| The harsh field service conditions of automobile high-strength plate require Cr8 die steel to have high wear resistance and toughness.Domestic Cr8 cold working die steel is easy to crack and the service life of which does not match the requirement.High strength sheet forming puts forward high requirements for the performance of Cr8 steel die,which needs to be guaranteed by the internal quality of die raw material structure,heat treatment,surface strengthening and subsequent process treatment.This paper systematically studies the original structure and properties,the evolution law of carbides after vacuum gas quenching,the surface strengthening technology and so on of domestic HNC53 steel.This paper also puts forward quality control key technology and die life control strategy of domestic Cr8 cold working die steel through die friction and wear numerical simulation combined by practice.A collaborative technology of Die material-Vacuum heat treatment-Surface Strengthening-Numerical simulation is formed to prolong die service life.In this paper,three kinds of Cr8 cold working die steel materials are studied: HNC53 made by Shanghai Baosteel Group and imported products of KD11 max and DC53.The microstructure and properties of their raw materials are systematically compared and analyzed,the distribution characteristics and particle size of carbides are studied and the effects of morphology,size,distribution and proportion of carbides on the directionality of mechanical properties are revealed.The structure of HNC53 steel is evenly distributed,and the size of bulk eutectic carbide in Y direction is 14 ~ 44 μm in length and 7 ~ 14 μm in width.Its small spherical carbides have little size difference and are evenly dispersed in the matrix;The mechanical properties of the material have good isotropy in X,Y and Z directions.This paper studies the effect of vacuum gas quenching + high temperature tempering heat treatment process on the microstructure and properties of HNC53,KD11 max and DC53 die steels,analyzes the dimensional stability of the materials,and studies the evolution law and growth kinetics of carbides during heat treatment.The results show that after vacuum gas quenching + high temperature tempering heat treatment process,the uneven distribution of carbides in the matrix is improved,the particle size of bulk eutectic carbides becomes smaller and the volume proportion decreases.At the same time,the spherical carbides in the original structure dissolve back into the matrix,precipitate small granular secondary carbides,and the martensite in the matrix is refined.The growth kinetics of carbide is: At the fore-end of M3 C precipitation,the driving force of C is dominant and with the progress of the reaction,Cr element diffuses to the matrix.Due to the limited solid solubility of Cr in the matrix,the driving force of C increases at the foreend of M23C6 precipitation until M3 C is completely transformed into M23C6.The secondary carbide particles precipitated by HNC53 and KD11 max are small and evenly distributed on the martensite matrix,which is beneficial to improve the toughness of the material.The hardness of HNC53 and KD11 max reaches 58 HRC,and their hardness,impact toughness and dimensional stability are slightly better than DC53.In this paper,three surface strengthening treatments ordinary PVD,TD and HKS are carried out on HNC53 cold working die steel and the effects of these strengthening treatment process on phase,chemical composition,hardness and coating substrate adhesion are analyzed.The results show that the TD strengthening treatment has good coating substrate adhesion which grade is HF1 and coating hardness of 2694HV;HKS-G also has good coating substrate adhesion which grade is HF2 and coating hardness of3326HV;ordinary PVD in comparison has relatively poor coating substrate adhesion and hardness.Because the TD surface strengthening treatment is a chemical method,it has best coating substrate adhesion.By comprehensive analysis,TD method is used for large quantities of high-strength steel forming die with more stamping times and higher die service life,HKS-G method for small quantities of ultra-high strength steel with relatively few stamping times and lower die service life.By simulating the forming force and die friction and wear of vehicle longitudinal beam,this paper studies the influence of different surface strengthening treatment methods on die friction and wear,and compares the theoretical simulation results with the actual application.The simulation result is: HKS-G surface strengthening treatment has smallest maximum die wear which is 2.1×10-8mm and the predicted die fatigue life is470000 times which is 5 times higher than that of uncoated die.TD method has maximum die wear of 2.6×10-8mm and predicted die fatigue life of 380000 times.The actual service life of the die after TD surface strengthening treatment is 330000 times,and the theoretical service life is consistent with the practice.In order to improve service life of HNC53 die,based on original structure and properties of HNC53 steel,vacuum gas quenching + high temperature tempering heat treatment process,surface strengthening technology,and numerical simulation combined with field practical application,this paper summarizes the main influencing factors of this Cr8 cold working die,and establishes the collaborative technical diagram of Die material-Vacuum heat treatment-Surface Strengthening-Numerical simulation.HNC53 has small directional difference and uniform original structure and good toughness;After vacuum gas quenching + high temperature tempering,the material has high hardness and good impact toughness,which can reach the level of foreign Cr8 cold working die steel.Service life of HNC53 die has been verified by practice and reaches 330000 times. |
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