A taphole for discharging a metallurgical vessel of an electric arc furnace and associated furnace and discharging method

Abstract

A taphole for discharging a metallurgical vessel of an electric arc furnace and associated furnace and discharging method The invention relates to a taphole (14) for discharging a metallurgical vessel (12) of an electric arc furnace (10), the taphole (14) comprising a central conduit extending along a longitudinal axis (X) between a top end (32) opening into the vessel (12) and a bottom end (34) for discharging molten steel outside the vessel (12) into an enclosure (24), such as a ladle. The central conduit comprises successively an upstream portion and a downstream portion, and a throttle arranged between the upstream and the downstream portions. The throttle has a constricted cross-section compared to a cross-section of the upstream and downstream portions, so as to generate a Venturi effect when discharging the vessel (12) through the taphole (14).

Description

[0001] A taphole for discharging a metallurgical vessel of an electric arc furnace and associated furnace and discharging method

[0002] The present invention relates to a taphole of a metallurgical vessel of an electric arc furnace, the metallurgical vessel being suitable for containing molten metal, in particular molten steel. The present invention also relates to an electric arc furnace comprising such vessel and to a method for discharging such furnace. Steel can be currently produced through two mains manufacturing routes. Nowadays, most used production route named “BF-BOF route” consists in producing hot metal in a blast furnace, by use of a reducing agent, mainly coke, to reduce iron oxides and then transform hot metal into steel into a converter process or Basic Oxygen furnace (BOF). This route, both in the production of coke from coal in a coking plant and in the production of the hot metal, releases significant quantities of CO2.The second main route involves so-called “direct reduction methods”. Among them are methods according to the brands MIDREX®, FINMET®, ENERGIRON® / HYL, COREX®, FINEX® etc., in which sponge iron is produced in the form of HDRI (hot direct reduced iron), CDRI (cold direct reduced iron), or HBI (hot briquetted iron) from the direct reduction of iron oxide carriers. Sponge iron in the form of HDRI, CDRI, and HBI undergoes further processing in electric arc furnaces (EAF) to produce steel.

[0003] One of the main options chosen by steelmakers to reduce CO2 emissions is therefore to switch from the BF-BOF route towards the DRI-EAF route. However, use of DRI (Direct Reduced Iron) products in classical electrical furnaces together with ferrous scraps has some limitations. Indeed, scrap contain a lot of impurities and resulting liquid steel will need to be further processed to produce high quality steel grades. Moreover, electric arc furnaces were up to now used for production of specific grades, mostly for long products applications, which do not have the same constraints in terms of metallurgy that the grades used notably for automotive products. For example, liquid steel produced from a basic oxygen furnace contains 20-to-90 parts per million (ppm) of nitrogen, compared to 100-to-140 ppm of nitrogen in liquid steel produced in an electric arc furnace. The nitrogen content of current electric arc furnace (EAF) steel is thus much higher than that of basic oxygen furnace (BOF) steel and cannot meet the requirements of high-grade steel. High nitrogen content can result in inconsistent mechanical properties in hot rolled steels, embrittlement of the heat affected zone (HAZ) of welded steels, and poor cold formability.

[0004] This is also true for hydrogen whose content is much higher in liquid steel produced in an EAF than the one of BOF steel. Just a few parts per million of hydrogen dissolved in steel can cause hairline cracks (flakes), hydrogen embrittlement, hydrogen blistering and loss of tensile ductility, particularly in large steel castings ingots, blooms and slabs. One aim of the present invention is to propose a remedy for the drawbacks described, and to propose a system for removing dissolved gases from molten metal for an electric arc furnace that is efficient and does not require complex and expensive equipment.

[0005] To this aim, the invention relates to a taphole for discharging a metallurgical vessel of an electric arc furnace, the taphole comprising a central conduit extending along a longitudinal axis between a top end opening into the vessel and a bottom end for discharging molten steel outside the vessel into a second enclosure, such as a ladle, the central conduit comprising successively an upstream portion and a downstream portion, the central conduit comprising a throttle arranged between the upstream and the downstream portions, the throttle having a constricted cross-section compared to a cross-section of the upstream and downstream portions, so as to generate a Venturi effect when discharging the vessel through the taphole.

[0006] The local section reduction creates local low pressure to allow bubbles formation and the removal of dissolved gases from molten metal.

[0007] The taphole according to the invention may comprise one or more of the following features, taken solely, or according to any technical feasible combination:

[0008] - the taphole is an eccentric bottom tapping;

[0009] - the upstream portion and the downstream portion respectively comprise a convergent tube and a divergent tube that are coaxial;

[0010] - a ratio of the cross-section of the throttle to the cross-section of the upstream portion at the top end is from 0.10 to 0.50, preferably from 0.20 to 0.30;

[0011] - a ratio of the cross-section of the throttle to the cross-section of the downstream portion at the bottom end is from 0.10 to 0.50, preferably from 0.20 to 0.30;

[0012] - the throttle is configured to generate a local pressure reduction, with the local pressure downstream of the throttle being inferior to 0.5 atm, preferably inferior to 0.3 atm;

[0013] - the central conduit forms an angle with the vertical axis from 30° to 60°.

[0014] - the taphole further comprises an injection system for injecting an inert gas in the central conduit;

[0015] - the injected gas is chosen between argon and carbon monoxide;

[0016] - the taphole further comprises a closing device movable between an open position for discharging molten metal out of the vessel through the taphole and a closed position, into which molten metal cannot exit the vessel through the central conduit.

[0017] The invention also relates to an electric arc furnace comprising a metallurgical vessel comprising a taphole as described above.

[0018] According to a preferred embodiment, the electric arc furnace further comprises a tilting device configured to tilt the vessel between a resting position and a pouring position. The invention also concerns a method for discharging an electric arc furnace as described above, the metallurgical vessel containing molten metal, the method comprising the step of tapping the molten metal into a second enclosure, preferably a ladle, through the taphole of the vessel.

[0019] The method according to the invention may comprise one or more of the following features, taken solely, or according to any technical feasible combination:

[0020] - the method further comprises a step of tilting the electric arc furnace into the pouring position;

[0021] - the method, further comprises a step of injecting an inert gas in the central conduit.

[0022] The invention will be better understood, based on the following description, given solely as an example and made in reference to the appended drawings, in which:

[0023] Figure 1 is a schematic view of an electric arc furnace comprising a taphole according to the invention;

[0024] Figure 2 is a schematic sectional view of the taphole of Figure 1.

[0025] An electric arc furnace 10 according to the invention is shown on Figure 1.

[0026] The electric arc furnace 10 comprises a metallurgical vessel 12 comprising a taphole 14, which connects an interior 15 of the vessel 12 to an exterior of the vessel 12.

[0027] Preferably, the furnace 10 further comprises a tilting device 16 for tilting the vessel 12 between a resting position (as shown on Figure 1) and a pouring position.

[0028] The electric arc furnace 10 of the present disclosure is a furnace that heats material by means of an electric arc created by electrodes 18.

[0029] Electric arc furnaces having various shapes and sizes may be suitable for use in connection with the present disclosure. Other components of an electric arc furnace are well known to those of ordinary skill in the art and will thus not be described.

[0030] The vessel 12 comprises a bottom wall 19 defining an interior surface 20 and an exterior surface 21 of the vessel 12.

[0031] The vessel 12 is configured to allow the preparation of a bath 22 of molten metal, in particular of molten steel. Slag is also present in the vessel on the top of the molten metal. Dissolved gases, such as H2, O2 and N2, are also present in the molten metal.

[0032] As used herein, “tapping” refers to the transferring of molten metal from an electric arc furnace to another enclosure 24, such as a ladle, by an operation that involves tilting the furnace. Tapping may refer to a partial transfer or a complete transfer of the molten metal. Therefore, tapping may involve transferring at least 50%, preferably at least 90%, of the volume or weight of molten metal from the electric arc furnace to another enclosure. The taphole 14 defines an opening in the electric arc furnace 10 through which metal can exit the furnace 10 during a tapping operation.

[0033] In the following description, the terms “upstream” and “downstream” are defined based on the flow direction of the metal and / or slag through the taphole 14 during tapping.

[0034] According to the invention, the taphole 14 is a bottom taphole and defines an opening in the bottom of the vessel 12 through which molten metal can flow by gravity, and thus be drained from the furnace.

[0035] Advantageously, as visible on Figure 1 , the taphole 14 is an eccentric bottom taphole (also designated by the acronym EBT). The taphole 14 is off-center (e.g., disposed closer to a first side of the furnace than to a second side of the furnace that is opposite the first side) to facilitate tapping of the furnace through a tilting operation.

[0036] A taphole 14 according to the invention is shown in more details on Figure 2.

[0037] The taphole 14 comprises a central conduit 30 extending along a longitudinal axis X between a top end 32 opening into the vessel 12 and a bottom end 34 for discharging liquid steel outside the vessel 12 into the enclosure 24. The central conduit 30 is delimited laterally by sidewall(s) 35.

[0038] Preferably, the taphole 14 comprises or is lined with a refractory material (not shown). Such a refractory material is chosen for example between an oxide of aluminum, silicon, magnesium, zirconium, calcium, or a mixture thereof.

[0039] In the following description, the shape and / or dimensions of the central conduit 30 are given taking into account such refractory material.

[0040] According to a preferred embodiment, as shown on Figure 2, the central conduit 30 forms an angle different from 90° with the bottom of the vessel 12.

[0041] The angle of the taphole 14 may be such that the longitudinal axis X of the central conduit 30 forms with the vertical axis an angle a from 30° to 60°, when the vessel 12 is in its resting position.

[0042] A cross-section of the central conduit 30 may be defined as a section of the conduit 30 according to a transverse plane normal to the longitudinal axis X.

[0043] In the embodiment shown on Figure 2, the central conduit 30 has sensibly round cross-sections. Alternatively, the central conduit 30 has oval cross-sections, orthogonal cross-sections or more complex shaped cross-sections

[0044] As visible on Figure 2, the central conduit 30 comprises successively an upstream portion 40 and a downstream portion 42, the upstream and downstream portions 40, 42 being arranged on either side of a throttle 44. The throttle 44 has a constricted cross-section compared to the cross-sections of the upstream and downstream portions 40, 42, thus generating a Venturi effect when discharging the vessel 12 through the taphole 14.

[0045] According to embodiment shown on Figure 2, each of the upstream portion 40 and of the bottom portion 42 comprises a tube portion in the form of a truncated cone.

[0046] The upstream portion 40 defines a convergent tube, the top end 32 having a diameter that is larger than a diameter of the throttle 44. The downstream portion 42 defines a divergent tube, the bottom end 34 having a diameter that is bigger than a diameter of the throttle 44. The convergent and divergent tubes are coaxial.

[0047] The sidewalls of the central conduit 30 present, along a transverse plane normal to the longitudinal axis X, a profile such that the transverse cross-section of the conduit decreases from the top end 32 towards the throttle 44 and then increases from the throttle 44 towards the bottom end 34.

[0048] More precisely, the longitudinal evolution of the transverse cross-section of the central 30 conduit varies in a manner so as to generate an acceleration of the flow by means of the Venturi effect in the throttle 44.

[0049] The upstream portion 40 thus presents a transverse cross section whose surface decreases from the top towards the bottom. The variation in the surface of the transverse cross section may be continuous from the top end 32 up to the throttle 44.

[0050] Alternatively, as shown on Figure 2, the surface may first be constant from the top end 32 up to an intermediary point and then decreases continuously from said intermediary point up to the throttle 44.

[0051] The ratio between the surface of the transverse cross section of the upstream portion 40 at the level of the throttle 44 and the surface of the transverse cross section of the upstream portion 40 at the level of the top end 32 is from 0.10 to 0.50, preferably from 0.20 to 0.30.

[0052] For example, the upstream portion 40 has a diameter ranging from 10 cm to 30 cm, preferably around 15 cm, at the level of the top end 32 and a diameter ranging from 3 cm to 10 cm, preferably from 7 cm to 9 cm, preferably around 7 cm, at the level of the throttle 44.

[0053] The downstream portion 42 presents a transverse cross section whose surface increases from the top towards the bottom. The variation in the surface of the transverse cross section may be continuous from the throttle 44 up to the bottom end 34.

[0054] Alternatively, as shown on Figure 2, the surface may first decrease continuously from the throttle 44 up to an intermediary point and then be constant from said intermediary point up to the bottom end 34. The ratio between the surface of the transverse cross section of the downstream portion 42 at the level of the throttle 44 and the surface of the transverse cross section of the downstream portion 42 at the level of the bottom end 34 is from 0.10 to 0.50, preferably from 0.20 to 0.30.

[0055] For example, the downstream portion 42 has a diameter ranging from 10 cm to 30 cm, preferably around 15 cm, at the level of the bottom end 34 and a diameter ranging from 3 cm to 10 cm, preferably from 7 cm to 9 cm, preferably around 7 cm, at the level of the throttle 44.

[0056] Preferably, the throttle 44 is arranged between the upstream portion 40 and the downstream portion 42, so that a ratio of a length L40 of the upstream portion 40 by a length L42 of the downstream portion 42 is comprised between 0.5 and 2, preferably about 1 .

[0057] In the embodiment shown on Figure 2, the throttle 44 defines a localized constriction. In other words, the throttle 44 corresponds to the junction of the upstream portion 40 with the downstream portion 42.

[0058] Alternatively (not shown), the throttle 44 defines an extended neck portion extending along the longitudinal axis X.

[0059] Alternatively (not shown), the central conduit 30 is in the form of a cylinder, where the top end 32 and the bottom end 34 have substantially the same diameter. For example, the central conduit 30 has a diameter ranging from 10 cm to 30 cm, preferably around 15 cm. The throttle 44 is formed here by an internal collar protruding axially into the central conduit 30.

[0060] Alternatively, the central conduit 30 may have other shapes and / or dimensions, in which the throttle 44 defines a local constriction of said central conduit 30.

[0061] The reduction of the cross-section at the throttle 44 allows a speeding up of the molten metal, thus creating a local reduction of the static pressure.

[0062] The local pressure just downstream of the throttle 44 is preferably inferior to 0.5 atm (ie. 50662.5 Pa), even more preferably inferior to 0.3 atm (ie. 30397.5 Pa).

[0063] At such a pressure, gas bubbles are locally generated.

[0064] Advantageously, the local pressure is below the partial pressure of the dissolved gases in the liquid metal. A cavitation phenomenon thus happens at the throttle 44 and bubbles appear. When the liquid metal is received in the enclosure 24, notably in the ladle, the bubbles coalesce and rise to the surface of the liquid metal. Dissolved gases are thus extracted from the liquid metal and released to the surrounding atmospheric.

[0065] In a preferred embodiment, as shown on Figure 2, the taphole 14 further comprises an injection system 50 for injecting a gas in the central conduit 30. The injection system 50 comprises at least one gas inlet gas 52 emerging in the central conduit 30. As visible on Figure 2, the inlet 52 emerges in the central conduit 30 at the throttle 44 level or adjacent to the throttle 44 but downstream thereof.

[0066] Preferably, the taphole 14 comprises a plurality of gas inlets surrounding the throttle 44.

[0067] The injected gas is preferably an inert or reducing gas, for example chosen between argon or carbon monoxide.

[0068] The injection of the inert gas facilitates and / or boosts bubbles generation by creating a high turbulent flow and by increasing the reaction surface, thus maximizing outgassing kinetics.

[0069] It also protects the liquid metal jet from the surrounding atmosphere and reduces N, O and / or H pick-up.

[0070] Advantageously, the taphole 14 further comprises a closing device (not shown) movable between an open position for discharging molten metal out of the vessel 12 through the taphole 14 and a closed position, into which molten metal cannot exit the vessel 12 through the central conduit 30.

[0071] For example, the closing device may be a bidirectional slide gate valve configured to obstruct the opening of the taphole 14 at the top end 32 or at the bottom end 34.

[0072] A method for discharging the electric arc furnace 10 as defined above will now be described.

[0073] As known per se, molten metal is made in the metallurgical vessel 12.

[0074] Molten metal is then transferred to another enclosure 24, in particular to a ladle.

[0075] Preferably, the vessel 12 is first tilted from an initial resting position to a pouring position and, when present, the closing device is put into its open position.

[0076] Molten metal is then tapped from the vessel 12 through the taphole 14. Molten metal successively flows through the upstream portion 40, the throttle 44 and the downstream portion 42.

[0077] The reduction of the cross-section at the throttle 44 generates a locally reduced pressure, allowing bubbles generation and the removal of the dissolved gas present in the molten metal.

[0078] Advantageously, an inert gas is injected in the central conduit 30 at the throttle 44. The injection of the inert gas boosts the generation of bubbles and protects the liquid metal jet from the surrounding atmosphere. The taphole 14 and the method for discharging the electric arc furnace 10 according to the invention allows an efficient and easy removal of dissolved gases from the molten metal, facilitating obtaining low H, low N and / or low O content in the molten metal recovered in the enclosure 24, notably in the ladle.

Claims

CLAIMS1.- A taphole (14) for discharging a metallurgical vessel (12) of an electric arc furnace (10), the taphole (14) comprising a central conduit (30) extending along a longitudinal axis (X) between a top end (32) opening into the vessel (12) and a bottom end (34) for discharging molten steel outside the vessel (12) into an enclosure (24), the central conduit (30) comprising successively an upstream portion (40) and a downstream portion (42), the central conduit (30) comprising a throttle (44) arranged between the upstream and the downstream portions (40, 42), the throttle (44) having a constricted cross-section compared to a cross-section of the upstream and downstream portions (40, 42), so as to generate a Venturi effect when discharging the vessel (12) through the taphole (14).2.- The taphole (14) according to claim 1 , wherein the upstream portion (40) and the downstream portion (42) respectively comprise a convergent tube and a divergent tube that are coaxial.3.- The taphole (14) according to any of the preceding claims, wherein a ratio of the cross-section of the throttle (44) to the cross-section of the upstream portion (40) at the top end (32) is from 0.10 to 0.50, preferably from 0.20 to 0.30.4.- The taphole (14) according to any of the preceding claims, wherein a ratio of the cross-section of the throttle (44) to the cross-section of the downstream portion (42) at the bottom end (34) is from 0.10 to 0.50, preferably from 0.20 to 0.30.

5. The taphole (14) according to any of the preceding claims, wherein the throttle (44) is configured to generate a local pressure reduction, with the local pressure downstream of the throttle (44) being inferior to 0.5 atm, preferably inferior to 0.3 atm.6.- The taphole (14) according to any of the preceding claims, wherein the central conduit (30) forms an angle (a) with the vertical axis from 30° to 60°.7.- The taphole (14) according to any of the preceding claims, further comprising an injection system (50) for injecting an inert or reducing gas in the central conduit (30).8.- The taphole (14) according to claim 7, wherein the injected gas is chosen between argon and carbon monoxide.9.- The taphole (14) according to any of the preceding claims, further comprising a closing device movable between an open position for discharging molten metal out of the vessel (12) through the taphole (14) and a closed position, into which molten metal cannot exit the vessel (12) through the central conduit (30).10.- An electric arc furnace (10) comprising a metallurgical vessel (12) comprising a taphole (14) according to any of the preceding claims.11.- The electric arc furnace (10) according to claim 10, wherein the taphole (14) is an eccentric bottom tapping.12.- The electric arc furnace (10) according to claim 10 or 11 , further comprising a tilting device (16) configured to tilt the vessel (12) between a resting position and a pouring position.13.- A method for discharging an electric arc furnace (10) according to any of claims 10 to 12, the metallurgical vessel (12) containing molten metal, the method comprising the step of tapping the molten metal into an enclosure (24), preferably a ladle, through the taphole (14) of the vessel (12).14.- The method according to claim 13, the electric arc furnace (10) being according to claim 12, further comprising a step of tilting the electric arc furnace (10) into the pouring position.15.- The method for discharging an electric arc furnace according to claim 13 or 14, the taphole (14) being according to claim 7 or 8, further comprising a step of injecting an inert or reducing gas in the central conduit (30).

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