Study On Control Technology Of Carbides In High-carbon Martensitic Stainless Steel 8Cr13MoV Used As Knives And Shears

Carbides are important phases which have remarkable effect on the performance of high-carbon martensitic stainless steel used as knives and shears.Controlling technology of carbides in china had obvious gap with some advanced countries,and the study about controlling technology of carbides was very few.In this thesis,high-carbon martensitic stainless steel used as knives and shears was regarded as the research object.The study focused on the carbides controlling in the smelting,rolling and heat treatment process.The purpose was to develop some effective methods for carbides controlling,provide detailed reference data and theoretical support for industrial production and make the material quality to meet foreign advanced level.The main research content and conclusion was described as below:The precipitation rule of each phase during solidification of 8Cr13MoV steel was calculated by using Thermo-calc software.Multiple etching methods and anode electrolytic method were employed to analyze the practical phase composition.Results indicated that the cast micro structure of 8Cr13MoV steel was constituted by martensite,retained austenite,ferrite,primary carbides and second carbides.Among the carbides,primary carbides were in majority,while secondary carbides were very few.The effect of electroslag remelt parameters on the carbides of 8Cr13MoV steel was studied.Results indicated that low current intensity and high cooling intentity were in favour of homogenizing the distribution of primary carbides and decreasing its size and total volume fraction.However,these two parameters had little effect on the carbides type.Current intensity can not affect the morphology of single primary carbides,while high cooling intensity can affect the morphology of primary carbides.High cooling intensity tended to make the eutectic fiber of primary carbides more conpact,and change the rodlike fiber to cerebriform structure.Different amount of Titanium was added into 8Cr13MoV steel.Results indicated that Titanium and carbon combined to form TiC,and TiC can act as heterogeneous nucleus to refine grain and M7C3 primary carbides.Ti addition can suppress the formation of M7C3 primary carbides,and promote the transformation of M7C3 to M23C6.Ti can refine the martensite acicular structure in as-cast microstructure,the number of martensite increased and tensile strength of steel increased obviously.In the annealing process,Ti can suppress the precipitation of secondary carbides.Moderate amount of Ti can decrease the size and total amount of carbides,and inprove the plasticity of as-annealed steel.Thermocompression experiment was employed on 8Cr13MoV steel.Results indicated that the higher deformation temperature was,the earlier dynamic recrystallization happened.The maximum stress decreased almost linearly with increasing deformation temperature and decreasing deformation rate.Low deformation temperature and high deformation rate contributed to the primary carbides breaking.Along with decreasing deformation temperature,increasing deformation temperature and cooling rate,the microstructure of steel was refined.The results of experiments and simulation coincided well.Low rolling start temperature and finish temperature were in favour of primary carbides breaking,and the microstructure was more fine.Increasing rolling deformation can realize the same effect.The study also discovered that primary carbides were more easily to dissolved at low start rolling temperature,hence the amount of primary carbides were lower.The spheroidizing annealing process was studied.Results indicated that carbides dissolved into steel matrix during austenitizing stage.The amount of carbides decreased and the element content in matrix increased.The combined influence resulted in hardness decrease first and then increase with increasing austenitizing time.In spheroidizing stage,carbdies grew up gradually and the hardness of steel decreased.Therefore,with increasing time of spheroidizing stage,the hardness of steel decreased gradually.In this study,the minimum hardness can be obtained when the austenitizing time and spheroidizing holding time were controlled at 90 minutes and 135 minutes,and this was in favour of subsequent process.The study also discovered that the longer spheroidizing holding time was,the higher spheroidization rate was in the initial spheroidization stage.However,this tendency was on the contrary in the later spheroidization stage.With increasing cooling rate,the size of carbides decreased and the number of lamelliform carbides increased.This will increase the hardness of steel and cooling rate should be controlled below 25?/h.