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KR-102963509-B1 - HIGH STRENGTH STEEL SHEET, HIGH STRENGTH COATED OR PLATED STEEL SHEET, METHODS OF PRODUCING SAME, AND MEMBER

KR102963509B1KR 102963509 B1KR102963509 B1KR 102963509B1KR-102963509-B1

Abstract

The present invention provides a high-strength steel plate capable of manufacturing parts with excellent elongation flangeability, bendability, and LME resistance, as well as high dimensional accuracy. The high-strength steel sheet of the present invention has a composition in which C, Si, Mn, P, S, Al, N, Nb, Ti, and B are contained in predetermined amounts, [%Si]/[%Mn] is 0.10 or more and 0.60 or less, the amount of free Ti is 0.001 mass% or more, and the remainder is Fe and unavoidable impurities; the sum of the solid solution content of Nb and the amount of precipitation of 100 nm or less in the surface layer of the steel sheet is 0.002 mass% or more; at the 1/4 thickness position, the area percentage of martensite is 78% or more, the area percentage of ferrite is 10% or less, the volume percentage of retained austenite is less than 10.0%, and the ratio of the grain size in the rolling direction to the grain size in the thickness direction of the former austenite grains is 2.0 or less; and the hardness variation per 1,500 μm in the width direction at the thickness of the sheet is 200 μm. It is a steel plate having a steel structure with a frequency of 20 times or less, and a tensile strength of 1180 MPa or more.

Inventors

  • 미나미 히데카즈
  • 와다 유스케
  • 도지 유키
  • 니시야마 다케시
  • 사에키 미치토시

Assignees

  • 제이에프이 스틸 가부시키가이샤

Dates

Publication Date
20260512
Application Date
20220531
Priority Date
20210709

Claims (13)

  1. In mass %, C : 0.090% or more and 0.390% or less, Si: 0.01% or more and 2.50% or less, Mn : 2.00% or more, 4.00% or less, P : 0.001% or more and 0.100% or less, S : 0.0001% or more and 0.0200% or less, Al : 0.001% or more and 0.100% or less, N : 0.0005% or more and 0.0100% or less, Nb : 0.002% or more and 0.100% or less, Ti : 0.005% or more and 0.100% or less and B: Containing 0.0002% or more and 0.0100% or less, [%Si]/[%Mn] satisfies the relationship between 0.10 and 0.60, and The relationship is satisfied such that the amount of free Ti obtained from the following (1) is 0.001 mass% or more, and A composition in which the remainder consists of Fe and unavoidable impurities, and On the surface of the steel plate, the sum of the solid solution content of Nb and the precipitation amount of 100 nm or less is 0.002 mass% or more, and At the 1/4 position of the plate thickness, Area ratio of martensite is 78% or more, The area percentage of ferrite is 10% or less, Volume percentage of retained austenite is less than 10.0%, The average value of the ratio of the grain size in the rolling direction to the grain size in the thickness direction of the old austenite grains is 2.0 or less, and Having a steel structure in which the frequency of hardness variation per 1,500 μm in the plate width direction at a depth of 200 μm from the surface of the steel plate is 20 times or less, High-strength steel plate with a tensile strength of 1180 MPa or higher. Free Ti Amount (%) ＝ [%Ti] － (47.9/14.0) × [%N] － (47.9/32.1) × [%S] … (1) Also, [%X] in (1) represents the content (mass%) of element X in the steel, and is set to 0 if it is not contained. In addition, the frequency of hardness variation per 1500 μm in the plate width direction is determined by creating a hardness distribution by measuring 100 Vickers hardness points at intervals of 15 μm along the plate width direction from the center of the plate width using a Vickers hardness tester under a load of 50 gf for an observation surface located 200 μm in the depth direction from the surface of the steel plate, and in the hardness distribution, the value of {(maximum hardness value Hv max ) - (minimum hardness value Hv min )}/2 is set as a reference variation amount, and the case where the Vickers hardness fluctuates up or down by more than the reference variation amount is counted as one instance, thereby measuring the number of hardness variations in the area where the Vickers hardness was measured.
  2. In Article 1, The above steel structure is also a high-strength steel plate having a surface softening thickness of 10 μm or more and 100 μm or less.
  3. In Article 1, The above component composition is additionally, in mass %, O : 0.0100 % or less, V : 0.200 % or less, Ta : 0.10% or less, W : 0.10 % or less, Cr : 1.00 % or less, Mo: 1.00% or less, Ni: 1.00% or less, Co : 0.010 % or less, Cu: 1.00% or less, Sn : 0.200 % or less, Sb : 0.200 % or less, Ca : 0.0100 % or less, Mg: 0.0100% or less, REM : 0.0100 % or less, Zr : 0.100% or less, Te : 0.100% or less, Hf: 0.10% or less and High-strength steel sheet containing at least one element selected from the group consisting of Bi: 0.200% or less.
  4. In Article 2, The above component composition is additionally, in mass %, O : 0.0100 % or less, V : 0.200 % or less, Ta : 0.10% or less, W : 0.10 % or less, Cr : 1.00 % or less, Mo: 1.00% or less, Ni: 1.00% or less, Co : 0.010 % or less, Cu: 1.00% or less, Sn : 0.200 % or less, Sb : 0.200 % or less, Ca : 0.0100 % or less, Mg: 0.0100% or less, REM : 0.0100 % or less, Zr : 0.100% or less, Te : 0.100% or less, Hf: 0.10% or less and High-strength steel sheet containing at least one element selected from the group consisting of Bi: 0.200% or less.
  5. A high-strength plated steel sheet having a plating layer on at least one side of the high-strength steel sheet described in any one of claims 1 to 4.
  6. A method for manufacturing a high-strength steel plate as described in any one of claims 1, 3, and 4, wherein A steel slab having the above compositional components is heated for at least 100 minutes at a slab heating temperature of 1150°C or higher, and Subsequently, the above steel slab is subjected to hot rolling with a rough rolling end temperature of 1050°C or higher and a finish rolling start temperature of 1000°C or higher to form a hot-rolled plate, and Next, the above hot-rolled plate is pickled, and Next, the above hot-rolled plate is cold-rolled with a cumulative reduction rate of 20% or more and 75% or less to form a cold-rolled plate, and A method for manufacturing a high-strength steel sheet, wherein the above cold-rolled sheet is heated to a heating temperature of 820°C or higher under conditions where the average heating rate in the temperature range of 250°C or higher and 700°C or lower is 10°C/s or higher and the residence time from 750°C to the heating temperature is 40 s or higher, and the annealing process is performed to cool to 150°C or lower with an average cooling rate in the temperature range of 250°C or higher and 400°C or lower is 1.0°C/s or higher to obtain a high-strength steel sheet.
  7. In Article 6, A method for manufacturing a high-strength steel sheet, wherein the oxygen concentration of the atmosphere at the heating temperature of the above annealing process is 2 volume ppm or more and 30 volume ppm or less, and the dew point of the atmosphere is -35°C or higher.
  8. A method for manufacturing a high-strength plated steel sheet, wherein, after the annealing process described in claim 6, a plating process is performed to obtain a high-strength plated steel sheet by performing a plating treatment on at least one side of the high-strength steel sheet.
  9. A method for manufacturing a high-strength plated steel sheet, wherein, after the annealing process described in claim 7, a plating process is performed to obtain a high-strength plated steel sheet by performing a plating treatment on at least one side of the high-strength steel sheet.
  10. A member formed by using a high-strength steel plate described in any one of claims 1 to 4 in at least a portion.
  11. A member formed by using the high-strength plated steel sheet described in claim 5 in at least a portion.
  12. In Article 10, A component for use as a structural frame component of an automobile, or as a reinforcing component of an automobile.
  13. In Article 11, A component for use as a structural frame component of an automobile, or as a reinforcing component of an automobile.

Description

High-strength steel sheet, high-strength coated or plated steel sheet, methods of producing the same, and member The present disclosure relates to high-strength steel sheets, high-strength plated steel sheets, methods for manufacturing the same, and members. With the aim of achieving both a reduction in CO2 emissions through vehicle weight reduction and an improvement in crash resistance performance through vehicle body weight reduction, the strength of thin steel sheets for automobiles is being increased, and new legal regulations are being introduced one after another. Consequently, in order to increase body strength, the application of high-strength steel sheets with a tensile strength (TS) of 1180 MPa or higher is increasing in major structural components that form the frame of the automobile cabin. High-strength steel sheets used in automotive reinforcement or frame structure components are required to possess excellent formability. Furthermore, the formed parts are required to have excellent dimensional accuracy. For example, since parts such as crash boxes have stamped ends or bent sections, steel sheets with high elongation flangeability or bendability are desirable from the perspective of formability. Additionally, from the perspective of part performance, increasing the yield ratio (YR = yield strength YS / tensile strength TS) of the steel sheet enables an increase in shock absorption energy during a collision. Moreover, from the perspective of part dimensional accuracy, controlling the yield ratio (YR) of the steel sheet within a certain range suppresses springback after forming, thereby enabling control of the part's dimensional accuracy. To increase the application rate of high-strength steel sheets for automotive parts, it is required that these characteristics be comprehensively satisfied. Furthermore, it has recently been confirmed that when spot welding high-strength galvanized steel sheets, zinc from the plating layer diffuses into the grain boundaries of the steel sheet's surface layer, causing Liquid Metal Embrittlement (LME) and resulting in intergranular cracking (LME cracking). Since LME cracking can occur even in high-strength steel sheets without a galvanizing layer if the welding partner is also galvanized steel, it is considered a problem for any type of high-strength steel sheet. Therefore, when applying high-strength steel sheets to skeletal components, high-strength steel sheets with excellent LME resistance are required. In response to these requirements, for example, Patent Document 1 provides a high-strength steel sheet having a tensile strength of 980 MPa or more, which has excellent ductility, elongation flangeability, bendability, and LME resistance, and makes it possible to manufacture parts with high dimensional accuracy. Figure 1 is a schematic diagram illustrating the measurement of the plate thickness reduction amount. Figure 2 is a diagram illustrating the measurement of the frequency of longitude fluctuation. Embodiments of the present disclosure are described below. Furthermore, the present disclosure is not limited to the following embodiments. First, the appropriate range of the component composition of the steel sheet and the reasons for such limitation will be explained. Furthermore, in the following description, "%" indicating the content of the component elements of the steel sheet means "mass %" unless otherwise specified. "ppm" means "mass ppm" unless otherwise specified. Also, in this specification, a numerical range indicated by "~" means a range that includes the values described before and after "~" as lower and upper limits. [C : 0.090% or more, 0.390% or less] Carbon (C) is one of the important basic components of steel and, particularly in this disclosure, is an important element that affects the area percentage of martensite and ferrite, as well as the volume percentage of retained austenite. If the C content is less than 0.090%, the area percentage of martensite decreases, and the area percentage of ferrite increases, making it difficult to achieve a TS of 1180 MPa or higher. On the other hand, if the C content exceeds 0.390%, the carbon concentration in the retained austenite increases excessively, causing a significant increase in the hardness of the martensite generated from the retained austenite during stamping. As a result, crack propagation during hole expansion is accelerated, and λ decreases. Additionally, bendability is reduced. Therefore, the C content is set to be 0.090% or more and 0.390% or less. The C content is preferably 0.100% or more. The content of C is preferably 0.360% or less. The content of C is more preferably 0.110% or more. The content of C is more preferably 0.350% or less. [Si : 0.01% or more, 2.50% or less] Si is one of the important basic components of steel, and in particular in this disclosure, it is an element that affects the volume percentage of retained austenite in that it suppresses the formation of carbides