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A low-carbon TRIP seamless steel tube,which is expected to be used in the hydroforming process,was successfully fabricated using piercing,cold-drawing and two-stage heat treatment process.The two-stage heat treatment is one crucial step because it significantly affects the microstructure and mechanical properties of TRIP seamless steel tube.In order to obtain the TRIP seamless steel tube with high hydroformability,several different heat treatment processes were conducted.The effects of heat treatment conditions(intercritical annealing(IA)and isothermal bainite treatment(IBT))on the TRIP seamless steel tube hydroformability which was determined by free hydraulic bulge test were analyzed.Two different internal pressure boosting velocities of 0.2and 0.5MPa/s of free hydraulic bulge tests were adopted to determine the effective stress vs.effective strain curve of TRIP seamless steel tube.The results showed that for the predetermined IA condition,the maximum bulge height increased,but the maximum burst internal pressure decreased,with the increase of IBT holding time from 4to 6min.For the predetermined IBT condition,the maximum bulge height decreased,but the maximum burst internal pressure increased,with the increase of IA holding time from 5to 10min.By analyzing the free hydraulic bulge test results,it was found that the maximum bulge heights of TRIP seamless steel tubes with the internal pressure boosting velocity of 0.5 MPa/s were higher than those when the internal pressure boosting velocity was 0.2MPa/s.This means that an appropriate deformation rate should be chosen to obtain the optimal hydroformability of TRIP seamless steel tube.In addition,the effective stress vs.effective strain curves of TRIP seamless steel tubes were obtained with free hydraulic bulge test.
A low-carbon TRIP seamless steel tube, which is expected to be used in the hydroforming process, was successfully fabricated using piercing, cold-drawing and two-stage heat treatment process. Two-stage heat treatment is one crucial step because it affects the microstructure and mechanical properties of TRIP seamless steel tube. order to obtain the the TRIP seamless steel tube with high hydroformability, several different heat treatment processes were conducted. these effects of heat treatment conditions (intercritical annealing (IA) and isothermal bainite treatment ( IBT)) on the TRIP seamless steel tube hydroformability which was determined by free hydraulic bulge test was analyzed. Two different internal pressure boosting velocities of 0.2 and 0.5 MPa / s of free hydraulic bulge tests were adopted to determine the effective stress vs. efferent strain curve of TRIP seamless steel tube. The results showed that for the predetermined IA condition, the maximum bulge height increased, but the m with the increase of IBT holding time from 4to 6min.For the predetermined IBT condition, the maximum bulge height decreased, but the maximum burst internal pressure increased, with the increase of IA holding time from 5to 10min.By analyzing the free hydraulic bulge test results, it was found that the maximum bulge heights of TRIP seamless steel tubes with the internal pressure boosting velocity of 0.5 MPa / s were higher than those when the internal pressure boosting velocity was 0.2 MPa / s.This means that an appropriate deformation rate should be chosen to obtain the optimal hydroformability of TRIP seamless steel tube. In addition, the effective stress vs.ffective strain curves of TRIP seamless steel tubes were obtained with a free hydraulic bulge test.