High tensile cold-rolled steel sheet having excellent strain aging hardening properties

A high tensile strength cold rolled steel sheet having excellent strain age hardening characteristics with tensile strength of 440 MPa or above, characterized in that the sheet consists of a composition containing, by mass %:

• 0.15% or less of C;
• 2.0% or less of Si;
• 0.65% to 3.0% or less of Mn;
• not more than 0.05% of P;
• 0.02% or less of 5;
• 0.02% or less of Al;
• and
• 0.0050 to 0.0250% of N;
• having 0.3 or more of N/Al and 0.0010% or more of N in a solid solution state, and having the balance of Fe and inevitable impurities;
• and that the steel sheet has a structure containing a ferritic phase having an average crystal grain size of 10 μm or less at an area ratio of 50% or more.

Production of a high tensile strength cold rolled steel sheet having excellent strain age hardening characteristics with tensile strength of 440 MPa or more, characterized in that sequentially carried out are:

• a hot rolling step wherein a steel slab that has a composition containing, by mass %:
• 0.15% or less of C;
• 2.0% or less of Si;
• 0.65% to 3.0% or less of Mn;
• not more than 0.05% of P;
• 0.02% or less of S;
• 0.02% or less of Al;
• and
• 0.0050 to 0.0250% of N;
• and having N/Al of 0.3 or higher is heated at a slab heating temperature of 1 ,000° C. or higher and is roughly rolled to form a sheet bar, and the sheet bar is finish rolled at a finish rolling deliver-side temperature of 800° C. or higher and is coiled at a coiling temperature of 650° C. or below to form a hot-rolled sheet;
• a cold rolling step in which the hot rolled sheet is pickled and cold rolled to form a cold rolled sheet;
• and
• a cold rolled sheet annealing step of primary cooling in which the cold rolled sheet is annealed at a temperature between the recrystallization temperature and 900° C. for a holding time of 10 to 60 seconds, and
• the cold rolled sheet is also cooled at a cooling rate of 10 to 300° C./s to a temperature of 500° C. or below, with a secondary cooling at a residence time of 300 seconds or less in a temperature range between the stopping temperature of the primary cooling and 400° C.