In what way are carbon steel castings crystallized? What kind of organization is ultimately obtained? This is determined on the one hand by the carbon content (related to the state diagram), and on the other hand, it is closely related to the cooling rate. Different crystallization conditions lead to different crystallization methods. When crystalline cores are formed and grow simultaneously over a large area, volumetric crystallization is formed. When the cooling rate is not too fast and the degree of subcooling is not large, after the latent heat of crystallization is released, as the heat is transferred outward, the crystallization proceeds layer by layer inward, thus forming sequential crystallization.

　　On the cross-section of carbon steel castings, there are generally three crystal bands: a fine equiaxed crystal band on the surface, a coarse equiaxed crystal band in the center, and a columnar crystal band between them. The reasons for the formation of these three crystal bands are as follows:When the molten steel is injected into the cavity and comes into contact with it, the temperature of the surface layer of the molten steel can immediately drop below the melting point due to the rapid cooling effect of the mold, resulting in a significant degree of undercooling. Due to the rapid cooling rate, the latent heat of crystallization released during crystallization can be quickly transferred to the mold without affecting the progress of the crystallization process. At this point, a large number of crystalline cores are formed over a relatively large area, growing simultaneously without being affected by the direction of heat flow. Crystallization occurs in a volumetric crystallization manner, resulting in a very fine equiaxed crystal structure. In the surface layer, a large number of cores are formed simultaneously, developing into a fine equiaxed crystal structure. Very fine equiaxed crystal structures and fine equiaxed crystal structures are called fine equiaxed crystal bands. Crystallization proceeds from the surface of the casting towards the center. When the cooling rate of the molten steel slows down and the degree of subcooling is not large, the directivity of heat transfer becomes obvious, and crystallization proceeds inward along the layer (i.e., sequential crystallization). At this time, if the number of formed nuclei is not large but the growth rate is relatively fast, and the cooling rate and growth rate are adapted, it is easy to develop inward in a single direction (against the direction of heat flow) along the main axis of the dendrites.Finally, columnar crystal bands are formed. From this, crystallization is carried out towards the center of the casting. The cooling rate is extremely slow, and the temperature distribution at the center of the casting is very uniform. The latent heat of crystallization released during crystallization is transferred very slowly, and nuclei are formed over a wide range. However, due to the small degree of subcooling, the number of nuclei formed is the least, and the directionality of heat transfer is not strong. It proceeds in the form of volume crystallization and eventually develops into large equiaxed crystal bands.

　　The crystallization intervals of low-carbon steel are small. As the carbon content increases, the crystallization intervals become wider. Under the same cooling conditions, the distribution ratios of the three types of crystal bands on the cross-sections of low-carbon steel and medium carbon steel castings are different. The columnar crystal band of low-carbon steel is wide, while the coarse equiaxed crystal band at the center is very narrow. The columnar crystal band of medium carbon steel is relatively narrow, while the coarse equiaxed crystal band at the center is relatively wide.

　　For carbon steel castings with a certain carbon content, as the cooling rate increases, the solidification zone Narrows, often promoting the development of columnar crystal bands. For instance, when some castings that were cast in sand molds are changed to be cast in metal molds, columnar crystals are prone to form on the cross-section. This is the reason. The crystallization directionality of columnar crystals is strong, and the crystallization proceeds layer by layer, which is conducive to the replenishment of molten steel and the uniformity of the crystallization

Mi. Its drawback is that the intergranular performance is relatively poor. During the crystallization process of coarse equiaxed crystals, as dendrites grow, the unsolidified liquid between dendrites is isolated, forming small closed metal pools that are isolated from the external liquid. When this closed molten steel solidifies without replenishment of the outside liquid, it shrinks between dendrites 躭 and forms shrinkage porosity. Ideally, the entire cross-section of the casting is composed entirely of fine equiaxed grains, but this is very difficult to achieve in production.

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