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Hot-rollability and Machinability of Free-machining Steels

Hot-rollability and Machinability of Free-machining Steels
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Free-machining steels are plain carbon steels containing lots of non-metallic or metallic inclusions to improve machinability, and are mainly used for machining automotive parts or precision machinery parts. Typical examples of non-metallic and metallic inclusions are MnS and soft additives such as Pb, Bi, Sn, and Sb respectively. These inclusions work as stress concentrators during the machining process, which promote the void initiation and crack propagation at interfaces between inclusions and steel matrix, and enhance the machinability by reducing the force required for machining. Free-machining steels containing Pb and S have been widely used up to now, and needs for their replacement have been strongly raised because they contain a harmful element of Pb for human bodies. In European Union and Japan, the use of Pb in free-machining steels is recently restricted, but the usage of alternative free-machining steels such as Bi-S-based free-machining steels have been delayed. This is because the hot rolling of Bi-S-based steels is more difficult than that of Pb-S-based steels as the hot-rolling cracking occurs more frequently. In fact, low-melting-temperature elements can play a role in initiating cracks as they are melted and segregated during hot rolling and MnS can also affect the cracking as they are brittle and heavily elongated along the rolling direction. Another reason is that the machinability of Bi-S-based steels isn’t investigated than that of Pb-S-based steels. So, various machinability data need to replace Pb-S free-machining steel with Bi-S free-machining steel by comparing them. In this study, hot-rollability and machinability of free-machining steels are investigated. First, 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 non-cracked region were parallel to the surface. This was because the alignment direction of pearlite bands was irregularly deviated up to 45 degrees from the normal direction parallel to the surface, while the billet was rolled and rotated at 90 degrees in the same direction between rolling passes. In 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, thus, 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. Second, cracks or scabs formed during hot rolling of Bi-S-based free-machining steel wire rods were analyzed, and their formation mechanisms were clarified in relation with microstructure. Detailed microstructural analyses of large-diameter rods showed that the rod having low carbon content was cracked, whereas the rod having higher carbon content was not, because oxides formed during hot rolling were penetrated into the relatively soft surface, thereby leading to the surface cracking. While the crack-free, large-diameter rod containing high carbon content was subsequently rolled to make a small-diameter rod, a few scabs of 1~2 mm in size were formed on the surface as some protrusions were folded during hot rolling. In order to prevent the cracking or scab formation in wire rods, thus, 1) the increase in hot-rolling temperature for homogeneous rolling of rods, 2) the minimization of temperature drop of rolled rods upon the descaling treatment, and 3) the increase of rolling passes and the decrease of reduction ratio of each pass were suggested. Using these methods, crack- or scab-free wire rods could be successfully fabricated. Lastly, five kinds of free-machining steels were prepared by the control of the volume fractions of MnS and the volume fractions and types of soft additives such as Pb and Bi, and analyzed their microstructures by OM and SEM. Mechanical properties for five kinds of free-machining steels were investigated by dynamic torsional test. Then, deformation and fracture mechanism of these specimens are analyzed and it is tried to find the correlation between microstructure and mechanical properties. Addition soft additives decrease τmax and increases γmax, but the increase of MnS volume fraction didn’t influence much to the mechanical properties of free-machining steels. Chip properties are analyzed by observing the variation of thickness and ridge shape and these results indicate that soft additives play a good role to decrease the variation of thickness and the irregularity of ridge shape even after repetitive machining process. The chip properties are closely related with the dynamic torsional properties and these two kinds of results show Bi-S-based free-machining steel can retain the competitive machinability of Pb-S-based free-machining steel.
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