Structure and properties of low-carbon steel after twist extrusion

Transcription

1 ice science Research Article Received 2/7/24 Accepted 7//24 Published online 2//24 Keywords: mechanical properties/metals/microscopy ICE Publishing: All rights reserved A. Zavdoveev PhD* Research Associate, Donetsk Institute for Physics and Engineering NAS of Ukraine, Donetsk, Ukraine 2 E. Pashinska EngD Leading Scientist, Donetsk Institute for Physics and Engineering NAS of Ukraine, Donetsk, Ukraine 3 S. Dobatkin EngD Leading Scientist, Baikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences, Moscow, Russia N. N. Belousov PhD Senior Scientist, Donetsk Institute for Physics and Engineering NAS of Ukraine, Donetsk, Ukraine 6 A. Maksakova MSc PhD Student, Donetsk Institute for Physics and Engineering NAS of Ukraine, Donetsk, Ukraine 7 F. Glazunov MSc Research Assistant, Donetsk Institute for Physics and Engineering NAS of Ukraine, Donetsk, Ukraine 4 V. Varyukhin DSc Corr. Member of NAS of Ukraine, Director of DonPhTI NASU, Donetsk Institute for Physics and Engineering NAS of Ukraine, Donetsk, Ukraine The new developing technologies of metal formation, based on simple shear, are known as severe plastic deformation (SPD) technologies. They are actively used for improving the mechanical properties of metals. Even complex systems such as steels have not been adequately investigated for SPD process. However, studies are in progress as is reflected from the publications in the literature. Due to insufficient knowledge of the structural changes, occurring in low-carbon steels during SPD, there is a need for a more detailed consideration by modern methods. This paper discusses the characteristics of the formation of structure and texture of low-carbon steels subjected to warm twist extrusion (TE). A relatively new method, which makes possible a detailed study of the structure of metals, is electron backscattered diffraction (EBSD). EBSD has shown that warm TE increases the structure isotropy, thus leading to significant fragmentation and activation of the mechanisms of dynamic polygonization, recrystallization and grain-boundary sliding. These structural features lead to hardening of the material by times, while maintaining a high level of plasticity.. Introduction An advantage of severe plastic deformation (SPD) is that it permits the processing of materials with unique complex properties by combining high strength and plasticity as distinct from classical methods of metal forming. Numerous structural studies have been carried out for SPD-processed materials, such as Al, Ti, Cu and their alloys. *Corresponding author address: zavdoveev@fti.dn.ua

2 Emerging Materials Research The production of bulk submicrocrystal-line and nanocrystalline materials with unique operational, physical and mechanical properties 4 has become one of the most important problems. In particular, this concerns low-carbon steels that are widely used for industrial production and construction. SPD is a promising method for the production of submicrocrystal and nanocrystal materials. A well-known SPD method is twist extrusion (TE). The cases when TE is applied to steels are isolated incidents,6 contrary to equal-channel angular pressing.4 It has been shown earlier7,8 that warm TE results in the reduction of texture at the cross-section, formation of highly dispersed structure with the prevailing uniaxial grains and well-defined unequal grain sizes. However, structure anisotropy, at the longitudinal cross-section, has not yet been studied. µm This work aims to study the specific features of formation of the structure, texture and properties of low-carbon steels in various directions relative to the deformation axis during warm TE. µm (d) µm (e) Figure. Microstructure of low-carbon steel 2G2S. (a,c,e) EBSD maps of band contrast; (b,d,f) TEM, ; (a,b) initial state; (c,d) warm TE, cross-section; (e,f) warm TE, longitudinal section 2 (f)

3 2. Experimental procedure Low-carbon construction steel was used as a test material (weight %: 24 C, 66 Mn, 2 Si, 4 Cr, 24 Ni, Al, 6 Cu, 4 S and 96 4 Fe); it was annealed at 9 С ( h) and subjected to warm TE. The deformation method of TE was used in which a prismatic sample is extruded through a die with a twist channel. TE was performed at a billet temperature of Т = 4 С and the temperature of the equipment was Т = 32 С. The extrusion pressure (P) did not exceed 2 МРа and the applied backpressure was МРа. The value of strain accumulated per pass was е 2, and the value of strain accumulated per three passes was е 6; a detailed description of the conditions of deformation has been presented in Pashinska et al. 7 Mechanical tests were carried out by using the method of upset. A microstructural analysis was performed using transmission electron microscopy (TEM) of a carbon replica and EBSD. Preparation of the samples for EBSD is reported in detail in Pashinska et al Results and discussion The analyses of evolution of structure, presented in Figures (f), demonstrate that warm TE results in substantial fragmentation of ferrite and pearlite structural components. The average grain size of ferrite, after deformation, is reduced from to μm. According to EBSD contrast maps, (Figures (а) (e)), it is seen that the structure, after warm TE, is modified. Along with fine grains, coarse uniaxial grains are observed at both longitudinal and transversal sections. TEM data (Figures (f)) show that warm TE results in the modification of perlite morphology. Cementite plates are substantially fragmented after the deformation Frequency: % TE longitudinal section Number of grains TE longitudinal d: µm Aspect ratio i: arbitrary units 4 2 TE, grains > µm, grains > µm, grains < µm, grains < µm Frequency: % TE longitudinal section 2 3 d/<d> Misorientation angle: (d) Figure 2. Quantitative data of the structure analysis of low-carbon steel 2G2S. Distribution of grain sizes; aspect ratio of grains; distribution of grains in normalized coordinates; (d) misorientation angle distribution 3

4 Emerging Materials Research (Figure ); at the longitudinal cross-section, the rest of the plates are directed along the slip bands (Figure (f)). A quantitative analysis of the size distribution of ferrite grains, presented in Figure 2(а), indicates the presence of two types of grains, that is, coarse grains of 3 μm and fine grains of μm at both longitudinal and transversal sections. At the longitudinal section, the number of grains, with the size exceeding μm, was higher than that at the cross-section. The analysis of form factor has shown that elongated grains prevail at the longitudinal section (the form factor is equal to 2), whereas the cross-section is characterized mostly by uniaxial grains (Figure 2). This fact is an evidence of inhomogeneity of the structure at the longitudinal and transversal sections of the low-carbon steel after warm TE that should affect the mechanical characteristics in the course of further deformation. Figure 2(d) shows the distribution of grain-boundary angles. It is seen that, in addition to the formation of a large share of low-angle boundaries (LABs), a substantial amount of high-angle boundaries (HABs) are formed in the course of warm TE. It is seen that the number of HABs at warm TE is comparable to the share of HABs at the annealed state. These data indicate active processes of both fragmentation and polygonization, that is, increasing of LABs corresponds to polygonization processes and the remaining of HABs The analyses of statistics of the grain size were conducted for two groups (below μm and above μm) in normalized coordinates (grain size was divided by the average value, and the frequency of occurrence of a grain size was divided by the total number of grains). This gives some indications of the mechanisms of structure formation in the course of warm TE (Figure 2). The choice of the separation of grains by size was determined by the fact that the presence of uniaxial grains of less than μm in size is a necessary criterion for the development of the mechanism of grain-boundary sliding.9 It is seen in Figure 2 that coarse and fine grains are similar in distribution at the original state. After the deformation, the distribution of coarse grains is substantially modified and is distinct from the distribution of fine grains, because the fraction of the grains with size below the average increases, due to the process of fragmentation of coarse grains. Stable characteristics of the distribution of fine grains are indicative of the processes of grain-boundary sliding during warm TE. Increasing volume fraction of grains with ratio d/<d> = indicates active processes of grain refinement. True stress: MPa ε: % 2 2 Figure 3. Stress strain curve for low-carbon steel 2G2S, upset tests: initial state, 2 TE axis is perpendicular to the axis of upsetting, 3 TE axis is parallel to the axis of upsetting 4 Figure 4. TEM microstructure of low-carbon steel after TE. Magnification,. upset test; TE with subsequent upset test, TE axis perpendicular to upset test axis; TE with subsequent upset test, TE axis parallel to upset test axis

5 corresponds to fragmentation processes. Besides, EBSD allows quantitative analysis of the distribution of recrystallized grains. 7 ly, the share of recrystallized grains is 3%, which is reduced to 7 % and % after warm TE at the cross-section and the longitudinal section, respectively. Because the deformation was performed at the temperature well below the threshold of recrystallization in steel, the existence of recrystallized grains, after warm TE, implies current dynamic recrystallization. 9 According to Pashinska et al., 8 warm TE results in the reduction of the intensity of texture and texture blurring occurs at the crosssection. Texture tests at the longitudinal section demonstrate that warm TE was followed by the reduced intensity of the texture peaks. These structural peculiarities enhance the strength by times along with high level of plasticity (Figure 3). It was found that, at the subsequent deformation by the upset method along the extrusion direction, a traditional behavior of the hardening curve was observed, and the less-slope strengthening curve was a characteristic of the deformation across the extrusion direction (Figure 3, curve 2). This behavior can be possibly attributed to Bauschinger effect, multiple-dislocation sliding and features of the structure formed in the course of TE (Figures 4 4). Microhardness tests of low-carbon steel, after warm TE and consequent upset, reveal additional strain hardening from H µ = 2 MPa (after TE) to H µ = 299 MPa (after TE and upset). Furthermore, the deformation of low-carbon steel, after warm TE, is more appropriate in the section perpendicular to TE axis. 4. Conclusions It has been demonstrated in this work that warm TE results in the fragmentation of the structure components of low-carbon steel (i.e. ferrite and pearlite). Ferrite grains, after deformation, are reduced by a factor of 3 (from to μm) and characterized by a substantial share of HABs. The structure is distinguished by an inequigranular structure with two types of grains: coarse grains of 3 μm in size and fine grains of μm. At the same time, structural inhomogeneity is observed where the elongated grains, with the form factor equal to 2, prevail at the longitudinal section contrary to the cross-section with the prevailing uniaxial grains. It has been shown that, in the course of warm TE, the structure of low-carbon steel formed is affected by the sequential development of the mechanisms of dynamic recrystallization and polygonization, grain-boundary sliding and fragmentation. The structural features lead to an increase in the strength by a factor of with the conserved high level of plasticity. A combination of SPD through TE and upset method gives an opportunity to achieve additional improvements in the mechanical properties. Acknowledgments The author is very grateful to Burkhovetskii V. V. for his help in experiments and discussions. The work was done with the support of the grant of NAS of Ukraine for young researchers Peculiarities of formation of submicrocrystal structure, texture and properties of low-carbon steel produced by twist extrusion number 3U3684. REFERENCES. Valiev, R.; Alexandrov, I. Nanostructured Materials Obtained by Severe Plastic Deformation. Moscow: Logos, Pashinska, E. Physical and Mechanical Base of Structure Evolution at Combined Plastic Deformation. Donetsk: Veber, Estrin, Y.; Vinogradov, A. Extreme grain refinement by severe plastic deformation: a wealth of challenging science. Acta Materialia 23, 6, Dobatkin, S.; Odesskiy, P.; Pippan, R. Warm and hot ECAP of low-carbon steels. Metals 24,, 9.. Beygelzimer, Y.; Varyukhin, V.; Synkov, S.; Orlov, D. Useful properties of twist extrusion. 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