JP7087724B2 - Steel manufacturing method - Google Patents

Description

The present invention relates to a method for producing steel, and more specifically, to a method for producing steel having few alumina clusters, which is suitable for materials such as steel plates for automobiles, thick plates for structural and wear resistance, and steel pipes for oil pipes.

Steel materials such as steel plates are usually made of undeoxidized molten steel that has been decarburized and smelted under atmospheric pressure in a primary smelting furnace such as a converter. It is produced as Al-Kild steel or Al-Si-Killed steel in which oxygen is deoxidized with Al or Al—Si in, for example, an RH vacuum degassing device.

Alumina, which is inevitably formed during deoxidation, is hard, easily aggregates and clusters, and remains in the steel as inclusions having a size of several hundred μm or more. Therefore, if the removal of alumina from the molten steel is insufficient, the dipping nozzle of the tundish will cause nozzle clogging due to adhesion in the nozzle hole during continuous casting.

If alumina remains in the final steel product, for example, sliver defects (linear defects) in hot rolling or cold rolling in thin plates, material defects in structural thick plates, and low temperature toughness in wear resistant thick plates. In the steel pipe for oil wells, inclusion defects caused by alumina clusters such as UST defects in the welded part occur.

As a method of removing alumina from molten steel,
(A) A method in which Al, a deoxidizing agent, is added at the time of steel ejection in a converter in order to ensure the aggregation of alumina, the floating from molten steel by coalescence, and the separation time as long as possible after deoxidation.
(B) A method in which the molten steel is strongly agitated by the CAS method or the RH vacuum degassing method, which is one of the secondary refining methods, to promote the floating and separation of alumina.
(C) A method of morphologically controlling alumina to CaO-Al ₂ O ₃ which is a low melting point inclusion by adding Ca to molten steel to make it harmless has been carried out.

However, there is a limit to the measures against the floating separation of alumina by the methods (a) and (b), and there is a problem that sliver flaws cannot be prevented because inclusions having a size of several hundred μm or more cannot be completely removed.

The modification of the oxide-based inclusions by Ca in (c) can prevent the formation of alumina clusters by lowering the melting point of the inclusions, and makes the inclusions finer. However, according to Non-Patent Document 1, in order to convert alumina into a liquid phase calcium aluminate in molten steel, [Ca] / [T. O] needs to be controlled in the range of 0.7 to 1.2. For this purpose, for example, T.I. It is necessary to add a large amount of Ca with O of 40 ppm and 28 to 48 ppm.

On the other hand, in steel cords and valve spring materials for tires, inclusions are generally controlled to a low melting point CaO-SiO ₂ -Al _{2O 3} (-MnO) system that is easily deformed during rolling to make them harmless. Widely known for.

However, in these methods, Ca is usually added by an inexpensive CaSi alloy, so that it has not been put into practical use in steel sheets for automobiles and cold-rolled steel sheets for cans, in which the upper limit of the Si content is strictly controlled.

In the deoxidation of molten steel using REM (rare earth element) such as Ce and La, (i) Al killed steel or Al-Si killed steel is premised, and after Al or Al-Si deoxidation, REM is used as an alumina modifier. (Ii) A method of deoxidizing REM alone or in combination with Ca, Mg, etc. without using Al is known.

In Patent Document 1, one or more of Se, Sb, La and Ce are used after Al deoxidation or Al—Si deoxidation as a method premised on Al killed steel or Al—Si killed steel from 0.001 to 0. By adding 05% by mass, or by combining this with molten steel stirring, the interfacial tension between the molten steel / alumina clusters is controlled to float and separate the alumina clusters in the molten steel to remove non-metal inclusions. A method for producing less clean steel is disclosed.

In Patent Document 2, the molten steel is deoxidized with Al and Ti, and then Ca and / or REM is added to reduce the size of the oxide-based inclusions to 50 μm or less and to make the composition from Al ₂ O ₃ : 10 to. Disclosed is a method for producing a cold-rolled steel sheet having excellent surface texture and internal quality by using 30% by mass, Ca and / or REM oxide: 5 to 30% by mass, and Ti oxide: 50 to 90% by mass. There is.

Further, Patent Document 3 discloses a method for producing a clean Al killed steel having no alumina clusters and few defects by compound deoxidation of Al, REM and Zr.

However, with the methods disclosed in Patent Documents 1 to 3, it is difficult to reliably levitate and separate alumina clusters, and inclusion defects cannot be reduced to the required quality level.

In Patent Document 4, as a method using no Al, after deoxidizing molten steel with a CaO-containing flux, for example, 100 to 200 ppm of an alloy containing one or more of Ca, Mg and REM is added to lower the melting point of inclusions and lower the melting point. A method for producing steel for steel cord by softening is disclosed.

In Patent Document 5, T.I. Disclosed is a method for producing a wire rod having good ultrafine linearity by adding 50 to 500 ppm of REM for the purpose of preventing air oxidation after adjusting O ≦ 100 ppm.

However, in the methods disclosed in Patent Documents 4 and 5, there is a problem that the cost of the deoxidizing agent increases because inexpensive Al is not used for deoxidizing. Further, in the case of deoxidizing with Si, it is difficult to apply it to a thin plate material in which the upper limit of the Si content is strictly controlled.

On the other hand, some generation mechanisms are disclosed regarding the clustering of alumina particles. For example, in Patent Document 6, it is assumed that P ₂ O ₅ in molten steel promotes the aggregation and coalescence of Al ₂ O ₃ particles, and Ca is added to make P ₂ O ₅ nCaO · mP ₂ O ₅ and Al. It is disclosed that the adhesion of Al ₂ O ₃ to the immersion nozzle can be prevented by reducing the binding force of P ₂ O ₅ which is a binder of ₂ O ₃ .

In Non-Patent Document 2, it is presumed that the alumina particles trapped in the Ar gas used for preventing the blockage of the dipping nozzle of the tundish in continuous casting are the cause of the sliver flaw generated in the cold-rolled steel sheet. Is disclosed.

Further, Non-Patent Document 3 discloses an observation result that alumina particles trapped in bubbles aggregate and coalesce on the surface of bubbles due to a capillary effect. As described above, the microscopic formation mechanism of alumina clusters is being elucidated, but the specific method for preventing clustering has not been clarified. For this reason, it has been difficult to reduce inclusion defects due to alumina clusters to the required quality level.

According to Patent Document 7, the present inventors have made the composition of inclusions in alumina 0.5 to 15% by mass by adding a small amount of REM, thereby sliver defects and structures of thin plates for automobiles and home appliances. We disclosed steel materials with few coarse alumina clusters that cause surface defects and internal defects such as defective materials of thick plates for wear, deterioration of low temperature toughness of thick plates for wear resistance, and UST defects in welded parts of steel pipes for oil pipes.

Further, the present inventors, according to Patent Document 8, REM / T.I. A method of reducing surface defects and internal defects in the product and preventing blockage of the tundish immersion nozzle during continuous casting by setting O in the range of 0.05 to 0.5 and suppressing the formation of alumina clusters. Disclosed.

However, low T. When it is O and the amount of REM added is very small, the formation of alumina clusters is suppressed on average and product defects and nozzle blockage are improved, but the content of REM oxide in alumina becomes large and coarse. In some cases, the above-mentioned improvement effect could not be obtained because the formation of various alumina clusters could not be stably suppressed.

Furthermore, as a result of repeated experiments and studies, the present inventors have found that by adjusting the REM concentration in the alloy, the variation in the content of REM oxide in the alumina of the obtained steel material can be reduced.

Therefore, according to Patent Document 9, the present inventors have made an Al killed steel containing 0.00001 to 0.0010% of one or more rare earth metals (REM) such as Ce, La, Pr and Nd by mass%. Alternatively, it has been disclosed that in producing Al—Si killed steel, a Fe—Si—REM alloy having a REM content of 1 to 50% is added after Al or Al—Si deoxidation. The rare earth element in the invention disclosed in Patent Document 9 means La of atomic number 57 to Lu of atomic number 71.

According to the invention disclosed in Patent Document 9, by preventing the formation of coarse alumina clusters that cause product defects in steel materials in molten steel and on the surface of Ar bubbles, there are few alumina clusters, and surface defects and internal defects are present. A small amount of steel can be manufactured.

Further, according to the invention disclosed in Patent Document 9, it is possible to prevent the operation trouble due to the blockage of the dipping nozzle during continuous casting, and to prevent the reduction of the refractory cost and the decrease of the productivity due to the replacement of the dipping nozzle. can.

Japanese Unexamined Patent Publication No. 52-70918 Japanese Unexamined Patent Publication No. 2001-26842 Japanese Unexamined Patent Publication No. 11-323426 Japanese Unexamined Patent Publication No. 56-5915 Japanese Unexamined Patent Publication No. 56-47510 Japanese Unexamined Patent Publication No. 9-192799 Japanese Unexamined Patent Publication No. 2004-52076 Japanese Unexamined Patent Publication No. 2004-52077 Japanese Unexamined Patent Publication No. 2005-2422

Materials and Processes, 4 (1991), p. 1214 (Shirota et al.) Iron and Steel, (1995), p. 17 (Annaka et al.) ISIJ Int. , 37 (1997), p. 936 (H. Yin et al.)

According to the invention disclosed in Patent Document 9, it is possible to certainly provide a steel material having a small amount of alumina clusters.

In recent years, the demand for reducing inclusion defects due to alumina clusters has been increasing more than ever due to the growing demand for defect-free orientation and improvement of processing characteristics for improving productivity of steel consumers. , There is a strong demand for further reduction of inclusion defects due to alumina clusters.

For this reason, although various measures have been taken such as thorough cleaning of molten steel in the steelmaking process and heavy maintenance of slabs, inclusion defects due to alumina clusters are sufficiently reduced to the extent currently required. It has not been possible to reduce it.

INDUSTRIAL APPLICABILITY The present invention has been made in view of this problem of the prior art, and provides a method for producing steel while sufficiently reducing inclusion defects caused by alumina clusters to the extent currently required. The purpose is.

As disclosed by the present inventors in Patent Document 8, FeO, which is a low melting point oxide, does not originally exist in the molten steel in an equilibrium state deoxidized by Al. However, when molten steel having an O concentration of about 0.2% by mass exists in a non-equilibrium part of the molten steel at about 1600 ° C. (O concentration: about 6 to 8 ppm), Al ₂ O ₃ and liquid FeO simultaneously coexist. Alumina clusters are generated by the formation of liquid FeO intervening between Al ₂ O ₃ as a binder.

As a result of diligent studies on means for preventing the generation of alumina clusters based on this generation mechanism of alumina clusters, the present inventors have obtained the findings (A) to (F) listed below.

(A) MeMn (meaning "metal Mn") added after Al or Al—Si deoxidation for adjusting the Mn concentration in the steelmaking process. In the present specification, "Mn" in the alloy for adjusting the component concentration is described below. "Metal" is also expressed as "Me"), for example, about 0.5% by mass, which is a very small amount, but contains O. The total amount of O that MeMn containing O brings to the molten steel is, for example, 15 ppm or more.

Therefore, when MeMn is added after deoxidizing Al or Al—Si as in the conventional case, the molten steel is locally oxygen-contaminated by O brought in from MeMn, whereby FeO in a liquid state becomes Al ₂ O ₃ . It is generated at the same time, and the generated FeO becomes a binder between Al ₂ O ₃ to generate an alumina cluster.

(B) In a steel type having a large amount of MeMn input, that is, a steel type having a large amount of brought-in oxygen of 15 ppm or more, MeMn is not charged into the molten steel after Al or Al—Si deoxidation as in the conventional case, but Al or FeO in a liquid state is generated at the same time as Al ₂ O ₃ by putting it in molten steel having an amount of dissolved oxygen of 50 ppm or more before Al-Si deoxidation and putting it in molten steel after Al-Si deoxidation. Since it is possible to suppress the formation of alumina clusters, it is possible to reduce inclusion defects due to the alumina clusters.

(C) By reducing the amount of oxygen brought in after deoxidation to 10 ppm or less, inclusion defects due to alumina clusters can be reduced.

(D) The amount of oxygen brought into the molten steel from MeMn introduced before deoxidation with Al or Al—Si and the deoxidation brought into the molten steel from MeMn introduced after deoxidation with Al or Al—Si. By increasing the ratio of the amount of oxygen brought in after deoxidation (the amount of oxygen brought in before deoxidation / the amount of oxygen brought in after deoxidation) to 2 or more, inclusion defects due to alumina clusters can be reduced.

(E) By charging MeMn before Al or Al—Si deoxidation, the yield of Mn charging is slightly reduced, but inclusion defects due to alumina clusters can be sufficiently reduced to the quality level currently required. Therefore, the productivity and quality of the steel material, which is the final product, can be remarkably improved, and the manufacturing cost of the steel material can be significantly suppressed.

(F) In addition to MeMn, the alloys for adjusting the components of molten steel include MeTi, MeCu, MeNi, FeMn, FeP, FeTi, FeS, FeSi, FeCr, FeMo, FeB, FeNb, and the like. Contains O. Therefore, by adding these component adjusting alloys before and after Al or Al—Si deoxidation as described in the above item B, it is possible to prevent the generation of alumina clusters.

The present invention is based on these findings (A) to (F), and is as listed below.

(1) After the undeoxidized molten steel melted in the converter is discharged into a ladle, the discharged molten steel is deoxidized with Al or Al—Si, and Al killed steel or Al—Si killed steel is used. Is a method of manufacturing
An oxygen-containing component adjusting alloy is put into molten steel having a dissolved oxygen content of 50 ppm or more during or after steel ejection to the pan and before deoxidation with Al or Al—Si. And put it into the molten steel after deoxidation with the Al or Al—Si.
The amount of oxygen (ppm) brought into the molten steel from the component adjusting alloy that is charged before deoxidation with Al or Al—Si, and the oxygen amount (ppm) that is charged after deoxidation with Al or Al—Si. The ratio (the amount of oxygen brought in before deoxidation / the amount of oxygen brought in after deoxidation) to the amount of oxygen brought in after deoxidation (ppm) brought into the molten steel from the component adjustment alloy is set to 2 or more.
The amount of oxygen brought in after deoxidation is set to 10 ppm or less.
The total amount of oxygen brought in before deoxidation and the amount of oxygen brought in after deoxidation should be 15 ppm or more, and at the same time.
A method for producing steel, wherein a REM alloy having a REM content of 1 to 50% by mass is added to a molten steel that has been deoxidized with the Al or Al—Si and then charged with the component adjusting alloy.

(2) The component adjusting alloy is one or more selected from MeMn, MeTi, MeCu, MeNi, FeMn, FeP, FeTi, FeS, FeSi, FeCr, FeMo, FeB, and FeNb. The method for manufacturing steel according to.

(3) The chemical composition of the Al killed steel or the Al—Si killed steel is mass%.
C: 0.0005-1.5%,
Si: 0.005-1.2%,
Mn: 0.05-3.0%,
P: 0.001 to 0.2%,
S: 0.0001 to 0.05%,
T. Al: 0.005 to 1.5%,
Cu: 0-1.5%,
Ni: 0 to 10.0%,
Cr: 0 to 10.0%,
Mo: 0-1.5%,
Nb: 0 to 0.1%,
V: 0-0.3%,
Ti: 0 to 0.25%,
B: 0 to 0.005%,
REM: 0.1-10ppm,
T. O: 5-50ppm,
The method for producing steel according to (1) or (2) above, wherein the balance is Fe and impurities.

(4) The chemical composition of the Al killed steel or the Al—Si killed steel is by mass%.
Cu: 0.1-1.5%,
Ni: 0.1 to 10.0%,
Cr: 0.1 to 10.0%, and Mo: 0.05 to 1.5%,
The method for producing steel according to (3) above, which contains one or more selected from the above.

(5) The chemical composition of the Al killed steel or the Al—Si killed steel is by mass%.
Nb: 0.005 to 0.1%,
V: 0.005 to 0.3%, and Ti: 0.001 to 0.25%,
The method for producing steel according to (3) or (4) above, which contains one or more selected from the above.

(6) The chemical composition of the Al killed steel or the Al—Si killed steel is mass%.
B: 0.0005-0.005%,
The method for producing steel according to any one of (3) to (5) above, which comprises.

(7) The method for producing steel according to any one of (1) to (6) above, wherein the steel has a maximum diameter of alumina clusters of 100 μm or less obtained by extracting slime from slabs.

(8) The method for producing steel according to (7) above, wherein the steel has 2 or less alumina clusters of 20 μm or more obtained by extracting slime from slabs.

According to the present invention, Al ₂ O ₃ and liquid FeO are simultaneously generated and alumina clusters are generated due to the addition of an oxygen-containing component adjusting alloy into the molten steel after deoxidization of Al or Al—Si. Can be prevented.

Therefore, according to the present invention, the generation of coarse alumina clusters that cause surface defects and internal defects in the final product made of Al-killed steel or Al-Si-killed steel is sufficiently reduced to the extent currently required. However, it will be possible to manufacture molten steel.

According to the present invention, in producing Al killed steel or Al—Si killed steel, after deoxidizing Al or Al—Si, a REM alloy having a REM content of 1 to 50% by mass is added to the final product. It is possible to reliably prevent the formation of coarse alumina clusters that cause surface defects and internal defects.

Therefore, the present invention eliminates the problems in the conventional Al-killed steel or Al-Si-killed steel, can surely produce a steel material having a small amount of alumina clusters, and contributes to the development of the industry.

FIG. 1 is a graph showing the relationship between the amount of oxygen brought in before deoxidation and the amount of oxygen brought in after deoxidation in the examples.

The present invention will be described. In the following description, the chemical composition is based on mass% unless otherwise specified.

1. 1. Outline of the present invention In the present invention, basically, after the undeoxidized molten steel melted in the converter is dispensed into a ladle, the ejected molten steel is subjected to, for example, Al or in an RH vacuum degassing apparatus. Al-sild steel or Al-Si killed steel is melted by deoxidizing with Al—Si.

At this time, the component adjusting alloy containing oxygen is put into the molten steel having a dissolved oxygen content of 50 ppm or more during or after the steel is discharged to the pan and before the deoxidation by Al or Al—Si. At the same time, it is put into molten steel after deoxidation with Al or Al—Si. The amount of dissolved oxygen in the molten steel is preferably 500 ppm or less.

Further, in the present invention, after deoxidizing with Al or Al—Si, and then adding the component adjusting alloy, the REM alloy having a REM content of 1 to 50% by mass is added. By using the REM alloy having a REM content of 50% or less as described above, the variation in the concentration of the REM oxide in the molten steel is reduced, and the formation of coarse alumina clusters can be stably suppressed.

Here, REM is a general term for 17 elements in which Y and Sc are combined with 15 elements of lanthanoids. One or more of these 17 elements can be contained in the steel material, and the REM content means the total content of these elements.

In the present invention, the component adjusting alloy and the REM alloy are charged into the molten steel in the order shown below, for example.
(I) Converter undeoxidized molten steel (Neither alloy for component adjustment nor REM has been added)
(Ii) Converter or RH vacuum degassing device (addition of alloy for pre-deoxidation component adjustment)
(Iii) RH vacuum degassing device (Al or Al-Si deoxidation → alloy for component adjustment after deoxidation → REM alloy injection)

In the present invention, in this way, while preventing the simultaneous generation of Al ₂ O ₃ and liquid FeO and the generation of alumina clusters, the generation of inclusion defects caused by the alumina clusters is sufficiently suppressed to the extent currently required. Reduce and manufacture molten steel.

2. 2. Alloy for component adjustment In the present invention, an alloy for component adjustment containing oxygen is charged into molten steel during or after steel ejection to a ladle and before and after deoxidation with Al or Al—Si. do. That is, the timing of charging the oxygen-containing component adjusting alloy into the molten steel is not only after deoxidation with the conventional Al or Al—Si, but also during or after steel ejection to the pan, and Al or Al. -Change before and after deoxidation with Si.

In the present invention, the amount of oxygen brought in before deoxidation (ppm) brought into molten steel having a dissolved oxygen amount of 50 ppm or more from the component adjusting alloy charged before deoxidation with Al or Al—Si, and Al or Al—Si The ratio (the amount of oxygen brought in before deoxidation / the amount of oxygen brought in after deoxidation) to the amount of oxygen brought in after deoxidation (ppm) brought into the molten steel from the component adjusting alloy introduced after deoxidation is set to 2 or more. The ratio is preferably 2.5 or more and 130 or less.

The amount of oxygen brought in (the amount of oxygen brought in before deoxidation and the amount of oxygen brought in after deoxidation) is the amount of oxygen brought in from each component adjusting alloy (mass ppm), and the amount of component adjusting alloy input (kg) x the relevant component. It can be calculated by the oxygen concentration (%) / 100 / molten steel amount (kg) × ¹⁰⁶ in the adjusting alloy, and the total amount of oxygen brought in from all the component adjusting alloys can be calculated. Further, in the present invention, the amount of oxygen brought in after deoxidation is 10 ppm or less, preferably 0.2 ppm or more and 5 ppm or less.

As a result, Al ₂ O ₃ and liquid FeO can be prevented from being simultaneously generated in the molten steel, and the generation of alumina clusters can be prevented. Therefore, molten steel can be manufactured while sufficiently reducing inclusion defects due to alumina clusters to the quality level currently required.

In the present invention, the total amount of oxygen brought in before deoxidation and the amount of oxygen brought in after deoxidation is 15 ppm or more. When the total amount of oxygen brought in before deoxidation and the amount of oxygen brought in after deoxidation is less than 15 ppm, only a small amount of Al ₂ O ₃ and liquid FeO are generated, and the alloy for component adjustment containing oxygen is generated. This is because there is no harmful effect of putting oxygen into molten steel. The total amount of oxygen brought in is preferably 170 ppm or less.

The timing of charging the component adjusting alloy into the molten steel is not particularly limited as long as it is during or after steel ejection to the ladle and before and after deoxidation with Al or Al—Si. However, if it is added as soon as possible before deoxidation by Al or Al—Si, for example, immediately after the steel is discharged into the ladle, the liquid FeO once generated before deoxidation by Al or Al—Si is surely contained in the molten steel. It is preferable because it will dissolve.

Examples of the oxygen-containing component adjusting alloy include one or more selected from MeMn, MeTi, MeCu, MeNi, FeMn, FeP, FeTi, FeS, FeSi, FeCr, FeMo, FeB, and FeNb.

The oxygen concentration of each component adjusting alloy is MeMn: about 0.5%, MeTi: about 0.2%, MeCu: about 0.04%, MeNi: about 0.002%, FeMn: about 0.4%. , FeP: about 1.5%, FeTi: about 1.3%, FeS: about 6.5%, FeSi: about 0.4%, FeCr: about 0.1%, FeMo: about 0.01%, FeB : About 0.4%, FeNb: about 0.03% are exemplified.

3. 3. REM
In the present invention, after deoxidizing Al or Al—Si and adding the component adjusting alloy, the REM alloy having a REM content of 1 to 50% is added. By using a REM alloy having a REM content of 50% or less, the variation in the concentration of the REM oxide in the molten steel becomes small, and the formation of coarse alumina clusters can be stably suppressed.

In the present invention, as will be described later, the REM content of the Al killed steel or the Al—Si killed steel is 0.1 to 10 ppm. The reason for this is that if the REM content is less than 0.1 ppm, there is no effect of suppressing the formation of alumina clusters, while if it exceeds 10 ppm, the variation in the content of REM oxide in alumina is small regardless of which REM alloy is used. Is.

Further, if the REM content in the alloy added to the molten steel is less than 1%, a large amount of alloy is required to obtain a predetermined REM concentration, so that the temperature drop of the molten steel becomes too large, while in the alloy added to the molten steel. When the REM content exceeds 50%, the variation in the content of the REM oxide in the alumina becomes large as in the conventional case. Therefore, the REM content in the alloy is set to 1 to 50%.

The addition of REM into the molten steel is carried out, for example, by deoxidizing Al or Al—Si of the molten steel using a CAS or RH vacuum degassing device of a secondary refining device, and then adding an alloy for component adjustment. ,conduct. The shape of the REM may be lumpy, granular, wire or the like. Since the amount of REM added is extremely small, it is desirable to add it to the reflux molten steel in the RH vacuum degassing tank or to stir with Ar gas after adding the ladle in order to make the REM concentration in the molten steel uniform. Further, REM can be added to the tundish and the molten steel in the mold.

4. Chemical Composition of Al Killed Steel or Al—Si Killed Steel Produced by the Present Invention The chemical composition of the Al killed steel or Al—Si killed steel produced by the present invention is, in mass%, C: 0.0005 to 1. 5%, Si: 0.005 to 1.2%, Mn: 0.05 to 3.0%, P: 0.001 to 0.2%, S: 0.0001 to 0.05%, T.I. Al: 0.005 to 1.5%, Cu: 0 to 1.5%, Ni: 0 to 10.0%, Cr: 0 to 10.0%, Mo: 0 to 1.5%, Nb: 0 ~ 0.1%, V: 0 to 0.3%, Ti: 0 to 0.25%, B: 0 to 0.005%, REM: 0.1 to 10 ppm, T.I. O: Carbon steel or alloy steel having a chemical composition of 5 to 50 ppm, the balance Fe and impurities is preferable. It can be applied to thin plates, thick plates, steel pipes, shaped steel, steel bars, etc. by adding necessary processing to steel materials. The reason why this range is preferable is as follows.

C: 0.0005-1.5%
C is a basic element that most stably improves the strength of steel materials. The C content is preferably 0.0005% or more in order to secure the strength or hardness of the steel material. However, if the C content exceeds 1.5%, the toughness and workability of the steel material are impaired. Therefore, the C content is preferably adjusted in the range of 0.0005 to 1.5% depending on the desired strength of the steel material.

Si: 0.005-1.2%
If the Si content is less than 0.005%, it becomes necessary to perform hot metal pretreatment, which imposes a heavy burden on refining and impairs economic efficiency. On the other hand, if the Si content exceeds 1.2%, plating defects occur and the surface properties and corrosion resistance of the steel material deteriorate. Therefore, the Si content is preferably 0.005 to 1.2%.

Mn: 0.05-3.0%
If the Mn content is less than 0.05%, the refining time becomes long and the economic efficiency is impaired. On the other hand, if the Mn content exceeds 3.0%, the workability of the steel material is significantly deteriorated. Therefore, the Mn content is preferably 0.05 to 3.0%.

P: 0.001 to 0.2%
If the P content is less than 0.001%, the time and cost of the hot metal pretreatment will increase and the economic efficiency will be impaired. On the other hand, if the P content exceeds 0.2%, the workability of the steel material is significantly deteriorated. Therefore, the P content is preferably 0.001 to 0.2%.

S: 0.0001 to 0.05%
If the S content is less than 0.0001%, the time and cost of the hot metal pretreatment are long and the economic efficiency is impaired. On the other hand, when the S content exceeds 0.05%, the workability and corrosion resistance of the steel material are significantly deteriorated. Therefore, the S content is preferably 0.0001 to 0.05%.

T. Al: 0.005 to 1.5%
In the present invention, the Al amount, which is the total amount of the solid-dissolved Al (sol.Al) amount that affects the material with respect to the Al content and the Al (insol.Al) amount derived from the inclusion Al ₂ O ₃ , is used. T. It is defined as Al (Total.Al). In other words, T. Al = sol. Al + insol. It means Al.

T. If the Al content is less than 0.005%, N is trapped as AlN and the solid solution N cannot be reduced. On the other hand, T.I. If the Al content exceeds 1.5%, the surface texture and workability of the steel material deteriorate. Therefore, T.I. The Al content is preferably 0.005 to 1.5%.

The above are essential elements, but in the present invention, in addition to these, one or more selected from (i) Cu, Ni, Cr and Mo, (ii) Nb, as optional elements according to their respective uses. It may contain one or more selected from V and Ti, and (iii) B.

One or more Cu, Ni, Cr, Mo selected from Cu: 0 to 1.5%, Ni: 0 to 10.0%, Cr: 0 to 10.0% and Mo: 0 to 1.5% , All of which are elements that improve the hardenability of steel, and one or more selected from the above elements may be contained, if necessary.

However, if Cu and Mo are contained in an amount of more than 1.5% and Ni and Cr are contained in an amount of more than 10.0%, the toughness and workability of the steel material are impaired. Therefore, Cu: 1.5% or less, Ni: 10.0% or less, Cr: 10.0% or less, Mo: 1.5% or less are preferable. In order to surely obtain the effect of improving the hardenability of the steel material, the Cu content, the Ni content and the Cr content are each preferably 0.1% or more, and the Mo content is preferably 0.05%. That is all.

One or more selected from Nb: 0 to 0.1%, V: 0 to 0.3% and Ti: 0 to 0.25% All of Nb, V and Ti increase the strength of steel by precipitation strengthening. It is an element to be improved, and may contain one or more kinds as needed.

However, if Nb exceeds 0.1%, V exceeds 0.3%, and Ti exceeds 0.25%, the toughness of the steel material is impaired. Therefore, it is preferable that Nb: 0.1% or less, V: 0.3% or less, and Ti: 0.25% or less. In order to surely obtain the effect of improving the strength of the steel material, the Nb content and the V content are each preferably 0.005% or more, and the Ti content is preferably 0.001% or more.

B: 0 to 0.005%
B is an element that improves the hardenability of steel materials and enhances their strength. Therefore, it may be contained as needed. However, if it is contained in excess of 0.005%, the precipitate of B may be increased and the toughness of the steel material may be impaired. Therefore, the B content is preferably 0.005% or less. In order to surely obtain the effect of improving the hardenability of the steel material, the B content is more preferably 0.0005% or more.

REM: 0.1-10ppm
If the REM content of the Al killed steel or the Al—Si killed steel is less than 0.1 ppm, the effect of preventing the clustering of the alumina particles cannot be obtained. On the other hand, if the REM content is more than 10 ppm _, a coarse cluster composed of a composite oxide of REM oxide and _Al2O3 may be formed. In addition, the reaction with slag produces a large amount of composite oxide, which deteriorates the cleanliness of molten steel and may block the dipping nozzle of the tundish. Therefore, the REM content is preferably 0.1 to 10 ppm. The REM content is more preferably 8 ppm or less.

T. O: 5-50ppm
In the present invention, the amount of O, which is the total amount of the amount of solid solution O (sol. O) that affects the material and the amount of O (insol. O) present in the inclusions, is defined as T.I. It is defined as O (Total.O). Al-Killed Steel or Al-Si Killed Steel T.I. If O is less than 5 ppm, the processing time in the secondary refining, for example, a vacuum degassing device is significantly increased, which is costly and impairs economic efficiency. On the other hand, T.I. This is because if O exceeds 50 ppm, the collision frequency of the alumina particles increases and the cluster may become coarse. In addition, since the amount of REM and Mg added required for reforming alumina is increased, the cost is high and the economic efficiency is impaired. Therefore, T.I. O is preferably 5 to 50 ppm.

5. Maximum diameter and number of alumina clusters 5-1. Maximum diameter The maximum diameter of the alumina cluster obtained by slime extraction of the slab is preferably 100 μm or less. This is because if the maximum diameter of the alumina cluster is larger than 100 μm, it leads to surface defects and internal defects of the steel.

5-2. Number The number of alumina clusters of 20 μm or more obtained by slime extraction of slabs is preferably 2 / kg or less. This is because if the number of alumina clusters of 20 μm or more is more than 2 / kg, it leads to surface defects and internal defects of steel after processing.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

The undeoxidized molten steel melted in a 270 ton converter is discharged into a pan, and then the discharged molten steel is deoxidized with Al or Al—Si in an RH vacuum degassing device. Alternatively, Al-Si killed steel was melted.

At this time, an oxygen-containing component adjusting alloy is added to the molten steel during or after the steel is discharged to the ladle and before the deoxidation with Al or Al—Si, and the molten steel after the deoxidation. After that, REM was added. Table 1 shows the alloy concentration and oxygen concentration of the component adjusting alloy.

Tables 2 to 4 show the alloy charging conditions (dissolved oxygen amount, charging timing (elapsed time from the start of steel removal when not deoxidized)), input alloy type (before deoxidation, after deoxidation), and the amount of oxygen brought in (deoxidation). The amount of oxygen brought in before acid, the amount of oxygen brought in after deoxidation), the ratio (the amount of oxygen brought in before deoxidization / the amount of oxygen brought in after deoxidization), the amount of oxygen brought in before deoxidization and the amount of oxygen brought in after deoxidization) are shown. .. Further, FIG. 1 graphically shows the relationship between the amount of oxygen brought in before deoxidation and the amount of oxygen brought in after deoxidation.

The alloy charging conditions in Tables 2 to 4 show the loading conditions of the component adjusting alloy having a large amount of oxygen brought in in comparison before and after deoxidation.

The amount of oxygen brought in in Tables 2 to 4 (the amount of oxygen brought in before deoxidation and the amount of oxygen brought in after deoxidation) is the amount of oxygen brought in from each alloy (mass ppm), and the amount of oxygen input from the alloy (kg) x the oxygen concentration in the alloy. It was calculated by (%) / 100 / amount of molten steel (kg) × ¹⁰⁶ , and the amount of oxygen brought in from all alloys was totaled.

REM in Tables 5 to 7 is the total content of each REM element such as Ce, La, Pr and Nd. The added metals in Tables 5 to 7 are MM: Mish metal (Alloy consisting of Ce: 45%, La: 35%, Pr: 6%, Nd: 9%, and other unavoidable impurities), 2REM: Fe-45% Si-. 2% REM alloy, 10REM: Fe-40% Si-10% REM alloy, 33REM: Fe-30% Si-33% REM alloy, 49REM: Fe-20% Si-49% REM alloy, 59REM: Fe-10% Si-59% REM alloy, 78REM: Fe-5% Si-78% REM alloy is shown.

Molten steel was collected 1 minute after the addition of the REM alloy, and the REM oxide concentration in the inclusions was analyzed by SEM / EDS. Alumina was arbitrarily extracted from the cross section of each sample for 10 samples, and converted so that the total amount of oxides excluding Fe was 100%. The difference between the maximum REM oxide concentration and the minimum REM oxide concentration was defined as the variation Δ of the inclusion composition. The REM oxide concentration is the total concentration of Ce ₂ O ₃ , La ₂ O ₃ , Pr ₂ O ₃ , and Nd ₂ O ₃ .

Molten Al-killed steel or Al-Si-killed steel molten steel is cast by a vertical bending type continuous casting machine. Continuous casting was performed under the condition of molten steel temperature in the dish: 1520 to 1580 ° C. to produce continuously cast slabs.

Then, the continuously cast slabs are (a) hot-rolled and pickled to produce thick plates having the chemical compositions shown in Tables 5 to 7, and (b) hot-rolled, pickled and cold-rolled. To produce a thin plate having the chemical composition shown in Tables 5 to 7, or (c) a thick plate produced by hot rolling and pickling, and having the chemical composition shown in Tables 5 to 7. Manufactured welded steel pipes. The plate thickness after hot rolling was 2 to 100 mm, and the plate thickness after cold rolling was 0.2 to 1.8 mm.

Tables 5 to 7 show the maximum cluster diameter, the number of clusters, the defect occurrence rate, the pot nozzle blockage status, etc. of the sample collected from the slab.

The "maximum cluster diameter" in Tables 5 to 7 is the major axis and minor axis of the inclusions photographed by taking a photograph (40 times) with a stereomicroscope of inclusions extracted with a slime electrode having a mass of 1 kg (using a minimum mesh of 20 μm). The average value of the above was obtained for all inclusions, and the maximum value of the average value was taken as the maximum inclusion diameter.

The "number of clusters" in Tables 5 to 7 are inclusions extracted from a slime electrode having a mass of 1 kg (using a minimum mesh of 20 μm), and the number of all inclusions of 20 μm or more observed with an optical microscope (100 times) is in units of 1 kg. It was measured by converting it into the number of pieces.

The "defect occurrence rate" in Tables 5 to 7 is the sliver defect occurrence rate (= sliver defect total length / coil length x 100,%) on the plate surface in the case of a thin plate, and the product in the case of a thick plate. The UST defect occurrence rate or separation occurrence rate on the plate (= number of defect generation plates / total number of inspection plates x 100,%), and the presence or absence of separation was confirmed by observing the fracture surface after the Charpy test.

In the case of steel pipes, it is the UST defect occurrence rate (= number of defect generation pipes / total number of inspection pipes x 100,%) at the welded part of the oil country tubular goods. Further, in the case of a thick plate, in the defect occurrence rate of the thick plate material in Tables 2 to 4, when the defect is a UST defect, it is described as UST, and when the defect is a separation defect, it is described as SPR.

The “impact absorption energy” in Tables 5 to 7 is a V-notch Charpy impact test value having a width of 10 mm in the rolling direction at −20 ° C., and is an average value of five test pieces. The "thickness direction drawing value" in Tables 2 to 4 is the plate thickness direction drawing value of the product plate at room temperature (= cross-sectional area of the fractured portion after the tensile test / cross-sectional area of the test piece before the test × 100,%).

Further, in the "nozzle blockage status" in Tables 5 to 7, the adhesion thickness of inclusions on the inner wall of the immersion nozzle is measured after casting, and the nozzle blockage status is determined from the average value of 10 points in the circumferential direction. Δ: Adhesive thickness of 1 to 3 mm, ×: Adhesive thickness of more than 3 mm.

No. in Table 5 A1 to A31 are examples of the present invention that satisfy all the scope of the present invention, and No. 1 in Table 6 shows. B1 to B16 are comparative examples in which the ratio (the amount of oxygen brought in before deoxidation / the amount of oxygen brought in after deoxidation) does not satisfy the range of the present invention, and No. 1 in Table 7 shows. C1 to C13 are comparative examples in which the REM content of the added REM alloy exceeds 50%.

As shown in Tables 5 to 7, according to the example of the present invention, Al ₂ O ₃ is caused by charging an oxygen-containing component adjusting alloy into Al or Al—Si deoxidized steel after Al deoxidation. And it is possible to prevent the simultaneous generation of liquid FeO and the generation of alumina clusters, which enables the production of molten steel while sufficiently reducing the generation of inclusion defects caused by the alumina clusters to the extent currently required. , Surface defects and internal defects caused by coarse alumina clusters in the final product steel can be reduced.

Claims (8)

After the undeoxidized molten steel melted in the converter is discharged into a ladle, the discharged molten steel is deoxidized with Al or Al—Si to produce Al killed steel or Al—Si killed steel. It ’s a method,
An oxygen-containing component adjusting alloy is put into molten steel having a dissolved oxygen content of 50 ppm or more during or after steel ejection to the pan and before deoxidation with Al or Al—Si. And put it into the molten steel after deoxidation with the Al or Al—Si.
The amount of oxygen (ppm) brought into the molten steel from the component adjusting alloy that is charged before deoxidation with Al or Al—Si, and the oxygen amount (ppm) that is charged after deoxidation with Al or Al—Si. The ratio (the amount of oxygen brought in before deoxidation / the amount of oxygen brought in after deoxidation) to the amount of oxygen brought in after deoxidation (ppm) brought into the molten steel from the component adjustment alloy is set to 2 or more.
The amount of oxygen brought in after deoxidation is set to 10 ppm or less.
The total amount of oxygen brought in before deoxidation and the amount of oxygen brought in after deoxidation should be 15 ppm or more, and at the same time.
A method for producing steel, wherein a REM alloy having a REM content of 1 to 50% by mass is added to a molten steel that has been deoxidized with the Al or Al—Si and then charged with the component adjusting alloy. The steel according to claim 1, wherein the component adjusting alloy is one or more selected from MeMn, MeTi, MeCu, MeNi, FeMn, FeP, FeTi, FeS, FeSi, FeCr, FeMo, FeB, and FeNb. Manufacturing method. The chemical composition of the Al killed steel or the Al—Si killed steel is, by mass%,
C: 0.0005-1.5%,
Si: 0.005-1.2%,
Mn: 0.05-3.0%,
P: 0.001 to 0.2%,
S: 0.0001 to 0.05%,
T. Al: 0.005 to 1.5%,
Cu: 0-1.5%,
Ni: 0 to 10.0%,
Cr: 0 to 10.0%,
Mo: 0-1.5%,
Nb: 0 to 0.1%,
V: 0-0.3%,
Ti: 0 to 0.25%,
B: 0 to 0.005%,
REM: 0.1-10ppm,
T. O: 5-50ppm,
The method for producing steel according to claim 1 or 2, wherein the balance is Fe and impurities. The chemical composition of the Al killed steel or the Al—Si killed steel is, by mass%,
Cu: 0.1-1.5%,
Ni: 0.1 to 10.0%,
Cr: 0.1 to 10.0%, and Mo: 0.05 to 1.5%,
The method for producing steel according to claim 3, which comprises one or more selected from the above. The chemical composition of the Al killed steel or the Al—Si killed steel is, by mass%,
Nb: 0.005 to 0.1%,
V: 0.005 to 0.3%, and Ti: 0.001 to 0.25%,
The method for producing steel according to claim 3 or 4, which comprises one or more selected from. The chemical composition of the Al killed steel or the Al—Si killed steel is, by mass%,
B: 0.0005-0.005%,
The method for producing steel according to any one of claims 3 to 5, which comprises. The method for producing steel according to any one of claims 1 to 6, wherein the steel has a maximum diameter of 100 μm or less of alumina clusters obtained by extracting slime from slabs. The method for producing steel according to claim 7, wherein the steel has a number of alumina clusters of 20 μm or more obtained by extracting slime from slabs of 2 pieces / kg or less.

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