High-strength steel composition system and annealing process - Database & Sql Blog Articles

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High-strength auto 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 the conventional performance indexes such as strength and plasticity, high-strength steel also has some personality requirements: for example, TRIP steel requires ultra-high strength, TWIP steel requires high resistance to delayed fracture and high yield strength, and Q&P steel requires high hole expansion. These properties are related to their constituent systems and annealing processes.

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. The basic principle is to use the characteristics of metastable austenite to induce phase transformation and phase transformation induced plasticity to improve the steel plate. Strong plastic product. Commonly used components 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 of heating, two-phase zone insulation, slow cooling, rapid cooling, and bainite isothermal transformation. Among them, the slow cooling and bainite isothermal transformation process are the most critical, and the two processes can Control the carbon content of austenite and improve the stability of austenite.

2, TWIP steel

The second generation of 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 are: Fe-18% Mn-1.5% Al-0.6% C, Fe-18% Mn-0.26% V-0.8% C, and the like.

TWIP steel is usually produced by a water tough process, and a continuous annealing line is required to be equipped with a water quenching device. The fast cooling rate controls 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 a quenching and partitioning process. The quenching and partitioning process is a temperature TQ between the rapid quenching of the steel after austenitizing to the martensite transformation starting temperature (Ms) and the martensite transformation end temperature (Mf), and then at this temperature or up to Ms At a temperature Tp above the temperature, carbon is distributed from the supersaturated martensite to the undecomposed austenite, and the carbon-rich retained austenite is stably present during the subsequent cooling to room temperature.

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