Study On Hot Deformation Performance And Processing Maps Of 38MnVTi Steel

The material of non-quenched and tempered steel using controlled forging and cooling technique can reach the mechanical properties of quenching and tempering process. 38 MnVTi non-quenched and tempered steel was the common forging material for front axle and steering knuckle of automobile and it had high requirement of the forging mechanical property. Deformation behavior of hot deformation for 38 MnVTi non-quenched and tempered steel had been studied according to the experiment of isothermal compression. The processing maps of Parasd and Murty criterions were obtained by those means of constitutive equation 、 artificial neural networks and microstrueture. The main research contents and conclusions in this paper were as follows:(1) The characteristics of stress-strain curves for 38 MnVTi non-quenched and tempered steel were analysed. The speed of reaching peak stresses was related to the values of the deformation temperatures.The higher the temperature, the faster peak values of stress arrived. Gentle or falling characteristics of stress-strain curves were related to the softening mechanisms such as dynamic recovery and dynamic recrystallization, which could impact the change of stress-strain curves.(2)The relationship between deformation parameters and flow stresses was also studied.Constitutive equation of peak stresses aiming to describe the biggest property of strength for deformation material was gained by regression analysis. In order to expand the application scope of constitutive equation, the polynomial of strain made of material constants was taken into the Arrhenius-type constitutive models.(3) Comparing of the accuracy of stress prediction between constitutive equation and BP artificial neural networks has been carried out. The input information included strain 、temperature and strain rate while the output information was stress, and artificial neural networks were gained by the input and output information. The predict stress values of artificial neural networks after training were more accurate than that of constitutive equation. Using the trained neural network had a prediction of stresses for different strain rate at the deformation temperatures of 1050℃ and 1150℃.(4)The processing maps of Parasd criterion and Murty criterion were built and their applicability was tested. The processing maps with different strain were analysed by characteristics of power dissipation efficiency and instability areas. Many verifications such as microstrueture、hardness and tensile test were adopted to verify the processing maps. The results show that the drawing of processing maps should choose the strain of stable stage and strain in unstable stage is not able to ensure the accuracy of processing maps. These two types of criteria have similarity, and the instability areas which use Parasd criteria relate to low power dissipation. For the process designing, the instability areas and characteristic of power dissipation are considered together when use processing maps of the two criterions. Besides,for the non-quenched and tempered steel of 38 MnVTi, It is not allowed for deformation at the condition of high strain-rate with large strain,or the instability will be appear in the microstrueture.The optimum processing condition of this material for hot deformation shows that the temperature range is 1100~1200℃ and strain rate of 0.1 ~ 1s-1 after process optimization.