Effects of Intercritical Annealing Temperature on Mechanical Properties of Fe-7.9Mn-0.14Si-0.05Al-0.07C Steel

Abstract

A medium Mn steel has been designed to achieve an excellent combination of strength and ductility based on the TRIP (Transformation Induced Plasticity) concept for automotive applications. Following six passes of hot rolling at 850 °C, the Fe-7.9Mn-0.14Si-0.05Al-0.07C (wt.%) steel was warm-rolled at 630 °C for seven passes and subsequently air cooled to room temperature. The sample was subsequently intercritically annealed at various temperatures for 30 min to promote the reverse transformation of martensite into austenite. The obtained results show that the highest volume fraction of austenite is 39% for the sample annealed at 600 °C. This specimen exhibits a yield stress of 910 MPa and a high ultimate tensile stress of 1600 MPa, with an elongation-to-failure of 0.29 at a strain rate of 1 × 10−3/s. The enhanced work-hardening ability of the investigated steel is closely related to martensitic transformation and the interaction of dislocations. Especially, the alternate arrangement of acicular ferrite (soft phase) and ultrafine austenite lamellae (50–200 nm, strong and ductile phase) is the key factor contributing to the excellent combination of strength and ductility. On the other hand, the as-warm-rolled sample also exhibits the excellent combination of strength and ductility, with elongation-to-failure much higher than those annealed at temperatures above 630 °C.

Ultrafinegrained Microstructure in a Medium Manganese Steel after Warm Rolling without Intercritical Annealing

“…Section of iron carbon-manganese (8%Wt Mn) phase diagram. similar microstructure was exhibited by Zhao et al 9) on a 7.9Mn annealed steel at 600°C for 30 min after warm rolling at 630°C. Another suchlike result was obtained by Zhan et al 10) studying the intercritical rolling effects (650°C for 6 h) on the microstructure of a 5Mn-0.10C steel.…”

“…6) Usually, these steels are produced by hot and subsequent cold rolling, with long intercritical annealing. 3,5,[6][7][8][9] Nevertheless, the warm rolling has obtained bigger interest in reducing operating costs and time. 6,[9][10][11][12] Some papers report the warm rolling of medium manganese steels.…”

“…3,5,[6][7][8][9] Nevertheless, the warm rolling has obtained bigger interest in reducing operating costs and time. 6,[9][10][11][12] Some papers report the warm rolling of medium manganese steels. [7][8][9][10] In all of them, the application of the intercritical annealing before or after the mechanical conformation is considered to obtain an adequate fraction of stabilized austenite to optimize the steel mechanical performance by the TRIP effect.…”

“…6,[9][10][11][12] Some papers report the warm rolling of medium manganese steels. [7][8][9][10] In all of them, the application of the intercritical annealing before or after the mechanical conformation is considered to obtain an adequate fraction of stabilized austenite to optimize the steel mechanical performance by the TRIP effect. In contrast, the present work proposes the use of warm rolling, without the hot rolling and intercritical annealing steps.…”

“…In this work, the effects using the knowledge of previous works on warm rolling 11,12) and the results recently published in literature, [7][8][9][10] the authors decided to apply this approach in order to produce ultrafine grains microstructure in a medium manganese low carbon steel (8Mn-0.08C). The main difference is that the warm working was applied directly as casting and homogenized samples without hot rolling and long intercritical annealing.…”