Effect Of Tempering On Microstructure And Mechanical Properties Of Low Carbon Microalloyed Bridge Steel Q420qE

In order to investigate the effect of tempering on microstructure and mechanical properties oflow carbon microalloyed bridge steel Q420qE, a series of tempering tests at different temperatureand time are carried out on two batches of Q420qE steel produced through different return redprocess system. The microstructure observation and mechanical properties test of rolled andtempered samples are carried out, and the changes of microstructure and mechanical properties ofQ420qE steel before and after tempering are analyzed. The main results obtained are as follows:A large number of acicular ferrite, granular bainite and M-A islands formed at lowre-reddening temperature during the controlled cooling process after rolling of Q420qE steel,resulting in high strength and yield ratio. The microstructure consisted of acicular ferrite, M-Aislands and degenerated pearlite at high return red temperature. Compared with the steel plate atlow re-reddening temperature, the microstructure become coarse, the number and the size of hardphase of M-A islands organization increased, the degenerated pearlite began to appear, theproportion of hard phase increased and the yield ratio decreased significantly.The decomposition of hard phase of M-A islands resulted in the decrease of tensile strengthafter the two kinds of Q420qE steel plate tempering at550℃for a short time. The tensilestrength increased due to the slow decomposition of M-A islands and the precipitationstrengthening effect of precipitated phase as the tempering time prolonged. The yield strength andyield ratio showed a rising trend in the whole process of tempering because that the precipitationof fine precipitated phase increased the yield strength continuously. The tensile strength increasedfirstly and then reduced and the yield strength and yield ratio showed a rising trend when twokinds of steel plate tempered under different temperatures(450~650℃), resulted from the M-Aislands decomposition and the strengthening effect of fine precipitates precipitation.The stable M-A islands and degenerated pearlite of the microstructure Q420qE steel plateprocessed at high re-reddening temperature, played a good role of hard phase, maintaining arelatively low yield strength ratio. However, because of a large number of M-A islands with largesize, the impact energy of rolled steel plate was low. And the impact energy increasedsignificantly because that a large number of M-A islands decomposed after tempering. The results of this thesis provided a theoretical basis for making return red and temperingprocess in actual production for the low yield ratio of Q420qE bridge steel.