High tensile strength cold rolled steel sheet having excellent strain age hardening characteristics and the production thereof

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;
• 3.0% or less of Mn;
• 0.08% or less of P;
• 0.02% or less of S;
• 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;
• 3.0% or less of Mn;
• 0.08% or less 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.

A high yield ratio type high tensile strength cold rolled steel sheet having excellent strain age hardening characteristics with tensile strength of 440 MPa or higher and a yield ratio of 0.7 or above, characterized in that the sheet has a composition containing, by mass %:

• 0.15% or less of C;
• 2.0% or less of Si;
• 3.0% or less of Mn;
• 0.08% or less of P;
• 0.02% or less of S;
• 0.02% or less of Al;
• 0.0050 to 0.0250% of N;
• and
• 0.007 to 0.04% of Nb;
• having 0.3 or more of N/Al and 0.0010% or more of N in a solid solution state, and
• furthermore containing deposited Nb at 0.005% or more, 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, and mainly peaniite as a residual portion.

A production of a high yield ratio type high tensile strength cold rolled steel sheet having excellent strain age hardening characteristics with tensile strength of 440 MPa or more and a yield ratio of 0.7 or above, 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;
• 3.0% or less of Mn;
• 0.08% or less of P;
• 0.02% or less of S;
• 0.02% or less of Al;
• 0.0050 to 0.0250% of N;
• and
• 0.007 to 0.04% of Nb;
• and having N/Al of 0.3 or more is heated at a slab heating temperature of 1,100° C. or higher, and
• is roughly rolled to form a sheet bar, and
• the sheet bar is finish rolled at a final pass draft of 25% or more at a finish rolling delivery-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 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 90 seconds, and
• the cold rolled sheet is cooled at a cooling rate of 70° C./s or below to a temperature of 600° C. and below.

A high tensile strength cold rolled steel sheet having excellent strain age hardening characteristics, formability and impact resistance with tensile strength of 440 MPa or higher, characterized in that the sheet has a composition containing, by mass %:

• 0.15% or less of C;
• 3.0% or less of Mn;
• 0.02% or less of S;
• 0.02% or less of Al;
• and
• 0.0050 to 0.0250% of N;
• and furthermore,
• having one or two elements of 0.05% to 1.0% of Mo and 0.05 to 1.0% of Cr, and
• 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, and furthermore, a martensitic phase at an area ratio of 3% or more.

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

• a hot rolling step wherein a steel slab has a composition containing, by mass %:
• 0.15% or less of C;
• 3.0% or less of Mn;
• 0.02% or less of S;
• 0.02% or less of Al;
• and
• 0.0050 to 0.0250% of N;
• and furthermore,
• containing one or two elements of 0.05 to 1.0% of Mo and 0.05 to 1.0% of Cr, and
• having N/Al of 0.3 or higher, or furthermore,
• containing one group, or two or more groups of the following e to h:
• Group e:
• one, or two or more elements of Si at 0.05 to 1.5%, P at 0.03 to 0.15%, and B at 0.0003 to 0.01%;
• Group f:
• one, or two or more elements of Nb at 0.01 to 0.1%, Ti at 0.01 to 0.2%, and V at 0.01 to 0.2%;
• Group g:
• one or two elements of Cu at 0.05 to 1.5% and Ni at 0.05 to 1.5%;
• and
• Group h:
• one or two elements of Ca and REM at a total of 0.0010 to 0.010% is heated at a slab heating temperature of 1,000° C. or above and is roughly rolled to form a sheet bar, and
• the sheet bar is finish rolled at finish rolling delivery-side temperature of 800° C. or above and
• is coiled at coiling temperature of 750° 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 in which the cold rolled sheet is annealed at a temperature between (Ac1 transformation point) and (Ac3 transformation point) for a holding time of 10 to 120 seconds, and
• is cooled at an average cooling rate that is a critical cooling rate CR defined by the following formula (1) or (2), or above from 600 to 300° C.:
• when B<0.0003%, log CR=−1.73[Mn+2.67Mo+1.3Cr+0.26Si+3.5P+0.05Cu+0.05Ni]30 3.95 (1);
• and
• when B≧0.0003%, log CR=−1.73[Mn+2.67Mo+1.3Cr+0.26Si+3.5P+0.05Cu+0.05Ni]+3.40 (2)
• wherein CR is a cooling rate (° C./s);
• and Mn, Mo, Cr, Si, P, Cu and Ni are contents of each element (mass %).