Key points of the production process of high-strength automotive steel

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, Q&P steel requires high hole expansion, etc. These properties are related to its component system and annealing process.

The composition systems and annealing processes 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 deformation-induced phase transformation and phase transformation-induced plasticity characteristics of metastable austenite to improve the strength of the steel plate. Strong plastic accumulation. Commonly used composition systems for producing TRIP steel include 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 zone insulation, slow cooling, rapid cooling, and bainite isothermal transformation. Among them, the slow cooling and bainite isothermal transformation processes are the most critical. These two processes can Regulate the carbon content of austenite and 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 energy. The composition system of TWIP steel prototype steel is Fe-25%Mn-3%Al-3%Si. The developed component systems include: Fe-18%Mn-1.5%Al-0.6%C, Fe-18%Mn-0.26%V-0.8%C, etc.
TWIP steel is usually produced using the water toughening process, and the continuous annealing line needs to be equipped with a water quenching device. Fast cooling rate can control carbide precipitation and metastable austenite grain growth.

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 using the quenching and partitioning process. The quenching distribution process is to austenitize the steel and then quickly quench it to a certain temperature TQ between the martensite transformation starting temperature (Ms) and the martensite transformation end temperature (Mf), and then rise to Ms at this temperature. During insulation at a certain temperature Tp above the point, carbon is distributed from supersaturated martensite to undecomposed austenite, and the carbon-rich residual austenite exists stably during subsequent cooling to room temperature.

By controlling the quenching temperature TQ, partitioning temperature Tp and partitioning time tp, a multiphase structure composed of C-rich metastable austenite and martensite is obtained, which has higher strength and better plasticity.