Cold rolled steel sheet and hot dipped steel sheet with superior strength and bake hardenability and method for manufacturing the steel sheets

A bake-hardenable high-strength cold-rolled steel sheet manufactured through hot rolling, cold rolling and continuous annealing of a steel, comprising:

• 0.0016-0.01% of C;
• 0.1% or less of Si;
• 0.2-1.5% of Mn;
• 0.05-0.15% of P;
• 0.01% or less of S;
• 0.08-0.5% of (soluble) Al;
• 0.0025% or less of N;
• 0.003-0.1% of Nb;
• 0.003% or less of Ti;
• 0.01-0.4% of Mo;
• 0.0005-0.005% of B;
• and the balance of Fe and other unavoidable impurities, in terms of weight %, while satisfying an Nb/C ratio of 0.3-0.7, wherein the steel sheet has fine AlN precipitates formed upon hot rolling the steel, and a grain size (ASTM No.) of 9 or more, the fine AlN precipitates having a size acting as a barrier for suppressing grain growth during annealing of the steel sheet.

A bake-hardenable high-strength cold-rolled steel sheet, comprising:

• 0.0016-0.0025% of C;
• 0.02% or less of Si;
• 0.2-1.2% of Mn;
• 0.05-0.11% of P;
• 0.01% or less of S;
• 0.08-0.12% of (soluble) Al;
• 0.0025% or less of N;
• 0.003-0.011% of Nb;
• 0.01-0.1% of Mo;
• 0.0005-0.0015% of B;
• and the balance of Fe and other unavoidable impurities, in terms of weight %, while satisfying an Nb/C ratio of 0.3-0.7, wherein the steel sheet has fine AlN precipitates having a grain size of 20 μm or less, a grain size (ASTM No.) of 9 or more, a bake hardening (BH) value of 30 MPa or more, an aging index (Al) of 30 Mpa or less, a DBTT of −30° C. or less at a drawing ratio of 2.0, and a tensile strength of 340-390 MPa.

A bake-hardenable high-strength cold-rolled steel sheet, comprising:

• 0.0016-0.0025% of C;
• 0.02% or less of Si;
• 0.2-1.2% of Mn;
• 0.05-0.11% of P;
• 0.01% or less of S;
• 0.08-0.12% of (soluble) Al;
• 0.0025% or less of N;
• 0.003% or less of Ti;
• 0.003-0.011% of Nb;
• 0.01-0.1% of Mo;
• 0.0005-0.0015% of B;
• and the balance of Fe and other unavoidable impurities, in terms of weight %, while satisfying an Nb/C ratio of 0.3-0.7, wherein the steel sheet has fine AlN precipitates having a grain size of 20 μm or less, a grain size (ASTM No.) of 9 or more, a bake hardening (BH) value of 30 MPa or more, an aging index (AI) of 30 MPa or less, a DBTT of −30° C. or less at a drawing ratio of 2.0, and a tensile strength of 340-390 MPa.

A hot-dipped steel sheet manufactured through hot rolling, cold rolling, continuous annealing, hot dipping, and temper rolling of a steel, comprising:

• 0.0016-0.01% of C;
• 0.1% or less of Si;
• 0.2-1.5% of Mn;
• 0.05-0.15% of P;
• 0.01% or less of S;
• 0.08-0.5% of (soluble) Al;
• 0.0025% or less of N;
• 0.003-0.1% of Nb;
• 0.01-0.4% of Mo;
• 0.0005-0.005% of B;
• and the balance of Fe and other unavoidable impurities, in terms of weight % while satisfying an Nb/C ratio of 0.3-0.7, wherein the steel sheet has fine AlN precipitates formed upon hot rolling the steel, and a grain size (ASTM No.) of 9 or more, the fine AlN precipitates having a size acting as a barrier for suppressing grain growth during annealing of the steel sheet.

A hot-dipped steel sheet manufactured through hot rolling, cold rolling, continuous annealing, hot dipping, and temper rolling of a steel, comprising:

• 0.0016-0.01% of C;
• 0.1% or less of Si;
• 0.2-1.5% of Mn;
• 0.05-0.15% of P;
• 0.01% or less of S;
• 0.08-0.5% of (soluble) Al;
• 0.0025% or less of N;
• 0.003-0.1% of Nb;
• 0.003% or less of Ti;
• 0.01-0.4% of Mo;
• 0.0005-0.005% of B;
• and the balance of Fe and other unavoidable impurities, in terms of weight %, while satisfying an Nb/C ratio of 0.3-0.7, wherein the steel sheet has fine AN precipitates formed upon hot rolling the steel, and a grain size (ASTM No.) of 9 or more, the fine AlN precipitates having a size acting as a barrier for suppressing grain growth during annealing of the steel sheet.

A hot-dipped steel sheet, comprising:

• 0.0016-0.0025% of C;
• 0.02% or less of Si;
• 0.2-1.2% of Mn;
• 0.05-0.11% of P;
• 0.01% or less of S;
• 0.08-0.12% of (soluble) Al;
• 0.0025% or less of N;
• 0.003-0.011% of Nb;
• 0.01-0.1% of Mo;
• 0.0005-0.0015% of B;
• and the balance of Fe and other unavoidable impurities, in terms of weight %, while satisfying an Nb/C ratio of 0.3-0.7, wherein the steel sheet has fine AlN precipitates having a grain size of 20 μm or less, a grain size (ASTM No.) of 9 or more, a bake hardening (BH) value of 30 MPa or more, an aging index (Al) of 30 Mpa or less, a DBTT of −30° C. or less at a drawing ratio of 2.0, and a tensile strength of 340-390 MPa.

A hot-dipped steel sheet, comprising:

• 0.0016-0.0025% of C;
• 0.02% or less of Si;
• 0.2-1.2% of Mn;
• 0.05-0.11% of P;
• 0.01% or less of S;
• 0.08-0.12% of (soluble) Al;
• 0.0025% or less of N;
• 0.003% or less of Ti;
• 0.003-0.011% of Nb;
• 0.01-0.1% of Mo;
• 0.0005-0.0015% of B;
• and the balance of Fe and other unavoidable impurities, in terms of weight %, while satisfying an Nb/C ratio of 0.3-0.7, wherein the steel sheet has fine AlN precipitates having a grain size of 20 μm or less, a grain size (ASTM No.) of 9 or more, a bake hardening (BH) value of 30 MPa or more, an aging index (AI) of 30 MPa or less, a DBTT of −30° C. or less at a drawing ratio of 2.0, and a tensile strength of 340-390 MPa.

A method for manufacturing a bake-hardenable high-strength cold-rolled steel sheet, comprising the steps of:

• hot-rolling a steel slab with finish rolling at or above an Ar3 transformation temperature to provide a hot rolled steel sheet after heating the steel slab to a temperature of 1,200° C. or more, the steel slab comprising 0.0016-0.01% of C, 0.1% or less of Si, 0.2-1.5% of Mn, 0.05-0.15% of P, 0.01% or less of S, 0.08-0.5% of (soluble) Al, 0.0025% or less of N, 0.003-0.1% of Nb, 0.01-0.4% of Mo, 0.0005-0.005% of B, and the balance of Fe and other unavoidable impurities, in terms of weight %, while satisfying an Nb/C ratio of 0.3-0.7;
• coiling the hot-rolled steel sheet;
• cold rolling the hot-rolled steel sheet;
• and
• continuous annealing the cold-rolled steel sheet.

A method for manufacturing a bake-hardenable high-strength cold-rolled steel sheet, comprising the steps of:

• hot-rolling a steel slab with finish rolling at or above an Ar3 transformation temperature to provide a hot rolled steel sheet after heating the steel slab to a temperature of 1,200° C. or more, the steel slab comprising 0.0016-0.01% of C, 0.1% or less of Si, 0.2-1.5% of Mn, 0.05-0.15% of P, 0.01% or less of S, 0.08-0.5% of (soluble) Al, 0.0025% or less of N, 0.003-0.1% of Nb, 0.003% or less of Ti, 0.01-0.4% of Mo, 0.0005-0.005% of B, and the balance of Fe and other unavoidable impurities, in terms of weight %, while satisfying an Nb/C ratio of 0.3-0.7;
• coiling the hot-rolled steel sheet;
• cold rolling the hot-rolled steel sheet;
• and
• continuous annealing the cold-rolled steel sheet.

A method of manufacturing a bake-hardenable high-strength cold-rolled steel sheet, comprising the steps of:

• hot-rolling a steel slab with finish rolling at a temperature of 900-950° C. to provide a hot rolled steel sheet, after homogenizing the steel slab at a temperature of 1,200° C. or more, the steel slab comprising 0.0016-0.0025% of C, 0.02% or less of Si, 0.2-1.2% of Mn, 0.05-0.11% of P, 0.01% or less of S, 0.08-0.12% of (soluble) Al, 0.0025% or less of N, 0.003-0.011% of Nb, 0.01-0.1% of Mo, 0.0005-0.0015% of B, and the balance of Fe and other unavoidable impurities, in terms of weight %, while satisfying an Nb/C ratio of 0.3-0.7;
• coiling the hot-rolled steel sheet at a temperature of 600-650° C.;
• cold rolling the hot-rolled steel sheet at a reduction rate of 75-80%;
• and
• continuous annealing the cold-rolled steel sheet at a temperature of 770-830° C.

A method of manufacturing a bake-hardenable high-strength cold-rolled steel sheet, comprising the steps of:

• hot-rolling a steel slab with finish rolling at a temperature of 900-950° C. to provide a hot rolled steel sheet, after homogenizing the steel slab at a temperature of 1,200° C. or more, the steel slab comprising 0.0016-0.0025% of C, 0.02% or less of Si, 0.2-1.2% of Mn, 0.05-0.11% of P, 0.01% or less of S, 0.08-0.12% of (soluble) Al, 0.0025% or less of N, 0.003% or less of Ti, 0.003-0.011% of Nb, 0.01-0.1% of Mo, 0.0005-0.0015% of B, and the balance of Fe and other unavoidable impurities, in terms of weight %, while satisfying an Nb/C ratio of 0.3-0.7;
• coiling the hot-rolled steel sheet at a temperature of 600-650° C.;
• cold rolling the hot-rolled steel sheet at a reduction rate of 75-80%;
• and
• continuous annealing the cold-rolled steel sheet at a temperature of 770-830° C.

A method for manufacturing a hot-dipped steel sheet, comprising the steps of:

• hot-rolling a steel slab with finish rolling at or above an Ar3 transformation temperature to provide a hot rolled steel sheet after heating the steel slab to a temperature of 1,200° C. or more, the steel slab comprising 0.0016-0.01% of C, 0.1% or less of Si, 0.2-1.5% of Mn, 0.05-0.15% of P, 0.01% or less of S, 0.08-0.5% of (soluble) Al, 0.0025% or less of N, 0.003-0.1% of Nb, 0.01-0.4% of Mo, 0.0005-0.005% of B, and the balance of Fe and other unavoidable impurities, in terms of weight %, while satisfying an Nb/C ratio of 0.3-0.7;
• coiling the hot-rolled steel sheet;
• cold rolling the hot-rolled steel sheet;
• continuous annealing the cold-rolled steel sheet;
• hot dipping the annealed steel sheet;
• and
• temper rolling the hot-dipped steel sheet.

A method for manufacturing a hot-dipped steel sheet, comprising the steps of:

• hot-rolling a steel slab with finish rolling at or above an Ar3 transformation temperature to provide a hot rolled steel sheet after heating the steel slab to a temperature of 1,200° C. or more, the steel slab comprising 0.0016-0.01% of C, 0.1% or less of Si, 0.2-1.5% of Mn, 0.05-0.15% of P, 0.01% or less of S, 0.08-0.5% of (soluble) Al, 0.0025% or less of N, 0.003-0.1% of Nb, 0.003% or less of Ti, 0.01-0.4% of Mo, 0.0005-0.005% of B, and the balance of Fe and other unavoidable impurities, in terms of weight %, while satisfying an Nb/C ratio of 0.3-0.7;
• coiling the hot-rolled steel sheet;
• cold rolling the hot-rolled steel sheet;
• continuous annealing the cold-rolled steel sheet;
• hot dipping the annealed steel sheet;
• and
• temper rolling the hot-dipped steel sheet.

A method for manufacturing a hot-dipped steel sheet, comprising the steps of:

• hot-rolling a steel slab with finish rolling at a temperature of 900-950° C. to provide a hot rolled steel sheet, after homogenizing the steel slab at a temperature of 1,200° C. or more, the steel slab comprising 0.0016-0.0025% of C, 0.02% or less of Si, 0.2-1.2% of Mn, 0.05-0.11% of P, 0.01% or less of S, 0.08-0.12% of (soluble) Al, 0.0025% or less of N, 0.003-0.011% of Nb, 0.01-0.1% of Mo, 0.0005-0.0015% of B, and the balance of Fe and other unavoidable impurities, in terms of weight %, while satisfying an Nb/C ratio of 0.3-0.7;
• coiling the hot-rolled steel sheet at a temperature of 600-650° C.;
• cold rolling the hot-rolled steel sheet at a reduction rate of 75-80%;
• continuous annealing the cold-rolled steel sheet at a temperature of 770-830° C.;
• hot dipping the annealed steel sheet;
• and
• temper rolling the hot-dipped steel sheet at a reduction rate of 1.2-1.5%.

A method for manufacturing a hot-dipped steel sheet, comprising the steps of:

• hot-rolling a steel slab with finish rolling at a temperature of 900-950° C. to provide a hot rolled steel sheet, after homogenizing the steel slab at a temperature of 1,200° C. or more, the steel slab comprising 0.0016-0.0025% of C, 0.02% or less of Si, 0.2-1.2% of Mn, 0.05-0.11% of P, 0.01% or less of S, 0.08-0.12% of (soluble) Al, 0.0025% or less of N, 0.003% or less of Ti, 0.003-0.011% of Nb, 0.01-0.1% of Mo, 0.0005-0.0015% of B, and the balance of Fe and other unavoidable impurities, in terms of weight %, while satisfying an Nb/C ratio of 0.3-0.7;
• coiling the hot-rolled steel sheet at a temperature of 600-650° C.;
• cold rolling the hot-rolled steel sheet at a reduction rate of 75-80%;
• continuous annealing the cold-rolled steel sheet at a temperature of 770-830° C.;
• hot dipping the annealed steel sheet;
• and
• temper rolling the hot-dipped steel sheet at a reduction rate of 1.2-1.5%.