The Development Of Cold Rolling Process For Fine Grain High-strength IF Steel

With the development of automobile industry, the automobile plate plays more and more important role on the steel production day by day, and the cold rolling automobile sheet has attracted a great deal of attention because of its good deep-drawing performance. In order to protect the environment and save energy, the development of the high-strength cold automotible plate with excellent deep-drawing performance is very important.This paper is based on the study of fine grain high-strength IF steel. The raw materials were the hot rolling plate rolled byΦ450 rolling mills of North-eastern University. The cold rolling exerpriments were carried out by theΦ160 reversing two-high cold rolling mill and the cold rolling plate were annealed by SX2-10 box furnace, finally the microstructure were observed and the properties were tested. The results are follows:1. The microstructure and properties under different cold rolling reduction ratio were studied. It is found that when the samples were annealed at the temperature of 820℃, with the increasing of deformation, ferrite grains became finer, tensile and yield strength increased and elongation and r value increased firstly and then decreased; While at the temperature of 850℃, ferrite grains, elongation and r value were increased firstly and then decreased, but tensile and yield strength were decreased firstly and then increased. Through the contrast of microstructure and properties under different reduction ratio at two annealing temperatures, the top value of r was reached at the reduction from 77.5% to 80%.2. The microstructure and properties with different annealing process were studied and it is found that with the higher the annealing temperature and the longer the annealing time, ferrite grains became bigger and tensile and yield strength is hardly chang, elongation and r value are increased. Properties of experimental steel were effected largely by annealing temperature, but the soaking time is hardly effected it after complete recystallization.3. By the way of observation the morphology of second phase particle, analysis the composition and measurement of the ferrite grain size, it is found that there is a few of small and disperse NbC precipitates and a little large NbNC precipitates in the steel, the ferrite grain size grade is about 7.5-11, the grain size grade of excellent performance steel is about 9.5.4. According to the study on fine grain high-strength IF steel's cold-rolling and annealing experiments, the cold rolling process in this paper are follows: reduction is 80%, the annealing temperature is 820℃and the holding time is 90min. The tension strength of the steel reaches 359MPa, the yield and tensile strength ratio is 0.48, the elongation ratio is 46%, the work hardening index n is 0.28 and plastic strain ratio r=1.48.