Study On Phase Transition Properties And Heat Treatment Process Of High Strength And Toughness Cold Working Die Steel Caldie

With the development of high strength steel sheet and its application in automobile panel,the traditional cold working die material can not meet the requirements of cold stamping forming of automotive high strength steel sheet?above800 MPa?.Based on the above engineering background and the development direction of cold work die steel proposed by the 13th Five-Year Plan,it is of great significance to research and develop a new type of high strength and toughness cold working die steel.Caldie steel is a new type of high strength and toughness cold working die steel abroad.In order to promote the localization of Caldie steel,phase transition properties,heat treatment process and service properties of this material are studied.In this paper,by means of dilatometer,scanning electron microscope?SEM?,X ray diffraction?XRD?,transmission electron microscopy?TEM?,phase transition properties,solid solution spheroidization process,quenching and tempering process of the steel Caldie were studied,and strength,toughness,wear resistance and the evolution of carbide were studied and analyzed in detail.The main results can be summarized as follows:?1?In the course of continuous cooling transformation of Caldie steel,the critical cooling rate of the complete martensite is 0.60?/s,which indicates that the hardenability is admirable.In the course of time temperature transformation,the nose temperature of pearlite transformation is about 700?.According to the degree of carbide spheroidization,the recommended spheroidizing annealing process is:840?×2 h+725?×8 h.In this state,the shape and distribution of carbide are more uniform,and the hardness is 182 HBW.?2?4Cr5Mo2V?5Cr5Mo2V and Caldie are three kinds of Cr-Mo-V die steels with different carbon contents?namely,the carbon contents of 0.36,0.46 and 0.74%?.With the increasing of carbon content,the Ac_cm and M_s temperatures are decreased obviously.The Ac_cm temperatures of 0.46C and 0.74C steels are decreased by 24?and 62?,and the Ms temperatures are decreased by 39?and 109?compared to0.36C steel,respectively.The phase regions of pearlite and bainite in CCT curves shift to the left,the critical cooling rate of the complete martensite of 0.36C steel is 0.4?/s,and increase to 0.6?/s when the carbon content is 0.74%.In addition,the microhardness also increases.?3?Caldie steel has good hardening capacity quenched between 1000¹030?and the hardness is above 65 HRC.When quenched at 1020?,the hardness is the highest,which is 65.7 HRC,and the grain size is grade 9.6.Carbide in quenched state is V-rich MC carbide which is spherical and has high strength,fine carbide can play a role in dispersion strengthening.With the increase of tempering temperature,the hardness of Caldie steel first decreases then increases and finally decreases,and the secondary hardening peak is between 500⁵30?.After quenching at different temperature,the trend between tempering hardness and tempering temperature is basically identical,and there is no significant shift in the secondary hardening peak temperature.The hardness of the sample after quenching at 1020?and tempering at 520?is 58.7 HRC.?4?When tempered at the secondary hardening peak temperature,the carbide is Fe/Cr-rich M₃C and Fe/Cr-rich M₇C₃.M₃C carbide is rod-shaped,larger and densely distributed relative to low temperature tempering and medium temperature tempering.However,Mo₂C carbide is not observed,mainly due to the short tempering time,and Mo₂C carbide is very fine and diffusely distributed.