Effect of Common Impurity Elements in 1. Steel

Common impurity elements in steel are P, S, H, N, O and so on. These elements play a harmful role in the performance of steel under normal circumstances, but some of them can also play a beneficial role under certain conditions and become specially added alloying elements.

Phosphorus is slightly soluble in steel, when the phosphorus content in steel, the formation of Fe 2P in the grain perimeter precipitation, reduce the plasticity and toughness of steel, increase the tendency of cold and hot cracking.

1, sulfur in steel to FeS-Fe eutectic exists in the steel grain perimeter, reduce the mechanical properties of steel, the sulfur content of the superior steel should generally be limited to less than 0.04.

2. In mechanical manufacturing, sometimes in order to improve the cutting performance of some steels, the sulfur content is artificially increased to form sulfides, which plays the role of interrupting the continuity of the matrix.

The increase of sulfur content increases the tendency of hot cracking of castings.

The solubility of hydrogen in molten steel increases with the increase of temperature. Under the condition of slow solidification, hydrogen precipitates in the form of pinholes. Upon rapid solidification, hydrogen evolution creates a highly stressed state within the iron lattice, leading to brittleness.

Absorption of nitrogen by molten steel from furnace gas during steelmaking

The nitrogen dissolved in the molten steel is precipitated due to the decrease of solubility during the solidification process, and combines with Si, Al, Zr and other elements in the steel to generate nitrides such as SiN, AlN and ZrN. A small amount of nitrides can refine the grains of steel. When nitrogen is more, it will reduce the plasticity and toughness of the steel.

Nitrogen is an element that expands the austenitic zone, which can partially replace the role of nickel in steel, and is an alloying element in chromium-manganese-nitrogen stainless steel. In ultra-low carbon stainless steel, it can replace the role of carbon and improve the strength of steel.

Effect of non-metallic inclusions in 2. steel

Non-metallic inclusions in steel include oxides, sulfides, sulfur and oxygen compounds, silicate compounds and nitrides. The sources of these inclusions are foreign and autogenous. Foreign inclusions include impurities entrained from the charge during the steelmaking process, refractory materials that are often eroded and shed from the lining, etc. Autogenous inclusions are generated during the steelmaking process and during the pouring of molten steel due to the oxidation of elements in the molten steel or other chemical reactions. Inclusions are harmful to the mechanical properties of steel, especially to weaken the toughness. To mitigate the harmful effects of inclusions, two approaches can be taken:

(1) Removal of inclusions

For example, in the steel-making oxidation period, the molten steel is well boiled to effectively remove inclusions, and the liquid steel is used in the ladle for a period of time (5~10min) before pouring after tapping, so that the inclusions float from the liquid steel. The inclusions in the molten steel can be effectively removed by refining with argon blowing outside the furnace or filtering the molten steel.

(2) improve the morphology of inclusions

The degree of weakening effect of inclusions on steel depends on its shape and distribution: polygonal inclusions with sharp corners cause large stress concentration in steel, and are easy to form crack sources under the action of external force, while granular and spherical inclusions are less harmful. When strip inclusions are distributed in mesh or intermittent mesh along the grain perimeter, they have a greater splitting effect on steel, the fragmentation of the inclusions in the island arc vertical distribution is less.

If the rare earth alloy is used to treat the molten steel so that the polygonal oxide and the strip sulfide are spherical rare earth oxysulfide, the weakening effect of the inclusions on the steel can be reduced.

Variation of 3. Inclusions Content in Electric Furnace Steel Production

1. The furnace charge is fully melted and the content of inclusions is high.

2. During the oxidation period, the general trend of the content of inclusions decreased.

3. During the reduction period, pre-deoxidizer and alloy are added, and the impurity content increases, and then slowly decreases.

4. Calm down: inclusions float up.

5, blowing argon: oxide, sulfide impurity content is reduced, but the nitride is almost unchanged.

6, pouring: due to the secondary oxidation, molten steel on the pouring system of the erosion, oxygen precipitation and reaction with other elements, inclusion content increased.

7, solidification: temperature drop, micro segregation, dendrite liquid enrichment of solute elements prone to a series of reactions, the formation of oxides, sulfides.