Study On Rolling And Heat Treatment Processing Of Low-carbon And Low-alloy Plastic Mould Steel

With the development of plastic industry, the larger need for plastic mould steel, the higher requirement to the performance of plastic mould steel. At present, P20 steel made by USA and 718 steel made by Swedish are the most widely employed mould steel around the world. On the basis of native standards, mould steel producers of different countries developed their own brands through improving P20 and 718 steel. Therefore, the development of new steel brand on the basis of P20 and corresponding hot rolling and heat treatment processing is meaningful to the development of our nation's plastic mould steel industry.The chemical composition of tested steel has been adjusted on the base of P20 steel for developing new plastic mould steel with low-carbon and low-alloy. Through researching the TMCP process, quenching and tempering treatment processing, the differences on microstructure and mechanical properties were analysed between rolling condition and quenching and tempering treatment, which can offer experimental guide for designing proper chemical composition and determining the optimum rolling and heat treatment process. The contents of the study include the following main parts:(1) On the basis of chemical constitution of P20, the No.1 test steel is fixed by lowering the carbon content, increasing manganese content and adding some micro alloying elements. The TMCP result shows that the rolled steel has higher hardness and strength, but lower ductility. Generally, the performance of rolled steel doesn't reach the standard of P20.(2) The result of quenching and tempering treatment on No.1 test steel indicates that the quenching temperature is relatively low, austenite has not fully transformed, quenching microstructure is martensite, bainite and ferrite, and tempering microstructure is tempering troostite plus tempering sorbite; With the increasing of tempering temperature, the ductility increases, but hardness decreases. When the tempering temperature is in the temperature range of 590-650℃, the ductility and hardness of the test steel can satisfy the requirement.(3) By adjusting the chemical constitutions of No.1 test steel, the chemical constitution of No.2 test steel is determined. The TMCP result indicates that the strength, hardness and ductility of No.2 test steel satisfy the requirement. The cooling speed has a remarkable influence on the microstructure and mechanical property of test steel. When the cooling rate is too high, the hardness of steel is higher than normal, and lower when the cooling speed is too small.(4) The research result on quenching and tempering treatment of fast-cooling and slow-cooling test steel indicates that when the quenching temperature is around 970℃, austenization can be fully achieved, quenching microstructure is slab martensite plus residual austenite, tempering microstructure is tempering sorbite. With the increasing of tempering temperature, the hardness of the test steel decreases and the ductility increases. The hardness of test steel can satisfy the requirement when the temperature around 590℃.(5)By the comparison and analysis of two round rolling and heat treatment experiments, the proper chemical composition of test steel and optimum rolling and quenching and tempering treatment process has been gained. The strength, hardness and ductility of test steel all reach or exceed the standards of P20 steel.