The Effect Of Pre-strain Style On Bake-hardening For Ultra-low Carbon Steel

The effect of pre-strain, under different loading path, on the baking hardening property for ultra-low carbon steel was studied in this paper. The samples were baked at 170℃for 20min in the oven, the pre-strained applied and bake-hardening performance measurement was carried out on the MTS810 universal test machine, and the dislocations morphology with different pre-strain style and baking was observed by the transmission electron microscopy. at the same time, the stress and strain with different pre-strain method was simulated with ANSYS software. The conclusions are as follows:(1) Under the condition of uniaxial tensile deformation that standard tensile specimen was adopted: in the range of 0% 4%, the BH value increased along with pre-strain increased; In the range of 4% 8%, the BH value decreased along with the increase of the pre-strain, and the BH value corresponding to 4% was highest; And in the range of 8% 15%, BH value improved along with the increase of the pre-strain, and the BH value corresponding to 8% reached minimum; The dislocation density of 4% was significantly larger than that of 8%, and the greater the deformation was, the greater of dislocation distribution uniformity was; And the smaller the pre-strain was, the closer of the simulated results to true ones, which was showed in the numerical simulation, however the higher the pre-strain became, the bigger the error was, but stress and strain distribution within the gauge was uniform.(2) In the case of wide plate sample (b/t > 30, b represent for width, t represent for thickness) to generate the plane deformation: the BH value increased with the increase of the pre-strain, but the magnitude was smaller than uniaxial tensile results; And the dislocation density was obviously less than that of uniaxial tension; Numerical simulation results showed that although the real pre-strain applied in the wide plate sample was similar to the uniaxial tension, the plastic deformation generated in the material was far less than the actual load, and also less than that of the uniaxial tensile samples.(3) In the case that cross tensile samples used to produce two-way deformation, 2% pre-strain always applied on one direction, and different pre-strain applied on another one, 1%, 2%, 3% and 4% respectively, so that the different loading ratio achieved. and two values, BH0 and BH₂ were defined according to uniaxial tensile of BH value, the results showed that, both BH0 and BH₂ increased with the increase of loading ratio; and dislocation density of sample was increased; Numerical simulation results showed that the distribution of stress and strain was directivity, namely the larger the load was,the higher the strain was.