Phase transformation and mechanical properties of si-free CMnAl transformation-induced plasticity-aided steel

Abstract

Conventional CMnSi transformation-induced plasticity (TRIP)-aided steels are a promising solution for producing lighter, crash-resistant car bodies, due to their high-strength and large uniform elongation. The CMnSi TRIP-aided steels, with more than 1 mass pct Si, have the drawback of poor galvanizability due to the presence of complex Si-Mn oxides on the surface. The full substitution of the Si by Al in cold-rolled and intercritically annealed TRIP-aided steels, therefore, was evaluated in detail. The phase-transformation kinetics during the intercritical annealing and the isothermal bainitic transformation were investigated by means of dilatometry. The allotropic phase-boundary was determined both by thermodynamic calculations and the experimental determination of the C content in the retained austenite. The results imply that short isothermal bainitic transformation times are sufficient to obtain the TRIP microstructure and that the processing of CMnAl TRIP-aided steels in a continuous annealing line not equipped for overaging is possible. The mechanical properties were evaluated for CMnAl TRIP-aided steels obtained using an industrial thermal cycle: the properties matched those of the conventional CMnSi TRIP-aided steels, where it was found that the high-Al CMnAl TRIP-aided steel had a high strain-hardening coefficient of 0.25, which was stable up to a true strain of 0.25.

Design of Composition in (Al/Si)-alloyed TRIP Steels

“…22) The effect of the partial or full substitution of Si by Al on the microstructure and mechanical properties of CMn TRIP steels has been studied by several authors. [22][23][24][25][26][27][28] Excellent mechanical properties have been found and Al alloyed TRIP steels are presented as promising candidates for processing in continuous galvanizing lines due to their lower Si contents. Besides, several authors have noted that Al TRIP steels need shorter holding times (less than 60 s at temperatures of 450-465°C) than Si steels to obtain maximal values in the amount of retained austenite and optimal values of mechanical properties.…”

Austenite Thermal Stabilization through the Concentration of Manganese and Carbon in the 0.2C–5Mn Steel

“…Here, the Ms temperature can be estimated by the formula proposed by Mahieu et al: 19) ........ (3) where the C (wt.%) and Mn (wt.%) are carbon content and manganese content in retained austenite (or reversed austenite, they are the same because of the high cooling rate).…”

Effect of Microstructure before Intercritical Annealing on Mechanical Properties of 5% Mn Steel

“…(5). 29) Cγ, Mnγ, Siγ and Alγ are C, Mn, Si and Al concentration in retained austenite. Cγ and Mnγ are used as the value Fig.…”