Study On Low Temperature Hot Deformation Behavior Of High Strength Steel Bars

High strength steel bar is the most widely used in the domestic and international construction of steel. How to produce high quality steel bar with low energy consumption and low cost has become the key for enterprise survival and the development of China’s iron and steel industry in recent years. Among them, the application of low temperature rolling technology is the most widely used.In this paper, the 20MnSi high strength steel bar rolling at low temperature and the 20MnSiV high strength steel bar produced by conventional processing technology are studied. The true stress-true strain curves, the heat treatment and the different microstructure of the two materials under high temperature and low strain rate were compared through thermal simulation compression experiment. The corresponding constitutive model was established, and the advantages and disadvantages of the two kinds of production processes were analyzed. With the increase of temperature and strain rate, the deformation resistance of 20MnSi steel is higher than that of 20MnSiV steel. When the temperature is 950℃ and 1000℃ and strain rate is 7s-1, the deformation resistance of 20MnSi steel is almost 60MPa higher than that of 20MnSiV steel. This shows that the 20MnSi steel which is not added with direct rolling has higher strength than that of the 20MnSiV steel under the same conditions. Combined with the processing maps, it can also be found that 20MnSi steel is more stable than 20MnSiV steel at high temperature.Based on the results obtained, the influence of different deformation temperatures and deformation rates on the deformation resistance and microstructure of 20MnSi steel under lower temperature conditions were further studied. The constitutive model was established and the hot processing maps were drawn during 700～850℃ deformation, and the optimal deformation parameters are determined. When the 20MnSi hot rolled steel bars deformed under low temperature and high strain rate conditions, all the true stress-true strain curves are dynamic recovery. And the effect of deformation temperature and deformation rate on the deformation resistance and microstructure of the material is similar to that at high temperature. Using the constitutive model of Zhou Jihua and Guan Kezhi, the constitutive model of 20MnSi hot rolled steel bar under the condition of 700～850℃ and 1-50s-1 was established: theoretical value and measured value fit well. By drawing the hot processing maps and the combination of different deformation conditions, the optimum deformation parameters of 20MnSi steel are determined:the deformation temperature is about 750℃ and the deformation rate is 2～10s-1.It is found that the deformation of 20MnSiV hot rolled steel bars has different deformation characteristics in the three areas, and the low temperature deformation resistance curve of 20MnSiV steel is determined and analyzed. The effect of fine grain strengthening and phase transformation hardening can ensure that the material deformed in the double phase region has higher strength and toughness than that of the austenite single phase region. Specially, when the temperature is 760℃, the deformation resistance decreases and the microstructure is excellent.