The main purpose of heating the ingot is to make the steel heated to the forging temperature range (that is, in the plastic deformation state or austenitizing temperature range) through reasonable heating process specifications, and then processed into the required forging blank under the action of the press. Because it is necessary to eliminate or improve the precipitated carbides in cold-rolled steel 9Cr2Mo, it is necessary to carry out high temperature diffusion annealing on the ingot at the same time during heating. According to the characteristics of box reverberatory furnace for 1250 ton hydraulic press, its heating process specification is shown in Figure 5.
The main purpose of ingot preheating is to make the material in the so-called blue brittle stage for a long time when the temperature head of the material is 250 ~ 300℃ at the end of the waiting period, so that the temperature stress gradient inside the material is too large in the subsequent heating process, forming cracks or breaking the material. Insulation at 800 ~ 850℃ can further reduce the temperature head of the ingot, and the insulated ingot will enter the category of hot ingot in the usual sense, and then it can be heated at a faster heating speed and enter the forging insulation stage. This operation can not only ensure the heating requirements of the ingot, but also greatly improve the working efficiency. At the same time, the heat preservation time of hot ingot before forging is 65438 0.5 ~ 2 times longer than that of conventional heat preservation time, so that the residual liquid carbide is basically eliminated after solid solution recrystallization of ingot, and the purpose of high temperature diffusion annealing of ingot is achieved.
2 forging process specification
Because the anti-failure ability of 9Cr2Mo cold roll is very sensitive to the inhomogeneity of steel structure and properties, there are strict requirements on the structure and original structure in use. There are two main requirements for the original structure: first, high metallurgical quality; The second is the unity of the organization. High metallurgical quality means that the content of residual nonmetallic inclusions, especially brittle oxides, in cold-rolled steel should be as low as possible, and white spots are not allowed; Uniform microstructure requires not only that the martensite structure itself should be fine and uniform, but more importantly, the broken banded carbide particles should be as fine (0.5 ~ 2 microns) and dispersed as possible, which can reduce the content of residual austenite in the working layer of the roll body and reasonably control the residual stress in all parts of the roll body.
Based on the above reasons, reasonable selection of upsetting ratio and reasonable distribution of upsetting ratio (i.e. upsetting times) in forging process can solve this problem to a great extent. The total technological process of 6 tons φ800 diameter circular 9Cr2Mo electroslag remelting ingot is as follows: bottom cutting, claw pressing, upsetting, φ drawing, blanking, forming, shaping and preliminary heat treatment. In this process, there are the following specific requirements in some work steps.
3 Preparatory heat treatment
The purpose of preliminary heat treatment for forging 9Cr2Mo cold roll steel is not only to eliminate the structural inhomogeneity caused by casting and forging, to improve the morphology and distribution characteristics of carbides, to diffuse the hydrogen remaining in the steel due to melting methods, and to prevent the generation of white spots, but also to prepare the necessary original structure for the final heat treatment, so as to ensure the good mechanical properties of the core and neck of the roll body. Therefore, it is very necessary to formulate reasonable pretreatment technical specifications.