Research On Microstructure And Performance Of The High-strength Bolt Steel

With the development of automobile and other mechanical industry, higher requirements for fasteners of the structural materials are needed. But the conventional bolts, especially the engine mounting bolts, cannot satisfy the requirement of lightweight and high strength. In response to this development tendency, no-modulation steel, dual-phase steel, and boron steel have been vigorously developed, but high-quality engine mounting bolts still adopt Cr-Mo steel series.In the paper, some key links of microstructure and performance were research objects for the high-strength bolt steel. The solidification and heating process of continuous casting billet were analyzed. The influencing factors for carbon diffusion were summarized, and deformation resistance models at room temperature and high temperature were established. Through study on the solid-state phase transformation process, organization transformation process was controlled with finite element technique. Moreover, the effect of heating process on the grain growth was discussed, and the law of dynamic and static recrystallization of high-strength bolt steel was also revealed. The main research achievements were as follows:(1) When the casting temperature decreased from 1551℃ to 1511℃,the maximum grain area decreased 6.6 times, the average radius decreased by 28%, the proportion of columnar grain zone decreased from 79.16% to 7.1%, the columnar grains became thin and short, and meanwhile the grain of central equiaxed grain zone increased from 13.79% to 69.1%. With the casting temperature decreasing, the average grain radius of central equiaxed grain zone first decreased and then increased. Casting rate had a great effect on the solidification position. The solidification microstructure could be optimized by adjusting the nucleation number and composition.(2) Through simulating the heating process of the continuous casting billet under the existing heating system, it was found that the maximum strain gradually decreased with the increase of hot feeding temperature. There was a inflexion point in the stress curve at 700℃, and two stress peaks would appear gradually. To reduce the fluctuation level of stress and strain curves, the hot feeding temperature should be controlled at around 600℃.(3) The effect of micro stress state at 550-1250℃ on the decarburization was obtained through experimental study. The thickness of decarburized layer did not increase monotonously with temperature increasing. The diffusion coefficient of carbon at 750℃ was solved to be 1.1×x10-12m2/s, and the diffusion activation energy was 415.9kJ/mol. The thickness of decarburized layer in the absence of stress and in the temperature range of 900～1200℃ was also measured, and it was found that there existed two peaks at 950℃ and 1100℃. In the 900～1100℃ austenite zone, the diffusion constant of high-strength bolt steel in the absence of stress was 6.62×10-4m2/s, and the diffusion activation energy was 189.5kJ/mol. When grain size decreased from 92.8μm to 15.8μm, the diffusion coefficient of carbon at 750℃ increased from 6.8×10-13m2/s to 1.25×10-12m2/s. However, when the original microstructure was martensite or bainite, the diffusion coefficient of carbon changed little at different temperatures.(4) The maximum uniform true strain of high-strength bolt steel at room temperature was 12.4%. A high-temperature constitutive model coupling strain for high-strength bolt steel was established, and the relationships between material constants and strain were described with a new quintic polynomial.(5) Phase transition critical points were obtained through thermal simulation experiments, and the dynamic and static CCT curves were plotted. When the cooling rate was 0.05, bainite already appeared in the microstructure of the steel. Phase transition model and phase variable model were obtained by regression analysis method, thereby working out the temperature rise of each time rolling. The combination of experiment and numerical simulation was adopted to control the cooling process, as a result, the formation of ferrite and pearlite was controlled, and the transition time and the phase variable of bainite were increased, thereby avoiding the formation of massive martensite.(6) The austenite grain size in the three stages of grain growth with different holding temperature and holding time were measured, i.e. initial stage, growth stage, and stabilization stage. A grain growth model for high-strength bolt steel was regressed in the growth stages under different grain heating temperatures.(7) The dynamic recrystallization activation energy of high-strength bolt steel was determined to be 308.066kJ/mol by regressing calculation. The critical strain values of dynamic recrystallization were also identified by mathematic ways. Moreover, the equations for describing the relationship between the critical strain, peak strain, critical stress, peak stress and Z parameter were built. The time exponent in the Avrami kinetic equation for dynamic recrystallization was determined to be 1.355,thereby regressing calculation an equation for describing the relationship between Avrami constant and strain rate. The static recrystallization activation energy of high-strength bolt steel was determined to be 182.8kJ/mol, and the static recrystallization kinetics equation was also derived.