Study On The Microstructural Control And Mechanical Properties In ZG24SiMn2CrB Steel

Nowadays, with the rapidly development of automobile industry, the demand for lightweight auto-materials is more and more increasing. Traditional steels can no longer meet the need of automobile, so it is no time to improve the steels. So far, Advanced High-Strength Steel(AHSS) has developed to the third generation, which has high strength and ductility at low cost. The third generation of AHSS meets both property and economy well. Based on the third generation of AHSS, the aim of the paper is to design a kind of cast steels with low content alloys at low cost. Having a known about the function and the price of alloys in the steel, carbon, silicon, manganese, chromium and other alloys with few amount are put into the steel, whose name is ZG24 Si Mn2Cr B.In this paper, the heat treatment parameter of the test steel has been optimized, so that we can acquire a kind of steel with high strength and ductility.First, intercritical temperature has been discussed. At this experimental condition, the test steel is heated to 770 °C、790 °C、810 °C、830 °C、850 °C after pre-heat treatment, and then cooled by wind. The steel has to be tempered at 340 °C right now. What has been observed in the steel are ferrite, bainite, martensite and retained austenite. Along with the increasing of the intercritical temperature, the amount of ferrite declines, and the amount of bainite and martensite has been improved. In a word, we can adjust the intercritical temperature to control the proportion of each phase, so that we can acquire the steels with different comprehensive mechanical properties.Second, the ways of cooling has been discussed. At this experimental condition, the test steels are cooled down to the room’s temperature though air, wind, spray and NJ. The steel cooled by wind has the best mechanical properties that (7≥ 1500 (6, ≥16%,Hardness ≥ 38 HRC. The microstructure of the steel cooled by NJ obtains more bainite than three other ways of cooling, so the way of NJ cooling has the property of cooling fast in high temperature and cooling slow in medium temperature.Finally, the tempering temperature has been discussed. At this experimental condition, the test steels are tempered by 300 °C, 340 °C, 380 °C, 420 °C after every way of cooling. And we can obtain it that with the rising of tempering temperature, whatever way of cooling, the content of bainite and martensite is increasing in the microstructure of test steel at room temperature. The test steel has the best property at 380 °C when it is cooled by air. When the test steel is cooled by wind spray and NJ, the tempering temperature that the test steel has the best property is 340 °C. When the test steel is cooled by air and wind at 340 °C and by spray and NJ at 300 °C, it all can have a optimum combination of temperature and cooling way, which meets the need of property. Therefore, with the improvement of the cooling rate, at the lower tempering temperature, the test steel can obtain higher mechanical properties...