Patent Application
20250065397
This present invention relates to a method and apparatus for purifying liquid metal and is particularly applicable to purifying molten steel during a continuous casting process. More particularly, the present invention relates to the removal of non-metallic particles from liquid steel as the liquid steel flows through an intermediate vessel prior to the entry of the liquid steel into one or more casting molds.
In the continuous casting of steel, a tundish is typically used to facilitate the continuous transfer of liquid steel to one or more molds, from a sequence of arriving ladles, each of which carries a batch or ‘heat’ of liquid steel. A tundish is a trough-like vessel lined with refractory material and is designed so as to receive liquid steel from a ladle. A tundish is equipped with one or more outlets that control the flow of the liquid steel exiting the tundish and subsequently entering into one or more casting molds.
The sources of non-metallic particles in liquid steel are both indigenous and exogenous in nature. Reoxidation refers to the process of oxygen pick-up by liquid steel from environmental sources after said steel has been deoxidized by conventional metallurgical methods. Reoxidation is of particular interest, as this reoxidation can lead to significant contamination of the liquid steel by harmful non-metallic particles formed when exogenous oxygen reacts with readily oxidizable solute elements in the liquid steel. These non-metallic particles can take the form of globules, if the particles are molten or partially molten at liquid steel temperatures, or take the form of regular or irregular-shaped solids. Reoxidation is very difficult to prevent, since active oxygen levels greater than a liquid steel's chemical equilibrium oxygen level are ubiquitous in the steelmaking and casting environment. Thus, tundish transfer operations, particularly liquid steel transfer from ladle-to-tundish, can readily cause reoxidation of the liquid steel in the tundish, resulting in elevated levels of large non-metallic particles. The harmful effects of these particles in the liquid steel include restriction and even total clogging of flow passages in the tundish and tundish-to-mold flow-control systems, and the occurrence of numerous types of surface and internal defects related to the presence of non-metallic inclusions in the semi-finished continuously-cast steel, such as slabs and billets, as well as a wide-range of inclusion-related defects in the great variety range of steel products produced from the semi-finished steel, such as steel sheets, plates, bars, beams, rods, etc.
A conventional means employed for the separation of non-metallic particles from liquid steel involves exposing the steel to a covering layer of molten slag and/or flux that can absorb the particles. Flux is a synthetic mixture of slag-making materials for use in a tundish or other liquid metal containing vessel, which is designed so as to have optimized properties with respect to non-metallic particle absorption and thermal insulation. Both the harmful non-metallic particles and the absorbent flux/slag (i.e., flux, slag, or flux and slag mixture) are less dense than the liquid steel. Thus, the molten flux/slag floats upon and covers the liquid steel and non-metallic particles in the steel, being somewhat buoyant, will tend to slowly rise to join with the floating flux/slag. A well-known method, often called ‘gas-purging’, for the promotion of non-metallic particle separation into a floating flux/slag cover is to expose the liquid steel to a rising stream of gas bubbles. Various gases of an inert or of a low-reactivity with liquid steel nature, such as argon or nitrogen have been utilized for this purpose. Particles in the liquid steel tend to become attached to the highly buoyant, and thus much more rapidly rising, gas bubbles. In this way, non-metallic particles may be carried more-quickly, and more efficiently, upward to the flux/slag cover that floats upon the surface of the liquid steel where the particles can be absorbed and thereby removed from the liquid steel.
In the gas-purging of liquid steel in a tundish, the purging gas (or purge-gas) is typically introduced into the liquid steel via purging bars located at the bottom of the tundish. A vertical cross-section of a typical tundish is a trapezoid, with the bottom edge shorter than the top edge. A purge bar is arranged at the bottom of the tundish parallel to the shorter edge of the trapezoid and thus does not extend the entire width of the liquid steel column above. Gas bubbles tend to float straight up from a purge bar and therefore, due to the sloped sidewalls of the tundish, gaps are formed towards the sides of the tundish where the curtain of bubbles does not efficiently penetrate the liquid steel. As a result, the liquid steel flow passing a purge bar may not be uniformly exposed to the gas. In addition, a purge bar provides a slim curtain of rising gas bubbles that can be easily disrupted by the flowing steel. This disruption further exacerbates the problem of inhomogeneous effectiveness of the gas bubbling.
A purge bar system suffers from factors that severely limit the gas flow rate that can be applied to a purge bar without inducing detrimental effects on the casting process. Increasing the rate of gas flow to a purge bar has the potential to increase contact between the liquid steel and the gas bubbles so as to increase the probability of particle/bubble attachment and thereby increase the rate and efficiency of particle removal to the flux/slag. However, the gas bubbles introduced to a tundish by the gas flow to a prior-art purge bar can readily, and adversely, disrupt the floating flux/slag layer and can also strongly affect the pattern of liquid steel flow developed in the tundish in ways that can be detrimental to liquid steel quality and casting process performance.
The gas bubbles created by a gas-purging process not only rise through the liquid steel in the tundish but then must also rise through the floating layer of flux/slag. This layer is not only less dense than the liquid steel, but typically also has much less depth than the liquid steel, since it is a necessary condition for smooth tundish operations and continuously-cast product quality that liquid steel mass in the tundish be maximized. Therefore, the static pressure developed within the flux/slag layer in a tundish is generally low, allowing the region or portion of the molten flux/slag layer above the purge bar to be easily disrupted by rising gas bubbles. Also, the floating flux/slag cover will become progressively more highly disrupted as gas flow is increased. The rising flow of bubbles entrains the surrounding liquid metal causing an up-welling flow of bubbly liquid beneath the flux/slag. The up-welling bubbly liquid and the evolution of bubbles from the liquid steel acts to push the flux/slag laterally away from the bubbly region, thereby thinning-out the flux/slag layer in the region where the bubbles are evolved from the liquid steel. These effects can significantly impede the absorption of particles into the flux/slag, and if sufficiently disrupted and/or thinned-out the flux/slag layer can no longer properly protect the steel from oxygen in the ambient environment leading to reoxidation of the liquid steel and to the creation of harmful non-metallic particles rather than to their removal.
High rates of gas flow and small bubble size are desirable to increase the efficiency of particle removal, as these two factors act to maximize both the area of gas/liquid metal interface available for particle/gas contact and the probability of particle/gas contact. However, these two factors can also act to profoundly affect the liquid steel flow pattern developed in the tundish, as the rising bubbles more effectively entrain the surrounding liquid steel. In this way, gas-purging can induce an extensive stirring activity into the liquid flow in the tundish, wherein the stirring energy or stirring intensity is proportional to the gas flow rate. Although the stirring flow will proceed generally upward in the bubbly region, the principle of mass conservation is only satisfied if this flow is balanced by flows that are generally outward relative to the upper part of the bubbly region and generally inward with respect to the lower part of the bubbly region. The stirring action induced by gas-purging must be properly controlled to avoid detrimental effects on the casting process that include short-circuiting of the flow between that incoming from the ladle and that out-going to the mold or molds, flux/slag entrainment into the liquid metal as the result of flux/slag layer disruption, and steel reoxidation.
According to one aspect of the invention, an apparatus and method are provided for the gas-purging of liquid steel that is covered by a layer of molten flux, slag, or a mixture of slag and flux, as the liquid steel flows through an intermediate vessel, such as a tundish. The apparatus and method in accordance with aspects of the invention enhance removal of non-metallic particle from liquid steel flowing through a tundish during a process for the continuous casting of steel. Advantageously, the device and method in accordance with the invention overcomes one or more problems and limitations of conventional gas-purging devices.
In accordance with one aspect of the invention, the apparatus spans an interior dimension of the vessel, upstream of one or more casting outlets, and is devised such that the entirety of the liquid steel flowing to the outlet(s) has passed through the apparatus. Thus, the liquid steel flow passing through the apparatus in accordance with the invention is uniformly exposed to the gas. The apparatus can include means for the introduction of gas bubbles into the liquid steel and unique means for venting and confining the bubbly flow of gas and liquid steel. A unique vent channel arrangement in accordance with the invention impedes deleterious disruption or thinning of the floating flux/slag layer covering the liquid steel, thereby enhancing the absorbance and capture of non-metallic particles. The inventive venting arrangement also acts to confine and control the liquid steel stirring action induced by gas-purging such that the potentially harmful effects of such stirring, such as short-circuit flows, flux/slag entrainment, and steel reoxidation, are strongly constrained or entirely avoided. The venting arrangement confines the inert or low-reactivity purge-gas evolving from liquid steel and flux/slag, such that a more chemically-inert atmosphere, as compared to ambient air, is formed in the upper portion of the vent channel. As a result, the oxygen contained in the venting arrangement is expelled from this region by the emerging purge-gas, reducing or eliminating the potential for reoxidation of the liquid steel. The venting arrangement allows the gas flow rate to be readily adjusted to levels that maximize non-metallic particle removal without adverse effects on liquid steel quality.
According to one aspect of the invention, a purifying device for removing impurities from liquid metal present in a vessel includes: a baffle including a first surface and a second surface disposed opposite the first surface; a vent channel formed between the first surface and the second surface, the vent channel having a proximal end and a distal end opposite the proximal end, the vent channel configured to receive a stream of gas at the proximal end and vent at least some of the stream of gas at the distal end; at least one first vent hole, arranged on one of the first surface or the second surface, the at least one first vent hole extending into the vent channel; and a passage for liquid metal flow, the passage having a proximal surface and a distal surface, where the distal surface is disposed adjacent to the proximal end of the vent channel.
In one embodiment, the baffle further includes a gas supply channel including an input port for receiving a gas, an output port for outputting the gas, and conduit connecting the input port to the output port, wherein the output port is disposed at the proximal surface of the passage.
In one embodiment, the device includes a porous element disposed at the proximal surface of the passage.
In one embodiment, the porous element is fluidically coupled to the output port of the gas channel.
In one embodiment, the porous element spans an entire width of the passage.
In one embodiment, the device includes at least one second vent hole arranged opposite the at least one first vent hole on the other of the first surface or the second surface.
In one embodiment, the distal end of the vent channel opens to the ambient environment above a free surface of the liquid metal.
In one embodiment, the at least one first vent hole comprises a plurality of first vent holes, and the at least one second vent hole comprises a plurality of second vent holes, wherein the plurality of first vent holes are arranged along a first plane and the plurality of second vent holes are arranged along a second plane.
In one embodiment, the device includes a divider wall arranged within the vent channel, the divider wall splitting the vent channel into two separate channels.
In one embodiment, the dividing wall spans between the proximal end and the distal end of the vent channel.
In one embodiment, the passage for liquid metal flow spans an entire width of the proximal end of the vent channel.
According to another aspect of the invention, a tundish includes: a floor; a wall attached to the floor, wherein the wall and the floor define a container; and the purifying device as described herein arranged with the container.
In one embodiment, the wall includes a first side wall and a second sidewall arranged opposite the first sidewall, the first and second sidewalls attached to the floor; and a first end wall and a second end wall arranged opposite the first end wall, the first end wall and second end wall attached to the floor and to respective ends of the first and second sidewalls to define a container.
In one embodiment, the tundish includes an inlet for receiving liquid metal into the tundish, and an outlet for dispensing liquid metal from the tundish, wherein the purifying device is disposed between the inlet and outlet.
In one embodiment, the tundish has a predetermined maximum level for liquid metal, and the distal end of the vent channel extends above the predetermined maximum level for liquid metal.
In one embodiment, the purifying device spans an entire width of the container between a floor of the container and a maximum liquid metal level of the container.
According to another aspect of the invention, a method for removing impurities from liquid metal present in a vessel is provided, where the liquid metal is covered by a top layer floating on a free surface of the liquid metal, the vessel including a baffle defining a first vessel section and a second vessel section, the baffle having a passage for liquid metal flow arranged proximate to a floor of the vessel and fluidically coupling the first vessel section to the second vessel section. The method includes: subjecting liquid metal flowing within the passage to a stream of gas; receiving at least some of the stream of gas and a portion of the liquid metal flowing within the passage at a vent channel disposed within the baffle, the vent channel guiding gas and liquid metal toward the top layer; expelling the liquid metal received by the vent channel out of the vent channel prior to the liquid metal reaching the top layer; and venting the gas received by the vent channel out of the vent channel.
In one embodiment, the top layer includes of at least one of flux or slag.
In one embodiment, venting includes guiding the stream of gas through a portion of the top layer that is constrained within the vent channel.
In one embodiment, receiving includes using a vent channel having a first vent section and a second vent section adjacent to and isolated from the first vent section.
In one embodiment, expelling includes laterally expelling the liquid metal from the vent channel.
In one embodiment, expelling further includes creating a clockwise flow of liquid metal in the second vessel section, the clockwise flow beginning at the floor adjacent to the passage for liquid metal flow, moving adjacent to the baffle and toward the top layer, and then returning toward the floor.
In one embodiment, venting the gas received by the vent channel out of the vent channel includes venting the gas over the top layer.
Examples of the specific embodiments are illustrated in the accompanying drawings. While the invention will be described in conjunction with these specific embodiments, it will be understood that it is not intended to limit the invention to such specific embodiments. On the contrary, it is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. The present invention may be practiced without some or all of these specific details. In other instances, well-known process operations have not been described in details so as to not unnecessarily obscure the present invention.
The invention may take physical form in certain parts and arrangement of parts, a preferred embodiment of which will be described in detail in the specification and illustrated in the accompanying drawings which form a part hereof, and wherein:
Various aspects now will be described more fully hereinafter. Such aspects may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey its scope to those skilled in the art.
The word “about” when immediately preceding a numerical value means a range of plus or minus 10% of that value, e.g., “about 50” means 45 to 55, “about 25,000” means 22,500 to 27,500, etc., unless the context of the disclosure indicates otherwise, or is inconsistent with such an interpretation. For example, in a list of numerical values such as “about 49, about 50, about 55, “about 50” means a range extending to less than half the interval(s) between the preceding and subsequent values, e.g., more than 49.5 to less than 52.5. Furthermore, the phrases “less than about” a value or “greater than about” a value should be understood in view of the definition of the term “about” provided herein.
In accordance with the present invention, a purifying device is disclosed that can be placed within a vessel, such as a tundish, the purifying device operative to remove impurities from liquid steel present in the container. The purifying device according to the invention utilizes gas purging, where a majority of gas bubbles released into the liquid steel are confined within a vent channel that releases the gas into the ambient environment. In this manner, the effusion of the mixing energy introduced to the tundish by the gas-bubbling of the liquid steel is constrained.
With reference to
The tundish 100 also includes a ladle shroud 114 having an inlet 114a for receiving an inflow of liquid steel and an outlet 114b for providing the liquid steel received at the input 114a to the container volume 106. The inlet 114a is arranged above a maximum liquid steel level of the tundish 100, and in the exemplary embodiment of
Arranged in the tundish 100 is a purifying device 130 in accordance with an embodiment of the invention, the purifying device 130 spanning between side walls of the tundish 100 so as to divide the tundish 100 into a first tundish section 100a and a second tundish section 100b. The purifying device 130 spans the tundish 100 at a location between the ladle shroud 114 (the position of liquid entry) and the outlet nozzle 116 such that the liquid steel exiting the tundish downstream of the purifying device will have passed therethrough. While the purifying device 130 is shown to be centrally located in the tundish, such location is merely exemplary and the purifying device 130 may be located at other locations within the tundish 100 between the ladle shroud 114 and the outlet nozzle 116.
With continued reference to
The purifying device 130 further includes a means for providing gas to the porous refractory element 136, such as a gas supply channel 138. In the exemplary embodiment of
Formed in a lower region of the baffle 132 is a lower opening 140 (also referred to as a passage) arranged near a floor of the tundish 100 (e.g., immediately adjacent to the bottom wall 104 or slightly above the bottom wall 104). The lower opening 140, which in the exemplary embodiment has a width that spans between the first surface 132a and the second surface 132b, is in the form of a through-hole that fluidically couples the first tundish section 100a to the second tundish section 100b to enable the flow of liquid steel from the first tundish section 100a to the second tundish section 100b. A length of the lower opening 140 preferably corresponds to a length of the porous refractory element 136. For example, if the porous refractory element 136 spans the entire width of the tundish 100 along a floor of the tundish (i.e., between side walls that are not shown in
An upstream portion of the lower opening 140 is located on the side of the baffle 132 towards the flow incoming to the tundish from the ladle shroud 114, while the downstream side of said lower opening is located on the side of the baffle 132 towards the flow exiting the tundish via the outlet nozzle 116. A vent channel 142 is disposed within the baffle 132 between the first surface 132a and the second surface 132b, the vent channel 142 having a proximal end 142a and a distal end 142b opposite the proximal end. The proximal end 142a of the vent channel 142 opens into the lower opening 140, while the distal end 142b opens to the ambient environment above a free surface of the liquid steel (above a predetermined maximum level of liquid steel in the tundish 100). Thus, the vent channel 142 fluidically couples the lower opening 140 to the ambient environment. Both the lower opening 140 and vent channel 142 are disposed relative to the porous refractory element 136 such that a stream of gas emitted by the porous refractory element 136 forms a gas curtain that passes vertically along the lower opening 140 and into the proximal end 142a of the vent channel 142. The vent channel 142 guides the stream of gas upwards toward a surface of the liquid steel and at least some of the gas is emitted at the distal end of the vent channel 142.
The baffle 132 includes a first lateral vent hole 144a arranged on the first surface 132a of the baffle 132, and second lateral vent hole 144b arranged on the second surface 132b of the baffle 132, where both the first and second vent holes 144a, 144b extend into the vent channel 142. In one embodiment, there are multiple first vent holes 144a arranged on the first surface 132a and/or multiple second vent holes 144b arranged on the second surface 132b. At least some of the first vent holes 144a may be arranged directly opposite a respective second vent hole 144b. Alternatively, or additionally, at least some of the first vent holes 144a may be offset from a respective second vent hole 144b, e.g., a first vent hole 144a is arranged vertically and/or horizontally offset from a respective second vent hole 144b. While vent holes 144a, 144b are shown on both sides 132a, 132b of the baffle 132, in some embodiments one or more vent holes may be arranged only on one side of the baffle 132. The slag and/or flux layer 120, in addition to floating upon the liquid steel on each side of the baffle 132, also floats upon the liquid steel that is formed within the vent channel 142 of the present invention.
In operation, liquid steel 118 is provided to the tundish 100 through the ladle shroud 114, and the liquid steel level within the tundish 100 rises to a nominal level. An inert, or low reactivity, purge-gas is provided to the porous refractory element 136 via the gas supply channel 138 and the outlet nozzle 116 is opened to permit the liquid steel 118 to flow out of the tundish 100. The porous refractory element 136 releases the gas, in the form of gas bubbles, from an upper surface of the porous refractory element 136 that contacts the liquid steel 118. The released gas is dispersed into the liquid steel 118 within the lower opening 140 as the liquid steel 118 flows therethrough. Buoyancy force causes a majority of the gas bubbles emitted from the porous refractory element 136 to rapidly rise upward into the vent channel 142 located above the opening 140.
An upstream gas trajectory 150a and a downstream gas trajectory 150b are depicted in
The flux/slag layer 120 within a vent channel 142 is not readily pushed laterally away from the bubbly region, as the vent channel 142 is substantially filled with bubbly liquid steel 118, and the flux/slag layer 120 is confined within the vent channel 142. Thus, disruption and thinning of the flux/slag layer 120 within the vent channel 142 is highly diminished or eliminated, as compared to the flux and/or slag layer disruption and thinning occurring with conventional purifying devices.
The liquid steel rising upward in the vent channel 142 is released from the vent channel 142 through the lateral vent holes 144a, 144b, which pierce the vent channel walls below the normal location the floating flux/slag layer 120, thereby reducing dynamic pressure at the flux/slag/liquid steel interface. In this way, turbulent interactions between the rising bubbly liquid in the vent channel 142, gas evolved from the bubbly liquid, and the flux layer 120 in the vent channel 142, are controlled. Thus, the lateral vent holes 144a, 144b provide an additional means to diminish disruption of the flux layer 120.
The purifying device 130 influences the downstream liquid steel flow in the tundish 100 to adopt a desirable flow pattern and flow behavior. The solid-line arrows of
In contrast to the purifying device 130 in accordance with the invention, conventional gas-purging devices do not confine the majority of the gas bubbles released into the liquid steel within a vent channel, and therefore the conventional gas purging devices do not constrain the effusion of the mixing energy introduced to the tundish by the gas-bubbling of the liquid steel. For this reason, conventional tundish gas-purging devices induce an extensive, more wide-spread, stirring energy into the liquid steel that has potentially harmful effects, such as short-circuit flows, flux/slag entrainment, and steel reoxidation. Using a purifying device in accordance with the invention, the mixing energy and turbulent stirring induced by gas-purging is largely confined within the apparatus so as to avoid harmful effects on liquid steel flow behavior and liquid steel quality.
Moving now to
A wide variety of shapes and sizes of tundish vessels are in use in order to suit particular casting operating conditions. Many alternate embodiments of the purification device 130, 130′ are possible to best befit these vessels and conditions. Alternate embodiments may include vent holes only on the downstream side, or only on the upstream side, of the purification device 130, 130′ in order to best influence liquid steel flow behavior in a particular application, or for the same reason an alternate embodiment may include no vent holes. Alternate embodiments may include multiple (i.e., three or more) vent channels to ensure strength and stiffness of the purification device 130, 130′, as well as to even more highly constrain disruptions of the flux/slag layer, better confine the purge-gas flow evolving from the top of each flow channel, and thereby better protect the turbulent liquid steel in a channel from reoxidation. Also, an alternate embodiment could comprise two or more lower openings in combination with two or more means to supply purge gas to two or more porous elements in order to best span a large tundish vessel.
Moving to
Beginning at step 402, molten metal 118 is provided to an input section (a first tundish section 100a) of the vessel, e.g., a tundish 100, the input section fluidically coupled to an output section (a second volume 100b) of the tundish. Such molten metal may be provided to the tundish via a ladle shroud 114 as is conventional. A flux/slag layer 120 is formed over the molten metal 118 as indicated at step 404, such flux/slag layer formed via conventional means.
Next at step 406 the molten metal flows from the first tundish section to the second tundish section through a lower opening 140 (e.g., a passage) formed in a baffle 132 of the purifying device 130, 130′ according to the invention, where the molten metal 118 is subjected to a stream of gas 150a, 150b as the molten metal 118 moves to the second tundish section. At step 408 at least some of the stream of gas 150a. 150b and a portion of the liquid steel 118 flowing within the passage 140 is received at a vent channel 142 disposed within the baffle 132, the vent channel 142 guiding the received gas and liquid steel toward the top layer 120 as indicated at step 410. As the column of liquid steel 118 and gas 150a. 150b travel upward through the vent channel 142, the liquid steel 118 is expelled from the vent channel 142 prior to the liquid steel 118 reaching the top layer 120 as indicated at step 412. For example, as the liquid steel 118 travels upward in the vent channel 142 it is laterally expelled through vent holes 144a, 144b. Such lateral expelling of the liquid steel 118 creates a clockwise flow of liquid steel in the second vessel section, the clockwise flow beginning at the floor adjacent to the lower opening 140, moving adjacent to the baffle 132 and toward the top layer 120, and then returning toward the floor near the outlet nozzle 116. Additionally, and as indicated at step 414, the gas received by the vent channel 142 is released out of the vent channel 142 above the top layer 120. More specifically, the stream of gas is guided through a portion of the top layer 120 that is constrained within the vent channel 142 (e.g., the gas is vented over the top layer).
The purification method is advantageous in that it minimizes disturbance to the top layer 120, which reduces the risk of oxidation. Also, the method provides a beneficial flow of the liquid steel in the second tundish section prior to flowing out of the tundish.
The foregoing description is a specific embodiment of the present invention. It should be appreciated that this embodiment is described for purposes of illustration only, and that numerous alterations and modifications may be practiced by those skilled in the art without departing from the spirit and scope of the invention. It is intended that all such modifications and alterations be included insofar as they come within the scope of the invention as claimed or the equivalents thereof.
