Research On Microstructure And Mechanical Properties Of Thermal Simulation DP780 High Strength Steel CGHAZ Based On RSM And QGA

Dual-phase high strength steel has been widely used in automobile manufacturing to achieve the purpose of energy conservation and emission reduction due to its light weight and high strength.However,the performance of the coarse grained heat affected zone(CGHAZ)were reduced severely after welding due to the poor weld ability caused by the large carbon equivalent.The thermal simulation method was adopted to simulate the CGHAZ of DP780 high strength steel;the microstructure and mechanical properties of the CGHAZ were evaluated by combining response surface method(RSM)and quantum genetic algorithm(QGA).The factors mainly included the peak temperature,heating rate and cooling rate and the ranges were:1100?1250?,55?80?/min and 1000?1500?/min,respectively.The parameters were divided into 12 groups by the uniform design.The metallographic analysis system Olympus BX51M was adopted to observe the microstructure of the simulated CGHAZ.Results showed that the simulated CGHAZ mainly includes the original austenite grain boundaries(AGB),ferrite(F),martensite(M),upper bainite(UB),and tempered martensite(TM).When the peak temperature,heating rate,and cooling rate were 1100?,60?/min,and 1300?/min,respectively,the minimum average grain size of CGHAZ was 42.89?m,the average grain size of CGHAZ increased with the increasing of the peak temperature.At the same time,the heating rate and cooling rate had less effect on the average grain size.In terms of mechanical properties,when the peak temperature,heating rate and cooling rate were 1195?,75?/min and 1500?/min,respectively,the tensile strength reached a maximum of 747.08 MPa;when the peak temperature,heating rate,and cooling rate were 1100?,60?/min,and 1300?/min,respectively,the elongation reached a maximum of 6.74%,at this time,the maximum impact energy was7.83J;the hardness of simulated CGHAZ increased relative to the base material;as the grain size increased,the tensile strength of CGHAZ showed constant first and then decreased,while the elongation and impact energy decreased with grain size increasing.However,the hardness was not affected by grain size.The fracture SEM morphology indicated that the fracture type of CGHAZ was brittle fracture,and the fractures mainly included quasi-cleavage fractures and intergranular fracture.The mass fraction of carbon in the CGHAZ increased significantly compared with the base material,according to EDS analysis.The RSM was used to establish response surface models of the parameters and the mechanical properties.The results showed that the response surface models fitted well with the original data,and the smallest value of R~2 was still 0.9728.It can be seen from the response surface model and the analysis of variance(ANOVA)that the peak temperature had the most significant effect on the mechanical properties of CGHAZ,followed by cooling rate,and the heating rate had the least.The response surface models were solved by the QGA,it can be seen that when the peak temperature,heating rate and cooling rate were 1100?,55?/min and 1000?/min,respectively,the maximum impact energy is 7.89J,which increased 0.71%,related to the maximum value obtained from simulation;when the peak temperature,heating rate and cooling rate were 1146?,63.4?/min and 1105?/min,respectively,the maximum tensile strength was 756.29 MPa,which increased 2.6%,related to the maximum simulation value.The verification results showed that the error between the solution and the verification were only1.93%and1.99%.They also provided theoretical guidance for actual production.