Martensitic Transformation Behavior of B-Bearing Steel During Isothermal Deformation Sina Salari, Alireza Saeed-Akbari, Malek Naderi,  and Wolfgang Bleck The purpose of the present research is to study the martensitic transformation in 22MnB5 steel under thermomechanical conditions by means of dilatation data. To reach this aim, the effects of deformation temperature and strain rate on the martensitic dilatation as well as martensite start temperature (M s ) were investigated. Thermomechanical treatments were performed in a deformation dilatometer including the isothermal deformation of samples in the temperature range of 550–9008C up to the final strain of 0.5 in three strain rates of 0.1, 1, and 10 s 1 . Finally, deformation temperatures were divided into two regimes of lower and higher than 8008C. In the former, strain-induced phase transformations, while in the latter, occurrence of dynamic recovery against mechanical stabilization of austenite influenced martensitic transformation. 1. Introduction Dilatometer is one of the most powerful thermal analysis methods used to determine the microstructure of steels. [1–3] It is possible to study martensitic transformation using this technique, [4–6] since transformation of austenite to marten- site causes a plastic deformation in the transformed region, and yields a specific amount of dilatation. The more amount of martensite, the more dilatation is observed. [7,8] In last decades, thermomechanical treatment has been employed in order to optimize the mechanical properties of steels. [9–11] Hot and warm deformation of austenite increases the lattice defects such as dislocations, leading to work hardening of austenite. Thus, growth of martensite plates by shear mechanism is hindered when their glissile interface encounters the introduced defects into the aus- tenite lattice causing a reduction in the fraction of mar- tensite. This phenomenon is known as mechanical stabilization of austenite (MSA). [12–14] 22MnB5 boron alloy is a quenchable high strength steel employed in the manufacturing of automobiles com- ponents subjected to sudden crashes. This steel undergoes hot stamping process after which its yield and tensile strength increase up to 1100 and 1500 MPa, respectively. These high values are due to the microstructure exhibiting lath martensite or lath martensite with small amounts of bainite and retained austenite. [15–17] Thermomechanical deformation of cylindrical steel samples is heterogeneous, that is, the effective strain along the horizontal centerline of the samples varies. Consequently, distribution of the produced phases is not homogeneous. This means that light optical microscopy investigations on only a few fields of the heterogeneously deformed surfaces is not the proper technique to identify the phase fractions within the experimental specimen. Hence, dilatation data can be reliable to identify the present phases. [5] In addition, in contrast to isothermal deformation con- dition, effect of non-isothermal deformation on marten- sitic transformation has been previously investigated, in details. [6] Thus, this paper aims to study the effects of different isothermal deformation parameters involving deformation temperature and strain rate on martensitic dilatation and M s temperature, using the dilatation data. Furthermore, metallographic photos and hardness tests are provided to investigate the observed results. 2. Experimental Figure 1 shows the CCT diagram of the studied 22MnB5 steel with the chemical composition given in Table 1. All the thermomechanical tests were performed in DIL Baehr 805 deformation dilatometer on Rastegaev samples with a height of 10 mm and diameter of 5 mm. A Pt/Pt-10% Rh thermocouple was surface mounted in the mid-length of the sample in order to measure the temperature. The samples were austenitized at 9008C for 5 min. Then, they [  ] S. Salari, M. Naderi Department of Mining and Metallurgy, AmirKabir University of Technology, Hafez Av., Tehran, Iran Email: mnaderi@aut.ac.ir A. Saeed-Akbari, W. Bleck Department of Ferrous Metallurgy, RWTH Aachen University, Intzestr1, 52072 Aachen, Germany DOI: 10.1002/srin.201100331 www.steel-research.de ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim steel research int. 83 (2012) No. 8 733 FULL PAPER