Combined blowing process in converter steel making
Combined blowing process in converter steel making
Inhomogeneities in chemical composition and temperature are created in the melt during the oxygen blow in the top blown converters due to the lack of the mixing in the metal bath. There is a relatively dead zone directly under the jet cavity in the converter. The necessity to improve the steel making process in the top blown converter has led to the development of the combined blowing process. The first combined blowing practice to be commercially accepted was the LBE (Lance Bubbling Equilibrium) process developed by ARBE-IRSID. This process is much more closely related to the BOF process in that all the oxygen is supplied from the top lance. The combined blowing aspect is achieved by a set of porous elements installed in the bottom of the converter through which argon or nitrogen is blown. In LBE process the nitrogen gas is typically used almost exclusively for the majority of the blow in the range of 3 -11 N Cum/min. However in the later part of the blow when nitrogen absorption can create a problem, argon gas is used for stirring. In addition, argon is used almost exclusively as the inert gas for post blow stirring, at this time the rate is increased to 10-17 N Cum/min.
The combined blowing process is shown in Fig.1.
Fig 1 Combined blowing processes
The profile of a porous element is shown in Fig 2
Fig 2 Profile of a porous element for the LBE process
The bottom buildup and the subsequent loss of the porous element is the major problem associated with this process.
The difficulties in maintaining the LBE elements operational have led to pursue the application of the non cooled tuyeres. Here also the oxygen is delivered through a top lance while the inert gas is introduced to the bath from the converter bottom through the elements of tubular design generally consisting of six small pipes set in a refractory matrix (Fig 3). Because of larger cross sectional area available larger flow rates are required to be maintained for keeping tuyeres operational.
Fig 3 Typical structure of tubular element
The combined blowing process
In the combined steel making process oxygen, required to refine the steel, is blown through the top mounted lance while the inert gas needed for improving the mixing of the metal bath is blown through the bottom mounted tuyeres or permeable elements. The process is shown in Fig 4. This stirring is performed with nitrogen gas in the high carbon range of the melt in the bath and with argon gas once lower carbon levels have been reached. The bottom flow rates are normally lower than 0.2 N Cum/t minute. In typical practice nitrogen gas is introduced through the bottom in the first 60 % to 80 % of the oxygen blow and argon gas is switched on in the last 40 % to 20 % of the blow. The rapid evolution of the CO gas in the first part of the oxygen flow prevents nitrogen pick up in the steel.
Fig 4 Combined blowing process
The configurational differences in combined blowing lie principally in the bottom tuyeres. These include fully cooled tuyeres, uncooled tuyeres and permeable elements. This process utilizes the advantages of both pure top and pure bottom blowing processes.
Like in the top blowing process, oxygen is injected through multi holes lance to the molten bath in the combined blowing process. The metal droplets are generated as a result of jet impact and the shearing action of the gas flow from the impact region where the jet strikes the metal surface and the gases are deflected upwards. This effect of jet liquid interaction is described in terms of three modes: dimpling, splashing and penetrating.
The amount of iron droplets splashed into the gas and the slag influences metallic yield, refractory wear and the progress of decarburization. There is an effect of gas and liquid properties on the depth of depression of the bath and the critical depth marks the onset of splashing. The splashing increases up to a certain jet momentum beyond which it decreases. The direction of splashes is dependent on lance nozzle angle, lance height, profile of the jet cavity estimated from its depth and diameter and overlap of the oxygen jet.
Many experiments have been carried out to modify the lance tips in order to control splashing or spitting in the LD converter. The importance of proper design of nozzle diameters and inclination angles is necessary for an optimum pressure distribution of the oxygen jet.
Different studies have shown that the top blowing with bottom stirring of the converter bath gives superior performance than only the top blowing in LD converter with respect to splashing and spitting.
Various methods of bottom blowing for stirring have been adopted. A ceramic plug with embedded multiple small pipes or multiple slits is used in the bottom tuyeres. The stirring is performed with special refractory stirring elements or through small, unprotected tuyeres that are arranged in the converter bottom.
The process of bottom blowing effectively raise bath height, and show different refractory wear profiles as compared to the wear profiles obtained in the top blown LD converter. Wear of the tuyeres and surrounding areas is often severe in this type of process, and requires the use of erosion resistant high density materials to resist the turbulent flow of molten metal.
For use of slag splashing along with combined blowing, the combined blowing steel making process has been developed so as to have higher life of the bottom tuyeres. Slag splashing technique is used to form a gas penetrating “slag mushroom” on the top of bottom tuyeres to protect the nozzle. Further development has also been done to protect the converter lining by preventing the bottom build up so as to have the effective blowing through bottom tuyeres. Due to this development the life of bottom tuyeres has been substantially enhanced.
Control of inert gas flow
The combined blowing process uses expensive gases (Argon, Oxygen and Nitrogen) and the accurate measurement and totalization of these gases assist economic operation and tight quality control by using these values in the generation of daily reports for management control.
To stir the converter bath, Argon or Nitrogen gas is injected through a number of tuyeres in the converter bottom. The total flow and type of gas for each sequence step are predetermined from the loaded recipe for the current blow.
The total flow is divided equally to a number of controllers, one for each tuyere to maintain an even distribution, and becomes the controller remote setpoint. The measured flow is mass-compensated for temperature and pressure for each tuyere and gas type and input to the control module. The 4-20mA control output then modulates the valve position.
If the tuyere is covered with heavy slag, the downstream pressure increases. In case it increases beyond a preset limit, control changes from flow control to pressure control and the control valve then responds to a different control algorithm. On reduction of pressure (less than a hysteresis value), control reverts to flow control. Changeover between control modes is to be automatic, as the non active loop tracks the output of the active loop.
Metallurgical effects of combined blowing
The following are the effects of the bottom stirring in the combined blowing process
- Decreased FeO content in the slag – Better mixing conditions in the converter makes FeO content in the slag closer to the equilibrium conditions which results into lower percentage of FeO in the slag. It has been experienced in many plants that during production of low carbon heats, FeO levels in the slag have been reduced by around 5 %. This in turn results into better metallic yield, lower FeO level in the ladle slag and reduced slag attack on the refractories. Around 1.5 % of improvement in iron yield has been reported by many plants.
- Reduced dissolved oxygen in the metal – It has been reported that bottom stirring can reduce the dissolved oxygen level in a low carbon heat by around 225 ppm. Lower oxygen level results in lower aluminum consumption. A saving of about 0.13 Kg of aluminum per ton of steel due to bottom stirring has been reported.
- Higher manganese content in the steel during tapping – Manganese content in the steel at the time of turn down is about 0.03 % higher with bottom stirring. This means lower consumption of Fe-Mn or Si-Mn in the ladle.
- Sulphur removal – Bottom stirring improves the mixing of the bath and hence enhances the desulphurization potential.
- Phosphorus removal – It has been reported that dephosphorization is not substantially improved due to bottom stirring even though the bottom stirring drives the reaction towards the equilibrium. This is because the reduced level of FeO in the slag tends to decrease the equilibrium phosphorus partition ratio.
Introduction of bottom blowing significantly increases the splashing specially in the lower part of the converter. At the same time this reduces metal losses and skulling of the cone. The success of the combined blowing process depends on the effectiveness of the bottom stirring devices. These devices should be reliable, should cause effective stirring, should have a reasonably long life and should not get blocked during converter operation.
Advantages of combined blowing
The advantages of combined blowing over top blowing in an LD converter are:
- Acceleration of blowing cycle
- Higher yield
- Less FeO in the slag
- Improved converter lining life
- Increased accuracy in achieving specific composition
- Reduced splashing and spitting