بررسی رفتار فولادهای نیمه ساخته و وانادیم ریز آلیاژی به وسیله آزمایش آهنگری گرم Investigation on the behaviour of medium carbon and vanadium microalloyed steels by hot forging test

Medium carbon steel (MC, AISI 1040) and medium carbon microalloyed steel (MC–MA, 38MnVS6) are being widely used for machinery structural parts [1]. In particular, MC–MA steels do not require heat treatment after they are shaped into parts, as the mechanical properties are obtained directly at the end of the process, so an important saving of costs and energy can be reached by reducing the number of operations. Also, these steels present very good characteristics of toughness and weldability. These beneficial properties have been achieved by a careful control of chemical composition and by adopting suitably controlled thermo-mechanical processes [2]. During hot working of plain carbon steels, the microstructure development is not as pronounced, as can be observed in the case of microalloyed steels that contain small amounts of Ti, Nb, Al or V singly or in combination [3]. In recent years, many papers demonstrated that microalloyed steels, containing 0.30–۰٫۵۰ wt.% of C, could satisfactorily replace conventional quenched and tempered steels. The driving force behind the development of microalloyed steels has been the need to reduce manufacturing costs. High strength steels achieve the desired strength and toughness by a sequence of thermal treatments, i.e., quenching and tempering after high temperature deformation. MC–MA steels, instead, are able to achieve high mechanical properties thanks to a simplified thermo-mechanical treatment, based on controlled cooling after hot deformation. Consequently, the desired properties can be obtained without the separate quenching and tempering treatments required by conventional carbon steels. The reduction of the cost for the production process and the improvements in properties and performance obtainable with microalloyed steels therefore led to an increase in their use [4,5]. The addition of alloying elements offers an important cost-effective approach to obtain a good combination of excellent toughness and strength through grain size control and precipitation hardening [6,7]. In microalloyed steels, strength increases are primarily achieved through increase of the pearlite volume fraction or by grain refinement and precipitation strengthening of the ferrite matrix as controlled with microalloy additions (e.g., Ti, Nb, or Al for grain size control and V for precipitation strengthening) [8]. However, hot deformation is an important parameter in grain refinement as well as microalloying elements. Rough deformation in austenite recrystallization region refines coarse austenite grains by repeated deformation and recrystallization. However, deformation in non-recrystallization region increases ferrite nucleation sites through pancaking of austenite grains and creation of deformation bands [9,10]. In this way, fine ferrite grains structure will be produced after transformation. These achievements are maintained when higher cooling rates are applied. Among hot deformation processes, forging has become a competitive technique for processing such steels [11,12]. The cooling rate after finishing deformation stage has a signifi- cant effect on the mechanical properties through engendering a variety of microstructure constituents that alter significantly the mechanical properties [13]. Higher cooling rates lead to a decrease of ferrite grain size and formation of high strength, hardness, dislocation density, and fine phases because it suppresses the atomic diffusion [14]. In contrast for lower cooling rates where, slow cooling rates lead to transformation into soft, coarse, and less dislocated phases like polygonal ferrite [12,15]. The size and percentage distribution of ferrite and pearlite within the microstructure play an important role on the final mechanical properties. Each of the microstructure variables is highly influenced by the composition of the microalloyed steels, the forging parameters utilized, and the post-forging cooling rate. Variation in the cumulative amount of deformation, working temperatures and post cooling rates can engender a variety of microstructure [6]. The present work is aimed to study the effect of cooling rate after controlled hot forging on the mechanical properties of MC and MC–MA steels. This paper also envisages to find out the influence of vanadium concentrations on the microstructures and mechanical properties of MC–MA steel forged and then cooled at different cooling rate.