X2CrNi12 ferritic stainless steel has a wide range of application prospects in the railway transportation, construction, and automobile fields due to its excellent properties. The properties of X2CrNi12 ferritic stainless steel can be further improved by cold-rolling and subsequent annealing treatment. The purpose of this work is to investigate the effect of cold-rolling reduction on the microstructure, texture and corrosion properties of the recrystallized X2CrNi12 ferritic stainless steel by using SEM, TEM, EBSD and electrochemical testing technology. The results show that the crystal orientation characteristics of the cold-rolled sheet could be inherited into the annealed sheet. The higher cold-rolling reduction could promote the deformed grains rotating into the {111}<uvw> orientation, increasing storage energy and driving force for recrystallization, which could reduce the recrystallized grain size. The orientation densities of α-fiber and γ-fiber were low at 50% cold-rolling reduction. After recrystallization annealing, a large number of grains with random orientation could be produced, and the texture strength was weakened. When the cold-rolling reduction rose to 90%, the γ-fiber texture at {111}<110> was strengthened and the α-fibers, particularly the {112}<110> component, were weakened after recrystallisation annealing, which could improve the formability of the steels. The proportions of special boundaries, i.e., low-angle grain boundaries and low-Σ CSL boundaries, among the grain boundary distribution of the recrystallized X2CrNi12 stainless steel were higher when the reduction was 90%, especially when the annealing temperature was 770 °C. Additionally, the proportion of LAGBs and low-Σ CSL boundaries were 53% and 7.43%, respectively, which improves the corrosion resistance of the matrix, showing the best corrosion resistance.
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It can be seen that rolling and annealing treatments have important effects on microstructure evolution and properties. However, the effect of cold-rolling reduction on the recrystallization structure, texture evolution, grain boundary distribution and corrosion resistance of low Cr ferritic stainless steel is rarely studied, especially for X2CrNi12 ferritic stainless steel. In this paper, the microstructure, texture evolution and grain boundary distribution of different recrystallization-annealed specimens with 50% and 90% cold-rolling reductions were systematically studied, and the corresponding corrosion properties were analyzed. This has important theoretical and practical significance to improve corrosion resistance and expand the range of applications.
In our previous work, the effects of annealing temperature on the microstructure, mechanical property and anticorrosion behavior of X2CrNi12 ferritic stainless steel were systematically investigated [ 6 ]. The precipitation behavior of (Fe, Cr) 23 C 6 carbides and martensite and their effects on mechanical and corrosion properties were revealed. However, the formability was not involved. The formability of ferritic stainless steel closely depends on the texture evolution during the annealing process, and the strong γ-fiber (<111>//ND) texture could improve the formability of the stainless steels [ 7 , 8 , 9 ]. Abreu et al. [ 10 ] studied AISI-444 ferritic stainless steel and found that high temperature annealing was beneficial to improve the strength of γ-fiber texture at {111}<112> and reduce the surface ridging. Huh et al. [ 11 ] introduced intermediate annealing after cold-rolling, which led to a weaker rolling texture in the bcc metals, which had less pronounced texture evolution. Thus, the recrystallization annealing process could promote the typical γ-fiber texture, improving the formability of the steel. Zhang et al. [ 12 ] claimed that the existence of shear bands could enhance the recrystallization nucleation rate during the hot-rolling and annealing process, which intensified the {111} textures in the final sheet. Rodrigues et al. [ 13 ] pointed out that reducing the size of the original grains of the specimen can enhance the γ-fiber component among the recrystallization textures. In addition, the distribution of the grain boundary has a great influence on the mechanical and corrosion properties of materials [ 14 , 15 , 16 , 17 ]. Yan et al. [ 14 ] found that the fractions of the Σ3 and Σ13b grain boundaries of Nb + Ti-stabilized ferritic stainless steel increased by two-step cold-rolling and annealing treatment, which improved the corrosion resistance of stainless steel. Han et al. [ 15 ] reported that adding low-Σ CSLs, e.g., Σ1, Σ3, and 2-CSL and 3-CSL triple junctions, through breaking up the connectivity of the random boundary networks, could improve the mechanical properties of materials. Shimada et al. [ 16 ] increased the proportion of grain boundaries with Σ ≤ 29 to make the material have high resistance to intergranular corrosion.
As a kind of prospective low chromium ferritic stainless steel, X2CrNi12 is inexpensive compared to austenitic stainless steels and medium-high chromium ferritic stainless steels, while it shows excellent corrosion resistance compared with carbon steels and weathering steels [ 1 , 2 ]. Besides, it has brought increased applications in railway transportation, building structures and automobile exhaust systems with its excellent weldability and mechanical properties [ 3 , 4 , 5 ]. However, the lower Cr in the matrix leads to the poor corrosion property of X2CrNi12 compared to austenitic stainless steel and medium- and high-chromium ferritic stainless steels, and its application is restricted in environments requiring high corrosion resistance. In addition, the formability of the ferritic stainless steels is limited compared to the austenitic and low-carbon steels, which leads to easy thinning during deep drawing, which is not conductive to the degree of deep drawing deformation and product surface quality. Therefore, it is of great significance to carry out extensive work to improve the corrosion performance and formability of X2CrNi12 ferritic stainless steel.
The tested specimen (working electrode) was first immersed in the 3.5 wt.% NaCl solution for 1800 s to reach a stable open circuit potential (OCP). The potentiodynamic polarization test was then started. The potential changed from −1.5 V to 0.5 V, and the scan rate of the polarization test was 1 mV/s. The scan frequency of electrochemical impedance spectroscopy (EIS) was 10 −2 Hz–10 5 Hz, and the disturbance amplitude was 10 mV. The ZsimpWin V3.61 software (AMETEK Scientific Instruments, Berwyn, IL, USA) was used to analyze the impedance data. In order to ensure the accuracy of the experiment, three specimens were measured at each condition.
The surfaces of the test specimens were sanded with SiC sandpaper to 2000#, and then polished and ultrasonically cleaned in alcohol. Before the electrochemical test, the specimens were encapsulated in epoxy resin; the test surface was composed of RD and TD, leaving a 1 cm 2 working surface. The electrochemical test was performed by using a CHI660D three-electrode electrochemical workstation. The counter electrode (CE) was the Pt electrode, the reference electrode (RE) was the saturated calomel electrode (SCE), and the working electrode (WE) was the specimen. Since the stainless steel is prone to localized corrosion in solutions containing Cl − , such as marine environments, a 3.5 wt.% NaCl electrolyte was used to obtain the marine environment and to have a deeper understanding of the actual corrosion of the alloy in the real working environment.
The microstructures of specimens were observed by a JSM-6510 scanning electron microscope (SEM, JEOL, Tokyo, Japan) with an accelerating voltage of 20 kV and a Tecnai G2 F30 transmission electron microscope (TEM, FEI, Amsterdam, The Netherlands). The SEM specimens were ground, polished and then etched by 3 mL HNO 3 + 9 mL HCl + 12 mL H 2 O. The TEM specimens were ground to 50 μm, punched to Φ3 mm discs and ion-milled. To further analyze the textures and microstructures, the rolling surface as defined by rolling direction (RD) and transverse direction (TD) were studied by electron backscattered diffraction (EBSD). Specimens were electro-polished by an applied potential of 30 V with 5% perchloric acid + 95% alcohol at −25 °C for 60 s. EBSD measurements were carried out by using the JSM-7200F field emission scanning electron microscope (FESEM, JEOL, Tokyo, Japan) with EBSD detector. The step of EBSD was 0.5 µm, and the EBSD data were analyzed by using the Channel 5 software (Oxford Instruments, Oxford, UK). The low-angle grain boundaries (LAGBs) were defined as 2° ≤ θ ≤ 15°, while the high-angle grain boundaries (HAGBs) were defined as θ > 15°.
The material used was commercial hot-rolled X2CrNi12 ferritic stainless steel plates with a thickness of 6.2 mm; the composition is shown in . It was cold-rolled to a thickness of 3.2 mm and 0.46 mm on a Φ200 cold-rolling mill with hydraulic tension, with a corresponding reduction of 50% and 90%, respectively. Then, the cold-rolled sheets were heated to 720 °C, 740 °C and 770 °C for 30 min, followed by air cooling. The specimens were expressed as 50%-deformed, 50%-720 °C, 50%-740 °C, 50%-770 °C, 90%-deformed, 90%-720 °C, 90%-740 °C and 90%-770 °C, respectively.
The orientation maps of the specimens obtained by EBSD at 50% and 90% cold-rolling reduction followed by annealing treatment with different temperatures are shown in . It can be seen that the grains in the cold-rolling sheet are mainly blue and red, which indicates that it is mainly composed of deformed grains with {111}<uvw> and {001}<uvw> orientations ( a,e). However, there are more deformed grains with {111}<uvw> orientation in the 90% deformed cold-rolling sheet, and there are some deformed grains with {112}<uvw> orientation. This is because uneven deformation gives the grains different degrees of fragmentation during the cold-rolling process, which produced a lot of orientation gradient area. In addition, shear bands and broken grains preferentially appear in the {111}<uvw> orientation [20], and the addition of nucleation sites is beneficial for grain refinement. b–d show that ferrite grains recrystallized after heat treatment. In the 50% reduction cold-rolled annealing plate, grains are mainly randomly distributed, and the number of recrystallized grains in the {111}<uvw> orientation is relatively small. As shown in f–h, the crystal orientation in the cold-rolled annealed plate with 90% reduction is mainly composed of {111}<uvw> and partial {112}<uvw> components, and the grain size is small. Ray et al. [21] illustrated that the stored energy (E) in a cold-rolling sheet could be arranged in the order: E{111}<uvw> > E{112}<uvw> > E{110}<110> > E{110}<001>. The large cold-rolling reduction can promote the deformed orientation of {111}<uvw>, which provides greater storage energy and driving force for recrystallization. It shows that the orientation of the sheets could be inherited by the cold-rolled and annealed sheets in some extent, and it has a positive influence on the recrystallized grain orientation.
Open in a separate windowis a schematic of the φ2 = 45° orientation distribution function (ODF) section that contains the texture components discussed in this work. The main texture components in the rolled and annealed ferritic stainless steel sheets are distributed on the α and γ orientation lines. All the orientations that belong to the α-fiber have their <110> axes parallel to the RD, where the main texture components on the α orientation line (φ1 = 0°, Φ = 0~90°, φ2 = 45°) are (001)[11¯0], (112)[11¯0], (223)[11¯0], (332)[11¯0] and (111)[11¯0], etc. In addition, the texture components related to deep drawing properties, e.g., (334)[48¯3] and (554)[22¯5¯], are also easily formed during the annealing process [10].
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Open in a separate windowThe properties of ferritic stainless steel are closely related to the recrystallization texture, and the cold-rolling textures have a great influence on the recrystallization texture [22]. The aggregation state of the grain orientation can be seen from the ODF. The ODFs (φ2 = 45° sections) of specimens at 50% and 90% deformation with different annealing temperatures are reproduced in a–h, respectively. The cold-rolling texture of ferritic stainless steels is mainly composed of the weak γ-fiber and strong α-fiber components. Evidently, the specimen of the 50%-deformed mainly contains (001)[11¯0], (001)[1¯1¯0], (334)[483¯] and (554)[22¯5¯] components, and the maximum intensity f(g)max = 7.16 ( a). The specimen of the 50%-720 °C mainly contains the weak α-fiber at (001)[11¯0] orientation and the weak γ-fiber at (111)[121¯] ( b) orientation. The specimen of the 50%-740 °C mainly contains the strong α-fiber at (112)[11¯0] and the weak γ-fiber at (111)[123¯] ( c). According to the research results of Raabe et al. [23], {111}<112> and {334}<483¯> have an orientation relationship of 26° <110>, which is close to the orientation relation of 27° <110> of the Σ19a coincidence site lattice (CSL) boundary, and the Ʃ19a grain boundary has a high moving rate. Owing to the high mobility of the Ʃ19a boundary, the {111}<112> nuclei can selectively grow into the {111}<112> texture. The specimen of the 50%-770 °C is mainly composed of weak textures at (001)[11¯0], (001)[1¯1¯0] and (112)[11¯1¯] orientations, and the maximum intensity f(g)max = 4.62 ( d).
Open in a separate windowWhen the cold-rolling reduction is 90%, the specimen of the 90%-deformed steel is mainly composed of the strong α-fiber at (112)[11¯0] with f(g)max = 12.1 and the weak γ-fiber at (111)[12¯1] and (111)[11¯2], and the maximum intensity f(g)max = 6.56 ( e). When the annealing temperature is 720 °C, the textures of (112)[11¯0] and (001)[1¯1¯0] disappear, forming the (001)[11¯0] and (114)[2¯2¯1] textures ( f). The α-fiber texture is weakened and the γ-fiber texture is enhanced. The highest intensity among the γ-fiber texture is at {111}〈110〉. As shown in g, the texture of the 90%-740 °C steel is consistent with the specimen at 90%-720 °C, but the maximum intensity of the α-fiber texture at (001)[11¯0] increases to 8.37, and the intensity of the γ-fiber textures increase to 13.1 for (111)[11¯0] and 12.6 for (111)[01¯1], respectively. According to the literature, the formation of high-density γ-fiber texture in a stainless steel plate is beneficial to improve the formability of the material [24,25]. As annealing temperature increased to 770 °C, the α-fiber texture disappeared and the strength of the γ-fiber texture decreased gradually. The specimen of the 90%-770 °C is mainly composed of the strong γ-fiber at (111)[12¯1] and (111)[1¯1¯2].
presents the orientation densities of the texture α and γ orientation lines at 50% and 90% cold-rolling reduction with different annealing temperatures. On the whole, the orientation density of the α-fiber and γ-fiber textures is smaller under 50% cold-rolling reduction, a great deal of randomly oriented grains will be produced after annealing, and the overall strength of the texture is low. The α-fiber and γ-fiber textures of cold-rolling sheet at 90% reduction are larger. After the recrystallization annealing, the texture strength increased, and the orientation density of α-fiber and γ-fiber recrystallization texture also improved. When heavily cold-rolling steels is recrystallisation-annealed, the γ-fiber texture is strengthened, while the α-fiber (particularly the {112}<110> component) is decreased. According to the literature [26,27], the density of the α-fiber texture after the recrystallization annealing is weakened at {001}<110> and {112}<110>, γ-fiber texture strength increases, and the maximum orientation density values appear around {111}<112> and {111}<110>. This is basically consistent with the findings of our work.
Open in a separate windowWith the rapid development of social economy in recent years, the demands for austenitic stainless steels have shown an explosive growth trend worldwide. Meanwhile, with their own unique advantages, such as high plasticity and toughness, strong corrosion resistance and good cold-forming performance at room temperature, austenitic stainless steels have been widely used in various fields [ 1 ], such as medical equipment, aerospace, daily life, and the petrochemical industry. Particularly, some austenitic stainless steels, such as 305 austenitic stainless steel, have been adopted in the production of high-end electronic products. Therefore, these kinds of steels have attracted much attention worldwide, especially in China where some high-end electronic products need to be imported, and among which the 5G communications metal mask templates are all dependent on imports.
However, what needs to be pointed out is that due to the low yield strength, the applications of 305 austenitic stainless steel in the production of high-end electronic products are extremely limited. As a consequence, the improvement of yield strength is the first concern in the research of 305 austenitic stainless steel for high-end electronic product applications.
It is well known that cold rolling (during which severe plastic deformation usually occur) is a feasible and effective method to improve the yield strength of metals and alloys. Therefore, a lot of attention has been paid on the research of the cold rolling of austenitic stainless steels by the scholars all over the world [ 2 3 ]. The results indicate that deformation twins and slip lines could be formed during cold rolling, and micro-twins could impede dislocation movement, resulting in the increase in yield strength. The development of ultrafine grain structure in metastable austenitic stainless steel grades is of utmost importance as it provides the potential for utilizing high-strength austenitic stainless steels in structural applications. Therefore, the origin and morphology of martensite in cold-rolled austenitic stainless steels have also been extensively studied. Olson et al. and Tamura [ 4 ] discussed a phenomenological model that could be used to explain the formation of strain-induced martensite caused by cold rolling; also the results of the study showed that the martensitic morphology in low-carbon steel, special steel, and austenitic stainless steel after cold rolling deformation mainly consists of dislocated forests, dislocated walls and tangles, large deformed lath martensite, and dislocated cytoplasmic martensite. Jana, Weyman, Coleman and West discussed the mechanism of phase transformation leading to changes in the magnetic properties of alloys [ 5 ]. Guy et al. investigated the phase transformation mechanism from a crystallographic point of view, while Breedis and Krauss reported the formation of twins and laminar dislocations due to phase transformation [ 6 ]. Recent studies have demonstrated that the formation of ultrafine austenite grains in austenitic stainless steels can be achieved by cold roll annealing, which improves the strength of the material, while other studies on substable austenite have demonstrated the feasibility of this research method [ 7 ]. Lee et al. [ 8 ] investigated pure nickel by using a cold rolling and annealing process, and found that annealing experiments carried out on it at the appropriate temperatures were effective in refining the grains to improve the strength. Rangaraju et al. [ 9 ] similarly refined the grains by annealing experiments. There are many studies on grain refinement of austenitic stainless steel using cold rolled annealing process to improve the strength of the material, but very few studies have been carried out to regulate both strength and permeability during the annealing process.
Based on the results of the above studies, it can be seen that the strength of austenitic stainless steel can be effectively improved by cold rolling, but the resulting martensitic phase will have an impact on the magnetic permeability. Although there are many studies on the cold rolling and annealing process of austenitic stainless steel, there are few studies on the regulation of strength and magnetic permeability at the same time, so a more in-depth study is necessary. Also, there is no or very little research on cold roll annealing of 305 stainless steel, so there is a need for research on 305 stainless steel. In addition, due to the special chemical composition of 305 austenitic stainless steel, deformation-induced martensitic transformation will occur during cold processing, resulting in changes in the magnetic permeability and mechanical properties of this material. If the magnetic permeability exceeds a certain limit, the transmission of signals will be shielded, thus it is also essential to regulate the magnetic properties of 305 stainless steel material during cold deformation process.
In this paper, the cold rolling and annealing experiments of 305 austenitic stainless steel were carried out to study the microstructure evolution, mechanical properties and magnetic properties of this steel under different conditions, so as to provide theoretical basis and technical support for the development of new stainless steel.
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