JP2009019221A - STEEL CONTAINING LITTLE Al AND HAVING HIGH CLEANLINESS, AND MANUFACTURING METHOD THEREFOR - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel containing little Al, which has high cleanliness in spite of containing Al in a small amount of 0.004 to 0.01% for securing high weldability and superior toughness. <P>SOLUTION: The steel containing little Al and having the high cleanliness has a chemical composition comprising 0.0015 to 0.8% C, 0.01 to 0.8% Si, 0.1 to 2% Mn, 0.01 to 11% Ni and Cr in total, 0.004 to 0.01% Al, 0.0025% or less O, less than 0.0035% B, less than 0.1% Nb, less than 0.015% P, less than 0.0035% S, and the balance Fe with impurities; and contains inclusions which comprise, by mass%, 1 to 12% SiO<SB>2</SB>, and the balance one or more oxides of Al oxide and an Mn oxide. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a low Al content steel having high cleanliness and a method for producing the same, and more particularly, a low Al content steel having improved cleanliness and improved cleanliness, and a molten steel capable of stably producing the steel. Relates to the refining method.

  Molten iron, particularly Al in molten steel, is widely used as a deoxidizing element, and contributes to the reduction of oxygen in steel as a product (hereinafter also referred to as “steel material”). However, in order to perform sufficient deoxidation with Al, the Al content in the molten steel (hereinafter, the Al content in the molten steel is sometimes referred to as “Al concentration.” Similarly, the content of the element X in the molten steel is It is common to use Al in a range of 0.02 to 0.06%. This is to stabilize deoxidation by Al.

  Furthermore, Al is widely used for heat treatment of molten steel. That is, Al is added to the molten steel, and oxygen is further added to the molten steel to oxidize Al in the molten steel, and the temperature of the molten steel is raised using this oxidation heat. The above technique utilizes the strong deoxidizing power of Al.

  As described above, Al is used for deoxidation and heating of molten steel, but as described above, the Al concentration is generally in the range of 0.02 to 0.06%.

  The Al concentration control and the molten steel heat treatment are often performed by an RH vacuum degassing apparatus (hereinafter referred to as “RH”) having two dip tubes and circulating the molten steel. The technique is disclosed in Document 1 and Patent Document 2.

  However, not all steel types are allowed to contain a high amount of Al in the steel material by performing the treatment at the above-described general Al concentration of 0.02 to 0.06%.

  For example, for steel materials intended to improve weldability and toughness, and further to reduce defects starting from inclusions, when the Al content is required to be as low as 0.004 to 0.01% In addition to the low Al content, high cleanliness is often required at the same time.

  Even in the case of producing a steel material containing a low amount of Al of 0.004 to 0.01%, a method of adding oxygen to molten steel in RH is used, but the Al concentration, that is, Al content in the molten steel is used. Since the amount is reduced, the deoxidation action by Al becomes unstable.

  Therefore, in the production of the low Al content steel as described above, in general, in order to eliminate the instability of deoxidation by Al, the treatment time in RH is longer than that of the so-called “Al deoxidized steel”. In addition, the amount of oxygen gas blown up is increased.

JP 2002-30330 A Japanese Patent Laid-Open No. 9249910

  Even when the techniques proposed in Patent Document 1 and Patent Document 2 are applied, when a steel material containing a low amount of Al of 0.004 to 0.01% is manufactured, the processing time in RH is lengthened. However, it has become necessary to increase the amount of oxygen gas blown up, and an increase in manufacturing cost is often unavoidable.

  Moreover, in the production of low Al content steel, in order to perform a stable deoxidation treatment, the circulation time of the molten steel in RH will be extended, but even if this is done, the cleanliness of the steel as a product, that is, The cleanliness of the steel material varied, and the cleanliness was not stable.

  That is, in the case of the same Al concentration, if the reflux time in RH is ensured for a certain time, deoxidation is stabilized and the floating removal of inclusions is promoted, so the cleanliness is thought to be stable. Even if the treatment is performed, in the case of a low Al content steel, it is not possible to stably ensure good cleanliness, and therefore 0.004 to 0.01% having high weldability and excellent toughness. It has been difficult to stably and inexpensively supply a steel material having a low Al content.

  Accordingly, an object of the present invention is to achieve high weldability and excellent toughness with a low Al content having high cleanliness despite the low Al content of 0.004 to 0.01%. It is to provide a method for refining molten steel that can stably produce the contained steel and the steel.

  In general, cleanliness is determined by the sum of the oxygen content (oxygen concentration) in the molten steel determined by deoxidation equilibrium and the residual residues in the molten steel resulting from the deoxidation reaction. Therefore, if the content of deoxidizing element in the molten steel, specifically, the Al concentration is constant and the treatment time in RH that governs inclusion removal is constant, the cleanliness varies and the cleanliness is not good. Stabilization does not occur in principle.

  However, in reality, even if the treatment time with Al concentration and RH is constant, the cleanliness varies, and instability is observed in cleanliness. When the concentration is reduced, it is considered that the deoxidation equilibrium is unstable due to some cause.

  Therefore, the present inventors have studied deoxidation reaction and oxygen concentration change in a low Al concentration region, and as a result, obtained various findings and completed the present invention.

  The gist of the present invention resides in the low Al-containing steel having high cleanliness shown in the following (1) and (2) and the low Al-containing steel having high cleanliness shown in (3) to (7).

(1) By mass%, C: 0.0015 to 0.8%, Si: 0.01 to 0.8%, Mn: 0.1 to 2%, the total of Ni and Cr: 0.01 to 11% , Al: 0.004 to 0.01%, O: 0.0025% or less, B: less than 0.0035%, Nb: less than 0.1%, P: less than 0.015%, S: 0.0035% The balance is a chemical composition composed of Fe and impurities, the inclusions in the steel contain 1 to 12% of SiO ₂ by mass%, and the balance is one of Al oxide and Mn oxide. A low Al-containing steel having high cleanliness, characterized by comprising more than seeds.

(2) By mass%, C: 0.0015 to 0.8%, Si: 0.01 to 0.8%, Mn: 0.1 to 2%, the total of Ni and Cr: 0.01 to 11% , Al: 0.004 to 0.01%, O: 0.0025% or less, B: less than 0.0035%, Nb: less than 0.1%, P: less than 0.015%, S: 0.0035% less, Ca: containing 0.0028% or less, the balance in the chemical composition of Fe and impurities, inclusions during steel, by mass%, containing SiO ₂ 1 to 12%, the balance Ca-Al A low Al-containing steel having high cleanliness, characterized by comprising at least one of oxide and Ca-Mn oxide.

  (3) In an RH type vacuum degassing apparatus having two dip tubes and circulating the molten steel, Al is added to the molten steel, and subsequently oxygen gas is blown up to reduce the Al content in the molten steel to 0.004. A method for producing a low Al-containing steel with a content of ~ 0.01%, wherein 0.005 to 0.02 kg of Al per ton of molten steel is added to the molten steel 1 to 2 minutes before the end of the oxygen gas top blowing process. A method for producing a low Al content steel having high cleanliness characterized by the following.

(4) The high amount as described in (3) above, wherein the amount of Al added per ton of molten steel satisfies the following formula (1) 1 to 2 minutes before the end of the oxygen gas top blowing treatment A method for producing a low Al content steel having cleanliness.
−0.833 [Al] + 0.022 ≧ M ≧ −0.833 [Al] +0.015 (1).
In the formula (1), M and [Al] are respectively
M: Al amount (kg) to be added per ton of molten steel 1 to 2 minutes before the end of the oxygen gas top blowing process,
[Al]: The target value of the Al content in the molten steel after completion of the oxygen gas top blowing treatment, in the range of 0.004 to 0.01% by mass%,
Represents.

  (5) In the above (3) or (4), 0.05 to 0.17 kg of Ca or Ca alloy is added to the molten steel in an amount of pure Ca per ton of molten steel after completion of the oxygen gas top blowing treatment. A method for producing a low Al-containing steel having high cleanliness as described.

  (6) A method for producing a low Al-containing steel having high cleanliness described in (1) or (2) above, wherein the chemical composition excluding Al and O is as described in claim 1 Is processed with a RH type vacuum degassing apparatus, The manufacturing method of the low Al content steel which has the high cleanliness as described in said (3) or (4) characterized by the above-mentioned.

  (7) A method for producing a low Al-containing steel having high cleanliness described in (2) above, wherein the chemical composition excluding Al, O and Ca is the one described in claim 2, The method for producing a low Al-containing steel having high cleanliness as described in (5) above, wherein the treatment is performed by an RH type vacuum degassing apparatus.

  Hereinafter, the invention related to the low Al-containing steel having high cleanliness shown in the above (1) and (2) and the invention related to the method for producing the low Al-containing steel having high cleanliness shown in (3) to (7). These are referred to as “present invention (1)” to “present invention (7)”, respectively. Also, it may be collectively referred to as “the present invention”.

  The low Al-containing steel of the present invention has high cleanliness despite the low Al content of 0.004 to 0.01%. For this reason, it is excellent in weldability and toughness. Among the low Al-containing steels of the present invention, those containing Ca have much better characteristics because inclusions are spheroidized while maintaining high cleanliness. These low Al-containing steels of the present invention can be produced at low cost by the method of the present invention.

  Hereinafter, each requirement of the present invention will be described in detail.

(A) Chemical composition of steel First, the chemical composition in the low Al content steel which has the high cleanliness of this invention, and its reason for limitation are described. In the following description, “%” display of the content of each element means “mass%”.

C: 0.0015 to 0.8%
C is an element having a weak deoxidizing action. If the content is less than 0.0015%, the deoxidation action is not sufficient, and the preliminary deoxidation becomes unstable. On the other hand, when the content of C increases, the deoxidizing power becomes too strong. In particular, when it exceeds 0.8%, the influence cannot be ignored in a reduced pressure reaction apparatus such as RH, and inclusion control becomes unstable. Therefore, the content of C is set to 0.0015 to 0.8%. In addition, it is preferable that content of C shall be 0.01 to 0.35%.

Si: 0.01 to 0.8%
Si is an element having a deoxidizing action. However, in the case of the low Al content steel as in the present invention, if the Si content is less than 0.01%, a sufficient deoxidation effect cannot be obtained, and conversely, the Si content is less than 0.8%. exceeds the, the high inclusions of the SiO ₂ content in the effect increases. Therefore, the Si content is set to 0.01 to 0.8%. Note that the Si content is preferably 0.05 to 0.35%.

Mn: 0.1 to 2%
Mn has a strength improving action and a deoxidizing action. However, if the Mn content is less than 0.1%, the steel cannot have the desired strength. On the other hand, although the deoxidizing power of Mn is weaker than that of Si, the content of Mn increases, and in particular, if it exceeds 2%, the influence on the deoxidation reaction increases, so that a large amount of MnO is generated in the inclusions. As a result, the degree of cleanliness increases and the cleanliness decreases. Therefore, the Mn content is set to 0.1 to 2%. In addition, it is preferable that content of Mn shall be 0.3-1.5%.

Total of Ni and Cr: 0.01-11%
Ni and Cr have the effect of enhancing the mechanical properties and corrosion resistance of steel. However, when the total content of Ni and Cr is less than 0.01%, no effect is obtained. In addition, since Ni and Cr do not affect the deoxidation effect, but affect the oxygen activity, the total content of both increases, and in particular, when it exceeds 11%, it is the oxygen content in the molten steel. As the oxygen concentration increases, cleanliness decreases. Therefore, the total content of Ni and Cr is set to 0.01 to 11%.

Al: 0.004 to 0.01%
Al has a deoxidizing action. However, if the Al content is less than 0.004%, the effect cannot be obtained. On the other hand, when the content of Al increases, particularly when it exceeds 0.01%, weldability and toughness may be deteriorated, and defects starting from inclusions may be caused. Therefore, the Al content is set to 0.004 to 0.01%. The Al content is preferably 0.006 to 0.009%.

O: 0.0025% or less When the content of O (oxygen) exceeds 0.0025%, the number of inclusions increases, the cleanliness increases, and the cleanliness decreases. Therefore, the content of O is set to 0.0025% or less. The O content is preferably 0.0018% or less.

B: Less than 0.0035% B is an element having a relatively strong deoxidizing power, and its content becomes high. In particular, when the content is 0.0035% or more, the inclusion form changes and the inclusion is contained in the inclusion. In some cases, the SiO ₂ content may be excessively reduced. Therefore, the B content is less than 0.0035%. The B content is preferably 0.0015% or less.

Nb: less than 0.1% Nb is an element that forms carbonitrides during the solidification process. When the content of Nb is increased, especially when the content is 0.1% or more, the number of the inclusions is increased, the cleanliness is increased and the cleanliness is lowered, and the mechanical properties are also lowered. There is a case. Therefore, the Nb content is less than 0.1%. The Nb content is preferably 0.05% or less.

P: Less than 0.015% P is an element that easily segregates in the center. When the content becomes high, particularly 0.015% or more, the center segregation becomes remarkable, and mechanical properties, particularly toughness, are greatly reduced. Therefore, the P content is less than 0.015%. In addition, it is preferable that content of P shall be 0.01% or less.

S: Less than 0.0035% S is an element that forms a sulfide during the solidification process. When the S content is high, especially 0.0035% or more, the number of the inclusions increases, the cleanliness increases and the cleanliness decreases, and the corrosion resistance and mechanical properties decrease. May be invited. Therefore, the S content is less than 0.0035%. The S content is preferably 0.0015% or less.

  For the above reasons, the low Al-containing steel having high cleanliness according to the present invention (1) contains C, Si, Mn, Ni and Cr, Al, O, B, Nb, P, S in the above-described range. The balance is defined as a chemical composition composed of Fe and impurities.

  The low Al-containing steel having high cleanliness according to the present invention shall contain Ca: 0.0028% or less in place of part of Fe in the present invention (1) as necessary. Can do. That is, in order to obtain inclusions with better characteristics by spheroidizing inclusions, the above amount of Ca is contained instead of a part of Fe in the low Al-containing steel having high cleanliness of the present invention (1). May be. Hereinafter, this will be described.

Ca: 0.0028% or less Ca has an action of spheroidizing inclusions and may be contained for this purpose. However, when the content of Ca increases, especially when it exceeds 0.0028%, the reduction of SiO ₂ by Ca proceeds and the content of SiO _{2 in} the inclusions decreases. Becomes higher, resulting in a decrease in cleanliness. Therefore, when Ca is added, the content of Ca is set to 0.0028% or less.

  In order to reliably obtain the effect of Ca described above, the Ca content is preferably 0.0004% or more. For this reason, the more desirable Ca content in the case of adding is 0.0004 to 0.0028%.

  For the reasons described above, the low Al-containing steel having high cleanliness according to the present invention (2) has already described C, Si, Mn, Ni and Cr, Al, O, B, Nb, P, S, and Ca. It was specified that the chemical composition was comprised of Fe and impurities.

  If the contents of Ti, V, Mo, W and Cu are less than 0.025%, less than 0.1%, less than 0.3%, less than 2% and less than 0.5%, respectively, The inclusion composition is not affected. Therefore, one or more elements selected from Ti: less than 0.025%, V: less than 0.1%, Mo: less than 0.3%, W: less than 2% and Cu: less than 0.5% You may make it contain in order to improve various characteristics, such as a mechanical property and corrosion resistance of the low Al content steel which has the high cleanliness of invention (1) and this invention (2).

(B) Inclusions in Steel In the low Al-containing steel having high cleanliness of the present invention (1), the inclusions in the steel are 1% by mass and contain 1 to 12% of SiO ₂ with the balance being Al. It must consist of one or more of oxides and Mn oxides.

Further, the low Al-containing steel having high cleanliness of the present invention (2), inclusions in the steel, by mass%, the SiO ₂ containing 1-12%, the remainder Ca-Al oxide and Ca- It must consist of one or more of the Mn oxides.

  The above rules are based on the results of investigation conducted by the present inventors, and will be described in detail below by taking the case of the present invention (1) as an example.

The inventors first changed various contents of Al between 0.004 to 0.01%, and various steels having the chemical composition of the present invention (1) described in the above section (A). Was dissolved in a 150 kg crucible. And about the steel after solidification, the relationship between the quantity of inclusions in steel and a composition was investigated in detail using the scanning electron microscope. The amount of inclusions was evaluated by a method in which the magnification was 1000 times, the surface of a sample of about 7 cm ² was observed with a scanning electron microscope and the number of inclusions was counted, and the composition was evaluated by EPMA. Furthermore, for comparison of the amount of inclusions, an “inclusion number index” was obtained in which the number of inclusions in a steel having an Al content of 0.035% was 1.

  As a result, the following items (a) to (c) became clear.

  (A) Even if the Al content is the same, a difference is observed in the number of inclusions. This confirms that the cleanliness varies in the low Al content range of 0.004 to 0.01%.

(B) In the case of the same Al content, there is a difference in the composition of inclusions between steel with a lot of inclusions and steel with little inclusions. That is, inclusions often inclusions present in the steel is very low Al entities of the content of SiO ₂ Al-Si-Mn based or Al-O system, or the SiO ₂ content of more Si principal Si- In contrast to Al—Mn—O, inclusions present in steel with few inclusions are Al—Si—Mn—O system containing a specific range of SiO ₂ .

(C) When SiO ₂ in the inclusions present in the steel having a small amount of inclusions in (b) is quantified, the content of SiO ₂ is 1% to 12% by mass%. That is, in the case of the chemical composition of the present invention (1) described in the above section (A), the cleanliness is increased by setting the content of SiO _{2 in} the inclusions to 1 to 12%. When the content of SiO _{2 in} the inclusions was as low as less than 1% or as high as more than 12%, the cleanliness deteriorated.

FIG. 1 shows an example of the relationship between the content (mass%) of SiO ₂ in inclusions and the inclusion number index. In FIG. 1, the unit of the content of SiO _{2 in} the inclusion is represented by “%”.

  The above matters can be understood as follows.

  That is, when the Al content in the steel as a product is higher than 0.1%, the deoxidation reaction is stabilized because the Al concentration, which is the Al content in the molten steel, is high, and the same oxygen is always present in the molten steel. Concentration and amount of inclusions. However, when the Al content in the steel as a product decreases, the Al concentration also decreases, so that it is easily affected by the Si concentration, which is the Si content in the molten steel. Note that the degree of influence of the Si concentration is poor in reproducibility and may be strongly affected or may be small, but this is considered to be caused by a slight difference in the amount of inclusions before Al deoxidation.

That is, even when the Al concentration is low, Al ₂ O ₃ is stable in terms of equilibrium, but in this case, the deoxidation equilibrium becomes the “Al—O” equilibrium, and therefore, as the Al concentration decreases, the oxygen concentration Increases monotonically.

On the other hand, when the amount of inclusions before Al deoxidation is large, since Al is generally insufficient, the “Al—Si—O” equilibrium is obtained instead of the “Al—O” equilibrium in terms of kinetics. At this time, inclusions change from Al ₂ O ₃ to “Al ₂ O ₃ —SiO ₂ ”, and the activities of Al ₂ O ₃ and SiO ₂ decrease. As a result, the amount of inclusions is reduced because the oxygen concentration is lowered.

Further, when Al is totally insufficient, the “Si—O” equilibrium becomes dominant, and the content of SiO _{2 in} the inclusion increases, so that the activity of SiO ₂ increases. For this reason, the oxygen concentration increases, and as a result, the amount of inclusions begins to increase again.

The above should be considered not only for Al deoxidation, but also for “Al-Si” complex deoxidation in the case of steel with a low Al content of 0.004 to 0.01%, and The composition of the inclusion when the oxygen concentration is the lowest due to the combined deoxidation indicates that the content of SiO _{2 in} the inclusion is 1 to 12%. Therefore, if the inclusions in the steel contain 1 to 12% by mass of SiO ₂ and the balance is composed of one or more of Al oxide and Mn oxide, the present invention (1) Thus, a low Al-containing steel having high cleanliness can be obtained.

For the same reason, in the case of Ca treatment, the inclusions in the steel contain 1% to 12% by mass of SiO ₂ with the balance being one of Ca—Al oxide and Ca—Mn oxide. If it consists of the above, the low Al content steel which has the high cleanliness based on this invention (2) will be obtained. Although it is possible to reduce the oxygen activity of Ca, it can be understood from the interaction coefficient that it is difficult to greatly reduce the oxygen concentration. Therefore, in order to reduce the oxygen concentration even when Ca is used, that is, to reduce the amount of inclusions, it is necessary to utilize the Si—Al—O equilibrium described above. Therefore, it is considered that the appropriate range of the content of SiO _{2 in} the inclusion does not change even when Ca treatment is performed.

(C) Manufacturing method Low Al-containing steel having high cleanliness is obtained by observing the inclusions in each steel material obtained and selecting only those having excellent cleanliness below the target value of cleanliness. Although the yield can be obtained, the yield in this case is extremely low.

  Moreover, as a method of more actively controlling inclusions, there is a method of performing ladle refining using slag or flux, but this leads to an increase in manufacturing cost.

  Therefore, the present inventors have ensured good productivity and a high yield, and as a method for stably obtaining a low Al-containing steel excellent in cleanliness with a cleanliness value equal to or less than a target value, Al The treatment using RH that can control the concentration most easily was examined. As a result, the following items (d) and (e) became clear.

  (D) As already described in the section “(B) Inclusions in Steel”, the variation in cleanliness is caused by deoxidation equilibrium in the low Al concentration region, kinetically “Al-O” or This is because it is deflected to one of “Al—Si—O”. For this reason, if the reaction rate is controlled to make the above-mentioned deoxidation equilibrium constant, it is possible to prevent variations in cleanliness.

  (E) Although there are several methods for controlling the reaction rate, the simplest method in refining steel is to control the content of reactants in the molten steel. However, in the case of the low Al-containing steel targeted by the present invention, the Al concentration, which is the Al content in the molten steel, cannot be increased.

  As a result of the examination, the following item (f) became clear.

(F) Depolarization of the deoxidation equilibrium is a phenomenon peculiar to a low Al concentration region. For this reason, the deflection of deoxidation can be controlled by adding an appropriate amount of Al to the molten steel at the time when the low Al concentration region is reached, that is, at the appropriate time immediately before the end of the oxygen gas top blowing. That is, if the amount of Al added to the molten steel is too large, not only low Al-containing steel cannot be produced, but also the inclusions become Al ₂ O ₃ . Further, if too small, the addition amount of Al in the molten steel, the content of SiO ₂ in the inclusions becomes extremely small. Furthermore, if the Al addition time to the molten steel before the end of oxygen gas blowing is too early, the effect of the added Al disappears. Conversely, if it is too late, the added Al remains in the steel as it is. And in any of the above-mentioned cases, the cleanliness of the steel becomes high, and the steel having the desired high cleanliness cannot be obtained. Therefore, the deoxidation equilibrium does not completely change to the “Al—O” system, but it is necessary to improve the cleanliness by deviating to the “Al—Si—O” system in which a small amount of Si is affected.

  Therefore, in order to examine the amount of Al to be added to the molten steel immediately before the end of the oxygen gas top blowing treatment and the timing of the addition, the present inventors further added oxygen to 1 ton of molten steel using a molten steel experimental apparatus. The amount of Al added to the molten steel is 0.001 to 0.03 kg, that is, in the range of 0.001 to 0.03 kg per ton of the molten steel before the oxygen gas top blowing treatment is completed. The addition time was changed in the range of 5 to 0 min before the end of the oxygen gas top blowing treatment, and the amount of inclusions in the steel was investigated for the solidified steel. In addition, Al content (Al concentration) in the molten steel after completion | finish of oxygen gas top blowing process was 0.005-0.0075%.

The amount of inclusions in the steel was evaluated by a method in which the magnification was set to 1000 and the surface of a sample of about 7 cm ² was observed with a scanning electron microscope and the number of inclusions was measured. For comparison of the amount of inclusions, the number of inclusions in the steel with an Al content of 0.035% already described in the section “(B) Inclusions in steel” is “1. The index was determined.

  FIG. 2 shows the results of the above investigation. In FIG. 2, the time before the end of the oxygen gas top blowing process is expressed as “time until the top blowing end”. This time “0” means that the end of the oxygen gas top blowing treatment and the addition of Al to the molten steel were simultaneous. Each numerical value on the right in FIG. 2 indicates that for each mark, the amount of Al added to the molten steel is 0.001 to 0.03 kg, that is, 0.001 to 0.03 kg per ton of molten steel. Show.

  From FIG. 2, the following items (g) and (h) became clear.

  (G) Regardless of the addition amount of Al, when Al is added to the molten steel earlier than 2 minutes before the end of the oxygen gas top blowing process, or when the Al addition time to the molten steel is 1 min before the end of the oxygen gas top blowing process In the case of a lower speed, the inclusion number index is large and the cleanliness is poor.

  The above (g) shows that the deoxidation deflection cannot be affected if the Al addition time to the molten steel is too early or conversely too late.

  (H) When the Al addition time to the molten steel is 1 to 2 minutes before the end of the oxygen gas top blowing process, a decrease in the inclusion number index is observed, but the decrease is caused by the Al added to the molten steel. Only when the amount is in the range of 0.005 to 0.02 kg, that is, 0.005 to 0.02 kg per ton of molten steel.

In the case of (h), as already described, if the amount of Al added to the molten steel is too large, the inclusion becomes Al ₂ O _3, and if the amount of Al added to the molten steel is too small. This is because the content of SiO _{2 in} the inclusions is extremely reduced, and in any case, the cleanliness of the steel is increased.

  From the above, in the present invention (3), the treatment is performed by adding Al to the molten steel and then continuously blowing oxygen gas in the RH vacuum degassing apparatus having two dip tubes and circulating the molten steel. At that time, it was defined that “0.005 to 0.02 kg of Al per ton of molten steel should be added to the molten steel 1 to 2 minutes before the end of the oxygen gas top blowing process”.

  Next, in FIG. 2, the present inventors found that the inclusion number index obtained by setting the number of inclusions in steel having an Al content of 0.035% as 1 is about 0.8 to 0.9, May be superior to the so-called “Al deoxidized steel”, and even if it is a low Al-containing steel, it is even higher if the deoxidation equilibrium is appropriately controlled to the “Al—Si—O” system. It came to guess that the cleaning effect was acquired.

  Since the present invention (3) is characterized by adding a small amount of Al to the molten steel in a low Al concentration region, strictly speaking, Al to be added to the molten steel depending on the Al concentration after the oxygen gas top blowing treatment. The appropriate amount of should change.

  Therefore, the inventors of the present invention performed the oxygen gas top blowing process in the same manner as described above, that is, after adding Al to 1 ton of molten steel using a molten steel experimental apparatus, and then the oxygen gas top blowing process. Before completion, the amount of Al added to the molten steel is 0.005 to 0.02 kg, that is, 0.005 to 0.02 kg per ton of molten steel, and the addition timing is 2 to 2 before the end of the oxygen gas top blowing treatment. In the range of 1 min, the Al content (Al concentration) in the molten steel after completion of the oxygen gas top blowing treatment was variously changed, and the amount of inclusions in the steel was investigated for the steel after solidification.

The amount of inclusions in the steel was evaluated by a method in which the magnification was set to 1000 and the surface of a sample of about 7 cm ² was observed with a scanning electron microscope and the number of inclusions was measured. Further, for comparison of the amount of inclusions, an “inclusion number index” in which the number of inclusions in the steel having the Al content of 0.035% described above was 1 was determined.

  As a result, the following item (i) became clear.

  (I) Inclusion number index of 0.8 to 0.95 may be obtained depending on the Al concentration after the oxygen gas top blowing treatment and the amount of Al added to the molten steel.

  Therefore, regression analysis was performed to obtain FIG. In FIG. 3, the Al concentration after the oxygen gas top blowing process is “final Al concentration”, the unit is “%”, the “inclusion number index” is “inclusion index”, and the Al addition amount is The unit was expressed as “kg / ton”. In the figure, the solid line and the broken line respectively represent the amount of Al (kg) added per 1 ton of molten steel before the end of the oxygen gas up-blowing process, and [Al] in mass%, 0.004 to 0.00. As a target value of Al content in the molten steel after completion of the oxygen gas top blowing treatment in the range of 01%, a line of M = −0.833 [Al] +0.022 and M = −0.833 [Al] +0.015 line is shown.

  From FIG. 3, the following item (j) became clear.

  (J) The inclusion number index of 0.8 to 0.95 can be obtained by the formula (1), “−0.833 [Al] + 0.022 ≧ M ≧ −0.833 [Al] +0.015. Is satisfied.

  From the above, in the present invention (4), it is specified that the amount of Al added per ton of molten steel before the end of the oxygen gas top blowing process satisfies the above formula (1). did.

  As described above, according to the present invention (3) or the present invention (4), a low Al-containing steel having improved cleanliness and improved cleanliness can be obtained. Al-Si-O inclusions. However, spheroidization of inclusions may be required depending on the steel type.

  In general, Ca treatment is widely used for spheroidization of inclusions, and many optimum conditions have already been developed. And since the deoxidation and inclusion state after the oxygen gas top blowing treatment in the present invention (3) and the present invention (4) are stable, the Ca treatment can be applied.

However, as already described in the section (f), the deoxidation equilibrium is not completely changed to the “Al—O” system, but is changed to the “Al—Si—O” system in which a small amount of Si is influenced. Therefore, the Ca concentration, which is the Ca content and the Ca content in the molten steel, is not arbitrary. For example, if the amount of Ca added is too large, the Ca concentration becomes too high, so that SiO ₂ reduction by Ca proceeds and the content of SiO _{2 in} the inclusions becomes extremely small. On the other hand, when the Ca concentration is too low, inclusions cannot be spheroidized.

Therefore, the inventors of the present invention performed the oxygen gas top blowing process in the same manner as described above, that is, after adding Al to 1 ton of molten steel using a molten steel experimental apparatus, and then the oxygen gas top blowing process. Before completion, the amount of Al added to the molten steel is 0.005 to 0.02 kg, that is, 0.005 to 0.02 kg per ton of molten steel, and the addition timing is 2 to 2 before the end of the oxygen gas top blowing treatment. In the range of 1 min, the Al content (Al concentration) in the molten steel after completion of the oxygen gas top blowing treatment is variously changed, and the Ca or Ca alloy is changed so that the pure content of Ca changes after the oxygen gas top blowing treatment is finished. In the steel after solidification, the Ca content, the content (mass%) of SiO ₂ in inclusions and the spheroidization rate of inclusions were investigated.

The content (mass%) of SiO ₂ in inclusions was measured by EPMA, and the amount of inclusions was set to 1000 times, and the surface of a sample of about 7 cm ² was observed with a scanning electron microscope. The number of inclusions was determined by a method, and the spheroidization rate of inclusions was evaluated by the following formula. In addition, the “spherical inclusion” in the formula refers to an inclusion having a value of “major axis / minor axis”, which is a ratio of the minor axis to the major axis, of less than 1.3.
Inclusion spheroidization rate (%) = (number of spherical inclusions) / (number of observed inclusions) × 100.

FIG. 4 shows the influence of the amount (kg) of pure Ca added per 1 ton of molten steel on the spheroidization rate of inclusions. FIG. 5 shows the influence of the amount (kg) of pure Ca added per ton of molten steel on the Ca content and the content (mass%) of SiO ₂ in inclusions.

  4 and 5, the amount of pure Ca added per ton of molten steel is expressed as “Ca addition amount (kg / ton)”.

  The following item (k) is clarified from FIG. 4, and the following item (l) is clarified from FIG.

  (K) By adding 0.05 kg or more of Ca or Ca alloy in the amount of pure Ca per ton of molten steel, the spheroidization rate of inclusions can be made 100%.

(L) By adding 0.17 kg or less of Ca or Ca alloy in a pure amount of Ca per ton of molten steel to the molten steel, the content of SiO _{2 in} the inclusion is changed to that of “(B) inclusion in steel”. 1% by mass or more as described in the section. And Ca content can be made into 0.0004 to 0.0028% by adding 0.05 to 0.17 kg / ton of Ca or Ca alloy to the molten steel with the said pure Ca content.

  Therefore, in order to satisfy the spheroidization rate of inclusions, the inclusion composition, and the Ca content at the same time, 0.05 to 0.17 kg of Ca or Ca alloy in terms of pure Ca per ton of molten steel should be added to the molten steel. is necessary.

  From the above, in the present invention (5), in the production method of the present invention (3) or the present invention (4), 0.05 to 0 in terms of pure Ca per ton of molten steel after completion of the oxygen gas top blowing treatment. It was specified that .17 kg of Ca or Ca alloy was added to the molten steel.

  In addition, the low Al-containing steel having high cleanliness according to the present invention (1) or the present invention (2) is a RH which has a chemical composition excluding Al and O described in the present invention (1). It can manufacture by this invention (6) which is a manufacturing method of this invention (3) or this invention (4) characterized by processing by a type | formula vacuum degassing apparatus.

  Further, the low Al content steel having high cleanliness according to the present invention (2) is an RH vacuum degassing of a molten steel whose chemical composition excluding Al, O and Ca is described in the present invention (2). It can manufacture by this invention (7) which is a manufacturing method of this invention (5) characterized by processing with an apparatus.

  Hereinafter, the manufacturing method of the present invention will be specifically described in detail using a process of “converter → RH → continuous casting”.

  First, after finishing the processing in the converter, the molten steel is taken out into the ladle. The addition of Al may be performed at the time of steel output or may be performed by RH in the next step.

  Next, the ladle is transferred to the RH, and the treatment with the RH is started. In RH, component adjustment, degassing, and the like may be performed as appropriate. However, before the oxygen gas is blown over the molten steel, in the case of the present invention (3) or the present invention (4), the chemical composition of the molten steel. In the case of the present invention (5), the chemical composition of the molten steel is adjusted to the desired steel composition excluding the Al concentration, O concentration and Ca concentration. Adjust to.

  Further, in the case of the present invention (6), the chemical composition of the molten steel is specifically adjusted to the steel composition of the present invention (1) or the present invention (2) except for the Al concentration and the O concentration. ), The chemical composition of the molten steel is specifically adjusted to the steel composition of the present invention (2) except for the Al concentration, O concentration and Ca concentration.

  Thereafter, Al is added to the molten steel, and then an oxygen gas top blowing treatment with RH is performed.

  The amount of Al added to the molten steel is determined by the amount required to oxidize Al in the molten steel and increase the molten steel temperature to a desired temperature using this oxidation heat, and the target after the oxygen gas top blowing treatment. It can be determined from the Al concentration which is the Al content in the molten steel. In this case, although the reaction efficiency of Al and oxygen and the heating rate are different for each RH, they can be easily obtained from the operation results in each RH. And based on the operation performance, Al addition amount is determined and added to molten steel. However, as described above, the Al addition to the molten steel must be performed before the oxygen gas top blowing treatment.

  After the addition of Al to the molten steel, the oxygen gas top blowing process is started.

  The oxygen gas top blowing treatment is performed through the top blowing lance, but the shape of the lance nozzle need not be particularly defined and may be any type. However, in order to stabilize the reaction efficiency of oxygen and Al concentration, which is the Al content in molten steel, it is desirable to use a nozzle having a high jet dynamic pressure.

The oxygen flow rate is preferably 0.06 to 0.18 m ³ (normal) / min per ton of molten steel. This is because the smaller the oxygen flow rate, the more stable the deoxidation. If the flow rate is less than 0.06 m ³ (normal) / min per ton of molten steel, the treatment time becomes longer and the heating rate becomes slower. This is because the cost increases. On the other hand, as the oxygen flow rate is large processing time can be shortened, deoxidation equilibrium of the oxygen gas on the blowing process end becomes unstable, especially when more than molten steel per ton 0.18m ³ (Normal) / min, oxygen This is because the deoxidation equilibrium at the end of the gas top blowing process may become extremely unstable.

  Furthermore, it is desirable that the vertical distance between the lance and the surface of the molten steel in the vacuum chamber is 1 to 4 m. This is because when the distance is less than 1 m, the lance is easily worn by splashing from the molten steel, and when it is greater than 4 m, the jet dynamic pressure on the molten metal surface is reduced. It is because reaction efficiency may fall.

  As described above, after adding Al to the molten steel, oxygen gas is sprayed onto the surface of the molten steel through an upper blowing lance. Next, 0.005 to 0.02 kg of Al per ton of molten steel is added to the molten steel 1 to 2 minutes before the end of the oxygen gas top blowing process. Thereafter, the oxygen gas top blowing process is terminated.

If the continuous casting is performed after the above treatment, inclusions in the steel as a steel after solidification are contained in mass% and contain SiO ₂ in an amount of 1 to 12%, with the balance being Al oxide and Mn oxide. A low Al-containing steel having high cleanliness, which is composed of one or more kinds and has Al and O contents of 0.004 to 0.01% and 0.0025% or less, respectively, is obtained.

  In the above case, if the amount of Al added per ton of molten steel satisfies the above equation (1) 1-2 minutes before the end of the oxygen gas top blowing treatment, the cleanliness is improved and the cleaner is further purified. A low Al-containing steel with high properties can be obtained.

  Therefore, it is preferable that the amount of Al added per ton of molten steel 1 to 2 minutes before the end of the oxygen gas top blowing process satisfies the above formula (1).

  In addition, it is not preferable to add Al again after completion of the oxygen gas top blowing process. This is because the deoxidation equilibrium deflected to the “Al—Si—O” system is destabilized again by the addition of Al, the degree of cleanliness becomes high, and good cleanability cannot be secured.

  For the same reason, it is not preferable to add Si after the end of the oxygen gas top blowing process. However, other component elements may be adjusted by adding them at the end of the oxygen gas top blowing process.

  It should be noted that after the oxygen gas top blowing process is completed, it is desirable to continue the reflux process. This is because the inclusions generated by the reaction with oxygen are floated and separated by performing the reflux treatment after the oxygen gas top blowing treatment is completed, so that better cleanliness can be obtained.

  In particular, if the reflux treatment time is 5 min or longer, more excellent cleanliness can be secured, and if it is 12 min or longer, extremely good cleanability can be secured.

  In addition, what is necessary is just to make 20 minutes the upper limit of the time of the said recirculation | reflux processing from the surface of production efficiency or cost.

  After completion of the oxygen gas top blowing process, 0.05 to 0.17 kg of Ca or Ca alloy may be added to the molten steel in terms of pure Ca per ton of molten steel. The addition method at this time may be a general injection method or a wire method.

  Said Ca alloy refers to what contains metal Ca or Ca alloy mixed with fluxes, such as CaO, in addition to Ca alloys, such as CaSi, CaAl, and FeCa.

  The addition rate of Ca or Ca alloy is preferably 0.03 to 1.0 kg / min per ton of molten steel in terms of pure Ca. If the rate of addition is less than 0.03 kg / min per ton of molten steel, the Ca activity in the molten steel cannot be sufficiently increased, and if greater than 1.0 kg / min, Since the Ca activity becomes too large, the amount and composition of inclusions in the steel may deviate from appropriate conditions.

  Next, slag will be described.

  In general, when manufacturing steel having high cleanliness, it is well known that lower oxide concentrations such as FeO and MnO in the slag are desirable, but also in the present invention, FeO in the slag The total concentration of MnO is desirably 6% or less.

In the present invention, the mass ratio of CaO to Al ₂ O ₃ in the slag is preferably 1 to 2.5.

  The present invention utilizes the reaction in molten steel, and since the reaction between slag and molten steel is not used, the influence of slag is small. However, except for the above slag composition range, the slag undergoes reoxidation during casting and is clean. The degree of cleanliness may decrease and cleanliness may decrease. In addition, as a method for adjusting the slag composition, there are a method of adding a slag modifier at the time of converter steelmaking, and a method of stirring molten steel-slag with a ladle refining device before RH treatment.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

  Twenty kinds of molten steel (250 tons) were refined in a converter, the ladle was transferred to RH, and the treatment was started with RH.

In Steels 1 to 8 and Steels 12 to 19 shown in Table 1, various alloys were added immediately after the start of the RH treatment, and components other than Al and O were adjusted to predetermined values. Then, after addition of Al in the molten steel was blown treated on inter 4~10min at 38m oxygen gas from the top lance ³ (Normal) / min (molten steel per ton ^{0.152m 3 (Normal) / min)} . During the oxygen gas top blowing process, Al was added to the molten steel by changing the amount and addition time of Al under the conditions shown in Table 2, and after the oxygen gas top blowing process was completed, the mixture was refluxed for 8 minutes, and then continuously. A slab was produced by casting.

Further, in Steels 9 to 11 and Steel 20 shown in Table 1, various alloys were added immediately after the start of the RH treatment, and components other than Al, O, and Ca were adjusted to predetermined values. Next, as in the case of steels 1 to 8 and steels 12 to 19, after adding Al to the molten steel, oxygen gas was blown from the top blowing lance at 38 m ³ (normal) / min for 4 to 10 minutes. . In addition, during the oxygen gas top blowing treatment, Al was added to the molten steel while changing the amount of Al and the addition timing under the conditions shown in Table 2. After the oxygen gas top blowing treatment was completed, as shown in Table 2, A pure content of 0.01 to 0.20 kg of Ca or Ca alloy was added to the molten steel, and after refluxing for 8 minutes, continuous casting was performed to produce a slab.

About each slab obtained as mentioned above, the relationship between the amount of inclusions in steel and the composition was investigated using a scanning electron microscope. That is, the amount of inclusions was evaluated by a method in which the magnification was 1000 times, the surface of a sample of about 7 cm ² was observed with a scanning electron microscope and the number of inclusions was counted, and the composition was evaluated by EPMA.

Furthermore, for comparison of the amount of inclusions, the number of inclusions in the so-called “Al deoxidized steel” having an Al content of 0.035% already described in the section “(B) inclusions in steel” is 1 The inclusion inclusion index was determined, and the inclusions spheroidization rate was determined from the following formulas for test numbers 9 to 11 and test number 20 treated with Ca.
Inclusion spheroidization rate (%) = (number of spherical inclusions) / (number of observed inclusions) × 100.

  As described above, the “spherical inclusion” refers to an inclusion having a value of “major axis / minor axis”, which is a ratio of the minor axis to the major axis, of less than 1.3.

  Table 3 summarizes the above test results.

  From Table 3, in the case of test numbers 1 to 11 satisfying the conditions of the present invention (1) or the present invention (2), the inclusion number index is 0.81 to 1.03 even for a low Al-containing steel. It is clear that it has cleanliness equivalent to or better than so-called “Al deoxidized steel”.

  And among the said test numbers 1-11, it is clear that the test numbers 4-11 whose manufacturing conditions satisfy | fill the conditions of this invention (4) are excellent in especially cleanliness.

  Furthermore, among the above test numbers 4 to 11, when the conditions of Ca treatment are test number 9 and test number 10 that satisfy the condition of the present invention (5), the inclusion spheroidization rate is 100%, which is more excellent. Characteristics can be secured.

  In contrast, in the case of test numbers 12 to 20 that deviate from the conditions of the present invention (1) or the present invention (2), the inclusion number index is 1.22 to 2.10, so-called “Al deoxidized steel”. Is clearly inferior to

  The low Al-containing steel of the present invention has high cleanliness despite the low Al content of 0.004 to 0.01%. For this reason, it is excellent in weldability and toughness. Among the low Al-containing steels of the present invention, those containing Ca have much better characteristics because inclusions are spheroidized while maintaining high cleanliness. These low Al-containing steels of the present invention can be produced at low cost by the method of the present invention.

Is a diagram showing an example of the relationship between the content of SiO ₂ in the inclusions (mass%) and inclusions number index. In addition, the unit of the content of SiO _{2 in} the inclusion is expressed by “%”. It is a figure which shows the influence which the amount of Al added per ton of molten steel before completion | finish of oxygen gas top blowing process, and its addition time have on the number index of inclusions. In addition, the time before the end of the oxygen gas top blowing process was expressed as “time until the top blowing end”. Each numerical value on the right side of FIG. 2 indicates that for each mark, the amount of Al added to the molten steel is 0.001 to 0.03 kg, that is, 0.001 to 0.03 kg per ton of molten steel. Indicates. It is a figure which shows the influence which the amount of Al added per ton of molten steel 1-2 minutes before completion | finish of oxygen gas top blowing process and Al concentration after completion | finish of oxygen gas top blowing process has on the number index of inclusions. In the figure, the Al concentration after finishing the oxygen gas top blowing process is “final Al concentration”, the unit is “%”, the “inclusion number index” is “inclusion index”, and the Al addition amount is The unit was expressed as “kg / ton”. It is a figure which shows the influence which the quantity (kg) in Ca pure added per ton of molten steel has on the spheroidization rate of inclusions. In the figure, the amount of pure Ca added per ton of molten steel was expressed as “Ca addition amount (kg / ton)”. Molten steel 1 amount in Ca pure content added per ton (kg) is a diagram showing the effect on Ca content and the content of SiO ₂ in the inclusions (mass%). In the figure, the amount of pure Ca added per ton of molten steel was expressed as “Ca addition amount (kg / ton)”.

Claims (7)

In mass%, C: 0.0015 to 0.8%, Si: 0.01 to 0.8%, Mn: 0.1 to 2%, Ni and Cr total: 0.01 to 11%, Al: 0.004 to 0.01%, O: 0.0025% or less, B: less than 0.0035%, Nb: less than 0.1%, P: less than 0.015%, S: less than 0.0035% The balance is a chemical composition composed of Fe and impurities, the inclusions in the steel are in mass% and contain 1 to 12% of SiO ₂ , and the balance is from one or more of Al oxide and Mn oxide. A low Al-containing steel having high cleanliness, characterized in that In mass%, C: 0.0015 to 0.8%, Si: 0.01 to 0.8%, Mn: 0.1 to 2%, Ni and Cr total: 0.01 to 11%, Al: 0.004 to 0.01%, O: 0.0025% or less, B: less than 0.0035%, Nb: less than 0.1%, P: less than 0.015%, S: less than 0.0035%, Ca : The content is 0.0028% or less, the balance is a chemical composition composed of Fe and impurities, the inclusions in the steel are 1% by mass and contain SiO ₂ in an amount of 1 to 12%, and the balance is Ca-Al oxide and A low Al-containing steel having high cleanliness, characterized by comprising at least one of Ca-Mn oxides.   In an RH-type vacuum degassing apparatus having two dip tubes and circulating the molten steel, Al is added to the molten steel, and then oxygen gas is blown up to reduce the Al content in the molten steel from 0.004 to 0.00. A method for producing a low Al content steel of 01%, characterized in that 0.005 to 0.02 kg of Al per ton of molten steel is added to the molten steel 1 to 2 minutes before the end of the oxygen gas top blowing process. A method for producing a low Al content steel having high cleanliness. The high cleanliness according to claim 3, wherein the amount of Al added per ton of molten steel 1 to 2 minutes before the end of the oxygen gas top blowing treatment satisfies the following formula (1): Manufacturing method of low Al content steel.
−0.833 [Al] + 0.022 ≧ M ≧ −0.833 [Al] +0.015 (1).
In the formula (1), M and [Al] are respectively
M: Al amount (kg) to be added per ton of molten steel 1 to 2 minutes before the end of the oxygen gas top blowing process,
[Al]: The target value of the Al content in the molten steel after completion of the oxygen gas top blowing treatment, in the range of 0.004 to 0.01% by mass%,
Represents.   The high cleanliness according to claim 3 or 4, wherein 0.05 to 0.17 kg of Ca or Ca alloy is added to the molten steel in terms of pure Ca per ton of molten steel after the oxygen gas top blowing treatment. The manufacturing method of the low Al content steel which has.   A method for producing a low-Al-containing steel having high cleanliness as claimed in claim 1 or 2, wherein the molten steel having a chemical composition excluding Al and O as described in claim 1 is subjected to RH vacuum desorption. The method for producing low Al-containing steel having high cleanliness according to claim 3 or 4, wherein the treatment is performed by a gas apparatus.   A method for producing a low-Al-containing steel having high cleanliness as described in claim 2, wherein the molten steel having a chemical composition excluding Al, O and Ca described in claim 2 is subjected to RH vacuum desorption. The method for producing a low Al-containing steel having high cleanliness according to claim 5, wherein the treatment is performed by a gas device.

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