Microstructural evolution and deformation behavior of twinning-induced plasticity (TWIP) steel during wire drawing

Abstract

The effect of wire drawing on the microstructural evolution and deformation behavior of Fe-Mn-Al-C twinning-induced plasticity (TWIP) steel has been investigated. The inhomogeneities of the stress state, texture, microstructure, and mechanical properties were clarified over the cross section of the drawn wire with the aid of numerical simulation, Schmid factor analysis, and electron backscatter diffraction (EBSD) techniques. The analysis of texture in drawn wire shows that a mixture of <111> and <100> fiber texture was developed with strain; however, the distribution of <111> and <100> fibers was inhomogeneous along the radial direction of wire due to uneven strain distribution and different stress state along the radial direction. It has also been shown that the morphology, volume fraction, and variant system of twins as well as twinning rate were dependent on the imposed stress state. The surface area was subjected to larger strain and more complex stress state involving compression, shear, and tension than the center area, resulting in a larger twin volume fraction and more twin variants in the former than in the latter at all the strain levels. While the surface area was saturated with twins at an early stage of drawing, the center area was not saturated with twins even at fracture, implying that the cracks were initiated at the surface area due to the exhaustion of ductility, especially due to twinning. Based on these results, it is suggested that imposing uniform strain distribution along the radial direction of wire by the control of processing conditions such as die angle and amount of reduction per pass is necessary to increase the drawing limit of TWIP steel. Wire were drawn at the different reduction of area (RA) per pass, 10%, 20%, and 30% to understand the effect of RA per pass. It shows that a larger RA per pass resulted in a higher yield strength, smaller elongation, and higher <111> texture than a smaller RA per pass at all strain levels. Although inhomogeneities in microstructure and mechanical properties along the radial direction decreased with increasing RA per pass, there existed an optimum RA per pass for maximum drawing limit: 20% RA per pass improved the drawing limit by about 30% as compared to the 10% and 30% RAs per pass. Insufficient penetration of strain from surface to center at small RA per pass (e.g., 10%) and high friction and unsound metal flow at large RA per pass (e.g., 30%) all resulted in heterogeneous microstructure and mechanical properties along the radial direction of drawn wire. The caliber rolling has been conducted to find alternative methods of wire drawing process for wire rod products. Caliber rolling process had higher LABs and twin density than wire drawing process. Moreover, caliber rolling process imposed higher stain with slightly uniform strain distribution on wire than wire drawing process, indicating that caliber rolling can manufacture high strength wires more effectively. Caliber rolling process was suitable to make materials of high strength and fine microstructures for materials deformed and hardened by twinning mechanism such as TWIP steels due to the characteristics of imposing severe strain with multi-direction, multi-pass with different shape at each pass, and alternating the loading direction between passes. Therefore, we proposed that complex stress states were needed to gain high strength in TWIP steels because twinning behaviors are more susceptible to stress state than slip.