JP2020002412A - Manufacturing method of steel - Google Patents

Abstract

To provide a manufacturing method of steel reducing generation of inclusion defect due to alumina cluster, and manufacturing a steel while suppressing nozzle jamming during continuous casting without using a large amount f slag modifier.SOLUTION: There is provided a manufacturing method of a steel for manufacturing a killed steel by deoxidizing a tapped molten steel by Al or the like after tapping an un-deoxidized molten steel ingot by a converter into a ladle, in which an alloy for adjusting components containing oxygen is input into a molten steel with molten oxygen amount during or after tapping into the ladle and before deoxidization by Al or the like of 50 ppm or more as well as into the molten steel after deoxidization by Al or the like, (carried oxygen amount before deoxidization/carried oxygen amount after oxidization) is 2 or more, carried oxygen amount after deoxidization is 10 ppm or less, total of carried oxygen amount before deoxidization and carried oxygen amount after deoxidization is 15 ppm or more, one or more kind of REM is added to the molten steel which is deoxidized by Al or the like and then into which the alloy for adjusting components is input, and the molten steel to which the REM is added is stirred over time at 2.0 to 10 times of uniform mixing time.SELECTED DRAWING: Figure 5

Description

  The present invention relates to a method for producing steel.

  Steel materials such as steel plates are usually produced in a ladle with undeoxidized molten steel that has been decarburized and refined under atmospheric pressure by a primary refining furnace such as a converter, and then increased in the molten steel by decarburization refining. It is manufactured as Al-killed steel or Al-Si-killed steel in which oxygen is deoxidized by, for example, Al or Al-Si in an RH vacuum degasser.

  Alumina inevitably generated during deoxidation is hard, easily aggregated and clustered, and remains in the steel as inclusions having a size of several hundred μm or more. For this reason, if alumina is not sufficiently removed from the molten steel, nozzle clogging due to adhesion in the nozzle hole occurs in the tundish immersion nozzle during continuous casting.

  Also, if alumina remains in the steel product as the final product, for example, sliver flaws (linear flaws) in hot rolling or cold rolling for thin sheets, poor material quality for structural thick plates, and low-temperature toughness for wear-resistant thick plates Inclusion defects due to alumina clusters, such as a decrease in the diameter of the steel pipe for oil wells and UST defects in the welded portions, occur.

  For example, even in the production of an ultra-low carbon steel (hereinafter, referred to as "340BH / HiMn-SULC") for a paint bake hardening type steel sheet having a tensile strength of 340 MPa, inclusion defects caused by alumina clusters become a problem.

  340BH / HiMn-SULC has a high Mn chemical composition with a Mn content of about 0.60% by mass. For this reason, metal Mn (hereinafter, referred to as “MeMn”, and similarly, “metal” in a later-described concentration adjusting alloy is described as “Me”) serving as a supply source of Mn in the steelmaking process is put into molten steel. Therefore, it is necessary to increase the Mn concentration of the molten steel.

  On the other hand, Mn added to the molten steel is oxidized to generate MnO and easily floats and separates, so that the yield of Mn input tends to decrease. For this reason, MeMn has previously produced undeoxidized molten steel melted in a converter into a ladle, deoxidized with Al or Al-Si using an RH vacuum degasser, and then converted into molten steel. Had been thrown.

  In the production of 340BH / HiMn-SULC, inclusion defects such as nozzle clogging and sliver flaws during cold rolling occur frequently in the tundish immersion nozzle during continuous casting, and the paint bake hardening type. The productivity and quality of the steel sheet were reduced.

  According to Patent Document 1, the present inventors add one or more rare earth metals of Ce, La, Pr, and Nd to Al or Al-Si deoxidized molten steel, so that C: 0 0.0005 to 1.5%, Si: 0.005 to 1.2%, Mn: 0.05 to 3.0%, P: 0.001 to 0.1%, S: 0.0001 to 0.05 %, Al: 0.005 to 1.5%, balance being Fe, having a steel composition in which the total rare earth element is 0.1 ppm or more and less than 10 ppm, and the solid solution rare earth element is less than 1 ppm, an alumina cluster. Has disclosed few steel materials.

  The steel material disclosed in Patent Document 1 prevents the formation of coarse alumina clusters that cause inclusion defects, in molten steel and on the surface of Ar bubbles, sliver flaws on thin plates for automobiles and home appliances, and thicknesses for structures. Inclusion defects such as poor material quality of the plate, lowering of the low-temperature toughness of the wear-resistant thick plate, and UST defects in the welded portion of the steel pipe for oil country tubular goods can be significantly suppressed.

  Also, when the molten steel subjected to Al deoxidation or Al-Si deoxidization is tapped into a ladle, Al in the molten steel is reoxidized by lower oxides such as FeO and MgO contained in the slag on the surface of the molten steel, and alumina is formed. Is done. As a result, the cleanliness of the steel deteriorates, the toughness and surface quality of the steel deteriorate, and nozzle clogging during continuous casting occurs.

Therefore, as disclosed in Patent Documents 2 to 5, a lower oxide in the slag is reduced or diluted by adding a slag modifier to the slag of the ladle, and the Al in the molten steel is recycled. A method of suppressing oxidation has been used. Patent Document 2 to 5, as a slag modifier, Al, Si, Ti, Ca , Mg, Zr, MgO, etc. _CaO-Al 2 _{O 3} is disclosed.

JP 2005-2425 A JP-A-2003-3209 JP 2001-234229 A JP 2001-254118 A JP-A-6-10025

  According to the invention disclosed in Patent Document 1, it is possible to provide a steel material with few alumina clusters.

  In recent years, the demand for reducing inclusion defects due to alumina clusters has increased even more than before due to the growing demand for defect-free orientation and improved processing characteristics for steel users to improve productivity. There is a strong demand for further reducing inclusion defects due to alumina clusters.

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

  Moreover, the reducing power of the slag modifier disclosed in Patent Documents 2 to 5 is insufficient, and a large amount of addition is required for slag reforming. In addition, since the cleanliness of molten steel deteriorates mainly at the interface between the molten steel and the slag in contact with the molten steel, a large amount of slag is required when the slag modifying material is introduced from the upper surface of the slag. As described above, in the conventional reforming method, an increase in the production cost due to the addition of a large amount of the slag reforming material cannot be avoided.

  The present invention has been made in view of this problem of the prior art, and sufficiently reduces inclusion defects due to alumina clusters to the extent currently required, without using a large amount of slag modifier. It is an object of the present invention to provide a method of manufacturing steel while improving the cleanliness and toughness of steel, and further improving the surface quality and suppressing nozzle clogging during continuous casting.

As disclosed by the present inventors in Patent Document 1, FeO, which is a low melting point oxide, is not originally present in molten steel in an equilibrium state deoxidized by Al. However, when the molten steel having an O concentration of about 0.2% by mass exists in a part of the molten steel at about 1600 ° C. (O concentration: about 6 to 8 ppm), Al ₂ O ₃ and liquid FeO are simultaneously generated. Alumina clusters are generated when liquid FeO is interposed between Al ₂ O ₃ as a binder.

  The present inventors have intensively studied means for preventing alumina cluster generation based on this alumina cluster generation mechanism, and as a result, the following findings (A) to (F) were obtained.

  (A) MeMn introduced after deoxidation of Al or Al-Si for adjusting the Mn concentration in the steelmaking process of 340BH / HiMn-SULC contains O although it is a trace amount, for example, about 0.5% by mass. I do. The total amount of O brought into the molten steel by MeMn containing O is, for example, 15 ppm or more.

For this reason, when MeMn is introduced after Al or Al-Si deoxidation as in the prior art, the molten steel is locally oxygen-contaminated by O brought in from MeMn, whereby FeO in a liquid state is simultaneously removed with Al ₂ O _3. The generated FeO becomes a binder between Al ₂ O ₃ and alumina clusters are generated.

(B) In a steel type such as 340BH / HiMn-SULC, in which the amount of MeMn is large, that is, a steel type in which the amount of oxygen brought in is as high as 15 ppm or more, MeMn is added to the molten steel after Al or Al-Si deoxidation as in the past. Instead of charging, Al is introduced into molten steel having a dissolved oxygen amount of 50 ppm or more before deoxidation of Al or Al-Si, and is injected into molten steel after deoxidation of Al or Al-Si. Thereby, the formation of alumina clusters can be suppressed by preventing the formation of FeO in the liquid state simultaneously with Al ₂ O _3, so that inclusion defects due to alumina clusters can be reduced.

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

  (D) Deoxygenation carried into molten steel from MeMn introduced before deoxidation by Al or Al-Si, and deoxidation brought into molten steel from MeMn introduced after deoxidation by Al or Al-Si introduced into molten steel. Inclusion defects due to alumina clusters can be reduced by increasing the ratio to the amount of oxygen carried after (oxygen amount before deoxidation / oxygen amount after deoxidation) to 2 or more.

  (E) By introducing MeMn before deoxidation of Al or Al-Si, although the yield of Mn is slightly reduced, inclusion defects due to alumina clusters can be sufficiently reduced to the currently required quality level. The productivity and quality of the steel product as the final product can be significantly improved, and the production cost of the steel material can be significantly reduced.

  (F) In addition to MeMn, alloys for adjusting the composition of molten steel include MeTi, MeCu, MeNi, FeMn, FeP, FeTi, FeS, FeSi, FeCr, FeMo, FeB, FeNb, and the like. Contains O. Therefore, by introducing these component adjusting alloys before and after Al or Al-Si deoxidation as described in the above section B, the generation of alumina clusters can be prevented.

The present invention is based on these findings (A) to (F), and is as listed below.
(1) After the undeoxidized molten steel produced in the converter is tapped into a ladle, the tapped molten steel is deoxidized with Al or Al-Si, and Al-killed steel or Al-Si-killed steel is produced. A method of manufacturing
The oxygen-containing component adjusting alloy is introduced into molten steel having a dissolved oxygen amount of 50 ppm or more during or after tapping into the ladle and before deoxidation by the Al or the Al-Si. Together with the Al or the Al-Si into the molten steel after deoxidation,
The amount of oxygen (ppm) carried before deoxidation brought into the molten steel from the component adjusting alloy introduced before the deoxidation by Al or the Al-Si, and the oxygen introduced after deoxidation by the Al or Al-Si. The ratio of the amount of oxygen brought in after deoxidation (ppm) brought into the molten steel from the component adjusting alloy to the molten steel (the amount of oxygen brought in before deoxidation / the amount of oxygen brought in after deoxidation) is 2 or more,
The amount of oxygen brought in after the deoxidation is 10 ppm or less,
The total amount of oxygen brought in before and after deoxidation should be 15 ppm or more,
One or more REMs are added to the molten steel that has been deoxidized by the Al or Al-Si and then the alloy for component adjustment is added, and the molten steel to which the REMs are added is mixed for a uniform mixing time of 2.0%. A method for producing steel in which stirring is performed for 10 to 10 times.

  (2) The component adjusting alloy is at least one selected from MeMn, MeTi, MeCu, MeNi, FeMn, FeP, FeTi, FeS, FeSi, FeCr, FeMo, FeB, and FeNb. 3. The method for producing steel according to item 1.

(3) The chemical composition of the Al-killed steel or the Al-Si-killed steel is represented by mass%,
C: 0.0005 to 1.5%,
Si: 0.005 to 1.2%,
Mn: 0.05-3.0%,
P: 0.001-0.2%,
S: 0.0001-0.05%,
T. 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 to 0.1%,
V: 0 to 0.3%,
Ti: 0 to 0.25%,
B: 0 to 0.005%,
REM: 0.1-20 ppm,
T. O: 5 to 50 ppm,
The method for producing steel according to the above (1) or (2), wherein the balance is Fe and impurities.

(4) The chemical composition of the Al-killed steel or the Al-Si-killed steel is represented 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 the above (3), comprising at least one selected from the group consisting of:

(5) The chemical composition of the Al-killed steel or the Al-Si-killed steel is represented 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 the above (3) or (4), comprising at least one selected from the group consisting of:

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

  (7) The steel according to any one of (1) to (6), wherein the time when the tracer reaches ± 5% of the equilibrium concentration is defined as the uniform mixing time, using S added to the molten steel as a tracer. Manufacturing method.

  (8) The method for producing steel according to any one of (1) to (7), wherein the stirring of the molten steel is performed by any one of a CAS method, an RH vacuum degassing method, and bottom bubbling.

Prevented by the present invention, the component adjustment for alloys containing oxygen due to put into the molten steel after the Al or Al-Si deoxidation, Al ₂ O ₃ and concurrent liquid FeO, and the occurrence of alumina cluster This makes it possible to produce molten steel while sufficiently reducing the occurrence of inclusion defects due to alumina clusters to the extent currently required.

  Further, according to the present invention, REM having a much stronger deoxidizing power than Al is added to the molten steel, and stirring is performed for 2.0 to 10 times the uniform mixing time τ, whereby REM and The contact property of the slag can be enhanced, and the interface of the slag on the molten steel side can be preferentially reformed. For this reason, slag can be reformed at low cost without using a large amount of slag modifier as in the past, improving the cleanliness, toughness and surface quality of steel, and reducing nozzle clogging during continuous casting. Suppression can be achieved.

FIG. 1 is an explanatory diagram showing a situation in which REM is added to molten steel subjected to Al deoxidation or Al-Si deoxidation according to the present invention. FIG. 2 is a graph showing a change in the degree of slag oxidation with stirring time. FIG. 3 is a graph showing the relationship between the deoxidizing element concentration and the slag oxidation degree index. FIG. 4 is a graph showing the vicinity of the origin of the graph of FIG. 3 in an enlarged manner. FIG. 5 is a graph showing the relationship between the amount of oxygen carried before deoxidation and the amount of oxygen carried after deoxidation in the example.

  The present invention will be described. In the following description, “%” relating to chemical composition or concentration means “% by mass” unless otherwise specified.

1. Outline of the present invention In the present invention, basically, after undeoxidized molten steel produced in a converter is tapped into a ladle, the tapped molten steel is deoxidized with Al or Al-Si. Produces Al-killed steel or Al-Si-killed steel.

  At this time, the oxygen-containing component adjusting alloy is introduced into molten steel having a dissolved oxygen amount of 50 ppm or more during or after tapping into a ladle and before deoxidation by Al or Al-Si. Together with the molten steel after deoxidation by Al or Al-Si. It is preferable that the amount of dissolved oxygen in the molten steel is 500 ppm or less.

  Further, after being deoxidized by Al or Al-Si, and then charged with a component adjusting alloy, one or more REMs (rare earth elements) are added. Then, the slag in the ladle is reduced by the REM by stirring the molten steel to which the REM is added for 2.0 to 10 times the uniform mixing time τ.

  Here, REM is a collective term for 17 elements of Y and Sc combined with 15 elements of lanthanoid. 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 are charged into molten steel in the following charging order, for example.
(I) Converter Non-deoxidized molten steel (All alloys for component adjustment and REM are not charged)
(Ii) Converter or RH vacuum degassing equipment (injecting alloy for component adjustment before deoxidation)
(Iii) RH vacuum degassing device (Al or Al-Si deoxidation → Input of alloy for component adjustment after deoxidation → Input of REM alloy

  This makes it possible to produce molten steel while sufficiently reducing the occurrence of inclusion defects due to alumina clusters to the extent currently required.

  Further, since the contact between the REM and the slag can be enhanced and the interface of the slag on the molten steel side can be preferentially reformed, the slag can be reformed at low cost without using a large amount of slag reforming material.

  Therefore, according to the present invention, it is possible to improve the cleanliness and toughness of the steel, as well as the surface quality, and to suppress nozzle clogging during continuous casting.

2. In the present invention, during or after tapping into a ladle, and before and after deoxidation by Al or Al-Si, a component-adjusting alloy containing oxygen is added to molten steel. I do. The injection timing of the oxygen-containing component adjusting alloy into molten steel is determined not only after deoxidation by conventional Al or Al-Si, but also during or after tapping into a ladle. Before and after deoxidation.

  In the present invention, from the alloy for component adjustment introduced before deoxidation by Al or Al-Si, the amount of oxygen (ppm) carried before deoxidation brought into the molten steel having a dissolved oxygen amount of 50 ppm or more; The ratio (the amount of oxygen carried before deoxidation / the amount of oxygen carried after deoxidation) with respect to the amount of oxygen carried after deoxidation (ppm) carried into the molten steel from the alloy for component adjustment introduced after the deoxidation is set to 2 or more. Further, 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 calculated by dividing the amount of oxygen brought in from each alloy for component adjustment (mass ppm) by the input amount of alloy for component adjustment (kg) x the relevant component It can be obtained from oxygen concentration (%) in adjustment alloy / 100 / amount of molten steel (kg) × 10 ⁶ , and can be obtained by summing up the amount of oxygen brought in from all the component adjustment alloys. Further, in the present invention, the amount of oxygen brought in after deoxidation is set to 10 ppm or less, preferably 0.2 ppm or more and 5 ppm or less.

Thereby, it is possible to prevent Al ₂ O ₃ and liquid FeO from being simultaneously generated in the molten steel, and to prevent generation of alumina clusters. Therefore, molten steel can be produced while sufficiently reducing inclusion defects due to alumina clusters to the currently required quality level.

In the present invention, the total of the 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 the deoxidation and the amount of oxygen brought in after the deoxidation is less than 15 ppm, only a small amount of Al ₂ O ₃ and liquid FeO is generated, and the oxygen-containing component adjusting alloy is used. This is because there is no adverse effect caused by charging the molten steel into the molten steel. The total amount of oxygen carried is preferably 170 ppm or less.

  The timing of charging the component adjusting alloy to molten steel is not particularly limited as long as it is during or after tapping into the ladle and before and after deoxidation with Al or Al-Si. However, if it is charged as soon as possible before deoxidation by Al or Al-Si, for example, immediately after tapping into a ladle, liquid FeO once formed before deoxidation by Al or Al-Si is surely introduced into molten steel. It is preferable because it can be dissolved.

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

  As the oxygen concentration of each component adjusting alloy, 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: 0 About 0.4% and about 0.03% of FeNb.

3. REM
FIG. 1 is an explanatory diagram showing a situation in which REM 3 is added to molten steel 2 that has been Al deoxidized or Al—Si deoxidized according to the present invention.

  As shown in FIG. 1, in the present invention, one or more REMs 3 are added to the Al-deoxidized or Al-Si-deoxidized molten steel 2 in the molten steel ladle 1 after the alloy for component adjustment is added, The slag 4 is reformed. As described above, REM in the present invention means a general term for 17 elements in which Y and Sc are combined with 15 elements of lanthanoid.

  In the present embodiment, the molten steel 2 in the molten steel ladle 1 is agitated by the RH vacuum degassing device 5. However, as a method of stirring the molten steel 2, in addition to the RH vacuum degassing device 5 shown in FIG. 1, means such as CAS (component fine adjustment equipment) and BB (bottom bubbling) can be used. By using these, uniform stirring of the added alloy becomes possible.

  When a normal alloy component is added to the molten steel 2, it is usual to perform stirring for a time corresponding to the uniform mixing time τ. The uniform mixing time τ is defined as the time when the concentration of the tracer reaches ± 5% of the equilibrium concentration when the tracer concentration is added to the molten steel, usually with S as the tracer in some cases.

  However, in the present invention, since the purpose is not to uniformly disperse the REM 3 in the molten steel 2, the stirring time after the addition is 2.0 to 10 times the uniform mixing time τ. The reason why S is used as a tracer is that S is usually introduced as an FeS alloy and has a density similar to that of molten steel, so that there is no influence of floating or sedimentation due to a difference in density, and a uniform mixing time can be estimated more accurately. .

Thus, by performing a prolonged agitation after addition of REM3, REM3 is in contact with the slag 4 of the surface of the molten steel _{2, 3MO + 2R → R 2} O 3 + 3M reaction scheme (where M: Fe, Mn, R: According to each REM element), the slag 4 is reformed by reducing FeO and MnO in the slag 4.

  In this way, in order to modify the slag 4 by the REM 3 added to the molten steel 2, the stirring is continued for 2.0 times or more of the uniform mixing time τ.

  FIG. 2 is a graph showing a change in the degree of slag oxidation with stirring time. As shown in the graph of FIG. 2, it can be seen that the degree of slag oxidation decreases most when the stirring time is 2.0 to 10 times the uniform mixing time τ. If the stirring time is too long, not only does the temperature of the molten steel 2 drop significantly, but also the refractory such as the molten steel pot 1 is liable to be damaged, thereby increasing the production cost.

  Here, FIG. 3 is a graph showing the relationship between the deoxidizing element concentration and the slag oxidation degree index, and FIG. 4 is a graph showing the vicinity of the origin of the graph of FIG. 3 in an enlarged manner.

  Al is generally used as a deoxidizing element for molten steel. However, as shown in the graphs of FIGS. 3 and 4, REM (Ce in FIGS. 3 and 4) shows an excellent deoxidizing effect in a very small amount as compared with Al. The vertical axis of the graphs of FIGS. 3 and 4 is the slag oxidation degree index, which indicates the concentration of lower oxides such as FeO and MnO in the slag as an index.

  That is, in order to set the slag oxidation degree index in molten steel to, for example, 6, when Al is used as the deoxidizing element, the Al concentration needs to be 500 ppm as shown in the graph of FIG. When Ce, which is a kind of REM, is used, the Ce concentration may be set to 3 ppm as shown in the graph of FIG. 4, and the amount of addition can be greatly reduced. Therefore, in the present invention, the amount of REM3, which is a deoxidizing element, can be reduced to a very small amount as compared with the conventional case.

  Further, according to the present invention, the slag reforming is performed by contact with the molten steel 2, instead of introducing the slag reforming material from the upper surface of the slag 4 as in the related art. Therefore, it is possible to preferentially modify the interface of the slag 4 on the molten steel side which affects the properties of the steel. From this point, too much slag reforming material is not required.

4. Chemical composition of Al-killed steel or Al-Si-killed steel manufactured according to the present invention The chemical composition of Al-killed steel or Al-Si-killed steel manufactured according to the present invention is 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.P. 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 20 ppm, T.O. O: 5 to 50 ppm, the balance is preferably carbon steel or alloy steel having a chemical composition of Fe and impurities. By applying necessary processing to steel, it can be applied to thin plates, thick plates, steel pipes, shaped steels, steel bars, and the like. The reason why this range is preferable is as follows.

C: 0.0005 to 1.5%
C is a basic element that most stably improves the strength of steel. The C content is preferably 0.0005% or more for securing the strength or hardness of the steel. However, if the C content exceeds 1.5%, the toughness of the steel is impaired. Therefore, the C content is preferably adjusted in the range of 0.0005 to 1.5% according to the desired strength of the material.

Si: 0.005 to 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 failure occurs, and the surface properties and corrosion resistance of the steel deteriorate. For this reason, the Si content is preferably 0.005 to 1.2%.

Mn: 0.05-3.0%
When the Mn content is less than 0.05%, the refining time is prolonged, and the economic efficiency is impaired. On the other hand, if the Mn content exceeds 3.0%, the workability of the steel is greatly 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 hot metal pretreatment increase, and economic efficiency is impaired. On the other hand, if the P content exceeds 0.2%, the workability of the steel is significantly deteriorated. For this reason, the P content is preferably 0.001 to 0.2%.

S: 0.0001-0.05%
If the S content is less than 0.0001%, the time and cost for hot metal pretreatment are increased, and the economy is impaired. On the other hand, if the S content exceeds 0.05%, the workability and corrosion resistance of steel are significantly deteriorated. For this reason, the S content is preferably 0.0001 to 0.05%.

T. Al: 0.005 to 1.5%
Al is the total amount of Al in the molten steel, which is the total amount of solid solution Al (sol. Al) affecting the material and the amount of Al (insol. Al) derived from Al ₂ O ₃ as inclusions. Al = sol. Al + insol. It means Al.

  T. If the Al amount is less than 0.005%, N cannot be trapped as AlN to reduce solid solution N. On the other hand, T. If the Al content exceeds 1.5%, the surface properties and workability of the steel deteriorate. For this reason, T. The amount of Al is preferably 0.005 to 1.5%.

  In the above exemplified steels, in addition to these essential elements, depending on the intended use, (i) one or more elements selected from Cu, Ni, Cr and Mo, and (ii) Nb, V and Ti And (iii) B.

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

  However, if Cu and Mo exceed 1.5% and Ni and Cr exceed 10.0%, respectively, the toughness and workability of the steel are impaired. Therefore, Cu is preferably 1.5% or less, Ni is 10.0% or less, Cr is 10.0% or less, and Mo is 1.5% or less.

  In order to reliably obtain the effect of improving the hardenability of steel, 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% or more. % Or more.

One or more types selected from Nb: 0 to 0.1%, V: 0 to 0.3%, and Ti: 0 to 0.25% All of Nb, V, and Ti are steel strengths due to precipitation strengthening. And may contain one or more kinds as necessary.

  However, if Nb exceeds 0.1%, V exceeds 0.3%, and Ti exceeds 0.25%, the toughness of the steel is impaired. Therefore, Nb: preferably 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 steel, 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 the steel and increases the strength of the steel. For this reason, you may make it contain as needed. However, when the B content exceeds 0.005%, the precipitates of B increase, and the toughness may be impaired. Therefore, the B content is preferably 0.005% or less. In order to reliably obtain the effect of improving the hardenability of steel, the B content is more preferably 0.0005% or more.

REM: 0.1-20 ppm
If the REM content is less than 0.1 ppm, the effect of preventing the clustering of alumina particles cannot be obtained, while if the REM content is more than 20 ppm, the composite of REM oxide and Al ₂ O ₃ There is a risk that coarse clusters composed of oxides may be generated, and a large amount of complex oxides may be generated by reaction with slag, which may deteriorate the cleanliness of molten steel and block the tundish immersion nozzle. That's why. For this reason, the REM content is preferably set to 0.1 to 20 ppm. More preferably, the REM content is 15 ppm or less.

T. O: 5 to 50 ppm
In the present invention, the O content, which is the total amount of the solid solution O (sol. O) that affects the material with respect to the O content and the O (insol. O (Total.O). T. of Al killed steel or Al-Si killed steel When O is less than 5 ppm, the processing time in secondary refining, for example, in RH, is significantly increased, so that cost is increased and economy is impaired. On the other hand, T. If O is more than 50 ppm, the frequency of collision of the alumina particles increases, and the clusters may become coarse. Further, since the amount of REM required for reforming alumina increases, the cost increases and the economy is impaired. For this reason, T. O is preferably 5 to 50 ppm.

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

  After undeoxidized molten steel melted in a 270 ton converter is tapped into a ladle, the tapped molten steel is deoxidized with Al or Al-Si in an RH vacuum degassing apparatus to remove Al. Killed steel or Al-Si killed steel was produced.

  At this time, the molten steel having a dissolved oxygen amount during or after tapping into the ladle and before the deoxidation by Al or Al-Si, and the molten steel after deoxidation, to adjust the composition containing oxygen. Alloys were put in. Table 1 shows the alloy concentration and the oxygen concentration of the component adjusting alloys charged.

  Tables 2 to 4 show alloy input conditions (dissolved oxygen amount, input timing (elapsed time from start of tapping when not deoxidized)), input alloy type (before and after deoxidation), and carried-in oxygen amount (deoxidized amount). Shows 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 deoxidation / the amount of oxygen brought in after deoxidation), and the sum of the amount of oxygen brought in before deoxidation and the amount of oxygen brought in after deoxidation. . FIG. 5 is a graph showing the relationship between the amount of oxygen brought in before deoxidation and the amount of oxygen brought in after deoxidation.

  The “alloy loading conditions” in Tables 2 to 4 show the loading conditions of alloys with a large amount of oxygen brought in before and after deoxidation.

The amount of oxygen brought in (the amount of oxygen brought in before deoxidation and the amount of oxygen brought in after deoxidation) is calculated by dividing the amount of oxygen brought in from each alloy for component adjustment (mass ppm) by the input amount of alloy for component adjustment (kg) x the relevant component The oxygen content in the alloy for adjustment (%) / 100 / the amount of molten steel (kg) × 10 ⁶ was obtained, and the total amount of oxygen brought in from all the alloys for component adjustment was obtained.

  Further, after the alloy for component adjustment was introduced into the molten steel of 1600 ° C. ultra-low carbon steel deoxidized at Al or Al-Si in the molten steel pot 1 shown in FIG. Or Al (REM alloy is 40% Fe-30% Si-15% Ce-9% La-4.5% Nd-1.5% Pr, Al is shot Al and 92% Al-2% Si-1.5 % Fe-2% Zn) and stirred with a RH vacuum degasser. Tables 5 to 7 also show "REM / stirring time after Al introduction / uniform mixing time [tau]". The above “REM / Al” means “REM alloy or Al”.

  The molten steel of the smelted Al or Al-Si killed steel was continuously cast by a vertical bending type continuous casting machine to produce a continuously cast slab. Thereafter, the continuous cast slab is subjected to (a) hot rolling and pickling to produce a thick plate having a chemical composition shown in Tables 5 to 7, and (b) hot rolling, pickling and cold rolling. To produce a thin plate having the chemical composition shown in Tables 5 to 7, or (c) using a thick plate produced by performing hot rolling and pickling as a material and having a chemical composition shown in Tables 5 to 7 Welded steel pipe was manufactured. The thickness after hot rolling was 2 to 100 mm, and the thickness after cold rolling was 0.2 to 1.8 mm.

  REM in the column of “Components of steel” in Tables 5 to 7 indicates the total content of each REM element. “Concentration / equilibrium concentration × 100%” in Tables 5 to 7 indicates the absolute value of the concentration / average concentration × 100 (%). Further, "T-Fe + MnO" in Tables 5 to 7 indicates the total concentration of FeO and MnO in the molten steel interface slag.

  Tables 5 to 7 show the maximum cluster diameter, the number of clusters, the defect occurrence rate, the state of nozzle clogging, and the like of the sample collected from the slab.

  The “maximum cluster diameter” in Tables 5 to 7 means that the inclusions extracted from a slab having a mass of 1 kg (using a minimum mesh of 20 μm) were photographed with a stereoscopic microscope (× 40) and the major diameter of the photographed inclusions And the average value of the minor axis was determined for all the inclusions, and the maximum value of the average value was determined as the maximum inclusion diameter.

  The “number of clusters” in Tables 5 to 7 is inclusions extracted with a mass of 1 kg of slime electrode (using a minimum mesh of 20 μm), and the number of all inclusions having a size of 20 μm or more observed by an optical microscope (× 100) is represented by 1 kg. It was measured by converting to the number.

  The “defect generation rate” in Tables 5 to 7 is the sliver flaw generation rate on the plate surface (= sliver flaw total length / coil length × 100,%) in the case of a thin plate, and the product in the case of a thick plate. This is the UST defect occurrence rate or separation occurrence rate (= number of defective sheets / total number of inspected sheets × 100,%) in steel plates. In the case of steel pipes, the UST defect occurrence rate (= defect occurrence Number of tubes / total number of tubes x 100,%).

  In the case of a thick plate, the presence or absence of separation was confirmed by fracture observation after the Charpy test. In addition, in the defect occurrence rate of the thick plate material in Tables 5 to 7, when the defect is a UST defect, it is described as UST, and when it 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. “Impact absorption energy” in Tables 5 to 7 is a V-notch Charpy impact test value in the rolling direction at −20 ° C., and is an average value of five test pieces.

The “thickness value in the thickness direction” in Tables 5 to 7 is the thickness value in the thickness direction of the product sheet at room temperature (= cross-sectional area of fractured portion after tensile test / cross-sectional area of test specimen before test × 100,%).
Further, the nozzle clogging state was determined by measuring the thickness of the inclusions on the inner wall of the immersion nozzle after casting, and the nozzle clogging state was determined from the average value at 10 points in the circumferential direction. Thickness 1 to 3 mm, ×: adhering thickness more than 3 mm, were classified into levels.

  No. 5 in Table 5. A1 to A31 are examples of the present invention satisfying all the scope of the present invention. B1 to B16 are comparative examples in which the ratio (the amount of oxygen carried in before deoxidation / the amount of oxygen carried in after deoxidation) does not satisfy the range of the present invention. C1, C3 to C5, C7, C9, C11, and C13 to C14 are comparative examples in which the stirring time / homogeneous mixing time τ after charging REM / Al does not satisfy the range of the present invention, and otherwise, after charging REM / Al. Is a comparative example in which the stirring time / uniform mixing time τ was satisfied but no REM was added.

As shown in Tables 5 to 7, according to the examples of the present invention, Al ₂ O ₃ caused by introducing an oxygen-containing component adjusting alloy into Al or Al—Si deoxidized steel after Al deoxidation. And the simultaneous generation of liquid FeO and the generation of alumina clusters, thereby making it possible to produce molten steel while sufficiently reducing the occurrence of inclusion defects caused by alumina clusters to the extent required today. In addition, surface flaws and internal defects due to coarse alumina clusters in the steel product as a final product can be reduced.

1 Molten steel pot 2 Molten steel 3 REM
4 Slag 5 RH vacuum degassing device

Claims (8)

After the undeoxidized molten steel melted in the converter is tapped into a ladle, the tapped molten steel is deoxidized with Al or Al-Si to produce Al-killed steel or Al-Si-killed steel. The method,
The oxygen-containing component adjusting alloy is introduced into molten steel having a dissolved oxygen amount of 50 ppm or more during or after tapping into the ladle and before deoxidation by the Al or the Al-Si. Together with the Al or the Al-Si into the molten steel after deoxidation,
The amount of oxygen (ppm) carried before deoxidation brought into the molten steel from the component adjusting alloy introduced before the deoxidation by Al or the Al-Si, and the oxygen introduced after deoxidation by the Al or Al-Si. The ratio of the amount of oxygen brought in after deoxidation (ppm) brought into the molten steel from the component adjusting alloy to the molten steel (the amount of oxygen brought in before deoxidation / the amount of oxygen brought in after deoxidation) is 2 or more,
The amount of oxygen brought in after the deoxidation is 10 ppm or less,
The total amount of oxygen brought in before and after deoxidation should be 15 ppm or more,
One or more REMs are added to the molten steel that has been deoxidized by the Al or Al-Si and then the alloy for component adjustment is added, and the molten steel to which the REMs are added is mixed for a uniform mixing time of 2.0%. A method for producing steel in which stirring is performed for 10 to 10 times.   The steel according to claim 1, wherein the component adjusting alloy is at least one 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, in mass%,
C: 0.0005 to 1.5%,
Si: 0.005 to 1.2%,
Mn: 0.05-3.0%,
P: 0.001-0.2%,
S: 0.0001-0.05%,
T. 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 to 0.1%,
V: 0 to 0.3%,
Ti: 0 to 0.25%,
B: 0 to 0.005%,
REM: 0.1-20 ppm,
T. O: 5 to 50 ppm,
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, in 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, comprising one or more kinds selected from the group consisting of: The chemical composition of the Al-killed steel or the Al-Si-killed steel is, in 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, comprising at least one selected from the group consisting of: The chemical composition of the Al-killed steel or the Al-Si-killed steel is, in mass%,
B: 0.0005 to 0.005%,
The method for producing steel according to any one of claims 3 to 5, comprising:   The method for producing steel according to any one of claims 1 to 6, wherein S added to the molten steel is used as a tracer, and a time when the tracer reaches ± 5% of the equilibrium concentration is defined as the uniform mixing time.   The method for producing steel according to any one of claims 1 to 7, wherein the stirring of the molten steel is performed by any one of a CAS method, an RH vacuum degassing method, and bottom bubbling.

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