Effect Of Annealing Temperature On Microstructure, Textures And Properties Of High-strength IF Steel

High strength IF steel is a kind of high strength deep-drawn steel sheet for automobile which is strengthened by adding solid solution strengthening elements, such as P, Si and Mn in the interstitial-free steel. It is one of the most widely used automotive steels because it is a typical steel sheet with good formability and higher strength..Due to its special performance, it is very important to research and develop high strength IF steel for saving energy and protecting the environment which is advocated by modern society.In this paper, two kinds of hot rolled steel plate of high strength IF steels(P210P1 and P250P1) which are produced by Pansteel are used as the experimental material. The research focus on the effect of cold reduction ratio and Annealing process(annealing temperature and holding time) on the microstructure、texture and mechanical properties of high-strength IF steel. Firstly, the hot-rolled steel plates was pickled and cold rolled with four different cold reduction ratio(55%, 65%, 75% and 85%, respectively) in laboratory. Secondly, the cold-rolled steel sheets were annealed by salt-bath annealing method with different annealing temperature(780℃,800℃,820℃,840℃,860℃,880℃,resectively)and different holding time(30S、60S、90S、120S,respectively). Lastly, The microstructure, texture and mechanical properties were evaluated to study the effect of cold-rolled reduction ratio and annealing process.Microstructure studies show that the recrystallization grains become finer with the in-c rease of cold reduction ratio and the irregularly shaped grains gradually become equiaxed ones resulting in the uniform distribution of the grains. The grain size increases with incr easing annealing temperature and the grain shape have a tendency to be from the pancake d into the equiaxed. But the effect of holding time on the recrystallization grain size is not evident. The test results of mechanical properties show that the increase of cold reduction ratio is helpful to improve the strength and formability of high strength IF steel. Althoug h the strength of high strength IF steel is decreased, it is absolutely beneficial to improve the r value and formability of high strength IF steel. When the cold reduction ratio is75%~85% and the annealing temperature is around 860℃, optimally matching of strengthand plasticity is obtained. The research of texture shows that {112}<110>, {111}<110>, {111}<112> and {001}<110> are the main components of texture in the cold rolled high streng th IF steel. The {111}<112> orientation density of γ fiber texture is almost the strongest after annealing. Increasing the annealing temperature in the proper way is beneficialto im prove the recrystallization texture of high strength IF steel and to obtain optimally formab ility. The above research results can partly guide the industrial production of high strengt h IF steel.