Knowledge

 

Section 1: Common Impurity Elements in Steel

 

Common impurity elements in steel include phosphorus, sulfur, hydrogen, nitrogen and oxygen. Under normal circumstances, these elements have a detrimental effect on the properties of steel. However, some of them can play a beneficial role under specific conditions and become alloying elements.

 

Phosphorus (P)

Source: Introduced from raw materials during the steelmaking process.

Function: Phosphorus is slightly soluble in steel. When the phosphorus content in steel is relatively high (mass fraction w(P) > 0.1%), Fe2P will precipitate at the grain boundaries, which reduces the plasticity and toughness of the steel, increases the cold cracking tendency of the steel, and hurts the performance of cast steel. Therefore, the phosphorus content needs to be controlled in cast steel production.

 

Sulfur (S)

Source: Introduced into the steel from the raw materials during the steelmaking process.

Function: Generally, sulfur exists in steel as FeS or FeS - Fe eutectic at the grain boundaries of steel, which reduces the mechanical properties of steel. Usually, the mass fraction of sulfur w(S) is required to be less than 0.04%, with specific values depending on the steel grade. This is because these sulfur compounds at the grain boundaries disrupt the continuity of the steel matrix and affect mechanical properties.

In mechanical manufacturing, for certain steel grades that require improved machinability (free-cutting steels), an appropriate amount of sulfur (w(S) = 0.1% - 0.4%, depending on the steel grade) can be added to the steel. The formed sulfides ((Mn, Fe)S) can play a role in interrupting the continuity of the matrix (breaking chips), but this will increase the tendency of the steel to hot cracking. This is an application that utilizes the negative effects of sulfur to meet specific processing performance requirements.

Hydrogen (H)

Source: Steel liquid absorbs hydrogen from furnace gas during the steelmaking process.

Function: Dissolved hydrogen in the steel liquid precipitates during the solidification process due to a decrease in solubility. Under slow solidification conditions, hydrogen precipitates in the form of pinholes, affecting the density of cast steel, etc. When solidifying rapidly, the precipitated hydrogen causes a high-stress state within the iron lattice, leading to brittleness in steel, which is highly detrimental to the quality and performance of cast steel. Measures must be taken in cast steel production to remove hydrogen and control its content to avoid hydrogen-induced defects.

Nitrogen (N)

Source: Steel liquid absorbs nitrogen from furnace gas during the steelmaking process.

Function: Dissolved nitrogen in the steel liquid precipitates during solidification due to a decrease in solubility and combines with elements such as Si, Al, and Zr in the steel to form nitrides such as SiN, AlN, and ZrN. A small amount of nitrides can refine the grains of steel and have a certain positive effect on the properties of cast steel; however, when there are too many nitrides, the plasticity and toughness of the steel will decrease, affecting the mechanical properties of cast steel. In production, the nitrogen content and related elements should be reasonably controlled to take advantage of the beneficial effects of nitrides and avoid the adverse effects.

Oxygen (O)

Source: Generated as FeO during the oxidation of molten steel in the steelmaking process.

Function: The FeO dissolved in the molten steel reacts with carbon in the steel during the temperature drop before solidification, forming CO bubbles, which cause porosity in cast steel parts and affect the density and quality of the cast steel.

During the solidification of molten steel, FeO precipitates at the grain boundaries of the steel due to a decrease in solubility, which can reduce the performance of the steel, disrupt the continuity of the grain boundary region, and have a negative impact on the mechanical properties of the cast steel. In cast steel production, it is necessary to control the oxygen content and reduce FeO-related defects.

 

Section 2: The "Five Harmful Elements" in Steel

 

In the composition system of steel, lead, tin, antimony, bismuth and arsenic, these five elements are collectively referred to as the "five harmful elements" due to their significant adverse effects on the properties of steel. Their behaviors and influences in steel are each distinctive.

I. Characteristics of Each Element and Their Impact on Steel

1. Lead (Pb)

Lead is bluish-gray in color, soft and malleable, with a melting point of only 327.5°C and a boiling point of 1755°C. Due to its relatively low boiling point, in the high-temperature environment of steelmaking, it is difficult for lead to dissolve in the molten steel, and most of it will turn into vapor and escape. Therefore, the mass fraction of lead in the finished steel is usually only about 0.001%. However, once the lead content exceeds the standard, its harmful effects will become apparent: it will significantly reduce the impact toughness of steel, making it more prone to fracture when subjected to impact loads; during hot working processes such as rolling and forging, it can easily cause surface cracks and other metallurgical defects in steel, directly leading to the scrapping of steel parts and having a significant impact on product quality.

2. Tin (Sn)

Tin is silvery-white, soft and malleable, with a melting point lower than lead, at 232°C, and a boiling point of 2275°C. During steelmaking, tin enters the steel along with raw materials, scrap steel, alloy materials, and deoxidizers. However, during the oxidation process of steelmaking, only a small amount of tin enters the slag, while most of it remains as impurities in the steel. When tin accumulates to a certain level in steel, it can cause hot brittleness problems. For example, in heat-resistant alloys, tin can severely reduce the high-temperature mechanical properties of the alloy, such as the creep strength of chromium-molybdenum-vanadium heat-resistant steel, which will significantly decrease due to the presence of tin; in nickel-chromium-molybdenum-vanadium rotor steel, tin tends to accumulate at grain boundaries, becoming a potential factor for tempering brittleness.

3. Antimony (Sb)

Antimony is a white and lustrous metal, hard and brittle, easily breaking into powder, with a melting point of 603°C and a boiling point of 1440°C. It has a significant detrimental effect on the strength and toughness of steel, significantly reducing the strength and toughness of steel and increasing its high-temperature brittleness. Generally, the mass fraction of antimony in steel should be controlled to be less than 0.1%, and for some steel alloy materials with strict requirements for impurities, the mass fraction of antimony should be below 0.001%.

4. Bismuth (Bi)

Bismuth is silvery-white and lustrous, extremely brittle and has no ductility. Its melting point is 273°C and boiling point is 1560°C. In high-temperature smelting environments, bismuth is prone to volatilization, so its residual content in steel is very low, mostly existing in a free state, with a mass fraction usually not exceeding 0.001%. When the bismuth content is too high, it will reduce the plasticity of steel and affect its high-temperature strength. However, it has a "dual nature". In certain scenarios, it can improve the machinability of steel and stabilize the carbides in cast iron, thus having potential application value in the production of cutting tools steel and cast iron to a certain extent.

5. Arsenic (As)

Arsenic is a brittle, crystalline substance with a light grayish-white color and strong metallic luster, with a melting point of 817°C. The arsenic in steel mainly comes from the raw materials used in steelmaking and is difficult to remove during the smelting process. In steel, arsenic mainly exists in the form of solid solutions (such as Fe3As, Fe3As2, FeAs, etc.). As the arsenic content increases, the impact toughness of steel decreases, brittleness increases, and severe segregation problems may occur. It is a typical harmful element. However, everything has two sides. When arsenic is added to steel, it can improve the corrosion resistance and oxidation resistance of steel to a certain extent. In some scenarios where there are special requirements for corrosion resistance and other properties and the negative effects can be tolerated, it may be considered for use.

II. Common Characteristics of the "Five Harmful Elements"

Low melting point: The melting points of these five elements are significantly lower than that of steel (generally around 1300 - 1500 degrees Celsius). When steel gradually solidifies, these elements remain in a liquid state and are thus called low-melting-point elements. This characteristic makes them prone to accumulate at grain boundaries and other locations during the solidification of steel, disrupting the continuity of the steel matrix.

Common performance degradation: When their content in steel exceeds a certain limit, they will all have a significant negative impact on the high-temperature mechanical properties of steel, increase the high-temperature brittleness of steel, reduce the strength and toughness of steel, make steel brittle, and affect the performance of steel in high-temperature environments. In industrial components subjected to high-temperature stress (such as boilers, turbine blades, etc.), the overlimit of these elements will greatly shorten the service life of the components.

Symbiotic mechanism and hazard amplification: They are often not present alone in steel but coexist and co-segregate. This symbiotic characteristic further amplifies their destructive effect on steel. The accumulation of multiple harmful elements at critical locations such as grain boundaries will more severely damage the microstructure of steel, reduce its comprehensive performance, and increase the risk of quality issues in steel (such as cracks, brittle fractures, etc.).

 

III. Response Strategies and Quality Control

Residual harmful elements in steel pose significant risks to the quality of steel products. Therefore, formulating and strictly implementing standards for the limits of these elements in steel becomes a crucial measure to ensure the quality of steel products. Taking general-purpose steel as an example, the content of elements such as tin (w(Sn) ≤ 0.05%), antimony (w(Sb) ≤ 0.01%), and arsenic (w(As) ≤ 0.045%) need to be controlled. Throughout the entire steel production process, from raw material procurement (strictly controlling the impurity content of furnace materials, scrap steel, etc.), to optimizing the smelting process (such as strengthening oxidation and refining to remove harmful elements as much as possible), and to product inspection (precisely detecting the content of harmful elements), full attention should be paid to the control of residual harmful elements. A comprehensive quality control system should be established to ensure that steel meets the performance requirements of various application scenarios.

 

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