Numerical Simulation Research On The Longitudinal Rolling Process In Low Temperature For Large High-strength Stell Bars

Bar is kind of steel that widely used in construction, infrastructure construction and automotive industries, playing an irreplaceable role in the development of the national economy and social construction. However, the insufficient of toughness and ductility has become an obstacle to the process of making it reach a new level of high-strength.The dynamic recovery / recrystallization conversion mechanism occurs in steel substrate under large strain and high value of parameter Zener-Hollomon conditions makes it obtain ultra-fined ferrite grains without adding alloying elements, which doubles the strength under the premise of remaining a good fit of strength and toughness. It is an ideal way tof grain refinement. In this paper, a deep research on microstructure refinement theory for large medium carbon steel is carried out by the way of using a combination of experiments and numerical simulation, considering both the product size and mechanical properties of the bar. On this basis, a rolling pass system suitable for producing large high-strength bars with ultra-fined ferrite grains is developed.In order to obtain the process parameters of mid-carbon steel rolling to produce ultra-fined ferrite grains, experiments are carried out to study the deformation characteristics and evolution of the structure under large deformation and low temperature conditions with the help of Gleeble-3800 thermal simulation testing machine and field emission scanning electron microscope, confirming that large deformation process under high value of parameter Zener-Hollomon(low temperature & high strain rate) processing conditions agree with the demands of obtaining mid-carbon steel bars with ultra-fined ferrite grains.According to the results of the experimental data, the plastic deformation constitutive equation of mid-carbon steel is derived by studying the relationships between the peak stress and peak strain in the flow curves and the parameter Zener-Hollomon; With the secondary development of finite element technique, the average sizes of ferrite grains under different deformation conditions are simulated and obtained and are used to construct a formula to predict the average sizes of ferrite grains obtained in actual deformation conditions. Taking advantages of computer-aided design and finite element simulation technology, a rolling pass system suitable for producing large high-strength bars with ultra-fined ferrite grains is developed. A finite elemental model of mid-carbon steel rolling process, coupling of strain, deformation temperature, strain rate, and other conditions, is built and used to simulate the strain and temperature distribution and the evolution of structure in the cross-section of rolled piece, obtaining good results. The agreement of the experimental and numerical simulation results confirms that the new type of rolling process is suitable for producing large high-strength bars with ultra-fined ferrite grains.