The uneven sign of chemical composition and impurity diffusion in steel is called segregation. Ordinary will be higher than the balance component, called positive segregation, lower than the balance component, called negative segregation. There are still macro segregation, such as regional segregation and microsegregation, such as dendrite segregation, intergranular segregation.
Segregation in large forgings is closely related to segregation of steel ingots, and the degree of segregation of steel ingots is related to steel grade, ingot type, smelting quality and casting conditions. Alloying elements, impurity content, and gas in steel all exacerbate the development of segregation. The larger the ingot, the higher the pouring temperature, the faster the casting rate, and the more segregation.
(1) Regional segregation It is a macrosegregation, which is caused by the selection of crystals in the agglomeration process of molten steel, the change in solubility and the difference in specific gravity.
The countermeasures are:
1) The content of segregation elements and good bodies such as sulfur and phosphorus in low steel, such as refining outside the furnace, vacuum carbon deoxidation (VCD) treatment and ingot bottom blowing argon.
2) Accept multi-furnace pouring, riser refilling, vibrating pouring and heat-insulating riser, and strengthen measures such as feeding and refining.
3) Strictly control the temperature injection and injection speed, accept short and thick ingot type, and improve crystallization conditions.
(2) Dendritic segregation belongs to microsegregation. The unevenness of dendritic crystals and the intergranular micro-area components probably cause the uneven distribution of structural properties. Acceptance of scanning electron microscopy (SEM), spectrometer (WDS), energy spectrometer (EDS) for micro-area investigation and composition description can detect and clarify the reason, ordinary experience high temperature dispersion heating, forging reasonable deformation and uniform heat treatment can eliminate or reduce Its adverse effects.