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The Role And Content Selection Of Main Alloying Elements In Steel
Carbon: Mainly affects the strength and hardness of steel. The strengthening of carbon is mainly achieved by solid solution strengthening, reduction of phase transformation point refinement grains, and phase transformation strengthening. Too high carbon will reduce the toughness and weldability of the steel. Carbon has a strong solid solution strengthening effect with a strengthening coefficient of 4 410 M Pa/Wt%. Although the increase of carbon content in steel can improve the strength and wear resistance of the part, it damages the plastic toughness and processability of the steel. Therefore, when designing the carbon content of steel, it is necessary to determine the carbon content according to different conditions. The steel for the head body is required to have a higher strength and wear resistance and a certain degree of plastic toughness. The carbon content is preferably between 0.42% and 0.48%.
Silicon: Silicon is generally present in the form of solid solution in ferrite or austenite. It has a strong solid solution strengthening effect and a strengthening coefficient of 77 M Pa /W t%, which can improve the strength and wear resistance of steel. Silicon is an element that shrinks the austenite phase region, but increases the stability of supercooled austenite and the content of retained austenite in the steel. Silicon increases the activity of carbon in austenite and strongly hinders cementite. Precipitate to improve the anti-tempering stability of steel. The influence of silicon on the toughness of steel is extremely complicated. As the silicon content increases, it not only increases the strength of the steel, but also improves the toughness of the steel and reduces the toughness-brittle transition temperature. The most striking effect of silicon on martensite and bainite structure is the use of silicon to improve the austenite stability and hinder the precipitation of carbides, so that the steel obtains a certain amount of retained austenite composition and improves the strength of the steel. toughness. We have a silicon content of from 0.15% to 0.3%.
Manganese: Manganese is present in ferrite and austenite solid solution, partially forming cementite with iron carbon, and the strengthening coefficient is 24. 5M Pa / W t%. The strengthening of manganese can increase the martensite and bainite content by increasing the hardenability of martensite and bainite, and lower the phase transition temperature of steel to refine the grain and achieve the purpose of strengthening. Manganese is an element that enlarges the austenite region, improves the stability of supercooled austenite and the amount of retained austenite, which is beneficial to the improvement of the toughness of steel. In the design of general steel, higher content of manganese is added. We chose it as 0. 60% ~ 0. 90%.
Molybdenum: Molybdenum has a strong solid solution strengthening effect in steel, and the strengthening coefficient is 80. 4 M Pa / W t% . In steels containing tempered brittle elements (such as Mn, Cr, etc.), it can prevent or reduce the temper brittleness tendency of steel, improve the toughness of steel, and increase the tempering resistance. Molybdenum is dissolved in ferrite, increasing the self-diffusion activation energy of iron and increasing the recovery and recrystallization temperature of steel. Molybdenum increases the stability of supercooled austenite and shifts the C curve to the right. However, the effect of molybdenum retarding the high temperature F-P transition is much greater than the delayed action on the bainite transformation, thus separating the pearlite and bainite transformation curves. And make the latter relatively prominent, greatly improving the hardenability of bainite. However, since molybdenum is less abundant in the world, it is a strategic resource and should be used sparingly. We chose it to be 0. 90% ~ 1. 10%.
Chromium: Chromium can effectively slow the decomposition rate of austenite in steel and improve the hardenability, carburizing performance and tempering stability of steel. In low carbon steel, chromium and molybdenum have similar effects, but not as good as molybdenum. strong. The effect of chrome delaying the F-P transition is significantly greater than the effect of delaying the bainite transformation, while at the same time separating the F-P transition curve from the bainite transformation. The addition of chromium to steel can significantly improve the oxidation resistance of steel and increase the corrosion resistance of steel. In general, steel must be added or added with other alloying elements such as Mn, M o, etc. in order to show better performance. We chose to add chromium content of 0.90% ~ 1 20%.
Nickel: Nickel can improve the hardenability and carburizing properties of steel, greatly improve the toughness and low-temperature impact toughness of steel. It is one of the important alloying elements, but due to its high price, it should be minimized in the design of steel. Usage amount. The content we chose is 0. 40% ~ 0. 70%.