Properties of materials especially its mechanical properties are strongly affected by grain size. Hall-Petch equation represents this effect. That’s why; nowadays grain refinement is one of the important subjects in enhancing mechanical properties. This method is the only way that improves strength and toughness simultaneously. Steels are attractive alloys in this area because of their wide use in industry. Different researchers invented many process for grain refining of steel. Between this those, severe plastic deformation and thermomechanical treatments gained more attention. In theromechanical treatment control of rolling and microstructure are done in specific procedures to create ultrafine grain size. One of these processes is cold rolling and annealing of product phase. Cold working increases the stored energy and with subsequent annealing static recrystallization will take place that will lead to a significant grain refinement. Martensite is a phase used in this process (martensite process). But it is a brittle phase that induces cracks near the edge of the specimen during rolling. Bianite is similar to martensite in formation mechanism but is tougher than martensite. Therefore, in this study we have tried to represent a simple process for grain refining. For this purpose thermomechanical treatment was done on low alloy low carbon steel. Thermal treatments were austenizing at 925 o C for 45-60 minute and isothermal transformation in 3 different temperatures in molten salt bath for 30 minute. The best temperature for isothermal transformation was 410 o c that produced 90% bainite. Mechanical treatment was included 90% cold rolling of the bainite. Rolling induced a lamellar structure with thickness of 410 nm. Then Annealing was done in 500-700 o C with temperature interval of 50 o C. Results showed that recovery was the main softening process during annealing at 500 and 550 o C. With annealing of lamellar structure at 650 o C for 120 minute and 700 o C for 10 minute, static recrystallization take placed and grains with mean grain size of 550 and 560 nm were obtained respectively. Keywords Low alloy low carbon steel; Ultrafine; thermo-mechanical treatment; Bainite; Cold rolling; Annealing.
