Preheating method for cold iron source

Abstract

We provide a technology that utilizes the heat energy of high-temperature slag to preheat a cold iron source. This is a method for preheating a cold iron source in which part or all of the total amount of cold iron source Wt (kg) is introduced into slag that remains partially or completely in a reaction vessel, and the ratio Wsc / Wsl(-) of the amount of slag remaining in the reaction vessel Wsl (kg) to the amount of cold iron source Wsc (kg) introduced into the reaction vessel using a scrap chute is controlled within a range that satisfies the following formula (1) or (2), which uses the temperature Tf (°C) of the hot material remaining in the furnace during the process with the slag remaining. (1) Formula 5.224×10 -7 ×Tf 2 -1.779×10 -4 ×Tf-0.4321 <Wsc / Wsl≦8.64×10 -7 ×Tf 1.947 Equation (2) Wsc / Wsl≧6.591×10 -6 ×Tf 1.695

Description

[Technical Field]

[0001] The present invention relates to a method for preheating a cold iron source by using the heat of the slag by adding the cold iron source into slag remaining in a reaction vessel such as a converter.

[0002] In this specification, the expression "x~y" means "greater than or equal to x and less than or equal to y" and includes boundary values. Furthermore, chemical formulas such as "SiO2" represent compounds of the specified composition, while notations such as "iron" and "phosphorus" indicate the inclusion of the element regardless of its form. T.Fe means total iron, representing the total amount of iron regardless of its form. M.Fe means metallic iron. The notation [M] indicates that element M is contained in molten iron. (R) indicates that a compound with the chemical formula R is contained in slag. "Moltane" refers to a molten metal primarily containing iron, and includes "molten pig iron" containing approximately 3-5 mass% C and "molten steel" containing approximately 2 mass% or less C. The unit of mass, "t," represents 1000 kg. [Background technology]

[0003] In response to the recent growing need to reduce CO2 emissions, there is a demand for an increase in the amount of cold iron used in the steelmaking process. By charging the cold iron into a reactor vessel in addition to the molten iron tapped from the blast furnace, it is possible to reduce the ratio of molten iron to the molten iron being filled and refined in the reactor vessel (hereinafter referred to as the molten iron blending ratio). As a result, it is possible to reduce the amount of molten iron used per unit of crude steel produced.

[0004] There are various types of cold iron sources. For example, iron scrap is stored in a yard, and reduced iron to be fed into the furnace is stored in an underground bunker and is generally kept at room temperature. These cold iron sources are charged into the reactor vessel through a scrap chute or a hopper above the furnace. To completely dissolve the charged cold iron source in the molten iron, the cold iron source must be heated and the heat required to melt it must be supplied from another source.

[0005] Therefore, if more cold iron is used, the heat that cannot be compensated for by the heat of the molten iron itself or the heat of combustion of impurities must be compensated for by adding a heat raising material or by a forced heat application means from outside. Examples of heat raising materials include graphite, ferrosilicon, and silicon carbide. Examples of heat application means include a burner or arc discharge.

[0006] In the reactor, slag is added to the molten iron, and impurities are removed by the slag that forms. This slag contains a large amount of heat, reaching temperatures of 1300-1400°C for dephosphorization and 1600-1700°C for decarburization. Typically, the slag after processing is solidified by adding a coolant such as dolomite and left in the reactor, with the remaining slag being discharged. This is because if the slag is not solidified, the iron oxide in the slag will react rapidly with the carbon in the molten iron being charged, causing the slag to bump.

[0007] However, if the heat of the slag itself, which is usually not utilized effectively because it is discharged outside the vessel, can be used to preheat the scrap, the amount of heat required to heat and melt the cold iron source can be reduced, and the molten iron blending ratio can be reduced even more than when the cold iron source is at room temperature.

[0008] Therefore, several methods have been disclosed that can preheat the cold iron source using the residual slag remaining in the reaction vessel while preventing this bumping. For example, Patent Document 1 discloses a technique in which steel is tapped while leaving the decarburized slag generated by the decarburization treatment in the reaction vessel, and a solid iron source is charged into the remaining slag. When charging molten iron after charging the solid iron source, Wsl / Wsc·(2-N -2 A method for charging molten iron under the condition that satisfies the following equation is shown. In the formula, Wsc is the amount of solid iron charged (t / ch), Wsl is the amount of decarburized slag remaining (t / ch), and N is the number of times the refining vessel is tilted back and forth.

[0009] Furthermore, Patent Document 2 discloses a method in which a solid iron source is added to the residue in the converter after steel has been tapped, such as slag or molten iron, to solidify the remaining molten iron.

[0010] The technology disclosed in Patent Document 1 recycles decarburization slag, while the technology disclosed in Patent Document 2 prevents the outflow of molten iron remaining in the converter during slag removal. Although their purposes are somewhat different, they can both be applied to preheating a cold iron source using slag in a reaction vessel. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-256839 [Patent Document 2] Japanese Patent Application Laid-Open No. 2009-228102 Summary of the Invention [Problem to be solved by the invention]

[0012] However, the above-mentioned conventional techniques have the following problems to be solved. That is, the techniques described in Patent Documents 1 and 2 have a problem in that they are unable to deal with the bumping of slag that can occur when a cold iron source containing moisture gets into the high-temperature slag in the reaction vessel due to the difference in specific gravity. Possible examples of a cold iron source containing moisture include water that gets into depressions in plate-shaped scrap, which can unavoidably occur during rainy weather, and wet powdery scrap.

[0013] For ease of transportation, cold steel sources are often stored in buildings that are usually covered but not airtight. In such storage areas, it is impossible to prevent the cold steel sources from getting wet due to wind and rain, so to remove moisture, the cold steel sources must be preheated in the cold steel source storage area. This requires the introduction of equipment for preheating, and goes against the purpose of utilizing unused heat.

[0014] Furthermore, the above-mentioned prior art also has problems as a measure to prevent reaction with molten iron. The amount of solid iron source charge Wsc in the formula shown in Patent Document 1 is a value determined based on a production plan and is an uncontrolled parameter, assuming that production volume is ensured. This means that either the amount of residual slag Wsl can be reduced or the number of reciprocating tilting movements N can be increased. The former approach fails to achieve the purpose of preheating the cold iron source because the total heat content of the slag is reduced, while the latter approach reduces productivity by increasing non-steelmaking time.

[0015] In Patent Document 2, the solid iron source is added to the reactor for the purpose of solidifying the remaining molten iron in the reactor, and the slag is discharged from the furnace in a molten state. Therefore, the method described in Patent Document 2 does not achieve the purpose of preheating the cold iron source with the remaining slag. Although it is described that the solid iron source can be retained in the molten slag by reducing the size of the solid iron source to less than 1 mm, it is difficult to secure a sufficient amount of such a solid iron source.

[0016] The present invention has been made to solve the above problems, and its purpose is to propose conditions under which the heat of the high-temperature slag remaining in the reaction vessel can be used to preheat the cold iron source in a safe and stable manner without bumping of the slag, without loss of production time. [Means for solving the problem]

[0017] In consideration of these problems, the inventors have conducted extensive research and have found that the conditions under which slag bumping does not occur can be determined by the ratio of the amount of slag remaining in the reaction vessel to the amount of cold iron source charged into the reaction vessel and the temperature of the remaining high-temperature material in the furnace in the process in which slag remains.

[0018] The method for preheating a cold iron source according to the present invention, which advantageously solves the above-mentioned problems, involves discharging, from a reaction vessel, molten iron refined with slag formed by adding it to the molten iron filled in the reaction vessel, and then introducing part or all of a total amount of cold iron source Wt (kg) into the slag that remains partly or entirely in the reaction vessel, and is characterized in that the ratio Wsc / Wsl(-) of the amount of slag remaining in the reaction vessel Wsl (kg) to the amount of cold iron source Wsc (kg) introduced into the reaction vessel using a scrap chute is controlled within a range that satisfies the following equation (1) or (2), which uses the temperature Tf (°C) of the hot material remaining in the furnace during the process in which the slag remains: (1) Formula 5.224×10 -7 ×Tf 2 -1.779×10 -4 ×Tf-0.4321 <Wsc / Wsl≦8.64×10 -7 ×Tf 1.947 Equation (2) Wsc / Wsl≧6.591×10 -6 ×Tf 1.695

[0019] The method for preheating a cold iron source according to the present invention includes the steps of: (a) disposing a cold iron source that is likely to contain moisture, including a powdered cold iron source and a cold iron source having voids, at a position X in the longitudinal direction of the scrap chute within a range that satisfies the following formulas (3) and (4) with respect to the ranges of the formulas (1) and (2), respectively; (b) when the amount of cold iron source Wsc charged using the scrap chute is a part of the total amount of cold iron source Wt, after the cold iron source is charged from the scrap chute, the remaining cold iron source is charged from an upper furnace hopper; This would be a more preferable solution to the problem. Equation (3) X≧(1.226×10 -5 ×Tf 2 -2.589×10 -2 ×Tf+17.75)×(Wsl / Wsc) (4) Formula X≧8.618×10-5 ×Tf 1.947 ×(Wsl / Wsc) Here, X (%) is the scrap loading position in the longitudinal direction of the scrap chute expressed as a percentage, with the tip of the scrap chute on the side of the reaction vessel being 0% and the end of the scrap chute on the side opposite to the reaction vessel being 100%. [Effects of the Invention]

[0020] According to the present invention, the heat of the high-temperature slag remaining in the reaction vessel can be utilized to preheat the cold iron source, and the cold iron source can be preheated safely and stably without any machine loss, without any bumping of the slag. [Brief explanation of the drawings]

[0021] [Figure 1] 1 is a schematic conceptual diagram showing a suitable equipment configuration for application to a method for preheating a cold iron source according to the present invention. [Figure 2] 1 is a graph showing the results of a slag bumping confirmation test in which a cold iron source wetted with water is added to slag. [Figure 3] 1 is a graph showing the results of a molten iron bumping confirmation test in which a cold iron source wetted with water is poured into molten iron. [Figure 4] FIG. 2 is a schematic cross-sectional side view showing how a cold iron source is loaded into a scrap chute. [Figure 5] 3 is a graph showing the influence of the position X of the water-containing cold iron source on the scrap chute on slag bumping under the condition that the total amount of the cold iron source corresponds to region B2 in FIG. 2. [Figure 6] 10 is a graph showing the influence of the initial temperature of the remaining high-temperature materials in the furnace on the threshold for determining whether or not slag bumping occurs, in relation to the loading position X of the cold iron source containing moisture in the scrap chute and the mass ratio of the input cold iron source to the remaining slag. [Figure 7] 1 is a graph showing the effect of the relationship between the minimum molten iron temperature and the C concentration in the molten iron on the clogging of the bottom blowing outlet. DETAILED DESCRIPTION OF THE INVENTION

[0022] The following describes in detail embodiments of the present invention. Note that the drawings are schematic and may differ from the actual embodiments. Furthermore, the following embodiments exemplify devices and methods for embodying the technical concept of the present invention, and are not intended to limit the configuration to those described below. In other words, the technical concept of the present invention can be modified in various ways within the technical scope described in the claims.

[0023] FIG. 1 shows an apparatus configuration suitable for carrying out the cold iron source preheating method of the present invention. A reaction vessel 1, such as a converter lined with refractory material 2, is tilted, and molten iron is discharged from the reaction vessel 1 through a tap hole 3. Then, as shown in FIG. 1(a), the reaction vessel 1 is tilted toward a scrap chute 6 loaded with a cold iron source 5, while leaving some or all of the slag 4 remaining in the reaction vessel 1. The tip of the scrap chute 6 is inserted into the reaction vessel 1. Then, a crane 7 is used to tilt the scrap chute 6, and the cold iron source 5 is charged into the reaction vessel 1. There may be approximately one ton of pre-processed molten iron, or so-called residual molten iron 8, in the vessel.

[0024] After the cold iron source 5 is charged, the reaction vessel 1 is tilted again toward the crane 7 as shown in FIG. 1(b). At the same time, the molten iron ladle 9 is tilted to charge molten iron 10 into the reaction vessel 1. After the molten iron 10 is charged, the reaction vessel 1 is turned upright as shown in FIG. 1(c). Then, oxygen gas 12 is sprayed from an oxygen supply lance 11 onto the molten iron 10 or slag 4, while the molten iron 10 is stirred with gas blown from a bottom gas system 13. The process then moves to the refining process, in which impurities are oxidized and removed. Before or during the refining process, a furnace charge material 15 may be added from an upper furnace hopper 14.

[0025] The material 15 to be charged onto the furnace may be a slag-forming material such as lime or a cooling material such as ore, or may also be a source of cooled iron on the furnace that is large enough to be wound up into a hopper on the furnace.

[0026] Although the composition of the slag 4 is not particularly specified, it is generally a steelmaking slag whose main components are CaO, SiO2, and FetO, and which also contains Al2O3, MgO, PO5, MnO, S, and M.Fe (metallic iron). Here, FetO represents iron oxide, including FeO and Fe2O3. The basicity C / S, which is the mass ratio of CaO to SiO2 in the slag, is often 0.5 to 4.5, and the FeO concentration in the slag is often 3 to 40 mass%. A slag formation promoter such as TiO2 may be added to the slag 4 as a furnace-charged material 15 to lower the melting point of the slag.

[0027] The refractory 2 lining the reaction vessel is made of a material that has sufficient corrosion resistance against the slag 4. MgO-C bricks are generally used. The treatment temperature may be adjusted to prevent the refractory from dissolving into the slag, and an MgO source such as lightly burned dolomite may be added as a furnace charge material 15 depending on the composition of the slag 4.

[0028] The cold iron source 5 may be carbon steel scrap, pig iron scrap, or granulated pig iron or reduced iron, which can be wound up into the furnace hopper 14, and is loaded into the scrap chute 6. Examples of carbon steel scrap include heavy scrap, press scrap, shredder scrap, new scrap, and steel turnings. Examples of pig iron scrap include old pig iron and pig iron turnings. Powdery scrap and scrap with voids, such as turnings, pipes, motors, and press scrap, are prone to contain moisture.

[0029] After the cold iron source 5 is charged, the reaction vessel 1 may be tilted back and forth to mix the cold iron source 5 with the slag 4. While tilting the reaction vessel 1 back and forth is not necessarily required, it is more preferable to perform tilting back and forth from the viewpoint of dispersing the cold iron source 5 and promoting solidification of the slag 4.

[0030] In this embodiment, the amount of cold iron source 5 charged from the scrap chute is determined based on the temperature of the residual high-temperature material in the furnace from the process that left slag behind, for example, the final temperature of the residual molten iron 8 and the amount of residual slag. As a result, the amount of cold iron source charged from the scrap chute 6 may be less than the total amount of cold iron source required. In this case, after the cold iron source 5 has been charged from the scrap chute 6, the reaction vessel 1 is turned upright, and the furnace cold iron source is charged from the furnace hopper 14 as the furnace charge material 15. The furnace cold iron source refers to a cold iron source that can be lifted up into the furnace hopper 14, such as granulated pig iron or reduced iron.

[0031] The cold iron source 5 can be introduced into the furnace after the scrap chute, regardless of whether it is before or after the start of the refining process. However, if the molten iron temperature is low, as it is before the start of refining, the molten iron temperature drops significantly in the early stages of refining due to the heat absorption associated with the melting of the cold iron source. Therefore, when the molten iron temperature drops to a temperature approximately 60°C above the solidification temperature determined from the C-Fe binary phase diagram, the gas outlet of the bottom-blown gas system 13 becomes clogged due to the solidification of the molten iron or the growth of the solidified phase of the cold iron source. In such cases, ineffective agitation of the bath can lead to adverse effects, such as slag overoxidation, reduced iron yield, and poor refining performance. In addition, gas can escape through gaps in the furnace wall via refractory joints, leading to equipment problems such as refractory detachment. Therefore, it is preferable to introduce the cold iron source into the furnace after the start of the refining process, when the molten iron temperature can be increased by the heat generated by the combustion of impurities using oxygen gas.

[0032] When supplying stirring gas from the bottom-blowing gas system 13, a certain amount of gas continues to flow as a standby flow rate even before the refining process to prevent nozzle clogging. The standby flow rate is determined based on the amount of residual molten iron 8 and slag 4 and the diameter of the gas outlet port of the bottom-blowing gas system 13, and is determined so that the gas discharge pressure does not fall below the static pressure. The type of gas injected from the bottom-blowing gas system 13 is generally an inert gas, such as N2 or Ar.

[0033] Next, a preferred embodiment of the present invention will be described in detail, including the background. (First embodiment) <Slag bumping confirmation test> The first embodiment was developed in response to the need to clearly and quantitatively demonstrate the conditions under which slag bumping occurs, as testing within the scope of the examples in Patent Document 1 did not consistently suppress slag bumping. A CaO-SiO2-FetO ternary slag was melted in a small-scale high-frequency melting furnace meeting the structural requirements of Figure 1, and the slag temperature was adjusted. After that, the melting furnace was turned off, and a cold iron source immersed in water was simultaneously poured into the molten slag to check for slag bumping. Then, using a ladle made from a processed graphite crucible, C-saturated molten iron at 1200-1300°C, which had been melted in a separate furnace, was scooped and charged into the furnace above the slag.

[0034] The slag composition was 20–50 mass% CaO, 10–40 mass% SiO2, and 10–40 mass% FeO. The slag temperature Tf' was 1200–1700°C, and the ratio Wsc' / Wsl' (-) of the amount of cold iron source charged (Wsc' (kg)) to the amount of melted slag (Wsl' (kg)) was in the range of 0.1–50. The cold iron source was iron flakes with lengths and widths ranging from 3–15 mm, thicknesses ranging from 3–7 mm, and C contents of 30 mass ppm or less and O contents of 150 mass ppm or less. The cold iron source was immersed in room-temperature water for approximately 1 minute. The cold iron source was then lightly drained immediately before being introduced into the melting furnace.

[0035] The results of a slag bumping confirmation test using slag and a water-wet cold iron source are shown in Figure 2. The symbols "●" in Figure 2 plot conditions under which slag bumping did not occur, and the symbols "x" plot conditions under which slag bumping occurred. From Figure 2, the inventors discovered that regions A and C where slag bumping occurred and regions B1 and B2 where it did not occur can be organized by the ratio Wsc' / Wsl'(-), the ratio of the amount of cold iron source charged Wsc' (kg) to the amount of melted slag Wsl' (kg), and the slag temperature Tf'.

[0036] In the slag bumping region A, where the mass ratio of the cold iron source to the residual slag, Wsc' / Wsl', is low, the slag does not bump immediately after the cold iron source is charged, or even after a while. This region is characterized by the gas generated when the carbon-saturated molten iron is subsequently charged, causing the slag to bump. It was found that the region B1, where bumping does not occur, can be approximated by the following equation (1A). (1A) formula Wsc' / Wsl'>5.224×10 -7 ×Tf 2 -1.779×10 -4 ×Tf-0.4321

[0037] However, if the mass ratio Wsc' / Wsl' of the cold iron source to the residual slag is increased beyond the value that can be calculated using the right-hand side of equation (1A), bumping will occur, causing the slag to bubble up to the top of the melting furnace immediately after the cold iron source is added, or shortly thereafter.It has been found that region C, where bumping begins to occur, can be approximated by the following equation (1B). (1B) formula Wsc' / Wsl'>8.64×10 -7 ×Tf 1.947

[0038] Furthermore, if the mass ratio of the cold iron source to the residual slag, Wsc' / Wsl', is increased, the slag bumping no longer occurs. It was found that this region B2 can be approximated by the following equation (2). Equation (2) Wsc' / Wsl'≧6.591×10 -6 ×Tf 1.695

[0039] The distinction between slag bumping regions A and C and regions B1 and B2 where slag bumping does not occur was also found to be independent of slag composition, at least within the range of the test. The theory behind the thresholds for slag bumping regions A and C and regions B1 and B2 where slag bumping does not occur is unclear. The inventors believe that in bumping region A, where the Wsc' / Wsl' ratio is lower, the slag does not solidify sufficiently due to a small amount of cold iron source charged relative to the slag volume, resulting in a rapid reaction between the C contained in the charged molten iron and the FeO in the slag. In other words, the reaction [C] + (FeO) = CO↑ + Fe is presumed to be a liquid-liquid reaction. Furthermore, in bumping region C, where the mass ratio of cold iron source to residual slag, Wsc' / Wsl', is higher, the slag sometimes appeared to be exploding upward. Based on this, we presume that the moisture in the scrap was rapidly vaporized by the high-temperature slag, causing a steam explosion.

[0040] <Molten iron bumping confirmation test> In addition to slag, residual steel from pretreatment may also be present in the reactor. Therefore, we also conducted a test to confirm whether or not molten iron would bump when a water-wetted cold iron source was added to the remaining molten iron in the vessel. The results of the molten iron bumping confirmation test are shown in Figure 3. The symbol "●" in Figure 3 plots conditions under which molten iron did not bump, and the symbol "×" plots conditions under which molten iron bumping occurred. Molten iron was melted in a small high-frequency melting furnace and its temperature was adjusted. After the furnace power was turned off, a cold iron source immersed in water was added to the molten iron. A test to confirm whether or not molten iron would bump was then conducted. The molten iron temperature, Tf'', was 1200°C to 1700°C, and the ratio, Wsc'' / Wrm''(-), of the amount of cold iron source added, Wsc'' (kg), to the amount of molten iron melted, Wrm'' (kg), was in the range of 0.1 to 3.0. The cold iron source used in the slag bumping confirmation test was the same as that used in the test described above, and the test was conducted in the same manner.

[0041] As shown in Figure 3, the inventors discovered that, similar to slag, the region D where molten iron bumps and the region E where it does not bump can be organized by the molten iron temperature Tf'' and the ratio Wsc'' / Wrm''(-) of the amount of cold iron source Wsc'' (kg) added to the amount of molten iron Wrm'' (kg).

[0042] In region D, where molten iron bumps, bumping was observed immediately after the cold iron source was added or shortly thereafter. The range of the mass ratio of the cold iron source to the residual molten iron is significantly narrower than regions A and C in the slag bumping confirmation test. Furthermore, in actual operation, the residual molten iron in the reactor is approximately 1 ton, while the slag amount exceeds 10 to 20 tonnes. Therefore, if the amount of cold iron source is sufficient to satisfy the lower limit of equation (1A) in Figure 2, the mass ratio Wsc'' / Wrm'' of the cold iron source to the residual molten iron will automatically avoid region D in Figure 3. Therefore, even if both slag and residual molten iron are actually present in the reactor, there is no problem as long as equations (1)' and (2)' below, which represent regions B1 and B2 that prevent slag bumping, are satisfied. (1)' expression 5.224×10 -7 ×Tf' 2 -1.779×10 -4 ×Tf'-0.4321 <Wsc’ / Wsl’≦8.64×10 -7 ×Tf' 1.947 (2)' expression Wsc' / Wsl'≧6.591×10 -6 ×Tf' 1.695

[0043] (Second embodiment) The second embodiment was developed to address the need to prevent slag bumping without problems, even when a powdery or porous cold iron source, which is particularly prone to moisture content, is used. A small amount of molten iron and a CaO-SiO2-FetO ternary slag were melted in a small-scale high-frequency melting furnace satisfying the structural requirements of Figure 1, and the molten iron and slag temperatures were adjusted. The melting furnace was then turned off, and the cold iron source immersed in water was simultaneously poured into the molten slag to check for slag bumping. Next, carbon-saturated molten iron at 1200-1300°C, which had been melted in a separate furnace, was loaded into the furnace on top of the slag using a ladle made from a modified graphite crucible.

[0044] The slag composition was 20–50 mass% CaO, 10–40 mass% SiO2, and 10–40 mass% FeO. The slag and metal temperatures, Tf''', were 1200–1700°C. The ratio of the amount of cold iron source charged, Wsc''' (kg), to the amount of melted slag, Wsl''' (kg), Wsc''' / Wsl'''(-), was in the range of 0.1–50. The ratio of the amount of cold iron source charged, Wsc''' (kg), to the amount of melted molten iron, Wrm''' (kg), Wsc''' / Wrm'''(-), was in the range of 0.1–3. The cold iron sources used were iron flakes 5A with lengths and widths in the range of 3 to 15 mm, thicknesses in the range of 3 to 7 mm, C concentrations of 30 massppm or less, and O concentrations of 150 massppm or less, as well as pure iron pieces 5B cut to 1 cm on each side with multiple indentations of about 2 mm deep drilled on each side.These pieces were immersed in water at room temperature for about 1 minute and then lightly drained immediately before being placed in the melting furnace.

[0045] In the slag bumping confirmation test described above, when a cold iron source that easily contains moisture was used, slag erupted, likely due to a steam explosion, even when the total amount of cold iron source was within region B2 in Figure 2, where slag bumping does not occur. After extensive analysis of the cause, the inventors determined that the use of a cold iron source that easily contains moisture did not cause a change in region C of slag bumping. In other words, when the cold iron source was added continuously, rather than all at once, the mass ratio Wsc''' / Wsl''' of the added scrap to the residual slag entered region C, where slag bumping occurs, as it increased from region B1 in Figure 2, where slag did not occur. The inventors determined that the addition of a cold iron source that easily contains moisture at that time caused slag to erupt before the mass ratio Wsc''' / Wsl''' reached region B1, even after the cold iron source had finished being added.

[0046] For this verification, as shown in Figure 4, the position X (%) at which the cold iron source (pure iron pieces 5B), which is particularly prone to containing moisture, was loaded was variously changed within the range of the longitudinal length of the scrap chute 6, where the position 6A at the tip of the chute was set to 0 (%) and the position 6B at the end of the chute was set to 100 (%). Then, the verification was carried out using the same procedure as in the slag bumping confirmation test.

[0047] As an example, Figure 5 shows a test in which the mass ratio of scrap to residual slag Wsc''' / Wsl''' when all cold iron sources were added was within the slag bumping-free region B2 shown at the top of Figure 2. The slag and molten iron temperatures, Tf''', were set to 1600°C, and the presence or absence of slag bumping was plotted in Figure 5. The symbols "●" in Figure 5 indicate conditions where slag bumping did not occur, and the symbols "×" indicate conditions where slag bumping occurred. In particular, when cold iron sources 5, which tend to contain moisture, are used in combination, even if the mass ratio of scrap to residual slag Wsc''' / Wsl''' when all cold iron sources were added was within the slag bumping-free region B2, slag bumping may or may not occur. It was found that this region can be determined by the position X at which the cold iron sources, which tend to contain moisture, were loaded and the mass ratio of scrap to residual slag Wsc''' / Wsl'''.

[0048] Figure 6 shows the slag bumping threshold as a function of position X at various temperatures and the mass ratio Wsc''' / Wsl''' of the scrap charged to the residual slag when all the cold iron sources are charged. Here, the slag and molten iron temperatures Tf''' were set to a range of 1200-1700°C. Furthermore, the mass ratio Wsc''' / Wsl''' of the scrap charged to the residual slag when all the cold iron sources are charged was set to fall within the upper and lower regions B1 and B2 in Figure 2 where slag bumping does not occur. From the results in Figure 6, it was found that these thresholds can be approximated by the following equations. (A) When the upper slag bumping area B2 in Figure 2 does not occur (2)'' expression Wsc''' / Wsl'''≧6.591×10 -6 ×Tf''' 1.695 (4)' expression X≧8.618×10 -5 ×Tf''' 1.947 ×(Wsl''' / Wsc'') (A) When the lower slag bumping area B1 in Figure 2 does not occur (1)'' expression 5.224×10 -7 ×Tf''' 2 -1.779×10-4 ×Tf'''-0.4321 <Wsc’’’ / Wsl’’’≦8.64×10 -7 ×Tf''' 1.947 (3)' expression X≧(1.226×10 -5 ×Tf 2 -2.589×10 -2 ×Tf+17.75)×(Wsl''' / Wsc''')

[0049] The slag temperature Tf', the molten iron temperature Tf'', and the slag and molten iron temperature Tf''' are synonymous in that they are the temperatures of the high-temperature objects that come into contact with water, and so these can be collectively referred to as the temperature of the high-temperature object Tf (°C). When applied to operation, it is difficult to measure the temperature of the slag remaining in the furnace each time, the slag temperature and the molten iron temperature are almost the same, and in cases where there is a long gap between the end of a process and the start of the next process, the slag temperature drops so much that it is difficult to achieve the object of the present invention. Therefore, the temperature of the high-temperature object Tf (°C) can be used as the final molten iron temperature of a process that leaves slag behind. However, if a coolant is added to the remaining slag from the scrap chute before the cold iron source is added, it is preferable to determine Tf taking into account the temperature drop expected from the cooling capacity of the coolant and its amount, or to measure the slag temperature.

[0050] Wsc', Wsc'', Wsc''', Wsl', Wsl'', Wsl''', and Wrm'' are simply separate terms used to distinguish between different test conditions. Therefore, since they all refer to the amount of cold iron source (subscript sc) fed through the scrap chute, the amount of slag remaining in the furnace (subscript sl), and the amount of molten iron remaining in the furnace (subscript rm), they are all synonymous and can be referred to as Wsc, Wsl, and Wrm, respectively.

[0051] (Third embodiment) The third embodiment was developed to address the situation where the amount of cold iron source introduced through the scrap chute is less than the total amount of cold iron source to be introduced into the process. This is because, in the present invention, the amount of cold iron source introduced through the scrap chute is determined based on the final temperature of the molten iron and the amount of residual slag in the process in which slag remains. Using a converter satisfying the configuration requirements of FIG. 1 , a test was conducted in which the cold iron source was introduced through the scrap chute in a manner that satisfied the requirements of the first and second embodiments. After a refining process in which impurities were removed using oxygen gas, the molten iron was discharged into a ladle through the converter tap hole, and the slag was carried over to the next process. The next process was also treated in the same manner. In this case, if the amount of cold iron source introduced through the scrap chute (Wsc) specified in the first embodiment was insufficient compared to the total amount of cold iron source to be introduced into the process (Wt), the shortage of cold iron source was introduced from the furnace hopper before the introduction of the cold iron source through the scrap chute.

[0052] In the above test, the incidence of slopping increased with the number of consecutive runs. The inventors investigated the cause and found that the residual molten iron and the slag that had been added to the furnace before the cold iron source was introduced from the scrap chute solidified into a slag mixture containing iron directly above the bottom-blowing gas outlet, causing the outlet to clog. The inventors determined that the resulting overoxidation of slag was the cause of slopping. As shown in Figure 7, this bottom-blowing outlet clog occurred when the minimum bath temperature (Tmin) reached during processing, Tliq (°C), was below 60°C plus the liquidus temperature (Tliq) calculated from the C-Fe binary phase diagram for the C concentration (Cc) of the molten iron being charged. In Figure 7, the symbol "◯" indicates conditions without nozzle clogging, and the symbol "×" indicates conditions with nozzle clogging.

[0053] Therefore, the inventors decided to change the timing of charging the cold iron source onto the furnace, which had previously been before charging the cold iron source from the scrap chute, to charging the cold iron source from the scrap chute after the cold iron source was charged. Specifically, first, the minimum bath temperature Tmin was calculated using the following formula using the temperature Tin (°C) of the charged molten iron and the temperature change ΔT (°C). Tmin=Tin+ΔT

[0054] The temperature change ΔT (°C) was calculated using the calculated Si concentration Csi (mass%), the hot metal blending ratio Y (%), and the charged molten iron temperature Tin using the following formula: The calculated Si concentration Csi was set to the value of the Si concentration Csi in the hot metal, taking into account the Si combustion efficiency in an actual furnace and the increase due to Si sources such as FeSi, when the value was less than 0.4 mass%, and to 0.4 mass% when it was 0.4 mass% or more. ΔT={76930×Csi-(100-Y)×2000}÷215

[0055] Next, the nozzle clogging threshold temperature Tn (°C) was calculated by adding 60°C to the liquidus temperature Tliq (°C) at the C concentration Cc (mass%) of the molten iron being charged. The maximum amount of cold iron source Wschn (kg) that could be added to the furnace without dropping below the nozzle clogging threshold temperature Tn was calculated, taking into account the cooling capacity (kcal / kg). The amount of cold iron source added to the furnace after the introduction of the cold iron source from the scrap chute and before the start of the refining process was limited to Wschn (kg). After that, the amount of cold iron source Wt that was insufficient to meet the total amount of cold iron required after the start of the refining process was added as cold iron source to the furnace.

[0056] After changing the timing of charging the cold iron source onto the furnace as described above, a continuous operation test was conducted while satisfying the requirements of embodiments 1 and 2. As a result, no nozzle clogging occurred, and it became possible to stably leave slag in the furnace. [Industrial Applicability]

[0057] The method for preheating a cold iron source according to the present invention is industrially useful because it can be applied to a blast furnace-converter process to reduce the molten iron blending ratio and contribute to reducing CO2 emissions. [Explanation of symbols]

[0058] 1 reaction vessel 2 Refractories 3 Steel tapping hole 4. Slug 5 cold iron source 5A Iron Flakes 5B Pure iron piece 6. Scrap Chute 6A (Scrap chute) tip position 6B (Scrap chute) End Position 7 Crane 8 Residual molten iron 9. Hot Metal Ladle 10 Molten Iron 11 Oxygen supply lance 12 Oxygen gas 13 Bottom blown gas system 14 Furnace hopper 15 Materials charged onto the reactor X (Loading cold iron source that is prone to moisture content) position

Claims

1. A method for preheating a cold iron source, comprising the steps of: discharging molten iron refined by slag added to and formed on molten iron charged in a reaction vessel from the reaction vessel; and then introducing a part or all of a total amount of cold iron source Wt (kg) into slag that is partly or entirely remaining in the reaction vessel; A method for preheating a cold iron source, wherein a ratio Wsc / Wsl(-) of an amount of slag remaining in the reaction vessel Wsl (kg) to an amount of cold iron source Wsc (kg) to be introduced into the reaction vessel using a scrap chute is controlled within a range that satisfies the following formula (1) or (2), which uses a temperature Tf (°C) of a hot material remaining in a furnace in a treatment in which the slag remains: (1) Formula 5.224×10 －７ ×Tf ２ -1.779×10 －４ ×Tf-0.4321<Wsc / Wsl≦8.64×10 －７ ×Tf １．９４７ (2) Formula Wsc / Wsl≧6.591×10 －６ ×Tf １．６９５

2. 2. The method for preheating a cold iron source according to claim 1, wherein a cold iron source that is likely to contain moisture and that includes a powdered cold iron source and a cold iron source having voids is disposed at position X in the longitudinal direction of the scrap chute within a range that satisfies the following formulas (3) and (4), respectively, relative to the ranges of formulas (1) and (2): (3) Formula X≧(1.226×10 －５ ×Tf ２ -2.589×10 －２ ×Tf+17.75)×(Wsl / Wsc) (4) Formula X≧8.618×10 －５ ×Tf １．９４７ ×(Wsl / Wsc) Here, X (%) is the scrap loading position in the longitudinal direction of the scrap chute expressed as a percentage, with the tip of the scrap chute on the reaction vessel side being 0% and the end of the scrap chute on the opposite side to the reaction vessel being 100%.

3. 3. The method for preheating a cold iron source according to claim 1, wherein, when an amount Wsc of the cold iron source charged using the scrap chute is a portion of the total amount Wt of the cold iron source, after the cold iron source is charged from the scrap chute, the remaining cold iron source is charged from an upper furnace hopper.

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