High-strength automotive steel can be divided into three generations: the first generation represented by TRIP steel, the second generation represented by TWIP steel, and the third generation steel represented by Q&P steel. In addition to meeting conventional performance indicators such as strength and plasticity, high-strength steel also has some individual requirements: TRIP steel requires ultra-high-strength, TWIP steel requires high delayed fracture resistance and high yield strength, and Q&P steel requires high hole expandability. These properties are related to its composition system and annealing process.
The composition system and annealing process of each generation of steel are:
1. TRIP steel
TRIP steel is a low-carbon low-alloy steel containing ferrite, bainite, and metastable austenite. Its basic principle is to use the characteristics of metastable austenite to induce phase transformation and phase transformation induced plasticity to improve the performance of the steel plate. Strong plastic product. The commonly used composition systems for the production of TRIP steel are 0.20%C-1.5%Si-1.5%Mn series, 0.20%C-0.30%Si-1.8%Mn-1.2%Al (low silicon) series, 0.20%C-0.30%Si -1.8%Mn-0.06%P (low silicon) series.
The annealing process of TRIP steel mainly includes six stages: heating, dual-phase insulation, slow cooling, rapid cooling, and bainite isothermal transformation. Among them, slow cooling and bainite isothermal transformation are the most critical. These two processes can Adjust the carbon content of austenite to improve the stability of austenite.
2. TWIP steel
The second-generation TWIP steel has excellent properties such as high strength, high plasticity, and high impact absorption. The composition system of TWIP steel prototype steel is Fe-25%Mn-3%Al-3%Si. The developed component systems are: Fe-18%Mn-1.5%Al-0.6%C, Fe-18%Mn-0.26%V-0.8%C, etc.
TWIP steel is usually produced by the water toughening process, and the continuous annealing line needs to be equipped with a water quenching device. The fast cooling rate can control the precipitation of carbides and the growth of metastable austenite grains.
3. Q&P steel
The composition system of the third-generation Q&P steel is C-Si-Mn or C-Si-Mn-Nb, which is produced by the quenching and distribution process. The quenching distribution process is to quickly quench the steel to a certain temperature TQ between the martensite transformation start temperature (Ms) and the martensite transformation end temperature (Mf) after austenitizing, and then increase to Ms at this temperature At a certain temperature above Tp, carbon is distributed from the supersaturated martensite to the undecomposed austenite, and the carbon-rich retained austenite exists stably during the subsequent cooling to room temperature.
By controlling the quenching temperature TQ, the distribution temperature Tp and the distribution time tp, a multiphase structure composed of C-rich metastable austenite and martensite is obtained, which has higher strength and better plasticity.