JP2009084672A - Method of heating molten steel, and method for production of rolled steel material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a molten steel-heating method for efficiently elevating the temperature of the molten steel using a circulating flow type vacuum degassing apparatus, and to provide a method for production of a steel material using the heating method. <P>SOLUTION: The molten steel-heating method using the vacuum degassing apparatus generating the circulating flow of the molten steel between a vacuum vessel and a ladle via an immersion tube includes: adding Al to the molten steel having the oxygen concentration before temperature-elevating operation of 0.010-0.070 mass%; blowing gaseous oxygen on the surface of the molten steel in the vacuum vessel; and reacting the gaseous oxygen and Al to heat the molten steel. The operation of Al addition is performed at least one time during a gaseous oxygen feeding process, and the Al amount per one Al addition operation is 0.1 to 0.5 kg based on 1 ton of the molten steel. The initial Al addition is performed after at least the lapse of a prescribed time τ which is decided based on the amount (t) and reflux flow (t/s) of the molten steel from the start of gaseous oxygen supply. A second addition or further addition is performed after the lapse of the time τ from the last addition. This method enables suppression of the remaining amount of an Al<SB>2</SB>O<SB>3</SB>-based inclusion in the molten steel. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

The present invention relates to a method of heating molten steel by adding Al and supplying oxygen gas to molten steel containing a certain amount of dissolved oxygen in a circulating vacuum degassing apparatus, by reducing the amount of Al ₂ O ₃ inclusions remaining in the molten steel. relates to a technique to as much as possible a small amount of molten steel and Al ₂ O ₃ based inclusions amount contained in the steel material obtained therefrom.

  One of the purposes of so-called secondary refining, in which molten steel is extracted from a steelmaking furnace such as a converter and stored in a ladle, is adjustment of the molten steel temperature. The reason is to adjust the temperature to an appropriate temperature for a casting process such as continuous casting which is the next process of secondary refining.

  If the temperature is higher than the temperature suitable for casting, it is possible to easily lower the molten steel temperature by extending the refining time or introducing a cold iron material, but the cost increases. On the other hand, when the temperature is lower than the temperature suitable for casting, casting may be impossible in the worst case, and the molten steel temperature must be raised.

  On the other hand, in order to reduce the oxygen concentration in the steel and to obtain the effect of the alloy component having an affinity for oxygen, in general, steel having a strong affinity for oxygen in the molten steel stage is added to reduce the oxygen concentration in the steel. So-called Al deoxidized steel is widely used. For example, in ultra-low carbon steel that is widely used as a thin plate, a steelmaking furnace such as a converter is undeoxidized and further decarburized with a vacuum degasser, and then a deoxidizer such as Al is added as appropriate. And cast as Al deoxidized steel.

In recent years, by deoxidizing weaker deoxidized states other than Al deoxidation, steel types that obtain steel material characteristics different from Al deoxidized steel are increasing. In such steel types, among non-metallic inclusions inevitably included in the steel making process, inclusions mainly composed of Al ₂ O ₃ oxide (hereinafter also referred to as Al ₂ O ₃ -based inclusions) There is a case where it causes surface flaws and is harmful to the desired steel quality, and a steelmaking method that does not leave the Al ₂ O ₃ inclusions is desired.

Patent Document 1 discloses a method using a heat increasing agent composed of aluminum and iron oxide as a method for increasing the temperature of aluminum-free high-quality molten steel. This is because a heat-up agent mainly composed of 15 to 25% by weight of aluminum and 65 to 85% by weight of iron oxide is added to the molten steel, and the rapid removal of Al ₂ O ₃ produced along with the molten steel is expected to be rapidly removed. To do. However, this technique is presumed to have low heating efficiency due to decomposition of iron oxide. Moreover, it not mentioned in the removal mechanism of the generated Al ₂ O _3.

  Patent Document 2 discloses a heating method in which a ladle refining apparatus limits the amount of inert gas introduced from the bottom into a dip tube and continuously adds Al or Si that undergoes an oxidation reaction. In this method, the sprayed oxygen is reacted with a metal such as Al which is oxidized near the surface of the molten steel, thereby suppressing the reaction of valuable metals in the molten steel and shortening the processing time. However, this method is premised on Al killed steel and Al-Si killed steel, and only mentions that the molten steel stirring operation is performed after the heat-up operation with regard to cleaning.

In Patent Document 3, in the production of Al-less Ti deoxidized steel, the inclusion composition is surely controlled to prevent the occurrence of surface defects in the thin plate material, and the cost of the deoxidizer and the addition yield of Ti are reduced. In order to achieve stabilization, a method is disclosed in which prior deoxidation is performed using a small amount of Al and Ti is added after the concentration of free oxygen in the molten steel is lowered in advance. That is, when producing molten steel containing C: 0.06% or less, Al: 0.01% or less, and Ti: 0.01-0.40% by weight ratio, molten steel deoxidized only with Si or deoxidized. A predetermined amount of Al determined with respect to the free oxygen concentration in the molten steel is added, the total Al concentration in the molten steel is adjusted to 10 to 80 ppm, and then Ti or a Ti alloy is added. This method aims to reliably change the Al ₂ O ₃ -based inclusions to TiO _x -based inclusions, but does not mention anything about the temperature rise of molten steel due to Al.

  On the other hand, a method for producing low-carbon and extremely low-carbon weakly deoxidized steel in a large-scale steelworks has the following characteristics in the refining process as compared with a normal steelmaking method. Rough decarburization excluding carbon is performed in a steelmaking furnace such as a converter, and steel is produced in a non-deoxidized or weakly deoxidized state as a low-carbon molten steel having a carbon concentration of 0.03% to 0.07% by mass. The discharged steel is further subjected to vacuum decarburization treatment as necessary in a refining process having a vacuum degassing device, and becomes a low carbon molten steel containing a carbon concentration of 0.025% by mass or less. In this case, it is necessary for the decarburization reaction to contain oxygen that reacts with carbon in the molten steel. If the oxygen concentration at this time is indicated, about 0.03 mass% to 0.08 mass% is included. .

  In recent large-scale steelworks, the vacuum degassing apparatus has two dip tubes, and is also referred to as a reflux-type vacuum degassing apparatus (hereinafter referred to as “RH apparatus”) that circulates molten steel between a vacuum tank and a ladle. ) Is often performed. The feature of this apparatus is that it is a refining apparatus that gives strong stirring to molten steel.

By the way, in order to raise the molten steel temperature using this apparatus, a method of adding metal Al and supplying oxygen gas and applying heat generated by the oxidation reaction to the molten steel is generally used. When such a method is used, non-metallic inclusions containing Al ₂ O ₃ having a diameter of several μm to several hundred μm are suspended by the oxidation reaction. When the non-metallic inclusions are Al ₂ O ₃ type, as described above, the desired steel quality may be adversely affected.

As a method for avoiding the generation of such Al ₂ O ₃ inclusions, there is an electric heating method in which an electric arc is generated from a graphite electrode to energize molten steel and heat is supplied by its Joule heat. Although this method does not cause a large amount of suspension of Al ₂ O ₃ inclusions as described above, there are problems such as power costs, cost of electrodes, and long total refining time. In addition, since it is strongly affected by ladle slag during arc heating, it becomes difficult to control components in a weakly deoxidized or non-deoxidized state, or slag inclusions generated by entrainment of the slag itself, etc. It was thought that this would cause problems.

Therefore, on the premise of this reflux type degassing apparatus, even in the case of non-deoxidized or weakly deoxidized steel that does not finally contain Al, even if Al is temporarily added and the molten steel is heated up, the steel material is finally obtained. The inventors have earnestly studied about the requirement not to leave Al ₂ O ₃ genus inclusions so that the inherent properties can be exhibited, and have found the conditions. Furthermore, the present invention was completed by establishing steelmaking conditions based on a mass production method of steel that realizes the conditions.

Furthermore, using the above method, [sol. Al] ≦ 0.003 mass% and [O] = 0.002 to 0.008 mass%, [Si] = 0.01 to 0.05 mass%, [Mn] = 0.10 to 2.0 mass% Is a weakly deoxidized steel containing, as a basic component, a kind further selected from the group consisting of weakly deoxidized components and concentrations Ti ≦ 0.05 mass%, Nb ≦ 0.20 mass%, and Zr ≦ 0.0030 mass% Alternatively, it contains two or more kinds, and has a chemical composition that satisfies at least one of the conditions of Ti ≧ 0.01 mass%, Nb ≧ 0.02 mass%, and Zr ≧ 0.0001 mass%, and the amount of Al ₂ O _{3 in the} steel Of 0.001% by mass or less and other impurities are included, and the manufacturing method technology of the rolled steel material made of the balance Fe has been completed.
JP-A-7-252518 Japanese Patent Laid-Open No. 9-3523 JP-A-10-219336

The present invention is a weakly deoxidized steel or non-deoxidized steel that can be manufactured by a mass production method of steel and does not substantially contain Al, and performs Al addition and oxygen gas supply using a reflux type vacuum degassing apparatus. To provide a method for heating molten steel that enables the amount of residual Al ₂ O ₃ -based inclusions to be made as small as possible even when the molten steel heating method is performed, and a method for producing rolled steel using the heating method With the goal.

The present invention provided to solve the above problems is as follows.
(1) In a vacuum degassing apparatus that circulates molten steel between a vacuum vessel and a ladle via a dip tube, the oxygen concentration before the heating operation is in the range of 0.010 to 0.070 mass%. In this method, oxygen gas is sprayed onto the surface of the molten steel in the vacuum chamber to react the oxygen gas with Al to heat the molten steel, and the Al addition operation is performed once during the process of supplying oxygen gas. In this Al addition operation, the Al addition amount per one time is 0.1 kg or more and 0.5 kg or less per 1 ton of molten steel, and the Al addition operation is at least the following formula (I) from the start of the supply of oxygen gas A method for heating molten steel, which is performed after elapse of time τ represented by:
τ (s) = 0.3W / Q (I)
Here, W is the molten steel amount (t), and Q is the ring flow rate (t / s) that satisfies the following formula (II).
Q = K ・ B ^1/3 D ^4/3 {ln [P1 / P2]} ^1/3 (II)
Here, constant K = 0.19, B: gas flow rate (Nl / min), D: dip tube diameter (m), P1: pressure of gas injection point (torr), and P2: vacuum chamber pressure (torr).

  In addition, it is preferable that the molten steel heated is 0.003 mass% or less by Al concentration before Al addition. Moreover, it is preferable to contain Si and Mn in the range of Si: 0.01-0.05 mass% and Mn: 0.10-2.0 mass%.

(2) With respect to the molten steel in which the oxygen concentration before the heating operation is in the range of 0.010 to 0.070% by mass in a vacuum degassing apparatus that circulates the molten steel between the vacuum vessel and the ladle through the dip tube. In this method, oxygen gas is sprayed onto the molten steel surface in the vacuum chamber and oxygen gas and Al are reacted to heat the molten steel, and the Al addition operation is performed a plurality of times during the process of supplying oxygen gas. The amount of Al added at each time in the Al addition operation is 0.1 kg or more and 0.5 kg or less per 1 ton of molten steel, and the first time of the Al addition operation is at least the following formula (I) from the start of supply of oxygen gas A method for heating molten steel, characterized in that the time interval from the last addition is set to be equal to or greater than the time τ after the second time τ has elapsed.
τ (s) = 0.3W / Q (I)
Here, W is the molten steel amount (t), and Q is the ring flow rate (t / s) that satisfies the following formula (II).
Q = K ・ B ^1/3 D ^4/3 {ln [P1 / P2]} ^1/3 (II)
Here, constant K = 0.19, B: gas flow rate (Nl / min), D: dip tube diameter (m), P1: pressure of gas injection point (torr), and P2: vacuum chamber pressure (torr).

  (3) The method for heating molten steel described in the above (1) or (2) is used. Al ≦ 0.003% by mass and O = 0.002 to 0.008% by mass, Si = 0.01 to 0.05% by mass, Mn = 0.10 to 2.0% by mass as basic components And further containing one or more selected from the group consisting of Ti ≦ 0.05 mass%, Nb ≦ 0.20 mass% and Zr ≦ 0.0030 mass%, and Ti ≧ 0.01 mass% , Nb ≧ 0.02 mass% and Zr ≧ 0.0001 mass%, a method for producing a rolled steel material having a chemical composition satisfying at least one condition.

(4) The molten steel is heated using the method described in any one of (1) to (3) above, and sol. Al ≦ 0.003% by mass, O: 0.002 to 0.008% by mass, Si: 0.01 to 0.05% by mass, Mn: 0.10 to 2.0% by mass as basic components and, wherein the Al ₂ O ₃ content in the inclusions is 50% or less, the production method of the rolled steel.

According to the present invention, for weakly deoxidized steel or non-deoxidized steel substantially not containing Al, a reflux type vacuum degassing apparatus, which is a production facility in a steel mass production method, is used to add Al and oxygen. Even if the molten steel heating method for supplying gas is performed, the amount of remaining Al ₂ O ₃ -based inclusions can be made as small as possible.

Further, by obtaining a weakly deoxidized rolled steel material in which the remaining Al ₂ O ₃ concentration is reduced, the surface quality of the rolled steel material can be made equivalent to that of the conventional steel while enjoying the properties of the weakly deoxidized steel.

The best mode of the present invention, the range of manufacturing conditions, and the reasons for setting them will be described below.
1. Study on Al ₂ O ₃ source In the production method of low- and ultra-low-carbon weakly deoxidized steel or non-deoxidized steel at large-scale steelworks, rough decarburization is performed by removing carbon in a steelmaking furnace such as a converter. As a low-carbon molten steel containing 0.03% by mass to 0.07% by mass of carbon, the steel is discharged into a container such as a ladle without being deoxidized. The discharged steel is further transported to a vacuum degassing device such as an RH device and subjected to a vacuum decarburization treatment, and if necessary, a low carbon molten steel containing a carbon concentration of 0.025% by mass or less is obtained. In this decarburization reaction, it is necessary that the molten steel contains oxygen that reacts with carbon, and the oxygen concentration before decarburization treatment is about 0.03 mass% to 0.08 mass%. Yes.

In order to compensate for the temperature drop of the molten steel during the processing time required for this vacuum decarburization and subsequent component adjustment, the molten steel is heat-treated before and after the vacuum decarburization treatment. In this heat treatment, the molten steel is heated by an oxidation reaction between a metal such as Al and oxygen gas. However, this method causes a large amount of suspension of Al ₂ O ₃ inclusions caused by Al combustion. Al ₂ O ₃ inclusions once generated are difficult to remove. In addition, the removal requires a method such as a recirculation operation with a long-time RH device, which further increases the cost and the probability that the cleanliness of the molten steel deteriorates.

First, an Al ₂ O ₃ source that causes the possibility of the formation of Al ₂ O ₃ -based inclusions other than metal Al added for heating the molten steel was examined.
The first is the metal Al content contained in the alloy iron such as FeSi. The second is Al ₂ O ₃ minutes contained in the ladle slag. And third, the Al ₂ O ₃ source adhering to the ladle inner wall brought from the previous molten steel treatment, and the Al ₂ O ₃ source adhering to the inside of the vacuum degassing device brought from the previous molten steel treatment It is.

As for the first, the Al concentration contained in FeSi and FeNb was at most about 1 to 3% by mass, and it was considered that there was no significant influence. On the other hand, the second is different in decarburization furnaces such as converters and electric furnaces, but most of them are hot metal coexisting with basic components such as CaO, forming a single Al ₂ O ₃ inclusion. It was thought that it could not be a factor. The third was considered to be a considerable amount of Al ₂ O ₃ source, and the range of variation was also considered large. However, it was considered that these Al ₂ O ₃ sources adhered to the refractory wall surface, and even if they separated and suspended in the molten steel, they floated and separated and absorbed by slag and the like. In addition, it was concluded that the second and third Al ₂ O ₃ sources do not react with undeoxidized or weakly deoxidized molten steel and do not become a major factor.

Therefore, in undeoxidized steel or weakly deoxidized steel that does not finally contain substantial Al, the main factor that Al ₂ O ₃ inclusions remain is Al ₂ resulting from molten steel heating due to Al addition and oxygen top blowing. This is thought to be due to the generation of O ₃ . Incidentally Al addition is not molten steel Noborinetsu only also be useful for rapid removal of the molten steel in the oxygen concentration remaining molten steel after vacuum (decarburization) processing of the molten steel, even Al ₂ O ₃ based inclusions in the Large suspension of things is inevitable.

2. Examination of method for suppressing residual of Al ₂ O ₃ inclusions On the other hand, Al ₂ O ₃ inclusions once suspended in molten steel in a reflux type vacuum degassing apparatus are in a weakly deoxidized state, that is, molten steel containing oxygen. In some cases, melts may be formed with oxides of Si and Mn, which are weak deoxidation elements in molten steel, and such MnO—SiO ₂ —Al ₂ O ₃ oxides are not formed. For example, it is considered that the agglomeration of inclusions is promoted by vigorous stirring to the molten steel, thereby enabling separation and removal.

Moreover, if it is the said oxide type inclusion, even when it makes it a weak deoxidation state, it will anticipate that it will be in the deoxidation state which can react with them rapidly and can obtain desired steel quality.
If the weak deoxidation state is illustrated here, [sol. Al] ≦ 0.003 mass%, [O]: 0.002-0.008 mass%, [Si]: 0.01-0.05 mass%, and [Mn]: 0.10-2.0 mass And [Ti]: 0.01 to 0.05% by mass, [Nb]: 0.02 to 0.20% by mass, and [Zr]: 0.0001 to 0.0030% by mass. It contains one or more selected from the group.

Focusing on such functions, investigations were made in consideration of the method of Al addition and oxygen top blowing, and further the formation reaction of inclusions.
The basic idea is that in order to prevent Al ₂ O ₃ generated by Al heating in a state in which the molten steel contains oxygen from remaining in the molten steel as much as possible, (1) Al is brought into contact with the molten steel surface in a high oxygen state. And (2) production of Al ₂ O ₃ is carried out in a vacuum tank, that is, Al ₂ O ₃ produced in the vacuum tank agglomerates when passing through a downcomer pipe from which the molten steel is discharged from the vacuum tank, It is at two points that it is easily separated to the upper surface of the ladle by the reverse flow in the ladle. In other words, if a state in which the Al concentration is high is formed in the vacuum chamber and the molten steel having a high Al concentration is mixed and reacted in the ladle bulk with the molten steel having a high oxygen content in the ladle, Al _{2 is} contained in the bulk. O ₃ is formed, which means that there is no opportunity for separation to the upper surface of the ladle molten steel by flotation promotion or reversal flow due to cohesive enlargement.

  Here, in order to examine the above idea, a circulating degassing apparatus in which about 20 t of molten steel was accommodated in a vacuum tank and about 250 t of molten steel was accommodated in a ladle molten steel was considered. About 100 kg of Al is added to the molten steel having a dissolved oxygen concentration of 0.050 mass% to 0.060 mass%, which is about 1/2 of the amount necessary for deoxidation of the contained oxygen, and the oxygen concentration in the molten steel And how the Al concentration changes. Here, the molten steel ring flow rate Qs: 1.67 t / s, Al was gradually added through the hopper and diffused and dissolved in the molten steel, and the concentration increased over 30 seconds.

At this time, the formation reaction of _Al 2 _{O 3} is represented by _{2Al + 3O = Al 2 O 3} (s), in accordance idea that supra, _Al 2 _{O 3} formed in a vacuum chamber was excluded from the system . Thereby, the Al concentration in the vacuum chamber is related to the amount of Al ₂ O ₃ produced in the ladle bulk.

FIG. 1 schematically shows the behavior of Al concentration and oxygen concentration in the vacuum chamber at that time. Since the RH type degassing apparatus has a small amount of molten steel in the vacuum tank, the Al concentration in the molten steel in the tank is temporarily extremely high when a large amount of Al is temporarily added. In other words, when Al is deoxidized, molten steel with high oxygen enters the vacuum chamber in the vacuum chamber, and molten steel with high oxygen enters the ladle within the ladle, and Al ₂ O ₃ series intervenes. It was thought that product formation occurred. That is, all the Al added in an excess state in the system becomes Al ₂ O ₃ inside the molten steel, so the area surrounded by the X axis and the curve in FIG. 1 is the Al ₂ O ₃ inclusions. It was considered to correlate with the production amount.

Therefore, Al concentration and oxygen in the vacuum tank when 100 kg of Al was added to the same undeoxidized molten steel in advance while oxygen gas by oxygen top blowing or the like was 0.21 kg / t · s. The concentration behavior is schematically shown in FIG. In this case, the oxygen concentration in the vacuum chamber is naturally increased because oxygen gas has been applied in advance, but even if Al is added thereafter, the time during which the Al concentration in the vacuum chamber is increased is shortened and the concentration is increased. Will decline. That is, if the molten steel containing a high Al concentration in the vacuum chamber is subsequently produced as Al ₂ O ₃ inclusions in contact with the non-deoxidized molten steel, the amount of Al ₂ O ₃ inclusions produced is as shown in FIG. It was thought that the amount correlated with the area surrounded by the X-axis and the curve decreased.

  FIG. 3 shows the behavior of the Al concentration in the vacuum chamber when oxygen gas is applied with the same Al addition start time and when oxygen gas is not applied. It was considered that the Al concentration in the vacuum chamber was kept lower when applied.

In this study, it is necessary to investigate the actual inclusion formation behavior only by considering the mass conservation of Al and oxygen. Then, the following investigation was performed using the recirculation type degassing apparatus in which about 20 tons of molten steel was accommodated in the ladle and about 220 to 255 tons of molten steel was accommodated in the ladle molten steel. Oxygen gas was sprayed on the molten steel containing dissolved oxygen. The oxygen gas blowing speed G is 0.20 to 0.22 kg / t · s. It is about 1/3 to 1/2 of the stoichiometric ratio required for removing oxygen contained in molten steel and sprayed oxygen, and finally the dissolved oxygen concentration is 0.005 mass% to 0.020 mass. Al was added so that% remained. About 120 seconds after the addition of oxygen gas and Al was completed, a molten steel sample was taken. Whether or not Al ₂ O ₃ -based inclusions are generated in the ladle by the heat treatment was determined by the inclusion composition in steel. That is, the oxide inclusions contained in the sample were examined with a scanning electron microscope attached with an X-ray microanalyzer. Inclusions of about several μm to several tens of μm were selected at random from about n = 10 to 20, and the average value of the Al ₂ O ₃ concentration therein was taken. Although depending on the molten steel composition, in the case of molten steel containing dissolved oxygen, a small amount of Mn and Si called so-called weak deoxidation elements may be contained. For example, the concentration range is [Mn]: 0.10 to 2.0 mass% and [Si]: 0.01 to 0.05 mass%. The observed inclusions are Al ₂ O ₃ , Al ₂ O ₃ contains a small amount of MnO, SiO ₂ , MnO and Al ₂ O ₃ , MnO, SiO ₂ and Al ₂ O ₃ However, by using the average value of the Al ₂ O ₃ concentration in the inclusions, the molten steel with a high Al concentration in the vacuum tank flows into the molten steel with a high oxygen concentration in the ladle. It was considered that whether or not Al ₂ O ₃ inclusions were generated could be grasped. Here, considering the Al ₂ O ₃ —MnO—SiO ₂ phase diagram, the oxide that balances with the Al ₂ O ₃ phase is the Al ₂ O ₃ —MnO—SiO ₂ liquid phase and the Al ₂ O ₃ concentration is It becomes around 50 mass%. Therefore, the technique of the present invention was examined using an average Al ₂ O ₃ concentration of 50% by mass as a critical index.

Here, the molten steel ring flow rate Q (t / s) in the RH apparatus and the time τ associated with the uniform mixing by the circulation will be described.
The molten steel flow rate Q (t / s) in the RH apparatus is already known by several researchers, but in the present invention, the following formula (II) is used.

Q = K ・ B ^1/3 D ^4/3 {ln [P1 / P2]} ^1/3 (II)
Here, a constant K = 0.19, B is a gas flow rate (Nl / min), D is a dip tube diameter (m ² ), P1 and P2 are a gas blowing point and a vacuum chamber pressure (torr), respectively.

Next, for the time τ associated with uniform mixing, the following equation was used.
τ (s) = 0.3W / Q (I)
Here, W: molten steel amount (t), Q: ring flow rate (t / s).

FIG. 4 shows the relationship between the time τ from the start of the oxygen top blowing time to the addition of Al and the average value of the Al ₂ O ₃ concentration in the inclusions. The error range is 1σ. This test was performed under the condition that the molten steel amount W was 242 t to 274 t and the ring flow rate Q = 1.67 t / s. Therefore, the value of τ calculated from the above equation is 43.5 to 49.2. As shown in the figure, when the average Al ₂ O ₃ concentration in the inclusion is read as 50% by mass, it is 48 (s). By setting the Al addition time after τ = 48 seconds after the oxygen top blowing start time, the average Al ₂ O ₃ concentration in the inclusions could be made less than 50 mass%. That is, such inclusions are mainly composed of SiO ₂ —MnO—Al ₂ O ₃ -based liquid phase inclusions having a relatively low melting point.

FIG. 5 shows the relationship between the amount of Al added per ton of molten steel and the average value of Al ₂ O ₃ concentration in inclusions when oxygen is blown over and Al is added once after τ seconds. . As shown in the figure, when the Al addition amount was 0.5 kg / t or less, the average Al ₂ O ₃ concentration in the inclusions could be less than 50% by mass.

FIG. 6 shows the addition interval time and Al ₂ O ₃ concentration in inclusions when oxygen was blown over and Al addition 0.3 to 0.4 kg / t was performed twice after time τ seconds. The average value relationship is shown. Here, time 0 (s) is 0.72 kg / t added at once. As shown in the figure, the average Al ₂ O ₃ concentration in the inclusions could be made less than 50% by mass by taking the Al addition interval time of τ seconds or more.

3. Scope of application of the present invention The limitation of the scope of application of the present invention will be described below.
The molten steel containing dissolved oxygen in the present invention is the oxygen concentration in the molten steel accommodated in the ladle and is in the range of 0.010 mass% to 0.070 mass% before the Al heating operation. The reason for limiting the oxygen concentration before heating to this range is that the oxygen concentration is less than 0.010% by mass, because it already contains some deoxidizing element, and is not applicable to the present invention. Because it is considered. Further, when the oxygen concentration exceeds 0.070% by mass, the coexisting steelmaking slag and the like are in an extremely peroxidized state, and it is considered difficult to obtain the stable effect of the present invention. If this oxygen concentration is related to the Al concentration in the molten steel, the Al concentration is 0.003% by mass or less before the Al heating operation. When supplying oxygen gas, the molten steel in the vacuum chamber may temporarily exceed this.

Although the amount of molten steel is not particularly limited, it is premised on a mass molten steel treatment process in which RH treatment is performed, and the amount is about 50 t to 400 t. The amount of molten steel in the RH tank is not particularly limited depending on the apparatus configuration, but is about 3 to 30 t. The example of the calculation formula for the reflux velocity Q has been described above, but the range is 0.5 t / s to 4.0 t / s. The oxygen supply rate G depends on the oxygen supply capacity, the lance / gas tuyere shape, and the like, and is 0.0010 Nm ³ / t · s to 0.0040 Nm ³ / t · s, for example.

  About Al to add, the metal Al used in a steelmaking process, the alloy containing it, and a metal Al containing material are meant. In the case of an alloy or a metal Al-containing material, the metal Al content may be converted. In consideration of a rapid reaction with molten steel, it is desirable to use a so-called metal Al for steel making having a pure metal Al content of 95% by mass or more.

  In the present invention, it is necessary to add at least 0.1 kg / t or more of Al. This is because if the amount of Al added is less than 0.1 kg / t, the effect of adding Al to raise the temperature is poor.

  An RH type vacuum degassing apparatus which is a premise of the present invention will be described. The outline of this device is that the vacuum tank is depressurized to suck up the molten steel into the tank, and an inert gas is flowed to one side of the two dip tubes for sucking up the molten steel, and the opposite side is By forming a downward flow, the molten steel is circulated between the vacuum chamber and the ladle. The molten steel is vigorously stirred by the recirculation of the molten steel, and the decarburization reaction and the deoxidation reaction are promoted.

4). Next, the steel material obtained by such operation will be described. The present invention can be applied to steel having an [Al] concentration of 0.003% by mass or less in steel, and is a weakly deoxidized steel having a very low real Al concentration. The oxygen concentration is in the range of 0.002 to 0.008 mass% in terms of total oxygen concentration. Since it is not made into the state of strong deoxidation, 0.003-0.005 mass% oxygen is typically contained.

By the operation of the present invention, the amount of Al ₂ O ₃ inclusions is reduced even if Al heating is performed, and the amount of Al ₂ O ₃ is 0.001% by mass or less. The amount of Al ₂ O ₃ may be obtained by a conventional method, and for example, a value obtained by assuming that the chemical composition is Al ₂ O ₃ from a commonly performed acid-insoluble Al concentration can be used.

As described above, in the rolled steel material having a low Al ₂ O ₃ content, the quality of the steel material surface is good even if the oxygen concentration is high. That is, sliver defects are reduced in the cold rolled steel strip, and inclusion physical surface defects can be suppressed in the thick steel plate.

  In steel [sol. In order to obtain an Al] concentration of 0.003% by mass or less and a total oxygen concentration of 0.002% by mass or more and 0.008% by mass or less, the Si and Mn content is within the range specified below. About Ti, Nb, and Zr, it is necessary to contain these 1 type, or 2 or more types by content of the range prescribed | regulated below.

“Si”: 0.01 to 0.05 mass%
Since Si has an affinity for oxygen, it can be deoxidized without using Al. In order to make the total oxygen concentration 0.008% by mass or less, [Si] is required to be 0.01% by mass or more. When [Si] exceeds 0.05 mass%, the oxygen concentration becomes less than 0.002 mass%.

“Mn”: 0.10 to 2.0 mass%
Mn has an affinity for oxygen and is an element that enables solid solution strengthening of steel. In particular, coexistence with Si enables stable deoxidation although it is weak. In order to make the total oxygen concentration 0.008% by mass or less, [Mn] needs to be 0.10% by mass or more. When [Mn] exceeds 2.0 mass%, the oxygen concentration becomes less than 0.002 mass%.

[Ti]: 0.01 to 0.05% by mass
Ti has an affinity for oxygen and has an affinity for carbon and nitrogen in the rolling temperature range, so it is an element added to improve steel strength and elongation.

  When Ti is added to make the total oxygen concentration 0.008% by mass or less, and [Nb] and [Zr] are not included exceeding the impurity concentration, [Ti] is 0.01% by mass. % Or more is necessary. However, even when [Ti] alone exceeds 0.05 mass%, the oxygen concentration becomes less than 0.002 mass%.

[Nb]: 0.02 to 0.20 mass%
Since it has an affinity for carbon and nitrogen in the Nb rolling temperature range, it is an element added to improve the strength and elongation of the steel material.

  Nb is an element having a smaller affinity for oxygen than Si and Ti in molten steel, but Nb is involved in oxygen concentration in weakly deoxidized steel such as the present invention. When Nb is added to make the total oxygen concentration 0.008% by mass or less and [Ti] and [Zr] are not included exceeding the impurity concentration, [Nb] is 0.02% by mass. % Or more is necessary. However, when [Nb] alone exceeds 0.20% by mass, the steel quality is remarkably changed due to precipitates such as carbides and nitrides, and is not suitable for a desired rolled material.

[Zr]: 0.0001 to 0.0030 mass%
On the contrary, Zr is a strong deoxidizing element, but if it is added in a small amount, there are cases where an effective property can be obtained for the steel.

  Zr is an element having an affinity for oxygen equivalent to that of Al in molten steel, and therefore, in weakly deoxidized steel such as the present invention, it is involved in a small amount of oxygen concentration. When Zr is added to make the total oxygen concentration 0.008% by mass or less and [Ti] and [Nb] are not included exceeding the impurity concentration, [Zr] is 0.0001% by mass. % Or more is necessary. However, even if [Zr] alone exceeds 0.003% by mass, the oxygen concentration becomes less than 0.002% by mass.

By satisfying such conditions and satisfying the above-mentioned requirements for Al heating, Al heating is performed, and [sol.Al] ≦ 0.003 mass%, [% O] = 0.002. A steel material in a range of ˜0.008 mass% can be stably produced, and the amount of Al ₂ O ₃ remaining in the steel material can be 0.001 mass% or less. If the amount of residual Al ₂ O ₃ exceeds 0.001%, even if it is weak deoxidized steel, surface flaws caused by Al ₂ O ₃ inclusions occur, and one of the advantages of weak deoxidized steel is You can not enjoy it.

  When [C] density | concentration suitable for this invention is illustrated, it is the range of [C]: 0.001-0.025%. The reason why this range is suitable is that when [C] exceeds 0.025%, weak deoxidized steel has manufacturing problems such as formation of blow holes in the slab. If [C] is less than 0.001%, the production cost for removing carbon increases.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to these examples.
The molten steel 270 ton was decarburized and refined in a converter, the ladle containing the undeoxidized molten steel was transferred to the RH apparatus, and vacuum decarburization processing was performed with the RH apparatus.

In the RH apparatus, about 20 t of molten steel was accommodated in a vacuum tank, and the recirculation speed was adjusted to a range of 1.6 to 2.0 t / s by adjusting the amount of blown Ar gas and the degree of vacuum. After the vacuum decarburization was completed in the RH apparatus, a necessary amount of metal Al was added to both the preliminary deoxidation of the undeoxidized molten steel and the temperature raising operation of the molten steel. Prior to the addition of Al, a necessary amount of oxygen gas was added by spraying from the lance provided above the vacuum chamber of the RH device toward the molten steel in the vacuum chamber in advance as the oxygen remaining in the molten steel. From the top blowing lance, oxygen gas was blown up at 38 Nm ³ / min for 4 to 10 minutes. The necessary amount of heat and excess oxygen concentration in the molten steel were removed by such an operation, and a molten steel containing an oxygen concentration of 0.010 to 0.070 mass% was obtained.

After that, various alloys were further added as necessary to adjust to predetermined components. If the main molten steel component at that time is described, [C] = 0.002 to 0.004 mass%, [Si] = 0.01 to 0.02 mass%, [Mn] = 0.36 to 0.44 mass %, [P] = 0.012-0.018 mass%, [S] = 0.002-0.005 mass%, [Ti] = 0.01-0.014 mass%, [Nb] = 0. It was 01-0.02 mass%.
In addition, after adding oxygen and blowing oxygen and stopping oxygen blowing, a molten steel sample was collected after about 120 to 240 seconds, and oxide inclusions having a diameter of 5 to 20 μm contained therein were randomly selected. About 10 to 20 pieces were selected and the average value of the Al ₂ O ₃ concentration was obtained. A sample having an Al ₂ O ₃ concentration of 50% by mass or less was marked with ◯, a sample with 30% by mass or less was marked with ◎, and a sample having an Al ₂ O ₃ concentration of 50% by mass or less was marked with ×.

  Table 1 shows various conditions and results of the examples of the present invention, and comparative examples and results outside the conditions of the present invention.

As shown in Examples 1 and 2 of Table 1, the oxidation reaction of Al is utilized by setting the average composition of (% Al ₂ O ₃ ) concentration in inclusions to 50% or less by the method of claim 1 It can be seen that the formation of Al ₂ O ₃ inclusions can be suppressed even when the molten steel is heated. Further, as shown in Example 3 of Table 1, even when Al is added a plurality of times and the total amount of added Al exceeds 0.50 kg / t, the method according to claim 2 allows Al to be added at predetermined time intervals. Is added, the average composition of the inclusions (% Al ₂ O ₃ ) concentration becomes 50% or less, which indicates that the formation of Al ₂ O ₃ inclusions can be suppressed.

On the other hand, as shown in Comparative Examples 4 and 5, when sufficient Al addition time is not secured, as shown in Comparative Examples 6 and 7, when the amount of Al addition exceeds 0.5 kg / t, both are Al. ₂ O ₃ inclusions remain, and the average (% Al ₂ O ₃ ) concentration in the inclusions exceeds 50%. Further, it can be seen from Comparative Example 8 that even if the first Al addition is within the range of the present invention, if the second addition interval is short, Al ₂ O ₃ inclusions remain.

  Furthermore, using the present invention according to claim 1 or claim 2 obtained in the above embodiment, Al heating is carried out, and then C, Si, Mn which are insufficient during the molten steel treatment in the RH apparatus are adjusted, Further, if necessary, Ti, Nb, and Zr were added as alloy components to adjust the concentration, and a slab was produced by a continuous casting method according to a conventional method. Using this slab as a raw material, hot rolling and cold rolling were performed to obtain a cold-rolled steel strip. The results are shown in Table 2. In addition, the oxygen concentration requirement in Table 2 is whether it is in the category of weakly deoxidized steel, that is, the oxygen content in the steel is in the range of 0.002 to 0.008% by mass, and this is satisfied In the case of not satisfying, it was marked as x.

In Examples 11 to 15, Al ₂ O ₃ was also obtained by carrying out the Al heating of the present invention shown in Table 3 in a weakly deoxidized steel with an oxygen concentration of 0.002 to 0.008% by mass. A non-residual rolled steel was obtained.

  On the other hand, Comparative Example 16 has a Si concentration exceeding the range of the present invention, Comparative Example 17 has a Mn concentration exceeding the range of the present invention, Comparative Example 18 has a Ti concentration exceeding the range of the present invention, and Comparative Example 19 has a Zr concentration. However, since it exceeded the scope of the present invention, it became a strong deoxidation state and no longer satisfies the requirements as a weakly deoxidized steel.

It is a graph which shows a time-dependent change of Al concentration in a vacuum chamber, and oxygen concentration at the time of adding Al to undeoxidized molten steel. It is a graph which shows the time-dependent change of Al concentration in a vacuum chamber, and oxygen concentration at the time of adding Al, spraying oxygen on undeoxidized molten steel. It is a graph which shows the comparison with the case where it does not carry out the oxygen top blowing of the Al concentration behavior in a vacuum chamber. It is a graph showing the time relationship between the start-blown oxygen on on the concentration of Al ₂ O ₃ in inclusions to Al addition. It is a graph showing the Al addition amount of relations in the blowing oxygen onto on the concentration of Al ₂ O ₃ in inclusions. Is a graph showing the relationship between the addition time interval when the two Al amount when blowing oxygen onto on the concentration of Al ₂ O ₃ in inclusions.

Claims (4)

Al is added to molten steel in the range of oxygen concentration in the range of 0.010 to 0.070 mass% before the heating operation in a vacuum degassing device that circulates molten steel between the vacuum tank and ladle through a dip tube. And a method of heating the molten steel by reacting oxygen gas and Al by spraying oxygen gas on the surface of the molten steel in the vacuum chamber,
During the process of supplying oxygen gas, the Al addition operation is performed only once,
Al addition amount in the Al addition operation is 0.1 kg or more and 0.5 kg or less per 1 ton of molten steel,
The method for heating molten steel, wherein the Al addition operation is performed at least after a time τ represented by the following formula (I) has elapsed from the start of supply of oxygen gas.
τ (s) = 0.3W / Q (I)
Here, W is the molten steel amount (t), and Q is the ring flow rate (t / s) that satisfies the following formula (II).
Q = K ・ B ^1/3 D ^4/3 {ln [P1 / P2]} ^1/3 (II)
Here, constant K = 0.19, B: gas flow rate (Nl / min), D: dip tube diameter (m), P1: pressure of gas injection point (torr), and P2: vacuum chamber pressure (torr). Al is added to molten steel in the range of oxygen concentration in the range of 0.010 to 0.070 mass% before the heating operation in a vacuum degassing device that circulates molten steel between the vacuum tank and ladle through a dip tube. And a method of heating the molten steel by reacting oxygen gas and Al by spraying oxygen gas on the surface of the molten steel in the vacuum chamber,
During the process of supplying oxygen gas, the Al addition operation is performed a plurality of times,
The amount of Al added each time in the Al addition operation is 0.1 kg or more and 0.5 kg or less per 1 ton of molten steel,
The first Al addition operation is performed after at least the time τ represented by the following formula (I) has elapsed from the start of the supply of oxygen gas, and the second and subsequent times have a time interval from the previous addition equal to or greater than the time τ. A method for heating molten steel, characterized by:
τ (s) = 0.3W / Q (I)
Here, W is the molten steel amount (t), and Q is the ring flow rate (t / s) that satisfies the following formula (II).
Q = K ・ B ^1/3 D ^4/3 {ln [P1 / P2]} ^1/3 (II)
Here, constant K = 0.19, B: gas flow rate (Nl / min), D: dip tube diameter (m), P1: pressure of gas injection point (torr), and P2: vacuum chamber pressure (torr). The method for heating molten steel according to claim 1 or claim 2 is used.
sol. Al ≦ 0.003% by mass, O: 0.002 to 0.008% by mass, Si: 0.01 to 0.05% by mass, Mn: 0.10 to 2.0% by mass as basic components And
Furthermore, it contains one or more selected from the group consisting of Ti ≦ 0.05 mass%, Nb ≦ 0.20 mass%, and Zr ≦ 0.0030 mass%, and Ti ≧ 0.01 mass%, Nb A method for producing a rolled steel material having a chemical composition satisfying at least one condition of ≧ 0.02 mass% and Zr ≧ 0.0001 mass%. The molten steel is heated using the method according to any one of claims 1 to 3, and sol. Al ≦ 0.003% by mass, O: 0.002 to 0.008% by mass, Si: 0.01 to 0.05% by mass, Mn: 0.10 to 2.0% by mass as basic components and, wherein the Al ₂ O ₃ content in the inclusions is 50% or less, the production method of the rolled steel.

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