Study On New Style Die Steel For Copper Alloy

Die-casting is one of the most advanced metal forming technologies, and it is used widely and developing fast. It is usually used in severe rugged environment. But the point of pouring of copper alloy is very high, so the operational life span is very short. The die steel contains high carbon and a large amount of alloy elements, which results in different kinds of carbides. The carbides segregate due to density variation, so the properties of the steel would become unhomogeneous radially. Meanwhile, some carbides distribute along the boundary of austenite grains while they are precipitating from the liquid phase, which leads to the emergence of continuous network of carbides at room temperature, and this would cause a severe decline of die cast dies' properties. Besides, phosphorus is usually segregated, and this can reduce the properties of steel. Thus it is an efficient approach to improve die cast dies' properties by preventing segregation of phosphorus and breaking the network of carbides distributing along the grain boundaries. And reasonable selection of materials, modification and heat treatment could save a large number of die cast dies materials, reduce production cost of die cast dies and improve quality and output of die cast dies.In this thesis, we investigated the effects of chemical composition, modification treatment and heat treatment of die cast dies on the microstructure and mechanical. The effect of zinc on the segregation of phosphorus was investigated utilizing optical microscope,Hi-scope video microscope (HSVM) and electroc probe microanalysis(EPMA). We find that, zinc can reduce the segregation of phosphorus, so the mechanical properties of steel can be improved. We also investigated the effect of K/Na and RE compound modification on the microstructure of die cast dies. The results showed that the grain size was refined, and the morphology of carbides was changed greatly after compound modification. The network of carbides was broken. The result show that the die cast dies modified by 0.5%K/Na+0.5%RE has the best structure and mechanical properties.In this thesis, the effects of quenching temperature, temper temperature and temper thermal retardation time have also been researched. The result show that, with the development of quenching temperature, the crystal grains are bigger. The crystal grains largen slowly under 1100℃,but the grain largens extremely above 1150℃. So the quenching temperature should be under 1100℃. With the development of quenching temperature, the hardness increases , and the thermal fatigue property of the die steel increased. The hardness after 550℃temper reach the maximizing. With the lengthen of temper thermal retardation time, the hardness first largens and then dimimishes. The hardness after 5 hours temper thermal retardation reach the maximizing.It was found that the optimum heat treatment was oil quenching thrice from 1050～1100℃for 50 minutes and tempering twice at 550℃for 3 to 5 hours, then air-cooling.