TWI699436B - Method for producing steel - Google Patents

Abstract

The present invention is a method for manufacturing steel, which includes the steps of tapping molten steel from a self-melting steel furnace and receiving it to a ladle, and discharging the molten steel received by the ladle from the ladle. The step of casting; when the molten steel tapped from the steel melting furnace is received to the ladle, the slag thickness T calculated according to the following formula (1) will satisfy the amount W of 0.02m or more and be composed of oxides The secondary raw materials are placed at the bottom of the ladle before receiving the molten steel, or put into the ladle at the same time as the molten steel is received, and the steel tapped from the molten steel furnace The liquid is received to the aforementioned ladle. The composition of the aforementioned auxiliary materials should satisfy CaO/Al ₂ O ₃ : 0.8～4.0, 5%≦SiO ₂ ≦10%, and MgO≦10%. T=(W/ρ)/((π·D ² )/4) (1) Among them, D: ladle diameter (m), ρ: molten oxide density (=3000kg/m ³ ), W: vice The amount of raw materials (kg).

Description

Steel manufacturing method

Invention field The present invention relates to a method for manufacturing steel.

Background of the invention The material properties required by steel have been continuously improved, and it is required to improve the properties of steel, which is the toughness. In particular, if nitrogen as a gas component is contained in the steel material, the toughness usually decreases. Therefore, in order to detoxify the nitrogen in steel materials, the addition of nitride-forming elements such as Ti, Nb, V, Zr, and Al is used to achieve detoxification. However, if alloys are added to steel materials, in addition to the increase in alloy costs, It will also affect other properties such as material strength. Therefore, it is desirable to have as low nitriding as possible in the manufacturing stage.

In the case of using the blast furnace-converter method to manufacture steel, the carbon content of the blast furnace will be 4 to 5% (as long as there is no special description in this manual, it means "%" and "ppm" of the amount of elements or compounds All refers to the mass ratio) molten iron is charged into the converter and decarburized in the converter. At this time, in the converter, a large amount of oxygen is blown from the top blowing spray gun to the molten steel. The converter is filled with CO gas generated by the decarburization reaction, and the partial pressure of nitrogen in the ambient gas is reduced. The gas jet stirs the molten steel drastically, so the denitrification reaction proceeds. In the converter, there is also a situation where the molten steel is stirred vigorously by bottom blowing, and the nitrogen concentration in the molten steel at the end of converter blowing is reduced to about 10 ppm. However, in order to transfer the molten steel to the next step, the molten steel is tapped into a ladle from the converter. However, during tapping, the tapping stream will entrain the atmosphere, resulting in the nitrogen concentration in the molten steel rise.

When using a vacuum degassing device to decompress the molten steel as the next step, although the nitrogen concentration in the molten steel will decrease during the decompression treatment, the nitrogen concentration in the molten steel decreases slowly. High-speed processing cannot rely on reduced pressure processing, and only using a vacuum degassing device will not achieve economical and stable production of low nitrogen steel.

Therefore, in order to produce low-nitrogen steel economically and stably, it is better to make molten steel whose nitrogen concentration has been reduced to about 10ppm in the converter, tap the steel into a ladle without absorbing nitrogen, and place it in a vacuum The degassing device maintains the state of suppressing nitrogen absorption and moves to the next step of continuous casting.

From the viewpoint of manufacturing low nitrogen steel, as shown below, a method of suppressing nitrogen absorption during tapping is proposed. In order to suppress the nitrogen absorption of molten steel during tapping, the following methods have been thought of: (1) blocking the part that produces nitrogen from the atmosphere; (2) reducing the partial pressure of nitrogen in the atmosphere; (3) delaying the nitrogen absorption reaction; (4) Reduce the area of the reaction boundary, etc.

Among these, (1) and (2) are techniques for introducing non-nitrogen gas into the tapping stream or ladle during tapping, and are proposed in Patent Documents 1 to 3 below. In Patent Literature 1, a technique is proposed in which the low nitrogen molten steel after denitrification is sealed with an inert gas while tapping is performed. Patent Document 2 proposes the following technology, which is characterized by preheating the ladle for receiving molten steel by burning fuel with oxygen-enriched air in a ladle with a lid. , And use the combustion exhaust gas to replace, thereby reducing the nitrogen in the ambient gas in the ladle for receiving molten steel, it is self-installed on the lid of the ladle for receiving molten steel when the converter is tapped and surrounded by The circular nozzle of the molten steel stream blows argon gas into the molten steel stream. In Patent Document 3, the following method is disclosed: tapping molten steel into a ladle containing calcium carbonate, and setting the atmosphere in the ladle at the time of tapping and during the tapping process as a CO ₂ atmosphere Gas, and restrain the molten steel from contacting air.

In addition, (3) is also described in Patent Document 4, a method of tapping steel in an undeoxidized or semi-deoxidized state during tapping, and it is a common method commonly used in most prior art documents.

It is considered that the part where the molten steel absorbs nitrogen when the molten steel is tapped into the ladle is as described in Non-Patent Document 1, and it is the waterfall formed when the molten steel is tapped into the ladle. However, regarding (4) the method of reducing the area of the reaction zone, that is, an invention focusing on reducing the area of the reaction zone in the waterfall pool, there is no other record other than Patent Document 5. In Patent Document 5, the following technology is proposed: the tapping flow is received to the ladle along the wall of the inclined ladle, and at the same time, inert gas is supplied to the tapping port of the steelmaking furnace such as a converter to mix the inert gas in and out. Steel flow.

[Patent Document 1] Japanese Patent Laid-Open No. 60-26611 [Patent Document 2] Japanese Patent Laid-Open No. 2-285020 [Patent Document 3] Japanese Patent Laid-Open No. 2003-293022 [Patent Document 4] Japanese Patent Laid-Open No. 59-190314 [Patent Document 5] Japanese Patent Laid-Open No. 61-166911

[Non-Patent Document 1] Chang Long Lang et al. "Calculation of Oxygen and Nitrogen Absorption in Molten Steel During Converter Tapping", Iron and Steel, 69 (1983), p.767-774 [Non-Patent Document 2] Okayama Atsushi et al. "Water model test related to gas absorption behavior of injected stream", Iron and Steel, 102 (2016), p.607-613

Summary of the invention [The problem to be solved by the invention] The technology disclosed in Patent Document 5 is a method for reducing the size of the waterfall pool itself. If the size of the waterfall pool is reduced, the area of the reaction zone that produces nitrogen absorption will also be reduced, so the nitrogen absorption inhibition effect can be obtained, but when the tapping flow is along the wall of the ladle, the risk of refractory melting loss is higher. Big. Therefore, even if a waterfall pool is formed, a technology that can reduce the area of the nitrogen absorption boundary in the waterfall pool is required.

The object of the present invention is to provide a method for manufacturing steel, which can effectively inhibit the nitrogen absorption of the waterfall formed by the tapping stream when the molten steel is tapped into the ladle. [Means to solve the problem]

That is, the gist of the present invention is as follows. <1> A method for manufacturing steel, which includes the steps of receiving molten steel from a self-melting steel furnace tapped into a ladle, and discharging the molten steel received by the ladle from the ladle. The step of casting; when the molten steel tapped from the steel melting furnace is received to the ladle, the slag thickness T calculated according to the following formula (1) will satisfy the amount W of 0.02m or more and be oxidized The auxiliary raw materials constituted by the material are placed in the bottom of the ladle before receiving the molten steel, or are poured into the ladle at the same time as the molten steel is received, and the steel will be tapped from the molten steel furnace. The molten steel is received to the aforementioned ladle. T=(W/ρ)/((π·D ² )/4) (1) T: slag thickness (m) D: ladle diameter (m) ρ: molten oxide density (=3000kg/m ³ ) W: Amount of secondary raw material (kg) <2> The method of manufacturing steel as described in <1>, wherein the composition of the aforementioned secondary raw material satisfies: CaO/Al ₂ O ₃ : 0.8～4.0 (2) 5%≦SiO ₂ ≦10% (3) MgO≦10% (4) CaO+Al ₂ O ₃ +SiO ₂ +MgO ≧90% (5); where the molecular symbols in the formulas (2)～(5) mean Content (mass%). <3> The method for producing steel as described in <1> or <2>, wherein the amount of the auxiliary raw material W is such an amount that the slag thickness T calculated according to the above formula (1) satisfies 0.1 m or less. <4> The method for manufacturing steel as described in any one of <1> to <3>, wherein before starting to receive the molten steel, the auxiliary raw material of the amount W is placed in the bottom of the ladle in advance. <5> The method for manufacturing steel as described in <4>, wherein the auxiliary raw materials placed in the steel ladle are preheated, and the molten steel is received under the state where the temperature of the auxiliary raw materials is above 800°C The aforementioned ladle. [Effects of Invention]

According to the present invention, a method for manufacturing steel can be provided, which can effectively suppress the nitrogen absorption of the waterfall formed by the tapping flow when the molten steel is tapped into the ladle.

Detailed description of the preferred embodiment The meaning of the terms used in the present invention will be explained. The so-called steel melting furnace (steel making furnace) refers to a converter, AOD (Argon Oxygen Decarburization, argon oxygen decarburization) furnace, electric furnace, etc., for the holding vessel for molten steel. The so-called tapping refers to the action of transferring the molten metal (liquid steel) held in the steelmaking furnace to the self-made steel furnace of the ladle. In addition, the so-called receiving molten steel means that a ladle is used to receive molten steel from a melting furnace, and tapping and receiving molten steel are performed at the same time. The so-called secondary raw materials refer to additives other than iron necessary for refining molten steel. In the present invention, secondary raw materials composed of oxides are targeted, and those composed of oxides containing components other than iron are used as secondary raw materials. Specifically, quicklime, silica sand, calcium aluminate-based slag forming agent, alumina brick scraps, calcined dolomite, etc. can be used. The so-called ladle diameter D means the inner diameter of the ladle. Normally, the inner diameter of the bottom and the upper part (opening) of the ladle will be the same, but when the inner diameters of the bottom and upper part are different, set the diameter (inner diameter) of the bottom and upper part of the ladle The average value. Moreover, 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 long diameter and the short diameter is the ladle diameter D.

In order to solve the aforementioned problems of the present invention, the inventors conducted gas absorption experiments using a dissolved oxygen concentration meter and a water model device, and conducted detailed investigations on bubble entrainment behavior and gas absorption behavior in the waterfall pool. About 8ppm of oxygen is usually dissolved in water, and it can be measured with a dissolved oxygen meter. Prepare a water model device to simulate the tapping from the converter to the ladle. For the water imitating molten steel in the converter, Ar is blown in advance to reduce the dissolved oxygen to 0.8 ppm. The amount of dissolved oxygen in the converter and the ladle of the water model device is continuously measured (see Non-Patent Document 2). It is inferred based on the tendency of oxygen to be absorbed from the ambient gas into the water in the water model experiment, which can simulate the tendency of nitrogen to be absorbed into the molten steel from the ambient gas during the actual melting of molten steel. That is, the conditions under which the amount of dissolved oxygen in the water in the ladle will increase in the water model experiment shows that a large amount of oxygen in the ambient gas is absorbed during tapping. If it is the same as the actual spinning furnace tapping Conditions, it can be estimated that nitrogen is easily absorbed in molten steel.

In the water model experiment, a comparative test was carried out between the situation where there is no float on the water surface of the ladle and the situation where there is oil floating on the water surface. As a result, it was discovered that when an injection flow is formed with oil floating on the water surface, oil and air are entrained in the waterfall pool, and if the entrained oil comes into contact with the bubbles, it stays on the surface of the bubbles and uses the The state floats. Investigating the gas absorption behavior at this time, the results show that when there is no floating situation on the water surface of the steel bucket, the dissolved oxygen content of the water in the steel bucket increases. When floating with oil, the increase of dissolved oxygen in the water in the ladle is suppressed. According to the experimental results, in the state of floating oil, the oil will cover part of the surface of the bubble forming the waterfall pool, thereby reducing the area of the reaction interface with the entrained air, and the gas absorption during the injection process is obtained inhibition.

Based on this insight, it is predicted that by forming a film with good fluidity on the surface of the molten steel in the ladle during tapping, the absorption of nitrogen in the molten steel in the waterfall can be prevented. In addition, the secondary raw materials that have been blended into the ladle are placed in the ladle and the molten steel is tapped to the ladle, or the secondary raw materials are tapped into the ladle at the same time Put it into the ladle, so that the high temperature molten steel just after tapping can be used to melt the auxiliary raw materials. Therefore, the molten oxide can be formed on the liquid surface immediately after tapping. If tapping is performed in this state, the molten oxide entrained in the waterfall can be intentionally generated, thereby suppressing nitrogen absorption. Furthermore, at this time, the entrainment (secondary raw material) in the waterfall basin is preferably in a molten state, but even if the solid phase remains, the condition of covering a part of the gas-liquid interface is not changed, so the nitrogen absorption suppression effect can be expected.

The present invention is based on the aforementioned concept and obtained by using molten steel experiments to confirm its effects. The inventors further found out the composition of the auxiliary materials placed or put into the ladle before or during tapping. Appropriate conditions such as, quantity, temperature, etc., complete the steel manufacturing method of the present invention. That is, the method of manufacturing steel of the present invention includes the steps of receiving molten steel from a self-melting steel furnace tapped into a ladle, and discharging the molten steel received by the ladle from the ladle for casting When the molten steel tapped from the steel melting furnace is received to the ladle, the slag thickness T calculated according to the following formula (1) satisfies the amount W of 0.02m or more and is composed of oxides The secondary raw materials are placed at the bottom of the ladle before receiving the molten steel, or put into the ladle at the same time as the molten steel is received, and the steel tapped from the molten steel furnace The liquid is received to the aforementioned ladle. T=(W/ρ)/((π·D ² )/4) (1) T: slag thickness (m) D: ladle diameter (m) ρ: molten oxide density (=3000kg/m ³ ) W: The amount of auxiliary materials (kg)

Previously, a large number of methods of adding secondary raw materials during tapping have been proposed. However, most of them focus on the modification of the lower oxides in the slag rather than the suppression of nitrogen absorption. Most of the by-products added are quicklime. In addition, in most cases the addition period is during or after tapping. Rear. According to the method of the present invention, the composition of the slag after tapping is the same as the previous method, but it is quite different from the previous method in the following aspects, namely, in order to effectively suppress the nitrogen absorption in the initial stage of the tapping, the auxiliary The period of raw materials is earlier than before. Place a certain amount of secondary raw materials in the ladle before tapping, or put the secondary raw materials into the ladle at the same time as the steel is tapped. More than a certain amount of auxiliary materials are melted.

In order to confirm the effect of suppressing nitrogen absorption after the secondary raw materials were placed, a 2ton-scale molten steel experiment was conducted and its behavior was studied. The low-nitrogen deoxidized molten steel 2 ton melted in the induction furnace is tapped into a preheated ladle in about 50 seconds, and the nitrogen concentration before and after tapping is investigated. At this time, the various conditions such as the composition and temperature of the molten steel are set to be the same, and the composition-adjusted synthetic flux (oxide) is placed in the ladle, and the steel is tapped in this state. At this time, the influence of the composition of the synthetic flux, the amount of flux, and the preheating temperature in the ladle was investigated. At this time, the amount of nitrogen absorption (hereinafter, Δ[N]) before and after steel tapping was studied. Compared with the Δ[N] under the condition (run1) where no synthetic flux was placed, it was improved by more than 4ppm. It is judged that there is nitrogen absorption suppression effect. Hereinafter, the component content in the synthetic flux means mass %. The test conditions and test results are shown in Table 1.

[Table 1]

First, the Δ[N] under the condition of no synthetic flux is 26ppm. Relative to this result, put 50kg (=basic conditions) of synthetic flux (CaO/Al ₂ O ₃ =2.0) with CaO=60%, Al ₂ O ₃ =30%, SiO ₂ =10% in a ladle The molten steel was tapped in the inner state, and the result Δ[N] was 21 ppm, confirming a significant nitrogen absorption suppression effect. The situation around the waterfall pool was photographed to investigate the conditions in the ladle at the time of tapping. The results confirmed the following: the synthetic flux was melted by contact with the molten steel injected into the ladle and existed around the waterfall pool And the molten slag mixed with solid phase and liquid phase will be entrained by the waterfall pool. There is no difference in the conditions except for the presence or absence of synthetic flux. Therefore, it is presumed that the main reason for the suppression of nitrogen absorption is that the molten slag entrained in the waterfall pool covers a part of the surface of the bubbles, thereby making the molten steel and air react. cut back. On the other hand, the synthetic flux is suspended on the wall slightly raised from the bottom of the ladle, and added to the steel surface 15 seconds after the start of tapping. Under this condition, the result is Δ[N] At 24 ppm, no significant nitrogen uptake inhibitory effect was found. At this time, it was confirmed that the synthetic flux did not melt until the end of tapping. However, the added synthetic flux was not found to melt during the first half to the middle of tapping, where the amount of nitrogen absorption was the largest. It is assumed that the reduction of molten steel and air was not achieved The area of the reaction boundary.

Next, with respect to the basic conditions (fixed flux composition, no preheating), the amount of synthetic flux to be placed was changed to investigate the nitrogen absorption suppression effect. The result is shown in Figure 1. Based on the amount of synthetic flux to be placed W and the size of the ladle (the ladle diameter D), the slag thickness T in the ladle is calculated according to the aforementioned formula (1), When the thickness T of the slag did not reach 0.02 m, as a result, no significant nitrogen absorption suppression effect was found. On the other hand, if the thickness of the slag exceeds 0.05 m, the effect of suppressing nitrogen absorption becomes saturated. Based on this situation, it is presumed that when the liquid phase or the liquid phase containing the solid phase entrained in the waterfall pool is less than a certain amount, the area of the reaction interface between the molten steel and the air cannot be sufficiently covered, and the nitrogen absorption suppression effect cannot be obtained. Furthermore, even if the liquid phase entrained in the waterfall pool or the liquid phase containing the solid phase is too much, the nitrogen absorption inhibitory effect is still saturated. Therefore, it is considered that there is a suitable upper limit for the amount of synthetic flux W placed in the ladle.

Also, set the amount of synthetic flux to be placed at a certain level (50kg), and change the composition of the synthetic flux to be placed with the composition shown in Table 1, and investigate the nitrogen absorption suppression effect without preheating. As a result, the composition of the synthetic flux is CaO/Al ₂ O ₃ : 0.8 to 4.0 ((2) formula), 5%≦SiO ₂ ≦10% ((3) formula), MgO≦10% ((4) Under the condition of formula), stable nitrogen absorption inhibition effect can be obtained. The composition of the synthetic flux when a stable nitrogen absorption suppression effect can be obtained is consistent with the condition that the liquid phase ratio near the molten steel temperature is higher. It is believed that the higher the liquid phase ratio, the greater the coating effect on the bubble surface in the waterfall pool.

Furthermore, with respect to the basic conditions, the synthetic flux placed in the ladle was preheated with a burner, and the temperature of the synthetic flux immediately before tapping was changed to investigate the nitrogen absorption suppression effect. In addition, the temperature of the synthetic flux was investigated by a thermocouple set in the steel drum. As a result, as shown in Figure 2, when the temperature of the synthetic flux is heated to above 800°C, a significant nitrogen absorption suppression effect is obtained. On the other hand, if the preheating temperature of the synthetic flux exceeds 1150°C, the nitrogen absorption suppression effect will be saturated. It is believed that the reason is that by preheating, the time until the flux reaches the start of melting is shortened, and nitrogen absorption immediately after tapping is suppressed.

Hereinafter, the embodiment of the steel manufacturing method of the present invention will be described in further detail. In the case of low-nitrogen steel, the molten iron with higher carbon concentration transported from the blast furnace or electric furnace is charged into the converter and other steel melting furnaces, and the carbon in the steel is removed in the form of CO gas by oxygen blowing . At this time, in the steel melting furnace, the partial pressure of nitrogen in the furnace is reduced by the C+O=CO reaction. In addition, combined with the stirring effect of bottom blowing and top blowing, the nitrogen concentration in the steel is reduced to About 10ppm. Regarding the molten steel after decarburization, in order to adjust the composition or degassing, the steel is tapped from the melting furnace to the ladle. Thereafter, the molten steel whose composition or temperature has been adjusted is supplied to the casting process, and after casting, it is shipped as a product after heating, rolling, heat treatment, surface treatment and other steps.

Usually, after the ladle is preheated by the burner, it is transported by the transfer trolley to the steel melting furnace to receive the molten steel. Usually, secondary raw materials such as quicklime are mostly added to the molten steel after tapping. However, when applying the steel manufacturing method of the present invention, it is necessary to place a certain amount of secondary raw materials in the ladle before receiving the molten steel. Or put a certain amount of secondary raw materials into the ladle while receiving molten steel. It is suitable to put the auxiliary materials into the steel drum before or during preheating. The form of the secondary raw material is preferably granular, so that it will not escape due to the updraft during preheating or during tapping; however, when preheating is performed, it is usually carried out with a lid covering the upper part of the ladle. Hot, so powdery auxiliary materials can also be used. It is appropriate to use the slag thickness as shown in formula (1) at the time when the ladle is transported directly below the melting furnace, at the latest when the molten steel starts to tap from the melting furnace (beginning to receive molten steel) The auxiliary raw material of the amount W determined so that T becomes 0.02m or more (preferably 0.1m or less, more preferably 0.05m or less) is put into the ladle. In addition, after the tapping starts, it is necessary to quickly melt the secondary raw materials. In addition, when starting to receive molten steel and at the same time put the auxiliary raw materials into the ladle, it is better to start the molten steel into the ladle within 10 seconds after starting the self-melting furnace It is more preferable to start the injection of auxiliary materials into the ladle within 5 seconds, and it is more desirable to start the injection of auxiliary materials into the ladle at the same time as the molten steel is injected. In addition, when starting to receive molten steel and at the same time put the auxiliary raw materials into the ladle, after starting to receive the molten steel, it should be completed within 60 seconds, more preferably within 40 seconds, and further preferably within 20 seconds. The thickness T of the injected slag becomes the secondary raw material in the amount W of 0.02m or more. In addition, the secondary raw materials may also be a combination of the following two: placing the secondary raw materials in the ladle before receiving the molten steel, and putting the secondary raw materials into the ladle at the same time as the receiving of the molten steel starts. That is, it is also possible to pre-place the secondary raw material of the amount W1 in the ladle before starting to receive the molten steel, and further put the secondary raw material of the amount W2 into the ladle when the molten steel is started, thereby making the secondary raw material The total amount (W1+W2) reaches the amount W calculated so that the slag thickness T shown in equation (1) satisfies 0.02 m or more. In addition, some minutes after the start of receiving molten steel, Al alloys are sometimes added for the purpose of deoxidation, etc., but the ingredients added for this purpose and timing are not included in the slag shown in formula (1) In the secondary raw material whose thickness T satisfies the amount W obtained by the method of 0.02m or more.

In order to obtain the low nitriding effect brought by the steel manufacturing method of the present invention, it is necessary to have molten slag in the waterfall pool during the process of receiving molten steel. The so-called process of receiving molten steel refers to the period from the start of injecting the molten steel into the ladle from the melting furnace to the completion of the injection after at least 1 minute, preferably the period from 30 seconds after the start of the injection of the molten steel to the completion of the injection. The so-called molten slag refers to the state in which the auxiliary materials placed or thrown into the ladle melt and become a liquid phase or a liquid phase containing a solid phase. In the present invention, by calculation using general-purpose thermodynamic calculation software, etc., a state where the liquid phase ratio is 50% or more is regarded as the liquid phase slag. The so-called waterfall pool refers to the entrainment and rise of bubbles generated by the gas phase around the injection stream when the injection stream enters the molten steel in the ladle, usually in the injection stream and the ladle. Produced directly below the part in contact with molten steel. If the waterfall basin is covered by molten slag during the tapping process, the low nitriding effect of the present invention can be obtained. When the molten steel tapped from the self-melting steel furnace is received to the ladle, the slag thickness T shown in formula (1) shall meet the requirements of 0.02m or more before or when the molten steel is received. 0.1m or less, more preferably 0.05m or less). Place or throw the secondary raw material composed of oxides in the ladle, and take the molten steel from the self-melting furnace to The ladle can make molten slag exist in the waterfall pool during the process of receiving molten steel.

In the present invention, the auxiliary materials to be placed or put into the ladle are auxiliary materials composed of oxides. Therefore, it does not contain carbon oxides, fluorides, carbides, etc. For example, Patent Document 3 discloses an invention in which calcium carbonate is placed in order to reduce the nitrogen concentration in the ambient gas in a steel ladle. In contrast, the purpose of the present invention is to prevent the phenomenon of nitrogen absorption in the waterfall basin by the molten slag on the surface of the molten steel, so calcium carbonate is not added. Calcium carbonate is accompanied by an endothermic reaction during decomposition, so that the temperature of the molten steel is lowered, which is also unfavorable in this respect. In addition, if fluoride or other fluoride is added, it hinders the recovery of the generated slag, so fluoride is not added. Furthermore, since it is not for dephosphorization or desulfurization, carbides such as calcium carbide are not added.

In addition, the secondary raw materials composed of oxides to be placed or put into the ladle should be adjusted in advance to the composition of CaO/Al ₂ O ₃ : 0.8～4.0 ((2) formula), 5%≦SiO ₂ ≦ Add 10% ((3) formula) and MgO≦10% ((4) formula). By setting it as such a composition range, the melting temperature of the auxiliary raw materials can be suitably reduced. More preferably, the MgO content is set to 5% or more. Furthermore, in addition to the aforementioned CaO, Al ₂ O ₃ , SiO ₂ , and MgO, the components contained in the secondary raw materials are also allowed to contain less than 5% of oxide components such as MnO and FeO. In addition, volatile matter or impurities are also allowed. That is, it is suitable as long as it satisfies the aforementioned formula (5).

The secondary raw materials placed in the ladle are preferably preheated together with the ladle, and are suitable to be preheated to above 800°C. The preheating temperature of the secondary raw materials can be evaluated by measuring the surface temperature of the secondary raw materials placed in the ladle with a radiation thermometer.

By using the steel manufacturing method of the present invention as described above, the increase in nitrogen concentration can be suppressed during tapping, and therefore low nitrogen steel can be manufactured economically and stably. Furthermore, according to the steel manufacturing method of the present invention, the increase in nitrogen concentration during tapping can be effectively suppressed, but the nitrogen concentration in the manufactured steel is not limited. The method of manufacturing steel of the present invention is very effective for carbon steel, but it is also effective for manufacturing stainless steel and alloy steel other than carbon steel. [Example]

According to the conditions of the examples and comparative examples of the molten steel shown below, the nitrogen absorption behavior evaluation test during tapping was performed to confirm the nitrogen absorption suppression effect. The molten iron (equivalent to 4.5% carbon content) transported from the blast furnace is charged into the converter and subjected to oxygen conversion. The composition after converter blowing is [C]=0.06～0.14%, [Si]=0.01～0.05%, [Mn]=0.1～0.4%, [P]=0.01～0.03%, [N]=9～12ppm , And the remainder is Fe and impurities. The processing capacity is 300ton scale, the ladle diameter (inner diameter) is 3.9m, and the tapping time is about 5 minutes. Before tapping, before preheating the ladle, or after preheating the ladle, place a specific amount of secondary raw materials adjusted in composition at the bottom of the ladle, and after the ladle is transported directly below the converter, Undertake molten steel. Alternatively, the auxiliary raw materials are poured into the ladle while receiving molten steel. When tapping, 2 minutes after the start of tapping, the alloy containing Al is poured in by entraining the tapping stream. In addition, 3 to 4 minutes after the start of tapping, the auxiliary raw materials (oxides) were added to the ladle, thereby creating the "final slag thickness t" shown in Table 2.

In order to confirm the effect of suppressing nitrogen absorption, samples of molten steel in the converter before tapping and the ladle after tapping were taken, and the nitrogen concentration change Δ[N](ppm) before and after tapping was taken as the amount of nitrogen absorption To evaluate. The test conditions are listed in Table 2. In the column of "Nitrogen Uptake Suppression Effect" in Table 2, if Δ[N] exceeds 17ppm and is 20ppm or less, it is deemed to have nitrogen uptake suppression effect and set to "C", Δ[N] exceeds 15ppm and is 17ppm or less If it was judged to have an excellent nitrogen absorption suppression effect, it was set as "B". In addition, if Δ[N] is 15 ppm or less, it is judged that there is a significant nitrogen uptake suppression effect and is set to "A". When Δ[N] exceeds 20 ppm, it is deemed that no nitrogen absorption suppression effect is found, and it is set to "D".

[Table 2]

Test No.1 is the condition that the secondary raw material is not placed in the ladle, and Test No.2 and No.3 are the conditions under which the thickness of the slag deviates from the scope of the present invention even though the secondary raw material is placed in the ladle. Comparative example. The Δ[N] of Test No. 2 and Test No. 3 with insufficient auxiliary materials to be placed was 23 to 24 ppm, and no nitrogen absorption inhibitory effect was found.

Test No. 4 to Test No. 17 are examples satisfying the requirements of the present invention, and Δ[N] is 20 ppm or less, and the effect of suppressing nitrogen absorption is confirmed. Test No.10 to Test No.13 are the conditions under which the composition of the secondary raw materials placed in the ladle is adjusted to a suitable range, and the Δ[N] is below 17 ppm, which is judged to have an excellent nitrogen absorption suppression effect. Test No. 3, 5, 9 and Test No. 14 to Test No. 16 are the conditions under which the preheating temperature of the auxiliary raw materials placed is changed. Comparing Test No.5 with Test No.7, Test No.5, which has a higher preheating temperature of the secondary raw materials, has a greater nitrogen absorption suppression effect. It can be seen that by increasing the preheating temperature of the secondary raw materials, excellent results can be obtained. Nitrogen absorption inhibition effect. The comparison between Test No. 11 and Test No. 14 also clarified the situation. Regarding Test No. 14, the composition of the secondary raw materials was controlled within the appropriate range of the present invention, and the preheating temperature of the secondary raw materials was set to 800°C or higher. Obtain a significant nitrogen uptake inhibitory effect. The same applies to test Nos. 15 and 16. Test No. 18 is an example in which the secondary raw materials are injected while receiving molten steel in a ladle. Δ[N] is 20 ppm, which is lower than the comparative example, and the effect of suppressing nitrogen absorption is confirmed. [Industrial availability]

Since it can effectively suppress the nitrogen absorption of molten iron during tapping, it is beneficial to the manufacturing method of low nitrogen steel.

Regarding the invention of Japanese Patent Application 2018-122844 filed on June 28, 2018, the entirety is incorporated into this specification by reference. All the documents, patent applications, and technical standards described in this specification are incorporated into this specification by reference to the same extent as when each document, patent application, and technical standards are specifically and separately described.

Figure 1 is a graph showing the relationship between the thickness of the slag in the ladle and the amount of nitrogen absorbed. Figure 2 is a graph showing the relationship between the temperature of the synthetic flux immediately before tapping and the amount of nitrogen absorbed.

Claims (5)

A method for manufacturing steel, comprising the steps of receiving molten steel from a self-melting steel furnace tapping into a ladle, and discharging the molten steel received by the ladle from the ladle to perform casting ; When the molten steel tapped from the steel melting furnace is received to the ladle, the slag thickness T calculated according to the following formula (1) meets the amount W of 0.02m or more and is composed of oxides The auxiliary raw materials of, are placed at the bottom of the ladle before receiving the molten steel, and the molten steel tapped from the molten steel furnace is taken to the ladle; T=(W/ρ)/( (π．D ² )/4) (1) T: slag thickness (m) D: ladle diameter (m) ρ: molten oxide density (=3000kg/m ³ ) W: amount of auxiliary materials (kg ). Such as the steel manufacturing method of claim 1, where the composition of the aforementioned secondary raw materials satisfies: CaO/Al ₂ O ₃ ：0.8~4.0 (2) 5%≦SiO ₂ ≦10% (3) MgO≦10% (4) CaO +Al ₂ O ₃ +SiO ₂ +MgO≧90% (5); among them, the molecular symbol in the formulas (2)~(5) means the content (mass%) of the molecule. Such as the method of manufacturing steel in claim 1, wherein the aforementioned sub The amount W of the raw material is the amount that the slag thickness T calculated according to the above formula (1) satisfies 0.1 m or less. The steel manufacturing method of claim 2, wherein the amount W of the aforementioned secondary raw materials is an amount that the aforementioned slag thickness T calculated according to the aforementioned formula (1) satisfies 0.1 m or less. The method for manufacturing steel according to any one of claims 1 to 4, wherein the auxiliary raw materials placed in the steel ladle are preheated, and the molten steel is heated in a state where the temperature of the auxiliary raw materials is above 800°C To undertake to the aforementioned ladle.

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