Highly impact-resistant steel pipe and method for producing the same

Abstract

A highly impact-resistant member has a round or square sectional shape, that is excellent in strength and toughness, does not undergo the deterioration of toughness in the vicinity of the welded portion, and a highly impact-resistant steel pipe has a tensile strength TS of 1,700 MPa or more and a yield ratio YR of 72% or less, the yield ratio being the ratio of a 0.1%-proof stress YS to a tensile strength TS (YS / TS). The toughness of the welded portion of the steel pipe is enhanced by controlling the Si amount in the steel of the steel pipe in the range from Mn / 8−0.07 to Mn / 8+0.07. The steel contains, in mass, 0.19 to 0.35% C, 0.10 to 0.30% Si, 0.5 to 1.60% Mn, not more than 0.025% P, not more than 0.01% S, 0.010 to 0.050% Al, 2 to 35 ppm B and 0.005 to 0.05% Ti as indispensable components.

Description

BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention relates to a highly impact-resistant steel pipe used as a member, such as a material for a door impact beam, a bumper, a bumper reinforcement or the like of an automobile, that requires an impact absorption energy, and a method for producing the steel pipe. Though the term “a steel pipe” is generally regarded as a steel pipe having a circular sectional shape, it is regarded as a steel pipe having a round or square sectional shape in this specification. Here, the round shape includes a circular shape, an elliptical shape, etc. and the square shape includes not only a polygonal shape such as a triangular shape, a tetragonal shape, a pentagonal shape, etc. but also an irregular sectional shape. [0003] 2. Description of the Related Art [0004] A high strength electric-resistance-welded steel pipe is mostly used for a member of a door impact beam disposed for absorbing the shock of a lateral collisio...

AI Technical Summary

Benefits of technology

[0007] The object of the present invention is, by solving the aforementioned conventional problems, to provide: a highly impact-resistant steel pipe having the material properties of a higher strength and a lower yield ratio than a conventional steel pipe, not undergoing the deterioration of toughness in the vicinity of an electric-resistance-welded portion in the case of an electric-resistance-welded steel pipe, and further having an ultra-low weight and a high collision safety; and a method for producing the steel pipe.

[0009] Further, the present inventors: measured the properties of Charpy absorbed energy in the vicinity of electric-resistance-welded portions in an attempt to solve the aforementioned problems; found that oxides containing Si and Mn remained on the fractured surface at a portion where Charpy absorbed energy decreased and those components were one of the causes of the deterioration of toughness; and confirmed that the deterioration of toughness in the vicinity of an electric-resistance-welded portion could be prevented when a specific relationship between Si and Mn, namely according to the expression Mn / 8−0.07≦Si≦Mn / 8+0.07, was secured.

[0013] As explained above, the present invention has been developed with intent to secure a material having more higher TS and a lower YR than a conventional material. A material having such a high TS is generally obtained by subjecting it to water quenching after heating and thus making the structure thereof composed of martensite. In prior technologies, the material property of a low yield ratio has been obtained by making soft austenite and / or ferrite remain partially in a hard martensite structure and thus lowering a proof stress. However, with such prior technologies, as has been explained above, the tensile strength and the yield ratio have been 1,500 to 1,600 MPa and 70 to 80%, respectively, at the least.

[0014] In contrast, in the present invention, while more higher TS than ever is secured by eliminating retained austenite and / or retained ferrite in a martensite structure, YS and YR are lowered by increasing the dislocation density in a hard martensite structure more than ever and thus causing the deformation to occur easily under a stress. A dislocation density in a steel pipe according to the present invention is in the range from 1010 to 1014 / mm−2 and is extremely high whereas a dislocation density in a conventional steel pipe is in the range from 108 to 109 / mm−2. By securing such a high dislocation density, a highly impact-resistant steel pipe according to the present invention can have a lower YR than a conventional steel pipe while more higher TS is maintained. Moreover, by employing a 0.1%-proof stress that has not been used as a YS value for calculating a YR value, an impact absorbing capacity can be evaluated more properly and the weight of a door impact beam can be reduced, almost to the limit.

[0015] Further, as explained above, it is thought that the oxides containing Si and Mn, the oxides being formed at an electric-resistance-welded portion, cause the toughness of the welded portion to deteriorate, and the present invention makes it possible to exclude the oxides from an electric-resistance-welded portion and completely prevent the toughness of the welded portion from deteriorating, while a high tensile strength of 1,700 MPa or more is maintained as shown in the data of the after-mentioned examples, by controlling an Si amount in the range from Mn / 8−0.07 to Mn / 8+0.07.

Problems solved by technology

However, most electric-resistance-welded steel pipes tend to undergo deterioration of strength and toughness in the vicinity of the electric-resistance-welded portions because the steel pipes are produced by electric-resistance-welding.

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