Microstructure Evolution Characteristics And Bending Orthopedic Of DP980 Dual Phase Steel Welded Pipe By Resistance Heating

In order to solve the energy crisis by energy conservation and emission reduction,the automotive industry and the aerospace industries start to accelerate the process of lightweight,and lightweight materials such as high-strength steels,aluminum alloys,magnesium alloys,and titanium alloys have been rapidly used and developed.Dual-phase steel is widely used in the production and processing of automobile complex parts and components,for the good forming performance and high strength plastic.In order to reduce spring back during the production process of dual phase steel parts,this paper proposes a method of thermal orthopedic by resistance heating,combining thermal orthodontic and heat treatment together in one operation to control the shape and performance of parts.In this paper,temperature distribution of the pipe,microstructure evolution and characters of the DP980 dual phase steel base metal and welded seam during the process of resistance heating will be investigated.Spring back simulation was explored using ABAQUS,and thermal orthopedic experiments results of bending pipe were analyzed.The influence of current density on the heating rate was studied by resistance heating experiments of DP980.The effect of electrode arrangement,heating rate,heating temperature,and the length of the pipe on the axial and circumferential temperature distribution of the pipe was investigated.The experiments adopts the conventional electrode arrangement.The faster the heating rate,the longer the pipe length,the more uniform axial temperature field was obtained.The temperature of the outside of the bending pipe is lower than that of the inside during the resistance heating process,increasing the current density can reduce the temperature difference.When the outer temperature was heated to 630°C using current density 16A/mm2,the temperature difference of circumferential direction reaches the minimum(33°C).The effects of heating rate,heating temperature,quenching temperature and cooling methods on microstructure and character of base metal and welded seam of DP980 dual phase steel welded pipe during resistance heating were invested by tensile,microhardness and metallographic observation.The mechanism of influence of microstructure evolution on the mechanical properties was obtained.When the heating temperature is lower than Ac1(~710℃),the martensite in the base metal was converted to tempered sorbite,the strength was decreased,and the weld center Vickers hardness is higher than in the base metal.When the heating temperature is higher than Ac1,the content of martensite in the base metal increases,the strength increases,the elongation decreases slightly,the hardness in the base metal increases,and the weld center hardness is lower than the base metal,but higher than that in original base metal.The yield strength of the material heated to 750℃ and quenched at 700℃ is 1004.46 MPa and the elongation is 13.36% which is closest to the original pipe.The effects of heating temperature and strain rate on deformation behaviors were investigated by hot tensile test.Spring back of the thermal orthopedic of the bending pipe under uniform and non-uniform temperature was simulated by ABAQUS.As the heating temperature increases,the springback of the bending pipe in uniform temperature decreases.In the circumferential non-uniform temperature,the springback decreases first and then increases as the circumferential temperature difference increases.The accuracy of the bending pipe with different bending angles(35-95°)was analyzed through the thermo orthopedic experiment.The effect of cooling methods on the mechanical properties of the bending pipe after thermal orthopedic was studied.A reasonable cooling method for the control of microstructure and mechanical properties evolution was obtained.The springback angel of 35° bending pipe is 3.425°,which can be improved by changing the die.The springback angles of the 65° and 95° bending pipe are 0.285° and 0.415°,respectively.The deflection of the inner and outer bend radius is less than 1mm.