The problem of residual elements in steel is one of the important problems faced by the metallurgical industry. In the process of steelmaking, raw materials for steelmaking (including molten iron, scrap steel, ferroalloys, etc.) will bring a large number of impurity elements into the steelmaking furnace. Some of the impurity elements can be removed, but some impurity elements will remain in the steel. This part of impurities (unintentionally added alloying elements) is collectively referred to as residual elements.
These residual elements are one of the main factors for the instability of steel quality. Certain residual elements are prone to segregation and, even at very low levels, can have a strong negative effect on steel properties.
For example, residual titanium in bearing steel is a typical case. Ti is easy to react with N to produce high hardness inclusions, which greatly affects the service life of bearing steel.
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1. Classification of residual elements
The residual elements in a part of the known steel are divided into three categories according to their oxidation potential, as shown in the following table. They show complete retention, partial retention, and very little retention, respectively, in the steelmaking process.
Residual elements in molten steel are classified according to their oxidation potential
| Fully reserved (The oxidation potential is less than iron) | Partially reserved (The oxidation potential is close to that of iron) | little reservation (The oxidation potential is greater than iron) |
| Cu | S | Pb |
| Ni | P | Zn |
| Co | Mn | V |
| As | Cr | Ti |
| W | C | Si |
| Mo | H | Al |
| Sn | N | Zr |
| Sb | Mg | |
| Ca | ||
| Nb |
In the above table, the oxidation potential of the first type of elements is lower than that of iron, and they do not participate in the oxidation reaction during steelmaking, and eventually, almost all of them accumulate in the steel product.
The oxidation potential of the second type of residual element is close to that of iron. During the blowing process of steelmaking, only a part is removed by oxidation, and the degree of removal is related to the characteristics of the element itself.
The oxidation potential of the third type of element is higher than that of iron. During the molten steel blowing process, they are first oxidized into the slag to be removed, and only a very small part of the remaining elements enter the product.
Therefore, the problem of residual elements in steel is only 15 elements contained in the first and second categories. Among them, 8 elements are fully reserved elements, and 7 elements are partially reserved elements.
2. Source of residual elements in steel
China is a country with much symbiotic iron ore. The symbiotic iron ore includes V, Ti, P, As, Sn, Sb, Re (rare earth elements), etc., which are brought into the steel during smelting.
In addition to the residual elements brought into the molten iron by primary iron ore, the largest source of residual elements in molten steel is scrap steel, which is mainly divided into:
(1) Alloy steel in scrap steel. At present, there is no cost-effective technology in steelmaking plants to separate alloy steel and plain carbon steel, and some medium and high alloy steels contain a wide variety of alloying elements. During the recycling of the steel, these alloying elements will enter the steel as residual elements;
(2) Surface coating or plating in scrap steel. Among them, the most problematic is tinplate, which enters the scrap steel cycle as a can. Other coatings include copper, nickel, chromium, etc.; galvanized sheet is also widely used, but zinc can be removed in steelmaking without consideration ;
(3) Non-ferrous metals entrapped in scrap raw materials. The most important is automobile scrap, which contains some micro-motors, and the main impurity is copper.
On the market, the most residual element content is copper, and copper is mainly entered into the steelmaking furnace from automobile scrap. It is estimated that the average copper content of mixed scrap in steelmaking plants is about 0.3% at present, and the specific content depends on the source and proportion of alloy steel.
Residual Sb and As in steel is mainly derived from primary iron ore, and when scrap steel containing these impurities is recycled, they can be diluted, but the residual amount will gradually accumulate in the steel.
The H and N in steel mainly come from the workshop atmosphere during steelmaking, and their content mainly depends on the composition of different steel grades and the steel-making process.
3. Segregation of residual elements in steel
Many residual elements exist and function in the form of segregation in steel. Most residual elements have a strong segregation ability in steel; the segregation process of this element can occur either in the solidification process of molten steel or in the subsequent solid phase transformation, but it requires a long diffusion time.
The main segregation elements in the riser part of the ingot are S, P, and C, followed by Sb, N, As, H, and Sn. After segregation to form inclusions, the hardness of this part of the material is also higher than that of other parts of the ingot.
Compared with solidification segregation, residual elements will produce grain boundary segregation during solid-phase transformation or heating. For example, the second type of temper brittleness of steel is mainly caused by P, Sn, As, and Sb grain boundary segregation.
4. Brief description of the role of residual elements
① 8 fully reserved elements
Ni, Co, W, Mo can improve the hardenability of steel and are beneficial elements;
On the one hand, Cu can cause copper embrittlement during high-temperature hot working of steel, but on the other hand, it can improve the ability of steel to resist atmospheric corrosion;
Residual elements Sn, As, and Sb are harmful elements, which not only strengthen copper brittleness in steel but also lead to the second type of temper brittleness of alloy steel;
Sn is one of the extremely harmful residual elements in steel, and Sn will greatly reduce the high-temperature mechanical properties of steel and alloys.
② 7 partial reserved elements
C, Mn, S, and P are conventional control elements;
Cr can improve the oxidation resistance of steel, increase the corrosion resistance and hardenability of steel, but also increase the temper brittleness of steel;
N is beneficial to control the grain size of austenite, but at the same time it can also cause strain aging of steel;
H in steel is a harmful and unhelpful element, which can cause white spots, cracks in low-alloy high-strength steels, etc.
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