軸受鋼巨大炭化物の均熱拡散

抄録

On the cost down of manufacturing bearing steel tubes, Hikari Works has adapted the continuously cast process and then studied the soaking process. The studies were mainly made with continuously cast blooms which have 260 mm square section and 180 mm octagonal section. A 350mm square section conventional cast ingot was also examined for comparison.
Soaking was the necessary process to dissolve massive carbides in bearing steels. Without this process these coarbides were decomposed by hot working but composed again in the annealing process.(Photo.2, Fig.6) The massive carbides appeared much at center parts of cross section of blooms or ingots, and size of the carbides was larger in larger cross section.(Fig.2, Fig.3) It seems that these massive carbides formed as the eutectics at the grain boundary segregate parts in non equilibrium solidification.
The massive carbides were dissolved by soaking. Experiments of soaking showed that the process was diffusional (Fig.7). The soaking time became shorter at higher soaking temperature to 1260°C. Above this temperature the steel would be overheated.
The dissolution conditions depended on amount and size of the massive carbides.(Fig.8) The carbides were analyzed as (Fe, Cr) ₃C. In these constituents Cr is most difficult to diffuse. Therefore the dissolution conditions of the massive carbides were considered to be determined by diffusion of Cr. Single Cr sphere which had same size as maximum size of the massive carbides in blooms or ingots, was watched. During the soaking process Cr concentration of the sphere would be decreased to same concentration in matrix. These considerations gave eq.(4). From this equation and experimental roults, the diffusion constants of the massive carbides in bearing steels were shown in table 2 and the activity energy to diffuse was 1.63-1.79×10⁵ Cal/g atom. Obtained diffusion constants were slightly lower and activity energy was slightly higher as compared to the case of diffusion of Cr in pure austenite. Therefore in the dissolution of the massive carbides was considered that the constituent atoms of the carbides separated out of molecular bond of cementite and diffused in segregate parts of the steels. From es:(4), the relation of the diffusion constant and the activity energy eq.(5) and the experimental results, the dissolution conditions were derived as eq.(7). This equation showed that the conditions were determined by maximum size of the massive carbides in blooms or ingots. Calculation examples are shown in Fig.7.
Finally, we approved that experimental results were able to apply the process of mass production of bearing steel.