CA2665220C - Refinement of steel - Google Patents
Refinement of steel Download PDFInfo
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- CA2665220C CA2665220C CA2665220A CA2665220A CA2665220C CA 2665220 C CA2665220 C CA 2665220C CA 2665220 A CA2665220 A CA 2665220A CA 2665220 A CA2665220 A CA 2665220A CA 2665220 C CA2665220 C CA 2665220C
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
- C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
- C21C7/0006—Adding metallic additives
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
- C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
- C21C7/04—Removing impurities by adding a treating agent
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
- C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
- C21C7/04—Removing impurities by adding a treating agent
- C21C7/06—Deoxidising, e.g. killing
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
- C21C2300/00—Process aspects
- C21C2300/08—Particular sequence of the process steps
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Abstract
A method of forming and refining molten silicon-bearing steel by addition of a calcium-containing silicon additive. Determine if the amount of calcium in the calcium-containing silicon additive is more or less than the amount of calcium desired in the finished steel. If it is more than the amount of calcium desired in the finished steel, add the amount of calcium-containing silicon additive corresponding to the excess calcium early during steel deoxidation or in refining to combine with oxygen, sulfur and other impurities in refining, and add the calcium-containing silicon additive containing the total amount of calcium desired in the finished steel after the desulfurization of the molten steel and before casting. If the amount of calcium in the calcium-containing silicon additive does not provide the total amount of calcium desired in the finished steel, adding an additional amount of calcium after desulfurization of the molten steel and before casting to the molten steel during refining.
Description
REFINEMENT OF STEEL
Background of the Invention [0001] This invention relates generally to refining of steel. More particularly, this invention relates to processes for refinement of silicon-bearing Al-killed and Al-Si dual killed steel to be directly cast in a continuous slab caster.
Background of the Invention [0001] This invention relates generally to refining of steel. More particularly, this invention relates to processes for refinement of silicon-bearing Al-killed and Al-Si dual killed steel to be directly cast in a continuous slab caster.
[0002] In continuous slab casting, the continuous caster is comprised of a tundish and an oscillating mold, in addition to a shroud and submerged entry nozzle.
The molten steel in the ladle is poured into a tundish and then poured vertically through the submerged entry nozzle into a hollow water-cooled oscillating mold, and continuously cast slabs are withdrawn horizontally from the bottom of the mold.
Refractory shrouds are used to transfer the molten steel from the ladle to the tundish, and then to the submerged entry nozzle and the mold, to avoid oxidation of the molten steel through contact with air. . The shroud between the tundish and the mold feeds through the submerged entry nozzle, and is regulated by a stopper rod.
The molten steel in the ladle is poured into a tundish and then poured vertically through the submerged entry nozzle into a hollow water-cooled oscillating mold, and continuously cast slabs are withdrawn horizontally from the bottom of the mold.
Refractory shrouds are used to transfer the molten steel from the ladle to the tundish, and then to the submerged entry nozzle and the mold, to avoid oxidation of the molten steel through contact with air. . The shroud between the tundish and the mold feeds through the submerged entry nozzle, and is regulated by a stopper rod.
[0003] The continuous slab caster produces wide rectangular strands of large cross-section, which are cut off into slabs to be hot rolled and cold rolled for use as material for sheet and plate. Thick slabs for flat-rolled products usually have an as-cast thickness of 100 to 250 mm. Thin slabs for flat-rolled products usually have an as-cast thickness of 30 to 100 mm. The slab caster is usually used in conjunction with an electric arc furnace or basic oxygen furnace, where the hot metal in produced for the caster.
[0004] Steel for continuous casting may be subjected to deoxidation treatment usually in a ladle prior to casting. Deoxidizing the molten steel in a ladle metallurgy furnace (LMF) or Vacuum Tank Degassed (VTD) to a desired oxygen level is typical.
Aluminum (or a combination of Al and Si) has been widely used as a deoxidizer and grain size controller in the manufacture of steels. Aluminum acts as a sacrificial metal which combines with oxygen to form a stable aluminum oxide, which migrates into the slag. Aluminum is a particularly desirable material for this purpose because it can be safely stored, handled and transported at ambient temperature, and, it is reactive as an oxidizing agent with steel at steelmaking temperatures.
Aluminum (or a combination of Al and Si) has been widely used as a deoxidizer and grain size controller in the manufacture of steels. Aluminum acts as a sacrificial metal which combines with oxygen to form a stable aluminum oxide, which migrates into the slag. Aluminum is a particularly desirable material for this purpose because it can be safely stored, handled and transported at ambient temperature, and, it is reactive as an oxidizing agent with steel at steelmaking temperatures.
[0005] Most thin slab casting and plating grades of steel are typically Al-killed steels. In some cases a combination of Al and Si is used to kill the steel. While this steel can be cast "as is" in large slab casters, further treatment is required in thin slab casters to avoid clogging or choking of submerged entry nozzles. One established practice in thin slab casting is to modify alumina and spinel inclusions by treatment with calcium to provide for more liquidity. With proper calcium treatment, the majority of the solid alumina (A1203) and/or spinel (MgA1204) inclusions are modified to liquid inclusions and casting is performed with acceptable surface quality to the cast slab. For continuous casting in a thin slab caster, 600 feet (182.9 m) of calcium wire has been found sufficient for a 170 ton (154 tons metric) ladle to add the calcium to avoid nozzle clogging (about 0.134 lb/ton, 0.067 kg/ton metric).
600 feet (182.9 m) of calcium wire contains about 22.5 lbs (10.2 kg) of calcium and is equivalent to about 16.8 ppm effective calcium in the refined steel. The recovery of calcium in the steel from calcium wire is less than 100% so that the effective calcium will be less than the amount added.
600 feet (182.9 m) of calcium wire contains about 22.5 lbs (10.2 kg) of calcium and is equivalent to about 16.8 ppm effective calcium in the refined steel. The recovery of calcium in the steel from calcium wire is less than 100% so that the effective calcium will be less than the amount added.
[0006] There are two main grades of silicon-bearing steels for sheets and plate steels made in a thin slab caster:
= Silicon-restricted steel typically with less than 0.03% silicon Generally ferrosilicon or silicomanganese is not added = Silicon-bearing steel typically with about 0.1 % to 1.5% silicon Silicomanganese and/or ferrosilicon is added to achieve the desired silicon content.
= Silicon-restricted steel typically with less than 0.03% silicon Generally ferrosilicon or silicomanganese is not added = Silicon-bearing steel typically with about 0.1 % to 1.5% silicon Silicomanganese and/or ferrosilicon is added to achieve the desired silicon content.
[0007] Problems with stopper rod wear associated with excessive Ca-addition have been observed in silicon-bearing steels where ferrosilicon and/or Silicomanganese have been added to achieve the desired silicon concentration in the finished steel. In a "Study of Casting Issues using Rapid Inclusion Identification and Analysis", Story, et al., AISTech 2006 Proceedings, Vol. 1, pp. 879-889, it was determined that ferrosilicon can contain calcium in addition to silicon and other alloying elements. To address stopper rod wear, Story et al. discussed using high purity ferrosilicon containing about 0.024% calcium.
Summary of the Invention I
Summary of the Invention I
[0008] A method of making silicon-bearing steel comprising the steps of:
a) refining molten steel to make a silicon-bearing steel having a silicon content between 0.1% and 1.5% by weight by addition of a calcium-containing silicon additive, b) determining the amount of calcium content in the calcium-containing silicon additive, c) determining if the amount of calcium in the calcium-containing silicon additive is more or less than the amount of calcium desired in the finished steel, d) if the amount of calcium in the calcium-containing silicon additive is more than the amount of calcium desired in the finished steel, adding the amount of calcium-containing silicon additive corresponding to the excess calcium during steel deoxidation or early in the refining step to combine with oxygen, and sulfur and other impurities in the steel during the refining, e) adding the calcium-containing silicon additive containing the total amount of calcium desired in the finished steel after desulfurization of the molten steel and before casting, and 0 if the amount of calcium in the calcium-containing silicon additive does not provide the total amount of calcium desired in the finished steel, adding an additional amount of calcium using Ca wire during refining after desulfurization of the molten steel and before casting to the molten steel.
a) refining molten steel to make a silicon-bearing steel having a silicon content between 0.1% and 1.5% by weight by addition of a calcium-containing silicon additive, b) determining the amount of calcium content in the calcium-containing silicon additive, c) determining if the amount of calcium in the calcium-containing silicon additive is more or less than the amount of calcium desired in the finished steel, d) if the amount of calcium in the calcium-containing silicon additive is more than the amount of calcium desired in the finished steel, adding the amount of calcium-containing silicon additive corresponding to the excess calcium during steel deoxidation or early in the refining step to combine with oxygen, and sulfur and other impurities in the steel during the refining, e) adding the calcium-containing silicon additive containing the total amount of calcium desired in the finished steel after desulfurization of the molten steel and before casting, and 0 if the amount of calcium in the calcium-containing silicon additive does not provide the total amount of calcium desired in the finished steel, adding an additional amount of calcium using Ca wire during refining after desulfurization of the molten steel and before casting to the molten steel.
[0009] The calcium-containing silicon additive may be ferrosilicon and cheap ferrosilicon additive since the percent of calcium in the additive need not be kept low.
The calcium-containing silicon additive may include additives having less than about 1.8% calcium, and further includes additives with less than about 1% calcium.
The calcium-containing silicon additive may include additives having less than about 1.8% calcium, and further includes additives with less than about 1% calcium.
[0010] The low carbon steel may have a carbon content between about 0.003% and about 0.5% by weight. The disclosed method of refining silicon-bearing steel includes low carbon steels.
[0011] The disclosed refining of silicon-bearing steel may occur in a ladle metallurgical furnace or vacuum tank degasser.
[0012] A cast steel is made by a method comprising the steps of:
a) refining molten steel to make a silicon-bearing steel having a silicon content between 0.1% and 1.5% by weight by addition of a calcium-containing silicon additive, b) determining the amount of calcium content in the calcium-containing silicon additive, c) determining if the amount of calcium in the calcium-containing silicon additive is more or less than the amount of calcium desired in the finished steel, d) if the amount of calcium in the calcium-containing silicon additive is more than the amount of calcium desired in the finished steel, adding the amount of calcium-containing silicon additive corresponding to the excess calcium early in the refining to combine with oxygen, sulfur and other impurities in the steel during the refining;
e) adding the calcium-containing silicon additive containing the total amount of calcium desired in the finished steel after desulfurization of the molten steel and before casting, and 0 if the amount of calcium in the calcium-containing silicon additive does not provide the total amount of calcium desired in the finished steel, adding an additional amount of calcium after desulfurization of the molten steel and before casting to the molten steel during refining; and g) casting the molten steel into steel slabs.
a) refining molten steel to make a silicon-bearing steel having a silicon content between 0.1% and 1.5% by weight by addition of a calcium-containing silicon additive, b) determining the amount of calcium content in the calcium-containing silicon additive, c) determining if the amount of calcium in the calcium-containing silicon additive is more or less than the amount of calcium desired in the finished steel, d) if the amount of calcium in the calcium-containing silicon additive is more than the amount of calcium desired in the finished steel, adding the amount of calcium-containing silicon additive corresponding to the excess calcium early in the refining to combine with oxygen, sulfur and other impurities in the steel during the refining;
e) adding the calcium-containing silicon additive containing the total amount of calcium desired in the finished steel after desulfurization of the molten steel and before casting, and 0 if the amount of calcium in the calcium-containing silicon additive does not provide the total amount of calcium desired in the finished steel, adding an additional amount of calcium after desulfurization of the molten steel and before casting to the molten steel during refining; and g) casting the molten steel into steel slabs.
[0013] Further the silicon content may be between 0.1% and 1.5% by weight.
Brief Description of the Drawings
Brief Description of the Drawings
[0014] FIG. 1 is a diagrammatic illustration making of silicon-bearing steel through a refining and casting process;
[0015] FIG. 2 is a schematic side view of a portion of the continuous slab caster of FIG. 1;
[0016] FIGS. 3A-3C illustrate a spreadsheet showing one embodiment of continuous casting process of the present invention.
Detailed Description 100171 Referring now to l'IG. 1, silicon-bearing steel is refined and casting in process 10 as shown. Process 10 includes an electric arc furnace 12 (EAF) in which molten steel is produced. From the EAT 12, the molten steel is transferred by ladle to a ladle metallurgical furnace or vacuum tank degassed 14 (EMI' or VTD), wherein the refining of molten steel is completed before continuous casting into a slab.
Ladles of molten steel suitable for casting are then transferred from LMF or VTD 14 to a continuous slab caster 16 wherein the refined molten steel is cast into continuous steel slabs.
100181 The ladle 18 of unrefined molten steel is routed from the EAF 12 to the EMI; or VTD 14 to reline the molten steel into a form suitable for casting by the continuous slab caster apparatus 16. In general terms, as seen in FIG. 2, casting steel continuously in such a slab caster involves introducing molten metal that is supplied during a casting operation by gravity from ladle 18 to a tundish 43, through a slide gate 44 and outlet nozzle 45. From tundish 43, the molten metal is supplied by gravity through slide gate 46 and outlet nozzle 47 to a submerged entry nozzle (SEN) 48 into continuous slab caster 16. Molten metal is introduced into the left-hand end of the tundish from the ladle 18 via an outlet nozzle 45 and slide gate valve 44.
At the bottom of tundish 43, there is an outlet 46 in the floor of the tundish to allow molten metal to flow from the tundish via an outlet nozzle 47 to the SEN 48. The tundish 43 is fitted with a stopper rod 42 and slide gate valve to selectively open and close the tundish outlet and effectively control the flow of metal through the outlet.
From the SEN 48, molten steel flows first through a mold 55 and then through a series of support rollers 53 and cooling sprays 51.
10019] In slab casting described herein, the steel is generally subjected to aluminum deoxidization, which results in the formation of solid A1703 inclusions in the steel. Following in the refining process, the deoxidized molten steel in ladle 18 is desulfurized. After desulfurization, the steel is treated with calcium to modify the solid A1203 and/or spinel inclusions to liquid Ca-alumina inclusions.
Following refining, the deoxidized, desulfurized and calcium treated molten steel in ladle 18 is transferred to the continuous steel slab casting apparatus 16.
100201 In the disclosed method, the amount of calcium in the required ferrosilicon (silicon additive) is taken into account during the refining of the molten steel. The following will consider FeSi as the silicon additive.
100211 First, the concentration of calcium in the source of ferrosilicon is determined. Next, the amount of ferrosilicon that is needed for addition to the molten steel to achieve the desired silicon concentration in the finished steel, and, the quantity of calcium in the required amount of ferrosilicon is calculated. If the amount of calcium is greater than the required amount (e.g., 16.8 ppm during normal non-startup operations), the required amount of ferrosilicon is divided into two portions, a early portion and a late portion. The late portion is the amount of ferrosilicon that contains the desired amount of calcium in the finished steel. The early portion is the amount of ferrosilicon containing the excess amount of calcium not wanted in the finished molten steel. In general, desired sources of ferrosilicon contain less than 1.8% calcium or less than 1% calcium; although this is desired, other concentrations, greater than 1.8%, can also be used in this disclosed method of forming and refining silicon-bearing steel.
100221 The early ferrosilicon portion, FeSiearly is added early during steel deoxidation with Al or early during refining in the ladle metallurgical furnace (LMF) or vacuum tank dcgasser (VTD), typically before or during desulfurization, so that the calcium in the early added ferrosilicon can combine with sulfur and other impurities, and migrate to the slag. For example, the calcium in the early added ferrosilicon can react with sulfur forming CaS that migrates to and is removed as part of the slag that is formed during refining. The late ferrosilicon portion, FeSilate, is added late in the refining process, after desulfurization has completed, typically to less than 0.01% S
by weight. The calcium added to the LW' or V I'D from the FeSiiate portion modifies the solid alumina inclusions into liquid inclusions and reduces the incidence of nozzle clogging or choking in the submerged entry nozzle. Since any excess calcium present in the total amount of ferrosilicon added to the LMF or VTD was removed during desulfurization by adding the excess portion, FeSiearly, during desulfurization, the incidence of excess stopper rod wear is reduced.
10023] Where the calcium present in the required quantity of FeSi is equal to or less than the required amount of calcium in the finished steel, only one addition of ferrosilicon, FeSiiõ,,,, is made during refining. This single late addition of ferrosilicon is done after desulfurization. In the event that the calcium present in the required quantity of FeSi is less than the required amount, an additional amount of calcium, typically in the form of calcium wire, is added with the required quantity of FeSi.
[0024] In casting campaigns using the method of forming and refining silicon-bearing steels described, It has been found that the casting campaigns have been extended to 18 heats, which is the typical limit for the submerged entry nozzle (SEN) before replacement. Using the early processes of adding the required amount of ferrosilicon after desulfurization and followed by adding the required amount of calcium, also added after desulfurization, stopper rod wear would usually be the limiting factor and limited the casting campaign to 10 heats.
100251 FIGS. 3A-3C show an Excel*) spreadsheet illustrating an embodiment of this method of refining silicon-bearing steel in accordance with the present invention. An initial step in this process is determining the concentration of calcium in the source of ferrosilicon. Five standards of ferrosilicon containing known concentrations of calcium, 0.064%, 0.14%, 0.43 %, 0.65% and 1.8 %, were obtained. These standards were used to calibrate an on-site slag analyzer permitting rapid in-house analysis of ferrosilicon when ferrosilicon was received. This calibration permitted more rapid processing of ferrosilicon as received, so that ferrosilicon quantities could be readily stored and used as needed without waiting for off-site analysis before use.
[0026] Once the concentrations of calcium and silicon in the ferrosilicon are known, the concentrations are entered into the spreadsheet at 101 and 103, respectively. The desired concentration of silicon in the finished steel is entered at 105. A total quantity 107 of required ferrosilicon is then calculated. The total quantity 107 of ferrosilicon required, FeSi i based on the heat size 102, multiplied -req, .S
by the target % ferrosilicon 105 and adjusted to account for the silicon concentration 103 in the ferrosilicon and the recovery factor 121 for ferrosilicon as follows:
Ileat Size* 'A Si target , .
(1.0) FeSireq % FeSi recovery % Si in FeSi 100271 The total ferrosilicon required, FeSireq, is then divided into a first or early ferrosilicon addition 111, FeSicariy, and a second or late ferrosilicon addition 109, FeSilate= The late ferrosilicon addition, FeSibte, is the amount of ferrosilicon that contains the target quantity Caturget, 123, of calcium from the total ferrosilicon required, FeSireq. The target quantity of calcium, Catarget, is that amount of calcium which results in 16.8 ppm calcium continuous operation, (22,4 ppm calcium startup), in the refined metal at the time of casting. lithe calcium available, Caavaii, in the total ferrosilicon required. FeSireq, is equal to or less than the target quantity of calcium, Catõrget, then FeSiiõ,,, is equal to FeSireq and there is no early addition of ferrosilicon. If the calcium available, Caavail, in the total ferrosilicon required is greater than the target quantity of calcium, Catarget, then FeSilate is that amount of FeSi that contains the target quantity of calcium, Catarget. Specifically, this amount can be calculated by dividing the target calcium, Cathrget by the calcium available, Caayaii, multiplied by the total ferrosilicon required, FeSi RN =
(2.0) If Caavaii < Catargeb FeShate FeSireq; FeSieõriy = 0 (2.1) Caavaii = FeSireq * concentration of Ca in ferrosilicon * % Ca ferrosilicon recovery (3.0) If Caavaii >Larget Catargeb Ca FeSitate - * l'eSireq; -early ¨ FeSireq - FeSikite CA
100281 In the event that the calcium, Caõvaii, present in the total ferrosilicon required, FeSireq, is less than the amount of calcium required, Catarget, 123, additional calcium is added, usually in the form of calcium wire with the FeSiiate portion ferrosilicon. For convenience, the additional calcium required 113, Caadd, is calculated in feet of calcium wire, because atypical way of adding any additional calcium is by adding calcium wire. Other units of measurement, such as pounds, kilograms, etc. could also be used.
(4.0) Caadd ¨ Catarget Caa, ail 100291 FIG. 3A illustrates a situation where the calcium available, Caavad, is greater than the calcium required Catõrget. In this situation, the ferrosilicon required, FeSireq is divided into a late portion, FeSiiõte of 1226 lbs (556 kg) and an early portion, eS
icar.y, o1252 pounds (114.3 kg). FIG. 313 illustrates a situation where the calcium available from the ferrosilicon, Caavait, is less that the calcium required, Catargei. In this situation, there is no early portion, FeSi early of ferrosilicon, and additional calcium, Caadd, of 118 feet (35.97 in) of calcium wire is required. This additional calcium is added to the molten metal when the late portion of ferrosilicon, FeSitate is added. FIG.
3C shows a situation where the calcium available in the total ferrosilicon required, Caavad, is equal to the calcium required, Catõrg,. In this situation, no additional calcium. Caadd, is required, and the early portion of ferrosilicon, FeSieariõ
is zero.
100301 The disclosed methods of making silicon-bearing steel reduce the cost of making the steel by replacing calcium wire with calcium containing ferrosilicon and by extending the length of a casting campaign to about 18 heats. It has been estimated the cost savings per ton of steel using the disclosed methods is about $2 per ton, about half due to reduced calcium wire usage and about half due to extending the length of the casting campaign.
100311 Although preferred embodiments of the invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that many additions, modifications, and substitutions are possible and that the scope of the claims should not be limited by the embodiments set forth herein, but should be given the broadest interpretation consistent with the description as a whole.
Detailed Description 100171 Referring now to l'IG. 1, silicon-bearing steel is refined and casting in process 10 as shown. Process 10 includes an electric arc furnace 12 (EAF) in which molten steel is produced. From the EAT 12, the molten steel is transferred by ladle to a ladle metallurgical furnace or vacuum tank degassed 14 (EMI' or VTD), wherein the refining of molten steel is completed before continuous casting into a slab.
Ladles of molten steel suitable for casting are then transferred from LMF or VTD 14 to a continuous slab caster 16 wherein the refined molten steel is cast into continuous steel slabs.
100181 The ladle 18 of unrefined molten steel is routed from the EAF 12 to the EMI; or VTD 14 to reline the molten steel into a form suitable for casting by the continuous slab caster apparatus 16. In general terms, as seen in FIG. 2, casting steel continuously in such a slab caster involves introducing molten metal that is supplied during a casting operation by gravity from ladle 18 to a tundish 43, through a slide gate 44 and outlet nozzle 45. From tundish 43, the molten metal is supplied by gravity through slide gate 46 and outlet nozzle 47 to a submerged entry nozzle (SEN) 48 into continuous slab caster 16. Molten metal is introduced into the left-hand end of the tundish from the ladle 18 via an outlet nozzle 45 and slide gate valve 44.
At the bottom of tundish 43, there is an outlet 46 in the floor of the tundish to allow molten metal to flow from the tundish via an outlet nozzle 47 to the SEN 48. The tundish 43 is fitted with a stopper rod 42 and slide gate valve to selectively open and close the tundish outlet and effectively control the flow of metal through the outlet.
From the SEN 48, molten steel flows first through a mold 55 and then through a series of support rollers 53 and cooling sprays 51.
10019] In slab casting described herein, the steel is generally subjected to aluminum deoxidization, which results in the formation of solid A1703 inclusions in the steel. Following in the refining process, the deoxidized molten steel in ladle 18 is desulfurized. After desulfurization, the steel is treated with calcium to modify the solid A1203 and/or spinel inclusions to liquid Ca-alumina inclusions.
Following refining, the deoxidized, desulfurized and calcium treated molten steel in ladle 18 is transferred to the continuous steel slab casting apparatus 16.
100201 In the disclosed method, the amount of calcium in the required ferrosilicon (silicon additive) is taken into account during the refining of the molten steel. The following will consider FeSi as the silicon additive.
100211 First, the concentration of calcium in the source of ferrosilicon is determined. Next, the amount of ferrosilicon that is needed for addition to the molten steel to achieve the desired silicon concentration in the finished steel, and, the quantity of calcium in the required amount of ferrosilicon is calculated. If the amount of calcium is greater than the required amount (e.g., 16.8 ppm during normal non-startup operations), the required amount of ferrosilicon is divided into two portions, a early portion and a late portion. The late portion is the amount of ferrosilicon that contains the desired amount of calcium in the finished steel. The early portion is the amount of ferrosilicon containing the excess amount of calcium not wanted in the finished molten steel. In general, desired sources of ferrosilicon contain less than 1.8% calcium or less than 1% calcium; although this is desired, other concentrations, greater than 1.8%, can also be used in this disclosed method of forming and refining silicon-bearing steel.
100221 The early ferrosilicon portion, FeSiearly is added early during steel deoxidation with Al or early during refining in the ladle metallurgical furnace (LMF) or vacuum tank dcgasser (VTD), typically before or during desulfurization, so that the calcium in the early added ferrosilicon can combine with sulfur and other impurities, and migrate to the slag. For example, the calcium in the early added ferrosilicon can react with sulfur forming CaS that migrates to and is removed as part of the slag that is formed during refining. The late ferrosilicon portion, FeSilate, is added late in the refining process, after desulfurization has completed, typically to less than 0.01% S
by weight. The calcium added to the LW' or V I'D from the FeSiiate portion modifies the solid alumina inclusions into liquid inclusions and reduces the incidence of nozzle clogging or choking in the submerged entry nozzle. Since any excess calcium present in the total amount of ferrosilicon added to the LMF or VTD was removed during desulfurization by adding the excess portion, FeSiearly, during desulfurization, the incidence of excess stopper rod wear is reduced.
10023] Where the calcium present in the required quantity of FeSi is equal to or less than the required amount of calcium in the finished steel, only one addition of ferrosilicon, FeSiiõ,,,, is made during refining. This single late addition of ferrosilicon is done after desulfurization. In the event that the calcium present in the required quantity of FeSi is less than the required amount, an additional amount of calcium, typically in the form of calcium wire, is added with the required quantity of FeSi.
[0024] In casting campaigns using the method of forming and refining silicon-bearing steels described, It has been found that the casting campaigns have been extended to 18 heats, which is the typical limit for the submerged entry nozzle (SEN) before replacement. Using the early processes of adding the required amount of ferrosilicon after desulfurization and followed by adding the required amount of calcium, also added after desulfurization, stopper rod wear would usually be the limiting factor and limited the casting campaign to 10 heats.
100251 FIGS. 3A-3C show an Excel*) spreadsheet illustrating an embodiment of this method of refining silicon-bearing steel in accordance with the present invention. An initial step in this process is determining the concentration of calcium in the source of ferrosilicon. Five standards of ferrosilicon containing known concentrations of calcium, 0.064%, 0.14%, 0.43 %, 0.65% and 1.8 %, were obtained. These standards were used to calibrate an on-site slag analyzer permitting rapid in-house analysis of ferrosilicon when ferrosilicon was received. This calibration permitted more rapid processing of ferrosilicon as received, so that ferrosilicon quantities could be readily stored and used as needed without waiting for off-site analysis before use.
[0026] Once the concentrations of calcium and silicon in the ferrosilicon are known, the concentrations are entered into the spreadsheet at 101 and 103, respectively. The desired concentration of silicon in the finished steel is entered at 105. A total quantity 107 of required ferrosilicon is then calculated. The total quantity 107 of ferrosilicon required, FeSi i based on the heat size 102, multiplied -req, .S
by the target % ferrosilicon 105 and adjusted to account for the silicon concentration 103 in the ferrosilicon and the recovery factor 121 for ferrosilicon as follows:
Ileat Size* 'A Si target , .
(1.0) FeSireq % FeSi recovery % Si in FeSi 100271 The total ferrosilicon required, FeSireq, is then divided into a first or early ferrosilicon addition 111, FeSicariy, and a second or late ferrosilicon addition 109, FeSilate= The late ferrosilicon addition, FeSibte, is the amount of ferrosilicon that contains the target quantity Caturget, 123, of calcium from the total ferrosilicon required, FeSireq. The target quantity of calcium, Catarget, is that amount of calcium which results in 16.8 ppm calcium continuous operation, (22,4 ppm calcium startup), in the refined metal at the time of casting. lithe calcium available, Caavaii, in the total ferrosilicon required. FeSireq, is equal to or less than the target quantity of calcium, Catõrget, then FeSiiõ,,, is equal to FeSireq and there is no early addition of ferrosilicon. If the calcium available, Caavail, in the total ferrosilicon required is greater than the target quantity of calcium, Catarget, then FeSilate is that amount of FeSi that contains the target quantity of calcium, Catarget. Specifically, this amount can be calculated by dividing the target calcium, Cathrget by the calcium available, Caayaii, multiplied by the total ferrosilicon required, FeSi RN =
(2.0) If Caavaii < Catargeb FeShate FeSireq; FeSieõriy = 0 (2.1) Caavaii = FeSireq * concentration of Ca in ferrosilicon * % Ca ferrosilicon recovery (3.0) If Caavaii >Larget Catargeb Ca FeSitate - * l'eSireq; -early ¨ FeSireq - FeSikite CA
100281 In the event that the calcium, Caõvaii, present in the total ferrosilicon required, FeSireq, is less than the amount of calcium required, Catarget, 123, additional calcium is added, usually in the form of calcium wire with the FeSiiate portion ferrosilicon. For convenience, the additional calcium required 113, Caadd, is calculated in feet of calcium wire, because atypical way of adding any additional calcium is by adding calcium wire. Other units of measurement, such as pounds, kilograms, etc. could also be used.
(4.0) Caadd ¨ Catarget Caa, ail 100291 FIG. 3A illustrates a situation where the calcium available, Caavad, is greater than the calcium required Catõrget. In this situation, the ferrosilicon required, FeSireq is divided into a late portion, FeSiiõte of 1226 lbs (556 kg) and an early portion, eS
icar.y, o1252 pounds (114.3 kg). FIG. 313 illustrates a situation where the calcium available from the ferrosilicon, Caavait, is less that the calcium required, Catargei. In this situation, there is no early portion, FeSi early of ferrosilicon, and additional calcium, Caadd, of 118 feet (35.97 in) of calcium wire is required. This additional calcium is added to the molten metal when the late portion of ferrosilicon, FeSitate is added. FIG.
3C shows a situation where the calcium available in the total ferrosilicon required, Caavad, is equal to the calcium required, Catõrg,. In this situation, no additional calcium. Caadd, is required, and the early portion of ferrosilicon, FeSieariõ
is zero.
100301 The disclosed methods of making silicon-bearing steel reduce the cost of making the steel by replacing calcium wire with calcium containing ferrosilicon and by extending the length of a casting campaign to about 18 heats. It has been estimated the cost savings per ton of steel using the disclosed methods is about $2 per ton, about half due to reduced calcium wire usage and about half due to extending the length of the casting campaign.
100311 Although preferred embodiments of the invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that many additions, modifications, and substitutions are possible and that the scope of the claims should not be limited by the embodiments set forth herein, but should be given the broadest interpretation consistent with the description as a whole.
Claims (23)
1. A cast slab of silicon-bearing Al-Si dual killed steel having a silicon content between 1.0% and 1.5% by weight and between 0.003 and 0.5% of carbon, produced by a method comprising the steps of:
a) refining molten steel, said molten steel having a carbon content between 0.003%
and 0.5% by weight, b) providing calcium-containing silicon additive;
c) determining what amount of calcium content is in the calcium-containing silicon additive, d) comparing the amount of calcium in the calcium-containing silicon additive to a datum amount of calcium desired in the silicon-bearing steel as finished, and, accordingly where the amount of calcium in the calcium-containing silicon additive is more than the datum amount of calcium desired in the silicon-bearing steel as finished, adding such amount of the calcium-containing silicon additive by which the calcium containing silicon additive exceeds the datum amount prior to desulfurizing, to combine with sulfur and other impurities in the steel during the refining;
desulfurizing the molten steel;
adding the balance of the amount of the calcium-containing silicon additive containing the total amount of calcium desired in the finished steel after desulfurization of the molten steel and before casting, and ii) where the amount of calcium in the calcium-containing silicon additive is less than or equal to the datum amount of calcium desired in the finished steel, adding to the molten steel an additional amount of calcium after desulfurization of the molten steel and before casting the molten steel to provide the desired datum amount of calcium in the finished steel; and e) determining an amount of aluminum in the calcium-containing silicon additive, f) utilizing the amount of aluminum in the calcium-containing silicon additive in deoxidizing the molten silicon-bearing steel in refining and for killing the steel prior to casting, and g) casting a silicon-bearing Al-Si dual killed steel having an effective amount of calcium of about 16.8 ppm.
a) refining molten steel, said molten steel having a carbon content between 0.003%
and 0.5% by weight, b) providing calcium-containing silicon additive;
c) determining what amount of calcium content is in the calcium-containing silicon additive, d) comparing the amount of calcium in the calcium-containing silicon additive to a datum amount of calcium desired in the silicon-bearing steel as finished, and, accordingly where the amount of calcium in the calcium-containing silicon additive is more than the datum amount of calcium desired in the silicon-bearing steel as finished, adding such amount of the calcium-containing silicon additive by which the calcium containing silicon additive exceeds the datum amount prior to desulfurizing, to combine with sulfur and other impurities in the steel during the refining;
desulfurizing the molten steel;
adding the balance of the amount of the calcium-containing silicon additive containing the total amount of calcium desired in the finished steel after desulfurization of the molten steel and before casting, and ii) where the amount of calcium in the calcium-containing silicon additive is less than or equal to the datum amount of calcium desired in the finished steel, adding to the molten steel an additional amount of calcium after desulfurization of the molten steel and before casting the molten steel to provide the desired datum amount of calcium in the finished steel; and e) determining an amount of aluminum in the calcium-containing silicon additive, f) utilizing the amount of aluminum in the calcium-containing silicon additive in deoxidizing the molten silicon-bearing steel in refining and for killing the steel prior to casting, and g) casting a silicon-bearing Al-Si dual killed steel having an effective amount of calcium of about 16.8 ppm.
2. The cast slab of steel as claimed in claim 1 where the calcium-containing silicon additive is selected from the group consisting of (i) ferrosilicon, (ii) low-C
Silicomanganese, and mixtures thereof
Silicomanganese, and mixtures thereof
3. The cast slab of steel as claimed in any one of claims 1 and 2, wherein the method further comprises adding a manganese-containing additive in the refining.
4. The cast slab of steel as claimed in any one of claims 1 to 3 where the step of refining molten steel occurs in a ladle metallurgical furnace or vacuum tank degasser.
5. The cast slab of steel of any one of claims 1 to 4 further comprising manganese.
6. A cast slab of silicon-bearing Al-Si dual killed steel having a silicon content between 1.0% and 1.5% by weight, and between 0.003 and 0.5% of carbon, by weight, produced by a method comprising the steps of:
refining a molten steel;
determining a datum amount of calcium desired in the killed steel as finished;
obtaining a first amount of calcium containing silicon additive;
comparing calcium content of said calcium-containing silicon additive to said datum amount of calcium;
and, according to that comparison, (a) where calcium content of said silicon additive exceeds said datum amount, dividing said first amount into a second amount and a third amount, said second amount being equal to said datum amount, and said third amount being equal to an excess amount defined by subtracting said datum amount from said first amount;
adding said excess amount to the molten steel;
deoxidizing the molten steel to kill said steel;
desulfurizing the molten steel; and, after desulfurizing, adding said second amount to said molten steel;
(b) where said first amount of calcium content of said silicon additive is less than said datum amount, deoxidizing the molten steel to kill said steel;
clesulfurizing the molten steel; and, after desulfurizing, adding said first amount of calcium containing silicon additive to the molten steel and further providing additional calcium to the molten steel to make up for any difference between said first amount and said datum amount; and (c) where said datum amount is equal to said first amount of calcium content of said silicon additive, deoxidizing the molten steel to kill said steel;
desulfurizing the molten steel; and, after desulfurizing, adding said first amount of calcium containing silicon additive to the molten steel;
and determining an amount of aluminum in the calcium-containing silicon additive, utilizing the amount of aluminum in the calcium-containing silicon additive in deoxidizing the molten silicon-bearing steel in refining and for killing the steel prior to casting, and casting said silicon-bearing Al-Si dual killed steel having an effective amount of calcium of about 16.8 ppm.
refining a molten steel;
determining a datum amount of calcium desired in the killed steel as finished;
obtaining a first amount of calcium containing silicon additive;
comparing calcium content of said calcium-containing silicon additive to said datum amount of calcium;
and, according to that comparison, (a) where calcium content of said silicon additive exceeds said datum amount, dividing said first amount into a second amount and a third amount, said second amount being equal to said datum amount, and said third amount being equal to an excess amount defined by subtracting said datum amount from said first amount;
adding said excess amount to the molten steel;
deoxidizing the molten steel to kill said steel;
desulfurizing the molten steel; and, after desulfurizing, adding said second amount to said molten steel;
(b) where said first amount of calcium content of said silicon additive is less than said datum amount, deoxidizing the molten steel to kill said steel;
clesulfurizing the molten steel; and, after desulfurizing, adding said first amount of calcium containing silicon additive to the molten steel and further providing additional calcium to the molten steel to make up for any difference between said first amount and said datum amount; and (c) where said datum amount is equal to said first amount of calcium content of said silicon additive, deoxidizing the molten steel to kill said steel;
desulfurizing the molten steel; and, after desulfurizing, adding said first amount of calcium containing silicon additive to the molten steel;
and determining an amount of aluminum in the calcium-containing silicon additive, utilizing the amount of aluminum in the calcium-containing silicon additive in deoxidizing the molten silicon-bearing steel in refining and for killing the steel prior to casting, and casting said silicon-bearing Al-Si dual killed steel having an effective amount of calcium of about 16.8 ppm.
7. The cast slab of steel as claimed in claim 6 wherein said deoxidising is aluminum de-oxidising.
8. The cast slab of steel of claim 7 wherein said deoxidising employs aluminum provided in said first amount of calcium containing silicon additive.
9. The cast slab of steel as claimed in any one of claims 6 to 8, wherein the method further comprises adding a manganese-containing additive prior to desulfurizing.
10. The cast slab of steel of any one of claims 6 ¨ 9 wherein the calcium-containing silicon additive is selected from the group consisting of (i) ferrosilicon, (ii) low-carbon silicomanganese, and mixtures thereof
11. The cast slab of steel of any one of claims 6 to 10 wherein the step of refining molten steel occurs in a ladle metallurgical furnace or vacuum tank degasser.
12. The cast slab of steel of any one of claims 6 to 11 wherein said datum amount of calcium is about 16.8 ppm of said silicon-bearing killed steel.
13. A method of making silicon-bearing steel comprising the steps of:
a) refining molten steel, said molten steel having a carbon content between 0.003%
and 0.5% by weight, b) providing calcium-containing silicon additive;
c) determining what amount of calcium content is in the calcium-containing silicon additive, d) comparing the amount of calcium in the calcium-containing silicon additive to a datum amount of calcium desired in the silicon-bearing steel as finished, and, accordingly (i) where the amount of calcium in the calcium-containing silicon additive is more than the datum amount of calcium desired in the silicon-bearing steel as finished, adding such amount of the calcium-containing silicon additive by which the calcium containing silicon additive exceeds the datum amount prior to desulfurizing, to combine with sulfur and other impurities in the steel during the refining;
desulfurizing the molten steel;
adding the balance of the amount of the calcium-containing silicon additive containing the total amount of calcium desired in the finished steel after desulfurization of the molten steel and before casting, and ii) where the amount of calcium in the calcium-containing silicon additive is less than or equal to the datum amount of calcium desired in the finished steel, adding to the molten steel an additional amount of calcium after desulfurization of the molten steel and before casting the molten steel to provide the desired datum amount of calcium in the finished steel; and e) determining an amount of aluminum in the calcium-containing silicon additive, f) utilizing the amount of aluminum in the calcium-containing silicon additive in deoxidizing the molten silicon-bearing steel in refining and for killing the steel prior to casting, and casting a silicon-bearing Al-Si dual killed steel having an effective amount of calcium of about 16.8 ppm, and a silicon content between 1.0 % and 1.5 % by weight.
a) refining molten steel, said molten steel having a carbon content between 0.003%
and 0.5% by weight, b) providing calcium-containing silicon additive;
c) determining what amount of calcium content is in the calcium-containing silicon additive, d) comparing the amount of calcium in the calcium-containing silicon additive to a datum amount of calcium desired in the silicon-bearing steel as finished, and, accordingly (i) where the amount of calcium in the calcium-containing silicon additive is more than the datum amount of calcium desired in the silicon-bearing steel as finished, adding such amount of the calcium-containing silicon additive by which the calcium containing silicon additive exceeds the datum amount prior to desulfurizing, to combine with sulfur and other impurities in the steel during the refining;
desulfurizing the molten steel;
adding the balance of the amount of the calcium-containing silicon additive containing the total amount of calcium desired in the finished steel after desulfurization of the molten steel and before casting, and ii) where the amount of calcium in the calcium-containing silicon additive is less than or equal to the datum amount of calcium desired in the finished steel, adding to the molten steel an additional amount of calcium after desulfurization of the molten steel and before casting the molten steel to provide the desired datum amount of calcium in the finished steel; and e) determining an amount of aluminum in the calcium-containing silicon additive, f) utilizing the amount of aluminum in the calcium-containing silicon additive in deoxidizing the molten silicon-bearing steel in refining and for killing the steel prior to casting, and casting a silicon-bearing Al-Si dual killed steel having an effective amount of calcium of about 16.8 ppm, and a silicon content between 1.0 % and 1.5 % by weight.
14. The method of refining steel as claimed in claim 13 where the calcium-containing silicon additive is selected from the group consisting of (i) ferrosilicon, (ii) low-C
Silicomanganese, and mixtures thereof.
Silicomanganese, and mixtures thereof.
15. The method of refining steel as claimed in any one of claims 13 and 14, wherein the method further comprises adding a manganese-containing additive in the refining.
16. The method of refining steel as claimed in any one of claims 13 to 15 where the step of refining molten steel occurs in a ladle metallurgical furnace or vacuum tank degasser.
17. A method of casting a silicon-bearing killed steel comprising the steps of:
refining a molten steel;
determining a datum amount of calcium desired in the killed steel as finished;
obtaining a first amount of calcium containing silicon additive;
comparing calcium content of said calcium-containing silicon additive to said datum amount of calcium;
and, according to that comparison, (a) where calcium content of said silicon additive exceeds said datum amount, dividing said first amount into a second amount and a third amount, said second amount being equal to said datum amount, and said third amount being equal to an excess amount defined by subtracting said datum amount from said first amount;
adding said excess amount to the molten steel;
deoxidizing the molten steel to kill said steel;
desulfurizing the molten steel; and, after desulfurizing, adding said second amount to said molten steel;
(b) where said first amount of calcium content of said silicon additive is less than said datum amount, deoxidizing the molten steel to kill said steel;
desulfurizing the molten steel; and, after desulfurizing, adding said first amount of calcium containing silicon additive to the molten steel and further providing additional calcium to the molten steel to make up for any difference between said first amount and said datum amount; and (c) where said datum amount is equal to said first amount of calcium content of said silicon additive, deoxidizing the molten steel to kill said steel;
desulfurizing the molten steel; and, after desulfurizing, adding said first amount of calcium containing silicon additive to the molten steel;
and determining an amount of aluminum in the calcium-containing silicon additive, utilizing the amount of aluminum in the calcium-containing silicon additive in deoxidizing the molten silicon-bearing steel in refining and for killing the steel prior to casting, and casting said silicon-bearing Al-Si dual killed steel having an effective amount of calcium of about 16.8 ppm, and a silicon content between 1.0% and 1.5% by weight.
refining a molten steel;
determining a datum amount of calcium desired in the killed steel as finished;
obtaining a first amount of calcium containing silicon additive;
comparing calcium content of said calcium-containing silicon additive to said datum amount of calcium;
and, according to that comparison, (a) where calcium content of said silicon additive exceeds said datum amount, dividing said first amount into a second amount and a third amount, said second amount being equal to said datum amount, and said third amount being equal to an excess amount defined by subtracting said datum amount from said first amount;
adding said excess amount to the molten steel;
deoxidizing the molten steel to kill said steel;
desulfurizing the molten steel; and, after desulfurizing, adding said second amount to said molten steel;
(b) where said first amount of calcium content of said silicon additive is less than said datum amount, deoxidizing the molten steel to kill said steel;
desulfurizing the molten steel; and, after desulfurizing, adding said first amount of calcium containing silicon additive to the molten steel and further providing additional calcium to the molten steel to make up for any difference between said first amount and said datum amount; and (c) where said datum amount is equal to said first amount of calcium content of said silicon additive, deoxidizing the molten steel to kill said steel;
desulfurizing the molten steel; and, after desulfurizing, adding said first amount of calcium containing silicon additive to the molten steel;
and determining an amount of aluminum in the calcium-containing silicon additive, utilizing the amount of aluminum in the calcium-containing silicon additive in deoxidizing the molten silicon-bearing steel in refining and for killing the steel prior to casting, and casting said silicon-bearing Al-Si dual killed steel having an effective amount of calcium of about 16.8 ppm, and a silicon content between 1.0% and 1.5% by weight.
18. The method of claim 17 wherein said deoxidising is aluminum de-oxidising.
19. The method of claim 18 wherein said deoxidising employs aluminum provided in said first amount of calcium containing silicon additive.
20. The method of any one of claims 17 to 19, wherein the method further comprises adding a manganese-containing additive prior to desulfurizing.
21. The method of any one of claims 17 to 20 wherein the calcium-containing silicon additive is selected from the goup consisting of (i) ferrosilicon, (ii) low-carbon silicomanganese, and mixtures thereof.
22. The method of any one of claims 17 to 21 wherein the step of refining molten steel occurs in a ladle metallurgical furnace or vacuum tank degasser.
23. The method of any one of claims 17 to 22 wherein said datum amount of calcium is about 16.8 ppm of said silicon-bearing killed steel.
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