Met. Mater. Int., Vol. 18, No. 2 (2012), pp. 317~320 doi: 10.1007/s12540-012-2015-5 Published 27 April 2012 Effect of Tensile Deformation of Austenite on the Morphology and Strength of Lath Martensite Zengmin Shi 1 , Kai liu 1 , Maoqiu Wang 2 , Jie Shi 2 , Han Dong 2 , Jian Pu 1 , Bo Chi 1 , Yisheng Zhang 1 , and Li Jian 1,* 1 School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China 2 National Engineering Research Center for Advanced Steel Technology, Central Iron and Steel Research Institute, Beijing 100081, PR China (received date: 3 May 2011 / accepted date: 14 June 2011) A hot-rolled steel, 22SiMn2TiB, was employed to study the effect of austenite deformation on the micro- structure and strength of the subsequently formed lath martensite. It was revealed that the sizes of the mar- tensite packet, block and lath were refined by the tensile deformation of austenite at temperatures above 850 ° C. With the increase of the deformation temperature, the packet size increased, whereas the block size decreased. The width of the lath was independent of the prior austenite grain size and the deformation temperature. The refinement of martensite blocks was considered to strengthen the ausformed martensite. Key words: deformation, microstructure, strength, lath martensite, EBSD 1. INTRODUCTION Lath martensite is one of the most widely used phases in steels due to its high strength and well-balanced toughness. Extensive studies have been done on the morphology and crystallography of lath martensite by means of various kinds of techniques, such as optical microscopy (OM), electron backscattering diffraction/scanning electron microscopy (EBSD/ SEM), transmission electron microscopy (TEM), etc. [1-5]. It is generally considered that lath martensite forms in vari- ous packets within an austenite grain; a packet contains blocks consisting of laths [1-6]. The laths within a packet have the same habit plane, and those within a block have similar crystallographic orientations. The packet size affects both the strength [7] and toughness [8] of low-carbon steels with a microstructure of lath martensite; and the block size is con- sidered a key factor determining the strength [9,10], ascribed to block boundary strengthening [11,12]. It is generally considered that the thermomechanical his- tory affects the morphology of lath martensite in steels, such as the sizes of the packet and block, and thus in turn influ- ences the strength and toughness of the steels. However, few efforts have been made to understand the relationship between thermomechanical processing, microstructure and mechani- cal properties of steels. It was reported that work-hardening of austenite leads to an increase in packet size and a decrease in block size of the lath martensite in an Fe-Ni alloy, com- pared to an alloy formed from undeformed austenite [6]. And, recently, it was found that the strength of ausformed martensite in 22SiMn2TiB steel is increased by tensile deformation of the austenite at temperatures above 850 ° C [13]. In the present study, lath martensite transformed from deformed austenite was examined in detail by OM, SEM, EBSD and TEM. The morphology of the microstructural units was analyzed to understand their contributions to the macroscopic hardness (strength). 2. EXPERIMENTAL PROCEDURES A hot-rolled steel, 22SiMn2TiB, with a composition of (wt%) 0.22C-0.87Si-1.64Mn-0.024Ti-0.0015B-0.0025N-(bal.) Fe was used in this study. Rod specimens of dimensions φ10×110 mm were prepared and austenitized at temperatures of 950, 1050 and 1150 ° C, respectively, for 5min, followed by cool- ing at a rate of 35 ° C/s to the deformation temperature in the range of 850~1050 ° C. The specimens were uniaxially tensile deformed isothermally to failure by using a Glee- ble 1500D thermo-simulation machine; samples were then air quenched. The average prior austenite grain size (PAGS) values of the specimens direct quenched from the three austenitization temperatures were 40.7, 50.4 and *Corresponding author: lijian@hust.edu.cn ©KIM and Springer