Process for producing a high tension steel sheet product having an excellent low-temperature toughness with a yield point of 40 kg/mm2 or higher

Abstract

The production process characterized in that a steel ingot or slab containing not less than 0.004% of TiN not larger than 0.02μ is heated to a temperature not higher than 1150° C. and rolled, and growth of the γ grains during this heating and rolling step is prevented by TiN to improve the toughness, and the resultant steel product has an excellent low-temperature toughness with a yield point of 40 kg/mm 2 or higher, and is useful as hot rolled or as heated at a temperature ranging from 300° to 750° C. after the hot rolling.

What is claimed is: .[. 
     
       1.  A process for producing a high tension steel sheet having excellent low-temperature toughness with a yield point not lower than 40 kg/mm 2  which comprises heating a steel ingot or slab to a temperature not higher than 1150° C., and rolling the steel ingot or slab thus heated with a total reduction amount not less than 50% in a temperature range not higher than 930° C. and a finishing temperature not higher than 830° C., said steel ingot or slab containing 0.01 to 0.13% C, 0.1 to 1.0% Si, 0.7 to 2.0% Mn, not more than 0.1% total Al, 0.004 to 0.03% Ti, 0.001 to 0.009% total N, 0.01 to 0.10% Nb, 0.01 to 0.15% V, 0.05 to 0.40% Mo, with the balance being unavoidable impurities and Fe, in which (Nb%)×(C%)≦5×10 -3  and TiN not larger than 0.02μ is not less than 0.004%..]. .[.2. A process for producing a high tension steel sheet having excellent low-temperature toughness with a yield point not lower than 40 kg/mm 2  which comprises heating a steel ingot or slab to a temperature not higher than 1150° C., and rolling the steel ingot or slab thus heated with a total reduction amount not less than 50% in a temperature range not higher than 930° C. and a finishing temperature not higher than 830° C.,   said steel ingot or slab containing 0.01 to 0.13% C, 0.1 to 1.0% Si, 0.7 to 2.0% Mn, not higher than 0.1% total Al, 0.004 to 0.03% Ti, 0.001 to 0.009% total N, 0.01 to 0.10% Nb, 0.05 to 0.40% Mo, 0.01 to 0.15% V, and one or both of 0.001 to 0.03% REM, 0.0005 to 0.03% Ca, with the balance being unavoidable impurities and Fe in which (Nb%)×(C%)≦5×10 -3 , REM/S=1.0 to 6.0 in case of REM addition, and TiN not larger than 0.02μ is not less than 0.004%..]. .[.3. A process for production a high tension steel sheet having excellent low-temperature toughness with a yield point not lower than 40 kg/mm 2  which comprises heating a steel ingot or slab to a temperature not higher than 1150° C. and rolling the steel ingot or slab thus heated with a total reduction amount not less than 50% in a temperature range not higher than 930° C. and a finishing temperature not higher than 830° C.,   said steel ingot or slab containing 0.01 to 0.13% C, 0.1 to 1.0% Si, 0.7 to 2.0% Mn, not more than 0.1% total Al, 0.004 to 0.03% Ti, 0.001 to 0.009% total N, 0.01 to 0.10% Nb, 0.01 to 0.15% V and 0.05 to 0.40% Mo, one or more of not more than 0.6% Cr, not more than 1.0% Cu, and not more than 4.0% Ni with the balance being unavoidable impurities and Fe in which   (Nb%)×(C%)≦5×10.sup.-3       (Cu+Ni)/5+Cr+Mo≦0.90%       
     
     
       and TiN not larger than 0.02μ is not less than 0.004%..]. .[.4.  A process for producing a high tension steel sheet having excellent low-temperature toughness with a yield point not lower than 40 kg/mm 2  which comprises heating a steel ingot or slab to a temperature not higher than 1150° C. and rolling the steel ingot or slab thus heated with a total reduction amount not less than 50% in a temperature range not higher than 930° C. and a finishing temperature not higher than 830° C., said steel ingot or slab containing 0.01 to 0.13% C, 0.1 to 1.0% Si, 0.7 to 2.0% Mn, not more than 0.1% total Al, 0.004 to 0.03% Ti, 0.001 to 0.009% total N, 0.01 to 0.10% Nb, 0.01 to 0.15% V, 0.05 to 0.40% Mo, one or both of 0.001 to 0.03% REM and 0.0005 to 0.03% Ca, and one or more of not more than 0.6% Cr, not more than 1.0% Cu and not more than 4.0% Ni with the balance being unavoidable impurities and Fe, in which   (Nb%)×(C%)≦5×10.sup.-3       REM/S=1.0 to 6.0 in case of REM addition       (Cu×Ni)/5+Cr+Mo≦0.90%     and TiN not larger than 0.02μ is not less than 0.004%..]. .Iadd. 5. A process for producing steel sheet suitable for pipe lines, having excellent low temperature toughness with a yield point of 40 kg/mm 2  or higher and a vTrs value of about -110° C. lower for a 50% reduction of the steel comprising continuously casting a molten steel into the form of a slab, said molten steel containing 0.01 to 0.13% C, 0.1 to 1.0% Si, 0.7 to 2.0% Mn, not more than 0.1% total Al, 0.004 to 0.03% Ti, 0.001 to 0.009% total N, 0.01 to 0.10% Nb, 0.05 to 127 0.40% Mo, with the balance being Fe and unavoidable impurities and satisfying the condition of (Nb%)×(C%)≦5×10 -3 , cooling the slab thus-obtained by continuous casting with an average cooling rate of 8° C./min. or more as measured in the central portion of the slab through the temperature range from the temperature at the time of pouring to 1100° C. to obtain a slab containing 0.004% or more TiN of a grain size not larger than 0.02μ, heating the slab at 1150° C. or lower and rolling the slab with a total reduction not less than 50% at temperatures not higher than 930° C. and with a finishing temperature not higher than 830° C. .Iaddend. .Iadd. 6. A process according to claim 5 in which the molten steel further contains one or more of a material selected from the group consisting of 0.01 to 0.15% V, up to 0.6% Cr, up to 1.0% Cu and up to 4.0% Ni within a range of (Cu+Ni)/5+Cr+Mo≦0.90%. .Iaddend. .Iadd. 7. A process according to claim 5 in which the molten steel further contains one or more of a material selected from the group consisting of 0.001 to 0.03% REM, and 0.0005 to 0.03% Ca, and wherein REM/S is from 1.0 to 6.0. .Iaddend.