Author: Wu Jia (senior engineer)
We often refer to iron-carbon alloys with WC <2.11% as steel. When discussing the structure and properties of steel, we only consider the influence of carbon. However, in the steelmaking process, it is impossible to remove all impurities, so in practice In the carbon steel used in practice, in addition to carbon, there are also small amounts of silicon (Si), manganese (Mn), sulfur (S), phosphorus (P), hydrogen (H), nitrogen (N), oxygen (O) and other elements. Their presence will also affect the performance of the steel.
Influence of Si and Mn
Si and Mn enter steel with deoxidizers during steelmaking. Small quantities can dissolve in ferrite, producing solid-solution strengthening and increasing ferrite strength and hardness, which benefits steel properties. However, Si has a strong affinity for O and can form SiO2, while Mn combines with the impurity S in steel to form MnS. Both are inclusions that adversely affect steel properties. The usual limits are WSi <0.5% and WMn <0.8%, allowing most Si and Mn to dissolve in ferrite and provide beneficial effects.
The influence of S and P
S and P are harmful elements introduced through pig iron that are not completely removed during steelmaking. S does not dissolve in Fe; instead, it forms FeS with a melting point of 1190°C. FeS and γ-Fe form a low-melting-point eutectic (980°C) distributed along austenite grain boundaries. When steel is forged or rolled at 1000–1200°C, this eutectic melts, making the steel brittle and causing cracking along austenite grain boundaries. This phenomenon is also known as hot shortness. An appropriate increase in Mn content allows it to combine preferentially with S to form high-melting-point MnS (1620°C), preventing hot shortness.
However, MnS is an inclusion in steel. It deforms into strips during forging or rolling and is distributed along the direction of stress to form streamlines, making the transverse plasticity and toughness of the steel significantly lower than the longitudinal one.
All P in steel dissolves in ferrite. Although it provides strong solid-solution strengthening, it drastically reduces steel's ductility and toughness, especially low-temperature toughness, making the steel brittle at low temperatures. This is known as cold brittleness. P also causes segregation in steel. In addition, both S and P reduce weldability. Because S and P are harmful elements, their contents are usually strictly controlled. Ordinary steel requires WS≤0.035%–0.050% and WP≤0.030%–0.035%; high-quality steel requires WS≤0.020% and WP≤0.030%.
Wind power parts have higher raw material requirements, generally requiring WS ≤ 0.015% and WP ≤ 0.020%. In addition, P also enhances the corrosion resistance of steel in the atmosphere. If a small amount of rare earth, titanium and other elements are added during steelmaking, the cold brittleness of P can be inhibited.
Effect of gas
The impact of gases in steel on the performance of steel is often ignored. With the occurrence of quality accidents, people are paying more and more attention to the impact of gas elements such as H, N, and O in steel. Some industries also set limits on the content of gas elements when purchasing raw materials.
The content of H in steel is very small, but it is extremely harmful to steel. Trace amounts of H (0.5~3mL/100g) in steel can cause "hydrogen embrittlement" and even produce a large number of cracks in the steel, significantly reducing the plasticity and toughness of the steel, causing parts to suddenly break during use. Extremely serious quality accidents caused by "hydrogen embrittlement" have occurred at home and abroad. In addition, H is detrimental to the welding performance of steel and can easily cause cracks at the weld.
Nitrogen dissolved in ferrite causes strain aging. This is the increase in strength and reduction in ductility and toughness of cold-worked low-carbon steel after storage at room temperature or heating for a period. Strain aging is unfavorable for boilers and chemical vessels because it increases brittleness and reduces safety and reliability. In this respect, N is harmful. However, N can combine with Al, V, Ti and Nb in steel to form fine dispersed nitrides, refining grains, strengthening the steel and reducing nitrogen-related strain aging. In these circumstances, N is beneficial.
O is a harmful element in steel. Although deoxidizers are added during the steelmaking process for deoxidation, it cannot be removed. When molten steel solidifies, O reacts with C to form CO, causing bubbles. In addition, O mainly exists in the form of impurities such as FeO, MnO, SiO2, Al2O3, etc. in steel, which reduces the strength and plasticity of steel, and especially has a serious impact on the fatigue strength and impact toughness of steel.
In order to improve the performance of steel, during steelmaking, in addition to ensuring that the C and alloying elements in the steel are within the specified range, the content of impurity elements must also be controlled. However, refining a furnace of qualified molten steel does not mean obtaining qualified steel ingots and steel products. Most of the metallurgical defects in steel are formed during the process of pouring into steel ingots. Therefore, the structure and quality of steel ingots are important links that affect the quality of steel.
(Source: Nanjing Avis Transmission Technology Co., Ltd.)