**1. Introduction**

Austenitic stainless steels (ASSs) that generally have excellent corrosion resistance, good plasticity, and a high strain hardening coefficient are widely used in food, petrochemicals, and the nuclear industry. However, their low yield strength limits their application in structural engineering and automotive industry [1,2]. The development of high strength-high ductility ASSs is considered an attractive approach to expanding their use and has been extensively studied.

There are various methods to improve the strength of ASSs, including cold deformation strengthening, bake hardening strengthening, grain size strengthening, et cetera. [3]. Due to the high strain hardening coefficient of ASSs, cold deformation strengthening with a simpler and easier process is considered an effective method and more suitable for industrial application [4,5]. The high yield strengths of 570–926 MPa and 554–735 MPa with reasonable elongations of ~43–22% and ~35–19% were achieved in 304 [6] and 316 L [7] ASSs through 10–30% cold rolling, respectively. However, the plastic straining induced ductility loss was more pronounced than the strength increase for the ASSs as the cold rolling reduction further increased, exhibiting a typical "banana-shaped" strength-ductility trade-off [8].

The mechanical properties of cold-rolled (CR) ASSs are determined by their deformation microstructures. Normally, the austenite phase in ASSs is not a stable phase and will transform into strain-induced martensite during the plastic deformation process [9]. Martensite phase generally exhibits a higher strength and lower plasticity. According to the mix law described by Huang et al [10] for two phase materials, the strengths of two phase materials are determined by the volume fractions

and the corresponding strengths of the two phases. The transformation from austenite phase into martensite phase will improve the strength of CR ASSs [6,11]. The phase transformation in ASSs is controlled by the stability of austenite that is determined by Md30 temperature (where 50% ά-martensite is present after 30% tensile deformation) [12]. Due to the higher content of alloy elements, some ASSs, like 316 ASSs, have a lower Md30 temperature and higher stability of austenite that inhibit the phase transformation during plastic deformation. Previous studies indicated that only 46–67% of austenite phase in 316 ASSs can be transformed into strain-induced martensite, even after 80–90% cold rolling [13–15]. Interestingly, these steels also exhibited the tendency of significant loss in plastic straining induced ductility [13–15], which cannot be explained by the mix law. Therefore, it is not enough to only consider the effect of volume fraction of the strain-induced martensite on the mechanical properties of ASSs, and the untransformed austenite evolution and its effect on mechanical properties should be considered at the same time. However, the evolution of untransformed austenite phase with increasing cold rolling reduction is less studied. The influence of untransformed austenite in CR ASSs on mechanical properties is not clear and needs in-depth studies.

In this study, the electron backscattered diffraction (EBSD) was chosen to study the evolution of untransformed austenite phase in CR 316LN ASS as the cold reduction increased from 10% to 40%. Evaluating the evolution of untransformed austenite in CR ASSs by using EBSD is rarely reported and is the innovation of this paper. Furthermore, transmission electron microscopy (TEM) was used to study the evolution of the deformation microstructures as the cold rolling reduction increased. The effect of cold deformation on the mechanical properties of 316LN ASS was studied by analyzing the relationship between deformation microstructures and mechanical properties.

#### **2. Experimental Procedures**

The chemical composition, the stacking fault energy (SFE) based equation of Brofman and Ansell [12], and the Md30 temperature based on Nohara's equation [16] of 3mm-thick 316LN ASS used in this study, are shown in Table 1. The strips were cold rolled in a pilot plant with thickness reductions from 10% to 90% reduction at room temperature. The volume fraction of martensite in CR samples was determined by X-ray diffraction (XRD, Panalytical, Almelo, The Netherlands). The Vickers hardness of samples was determined by measuring more than 20 points in different regions of the samples using a microhardness tester with a 0.5 kg load (HV-1000B, Matsuzawa, Tokyo, Japan). The structures in original 316LN ASS and CR samples were evaluated using an OM (Axioplan2 Imaging Zeiss, Göttingen, Germany). For the CR samples, an etchant of two solutions at a 1:1 ratio was used. The first solution consists of 0.20 g sodium-metabisulfate in 100 mL distilled water and the second consists of 10 mL hydrochloric acid in 100 mL distilled water. The original samples were mechanically polished and then electrochemically etched with 60% nitric acid solution. Microstructures of CR samples were examined by TEM (JEM-2100, JEOL, Tokyo, Japan). Thin foils were prepared by twin-jet electropolishing of 3 mm disks, punched from the specimens using a solution of 10% perchloric acid in acetic acid as the electrolyte. EBSD was utilized to evaluate the microstructures and grain boundary of 0–40% CR samples. For that purpose, the samples were electrochemically etched with 20% perchloric acid alcohol solution operated at 25 ◦C with an applied potential of 15 V. The size of the austenite structure in EBSD micrographs was determined by analyzing grain boundary misorientation over 2◦. The original and CR strips were machined to make tensile samples with a profile of 140 × 20 mm and a gage length of 65 mm. The uniaxial tensile tests were carried out at room temperature at an engineering strain rate of 5 × <sup>10</sup>−<sup>4</sup> <sup>s</sup>−<sup>1</sup> and each specimen was tested three times to obtain an average value for each mechanical property.

**Table 1.** Chemical compositions, stacking fault energy (SFE), and Md30 of the investigated steel (wt, %).

