage may occur at the bent portion during collision deformation, so the average block particle size should be 3.0 ⁇ m or less.
  • the average block particle size is preferably 2.7 ⁇ m or less, 2.5 ⁇ m or less, or 2.4 ⁇ m or less.
  • Non-Patent Document 2 p As shown in the table 223, martensite and bainite can be classified as being composed of 24 different crystalline units (variants) as their substructure. As one of the methods for grouping these 24 variants, p. There is a Bain group described in 223, which allows martensite and bainite to be classified into three types of crystalline units. The block particle size in the present invention indicates the average size of group grains when classified by this Bain group.
  • the average block particle size is measured by the following procedure. First, each steel plate is cut so that a surface parallel to the rolling direction and the thickness direction becomes an observation surface, and an area region of 5000 ⁇ m 2 or more is formed between the 1/4 position and the 1/2 position of the plate thickness of this cross section. Measure by the EBSD method. The measurement step size is 0.2 ⁇ m. Then, based on the crystal orientation information obtained by the EBSD measurement, the orientation is classified into three Bain group units, the image is displayed, and the size of this crystal unit is determined by the crossing line segmentation method described in Annex 2 of JIS G 0552. Ask for.
  • Average axial ratio of martensite and bainite 1.0004 to 1.0100
  • the axial ratio is a value represented by c / a when the lattice constants of the a-axis and the c-axis in the tetragonal structure are a and c, respectively.
  • the reason why the magnitude of the axial ratio c / a is related to the cracking behavior at high speed and large deformation during the collision test is not clear, but it is possible that the crystal lattice strain has some influence.
  • the average axial ratio is set to 1.0004 to 1.0100. From the viewpoint of ensuring stable yield stress, the average axial ratio is preferably 1.0006 or more. Further, in order to more reliably suppress cracking at the time of collision deformation, the average axial ratio is preferably 1.0007 or more. On the other hand, from the viewpoint of absorbing more impact energy, the average axial ratio is preferably 1.0080 or less.
  • the average axial ratio of martensite and bainite is measured by the following procedure by the X-ray diffraction method.
  • the average axial ratio c / a shall be obtained by one of the following two methods depending on the presence or absence of splitting of the diffraction line of tetragonal iron or cubic iron.
  • the area of the X-ray irradiation region on the sample is set to 0.2 mm 2 or more.
  • Average particle size of iron carbide 0.005 to 0.20 ⁇ m
  • Iron carbide may be contained in the metal structure of the steel sheet according to another embodiment of the present invention.
  • the average particle size of iron carbide exceeds 0.20 ⁇ m, fracture from the bent portion tends to be promoted during collision deformation, while when the average particle size of iron carbide is less than 0.005 ⁇ m, iron collision deformation tends to be promoted. Brittle fracture from the bend tends to be promoted inside. Therefore, the average particle size of the iron carbide is preferably 0.005 to 0.20 ⁇ m.
  • the iron carbide may contain an alloy element such as Mn or Cr.
  • the average particle size of iron carbides in martensite and bainite is measured by microstructure observation in an area region of 10 ⁇ m 2 or more by SEM and TEM. Fine iron carbides that cannot be identified by TEM are measured by the atom probe method. In this case, 5 or more iron carbides shall be measured.
  • the steel sheet according to another embodiment of the present invention may have a plating layer on its surface.
  • the composition of the plating is not particularly limited, and any of hot-dip plating, alloyed hot-dip plating, and electroplating may be used.
  • yield stress 1000 MPa or more If the yield stress is less than 1000 MPa, the merit of reducing the weight of the member by thinning the member cannot be obtained, so the yield stress is set to 1000 MPa or more.
  • the yield stress is the flow stress (0.2% proof stress) at a strain of 0.002 when a tensile test is performed in accordance with JIS Z 2241 2011.
  • the tensile strength is not particularly limited, but is preferably 1400 MPa or more from the viewpoint of enhancing the impact energy absorption characteristics.
  • the slab can be obtained from molten steel having the above chemical composition by a conventional method.
  • the slabs used for hot rolling are not particularly limited. That is, it may be manufactured by a continuously cast slab or a thin slab caster. It is also suitable for processes such as continuous casting-direct rolling, in which hot rolling is performed immediately after casting.
  • the Ms point (° C.), the Ac 3 point (° C.) and the Ar 3 point (° C.) are represented by the following formulas, and the element symbol in the formula is the content of each element in the steel plate. It represents (mass%), and if it is not contained, 0 is substituted.
  • Ms 546 x exp (-1.362 x C) -11 x Si-30 x Mn-18 x Ni-20 x Cu-12 x Cr-8 (Mo + W) ...
  • Ac 3 910-203 x C 0.5 + 44.7 (Si + Al) -30 x Mn + 700 x P-15.2 x Ni-26 x Cu-11 x Cr + 31.5 x Mo ...
  • Ar 3 910-310 x C + 33 x Si-80 x Mn-55 x Ni-20 x Cu-15 x Cr-80 x Mo ... (viii)
  • the slab is first heated.
  • the heating temperature is not particularly limited, but is preferably 1200 ° C. or higher in order to redissolve the alloy carbonitride precipitated during casting or rough rolling.
  • the average cooling rate between the rolling end temperature and 650 ° C. is set to 8 ° C./s or more. If the average cooling rate is less than 8 ° C./s, the block particle size of martensite in the final product becomes large, and the impact characteristics deteriorate. Then, the steel plate is wound up.
  • the winding temperature is not particularly limited, but is preferably 630 ° C. or lower. Then, after winding, it is further cooled to room temperature.
  • cold rolling is performed.
  • the conditions for cold rolling it is not necessary to specify the number of rolling passes and the rolling reduction ratio in particular, and the conventional method may be followed.
  • the steel sheet after cold rolling is held for 3 to 90 s in a temperature range of Acc 3 points to (Ac 3 points + 100) ° C. If the annealing temperature is less than 3 points of Ac, a predetermined amount of martensite cannot be obtained, and if it exceeds (3 points of Ac + 100) ° C., the block particle size becomes large. Further, if the holding time within this temperature range is less than 3 s, a predetermined amount of martensite cannot be obtained, and a yield stress of 1000 MPa or more cannot be obtained. On the other hand, when the holding time exceeds 90 s, the block particle size becomes large. From the viewpoint of reducing the block particle size, the annealing temperature is preferably low, and is preferably (Ac 3 points + 80) ° C. or lower. The holding time is preferably 10 s or more, and preferably 60 s or less.
  • the average cooling rate After holding for a predetermined time in the above temperature range, cool under the condition that the average cooling rate between 700 ° C. and (Ms point-50) ° C. is 10 ° C./s or more. If this average cooling rate is less than 10 ° C./s, a predetermined amount of martensite cannot be obtained, the yield stress is reduced, the block particle size is further increased, and cracks are likely to occur during impact deformation. ..
  • the average cooling rate is preferably 20 ° C./s or more.
  • the temperature at which this cooling is stopped may be (Ms-50) ° C. or lower, and is not particularly limited, but is preferably 100 ° C. or higher from the viewpoint of fracture resistance.
  • heat treatment is performed so as to have the following thermal history according to Ms calculated from the chemical composition of the steel sheet.
  • the following heat treatment may be continuously performed, or heating may be performed to such an extent that the upper limit of the temperature range of the following heat treatment step is not exceeded.
  • the residence time in the temperature range of (Ms point +50) to 250 ° C. is set to 100 to 10000 s. If the residence time is less than 100 s, the average axial ratio may exceed a predetermined value and brittle fracture may occur during a collision test, or a predetermined yield stress may not be obtained. On the other hand, if it exceeds 10000 s, the average axial ratio becomes less than a predetermined value, the iron carbide becomes coarser, and cracks are likely to occur at the time of collision.
  • the residence time is preferably 400 s or more, and preferably 5000 s or less. In particular, when the above-mentioned average axial ratio is set to 1.0007 or more and cracking at the time of collision deformation is desired to be suppressed more reliably, the residence time is more preferably 1500 s or less.
  • the residence time in the temperature range of (Ms point +80) to 100 ° C. is set to 100 to 50,000 s. If the residence time is less than 100 s, the average axial ratio exceeds a predetermined value, and there is a risk of brittle fracture during a collision test. On the other hand, if it exceeds 50,000 s, the average axial ratio becomes less than a predetermined value, the iron carbide becomes coarser, and cracks are likely to occur at the time of collision.
  • the residence time is preferably 400 s or more, preferably 30,000 s or less, and more preferably 10,000 s or less.
  • the cold rolling step is not performed in this step.
  • ferrite which is the matrix phase
  • the texture will develop even in austenite that exists in the temperature range of Ac 3 points to (Ac 3 points + 100) ° C. Due to the development of the texture, when the orientally biased austenite transforms into martensite, the crystals of martensite are formed and grow in a specific direction.
  • the steel sheet expands in a specific direction from a macro perspective.
  • the plate-passing property is deteriorated. Therefore, tension is usually applied to correct the shape of the steel plate and maintain the stability of the plate-passing property.
  • the steel sheet according to the embodiment of the present invention is preferably a hot-rolled steel sheet for the purpose of making the above random.
  • the slab is first heated.
  • the heating temperature is not particularly limited, but is preferably 1200 ° C. or higher in order to redissolve the alloy carbonitride precipitated during casting or rough rolling.
  • Hot rolling is performed after heating.
  • the average cooling rate between the rolling end temperature and 650 ° C. is set to 8 ° C./s or more. If the average cooling rate is less than 8 ° C./s, the block particle size of martensite in the final product becomes large, and the impact characteristics deteriorate.
  • the steel sheet may be wound up, or may be cooled to room temperature without being wound up. Further, after cooling, a treatment such as pickling may be performed, or a shape correction may be performed.
  • the steel sheet after hot rolling is held for 3 to 90 s in a temperature range of Acc 3 points to (Ac 3 points + 100) ° C. If the annealing temperature is less than 3 points of Ac, a predetermined amount of martensite cannot be obtained, and if it exceeds (3 points of Ac + 100) ° C., the block particle size becomes large. Further, if the holding time within this temperature range is less than 3 s, a predetermined amount of martensite cannot be obtained, and a yield stress of 1000 MPa or more cannot be obtained. On the other hand, when the holding time exceeds 90 s, the block particle size becomes large. From the viewpoint of reducing the block particle size, the annealing temperature is preferably low, and is preferably (Ac 3 points + 80) ° C. or lower. The holding time is preferably 10 s or more, and preferably 60 s or less.
  • the average cooling rate After holding for a predetermined time in the above temperature range, cool under the condition that the average cooling rate between 700 ° C. and (Ms point-50) ° C. is 10 ° C./s or more. If this average cooling rate is less than 10 ° C./s, a predetermined amount of martensite cannot be obtained, the yield stress is reduced, the block particle size is further increased, and cracks are likely to occur during impact deformation. ..
  • the average cooling rate is preferably 20 ° C./s or more.
  • the temperature at which this cooling is stopped may be (Ms-50) ° C. or lower, and is not particularly limited, but is preferably 100 ° C. or higher from the viewpoint of fracture resistance.
  • processing is performed so as to have the following thermal history according to Ms calculated from the chemical composition of the steel sheet.
  • the following heat treatment may be continuously performed, or heating may be performed to such an extent that the upper limit of the temperature range of the following heat treatment is not exceeded.
  • the residence time in the temperature range of (Ms point +50) to 250 ° C. is set to 100 to 10000 s. If the residence time is less than 100 s, the average axial ratio may exceed a predetermined value and brittle fracture may occur during a collision test, or a predetermined yield stress may not be obtained. On the other hand, if it exceeds 10000 s, the average axial ratio becomes less than a predetermined value, the iron carbide becomes coarser, and cracks are likely to occur at the time of collision.
  • the residence time is preferably 400 s or more, and preferably 5000 s or less. In particular, when the above-mentioned average axial ratio is set to 1.0007 or more and cracking at the time of collision deformation is desired to be suppressed more reliably, the residence time is more preferably 1500 s or less.
  • the residence time in the temperature range of (Ms point +80) to 100 ° C. is set to 100 to 50,000 s. If the residence time is less than 100 s, the average axial ratio exceeds a predetermined value, and there is a risk of brittle fracture during a collision test. On the other hand, if it exceeds 50,000 s, the average axial ratio becomes less than a predetermined value, the iron carbide becomes coarser, and cracks are likely to occur at the time of collision.
  • the residence time is preferably 400 s or more, preferably 30,000 s or less, and more preferably 10,000 s or less.
  • (C) Method including hot rolling step and heat treatment step The hot rolling step and heat treatment step are sequentially performed on the above slab.
  • the obtained steel sheet is a hot-rolled steel sheet.
  • the annealing step is not performed in this step.
  • interfacial movement of the martensite structure occurs during heating from room temperature to the annealing temperature in the annealing step.
  • the crystal interface in a specific orientation having a high degree of mobility is preferentially moved, so that the randomization of the crystal orientation is impaired, and a slight residual stress remains in the steel sheet that has undergone the annealing step. Therefore, from the viewpoint of reducing the residual stress as much as possible, it is preferable to omit the annealing step.
  • the slab is first heated.
  • the heating temperature is not particularly limited, but is preferably 1200 ° C. or higher in order to redissolve the alloy carbonitride precipitated during casting or rough rolling.
  • Hot rolling is performed after heating.
  • the rolling end temperature is set to Ar 3 points or more. If the rolling end temperature is less than 3 Ar points, ferrite is formed and it becomes difficult to obtain a predetermined yield stress.
  • the temperature at which this cooling is stopped may be (Ms-50) ° C. or lower, and is not particularly limited, but is preferably 100 ° C. or higher from the viewpoint of fracture resistance.
  • processing is performed so as to have the following thermal history according to Ms calculated from the chemical composition of the steel sheet.
  • the following heat treatment may be continuously performed, or heating may be performed to such an extent that the upper limit of the temperature range of the following heat treatment is not exceeded.
  • the residence time in the temperature range of (Ms point +50) to 250 ° C. is set to 100 to 10000 s. If the residence time is less than 100 s, the average axial ratio may exceed a predetermined value and brittle fracture may occur during a collision test, or a predetermined yield stress may not be obtained. On the other hand, if it exceeds 10000 s, the average axial ratio becomes less than a predetermined value, the iron carbide becomes coarser, and cracks are likely to occur at the time of collision.
  • the residence time is preferably 400 s or more, and preferably 5000 s or less. In particular, when the above-mentioned average axial ratio is set to 1.0007 or more and cracking at the time of collision deformation is desired to be suppressed more reliably, the residence time is more preferably 1500 s or less.
  • the residence time in the temperature range of (Ms point +80) to 100 ° C. is set to 100 to 50,000 s. If the residence time is less than 100 s, the average axial ratio exceeds a predetermined value, and there is a risk of brittle fracture during a collision test. On the other hand, if it exceeds 50,000 s, the average axial ratio becomes less than a predetermined value, the iron carbide becomes coarser, and cracks are likely to occur at the time of collision.
  • the residence time is preferably 1000 s or more, preferably 30,000 s or less, and more preferably 10,000 s or less.
  • temper rolling may be performed for shape correction.
  • the growth rate is not particularly limited.
  • the plating treatment may be performed during the heat treatment or after the heat treatment is completed within a range that satisfies the heat history.
  • the plating method may be a continuous annealing / plating line, or a dedicated plating facility may be used separately from the annealing line.
  • the composition of the plating is not particularly limited, and any of hot-dip plating, alloyed hot-dip plating, and electroplating may be used.
  • a steel having the composition shown in Table 1 was melted to produce a slab, and this slab was heated at 1220 to 1260 ° C. and roughly rolled hot. Subsequently, finish rolling was performed, and after cooling, winding treatment was performed at 500 to 620 ° C., and cooling was performed to room temperature. Then, as shown in Tables 2 and 3, the average cooling rate (CR1) between the rolling end temperature (FT) and 650 ° C. was changed.
  • Annealing changes the annealing temperature (ST), annealing holding time (t1), average cooling rate (CR2) between 700 ° C. and (Ms point-50) ° C., and in the heat treatment step, steel with Ms of 250 ° C. or higher.
  • the residence time (t2) between (Ms + 50) ° C. and 250 ° C. was changed, and the residence time (t3) between (Ms + 80) ° C. and 100 ° C. was changed for steels having Ms less than 250 ° C.
  • temper rolling for shape correction was performed.
  • the metallographic structure of the obtained steel sheet was observed, and the volume fraction of each structure was measured. Specifically, after mirror-polishing a 1/4 thickness portion of a surface parallel to the rolling direction and the thickness direction of the steel sheet, the surface corroded by nital was observed by SEM. Using the tissue photograph, the area ratio of each tissue was obtained by measuring by the point counting method, and the value was taken as the volume fraction of each tissue. At this time, the observation area was set to 2500 ⁇ m 2 or more. The volume fraction of retained austenite was measured by an X-ray diffraction method.
  • F in the residual structure column indicates ferrite
  • B indicates bainite
  • P indicates pearlite
  • fM and fA indicate the volume fractions of martensite and retained austenite with respect to the entire structure, respectively.
  • each steel plate is cut so that a surface parallel to the rolling direction and the thickness direction becomes an observation surface, and an area region of 5000 ⁇ m 2 or more is formed between the 1/4 position and the 1/2 position of the plate thickness of this cross section. It was measured by the EBSD method. The measurement step size was 0.2 ⁇ m.
  • the orientations were classified into the three Bain group units shown in Table 223.
  • the block grain size (db) was determined by the cross-line segmentation method described in Annex 2 of JIS G0552, with the boundary between these groups as the block grain boundary and the region surrounded by this boundary as the block grain. ..
  • the average axial ratio of martensite and bainite was measured by the following procedure by X-ray diffraction method. At this time, the axial ratio c / a was measured by the following two methods according to the presence or absence of splitting of the diffraction lines of tetragonal iron or cubic iron, and the average axial ratio was determined.
  • the tensile test piece described in JIS Z 2241 (2011) was collected from the obtained steel sheet with the rolling perpendicular direction (plate width direction) as the longitudinal direction. Then, using the tensile test piece, a tensile test was performed in accordance with JIS Z 2241 (2011), and the mechanical properties (yield stress YS, tensile strength TS) were measured.
  • the hat-shaped part A is formed by cold bending or roll-forming the steel plate, and then the hat-shaped part A and the lid B are joined by spot welding to obtain a test piece having the shape shown in FIG. Made.
  • the test piece was placed on the table D so that A was on the upper surface, and a cylindrical weight C having a weight of 500 kg was made to collide with the central part of the test piece from a height of 3 m. Then, the portion bent by the collision and the end face of the test piece were visually observed to evaluate the crack.
  • a score of 10 mm or more is given as E, 7 mm or more and less than 10 mm is given as D, 4 mm or more and less than 7 mm is given as C, 2 mm or more and less than 4 mm is given as B, and less than 2 mm is given as A. rice field.
  • Annealing changes the annealing temperature (ST), annealing holding time (t1), average cooling rate (CR2) between 700 ° C. and (Ms point-50) ° C., and in the heat treatment step, steel with Ms of 250 ° C. or higher.
  • the residence time (t2) between (Ms + 50) ° C. and 250 ° C. was changed, and the residence time (t3) between (Ms + 80) ° C. and 100 ° C. was changed for steels having Ms less than 250 ° C.
  • temper rolling for shape correction was performed.
  • the yield stress is 1000 MPa or more, and it can be seen that cracks do not occur after the collision test of the members. From this, it is clear that the steel sheet according to the present invention has excellent collision characteristics.
  • Example 5 For the obtained steel sheet, the metallographic structure and mechanical properties were measured and the collision resistance properties were evaluated in the same manner as in Example 1. The measurement results and evaluation results are shown in Table 5.
  • the yield stress is 1000 MPa or more, and it can be seen that cracks do not occur after the collision test of the members. From this, it is clear that the steel sheet according to the present invention has excellent collision characteristics.
  • the steel plate according to the present invention is suitable for use in skeleton parts, reinforcing parts of automobiles, and parts of construction industrial machinery.

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PCT/JP2021/001658 2020-01-22 2021-01-19 鋼板およびその製造方法 WO2021149676A1 (ja)

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KR1020227028020A KR20220127894A (ko) 2020-01-22 2021-01-19 강판 및 그 제조 방법
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WO2022209520A1 (ja) * 2021-03-31 2022-10-06 Ｊｆｅスチール株式会社 鋼板、部材およびそれらの製造方法
WO2023113387A1 (ko) * 2021-12-14 2023-06-22 주식회사 포스코 굽힘 특성이 우수한 초고강도 강판 및 이의 제조방법
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WO2023113387A1 (ko) * 2021-12-14 2023-06-22 주식회사 포스코 굽힘 특성이 우수한 초고강도 강판 및 이의 제조방법
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CN114875302A (zh) * 2022-03-25 2022-08-09 广东省科学院新材料研究所 一种低合金钢及其制备方法与应用

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