| Non-quenched and tempered steel is a kind of sulfur containing free-cutting steel with excellent machining property and mechanical properties. As a result, it has been widely used abroad for producing automobile parts and is expected to have a good prospect considering the fast development of domestic automobile industry. Sulfide inclusions usually form in non-quenched and tempered steel because of the addition of sulfur. Comparative analysis indicates that the sulfides in foreign steels are in appropriate shape and dispersive distribution. In contrast, the sulfides in domestic steels usually have elongated morphologies and distribute along grain boundaries, leading to worse machining property and mechanical properties of non-quenched and tempered steel. Consequently, the car manufacturers in our country have to import most of the high quality non-quenched and tempered steels from overseas. For the purpose of improving the level of sulfide in domestic steels, this systematic work has been conducted focusing on the effect of composition, solidification process, hot working and homogenization process on the precipitation behavior and morphology transition of sulfide inclusions.Thermodynamic calculations were carried out to assess the solidification process. The results show that83%of the sulfide inclusions precipitate at the end of solidification with the segregations of elements Mn and S in the remaining liquid. The primary precipitation temperature increases with decreasing C,Si and Al content or increasing Mn and S content. The ternary phase diagram of Fe-Mn-S system shows that MnS precipitates in either a eutectic or a monotectic mode.The effect of Al on the morphology of MnS in non-quenched and tempered steel was investigated at three different cooling rates (0.24℃/s,0.43℃/s, and200℃/s). The formation mechanisms of three types of MnS are elucidated based on phase diagram information combined with crystal growth models. A monotectic reaction and subsequent fast solidification lead to globular Type Ⅰ MnS. Type Ⅱ MnS inclusions with different morphological characteristics form as a result of a eutectic reaction followed by growth in Fe matrix. Type Ⅲ MnS presents a divorced eutectic morphology. At the cooling rate of0.24℃/s, the precipitation of dispersed Type Ⅲ MnS is significantly enhanced by the addition of0.044wt%acid-soluble Al. At the relatively higher cooling rates of200℃/s, the formation of Type Ⅱ MnS inclusions is promoted, which aggregate at the grain boundaries.Concerning the fluctuation of S content in the non-quenched and tempered steels from domestic steel plants, the effect of S contents on the morphology of sulfides and microstructure in steels were studied. The results show that with the increase of S content from0.025wt%to0.065wt%, the number of sulfide inclusions increases, and the aggregation of sulfide inclusions is promoted. However, the proportion of angular type Ⅲ MnS decreases with increasing S content. Both MnS and MnS-V(C, N) complex inclusions are found to act as effective nucleation sites to improve the formation of intragranular idiomorphic ferrite.Sulfides in as-cast condition are usually elongated along the rolling direction, leading to the anisotropy of tensile properties. In-situ observation shows that it is easier for sulfides in transverse tensile specimens to debond from the Fe matrix with the applied load perpendicular to elongated MnS. The detached places act as the initiation of cracks and the propagation along MnS is promoted. Therefore, the transverse properties are much worse than those of longitudinal direction and it is necessary to reduce the size of MnS inclusions by the following hot working processes.The effect of deformation parameters on the relative plasticity and distribution of MnS was studied by using Gleeble-1500thermal-mechanical simulator. Under slow strain rate (0.01s-1) and heavy deformation (80%), the relative plasticity of MnS is low and the dynamic recrystallization of steel is promoted. Under this condition, elongated MnS inclusions fully crack at the deformation temperatures of1050℃and1250℃; however the distribution of MnS is still uneven.High temperature homogenization treatment was used to further decrease the size and to disperse the distribution of deformed sulfides. It is found that the shape evolution of elongated MnS is as follows. First, MnS inclusions gradually coarsen and become cylindrical with the increase of isothermal holding time. Then the contraction happens at some places of the cylindrical sulfides and finally causes the fracture of MnS inclusions. The newly formed MnS fragments will turn into spheres further. The controlling factor of the coarsening and fracture of MnS inclusions is the diffusion rate of S in the steel. After being isothermally treated at the temperature of1250℃for3h, or at1050℃for5h, a large number of elongated MnS inclusions have broken up into small pieces, meanwhile, the distribution of sulfides becomes more dispersed. |
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