FIELD OF THE INVENTION

[0001] This invention relates to a tap hole closure for minimizing slag carryover while tapping a Basic Oxygen Furnace (BOF) converter during the production/manufacture of steel and optimize the runtime. It also concerns a process to increase the life time of a tap hole bore of a converter.

BACKGROUND OF THE INVENTION

A long-standing problem in the steel making industry has been the ability to control or minimize the carryover of slag during the tapping of a BOF converter. Tapping is the pouring of molten metal from a BOF converter into a corresponding ladle, with the metal flowing from the converter through a tap hole defined therein.

[0002] During the manufacture of steel, molten iron having impurities (e.g. C, Si, Mn, S, P, etc.) therein is typically introduced into a converter vessel known as a basic oxygen furnace (BOF). In the BOF converter, gaseous oxygen (02) is injected or jetted onto the hot metal in order to remove the impurities to desirable levels. During this purification process, fluxes such as lime (Cao) and MgO are added into the furnace and combine with oxides such as Si02, MnO, and FeO formed during the oxidation process to form molten "slag" in the converter. This slag floats on top of the molten steel in the BOF converter, because the slag's density is less than that of the molten steel.

After the oxygen is introduced into the BOF converter for an extended period of time (e.g. from about 16-25 minutes depending upon the volume of the BOF converter, the amount of molten iron therein, and the grade of the steel to be made) and the molten slag and steel have formed, the converter vessel is tilted and tapped. During tapping, molten steel is poured from a tap hole in the side of the BOF converter into a ladle located below the same. It is during this tapping that undesirable slag carryover can occur.

When the BOF converter vessel is properly tapped, a small amount of carryover may occur at the beginning of tapping, but the slag carryover of most concern occurs at the end of tapping when most of the substantially purified molten steel has already been poured into the ladle below, and mostly slag (instead of mostly steel) remains in the BOF converter. When a typical BOF converter is tilted to a pouring position for tapping, the molten steel is poured from the tap hole located in the side of the converter before the slag is poured, due to the different densities of the two molten materials. If the operator(s) tapping the converter does not stop tapping (or pouring) at about the precise instant when the molten slag begins to flow through the tap hole, the undesirable molten slag is also poured into the ladle below on top of the already poured molten steel. Any attempt to remove or minimize the effect of excess slag poured into the ladle is expensive, time-consuming, and/or labor intensive. In the meantime, the amount of liquid steel remaining in the converter must be as low as possible.

[0003] it has long been known to employ a refractory component in the form of a body of a density such that when deposited in a converter above the tap hole it floats at the slag/molten metal interface. During the tapping, the refractory component is drawn down by the vortex created in the molten steel until it locates in the mouth of the tap hole to close it. Slag balls or floats, as they are frequently referred to, are reasonably efficient in the prevention of slag being entrained in the molten metal flowing through the tap hole provided that they succeed to reach the tap hole. However, this is quite random.

[0004] Modified constructions of an elongate and tapered configuration have been evolved, so- called darts, to improve the surety that the component would locate properly in a tap hole. A common dart indeed comprises a conical body and a stem which guides the dart inside the tap hole. The dart is then pulled over the tap hole once a major part of the steel has been withdrawn. The dart is then lowered into the tap hole as the steel empties into the ladle. At an unpredictable moment, the dart is pulled down on the tap hole reducing the flow of molten steel to a significant degree. The throttling of the full flow of molten steel emerging from the tap hole is recognizable as indicating that slag is now approaching the tap hole. In a reduced flow, the first signs of slag are easier to detect. The operator terminates tapping ensuring that substantially all the useable metal is removed from the furnace and any entrained slag is minimal.

[0005] US-A-4,494,734 depicts a dart whose the conical shape body with grooves is adapted to insure a satisfactory seating in an irregular shaped tap hole due to erosion of the molten metal previously running therethrough.

[0006] GB-A-2,236,837 describes a tap hole closure in the shape of a dart comprising a refractory body of elliptical transverse cross-section longitudinally tapered. The advantage of the described dart is that when the body is located in a tap hole, a maximum of 35% of the area is available for the flow of molten metal around the closure means and through the tap hole. There is any more need of grooves which are cost effective.

[0007] Simonnet, M et al ^* Behavior and improved design of dart system in BOF- Process modelling in BOF and AOD steelmaking Session 3. Dusseldorf, 27 June - 1 July 201 1 studied the behavior of a dart through numerical model simulating tapping process. The dart volume, the rod length and diameter as well as the tap hole age influence the time at which the dart is pulled over the tap hole bore. With the consequence that a higher or lower amount of liquid metal remains in the converter. The influence of each parameter was however studied separately.

[0008] It is important to minimize the slag contamination but the tapping time should be reduced at maximum. A compromise between steel throughput, slag carryover and tapping time should be found.

[0009] Document CN 103 184 310 discloses a tap hole closure comprising an upstream portion, in the form of a floater, and a downstream portion, being in the form of a stem adapted to engage entirely in the tap hole of the converter. The tap hole closure further comprises an intermediate portion connected to upstream portion and adjacent to the downstream portion.

[0010] The object of the present invention is to reduce the tapping time while minimizing the contamination of slag and the remaining amount of liquid metal. The weight of tap hole closure is also reduced which improves then the handling of the tap hole closure. The tap hole life can also be increased by using tap hole closures with different widths over the campaign while maintaining a reduced taping time.

SUMMARY OF THE INVENTION

[0011] -The present invention is defined in the appended independent claims. Preferred embodiments are defined in the dependent claims. In particular, the present invention concerns a tap hole closure (1 ) for use in a tap hole bore of a converter, comprising an elongated refractory body (2) extending along a central longitudinal axis X, said body comprising:

- an upstream portion (16) extending over a height H3, said upstream portion having a maximum width W3m measured normal to the central longitudinal axis and being the upper end of the body in its used position, said upstream portion (16) being adapted to float at the slag/molten metal interface and to block the tap hole when introduced in the tap hole, said upstream portion (16) having an elliptical or circular transverse cross- section normal to the longitudinal axis X,

a downstream portion (14) being the lower end of the body in its used position and having a lowest end (b), said downstream portion being in the form of stem adapted to engage entirely in the tap hole, said downstream portion having an approximately constant width W1 measured normal to the central longitudinal axis, said downstream portion extending over a height H1 along the central longitudinal axis X and measured from the lowest end (b),

wherein

the width W1 of the downstream portion (14) is at most 40 mm and the length H1 of the downstream portion (14) is superior or equal to 600 mm;

- the elongated refractory body (2) further comprises an intermediate portion (15) directly connected to the upstream portion (16) and adjacent to a downstream portion (14), said intermediate portion being adapted to engage entirely in the tap hole, said intermediate portion (15) extending over a height H2 of at least 50 mm along the central longitudinal axis,

- said intermediate portion having a width W2 measured normal to the central longitudinal axis at any location such as W1 <W2 < W3m and such as the ratio W2/W3m <0.4, preferably < 0.3,

the width W2 of the intermediate portion (15) is comprised between 50 and 120 mm and the height H2 of the intermediate portion is at most equal to 500 mm,

- said intermediate portion has a width W2d measured at a distance H1 +10 mm along the central longitudinal axis X, from the lowest end (b) of the downstream portion (14), W2d being such as W2d > W1 + 10 mm.

[0012] The width of the upstream portion (16) is the diameter of the circular transverse cross- section measured normal to the longitudinal axis. In the case of an elliptical cross section, the width is the major axis of the ellipse.

[0013] The downstream portion (14), the intermediate portion (15) and the upstream portion (16) have generally a circular cross section shape measured normal to the central longitudinal axis and the width represents then the diameter of the cross section. The maximum width W3 m of the upstream portion (16) is larger than the tap hole diameter.

[0014] In a preferred embodiment, the width of the intermediate portion (15) is constant over the whole height H2 of the portion.

[0015] The referred dimensions (W1 , W2,W3, H1 , H2,H3) of the tap hole closure (1 ) depend of the size of the converter and the tap hole bore of the converter. The tap holes bores vary generally from 100 mm to 300 mm. The width W2 of the intermediate portion (14) depends the tap hole bore erosion. H2 is measured from the location (a) which is the closest location of the intermediate portion to the downstream portion, on the central longitudinal axis of the elongated refractory body wherein the width of the elongated body = W1 + 10 mm.

[0016] In a preferred embodiment, there are slots along the longitudinal length of the upstream portion which increase the flow rate of the molten metal leaving the converter.

[0017] The upstream portion (16) can be conical. In a preferred embodiment the upstream portion (16) has a vertical sigmoid profile as illustrated on Figure 3. Vertical sigmoid profile means a sigmoid profile rotated of 90°.The shape suits better to the tap hole which get eroded with the time.

[0018] In order to withstand high temperature, the tap hole closure is made of refractory material comprising chromite or alumina-silicate or zirconia.

[0019] The density of the tap hole closure depends on the molten steel. The tap hole closure density of the intermediate and the upstream portion usually varies between 2.5 g/cm^ and 4.0 g/cm3.

[0020] In order to facilitate the handling of the tap hole closure, a tail (3) is provided extending upwardly from the upstream portion.

[0021] The present invention also concerns a process for pouring a converter wherein a tap hole closure defined as above is dropped in the converter.

[0022] The unit of the measurements of H1 , H2, H3, W1 , W2, W3 and W2d are in mm.

BRIEF DESCRIPTION OF THE FIGURES

For a full understanding of the nature of the present invention, reference is made to the following detailed description taken in conjunction with the accompanying drawings in which:

Figure 1 : represents an embodiment of a tap hole closure according to the invention.

Figure 2: represents a general view of a dart from the prior art. Figure 3: represents another embodiment of a tap hole closure according to the invention.

Figure 4: represents an enlarge view of the junction between the intermediate portion (15) and the downstream portion (14) of the tap hole closure of Figure 3.

Figure 5: represents a schematic view of the friction force (Fg) versus the depth (D) of the stem of the tap hole closure inside the tap hole bore.

Figure 6: represents the Computational Fluid dynamics (CFD) results of the resulting vertical force versus the steel height in a converter.

The figures are not drawn to scale.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 illustrates a preferred embodiment of the tap hole closure wherein the upstream portion (16) has a vertical sigmoid profile. The maximum width of the upstream portion (16) depends on the tap hole diameter bore of the converter which increases with the age of the tap hole due to the erosion. The height (H3) of the upstream portion (16) being measured on the longitudinal axis is preferably 150 mm but can have a value of 250mm. The height (H2) of the intermediate portion (15) preferably varies between 50-250 mm. The diameter of the intermediate portion (14) usually varies between 50-80 mm depending the tap hole bore erosion. The downstream portion is a stem having a diameter comprised between 35-40 mm and a height (H1 ) comprised between 600-1600 mm.

[0023] Figure 2 illustrates a tap hole closure from the prior art. The diameter of the stem is usually around 40-45 mm. This relative wide diameter of the stem compared to the diameter of the tap hole bore can cause hit rate problem. When the tap hole closure misses the tap hole, the tap is completed without any dart used. This causes excess slag carryover. The diameter of the downstream portion (14) of the present invention is at most 40mm and preferably is comprised between 35-40 mm.

[0024] Figure 3 illustrates another preferred embodiment of a tap hole closure.

[0025] Figure 4 represents an enlarged view of the junction between the intermediate portion (15) and the downstream portion (14) of the tap hole closure represented of Figure 3. The reduction of the width of the elongated refractory body at the junction with the downstream portion is not straight (angle of 90°)as it is in the embodiment represented on Figure 1 but occurs on a maximum height of 10mm. This ensures an abrupt decrease of the width from the intermediate portion (15) to the downstream portion (14). The location (a) on the central longitudinal axis of the refractory body is the closest location of the intermediate portion to the downstream portion where W2=W1 +10mm, the location (a) being at the junction between the downstream portion and the intermediate portion.

[0026] During the tapping, three forces act on a tap hole closure: the gravity, the buoyancy and the friction due to steel flow on the stem. When the friction on the stem and the gravity exceed the buoyancy, the tap hole closure will be pulled over the tap hole. The remaining liquid quantity in the converter at the end of the tapping process depends on the moment that the tap hole closure is pulled over the tap hole. [0027] The friction force is given by

1 _S oc CfApU where Cf is the coefficient of friction, A the area of the stem inside the tap hole, p is the density of steel and U is the flow rate of the steel.

[0028] This flow rate in turn is given by:

U=a^l(2gh).f _D .y.60

where U is the flow rate, a a constant, g is gravity, h is the height of the ferrostatic head,†Q is the tap hole diameter and y is the density of the steel.

[0029] The friction force inside the tap hole is then not linear and the graph shape of this friction force depending on the depth of the tap hole closure inside the tap hole is shown on Figure 5.

[0030] The original idea of the invention is to add an intermediate region of a determined length and width above the stem representing the downstream portion which engages entirely in the tap hole bore. The initial friction force due to the stem is significantly increased for a certain period of time when the intermediate portion engages in the tap hole bore. The intermediate region which may also have the shape of a stem has a width of at least 10 mm greater than the width of the stem of the downstream portion and has a minimum height of 50 mm. The result is a tap hole closure which is pulled down quickly but at the last moment meaning when a minimum of steel is remaining in the converter.

[0031] Computational Fluid dynamics (CFD) were carried out to provide trends of tap hole closure behavior. Figure 6 shows the resulting vertical force resulting from the three forces versus the steel height in the converter. The figure was calculated from the formulas here-above. The calculation was carried out for two similar tap hole closures (same weight, same total length, same diameter of the upper portion, same diameter of the downstream portion), the only difference sets in the presence of an intermediate portion according to the invention: the behavior of the dart of the prior art (see Figure 2) is represented by a continuous line while the behavior of the tap hole closure according the present invention is represented by a dash line.

[0032] What is important to notice is that the resulting vertical force (on the left side of the figure) increases (in negative values) more abruptly than the resulting vertical force with the tap hole closure without the intermediate portion meaning that the tap hole closure is pulled down much quicker than the comparative tap hole closure. In addition, the resulting force on the tap hole closure of the present invention is greater (on the right side of the figure) than the resulting force on the tap hole of the prior art. This means that a lighter tap hole closure could also be used: Normally a lighter tap hole closure of the prior art is sucked later than a tap hole closure having the same dimensions. When a tap hole closure is sucked later, there is a risk than slag is carried over. In the present case as the tap hole closure according the present floats as long as the comparative tap hole closure but it is then sucked more quickly.

[0033] A heat which comprises the steps of filling the converter, blowing oxygen and tapping the converter typically lasts for 40 minutes. If 30 seconds is saved on each heat, an extra heat is obtained every 120 heats. On a typical steel plant, this is an extra heat every week. This time advantage is significant at the end of the campaign.

[0034] As a lighter tap hole closure will be used, health and safety issues related to the manual handling of this product are also reduced.

[0035] In addition, when the tap hole bore gets older, its diameter increases due to erosion. By using a tap hole closure of the prior art, the diameter of the cone must be increased leading to a heavier closure which in turn will be pulled over the tap hole sooner, decreasing the quantity of slag carried over but increasing the tapping time. The tap hole closure according to the invention provides more flexibility as lighter closures can be used preventing the tapping time from increasing.

[0036] List of references