| Three key technologies:refining, hot working and recrystallization, of Fe-Mn-Cu-C twinning induced plasticity (TWIP) steels were systematically studied by means of tension test, X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD).There are mainly four major inclusions in the TWIP steel, including oxide inclusions, silicate inclusions, sulfide inclusions as well as nitride inclusions. Sulfide inclusions resulted in a good plasticity in the temperature range of application and hot-working of the TWIP steel, while nitride inclusions, as hard particles, belong to brittle inclusions, the deformation behavior of the alloy with silicate inclusions and ferro-manganese oxide varied with the content and temperature. In this paper, the steel was prepared by vacuum melting and the smelting process of the calcium-manganese-silicon alloy in concert with high manganese steel slag remover. Inclusions in the solution-treated and thermal processed samples were identified and counted. The results show that the smelting process of the calcium-manganese-silicon alloy in concert with high manganese steel slag remover has a positive effect to reduce the nonmetallic content and refine the size of the nonmetallic inclusions. After forging and hot rolling, the mechanical properties of the steel along the rolling direction did not change evidently for different content and size of nonmetallic inclusions.This thesis regards Fe-20Mn-3Cu-1.4C TWIP steel as a research object. The results show that the steels obtained by forging and rolling revealed the same effect in the aspect of reducing shrinkage porosity, compared with the steel only deformed by rolling in the same deformation, while the strip defects of the former has a shorter size and a less content. Meanwhile, as steels hold a longer time at a high temperature during the forging and rolling, it increases the degree of decarburization. Based on the chemical measurement, the steels via forging plus rolling and rolling without forging are 1.22% and 1.27%, respectively. The lower the carbon contents, the lower the stacking fault energy to be and the poorer the mechanical property to be.A study of the heat treatment process after hot working found that the recrystallization of the Fe-20Mn-3Cu-1.4C TWIP steel occurs at a slow rate at 350℃. The recrystallization volume fraction is only 57.7% after holding for 20 days at 350℃. As the temperature increases, the recrystallization rate increased. It only takes 7 minutes to finish recrystallization at 900℃. Carbide precipitated at the temperature range from 400℃ to 800℃. Carbide could impede recrystallization to a certain extent and decrease the plasticity of the steel. So recrystallization for a short time above 900℃ is appropriate. The grain orientation of the fully recrystallizated TWIP steel is randomized.Summarily, this study gives the suitable preparation and forming process of the TWIP steel plate. The smelting process of the calcium-manganese-silicon alloy in concert with high manganese steel slag remover for enhancing the purification of steel is a better way to get Fe-20Mn-3Cu-1.5C TWIP steel ingots. The ingots were homogenized at 1100℃ for 18 h to eliminate the segregation of alloying elements. After forging, forging stock was hot-rolled at 1050℃ to produce 4 mm thickness sheets, followed by water quenching to prevent carbide precipitation. Hot-rolled sheets were subsequently annealed at 1050℃ for 10 min. High carbon Fe-20Mn-3.0Cu-1.39C TWIP steel has a comprehensive property. The tensile strength is 1171 MPa, the elongation is 92.81%, and the absorbed energy per unit volume achieves 1.175 J/mm3. |
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