JP2010132982A - Method of denitrizing molten steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of melting a steel, with which in a degassing treatment by using a vacuum-degassing treating apparatus, nitrogen concentration in the molten steel after treatment is simply reduced to ≤25 ppm. <P>SOLUTION: When the molten steel composed of, by mass, 0.002-0.4% C, 0.1-2% Mn, 0.001-1% Si, ≤0.0025% S, 0.005-1% Al, ≤0.007% N, and ≤0.003% O, is subjected to the degassing-treatment with the vacuum-degassing treating apparatus, lanthanoids of one or more selected from a group composed of La, Ce and Nd, are added to the molten steel, and total concentration of the lanthanoids during degassing-treatment, is made to be ≥0.0005 mass%. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for melting ultra-low sulfur ultra-low oxygen ultra-low nitrogen thick steel plate used for line pipes, gas tanks, steel structures, etc. The present invention relates to a steel melting method for reducing the sulfur concentration to an extremely low concentration range.

  Sulfur (hereinafter referred to as “S”), oxygen (hereinafter referred to as “O”), and nitrogen (hereinafter referred to as “N”) in steel cause various defects and a decrease in weldability. Therefore, many of these reduction techniques have been developed conventionally.

  However, in recent years, the required performance has further increased, and simple S reduction, O reduction, or N reduction has become insufficient, and it has become necessary to simultaneously reduce S, O, and N. In order to cope with this, it is necessary to improve conventional techniques for reducing impurities of S, O, and N, and techniques for that purpose have been proposed (for example, Patent Documents 1 to 3).

In addition, it has been considered that S, O, and N can be simultaneously reduced by using conventionally developed technologies for reducing S, O, and N, but it has been found that this is difficult.
The first reason is a productivity problem. If the treatments for reducing S, O, and N are simply combined, the treatment time is not only long, but such a long treatment is virtually impossible from the viewpoint of the temperature drop of the molten steel.

  The second reason is that chemical reactions of the reduction treatment of each element interfere with each other. For example, when S and O are reduced, it becomes difficult to reduce N because the molten steel easily absorbs nitrogen. Moreover, if molten steel stirring is weakened in order to reduce O by suppressing inclusion inclusion, desulfurization is delayed and S reduction becomes difficult.

For these reasons, there are many problems in industrially reducing S, O, and N at the same time and increasing the amount of reduction compared to the prior art.
JP-A-5-171253 JP 2000-297318 A JP 2001-181730 A

  In order to solve this problem, the present inventors previously described in Japanese Patent Application No. 2007-324915, by appropriately combining CaO-based flux desulfurization ability and REM ability, low O, low S and low N. Presented the technology to work at the same time. In this Japanese Patent Application No. 2007-324915, it is possible to manufacture a high-performance and high-performance product by simply reducing O, S, and N. On the other hand, when general N steel requires only low N or general In some cases, it is necessary to slightly reduce the N concentration in order to satisfy a typical N standard component. In these cases, since Japanese Patent Application No. 2007-324915 becomes excessive performance, a method for achieving the object at a lower cost is also necessary.

  In view of this, the present invention proposes a technique that enables denitrification of molten steel more easily and inexpensively and enables mass production of low nitrogen steel. That is, an object of the present invention is to provide a steel melting method that easily reduces the nitrogen concentration to 25 ppm or less in a degassing process using a vacuum degassing apparatus.

1. Technology for promoting denitrification by RH In the technology disclosed in Japanese Patent Application No. 2007-324915, in addition to high-efficiency desulfurization recently required in RH, low O and low N are simultaneously achieved. This method is based on the technical idea of reducing the N concentration by reducing the S concentration and the O concentration. On the other hand, as described above, the main purpose of the present invention is to simply reduce nitrogen, so the N concentration is rather selected regardless of the reduction of the S concentration or the O concentration by highly efficient desulfurization. It is a method to reduce it. Therefore, the present inventors examined a denitrification promoting technique for reducing the N with conventional RH.

2. Molten steel components In conducting the study based on the above policy, first, molten steel components other than iron to be treated in the present invention were specified for the following reasons. In the present specification, “%” in the steel composition and REM concentration (details will be described later) means “mass%” unless otherwise specified.

  C: C acts as a deoxidizing element under reduced pressure and affects the activities of S and N. For this reason, if C is less than 0.002%, the effect of reducing oxygen becomes unstable, and if it exceeds 0.4%, the activity of S and N changes greatly, and the reaction mechanism changes. Therefore, C is set to 0.002% or more and 0.4% or less.

  Mn: Mn is also a deoxidizing element and is an essential element because it improves various steel properties. Therefore, if it is less than 0.1%, deoxidation becomes unstable, and if it exceeds 2%, the activity of S is lowered and desulfurization is difficult. Therefore, the Mn concentration is set to 0.1% or more and 2% or less.

  Si: Si is an element indispensable for deoxidation stability as well as Mn. However, deoxidation is unstable if it is less than 0.001%, and if it exceeds 1%, N activity is increased to promote denitrification. . In the present invention, Si is set to 1% or less because the object is to efficiently reduce nitrogen-containing impurities in a component system that is not easy to reduce N.

  Al: Al is an element having the strongest deoxidizing power, and is essential for realizing low O, low S and low N. To obtain this deoxidation effect, 0.005% or more is necessary. On the other hand, if it exceeds 1%, the dissolved oxygen concentration becomes high again and it becomes difficult to realize low O, so 1% or less is necessary.

  S: S is an element to be removed, but if it exceeds 0.0025%, the amount of desulfurization agent used increases greatly in terms of material balance, and the cost increases. This tendency becomes more prominent when it exceeds 0.005%. Therefore, in the present invention, 0.0025% or less of molten steel is a treatment target.

  N: N is also an element to be removed as in S, but if it exceeds 0.007%, it becomes difficult to shorten the treatment time, so 0.007% or less of molten steel was treated.

  O: O is also an element to be removed. However, when Si, Al, and Mn are in the above-mentioned concentration range, a large amount of non-metallic inclusions (hereinafter, “ "Inclusions") will be present in the molten steel. If a CaO-based flux described later is used in this state, the desulfurization ability is reduced by collision of the CaO flux with inclusions, so the O concentration needs to be 0.005% or less. This suppression of desulfurization ability lowering becomes more effective when the O concentration is 0.003% or less.

  In addition to the above essential components, one or more kinds selected from Cr, Ni, Cu, Mo, V, B and Nb, W, etc. are optionally added for the purpose of securing other product characteristics. May be. Preferably, Cr: 20% or less, Ni: 10% or less, Cu: 0.1% or less, Mo: 2% or less, V: 2% or less, B: 0.003% or less, Nb: 0.1% or less It is. These elements do not affect the refining reaction intended by the present invention. Therefore, for the purpose of improving product properties such as strength, corrosion resistance, and weldability, these elements may be arbitrarily added to adjust the concentration in the product.

3. Basic Policy for Problem Solving Next, on the premise of the above steel composition, a technology capable of imparting low O and low N to the conventional RH desulfurization technology was specifically examined.

  Low S and O are advantageous for achieving low N, and low O is advantageous for achieving low S. Then, it is the highest priority to achieve low O, and there is a possibility that the effect may be improved if elements having strong affinity with S are used together. That is, it is desirable to use an element that has deoxidizing power and can achieve low O, and at the same time has an affinity for S. Since the present invention is premised on vacuum processing, it is desirable that the element has a low vapor pressure.

  In view of the above, rare earth elements such as La, Ce, Nd, and Y (hereinafter referred to as “REM”) are known as elements that satisfy such conditions. That is, if these REMs are used, it is expected that low S, low O, low N steel can be obtained by a simple method.

However, when REM is used, there are the following problems, which cannot be easily used in reality.
First, since the specific gravity of inclusions resulting from the reaction of REM with O and S is close to the specific gravity of the molten steel, they do not rise. Therefore, when REM is simply added, the dissolved S concentration and O concentration decrease, but the S concentration and O concentration in the steel do not change much, but rather the cleanliness deteriorates.

Secondly, the presence of these inclusions induces problems such as nozzle clogging during casting and significantly reduces productivity.
Thirdly, REM itself may affect steel properties, and it may be inappropriate to include these in steel.

  In other words, refining effects such as low N can be expected by using REM, but the above problem occurs even if trying to promote low N by reducing O and S by using only REM. It cannot be achieved. In order to solve this problem, the present inventors previously described in Japanese Patent Application No. 2007-324915, by appropriately combining CaO-based flux desulfurization ability and REM ability, low O, low S and low N. Presented the technology to work at the same time. This method has many advantages that by using REM and CaO top blowing in combination, cleanliness, castability, and avoiding changes in steel properties due to REM, it is possible to produce ultra-low sulfur, ultra-low nitrogen and high cleanliness.

Next, a method for simply reducing only the nitrogen concentration, which is the object of the present invention, will be described.
First, the denitrification rate, desulfurization and deoxidation rates are examined. Desulfurization is mainly due to the reaction with the CaO flux, and deoxidation is due to the reaction between the flux and inclusions or the floating of inclusions from the molten steel, whereas denitrification is the reaction between the molten steel and the gas phase. Then, it is thought that the speed of denitrification, which is a simple degassing reaction, is faster than the reaction with the flux and the rising from the molten steel. Next, since lanthanoids such as La and Ce are strong deoxidizing elements, the concentration gradually decreases during the treatment. As the concentration decreases, the O and S concentrations that balance with the lanthanoid gradually increase. In order to reduce S and O at the same time as reducing N, it is naturally necessary to use CaO flux or the like in order to suppress this increase in O and S concentration. In this case, it is only necessary to reduce S and O only during the denitrification reaction, and the S and O concentration may be increased after denitrification.
Then, when it is desired to promote only low N, CaO flux or the like becomes unnecessary, and only the lanthanoid may be added. That is, the lanthanoid is added only during denitrification, and S and O may be reduced only during denitrification.

However, it is believed that there is an appropriate concentration of lanthanoid during denitrification. If the lanthanoid concentration is too low, the denitrification promoting effect cannot be obtained. If the lanthanoid concentration is excessively high, not only the effect is saturated, but also the inclusions completely change to lanthanoid oxysulfides such as La ₂ O ₃ and LaS. As a result, castability deteriorates and product performance is affected, the number of inclusions increases, and cleanliness deteriorates.

  Therefore, it is necessary to control the denitrification to an appropriate lanthanoid that sufficiently promotes denitrification and at the same time does not completely change the inclusions to the lanthanoid system.

4). Investigation using molten steel Therefore, we conducted a molten steel experiment using lanthanoids and investigated the relationship between the lanthanoid concentration and the denitrification reaction.

  The survey was conducted as follows. 1500 kg of molten steel was heated to 1873K, the atmosphere was an Ar atmosphere, and the pressure was 133 to 1330 Pa. Thereafter, the molten steel components were adjusted to the above range, and one or more selected from the group consisting of La, Ce and Nd were mixed as lanthanoids, and a predetermined amount was added to the molten steel all at once. Note that REM includes La, Ce, Nd, Y, and the like, and since these properties are close to each other, similar effects are expected. However, in the present invention, investigation was performed using lanthanoids La, Ce, and Nd. .

Thereafter, the molten steel was held for a predetermined time, and the denitrification rate from the molten steel and the inclusion composition in the molten steel were measured. The S concentration immediately before the addition of the lanthanoid is 30 ppm, and the N concentration is 30 ppm.
FIG. 1 shows the relationship between the molten steel N concentration 20 minutes after the addition of the lanthanoid and the lanthanoid concentration in the molten steel (hereinafter referred to as “[LA]”). [LA] means the concentration when one of La, Ce and Nd is added, and the total concentration when two or more of these are added. As the [LA] increases, the nitrogen concentration decreases, and the effect becomes particularly apparent when [LA] ≧ 0.0005%. When [LA] is 0.0005% or more, the nitrogen concentration decreases, but no significant decrease is observed, and it can be seen that the effect is not so high even if the [LA] concentration is increased. Therefore, it turns out that the lanthanoid concentration in molten steel needs to be 0.0005% or more.

  Next, the state of the inclusion was measured. The number of inclusions in the sample was measured with an optical microscope (magnification 1000 times). FIG. 2 shows the result of indexing the number at each [LA] concentration by normalizing the number at other concentrations, with the number of inclusions at [LA] = 1 ppm (0.0001%) being 1. . When [LA] ≦ 0.05%, the number of inclusions hardly changes, but when [LA]> 0.05%, the number of inclusions increases remarkably. This is presumably because lanthanoid deoxidation and desulfurization became dominant and lanthanoid oxysulfides were produced. Therefore, the influence of lanthanoids on denitrification promotion does not change greatly, and if inclusions increase, it can be concluded that 0.05% or less of lanthanoids is appropriate.

  From the above, as described in claim 1 for promoting denitrification, the lanthanoid concentration during denitrification is 0.0005% or more, and in the case of promoting denitrification without deteriorating cleanliness, 0.05% or less is appropriate.

  Note that the lanthanoid concentration may be set to the above range during the denitrification treatment, but since the lanthanoid is a strong deoxidizing element, it may react with slag or a slight oxygen source in the gas phase, and its concentration may decrease. . In addition, it is assumed that the lanthanoid concentration exceeds the upper limit of 0.05% due to variations in the lanthanoid yield. In order to avoid such a situation, the lanthanoid concentration during the denitrification treatment can be narrowly controlled by dividing or continuously adding the lanthanoid during the denitrification treatment.

The present invention has been made based on the above findings, and the gist thereof is as follows.
(1) A denitrification method for molten steel in which one or two or more lanthanoids selected from the group consisting of La, Ce and Nd are added to molten steel using a vacuum degassing apparatus, wherein the components of the molten steel are %: C: 0.002% to 0.4%, Mn: 0.1% to 2%, Si: 0.001% to 1%, S: 0.0025%, Al: 0.00%. After adjusting to 005% or more and 1% or less, N: 0.007% or less, O: 0.003% or less, the lanthanoid is added to the molten steel, and the lanthanoid concentration in the molten steel is totaled. A denitrification method for molten steel, characterized by being 0.0005 mass% or more.

  (2) A denitrification method for molten steel in which one or more lanthanoids selected from the group consisting of La, Ce and Nd are added to the molten steel using a vacuum degassing apparatus, wherein the components of the molten steel are %: C: 0.002% to 0.4%, Mn: 0.1% to 2%, Si: 0.001% to 1%, S: 0.0025%, Al: 0.00%. 005% or more and 1% or less, N: 0.007% or less, O: 0.003% or less, and then adding the lanthanoid to the molten steel, the total lanthanoid concentration in the molten steel A denitrification method for molten steel, characterized by being 0.0005 mass% or more and 0.05 mass% or less.

  (3) The method for denitrifying molten steel according to (1) or (2) above, wherein the addition of the lanthanoid to the molten steel is performed in a plurality of times or continuously.

  According to the present invention, a denitrification reaction from molten steel can be promoted easily and inexpensively, and an extremely low nitrogen concentration molten steel can be produced efficiently and in one step.

The best mode will be described by taking the case where the present invention is carried out using a converter and RH as an example.
(1) After the converter process before the RH treatment is completed, the molten steel is discharged from the converter steel into the ladle. An alloy such as Si or Mn may be added at the time of steeling, or a faux-forming agent such as CaO may be added. Moreover, you may use a slag modifier and Al for the purpose of reducing a lower oxide in a slag at the time of steel production. At this time, the amount of slag is desirably 10 kg / ton or more. This is because when the amount of slag is small, the coating effect on the surface of the molten steel is reduced, and reoxidation and nitrogen absorption from the atmosphere are likely to occur. Moreover, it is preferable that the sum total of FeO and MnO in a slag is 3 mass% or less, More preferably, a slag composition is 1.5 mass% or less. This is because if the FeO and MnO concentrations in the slag are high, cleanliness deterioration due to re-oxidation from the refining process to the end of casting tends to proceed.

  Although the ladle moves to RH, preliminary treatment may be performed using a ladle refining device such as blowing an inert gas as necessary. In particular, when suppressing nitrogen absorption during steel output, it is effective to suppress the amount of Al and Si added during steel output. In this case, since it becomes difficult to control the slag composition, pretreatment is preferably performed using a ladle refining device such as blowing an inert gas before the RH treatment for the purpose of slag control. However, when the treatment using the ladle refining apparatus is performed for a long time, the slag control is facilitated, while the N concentration in the molten steel gradually increases and the treatment time becomes longer and the productivity is lowered. Therefore, even when using a ladle refining apparatus, the processing time is preferably within 10 minutes.

(2) Treatment with RH After the ladle is transferred to RH, the treatment is started immediately. The treatment with RH includes denitrification by adding a lanthanoid according to the present invention, in addition to normal vacuum degassing such as dehydrogenation, temperature adjustment of molten steel, and component adjustment. These treatments may be carried out in any order, but preferably the order of temperature adjustment, component adjustment, and denitrification by addition of the lanthanoid according to the present invention. This is due to the following reason. Since the higher the temperature, the higher the temperature of the denitrification speed, so the effect is higher when the present invention is implemented after increasing the molten steel temperature by temperature adjustment. Furthermore, the denitrification effect is more stable when an element having deoxidizing power such as Al is added before adding the lanthanoid. In addition, when a large amount of alloy is added to adjust the components after denitrification by addition of lanthanoid, nitrogen contamination may occur.

(3) Lanthanoid addition method and addition amount Next, the type of lanthanoid to be added will be described. The lanthanoid to be added may be a pure metal such as La, Ce, or Nd, or a mixture thereof such as misch metal. Moreover, you may use the alloy and mixture with metals other than a lanthanoid, for example, Al, Ca, etc.

  Next, a method for adding a lanthanoid will be described. The lanthanoid may be added all at once from the RH vacuum chamber. Moreover, you may add a metal particle or a metal powder continuously via an upper blowing lance. Moreover, you may add to a molten steel in a ladle using a wire etc. In the case of adding continuously, in addition to continuously adding for 30 seconds or more, including intermittently adding between them. However, it is necessary to maintain the above lanthanoid concentration (0.0005% or more) during the denitrification treatment. In order to shorten the treatment time, it is necessary to increase the lanthanoid concentration in the molten steel in a short time. Therefore, the addition in the vacuum chamber is more desirable than the wire addition method having a slow supply rate.

  Further, the atmospheric pressure in the vacuum chamber after the addition of the lanthanoid is desirably lower, preferably 1.3 kPa or less, more preferably 0.66 kPa or less. Furthermore, it is desirable that the reflux gas flow rate be larger, and the reflux gas flow rate is 4 or more and 15 Nl / (min · ton of molten steel) in Ar. If it is less than 4, the flow rate will be limited, and if it exceeds 15 the flow rate will be saturated.

  The denitrification time after the addition of the lanthanoid is preferably 10 minutes or longer, more preferably 15 minutes or longer and 20 minutes or shorter. If it is less than 10 minutes, considering the uniform mixing time of the molten steel, the substantial mixing time becomes several minutes, and sufficient denitrification time cannot be secured. If the time is longer than 20 minutes, the effect is saturated, and in addition, adverse effects such as a decrease in molten steel temperature occur.

  Next, the amount of lanthanoid added will be described. Although it may be added by the above method so as to satisfy the above lanthanoid concentration (0.0005% or more), it is necessary to consider the lanthanoid yield. The yield may be determined empirically for each apparatus, and the addition amount may be determined in consideration of this yield. In the experiments conducted by the present inventors, the lanthanoid yield is about 70%. However, [LA] after the denitrification treatment needs to be 0.0005% or more.

  According to the present technology, lanthanoid remains in the molten steel after the denitrification treatment, and there are the following methods for this residual lanthanoid. The first is a case where lanthanoid is used as a steel component, and in that case, the process may be terminated as it is. The second is a case where a lanthanoid is not necessary as a steel component. In this case, it is necessary to remove the lanthanoid. The lanthanoid can be easily removed by supplying oxygen to the molten steel. Since lanthanoids have a strong affinity for oxygen, supply of oxygen to molten steel selectively removes lanthanoids. As a method for supplying oxygen, there are a method in which oxygen gas is blown or blown into molten steel, and a method in which a solid oxygen source such as iron oxide is added. The amount of oxygen to be supplied may be an amount that satisfies the material balance. In addition, although this lanthanoid removal process produces | generates a lanthanoid oxide inclusion temporarily, if a lanthanoid in molten steel lose | disappears, this lanthanoid type inclusion will also disappear according to a thermodynamic deoxidation equilibrium.

(4) Other suitable conditions The weight concentration ratio CaO / Al ₂ O ₃ of CaO to Al ₂ O ₃ in the slag during the RH treatment is preferably 1.0 or more, and more preferably 1.3 or more 3 .5 or less. If the CaO / Al ₂ O ₃ ratio is less than 1.0, the equilibrium oxygen tends to be high, and the lanthanide effect may become unstable. If it exceeds 3.5, in addition to the increase in the amount of slag, there is a concern that the operability will deteriorate, such as solidification of the slag.

  According to the present invention, denitrification from molten steel is promoted, but when the RH nitrogen absorption prevention technique is used in combination with the present invention, the effect is further enhanced. As a method for preventing nitrogen absorption, a method of purging the space between the RH lower tank and the ladle with an inert gas or the like is known in addition to optimization of the dip tube structure and the refractory material.

  Further, the present invention may be combined with an inert gas top blowing. By blowing the inert gas on the surface of the molten steel in the vacuum chamber, in addition to reducing the gas phase mass transfer resistance, the effect can be further enhanced by reducing the nitrogen gas partial pressure in the vacuum chamber.

  In addition, when this invention is performed by VOD or a tank degassing apparatus, it becomes the same form as RH, but since the reflux is not performed in VOD or tank degassing, it is desirable that the molten steel gas stirring is strong. Furthermore, in the case of VOD or tank degassing, slag is present on the surface of the molten steel, but this desirable slag composition is the same as that of RH, but the amount of slag is desirably 100 kg / molten steel ton or less, more preferably 80 kg / molten steel ton or less. More desirable. When the amount of slag increases beyond 100 kg / molten steel ton, the molten steel surface pressure due to the slag weight decreases, and the denitrification reaction may be inhibited. When the amount of slag is 80 kg / molten steel ton or less, the influence of slag becomes small.

The hot metal that has been subjected to hot metal desulfurization and hot metal dephosphorization treatment in advance as needed is charged into a 250 ton scale upper bottom blowing converter, and the C content in the molten iron is 0.03 to 0.06%. Rough decarburization blowing is performed until the end temperature is 1630 to 1690 ° C., and the raw decarburized molten steel is taken out into the ladle, and various deoxidizers and alloys are added at the time of the outgoing steel to add the molten steel components in the ladle. , C: 0.03-0.2%, Si: 0.01-0.3%, Mn: 0.2-1.3%, P: 0.005-0.013%, S: 20-24ppm , Sol. Al: 0.007 to 0.05%. Furthermore, CaO was added at the time of steel production, and the CaO / Al ₂ O ₃ weight ratio in the slag was adjusted to 2 to 2.5, and the total concentration of FeO and MnO in the slag was adjusted to 5% by mass or less.

  After that, the ladle is transferred to RH, the components are adjusted and the temperature is adjusted, and then the lanthanoid (composition: metal Nd, metal La, metal Ce or Nd, La, Ce or 1: 1 by weight ratio is set to the molten steel. The mixture was added to the molten steel at a mass% of 1.35 times the target mass% concentration, and after 2 minutes of addition, the pressure in the vacuum chamber was set to 133 Pa and the mixture was refluxed for 15 minutes for denitrification.

  Table 1 shows the molten steel components before the lanthanoid addition, the N concentration in the molten steel after the denitrification treatment by the lanthanoid addition, the total lanthanoid concentration [LA], and the inclusion number index. In addition, the unit of the molten steel component in a table | surface is the mass%, and "0" in [LA] shows that it was below an analysis minimum concentration. Further, the inclusion number index is a value normalized by using the number of test number 1 by measuring the number of inclusions by the same method as in FIG. 2 (magnification 1000 times, visual field 10 cm × 10 cm).

  Test Nos. 1 to 15 satisfy the present invention, and the N concentration in the molten steel is reduced to 0.0025% or less. In particular, Test Nos. 1 to 10 in which the [LA] is 0.05% by mass or less. Then, the increase in the number of inclusions is not recognized. Among them, test numbers 9 and 10 are the results obtained by dividing the predetermined lanthanoid into two parts and adding them in two portions immediately before the start of the denitrification treatment and 6 minutes after the first addition, but more than the test numbers 1 to 8. It can be seen that denitrification is promoted.

  Test Nos. 11 to 15 are results of adding the lanthanoid so that the [LA] is maintained at a high concentration exceeding 0.05 mass% and 0.10 mass% or less during the denitrification treatment. . In Test Nos. 11 to 15, denitrification proceeds to satisfy the first aspect of the present invention. However, since they do not satisfy claim 2, inclusions are increased.

Test numbers 16 to 20, which are comparative examples, are results in which the [LA] is lower than the range of the present invention, and an increase in the number of inclusions is not observed, but denitrification has not progressed much.
From the above results, by setting the lanthanoid concentration during denitrification to 0.0005% by mass or more in total, denitrification is promoted to N concentration of 0.0025% or less in molten steel, and during denitrification Denitrification can be promoted by dividing the lanthanoid concentration to 0.0005 mass% or more and 0.05 mass% or less to suppress deterioration of cleanliness of molten steel, and by adding the lanthanoid in multiple steps. It was confirmed that the effect could be further enhanced.

It is a graph which shows the relationship between the molten steel N density | concentration 20 minutes after lanthanoid addition, and the lanthanoid density | concentration [LA] in molten steel. It is a graph which shows the relationship between the lanthanoid density | concentration in molten steel and the inclusion number index | exponent 20 minutes after lanthanoid addition.

Claims (3)

  A denitrification method for molten steel in which one or two or more lanthanoids selected from the group consisting of La, Ce, and Nd are added to molten steel using a vacuum degassing apparatus, wherein the components of the molten steel are expressed in mass%. C: 0.002% to 0.4%, Mn: 0.1% to 2%, Si: 0.001% to 1%, S: 0.0025% or less, Al: 0.005% or more After adjusting so that it may become 1% or less, N: 0.007% or less, O: 0.003% or less, the said lanthanoid is added to this molten steel, and the said lanthanoid density | concentration in this molten steel is 0.0005 in total. A denitrification method for molten steel, characterized by comprising at least mass%.   A denitrification method for molten steel in which one or two or more lanthanoids selected from the group consisting of La, Ce, and Nd are added to molten steel using a vacuum degassing apparatus, wherein the components of the molten steel are expressed in mass%. C: 0.002% to 0.4%, Mn: 0.1% to 2%, Si: 0.001% to 1%, S: 0.0025% or less, Al: 0.005% or more After adjusting so that it may become 1% or less, N: 0.007% or less, O: 0.003% or less, the said lanthanoid is added to this molten steel, and the said lanthanoid density | concentration in this molten steel is 0.0005 in total. A denitrification method for molten steel, wherein the denitration method is characterized in that the content of the molten steel is 0.05% by mass or more. The method for denitrifying molten steel according to claim 1 or 2, wherein the addition of the lanthanoid to the molten steel is performed in a plurality of times or continuously.

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