Production And Process of Stainless Steel

The production process of stainless steel mainly includes one-step, two-step, and three-step methods, as well as new integrated production methods. The one-step smelting process is: molten iron+AOD (argon oxygen refining furnace); The two-step method is: EAF (electric arc furnace)+AOD (argon oxygen refining furnace); The three-step method is: EAF (electric arc furnace)+AOD (argon oxygen refining furnace)+BOD (vacuum oxygen decarbonization furnace). Among them, five stainless steel smelting processes including AOD refining, KAWASAKI-BOP, CLU, metal refining, and Krupp composite blowing are all used for producing stainless steel, but each has different efficiency, cost, and risk.

01 AOD refining method

This method, also known as argon oxygen decarburization, is an advanced technology widely used in stainless steel smelting. This method is widely adopted due to its advantages of simple equipment, convenient operation, strong adaptability, low investment, and low production cost. In the specific operation process, the molten iron from the blast furnace and the molten steel from the electric furnace are injected into the AOD furnace through the ladle. During smelting, a mixture of O2, Ar, or N2 gas is blown in to decarburize the molten steel. At the same time, the feeding system will add reducing agents, desulfurizers, ferroalloys, or coolants to adjust the composition and temperature of the molten steel, ultimately producing qualified stainless steel materials. In addition, the theoretical basis of AOD method is basically the same as VOD method. Its method of reducing Pco is not to use vacuum method, but to use argon gas to dilute carbon monoxide gas in the furnace to reduce Pco, so that chromium can be removed to a very low level without oxidation at lower temperatures.

02 KAWASAKI-BOP method

This is a stainless steel smelting process that is similar to a BOF oxygen converter that blows oxygen from a furnace top oxygen lance. There are 7 bottom air vents that can blow oxygen, which use propane gas for cooling (gas cracking). In addition, lime powder can also be sprayed through the tuyere of the converter. In the specific operation process, the bottom blowing oxygen needs to be controlled below 40%, while the remaining oxygen is supplied to the molten pool by top blowing. This smelting method is widely adopted due to its advantages of simple equipment, convenient operation, strong adaptability, low investment, and low production costs.

03 CLU method

The full name Creusot Loire and Uddeholm process is an external refining technology jointly developed by Creusot Loire from France and Uddeholm from Sweden in 1972. This technology is similar in principle to the argon oxygen refining method, with the main difference being that the CLU method dilutes the carbon monoxide produced by the decarbonization reaction by using water vapor, rather than the more expensive argon gas. This method is widely adopted due to its advantages of simple equipment, convenient operation, strong adaptability, low investment, and low production cost.

04  Metal Refining Method

A technique for deep purification of metals, commonly used to improve metal purity and remove impurities. Common refining methods include pyrometallurgical refining, electrolytic refining, etc. Pyrometallurgical refining refers to the process of removing impurities from crude metals through various methods at high temperatures, such as additive method, melting method, distillation, etc. It is mainly used for refining heavy non-ferrous metals and certain light non-ferrous metals. For example, copper can be refined by displacement method: CuSO4+Fe==Cu+FeSO4.

Electrolytic refining is a technique that extracts pure metals by utilizing the differences in the ease of anodic dissolution or cathodic precipitation of different elements. It is commonly used for refining non-ferrous metals such as crude copper, crude silver, and crude nickel. In addition, the ladle argon blowing refining process is also a commonly used refining method. The process involves blowing argon gas from the bottom of the ladle into the ladle after the molten steel is discharged. By controlling the pressure and time of argon blowing, the molten steel is boiled and refined to achieve uniform chemical composition and temperature, accelerate chemical reactions, remove harmful gases and inclusions, and purify the molten steel.

05 Krupp Composite Smelting Method

A stainless steel smelting method developed by Krupp Steel Plant, which is an improved version of BOF converter. This method combines oxygen lance and tuyere on the side wall of the converter for composite blowing, and introduces process gas to improve decarburization rate. At the beginning of the smelting process, oxygen is blown simultaneously from the furnace top oxygen gun and side wall air vents. After reaching a certain temperature, iron alloy and scrap steel are added. In addition, according to specific needs, the Krupp composite blowing method can also add various alloy materials, lime or scrap steel in batches during the smelting process. It is worth noting that when the carbon content reaches the critical value, the Krupp composite blowing method will reduce the oxygen content of the process gas and add inert gases such as nitrogen or argon to gradually reduce the carbon content. This design makes the smelting method highly flexible and adaptable.

In the steel industry, we often hear the concepts of cold rolling and hot rolling. So, what are the cold rolling and hot rolling of stainless steel?, Next, we will briefly share with you the relevant knowledge about stainless steel cold rolling and hot rolling, so that you have a preliminary understanding.

06 Stainless Steel Hot Rolling

Hot rolling is relative to cold rolling, where cold rolling is carried out below the recrystallization temperature of the material, while hot rolling is carried out above the recrystallization temperature of the material. Hot rolled steel plates have poor toughness and surface smoothness, low hardness, are easy to process, and have excellent ductility. Its surface has oxide scale, low smoothness, generally medium thick plate, excellent wear resistance and corrosion resistance, relatively low price, and lower plate thickness; Hot rolled plates have a wide range of applications, mainly used in the production of petrochemical, aerospace, construction, steel structural components, bridges, ships, vehicles, and can also be used to produce various cold-rolled products.

The termination temperature of hot rolling is generally 800-900 ℃, and then it is usually cooled in air, so the hot rolling state is equivalent to normalizing treatment. Most steel is rolled by hot rolling method. Steel delivered in hot-rolled state has a certain corrosion resistance due to the formation of a layer of iron oxide scale on the surface due to high temperature, and can be stored outdoors.

advantage:

Fast molding speed, high output, and no damage to the coating, can be made into a variety of cross-sectional forms to meet the needs of usage conditions; Cold rolling can cause significant plastic deformation in steel, thereby increasing its yield point.

Disadvantages:

Although there is no hot plastic compression during the forming process, residual stresses still exist in the cross-section, which inevitably affects the overall and local buckling characteristics of the steel;

2. The style of cold-rolled steel is generally an open section, which reduces the free torsional stiffness of the section. It is prone to torsion when subjected to bending and bending and buckling when subjected to compression, resulting in poor torsional performance;

3. Cold rolled formed steel has a relatively small wall thickness and is not thickened at the corners where the plates are connected, resulting in weak ability to withstand localized concentrated loads.

07 Stainless Steel Cold Rolling

Cold rolled stainless steel plate is made from hot-rolled coils, which are rolled through a process of hot rolling, acid washing, and cold rolling below the recrystallization temperature of the material. Through cold rolling, cold-rolled steel strips and plates with thinner thickness and higher precision can be obtained. The flatness, surface oxidation free skin, and smoothness of the plates are higher, making them easier to coat and process. At the same time, they have high stamping performance and low yield point. However, cold-rolled plates have a certain degree of work hardening, low toughness, and are more expensive; Cold rolled steel plates are widely used in industries such as automobiles, electrical products, printing, construction, and building materials. They are also the best material for producing organic coated steel plates.

advantage:

It can destroy the casting structure of steel ingots, refine the grain size of steel, and eliminate defects in the microstructure, thereby making the steel structure dense and improving its mechanical properties. This improvement is mainly reflected in the rolling direction, so that the steel is no longer isotropic to a certain extent; Bubbles, cracks, and looseness formed during pouring can also be welded under high temperature and pressure.

Disadvantages:

After hot rolling, non-metallic inclusions (mainly sulfides, oxides, and silicates) inside the steel are compressed into thin sheets, resulting in delamination. Layering greatly deteriorates the tensile properties of steel along the thickness direction, and there is a possibility of interlayer tearing during weld shrinkage. The local strain induced by weld shrinkage often reaches several times the yield point strain, much larger than the strain caused by load;

2. Residual stress caused by uneven cooling. Residual stress is the stress that self balances internally in the absence of external forces, and various hot-rolled steel sections have such residual stresses. Generally, the larger the cross-sectional size of the steel section, the greater the residual stress. Although residual stress is self balanced, it still has a certain impact on the performance of steel components under external forces. It may have adverse effects on deformation, stability, fatigue resistance, and other aspects.

08 The difference between cold rolling and hot rolling  

As mentioned above, stainless steel plates can be divided into hot rolling and cold rolling. Simply put, cold rolling and hot rolling mainly involve different temperatures during the rolling process. Cold "refers to room temperature, while" hot "refers to high temperature (i.e. crystallization temperature)

Secondly, the production process is different. Hot rolling is carried out above the recrystallization temperature of the material, while cold rolling is carried out below the recrystallization temperature. The most obvious and simple difference is that the surface and thickness of the product are different. Hot rolled products have oxide scales and are relatively rough, and their processing performance such as heat treatment is not as good as cold rolling. The thickness of the plate is mostly 3.0mm and above.

Cold rolling is made by rolling hot-rolled coils as raw materials. Cold rolled products have high dimensional accuracy, no oxide skin on the surface, higher smoothness, better quality, and excellent post-processing performance. The thickness range is generally below 6mm, and the common thickness range is 0.05mm~3mm.

Secondly, cold-rolled steel allows for local buckling of the cross-section, thereby fully utilizing the bearing capacity of the bent member; Hot rolled steel does not allow local buckling of the section. And the reasons for the generation of residual stress are different, so the distribution on the cross-section also varies greatly.

Finally, the free torsional stiffness of hot-rolled steel is higher than that of cold-rolled steel, so the torsional resistance of hot-rolled steel is better than that of cold-rolled steel.