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The role and hazards of N element in steel

What is the role of N element in steel? Nitrogen element in steel can be called a "double-edged hand" that people love and hate. It can solid solution strengthen, refine grain size, enhance steel strength and toughness, and help the birth of high-performance steel; But it can also cause aging brittleness, form pores, interfere with welding, and threaten the quality of steel. The precise control of the content of both nitrogen and nitrogen elements has become the key to unlocking the excellent performance of steel.

effect:

Strengthening effect: N can dissolve into the lattice of iron, producing a solid solution strengthening effect and improving the strength and hardness of steel. For example, in some high-strength alloy steels, an appropriate amount of N can effectively improve the yield strength and tensile strength of the steel.

Refining grain size: N can form small nitrides with certain elements in steel (such as Al, V, Ti, etc.), which can prevent grain growth during the solidification and heating process of steel, thereby refining the grain size of steel. Refined grains can not only improve the strength of steel, but also enhance its toughness, plasticity, and weldability.

Improving corrosion resistance: In some stainless steels, N can enhance the steel's resistance to pitting and crevice corrosion. N can enhance the stability of the passivation film on the surface of steel, making the passivation film denser, thereby improving the corrosion resistance of steel in corrosive media.

harm:

Aging brittleness: If steel contains a high amount of N, when left at room temperature for a long time or kept at a certain temperature, N will combine with elements such as carbon and manganese in the iron to form nitrides, leading to a decrease in the toughness and plasticity of the steel, resulting in aging brittleness. This phenomenon is more pronounced in low-carbon steel and can affect the long-term performance of the steel.

Reduce the high-temperature strength and plasticity of steel: In high-temperature environments, nitrogen and microalloying elements (such as Nb) form nitride precipitates. For example, in Nb containing steel, when the casting temperature drops below 950 ℃, a large amount of fine Nb (C, Nb) precipitates from the γ phase, causing a sharp decrease in the ductility of the steel; When the temperature is below 900 ℃, AlN precipitates at the γ - grain boundaries, exacerbating the brittleness of steel. This is extremely unfavorable for steel components that need to withstand loads at high temperatures, limiting the application of steel in high-temperature industrial fields.

Formation of pores and looseness: During the steelmaking process, if the N content is too high, N will precipitate in the form of gas when the steel solidifies, which can easily form defects such as pores, bubbles, and looseness in the steel ingot or casting. These defects can damage the internal structural integrity of steel, reduce its density, and thus affect the strength, toughness, and fatigue resistance of steel. When steel products are subjected to loads such as pressure and tension, they are prone to rupture or failure from these defects, seriously affecting product quality and service life.

Reduced welding performance: N will increase the sensitivity to hot cracks during welding. During the welding process, N will react with alloy elements in the steel, producing welding defects such as gas and cracks. It will also form high melting point nitrides, which will form inclusions in the weld, reduce the toughness and plasticity of the weld, and also affect the density of the weld, leading to a decrease in welding quality.

Here are some practical application cases of controlling the content of N element in steel found on the internet:

Large scale ultra-low N stainless steel production: The patent for "a production process for large ultra-low N stainless steel" applied by Erzhong (Deyang) Heavy Equipment Co., Ltd. can prepare large ultra-low N stainless steel with N ≤ 0.010wt% through internal control composition design, electric furnace initial refining, ladle refining, pouring, and electric slag remelting processes, especially multiple alloying treatments and VD vacuum refining treatments during ladle refining. Controlling the N content below 0.010wt% is beneficial for achieving better toughness and fatigue performance of the new 9Cr-3W-3Co stainless steel, overcoming the problem of strong interaction between other elements and N elements during the smelting process of this type of stainless steel material, which makes it difficult to remove N. It can achieve the production of large ultra-low N stainless steel ingots with N content<0.008wt%.

HiB Steel Production: After five years of hard work, the young team of silicon steel at Taiyuan Iron and Steel Second Steelmaking Plant has established the key operational points for the entire process from pretreatment to continuous casting in HiB steel production. The control range of key elements such as Al, N, Ti in HiB steel is on the order of a few ppm, and the precision requirement is extremely high. The team used a stopwatch to record the bottom blowing time, sampling time, and continuous casting time of the steel ladle, identified the sampler, and tracked the relevant operations, forming measures such as steel sample sampling and cooling operation standards, successfully improving the quality of steel samples, reducing analysis fluctuations, and achieving precise control of key elements such as N, making the 80t carbon steel line in the southern area of Steelmaking Plant 2 the only small converter production line in China that can stably supply HiB steel billet raw materials.

High strength welding wire steel production: During the research and development of 1100MPa solid high-strength welding wire steel by Nangang Special Steel Division and New Materials Research Institute, continuous technological breakthroughs and independent innovation were carried out to adjust product composition and optimize production processes. In this process, precise control of various elements such as N is necessary to ensure that the mechanical properties of the deposited metal, such as yield strength and tensile strength, fully meet the strength matching requirements of 1100MPa high-strength steel plates, successfully filling the gap of 1100MPa high-strength solid welding wire in China. High level silicon steel production: Baotou Iron and Steel (Group) Co., Ltd. applied for a patent for "a method for precise control of N element in high-level silicon steel", including converter, RH refining, and continuous casting processes. In the converter smelting process, a full nitrogen blowing mode is adopted for underwater blowing of steel, weak deoxidation is used in the steelmaking process, and measures such as strictly prohibiting large argon gas stirring after steelmaking are taken to accurately control the N element content; Accurately control N at 65-85ppm in the RH refining process to reduce fluctuations in the N content of the molten steel after refining; Proper protection pouring should be carried out in the casting process to ensure that the N content in the finished product meets the requirement of 65-90ppm, in order to guarantee the performance and quality stability of the final product.

Preparation of Ultra Pure Iron: The high-performance steel materials team at Shanghai University used a combination of electrolysis, vacuum melting, and vacuum vertical zone melting to prepare 5N grade ultra pure iron. The gas impurity elements, including N, were strictly controlled, resulting in ultra pure iron with a purity of up to 5N2 (99.9992%). The gas impurity element content (carbon, nitrogen, oxygen, and hydrogen) was only 4.5ppm.