Effect Of Rare Earth On The Microstructure Evolution In The Coiling Process Of 600MPa Low Carbon Microalloy Steel

In recent years,low-carbon microalloyed high-strength steels have been widely used in the automotive field,especially in the environment of environmental protection,energy conservation and emission reduction.Therefore,the use of microalloyed high-strength steel sheets instead of low-strength thick steel sheets is the only way to reduce the weight of automobiles,which not only reduces the fuel consumption of automobiles but also reduces their load capacity.However,since the research and development,there are still some shortcomings in the research and development of low-carbon microalloyed steel in China,which are embodied in the following aspects: insufficient process optimization,high smelting cost and unstable organization.On the one hand,the addition of rare earths forms high-melting-point rare earth inclusions,which acts as an effective nucleation substrate to promote non-uniform nucleation of molten steel: on the other hand,rare earths will affect the redistribution of solute,causing the components to be too cold;The evolution of tissue during processing.Therefore,this paper studies the influence of rare earth on the microstructure evolution of hot-rolled coiling process of low-carbon microalloyed highstrength steel,so as to optimize the actual production process and have important significance for improving the performance of the material.In this paper,the microalloyed high-strength steel with rare earth(1#)and rare earth(2#)was added as the research object.The 25 kg vacuum induction furnace was used in the Hebei Iron and Steel Research Institute to use the thermal expansion instrument to change the phase transition point at different cooling rates.The measurement was carried out,and the microstructure at different cooling rates was analyzed by OM and SEM,and the hardness at different cooling rates was measured by a hardness tester.Combined with phase transition point and tissue analysis,the effect of rare earth on microstructure evolution at different cooling rates was obtained.Secondly,the inclusions were counted at different positions of the forged sample,and the inclusions were scanned at the middle(1/2)to study the effect of rare earth on the metallurgical quality of the material.Finally,combined with the on-site production process,using thermal simulation methods,different hot rolling crimpingprocesses were developed,and the high-strength La low-carbon microalloys were analyzed by modern analytical methods such as metallographic microscope,scanning electron microscope,transmission electron microscope and X-ray diffractometer.The influence of the law of tissue evolution during the coiling process of steel and the role of rare earths.The results show that in the CCT curve,the addition of rare earth La inhibits the formation of pearlite,prolongs the incubation period of pearlite transformation,and reduces the pearlite and bainite transformation zones,making the ferrite and pearlite regions as a whole.Moving to the right,the bainite region as a whole moves to the left;after the addition of rare earth La,the rare earth sulfide and oxide composite inclusions are formed in the steel,and the morphology of the inclusions in the steel also changes significantly,from the original irregular spindle shape.It is transformed into a circular shape and an elliptical shape.Compared with the four coiling temperatures of 500 ??550 ??600??and 650 ?under the same holding time,it is found that the microstructure changes from pearlite +bainite as the temperature increases.Ferrite + pearlite + bainite: Since the microstructure has the least pearlite at 500 °C,it has little effect on the morphology and type of precipitates of the second phase.After carbon film replicating,it is found that there is no NbC and Ti(C,N)precipitates exist in the rare earth steel,but the NbC and TiN composite phases are precipitated in the rare earth steel(1#).