Effect of Heat Treatment Process on Microstructure and Properties of Hot Rolled Sheet of Automobile Steel 20Mn2Cr

A new type of automobile steel 20Mn2Cr was simulated and designed with JMatPro software. The hot rolled sheet of 20Mn2Cr steel was heat treated by different processes with Gleeble thermal simulator. The effects of cooling rate, holding time of isothermal quenching and austenitizing temperature on the microstructure and properties of hot rolled sheets of automobile steel 20Mn2Cr were analyzed with optical microscope, scanning electron microscope (SEM) and microhardness tester. The results show that 20Mn2Cr steel exhibits excellent hardenability, and with the decrease of cooling rate, the microhardness of tested steel decreases as a result of the formation of a few bamboo-leaf-like lower bainite tissue in the microstructure and the increase of the size of original austenite grain. The holding time of isothermal quenching hardly affects the microstructure morphology of 20Mn2Cr steel, while the microhardness of matrix increases with the increasing of holding time. When the austenitizing treatment is adjusted from 950℃ in the single-phase region to 800℃ in the dual-phase region, a small amount of ferrite appears in the microstructure, and the carbon content in martensite matrix increases, resulting in the increase of microhardness.