Refining method of molten iron

Abstract

To provide a novel method capable of reducing a tramp element in molten iron.SOLUTION: A method according to the preset disclosure comprises a process for refining molten iron, in which the molten iron satisfying formula (1): 1+161×[%O]+63.4×[%S]≥2.1 where [%O] is oxygen concentration in molten iron (mass%), and [%S] is sulfur concentration in molten iron (mass%), is irradiated with an arc in an atmosphere containing nitrogen to reduce a tramp element in the molten iron.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] This application discloses a method for refining molten iron. [Background technology]

[0002] Due to the growing interest in carbon neutrality worldwide, steelmaking processes using large amounts of scrap as an iron source may become mainstream in the future. Scrap contains relatively large amounts of so-called tramp elements, such as Cu and Sn, which are difficult to remove by normal oxidation refining, and there are concerns about the effects of their concentration. Copper in particular is known to affect the red embrittlement of steel and causes cracks in the continuous casting and rolling processes, so it is necessary to remove it.

[0003] Patent Document 1 discloses a method for removing tramp elements by injecting a sulfide or sulfate of an alkali metal or alkaline earth metal into molten iron with a nozzle. Patent Document 2 discloses a method for removing tramp elements by bringing molten silver into contact with molten iron to transfer oxides of the tramp elements to the molten silver side. Patent Document 3 discloses a method for removing tramp elements by an electrochemical method using molten iron as an anode electrode. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 5-306408 [Patent Document 2] JP 2013-119666 A [Patent Document 3] JP 2022-129591 A Summary of the Invention [Problem to be solved by the invention]

[0005] A new method capable of reducing the amount of tramp elements in molten iron is needed. [Means for solving the problem]

[0006] The present application discloses the following aspects as one of the means for solving the above problems. <Aspect 1> In an atmosphere containing nitrogen, molten iron satisfying the following formula (1) is irradiated with an arc to reduce tramp elements in the molten iron. A method of refining molten iron. 1+161×[%O]+63.4×[%S]≧2.1 ···(1) [%O]: Oxygen concentration in the molten iron (% by mass) [%S]: sulfur concentration in the molten iron (mass%) <Aspect 2> The atmosphere contains 20% by volume or more of nitrogen. 2. The method of refining molten iron according to claim 1. <Aspect 3> The molten iron contains one or two of copper and tin as the tramp element, 3. The method for refining molten iron according to embodiment 1 or 2. Effect of the Invention

[0007] According to the method for refining molten iron disclosed herein, it is possible to reduce the amount of tramp elements in the molten iron. [Brief description of the drawings]

[0008] [Figure 1] The graph shows the relationship between the α value of molten iron (= 1 + 161 × [%O] + 63.4 × [%S]) and the copper concentration ratio (copper concentration after arc irradiation / copper concentration before arc irradiation) and the tin concentration ratio (tin concentration after arc irradiation / tin concentration before arc irradiation). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Hereinafter, one embodiment of the method for refining molten iron according to the present disclosure will be described. However, the method for refining molten iron according to the present disclosure is not limited to the following embodiment.

[0010] According to "Tetsu to Hagane, Vol. 82 (1996), No. 2, p. 135" and "Tetsu to Hagane, Vol. 102 (2016), No. 1, p. 17," highly active nitrogen is thought to be involved in copper removal and tin removal from molten iron by spraying ammonia gas. However, ammonia gas is a flammable and toxic gas, and is not easy to handle. Therefore, the inventors of the present invention have considered reacting molten iron with highly active nitrogen by another means. As a result, they came up with the idea of ​​generating an arc in an atmosphere containing nitrogen and reacting the nitrogen plasma generated by the arc with the molten iron.

[0011] Based on the above-mentioned idea, the inventors have further conducted intensive research and have found that increasing the nitrogen concentration in the molten iron during arc irradiation is important in reducing tramp elements such as copper. They have also found that by controlling the concentrations of oxygen and sulfur, which act as surface active elements in the molten iron, within appropriate ranges, denitrification from the molten iron during arc irradiation can be suppressed and the reduction of tramp elements can proceed more efficiently.

[0012] Based on the above findings, the method for refining molten iron according to the present disclosure has the following configuration: That is, the method for refining molten iron according to one embodiment includes reducing tramp elements in the molten iron by irradiating an arc to the molten iron that satisfies the following formula (1) in an atmosphere containing nitrogen.

[0013] 1+161×[%O]+63.4×[%S]≧2.1 ···(1) [%O]: Oxygen concentration in the molten iron (% by mass) [%S]: sulfur concentration in the molten iron (mass%)

[0014] The nitrogen concentration in molten iron is determined by the balance between the absorption of nitrogen into the molten iron and the desorption of nitrogen from the molten iron. This balance is largely influenced by oxygen and sulfur, which act as surface active elements in molten iron. The nitrogen absorption and desorption rate constant k N is given by the following equation (2).

[0015]

number

[0016] where f N is the activity coefficient of nitrogen, and [%O] and [%S] are as described above. According to equation (2), as the concentrations of oxygen and sulfur in molten iron increase, k N In other words, as the concentrations of oxygen and sulfur in the molten iron increase, the rate at which nitrogen is absorbed into the molten iron and the rate at which nitrogen is desorbed from the molten iron decrease.

[0017] On the other hand, the above-mentioned nitrogen adsorption and desorption rate constant k N Nitrogen absorption deviates from the normal range. The causes of this are: (i) The nitrogen plasma generated by the arc is in a highly active state and can be easily dissolved in molten iron. (ii) The arc irradiated area is heated to a very high temperature (several thousand degrees), which causes the surfactant effect to be inactivated. (iii) The high temperature of the arc jet causes the surface active elements in the arc-irradiated area to move out of the arc-irradiated area. Therefore, when considering the nitrogen concentration in molten iron under arc irradiation conditions, it is necessary to take into account both the nitrogen absorption by the arc and the denitrification from the molten iron. That is, in order to increase the nitrogen concentration in molten iron, it is important to (I) increase the rate of nitrogen absorption by arc irradiation and (II) decrease the rate of nitrogen denitrification from the molten iron.

[0018] First, (I) the improvement of the nitrogen absorption rate by arc irradiation will be described. In this embodiment, an arc is irradiated to the molten iron in an atmosphere containing nitrogen. The atmosphere during the arc irradiation may contain nitrogen, and the concentration is not particularly limited. However, according to the knowledge of the present inventors, the lower the nitrogen concentration in the atmosphere during the arc irradiation, the smaller the amount of nitrogen plasma generated by the arc, and the smaller the amount of nitrogen dissolved in the molten iron, resulting in a slower reaction rate of the tramp element. On the other hand, the higher the nitrogen concentration, the more likely the arc is to become unstable and the more likely the amount of power used is to increase. This is because, when an arc is generated, a dielectric breakdown in the gas phase is necessary, but the voltage required for dielectric breakdown of nitrogen is large, and a higher voltage is required to stably generate an arc in a situation where the nitrogen concentration is high. According to the knowledge of the present inventors, when the atmosphere during the arc irradiation contains 20% or more by volume of nitrogen, a more significant tramp element removal effect is likely to be obtained. More preferably, the atmosphere during the arc irradiation contains 20% or more by volume and 95% or less by volume of nitrogen.

[0019] Next, (II) the decrease in the denitrification rate from molten iron will be described. As described above, the denitrification rate from molten iron is determined by the concentrations of oxygen and sulfur in the molten iron. Specifically, it can be said that increasing the concentrations of oxygen and sulfur in the molten iron is effective in decreasing the denitrification rate. The present inventors have intensively studied the concentrations of oxygen and sulfur in molten iron and have found that when the concentrations of oxygen and sulfur contained in the molten iron satisfy the above formula (1), the tramp elements in the molten iron can be effectively decreased. That is, as shown in the above formula (1), when the value of 1+161×[%O]+63.4×[%S] (hereinafter referred to as α) for the oxygen and nitrogen concentrations in the molten iron is 2.1 or more, it is considered that the denitrification rate can be decreased, the nitrogen concentration in the molten iron can be increased by arc irradiation, and the tramp elements can be efficiently reacted and removed. In other words, when the value of α is smaller than 2.1, the nitrogen absorbed by the arc is diffused and then quickly desorbed from the surface of the molten iron, so that the nitrogen concentration in the molten iron is difficult to increase even during the arc irradiation, and the reaction rate with the tramp elements in the molten iron is accordingly reduced, so that the removal of the tramp elements is difficult to proceed. In addition, by increasing the value of α, that is, by increasing the concentration of oxygen and sulfur in the molten iron, the denitrification rate is reduced, which is advantageous for the reduction and removal of the tramp elements, but on the other hand, oxygen and sulfur in the molten iron are impurity elements and need to be removed in other processes. Therefore, the upper limit of α can be determined based on the series of refining processes. The upper limit of α may be, for example, 25 or less, 20 or less, 15 or less, or 10 or less.

[0020] The tramp elements that can be reduced by the method according to the present embodiment may include, for example, one or more of copper (Cu), tin (Sn), arsenic (As), bismuth (Bi) and lead (Pb). In particular, when the molten iron contains one or two of copper and tin as tramp elements, a more significant effect can be expected. The concentration of the tramp elements contained in the molten iron is not particularly limited.

[0021] The arc irradiation means and arc irradiation conditions employed in the method according to the present embodiment are not particularly limited, and may be any means and conditions capable of reducing the tramp elements in the molten iron by the mechanism described above. In the method according to the present embodiment, slag may or may not be present on the surface of the molten iron to which the arc is irradiated. If slag is present, it is preferable that the arc is not covered with slag, i.e., a so-called open arc.

[0022] As described above, the method according to the present embodiment can reduce the tramp elements in the molten iron. The method according to the present embodiment can also remove tramp elements that have escaped the screening process at the scrap stage or that have been mixed in the scrap due to being solid-solubilized therein during the refining process. The method according to the present embodiment can also use low-grade scrap that contains a large amount of tramp elements, which contributes to reducing production costs. In addition, the method according to the present embodiment does not require the use of special slag such as sulfides or sulfates, does not significantly affect the wear rate of the refractory in the refining vessel, and allows operation with the same repair frequency as in the past for refining vessels. EXAMPLES

[0023] The present invention will be further described below with reference to examples, but the present invention is not limited to the following examples. The present invention can adopt various conditions as long as it does not deviate from the gist of the invention and achieves its object.

[0024] In an electric resistance furnace capable of controlling the atmosphere, 1 kg of iron containing copper or tin was melted in an MgO crucible. At this time, under conditions in which the oxygen and sulfur concentrations in the molten iron were variously changed, an electric current was passed between an electrode placed on the bottom of the crucible and a graphite electrode placed on the top of the molten iron to generate an arc. The arc was generated for 10 minutes with a direct current of 30 A and an electrode height (distance between the molten iron and the electrode) of 10 mm. At this time, the electric current was passed under positive polarity conditions, with the electrode on the bottom of the crucible as the positive electrode and the graphite electrode on the top of the molten iron as the negative electrode. The atmosphere was an argon-nitrogen atmosphere, and the nitrogen concentration in the atmosphere was adjusted by changing the ratio of each.

[0025] First, the relationship between the oxygen and sulfur concentrations in the molten iron and the reduction rate of the copper or tin concentration before and after the arc irradiation of the molten iron is shown in Figure 1. The oxygen and sulfur concentrations in the molten iron are expressed by the above-mentioned value of α, and the reduction rate of the copper or tin concentration is the ratio of the concentration after the arc irradiation of the molten iron to the initial concentration of copper or tin. The nitrogen concentration in the atmosphere during the arc irradiation was controlled to 50 volume %.

[0026] As shown in Figure 1, when α was less than 2.1, no change was observed in the copper or tin concentration even when the molten iron was irradiated with an arc. On the other hand, in the region where α was 2.1 or more, it was confirmed that the copper or tin concentration significantly decreased when the molten iron was irradiated with an arc. This is thought to be because the desorption of nitrogen from the molten iron was inhibited by the surface active elements oxygen and sulfur, which increased the nitrogen concentration in the molten iron, and this reacted with the copper or tin in the molten iron, removing the copper or tin.

[0027] Next, the relationship between the nitrogen concentration in the atmosphere and the rate of decrease in copper concentration before and after arc irradiation of the molten iron was confirmed. In both cases, α was set to 5.4. First, when the nitrogen concentration in the atmosphere was 0% by volume, i.e., in an argon atmosphere only, no decrease in copper concentration was observed before and after arc irradiation of the molten iron. On the other hand, when the nitrogen concentration in the atmosphere was 20% by volume or more, the copper concentration decreased due to arc irradiation of the molten iron, and the rate of copper decrease also increased as the nitrogen concentration in the atmosphere increased. This is thought to be because the amount of nitrogen plasma generated by the arc increases as the nitrogen concentration in the atmosphere increases.

[0028] In this experiment, an arc did not occur stably when the nitrogen concentration was 100% by volume, i.e., in an atmosphere of nitrogen alone. This is because, when an arc occurs, a dielectric breakdown is required in the gas phase, but the voltage required for dielectric breakdown of nitrogen is large, so a higher voltage is required to generate an arc stably.

[0029] In addition, in none of the experiments was the nitrogen concentration in the molten iron increased to a supersaturated level before or after the arc irradiation. This is thought to be because, even if the nitrogen concentration in the molten iron was supersaturated during the arc irradiation, the nitrogen quickly escaped from the molten iron in the form of gas bubbles the moment the arc irradiation was stopped. In fact, for about 10 seconds after the arc irradiation, it was observed that the surface of the molten iron was violently shaking due to gas bubbles generated from the inside, which is thought to be due to some of the nitrogen dissolved in the supersaturated state becoming gas bubbles and escaping from the molten iron.

[0030] In this experiment, a method for reducing or removing copper or tin as a tramp element contained in molten iron was exemplified, but the tramp elements removed by the method disclosed herein are not limited to copper or tin. Similar effects can be expected for other tramp elements such as arsenic, bismuth, and lead.

[0031] As described above, it can be said that it is possible to reduce the tramp elements in the molten iron by irradiating an arc to the molten iron that satisfies the following formula (1) in an atmosphere containing nitrogen. 1+161×[%O]+63.4×[%S]≧2.1 ···(1) [%O]: Oxygen concentration in molten iron (mass%) [%S]: Sulfur concentration in molten iron (mass%)

Claims

1. In an atmosphere containing nitrogen, molten iron satisfying the following formula (1) is irradiated with an arc to reduce tramp elements in the molten iron. A method of refining molten iron. 1+161×[%O]+63.4×[%S]≧2.1...(1) [% O]: Oxygen concentration in the molten iron (% by mass) [%S]: sulfur concentration in the molten iron (% by mass)

2. The atmosphere contains 20% by volume or more of nitrogen. The method for refining molten iron according to claim 1.

3. The molten iron contains one or two of copper and tin as the tramp element. The method for refining molten iron according to claim 1 or 2.

Images