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.

1 . A highly impact-resistant steel pipe characterized in that: the tensile strength TS of said steel pipe is 1,700 MPa or more; and the YR thereof, said YR being the ratio of the 0.1%-proof stress YS to said tensile strength TS (YS/TS), is 72% or less.  
     
     
         2 . A highly impact-resistant steel pipe characterized in that: the tensile strength TS of said steel pipe is 1,800 MPa or more; and the YR thereof, said YR being the ratio of the 0.1%-proof stress YS to said tensile strength TS (YS/TS), is 70% or less.  
     
     
         3 . A highly impact-resistant steel pipe characterized in that: the tensile strength TS of said steel pipe is 1,900 MPa or more; and the YR thereof, said YR being the ratio of the 0.1%-proof stress YS to said tensile strength TS (YS/TS), is 68% or less.  
     
     
         4 . A highly impact-resistant steel pipe characterized in that: the tensile strength TS of said steel pipe is 2,000 MPa or more; and the YR thereof, said YR being the ratio of the 0.1%-proof stress YS to said tensile strength TS (YS/TS), is 66% or less.  
     
     
         5 . A highly impact-resistant electric-resistance-welded steel pipe characterized in that: the tensile strength TS of said steel pipe is 1,700 MPa or more; and the Si amount in the steel of said steel pipe is controlled in the range from Mn/8−0.07 to Mn/8+0.07 (mass %).  
     
     
         6 . A highly impact-resistant steel pipe according to any one of claims  1  to 4, characterized in that the dislocation density of said steel pipe is in the range from 10 10  to 10 14 /mm −2 .  
     
     
         7 . A highly impact-resistant steel pipe according to any one of claims  1  to 5, characterized in that the steel of said steel pipe 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.  
     
     
         8 . A highly impact-resistant steel pipe according to  claim 7 , characterized in that the steel of said steel pipe further contains, in mass, one or more components selected from among the group of 0.005 to 0.050% Nb, 0.005 to 0.070% V, 0.005 to 0.5% Cu, 0.1 to 0.5% Mo, 0.1 to 0.5% Ni, not more than 0.01% Ca, and not more than 0.1% rare earth metals (REMs).  
     
     
         9 . A highly impact-resistant steel pipe according to any one of claims  1  to 5, characterized in that 95% or more of the microstructure of said steel pipe is transformed into martensite by induction hardening and the prior austenite grain size number of said steel pipe is #6 or more.  
     
     
         10 . A highly impact-resistant steel pipe according to any one of claims  1  to 5, characterized in that said steel pipe has a round or square sectional shape.  
     
     
         11 . A method for producing a highly impact-resistant steel pipe according to any one of  claims 1  to  5 , characterized in that said steel pipe containing, 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, and further one or more components selected from among the group of 0.005 to 0.050%.Nb, 0.005 to 0.070% V, 0.005 to 0.5% Cu, 0.1 to 0.5% Mo, 0.1 to 0.5% Ni, not more than 0.01% Ca, and not more than 0.1% rare earth metals (REMs), is subjected to induction heating and then water quenching.  
     
     
         12 . A method for producing a highly impact-resistant steel pipe according to  claim 11 , characterized in that the cooling rate of said water quenching is 100° C./sec. or higher.  
     
     
         13 . A method for producing a highly impact-resistant steel pipe according to  claim 11  or  12 , characterized in that the cooling water temperature of said water quenching is 35° C. or lower.