Boron-containing steels are low-alloy steels based on manganese and boron instead of chromium steel or nickel steel. Boron added to the steel as an alloying element can significantly increase the hardenability, increase the hardenability of the steel, and increase the uniformity of the microstructure and properties in the steel section after quenching, as long as it is 0.001% to 0.002%.
Cold-heading steel has been used in medium-carbon steel or low-alloy steel. These steels have poor deformability (needs ball annealing and intermediate softening annealing), poor mechanical properties, and serious decarburization. While reducing the carbon content, improving the cold crucible deformation capacity, adding boron to improve the reduction of carbon and loss of hardenability, which is the basic meaning of the use of boron steel. Boron-containing steel uses a small amount of boron instead of a large number of other alloying elements. Therefore, compared with other alloy steels with comparable hardenability levels, the total alloy content is low, and high-temperature deformation resistance is small, and plastic deformation is easy.
It should be noted:
1Thermal processing has the greatest influence on the variation of various forms of boron in solid steel. When the content of boron in the steel exceeds 0.003%, a low-melting-point Fe-CB ternary eutectic occurs at the grain boundary and heat is generated. Brittleness degrades hot workability;
2 The main role of boron is to improve hardenability, so it is necessary to ensure that the content of solid solution of boron is in a suitable range, greater than 0.001%, so as not to impair hardenability. In addition, whether boron can play its due role, in a large The degree depends on the selection and operation of the heat treatment system;
3 Quenching temperature should be selected according to the boron content of steel, chemical composition and the state of boron before quenching, and should not be fixed, the tempering temperature in the heat treatment system decreases, will increase the delayed fracture tendency of boron steel bolts . Many researches and practical experiences have also proved that the increase of normalized boron before quenching is extremely beneficial, which can eliminate or reduce the boron phase and improve the performance of boron-containing steels after quenching and tempering.
10B21, 10B28, 10B33, 20MnTiB and ML20MnTiB are used most in bolts. The final mechanical properties of the bolts are determined by the quality of the materials used to make the fasteners, while the lower limits of the carbon content of 10B21, 20MnTiB and ML20MnTiB are all lower than 0.20% and cannot be satisfied. Standard requirements for materials; when the bolt strength ≥ 1040MPa, hardness in the 33 ~ 39HRC requirements, is achieved by lowering the tempering temperature, when using 425 °C and then tempering temperature test hardness and strength are reduced, can not meet the standard The minimum tempering temperature performance requirement.
It can be seen from the production practice that the mechanical properties of bolts are not only determined by the heat treatment process and the metallurgical structure, but more importantly they should have good chemical composition.
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