Cold Rolled Steel Sheet Having Aging Resistance and Superior Formability, and Process for Producing the Same

A cold rolled steel sheet having aging resistance and excellent formability, comprising in weight %:

• 0.003% or less of C;
• 0.003˜0.03% of S;
• 0.01˜0.1% of Al;
• 0.02% or less of N;
• 0.2% or less of P;
• at least one of 0.03˜0.2% of Mn and 0.005˜0.2% of Cu;
• and the balance of Fe and other unavoidable impurities, wherein, when the steel sheet comprises one of Mn and Cu, a composition of Mn, Cu, and S satisfies at least one relationship:
• 0.58*Mn/S≦10 and 1≦0.5*Cu/S≦10, and when the steel sheet comprises both Mn and Cu, the composition of Mn, Cu, and S satisfies both of the relationships:
• Mn+Cu≦0.3 and 2≦0.5*(Mn+Cu)/S≦20, and the steel sheet comprising one or more precipitates selected from the group consisting of MnS, CuS, and (Mn, Cu)S and having an average size of 0.2 μm or less.

A cold rolled steel sheet having aging resistance and excellent formability, comprising in weight %:

• 0.003% or less of C;
• 0.005˜0.03% of S;
• 0.01˜0.1% of Al;
• 0.02% or less of N;
• 0.2% or less of P;
• 0.05˜0.2% of Mn;
• and the balance of Fe and other unavoidable impurities, wherein a composition of Mn and S satisfies the relationship:
• 0.58*Mn/S≦10, and wherein the steel sheet comprises precipitates of MnS having an average size of 0.2 μm or less.

A cold rolled steel sheet having aging resistance and excellent formability, comprising in weight %:

• 0.0005˜0.003% or less of C;
• 0.003˜0.025% of S;
• 0.01˜0.08% of Al;
• 0.02% or less of N;
• 0.2% or less of P;
• 0.01˜0.2% of Cu;
• and the balance of Fe and other unavoidable impurities, wherein a composition of Cu and S satisfies the relationship:
• 1≦0.5*Cu/S≦10, and wherein the steel sheet comprises precipitates of CuS having an average size of 0.1 μm or less.

A cold rolled steel sheet having aging resistance and excellent formability, comprising in weight %:

• 0.0005˜0.003% or less of C;
• 0.003˜0.025% of S;
• 0.01˜0.08% of Al;
• 0.02% or less of N;
• 0.2% or less of P;
• 0.03˜0.2% of Mn;
• 0.005˜0.2% of Cu;
• and the balance of Fe and other unavoidable impurities, wherein a composition of Mn, Cu, and S satisfies the relationship:
• Mn+Cu≦0.3 and 2≦0.5*(Mn+Cu)/S≦20, and wherein the steel sheet includes precipitates of MnS, CuS, and (Mn, Cu)S having an average size of 0.2 μm or less.

A method of manufacturing a cold rolled steel sheet having aging resistance and excellent formability, comprising the steps of:

• hot-rolling a steel slab with finish rolling at an Ar3 transformation temperature or more to provide a hot rolled steel sheet, after reheating the steel slab to a temperature of 1,100° C. or more, the steel slab comprising in weight %:
• 0.003% or less of C;
• 0.005˜0.03% of S;
• 0.01˜0.1% of Al;
• 0.02% or less of N;
• 0.2% or less of P;
• 0.05˜0.2% of Mn;
• and the balance of Fe and other unavoidable impurities, wherein a composition of Mn and S satisfies the relationship:
• 0.58*Mn/S≦10;
• cooling the steel sheet at a speed of 200° C./min or more;
• coiling the cooled steel sheet at a temperature of 700° C. or less and then cold rolling the steel sheet;
• and continuous annealing the cold rolled steel sheet so as to obtain a cold rolled steel sheet comprising MnS precipitates having an average size of 0.2 μm or less.

A method of manufacturing a cold rolled steel sheet having aging resistance and excellent formability, comprising the steps of:

• hot-rolling a steel slab with finish rolling at an Ar3 transformation temperature or more to provide a hot rolled steel sheet, after reheating the steel slab to a temperature of 1,100° C. or more, the steel slab comprising in weight %:
• 0.0005˜0.003% of C;
• 0.003˜0.025% of S;
• 0.01˜0.08% of Al;
• 0.02% or less of N;
• 0.2% or less of P;
• 0.01˜0.2% of Cu;
• and the balance of Fe and other unavoidable impurities, wherein a composition of Cu and S satisfies the relationship:
• 1≦0.5*Cu/S≦10;
• cooling the steel sheet at a speed of 300° C./min;
• coiling the cooled steel sheet at a temperature of 700° C. or less and then cold rolling the wound steel sheet;
• and continuous annealing the cold rolled steel sheet so as to obtain a cold rolled steel sheet comprising CuS precipitates having an average size of 0.2 μm or less.

A method of manufacturing a cold rolled steel sheet having aging resistance and excellent formability, comprising the steps of:

• hot-rolling a steel slab with finish rolling at an Ar3 transformation temperature or more to provide a hot rolled steel sheet, after reheating the steel slab to a temperature of 1,100° C. or more, the steel slab comprising in weight %:
• 0.0005˜0.003% of C;
• 0.003˜0.025% of S;
• 0.01˜0.08% of Al;
• 0.02% or less of N;
• 0.2% or less of P;
• 0.03˜0.2% of Mn;
• 0.005˜0.2% of Cu;
• and the balance of Fe and other unavoidable impurities, wherein a composition of Mn, Cu, and S satisfies the relationships:
• Mn+Cu≦0.3 and 2≦0.5*(Mn+Cu)/S≦20;
• cooling the steel sheet at a speed of 300° C./min;
• coiling the cooled steel sheet at a temperature of 700° C. or less and then cold rolling the wound steel sheet;
• and continuous annealing the cold rolled steel sheet so as to obtain a cold rolled steel sheet comprising MnS, CuS, (Mn, Cu)S precipitates having an average size of 0.2 μm or less.