High strength steel sheet and manufacturing method therefor
a technology of steel sheets and manufacturing methods, applied in the field of high strength steel sheets, can solve the problems of increasing energy for heating steel sheets, imposing an excessively large burden on pressing machines, and deteriorating the surface state of steel sheets, and achieve excellent formability at room temperature. , the effect of high strength
Inactive Publication Date: 2019-03-21
KOBE STEEL LTD
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Benefits of technology
The present invention provides a strong steel sheet that is easy to form and has a high tensile strength of 980 MPa or higher. This sheet can be easily molded at room temperature, specifically in elongation and hole expansion, and requires less molding pressure. The invention also provides a method to manufacture this sheet.
Problems solved by technology
Though the elongation of the TRIP steel sheet is good in this manner, the strength of the steel sheet itself is high, so that the molding load during the press-forming is large, thereby imposing an excessively large burden on a pressing machine.
However, when the steel sheet is heated to such a high temperature, the surface state of the steel sheet is deteriorated by oxidation, and also the energy for heating the steel sheet increases.
Method used
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[0161]A steel material was produced by melting a steel containing the components shown in the following Table 1 with a balance being iron and inevitable impurities. The obtained steel material was heated and held at 1250° C. for 30 minutes and thereafter hot-rolled with the rolling reduction being set to be about 90% and with the finish rolling end temperature being set to be 920° C. Thereafter, the steel sheet was cooled down from this temperature to a coiling temperature of 600° C. at an average cooling rate of 30° C. / sec, followed by coiling. After the coiling, the steel sheet was cooled to room temperature, so as to produce a hot-rolled steel sheet having a sheet thickness of 2.6 mm.
[0162]After the obtained hot-rolled steel sheet was pickled to remove surface scale, cold rolling was carried out at a cold rolling rate of 46% to produce a cold-rolled steel sheet having a sheet thickness of 1.4 mm.
[0163]The obtained cold-rolled steel sheet was subjected to continuous annealing to p...
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Abstract
One aspect of the present invention is a high strength steel sheet having a specific component composition, wherein a metal structure of the steel sheet comprises polygonal ferrite, bainite, tempered martensite, and retained austenite; when the metal structure is observed with a scanning electron microscope, the metal structure satisfies polygonal ferrite: 10 to 50 area %, bainite: 10 to 50 area %, and tempered martensite: 10 to 80 area % with respect to the metal structure overall; and when the metal structure is measured by X-ray diffractometry, the metal structure satisfies retained austenite: 5.0 volume % or more, retained austenite with a carbon concentration of 1.0 mass % or less: 3.5 volume % or more, and retained austenite with a carbon concentration of 0.8 mass % or less: 2.4 volume % or less with respect to the metal structure overall.
Description
TECHNICAL FIELD[0001]The present invention relates to a high strength steel sheet and a manufacturing method therefor. In further detail, the present invention relates to a high strength steel sheet being excellent in formability at room temperature and having a tensile strength of 980 MPa or more in which a molding load for forming at a hot temperature of 100 to 350° C. is outstandingly reduced as compared with the molding load for forming at room temperature, as well as to a manufacturing method therefor.BACKGROUND ART[0002]In a steel sheet used for structural components in an automobile or the like, a high strength of 980 MPa or more is demanded for realizing collision safety for passengers and improvement in fuel cost performance. Meanwhile, the steel sheet is typically molded into a component shape at room temperature, and press-forming is carried out for this molding. Accordingly, it is demanded that the steel sheet has a good press-formability (which may hereafter be simply r...
Claims
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IPC IPC(8): C21D9/46C22C38/00C22C38/04C22C38/02C22C38/06C23C2/02C23C2/06C23C2/40
CPCC21D9/46C22C38/001C22C38/04C22C38/02C22C38/06C23C2/02C23C2/06C23C2/40C21D2211/005C21D2211/002C21D2211/008C21D2211/001C22C38/28C22C38/32C22C38/34C22C38/38C22C38/002C22C38/005C22C38/08C22C38/16C22C38/14C22C38/12C22C38/60C21D1/785C21D1/185C21D1/19C21D6/005C21D6/008C21D8/0247C21D8/0447C23C2/024C23C2/0224C21D9/0062C25D5/34
Inventor IKEDA, MUNEAKIKASUYA, KOJIMURATA, TADAOSAITO, KENJIMURAKAMI, TOSHIOFUTAMURA, YUICHI
Owner KOBE STEEL LTD