Thread Content
Element B1: When steel contains trace amounts of boron (0.001–0.005%), its hardenability can be doubled. 2. Adding a trace amount of boron to steel can improve its density and hot rolling properties, as well as enhance its strength. 3. Trace amounts of boron can improve the hardenability of steel, but as the carbon content in the steel increases, this improvement in hardenability gradually diminishes until it disappears altogether ； 4. Beneficial effects of boron: 1) Adding a small amount of boron (0.0005–0.005%) to steel significantly improves its hardenability, with little or no effect on other properties. ——To a certain extent, this can replace Ni (Cr, Mo). 2) Boron has little effect on the sensitivity of steel to quench cracking. 3) Boron in structural steel reduces the impact value of the steel after normalizing, but good impact values can be obtained after quenching + low-temperature tempering. 4) Low-carbon boron steel has good carburizing properties, and the surface carbon concentration does not tend to increase excessively. Therefore, it exhibits high strength and fatigue strength; it can be directly quenched after carburizing, and has very low notch sensitivity. For carburized boron steel, a carbon content of C≯1% is appropriate. 5) Medium-carbon boron steel exhibits good comprehensive mechanical properties after quenching and tempering. (Its temper stability, temper brittleness, the relationship between fatigue limit and strength/hardness, etc., are basically the same as those of boron-free steel.) 6) Boron steel has good hot working properties, similar to those of ordinary alloy structural steel. 7) Boron dissolves in the solid solution, causing the lattice to expand and thereby increasing strength. Boron at the grain boundaries prevents recrystallization diffusion, which allows for an increase in the heat resistance of steel. 5. Adverse effects of boron: 1) When the B content exceeds 0.007%, it can easily lead to brittleness (pearlite is said to reach this level, while other types of steel may tolerate higher levels). 2) It will lower the temperature at which grain coarsening occurs in the A phase, making grain coarsening more likely; however, adding aluminum can improve this situation. 3) During heat treatment of medium-grain boron steel, needle-like ferrite tends to form in the core, affecting mechanical properties. 4) Boron has a strong affinity for O and N, which leads to the formation of non-metallic inclusions; therefore, a higher amount of boron should be used. To overcome this defect, 0.1–0.12% Al and 0.06–0.04% Ti can be added during smelting for deoxidation and denitrogenation (the burn-off values of Al and Ti are not taken into account). The B content in ordinary alloy steels ranges from 0.001% to 0.005%. Currently, only alloy structural steels contain boron at levels as mentioned above; other boron-containing steels are less common. There are many types of boron steel abroad, but their B content never exceeds 0.005%, as otherwise the hardenability will decrease.