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High-yield ratio high-strength cold-rolled steel sheet and manufacturing method thereof

A technology of cold-rolled steel sheet and high yield strength ratio, applied in the field of high-strength cold-rolled steel sheet and its manufacturing, can solve the problems of poor hole expandability and achieve excellent manufacturing results

Active Publication Date: 2018-04-03
JFE STEEL CORP
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  • Summary
  • Abstract
  • Description
  • Claims
  • Application Information

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Problems solved by technology

However, the TRIP steel sheet has a disadvantage that the retained austenite transforms into martensite during punching, and therefore cracks occur at the interface with ferrite, resulting in poor hole expandability.

Method used

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  • High-yield ratio high-strength cold-rolled steel sheet and manufacturing method thereof
  • High-yield ratio high-strength cold-rolled steel sheet and manufacturing method thereof
  • High-yield ratio high-strength cold-rolled steel sheet and manufacturing method thereof

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Embodiment

[0113] Steel with the chemical composition shown in Table 1 was melted to cast a slab with a thickness of 230 mm, and the steel slab was hot-rolled at a hot rolling start temperature of 1250° C. under the conditions shown in Table 2 and Table 3 to obtain a slab. 3.2mm thick hot-rolled steel plate. In this hot rolling process, cooling is started within a predetermined time after finishing rolling, primary cooling is performed at a predetermined average cooling rate and cooled to a predetermined cooling temperature, and then secondary cooling is performed at a predetermined average cooling rate Then, it is cooled to a predetermined cooling temperature (the same temperature as the winding temperature), and the winding is performed.

[0114] After pickling the obtained hot-rolled steel sheet, cold rolling was performed to obtain a cold-rolled steel sheet with a plate thickness of 1.4 mm. Thereafter, continuous annealing was performed under the conditions shown in Table 2 and Tabl...

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Abstract

There is provided a high-strength cold-rolled steel sheet having excellent elongation and hole expansion formability and a high yield ratio. The high-strength cold-rolled steel sheet has a composite structure containing 0.15 to 0.25% by mass of C, 1.8 to 3.0% by mass of Mn, and 0.0003 to 0.0050% by mass of B, and having a ferrite volume fraction of 20% to 50%, a retained austenite volume fraction of 7% to 20%, a martensite volume fraction of 1% to 8%, and the balance containing bainite and tempered martensite, and in the composite structure, ferrite has an average crystal grain diameter of 5 µm or less, retained austenite has an average crystal grain diameter of 0.3 to 2.0 µm and an aspect ratio of 4 or more, martensite has an average crystal grain diameter of 2 µm or less, a metal phase containing both bainite and tempered martensite has an average crystal grain diameter of 7 µm or less, the ratio of the volume fraction of tempered martensite to the volume fraction of a metal structure other than ferrite is 0.60 to 0.85, and the average C concentration in retained austenite is 0.65% by mass or more.

Description

technical field [0001] The present invention relates to a high-strength cold-rolled steel sheet having a high yield ratio and a method for producing the same, and particularly relates to a thin steel sheet suitable for use as structural parts of automobiles and the like. In addition, the yield ratio (YR) is a value showing the yield stress (YS) relative to the tensile strength (TS), and is represented by YR=YS / TS. Background technique [0002] In the automotive field, as fuel efficiency improvement through weight reduction of car bodies is becoming an important issue, thinning of automobile parts through application of high-strength steel sheets is being promoted, and the application of steel sheets with a TS of 980MPa or more is being promoted. [0003] High-strength steel sheets used in automotive structural members and reinforcement members are required to have excellent formability and impact energy absorption characteristics. Especially for the molding of parts with co...

Claims

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Application Information

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Patent Type & Authority Patents(China)
IPC IPC(8): C22C38/00C21D9/46C22C38/14C22C38/58
Inventor 高岛克利小野义彦长谷川浩平
Owner JFE STEEL CORP
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