High Strength Thick-Gauge Electric-Resistance Welded Steel Pipe Excellent in Hardenability, Hot Workability and Fatigue Strength and Method of Production of the Same

Abstract

The present invention provides high strength thick-gauge electric-resistance welded steel pipe excellent in hardenability, hot workability, and fatigue strength and a method of production of the same, that is, thick-gauge electric-resistance welded steel pipe containing, by mass %, C: 0.25 to 0.4%, Si: 0.01 to 0.50%, Mn: 0.8 to 1.5%, P: 0.05% or less, S: 0.05% or less, Al: 0.05% or less, Ti: 0.005 to 0.05%, B: 0.0005 to 0.01%, N: 0.001 to 0.05%, and a balance of Fe and unavoidable impurities, having a critical cooling rate Vc expressed by equation <1> of less than 30° C. / s, and having a ratio of thickness t and outside diameter D, t / D, of over 0.15 to 0.30 in range:log Vc=2.94−0.75β  <1>where, β=2.7C+0.4Si+Mn

Description

TECHNICAL FIELD[0001]The present invention relates to high strength thick-gauge electric-resistance welded steel pipe excellent in hardenability, hot workability, and fatigue strength suitable for a hollow stabilizer for securing running stability of an automobile and method of production of the same.BACKGROUND ART[0002]As one measure for improvement of the fuel economy of automobiles, the chasses of automobiles are being reduced in weight. The stabilizer easing the rolling of the chassis at the time of cornering of the automobile and securing stability of the chassis at the time of high speed running may be mentioned as being part of this. In the past, stabilizers have been produced by working a steel bar or other solid material into the required shape, but to lighten the weight, they are increasingly being produced using seamless steel pipe, electric-resistance welded steel pipe, or other hollow members.[0003]As electric-resistance welded steel pipe for a stabilizer, WO2002 / 070767...

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Benefits of technology

[0013]The thick-gauge electric-resistance welded steel pipe of the present invention was devised to achieve this object. It increases the C as much as possible to improve the strength (hardness) to an extent not impairing the weldability and toughness, strictly limits the range of the N content to improve the hot workability and fatigue strength, and adjusts the composition of the steel material so that the critical cooling rate Vc becomes a specific range so as to thereby secure the hardenability. Further, in the production of the thick-gauge electric-resistance welded steel pipe of the present invention, the heating temperature and the cross-section reduction rate are set to specific ranges for the stretch reducing rolling of the electric-resistance welded steel pipe.

[0022]The thick-gauge electric-resistance welded steel pipe of the present invention is extremely excellent in hardenability, so in the production of a stabilizer or other automobile structural member, for example, it is possible to obtain a sufficient hardening effect immediately after hot forming. Further, the quenching means is not limited to water cooling. It is also possible to obtain a sufficient hardening effect by oil quenching which has a smaller cooling rate than water cooling.

[0023]Further, since the hot workability is excellent, cracks or flaws are difficult to occur even with hot forming when producing automobile members. In addition, the fatigue strength is excellent, so there is high durability against repeated load. Further, the strength is high, so the stabilizers and other automobile structural members may be lightened in weight more.

Problems solved by technology

Further, they cannot be said to be sufficient in terms of fatigue characteristics either.

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