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High Strength Thin-Gauge Steel Sheet Excellent in Elongation and Hole Expandability and Method of Production of Same

a thin-gauge steel sheet, high-quality technology, applied in the direction of manufacturing tools, furnaces, heat treatment equipment, etc., can solve the problems of obstructing the precision of parts, and sometimes demanding complicated shapes, etc., to achieve high strength, and excellent elongation and hole expandability

Inactive Publication Date: 2008-01-03
NIPPON STEEL CORP
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  • Summary
  • Abstract
  • Description
  • Claims
  • Application Information

AI Technical Summary

Benefits of technology

[0008] The inventors studied the methods of production of high strength thin-gauge steel sheet with excellent elongation and hole expandability and as a result discovered that to further improve the ductility and hole expandability of steel sheet, in the case of high strength cold rolled steel sheet with a tensile strength of steel sheet of 500 MPa or more, the form and balance of the metal structure of the steel sheet and the use of tempered martensite are important. Furthermore, they discovered steel sheet establishing a specific relationship between the tensile strength and Si and Al so as to secure a suitable ferrite area fraction and avoid deterioration of the chemical conversion ability and plating adhesion and controlling precipitates and other inclusions contained inside by the addition of Mg, REM, and Ca so as to improve the local formability and thereby improve the press formability to an unparalleled level and a method of production of the same.

Problems solved by technology

From the aesthetic design and chassis design requirements, complicated shapes are sometimes demanded.
On the other hand, the phenomenon of springback or other elastic recovery after working a part occurs more readily the higher the strength of the steel sheet and obstructs securing the precision of the part.
In this way, these working methods require stretch flangeability, hole expandability, bendability, and other local formability of the steel sheet, but conventional high strength steel sheet do not have sufficient performance, cracks and other defects occur, and stable working of the products is not possible.

Method used

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Examples

Experimental program
Comparison scheme
Effect test

example 1

[0080] Comparing Experiment No. [8] with the same operating conditions as the past as a comparative example and Experiment Nos. [2], [6], and [9] of invention examples, it is learned that the invention examples exhibit better values of the hole expansion rate and elongation.

[0081] Further, as a comparison of sheets with the same level of tensile strength and generally the same ingredients, but satisfying formula (A) and not satisfying it, among the steel types B and C, E and F, and K and L, the C, F, and L satisfying formula (A) exhibited larger ferrite area fractions and better elongation.

example 2

[0082] Further changing and comparing the tempering conditions, the drop in strength was large and the elongation also conversely dropped. The drop in elongation is believed due to the formation of pearlite. Experiment Nos. [1], [2], [5], [6], and [9] of the invention examples all exhibited good results.

TABLE 3(Example 1)Experiment No. [8] (Comparative Examples)Underlined, bold-face, italics indicate rejectionHoleTemperedSteelTSexpansionFerriteResidual γmartensiteOthertype(MPa)EL (%)TS × ELratearea (%)vol. (%)area (%)compositionClassA59830.9184784181.83.6≦0.1MainlyComp. Ex.B60230.2181804084.12.9≦0.1martensiteComp. Ex.C61332.3198004084.33.6≦0.1Comp. Ex.D66529.2194183873.02.7≦0.1Comp. Ex.E70327.1190513862.13.7≦0.1Comp. Ex.F72228.6206493866.82.7≦0.1Comp. Ex.G79924.7197353859.33.3≦0.1Comp. Ex.H81123.6191403758.62.9≦0.1Comp. Ex.I83621.8182253457.13.1≦0.1Comp. Ex.J87520.7181133352.32.7≦0.1Comp. Ex.K93119.6182483337.74≦0.1Comp. Ex.L95620.5195983244.33.4≦0.1Comp. Ex.M98418.8184993035.53.6...

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PUM

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Abstract

The present invention provides high strength thin-gauge steel sheet with excellent elongation and hole expandability having a tensile strength of 500 MPa or more and a method of production of high strength thin-gauge steel sheet with excellent elongation and hole expandability enabling production of this on an industrial scale, that is, high strength thin-gauge steel sheet comprised of, by mass %, C: 0.03 to 0.25%, Si: 0.4 to 2.0%, Mn: 0.8 to 3.1%, P≦0.02%, S≦0.02%, Al≦2.0%, N≦0.01%, and a balance of Fe and unavoidable impurities and having a microstructure comprised of ferrite with an area fraction of 10 to 85% and residual austenite with a volume fraction of 1 to 10%, an area fraction of 10% to 60% of tempered martensite, and a balance of bainite.

Description

TECHNICAL FIELD [0001] The present invention relates to high strength thin-gauge steel sheet excellent in elongation and hole expandability and a method of production thereof. BACKGROUND ART [0002] Recently, due to the need for reducing the weight of automobiles and improving collision safety, high strength steel sheet excellent in formability into chassis frame members and reinforcement members, seat frame parts, and the like are being strongly demanded. From the aesthetic design and chassis design requirements, complicated shapes are sometimes demanded. High strength steel sheet having superior working performance is therefore necessary. [0003] On the other hand, due to the increasingly higher strength of steel sheet, the working method is frequently shifting from the conventional drawing using wrinkle elimination to simple stamping and bending. Especially, when the bending ridge is an arc or other curve, stretch flanging where the end face of the steel sheet is elongated is somet...

Claims

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Application Information

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Patent Type & Authority Applications(United States)
IPC IPC(8): C22C38/06C21D9/46
CPCB21B3/02C22C38/14C21D8/041C21D8/0426C21D8/0436C21D8/0473C21D9/46C21D2211/001C21D2211/002C21D2211/005C21D2211/008C22C38/02C22C38/04C22C38/06C22C38/12C22C38/001C22C38/002C21D1/25C22C38/005C22C38/18C22C38/22C22C38/26C22C38/28C22C38/32C22C38/38
Inventor NONAKA, TOSHIKITANIGUCHI, HIROKAZUGOTO, KOICHI
Owner NIPPON STEEL CORP
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