Hot ductility behavior of high-Mn austenitic Fe–22Mn–1.5Al–1.5Si–0.45C TWIP steels microalloyed with Ti and V

Abstract

This research work studies the influence of microalloying elements (Ti and V) and the solidification route on the hot ductility behavior of high-manganese TWIP steels. Uniaxial hot tensile tests in the temperature range of 700–1100 °C under a constant strain rate of 10⁻³ s⁻¹ were carried out. Hot ductility as a function of reduction of area (RA) showed a significant improvement in the V-microalloyed TWIP steel, when compared to a non-microalloyed TWIP steel with a similar composition, in the intermediate temperature range of 800–900 °C. The highest value of 86% RA is attributed to the onset of dynamic recrystallization (DRX) near to the fracture tip. On the other hand, Ti addition to TWIP steel did not exhibit any improvement on the hot ductility, resulting in the worst hot ductility behavior, with a maximum value of 34% RA. The TWIP steels solidified in metallic ingot molds (MM) showed higher peak stress (σ_p) and ductility values than the sand mold (SM) cast ingots at low and intermediate temperatures, respectively, which is associated with the finer microstructure generated during solidification. Grain boundary sliding was recognized as the failure mechanism associated with second-phase particles precipitated at the grain boundaries, which play the role of nucleation and propagation sites of void-cracks.

Introduction

During the last decade, high-Mn austenitic twinning induced plasticity (TWIP) steels have been the object of intense worldwide scientific activity [1], because these new kind of advanced steels combine exceptional properties of strength and ductility, which are particularly promising [2]. TWIP steels have been widely considered for many industrial applications, particularly in the automotive sector, high speed-trains and building industries [3], [4].

The hot working behavior of high-Mn TWIP steels is of primary importance to assess manufacturing routes, consisting of different steps of hot forming and cooling until room temperature. However, their hot work hardening and microstructural evolution controlled by thermal activated processes have not drawn much attention compared to their cold-working behavior [5]. The development of a new technology for the production of high-Mn austenitic TWP steels requires knowledge about their behavior during solidification and hot plastic deformation [6]. Thus, grain growth and grain size control during solidification process is not only of intrinsic interest but also has a great technological significance. For metallic alloys the solidification involves the formation of crystals with a preferred growth direction due to heat flux from the system to the surroundings, where most of the casting processes result in a dendritic morphology [7]. In this way, the properties of metals and alloys are highly influenced by their microstructure, which can be modified by alloying/microalloying elements, heat treatment or plastic deformation. Accordingly with metal forming processes, in a microstructure with fine grains the dislocations can move only a short distance before encountering an obstacle, i.e. grain boundary. Therefore, a metal is stronger when it has finer grain microstructure [8]. It is well known [9], [10] that temperature (T), strain (ε) and strain rate $(\dot{\epsilon })$ are some of the most important parameters on determining the hot deformation behavior of metals and alloys, particularly in austenitic steels [11]. The hot deformation properties of austenitic alloys are mainly determined by the stacking fault energy (SFE). A high SFE facilitates dislocation cross-slip, which in turn promotes dynamic recovery (DRV) during hot deformation. The DRV process brings about high ductility and low deformation stresses during hot working. The high ductility is obtained after a monotonic hardening stage, which is followed by a steady-state plateau in the flow behavior when dislocation generation is compensated by dislocation annihilation. In contrast, low SFE promotes dynamic recrystallization (DRX), where the dislocations are eliminated and deformed grains are replaced by new grains with low dislocation density. The DRX occurs when sufficient driving force is present to start the recrystallization process, i.e., when the applied strain is larger than a critical strain (ε_c) [12].

On the other hand, it is well documented that the third generation of advanced high strength steels (AHSS) utilize complex interaction of solid solution hardening, precipitation hardening, grain refinement and microalloying [13]. Microalloying elements like V, Nb, Ti and B can be used to optimize the microstructure evolution and mechanical properties of AHSS [14]. This is particularly true in TWIP steels where the initial yield point (as annealed) can be relatively low. Microalloying elements are characterized by being added in small amounts (<0.2 wt%) and their ability to form carbides, nitrides and carbonitrides. They can increase strength by grain refinement and precipitation hardening, retard or accelerate phase transformations and affect the diffusion kinetics. Reyes-Calderon et al. [4] studied the effect of microalloying elements such as Nb, V and Ti on the hot flow behavior of high-Mn austenitic TWIP steel. They discussed their results in terms of peak stress (σ_p) and peak strain (ε_p) and its dependence on the strain rate $(\dot{\epsilon })$ and temperature (T). They found that the addition of microalloying elements generates a slight increase of the σ_p value, particularly at low strain rates, being Ti the element that provided larger hardening due the presence of TiC and Ti(C,N) particles. In addition to the hot flow behavior of the steels and its dependence on parameters such as the temperature and the strain rate, the hot ductility of the materials also dictate their high temperature workability [15]. However, the hot ductility behavior of the high-Mn TWIP steels has not received much attention [16], [17], [18], [19], [20]. This property is an important factor that determines the susceptibility of the steel to hot cracking and may affect the product surface quality, e.g., during the continuous casting process and subsequent hot rolling stages. Kang et al. [16] examined the hot ductility of six 0.6 wt%C TWIP steels containing 18–22 wt% Mn with N levels in the range 0.005–0.023 wt% and Al additions either low (<0.05 wt%) or high (1.5 wt%) in the temperature range of 650–1100 °C. Hot ductility was generally poor (<40%) and the 1.5 wt% Al containing steels had worse ductility than lower Al containing steels because of the presence of larger amounts of AlN particles precipitated at the austenitic grain boundaries. Hamada et al. [17] studied the hot ductility behavior of four high-Mn TWIP steels (with 20 wt% Mn) in the temperature range of 700–1300 °C. They reported a ductility trough between 700 and 900 °C, which was attributed to grain boundary sliding (GBS) mechanism. Additionally, it was shown that Al additions (up to 3% Al) would lead to the formation of ferrite grains at the austenitic grain boundaries at higher temperatures and this, in turn, would render a detrimental effect to the hot ductility behavior. Within the temperature range from 1000 to 1200 °C, DRX takes place and consequently the ductility is enhanced. Kang et al. [18] examined the hot ductility of high Al TWIP steels containing Nb and V, over the temperature range from 650 to 1150 °C, after melting and after solution heat treatment. Very low reduction of area values (10–20%) were obtained in the temperature range from 700 to 900 °C and adding Nb and V made the ductility even worse due to the additional precipitation of Nb(C,N) and VN particles. On the other hand, Kang et al. [19] evaluated the influence of S and AlN on the hot ductility of high Al TWIP steels. The testing temperature was in the range from 700 to 1100 °C. They reported a very poor ductility (<20% RA) for the two higher S steels between 950 and 1100 °C, but ductility increased up to 30–40% RA at lower temperatures of 700–900 °C. In contrast, the very low S steel exhibited a similar hot ductility from 700 to 900 °C, but it slightly improved in the higher temperature range from 950 to 1100 °C with values of 50–55% RA. The relatively low values in the reduction of area were attributed to the presence of long and coarse dendritic AlN rods located along the austenitic grain boundaries, which promote intergranular failure. More recently, Baradaran et al. [20] assessed the hot ductility behavior of a high-Mn TWIP steel with 30 wt% Mn in a wide range of temperatures (100–1000 °C). Their results indicate that, as a general trend, ductility decreases with temperature, although two regions of moderately improved hot ductility at 400 and 1000 °C were noticed and associated to the activation of dynamic restoration processes.

The aim of this research work is to determine the influence of microalloying elements, such as Ti and V, and the solidification route on the hot ductility behavior of high-Mn austenitic TWIP steel with the following nominal composition: Fe–22Mn–1.5Al–1.5Si–0.45C.