Influence of forging and cooling rate on microstructure and properties of medium carbon microalloy forging steel

Abstract

In view of their capacity to develop high strength following limited alloying and ease of processing medium carbon microalloyed (MA) steels are very cost-effective compared to quenched and tempered steels for the production of automotive components. To be able to substitute quenched and tempered steels, MA steels must be processed to similar strength levels and acceptable toughness [1]. The increased use of microalloyed forging steels in production applications should be supplemented with an increased understanding of not only the strengthening mechanisms that occur in these steels, but also the effects of the composition and forging parameters on these mechanisms. 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 [2–4]. The aim of the present study is to investigate the influence of different cooling rates on structure and properties of MA steel processed through forging route. The chemical compositions of the steels used in this study are shown in Table I. The steels are medium carbon microalloyed steel with different vanadium and aluminum contents. Specimens obtained from steels 1 and 2 were heated at 1100 ◦C for 30 min and forging operation was carried out. Thirty-six percent deformation was applied by repeated strokes in temperature range of 1000–1100 ◦C. Then forged steel samples were cooled either in water, air, or sand. Room temperature tensile strength was measured by using an Instron machine at a crosshead speed of 1 mm/min. The pearlite grain size, volume fraction of ferrite, and pearlite were determined by using mean linear intercept (mli) method and point counting. Hardness measurement was also carried out using the Vickers hardness test. Fig. 1 shows the evaluation of the microstructure for both of the microalloyed steel under various cooling condition. Table II also shows volume fraction of ferrite and pearlite and mli grain sizes of pearlite in as-received,