Control And Improvement Of Dendritic Segregation And Primary Carbide In H13 Steel Processed By Electroslag Remelting

The demand for high-end dies is growing with the developing industry level,which requires the higher-quality die steel,the most important raw materials of dies.Because of the good performance in hardness,strength and thermal stability,H13 steel is widely used for hot-working dies.The dominating failure of hot-working dies is thermal fatigue.The dendritic segregation and primary carbides caused by dendritic segregation are the important factors that leads to the thermal fatigue of H13 steel.The control and improvement of dendritic segregation and primary carbides are essential to the production of the high-quality H13 steel.The present work comprehensively explored the improvement methods for the dendritic segregation and primary carbides of H13steel based on the refining,modifier addition and heat-treatment process.In the present research,the dendritic segregation and primary carbides in different regions of H13 ingot processed by one-ton industry electroslag remelting furnace with slag composition of 70 wt.%Ca F₂-30 wt.%Al₂O₃ were investigated.Improvement effect of improving cleanliness of steel and rare earth modifier addition on the dendritic segregation and primary carbides in H13 steel were studied.The H13 steel was remelted in the industry electroslag remelting furnace with slag contains rare earth oxide,which achieved both the cleanliness improvement and rare earth modification.Finally,the grains growth behavior,dissolution of primary carbide and dendritic segregation evolution of H13 steel during high-temperature homogenization heat treatment were investigated.The conclusions could be drawn as follows:（1）The dendritic segregation degree of different regions in H13 ingot was different,namely,that in the upper part in ingot was more serious than the lower part and in the center part was more serious than the edge of the ingot.There were V-rich primary carbide and Mo-rich primary carbide in H13 steel.With the dendritic segregation got more serious,the size,number and area ratio of primary carbide also increased.According to the calculation,the Al₂O₃inclusion in H13 steel had disregistry with the V-rich primary carbide of 9.4%,and the Mn S inclusion in H13 steel had disregistry with Mo-rich primary carbide of 9.8%.Both Al₂O₃and Mn S inclusion could promote the nucleation and growth of primary carbide,and combined with the primary carbides to form large-sized complex precipitates.（2）The decrease of sulfur content in H13 steel contributed to the suppression for the formation of large-sized complex precipitates.The Mn S inclusions would precipitate later than the primary carbides as the sulfur content was 0.0007 wt.%,which prevented the formation of large-sized precipitates with core of Mn S inclusions.In addition,the decreasing sulfur content was also helpful to suppress the dendritic segregation.The reason might be that during the solidification process of H13 steel,the segregated sulfur was enriched in the residual liquid phase and kept decreasing the solidus temperature,which extended the local solidification time and promoted the dendritic segregation.The decreasing sulfur content weakened the effect of sulfur on solidus temperature,which therefore suppressed dendritic segregation.（3）The rare earth Ce addition achieved the modification of inclusions in H13 steel.As the Ce content were 0.0130 to 0.0190 wt.%,the Al₂O₃ and Mn S inclusion were modified into Ce₂O₃ and Ce₂O₂S inclusion in the investigated H13 steel.The modified inclusions had smaller size and more quantity.Ce₂O₃ and Ce₂O₂S inclusion had higher disregistry with primary carbides,but had lower disregistry withγ-Fe.It prevented the complex precipitates with cores of inclusions from formation.Besides,the fine and disperse modified inclusions prompted the nucleation ofγ-Fe during the solidification process,which finally refined the dendrite structures.The refined dendrite structures limited the growth of primary carbides both in the time and space,and therefore contributed to the refinement of primary carbides.The micro-hardness of carbides rich bands was higher than that of the carbides poor bands for 18.2%in the forged H13 steel without Ce addition.However,that of the H13 steel contained 0.0190 wt.%Ce was6.8%.The banded segregation of forged H13 steel was also improved after Ce modification.（4）The rare earth modification effect could be achieved by remelting H13 steel with slag contains rare earth oxide and the added reductant Si Ca.However,the Al₂O₃in slag would decrease the activity of rare earth oxide,which suppressed the reduction of rare earth oxide.The rare earth modification effect would be strengthened by decreasing the Al₂O₃content in slag,and then refined the dendrite structures and primary carbides.（5）By using the slag with initial composition of 52.2 wt.%Ca F₂-26.7 wt.%Ce O₂-17.8 wt.%Ca O-3.3 wt.%Al₂O₃ to remelt H13 steel in one-ton industry electroslag remelting furnace,and adding reductant Si Ca alloy during the remelting process,both the cleanliness improvement and rare earth modification were realized.The deoxidation rate and desulfurization rate of in H13 steel processed by electroslag remelting with slag composition of 70 wt.%Ca F₂-30 wt.%Al₂O₃were 10.9～23.9%and42.5～57.5%,and that of the H13 steel remelted with rare earth oxide contained slag were 30.3～39.1%and 73.8～92.5%respectively.Meanwhile,the H13 ingot contained0.0013 to 0.0065 wt.%Ce after remelted with rare earth slag.In addition,H13 steel remelted with rare earth slag had finer dendrite structures and primary carbides.Banded segregation of alloying elements in forged H13 steel became uniform and impact toughness was improved.The impact toughness was increased from 20 J to 26 J.The impact toughness was increased by 30%.（6）The austenite grain growth rate of H13 steel would increase sharply as the temperature increased from 1150 to 1200 ^oC,which leaded to the dramatical growth of austenite grain when soaked above 1200 ^oC.In the investigated H13 steel,the Mo-rich primary carbides completely dissolved at the investigated temperature in the range of1150 to 1250 ^oC,and the needed time for complete dissolution of the Mo-rich primary carbide when soaked at 1150,1200 and 1250 ^oC was 60 to 120 min,30 to 60 min and 10to 30 min,respectively.Most of the V-rich primary carbides dissolved after soaked for60 min,which however cannot dissolve completely in the investigated temperature range.The thermodynamic calculation results indicated that Ti and N content in H13steel would increase the thermal stability of primary carbides.The decreasing Ti and N content in H13 steel could reduce the complete dissolution temperature of primary carbides.The dendrite structures of H13 steel gradually disappeared during the homogenization process.The time needed for the elimination of dendrite structures at1250 ^oC was about 600 to 720 min.The time for 1150 and 1200 ^oC was estimated to be3.37 and 1.80 times as longer as that at 1250 ^oC.