| 摘要 |
The present invention provides: a highly impact-resistant member having 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 having a tensile strength TS of 1,700 MPa or more and a yield ratio YR of 72% or less, said 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 said steel pipe is enhanced by controlling the Si amount in the steel of said steel pipe in the range from Mn/8 - 0.07 to Mn/8 + 0.07. Said 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. Said steel pipe according to the present invention comprises a steel wherein 95% or more of the microstructure of said steel is transformed into martensite by subjecting said steel pipe to induction heating and then water quenching at a cooling rate of 100.degree.C or higher and the prior austenite grain size number of said steel is #6 or more. The present invention includes methods for producing said steel pipe.
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