Key Elements of Sulfide Modification in the Sulfur-based Free-cutting Steel

Abstract

Sulfides are beneficial inclusion in free-cutting steel, and can cause chips to break easily as an internal source of stress concentration when cutting steel, and also lubricate the tool and workpiece, thus reducing tool wear and improving machinability. Sulfide morphology directly determines the performance of the free-cutting steel, and it is possible to improve the machinability of the free-cutting steel by changing the MnS inclusions into small, dispersive, spherical or spindle-shaped inclusions, thus making them difficult to deform. In aluminum-killed steel treated with Ca or Mg, there are numerous small and dispersive complex inclusions with an oxide core (CaO· Al2O3 or MgO· Al2O3) and a sulfide surface layer ((Mn,Ca)S or (Mn,Mg)S). Mg can form simple oxides like MgO, MgS or Mg-O-S. Mg can also form complex inclusions with an oxide core (MgO) and sulfide surface layer (MnS or Mn-Mg-S). Overview of Sulfide in Sulfur-based Free-cutting SteelThe sulfide in sulfur-based free-cutting steel usually exists as (FeMn)S[1]. MnS are beneficial inclusions which can cause chips to be easily broken as an internal source of stress concentration when cutting steel, and can also lubricate the tool and workpiece and reduce tool wear, thus improving machinability [2,3]. MnS is one of the most common plastic nonmetallic inclusions in free-cutting steel, and is commonly used to improve machinability. The size, shape and distribution of MnS have a significant impact on the performance of the resulting steel. MnS can suppress the grain growth and promote the precipitation of intragranular ferrite, by improving w(Mn)/w(S) to generate more MnS while decreasing the generation of FeS, which has a low melting point, thus finally improving the high temperature ductility of the steel [4].MnS has good deformability and is easy to extend along the rolling direction in the rolling process, which causes anisotropy and significantly decreases the transverse properties of the material [5], and these elongated MnS inclusions are ready sources of cracks and expansion channels in slab steel, thus reduce the life of the material [6]. In order to suppress such harmful effects, it is helpful to form hard inclusions which are difficult to deform by adding an appropriate amount alloying elements, like Ca, Ti, Mg, RE and so on. These hard inclusions distribute around the MnS and suppress its deformation, and make the MnS spindle-shaped or spherical (L /W≤3). Such inclusions deform less during hot processing, and this steel has good machinability. Modification Mechanism and Route of Ca on SulfidesCa is the currently most widely used alloying element to modify sulfides. The works of Averin [7], Lou [8], and Jiang et al. [9] have shown that Ca can partially transform MnS into CaS, and they can form a solid solution with each other to generate a new phase of (Mn,Ca)S. The hardness of (Mn,Ca)S is relatively high and the ductility is relatively low, and thus it can improve the transverse impact

Effect of Magnesium on Sulfide and Microstructure in Microalloyed Steel

“…[8,9,11] As the magnesium content increases, the evolution order of magnesium deoxygenation products is Al 2 O 3 !MA!Mg-Al-O-S!Mg-O-S. [12] In addition, due to the similarity in chemical reactions between Mg or Ca and Mn, it has been found that Mg or Ca can dissolve in MnS to form (Mn,Mg)S or (Mn,Ca) S. When they serve as the outer layer of the oxide, their cores are found to be MA and calcium aluminum spinel (CaAl 2 O 4 , CA), respectively. [13] However, composite MnS-MgS-CaS inclusions in steel containing high magnesium content were found to be irregular, [14] which is not conducive to improving the morphology of sulfides, due to the importance of sulfide in forming composite inclusions with oxides. Similarly, it is worth noting that the composition of magnesium oxides and sulfides is closely related to the precipitation of acicular ferrite in steel.…”