Effect of microstructure on retained austenite stability and tensile behaviour in an aluminum-alloyed TRIP steel

Abstract

a b s t r a c tMicrostructures, retained austenite (RA) stability and tensile behaviour were investigated for an aluminum-alloyed TRIP steel. Two distinct microstructural variants (equiaxed and lamellar) were studied: (i) at constant volume percent RA and (ii) at constant UTS. The RA in the lamellar variant transforms more slowly with tensile strain than for the equiaxed variant and stabilizes at $ 80% transformed. The higher RA stability for lamellar microstructures is primarily due to the stress shielding effect of RA grains by the surrounding bainite. A secondary contributor is the elongated shape of the RA grains in the lamellar microstructure. The tensile work hardening behaviour correlates with RA transformation behaviour for the two model microstructural variants. The lamellar variant exhibits a relatively constant work hardening rate sustained up to the point of necking. By contrast, the equiaxed variant exhibits a maximum work hardening rate at low strain, followed by a monotonic decrease to the necking point. Comparing the results reported here with those of a previous study on silicon-alloyed TRIP steel shows that for both microstructural variants RA transforms more slowly with tensile strain and the UTS Â El values are higher for the aluminum-alloyed steel.

The Effect of Two-Step Heat Treatment Parameters on Microstructure and Mechanical Properties of 42SiMn Steel

“…The integrated intensities of (200) ferrite peak and (111), (002) and (022) austenite peaks were used for evaluation. Carbon content in retained austenite was calculated when considering the effect of alloying elements, according to [1,27]: a = 3.572 + 0.0012 Mn − 0.00157 Si + 0.0056 Al + 0.033 C…”

“…Higher carbon content means higher mechanical stability of the retained austenite against martensitic transformation, however it does not necessarily ensure better mechanical properties of the steel [1,35]. The soaking temperature had the most significant effect.…”

“…Carbon content is only one of the factors that are affecting stability, and thus also transformation behaviour of retained austenite during straining and higher carbon contents contribute to higher stability of retained austenite [1,[4][5][6]35,36]. The aim of TRIP steel processing is to achieve an optimal stability of retained austenite, which means that the austenite should be able to continuously transform to martensite during straining.…”

“…Continuous transformation of retained austenite was observed for several low carbon steels treated with bainitic holds around 400 • C [31,36]. Other important factors that influence the stability of the retained austenite are the retained austenite morphology (thin films and laths are more stable than bulky islands), the grain size (stability increases with decreasing size), and the effect of surrounding phases [1]. Retained austenite surrounded by ferrite will deform and transform to martensite only after the surrounding ferrite grains are deformed.…”

“…One of these steels is TRIP (Transformation Induced Plasticity) steel, which generally possesses high strengths above 800 MPa when combined with good total elongation of around 30%. These TRIP steels are typically low to medium carbon low alloyed materials with 0.2-0.4% carbon and 1-2% of manganese and silicon [1][2][3][4][5][6]. Various combinations of alloying elements have been proposed and tested to improve either the mechanical or technological properties of TRIP steels [6][7][8][9][10].…”