High-strength electric resistance welded steel tube and production method therefor

Abstract

A high-strength electric resistance welded steel tube has a composition including, in terms of percent by mass, C: 0.05 to 0.20%, Si: 0.5 to 2.0%, Mn: 1.0 to 3.0%, P: 0.1% or less, S: 0.01% or less, Al: 0.01 to 0.1%, N: 0.005% or less, and the balance being Fe and unavoidable impurities; and a structure which is a dual phase structure including a ferrite phase and a martensite phase, with a volume ratio of the martensite phase being 20 to 60%, wherein a tensile strength TS is 1180 MPa or more, an elongation El in a tube axis direction is 10% or more, and a yield ratio is less than 90%; and after application of a 2% prestrain and baking finishing that includes a heat treatment of 170° C.×10 min, a strength increase (BH value) is 100 MPa or more and a yield ratio is 90% or more.

Description

CROSS REFERENCE TO RELATED APPLICATIONS[0001]This application is the U.S. National Phase application of PCT International Application No. PCT / JP2011 / 057928, filed Mar. 23, 2011, and claims priority to Japanese Patent Application No. 2010-068499, filed Mar. 24, 2010, and 2011-015076, filed Jan. 27, 2011, the disclosures of each of which are incorporated herein by reference in their entireties for all purposes.FIELD OF THE INVENTION[0002]The present invention relates to high-strength electric resistance welded steel tubes suitable for use in crash members for automobiles such as door impact beams, cross members, and pillars, and, in particular, to a high-strength electric resistance welded steel tube having both excellent formability and shock absorption.BACKGROUND OF THE INVENTION[0003]In recent years, for the purposes of achieving enhanced safety of automobiles and in particular ensuring safety of occupants, shock absorbing members for absorbing impact energy upon collision are inst...

AI Technical Summary

Benefits of technology

The present invention provides a high-strength electric resistance welded steel tube that has excellent formability and shock absorption suitable for use in automotive shock absorbing members. The method for producing the high-strength electric resistance welded steel tube involves using a steel sheet with excellent formability and a desired bake hardenability, without significantly degrading its formability during tube production. After being worked to a desired shape, a heat treatment is performed to increase the strength and improve proof stress, ensuring reliable shock absorption. The high-strength electric resistance welded steel tube has excellent formability, can be used in various automotive parts, and provides low-cost industrial advantages.

Problems solved by technology

The technologies described in Patent Literatures 2 to 5 have problems in that, because of the low cooling rate after holding of heat during annealing, precipitation of carbides occurs, the solute C content in the ferrite becomes insufficient, the strength increase (bake hardening value or BH value) caused by a prestrain-baking finishing treatment is small, and a BH value of 100 MPa or more is not reliably achieved.

For example, when the time taken up to the start of water quenching is long due to the layout of the production line and thus the cooling rate is low, the C content distribution proceeds between ferrite and austenite and thus the amount of the solute C remaining in the ferrite presumably contributing to the bake hardenability is insufficient.

Accordingly, when a steel sheet produced by the technology described in Patent Literature 7 is formed into an electric resistance welded steel tube, the elongation will further decrease due to the processing strain applied during tube forming and the resulting steel tube does not reliably achieve an elongation of 10% or more.

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