Development of high strength micro-alloyed forged steel

Abstract

High strength micro-alloyed steel is in growing demand due to its application in automobiles, structural components, oil pipelines, heavy industrial equipments, power transmission towers, light poles, etc. An attempt has been made to produce high strength micro-alloyed steel with optimum ductility with the judicious addition of alloying elements. The micro-alloying elementsniobium, vanadium and titanium wereaddedfrom 0.1to0.2% during steel making in an induction furnace. Other additions included aluminium as deoxidizer and manganese for enhancing strength. Niobium, vanadium and titanium are carbide formers, they enhance the strength of steel with respective carbide formation. These steels acquire strength due to precipitation hardening, grain refinement and solid solution strengthening. The ductility strictly depends on carbon content, it decreases with the increase in carbon content. In present investigation high strength micro-alloyed steel was produced in an induction furnace making use of steel scrap with varying percentage of carbon. Cast samples were subjected to homogenisation at 900–950°C in order to reduce segregation and enhance uniformity in the structure. Homogenised samples were forged to about 30% reduction leading to grain refinement. The forged samples were normalised at 950°C for 2 hours in order to study the effect of forging and normalising. All forged samples as well as forged and normalised samples were subjected to tensile testing under ASTM. The Vickers hardness of all samples was measured along with the study of their microstructure. The values of their yield strength(YS) ranged from 415 to 724 MPa and ultimate tensile strength(UTS) from 510 to 1109 MPa, whereas Vickers hardness from 192 to 360 VHN and elongation from 4.2 to 7.5%. The percentage of carbon had significant effect on the values of YS, UTS and hardness. The micro-alloyed forged steel so produced had high strength, high hardness but low elongation.