Basic knowledge of converter steelmaking

Converter steelmaking is one of the mainstream methods of steelmaking currently.

2. Characteristics of Converter Steelmaking: Using hot molten iron as the main raw material, using injected gas as an oxidant to oxidize elements and impurities in iron, using the chemical heat generated by the oxidation of C, Mn, Si, P, etc., in molten iron and the physical heat in molten iron as the heat source, under given raw material conditions, according to the steel grade composition requirements, smelting qualified steel with qualified composition and temperature
3. Basic Tasks of Steelmaking
1. Decarbonization: Achieved by injecting oxygen
2. Heating: Achieved by injecting oxygen

3. Removal of sulfur and phosphorus: Achieved by adding lime to create a basic slag
4. Deoxidation and alloying: Achieved by adding deoxidizers and alloying materials

5. Removal of gases and inclusions: Achieved by secondary refining (the simplest is argon blowing in a ladle)

Basic Principles of Steelmaking
1. Chemical Equilibrium Principle
A chemical reaction is a process in which two or more elements combine to form one or more compounds, Fe+O=FeO±Q (heat)
Increasing the concentration of reactants and decreasing the concentration of products are conducive to the forward reaction. Low temperature is conducive to exothermic reactions, and vice versa.
2. Oxygen Transfer in Steelmaking Process
Direct oxygen transfer: Oxygen directly reacts with elements in the molten metal.
Indirect oxygen transfer: Oxygen reacts with iron to form FeO, which then reacts with elements in the molten pool through the slag.
FeO is a very unstable compound and may decompose into Fe and O at any time, so the oxidizing property of the slag is represented by the FeO content. Oxygen transfer between the slag and the metal is accomplished through FeO.

3. Oxidation-Reduction Reactions in Steelmaking Process
(1) Oxidation-Reduction Order of Elements
Si-Mn→C→P→Fe
Note: This order is not absolute and is also related to the concentration of elements and the reaction temperature.

(2) Oxidation-Reduction of Silicon and Manganese
After silicon is oxidized, it is basically no longer reduced, and only traces remain at the end of blowing.
Manganese is oxidized in the early stage, and part of it is reduced and enters the molten steel at the end.

(3) Oxidation of Carbon
Early stage: Mainly silicon and manganese oxidation, carbon oxidation is weak.
Middle stage: Carbon-oxygen reaction is intense, pay attention to dry-out and splashing.
Late stage: Carbon-oxygen reaction weakens, FeO in slag increases, and temperature rises rapidly.

(4) Phosphorus Removal
Conditions for phosphorus removal: High basicity, large slag volume, high FeO, low temperature.
There is a phenomenon of phosphorus returning during steel tapping, which should be noted, especially when smelting low-phosphorus steel.
(5) Sulfur Removal
Conditions for sulfur removal: High basicity, large slag volume, high temperature, low FeO. 5. Converter Slag
(1) Sources of Slag
Brought by the metal materials themselves, such as the oxidation products of elements in the metal materials.

Added slag-making materials.
Refractory erosion.

(2) Composition of Slag
CaO, MgO, MnO, FeO, SiO2, P2O5, Al2O3, etc. Note: CaO, SiO2, and FeO have a decisive influence on the properties of slag.

3) Functions of Slag: Removal of harmful impurities such as S and P; Indirect oxygen transfer during blowing; Protection of refractory lining.
(4) Properties of Slag
Basicity: The classic view is that R≈3 is appropriate, but now it is required to be lower.

Oxidation-reduction property: Depends on FeO, converter uses oxidizing slag, electric furnace uses reducing slag.

Viscosity and fluidity: High CaO and MgO lead to high viscosity, high FeO leads to good fluidity.

5. Removal of Gases and Non-Metallic Inclusions in Steel: Quenching, stirring, slag washing.
6. Material Balance and Heat Balance
(1) Material Balance
Input: Molten iron, scrap steel (iron blocks), iron-containing raw materials, slag-making materials, oxygen, alloying materials, deoxidizers, carburizers.
Output: Molten steel, slag (ladle slag), dust, gas.

(2) Heat Balance
Input: Chemical heat of molten iron, among which the heat released by carbon is the highest, followed by silicon, and the physical heat brought by molten iron, etc.
Output: Physical heat of final molten steel, physical heat of steel slag, etc.

Raw Materials for Converter Steelmaking

I. Metal Materials
1. Molten Iron
(1) Temperature of Molten Iron: The temperature of molten iron used for steelmaking generally requires more than 1250°C. (2) Composition of Molten Iron
Si: 0.30-0.50%; Mn: 0.20-0.40%; P≤0.150%; S≤0.070%

2. Scrap steel
Moderately sized pieces; no enclosed containers or explosives allowed.
Clean and dry; no non-ferrous metals or rubber contaminants.
3. Ferroalloys
Technical requirements: Composition and particle size must meet relevant standards or technical conditions; dry and clean.
Ferrosilicon: Si 27%; Mn ≤0.5%; P ≤0.04%; S ≤0.02%;
Ferromanganese: Mn 65%; Si 17%; P ≤0.25%; S ≤0.04%;

II. Slag-forming materials
1. Lime
Index requirements: CaO ≥85%; SiO2 ≤2.5%; S ≤0.10% The most important aspect of lime quality is the CaO and SiO2 content. Lime with an activity higher than 310 is called active lime.
2. Lightly calcined dolomite
Index: CaO+MgO ≥78%; MgO ≥30%; SiO2 ≤5.0% Lightly calcined dolomite mainly utilizes its MgO content, while also reducing the amount of lime needed.
3. Limestone
Index: CaO ≥50.0%; SiO2 ≤2.0% Limestone mainly utilizes its CaO content to replace some of the lime.