Study On The Mechanism Of Three Layers Rolling Clad Of Extra Thick Steel Plate Using Pre-Rolled Continuous Casting Billets As Intermediate Layer

Special thick plate is widely used in military and civil fields, and it has a great market demand. The clad rolling technology of continuous casting slabs uses the continuous casting slabs as raw material. And then hot rolling is conducted with the slabs combined by vacuum electron beam welding. It has advantages of high production efficiency and low cost. Moreover, the production of extra thick steel plate has good interface structure and mechanical properties, which has significant economic and social benefits. However, the hot rolling clad technology of double layers combination currently used, especially when producing the much thicker specification steel plate, in order to guarantee the bonding strength of the interface and the elongation of the rolling process, the requirements for the initial thickness of the slabs are usually higher, which limits the production capacity of extra thick plate in ordinary small steel enterprises with few thickness specifications of the continuous cast slabs. Therefore, this paper deeply studies the clad mechanism of hot rolling. The three layer clad rolling technology for extra thick plate using the pre-rolling billet as the middle layer is proposed. Using the good microstructure and properties of the pre-rolling billet, on the basis of satisfying the interface bonding strength, it can effectively reduce the initial thickness specifications, which has realistic significance to guide industrial production.Considering the large billet exists porosity, shrinkage hole and other cast defects, and the complex nonlinear relationship between the bonding strength of the hot rolling clad interface and the parameters of the rolling temperature, rolling reduction and rolling speed, this paper combines the numerical simulation method with the scale model of hot rolling experiment in the general conditions of two rolling mill. And the three layers rolling clad mechanism of the intermediate layer pre-rolling billet is studied. The specific research contents are as follows:(1) Interfacial shear deformation is an important difference between the rolling cladding and the traditional diffusion welding, but the shear can’t be considered in the physical simulation model. There exists the thermodynamic similarity between the process of producing extra thick plate by hot rolling with continuous casting slabs and the process of hot rolling under laboratory conditions. Using the finite element MARC software, the 2-D models of extra thick plate clad rolling both in industrial field and in the laboratory are established. And the thermodynamic condition similarity of them is analyzed. From the simulation results we can know the interface bonding effect of the physical simulation research in the laboratory is approximately equivalent to that of the clad rolling in the industrial field. By using the physical simulation experiment, the interface bonding effect of the clad rolling in the industrial field can be predicted;(2) Based on the experiment of double layers Q235 clad rolling in the laboratory, through the analysis of the microstructure, energy spectrum and the bonding strength of the interface, the effects of different ways of billet combination on the microstructure and the bonding strength of hot clad rolling were analyzed. And the appropriate scheme of billet combination is proposed to meet the physical simulation experiment.(3) The interfacial clad mechanism of traditional double layers clad rolling technology as well as the influence of the reduction on the macro and micro properties of the clad interface and the substrate are analyzed, Which lays the foundation for the new hot rolling clad process;(4) A new technique for reducing the thickness of initial billets is presented which is the three layers clad rolling process based on the intermediate layer pre-rolling billet. Using finite element DEFORM-2D software, the hot rolling clad models of the double layers and three layers of Q235 continuous casting billets are establish. And the loose closure situations of the center of continuous casting billets are compared in the two different ways of billet combination in the rolling process. Simulation results show the three layer rolling process based on the intermediate layer pre-rolling billet not only can solve the problem that the deformation is difficult to penetrate into the core and the large grain size in the core. And it is able to make full use of the large deformation of the edge. Thus fewer passes and smaller total compression rate can achieve the deformation to change the cast microstructure of the upper and lower layers of continuous casting billets, and then the core loose closure is realized.(5) The clad rolling experiment of three layer billets with different thickness was carried out. The influence of the thickness ratio of each layer and the reduction(rolling passes) on the bonding effect of the interface and the properties of the substrate are analyzed. Combined with the thermodynamic similarity conditions, the theoretical guidance is provided for the actual industrial production.