Cr8 Type Cold Work Die Steel High Performance Research

The research and development of Cr8 type cold work die steel with both good wear resistance and good toughness have great significance to meet the urgently requirement of our country to high quality die steel. Meanwhile, to enhance the wear resistance and toughness of Cr8 type cold work die steel is also an important development tendency of cold work die steel.In this work, alloying treatments were carried out to two typical Cr8 cold work die steels. The effect and action mechanism of Al, Si and Nb on Cr8 type cold work die steels were studied. Meanwhile, the difference of microstructure and mechanical properties of the two type steels were contrast studied, which can promote the serialization of Cr8 type cold work die steel. In the end, the hardening mechanism and phase transformation of Cr8 type cold work die steel were depth studied. The main conclusions are as follows.The comparative study on microstructure and mechanical properties of the two type steels shows that:the as cast microstructure of steel Cr8Mo2SiV contains M2C and M7C3 type eutectic carbides, whereas the steel Cr8WMo2V2SiNb contains MC, M6C and M7C3 type eutectic carbides; The peak quenched hardness of steel Cr8Mo2SiV is higher than that of steel Cr8WMo2V2SiNb, but the peak temperature is lower; The tempered hardness of steel Cr8Mo2SiV is higher than that of steel Cr8WMo2V2SiNb when tempering below 560℃, but if the temperature is exceeding 560℃, the result is opposite; The bending strength and sliding wear resistance of steel Cr8WMo2V2SiNb are higher than that of steel Cr8Mo2SiV. The direction of optimizing the composition of steel Cr8Mo2SiV to improve the impact toughness was discussed by combining the experiment results and that is suitable decreasing the carbon content.The study on effect of Al on microstructure and mechanical properties of steel Cr8WMo2V2SiNb shows that Al can refine the as cast microstructure, refine carbides, and refine austenite grain size; Al completely solutes in matrix, thus produces solution strengthening, and increases the tempered hardness, bending strength and wear resistance of steel Cr8WMo2V2SiNb. In addition, Al can remarkable increase the Ac1 temperature, narrow the y region. It is especially notice that the effect of Al on critical point when adding Al in high alloy steel. And its added amount must be considered to reasonable cooperate with Cr and V.The study on effect of Nb on microstructure and mechanical properties of steel Cr8WMo2V2SiNb indicates that the precipitation temperature of MC-eutectic carbides and the metatectic, eutectic temperature increase with the increase of Nb content, and the improvement of their properties is closely related with Nb content; The variation in Nb/V ratio with the increase of Nb content will change the type of MC-eutectic carbides, the type of MC-eutectic carbides from mainly VC change to VC and a small amount of (Nb, V)C and then to mainly NbC and (Nb, V)C. So the shape of ledeburite is more straight. If Nb content reaches to 1.32%, the substitution of Nb for V increases quenching hardness, the peak of quenching hardness moves to low temperature region; The substitution of Nb for V is beneficial for secondary hardening in tested steels, and higher hardness and temper softening resistance can be obtained in them. More importantly, it can decrease the Nb-V total content in tested steel, thus save resources.The study on effect of Si on microstructure and mechanical properties of steel Cr8Mo2SiV indicates that Si can simultaneously improve the low temperature tempering hardness, secondary hardening effect, bending strength and impact toughness of steel Cr8Mo2SiV; Meanwhile, the temperatures of Ac1, Ac3 and Ms of steel Cr8Mo2SiV were also increased with the increase of Si content. Thus, the quenched hardness of steel Cr8Mo2SiV increase with the increase of Si content when quenching at low temperature range and decrease with the increase of Si content when quenching at high temperature range.The study on secondary hardening mechanism of steel Cr8Mo2SiV indicates that the secondary hardening mechanism of steel Cr8Mo2SiV is the combination of the transformation of retained austenite and the early stage of Mo2C-carbide precipitation, and the role of transformation of retained austenite is more obvious. The secondary hardening carbide of steel Cr8Mo2SiV is Mo2C, but the steel Cr8WMo2V2SiNb also contains VC.The study on phase transformation and recovery and recrystallization mechanism of steel Cr8Mo2SiV during tempering process indicates that the transformation of steel Cr8Mo2SiV during tempering process contains the processes of carbides precipitated, pre-precipitated from martensite and own recovery of martensite. The order of precipitation of carbides is as follows:During the tempering process, both polygonization and formation of dislocation cell could occur for steel Cr8Mo2SiV. The process of formation for sub-grain:(a) dislocations cells→sub-grains; (b) deforming bands→boundaries of high dence dislocations→polygonization→dislocation walls→sub-grains. Sub-grains grew through the mechanisms of grain boundary bulge or consolidation; the original and precipitated carbides can pin dislocations and influence the cancel each other and re-arrange of dislocation and the formation of small angle interface, and then inhabit the recovery and recrystallization process. Distributed secondary particles can also pin sub-grain boundary, thus inhabit the growth of sub-grain and recrystal grain; the grain size after recovery and recrystallization is very fine, the growth of grain is very slowly with the holding time, there is the following relationship:S(μm)=0.1383+0.1619Int.The coarsening kinetic model of M23C6-carbide in steel Cr8Mo2SiV was also presented. The experiment results prove that the coarsening process of M23C6-carbide is mainly controlled by solute diffusion. It is exist the following relationship between carbide size with holding time that r(nm)= 55t1/3. It meets the t1/3 law of the model.