H, O, N, P, and S are called "five-hazard elements," and seriously harm cold-headed steel for fasteners. For fastener companies, the detection of fastener production steel is necessary. The reduction of the five harmful elements makes the number of sulfides, oxides, and nitrides in the cold crucible steel, which may be the source of the cracks, to be contained in the grains and the number of grain boundaries to be reduced. As a result, the grain boundary weakening effect is reduced, and the fracture toughness and impact toughness are improved. . Studies have shown that in the 0.60% to 0.90% Mn cold crucible steel, S mainly forms MnS inclusions, MnS inclusion in the steel matrix corresponds to the crack source, and segregation of S and P elements at grain boundaries weakens the grain boundary. Therefore, The increase of S and P content reduces the plasticity of the steel and reduces the reduction of the area of the steel.
In the steel, N mainly forms nitride inclusions. Therefore, the increase in N causes the reduction of the area of the steel to be significantly reduced, and the elongation after the fracture does not change much. O Major oxide inclusions are formed in the steel. O increases to increase oxide inclusions, and the reduction of the area of the steel significantly decreases. H produces defects such as white spots and spot segregation in the steel. As the H content decreases, defects decrease, and the area shrinkage increases significantly.
For large-scale high-strength bolt steels, reducing the five-hazard elements, thereby reducing the content of non-metallic inclusions and segregation of inclusion elements in the grain boundary, increasing the crack nucleation and expansion resistance, is very effective to ensure the normal service of the bolt . Classes, aluminas, and spherical oxides are not more than 1.5 (including coarse and fine lines). If the impact toughness is not enough, consider adding a 580 ~ 600 °C and then tempering process, and then fast cooling.
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