With the continuous advancement of mining technology and the increase in impact power of rock drills, the requirements for steel for rock drill bits are also getting higher and higher. The steel for high-quality rock drill bits is mainly Cr-Ni high-quality steel, such as 24SiMnCrNi2Mo in Sweden, 30Cr2Ni4Mo in Britain and the United States, etc. The high-quality steel used in rock drill bits contains many alloying elements. What are the functions of these different alloying elements?
Carbon
Carbon mainly affects the strength and hardness of steel. The strengthening of carbon is through solid solution strengthening, lowering the phase change point to refine the grain, and phase change reinforcement to achieve. The amount of carbon is too high, which will reduce the toughness and welding properties of steel. Carbon has an extremely high solid solution-strengthening effect. Although the increase of carbon content in steel can improve the strength and wear resistance of the parts, it damages the plastic toughness and processability of steel, so the design of steel carbon content, according to the different conditions to determine the carbon content, rock drill bit with steel needs higher strength and wears resistance and a certain plastic toughness.
Silicon
Silicon generally exists in the form of a solid solution in ferrite or austenite, solid solution strengthening effect is high and can improve the steel strength and wear resistance. Silicon is an element that shrinks the austenite phase region but increases the stability of subcooled austenite and the amount of residual austenite in steel. Silicon increases the activity of carbon in austenite and strongly hinders the formation of cementite. Precipitation improves the stability of steel against tempering. The influence of silicon on the strength and toughness of steel is very complex. The increase of silicon in a range of compositions, not only improves the strength of steel but also improves the toughness of steel and reduces the toughness-brittleness transition temperature. The influence of silicon on martensite and bainite structure is the most noticeable in the use of silicon to improve the stability of austenite and hinder the precipitation of carbides so that the steel can obtain a certain amount of retained austenite composition and improve the toughness of steel.
Manganese
Manganese exists in ferrite and austenite solid solution, and part of it forms cementite with iron-carbon. The strengthening of manganese can increase the content of martensite and bainite by improving the hardenability of martensite and bainite and reducing the phase transition temperature of the steel to refine the grain and achieve the purpose of strengthening. Manganese is an element that expands the austenite zone, improves the stability of the supercooled austenite and the amount of retained austenite, and is beneficial to the improvement of the toughness of the steel.
Molybdenum
Molybdenum in steel has a high solid solution-strengthening effect. Steel containing elements that lead to tempering brittleness (such as Mn, Cr.) can prevent or reduce the tendency to temper the steel brittleness, improve the steel toughness, and increase the tempering resistance. Molybdenum dissolved in ferrite, increase the self-diffusion activation of iron and increases the steel recovery and recrystallization temperature. Molybdenum increases the stability of subcooled austenite and shifts the C curve to the right, and the effect of molybdenum on delaying the high-temperature F-P transformation is much greater than that on bainite transformation so that the transformation curves of pearlite and bainite are separated, and the latter is relatively prominent, improving the hardenability of bainite.
Chromium
Chromium in steel can effectively slow down the decomposition rate of austenite and improve the hardenability of steel, carburizing properties, and tempering stability. In low-carbon steel, chromium and molybdenum have similar effects but are not as strong as molybdenum. Chromium added to steel can significantly enhance its antioxidant properties and corrosion resistance. General steel in the addition of chromium at the same time must also improve or add other alloying elements, such as Mn and Mo to show better performance.
Nickel
Nickel can improve steel hardenability and carburizing properties and greatly enhance the toughness and low-temperature impact toughness of the material. It is considered one of the most important alloying elements.
