Composition Optimization Of H13 Hot Work Die Steel And Microstructure And Properties Research

With the rapid development of advanced industries,such as aerospace,rail transportation,the H13 hot work die steel which is widely used worldwide is gradually unable to meet the increasing service performance requirements,especially high-temperature strength.The development of novel high-strength and long-life hot work die steels with independent intellectual property rights is the current significant research area.In the present work,we propose to take advantage of a rapid solidification technology“Spray forming”and alloy composition optimization to develop novel high-strength hot work die steel.The differences of yield strength contribution factors:grain size,solute atoms,dislocation density and precipitations in different materials were compared by Scanning Electron Microscope（SEM）,Electron Backscattered Diffraction（EBSD）,Transmission Electron Microscope（TEM）and Diffraction of X-rays（XRD）,and the correlation model between alloy composition,microstructure and mechanical properties was constructed to reveal the strengthening mechanism of the modified material.Firstly,a modified 2000 MPa SF01 steel with increased carbon content was prepared by spray forming.The yield strength（YS）of SF01 steel was at least 10%larger than H13 steel from 25 to650℃.Microstructural analysis and strength calculations indicate that the higher yield strength of SF01 steel is due to its finer grain size and the presence of diffusely distributed nano-sized V₈C₇in the matrix.The finer grain size ascribes to the combined effect of spray forming and the pinning of grain boundaries by VC during the 1040℃austenitization process in quenching.The brittle-to-ductile transition temperature of SF01 steel is about 200℃.Below,dislocation slip occurs at a very short distance.Microcracks initiate at the defects and propagate in a cleavage mode.Above200℃,the plastic deformation takes place mainly via dislocations glide.As the deformation temperature raised to around 400℃,twinning would act as a complementary deformation method when dislocation slip is difficult to carry out,allowing for increased toughness.As the temperature reaches above 600℃,dynamic recrystallization results in marked soften and a significant increase in toughness..Secondly,CXN02 steel and CXN03 steel with the addition of Tungsten,Molybdenum,and lowering of Chromium compared with H13 were prepared.The yield strength of CXN03 steel is1714 MPa at 25℃,which is 106%of CXN02 and～111%of H13 steel.With the test temperature increase,the yield strength of all materials decreased slowly from 25 to 400℃and rapidly above400℃.At 650℃,the yield strengths of CXN02 steel and CXN03 steel became similar to about700 MPa,which is about 64%higher than that of H13.The additional W and Mo in CXN03 steel allowed more M₆C to present in as-annealed CXN03 steel,thus more undissolved carbides that could pin prior austenite grains exist in as-quenched CXN03 steel,refining the grains.The tempering softening resistance（TSR）of CXN03 steel is significantly better than H13steel.After quenching at 1040℃,the hardness and strength of H13 steel were larger than those of CXN03.However,the hardness and strength of CXN03 steel exceeded those of H13 after 2 h tempering at 600℃.After 48 h tempering,CXN03 steel maintained 448.5 HV1,which is about157%of 285.3 HV1 to H13.The superior tempering softening resistance of CXN03 steel mainly results from the excellent stability of dispersive nano-sized M₂C,which could prevent dislocation recovery.Recrystallization occurred in H13 steel but not CXN03 steel.The recrystallization of H13 steel is driven by dislocation movement.The rearrangement of dislocations contributed to the formation of sub-boundaries.These sub-boundaries could divide martensite lath as well as form sub-grains.As the tempering time increased,sub-boundaries transformed into high-angle grain boundaries by absorbing the vicinal dislocations.Therefore,martensite lath collapsed,and massive recrystallized grains occurred.The massive stable M₂C in CXN03 steel hindered the dislocation rearrangement,thus preventing the recrystallization.Finally,the effects of isothermal quenching temperature and tempering treatment on the microstructure and performance of SF01 steel were investigated and revealed the influence mechanism,which provides a basic theory for the development of quenching process parameters of large-size SF01 steel.After isothermal quenching at 220°C（below M_S）,the microstructure of SF01 steel consists of martensite,auto-tempered martensite,and retained austenite.After isothermal quenching at 260°C and 300°C（above M_S）,the microstructure consisted of martensite,acicular bainite,and retained austenite,and the grain size increased compared to 220°C.In addition,bainite and martensite in all three specimens allowed the K-S relationship with the parent austenite.The impact toughness after isothermal quenching above the M_S is significantly lower than that below the M_S,the main reasons are as follow:（1）the content of retained austenite,which could improve toughness,is lower and the dislocation density in retained austenite is higher.（2）Stress might concentrate at the boundary between martensite and bainite,promoting crack sprouting and expansion.（3）Several twin-martensite with high hardness and brittleness presented in the microstructure,which impairs the toughness.After tempering at 350℃,the impact toughness of the martensitic and bainitic multiphase structure increased significantly.The reason is that the stress in the twin martensite decreased and part of the twin martensite decomposed after tempering at this temperature,and the dislocation density in the residual austenite decreased.