JP6806288B2 - Steel manufacturing method - Google Patents

Description

The present disclosure relates to a method for producing steel.

The material properties required for steel materials are becoming more sophisticated, and improvement of characteristic values such as toughness of steel materials is required. In particular, when nitrogen, which is a gas component, is contained in the steel material, the toughness generally decreases. Therefore, in order to detoxify nitrogen in the steel material, it is detoxified by adding nitride forming elements such as Ti, Nb, V, Zr, and Al, but adding an alloy to the steel material increases the alloy cost. In addition to the increase, it also affects other properties such as material strength. Therefore, it is desirable to reduce nitrogen as much as possible at the production stage.

When steel is produced by the blast furnace-converter method, 4 to 5% of carbon melted in the blast furnace (unless otherwise specified herein, "%" and "ppm" indicating the amount of an element or compound. "" Means the mass ratio.) The hot metal is charged into the converter and decarburized in the converter. At that time, a large amount of oxygen is blown from the top-blown lance to the molten steel in the converter, and the inside of the converter is filled with CO gas generated by the decarburization reaction, the partial pressure of nitrogen in the atmosphere decreases, and the top-blown gas jet Since the molten steel is vigorously agitated at, the denitrification reaction proceeds. In the converter, the molten steel may be strongly agitated by bottom blowing, and the nitrogen concentration in the molten steel at the end of the converter blowing drops to about 10 ppm. However, since the molten steel is transported to the next process, the molten steel is discharged from the converter to the ladle, but the nitrogen concentration in the molten steel rises because the steel discharge flow entrains the atmosphere during the steel ejection. ..

When the molten steel is decompressed using a vacuum degassing device as the next step, the nitrogen concentration in the molten steel decreases during the decompression process, but the rate of decrease in the nitrogen concentration in the molten steel is slow and high-speed processing is required. However, it is not possible to rely on decompression treatment, and it has not been possible to economically and stably produce low nitrogen steel using only a vacuum degassing device.

Therefore, in order to produce low nitrogen steel economically and stably, molten steel whose nitrogen concentration has been reduced to about 10 ppm in a converter is discharged to a ladle without sucking nitrogen, and sucked by a vacuum degassing device. Ideally, the state of suppressing nitrogen is maintained and the next process, continuous casting, is started.

From the viewpoint of producing low nitrogen steel, a method of suppressing nitrogen absorption at the time of steel ejection has been proposed as shown below. To suppress the nitrogen absorption of molten steel at the time of steel ejection, (1) shut off the part where nitrogen absorption occurs from the atmosphere, (2) reduce the partial pressure of nitrogen in the atmosphere, and (3) delay the nitrogen absorption reaction. , (4) A method of reducing the reaction boundary area can be considered.

Among these, (1) and (2) are techniques for introducing non-nitrogen gas into the steel ejection flow or the ladle at the time of steel ejection, and are proposed in the following Patent Documents 1 to 3.
Patent Document 1 proposes a technique for ejecting denitrified low-nitrogen molten steel while sealing it with an inert gas.
In Patent Document 2, in a steel ladle having a lid, fuel is burned by oxygen-enriched air to preheat the steel ladle, and the steel ladle is replaced with combustion exhaust gas to create an atmosphere in the steel ladle. A technique characterized by blowing argon gas onto the molten steel flow from a nozzle arranged in an annular shape surrounding the molten steel flow provided on the lid of the steel ladle at the time of steelmaking from the converter after reducing the nitrogen content of the steel. Has been proposed.
In Patent Document 3, molten steel is ejected into a ladle containing calcium carbonate, and the atmosphere in the ladle at the time of steel ejection and during steel ejection is set as a CO ₂ gas atmosphere to prevent the molten steel from coming into contact with air. The method is disclosed.

Further, (3) is a method of ejecting steel in a non-deoxidized or semi-deoxidized state at the time of steel ejection as described in Patent Document 4, and is a general method found in many prior art documents.

When the molten steel is discharged from the converter to the ladle, the place where the nitrogen absorption to the molten steel occurs is when the molten steel is discharged from the converter into the ladle, as described in Non-Patent Document 1. It is considered to be the waterfall basin that occurs in. However, (4) a method for reducing the reaction boundary area, which is also an invention focusing on the reduction of the reaction boundary area in the waterfall basin, is not found except for Patent Document 5. In Patent Document 5, the steel output flow is received by the ladle along the wall of the inclined ladle, and an inert gas is supplied to the steel output port of a steelmaking furnace such as a converter to supply the steel output flow. A technique has been proposed in which an inert gas is mixed into the steel.

Japanese Unexamined Patent Publication No. 60-26611 Japanese Unexamined Patent Publication No. 2-285020 Japanese Unexamined Patent Publication No. 2003-293022 JP-A-59-190314 Japanese Unexamined Patent Publication No. 61-166911

Chotakaro et al., "Estimation of Oxygen and Nitrogen Absorption of Molten Steel at the Time of Steelmaking from Converter", Iron and Steel, 69 (1983), p. 767-774 Atsushi Okayama et al., "Water Model Experiment on Gas Absorption Behavior of Injection Flow", Iron and Steel, 102 (2016), p. 607-613

The technique disclosed in Patent Document 5 is a method of reducing the size of the waterfall basin itself of the Izushi style. If the size of the basin is reduced, the area of the reaction boundary where nitrogen absorption occurs is also reduced, so the effect of suppressing nitrogen absorption can be obtained, but there is a large risk of melting of refractories along the wall of the ladle. For this reason, even if a waterfall basin is generated, there is a need for a different cutting technique that can reduce the boundary area where nitrogen absorption occurs in the waterfall basin.

It is an object of the present disclosure to provide a method for producing steel capable of effectively suppressing nitrogen absorption at a waterfall basin formed by a steel discharge flow when molten steel is discharged into a ladle.

That is, the gist of this disclosure is as follows.
<1> The process of receiving the molten steel discharged from the molten steel furnace into a ladle and
Including a step of discharging the molten steel received in the ladle from the ladle and casting it.
When the molten steel discharged from the molten steel furnace is received in the ladle, it can be melted by contact with the molten steel received in the ladle, and the slag thickness calculated by the following equation (1). T is the auxiliary materials of oxide amount W satisfying than 0.02 m, and placed placed in the bottom of the ladle before受鋼start of the molten steel, the said molten steel is tapped from the molten steel furnace A method of manufacturing steel that receives steel in a ladle.
T = (W / ρ) / ((π ・ D ² ) / 4) (1)
T: Slag thickness (m)
D: Ladle diameter (m)
ρ: Molten oxide density (= 3000 kg / m ³ )
W: Amount of auxiliary material (kg)
<2> The method for producing steel according to <1>, wherein the auxiliary raw material contains CaO, Al ₂ O ₃ , SiO _2, and MgO.
< 3 > The composition of the auxiliary raw material is
CaO / Al ₂ O ₃ : 0.8 to 4.0 (2)
5% ≤ SiO ₂ ≤ 10% (3)
MgO ≤ 10% (4)
CaO + Al ₂ O ₃ + SiO ₂ + MgO ≧ 90% (5)
The method for producing steel according to <1> or <2> , which satisfies the above.
However, the molecular symbol in the formulas (2) to (5) means the content (mass%) of the molecule.
<4> The amount W of adjuncts, wherein (1) said slag thickness T that is calculated by the formula is that amount which satisfies the 0.1m or less, according to any one of <1> to <3> Steel manufacturing method.
<5> Production of the steel according to <4>, wherein the auxiliary raw material placed in the ladle is preheated, and the molten steel is received in the ladle when the temperature of the auxiliary raw material is 800 ° C. or higher. Method.

According to the present disclosure, there is provided a method for producing steel capable of effectively suppressing nitrogen absorption at a waterfall basin formed by a steel discharge flow when molten steel is discharged into a ladle.

It is a figure which shows the relationship between the slag thickness in a ladle and the amount of nitrogen absorption. It is a figure which shows the relationship between the synthetic flux temperature just before steel out and the amount of nitrogen absorption.

The meanings of the terms used in the present disclosure will be described.
The molten steel furnace (steelmaking furnace) refers to a holding container for melting molten steel, such as a converter, an AOD (Argon Oxygen Decarburization) furnace, and an electric furnace.
Steelmaking refers to the operation of transferring molten metal (molten steel) held in a steelmaking furnace from a steelmaking furnace to a container for transportation such as a ladle. Further, the steel receiving means that the ladle receives the molten steel discharged from the molten steel furnace, and the steel ejection and the steel receiving are performed at the same timing.
Auxiliary raw materials refer to additives other than iron required for refining molten steel. In the present disclosure, an auxiliary raw material composed of an oxide is targeted, and a material composed of an oxide containing a component other than iron is used as an auxiliary raw material. Specifically, quicklime, silica sand, calcium aluminate-based slagifying agent, alumina brick scrap, calcined dolomite and the like can be used.
The diameter D of the ladle means the inner diameter of the ladle. Normally, the inner diameters of the bottom and top (opening) are the same inside the ladle, but if the inner diameters of the bottom and top are different, the average value of each diameter (inner diameter) at the bottom and top of the ladle To do. When the cross section of the inside of the ladle perpendicular to the height direction of the ladle is elliptical, the average value of the major axis and the minor axis is defined as the pan diameter D.

In order to solve the above-mentioned problems of the present disclosure, the present inventor conducted a gas absorption experiment using a dissolved oxygen concentration meter and a water model device, and investigated in detail the bubble entrainment behavior and the gas absorption behavior in the waterfall basin. Oxygen of about 8 ppm is usually dissolved in water, and it can be measured using a dissolved oxygen concentration meter. Prepare a water model device that simulates steel output from a converter to a ladle. For water imitating molten steel in a converter, the amount of dissolved oxygen was reduced to 0.8 ppm by injecting Ar in advance. The amount of dissolved oxygen in the converter of the water model device and in the ladle is continuously measured (see Non-Patent Document 2). From the tendency of oxygen absorption from the atmosphere to water in the water model experiment, it is inferred that the tendency of nitrogen absorption from the atmosphere to the molten steel in the actual melting of molten steel can be simulated. That is, in the water model experiment, the condition that the amount of dissolved oxygen in the water in the ladle increases shows that a large amount of oxygen in the atmosphere was absorbed at the time of steel removal, which is the same at the time of actual steel removal from the converter. Under the conditions, it can be estimated that nitrogen is easily absorbed in the molten steel.

In the water model experiment, a comparison test was conducted between the case where nothing floats on the water surface of the ladle and the case where oil floats on the water surface. As a result, it was found that when an injection flow is formed with the oil floating on the water surface, the oil is entrained with the air in the waterfall basin, and when the entrained oil comes into contact with the air bubbles, it stays on the air bubble surface and floats as it is. did. As a result of investigating the gas absorption behavior at this time, the amount of dissolved oxygen in the water in the ladle increased when nothing floated on the water surface of the ladle, whereas when oil floated on the water surface, the amount of dissolved oxygen increased. It was found that the increase in the amount of dissolved oxygen in the water in the ladle was suppressed. From the results of this experiment, when the oil is floating, the area of the reaction boundary with the entrained air is reduced by covering a part of the surface of the bubbles forming the waterfall basin, and the amount of gas absorbed during injection is suppressed. It is thought that.

Based on this knowledge, it is predicted that nitrogen absorption into the molten steel at the waterfall basin can be prevented by forming a film with good fluidity on the surface of the molten steel in the ladle at the time of steel removal. Then, the molten steel is put out in the ladle with the auxiliary raw material mixed so as to improve the slag property in the ladle, or the molten steel is put out in the ladle and the auxiliary raw material is put in the ladle. By doing so, the auxiliary raw material can be melted with the high-temperature molten steel immediately after the steel is ejected. For this reason, it is possible to form a state in which the molten metal surface is covered with molten oxide immediately after the steel is discharged, and if the steel is discharged in that state, a situation is intentionally created in which the molten oxide is involved in the waterfall basin, and the molten oxide is absorbed. Nitrogen can be suppressed. Furthermore, it is desirable that the substance (auxiliary raw material) caught in the waterfall basin at this time is in a molten state, but even if the solid phase remains, it still covers a part of the gas-liquid interface, so it absorbs. Expected to suppress nitrogen.

The present disclosure has been studied by confirming the effect by a molten steel experiment based on the above-mentioned idea, and the present inventor further puts it in a ladle before or at the time of steel removal. The method for producing steel according to the present disclosure was completed by finding favorable conditions such as the composition, amount, and temperature of the auxiliary raw materials to be added.
That is, the method for producing steel according to the present disclosure is as follows.
The process of receiving the molten steel discharged from the molten steel furnace into a ladle and
Including a step of discharging the molten steel received in the ladle from the ladle and casting it.
When the molten steel discharged from the molten steel furnace is received in the ladle, an auxiliary raw material composed of an oxide having an amount W such that the slag thickness T calculated by the following formula (1) satisfies 0.02 m or more is used. The molten steel is placed in the bottom of the ladle before the start of receiving the molten steel or put into the ladle at the start of receiving the steel, and the molten steel discharged from the molten steel furnace is received in the ladle. This is a method for manufacturing steel.
T = (W / ρ) / ((π ・ D ² ) / 4) (1)
T: Slag thickness (m)
D: Ladle diameter (m)
ρ: Molten oxide density (= 3000 kg / m ³ )
W: Amount of auxiliary material (kg)

Conventionally, many methods of adding auxiliary raw materials at the time of steel ejection have been proposed. However, most of them are aimed at reforming lower oxides in slag rather than suppressing nitrogen absorption, and in addition to the large amount of quicklime being added as an auxiliary raw material, the addition time is during or during steel removal. It was often after that. According to the method of the present disclosure, the slag composition after steel removal is the same as that of the conventional method, but the time for adding the auxiliary raw material in order to effectively suppress the nitrogen absorption at the initial stage of steel removal is earlier than before. It is necessary to put a certain amount or more of the auxiliary material in the ladle before steel, or put it in the ladle together with the steel removal and melt the auxiliary material of a certain amount or more immediately after the start of steel removal. , It is a big difference from the conventional method.

In order to confirm the effect of suppressing nitrogen absorption with the auxiliary raw materials, a 2-ton scale molten steel experiment was conducted and its behavior was examined. 2 tons of low nitrogen deoxidized molten steel melted in the induction furnace were put out in a preheated ladle in about 50 seconds, and the nitrogen concentration before and after the steel was put out was investigated. At this time, various conditions such as molten steel composition and temperature were assumed to be the same, and a synthetic flux (oxide) whose composition was adjusted was placed in a ladle, and steel was discharged in that state. At that time, the influence of parameters such as the synthetic flux composition, the amount of flux, and the preheating temperature placed in the ladle was investigated. At this time, the amount of nitrogen absorption before and after steel removal (hereinafter referred to as Δ [N]) is investigated, and if it is improved by 4 ppm or more from Δ [N] under the condition (run1) in which the synthetic flux is not placed, the effect of suppressing nitrogen absorption is achieved. I decided that there was. Hereinafter, the content of the component in the synthetic flux means mass%. Table 1 shows the test conditions and test results.

First, Δ [N] was 26 ppm under the condition that the synthetic flux was not placed. Based on this result, 50 kg (= base condition) of synthetic flux (CaO / Al ₂ O ₃ = 2.0) of CaO = 60%, Al ₂ O ₃ = 30%, and SiO ₂ = 10% was placed in a pan. When the molten steel was ejected in the placed state, Δ [N] was 21 ppm, and a clear effect of suppressing nitrogen absorption was observed. When the situation around the basin was photographed during steel removal and the situation inside the ladle was investigated, the synthetic flux melted when it came into contact with the molten steel injected into the ladle, and the solid existing around the basin It was confirmed that the slag, which was a mixture of the phase and the liquid phase, was caught in the basin. Since there is no difference in the conditions except for the presence or absence of synthetic flux, the reason why the effect of suppressing nitrogen absorption was obtained is that the slag caught in the basin covered a part of the bubble surface, and the molten steel and air. It was presumed that the reaction boundary area of the waterfall was reduced.
On the other hand, when the synthetic flux was hung on the wall surface slightly lifted from the bottom of the ladle and the steel was discharged under the condition that it was added to the molten steel surface 15 seconds after the start of steel removal, Δ [N] was 24 ppm, which was clear. No effect of suppressing nitrogen absorption was observed. In this case, it was confirmed that the synthetic flux was melted at the end of steel ejection, but the synthetic flux added was not melted from the first half to the middle of steel ejection, which has the highest amount of nitrogen absorption. It is estimated that the reaction boundary area between molten steel and air was not reduced.

Next, the effect of suppressing nitrogen absorption was investigated by changing the amount of synthetic flux to be placed under the base conditions (constant flux composition, no preheating). As a result, as shown in FIG. 1, the slag thickness T in the ladle obtained by the above equation (1) from the synthetic flux amount W to be placed and the size of the ladle (ladle diameter D) is less than 0.02 m. In some cases, no clear effect of suppressing nitrogen absorption was observed. On the other hand, when the slag thickness exceeds 0.05 m, the effect of suppressing nitrogen absorption is saturated. For this reason, when the amount of the liquid phase or the liquid phase containing the solid phase involved in the waterfall is less than a certain amount, the reaction boundary area between the molten steel and air cannot be sufficiently covered, and the effect of suppressing nitrogen absorption cannot be obtained. Was estimated. Further, even if there are too many liquid phases or liquid phases containing a solid phase involved in the waterfall, the effect of suppressing nitrogen absorption is saturated, so it is considered that there is a preferable upper limit of the amount of synthetic flux W to be placed in the ladle.

Further, the composition of the synthetic flux to be placed was changed according to the composition shown in Table 1 with the amount of the synthetic flux to be placed to be constant (50 kg), and the effect of suppressing nitrogen absorption under the condition without preheating was investigated. As a result, the synthetic flux composition is _{CaO / Al 2 O 3: 0.8~4.0} ((2) _{formula), 5% ≦ SiO 2 ≦} 10% ((3) formula), MgO ≦ 10% (( 4 )), A stable effect of suppressing nitrogen absorption was obtained. The synthetic flux composition when a stable nitrogen absorption suppressing effect is obtained matches the condition that the ratio of the liquid phase is high near the molten steel temperature, and the higher the ratio of the liquid phase, the more the bubble surface in the basin It is considered that the covering effect is large.

Furthermore, for the base conditions, the synthetic flux placed in the ladle was preheated with a burner, and the synthetic flux temperature immediately before steel removal was changed to investigate the effect of suppressing nitrogen absorption. The temperature of the synthetic flux was investigated with a thermocouple installed in the ladle. As a result, as shown in FIG. 2, when the temperature of the synthetic flux was heated to 800 ° C. or higher, a remarkable effect of suppressing nitrogen absorption was obtained. On the other hand, when the preheating temperature of the synthetic flux exceeds 1150 ° C., the effect of suppressing nitrogen absorption is saturated. It is considered that the preheating shortened the time until the flux melted and suppressed the nitrogen absorption immediately after the start of steel removal.

Hereinafter, embodiments of the steel manufacturing method according to the present disclosure will be described in more detail.
When producing low nitrogen steel, hot metal having a high carbon concentration transferred from a blast furnace or an electric furnace is charged into a molten steel furnace such as a converter, and carbon in the steel is removed as CO gas by oxygen blowing. At that time, in the molten steel furnace, the nitrogen partial pressure in the furnace is lowered by the C + O = CO reaction, and the nitrogen concentration in the steel is lowered to about 10 ppm in combination with the stirring action by the bottom blowing and the top blowing. The molten steel after decarburization is discharged from the molten steel furnace to a ladle in order to adjust the composition and degas. After that, the molten steel whose composition and temperature have been adjusted is subjected to a casting process, and after being cast, it is shipped as a product through processes such as heating, rolling, heat treatment, and surface treatment.

Normally, the ladle is preheated by a burner and then transported by a transport carriage to just below the molten steel furnace to receive the molten steel. Normally, auxiliary raw materials such as quicklime are often added to molten steel after steel is ejected, but when applying the steel manufacturing method according to the present disclosure, a certain amount or more of auxiliary raw materials are used before receiving the molten steel. It is necessary to put the ladle in the ladle or to receive the molten steel and put a certain amount or more of the auxiliary material into the ladle. Preferably, it is preferable to put the auxiliary raw material into the ladle before or during the preheating of the ladle.
The form of the auxiliary material is preferably granular so as not to dissipate due to the updraft during preheating or steel ejection, but when preheating is performed, the upper part of the ladle is usually covered with a lid. Powdery auxiliary materials can also be used. Preferably, when the ladle is transported to just below the molten steel furnace, the slag thickness T represented by Eq. (1) is formed in the ladle at the latest with the start of steel ejection from the molten steel furnace (start of receiving steel). It is necessary to add the auxiliary raw material in the amount W determined to be 0.02 m or more (preferably 0.1 m or less, more preferably 0.05 m or less). In addition, it is necessary to melt the steel immediately after the start of steel removal. When the auxiliary raw material is put into the ladle at the start of receiving steel, it is preferable to inject the molten steel within 10 seconds, more preferably within 5 seconds, and further preferably within 10 seconds after the molten steel starts to be injected from the molten steel furnace into the ladle. At the same time, the input of auxiliary ingredients into the ladle is started. When the auxiliary raw material is put into the ladle at the start of steel receiving, the slag thickness T is 0, preferably within 60 seconds, more preferably within 40 seconds, and even more preferably within 20 seconds after the start of steel receiving. Complete the input of the auxiliary material with an amount W of .02 m or more.
Further, the auxiliary raw material may be a combination of placing the auxiliary raw material in the ladle before the start of steel receiving and charging the auxiliary raw material into the ladle at the start of steel receiving. That is, the total amount of the auxiliary raw materials (W1 + W2) is obtained by placing the auxiliary raw material of the amount W1 in the pan before the start of steel receiving and further charging the auxiliary raw material of the amount W2 into the pan at the same time as the start of steel receiving. ) May be the amount W obtained so that the slag thickness T represented by the equation (1) satisfies 0.02 m or more.
A few minutes after the start of steel receiving, an Al alloy or the like may be added for the purpose of deoxidation, etc., and the component added at such a purpose and timing is the slag thickness shown in Eq. (1). It is not included in the auxiliary raw material of the amount W required so that T satisfies 0.02 m or more.

In order to obtain the effect of low nitrogen by the steel manufacturing method according to the present disclosure, it is necessary that molten slag is present in the waterfall basin during the steel receiving. “Receiving steel” refers to the period from at least 1 minute after the molten steel is injected into the ladle from the molten steel furnace until the injection is completed, preferably from 30 seconds after the injection of the molten steel is started until the injection is completed. .. The molten slag refers to a state in which the auxiliary raw material placed or put in the ladle is melted to form a liquid phase or a liquid phase containing a solid phase. In the present disclosure, the liquid phase slag is a state in which the liquid phase ratio is 50% or more in the calculation using general-purpose thermodynamic calculation software or the like.
The basin part refers to the part where bubbles are entrained and risen by entraining the gas phase around the injection flow when the injection flow enters the molten steel in the ladle, and the injection flow is usually inside the ladle. It occurs just below the part in contact with the molten steel. If the waterfall basin is covered with molten slag during steel ejection, the effect of reducing nitrogen according to the present disclosure can be obtained. When the molten steel discharged from the molten steel furnace is received in a ladle, the auxiliary raw material composed of oxide is used before the disclosure of the steel reception or at the start of the steel reception, and the slag thickness T represented by the formula (1) is 0.02 m or more. (Preferably 0.1 m or less, more preferably 0.05 m or less) is placed or put into a ladle in an amount W determined to satisfy the condition, and the molten steel discharged from the molten steel furnace is received in the ladle. As a result, molten slag can be present in the ladle portion during the steel receiving.

In the present disclosure, the auxiliary raw material placed or put in the ladle is an auxiliary raw material composed of oxides. Therefore, carbon oxides, fluorides, carbides, etc. are not included. For example, Patent Document 3 discloses an invention in which calcium carbonate is placed for the purpose of reducing the nitrogen concentration in the atmosphere in the ladle. On the other hand, in the present disclosure, since the purpose of the present disclosure is to prevent the nitrogen absorption phenomenon at the waterfall basin by the molten slag on the surface of the molten steel, calcium carbonate is not added. Calcium carbonate is not preferable from the viewpoint of lowering the temperature of molten steel because it involves an endothermic reaction during decomposition. In addition, if fluoride such as fluorite is added, it will hinder the recycling of produced slag, so fluoride is not added. Furthermore, since it is not intended for dephosphorization or desulfurization, it does not add carbides such as calcium carbide.

Further, the auxiliary raw material composed of oxide, which is placed or put in the ladle, has a composition of CaO / Al ₂ O ₃ : 0.8 to 4.0 (formula (2)), 5% ≤ SiO ₂ ≤. It is preferable to add the mixture after adjusting the range to 10% (Equation (3)) and MgO ≦ 10% (Equation (4)). By setting the composition range as such, the melting temperature of the auxiliary raw material can be preferably reduced. It is more preferable that the MgO content is 5% or more. In addition to the above-mentioned CaO, Al ₂ O ₃ , SiO ₂ , and MgO, it is permissible for the components contained in the auxiliary raw materials to contain oxide components such as MnO and FeO in an amount of less than 5%. It is also permissible to contain volatiles and impurities. That is, it is preferable as long as it satisfies the above equation (5).

It is desirable that the auxiliary raw material placed in the ladle is preheated together with the ladle, and it is preferable that the auxiliary material is preheated to 800 ° C. or higher. The preheating temperature of the auxiliary material can be evaluated by measuring the surface temperature of the auxiliary material placed in the ladle with a radiation thermometer.

By using the steel manufacturing method according to the present disclosure as described above, an increase in nitrogen concentration can be suppressed at the time of steel ejection, so that low nitrogen steel can be economically and stably manufactured. According to the method for producing steel according to the present disclosure, an increase in nitrogen concentration at the time of steel ejection can be effectively suppressed, but the nitrogen concentration in the produced steel is not limited.
Such a method for producing steel according to the present disclosure is very effective for carbon steel, but is also effective for producing stainless steel and alloy steel other than carbon steel.

Under the conditions of the examples and comparative examples of molten steel shown below, a nitrogen absorption behavior evaluation test was conducted at the time of steel ejection, and the effect of suppressing nitrogen absorption was confirmed.
The hot metal (equivalent to 4.5% carbon content) transported from the blast furnace was charged into the converter and oxygen was blown. The components after converter blowing are [C] = 0.06 to 0.14%, [Si] = 0.01 to 0.05%, [Mn] = 0.1 to 0.4%, [P]. ] = 0.01 to 0.03%, [N] = 9 to 12 ppm, and the balance is Fe and impurities. The processing amount is 300 ton scale, the diameter (inner diameter) of the ladle is 3.9 m, and the steel ejection time is about 5 minutes. Before steel removal, before preheating the ladle, or after preheating the ladle, place a predetermined amount of auxiliary material with adjusted components in the bottom of the ladle, transport the ladle to just below the converter, and then receive the molten steel. Steeled. Alternatively, the auxiliary raw material was put into the ladle together with the receiving steel of the molten steel. At the time of steel ejection, an alloy containing Al was charged in a form of being involved in the steel ejection flow 2 minutes after the start of steel ejection. Further, the auxiliary raw material (oxide) was additionally added into the ladle 3 to 4 minutes after the start of steel removal to obtain the “final slag thickness t” shown in Table 2.

In order to confirm the effect of suppressing nitrogen absorption, molten steel samples are taken in the converter before steel removal and in the ladle after steel removal, and the amount of nitrogen concentration change Δ [N] (ppm) before and after steel removal is the amount of nitrogen absorption. Evaluated as. The test conditions are shown in Table 2. In the column of "anti-digestion effect" in Table 2, when Δ [N] is more than 17 ppm and 20 ppm or less, it is regarded as “C” as having an anti-nitrogen effect, and Δ [N] is more than 15 ppm and 17 ppm or less. If there was, it was judged that there was an excellent anti-digestion effect and was rated as "B". When Δ [N] was 15 ppm or less, it was judged that there was a remarkable effect of suppressing nitrogen absorption, and the value was set to “A”. When Δ [N] was more than 20 ppm, it was rated as “D” because the effect of suppressing nitrogen absorption was not observed.

Test No. No. 1 is a condition in which no auxiliary material is placed in the ladle, and test No. 2 and No. No. 3 is a comparative example under the condition that the slag thickness is out of the scope of the present disclosure, although the auxiliary raw material is placed in the ladle. Test No. in which the auxiliary raw material to be stored was insufficient. 2 and test No. The Δ [N] of 3 was 23 to 24 ppm, and the effect of suppressing nitrogen absorption was not observed.

Test No. Test No. 4 from Up to 17, the examples satisfied the requirements of the present disclosure, Δ [N] was 20 ppm or less, and the effect of suppressing nitrogen absorption was recognized.
Test No. Test No. 10 Up to 13, the conditions were such that the composition of the auxiliary raw material to be placed in the ladle was adjusted to a suitable range, and Δ [N] was 17 ppm or less, and it was judged that there was an excellent effect of suppressing nitrogen absorption.
Test No. 3, 5, 9 and test No. Test No. 14 from Up to 16 are conditions in which the preheating temperature of the placed auxiliary material is changed. Test No. 5 and test No. Comparing No. 7, the preheating temperature of the auxiliary raw material is high. It can be seen that the case of 5 has a larger effect of suppressing nitrogen absorption, and an excellent effect of suppressing nitrogen absorption can be obtained by raising the preheating temperature of the auxiliary raw material. This means that Test No. 11 and test No. It is also clear by comparing 14 and the test No. In addition to controlling the composition of the auxiliary raw material within the preferable range of the present disclosure, 14 has a remarkable effect of suppressing nitrogen absorption by setting the auxiliary raw material preheating temperature to 800 ° C. or higher. Test No. The same applies to 15 and 16.
Test No. Reference numeral 18 denotes a reference example in which the auxiliary raw material is put into the ladle together with the receiving steel. Δ [N] was 20 ppm, which was lower than that of the comparative example, and the effect of suppressing nitrogen absorption was observed.

It is useful in the method for producing low nitrogen steel because it can effectively suppress the absorption of nitrogen at the time of steel ejection of molten iron.

The disclosure of Japanese patent application 2018-122844 filed June 28, 2018 is incorporated herein by reference in its entirety. All documents, patent applications, and technical standards described herein are referred to herein to the same extent as if individual documents, patent applications, and technical standards were specifically and individually stated. Is taken in by.

Claims (5)

The process of receiving the molten steel discharged from the molten steel furnace into a ladle and
Including a step of discharging the molten steel received in the ladle from the ladle and casting it.
When the molten steel discharged from the molten steel furnace is received in the ladle, it can be melted by contact with the molten steel received in the ladle, and the slag thickness calculated by the following equation (1). T is the auxiliary materials of oxide amount W satisfying than 0.02 m, and placed placed in the bottom of the ladle before受鋼start of the molten steel, the said molten steel is tapped from the molten steel furnace A method of manufacturing steel that receives steel in a ladle.
T = (W / ρ) / ((π ・ D ² ) / 4) (1)
T: Slag thickness (m)
D: Ladle diameter (m)
ρ: Molten oxide density (= 3000 kg / m ³ )
W: Amount of auxiliary material (kg) The auxiliary raw materials are CaO and Al. ₂ O ₃ , SiO ₂ The method for producing steel according to claim 1, which comprises and MgO. The composition of the auxiliary raw material
CaO / Al ₂ O ₃ : 0.8 to 4.0 (2)
5% ≤ SiO ₂ ≤ 10% (3)
MgO ≤ 10% (4)
CaO + Al ₂ O ₃ + SiO ₂ + MgO ≧ 90% (5)
The method for producing steel according to claim 1 or 2 , wherein the method for producing steel according to claim 1 or 2 .
However, the molecular symbol in the formulas (2) to (5) means the content (mass%) of the molecule. The amount W of auxiliary materials is an amount of the slag thickness T that is calculated by the equation (1) satisfies the 0.1m or less, the production of steel according to any one of claims 1 to 3 Method. The method for producing steel according to claim 4, wherein the auxiliary raw material placed in the ladle is preheated, and the molten steel is received in the ladle when the temperature of the auxiliary raw material is 800 ° C. or higher.