Formation Mechanisms of Cracks Formed During Hot Rolling of Free-Machining Steel Billets

In this study, cracks formed in the edge side of Bi-S–based free-machining steel billets during hot rolling were analyzed in detail, and their formation mechanisms were clarified in relation with microstructure. Particular emphasis was placed on roles of bands of pearlites or C- and Mn-rich regions and complex iron oxides present in the edge side. Pearlite bands in the cracked region were considerably bent to the surface, while those in the noncracked region were parallel to the surface. This was because the alignment direction of pearlite bands was irregularly deviated up to 45 deg from the normal direction parallel to the surface, while the billet was rolled and rotated at 90 deg in the same direction between rolling passes. On the edge side, where pearlite bands were bent, iron oxides intruded deeply into the interior along pearlite bands, which worked as stress concentration sites during hot rolling and, consequently, main causes of the crack initiation in the rolled billet. On the surface of the wire rod rolled from the cracked billet, a few scabs were found when some protrusions were folded during hot rolling. In order to prevent the cracking in billets and scab formation in wire rods, (1) the increase of rolling passes and the decrease of reduction ratio for homogeneous rolling of billets and (2) the reduction in sulfur content for minimizing the formation and intrusion of complex iron oxides were suggested.