Development Of 500MPa Grade Hot Rolled Wire Rod Of Fine Grains

The work of this dissertation was carried out integrating with the 863 project of 'The Advanced Industrial Manufacture Technology of 500MPa Carbon Steel' and the 'Research on Functional Linearization of Deformation Power and Its Application in Metal Forming'which sponsored by National Natural Science Foundation of China. The microstructure evolution and mechanical properties of plain C-Mn tested steel during controlled rolling and controlled cooling were investigated. In addition, industrial trials to develop HRBF500 wire rod were carried out. The main works are shown as following:(1) Temperature evolution and reheating behavior of hot rolled wire rod in the controlled cooling zonesThe transient temperature field model of hot rolled wire rod was developed by ANSYS software to calculate temperature distribution inside the workpiece from exit of the finishing mill to Stelmor fans cooling zone. The effects taken into account were radiation, forced convection and natural cooling. The temperature field and reheating behavior of the workpiece with time during controlled cooling were analyzed, which provided the foundation for the qualitative prediction of the microstructure evolution and mechanical properties.(2) Deformation and temperature jump of the workpiece in the finishing millIn order to calculate the inhomogeneous deformation of hot rolled wire rod, a new analytic method called OA (Overlapping Area) method to solve the effective strain without using rectangular sections conversion is therefore proposed on basis of the principle of the equivalent rectangular approximation method. The spread types and the range of effective strain in oval-round passes were determined by a parameter which is the ratio of overlapping area to the entry area for a given pass. To validate the OA method, it was used to calculate the temperature jump during finish rolling ofΦ6.5mm wire. The results show that the calculated values of accumulative temperature jump is in good agreement with the measured ones as the maximum relative error is no more than 10%. The effect of the entry temperatureθand finishing exit speed v on the total temperature jumpΔθwas analyzed. The results show thatΔθslightly decreases with lowering the speed by 10%, but it almost increases linearly with decreasingθ.(3) Room tensile properties and deformation behaviour of tested steelThe mechanical behaviors of several tested steels in uniaxial tension were compared including the relationship between work-hardening coefficient, work-hardening index, work-hardening rate and true strain. The results reveal that in uniform deformation stage the different yield style tested steels possess the similar strain hardening characteristics, and their characteristics have an obvious difference in small strain stage. From physical essence of yield phenomenon, the reason of no yield point with abnormal mechanical properties similar to dual phase steel for tested steel was analyzed with the colligation of several theories including dislocation multiplying, residual stress theory, the multiplication yield model of polycrystalline metal, and so on. Then, the corresponding countermeasures were put forward.(4) Process simulation and capacity of rolling mill checkThe microstructure evolution for plain C-Mn steel in controlled cooling process was investigated using single pass compression test and metallographic analysis. Moreover effect rule of cooling rate and Mn content on microstructure was obtained. The capacity of roughing mill was checked during controlled rolling by the functional linearization of deformation power method. The results show the roll strength and electric engine load of No.3 roughing mill are within the permissive range.(5) Industrial trials to develop the HRBF500 wire rodIndustrial trials were carried out on high speed wire mill based on the results of thermal-mechanical simulation and temperature field analysis. The process routine was determined by adjusting chemical composition, rolling temperature and cooling parameter in order to improve mechanical properties. After decreasing Mn content and increasing C content, HRBF500 wire rod was successfully produced.