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An automobile body reinforcing steel pipe having a wall thickness-to-outer diameter ratio, t/D, defined by: 0.09-4.8×10-5×L≤t/D≤0.16-6.0×10-5×L where L(mm) is a span of a bending load applied to the pipe. The pipe has a tensile strength of 120 kgf/mm2 or more and an elongation of 10% or more, and is

An automobile body reinforcing steel pipe having a wall thickness-to-outer diameter ratio, t/D, defined by: 0.09-4.8×10-5×L≤t/D≤0.16-6.0×10-5×L where L(mm) is a span of a bending load applied to the pipe. The pipe has a tensile strength of 120 kgf/mm2 or more and an elongation of 10% or more, and is preferably made of a steel consisting of 0.15-0.25 wt % C, 1.8 wt % or less Mn, 0.5 wt % or less Si, 0.04 wt % or less Ti, 0.0003-0.0035 wt % B, and the balance of Fe and unavoidable impurities including 0.0080 wt % or less N. A process for producing the steel pipe comprises: coiling a hot rolled steel sheet at a temperature of 600°C. or higher; electric welding the adjoining edges of the sheet to form a steel pipe; and quench hardening the pipe.

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A process for producing an automobile body reinforcing steel pipe having a wall thickness-to-outer diameter ratio, t/D, defined by the following formula; 0.09-4.8×10-5×L≤t/D≤0.16-6.0×10-5×L where L(mm) is a span of a bending load applied to the pipe, said process comprising the steps of: hot rolling

A process for producing an automobile body reinforcing steel pipe having a wall thickness-to-outer diameter ratio, t/D, defined by the following formula; 0.09-4.8×10-5×L≤t/D≤0.16-6.0×10-5×L where L(mm) is a span of a bending load applied to the pipe, said process comprising the steps of: hot rolling to form a steel sheet from a steel consisting of; C in an amount of from 0.15 to 0.25, Mn in an amount sufficient to prevent a self-tempering during quench hardening of said steel, but not more than 1.8 wt %, Si in an amount sufficient to obtain a sound weld-bonding of said pipe, but not more than 0.5 wt %, Ti in an amount sufficient to fix N in steel, so that B effectively improves the steel hardenability, but not more than 0.04 wt %, B in an amount from 0.0003 to 0.0035 wt %, and the balance of Fe and unavoidable impurities, including N in a minimum amount of not more than 0.0080 wt %; coiling said steel sheet in an as-hot rolled state at a temperature of 600°C. or higher; roll-forming said steel sheet to a pipe shape having adjacent edges; electric welding said pipe shape at said adjacent edges to form an electric welded steel pipe; and quench hardening said steel pipe.