High-strength steel sheets with excellent resistance to delayed fracture after forming, method for manufacturing the same, and high-strength automotive part manufactured of the same

Abstract

Steel sheets containing residual austenite of not more than 7 vol. %, crystallized and / or precipitated compounds with particle diameters of 0.01 to 5.0 μm of 100 to 100000 particle / mm2 and C of 0.05 to 0.3 mass %, Si of not more than 3.0 mass %, Mn of 0.01 to 3.0 mass %, P of not more than 0.02 mass %, S of not more than 0.02 mass %, Al of 0.01 to 3.0 mass %, N of not more than 0.01 mass % and Mg of 0.0002 to 0.01 mass %, with the remainder comprising iron and unavoidable impurities.

Description

[0001]This application is a divisional application under 35 U.S.C. §120 and §121 of U.S. application Ser. No. 10 / 558,579 filed Nov. 28, 2005, which is a 35 U.S.C. §371 of PCT / JP03 / 06617 filed May 27, 2003, which is incorporated by reference in its entirety.TECHNICAL FIELD[0002]The present invention relates to high-strength steel sheets, which inhibit delayed failures and delayed fractures that lead to problems, particularly with high-strength steel sheets, a method of manufacturing such steel sheets, and high-strength automotive part manufactured of such steel sheets.BACKGROUND ART[0003]While high-strength steels are often used for bolts, pre-stressed concrete (PC) wires, line pipes and other uses, it has been known that penetration of hydrogen into steel leads to delayed fractures when the strength exceeds 780 MPa.[0004]Meanwhile, there has been little awareness of delayed-fracture problems because (i) the penetrated hydrogen escapes from within steel in a short time because the sh...

AI Technical Summary

Benefits of technology

The patent text describes a method to improve the resistance to delayed fractures in high-strength steel sheets. The inventors discovered that by forming compounds or composite compounds of Mg and controlling the shape of these compounds, the steel sheets can have excellent resistance to delayed fractures even after being formed. The method can be applied to existing manufacturing equipment and does not compromise the formability of the steel sheets. The patent also describes the characteristics of the compounds and the distribution of the particles that contribute to the resistance. The resulting high-strength steel sheets have excellent post-forming delayed-fracture resistance.

Problems solved by technology

While high-strength steels are often used for bolts, pre-stressed concrete (PC) wires, line pipes and other uses, it has been known that penetration of hydrogen into steel leads to delayed fractures when the strength exceeds 780 MPa.

Meanwhile, there has been little awareness of delayed-fracture problems because (i) the penetrated hydrogen escapes from within steel in a short time because the sheet thickness is small and (ii) steel sheets whose strength is greater than 780 MPa were seldom used because of low workability.

Containing 0.4% or more C and large quantities of alloying elements, however, such steels do not have the workability and weldability required of steel sheets.

In addition, the need to apply hours of precipitation heat treatment to separate alloy carbides presents a problem in productivity.

Furthermore, no consideration is given to workability that is a basic property of steel sheets.

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