Multi-dimensional Deformation And Experimental Research Of The Heavy Steel Plate For Gradient Temperature Rolling

The heavy steel plate are widely used in offshore platforms,oil pipelines and engineering machinery.The symmetrical rolling for the thick steel plate will result in insufficient central deformation,coarse grains and low overall performance.The central deformation can not be improved by increasing the total compression ratio limited by the maximum thickness of the continuous casting slab and the opening of the rolling mill.The research object is the process of the gradient temperature rolling（GTR）for the heavy steel plate,aiming to improve the central deformation and microstructure,and reduce the flatness of the heavy steel plate.The multi-dimensional deformation influence law of the steel plate with GTR was studied by thermal-mechanical coupling finite element simulation,theoretical analysis and experimental verification.The main work of this paper is as follows:（1）The influence of different heat transfer coefficient,cooling time and initial thickness on the temperature field distribution and temperature gradient along the thickness direction of the steel plate was studied by ANSYS implicit analysis,which lays a foundation for the deformation process of the GTR.The heat transfer coefficient and cooling time have an important influence on the temperature field of the heavy steel plate,and the temperature drop mainly occurs in the surface layer when the heat transfer coefficient is1000～11000W·m^-2·K^-1and the cooling time is within 25 seconds.When the cooling conditions are the same,the temperature field distribution is less affected by the initial thickness when the plate thickness is range 100～350 mm.The temperature drop mainly occurs within 30 mm from the surface.（2）The GTR process was simulated based on the thermal-mechanical coupling and investigated the effects of different process and equipment parameters on the thickness direction and central deformation.The strain calculation model of the GTR is established by the flow function method based on the principle of minimum energy.Through the calculation results,the calculation model of the central strain with GTR is fitted by the least square method.The central deformation with GTR is easier to penetrate into the core compared with the uniform temperature rolling（UTR）.The central deformation gradually increases with the increase of the surface heat transfer coefficient,speed ratio,roll diameter and reduction rate,while the initial speed has little effect on the central deformation of heavy steel plate.（3）According to the thermodynamic effect of the materials,the asymmetrical cooling method was used to control the plate shape,and studied the influence of different heat transfer coefficient ratio,cooling time,initial thickness and initial curvature on the plate flatness.When other process parameters are constant,the plate flatness can be effectively reduced by adjusting the heat transfer coefficient ratio and cooling time.Under the same conditions,the plate flatness after asymmetrical cooling is also greater with the increase of the initial thickness and initial curvature.（4）The central microstructure of the 45 steel and 42Cr Mo was characterized by OM,SEM and EBSD based on the experiments of GTR.And the tensile mechanical properties of steel plate under different processes were measured.The results showed that there are a large number of acicular ferrite in the center and the grain size is refined compared with the UTR.During the uniform temperature rolling,the proportion of massive ferrite is larger,the intragranular substructure is smaller,and the proportion of small-angle grain boundaries is smaller.The steel plate has high mechanical properties with gradient temperature rolling.