| H13 steel contains about 0.4%of carbon elements and about 8%of Cr,Mo,V and other alloying elements in total.Therefore,it is very easy to generate large-sized primary carbides due to the segregation of alloying elements during the solidification process.The large-sized primary carbides cannot be completely eliminated in the subsequent heat treatment process and still remain in the final product.During the use of H13 steel,large-size primary carbides will act as crack sources to cause fatigue failure of die steel.The larger the size of primary carbides,the shorter the service life of die steel.Therefore,how to effectively reduce the size of large-sized primary carbides in H13 steel is the key to further improving its service life,and there are few studies on this aspect.Based on this,this paper,funded by the National Natural Science Foundation of China,studied the effect of elemental Ce on the behavior of large-size primary carbides in H13 steel.First,the three-dimensional characteristics of primary carbides were obtained by organic solution electrolysis,and the precipitation mechanism of primary carbides was obtained.Then,using high temperature experiments,the high temperature thermal stability of different types of primary carbides was obtained,and the effect of Ti content on primary carbides in H13 steel was further studied.In order to reduce the three-dimensional size of primary carbides in H13 steel smelted by conventional alloys,the influence mechanisms of Ce content,cooling rate and Ce-S inclusions on the three-dimensional characteristics of large-size primary carbides in H13 steel were studied respectively.The main findings of the full paper are as follows.(1)By studying the relevant characteristics of primary carbides in H13 steel,it is found that the segregation of Cr,Mo,V,and C alloying elements during the solidification process is the main reason for the formation of last-to-solidify region and large-sized primary carbides.The three-dimensional morphology of primary carbides is a typical "tree" structure,and the "trunk" is Ti-V-rich carbides,and the "branches" are V-rich carbides.The precipitation mechanism of primary carbides is as follows:at the end of solidification,alloying elements are enriched in the last-to-solidifiy region,and Ti-V-rich carbides on the "trunk" are first precipitated,then the V-rich carbides on the "branches" are precipitated with Ti-V-rich carbides as nucleation cores.(2)The thermal stability of primary carbides was studied through high temperature experiments,and it was found that the thermal stability of Ti-V-rich carbides was higher than that of V-rich carbides.When the heating temperature reaches 1200℃,most of the V-rich carbides dissolve,and the Ti-V-rich carbides still exist stably.As the heating temperature increases from 1150℃ to 1250℃,the three-dimensional size of primary carbides decreases significantly from 22.7μm to 12.4μm.By studying the effect of Ti content on the characteristics of primary carbides in H13 steel,it is found that the reduction of Ti content can completely inhibit the precipitation of Ti-V-rich carbides.As the Ti content decreased from 110 ppm to 15 ppm,the average three-dimensional size of primary carbides decreased from 31.7μm to 26 μm.When the Ti content is less than or equal to 24ppm,the precipitation of Ti-V-rich carbides in H13 steel can be completely suppressed.(3)By studying the influence mechanism of Ce content on the characteristics of primary carbides in H13 steel,it was found that with the increase of Ce content from 0%to 0.032%,the size of the last-to-solidify region decreased from 85 μm to 65 μm,and the morphology of primary carbides changed from "dendritic" to "flaky",and the average three-dimensional size of primary carbides decreased from 31 μm to 23 μm.There is a good positive correlation between the last-to-solidify region size and the three-dimensional size of primary carbides.The increase of Ce content increases the number density of Ce-O inclusions.Ce-O inclusions act as effective heterogeneous nucleation cores to promote the nucleation of ferrite and increase the nucleation rate of molten steel,which in turn reduces the size of the last-to-solidify region and the three-dimensional size of primary carbides.However,when the Ce content reaches 0.18%,the size of the last-to-solidify region increases to 80 μm,the three-dimensional size of the primary carbide increases to 38 μm,and the morphology becomes a "tree-like" structure again.(4)By analyzing the distribution of primary carbides from the edge to the core of the ingot in H13 steel containing elemental Ce,the comprehensive influence mechanism of cooling rate and Ce content on the three-dimensional size of primary carbides is obtained.When the Ce content is 0.0018%,the three-dimensional size of primary carbides increases from 8.6 μm at the the edge to 41.7 μm at the center.The increase of Ce content effectively reduces the three-dimensional size of primary carbides at various positions from the edge to the center.When the Ce content increased to 0.037%,the three-dimensional size of primary carbides at the edge decreased from 8.6 μm to 7.7μm,and the three-dimensional size of primary carbides at the center decreased from 41.7 μm to 26 μm.The optimization effect of rare earth Ce element on the primary carbide size at the center of the ingot is much better than that at the edge of the ingot.At the edge of the ingot,the cooling rate plays a leading role in the nucleation rate,and the optimization effect of Ce element is not obvious.At the center of the ingot,the Ce content plays a leading role in the nucleation rate of the molten steel,and the optimization effect of this position is remarkable(5)By studying the effect of different S content on the size of last-to-solidify region and the primary carbides in Ce-treated H13 steel,it shows that the size of the last-to-solidify region and the primary carbides are closely related to the number density of Ce-S inclusions.As the number density of Ce-S inclusions increases from 4.2 mm-2 to 36.4 mm-2,the size of the last-to-solidify region decreases from about 74 μm to about 65 μm,and the average three-dimensional size of primary carbides decreases from about 30 μm to about 20 μm.The proportion of larged-sized primary carbides(>50 μm)was reduced from 14.2%to 2.5%,effectively hindering the aggregation of primary carbides.Controlling the O content of the molten steel to be about 10ppm,the Ce content to be above 0.01%,and the S content to be above 30ppm can generate a large number of finely dispersed Ce-S inclusions... |
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