INFLUENCE OF AUSTENITE GRAIN SIZE TO DEVELOPMENT OF DECARBONIZATION IN PRODUCTION OF ROLLED STEEL

Common and undesirable defects in microstructures include surface decarburization which compromises the quality of HC wire rods hot-wound into coils (coil rod) and performance of products made there from. Resistance of metal to alternating loads is determined by the depth of the decarburized layer, i.e. actual difference between surface microstructure and base metal structure. As we know [1, 2], for decarburization process to occur on the metal surface when heated in a furnace, the gaseous atmosphere shall not produce a very strong oxidizing effect. If the rate of oxidation is higher than that of carbon diffusion in steel, it enhances scale build-up whereby oxygen can oxidize both carbon and iron. Although decarburized layer is mostly deemed to be detrimental, it has been argued that decarburization of wire rod surface can be advantageous to useful qualities of wire rods. Soft decarburized surface ensures improved flextural (bending) or torsional (twisting) dictility of metal resulting from low sensitivity to stress concentration factors, high resistance to crack propagation and higher corrosion resistance [3]. Compressive residual stresses occurring in a decarburized skin improve the fatigue life and increase the durability of steel-wire ropes [4]. Wire rods with more pronounced surface decarburization are less likely to form the hardened structures (martensite) in drawing operations due to surface cracking and cupping of the martensitic layer. However, in the process of cold drawing whereby metal is deformed by pulling wire rods through a serious of drawing dies (monolithic system), maximum stresses develop on the surface of rolled products[3?5]. Therefore uniform distribution of structure on the surface and in the boundary layers is essential. This means that minimum and uniform depth of decarburization on the metal surface is a prerequisite for the production of high-quality HC rolled stock [3?7]. The paper [11] acknowledges the influence of micro-additives of boron on changes in the austenite grain size and accordingly the length of borders in high-carbon steels with austenitizing temperature increase within the range of 900…1 100ºС.