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Combined Blowing Process in Converter Steelmaking

• satyendra
• April 30, 2013
• 2 Comments
• argon, bottom blowing, LBE process, nitrogen, porous plug, top lance, tubular element,

Combined Blowing Process in Converter Steelmaking 

Inhomogeneities in chemical composition and temperature are created in the liquid steel during the oxygen (O2) blow in the top blown converters because of the lack of mixing in the liquid steel bath. There is a relatively dead zone directly under the jet cavity in the converter. The necessity to improve the steelmaking process in the top blown converter has led to the development of the combined blowing process.

The combined blowing process also known as the top and bottom blowing or mixed blowing process is characterized by both a top blowing lance and a method of achieving stirring from the bottom. The configurational differences in mixed blowing lie principally in the bottom tuyeres or purging elements. These range from fully cooled tuyeres, to uncooled tuyeres, to permeable elements. The need of the bottom stirring system is necessary for the production of a range of high-quality demanding steel grades and is essential for the process to be economical. Hence, proper functionality of the stirring has to be ensured over the entire campaign of the basic oxygen converter (BOF). Fig 1 shows top blowing and combined blowing steelmaking processes.

Fig 1 Top blowing and combined blowing steelmaking processes

Presently, the top and bottom combined blown converter is commonly used in primary steel making plants. In the combined blowing converter, the agitating and the mixing of the bath are forced by the top blown O2 jets and the bottom inert gas streams,  which can achieve a high mixing efficiency for the bath. In rare cases, O2 is also injected from the bottom with concentric double tube tuyeres together with shrouding gases to control the temperature at the tuyere outlet and the wear of the bottom. However, since inert gas purging provides generally higher control of wear, lifetime of the purging elements and the bottom, the majority of the converters are equipped with the bottom stirring with gas purging plugs.

The first combined blowing practice to be commercially accepted was the LBE (Lance Bubbling Equilibrium) process developed by ARBED-IRSID. This process is much more closely related to the BOF process in that all the O2 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 (Ar) or nitrogen (N2) is blown. In LBE process, N2 gas is typically used almost exclusively for the majority of the blow in the range of 3 normal cubic meter per minute (N cum/min) to 11 N cum/min. However in the later part of the blow, when N2 absorption can create a problem, Ar gas is used for stirring. In addition, Ar is used almost exclusively as the inert gas for post blow stirring, at this time the rate is increased to 10 N cum/min to 17 N cum/min. Fig 2 shows an LBE converter with bottom blowing element.

In the combined blowing process, the bottom stirring is carried out using inert gases such as N2 and Ar are being used extensively to improve the mixing conditions in the BOF. The inert gases are introduced at the bottom of the furnace by means of permeable elements (LBE process) or tuyeres. In a typical practice, N2 gas is introduced through tuyeres or permeable elements in the first 60 % o 80 % of the O2 blow, and Ar gas is switched on in the last 40 % to 20 % of the blow. The rapid evolution of CO in the first part of the O2 blow prevents N2 pickup in the steel. The profile of a porous element is shown in Fig 2

Fig 2 LBE converter with the profile of a porous element and the types of plugs

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 O2 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. Because of larger cross sectional area available larger flow rates are required to be maintained for keeping tuyeres operational.

Configurations of bottom plug / nozzle

The initial development of combined blowing has been based upon basically three types of bottom plugs used for bottom blowing. First, there is a refractory element which behaves much like porous plugs. This unit is made of compacted bricks with small slits. Like most tuyeres, it needs sufficient gas pressure to prevent steel penetration. This unit is more penetrating than porous plugs. Second, an uncooled tuyere is used to introduce large amounts of inert gases per nozzle. This results in local heavy stirring, which can more easily penetrate the build-up. Air or O2 cannot be used because there is no coolant and the heat generated to make tuyere life too short to be practical. The third type is a fully cooled tuyere. Here, either inert gas or O2 can be blown, causing very strong stirring and almost no problems penetrating bottom build-up. In all cases the gas piping is routed through the furnace trunnions using rotary joints or seals to allow full rotation of the furnace. Various types of the bottom blowing plugs developed for the combined blowing are shown in Fig 2.

Present state of designs of the plugs used for the bottom blowing of the inert gas are based upon single-hole plug (SHP) designs and multi-hole plug (MHP) designs. These plug designs have been established as widely accepted state-of-the-art designs for the bottom blowing plugs. Both SHP and MHP purging plug are being designed with flow rate optimized pipe diameters and number of pipes. However, the MHPs for inert gas bottom purging are more popular. Both types of the purging plugs with SHP and MHP designs are based on magnesia-carbon (MgO-C) refractories and they are normally made from 100 % high grade fused magnesia, high grade graphite, optimized grain size distribution, and sometimes with additives.

Efficient purging until the end of the BOF lining campaign is the target of all gas purging plugs in BOF shops and is affected by the applied range of gas flow rates, the blockage potential, and the wear rate under particular process conditions. The highest safety standard is an essential requirement for the bottom purging.

Blockage potential – The reduced availability of the purging plugs due to bottom build-up is often the reason for low purging efficiency. This increases the cost of de-oxidation agent, lowers the yield, and leads to a less efficient blowing process. The main reasons for blockage are bottom build-up due to very sticky slag or high slag splashing frequencies, problems with the inert gas supply or non-adequate purging plug design. Whereas the high gas flow rate through an SHP can help to reduce slag blockage potential at low slag splashing rates, high slag splashing rates with potential bottom build-up or insufficient inert gas supply causes rather deep infiltration of an SHP with a very low probability of reopening. The purging efficiency of an MHP, however, is increased by numerous pipes with a flow-optimized number, diameter and arrangement. The reopening rate of an MHP is reported regularly and not prone to fluctuations in gas pressure and inert gas supply.

Safety – Highest safety standards are generally given for the MHP designs. The gas pipes are directly pressed into the MgO-C brick. If gas supply to the MHP is reduced or zero for any reason, steel infiltration applies only a few millimetres into the purging plug. The risk of steel breakout through the MHP is minimized.

Purging characteristics and wear rate – The flow regime of a SHP is in the transition zone between bubbling and jetting or entirely in the jetting zone, results into large gas volumes above the single pipe and subsequent decay into smaller gas bubbles with a large size distribution. This flow regime is characterized in general by increased wear rates, e.g. 0.4 mm/heat to 0.7 mm/heat. The MHP design provides a much more appropriate gas bubble distribution above the purging plug with a higher share of small gas bubbles. The higher specific surface of the small gas bubbles increases the gas purging and metallurgical efficiency. The wear rate is generally lower as a result of decreased back-attack phenomena and turbulence at lower gas velocities. Fig 3 shows SHP and MHP with the gas bubble evolution in the water model.

Fig 3 SHP and MHP with the gas bubble evolution in the water model

Process of combined blowing 

In the combined steel making process, O2 required to refine the steel is blown through the top mounted lance while the inert gas (N2 or Ar) needed for the bottom stirring process is introduced into the melt through bottom stirring bricks for improving the process conditions by optimized mixing. The flow rate and type of stirring gas depends on the process phase and steel grade. A faster and better approaching of the metal slag equilibrium is achieved because of the bottom stirring. Equilibrium and mixing time depend on type, number, location of stirrers, and flow rate. Stronger stirring shifts the thermodynamic equilibrium to the desired direction and reduces the mixing time. A shift-over from N2 to Ar is normally required, depending on the final steel chemistry. The valve station as the central part of the bottom stirring system allows individual flow control per individual purging plug.

Like in the top blowing process, O2 is injected through multi holes lance to the molten steel 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 namely (i) dimpling, (ii) splashing, and (iii) 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 O2 jet.

Many experiments have been carried out to modify the lance tips in order to control splashing or spitting in the BOF converter. The importance of proper design of nozzle diameters and inclination angles is necessary for an optimum pressure distribution of the O2 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 BOF 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 BOF 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 steel.

The combined blowing process uses expensive gases (O2, Ar, and N2) and the accurate measurement and totalization of these gases assist the economic operation and the tight quality control by using these values in the generation of daily reports for management control. To stir the converter bath, Ar or N2 gas is injected through a number of stirring plug bricks in the converter bottom. The total flow and type of gas for each sequence step are predetermined from the loaded menu for the current blow. The total flow is divided equally to a number of controllers, one for each stirring plug brick to maintain an even distribution, and becomes the controller remote set-point. The measured flow is mass-compensated for temperature and pressure for each stirring plug brick and gas type and input to the control module. The 4-20 mA control output then modulates the control valve position.

If the stirring plug brick 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. Change-over between control modes is to be automatic, as the non active loop tracks the output of the active loop.

Foe optimization of gas consumption and the flow control range, an additional inlet pressure control is installed. The combination of pressure control in the feeder line and individual flow control in the stirring lines maintains constant flow rates of the individual stirrers, thus avoiding blockage of porous plugs by viscous slag. Suitable instrumentation provides the operator with an indication of the porous plug condition. The process reliability is very important. Fail-safe philosophy is generally provided for the feeding lines (gas switch-over in case of low inlet pressure) as well as individual streams (fail-safe open in case of media and power failure). The schematic representation of the combined blowing process is shown in Fig 4.

Fig 4 Schematic representation of the combined blowing process

The converter bottom stirring system is controlled via a PLC (programmable logic controller) installed either as a stand-alone unit with individual HMI (human machine interface) station or provided for integration into new or existing networks.  The operation requires flexibility. Depending on the selected steel grade, the software follows stirring patterns (set-point parameter tables) for Ar and N2 flow rates during the complete heat as a function of the total blown O2 quantity. The set-point changes and control action take place in automatic mode without operator interaction, based on field signals.

During tapping, de-slagging, and charging, pre-defined flow rates are to be ensured for reduced refractory wear and high life-time of the porous plugs. The design of the stirring plug bricks is to ensure long service lives by having low erosion speed, advanced spalling resistance, and flexible brick length.

The positioning of the stirring plug bricks with respect to the O2 jets is very important for the effectiveness of the bottom stirring system. For the optimization of the location of the stirring plug bricks the points to be considered are (i) impact of the O2 jets under various process conditions (such as lance tip design and variability of lance height etc.), (ii) aspect ratio of melt height to converter diameter, and (iii) influence on refractory wear.  Use of the latest CFD (Computational Fluid Dynamics) simulations is normally made to optimize the location of the stirring plug bricks by considering the complex conditions in the BOF converter.

The latest development in the area is the patented alternating stirring technology. In the alternating stirring practice, groups of stirring elements are controlled with alternating high and low stirring gas flow rates. Statistical evaluation of process results over several campaigns, after implementation of this technology in a BOF shop, has shown that there is potential to reduce the Ar cost by 30 % without negative influence on the metallurgical results.

Metallurgical effects of combined blowing

Purging patterns, especially number of plugs, flow rates and the kind and quality of purging gases have a remarkable influence on the BOF metallurgy. Those parameters are to be strictly coordinated otherwise the process can go beyond control and aimed metallurgical results cannot be achieved. The following are the metallurgical effect of the combined blowing.

Carbon/O2 – As a result of bottom purging the kinetics for decarburization are improved and thus lower carbon (C) levels at the end of blow without steel bath over oxidation are achieved. The indicator for an efficient purging performance is the [C] x [O] product, which is compared to a top blown operated converter much lower and in the average range of 0.002 % to 0.0025 %. Due to the refining process there are non-equilibrium conditions in liquid steel bath existent and also between slag and liquid steel bath.

With an appropriate bottom purging program, the reactions can be driven closer to the equilibrium at the end of blow and hence the de-carburization effect is strengthened. The duration of post stirring intensifies that effect additionally. For aiming lowest C levels, the C content of the refractory lining is also a significant parameter.

In reference to a top blown operated converter dissolved [O] contents at equal [C] levels at tapping are lower resulting in a minimization of the de-oxidation agent consumption in the ladle. There is also the chance to release or save the expensive RH (Rurhstahl Heraeus) degassing treatment caused by lowest refined levels at tapping.

Iron yield – Bottom purging, hot metal composition ([Si] content), the slag practice, and the blowing programs influence the FeO level in the slag and hence the chemical reaction potential between slag and lining and the effect of post stirring. A BOF with bottom purging system is characterized with lower iron contents in slag and also lower slag volumes in comparison to a top blown BOF converter. Also the FeO level in slag at tapping is dependent on the dissolved C in the steel bath.

Manganese (Mn) – The Mn yield at equal C levels at tapping is higher than a conventional top blown BOF process. In this connection lesser Fe-Mn is needed for the secondary metallurgy alloying depending on the steel grades. Thus the adjustments of Mn levels are better controllable.

Phosphorus (P) – Bottom purging is characterized through a better intake capacity of P2O5 in the slag and quicker lime dissolution. According to the sprayed liquid iron drops during the refining process in the BOF converter, especially during the hard blowing phase, the temperature of the formed slag is higher than the melting bath. This results in weaker conditions for de-phosphorization. Through purging, the slag temperature is lowered considerably caused by the excellent bath agitation and the better temperature equilibration between slag and steel bath.

Influence of post stirring – The main purpose of post stirring is on the one hand the realization of lowest C and P levels at tapping and on the other hand the quick and precise adjustment of the tapping temperature (cooling effect). Purging time and intensity are the two decisive parameters for the achievement of certain element levels. Post stirring enhances the decarburization effect significantly by leading the dissolved C and O2 in steel bath closer to the equilibrium. Post stirring causes cooling of the liquid steel bath enhanced by additional charging of BOF slag. That means an enhancement of the P distribution at factor three and a decreasing of the P level at tapping to 0.005 %.

Influence of purging plug arrangement and number of plugs – The purging system influences the equilibrium conditions in the steel bath during the refining process and hence the metallurgical results. Bottom purging permits to get closer to or rather approach the equilibrium at the end of blowing. The effect of decarburization and dephosphorization is considerably improved. For effectiveness of the purging the parameter Rp has been established. Rp describes the ratio of the condition actual to the condition equilibrium. If the equilibrium is attained the parameter Rp is one. An increasing of the number of plugs means enhanced bath agitations and hence higher values of Rp closer to one. Fig 5 shows the consequence of various plug arrangements and numbers on the equilibrium approaching (defined by the purging parameter Rp).

Fig 5 influence of purging plug arrangement and number of plugs on equilibrium conditions

The indicator for bath agitation or mixing is the relative mixing time. A reduced mixing time means an improved bath mixing / kinetic and hence an acceleration of the chemical reactions (shortening of reaction ways). A further parameter for the description of the bath kinetic is the mixing energy. The mixing energy involves the lance height, geometry, blowing practice, the bath level of the liquid metal and for the top blown converter with bottom purging system the purging flow rates as well.

Key for a successful operating bottom purging system are primary the purging pattern, number of plugs, wear rates and the availability of each plug. The purging plug arrangement is almost irrelevant and just a design element.

Influence of purging intensity – The level of purging intensity plays a decisive role for attaining lowest [C] x [O] products and iron losses in steel bath. A minimum level of purging leads to a considerable decreasing of the [C] x [O] product, especially below a set flow rate of 0.06 N cum/ t min.

Operating benefits – A top blowing process with bottom purging system is also reflected in less turbulent refining and hence reduced slopping with the consequence of higher yields. Furthermore the total O2 consumption is around 2 % and the tapping temperature in average 10 deg C lower compared to the conventional BOF process. It is due to the result of the better bath agitation and homogenized conditions of the steel bath. The charged lime amount is reduced about 10 % to 15 % in comparison to a top blowing operated converter.

Ideal switching point from N2 to Ar

N2 levels at tapping are flexibly adjusted during the refining process by shifting the point of switching from N2 to Ar and the particularly purging flow rates. The normal practice is for lower N2 flow rates at refining start and a significant increasing of the Ar purging intensities after switching. Hence, it is desirable for realizing lowest [C] x [O] products, an intensive purging at the last one third of the refining period is adequate.

Normally, until 25 % of the refining process gas type and purging intensity does not have any influence on the N2 level in steel bath. A purging with Ar at this refining phase is not cost effective and without purpose. Ar is more expensive than N2. For aiming lowest N2 levels, it is necessary to switch from N2 to Ar between 25 % and 50 % of the blowing time. A retarded switching, especially over 50 % of refining, causes very high N2 levels at tapping.

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 are to be reliable, cause effective stirring, have a reasonably long life, and not to get blocked during converter operation.

Slag splashing and combined blowing

Slag splashing is a proven technique used for increasing the life of the BOF campaigns to very high levels. After tapping, the slag in the converter is splashed with N2 onto different areas of the lining during a period of ranging from 2 minutes to 5 minutes. Also there are practices such as slag coating and slag washing. This practice retains a small amount of liquid slag in the converter after tapping. The slag is enriched with dolomite or raw dolomite. Afterwards, the converter is rocked several times to cover the bottom and the neighboring areas with a thin layer of slag. Hot patching and gunning are other measures to enhancing the life of the refractory lining of the converter.

Slag splashing works best with a creamy and sticky slag. But sticky slag causes the bottom of the converter to build up with layers of slag hindering a free evolution of the gas jet from the tuyeres/plugs or even blocking them completely. This is normally not a malfunction of the bottom stirring system itself but it results in a significant deterioration of the metallurgical results due to an unfavourable distribution of the gas.

With a slag layer covering the bottom stirring elements, effective bottom stirring is not possible as the gas cannot be injected with a directed jet stream. It rather creeps between lining and slag layer until it finds a crack to escape. In this case the important stirring effect can no longer be fully established. With extremely thick layers of slag, the gas may even creep along the barrel to the upper cone or mouth, showing no stirring effect and no metallurgical effect at all. This phenomenon is shown in Fig 6. It has been verified by using natural gas, identifiable by a flame. It was detected that natural gas escaped at the areas described. The left side of Fig 6 shows the situation of a slag layer just covering the bottom. The right side shows the situation with a slag layer covering bottom, lower knuckle, and barrel as created in case of intensive slag splashing.

Fig 6 Influence of slag splashing on the stirring gas distribution

It can be seen that in extreme cases, the inert gases neither have contact with the melt nor with the slag at all. Hence, it is necessary to control the bottom thickness on a regular basis and start early counter-measure in order to maintain the function of the bottom stirring system.

Advantages of combined blowing 

The fundamental reasons for implementing a bottom purging system are on the one hand to improve metallurgical results and on the other hand to guarantee a high quality economical O2 steel production at lowest costs. The most important benefits of combined blowing converter over top blowing in a BOF converter are (i) acceleration of blowing cycle resulting into shortening of tap to tap time, (ii) shorter and quicker formation of slag and improved interaction between slag and steel bath (better conditions for scrap/ flux addition melting, higher scrap / hot metal ratio), (iii) reduction in the re-blows and increased hitting rate in composition and tempera­ture,  (iv) improved steel bath homogenization / agitation and temperature distribution, (v) Increased accuracy in achieving specific composition, (vi) improved process control (higher accuracy for tapping temperature and element levels), (vii) improved steel and flux addition yields (less slag volume and lower loss of iron to slag and melting dust), (viii) less splashing and spitting, and slopping, (ix) lower (FeO), [P] levels and [Mn] oxidation, hence lower consumption of O2,  (x) lower iron oxide in slag, (xi) improved blowing efficiency due to the strongly intensified melt stirring, (xii) lower final O2 content in steel so lower quantity of de-oxidizers (ferro-alloys and aluminum) are needed, (xiii) improved quality of steel, since inert gas blowing at the end of the procedure diminishes the concentration of gases in the metal, and (xiv) increased refractory lining life by avoiding over-heated FeO-rich slag.

The disadvantages of the combined blowing are (i) converter equipment for the combined-blown process is more complicated, which increases the cost of the shop, but this is more than compensated by the advantages mentioned above, (ii) high cost of Ar gas which is in many cases tried to replace at least partly with N2, (iii) availability of bottom stirring nozzles or bricks is often less than 100 % due to more severe wear of the bricks as compared with the other converter lining.