Research On The Forging Process Controlling Quality Of Cr5 Cold Roll Blank

According to elimination of a series of metallurgical defects in Cr5 electroslag remelt (ESR) ingot, such as liquation carbide, void defects, composition segregation and network carbide etc, a comprehensive research on forging process controlling the quality of cold roll blank, which is based on experiments, practice and simulation technology, is carried out in this paper. By establishing the corresponding model, performing calculation and analysis, the essential features of effect of forging parameters on the quality are disclosed. It provides the theoretical basis and design method for making forging process to improve roll blank quality, which is of important practical value.The carbide is extracted by electrolysis process and analyzed by XRD. The results show that liquation carbide is mainly M7C3 in Cr5 ESR ingot. Also, the morphology and quantity of liquation carbide changing with the holding time are counted. It shows that the diffusion of liquation carbide in Cr5 finished steel is more slowly than that in the ESR ingot. Therefore, liquation carbide, this kind of metallurgical defect, should be removed during the homogenization treatment before forging as much as possible.There are some void defects in Cr5 ESR ingot, and voids section shape is mainly roundness. However, any internal void from the initial cast ingot needs to be eliminated. On this basis, simulation software (DEFORM-3D) is used for the analysis of the evolution of void defects in Cr5 ESR ingot during the forging operation, and the effects of parameters on void closure are discussed. Then, the essence of void closure is further explored on the basis of simulated results. Limited by conditions of high temperature and high pressure, the x-ray diffraction method for determining residual strain of retained austenite is adopted to obtain the distributions of strain in the ingot. It shows that during single-pass forging, for models without void, distribution ofεz along the vertical direction can be given by Gaussian function fitting; for models with void, while distribution ofεz can be given by discontinuous Gaussian function fitting, cutoff bound of the discontinuous function is determined by equal-area method. Hereby, a kind of new researching method for solving void closure is proposed from the view of strain function. Then, the critical condition of void closure is presented by derivation and calculation of the strain function, which gives the essential description of void closing. For Cr5 steel, the criterion for spherical void closure is proposed, i.e.,εz0(z1)≥0.65. In the practical process, the region of void closure determined by the criterion is developed to express the closure of void located at any position, which simplifies the models with void. Also, with the region of void closure, a specific problem of the closing of void located at one or other position is transformed into a general one. It provides a new approach with extensive application prospect in terms of elimination of void defects in the ingot.During multiple-pass forging, the distributions of strain in the axial forging zones can be expressed by linearly superposition of the Gaussian functions. The optimal value of feed, i.e., x0=175.309 mm, which can assure uniform strain distributions in the center of forgings, is determined by analytic solution of the strain function. Hereby, an efficient forging process is presented to improve the void closing and to achieve the uniform forging effect.Pearlite is the dominant microstructure in the matrix of annealed Cr5 ESR ingot. And there is composition segregation inside the grains. Therefore, distribution of chromium in Cr5 ESR ingot under two different states (i.e., the homogenization treatment after forging and the homogenization treatment with no deformation) is measured in the present paper. On the basis of the experimental data, the mechanism is studied from the view of the Fick's first law. By establishing the diffusion model with different deformation conditions, using combined solution of the analytic and numerical solution, the quantitative differences of diffusivity of Cr pre and post deformation are calculated from the two aspects, the geometry change and recrystallization grain boundary. It shows effect of forging on diffusion is made of linear superposition of effect of the geometry change and recrystallization grain boundary, which proposes quantitative and theoretical explanation of forging enhancing the diffusion. In addition, an empirical formula to describe the relationship between the average diffusion concentration of Cr and the relative draft is presented, which has guiding significance for making the forging process.In order to avoid overburning, the initial forging temperature of Cr5 steel should not exceed 1200℃, hereby, it is set at 1150℃. The temperature field distribution of cross section is given by numerical simulation of temperature field of the ESR ingot forging process; by the mode of point tracking, the temperature variation on the surface and inner of the ingot are analyzed, and the difficulties that the temperature variation on the inner of the ingot can't be measured in the practical process are solved. On the other hand, the temperature field distribution of cross section at the final forging temperature provides initial conditions for calculating the temperature field of cooling after forging. The simulation results show that ingot surface temperature is decreased to 900℃, and the temperature at the core almost remains in 1150℃after forging. On this basis, the variation of cooling rate in different site of cross section is obtained. By comparing with the critical cooling rate restraining network carbide precipitation at that temperature, prediction for the distributions of network carbide is implemented. At current cooling conditions(i.e., with the environmental temperature of 55℃), there is no obvious network carbide precipitation within about 30 mm scale range apart from the surface of roll blank with the roll diameter 460 mm.