| The application of high strength steels in automotive industry has increased with the unceasing thorough of research on energy-saving, emission reduction and automobile lightweight program. All of these days, most of the high strength steels home and abroad are produced by adding expensive elements, such as Mo, Cr, Ni, Cu and V, causing the cost increase. In this paper, the Nb-Ti microalloying technology and TMCP process are used to develop690MPa grade high strength hot-rolled low carbon bainitic automotive wheel steel, according with the development theme of low-cost, high-performance and energy-saving. Through the study of phase transformation, the precipitating behavior of the second phase particles and strengthening-toughening mechanisms after different TMCP process, the processing technologies are optimized for providing the necessary theoretical basis and process reference. The main works of this paper are as follows:(1) The influence of different cooling rate on microstructure was studied by measuring the static CCT curve. The ferrite, pearlite and granular bainite were obtained at the cooling rate between0.1℃/s and0.5℃/s, the microstructure was granular bainite at the cooling rate between3℃/s and5℃/s, the martensite was observed as the cooling rate increasing to10℃/s, the volume fractions of martensite would increase with increasing cooling rate.(2) The influence of cooling rate, coiling temperature and holding time on microstructure after deformation were studied by the experiment of process simulation. The proeutectoid ferrite transformation was promoted by deformation, the bainite single phase would be obtained when the cooling rate between15℃/s and25℃/s and the coiling temperature between450℃and540℃.The grain was refined as the cooling rate increasing and coiling temperature decreasing, the M/A constituent changed from bulky into small, dispersive and granular. The incubation period of pearlite was very long, while the incubation period of bainite was short, the rate of transformation was fast.(3) The microstructure analysis of hot-rolled test steels showed that the proeutectoid ferrite was obtained when the finishing roll temperature was780℃, the bainite volume fraction increased as the cooling rate increasing and the coiling temperature decreasing.(4) The fine microstructure was studied by SEM, TEM, EPMA and Nanoindentor. As the cooing rate increasing and coiling temperature decreasing, the M/A constituent changed from bulky, strip and triangle into small, dispersive and granular, and the difference of carbon content and hardness between carbides and matrix would reduce, the formation of continuous cementite along the grain boundary was inhibited, the bainitic laths were refined, the dislocation density was increased, but the precipitation of (Nb,Ti)(C,N) particles would be suppressed, so the precipitation strengthen effect was weakened.(5) Through the analyzing the comprehensive mechanical properties of the test steels, the performance of tensile, cold-bending, hole-expansioning, impacting experiment were all qualified. The main strengthening mechanism were microstructure, grain refining, precipitation, dislocation, solid solution strengthen.(6) According to influence of TMCP parameters on the microstructures and mechanical properties, two sets of process routes are worked out:The reheated temperature is1200℃, the start temperature of rough roll is between1100-1150℃.(a) The way of acquiring high strength bainite single phase steel for wheel rim:The start temperature of fine roll is between920℃and950℃, the finishing roll temperature is between800℃and830℃, the cooling rate is between30℃/s and35℃/s, the coiling temperature is between450℃and500℃.(b) The way of acquiring high hole-expansioning ferrite-bainite steel for wheel spoke:The start temperature of fine roll is between900℃and930℃, the finishing roll temperature is between760℃and800℃, the cooling rate is between25℃/s and30℃/s, the coiling temperature is between550℃and600℃. |
PDF Full Text Request