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KR-102964359-B1 - High-strength steel plate and method of manufacturing the same

KR102964359B1KR 102964359 B1KR102964359 B1KR 102964359B1KR-102964359-B1

Abstract

Having a specified compositional composition, in terms of area percentage, ferrite is 30% or more and 80% or less, martensite is 5% or more and 35% or less, retained austenite is 8% or more, and the value obtained by dividing the area percentage of retained austenite with an aspect ratio of 2.0 or more and a uniaxial length of 1 μm or less by the total area percentage of retained austenite is 0.3 or more, the value obtained by dividing the average Mn amount (mass%) in retained austenite by the average Mn amount (mass%) in ferrite is 1.5 or more, the value obtained by multiplying the average aspect ratio of retained austenite by the value obtained by dividing the average Mn amount (mass%) in retained austenite by the average Mn amount (mass%) in ferrite is 3.0 or more, the value obtained by dividing the average C amount (mass%) in retained austenite by the average C amount (mass%) in ferrite is 3.0 or more, and the average C amount (mass%) in retained austenite is retain A high-strength steel sheet characterized by having a steel structure in which the value divided by the average Mn amount (mass%) in the austenite is less than 0.05.

Inventors

  • 엔도 카즈키
  • 가와사키 요시야스
  • 도지 유키

Assignees

  • 제이에프이 스틸 가부시키가이샤

Dates

Publication Date
20260513
Application Date
20201008
Priority Date
20191023

Claims (11)

  1. In mass %, C: 0.030% or more and 0.250% or less, Si: 0.01% or more and 3.00% or less, Mn: 3.10% or more and 8.00% or less, P: 0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, N: 0.0005% or more, 0.0100% or less, A composition containing Al: 0.001% or more and 2.000% or less, with the remainder consisting of Fe and unavoidable impurities, and In terms of area percentage, ferrite is 30% or more and 80% or less, martensite is 5% or more and 35% or less, retained austenite is 8% or more, and phases other than the ferrite, martensite, and retained austenite are 10% or less in area percentage, and The aspect ratio is 2.0 or higher, and the value obtained by dividing the area ratio of residual austenite with a shortening of 1 μm or less by the area ratio of the total residual austenite is 0.3 or higher, and The value obtained by dividing the average Mn amount (mass%) in the retained austenite by the average Mn amount (mass%) in the ferrite is 1.5 or higher, and the value obtained by multiplying the value obtained by dividing the average Mn amount (mass%) in the retained austenite by the average aspect ratio of the retained austenite is 3.0 or higher. The value obtained by dividing the average C content (mass%) in the retained austenite by the average C content (mass%) in the ferrite is 3.0 or higher, and It has a steel structure in which the value obtained by dividing the average C content (mass%) in the retained austenite by the average Mn content (mass%) in the retained austenite is less than 0.05, and High-strength steel plate with a yield ratio (YR) greater than 0.70.
  2. In paragraph 1, The above component composition, in mass %, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.500% or less, W: 0.500% or less, B: 0.0050% or less, Ni: 1.000% or less, Cr: 1.000% or less, Mo: 1.000% or less, Cu: 1.000% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ta: 0.100% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, Zr: 0.0050% or less, REM: 0.0050% or less High-strength steel plate additionally containing at least one element selected from among.
  3. In paragraph 1 or 2, High-strength steel sheet having an additional zinc plating layer on the surface.
  4. In paragraph 3, A high-strength steel sheet in which the above zinc plating layer is an alloyed zinc plating layer.
  5. A steel slab having the compositional component described in claim 1 or 2 is, after hot rolling, coiled at a temperature of 300°C or higher and 750°C or lower, maintained for more than 1800 seconds in a temperature range below the Ac 1 transformation point, then cold rolled, maintained for 20 seconds or more and 1800 seconds or less in a temperature range of -50°C or higher than the Ac 3 transformation point, then cooled to a cooling stop temperature below the martensite transformation start temperature, then reheated to a reheating temperature in a temperature range above the Ac 1 transformation point and below the Ac 1 transformation point +150°C, maintained for 20 seconds or more and 1800 seconds or less at the reheating temperature, and then cooled, wherein, in terms of area percentage, ferrite is 30% or more and 80% or less, martensite is 5% or more and 35% or less, retained austenite is 8% or more, and phases other than the ferrite, martensite, and retained austenite are 10% in terms of area percentage. A method for manufacturing a high-strength steel sheet having a steel structure in which the aspect ratio is 2.0 or higher, the value obtained by dividing the area ratio of the retained austenite with a shortening length of 1 μm or less by the total area ratio of the retained austenite is 0.3 or higher, the value obtained by dividing the average Mn amount (mass%) in the retained austenite by the average Mn amount (mass%) in the ferrite is 1.5 or higher, the value obtained by multiplying the average aspect ratio of the retained austenite by the value obtained by dividing the average Mn amount (mass%) in the retained austenite by the average Mn amount (mass%) in the ferrite is 3.0 or higher, the value obtained by dividing the average C amount (mass%) in the retained austenite by the average C amount (mass%) in the ferrite is 3.0 or higher, and the value obtained by dividing the average C amount (mass%) in the retained austenite by the average Mn amount (mass%) in the retained austenite is less than 0.05.
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  7. In paragraph 5, A method for manufacturing high-strength steel sheets by additionally performing zinc plating treatment.
  8. In Paragraph 7, A method for manufacturing a high-strength steel sheet, wherein, following the above zinc plating treatment, an alloying treatment of the zinc plating is performed in a temperature range of 450°C or higher and 600°C or lower.
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Description

High-strength steel plate and method of manufacturing the same The present invention relates to a high-strength steel sheet with excellent formability and a method for manufacturing, which is suitable as a component for use in industrial fields such as automobiles and electrical equipment. In particular, the present invention aims to obtain a high-strength steel sheet having a high yield ratio (YR) of greater than 0.70 and a tensile strength (TS) of 980 MPa or more, and having excellent ductility as well as stretch flangeability and bendability. Recently, improving the fuel efficiency of automobiles has become an important task from the perspective of preserving the global environment. For this reason, there is a growing movement to lighten the vehicle body itself by increasing the strength of body materials to achieve thinning. On the other hand, since increasing the strength of steel sheets leads to a decrease in formability, there is a demand for the development of materials that combine high strength and high formability. Furthermore, when steel sheets with a TS of 980 MPa or higher are applied as structural members for automobiles, a high YR (yield ratio) is required in addition to high formability to protect occupants. As a steel sheet with excellent high strength and ductility, a high-strength steel sheet utilizing the work-induced transformation of retained austenite has been proposed. This steel sheet exhibits a structure containing retained austenite, and while forming the steel sheet is easy due to the retained austenite, it is equipped with high strength because the retained austenite becomes martensite after forming. For example, Patent Document 1 proposes a high-strength steel sheet with very high ductility utilizing a process-induced transformation of retained austenite, having a tensile strength of 1000 MPa or more and an overall elongation (EL) of 30% or more. Such a steel sheet is manufactured by austenitizing a steel sheet with C, Si, and Mn as basic components, and then quenching it to the bainite transformation temperature range and isothermally holding it, a process known as austempering treatment. Retained austenite is generated by the concentration of C into austenite through this austempering treatment, but in order to obtain a large amount of retained austenite, it is necessary to add a large amount of C exceeding 0.3%. However, as the C concentration in the steel increases, spot weldability decreases, and especially at C concentrations exceeding 0.3%, the decrease is significant, making it difficult to commercialize as a steel sheet for automobiles. Furthermore, since the aforementioned patent document focuses on improving the ductility of high-strength thin steel sheets, hole expansion or bendability is not considered. In Patent Document 2, a high strength-ductility balance is obtained by using high-Mn steel and performing heat treatment in the two-phase region of ferrite and austenite. However, in Patent Document 2, the improvement of ductility due to Mn enrichment into the untransformed austenite is not considered, so there is room for improvement in machinability. In addition, Patent Document 3 improves overall elongation by using heavy Mn steel and performing heat treatment in the two-phase region of ferrite and austenite to enrich Mn into the untransformed austenite, thereby forming stable retained austenite. However, it does not consider the compatibility of yield ratio, elongation, hole expandability, and bendability. Furthermore, it does not examine the improvement of hole expandability or bendability by controlling the distribution of C as well as Mn in the second phase consisting of retained austenite or martensite. In the manufacturing method of Patent Document 3, since the heat treatment time is short and the diffusion rate of Mn is slow, it is inferred that the enrichment of Mn is insufficient to achieve the compatibility of yield ratio, hole expandability, and bendability in addition to elongation. In addition, Patent Document 4 describes how, by using heavy Mn steel and performing a long-term heat treatment in the two-phase region of ferrite and austenite on a hot-rolled plate, Mn enrichment into the untransformed austenite is promoted to form a retained austenite with a large aspect ratio, thereby improving uniform elongation and hole expansion. However, the above document examines the improvement of ductility and hole expansion of high-strength steel sheets based solely on Mn enrichment, and does not examine the improvement of yield ratio or bendability by controlling the distribution of C and Mn in the second phase consisting of retained austenite or martensite. (Form for carrying out the invention) The present invention will be described in detail below. Additionally, "%" indicating the content of a component element means "mass%" unless otherwise specified. (1) The reason for limiting the composition of the steel components in the present invention to the abov