Why is it necessary to test the oxygen, nitrogen, and hydrogen content in steel during the steelmaking process?

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I. Why test the oxygen, nitrogen, and hydrogen content in steel: that is, the hazards or effects of oxygen, nitrogen, and hydrogen on steel products. 1. Hazards of oxygen: Like hydrogen, oxygen can have an adverse effect on the mechanical properties of steel. Not only the oxygen concentration, but also the amount, type, and distribution of oxygen-containing inclusions have a significant impact as well. Such inclusions refer to metal oxides, silicates, aluminates, oxygen-containing sulfides, and similar inclusion compounds. Steelmaking requires deoxidation, as during solidification, the reaction between oxygen and carbon in the solution produces carbon monoxide, which can cause bubbles. Furthermore, during cooling, oxygen can precipitate from the solution as FeO, MnO, and other oxide inclusions, thereby reducing its hot or cold workability, as well as its ductility, toughness, fatigue strength, and machinability. Oxygen, along with nitrogen and carbon, can also cause aging or a spontaneous increase in hardness at room temperature. In cast iron, as the casting is solidifying, oxides can react with carbon, thereby causing pores in the product and increasing its brittleness. 2. The hazards or benefits of nitrogen: Nitrogen cannot be simply classified as a harmful gaseous element, as nitrogen is intentionally added to certain specialty steels. All steels contain nitrogen, and its amount depends on the method of steel production, the types and quantities of alloying elements and the way in which they are added, the casting method of the steel, and whether nitrogen is added intentionally. For certain grades of stainless steel, increasing the N content appropriately can reduce the amount of Cr needed; since Cr is relatively expensive, this method can help to lower costs. Most of the nitrogen in steel is in the form of metallic nitrides. For example, after being stored for some time, steel undergoes strain aging, and it can no longer be deep-drawn (for use in things like car trim panels), as the steel will tear and cannot be stretched evenly in all directions. This is due to large grains and the deposition of Fe4N at the grain boundaries. For another example, in stainless steel, the formation of chromium nitride (Cr2N) at the grain boundaries depletes the chromium present at those boundaries, leading to the so-called intergranular corrosion phenomenon. By adding titanium, which preferentially forms titanium nitride, such harmful effects can be prevented. 3. Hazards of hydrogen: When the hydrogen content in steel exceeds 2 ppm, hydrogen plays an important role in the so-called \"scale flaking\" phenomenon. This peeling phenomenon is generally more pronounced when internal cracks and fractures occur during the cooling process after rolling and forging, and it is seen more frequently in large cross-sections or high-carbon steels. Due to the presence of internal stresses, this defect can cause the large rotor to crack during the engine’s operation. When the hydrogen content in cast iron exceeds 2 ppm, pores or general porosity tend to form; the porosity caused by this hydrogen leads to the embrittlement of the iron. “\"Hydrogen embrittlement\" occurs mainly in martensitic steels; it is not very prominent in ferritic steels, while its occurrence in austenitic steels is actually unclear. Furthermore, hydrogen embrittlement generally increases along with hardness and carbon content. II. Forms of existence of oxygen, nitrogen, and hydrogen in steel 1. Forms of existence of oxygen Oxygen exists in both combined and free states; the free state is generally present in small amounts. It mainly occurs in the form of Fe2O3, Fe3O4, FeO, as well as metal oxide inclusions, silicates, aluminates, oxygen-containing sulfides, and similar inclusion compounds. The total oxygen content determined by instrumental testing is usually denoted by T. 2. Forms of nitrogen presence: Some of the nitrogen in steel exists in the form of metallic nitrides or carbonitrides ； Most of the elements added to special alloy steels today can form nitrides under appropriate conditions. These elements include manganese, aluminum, boron, chromium, vanadium, molybdenum, titanium, tungsten, niobium, tantalum, zirconium, silicon, and rare earths. Given that many elements that form nitrides have several simple or complex nitrides, as many as 70 different nitrides can form in steel under such conditions. Another portion of nitrogen is dissolved in the steel in the form of nitrogen atoms. In very rare cases, nitrogen is trapped in bubbles in molecular form or adsorbed on the surface of the steel. 3. Forms of hydrogen presence: In steel, hydrogen exists in the form of hydrogen atoms; at high temperatures, two hydrogen atoms easily combine to form a hydrogen molecule. Hydrogen atoms are highly reactive, and in their natural state they form hydrogen molecules that are released slowly. III. Sources of oxygen, nitrogen, and hydrogen in steel 1. Source of oxygen Oxygen is present in the molten steel in certain amounts at the end of the steelmaking process in various types of furnaces. Oxygen is added during the production process, as the steelmaking process is essentially an oxidation process; oxygen needs to be supplied to the molten iron in order to carry out various oxidation steps. However, as the steelmaking process progresses, despite the great variety of processes, there are common patterns in the relationship between the molten steel in the furnace pool. That is, as it gradually decreases, it gradually increases, and there is a corresponding balanced relationship between them. 2. Sources of nitrogen: The partial pressure of nitrogen in furnace gas is very high; the partial pressure of nitrogen in the atmosphere remains at around 7.8×10^4 Pa. Therefore, nitrogen in steel is primarily absorbed and dissolved during the period when the molten steel is exposed. Electric furnace steelmaking, including arc heating for secondary refining, accelerates the dissociation of gases, resulting in higher concentrations ； The long melting time in the open hearth increases nitrogen content ； Improper control of the converter’s reblowing, as well as untimely switching between nitrogen and argon, can also increase the nitrogen content ； Nitrogen in ferroalloys, scrap steel, and slag is also introduced into the molten steel along with the charge materials. 3. Sources of hydrogen: The partial pressure of hydrogen in furnace gas is very low; the partial pressure of hydrogen in the atmosphere is 0.053 Pa. Therefore, the hydrogen in steel is primarily determined by the partial pressure of water vapor in the furnace gas. The main pathway for hydrogen to enter the molten steel is through the rust on the surface of scrap steel (xFeO•yFe3O4•2H2O) ； Hydrogen in ferroalloys ； Carbide agents, deoxidizers, fluxes, insulating agents, slag formers (Ca(OH)2), and moisture in asphalt and tar ； Un-dried ladles, tundishes, and pouring nozzles ； Coating material for ingot molds ； Water seeps into the mold, and water from the atmosphere reacts with the molten steel or slag and enters the steel.