Hot deformation behavior and microstructure evolution of a stabilized high-Cr ferritic stainless steel

Abstract

The hot deformation behavior and static microstructure evolution of a 21Cr stabilized ferritic stainless steel was studied using axisymmetric hot compression tests on a Gleeble 1500 thermomechanical simulator. The deformation was carried out at 950–1050 °C to strains of 0.2 to 0.6 using strain rates of 0.01, 0.1 and 1 s⁻¹. The compression was followed by a holding period of 0 to 180 s in order to study the static recrystallization kinetics. The electron backscatter diffraction (EBSD) technique was used in analyzing the resultant microstructures. A constitutive equation that well describes the flow stress as a function of strain, strain rate and temperature was developed. The active dynamic restoration mechanism was found to depend on the Zener–Hollomon parameter, such that continuous dynamic recrystallization was observed under low Zener–Hollomon parameter conditions but under high Zener–Hollomon parameter microstructures were dynamically recovered, and no dynamic formation of new grains occurred. Static recrystallization resulted in little or no grain refinement, and further, strain did not have an accelerating effect on the static recrystallization kinetics beyond the strain of 0.4.

Introduction

The high and volatile prices of nickel and molybdenum have made the high chromium (Cr>18 wt%) ferritic stainless steels an attractive alternative to austenitic Cr–Ni and Cr–Ni–Mo and ferritic Cr–Mo grades in general stainless steel applications such as in building materials or kitchen appliances. These applications often require good deep drawability and resistance to roping or ridging. These properties can be improved by increasing the amount of the favorable recrystallization texture in the cold rolled and annealed sheet and by breaking up colonies of similarly oriented grains [1], [2]. Hot rolling and hot band annealing affect both of these. Because of their composition, these steels are ferritic at all temperatures below the solidification temperature, and the austenite–ferrite phase transformation does not occur during hot deformation. The detrimental textures inherited from the solidification structure can be weakened in the hot rolling stage only by recrystallization.

Recovery and recrystallization are competing restoration processes which take place during hot deformation and during the inter-pass times of hot rolling leading to the softening of the material. The stacking fault energy of ferritic stainless steels is relatively high and hence dislocations can cross-slip readily which promotes recovery over recrystallization [3]. Hence, dynamic recovery is very intense in these steels. Dynamic recrystallization can occur by three different mechanisms: discontinuous dynamic recrystallization, continuous dynamic recrystallization and geometric dynamic recrystallization [4]. Discontinuous dynamic recrystallization operates by nucleation and growth and is generally considered not to take place in metals with high stacking fault energy because of the intense recovery, even though it has been shown that its occurrence is dependent on the purity of the material, e.g. [5], and on the deformation conditions, i.e. the Zener–Hollomon parameter [6]. In continuous dynamic recrystallization, no nucleation phase is present and the new grains form by the gradual increase of the misorientation of low-angle boundaries. Geometric dynamic recrystallization operates at high strains ∼5–10, where the grains are fragmented into new grains during the deformation [7]. In ferritic stainless steels, due to intense dynamic recovery, conventional discontinuous dynamic recrystallization is usually considered as unfeasible. Instead, continuous dynamic recrystallization by the coalescence of subgrain boundaries has been shown by a number of authors to occur under certain conditions during hot deformation of carbon steels in the ferrite range and of ferritic stainless steels [8], [9], [10], [11], [12], [13].

The dynamic restoration mechanisms depend on the deformation conditions, as described above, and thereby the relations between flow behavior and dynamic microstructure evolution are very important factors affecting the hot rolling loads as well as the following static recrystallization and texture evolution during hot band annealing. The dislocation storage, i.e. the amount of stored energy, serves as the driving force for static recrystallization during the post hot rolling annealing. The amount of stored energy of a bulk material can be measured quantitatively by thermal analysis methods, such as differential scanning calorimetry (DSC) e.g. [14], [15], [16]. Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) [16], [17], microhardness e.g. [16], [18] and the line broadening of X-ray diffraction (XRD) [19] are methods for qualitatively analyzing the dislocation density and thereby the local stored energy. Further, the amount of stored energy can be estimated qualitatively from the amount of work hardening in the flow stress curves [20]. The dynamic restoration processes greatly affect the amount of stored energy and hence the static restoration mechanisms and kinetics. The flow behavior and the microstructure evolution of austenite in static recrystallization process during hot deformation are extensively studied subjects. Also the deformation of ferrite in interstitial free and low-carbon steels has been investigated by many authors in warm working temperature range, e.g. [21], [22], but in these studies, the deformation took place below the austenite–ferrite phase transformation temperature A₁. Therefore, the restoration mechanisms may differ from those experienced in the hot deformation temperature range of 950–1050 °C. Zhang et al. [23] investigated the effect of shear bands formed during hot rolling on static recrystallization rate in a 21%Cr ferritic stainless steel, however they did not pay attention to the flow behavior of the steel.

In this study, the hot deformation behavior of stabilized high-Cr ferritic stainless steel was investigated by hot compression tests with the final aim of clarifying the hot rolling schedules, which might improve the final product quality. The purpose was to identify the dynamic restoration processes taking place during hot deformation and to develop constitutive equations to predict the flow stress under various Zener–Hollomon parameter conditions. Also, the effect of Zener–Hollomon parameter on the static restoration kinetics was studied in order to explore ways to promote recrystallization after the hot rolling stage.