| Iron and steel industry is currently facing enormous pressure from overcapacity,resource shortages and environmental protection regulations.The development of HSLA steel with a high performance represents the most economical and effective way to solve these problems.HSLA steel has a high strength,which not only meets engineering strength requirements but also provides a weight reduction.Because it contains a small number of alloying elements,the consumption of nonrenewable resources is reduced.Compared to those of ordinary carbon steel,HSLA steel has a higher yield strength and toughness as well as better weldability and corrosion resistance.After rolling and forging,HSLA steel can be widely applied in numerous fields,such as pipelines,automobile manufacturing,shipbuilding and marine engineering.The grains in HSLA steel can be significantly improved by thermomechanical control process(TMCP)technology.In this paper,the hot deformation behavior of an as-cast HSLA steel was studied by hot compression testing.Strain-dependent constitutive model,dynamic recrystallization model and processing maps based on dynamic material model for the HSLA steel were established.The microstructure evolution mechanism of the HSLA steel under different deformation conditions was studied by TEM and EBSD.In addition,the rolling process for the HSLA steel was simulated using the finite element method.Finally,the TMCP experiment that used the HSLA steel was carried out with a hot rolling mill.The main results are as follows.The effect of hot deformation parameters on the flow stress of the HSLA steel was studied utilizing hot compression testing.A strain-dependent constitutive model based on the Arrhenius equation and a new constitutive model based on the Z-parameter were also established.The predictive ability of two constitutive models was compared using statistical measures.The results indicate that the new constitutive model based on the Z-parameter can more accurately predict the flow stress of HSLA steel under different deformation conditions than the strain-dependent constitutive model based on the Arrhenius equation.The processing maps of the HSLA steel at strains of 0.3,0.5 and 0.7 were established.The results indicate that the size of the processing window of the steel decreased as the strain increased.When the steel was deformed at a strain of 0.7,there were two areas that formed stable deformation regions.Domain I occurred at strain rates from 0.018-0.562 s-1 and at temperatures from 875-1025°C with a low power dissipation efficiency(17%-33%).Domain II occurred at strain rates from 0.018–0.562 s-1 and at temperatures from 1050-1200°C with a high power dissipation efficiency(33%-50%).The processing maps revealed that the optimal hot working parameters for this steel at a true strain of 0.7 are temperatures from 1050–1200°C and strain rates from 0.018–0.562 s-1.The dynamic recrystallization model for the HSLA steel was established at temperatures from 950-1150°C and strain rates from 0.001-1 s-1.The relationship between the deformation parameters and Z parameters was also studied.EBSD and TEM were conducted on the specimen with a deformation parameter set of 950°C/0.01 s-1 at different strains.The results indicated that the critical strain of dynamic recrystallization was 0.223.When the true strain increased from 0.105 to 0.916,the average grain size decreased from 23.90?m to 17.75?m,and the fraction of HAGBs increased from 16.2%to 85%,and the mechanism of hot deformation transformed from work hardening to dynamic recrystallization softening.The rolling process of the HSLA steel was simulated utilizing the finite element method.The effects of the rolling parameters on the equivalent strain field,temperature field and rolling force of the workpiece were studied.According to the simulation results,the optimal rolling reduction of the HSLA steel ranged from 10%-20%,and the rolling rate was 1 m/s.The results show that the maximum rolling force increased with a corresponding increase in the reduction,decreased with an increasing rolling temperature and increased slightly with an increasing rolling speed.According to the results of the finite element simulation and austenite continuous cooling transformation experiment,the TMCP processes of HSLA steel were carried out.The microstructure of the rolled HSLA steel was obviously finer than that of the as-cast HSLA steel.The average grain size of the rolled HSLA steel after rolling is 5.32?m,and its yield strength,tensile strength,elongation and impact energy are improved. |
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