Steel Making Process

The Main Task of Steel making

The main task of steel making is to smelt molten iron, scrap steel, etc. into steel with the required chemical composition, and make it have certain physicochemical properties and mechanical properties. The main tasks are summarized as “four deprivation, two removals, two adjustments”.

Four deprivation: decarburization, desulfurization, dephosphorization, deoxygenation;

Two removals: remove harmful gases, remove harmful impurities;

Two adjustments: adjust the temperature of the molten steel and adjust the composition of the alloy material.

1. Decarbonize and adjust its content to a certain range

When the carbon content in steel increases, the hardness, strength, and brittleness will increase, and the ductility will decrease; on the contrary, if the carbon content decreases, the hardness and strength will decrease, and the ductility will increase. Therefore, the steel making process must control the carbon to a certain range according to the steel grade specifications.

2. Remove impurities, mainly including:

Dephosphorization and desulfurization: For most steels, P and S are harmful impurities. P can cause cold brittleness of steel, and S can cause hot brittleness of steel. Therefore, it is required to remove it as much as possible during the steel making process.

Deoxidation: During the oxidation refining process, a large amount of oxygen is input to the melt to oxidize impurities, so that a certain amount of oxygen is put into the molten steel. It will greatly affect the quality of steel. Therefore, it is necessary to reduce the oxygen content in the steel. It is generally done by adding elements with greater affinity than iron (such as A1, Si, Mn and other alloys)

Removal of gas and non-metallic inclusions. The gas in steel mainly refers to hydrogen and nitrogen dissolved in steel. Non-metallic inclusions include oxides, sulfides, phosphides, nitrides, and their complex formation. In the general steel-making method, the escape of CO bubbles is generated during the carbon-oxygen reaction, and the molten pool boiling is caused to reduce the gas and non-metallic inclusions in the steel.

3. Adjust the composition and temperature of molten steel

In order to ensure the various physical and chemical properties of steel, in addition to controlling the carbon content of the molten steel and reducing the impurity content, an appropriate amount of alloying elements should also be added to make the content reach the steel specification range. In order to complete the above tasks and ensure that the molten steel can be poured smoothly, the molten steel must be heated and maintained within a certain high temperature range, and at the same time, the molten steel temperature should be continuously adjusted to an appropriate range according to the requirements of the smelting process.

4. Cast molten steel into good quality steel ingots or billets

Through the method of die casting or continuous steel casting, the molten steel is poured into steel ingots or billets of various shapes and different cross-sectional dimensions with good quality.

The main equipment of steel making

1. Molten Ironpretreatment station

The molten iron pretreatment mainly performs desulfurization and slag treatment on the molten iron to provide qualified molten iron for subsequent converter steel making and refining. It is also the main equipment for adjusting the supply and demand between the blast furnace and the converter. The molten iron is stored in the mixer, and the composition and temperature of the molten iron are evenly mixed.

2. Converter

The converter is the most important equipment in the modern iron and steel industry. Most of the world’s steel plants produce molten steel by the converter. Other parts of the short process and special steel may use electric arc furnace and other steel-making equipment.

The converter takes molten iron and scrap steel as raw materials, and blows oxygen into the molten iron to oxidize impurities and carbon elements in the molten iron, and drives the molten iron flow with the high-pressure gas blown in, which plays the role of inclusion floatation and molten iron decarburization. Generally, the carbon content of molten iron is about 4%, and the converter is an important part of decarburization.

3. Refining furnace

The refining furnace generally refers to the refining equipment outside the furnace. In the past 30 years, there have been more than 30 kinds of refining equipment outside the furnace. The main role of refining outside the furnace is to improve the quality of molten steel, expand the range of varieties, optimize the metallurgical production process, and improve production efficiency.

At present, the main refining equipment are LF and RH refining furnace. The main function of RH refining furnace is to remove harmful gases and impurities in molten steel, and carry out deep decarburization. LF furnace has the functions of heating, desulfurization, deoxidization and alloying.

4. Continuous casting

Continuous casting is the continuous casting of molten steel into billets by the continuous caster, and then sent to the subsequent rolling production line for rolling.