Cold-rolled high-tension steel sheet having superior deep drawability and method thereof

A high-tension steel sheet having a tensile strength not lower than 40 kgf/mm.sup.2, suitable for deep drawing and having superior surface treatment characteristics, said steel sheet being made of a steel consisting essentially of, by weight:

• 0.001 to 0.05% of C;
• not more than 1.0% of Si;
• not more than 2.5% of Mn;
• 0.05 to 1.0% of Mo;
• one or both of 0.001 to 0.2% of Nb and not more than 0.3% of Ti, wherein Ti*%+(48/93)Nb%.gtoreq.(48/12)C% in which Ti*%=Ti%-(48/32)S%-(48/14)N%, wherein, when Ti*%<0, Ti*% is regarded as being 0;
• 0.0005 to 0.01% of B;
• 0.01 to 0.10% of Al;
• not more than 0.15% of P;
• not more than 0.010% of S;
• not more than 0.006% of N;
• 0.05 to 2.0% of Ni alone or in combination with 0.05 to 2.0% of Cu;
• Si, Mn and P meeting the condition of 0.2<(Si%+10P%)/Mn%<3.3;
• and the balance substantially Fe and incidental impurities.

A method of producing a high-tension steel sheet having a tensile strength not lower than 40 kgf/mm.sup.2, suitable for deep drawing and having superior surface treatment characteristics, comprising the steps of:

• preparing a steel slab made of a steel consisting essentially of, by weight:
• 0.001 to 0.05% of C;
• not more than 1.0% of Si;
• not more than 2.5% of Mn;
• 0.05 to 1.0% of Mo;
• one or both of 0.001 to 0.2% of Nb and not more than 0.3% of Ti, wherein Ti*%+(48/93Nb%.gtoreq.(48/12)C% in which Ti*%=Ti%-(48/32)S%-(48/14)N%, wherein, when Ti*%<0,Ti*% is regarded as being 0;
• 0.0005 to 0.01% of B;
• 0.01 to 0.10% of Al;
• not more than 0.15% of P;
• not more than 0.010% of S;
• not more than 0.006% of N;
• Si, Mn and P meeting the condition of 0.2<(Si%+10P%)Mn%<3.3;
• and the balance substantially Fe and incidental impurities;
• hot-rolling said steel slab to obtain hot rolled steel strip a final hot-rolling temperature not lower than the Ar.sub.3 transformation temperature;
• coiling said steel strip at a temperature not lower than 300.degree. C. but not higher than 615.degree. C. when Nb is not contained and not lower than 500.degree. C. but not higher than 700.degree. C. when Nb is contained;
• cold-rolling said steel strip to obtain a cold rolled strip at a rolling reduction not smaller than 65%;
• and recrystallization-annealing said cold rolled strip at a temperature not lower than the recrystallization temperature but below the Ac.sub.3 transformation temperature.

A method of producing a high-tension steel sheet having a tensile strength not lower than 40 kgf/mm.sup.2, suitable for deep drawing and having superior surface treatment characteristics, said method comprising the steps of:

• preparing a steel slab made of steel consisting essentially of, by weight;
• 0.001 to 0.005% of C;
• not more than 1.0% of Si;
• not more than 2.5% of Mn;
• 0.05 to 1.0% of Mo;
• one or both of 0.001 to 0.2% of Nb and not more than 0.3% of Ti, wherein Ti+%+(48/93)Nb%.gtoreq.(48/12)C% in which Ti*%=Ti%-(48/32)S%-(48/14)N%, wherein, when Ti*%<0,Ti*% is regarded as being 0;
• 0.0005 to 0.01% of B;
• 0.01 to 0.10% of Al;
• not more than 0.15% of Pl not more than 0.010% of S;
• not more than 0.006% of N;
• 0.05 to 2.0% of Ni alone or in combination with 0.05 to 2.0% of Cu;
• Si, Mn and P meeting the condition of 0.2<(Si%+10P%)/Mn%<3.3;
• and the balance substantially Fe and incidental impurities;
• hot-rolling said steel slab to obtain a hot rolled steel strip at a final hot-rolling temperature not lower than the Ar.sup.3 transformation temperature;
• coiling said steel strip at a temperature not lower than 300.degree. C. but not higher than 615.degree. C. when Nb is not contained and not lower than 500.degree. C. but not higher than 700.degree. C. when Nb is contained;
• cold-rolling said steel strip to obtain a cold rolled strip at a rolling reduction not smaller than 65%;
• and recrystallization-annealing said cold rolled steel strip at a temperature not lower than the recrystallization temperature but below the Ac.sub.3 transformation temperature.