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

摘要

The present invention presents a high tensile strength cold rolled steel sheet having excellent formability, impact resistance and strain age hardening characteristics, and the production thereof. As a specific means, a slab having a composition which contains, by mass %, 0.15% or less of C, 0.02% or less of Al, and 0.0050 to 0.0250% of N at N/Al of 0.3 or higher, and has N in a solid solution state at 0.0010% or more, is first hot rolled at the finish rolling delivery-side temperature of 800 DEG C or above, and is subsequently coiled at the coiling temperature of 750 DEG C or below to prepare a hot rolled plate. Then, after cold rolling, the hot rolled plate is continuously cooled at a temperature from the recrystallization temperature to 900 DEG C at a holding time of 10 to 120 seconds, and is cooled by primary cooling in which the hot rolled plate is cooled to 500 DEG C or below at a cooling rate of 10 to 300 DEG C/s, and furthermore if necessary, by secondary cooling in which a residence time is 300 seconds or less in a temperature range of the primary cooling stopping temperature or below and 350 DEG C or higher. Provided is a steel sheet containing a ferritic phase having an average crystal grain size of 10 mu m or less at an area ratio of 50% or more, and if necessary, a martensitic phase at an area ratio of 3% or more as a second phase.