Study On Microstructures And Deep-Drawabilities Of Ferritic Rolled P-Added High Strength IF Steel Sheets

As a result of excellent deep drawing properties and high strength of high strength interstitial-free steels (referred to as HIF steels), they can dramatically reduce the weight of vehicles, decrease the product cost, save energy and environment friendly, therefore, the application of HIF steels in automobile industry has attracted much attention in recent twenty years. HIF steels are achieved by adding P, Si, Mn or other solution strengthening elements to traditional IF steels. Among these solution strengthening elements, P has the strongest solid solution strengthening function and the smallest damage to formability. P element exists in Fe matrix in the form of replacement solid solution. As a result of larger radius of P atoms than that of Fe atoms, the replacement of P atoms leads to lattice distortion of the matrix. The interaction of its stress field and the dislocation stress field blocks the movement of dislocations during the deformation of steels, and then results to the improvement of steel strength.Studies showed that Ferritic-Rolled-IF steels possessed a strong{111}//RD texture type comparable with cold-rolled-IF steels. The technology of ferritic-rolling simplifies the traditional process of cold-rolled deep drawing steel sheet and reduces production cost, leading to the hot-rolled plate with a good deep-drawing performance. Therefore, producing high-strength IF steel with ferritic-rolling technology is advanced and has significant economic benefits, furthermore, the development of ferritic-rolling technology has importantly theoretical meaning and practical value.The steel used in this experiment was smelt in CONSARC500 kg vacuum induction furnace in Baoshan Iron and Steel Company, and the ferritic-rolling test was carried out in Iron and Steel Research Institute. Optical microscope (OM), Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM), Electron Backscattered Diffraction (EBSD), Rockwell Hardness Tester and Uniaxial Tensile Tester were used to investigate the effect of metallurgical factors and ferrite-rolling process parameters on microstructure and mechanical properties of P-added high-strength IF steels.The effect of P element on the microstructures and mechanical properties of ferritic-rolled P-added high-strength IF steel were studied. Results showed that after adding P element, the strength of steel was significantly improved and deep drawing performance was decreased slightly. P element exists in the steel in two kinds of forms:P atoms that replace Fe atoms in matrix lattice and P atoms that form FeTiP compounds. The replacement of P atoms to Fe atoms causes lattice distortion of the matrix and impedes dislocation movement, which is a major factor on improving strength steel. FeTiP particles restrain the growth of favorable texture during annealing; meanwhile, the formation of FeTiP in the steel consumes Ti atoms that are supposed to eliminate C and N interstitial atoms, which leads to the existence of Fe4N, Fe3C, AlN, besides TiN, TiC, TiS and Ti4C2S2. The existence of C and N interstitial atoms in the steel is the other reason that the formability of high strength P added IF steel decreases.Different rolling parameters such as start-rolling temperatures (ST), finish-rolling temperatures (FT) and the rolling reduction (RED) were designed when ferrite regions were rolled. Comparing the microstructures, textures and mechanical properties between P-added high-strength IF steel sheets rolled with different rolling parameters, it is found that ferritic-rolling temperature and reduction have slight effect on yield strength (Rp), tensile strength (Rm), elongation (e) and n-value, but have significant effect on r-value. When the start-rolling temperature was at 840℃, the steel showed poor of formability,r<1; when the temperatures were at 700℃and 750℃, the steel showed good formability,r<1. Finish-rolling temperature and rolling reduction have great influence on r-value.r-value increases with increasing finish-rolling temperature, in contrast,r-value decreases with increasing rolling reduction. Meanwhile, from cross-comparison, it is found that, when finish-rolling happens at lower temperature, such as 540℃, higher r-value and good formability are obtained at lower rolling reduction; when finish-rolling happens at higher temperature, such as 660℃, higher r-value and good formability are obtained at higher rolling reduction.Comparing the microstructures, textures and mechanical properties of the steel sheets that are rolled in lubricate conditions and nonlubricate conditions, it is found that good lubricate condition plays an important role in obtaining good formability. Serious inhomogeneous deformation occured when the steels were rolled in ferrite region without lubricate and large amount of<110>//ND fine grains formed in the surface layer of the steel sheet, which finally leaded to weak texture condition. However, a small amount of{111} recrystallization grains form after annealing in the center layer, and the dis-orientation between surface and center layers is very close to random distribution. This texture in-homogeneity is the essential reason for the decrease of r-value. Lubrication condition leads to the uniformly pressed deformation along thickness direction. Long fibrous{001},{112} and{111} deformed grains were formed in both surface and center layer, and large amount of {111} recrystallized grains were formed after annealing treatment, which have excellent formability. Therefore, good lubricant condition is necessary to obtain excellent formability.