When using continuous annealing equipment to manufacture steel plates for cold stamping, the cold-rolled steel plates with surface rolling oil removed are heated in the ferrite + austenite two-phase region or the austenite single-phase region. The ferrite of the heated steel plate in the two-phase zone recrystallizes and the ductility is restored. During the subsequent rapid cooling process, austenite transforms into martensite. Rapid cooling methods include roller cooling, jet cooling, water quenching cooling, etc. Water quenching cooling has the largest cooling rate and can achieve uniform cooling, and is most suitable for the production of low-alloy high-strength steel plates.
The martensite generated by quenching is tempered during subsequent heating, making the steel plate tough and adjusting its hardness.
This method can be used to produce high-strength steel sheets with a dual-phase structure composed of martensite, which provides strength, and ferrite, which provides workability. In addition, if cooling is stopped below 500°C and kept warm during the cooling process to promote bainite transformation, room temperature stable austenite can remain in the steel. This steel containing retained austenite combines high strength with good elongation due to the TRIP (Transformation Induced Plasticity) effect. In order to retain a large amount of austenite, the formation of cementite must be suppressed, and for this reason, more Si and Mn must be added. For steel plates with these alloying elements added, steel plate application technology with guaranteed spot weldability characteristics is required. To further strengthen the steel sheet, a combination of increasing the continuous annealing heating temperature and adding austenite-generating elements increases the austenite fraction during heating.