| With the increasingly serious problems of global pollution and the gradual scarcity of oil resources,higher requirements have been put forward for the research of "lightweight" car bodies.Traditional light-weight high-strength steels such as dual phase steel(DP),transformation induced plasticity steel(TRIP),interstitial atom-free steel(IF)and austenitic steel,etc.,are restricted due to production costs,metallurgical and processing performance issues.application.The matrix structure of medium manganese TRIP steel(with Mn content of 4-12%)is composed of austenite and ferrite,and has excellent strength and plasticity.Reported studies have shown that its strong plastic product(tensile strength and elongation after fracture)Product)can reach30～60GPa%,which has extremely high application prospects.Resistance spot welding is the main method of thin plate connection in the automobile assembly process.The integrity and mechanical properties of the welding joint are the key to ensure the safety of the whole vehicle.Therefore,the resistance spot welding process,microstructure and performance of the medium manganese steel TRIP steel and the structure change after welding Sex has important reference and guiding significance for its application.This paper takes 0.13C-7Mn medium-manganese steel as the research object,carries out single and double pulse spot welding experiments,and studies on in-situ tempering after welding and tempering in heat treatment furnace.The microstructure evolution,microhardness distribution,segregation and redistribution behavior of the main alloying elements and the failure mechanism of the weld joint of the test steel were studied.The test results of this article can be summarized as follows:(1)In the single pulse test,the influence of the welding process on the nugget diameter and tensile-shear properties of the solder joint was studied.The welding current has the most significant effect on the comprehensive mechanical properties of the solder joints.The best tensile load of 14.3KN is obtained when the welding parameter combination is 4KN(pressure),320ms(time),and 11KA(current);by changing the high temperature residence time,nucleation is affected The content of liquid metal will affect the diameter and mechanical properties of the solder joint,but the peak load is not directly related to it under the condition of high welding energy;the size of the electrode pressure will affect the contact resistance and cooling rate.The test results show that under 4KN The comprehensive mechanical properties of the solder joints are better than 4.5KN.(2)In the double-pulse test,the ductility and nugget diameter of the solder joint are slightly improved within 9～10KA of the welding current.In the case of high current,the mechanical properties of the solder joint at 11 k A are equivalent to a single pulse,because it can be considered that the mechanical properties of the solder joint are affected in many ways under the high welding heat input,and the nugget diameter is only one of them.(3)In the single and double pulse tests,the failure behaviors of solder joints are both interfacial fracture(IF)and partial interfacial fracture(PIF).IF fracture is intergranular cracking and gradually forms macroscopic cracks;PIF fracture has intergranular cracking and transgranular fracture,but All are brittle fracture.The EDS spectrum results show that Mn enrichment occurs due to the massive accumulation of grain boundaries near the nugget line,and the columnar crystals in the nugget zone form segregation bands of C and Mn,which leads to the deterioration of the intergranular strength.The failure behavior of the solder joint is determined by the diameter of the nugget,the thickness of the plate,the alloy elements of the base metal and the weld structure.(4)The structure of the heat-affected zone after welding is mainly composed of retained austenite,martensite and a small amount of ferrite,and the nugget zone is almost entirely martensite.The hardness test shows that the heat-affected zone has a higher hardness than the nugget zone,the maximum is about 600±10Hv,and the average hardness of the nugget zone is about 540±20Hv.Large fluctuations in hardness indicate lower strength,which is consistent with the tensile results.(5)In the tempering test,the tempering process of the pulse tempering current cannot improve the mechanical properties of the weld.In contrast,the tempering process in the furnace significantly improves the comprehensive mechanical properties of the solder joints.After being kept at 250℃ for 10 min,20min and 30 min,the mechanical properties of the specimens were significantly improved compared with the welded samples,the increase was 98.1%,106% and 47.3% respectively.(6)After tempering in the furnace,the structure of the weld zone has changed accordingly.When the holding time is 10 min and 20 min,the structure of the heataffected zone and the nugget zone are mainly composed of tempered martensite,and the retained austenite is being tempered.When the bainite transformation occurs,the residual stress in the weld zone is basically eliminated.When the holding time is 30 minutes,cementite is precipitated,which reduces the mechanical properties of the solder joints.(7)After tempering,the failure mode of the solder joint has changed,from IF to PIF and PF mode.The PIF fracture still has a certain degree of brittleness in part of the heat-affected zone.The fracture shows a mixed fracture.The overall fracture strength is about 20 KN.The fracture morphology is composed of river patterns and shallow dimples.The PF mode has high toughness,the peak load is 28.57 KN,and the fracture morphology is dominated by "wine cup-shaped" dimples. |
