Inclusions in Calcium‐Treated and Resulfurized Al‐Killed Steels

Abstract

The resulfurized steel is widely used in automobiles, machine tools, and other fields due to its high strength, low energy consumption, and good machinability. The high sulfur content in the steel inevitably leads to the precipitation of a large amount of sulfide inclusions with complex morphology and uneven distribution in the steel during the cooling and solidification process. The size, distribution, and morphology of inclusions, especially the manganese sulfide (MnS) in the resulfurized steel, have a significant influence on the machinability of the steel. [1][2][3][4] Manganese sulfide inclusions in steel are classified by their morphology into three types: globular MnS (Type I), fine rod-like MnS (Type II), and angular MnS (Type III), as reported by Sims and Dahle in 1938. [5] Manganese sulfide inclusions may lead to the stress accumulation during the machining process of the resulfurized steel. It was reported that Type I and Type III MnS inclusions were beneficial to improve the machinability of the steel due to their small size and uniform distribution in the steel, which induced cleavage fracture during the machining process and acted as stress raisers in the shear plane at the tip of the tool. [6][7][8][9][10] Numerous studies have focused on the formation mechanism and the shape control of MnS in steel. [11][12][13][14][15][16][17][18][19][20] Sulfide inclusions were modified from MnS to CaS through calcium treatment due to the strong affinity between Ca and O and S elements in the resulfurized steel, [13,[21][22][23][24] which was widely used in the production process. However, a large number of CaS-CaO-rich inclusions with a high melting point were generated during excess or less calcium addition to the steel, leading to the clogging of the submerged entry nozzle during the continuous casting process. Extensive studies were performed to study the calcium treatment of inclusions in low sulfur Al-killed steels, including modification mechanism, [25][26][27][28][29][30][31][32][33] kinetics, [31,34,35] and thermodynamics. [21,[36][37][38] However, it was difficult to achieve the precise control of inclusions in the resulfurized steel by calcium treatment due to the narrow range of calcium content for good castability during the continuous casting process and the unstable yield of calcium. Kusano found that excess calcium addition to the steel with >0.02 wt% sulfur led to a high CaS content in inclusions, causing a poor ultrasonic testing result. [39] Li found that the calcium treatment significantly improved the morphology and spatial distribution of sulfides in the free-cutting steel. [40] Holappa reported that the deformability of (Mn, Ca)S composite inclusions after calcium treatment was significantly lower than that of pure MnS inclusions in the resulfurized steel. [41] Jiang revealed that the microhardness of (Mn, Ca)S composite inclusions increased with the increase of calcium content in sulfides in the steel after calcium treatment. [42,43] With the increase of calcium content in sulfides, the s...

Ductile Fracture Behavior of ASTM A516 Gr.70 Pressure Vessel Steel by ASTM and ISO Fracture Toughness Standards

“…Ju, Lee, and Jang [60] demonstrated the effect of crystallographic texture on the fracture toughness of the API 5L X65 pipeline steel using texture analysis through pole figure measurement of the volume fraction of crystallographic planes as a function of the angle to the rolling direction of the plates. Their results showed that all the planes had uniform distribution, but the {110} plane showed more density in the transverse to the rolling direction [61,62]. The {110} plane is one of the most densely packed planes of body-centered cubic (BCC) ferrite crystals, being, therefore, responsible for the plastic deformation that precedes ductile tearing at the upper shelf regime, and this was the reason the authors provided to explain the higher fracture toughness observed at the transverse direction in comparison to the rolling direction.…”