WO2020213393A1 - 含クロム溶鉄の製造方法 - Google Patents
含クロム溶鉄の製造方法 Download PDFInfo
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- WO2020213393A1 WO2020213393A1 PCT/JP2020/014961 JP2020014961W WO2020213393A1 WO 2020213393 A1 WO2020213393 A1 WO 2020213393A1 JP 2020014961 W JP2020014961 W JP 2020014961W WO 2020213393 A1 WO2020213393 A1 WO 2020213393A1
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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
- C21C5/00—Manufacture of carbon-steel, e.g. plain mild steel, medium carbon steel or cast steel or stainless steel
- C21C5/005—Manufacture of stainless steel
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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
- C21C5/00—Manufacture of carbon-steel, e.g. plain mild steel, medium carbon steel or cast steel or stainless steel
- C21C5/52—Manufacture of steel in electric furnaces
- C21C5/5264—Manufacture of alloyed steels including ferro-alloys
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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
- C21C5/00—Manufacture of carbon-steel, e.g. plain mild steel, medium carbon steel or cast steel or stainless steel
- C21C5/52—Manufacture of steel in electric furnaces
- C21C5/527—Charging of the electric furnace
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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
- C21C5/00—Manufacture of carbon-steel, e.g. plain mild steel, medium carbon steel or cast steel or stainless steel
- C21C5/52—Manufacture of steel in electric furnaces
- C21C5/54—Processes yielding slags of special composition
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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
- C21C7/0025—Adding carbon material
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
- C22B34/00—Obtaining refractory metals
- C22B34/30—Obtaining chromium, molybdenum or tungsten
- C22B34/32—Obtaining chromium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
- C22B7/00—Working up raw materials other than ores, e.g. scrap, to produce non-ferrous metals and compounds thereof; Methods of a general interest or applied to the winning of more than two metals
- C22B7/04—Working-up slag
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C33/00—Making ferrous alloys
- C22C33/04—Making ferrous alloys by melting
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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
- C21C5/00—Manufacture of carbon-steel, e.g. plain mild steel, medium carbon steel or cast steel or stainless steel
- C21C5/52—Manufacture of steel in electric furnaces
- C21C5/527—Charging of the electric furnace
- C21C2005/5276—Charging of the electric furnace with liquid or solid rest, e.g. pool, "sumpf"
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P10/00—Technologies related to metal processing
- Y02P10/20—Recycling
Definitions
- This disclosure relates to a method for producing chromium-containing molten iron.
- Patent Document 1 discloses a technique for returning converter slag to an electric furnace and reducing it in the electric furnace. According to this technique, it is stated that metallic chromium can be sufficiently recovered. Further, Patent Document 2 discloses a technique for reducing chromium in an electric furnace without using an F-containing substance such as fluorite. According to this technique, it is described that metallic chromium can be effectively reduced and recovered from chromium oxide.
- Patent Document 3 describes the size of slag, the temperature of molten iron after reduction treatment, the relationship between C concentration and Si concentration in molten iron, CaO concentration and SiO 2 concentration in slag after reduction treatment, and Al 2 O 3
- Patent Document 1 Japanese Patent Application Laid-Open No. 2010-242128
- Patent Document 2 Japanese Patent Application Laid-Open No. 2010-90428
- Patent Document 3 Japanese Patent Application Laid-Open No. 2016-194126
- Patent Documents 1 and 2 can suppress the amount of slag generated to the outside of the system, but the amount of slag reduction is insufficient from the viewpoint of social demand for environmental harmony.
- it is also required to be able to efficiently recover chromium.
- chromium is efficiently reduced from slag, but it is desirable that both efficient recovery of chromium and reduction of the amount of slag generated can be achieved at the same time.
- the present disclosure provides a method for producing chromium-containing molten iron capable of efficiently reducing unreduced slag containing chromium oxide generated by oxidative refining to recover chromium and suppressing the amount of slag generated.
- the purpose is to provide.
- a charge containing at least one metal raw material of ferrochrome containing metallic Si and ferrosilicon and unreduced slag containing Cr oxide generated by oxidative refining is defined by the amount of metallic Si / the amount of Cr oxide. It was charged into an electric furnace as a composition having a mass ratio of 0.30 to 0.40 and a C concentration of 2.0% by mass or more and a saturation concentration or less. When the charge is heated and melted in the electric furnace, the maximum average heating rate in an arbitrary 80 ° C. section from 1300 ° C. to the reached temperature is set to 15.0 ° C./1.
- a molten iron containing Cr in which the Cr oxide has been reduced is produced by setting the minimum average heating rate in an arbitrary 80 ° C. section from 1300 ° C. to the reached temperature to 3.0 ° C./min or more.
- a method for producing chromium-containing molten iron is produced by setting the minimum average heating rate in an arbitrary 80 ° C. section from 1300 ° C. to the reached temperature to 3.0 ° C./min or more.
- the container contains a carbon source, a silicon source, a CaO source, and an Al 2 O 3 source, and the container other than the metal raw material is an auxiliary raw material.
- the content of the auxiliary material with a mesh over 25 mm is 5% by mass or more and 30% by mass or less with respect to the entire container, and the content of the auxiliary material with a mesh under 3.15 mm is the entire container.
- the C concentration and Si concentration in the molten iron after the reduction treatment satisfy the condition of the following formula (1), and the relationship between the CaO concentration, the SiO 2 concentration and the Al 2 O 3 concentration in the slag after the reduction treatment is The method for producing chromium-containing molten iron according to ⁇ 1>, wherein the charged material is charged into the electric furnace so as to satisfy the condition of the following formula (2).
- [C] and [Si] are the C concentration (mass%) and Si concentration (mass%) in the molten iron after the reduction treatment, respectively, and (CaO), (SiO 2 ) and (Al 2 O 3 ) are respectively.
- T represents the ultimate temperature (° C.).
- the electric furnace has three electrodes and has three electrodes.
- the electric furnace is viewed in a plan view from the central axis direction, the center of the furnace is located at the center of a triangle whose apex is the center of each of the three electrodes, and the electric furnace is further viewed in a plan view from the central axis direction.
- the stirring is performed in the bottom region excluding the band region.
- ⁇ 6> The method for producing chromium-containing molten iron according to any one of ⁇ 1> to ⁇ 5>, wherein the stirring power density during operation of the electric furnace is set to 0.01 kW / ton or more and 1.0 kW / ton or less. ..
- chromium-containing molten iron capable of efficiently reducing unreduced slag containing chromium oxide generated by oxidative refining to recover chromium and suppressing the amount of slag generated. it can.
- the numerical range represented by using “-” means a range including the numerical values before and after “-” as the lower limit value and the upper limit value.
- the numerical range when “greater than” or “less than” is added to the numerical values before and after “to” means a range in which these numerical values are not included as the lower limit value or the upper limit value.
- the upper limit value or the lower limit value of the numerical range described stepwise may be replaced with the upper limit value or the lower limit value of the numerical range described stepwise. , May be replaced with the values shown in the examples.
- “%” means “mass%”.
- FIG. 1 is a vertical sectional view of an electric furnace for producing chromium-containing molten iron in the present embodiment.
- the detailed structure of the electric furnace used in the present embodiment will be described.
- the electric furnace 10 used in the present embodiment is an arc-type electric furnace having three electrodes as shown in FIG. Details will be described later, but in the present embodiment, unreduced slag containing chromium oxide generated by oxidative refining, for example, chromium-containing molten iron, is decarburized in a converter which is a subsequent step as a charge to the electric furnace. Separately charged converter slag, ferrochrome containing metallic Si and / or ferrosilicon as a metal raw material, and other auxiliary raw materials as needed are charged into the electric furnace 10 to form unreduced slag. Chromium-containing molten iron is produced by reducing the contained Cr oxide with a metal Si derived from ferrochrome and / or ferrosilicon.
- a plurality of agitating gas blowing plugs 13 are installed in the bottom 12 of the electric furnace 10, and the number of agitating gas blowing plugs 13 is 0.12 or more per 1 m 2 of the molten metal surface area, and further adjacent stirring gas. Assuming that the distance between the centers of the blowing plug 13 is L and the molten metal depth from the furnace bottom 12 to the molten metal surface is H, the L / H is preferably 0.50 or more. This makes it possible to further improve the efficiency of reduction and recovery of chromium oxide.
- the molten metal surface area is the area of the molten metal surface when the electric furnace 10 is viewed from above. Further, the distance L between the centers of the adjacent stirring gas blowing plugs 13 is a horizontal distance.
- the molten metal depth H is the average value of the molten metal depths at the two adjacent plug positions. Generally, the molten metal depth H is 50 cm or more, and the maximum value is about 2 m in a large electric furnace.
- Stirring of the molten metal appropriately promotes contact between the undissolved charge and the molten metal, and promotes heating and melting of the charged material.
- the electric furnace 10 is a shallow bath, the stirring efficiency is low, and there is a limit to the range in which the molten metal can be well stirred by one stirring gas blowing plug 13.
- the number of the stirring gas blowing plugs 13 per 1 m 2 of the molten metal surface area is within the above range.
- the "stirring gas blowing number of plugs per molten metal surface area 1 m 2” in which the number of stirring gas blowing plug 13 divided by the molten metal surface area at operation.
- the area of the weakly agitated region is reduced and the entire molten metal is well agitated, and as a result, the contact between the charged material and the molten metal is promoted, and the heating and melting of the charged material are promoted.
- There is a physical upper limit (installation location) for the number of agitated gas blowing plugs to be installed and it is generally considered that the upper limit is about 0.5 per 1 m 2 of the molten metal surface area.
- the upper limit of L / H does not need to be set in particular, but there is a physical upper limit of L / H, which is generally about 5.
- the electric furnace 10 is viewed in a plan view from the central axis direction, and the center of gravity of the triangle having each center of the three electrodes 15 as the apex is set.
- the furnace center 11 is arranged, and further, the electric furnace 10 is viewed in a plan view, the center line is a virtual line extending from the furnace center 11 to the center of the electrode 15 and extending to the refractory furnace wall 14, and the diameter of the electrode is defined as the width.
- the stirring gas blowing plug 13 it is preferable to arrange the stirring gas blowing plug 13 in the furnace bottom region excluding the band region 16.
- an electromagnetic force is generated between conductors in which current flows in parallel.
- an outward electromagnetic force acts on the arc with respect to the electrode circle.
- the arc is tilted toward the furnace wall rather than vertically.
- the arc directed to the furnace wall blows an arc jet stream of high-temperature gas onto the furnace wall along a virtual line extending from the center of the furnace through the center of the electrodes to the furnace wall.
- This arcjet flow flowing at high speed on the surface layer of the molten metal bath surface gives a shearing force to the molten metal bath surface, and the molten metal flow along the arcjet flow is generated.
- the stirring gas blowing plug has a region that does not hinder the flow of molten metal due to the arcjet flow, that is, a band region whose center line is a virtual line extending from the center of the furnace to the furnace wall through the center of the electrode and the width of the diameter of the electrode. It is preferably arranged in the bottom area to be excluded. In this case, at least one of the plurality of agitating gas blowing plugs may be arranged in the fire bottom region excluding the band region, but all the agitating gas blowing plugs may be arranged in the fire bottom region excluding the band region. More preferred.
- the wear of refractories in the electric furnace can be suppressed, the non-operating time of the electric furnace can be shortened, the time can be devoted to the dissolution and reduction of unreduced slag, and the reduction and recovery of chromium oxide can be further improved. Become.
- the electric furnace 10 When the electric furnace 10 is viewed in a plan view from the central axis direction, as shown in FIG. 3, for example, three electrodes 15 are arranged in the central portion of the furnace body 17 so as to form an equilateral triangle with each center as the apex.
- a refractory furnace wall 14 is provided on the inner surface of the main body 17. Further, the electric furnace 10 according to the present embodiment receives the radiant heat of the arc generated between the metal raw material and the electrode, and the surface of the refractory furnace wall 14 is located at a position facing the electrode 15 in the radial direction with the furnace body 17.
- a temperature measuring unit 30 for measuring the temperature of the above is provided.
- the temperature measuring unit 30 is composed of three thermocouples 31, 33, and 35, respectively.
- the thermocouples 31, 33, and 35 are provided so as to penetrate the furnace body 17 and the perm refractory 14a and the ware refractory 14b constructed on the inner surface thereof so that the tip portion is located inside the ware refractory 14b. ..
- the tips of the thermocouples 31, 33, and 35 are arranged so that the distances L1, L2, and L3 from the surface of the perm refractory 14a in the radial direction of the furnace body 17 are different. As a result, the temperature distribution on the inner wall surface of the furnace at the position measured by the temperature measuring unit 30 can be estimated.
- the temperature measurement value measured by the temperature measurement unit 30 is output to the control device 40 that controls the operation of the electric furnace 10.
- the temperature measuring unit 30 in the present embodiment is composed of three thermocouples 31, 33, and 35, but the present invention is not limited to this example, and the temperature measuring unit 30 may be composed of a plurality of thermocouples.
- the temperature measurement values measured by the thermocouples 31, 33 and 35 are output to the control device 40.
- the control device 40 calculates the temperature gradient in the refractory thickness direction based on these temperature measurement values, and estimates the surface temperature of the refractory furnace wall 14. By obtaining the temperature gradient in the refractory thickness direction in this way, the surface temperature of the refractory furnace wall 14 can be estimated more accurately based on the temperature gradient.
- the acquisition of the surface temperature of the refractory furnace wall 14 is not limited to such a method, and for example, a method of directly measuring the surface temperature or another appropriate surface temperature estimation method may be used.
- the electric furnace 10 in the present embodiment can also measure the heat flux from the surface of the refractory furnace wall 14 to the inside of the furnace body.
- the heat flux can be obtained by, for example, the temperature measuring unit 30 shown in FIG. 3, similar to the measurement of the surface temperature of the refractory furnace wall.
- the temperature measurement values measured by the plurality of thermocouples 31, 33, 35 installed so that the tip portion is located at different positions in the refractory furnace wall 14 in the refractory thickness direction are output to the control device 40.
- the control device 40 calculates the temperature gradient in the refractory thickness direction based on these temperature measurement values, and estimates the heat flux from the surface of the refractory furnace wall 14 to the inside of the furnace body 10.
- the temperature gradient is calculated at a predetermined sampling time (for example, an arbitrary time of 300 seconds or less) to estimate the heat flux.
- the heat flux may be calculated by performing an inverse problem analysis in heat transfer from the time course of the temperature measurement values at two points using any two temperature measurement values of the thermocouples 31, 33, and 35.
- the heat flux is not limited to this method, and other appropriate surface temperature estimation methods and heat flux estimation methods may be used.
- the heat flux is acquired based on the temperature measurement values of each of the three temperature measurement units 30. Then, the maximum heat flux in each charge is determined from the obtained heat flux.
- a separately charged converter slag generated by decarburization treatment (oxidation refining) in a converter, which is a subsequent process is loaded into an electric furnace as unreduced slag containing Cr oxide after oxidation refining. Enter. Therefore, the out-of-system discharge of converter slag becomes zero, and slag to be discharged to the outside of the system is generated only in the electric furnace, so that the total amount of slag generated in the entire melting process can be reduced.
- the decarburization treatment in the converter in the subsequent process can be performed under known conditions.
- the amount of metallic Si refers to the amount of metallic silicon contained in ferrosilicon and ferrochrome.
- ferrosilicon may be used as the metal Si source, from the viewpoint of reducing the ratio of the amount of metal Si / the amount of Cr oxide to 0.40 or less, only ferrochrome containing metal Si or ferrochrome containing metal Si is used. It is preferable to use ferrosilicon together.
- the mass ratio defined by the amount of metallic Si / the amount of Cr oxide is less than 0.30, the amount of metallic silicon is insufficient under the condition assuming the heating rate described later, and the chromium oxide in the slag produced in the electric furnace is relative. Chromium cannot be reduced efficiently.
- the mass ratio defined by the amount of metallic Si / the amount of Cr oxide exceeds 0.40, the amount of silicon in the chromium-containing molten iron to be produced increases, and it is produced in the oxidation refining process in the converter, which is a subsequent process. The amount of converter slag increases.
- both the amount of refluxed slag and the amount of refluxed chromium oxide are increased. ..
- the amount of metallic silicon used increases according to the increased amount of slag and the amount of chromium oxide, resulting in an increase in the amount of slag generated in the electric furnace, and as the number of operations increases. The inconvenience of increasing the amount of slag in the electric furnace occurs.
- ⁇ C concentration 2.0% by mass or more>
- the C concentration is set to 2.0% by mass or more for the purpose of improving the efficiency of reduction and recovery of chromium oxide.
- the upper limit of the C concentration is not particularly limited, but is substantially equal to or less than the saturation concentration according to the Cr concentration.
- the saturation concentration of carbon differs depending on the Cr concentration. For example, when the Cr concentration is 0% by mass, the saturation concentration of carbon is about 4.4% by mass, and in a normal chromium-containing steel having a Cr concentration of about 10%, it is about 5.5% by mass.
- the activity of silicon is a factor that affects the reduction reaction, but by setting the C concentration to 2.0% by mass or more, the activity of silicon is maintained at a high level.
- the reduction reaction can be suitably carried out.
- Carbon is contained in carbonaceous materials such as coke and coal, or ferrochrome, and the C concentration can be adjusted to 2.0% by mass or more by adjusting the charge amount thereof.
- the temperature conditions in the electric furnace will be explained.
- the ultimate temperature is set according to the components of the molten metal produced in the electric furnace and the convenience of processing after the electric furnace process. That is, the temperature reached by heating and melting in the electric furnace is approximately 1400 to 1700 ° C.
- the estimated value from the thermocouple measurement value embedded in the furnace wall refractory is used as the molten metal temperature, but the electricity in which the thermocouple meter is not embedded in the furnace wall refractory.
- values measured by a consumable thermocouple or a radiation thermometer can be adopted.
- ⁇ Maximum average heating rate 15.0 ° C / min or less>
- the maximum average heating rate in any 80 ° C. section from 1300 ° C. to the ultimate temperature is 15.0 ° C./min or less. That is, the average heating rate within the section of 80 ° C. should not exceed 15.0 ° C./min regardless of where the temperature is divided from 1300 ° C. to the reached temperature.
- the maximum average heating rate exceeds 15.0 ° C./min, the temperature is raised to near the reached temperature before the charged unreduced slag is sufficiently dissolved.
- the reduction treatment is performed in a high temperature region where the reduction of chromium oxide is difficult to proceed in terms of equilibrium theory, the reduction of chromium oxide becomes insufficient, and the recovery rate of chromium decreases. Furthermore, a large amount of metallic silicon that was not consumed in the reduction reaction remains in the chromium-containing molten iron, and the amount of converter slag generated in the oxidation refining process increases, resulting in slag in the electric furnace as described above. The amount will increase.
- the minimum average heating rate in an arbitrary 80 ° C. section from 1300 ° C. to the reached temperature is 3.0 ° C./min or more. That is, the average heating rate within the section of 80 ° C. should not be less than 3.0 ° C./min regardless of where the temperature is divided from 1300 ° C. to the reached temperature. If the minimum average heating rate is less than 3.0 ° C./min, it will take a lot of time to raise the temperature to the reached temperature. If a large amount of time is required, the metallic silicon will be oxidatively lost due to the oxygen in the air that inevitably enters the electric furnace.
- the mass ratio of the amount of metallic Si / the amount of Cr oxide in the container charged to the electric furnace and the C concentration are set within a predetermined range, and the average of an arbitrary 80 ° C. section from 1300 ° C. to the reached temperature is further set.
- the heating rate is set to a predetermined condition, chromium can be efficiently recovered and the amount of slag discharged to the outside of the system can be suppressed.
- sampling at a temperature in an "arbitrary 80 ° C. section" is used as a reference in order to avoid a rapid temperature rise and to achieve a gentle temperature rise.
- the chromium can be recovered more efficiently by the reduction of chromium oxide. be able to.
- auxiliary raw material refers to charged materials other than metal raw materials (ferrochrome, ferrosilicon, scrap, etc.), and in addition to converter slag (unreduced slag), oxides (fresh lime, silica stone, magnesia, alumina, decommissioning material) , Metal oxides), charcoal oxides (limestone, slagite), hydroxides (metal or semi-metal hydroxides). These can be charged into an electric furnace as needed.
- the mass ratio of the auxiliary material having a mesh size of over 25 mm (hereinafter, may be referred to as “lumpy auxiliary material”) that is difficult to heat and dissolve with respect to the entire charge, and the sieve that is easily heated and dissolved.
- the mass ratio of the auxiliary material with a mesh of 3.15 mm under (hereinafter, sometimes referred to as "fine powder auxiliary material") to the entire charge, the chromium can be recovered more efficiently. it can.
- the above-mentioned 25 mm over mesh and 3.15 mm under mesh are left on the sieve of JIS Z8801-2: 2000 with a nominal opening of 25 mm and a sieve with a nominal opening of 3.15 mm, respectively. Refers to the one under the sieve.
- ⁇ Mass ratio of auxiliary material over 25 mm mesh to the entire container 5 to 30% by mass>
- the mass ratio of the auxiliary raw material having a sieve mesh over 25 mm to the entire charge is preferably 5 to 30% by mass.
- the mass ratio of the auxiliary material having a sieve mesh of 25 mm or more with respect to the charged material is specified in order to specify the composition of the auxiliary material that is difficult to heat and dissolve. In addition, a part or all of the lumpy auxiliary raw material which is difficult to dissolve becomes unreduced slag.
- the chromium oxide in the slag is dissolved, and the reduction reaction of the chromium oxide by the molten iron changes from a solid-liquid reaction to a liquid-liquid reaction. change.
- the chromium reduction capacity coefficient which is an index of the reduction reaction of chromium oxide, is significantly improved from about 0.01 (1 / min) to 0.05 (1/min) or more, and the reduction reaction is efficient. You can proceed to.
- the chromium reduction capacity coefficient is a value representing a change in the concentration of chromium oxide per unit time, and is an index of the ease of progress of the reduction reaction.
- the mass ratio of the auxiliary raw material having a sieve mesh over 25 mm to the entire charge is less than 5% by mass, the effect is difficult to obtain. Further, if the mass ratio of the auxiliary raw material having a mesh over 25 mm to the entire charge exceeds 30% by mass, it takes a lot of time for heating and melting, and the efficiency of the reduction reaction tends to decrease.
- the mass ratio of the auxiliary raw material having a sieve mesh of 3.15 mm under to the entire charge is preferably 3.0% by mass or more.
- the auxiliary material having a sieve mesh of 3.15 mm under is easily dissolved after heating to promote the dissolution of the massive auxiliary material. If the mass ratio of the fine powder auxiliary material to the entire charge is 3.0% by mass or more, it will be present around the massive auxiliary material after the fine powder auxiliary material is dissolved, and the stirring action by the arc jet flow of the electric furnace It can also contribute to the promotion of heating and melting of the bulk auxiliary material.
- the mass ratio of the auxiliary raw material under the sieve mesh of 3.15 mm to the entire charge is less than 3.0% by mass, the effect cannot be sufficiently obtained. Further, although the upper limit is not particularly specified, even if the fine powder auxiliary material is added in an amount of more than 25% by mass with respect to the entire container, the effect is saturated.
- the fine powder auxiliary material may or may not contain unreduced slag.
- the metal raw material (scrap, ferroalloy, granular iron, solid or melt) is preferably contained in an amount of 45% by mass or more based on the total charge, considering the electrical conductivity in the electric furnace.
- the charged material is composed of the above-mentioned auxiliary raw material and the metal raw material
- the fine powder auxiliary material is 3% by mass or more and the massive auxiliary raw material is 5% by mass or more. Therefore, the metal raw material is the entire charged material. It is preferably 92% by mass or less.
- the heat dissolution of the bulk auxiliary material can be promoted by containing the fine powder auxiliary material in a certain ratio. Further, after the reduction treatment, when the ultimate temperature, C concentration, and Si concentration satisfying the formula (1) described later, and by satisfying the formula (2) described later, favorable viscosity conditions for the slag can be secured. , Chromium reduction can be more preferably realized.
- [C] and [Si] represent the C concentration and the Si concentration (mass%) in the molten iron, respectively, and T represents the ultimate temperature (° C.). It is possible to adjust the C concentration and the Si concentration by controlling the addition amounts of the carbon source and the silicon source in the container.
- the carbon source carbon materials such as coke and coal, carbon contained in ferrochrome, and the like can be used.
- silicon source ferrosilicon, metallic silicon, silicon contained in ferrochrome, or the like can be used.
- the chromium oxide can be reduced more efficiently.
- the slag composition proper region i.e., equation (2) range
- the CaO source CaO content contained in quicklime, limestone, dolomite and the like can be used.
- Al 2 O 3 source aluminum ash, high alumina brick, Al 2 O 3 minutes contained in secondary refining slag or the like can be used. 0.04 ⁇ (CaO) / ⁇ (SiO 2 ) ⁇ (Al 2 O 3 ) ⁇ ⁇ 0.20 ⁇ ⁇ ⁇ (2)
- (CaO), (SiO 2 ), and (Al 2 O 3 ) represent the CaO concentration, the SiO 2 concentration, and the Al 2 O 3 concentration (mass%) in the slag after the reduction treatment, respectively.
- the viscosity has a large effect on the solubility of the slag, and the viscosity decreases as the slag dissolves. It is important to try. It is generally known that the viscosity of this slag increases when Al 2 O 3 is added to basic slag and decreases when Al 2 O 3 is added to acidic slag (eg, No. 1). 3rd Edition Steel Handbook Vol. I, p.43).
- the viscosity of the slag can decrease as the amount increases. That is, M. S. I.
- the by 0.04 or more, the Cr 2 O 3 concentration in the pre-reduction treatment was high concentration of more than 30% by weight slag, to a low concentration of less than 10% by weight after the reduction treatment, more efficiently It is preferable because it can be reduced.
- M. S. I. When the value exceeds 0.20, the melting point of the slag is remarkably increased, the dissolution of the slag is inhibited, and the reduction rate of chromium is greatly reduced. Therefore, in order to secure the reduction rate of chromium, M.I. S. I. Is preferably 0.20 or less.
- fluorine is not substantially used means that the elution of fluorine is not significantly observed from the slag after the reduction treatment, and the slag composition after the reduction treatment is 0.5 in terms of CaF 2. It refers to the case where it is mass% or less, but more preferably 0.3 mass% or less.
- the Al 2 O 3 concentration in the slag after the reduction treatment is 5.0% by mass or more.
- the Al 2 O 3 concentration exceeds 30.0% by mass, the effect of promoting the reduction of chromium oxide by dissolving the slag cannot be expected, and the cost of the alumina source is high. Therefore, 30.0% by mass. The following is preferable.
- the stirring power density can be controlled by the stirring gas (bottom blowing gas) from the stirring gas blowing plug.
- the stirring power density By setting the stirring power density to 0.01 kW / ton or more, it is possible to obtain a stirring effect for efficient reduction of chromium oxide.
- the stirring power density exceeds 1.0 kW / ton, the stirring gas may blow through the molten metal and not contribute to stirring.
- the surface of the molten metal fluctuates sharply, and although the operation is possible, the refractory may be noticeably damaged. Therefore, the stirring power density is preferably 1.0 kW / ton or less.
- the stirring power density ⁇ for each stirring gas blowing plug is expressed by the following equation (3). Therefore, the total stirring power density for each stirring gas blowing plug is the stirring power density of the electric furnace.
- ⁇ (0.371 ⁇ Q ⁇ T l / W) ⁇ [ln ⁇ 1 + (9.8 ⁇ ⁇ l ⁇ h) / P a ⁇ + ⁇ (1-T n / T l )] ⁇ ⁇ ⁇ (3)
- ⁇ stirring power density (kW / ton)
- Q stirring gas flow rate (Nm 3 / sec)
- T l molten iron temperature (K)
- W molten iron mass (ton)
- T n stirring gas temperature (K)
- ⁇ l density of molten iron (kg / m 3)
- h depth bath in stirring gas blowing plug (m)
- P a pressure of the atmosphere (Pa)
- efficient stirring makes it possible to dissolve unreduced slag more quickly.
- the maximum temperature reached by the surface temperature of the refractory furnace wall in one charge is 1000 ° C. or higher and 1800 ° C. or lower.
- the surface temperature of the refractory furnace wall can be measured by the method described above. When the maximum temperature reached exceeds 1800 ° C., the surface temperature of the refractory furnace wall becomes close to the melting point of the refractory, and the strength is remarkably lowered. When the strength of the refractory furnace wall is reduced, the melting damage of the refractory furnace wall becomes remarkable due to the scattering of molten metal or slag.
- the surface temperature of the refractory furnace wall is preferably 1800 ° C. or lower.
- the maximum temperature reached is preferably 1000 ° C. or higher.
- the heat flux from the surface of the refractory furnace wall to the inside of the furnace body is preferably 150 Mcal / m 2 / hr or less.
- the heat flux from the surface of the refractory furnace wall to the inside of the furnace body can be measured by the method described above.
- the surface temperature of the refractory furnace wall is in the range of 1000 ° C. or higher and 1800 ° C. or lower, if the temperature suddenly rises due to contact with molten metal or slag, a difference in thermal expansion occurs locally, similar to spalling. Thermal stress is generated. Therefore, if the surface temperature of the refractory furnace wall rises sharply when the surface temperature of the refractory furnace wall is in the range of 1000 ° C. or higher and 1800 ° C. or lower, the refractory furnace wall is cracked and the wear is remarkable. It becomes.
- the surface temperature of the refractory furnace wall is in the range of 1000 ° C. or higher and 1800 ° C. or lower and the heat flux is 150 Mcal / m 2 / hr or less
- the wear of the refractory furnace wall due to the above-mentioned thermal expansion difference can be suppressed.
- the temperature gradient is gentle, so that the local thermal expansion difference is small and the generation of thermal stress is also small. Therefore, cracks are less likely to occur in the refractory furnace wall.
- the wear of the refractory furnace wall is suppressed and the power is as high as possible.
- the electric furnace can be operated with high electric power within the possible range, the operating time can be shortened.
- wear of the refractory furnace wall can be further suppressed, heat dissipation loss from the furnace wall and the like can be reduced, and manufacturing costs can be reduced.
- the conditions in the examples are one condition example adopted for confirming the feasibility and effect of the present disclosure, and the present disclosure is based on this one condition example. It is not limited.
- the present disclosure may adopt various conditions as long as the gist of the present disclosure is not deviated and the object of the present disclosure is achieved.
- Metal raw material 55 tons in total (55% by mass of total charge)
- scrap cast iron (solidified blast furnace hot metal), and alloy iron (ferrochrome), which are metal raw materials containing a carbon source and a silicon source, were used.
- Auxiliary material 45 tons in total (45% by mass of the total charge) Breakdown of unreduced slag containing Cr 2 O 3 generated by decarburization of another charge in the converter 33-39 wt% (converter slag): total 43 tons (43% by weight of the electric RoSo container ),
- the bulk auxiliary material having a mesh size of 25 mm over was 5 to 35 tons
- the fine powder auxiliary material having a sieve mesh of 3.15 mm under was 0.1 to 25 tons.
- the balance of the auxiliary raw material was an Al 2 O 3 source (aluminum ash) and a Ca O source (quick lime) having an intermediate particle size (over mesh 3.15 mm and under mesh 25 mm).
- the mass ratio defined by the amount of metallic Si / the amount of Cr oxide was set to 0.2 to 0.5.
- the ultimate temperature was set to 1400 to 1700 ° C.
- the maximum and minimum average heating rates in any 80 ° C. section from 1300 ° C. to the ultimate temperature were the conditions shown in Table 1 below.
- the slag composition “(CaO) / ⁇ (SiO 2 ) ⁇ (Al 2 O 3 ) ⁇ ” (described as C / SA in the table) after the reduction treatment was set to 0.01 to 0.25.
- the Al 2 O 3 concentration in the slag after the reduction treatment was set to 4.5 to 30% by mass.
- the stirring power density was set to 0.00 to 1.5 kW / ton.
- the stirring power density is the total value of the stirring power density for each stirring gas blowing plug of the above-mentioned formula (3). Further, "0 kW / ton" means a condition in which the stirring gas (bottom blown gas) is not blown.
- the evaluation results were evaluated as AA for less than 0.05, A for 0.05 or more and less than 0.2, B for 0.2 or more and 0.5 or less, and C for those exceeding 0.5.
- the total amount of slag generated after the reduction treatment in the electric furnace and the amount of slag generated after refining in the converter in the subsequent process was used as an evaluation index of the amount of slag generated.
- Such evaluation is based on Comparative Example 1 (100%), A when the total slag amount is less than 85% of the standard, B when 85% or more and 95% or less of the standard, and Comparative Example 1. When it was equivalent to (less than ⁇ 5%) or worsened, it was evaluated as C.
- Comparative Example 1 since the maximum average heating rate in an arbitrary 80 ° C. section from 1300 ° C. to the reached temperature was too large, the temperature was raised to near the reached temperature before the charged unreduced slag was sufficiently dissolved. It has been done. As a result, the reduction of chromium oxide becomes insufficient, the recovery rate of chromium decreases, and a large amount of metallic silicon that was not consumed in the reduction reaction remains in the chromium-containing molten iron, and after refining in the converter in the subsequent process. The amount of slag generated has increased.
- Comparative Example 3 since the amount of metallic silicon was relatively small, the reduction reaction was insufficient, the amount of chromium oxide in the slag produced in the electric furnace was relatively large, and the amount of slag generated was also increased accordingly. I have. In Comparative Example 4, since the amount of metallic silicon was relatively large, the reduction reaction was efficiently and sufficiently performed, but the Si concentration in the molten iron became high, and the amount of slag generated after refining in the converter in the subsequent process. Has increased.
- Example 2 The operation was repeated under the same conditions as the charges described in Example 1 and Comparative Example 1 in Table 1, and the electric furnace was operated for one month.
- the electric furnace was operated under the above operating conditions, and the wear state and productivity of the refractory furnace wall were evaluated.
- the maximum temperature of the refractory furnace wall surface was 1200 to 2000 ° C.
- the maximum heat flux of the refractory furnace wall surface was 20 to 150 Mcal / m 2 / hr.
- the results of verifying the effectiveness of the operating method of the electric furnace disclosed in the present disclosure are shown below.
- the evaluation criteria for the wear of the refractory furnace wall, the productivity of one charge, and the productivity of one month are as follows.
- Examples 31 to 33 are the same as Example 1 in Table 1
- Comparative Examples 11 to 13 are the same as Comparative Example 1 in Table 1.
- Example 1 there was a charge in which the maximum temperature reached exceeded 1800 ° C., and in Example 31, there was a charge in which the maximum heat flux exceeded 150 Mcal / m 2 / hr. Therefore, the productivity per charge was better than that of Example 33, but the rate of wear of the refractory furnace wall was high, and the refractory repair time per month increased.
- Example 33 the maximum temperature reached for each charge was 1000 ° C. or higher and 1800 ° C. or lower, and the maximum heat flux was 150 Mcal / m 2 / hr or lower.
Abstract
Description
効率良くステンレス鋼を溶製するため、酸化クロムの発生量を削減したり、スラグ発生量を削減したりすることが求められている。
また、特許文献2には、蛍石等のF含有物を用いずに電気炉でクロムを還元する技術が開示されている。この技術によれば、酸化クロムから金属クロムを有効に還元回収することできると記載されている。
さらに、特許文献3には、スラグのサイズ、還元処理後の溶鉄の温度及び溶鉄中のC濃度とSi濃度との関係、還元処理後のスラグ中のCaO濃度とSiO2濃度とAl2O3濃度の関係を規定したクロム含有スラグからのクロム回収方法が開示されている。
特許文献2:特開2010-90428号公報
特許文献3:特開2016-194126号公報
<1> 金属Siを含むフェロクロム及びフェロシリコンの少なくとも一方の金属原料と酸化精錬で発生したCr酸化物を含む未還元スラグとを含む装入物を、金属Si量/Cr酸化物量で定義される質量比が0.30~0.40、かつC濃度が2.0質量%以上飽和濃度以下となる配合として電気炉に装入し、
前記電気炉で前記装入物を加熱溶解する際に、1400~1700℃を到達温度として、1300℃から前記到達温度までの任意の80℃の区間の最大の平均加熱速度を15.0℃/分以下、かつ1300℃から前記到達温度までの任意の80℃の区間の最小の平均加熱速度を3.0℃/分以上として前記Cr酸化物が還元処理されたCrを含む溶鉄を製造する、
含クロム溶鉄の製造方法。
篩目25mmオーバーの前記副原料の含有量が前記装入物全体に対して5質量%以上30質量%以下、かつ、篩目3.15mmアンダーの前記副原料の含有量が前記装入物全体に対して3.0質量%以上であり、
前記還元処理後の溶鉄中のC濃度及びSi濃度が下記(1)式の条件を満たし、かつ、前記還元処理後のスラグ中のCaO濃度とSiO2濃度とAl2O3濃度との関係が下記(2)式の条件を満たすように前記電気炉に前記装入物を装入する<1>に記載の含クロム溶鉄の製造方法。
[C]≧-29.4+0.015×(T+273)-0.003×(T+273)×log[Si] ・・・(1)
0.04≦(CaO)/{(SiO2)×(Al2O3)}≦0.20 ・・・(2)
ここで、[C]と[Si]はそれぞれ還元処理後の溶鉄中のC濃度(質量%)とSi濃度(質量%)、(CaO)と(SiO2)と(Al2O3)はそれぞれ還元処理後のスラグ中のCaO濃度(質量%)とSiO2濃度(質量%)とAl2O3濃度(質量%)、Tは前記到達温度(℃)を表す。
前記電気炉を中心軸方向から平面視して、前記3本の電極の各中心を頂点とする三角形の重心に炉中心が位置し、さらに、前記電気炉を前記中心軸方向から平面視して、前記炉中心から前記3本の各電極の中心を通り炉壁まで延びる仮想線を中心線とし該電極の直径を幅とするバンド領域を想定し、前記バンド領域を除く炉底領域に前記撹拌ガス吹き込みプラグが位置する<1>~<4>のいずれか1つに記載の含クロム溶鉄の製造方法。
なお、本明細書中において、「~」を用いて表される数値範囲は、「~」の前後に記載される数値を下限値及び上限値として含む範囲を意味する。また、「~」の前後に記載される数値に「超」または「未満」が付されている場合の数値範囲は、これら数値を下限値または上限値として含まない範囲を意味する。
本明細書中に段階的に記載されている数値範囲において、ある段階的な数値範囲の上限値又は下限値は、他の段階的な記載の数値範囲の上限値又は下限値に置き換えてもよく、実施例に示されている値に置き換えてもよい。
また、成分の含有量について、「%」は「質量%」を意味する。
炉底に設置された撹拌ガス吹き込みプラグから溶湯に吹き込まれる撹拌ガスによる湯面の広がりは、図2に示すように、湯面の広がり半径が0.21×溶湯深さHであることが知られている。なお、鉛直線に対する撹拌ガスの広がり角度は12度であることが一般的である。
まず、金属Siを含むフェロクロム及びフェロシリコンの一方とCr酸化物及び未還元スラグとを含む装入物を電気炉に装入し、これらを溶解する際に、未還元スラグを金属シリコンで還元する前提で、金属Si量/Cr酸化物量で定義される質量比が0.30~0.40となるように配合する。なお、電気炉で装入物を溶解する際に、金属Siを含むフェロクロム及び/又はフェロシリコン、必要に応じてその他の副原料も装入されるが、副原料については後述する。また、金属Siを含むフェロクロム及びフェロシリコンを用いる場合、金属Si量は、フェロシリコン及びフェロクロム中に含まれる金属シリコンの量を指す。なお、金属Si源としてフェロシリコンのみを用いてもよいが、金属Si量/Cr酸化物量の比を0.40以下にする観点から、金属Siを含むフェロクロムのみ、又は、金属Siを含むフェロクロムとフェロシリコンを併用することが好ましい。
酸化クロムの還元回収の高効率化を目的として、C濃度は2.0質量%以上とする。C濃度の上限については特に限定しないが、実質的にはCr濃度に応じた飽和濃度以下となる。なお、炭素の飽和濃度は、Cr濃度によって異なる。例えば、Cr濃度が0質量%のときは炭素の飽和濃度は4.4質量%程度であり、Cr濃度が10%程度の通常の含クロム鋼では5.5質量%程度である。金属シリコンによるスラグ中の酸化クロムの還元反応において、シリコンの活量は還元反応に影響する因子となるが、C濃度を2.0質量%以上とすることで、シリコンの活量を高位に維持でき、還元反応を好適に進めることができる。炭素は、コークス及び石炭といった炭材、もしくはフェロクロム中に含まれており、これらの装入量を調整することによってC濃度を2.0質量%以上にすることができる。
前述した濃度条件を前提とした場合、酸化クロムの還元反応が起こりやすい温度域を規定する必要がある。そこで、昇温途中で温度を保定してもよいが、1300℃から到達温度(1400~1700℃)までの任意の80℃区間の平均加熱速度の条件を規定することによって、急速な加熱、および極端に長い時間の温度の保定を排除する。ここで、「到達温度」及び「平均加熱速度」における温度とは、溶湯温度のことを示す。溶湯温度を求める方法には種々あり、本実施形態では炉壁耐火物に埋め込んだ熱電対計測値からの推算値を溶湯温度として用いるが、炉壁耐火物に熱電対計が埋め込まれていない電気炉を用いる場合は、消耗式熱電対又は放射温度計による測定値を採用することができる。
1300℃から到達温度までの任意の80℃の区間の最大の平均加熱速度は15.0℃/分以下とする。つまり、1300℃から到達温度までの間で、どこで区切っても80℃の区間内の平均加熱速度が15.0℃/分を超えないようにする。最大の平均加熱速度が15.0℃/分超の場合、装入した未還元スラグが十分溶解しないうちに到達温度近傍まで昇温される。この場合、平衡論的にクロム酸化物の還元が進みにくい高温域で還元処理を行うこととなり、酸化クロムの還元が不十分となり、クロムの回収率が低下してしまう。更に、還元反応で消費されなかった金属シリコンが含クロム溶鉄中に多く残存することとなり、酸化精錬工程での転炉スラグの発生量が増加し、前述したように結果的に電気炉でのスラグ量が増加してしまう。
また、1300℃から到達温度までの任意の80℃の区間の最小の平均加熱速度は3.0℃/分以上とする。つまり、1300℃から到達温度までの間で、どこで区切っても80℃の区間内の平均加熱速度が3.0℃/分未満にならないようにする。最小の平均加熱速度が3.0℃/分未満の場合、到達温度までの昇温に多くの時間を要してしまう。多くの時間を要してしまうと、電気炉内に不可避的に侵入する空気中の酸素によって金属シリコンがその分酸化ロスされてしまう。その結果、金属シリコンが不足し、酸化クロムの還元が不十分となる。あるいは、酸化ロス分の金属シリコンを追加で装入すると、電気炉内でのシリコンの消費量が増加し、電気炉でのスラグ発生量が増加してしまう。
上記した副原料は、金属原料(フェロクロム、フェロシリコン、スクラップ等)以外の装入物を指し、転炉スラグ(未還元スラグ)のほか、酸化物(生石灰、珪石、マグネシア、アルミナ、廃炉材、金属酸化物)、炭酸化物(石灰石、ドロマイト)、水酸化物(金属又は半金属の水酸化物)、が含まれる。これらは必要に応じて電気炉に装入することができる。
篩目25mmオーバーの副原料の装入物全体に対する質量比は5~30質量%とすることが好ましい。装入物に対する篩目25mmオーバーの副原料の質量比を規定したのは、加熱及び溶解しにくい副原料の構成を規定するためである。なお、溶解しにくい塊状副原料の一部又は全部が、未還元スラグとなる。
一方、篩目3.15mmアンダーの副原料の装入物全体に対する質量比は3.0質量%以上とすることが好ましい。篩目3.15mmアンダーの副原料は、加熱後容易に溶解し、塊状副原料の溶解を促進する。装入物全体に対する微粉副原料の質量比が3.0質量%以上であれば、この微粉副原料が溶解した後に塊状副原料の周囲に存在することとなり、電気炉のアークジェット流による撹拌作用も含めて、塊状副原料の加熱や溶解の促進に寄与できる。篩目3.15mmアンダーの副原料の装入物全体に対する質量比が3.0質量%未満ではその効果が十分に得られない。また、上限については特に規定はしないが、微粉副原料を装入物全体に対して25質量%を超えて添加しても、その効果は飽和する。なお、微粉副原料には、未還元スラグが含まれてもよいが、含まれなくてもよい。
金属原料(スクラップ、合金鉄、粒鉄、の固形物あるいは溶融物)は、電気炉内の通電性を考えると、装入物全体に対して45質量%以上含むことが好ましい。ここで、装入物とは、上記した副原料と金属原料とからなることから、微粉副原料が3質量%以上で塊状副原料が5質量%以上であるため、金属原料は装入物全体に対して92質量%以下となることが好ましい。
クロムの回収率をより向上させるために、還元処理後の溶鉄中のC濃度およびSi濃度は、以下の(1)式の条件を満たしていることが好ましい。
[C]≧-29.4+0.015×(T+273)-0.003×(T+273)×log[Si] ・・・(1)
さらに、還元処理後のスラグ中のCaO濃度、SiO2濃度、及びAl2O3濃度が、以下の(2)式を満たすと、クロム酸化物をより効率良く還元することが可能となる。装入物としてCaO源およびAl2O3源を電気炉に添加することで、スラグ組成を適正な領域(即ち、(2)式の範囲)に制御することができる。ここでCaO源としては、生石灰、石灰石、ドロマイトなどに含まれるCaO分などが使用できる。また、Al2O3源は、アルミ灰及び高アルミナ質れんが、二次精錬スラグなどに含まれるAl2O3分などが使用できる。
0.04≦(CaO)/{(SiO2)×(Al2O3)}≦0.20 ・・・(2)
撹拌力の弱い電気炉内で、高濃度のクロム酸化物を含む未還元スラグからクロム分を効率よく回収するためには、スラグの溶解性と共に粘度の影響が大きく、スラグの溶解と共に粘度の低下を図ることが肝要である。このスラグの粘度は、塩基性スラグにAl2O3を添加した場合は増加し、酸性スラグにAl2O3を添加した場合は低下することが、一般的に知られている(例えば、第3版 鉄鋼便覧 第I巻、p.43参照)。
より好ましくは、スラグ性状の点から、還元処理後のスラグ中のAl2O3濃度を適正範囲とすることで、実質的にフッ素を使用せずにスラグを溶解し、クロム酸化物をより効率良く還元することが可能である。ここで、実質的にフッ素を使用しないとは、還元処理後のスラグからフッ素の溶出が顕著には認められないことを指すもので、還元処理後のスラグ組成において、CaF2換算で0.5質量%以下となる場合を指すが、0.3質量%以下であれば更に好ましい。
スラグ中のクロム酸化物を効率的に還元する際に、電気炉での操業時の撹拌動力密度に適正域が存在する。攪拌動力密度は撹拌ガス吹き込みプラグからの攪拌ガス(底吹きガス)によって制御することができる。この撹拌動力密度を0.01kW/トン以上とすることにより、クロム酸化物の効率的な還元のための撹拌効果を得ることが可能となる。一方、撹拌動力密度が1.0kW/トンを超えると、攪拌ガスが溶湯を吹抜けて撹拌に寄与しない場合がある。また、溶湯面の変動が激しくなり、操業は可能であるが耐火物の溶損が目立つ場合等がある。このため、攪拌動力密度は1.0kW/トン以下とすることが好ましい。
ε=(0.371×Q×Tl/W)×[ln{1+(9.8×ρl×h)/Pa}+η(1-Tn/Tl)] ・・・(3)
まず、1チャージにおける耐火物炉壁の表面温度の最高到達温度を1000℃以上1800℃以下とすることが好ましい。耐火物炉壁の表面温度は、前述した方法によって測定することができる。最高到達温度が1800℃を超えると耐火物炉壁の表面温度が耐火物の融点に近くなり、強度が顕著に下がる。耐火物炉壁の強度が低下すると、溶湯あるいはスラグの飛散により、耐火物炉壁の溶損が顕著となる。したがって、耐火物炉壁の表面温度は1800℃以下にすることが好ましい。一方、最高到達温度を1000℃未満とすると高い生産性を得られない。これより、最高到達温度は1000℃以上とすることが好ましい。
また、耐火物炉壁の表面温度が1000℃以上1800℃以下の範囲では、耐火物炉壁表面から炉本体内部への熱流束を150Mcal/m2/hr以下とすることが好ましい。耐火物炉壁表面から炉本体内部への熱流束は、前述した方法によって測定することができる。
本実験では、アーク式電気炉に金属原料を装入して溶鉄を溶製する際に、クロム酸化物を含むスラグを添加し、スラグ中のクロム酸化物を還元して、クロムを溶鉄中に還元回収した。なお、Al2O3源を添加するに際しては、アルミナ灰を通電開始前に添加し、CaO源として生石灰を添加するに際しては、溶解中に上方ホッパーより添加した。また、溶湯の撹拌を行う場合は、攪拌ガスとして底吹きArガスを吹き込み、撹拌動力密度を(3)式に基づいて算出した。この実験条件を以下に示す。
ここでは、図1~図4に示すような構成の100トンの溶湯が溶製できるアーク式電気炉(アーク式溶解炉)を用いて実験を行った。炉底に配置した撹拌ガス吹き込みプラグの数は、3個、4個、又は6個で実験を行い、実施例12では、すべての撹拌ガス吹き込みプラグをバンド領域以外の位置に設置して実験を行った。また、電極は24インチ径のものを3本用い、中心軸から平面視した場合の各電極の中心を通る円の直径(PCD)は1.8m、炉内直径は6.1mとした。溶鉄の静止湯面から電極先端までの距離(電極高さ)は、3本の平均で0.3mとした。
金属原料:合計55トン(装入物全体の55質量%)
実験には、炭素源と珪素源を含む金属原料である、スクラップ、鋳銑(高炉溶銑を凝固させたもの)、及び、合金鉄(フェロクロム)を用いた。
内訳は、転炉での別チャージの脱炭処理で生成されたCr2O3を33~39質量%含む未還元スラグ(転炉スラグ):総量43トン(電気炉装入物の43質量%)であり、上記未還元スラグのうち、篩目25mmオーバーの塊状副原料を5~35トンとし、篩目3.15mmアンダーの微粉副原料を0.1~25トンとした。
副原料の残部は、中間粒度(篩目3.15mmオーバー、かつ、篩目25mmアンダー)のAl2O3源(アルミ灰)とCaO源(生石灰)とした。
アーク電極3本、40MW、合計通電時間は60分で一定とした。
加熱溶解では、到達温度を1400~1700℃に設定した。1300℃から到達温度までの任意の80℃の区間の最大及び最小の平均加熱速度は、以下の表1に示す条件とした。なお、「到達温度」及び「平均加熱速度」における温度は、耐火物炉壁に埋め込んだ熱電対計測値からの推算される溶湯温度を用いた。
還元処理後のスラグ組成「(CaO)/{(SiO2)×(Al2O3)}」(表中はC/S・Aと記載)は、0.01~0.25に設定した。
還元処理後のスラグ中のAl2O3濃度は、4.5~30質量%に設定した。
撹拌動力密度は、0.00~1.5kW/トンに設定した。なお、撹拌動力密度は、前述の(3)式の撹拌ガス吹き込みプラグごとの撹拌動力密度の合計値である。また、「0kW/トン」とは、攪拌ガス(底吹きガス)を吹き込まない条件を意味する。
以上の条件にて実験を行い、クロム還元性及びスラグ発生量の2つの項目で評価した。
クロム還元性の良否判定の指標としては、「還元処理後のCr2O3/Cr(質量%比)」を用いた。これは、還元処理後の溶鉄中のCr濃度に対する、還元処理後のスラグ中のCr2O3濃度を算出した値であり、この値が小さいほど、効率的に還元処理ができていることを意味している。ここでは、0.5以下であれば、クロムを効率良く回収できたものとみなした。評価結果は、0.05未満をAA、0.05以上0.2未満をA、0.2以上0.5以下をB、0.5を超えたものをCと評価した。
電気炉での還元処理後のスラグ発生量と、後工程の転炉での精錬後のスラグ発生量との合計スラグ量を、スラグ発生量の評価指標とした。かかる評価は、比較例1を基準(100%)として、合計スラグ量が基準より85%未満に少なくなった場合にはA、基準より85%以上95%以下の場合にはB、比較例1と同等(±5%未満)あるいは悪化した場合にはCと評価した。
比較例4では、相対的に金属シリコンの量が多かったため、還元反応は効率良く十分に行われたが、溶鉄中のSi濃度が高くなり、後工程の転炉での精錬後のスラグ発生量が増加してしまった。
表1の実施例1及び比較例1に記載のチャージと同様の条件にて操業を繰り返し行い、1か月間、電気炉を操業させた。以下の表2に示す実施例及び比較例について、上記の操業条件で電気炉を操業し、耐火物炉壁の損耗状態及び生産性を評価した。なお、表1に示した実施例及び比較例では、耐火物炉壁表面の最高温度は1200~2000℃、耐火物炉壁表面の最大熱流束は20~150Mcal/m2/hrであった。
100~200ch操業後の最大損耗量から算出した1チャージあたりの損耗量から算出される損耗速度により評価
A:損耗速度1.5mm/ch未満
B:損耗速度1.5mm/ch以上
1チャージあたりの金属原料の溶解時間により評価
AA:通電開始から通電終わりまでの時間が75分未満
A:通電開始から通電終わりまでの時間が75分以上90分未満
B:通電開始から通電終わりまでの時間が90分以上
A:比較例1を基準とし、増産代が5%以上改善
B:比較例1を基準とし、増産代が5%未満
比較例1を基準(100%)として、合計スラグ量が基準より85%未満に少なくなった場合にはAA、基準より85%以上95%以下の場合にはA、比較例1と同等(±5%未満)あるいは悪化した場合にはBと評価した。
実施例33では、各チャージの最高到達温度は1000℃以上1800℃以下に収まっており、最大熱流束は150Mcal/m2/hr以下に収まっていた。このように電気炉を操業した結果、耐火物炉壁の損耗、1チャージの生産性、および1ヶ月の生産性はいずれも良好であり、その結果、1ヶ月でのスラグ発生量を実施例1よりも低減できた。
一方、比較例11~13では、最大の平均加熱速度が15.0℃/分を超えており、スラグ量としては比較例1と同等または悪化した。
11 炉中心
12 炉底
13 撹拌ガス吹き込みプラグ
14 耐火物炉壁
15 電極
17 炉本体
30 温度測定部
31、33、35 熱電対
40 制御装置
Claims (7)
- 金属Siを含むフェロクロム及びフェロシリコンの少なくとも一方の金属原料と酸化精錬で発生したCr酸化物を含む未還元スラグとを含む装入物を、金属Si量/Cr酸化物量で定義される質量比が0.30~0.40、かつC濃度が2.0質量%以上飽和濃度以下となる配合として電気炉に装入し、
前記電気炉で前記装入物を加熱溶解する際に、1400~1700℃を到達温度として、1300℃から前記到達温度までの任意の80℃の区間の最大の平均加熱速度を15.0℃/分以下、かつ1300℃から前記到達温度までの任意の80℃の区間の最小の平均加熱速度を3.0℃/分以上として前記Cr酸化物が還元処理されたCrを含む溶鉄を製造する、
含クロム溶鉄の製造方法。 - 前記装入物には、炭素源、珪素源、CaO源及びAl2O3源が含まれており、前記装入物のうち前記金属原料以外は副原料であり、
篩目25mmオーバーの前記副原料の含有量が前記装入物全体に対して5質量%以上30質量%以下、かつ、篩目3.15mmアンダーの前記副原料の含有量が前記装入物全体に対して3.0質量%以上であり、
前記還元処理後の溶鉄中のC濃度及びSi濃度が下記(1)式の条件を満たし、かつ、前記還元処理後のスラグ中のCaO濃度とSiO2濃度とAl2O3濃度との関係が下記(2)式の条件を満たすように前記電気炉に前記装入物を装入する請求項1に記載の含クロム溶鉄の製造方法。
[C]≧-29.4+0.015×(T+273)-0.003×(T+273)×log[Si] ・・・(1)
0.04≦(CaO)/{(SiO2)×(Al2O3)}≦0.20 ・・・(2)
ここで、[C]と[Si]はそれぞれ還元処理後の溶鉄中のC濃度(質量%)とSi濃度(質量%)、(CaO)と(SiO2)と(Al2O3)はそれぞれ還元処理後のスラグ中のCaO濃度(質量%)とSiO2濃度(質量%)とAl2O3濃度(質量%)、Tは前記到達温度(℃)を表す。 - 還元処理後のスラグ中のフッ素濃度がCaF2換算で0.5質量%以下、かつAl2O3濃度が5.0質量%以上30.0質量%以下となるようにする請求項2に記載の含クロム溶鉄の製造方法。
- 前記電気炉の炉底に、湯面面積1m2当たり0.12個以上の撹拌ガス吹き込みプラグが配置されており、隣接する前記撹拌ガス吹き込みプラグの中心間距離をL、前記炉底から湯面までの溶湯深さをHとすると、L/Hが0.50以上となるようにする請求項1~請求項3のいずれか1項に記載の含クロム溶鉄の製造方法。
- 前記電気炉は3本の電極を有し、
前記電気炉を中心軸方向から平面視して、前記3本の電極の各中心を頂点とする三角形の重心に炉中心が位置し、さらに、前記電気炉を前記中心軸方向から平面視して、前記炉中心から前記3本の各電極の中心を通り炉壁まで延びる仮想線を中心線とし該電極の直径を幅とするバンド領域を想定し、前記バンド領域を除く炉底領域に前記撹拌ガス吹き込みプラグが位置する請求項1~請求項4のいずれか1項に記載の含クロム溶鉄の製造方法。 - 前記電気炉の操業時の撹拌動力密度を、0.01kW/トン以上1.0kW/トン以下にする請求項1~請求項5のいずれか1項に記載の含クロム溶鉄の製造方法。
- 1チャージにおける前記電気炉の耐火物炉壁の表面温度の最高到達温度を1000℃以上1800℃以下とし、かつ、前記耐火物炉壁の表面温度が1000℃以上1800℃以下の範囲では、前記耐火物炉壁表面から炉本体内部への熱流束が150Mcal/m2/hr以下となるようにして、前記電気炉に装入された前記装入物を溶解する請求項1~請求項6のいずれか1項に記載の含クロム溶鉄の製造方法。
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