Regularities of deformation microstructures in ferritic stainless steels during large strain cold working

The recent studies on grain refinement in several Cr-Ni ferritic stainless steels during large strain deformation at room temperature are critically reviewed. The paper is focussed on the mechanism of structural changes that is responsible for ultrafine grain development. It is concluded that the development of submicrocrystalline structures results from a kind of strain-induced continuous reaction; that is formation of deformation subboundaries and gradual increase in their misorientations up to typical values of ordinary grain boundaries. Following the rapid reduction at an early deformation, the transverse grain/subgrain size smoothly approaches its final value of the order of 0.1-0.2 μm, depending on alloying extent. The increase in average subboundary misorientation upon processing can be related to the strain as Δθ ∼ Kε, with K of about 5°. Therefore, the average misorientation between deformation grains/subgrains is about 20° after processing to total strain of 4 and the corresponding microstructure is composed of 50 % of high-angle grain boundaries. The structural refinement of starting material can significantly accelerate the kinetics of ultrafine grain development during subsequent plastic working, while the processing method has a little effect on developing microstructures.