Investigation On Hot Deformation Behavior Of Ferritic Stainless Steel

Since stainless steels were discovered, austenitic stainless steel has been most commonly in use. With the global shortage of nickel, low-nickel or nickel-free ferritic stainless steel has been taken seriously and widely used in practice all over the world. Compared with austenitic stainless, there are advantages of ferritic stainless steel, for instance low-cost, low thermal dilatation, excellent high temperature oxidation and corrosion resistance. Ferritic stainless steel is used for construction, heat exchanger, turbine, superhigh pressure power generation, decorative trim, and household appliances.Not only is 430 ferritic stainless steel(430FSS) represent of ferrite grade, but also the most widely used type of ferrite grade. Because of fine low-temperature processing formability, it is applied to architectural decoration and kinds of pipe production. Because of its unique properties, such as low work-hardening tendency, high temperature resistance, chloride stress corrosion and pitting resistance,409L ferritic stainless steel(409LFSS) has become top-priority in car exhaust system.Thin strip casting has been the most concerned technology in many iron and steel industry, but the cracks in the stainless steel slab will seriously affect the following processing, as well as the quality of the final product. The following defects, in hot-rolled coil such as the edge damage, edge cracks, will also make obvious effect in yield and economic benefits. In this paper, to provide the basic date for the development of casting technology, the elevated-temperature mechanical properties and high temperature deformation behavior of the as-cast 409L ferritic stainless steel and 430 ferritic stainless steel were investiganted.The hot deformation behavior of ferritic stainless steel was investigated by means of thermo-mechanical simulator, using THERMORSTER-W simulator, at a temperature ranging from 950℃to 1150℃with stain rate 0.01 s-1,0.05 s-1,0.1 s-1,0.5 s-1,1.0s-1 and 2.5s-1 separatively. The effect of microstructure,temperature,strain rate and processing map on the flow behavior of ferritic stainless steel at reduction ratio 50% were discussed.The results show that:(1) The change of ferrite region in 409LFSS is similar with 430FSS's. There are three parts, recovery, part-recrystallization and recrystallization. And the trend is the higher the temperatrue is, the bigger the gain size of ferrite is; the higher the strain rate is, the smaller the gain size of ferrite is.(2) At the certain strain rate, there is a decline of peak stress with increase of temperature. However at the same deformation temperature, the trend between peak stress and strain rate is different completely:with the strain rate value increases the hot deformation peak stress increases. The flow curves of 409LFSS are different from the single-phase ferritic stainless steel or single-phase austenitic stainless steel.(3) The apparent activation energy and stress exponent are about 310 kJ/mol and 5.51, respectively. The hot deformation equation of 409LFSS is The apparent activation energy and stress exponent are about 344 kJ/mol and 1.08, respectively. The hot deformation equation of 409LFSS is(4) The stress exponent n and the apparent activation energy Q show that the hot deformation mechanism of 409LFSS is inter-grain climb spread, and 430FSS is grain boundary diffusion. The softening mechanism of ferritic stainless steel varies with Z parameter.(5) The differences of carbon content and chromium content lead to the differences of the strain influence on the shape of the hot processing map.