Cold-rolled steel sheet and galvanized steel sheet having strain age hardenability and method of producing the same

A cold-rolled steel sheet having excellent strain age hardenability comprising a composition, by mass %, comprising:

• C:
• less than 0.01%;
• Si:
• 0.005 to 1.0%;
• Mn:
• 0.01 to 1.5%;
• P:
• 0.1% or less;
• S:
• 0.01% or less;
• Al:
• 0.005 to 0.030%;
• and
• N:
• 0.005 to 0.040%;
• wherein N/Al is 0.30 or more, the amount of dissolved N is 0.0010% or more, and the balance is composed of Fe and inevitable impurities.

A method of producing a cold-rolled steel sheet having excellent strain age hardenability comprising hot-rolling a steel slab under conditions in which the steel slab is cooled immediately after the end of finish rolling and coiled at a coiling temperature of 400 to 800° C., cold-rolling the hot-rolled sheet with a rolling reduction ratio of 60 to 95%, and then recrystallizing and annealing the cold-rolled sheet at a temperature of 650 to 900° C., wherein the steel slab has a composition, by mass %, comprising:

• C:
• less than 0.01%;
• Si:
• 0.005 to 1.0%;
• Mn:
• 0.01 to 1.5%;
• P:
• 0.1% or less;
• S:
• 0.01% or less;
• Al:
• 0.005 to 0.030%;
• and
• N:
• 0.005 to 0.040%;
• wherein N/Al is 0.30 or more, and the balance is substantially composed of Fe.

A cold-rolled deep drawing steel sheet having excellent strain age hardenability comprising a composition, by mass %, comprising:

• C:
• 0.01% or less;
• Si:
• 1.0% or less;
• Mn:
• 0.0l to 1.5%;
• P:
• 0.1% or less;
• S:
• 0.01% or less;
• Al:
• 0.005 to 0.020%;
• and
• N:
• 0.0050 to 0.040%;
• wherein N/Al is 0.30 or more, the amount of dissolved N is 0.0010% or more, the balance is composed of Fe and inevitable impurities, and Ts×r value is 750 MPa or more.

A method of producing a cold-rolled deep drawing steel sheet having excellent strain age hardenability comprising heating a steel raw material to 950° C. or more, roughly rolling the raw material so that the finisher delivery temperature is Ar3 to 1000° C., finish-rolling the material while lubricating it in the temperature region of 600° C. to Ar3, coiling the rolled sheet in which the total reduction by rolling starting from rough rolling to finish rolling is 80% or more, recrystallizing and annealing the hot-rolled sheet, cold-rolling the rolled sheet with a rolling reduction of 60 to 95%, and then recrystallizing and annealing the resultant cold-rolled sheet, wherein the steel-raw material has a composition, by mass %, comprising:

• C:
• less than 0.01%;
• Si:
• 0.005 to 1.0%;
• Mn:
• 0.01 to 1.5%;
• P:
• 0.1% or less;
• S:
• 0.01% or less;
• Al:
• 0.005 to 0.030%;
• N:
• 0.005 to 0.040%;
• and
• at least one of the following:
• B:
• 0.0003 to 0.0030%;
• Nb:
• 0.005 to0.050%;
• Ti:
• 0.005 to0.070%;
• and
• V:
• 0.005 to 0.10%;
• wherein N/(Al+Nb+Ti+V+B) is 0.30 or more.

A high-tensile-strength cold-rolled steel sheet having excellent moldability, strain age hardenability and natural aging resistance comprising a composition, by mass %, comprising:.

• C:
• 0.0015 to 0.025%;
• Si:
• 1.0% or less;
• Mn:
• 2.0% or less;
• P:
• 0.1%. or less;
• S:
• 0.02% or less;
• Al:
• 0.02% or less;
• N:
• 0.0050 to 0.0250%;
• and
• one or both of the following:
• B:
• 0.0001 to 0.0050%;
• and
• Nb:
• 0.002 to 0.050%;
• wherein N/Al is 0.30 or more, the amount of dissolved N is 0.0010% or more, the balance is composed of Fe and inevitable impurities, the structure is composed of an acicular ferrite phase at an area ratio of 5% or more and a ferrite phase having an average crystal grain diameter of 20 μm or less, and the r value is 1.2 or more.

A method of producing a high-tensile-strength cold-rolled steel sheet having a r value of 1.2 or more, and excellent moldability, strain age hardenability and natural aging resistance comprising:

• the hot-rolling step of roughly rolling a steel slab by heating to a slab heating temperature of 1000° C. or more to form a sheet bar, finish-rolling the sheet bar so that the finisher delivery temperature is 800° C. or more, and coiling the finish-rolled sheet at a coiling temperature of 800° C. or less to form a hot-rolled sheet;
• the cold rolling step of pickling and cold-rolling the hot-rolled sheet to form a cold-rolled sheet;
• and
• the cold-rolled sheet annealing step of continuously annealing the cold-rolled sheet at a temperature in the ferrite-austenite two-phase region, and cooling the annealed-sheet to the temperature region of 500° C. or less at a cooling rate of 10 to 300° C./s;
• wherein the steel slab has a composition, by mass %, comprising at least one of:
• C:
• 0.0015 to 0.025%;
• Si:
• 1.0% or less;
• Mn:
• 2.0% or less;
• P:
• 0.1% or less;
• S:
• 0.02% or less;
• Al:
• 0.02% or less;
• N:
• 0.0050 to 0.0250%;
• and
• at least one of the following:
• B:
• 0.0001 to 0.0050%;
• and
• Nb:
• 0.002 to 0.050%;
• wherein N/Al is 0.30.or more.

A high-tensile-strength cold-rolled steel sheet having a high r value and excellent strain age hardenability and natural aging resistance comprising a composition, by mass %, comprising:

• C:
• 0.025 to 0.15%;
• Si:
• 1.0% or less;
• Mn:
• 2.0% or less;
• P:
• 0.08% or less;
• S:
• 0.02% or less;
• Al:
• 0.02% or less;
• and
• N:
• 0.0050 to 0.0250%;
• wherein N/Al is 0.30 or more, the amount of dissolved N is 0.0010% or more, the balance is composed of Fe and inevitable impurities, the structure is composed of a ferrite phase having an average crystal grain diameter of 10 μm or less at an area ratio of 80% or more and a martensite phase as a second phase at an area ratio of 2% or more, and the r value is 1.2 or more.

A method of producing a high-tensile-strength cold-rolled steel sheet having a r value of as high as 1.2 or more, and excellent strain age hardenability and natural aging resistance comprising:

• the hot-rolling step of roughly rolling a steel slab by heating to a slab heating temperature of 1000° C. or more to form a sheet bar, finish-rolling the sheet bar so that the finisher delivery temperature is 800° C. or more, and coiling the finish-rolled sheet at a coiling temperature of 800° C. or less to form a hot-rolled sheet;
• the cold rolling step of pickling and cold-rolling the hot-rolled sheet to form a cold-rolled sheet;
• and
• the cold-rolled sheet annealing step of box-annealing the cold-rolled sheet at an annealing temperature of the recrystallization temperature to 800° C., then continuously annealing the annealed sheet at an annealing temperature of Ac1 transformation point to Ac3 transformation point−20° C.), and then cooling the sheet to the temperature region of 500° C. or less at a cooling rate of 10 to 300° C./s;
• wherein the steel slab has a composition, by mass %, comprising at least one of:
• C:
• 0.025 to 0.15%;
• Si:
• 1.0% or less;
• Mn:
• 2.0% or less;
• P:
• 0.08% or less;
• S:
• 0.02% or less;
• Al:
• 0.02% or less;
• and
• N:
• 0.0050 to 0.0250%;.
• wherein N/Al is 0.30 or more.