PURPOSE: To secure sufficient absorbed energy and facilitate the machining of both end parts by setting the strength of the length parts 1/4L-5/8L of the center part higher than that of both end parts in the length L of a steel pipe, and setting the variable grade of strength in a region where strength between the center part and both end parts changes, within a specific range.

CONSTITUTION: In the whole length L of a steel pipe, the strength of the length parts 1/4L-5/8L of the center part is higher than that of both end parts. The high strength parts at the center do not have to be necessarily in complete symmetry. The dislocation from the center is allowed within a range of approxi mately 30% of the length of the high strength part. The strength is set to be over 80kg/mm2, and the variable grade of strength in a region where the strength between the center part and both end parts changes is set to be within a range of 0.2-2.0kg/mm2/mm. As a result, the machinability of pipe ends becomes desirable, and high bending strength and sufficient absorbed energy can be obtained.