4, white dot
Hot rolled steel billet and white point is more common in large forging defects, is a breakdown of internal steel mold making. The presence of white steel with a very negative impact on performance, which mainly affects the mechanical properties of the steel mold design decreases when the forging hardening heat treatment cracking, or use to develop into more serious accident damage, so in any case Next, we can not use a white point of the forgings. White point of the different sensitivity of steel is different, generally considered prone to white spots are chrome steel, steel, chrome molybdenum steel, manganese steel, manganese steel, molybdenum, chromium nickel molybdenum steel, chromium tungsten steel. Which contains W (C) greater than 0.30%, W (Cr) greater than 1%, W (Ni) 2.5% of the earth martensitic chrome-nickel steel and chrome-nickel molybdenum steel, the sensitivity of the biggest white spot. The reason is the formation of white steel, precipitation of hydrogen in the precipitation accumulation, vertical section of the steel to form crystalline silver bright white thick round or oval spots. It tends to make forgings and billet material internal cracks. Die steel 5CrNiMo, 5CrMnMo white as the most likely to occur if the increase in carbon elements Cr, Mo, and V can reduce the white point after the sensitivity. In the production of such steel degassing must pay attention to and strengthen the great relief after forging or cold forging dehydrogenation annealing.
5, the oxygen content
General did not provide for die steel in steel gas content allowed. With the increase of oxygen content, oxide particles and volume increased, decreased fatigue properties of steel, are also prone to hot cracking. Some people have been tested 4Cr5MoSiV1 steel, the oxygen content of no more than 1.5 * 10-5, which provides Japan's Sanyo Special Steel Company of high purity oxygen content of steel is not greater than 1.0 * 10-5. Therefore, in recent years, in order to improve the quality of the mold making supplier. Gradually die steel at home and abroad to the low oxygen content in the direction of development.
6, the carbide inhomogeneity
Carbide is the most essential components of die steel, in addition to the carbide can be dissolved in austenite, there will be some residual carbon is not soluble in austenite. Carbide size, shape, distribution of properties such as the use of die steel has a very important influence. On the carbide size, shape and distribution methods and steel smelting, steel ingot solidification conditions and heat processing of deformation conditions and so on. Eutectoid carbon steel may be formed at the grain boundaries or in the processing of the wind-shaped deformation of carbide carbide carbide is elongated to form zonal or both, ledeburite die steel, there is a carbide and secondary carbides in the hot deformation process, the mesh can be broken most of eutectic carbide, carbide be extended along the deformation direction, resulting in ribbon, with the increase of deformation, the carbide becomes uniform, fine . Carbide heterogeneity on quenching deformation, cracking, mechanical properties of steel greater.
Segregation of the steel composition and organization of the performance of non-uniformity, which is in the mold steel macrostructure often test the existence of a defect. Is the ingot formed in the solidification process, and steel, chemical composition and pouring temperature and so on. Generally divided into dendritic segregation, square segregation, dotted segregation. Because the presence of dendritic segregation, so that all negative then the mechanical properties in different directions showed significant differences. Square ingot segregation is due to crystallization, the end of the columnar crystals and equiaxed ingot heart interval, bringing together more impurities and pores formed. Square severe segregation, the impact on the quality of steel is significant, especially cutting the amount of large parts or the force of the Department of heart mold parts. Segregation in addition to the mechanical properties of die steel isotropic, the polishing of the mold have an impact on performance. Therefore, foreign-related standards have strict requirements.
Loose density of steel is not the performance. Most loose in the upper and central parts of ingots, because in these places more focus on the result of impurities and gases. Because osteoporosis defects, reducing the strength and toughness of steel, but also seriously affected the surface roughness after processing, in general the impact of mold steel is not particularly large, but if cold roll, a large block, punch and plastic molding mold and others have more stringent requirements. Such as deep cavity of the forging die and punch requires loose no more than 1 or 2, for the dial or light pieces of plastic mold steel, requires no more than one loose.