| At present,the high-speed guardrail steel uses the ordinary strength level steel with the yield strength within 345 MPa.This kind of steel has some problems such as low strength,thick material and poor safety performance and so on.Using new high-strength guardrail steel to instead of the ordinary strength guardrail steel can effectively solve these problems.This study takes high-strength Ti-Nb composite micro-alloyed high-speed guardrail steel as the research object,and mainly through the following research,in order to provide a theoretical basis for the industrial production of Ti-Nb micro-alloyed new high-speed guardrail steel.(1)The precipitation behavior of the microalloyed second phase in the test steel was investigated.By studying the precipitation behavior of the second phase of microalloy in test steel,it was found that the solid solution content of Ti,Nb and C in steel decreased with the decrease of temperature,in which a large amount of Ti precipitated at high temperature and Nb almost completely precipitated.The volume fraction of precipitated phase(Ti,Nb)C in the test steel increased with the decrease of temperature.When the(Ti,Nb)C particles nucleated on the ferrite dislocation line,the optimal nucleation temperature was 620 ℃.(2)The continuous cooling tissue transformation pattern of the test steel was revealed.By means of thermal simulation experiment and microstructure observation,the phase transition law of the test steel was studied.The results showed that different types of phase transition occurred in the test steel at different cooling rates.When the cooling rate was0.5 ℃/s,the ferritic-pearlite phase transition occurred.When the cooling rate was higher than 1 ℃/s,bainitic phase transition began.When the cooling rate was 10-20 ℃/s,there occurred both ferritic-bainitic and martensitic transitions.When the cooling rate was more than 30 ℃/s,bainite-martensite phase transition occurred.The hardness of the test steel increased with the increase of cooling rate.(Ti,Nb)C precipitates existed in steel at different cooling rates,and were dispersed in steel.At low cooling rates,the volume fraction of the steel was large and the size of the precipitated phase was small,which had a certain effect of precipitation strengthening.(3)The mechanism of the effect of coiling temperature on the tissue properties of the test steel was clarified.The effect of coiling temperature on the microstructure and properties of Ti-Nb microalloyed high-speed guardrail steel was systematically studied.It was found that when the coiling temperature increased from 570 ℃ to 670 ℃,the strength of the test steel increased first and then decreased.When the coiling temperature was 620 ℃,the test steel obtained the best comprehensive mechanical properties.At different coiling temperatures,the variation of the strength of the test steel was mainly due to the different contributions of fine grain strengthening and precipitation strengthening.There was an N-W orientation relationship between fine(Ti,Nb)C particles and ferritic matrix,and a large number of nano-sized(Ti,Nb)C particles were formed during and after coiling. |
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