Methods of manufacturing cold-rolled and hot-dip galvanized steel sheet excellent in strain age hardening property

A manufacturing method of a steel sheet excellent in press-formability and in strain age hardening property as typically represented by a ΔTS of 80 MPa or more, comprising the steps, when hot-rolling a steel slab having a chemical composition comprising, in weight percentage:

• 21 C:
• 0.15% or less, Si:
• 2.0% or less, Mn:
• 3.0% or less, P:
• 0.1% or less, S:
• 0.02% or less, Al:
• 0.1% or less, N:
• 0.02% or less, and Cu:
• from 0.5 to 3.0%,
• into a hot-rolled steel sheet having a prescribed thickness, carrying out said hot rolling with a finish rolling end temperature FDT of the Ar3 transformation point or more, then after the completion of the finish rolling, cooling the hot-rolled steel sheet to a temperature region from the (Ar3 transformation point) to the (Ar1 transformation point) at a cooling rate of 5° C./second or more, air-cooling or slowly cooling the sheet within said temperature region for a period of from 1 to 20 seconds, then cooling the sheet again at a cooling rate of 5° C./second or more, and coiling the sheet at a temperature of 550° C. or below.

A manufacturing method of a cold-rolled steel sheet excellent in press-formability and in strain age hardening property typically represented by a ΔTS of 80 MPa or more, comprising the steps of using a steel slab having a chemical composition containing, in weight percentage:

• 25 C:
• 0.15% or less, Si:
• 2.0% or less, Mn:
• 3.0% or less, P:
• 0.1% or less, S:
• 0.02% or less, Al:
• 0.1% or less, N:
• 0.02% or less, and Cu:
• from 0.5 to 3.0%
• as a material;
• a hot rolling step of applying hot rolling to said material into a hot-rolled steel sheet;
• a cold rolling step of applying cold rolling to said hot-rolled steel sheet into a cold-rolled steel sheet;
• and a recrystallization annealing step of applying recrystallization annealing into a cold-rolled annealed steel sheet;
• these steps being sequentially applied;
• wherein said recrystallization annealing is conducted in a ferrite+austenite dual phase region within a temperature range of from Ac1 transformation point to Ac3 transformation point.

A manufacturing method of a hot-dip galvanized steel sheet excellent in press-formability, and in strain age hardening property as typically represented by a ΔTS of 80 MPa or more, comprising the steps of using a steel sheet having a chemical composition containing, in weight percentage:

• 27 C:
• 0.15% or less, Si:
• 2.0% or less, Mn:
• 3.0% or less, P:
• 0.1% or less, S:
• 0.02% or less, Al:
• 0.1% or less, N:
• 0.02% or less, and Cu:
• from 0.5 to 3.0%,
• applying annealing comprising heating to a dual phase region of ferrite+austenite within a temperature range of from Ac3 transformation point to Ac1 transformation point to said steel sheet on a line for conducting continuous hot-dip galvanizing, and then, performing a hot-dip galvanizing treatment thereby forming a hot-dip galvanizing layer on the surface of said steel sheet.