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WO2021200169A1 - Feuille d'acier - Google Patents

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Publication number
WO2021200169A1
WO2021200169A1 PCT/JP2021/010890 JP2021010890W WO2021200169A1 WO 2021200169 A1 WO2021200169 A1 WO 2021200169A1 JP 2021010890 W JP2021010890 W JP 2021010890W WO 2021200169 A1 WO2021200169 A1 WO 2021200169A1
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WIPO (PCT)
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steel sheet
martensite
content
rolling
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English (en)
Japanese (ja)
Inventor
林　宏太郎
孝彦 神武
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Nippon Steel Corp
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Nippon Steel Corp
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

  • the present invention relates to a steel sheet.
  • Patent Document 1 proposes a steel containing Mn of 2.6% or more and 4.2% or less. Since the above steel also contains a larger amount of Mn than general high-strength steel, retained austenite is easily generated, has high elongation, and exhibits excellent formability and bendability.
  • Non-Patent Document 1 since the steel sheet disclosed in Non-Patent Document 1 has a high Mn content, weldability may become a problem when it is used for parts for automobile bodies. Therefore, considering the utility as automobile parts and the like, it is desired to improve both the strength and formability of the steel sheet with a smaller Mn content. Further, in the steel sheet disclosed in Patent Document 1, the retained austenite is not isotropic and is unevenly distributed, so that the Charpy impact characteristic at room temperature is deteriorated. As described above, when the impact characteristics are lowered, the steel material used cannot be thinned, and the weight reduction of the automobile body cannot be sufficiently achieved.
  • An object of the present invention is to solve the above problems and to provide a steel sheet having high strength and excellent strength-ductility balance, bendability and impact characteristics.
  • the present invention has been made to solve the above problems, and the following steel materials are the gist of the present invention.
  • the chemical composition of the steel sheet is mass%.
  • C More than 0.18% and less than 0.30%, Si: 0.001% or more and less than 2.00%, Mn: More than 2.50% and less than 4.20%, sol. Al: 0.001% or more and less than 1.00%, P: 0.030% or less, S: 0.005% or less, N: Less than 0.050%, O: Less than 0.020%, Cr: 0 to 0.50%, Mo: 0 to 0.50%, W: 0 to 0.30%, Cu: 0 to 0.30%, Ni: 0 to 0.50%, Ti: 0 to 0.100%, Nb: 0 to 0.100%, V: 0 to 0.100%, B: 0 to 0.010%, Ca: 0 to 0.010%, Mg: 0 to 0.010%, Zr: 0 to 0.010%, REM: 0-0.010%, Sb: 0 to 0.050%, Sn: 0 to 0.050%, Bi: 0 to 0.050%, Remain
  • Residual austenite 10% or more
  • Tempering martensite 60-80%
  • Martensite less than 20%
  • the grain boundary density of small angles with a crystal orientation difference of 2 degrees or more and less than 20 degrees is 0.20 to 1.0 ⁇ m -1
  • the grain boundary density of large angles with a crystal orientation difference of 20 to 50 degrees is 0.20 to 1.0 ⁇ m -1. It is 0.30 to 0.60 ⁇ m -1 .
  • the ratio A L / A N of the particle density A N in the particle density A L and the plate thickness direction in the rolling direction of the residual austenite is 0.80 to 1.0 Steel plate.
  • the chemical composition is mass%. Cr: 0.01-0.50%, Mo: 0.01-0.50%, W: 0.01-0.30%, Cu: 0.01 to 0.30%, and Ni: 0.01 to 0.50%, Contains one or more selected from, The steel sheet according to (1) above.
  • the chemical composition is mass%. Ti: 0.005 to 0.100%, Nb: 0.005 to 0.100%, and V: 0.005 to 0.100%, Contains one or more selected from, The steel sheet according to (1) or (2) above.
  • the chemical composition is mass%.
  • the chemical composition is mass%.
  • Sb 0.0005 to 0.050%
  • Sn 0.0005 to 0.050%
  • Bi 0.0005 to 0.050%
  • a hot-dip galvanized layer is provided on the surface of the steel sheet.
  • the steel sheet according to any one of (1) to (5) above.
  • An alloyed hot-dip galvanized layer is provided on the surface of the steel sheet.
  • the steel sheet according to any one of (1) to (5) above.
  • C More than 0.18% and less than 0.30% C is an element necessary to increase the strength of steel and secure retained austenite. On the other hand, when C is excessively contained, it becomes difficult to maintain the weldability of the steel sheet. Therefore, the C content is set to more than 0.18% and less than 0.30%.
  • the C content is preferably 0.20% or more.
  • the C content is preferably 0.28% or less, more preferably 0.25% or less.
  • Si 0.001% or more and less than 2.00%
  • Si is an element effective for strengthening tempered martensite and further improving moldability. Si also has the effect of suppressing the precipitation of cementite and promoting the formation of retained austenite.
  • the Si content is set to 0.001% or more and less than 2.00%.
  • the Si content is preferably 0.020% or more, preferably 0.10% or more, 0.30% or more, or 0.50% or more. More preferred.
  • the Si content is preferably 1.80% or less, more preferably 1.60% or less.
  • Mn More than 2.50% and less than 4.20% Mn is an element that stabilizes austenite. More than 2.50% Mn is required to stabilize austenite at room temperature. On the other hand, when Mn is excessively contained, the weldability is lowered. Further, by setting the Mn content to less than 4.20%, the non-uniformity of the Mn content due to solidification segregation can be reduced, and the bendability can be improved. Therefore, the Mn content is set to more than 2.50% and less than 4.20%.
  • the Mn content is preferably more than 3.00%, more preferably more than 3.20%.
  • the Mn content is preferably less than 3.80%, more preferably less than 3.50%.
  • Al 0.001% or more and less than 1.00%
  • Al is an antacid and needs to be contained in an amount of 0.001% or more.
  • Al also has an effect of improving material stability because it widens the temperature range of the two-phase region at the time of annealing. The higher the Al content, the greater the effect, but if the Al content is excessive, it becomes difficult to maintain the surface texture, paintability, and weldability. Therefore, sol.
  • the Al content is 0.001% or more and less than 1.00%. sol.
  • the Al content is preferably 0.005% or more, more preferably 0.010% or more, and further preferably 0.020% or more.
  • sol. The Al content is preferably 0.80% or less, and more preferably 0.60% or less.
  • sol.Al means "acid-soluble Al".
  • P 0.030% or less
  • P is an impurity, and if the steel sheet contains P in excess, the weldability is impaired. Therefore, the P content is 0.030% or less.
  • the P content is preferably 0.025% or less, more preferably 0.020% or less, and even more preferably 0.015% or less. Since the steel sheet according to the present invention does not require P, the P content may be more than 0%, 0.001% or more, or 0.003% or more, but the smaller the P content, the more preferable.
  • S 0.005% or less
  • S is an impurity, and if the steel sheet contains S in excess, the weldability is impaired. Therefore, the S content is set to 0.005% or less.
  • the S content is preferably 0.003% or less, and more preferably 0.002% or less. Since the steel sheet according to the present invention does not require S, the S content may be more than 0% or 0.0003% or more, but the smaller the S content, the more preferable.
  • N Less than 0.050% N is an impurity, and if the steel sheet contains 0.050% or more of N, the low temperature toughness is reduced. Therefore, the N content is set to less than 0.050%.
  • the N content is preferably 0.030% or less, more preferably 0.010% or less, and even more preferably 0.006% or less. Since the steel sheet according to the present invention does not require N, the N content may be more than 0%, 0.001% or more, or 0.002% or more, but the smaller the N content, the more preferable.
  • O Less than 0.020% O is an impurity, and if the steel sheet contains 0.020% or more of O, the low temperature toughness is reduced. Therefore, the O content is set to less than 0.020%.
  • the O content is preferably 0.010% or less, more preferably 0.005% or less, and even more preferably 0.003% or less. Since the steel sheet according to the present invention does not require O, the O content may be more than 0% or 0.001% or more, but the smaller the O content, the more preferable.
  • the steel sheet of the present invention includes the following amounts of Cr, Mo, W, Cu, Ni, Ti, Nb, V, B, Ca, Mg, Zr, REM, Sb, Sn and It may contain one or more elements selected from Bi.
  • Cr 0 to 0.50% Mo: 0 to 0.50% W: 0 to 0.30%
  • Cu 0 to 0.30%
  • Ni 0 to 0.50% Cr, Mo, W, Cu, and Ni are elements that improve the strength of the steel sheet. Therefore, one or more selected from these elements may be contained. However, if these elements are excessively contained, surface defects are likely to occur during hot rolling, and the strength of the hot rolled steel sheet may become too high, resulting in a decrease in cold rollability. Therefore, the Cr content is 0.50% or less, the Mo content is 0.50% or less, the W content is 0.30% or less, the Cu content is 0.30% or less, and the Ni content is 0.50%. It is as follows.
  • the above-mentioned elements it is preferable to contain at least 0.01% or more of the above-mentioned elements.
  • the contents of Cr, Mo, W, Cu, and Ni are more preferably 0.05% or more, respectively.
  • Nb 0 to 0.100%
  • V 0 to 0.100% Since Ti, Nb, and V are elements that produce fine carbides, nitrides, or carbonitrides, they are effective in improving the strength of the steel sheet. Further, when the content of Nb is 0.100% or less, it has an effect of promoting the miniaturization of the structure during hot rolling. Therefore, one or more selected from these elements may be contained. However, if these elements are excessively contained, the strength of the hot-rolled steel sheet may be excessively increased, and the cold rollability may be lowered. Therefore, the Ti content is 0.100% or less, the Nb content is 0.100% or less, and the V content is 0.100% or less.
  • the contents of Ti, Nb, and V are all preferably 0.050% or less, and more preferably 0.030% or less. In order to more reliably obtain the effect of the above-mentioned actions of these elements, it is preferable to contain at least one of the above-mentioned elements in an amount of 0.005% or more.
  • the contents of Ti, Nb, and V are more preferably 0.007% or more, and further preferably 0.010% or more, respectively.
  • B 0 to 0.010% Ca: 0 to 0.010% Mg: 0 to 0.010% Zr: 0 to 0.010% REM: 0-0.010%
  • B, Ca, Mg, Zr, and REM (rare earth metals) improve the local ductility and hole expandability of steel sheets. Therefore, one or more selected from these elements may be contained. However, if these elements are excessively contained, the moldability of the steel sheet may be deteriorated. Therefore, the B content is 0.010% or less, the Ca content is 0.010% or less, the Mg content is 0.010% or less, the Zr content is 0.010% or less, and the REM content is 0.010%. It is as follows.
  • the contents of B, Ca, Mg, Zr, and REM are all preferably 0.006% or less, and more preferably 0.003% or less.
  • the total content of one or more elements selected from B, Ca, Mg, Zr, and REM may be 0.050% or less, but is preferably 0.030% or less.
  • the content of at least one of the above-mentioned elements is preferably 0.0001% or more, more preferably 0.0005% or more. It is more preferably 0.001% or more.
  • REM is a general term for 17 elements including 15 elements of lanthanoids and Y and Sc, and one or more of these elements can be contained.
  • the content of REM means the total content of these elements.
  • Sb 0 to 0.050%
  • Sn 0 to 0.050%
  • Bi 0 to 0.050%
  • Sb, Sn, and Bi suppress that easily oxidizing elements such as Mn, Si, and / or Al in the steel sheet are diffused on the surface of the steel sheet to form an oxide, and improve the surface texture and plating property of the steel sheet. Therefore, one or more selected from these elements may be contained. However, even if these elements are excessively contained, the effect is saturated. Therefore, the Sb content is 0.050% or less, the Sn content is 0.050% or less, and the Bi content is 0.050% or less.
  • the contents of Sb, Sn, and Bi are all preferably 0.030% or less, 0.010% or less, 0.006% or less, or 0.003% or less.
  • At least one of the above-mentioned elements in an amount of 0.0005% or more.
  • the contents of Sb, Sn, and Bi are more preferably 0.001% or more, respectively.
  • 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 steel is industrially manufactured, and is allowed as long as it does not adversely affect the present invention. Means something.
  • the position at a depth of 1/4 of the plate thickness also referred to as "1/4 position” from the surface.
  • the metallographic structure contains 10% or more retained austenite, 60-80% tempered martensite, and martensite is limited to less than 20%.
  • the fraction of each structure changes depending on the heat treatment conditions and affects the material of the steel sheet such as strength, elongation, bendability, and impact characteristics. The reasons for the limitation of each organization will be explained in detail.
  • Residual austenite 10% or more
  • Residual austenite is a structure that enhances the strength-ductility balance of a steel sheet by transformation-induced plasticity. In order to obtain these effects, the steel sheet according to the present invention needs to contain 10% or more of retained austenite in the metal structure.
  • the area ratio of retained austenite is preferably 13% or more, more preferably 18% or more. When the area ratio of retained austenite is 13% or more, further 18% or more, both strength and elongation are compatible, and TS ⁇ El, which will be described later, becomes higher.
  • the upper limit of the area ratio of retained austenite is not particularly specified, but is substantially 30% or less.
  • Tempering martensite 60-80% Tempered martensite is also a hard phase, but it has a structure different from that of martensite, which will be described later, and has the effect of improving impact characteristics and ensuring the strength of the steel sheet.
  • the area ratio of tempered martensite shall be 60% or more.
  • the area ratio of tempered martensite is 80% or less.
  • the area ratio of tempered martensite is preferably 63% or more, and preferably 77% or less.
  • the steel sheet of the present invention has a metal structure mainly composed of tempered martensite by being manufactured through the steps described later.
  • a metal structure mainly composed of tempered martensite by being manufactured through the steps described later.
  • the steel sheet has a hot-dip galvanized layer or an alloyed hot-dip galvanized layer, a small amount of bainite may inevitably be mixed in the plating process.
  • the tempered martensite contains bainite.
  • bainite is also measured in addition to tempered martensite. That is, it means that the total area ratio of tempered martensite and bainite is 60 to 80%.
  • Martensite less than 20% Martensite is a hard phase with many dislocations in its structure and is an effective structure for obtaining the strength of steel sheets.
  • the area ratio of martensite is set to less than 20% because the impact characteristics are significantly deteriorated.
  • the area ratio of martensite is preferably 15% or less, more preferably 10% or less, still more preferably 0%.
  • martensite means martensite which has not been tempered.
  • the rest other than retained austenite, martensite, and tempered martensite is cementite, and its area ratio is preferably 1% or less. Further, if ferrite and pearlite are mixed in the metal structure even in a small amount, it becomes difficult to increase the strength, so that the metal structure cannot be included. That is, the area ratio of ferrite and pearlite is 0%.
  • the tempered martensite and the small-angle grain boundary density at the martensite are 0.20 ⁇ m -1 or more.
  • the small-angle grain boundary density is preferably 0.30 ⁇ m -1 or more.
  • the small-angle grain boundary density means the total length ( ⁇ m) of the boundaries observed per unit area ( ⁇ m 2 ) in which the crystal orientation difference between tempered martensite and martensite is 2 degrees or more and less than 20 degrees. do.
  • the large-angle grain boundary density means the total length ( ⁇ m) of the tempered martensite and the boundary where the crystal orientation difference between martensite is 20 to 50 degrees, which is observed per unit area ( ⁇ m 2).
  • the ratio A L / A N of the particle density A N in the particle density A L and the plate thickness direction in the rolling direction of the residual austenite is 0.80 or more.
  • the upper limit of A L / A N is substantially 1.0 or less.
  • Average crystal grain size of retained austenite 2 ⁇ m or less
  • the average crystal grain size of retained austenite is not particularly limited, but from the viewpoint of ensuring high strength, it is 2 ⁇ m or less. preferable.
  • the area ratio of each structure, the small-angle grain boundary density and the large-angle grain boundary density in tempered martensite and martensite, the dispersed state of retained austenite, and the average crystal grain size of retained austenite are measured by the following methods. It shall be.
  • the area ratio of retained austenite is calculated as follows. First, a test piece 25 mm in the rolling direction and 25 mm in the rolling perpendicular direction (width direction) is cut out from the steel sheet. At this time, the thickness of the test piece is the same as the thickness of the steel plate. Then, the test piece is mechanically polished and then chemically polished to reduce the plate thickness by 1/4 to obtain a chemically polished test piece having a strain-free surface. X-ray diffraction analysis with a measurement range of 2 ⁇ of 45 to 105 degrees is performed three times on the surface of the test piece using a Co tube.
  • the integrated intensities of the peaks (111), (200), and (220) are obtained, and for the bcc phase, the integrated intensities of the peaks (110), (200), and (211) are obtained.
  • the integrated intensities are analyzed, the volume fraction of retained austenite is obtained, and the results of three X-ray diffraction analyzes are averaged to obtain the value as the area fraction of retained austenite.
  • the area ratio of tempered martensite and martensite is calculated from microstructure observation with a scanning electron microscope (SEM).
  • SEM scanning electron microscope
  • the L cross section obtained by cutting the steel sheet in parallel with the thickness direction and the rolling direction is mirror-polished, and then the microstructure is revealed by 3% nital. Then, at a magnification of 5000 times, a microstructure in a range of 100 ⁇ m in length (length in the plate thickness direction) ⁇ 300 ⁇ m in width (length in the rolling direction) is observed around the 1/4 position from the surface.
  • Tempered martensite is identified as a gray underlying structure, and retained austenite and martensite are identified as a white structure.
  • the area ratio of tempered martensite includes the area ratio of bainite.
  • the gray parts those having a polygonal shape and not containing cementite are judged to be ferrite, and those having a lamellar structure are judged to be pearlite, and are distinguished.
  • the area ratio of martensite is calculated by subtracting the area ratio of retained austenite measured by the X-ray diffraction method from the total area ratio of retained austenite and martensite.
  • EBSP Backscattered electron diffraction
  • the EBSP measurement is performed in the range of 160 ⁇ m in length (length in the plate thickness direction) ⁇ 80 ⁇ m in width (length in the rolling direction) centered on the 1/4 position from the surface. Then, in the above measurement range, as shown by a dotted line in FIG. 1, two lines are drawn in the horizontal direction and one line is drawn in the vertical direction, and the number of residual austenite grains crossed by each line is counted.
  • the value obtained by dividing the number of retained austenite grains crossed by the two horizontal lines by the length of the lines is the particle density AL ( ⁇ m -1 ) in the rolling direction, and the residual austenite grains crossed by the vertical lines.
  • the value obtained by dividing the number of lines by the total length of the two lines is defined as the particle density AN ( ⁇ m -1 ) in the rolling direction.
  • AL / AN is obtained from those results.
  • the average crystal grain size of the retained austenite is obtained by calculating the average value of the circle-equivalent diameters of the retained austenite grains specified by the EBSP measurement.
  • the tensile strength (TS) of the steel sheet according to the present invention is preferably 980 MPa or more, more preferably 1180 MPa or more. This is because when a steel sheet is used as a material for automobiles, the thickness is reduced by increasing the strength, which contributes to weight reduction. Further, in order to use the steel sheet according to the present invention for press molding, the elongation at break (El) is preferably 15% or more, more preferably 17% or more.
  • TS ⁇ El is preferably 22000 MPa ⁇ % or more, more preferably 24,000 MPa ⁇ % or more, and 26000 MPa ⁇ %. The above is more preferable.
  • the steel sheet according to the present invention is also excellent in bendability and Charpy impact characteristics at room temperature.
  • (A) Melting step Steel having the above-mentioned chemical composition is melted by a conventional method and cast to prepare a slab or an ingot.
  • the molten steel may be melted by a normal blast furnace method, and the raw material is a large amount of scrap like the steel produced by the electric furnace method. It may include.
  • the slab may be manufactured by a normal continuous casting process or may be manufactured by thin slab casting.
  • (B) Hot-rolling step The above-mentioned slab or steel ingot is heated and hot-rolled to obtain a hot-rolled steel sheet.
  • the heating temperature of the steel material to be subjected to hot rolling is 1100 to 1300 ° C.
  • the heating temperature By setting the heating temperature to 1100 ° C. or higher, the deformation resistance during hot rolling can be further reduced.
  • the heating temperature By setting the heating temperature to 1300 ° C. or lower, it is possible to suppress a decrease in yield due to an increase in scale loss.
  • the above heating temperature means the surface temperature of a slab or a steel ingot.
  • the time for holding in the temperature range of 1100 to 1300 ° C. before hot rolling is not particularly specified, but in order to improve the material stability, it is preferably 30 minutes or more, and more preferably 1 h or more. Further, in order to suppress excessive scale loss, it is preferably 10 hours or less, and more preferably 5 hours or less.
  • hot rolling may be performed as it is without heat treatment.
  • Hot rolling may be performed on a normal continuous hot rolling line. Further, in hot rolling, finish rolling is performed after rough rolling.
  • the starting temperature in finish rolling shall be 1000 ° C or lower. By setting the finish rolling start temperature to 1000 ° C or lower, coarsening of the structure during finish rolling is prevented, subsequent structure control becomes easy, and deterioration of the surface texture of the steel sheet due to intergranular oxidation is suppressed. ..
  • the finish rolling start temperature is preferably 750 ° C. or higher. When the finish rolling start temperature is 750 ° C. or higher, the deformation resistance during rolling can be reduced and the structure control can be easily performed.
  • Cooling step After finish rolling, allow to cool for 1.2 to 4.0 s. By allowing to cool after finish rolling, the formation of ferrite is suppressed, the structure becomes uniform, and retained austenite is isotropically dispersed. Further, by setting the cooling time to 4.0 s or less, it is possible to prevent coarsening of retained austenite.
  • the cooling rate is preferably 500 ° C./s or less.
  • the cooling rate is 500 ° C./s or less, uneven cooling is less likely to occur and cold rollability is improved.
  • the structure of the hot-rolled steel sheet can be made into a bainite single-phase or martensite single-phase, or a composite structure of bainite and martensite, and the retained austenite grains of the steel sheet according to the present invention. Can be isotropically dispersed.
  • (F) Cold-rolled steel sheet The hot-rolled steel sheet produced through the above steps is pickled by a conventional method and then cold-rolled at a reduction rate of 30 to 70% to obtain a cold-rolled steel sheet. do.
  • the rolling reduction of cold rolling is 30% or more, recrystallization occurs uniformly, retained austenite is uniformly generated, and the mixture is isotropically dispersed.
  • the rolling reduction ratio is 70% or less, fracture is less likely to occur during cold rolling.
  • the rolling reduction of cold rolling is preferably 40% or more, and preferably 60% or less. Cold rolling may be carried out on a normal continuous cold rolling line.
  • light rolling with a reduction ratio of more than 0% and 5% or less may be performed before pickling. Modifying the shape by light rolling is advantageous in terms of ensuring flatness. In addition, light rolling before pickling improves pickling properties, promotes removal of surface-concentrating elements, and has the effect of improving chemical conversion treatment properties and plating treatment properties.
  • the holding time is preferably 2.0 h or more, and more preferably 4.0 h or more.
  • the holding time is preferably 10.0 h or less, and more preferably 8.0 h or less.
  • the average rate of temperature rise from 300 ° C. to the holding temperature is preferably 0.01 ° C./s or more and 5 ° C./s or less.
  • the average cooling rate from the holding temperature to 300 ° C. is preferably 0.001 ° C./s or more and 500 ° C./s or less.
  • the holding time means the time held in the temperature range of 600 ° C. or higher and lower than Ac 3.
  • (H) Second heat treatment step Following the first heat treatment step, a second heat treatment step is performed.
  • heat treatment is performed in which the temperature is maintained at 3 points or more and Ac 3 + 80 ° C. or lower for 30 to 180 seconds, and then cooled to a temperature of 300 ° C. or lower.
  • the holding temperature By setting the holding temperature to Ac 3 points or more, the area ratio of the tempered martensite finally produced can be increased. It is also effective for miniaturizing retained austenite.
  • the holding temperature to Ac 3 + 80 ° C. or lower, the generation of block boundaries and packet boundaries can be promoted, and the large-angle grain boundary density can be increased.
  • the holding time is preferably 60 s or more, and preferably 120 s or less.
  • the metal structure is once made into a martensite single phase after the second heat treatment step. From this point of view, the average cooling rate from the holding temperature to 300 ° C. is preferably 2 ° C./s or more and 2000 ° C./s or less.
  • the holding time means the time for holding in the temperature range of 3 points or more and 3 + 80 ° C. or less of Ac.
  • a third heat treatment step is performed.
  • the third heat treatment step holding 650 ° C. or higher Ac 3 -20 ° C. point below the temperature range at 5s above. By setting the holding temperature to 650 ° C. or higher, austenite is likely to be produced. The holding temperature by below Ac 3 -20 ° C. points, to promote the production of tempered martensite, the strength - thereby improving the ductility balance.
  • the holding time is preferably 30 s or more from the viewpoint of more reliably dissolving cementite and stably ensuring good low temperature toughness. Further, from the viewpoint of productivity, the holding time is preferably 300 s or less.
  • the retention time is meant a time that is maintained at a temperature range of 3 -20 ° C. point 650 ° C. or higher Ac.
  • the average cooling rate from the holding temperature to 520 ° C. is 0.1 ° C./s or more and 100 ° C./s or less.
  • the first to third heat treatments may be performed in a batch furnace such as a box annealing furnace (BAF), or may be performed using a continuous annealing line.
  • the atmosphere of the heat treatment is not particularly limited, for example, an inert atmosphere, or may be any of a reducing atmosphere containing H 2 or the like.
  • the steel sheet is not plated after the third heat treatment step, it may be cooled to room temperature as it is.
  • plating a steel sheet it is manufactured as follows.
  • Hot-dip galvanizing step When hot-dip galvanizing the surface of a steel sheet to produce a hot-dip galvanized steel sheet, cooling after the third heat treatment is stopped in the temperature range of 430 to 500 ° C., and then the cold-rolled steel sheet is used. Is immersed in a hot-dip galvanizing bath to perform hot-dip galvanizing. The conditions of the plating bath may be within the normal range. After the plating treatment, it may be cooled to room temperature.
  • (K) Alloyed hot-dip galvanizing step When the surface of a steel sheet is subjected to alloying hot-dip galvanizing to produce an alloyed hot-dip galvanized steel sheet, the steel sheet is subjected to hot-dip galvanizing treatment and then cooled to room temperature. Before this, the hot dip galvanizing process is performed at a temperature of 450 to 620 ° C.
  • the alloying treatment conditions may be within the usual range.
  • the obtained 30 mm-thick steel piece was heated at 1250 ° C. for 1 h, and then hot-rolled under the conditions shown in Table 2. Subsequently, the winding was simulated and held at a predetermined temperature corresponding to the winding temperature for 30 minutes, and then slowly cooled to room temperature at 20 ° C./h to prepare a 2.6 mm thick hot-rolled steel sheet.
  • the example in which the winding temperature is shown as “room temperature” indicates that the sample has been cooled to room temperature depending on the conditions of the accelerated cooling step.
  • the obtained hot-rolled steel sheet was pickled and then cold-rolled under the conditions shown in Table 2 to prepare a 1.6 mm-thick cold-rolled steel sheet.
  • the obtained cold-rolled steel sheet was subjected to the first to third heat treatments under the conditions shown in Table 3.
  • the heat treatment was carried out in a reducing atmosphere of 98% nitrogen and 2% hydrogen.
  • heating is performed under the condition that the average temperature rise rate from 300 ° C. to the holding temperature is 0.5 ° C./s, and the average cooling rate from the holding temperature to 300 ° C. is 0.5 ° C./s. It was cooled to a temperature of 300 ° C. or lower under the above conditions.
  • the temperature was cooled to 300 ° C. or lower under the condition that the average cooling rate from the holding temperature to 300 ° C. was 10 ° C./s.
  • test No. 1, 3 to 10, 13 to 15, 17, 19 to 22 and 24 to 29 are heated under the condition that the average heating rate in the temperature range of 500 to 600 ° C. is 5 ° C./s. It was cooled to room temperature under the condition that the average cooling rate up to 520 ° C. was 10 ° C./s.
  • Test No. The hot-dip cold-rolled steel sheets of 2, 12, 16, 18 and 23 were heated under the condition that the average temperature rise rate in the temperature range of 500 to 600 ° C. in the third heat treatment was 5 ° C./s.
  • Cooling at an average cooling rate of 10 ° C./s is stopped at 460 ° C., the cold-rolled steel sheet is held at that temperature for 10 seconds, and immersed in a hot-dip galvanizing bath at 460 ° C. for 2 seconds to perform hot-dip galvanizing treatment. rice field.
  • the conditions of the plating bath are the same as those of the conventional one.
  • test No. 16 after the hot-dip galvanizing treatment, the mixture was cooled to room temperature at an average cooling rate of 10 ° C./s.
  • Test No. The annealed cold-rolled steel sheets of 2, 12, 18 and 23 were subjected to hot-dip galvanizing treatment, then heated to 500 ° C. at 10 ° C./s without cooling to room temperature, and held at 500 ° C. for 5 s.
  • the alloying treatment was performed, and then the mixture was cooled to room temperature at an average cooling rate of 10 ° C./s.
  • the annealed cold-rolled steel sheet thus obtained was subjected to skin pass rolling with a reduction ratio of 0.5%, and the test No. Steel sheets 1 to 29 were produced.
  • a JIS No. 5 tensile test piece was taken in the width direction of the steel sheet, and a tensile test was performed to measure the tensile strength (TS) and the elongation at break (El). The tensile test was carried out by the method specified in JIS Z 2241: 2011 using a JIS No. 5 tensile test piece.
  • the bendability was evaluated by performing a bending test. Bending test pieces having a width of 20 mm and a length of 50 mm were collected from each of the annealed steel sheets. The width direction of the bending test piece is parallel to the bending axis. The case where the width direction of the bending test piece is parallel to the rolling direction of the steel sheet is called a rolling direction bending test, and the case where the width direction of the bending test piece is parallel to the width direction of the steel sheet is called a width direction bending test. The test piece was pushed into the mold with a V-shaped punch having a punch radius of 4.8 mm and an apex angle of 90 ° of 3.2 mm.
  • the bending test was performed according to the V block method of JIS Z 2248: 2006. Observe the sample surface after the bending test, and if cracks are not observed at both the punch radius of 4.8 mm and 3.2 mm, the bendability is further improved, and if cracks are not observed only at the punch radius of 4.8 mm, bending is observed. When the property was good and cracks were observed at both the punch radii of 4.8 mm and 3.2 mm, the bendability was considered to be poor.
  • the impact characteristics were evaluated by performing a Charpy impact test.
  • a V-notch test piece was taken from each steel sheet after annealing. At this time, the length direction of the V-notch test piece was made to coincide with the rolling direction of the steel sheet. After stacking four of the test pieces and screwing them together, they were subjected to a Charpy impact test according to JIS Z 2242: 2018.
  • the impact characteristics when the impact value at 20 ° C. was 30 J / cm 2 or more, the impact characteristics were good, and when it was less than that, the impact characteristics were poor.
  • Table 4 shows the above evaluation results.
  • TS of 980 MPa or more, TS ⁇ El of 22000 MPa ⁇ % or more, good impact characteristics, and good bendability has high strength, strength-ductility balance, and bending. It was evaluated as a steel sheet with excellent properties and impact characteristics.
  • Test No. 8 since the second heat treatment step was not performed, tempered martensite was not generated and mainly became ferrite, and the small-angle grain boundary density and the large-angle grain boundary density decreased. In addition, retained austenite was also coarsened. As a result, the strength, TS ⁇ El and impact characteristics deteriorated. Test No. In No. 9, the holding temperature in the third heat treatment step was high, the area ratio of tempered martensite was low, and the area ratio of martensite was high, so that the impact characteristics deteriorated.
  • Test No. 11 since the third heat treatment step was not performed, the martensite became a single phase, and TS ⁇ El, impact characteristics, and bendability deteriorated.
  • Test No. 13 the holding time in the second heat treatment step was long, and the large-angle particle size density was lowered, so that the impact characteristics were deteriorated.
  • Test No. 15 the holding temperature in the second heat treatment step was high, and the large-angle particle size density was lowered, so that the impact characteristics were deteriorated.
  • Test No. 19 the holding time in the first heat treatment step was short, the small angle particle size density was low, and the isotropic property of the retained austenite was lowered, so that TS ⁇ El, impact characteristics, and bendability were deteriorated.
  • Test No. 22 the take-up temperature was high and the isotropic property of retained austenite was lowered, so that the impact characteristics and bendability were deteriorated.
  • Test No. 24 the holding temperature in the first heat treatment step was low, the small angle particle size density was low, and the isotropic property of the retained austenite was lowered, so that TS ⁇ El, impact characteristics, and bendability were deteriorated.
  • Test No. 26 the holding temperature in the third heat treatment step was low, and the area ratio of tempered martensite was high, so that the strength deteriorated.
  • Test No. 29 since the first heat treatment step and the second heat treatment step were not performed, tempered martensite was not generated and was mainly ferrite, and the area ratio of martensite was further increased. In addition, the small-angle grain boundary density and the large-angle grain boundary density decreased, and the isotropic property of retained austenite also decreased. As a result, TS ⁇ El, impact characteristics, and bendability deteriorated.
  • the steel sheet of the present invention exhibits excellent results not only in width direction bendability but also in rolling direction bendability because retained austenite is isotropically dispersed.
  • the steel sheet of the present invention can be used for various purposes, and is particularly preferably used for structural parts of automobiles such as side sills, and contributes to weight reduction of automobiles.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Sheet Steel (AREA)

Abstract

La présente feuille d'acier a une composition chimique dans laquelle, exprimé en pourcentage en masse, C est supérieur à 0,18 % et inférieur à 0,30 % ; Si représente au moins 0,001 % et moins de 2,00 % ; Mn est supérieur à 2,50 % et inférieur à 4,20 % ; sol. Al représente au moins 0,001 % et moins de 1,00 % ; P représente 0,030 % ou moins ; S représente 0,005 % ou moins ; N est inférieur à 0,050 % ; O est inférieur à 0,020 % ; Cr représente 0 à 0,50 % ; Mo représente 0 à 0,50 % ; W représente 0 à 0,30 % ; Cu représente 0 à 0,30 % ; Ni représente 0 à 0,50 % ; Ti représente 0 à 0,100 % ; Nb représente 0 à 0,100 % ; V représente 0 à 0,100 % ; B représente 0 à 0,010 % ; Ca représente 0 à 0,010 % ; Mg représente 0 à 0,010 % ; Zr représente 0 à 0,010 % ; REM représente 0 à 0,010 % ; Sb représente 0 à 0,050 % ; Sn représente 0 à 0,050 % ; Bi représente 0 à 0,050 % ; et le reste étant du Fe et des impuretés, dans laquelle, dans une section transversale de la feuille d'acier, la section transversale étant parallèle à la direction de laminage et à la direction de l'épaisseur de la feuille, la structure métallographique à une position 1/4 de l'épaisseur de la feuille à partir de la surface comprend, en termes de pourcentage de surface, 10 % ou plus d'austénite résiduelle, 60 à 80 % de martensite revenue, moins de 20 % de martensite, et la densité de limite d'angle faible étant de 0,20 à 1,0 µm-1, la densité de limite d'angle élevée est de 0,30 à 0,60 µm-1, et AL/AN représente 0,80 à 1,0.
PCT/JP2021/010890 2020-04-02 2021-03-17 Feuille d'acier Ceased WO2021200169A1 (fr)

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KR20250140093A (ko) 2023-03-10 2025-09-24 닛폰세이테츠 가부시키가이샤 강판

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WO2014020640A1 (fr) * 2012-07-31 2014-02-06 Ｊｆｅスチール株式会社 Tôle d'acier galvanisée par immersion à chaud à résistance élevée qui présente une excellente aptitude au moulage et une excellente aptitude à la fixation de formes, ainsi que procédé de fabrication de cette dernière
WO2017208762A1 (fr) * 2016-05-30 2017-12-07 株式会社神戸製鋼所 Tôle d'acier à haute résistance et son procédé de production
WO2018105003A1 (fr) * 2016-12-05 2018-06-14 新日鐵住金株式会社 Tôle d'acier à résistance mécanique élevée
WO2018116155A1 (fr) * 2016-12-21 2018-06-28 Arcelormittal Tôle d'acier laminée à froid à haute résistance présentant une formabilité élevée et son procédé de fabrication
WO2018147400A1 (fr) * 2017-02-13 2018-08-16 Ｊｆｅスチール株式会社 Plaque d'acier à haute résistance et son procédé de fabrication
WO2019131188A1 (fr) * 2017-12-26 2019-07-04 Ｊｆｅスチール株式会社 Tôle d'acier laminée à froid à haute résistance et son procédé de fabrication

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Publication number Priority date Publication date Assignee Title
WO2014020640A1 (fr) * 2012-07-31 2014-02-06 Ｊｆｅスチール株式会社 Tôle d'acier galvanisée par immersion à chaud à résistance élevée qui présente une excellente aptitude au moulage et une excellente aptitude à la fixation de formes, ainsi que procédé de fabrication de cette dernière
WO2017208762A1 (fr) * 2016-05-30 2017-12-07 株式会社神戸製鋼所 Tôle d'acier à haute résistance et son procédé de production
WO2018105003A1 (fr) * 2016-12-05 2018-06-14 新日鐵住金株式会社 Tôle d'acier à résistance mécanique élevée
WO2018116155A1 (fr) * 2016-12-21 2018-06-28 Arcelormittal Tôle d'acier laminée à froid à haute résistance présentant une formabilité élevée et son procédé de fabrication
WO2018147400A1 (fr) * 2017-02-13 2018-08-16 Ｊｆｅスチール株式会社 Plaque d'acier à haute résistance et son procédé de fabrication
WO2019131188A1 (fr) * 2017-12-26 2019-07-04 Ｊｆｅスチール株式会社 Tôle d'acier laminée à froid à haute résistance et son procédé de fabrication

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
KR20250140093A (ko) 2023-03-10 2025-09-24 닛폰세이테츠 가부시키가이샤 강판

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