High strength thin steel sheet excellent in resistance to delayed fracture after forming and method for preparation thereof , and automobile parts requiring strength manufactured from high strength thin steel sheet

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

TECHNICAL FIELD [0001] 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 [0002] 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. [0003] 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. [0004] Recently, however, the need to reduce the weight and increase the pre-collision passive safety of automobiles have...

AI Technical Summary

Benefits of technology

[0017] That is to say, the inventors discovered that delayed-fracture resistance after forming of high-strength steel sheets can be improved without deteriorating the formability thereof by forming compounds or composite compounds of Mg and controlling the shape of such compounds.

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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