Secondary steel making operations
Secondary steel making involves some of the following options. The selection of which are available in any particular steel making shop depends on the types of steel being made and the availability of space within the factory.
Secondary steel making involves some of the following options. The selection of which are available in any particular steel making shop depends on the types of steel being made and the availability of space within the factory.
*Stirring
*Lance
1. Bottom porous plug
2. Electromagnetic Stirring (EMS)
*Ladle furnace
*Ladle injection
1. Powder
2. Wire
*Degassing
1. Tank Degasser
2. Stream Degasser
3. RH Degasser
4. DH Degasser
*CAS-OB
*AOD
Why deoxidize?
The oxygen content of liquid steel in the ladle at the start of the secondary steel making process is 400-1000 ppm (0.04 - 0.1%).
The solubility of oxygen in liquid steel is 0.16% but in solid steel it is only 0.003%.
Therefore, steps have to be taken to reduce the oxygen content (deoxidize) of the steel before it solidifies in order to prevent blowhole formation during casting and a porous product being created or large quantities of FeO being precipitated.
The oxygen content of liquid steel in the ladle at the start of the secondary steel making process is 400-1000 ppm (0.04 - 0.1%).
The solubility of oxygen in liquid steel is 0.16% but in solid steel it is only 0.003%.
Therefore, steps have to be taken to reduce the oxygen content (deoxidize) of the steel before it solidifies in order to prevent blowhole formation during casting and a porous product being created or large quantities of FeO being precipitated.
Sources of oxygen
Reducing oxygen content
The addition of a strong oxide forming element is the most commonly used method of reducing the oxygen content of liquid steel.
Use the Ellingham diagram to help you decide which of the following may be suitable (cheapest) choices for this.
Use the Ellingham diagram to help you decide which of the following may be suitable (cheapest) choices for this.
Deoxidation reactions
Aluminum, silicon and manganese are the most common deoxidisers used in steel making The chemical reactions associated with their use are:
Aluminum, silicon and manganese are the most common deoxidisers used in steel making The chemical reactions associated with their use are:
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Effectiveness of deoxidizers
Equilibrium concentrations of dissolved oxygen under different concentrations of manganese, silicon and aluminum
Steel cleanness
There are a variety of sources of inclusions:
Equilibrium concentrations of dissolved oxygen under different concentrations of manganese, silicon and aluminum
Steel cleanness
There are a variety of sources of inclusions:
* indigenous (small):
1. deoxidation product and MnS
* exogenous (large)
1. reoxidation (reaction with air or slag)
2. entrainment of slag
3. eroded refractories
1. deoxidation product and MnS
* exogenous (large)
1. reoxidation (reaction with air or slag)
2. entrainment of slag
3. eroded refractories
Inclusions are formed by chemical reactions (deoxidation, reoxidation and precipitation) or by physical conditions (turbulance or wear).
Effect of inclusions on downstream processing and properties and performance
Most inclusions have a detrimental effect on properties. Solid oxides (alumina or certain calcium aluminates) can cause nozzle blockage during continuous casting and disrupt the process and have to be burnt out. Some inclusions can cause cracking and defects, slivers and delamination in rolled products and also fracture during hot/cold forming and wire drawing.
Most inclusions have a detrimental effect on properties. Solid oxides (alumina or certain calcium aluminates) can cause nozzle blockage during continuous casting and disrupt the process and have to be burnt out. Some inclusions can cause cracking and defects, slivers and delamination in rolled products and also fracture during hot/cold forming and wire drawing.
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