Method for refining molten metal for iron and steel products, including cast iron.

Abstract

The present invention provides a method for refining molten metal for steel products, including cast iron, that efficiently removes Si, Mn, or Cr from the molten metal while minimizing carbon loss. [Solution] In the method for refining molten metal for steel products containing cast iron according to the present invention, an electric furnace is used to refine the molten metal to a level higher than the solidification point of any component, T EC (=-27,486 / {log[Si / C 2 The process comprises a first step of adding iron oxide to molten metal heated to a Si oxidation temperature range of 15.47 - 273 °C or lower, a second step of stirring the molten metal with the added iron oxide for a predetermined time until the iron oxide dissolves in the molten metal, and a third step of removing slag containing Si, Mn, or Cr generated by the reaction between the molten metal and the iron oxide. The removal rate of Si, Mn, or Cr in the molten metal is determined according to the number of times the first and second steps are repeated, and the first and second steps are repeated a number of times corresponding to the removal rate that corresponds to the amount of Si, Mn, or Cr to be removed from the molten metal from the determined removal rate.

Description

[Technical Field]

[0001] The present invention relates to a method for refining molten metal for steel products, including cast iron. [Background technology]

[0002] Traditionally, cast iron has been manufactured from scraps of automotive rigid sheets, but in recent years, manganese and other elements have been added to automotive rigid sheets to increase their strength. However, when manufacturing cast iron from scraps, it is desirable to remove Si, Mn, or Cr contained in the scraps, as these elements hinder the toughness of the resulting cast iron.

[0003] Therefore, a manganese removal treatment method has been proposed in which a demanganating agent containing sulfur is added to molten cast iron containing manganese, causing manganese sulfide to float to the surface and remove manganese from the molten metal (see, for example, Patent Document 1). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2003-105420 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] However, in the above technology, in order to improve the manganese removal effect, an inert gas such as Ar, nitrogen gas, or compressed air is blown in through a porous plug (porous refractory material) at the bottom of the ladle, which requires equipment capable of blowing in gases and gas blowing work.

[0006] Furthermore, in the manufacture of castings, it is preferable for cast iron to contain a certain amount of carbon to improve its castability, but the manganese removal process also reduces the amount of carbon, which is a problem. The above technology does not take into consideration the reduction of carbon.

[0007] Therefore, the present invention aims to provide a method for refining molten metal for steel products, including cast iron, that does not require the time and equipment for oxygen blowing, and efficiently removes Si, Mn, or Cr from the molten metal while minimizing the reduction of carbon. [Means for solving the problem]

[0008] To solve the above problems, the present invention is a method for refining molten metal for steel products including cast iron, wherein the molten metal is refined in an electric furnace to a temperature higher than the solidification point of any component, T EC (=-27,486 / {log[Si / C 2 The present invention provides a method for refining molten metal, comprising: a first step of adding iron oxide to molten metal heated to a Si oxidation temperature range of ] - 15.47 - 273})℃ or lower; a second step of stirring the molten metal containing the added iron oxide for a predetermined time until at least the entire surface of the iron oxide is wet with the molten metal and the iron oxide dissolves in the molten metal; and a third step of removing, if necessary, slag containing Si, Mn, or Cr generated by the reaction of the molten metal and the iron oxide after the second step, wherein the removal rate of Si, Mn, or Cr in the molten metal is determined according to the number of times the set of the first step and the second step is repeated, and the set of the first step and the second step is repeated a number of times corresponding to the removal rate corresponding to the amount of Si, Mn, or Cr to be removed from the molten metal from the determined removal rate. [Effects of the Invention]

[0009] According to the present invention's method for refining molten metal for steel products, including cast iron, it is possible to efficiently remove Si, Mn, or Cr from the molten metal without requiring the time and equipment for oxygen blowing, while minimizing the reduction of carbon. [Modes for carrying out the invention]

[0010] Embodiments of the present invention will be described below.

[0011] The present invention relates to a method for refining molten metal for steel products containing cast iron. By means of an electric furnace, the temperature is higher than the freezing point of the molten metal with any component and T EC (=-27,486 / {log[Si / C 2 - 15.47 - 273})°C or lower, a first step of introducing iron oxide into the molten metal heated in the oxidation temperature range of Si; a second step of stirring the molten metal into which the iron oxide has been introduced for a predetermined time until at least the entire surface of the iron oxide is wetted by the molten metal and the iron oxide is dissolved in the molten metal; and a third step of removing slag containing Si, Mn or Cr generated by the reaction of the molten metal and the iron oxide after the second step as necessary. The removal rates of Si, Mn or Cr in the molten metal corresponding to the number of times the set of the first step and the second step is repeated have been determined, and the set of the first step and the second step is carried out for the number of times corresponding to the removal rate corresponding to the amount of Si, Mn or Cr to be removed from the molten metal among the determined removal rates.

[0012] As the electric furnace, an induction furnace (high-frequency furnace, medium-frequency furnace, low-frequency furnace), an arc furnace, etc. can be considered. In particular, as the electric furnace, it is preferable to have a stirring function. For example, in the case of an induction furnace, electromagnetic stirring can be performed, and in the case of an arc furnace, bubbling stirring can be considered.

[0013] "Higher than the freezing point of the molten metal with any component and T EC (=-27,486 / {log[Si / C 2 - 15.47 - 273})°C or lower" is a temperature range (oxidation temperature range of Si) set to preferentially oxidize and remove Si, Mn, Cr while suppressing the oxidation (decarburization) of C as much as possible. Specifically, the temperature range is set in consideration of "when it becomes lower than the freezing point of the molten metal with any component, stirring cannot be performed" and "when it becomes higher than T EC (=-27,486 / {log[Si / C 2 - 15.47 - 273})°C, C is preferentially oxidized rather than Si". Note that T EC is derived from the formula in Table 1 below (the units of Si and C are wt% respectively).

Table 1

[0014] The molten metal is produced from general steel scrap, but for example, high-tensile steel which is high-Mn steel may be used. In that case, "C < 0.16 wt% (including the case of =), Si 0.20 - 0.55 wt%, Mn 0.80 - 1.60 wt%, P < 0.030 wt% (including the case of =), S < 0.015 wt% (including the case of =)", "C < 0.15 wt% (including the case of =), Si < 0.55 wt% (including the case of =), Mn < 2.00 wt% (including the case of =), P < 0.030 wt% (including the case of =), S < 0.015 wt% (including the case of =)", etc., compositions with higher Mn values than other steel scraps are considered.

[0015] Iron oxide is used as an oxidizing agent, and three types, Fe3O4, Fe2O3, and FeO, are considered.

[0016] The predetermined time for stirring can be arbitrarily set. However, in the experiment, since it took at least several seconds until the entire surface of the iron oxide was wetted by the molten metal and the iron oxide dissolved in the molten metal, it is preferable that stirring is performed for a longer time. On the other hand, as will be described later, stirring significantly longer than 120 seconds is not very effective, so the upper limit of the time within the predetermined range is preferably about 120 seconds. However, it does not exclude stirring for more than 120 seconds. Since the specific gravity of iron oxide is lighter than that of the molten metal, undissolved iron oxide floats on the molten metal and can be visually confirmed. However, the "dissolution" in the present invention means that at least the entire surface of the iron oxide is wetted and dissolved to such an extent that the iron oxide cannot be visually confirmed on the molten metal.

[0017] In addition, stirring can be considered to be carried out by generating an electromagnetic stirring phenomenon in an induction furnace. Specifically, by increasing the amount of electricity supplied to the induction furnace, the magnetic field is strengthened, and accordingly, the molten metal will automatically start to rotate. However, since it is not always possible to accurately end the stirring at a determined time, it is preferable to determine the predetermined time for stirring with a margin (for example, 40 seconds ± 5 seconds, etc.). Also, in order to perform accurate removal, it is preferable to stir with approximately the same stirring force in each set. However, in the experiment, even when the stirring force was changed, there was no significant difference in the removal rate. Therefore, in the second step, "stir for a predetermined time until at least the entire surface of the iron oxide is wetted by the molten metal and the iron oxide dissolves in the molten metal", and the effects of the present invention can be obtained.

[0018] As a result of the experiment, the removal rates of Si, Mn, or Cr corresponding to the number of repetitions of the sets of the first step and the second step can be determined as shown in Tables 2 and 3. The removal rate is not limited to those shown in Tables 2 and 3, and may be determined according to the molten metal refining environment performed by the user. For example, it can be considered to be determined according to the amount of iron oxide input per time desired by the user, the type of iron oxide, the predetermined time of stirring per time, etc.

Table 2

Table 3

[0019] [[ID=ID=19]]As slag, mainly oxides such as SiO2, MnO, and Cr2O3 are generated by the reaction of Si, Mn, Cr with iron oxide and float on the surface of the molten metal. The slag can be removed using a rod or the like. Among the iron oxides, the remaining Fe will dissolve in the molten metal.

[0020] Under the above configuration, by performing a set of the first and second steps a number of times corresponding to the removal rate that corresponds to the amount of Si, Mn, or Cr to be removed from the molten metal, it becomes possible to produce molten metal from which the desired amount of Si, Mn, or Cr has been removed. In other words, the present invention allows for the design of the amount of oxidizing agent, stirring time, and number of inputs according to the refining purpose. [Examples]

[0021] The following describes embodiments of the present invention. <Example 1>

[0022] First, molten cast iron was produced by adding a total of 300 kg of scrap iron and other raw materials to an electric furnace (Toshiba high-frequency induction furnace (450kW-500Hz-500kg) (nominal upper limit (upper limit before molten metal overflows) 500kg)). When the molten metal reached approximately 1340°C, 3.9 kg (approximately 1.3 wt% of the molten metal) of Fe3O4 was added, and the mixture was stirred by generating electromagnetic stirring in the electric furnace. Specifically, the magnetic field was strengthened by increasing the current supplied to the electric furnace, causing the molten metal to automatically begin to swirl. When 3.9 kg of Fe3O4 was added, the atomic weight of Fe was 55.845 and the atomic weight of O was 16.00, so theoretically the amount of oxygen in Fe3O4 was approximately 27.6 wt%, and the oxygen mass could be calculated to be approximately 1.08 kg.

[0023] Furthermore, while typically Si is added separately from the cast iron at the beginning of the molten metal treatment process to improve fluidity and control the microstructure during casting, in this embodiment as well, an additional amount of Si, amounting to approximately 0.5 wt% of the total, was added at the start of each set during the experiment.

[0024] In this example, the addition and stirring process was carried out under the following two conditions.

[0025] Condition A: 43.9 kg of Fe3O added 6 times, stirring for 40 seconds each time (total added 23.4 kg) Condition B: 3.9 kg of Fe3O4 added 6 times, stirred for 120 seconds each time (total added 23.4 kg)

[0026] Although the temperature of the molten metal decreases after the addition of Fe3O4, heating by the electric furnace was controlled so that it did not fall below the freezing point of the molten metal (approximately 1150°C in this experimental system). However, since stirring is extremely difficult at temperatures slightly above the freezing point, it is preferable that the temperature be above the liquidus line (approximately 1200°C in this experimental system). Furthermore, since the oxidation reaction takes a long time and is inefficient at temperatures near the liquidus line, heating was controlled in this embodiment to prevent the temperature from falling below 1250°C.

[0027] In this embodiment, after the stirring of each set was completed, the slag that had floated to the surface of the molten metal was removed (for about 30-60 seconds per set). After removing the slag, it was confirmed that the molten metal had risen to a temperature higher than its freezing point before starting the stirring of the next set.

[0028] Table 2 shows the removal rates of C, Si, Mn, and Cr when the experiment was conducted under condition A. Table 3 shows the removal rates of C, Si, Mn, and Cr when the experiment was conducted under condition B. Table 4 shows a comparison of the removal rates of C, Si, Mn, and Cr when the experiment was conducted under conditions A and B. The content of each element was confirmed by countback (emission spectroscopy / spark discharge emission spectroscopy) testing. Specifically, the content before stirring in the first set was measured as the initial value, and the content after each set was measured after slag removal and compared with the initial value to calculate the removal rate after each set. [Table 4]

[0029] Tables 2-4 show that condition B (120 seconds of stirring) resulted in a higher removal rate of Si, Mn, and Cr.

[0030] Table 5 shows the removal rate data when the first and second processes (120 seconds of stirring) are performed up to eight times. In Table 5, Si, Mn, and Cr can be reduced to almost their limit by the eighth set, indicating that performing the first and second processes more than eight times is not very effective. However, "how many sets it takes to reduce Si, Mn, and Cr to almost their limit" will vary depending on the amount of iron oxide added per cycle, the type of iron oxide, the specified stirring time per cycle, etc., as requested by the user. [Table 5]

[0031] Based on the experimental results above, for example, when the first and second processes are performed with 120 seconds of stirring, Table 1 shows that approximately 78% of Si remains after the first set, approximately 54% after the second set, approximately 37% after the third set, approximately 22% after the fourth set, approximately 13% after the fifth set, and approximately 4% after the sixth set. These values ​​are then determined as the "Si removal rate corresponding to the number of times the first and second processes are repeated." In the actual molten metal refining process, by performing the first and second processes a number of times corresponding to the removal rate that corresponds to the amount of Si, Mn, and Cr to be removed from the molten metal, it becomes possible to remove Si, Mn, and Cr at the desired removal rate.

[0032] Below, we will discuss the experimental results for conditions A and B.

[0033] (1) Reasons why the difference in removal rates is small in the first set

[0034] Immediately after the first set, there was no significant difference in the Si, Mn, and Cr removal rates between condition A (40 seconds of stirring) and condition B (120 seconds of stirring). At this stage, the concentrations of each element are high, and sufficient Si, Mn, and Cr are present near Fe3O4. The chemical reaction rate at the interface is dominant (chemical reaction rate-limiting) (because the influence of elemental movement due to stirring is small). Therefore, even if the stirring time is extended from 40 seconds to 120 seconds, the difference in removal rates is hardly apparent.

[0035] (2) Reasons why the gap widens from the second set onwards, especially from the third set onwards.

[0036] As the number of sets progresses and the Si·Mn·Cr concentration decreases, it becomes necessary to supply Si·Mn·Cr from sources other than the immediate vicinity of Fe3O4. At this stage, the speed at which Si·Mn·Cr diffuses and moves to the vicinity of Fe3O4 and the overall mixing state of the molten metal greatly influence the reaction. In other words, when the Si·Mn·Cr concentration decreases, 40 seconds of stirring leaves a larger amount of Si·Mn·Cr that is not yet close to Fe3O4, while 120 seconds of stirring allows Si·Mn·Cr and Fe3O4 to move through the molten metal for a longer period, resulting in less Si·Mn·Cr remaining that is not yet close to Fe3O4. As a result, it is thought that the removal rate of Si·Mn·Cr was significantly higher with 120 seconds of stirring from the second set onwards, especially from the third set onwards.

[0037] Furthermore, oxides such as SiO2, MnO, and Cr2O3 increase in particle size as the fine particles agglomerate immediately after formation, causing them to float as slag. However, this process takes a certain amount of time. With 40 seconds of stirring, the stirring is stopped during this agglomeration and floating process, which is thought to make it easier for fine oxides to remain in the molten metal. On the other hand, with 120 seconds of stirring, sufficient time is provided for agglomeration and floating, allowing for the removal of more oxides at the time of slag removal. (3) Reasons why C is difficult to remove

[0038] Comparing the standard Gibbs free energies of each oxidation reaction, the ease of oxidation at around 1340°C is generally Si > Mn > Cr > C. Therefore, in the initial to middle sets where the concentrations of each element are high, Si, Mn, and Cr are preferentially oxidized, while C is gradually oxidized by the excess oxygen. However, for example, as shown in Table 2, if stirring for 120 seconds is continued for up to 8 sets, Si, Mn, and Cr will be almost depleted, and thereafter, the excess oxygen will almost entirely move towards C oxidation (CO production). Therefore, beyond 8 sets, C will be significantly oxidized.

[0039] (4) Positioning of CO-SiO2 equilibrium and 1340°C operating conditions

[0040] First, as mentioned above, this experiment aims to preferentially oxidize and remove Si while minimizing the oxidation (decarburization) of C. Considering that "if the temperature is higher than around 1400°C, C will be preferentially oxidized over Si" and "if the temperature is lower than around 1340°C, the viscosity of the molten metal will become too high, making stirring and slag removal difficult," the reaction temperature was set to 1340°C for the experiment.

[0041] Comparing the standard Gibbs free energies of each oxidation reaction at around 1340°C, the free energy is approximately "SiO2 ≪ CO," indicating that Si is a much more easily oxidized element than C. Therefore, the oxygen from the introduced Fe3O4 is first used to oxidize Si, Mn, and Cr (the resulting SiO2, MnO, and Cr2O3 are removed as slag).

[0042] However, for example, if Si decreases, Si / C 2 As the (variable used when calculating equilibrium temperature) decreases, the relationship SiO2 ≪ CO breaks down (the CO-SiO2 equilibrium breaks down), and concerns arise that if the same 1340°C is maintained, "we might enter a region where the oxidation of carbon becomes dominant."

[0043] However, in this experiment, Si was preferentially oxidized throughout, and the decrease in C remained below approximately 5% (3.75 → 3.57 wt%). This is thought to be due to the influence of the "actual oxygen potential" and the "behavior based on the Gibbs free energy difference in the oxidation reaction of each element," which cannot be fully explained by the CO-SiO2 equilibrium line alone.

[0044] The CO-SiO2 equilibrium is based on the premise that the oxygen potential is uniquely determined by the partial pressure ratio of CO and CO2 (CO / CO2 ratio), and it represents the condition under which Si / SiO2 and C / CO are simultaneously in equilibrium under that oxygen potential. In other words, "how easily Si is oxidized" is determined by the CO / CO2 ratio, and the Si / C ratio mentioned above is also determined by the CO / CO2 ratio. 2 The relationship between this and temperature can be considered a guideline for a state where "the ease of oxidation and reduction of the molten metal is governed by this CO / CO2 ratio."

[0045] However, in this experiment, oxygen is supplied by the reduction of Fe3O4 (Fe3O4 → FeO → Fe), so the oxygen potential fluctuates. As a result, the oxidation of each element is mainly determined not by the CO / CO2 ratio, but by the "change in the oxidation state of Fe" and the "presence of easily oxidizable elements such as Si, Mn, and Cr."

[0046] In other words, the "equilibrium temperature" obtained from the CO-SiO2 equilibrium (SiO2 + 2C = Si + 2CO) is calculated from the composition (Si / C 2Because it changes depending on the amount of Si, the equilibrium temperature decreases as the amount of Si decreases. However, in this experiment, as the oxidation state of Fe changes (Fe3O4 → FeO → Fe), even as the Si / Mn / Cr ratio decreases, the oxygen potential near the interface is kept close to the value at which the "Si / SiO2 equilibrium" is established. This means that the oxygen potential is kept in a region where "the oxidation of Si / Mn / Cr proceeds preferentially over the oxidation of C (sufficiently lower than the oxygen potential corresponding to the C / CO equilibrium)." In particular, in this experiment, since Fe3O4 is continuously added in steps, a high oxygen potential is maintained. Furthermore, due to this effect of "Si consuming the oxygen potential first," sufficient oxygen does not reach the carbon dissolved throughout the molten metal, and the decarburization reaction is limited to a very limited range. Therefore, the CO-SiO2 equilibrium line is not a boundary line that strictly defines this experimental system, but rather "Si / C 2 This corresponds to a "reference line for evaluating the relationship between temperature and oxygen," and the actual behavior can be understood as a preferential oxidation of Si / Mn / Cr based on the Gibbs free energy relationship in the oxidation reaction of each element when Fe3O4 is used as the oxygen source.

[0047] In detail, as Si is removed, the molten metal composition shifts from "high-C cast iron" to "Fe-C-small amount of Si system, closer to steel." At this point, the critical temperature calculated from the CO-SiO2 equilibrium line is (Si / C 2 As the temperature decreases, it approaches 1340°C, and from the perspective of pure CO-SiO2 equilibrium, it approaches a region where "C oxidation may become dominant if the temperature is not lowered." However, in this experiment, by continuing to add Fe3O4 stepwise even at that stage, the reduction of Fe3O4 (Fe3O4 → FeO → Fe) always supplies "enough oxygen potential to preferentially oxidize Si, Mn, and Cr" locally. In addition, as long as Si, Mn, and Cr remain, their oxidation reactions are more advantageous than the C / CO system in terms of the Gibbs free energy of each oxidation reaction, so in the actual reaction pathway, the oxidation of Si, Mn, and Cr tends to precede the oxidation of C until the very end.

[0048] The experimental results show that, despite being carried out at a constant temperature of 1340°C, Si levels decreased to below the detection limit, and approximately 97% of Mn / Cr was removed, while the decrease in C remained below 5%. This contradicts the simple view based solely on the CO-SiO2 equilibrium line, which assumes that "if Si decreases, the oxidation of C should become dominant unless the equilibrium temperature is lowered." In reality, it can be interpreted that "the oxygen potential is controlled by the oxygen supply from Fe3O4 and the strong oxidation tendency of Si / Mn / Cr, and a state in which Si is always dominant in terms of the Gibbs free energy relationships of each oxidation reaction is maintained." In other words, this process "achieves conditions for preferentially removing Si, Mn, and Cr while suppressing the oxidation of C by absorbing the oxygen potential to the Si / Mn / Cr side just before the CO-SiO2 equilibrium line, even while being carried out at 1340°C."

[0049] The molten metal refining method according to the present invention, as described above, has the following advantages compared to conventional techniques. (1) Temperature conditions and processing time (reduced processing time and increased efficiency)

[0050] Most conventional Mn / Cr removal processes using iron oxide involve adding approximately 2-6 wt% of the reagent FeO or Fe2O3 (or black scale) in a single batch at a high temperature of around 1450°C, followed by a holding time of several minutes or more. Furthermore, when using a pure oxygen burner or air blowing, Mn removal is achieved by processing for several tens of minutes to an hour. All of these methods are fundamentally based on allowing the oxidation reaction to proceed through prolonged high-temperature holding.

[0051] In contrast, the molten metal refining method according to the present invention completes the process at a relatively low temperature of 1340°C. Specifically, without significantly heating the molten cast iron above its typical holding temperature, a predetermined amount (1.3 wt% in the above experiment) of iron oxide (Fe3O4 in the above experiment) is added multiple times, and stirring is performed for a predetermined time (approximately 40 to 120 seconds in the above experiment) each time, thereby achieving the removal of Si, Mn, and Cr to a practically sufficient level. Furthermore, the molten metal refining method according to the present invention does not require heating the entire molten metal to around 1450°C and holding it for a long time. Therefore, the desired composition (Si, Mn, Cr) can be adjusted in a shorter time than conventional methods in terms of both "temperature margin" and "holding time," enabling highly efficient refining in terms of both energy and operation. (2) Agitation method and equipment configuration (labor saving, no additional equipment required)

[0052] Conventional Mn removal technologies using rotary furnaces or ladle processes employ configurations that forcibly agitate the surface and interior of the molten metal using tilting and rotating mechanisms, pure oxygen or air blowing equipment, and gas bubbling devices such as N2, thereby simultaneously increasing the specific surface area of ​​the molten metal and supplying oxygen. Furthermore, even in iron oxide-added Mn removal processes, it is assumed that the metal is dissolved in a high-frequency induction furnace, and then reagents such as FeO are added all at once and held at a predetermined temperature, and the agitation conditions and number of agitations are not considered as control parameters.

[0053] In contrast, the molten metal refining method according to the present invention utilizes only the electromagnetic stirring action inherent in the high-frequency induction furnace itself. It simply involves gradually adding a predetermined amount (1.3 wt% in the above experiment) of iron oxide (Fe3O4 in the above experiment), stirring for a predetermined time (approximately 40 to 120 seconds in the above experiment), and repeating the operation of removing the generated slag. Therefore, there is no need to add new gas injection devices, rotating mechanisms, burner equipment, etc., and it can be operated simply by combining the measurement and addition of iron oxide, power setting in the existing furnace (management of stirring time), and normal slag removal work. For this reason, it is clearly superior to conventional technology in terms of both "no additional equipment required" and "labor saving," as it can add Si / Mn / Cr removal functionality without introducing additional large equipment or control systems to existing high-frequency melting equipment. (3) Elemental removal efficiency and carbon retention (high efficiency)

[0054] Conventional iron oxide-based processes assume that some loss of carbon (C) occurs in exchange for the removal of manganese (Mn) and chromium (Cr). In particular, oxidation of carbon (CO generation) is likely to proceed at temperatures around 1450°C, and many reports indicate that prioritizing the rate of Mn removal results in a significant decrease in carbon. Even in Mn removal technologies using sulfide-based fluxes, the unavoidable decrease in carbon in exchange for Mn removal remains a challenge.

[0055] In contrast, the molten metal refining method according to the present invention, for example, under conditions of 1340°C·F Fe3O4 1.3 wt% × 8 times with 120 seconds of stirring each time, reduces Si to below the detection limit, achieves a removal rate of approximately 97% for Mn and Cr, and suppresses the decrease in C concentration to less than approximately 5% (3.75 → 3.57 wt%). This can be explained by the fact that "oxygen derived from Fe3O4 is preferentially used first for the oxidation of Si, Mn, and Cr," and "the oxygen potential is consumed on the Si / Mn / Cr side before entering the C oxidation-dominant region from the perspective of CO-SiO2 equilibrium." Thus, by achieving both a high removal rate of Si / Mn / Cr and retention of C even in the low temperature range, a more efficient method than conventional methods has been realized in terms of both "element removal efficiency" and "product composition stability." (4) Operational advantages when implemented on-site (summary of labor savings, reduced time, and no need for additional equipment)

[0056] When the molten metal refining method according to the present invention is introduced on-site, it offers the following advantages:

[0057] Firstly, in terms of labor saving, it does not require special burner operation or gas flow control, and can be carried out simply by measuring and adding the oxidizer (Fe3O4 in the above experiment) and setting the power of the existing furnace (managing the stirring time). Furthermore, slag removal can be handled using the same procedure as that performed in a normal casting site.

[0058] Secondly, in terms of reducing the time required, compared to conventional methods that assume high temperature and long-term holding, this configuration involves multiple short-duration stirring cycles at around 1340°C, making it possible to shorten the total actual working time.

[0059] Thirdly, in terms of high efficiency, it achieves both high removal rates of Si / Mn / Cr and retention of C in the low-temperature range, resulting in high utilization efficiency of the oxidizing agent (Fe3O4 in the above experiment).

[0060] Fourthly, in terms of requiring no additional equipment, as long as an electric furnace is available, it does not require a new furnace body, rotating mechanism, burner, gas bubbling equipment, etc., and can be directly integrated into the existing line. This makes it possible to introduce the system while keeping capital investment low and without significantly changing the layout or operation flow of the existing factory.

[0061] From these points, the molten metal refining method according to the present invention can be positioned as a refining method that simultaneously satisfies four points compared to conventional technology: labor saving, time reduction, high efficiency, and no need for additional equipment. In particular, the practical feature of the iron molten metal refining method according to the present invention is that it can achieve the need to reduce Si, Mn, and Cr in the molten metal while retaining C by utilizing existing electric furnaces as they are.

[0062] Furthermore, the molten metal refining method of the present invention is not limited to the embodiments described above, and various modifications and improvements are possible within the scope described in the claims.

[0063] For example, in the above embodiment, approximately 1.3 wt% of iron oxide (Fe3O4) was added to the molten metal, but this is not limited to this amount. For example, it is possible to use the method of the present invention even if the amount of iron oxide is less than 1.3 wt%, although this may reduce the efficiency of removal as it may become necessary to increase the number of sets or stirring time. Also, it is possible to use the method of the present invention even if the amount of iron oxide is more than 1.3 wt%, although this may cause a sudden generation of slag or a decrease in the oxidation-reduction reaction due to a temperature drop. However, if the amount of iron oxide is too high, the work efficiency will decrease due to the generation of slag and the decrease in the oxidation-reduction reaction as described above, so in practice, it was preferable that the amount of iron oxide be up to about 5 wt% of the initial weight of the molten metal (300 kg in the above embodiment) or the weight before each set. In the experiment described above, the weight inside the furnace gradually increased by adding 3.9 kg of iron oxide each time. However, since the weight of iron oxide added each time remained constant (3.9 kg), the proportion of iron oxide in the total molten metal actually decreased and stayed well within 5 wt%.

[0064] Furthermore, slag removal (the third step) can be performed as needed and does not necessarily have to be done in every set.

[0065] Furthermore, the electric furnace used in this invention is not limited to those used in the above embodiments, but can be of various sizes and types. For example, not only high-frequency induction furnaces, but also medium-frequency induction furnaces, low-frequency induction furnaces, arc furnaces, etc., can be used. When using an arc furnace, bubbling agitation can be considered for stirring. Also, this invention does not exclude manual stirring.

Claims

1. A method for refining molten metal for iron and steel products, including cast iron, In an electric furnace, the molten metal of any component reaches a level higher than its freezing point, T EC (=-27,486 / {log[Si / C 2 ] - 15.47 - 273}) °C (units for Si and C are wt%) or less, a first step of adding iron oxide to the molten metal heated to the Si oxidation temperature range, A second step involves stirring the molten metal to which the iron oxide has been added for a predetermined time until at least the entire surface of the iron oxide is wetted by the molten metal and the iron oxide dissolves in the molten metal. A third step, if necessary, to remove the slag containing Si, Mn, or Cr generated by the reaction of the molten metal and the iron oxide after the second step, Equipped with, The removal rate of Si, Mn, or Cr in the molten metal is determined according to the number of times the set of the first and second steps is repeated. A method for refining molten metal, characterized by performing the first and second steps a number of times corresponding to the removal rate that corresponds to the amount of Si, Mn, or Cr to be removed from the molten metal from the determined removal rate.

2. The method for refining molten metal according to claim 1, characterized in that the iron oxide is within 5 wt% of the initial weight of the molten metal or the weight before each set.

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