High tensile steel excellent in high temperature strength and method for production thereof

Abstract

The present invention relates to a high-tensile-strength low alloy carbon steel (in the form of a steel sheet, a steel pipe, a section steel or a wire rod), for a building structure, said high-tensile-strength steel being excellent in high temperature strength during a relatively short span of about one hour in the temperature from 600° C. to 800° C. and being used in the field of building construction, civil engineering, an offshore structure, shipbuilding, a reservoir tank or the like; and, more specifically, is a high-tensile-strength steel excellent in high temperature strength, characterized by containing, in mass, C at not less than 0.005% to less than 0.08%, Si at not more than 0.5%, Mn at 0.1 to 1.6%, P at not more than 0.02%, S at not more than 0.01%, Mo at 0.1 to 1.5%, Nb at 0.03 to 0.3%, Ti at not more than 0.025%, B at 0.0005 to 0.003%, Al at not more than 0.06%, and N at not more than 0.006%, with the balance consisting of Fe and unavoidable impurities and satisfying the expression p≧−0.0029×T+2.48 when the steel temperature T (° C.) is within the range from 600° C. to 800° C., wherein p is a stress drop ratio (a yield stress at a high temperature/a yield stress at room temperature) that is obtained by converting a yield stress normalized by using a yield stress at room temperature.

1 . A high-tensile-strength steel excellent in high temperature strength, characterized by containing, in mass, C at not less than 0.005% to less than 0.08%, Si at not more than 0.5%, Mn at 0.1 to 1.6%, P at not more than 0.02%, S at not more than 0.01%, Mo at 0.1 to 1.5%, Nb at 0.03 to 0.3%, Ti at not more than 0.025%, B at 0.0005 to 0.003%, Al at not more than 0.06%, and N at not more than 0.006%, with the balance consisting of Fe and unavoidable impurities.  
   
   
       2 . A high-tensile-strength steel excellent in high temperature strength according to  claim 1 , characterized by said steel satisfying the expression p≧−0.0029×T+2.48 when the steel temperature T (° C.) is within the range from 600° C. to 800° C., wherein p is a stress drop ratio (a yield stress at a high temperature/a yield stress at room temperature) that is obtained by a yield stress normalized by using a yield stress at room temperature.  
   
   
       3 . A high-tensile-strength steel excellent in high temperature strength according to  claim 1 , characterized in that: said steel comprising a single structure composed of bainite or a composite structure composed of ferrite and bainite at room temperature; the temperature (Ac 1 ) at which said structure reversely transforms into austenite during high temperature heating corresponding to a fire higher than 800° C.; and said steel satisfies the expression p≧−0.0029×T+2.48 when the steel temperature T (° C.) is within the range from 600° C. to 800° C., wherein p is a stress drop ratio (a yield stress at a high temperature/a yield stress at room temperature) that is obtained by converting a yield stress normalized by using a yield stress at room temperature.  
   
   
       4 . A high-tensile-strength steel excellent in high temperature strength according to  claim 1 , characterized in that, in the high temperature range from 600° C. to 800° C.: said steel has such a strength as to satisfy the expression p≧−0.0029×T+2.48 when the steel temperature T (° C.) is within the range from 600° C. to 800° C., wherein p is a stress drop ratio (a yield stress at a high temperature/a yield stress at room temperature) that is obtained by converting a yield stress normalized by using a yield stress at room temperature; said steel has a structure wherein the temperature (Ac 1 ) at which a single structure composed of bainite or a composite structure composed of ferrite and bainite at room temperature reversely transforms into austenite during high temperature heating corresponding to a fire is higher than 800° C.; one or more of carbonitrides precipitated phases thermodynamically stable in said single structure composed of bainite or in said composite structure composed of ferrite and bainite is not less than 5×10 −4  in terms of a molar fraction; and the total amount of Mo, Nb and Ti that dissolve in the ferrite structure is not less than 1×10 −3  in terms of a molar concentration.  
   
   
       5 . A high-tensile-strength steel excellent in high temperature strength according to  claim 1 , characterized in that, in the high temperature range from 600° C. to 800° C.: said steel has such a strength as to satisfy the expression p≧−0.0029×T+2.48 when the steel temperature T (° C.) is within the range from 600° C. to 800° C., wherein p is a stress drop ratio (a yield stress at a high temperature/a yield stress at room temperature) that is obtained by converting a yield stress normalized by using a yield stress at room temperature; said steel has a structure wherein the temperature (Ac 1 ) at which a single structure composed of bainite or a composite structure composed of ferrite and bainite at room temperature reversely transforms into austenite during high temperature heating corresponding to a fire higher than 800° C.; the average circle equivalent diameter of prior austenite grains in said steel is not more than 120 μm; one or more of carbonitrides precipitated phases thermodynamically stable in said single structure composed of bainite or in said composite structure composed of ferrite and bainite is not less than 5×10 −4  in terms of a molar fraction; and the total amount of Mo, Nb and Ti that dissolve in the ferrite structure is not less than 1×10 −3  in terms of a molar concentration.  
   
   
       6 . A high-tensile-strength steel excellent in high temperature strength according to  claim 1 , characterized in that the weld cracking susceptibility index PCM of said steel defined by the following expression is not more than 0.20%;  
         PCM =C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15 +v/ 10+5×B.  
   
   
       7 . A high-tensile-strength steel excellent in high temperature strength according to  claim 1 , wherein the steel further containing, in mass, one or more of Ni at 0.05 to 1.0%, Cu at 0.05 to 1.0%, Cr at 0.05 to 1.0%, and V at 0.01 to 0.1%.  
   
   
       8 . A high-tensile-strength steel excellent in high temperature strength according to  claim 1 , wherein the steel further containing, in mass: one or more of Ni at 0.05 to 1.0%, Cu at 0.05 to 1.0%, Cr at 0.05 to 1.0%, and v at 0.01 to 0.1%; and additionally one or more of Ca at 0.0005 to 0.004%, REM at 0.0005 to 0.004%, and Mg at 0.0001 to 0.006%.  
   
   
       9 . A high-tensile-strength steel excellent in high temperature strength according to  claim 7 , characterized in that, in the high temperature range from 600° C. to 800° C.: said steel has such a strength as to satisfy the expression p≧−0.0029×T+2.48 when the steel temperature T (° C.) is within the range from 600° C. to 800° C., wherein p is a stress drop ratio (a yield stress at a high temperature/a yield stress at room temperature) that is obtained by converting a yield stress normalized by using a yield stress at room temperature; said steel has a structure wherein the temperature (Ac 1 ) at which a single structure composed of bainite or a composite structure composed of ferrite and bainite at room temperature reversely transforms into austenite during high temperature heating corresponding to a fire higher than 800° C.; the average circle equivalent diameter of prior austenite grains in said steel is not more than 120 μm; one or more of carbonitrides precipitated phases thermodynamically stable in said single structure composed of bainite or in said composite structure composed of ferrite and bainite is not less than 5×10 −4  in terms of a molar fraction; and the total amount of Mo, Nb and Ti that dissolve in the ferrite structure is not less than 1×10 −3  in terms of a molar concentration.  
   
   
       10 . A method for producing a high-tensile-strength steel excellent in high temperature strength, characterized by comprising the steps of: reheating a casting or a slab having a steel composition according to  claim 1  in the temperature range from 1,100° C. to 1,250° C.; hot rolling it in the temperature range of not lower than 850° C. while controlling the cumulative reduction ratio in the temperature range of not higher than 1,100° C. to not less than 30%; finishing the hot rolling, cooling the hot-rolled steel sheet at a cooling rate of not lower than 0.3 K/sec. from the temperature of not lower than 800° C. to the temperature of not higher than 650° C.; and thus making the microstructure of the steel comprising a single structure composed of bainite or a composite structure composed of ferrite and bainite.  
   
   
       11 . A high-tensile-strength steel excellent in high temperature strength, characterized by the steel comprising, in mass, C at not less than 0.005% to less than 0.08%, Si at not more than 0.5%, Mn at 0.1 to 1.6%, P at not more than 0.02%, S at not more than 0.01%, Mo at 0.1 to 1.5%, Nb at 0.03 to 0.3%, Ti at not more than 0.025%, B at 0.0005 to 0.003%, Al at not more than 0.06%, and N at not more than 0.006%, with the balance consisting of Fe and unavoidable impurities; having a structure wherein the temperature (Aci) at which a composite structure composed of ferrite and bainite, the composite structure having a bainite fraction being in the range from 20 to 95% at room temperature, reversely transforms into austenite during high temperature heating corresponding to a fire is higher than 800° C.; and having a low yield ratio.  
   
   
       12 . A high-tensile-strength steel excellent in high temperature strength according to  claim 11 , wherein the steel further containing, in mass, one or more of Ni at 0.05 to 1.0%, Cu at 0.05 to 1.0%, Cr at 0.05 to 1.0%, and v at 0.01 to 0.1%.  
   
   
       13 . A high-tensile-strength steel excellent in high temperature strength according to  claim 11 , wherein the steel further containing, in mass: one or more of Ni at 0.05 to 1.0%, Cu at 0.05 to 1.0%, Cr at 0.05 to 1.0%, and V at 0.01 to 0.1%; and additionally one or more of Ca at 0.0005 to 0.004%, REM at 0.0005 to 0.004%, and Mg of 0.0001 to 0.006%.  
   
   
       14 . A method for producing a high-tensile-strength steel excellent in high temperature strength, characterized by comprising the steps of: reheating a casting or a slab having a steel composition according to  claim 11  in the temperature range from 1,100° C. to 1,250° C.; hot rolling it in the temperature of not lower than 850° C. while controlling the cumulative reduction ratio in the temperature of not higher than 1,100° C. to not less than 30%; finishing the hot rolling, cooling the resultant hot-rolled steel sheet at a cooling rate of not lower than 0.3 K/sec. from the temperature of not lower than 800° C. to the temperature of not higher than 650° C.; thus making the microstructure of the steel comprising a single structure composed of bainite or a composite structure composed of ferrite and bainite; forming a microstructure wherein the temperature (Ac 1 ) at which a composite structure composed of ferrite and bainite, the composite structure having a bainite fraction being in the range from 20 to 95% at room temperature, reversely transforms into austenite during high temperature heating corresponding to a fire is higher than 800° C.; and securing a low yield ratio.