Study On Effect Of Heat Treatment On The Microstructures And Properties Of20Mn2Si2Cr Steel

So far, with the development of automotive industry, design to lightweight the body and improve the amount of energy absorbed when collision occurs has become the central topic of automotive engineering and the mechanical engineering throughout. Obviously, not only the strength of steel need to meet the requirements, as well as good ductility and formability, which makes the advanced high-strength steel (AHSS), with complex microstructures got more and more attention and application. DP and TRIP steel are representatives of them. The duplex structure in DP steel formed by ferrite and martensite makes its good strength and ductility, and the retain austenite in TRIP steel attribute to the higher energy absorption and better formability due to its TRIP effect. However, the application of these advanced design and processing technology was not widespread in thick materials. In this paper, hot rolled20Mn2Si2Cr as raw material, were made into DP and TRIP steels by different heat treatment; The microstructure of DP and TRIP steel was observed by LSCM, SEM, TEM and XRD, the tensile and impact tests at room temperature were implemented to test the mechanical performance of steel; The effect of heat treatment on microstructure and properties of20Mn2Si2Cr was analyzed by comparing the microstructure and mechanical properties of each tested steel under different heat treatment; The effect of second phase on the mechanical properties of steel was studied through analyzing the different of properties between duplex steel and transformation induced plasticity steel annealing at the same intercritical temperature. The results were as follows:1. The microstructure are quite different between steels subjected to intermediate quenching and intercritical quenching. The strip phases of steel subjected to intermediate quenching formed on the substrate of tempered martensite, inheriting the morphology of parent martensite phase, which is the result of the genetic organization; The ferrite was polygon in steel subjected to intercritical quenching. The strip ferrite has a higher yield strength compared to polygonal ferrite.2. The elongation and impact toughness of TRIP steel improved through pre-treatment, while the yield strength and tensile strength slightly decreased, this may be due to the refined organization caused by pre-treatment, and may also because the hardness of the matrix reduced after annealing. An appropriate increase of austenitizing temperature during pre-treatment helps to improve the amount of retained austenite in the final organization. Steel subjected to normalizing during pre-treatment had the best mechanical properties, its yield strength was584MPa, its tensile strength was1058MPa, and its product of strength and elongation reached to26344MPa%.3. The content of retained austenite and its carbon content of TRIP steel had a great impact on the mechanical properties of the material. Affected by many factors, the content of retained austenite did not changed unilateral with the increasing of intercritical annealing temperature, but it had the same trend with its own carbon content, this is because carbon is the most important element to improve the stability of the retained austenite. In the range of780℃～815℃, with the increasing of intercritical annealing temperature, the mechanical properties of the steels had improved by varying degrees.4. The second phase on ferritic matrix had great influence on the mechanical properties of steel. The DP and TRIP steel obtained through different heat treatment process based on20Mn2Si2Cr were quite different in the mechanical properties. DP steel had a243MPa higher yield strength than TRIP steel, and a328MPa higher tensile strength, at the same time, TRIP steel had a6.14%higher elongation than DP steel, and a35J higher energy absorption when impact occured. This is because the martensite had a higher strength than bainite, and a lower toughness compared to bainite. The TRIP effect produced by the retained austenite also had a great attribution to the higher elongation and energy absorption of TRIP steel.