Double-hit compression behavior of TWIP steels
Highlights
► The static restoration behavior of a new grade of austenitic TWIP steels has been investigated. ► An interpass time strengthening phenomenon was observed. ► The result was discussed based on the static strain aging phenomenon and the formation of high frequency annealing twins during interpass time.
Introduction
In general to reduce the fuel consumption and the CO2 emissions, the weight of vehicles should be reduced [1]. Subsequently the introduction of high strength materials with an excellent formability has been necessitated. This in turn has been followed by the development of new alloys in general and of steels in particular. Recently, a new class of high-Mn steels (15–30 wt% Mn) which are prone to the twinning induced plasticity effect has been developed [2], [3]. The presence of fully austenitic microstructure in these steels is due to their low stacking fault energy (SFE) [4], [5], [6].
The main explanations for the excellent balance between flow stresses and ductility of TWIP steels are an atypical strain aging mechanism and the occurrence of deformation mechanisms additional to dislocation gliding [7], [8], [9], [10]. Bake hardening (i.e., involving interactions between interstitial atoms and dislocations) is essentially a strain aging process and can be the main reason for increase in yield strength. Investigations on an ultra low carbon steel (with a total carbon content of 20 ppm) have established that the strengthening takes place in two stages: (a) a Cottrell atmosphere formation stage and (b) a precipitation stage. The yield stress first increase with aging time and reaches a maximum and then remains almost constant thereafter. Carbon segregation to the dislocations increases with increasing the applied pre-strain. As the dislocation density increases with the amount of applied pre-strain, the amount of carbon which contributes to Cottrell atmosphere formation on all the dislocations increases. Consequently, the amount of carbon contributing to the second stage of aging or precipitation decreases with higher pre-strain [8].
The microstructural evolution during recrystallization is highly dictated by the characteristics of the deformed matrix (including the stored energy content and the local chemistry), the orientation relationship between the deformed structure and the growing grain, the temperature, and so on. Therefore, twin boundaries may complicate the restoration processes in low SFE materials [11], [12]. In fact, the twinned structure may change the energy and the mobility of a mobile interface, thereby either enhancing or retarding the evolution of a given microstructure [11], [12], [13], [14].
To date the most research on high manganese TWIP steels has been conducted on their room temperature mechanical properties [1], [2], [3], [4], [5], [6]. In the present work, the static restoration behavior of a grade of TWIP steels with 29 wt% manganese has been studied through utilizing the double-hit compression testing technique. In addition, the role of static strain aging mechanism and twin formation during the interpass time on the mechanical properties and microstructural evolution of the experimental TWIP steel has been investigated.
Section snippets
Experimental methods
The chemical composition of the experimental steel which was cast under an argon atmosphere is given in Table 1. In order to eliminate the undesired solidified structure and to achieve a fine grain microstructure, the ingot was forged at 1150 °C. The forged material was annealed at 1100 °C for 45 min to chemically homogenize the microstructure. The material was then machined into the cylindrical specimens in accordance with the ASTM-F136 standard (12 mm in height and 8 mm in diameter).
The hot
Initial microstructure
The optical microstructure of the solution annealed steel (1100 °C, 45 min, water quenched) is shown in Fig. 1. A fully austenitic microstructure characterized by annealing twins (shown by A.T. symbol in Fig. 1) is observed. The average initial grain size after solution annealing is 80 μm.
Critical strain for initiation of dynamic recrystallization (ɛC)
The typical true stress–true strain curves of single-hit compression tests are shown in Fig. 2. As is seen the flow stress decreases with increasing temperature (decreasing Zener–Hollomon parameter) at constant
Conclusions
In the present study, the double-hit compression behavior of TWIP steels in the temperature range of 950–1150 °C and under strain rate of 0.1 s−1 has been investigated and the following conclusions have been drawn:
- 1.
After holding the TWIP steel for different interpass times, the yield stress of the second pass has been unexpectedly elevated in comparison to the first pass.
- 2.
It is suggested that static strain aging can play an important role in the hardening behavior observed after relaxation time.
- 3.
The
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