Slopping in ld converter

Since then, the converter is in full production mode. The converters are made of high-temperature creep-resistant steel. In conjunction with a combined air and water cooling system, this will achieve a long service life. The new converters will have a larger interior volume than the previously used models. This will reduce slopping. The remaining converters will be consecutively replaced until spring 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.

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.

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 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.

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.

It is due to the result of the better bath agitation and homogenized conditions of the steel bath. 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. A purging with Ar at this refining phase is not cost effective and without purpose. Ar is more expensive than N2. 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 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. Introduction of Gunning maintenance in the year , has led to a significant increase in lining life of Converter, despite simultaneous adverse affect of increased tapping temperatures due to use of cold tundishes and Billet Caster without ladle furnace. Gunning mass spraying on worn out refractories of LD-Converter.

The gunning machine is comprised of a telescopic gunning lance that is mounted on a Carcass frame with electric drive, a water pump, a material tank, water and material hosepipes and a regulation valve for the remote-controlled adjustment of MgO base gunning mass and amount of water. A gunning repair is a time consuming operation and takes 12 minutes. Excessive use of gunning material cannot be economical and increases cost per ton of steel.

Slag Washing Maintenance:. Slagging or slag washing maintenance is carried out frequently after tapping the steel and leaving a portion of slag congealed with dolomite on the bottom of converter. The vessel is rocked from one pad to another to coat a thin layer of about 2 inches of conditioned slag. Slagging maintenance takes 1- 2 minutes. Slag Splashing Maintenance:.

Slag splashing is carried out normally after steel tapping by retaining total or part of the slag on the converter bottom which is blown via a supersonic jet of nitrogen gas through the existing oxygen blowing lance. During slag splashing the molten slag is splashed on the sidewalls, cone and all over converter lining by regulating the blowing height and nitrogen flow rate. Any leftover slag is discharged. The coated slag layer acts as a consumable refractory lining which protects the erosion of Converter lining in subsequent heats.

Both of the lances are remained connected to Oxygen supply line to overcome any failure during oxygen blowing. So time needed to switch over from Oxygen to Nitrogen supply line should be taken into account during a process cycle and should be added to the slag splashing total process time. In order to maintain proper blowing height, the bottom is usually flushed down which hampers the productivity of Converter. As higher production rate means lesser time for preventive repairs. Slag splashing technique is being employed in various steel plants world over to increase refractory life of Converter.

Increase in lining life depends on the frequency of slag splashing which varies from plant to plant in accordance to the prevailing technological parameters and operating conditions of individual plant. The optimal tapping duration:. The purpose of the tap hole is to allow controlled tapping of steel only, leaving the slag in the Converter and directing the metal spurt into the ladle. De-oxidation and alloy additions are made to the ladle during tapping operation to bring the raw steel to the required chemical composition.

As per project refractory life of tap hole was as low as an average of 10 heats and after that it was frequently repaired by re-piping with a shuttering of steel pipe and filling MgO base mass for producing next heats.

Later in , imported Isojet Tap hole Block system pre-assembled MgO- C tap hole blocks was introduced, comprised of dual- ring tap hole assembly blocks system, in which the wear is retained within the inner concentric ring thus permitting the outer ring to maintain the integrity of the entire assembly.

The inside refractory rings are changed upon wearing normally after heats. The Isojet taphole block system has significantly improved the frequency of tap hole repairs and reduced down time. The CD nozzle design helps in accelerating the gas velocity to the supersonic velocities. The gas flow jet is sub-divided into potential core, supersonic, and subsonic regions.

Within the core region, the velocity is constant. At the end of supersonic region the velocity becomes Mach 1. Downstream the velocity is sub sonic. The jet interacts with the converter environment and produces a region of turbulent mixing. The entrainment process increases the mass flow rate and the jet diameter and decreases the mean axial velocity as the distance from the nozzle exit increases. The impact force on the slag melt surface is reduced with increasing lance height.

The jet length and the spreading angle are affected by the gas temperature and pressure as well condition of metal slag mix in the converter. Supersonic jets are produced with the CD nozzles Fig 1. A reservoir of stagnant O2 is maintained at a pressure P1.

The O2 accelerates in the converging section up to sonic velocity Mach number 1 in the cylindrical throat zone. The O2 then expands in the diverging section. The expansion decreases the temperature, density, and pressure of the O2 and the velocity increases to supersonic levels Mach number higher than 1.

As the O2 jet leaves the nozzle into the converter, at a pressure P2, it spreads and decays. A supersonic core remains for a certain distance from the nozzle. Supersonic jets spread at an angle of around 10 degrees to 14 degrees. Both the proper design of the nozzle and proper operation are necessary for getting efficiently the desired steelmaking reactions and to maximize the life of the nozzle.

If a nozzle is overblown, which means that the jet of O2 is not fully expanded at the time it leaves the nozzle, shock waves develop as the jet expands outside of the nozzle. Useful energy is lost in these shock waves, and an overblown jet impacts the molten bath with less force than an ideally expanded jet. Nozzles are under blown when the jet expands to a pressure equal to the surrounding pressure and then stops expanding before it leaves the nozzle.

In this case, the O2 flow separates from the internal nozzle surface. Hot gases from the converter then burn back or erode the nozzle exit area. This erosion not only decreases the nozzle life, but also results in a loss of jet force, leading to a soft blowing condition.

Under blowing and over blowing conditions are also shown in Fig 1. Oxygen lance is subjected to the heating load in the converter from radiation, convection, and conduction. It is subjected to continuous corrosion by high temperature slag and splashing. Further during the converter blowing molten slag particles gets solidified on the lance surface and sticks to the lance. These slag particles impact the transfer of heat to the lance. Fig1 Mechanics of supersonic jet formation and overblown and underblown conditions at the nozzle tip.

The lance tips can have either i single hole, or ii multiple holes.