Effect Of Multi-modification On Structures And Properties Of Cr8 Cold Working Die Steel

As a kind of high-carbon mid-chromium hypereutectoid cold working die steel, Cr8 steels contain higher content of C (0.8-1.2%) and of Cr, Mo, V etc. alloying elements. They are widely used due to their high hardness and good wear resistance. But the strength and toughness of Cr8 die steels are reduced seriously because of coarse dendritic grains and large number of network-like distribution of eutectic carbides, caused by the segregation of alloy elements during the solidification process, along the grain boundaries. Meanwhile, the existent of high content of phosphorus, which is harmful to the steel, can induce the cold brittleness of the die steel. So it is an efficient method to improve the performance of Cr8 cold working die steel by dephosphorization, grain refinement, breaking the network-like carbides and changing their morphology.In this work, under laboratory conditions, on the basis of Cr8 die steels, through reducing the phosphorus content in steel and rationalizing the chemical composition of die steel, W, Mo are added in right ratio to ensure enough secondary hardness and 1%Si addition is necessary, which can enhance the strength and toughness of the steel; Higher content of V and Nb are added to form many MC style carbides of high hardness, which can increase the wear resistance and refine grains to increase toughness. K/Na-RE multi-modificator is used to modify the steel after alloying. The microstructure and performance of test die steel were studied by methods of HSVM, SEM, XRD and DSC etc. The results show that after alloying and multi-modification, test die steel was refined evidently and primary austenite exists as isometric crystal style. In addition, the Cr7C3 carbides which connect to each other in the matrix are decreased and the morphology of those is transformed from network-like to bloke-like. And there are particle-like MC style carbides, including (NbV)C and NbC, which appear in the matrix and distribute evenly.The effects of annealing, quenching and tempering on microstructures and properties of test Cr8 cold working die steel have been researched. High temperature heat-treatment can induce the solution of carbides effectively, and lath martensite, which has fine toughness, also can be gained in this process. The hardness of test die steel increased with the rising of quenching temperature under 1050℃, in conditions of oil cooling. But above 1050℃, the hardness decreased sharply due to coarse grains and the increasing content of retained austenite. Tempered martensite and carbides can be gained after tempering at 480～550℃, and the hardness will decrease slightly.The results of mechanical properties tests show that, after quenching and high-temperature tempering, the hardness of test Cr8 die steel Nb-alloyed and modified by K/Na-RE can reach to 62-64HRC, and the impact ductility can also increase to 44J/cm2. Based the higher hardness and toughness, the wear resistance is also improved obviously.