Microstructure, Anisotropy and Formability Evolution of an Annealed AISI 430 Stainless Steel Sheet

Abstract

Stainless steel represents a large family of iron-based alloys of which the principal characteristic is its good corrosion resistance. Nowadays, ferritic stainless steels (FSSs) are widely accepted for use in a wide variety of applications, as, for example, structural framework and body panels for buses and cars, [1] not to mention that it is much cheaper than 300 series stainless steels. One of its disadvantages is that it is more susceptible to corrosion than the 300 series grades because its main alloying element is chromium. [2] The FSS has an excellent resistance to stress corrosion cracks, pitting, and crevice-type of corrosions. Furthermore, this stainless-steel grade shows a low strain hardening rate, which allows them to be easily deformed by any metal working process.Steels are strengthened by various possible mechanisms of hardening. Hardening by plastic deformation is one of the most important methods of strengthening metals. When a crystalline solid is deformed plastically, it becomes more resistant, and a greater stress is required for additional deformation. [3] The hardening in a crystalline structure occurs because these materials deform plastically by the movement of dislocations. When a polycrystal is deformed in rolling, forming, drawing, and so on, the randomly oriented grains will slip on their appropriate glide systems and rotate from their initial conditions, under the constraint from the neighboring grains. [3] This leads to a strong increase in dislocation density, which in turn produces a higher yield strength. Dislocation sources create new dislocations during plastic deformation and serve to increase the dislocation density. This hardens the material, a process called work hardening, strain hardening, or sometimes strengthening by cold working. [4] The formability test is a widely applied technique used in the manufacturing of metal sheet parts. The formability process includes a variety of types and operating conditions. An important tool used to evaluate the formability of metal sheets is the forming limit diagram (FLD). The FLD concept was introduced by Keeler and Backofen [5] and Goodwin. [6] The FLD represents the maximum values of the principal strains (major ε 1 and minor ε 2 ) that one metal sheet can withstand before the onset of necking. Currently, there are many different mechanical tests to determine the experimental FLD. Among these, the most commonly used tests are the Marciniak [7] and Nakazima [8] tests, whereby the sheet metal is plastically strained by a flat tip or a round-tip punch, respectively. [9] Both tests use different sample geometries to induce the different strain paths, whereby the principal difference is the punch geometry (see Appendix A).Various approaches are used to determine the onset of necking and the corresponding limit strain. In one approach, grid circles and square patterns are applied on the sample, as proposed by