JP7087725B2 - Steel manufacturing method - Google Patents

Description

The present invention relates to a method for producing steel.

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.

For example, inclusion defects caused by alumina clusters also become a problem in the melting of ultra-low carbon steel (hereinafter referred to as “340BH / HiMn-SULC”) for coating and baking hardened steel sheets having a tensile strength of 340 MPa class.

340BH / HiMn-SULC has a high Mn chemical composition with a Mn content of about 0.60% by mass. Therefore, in the steelmaking process, it is necessary to add a metal Mn (hereinafter referred to as “MeMn”), which is a source of Mn, into the molten steel to increase the Mn concentration of the molten steel.

On the other hand, since Mn added to the molten steel is oxidized to generate MnO and easily floats and separates, the input yield of Mn tends to decrease. For this reason, MeMn is used to dispense undeoxidized molten steel melted in a converter into a ladle, deoxidize it with Al or Al—Si using an RH vacuum degassing device, and then turn it into molten steel. It was thrown in.

Even in the melting of 340BH / HiMn-SULC, inclusion defects such as nozzle clogging due to adhesion in the nozzle hole and sliver flaws in cold rolling occur frequently in the dipping nozzle of the tundish during continuous casting. The productivity and quality of the steel sheet had declined.

According to Patent Document 1, the present inventors add one or more types of REM (rare earth metal) selected from Ce, La, Pr and Nd to Al or Al-Si deoxidized molten steel to obtain mass. %, C: 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 It has a steel composition of .0001 to 0.05%, Al: 0.005 to 1.5%, and the balance is Fe, and the total REM is 0.1 ppm or more and less than 10 ppm, and the solid solution REM is less than 1 ppm. A steel material with few alumina clusters was disclosed.

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, and sliver defects and structural thicknesses of thin plates for automobiles and home appliances. It is possible to significantly suppress inclusion defects such as defective plate material, deterioration of low temperature toughness of wear-resistant thick plate, and UST defect of welded portion of steel pipe for oil country tubular goods.

Normally, REM is added to the molten steel inside such as a molten steel pan and stirred to make it uniform. At this time, it is common sense in the industry to secure a uniform mixing time τ and stir. This uniform mixing time τ is defined as the time when S, in some cases Cu or the like, is added to the molten steel as a tracer and stirred, and the concentration of the tracer in the molten steel reaches ± 5% of the equilibrium concentration.

The uniform mixing time τ varies depending on the type and size of the molten steel pot, molten steel temperature, stirring energy, etc., but once these are determined, it is a time that can be objectively measured, and even after the addition of REM, this uniform mixing time τ or more. The molten steel was agitated for the same period of time. Therefore, a proposal for shortening the uniform mixing time τ itself has been conventionally made.

For example, in Patent Document 2, when an alloy component is added from the surface of molten steel that is agitated by bottom bubbling, the uniform mixing time τ is shortened by adding the alloy component from a plurality of positions sandwiching the gas blowing position. It has been disclosed.

Further, Patent Document 3 discloses a method of shortening the uniform mixing time τ by setting the nozzle position for blowing the stirring gas into the ladle and the diameter of the immersion tube so as to satisfy a predetermined formula. There is.

Japanese Unexamined Patent Publication No. 2005-2425 Japanese Unexamined Patent Publication No. 7-145421 Japanese Unexamined Patent Publication No. 2000-204421

According to the invention disclosed in Patent Document 1, 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.

Further, when the molten steel to which REM is added is stirred for a uniform mixing time of τ or more, the yield of REM in the molten steel is only about 30%. This is because the added REM is oxidized and consumed by FeO, MgO, etc. in the slag during stirring, and an excessive amount of REM is required to exert the original function of preventing alumina clusters. ..

Further, since the uniform mixing time τ of the large molten steel pan exceeds 60 seconds, a considerably long processing time is required to secure this uniform mixing time τ after the addition of REM. For this reason, various problems such as a decrease in the temperature of the molten steel, a progress of melting of the refractory, and an increase in the cost of the stirring utility have occurred during that period.

Therefore, regardless of the techniques disclosed in Patent Documents 2 and 3, conventionally, the molten steel has been stirred for a longer time than this uniform mixing time τ.

The present invention has been made in view of this problem of the prior art, in which inclusion defects due to alumina clusters are sufficiently reduced to the extent currently required, and the yield of added REM is improved. Another object of the present invention is to provide a method for producing steel while shortening the processing time and making the REM concentration in the molten steel uniform as in the conventional case.

As disclosed by the present inventors in Patent Document 1, 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 (G) listed below.

(A) MeMn (meaning "metal Mn") added after Al or Al-Si deoxidation for adjusting the Mn concentration in the steelmaking process of 340BH / HiMn-SULC. In the present specification, the following component concentration is used. The "metal" in the adjusting alloy of the above 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 is simultaneously charged with Al ₂ O ₃ . The generated FeO becomes a binder between Al ₂ O ₃ and an alumina cluster is generated.

(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, such as 340BH / HiMn-SULC, MeMn is used as a molten steel after Al or Al-Si deoxidation as in the conventional case. Instead of charging, FeO in a liquid state can be obtained by charging it into molten steel in which the amount of dissolved oxygen before Al or Al—Si deoxidation is 50 ppm or more and into the molten steel after Al or Al—Si deoxidation. Since the formation of alumina clusters can be suppressed by preventing the formation of Al ₂ O ₃ at the same time, inclusion defects due to the alumina clusters can be reduced.

(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. The productivity and quality of the final product, steel, can be significantly improved, and the manufacturing cost of steel can be significantly reduced.

(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 (B) above, it is possible to prevent the generation of alumina clusters.

(G) The specific densities of REM or REM alloy (Fe-Si-REM) added to the molten steel are 6.7 and 6.0, respectively, which are much heavier than 2.7 of Al and 2.3 of S. Since it is close to the specific gravity of molten steel, its behavior during stirring is different from that of S, which has a small specific gravity.

The present invention is based on these findings (A) to (G), 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.
One or more REMs are added to the molten steel deoxidized by the Al or Al—Si, and then the alloy for adjusting the composition is added, and the molten steel to which the REM is added is mixed with the molten steel having a uniform mixing time of 0. A method for producing steel, which is agitated for 10 to 0.9 times as long.

(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, in 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: 0.1-20ppm,
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, 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 (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 steel according to any one of (1) to (6) above, wherein S added to the molten steel is used as a tracer, and the time when the tracer reaches ± 5% of the equilibrium concentration is defined as the uniform mixing time. Manufacturing method.

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

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.

Further, according to the present invention, the yield of the added REM can be improved, the processing time can be shortened, and the steel can be manufactured while making the REM concentration in the molten steel uniform as in the conventional case.

According to the present invention, the molten steel may be stirred in a short time of 0.10 to 0.9 times the conventionally used uniform mixing time τ. Therefore, the possibility that the added REM reacts with the slag and is oxidized is reduced, and the yield of the REM can be increased to about 40 to 60%. Therefore, the amount of REM to be added in order to exert the same effect can be reduced.

Further, according to the present invention, it is possible to shorten the processing time as a matter of course, and problems of the prior art such as a decrease in the temperature of the molten steel, progress of melting of the refractory, and an increase in the cost of the stirring utility are problems. Can be resolved. Moreover, despite the shortening of the stirring time in this way, the REM concentration in the molten steel has reached the equilibrium concentration as in the conventional case, and sufficient homogenization is possible.

FIG. 1 is a cross-sectional view showing an embodiment of the present invention. FIG. 2 is a graph showing the relationship between the stirring time and the concentration in the case of continuous stirring. FIG. 3 is a graph showing the relationship between the stirring time and the concentration when stirring is stopped in the middle. FIG. 4 is a graph showing changes in REM yield with stirring time. FIG. 5 is a graph showing the change in time until the REM concentration becomes uniform depending on the stirring time. FIG. 6 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, "%" regarding the chemical composition or concentration means "% by mass" unless otherwise specified.

1. 1. Outline of the present invention In the present invention, basically, undeoxidized molten steel melted in a converter is dispensed into a ladle, and then the molten steel that has been melted is deoxidized with Al or Al—Si. Manufactures Al-killed steel or Al-Si killed steel.

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, the alloy for component adjustment is added to the molten steel after deoxidation with Al or Al—Si, and then REM is added, and then the mixture is stirred for a time of 0.10 to 0.9 times the uniform mixing time τ.

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 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 input for component adjustment after deoxidation → REM input)

In the present invention, the simultaneous generation of Al ₂ O ₃ and liquid FeO and the generation of alumina clusters can be prevented in this way, whereby the generation of inclusion defects caused by the alumina clusters is currently required. It can be sufficiently reduced to a certain extent.

Further, according to the present invention, the yield of the added REM can be improved, the processing time can be shortened, and the steel can be manufactured while making the REM concentration in the molten steel uniform as in the conventional case.

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, it is possible to prevent Al ₂ O ₃ and liquid FeO from being simultaneously generated in the molten steel, and it is possible to prevent the generation of alumina clusters. 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 the harmful effect of putting the oxygen into the molten steel does not occur. 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 the liquid FeO once generated before the deoxidation by Al or Al—Si is surely put into the molten steel at the timing as soon as possible before the deoxidation by Al or Al—Si, for example, immediately after the steel is discharged into the ladle. 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 addition and subsequent stirring time FIG. 1 is a diagram showing a state in which the molten steel 2 in the molten steel pan 1 is stirred by the RH (vacuum degassing equipment) 3. The molten steel 2 is Al deoxidized or Al—Si deoxidized in the previous step. One or more REM4s are added from the surface of the molten steel 2 inside the RH3. As described above, REM in the present invention means a general term for 17 elements including 15 elements of lanthanoids, Y and Sc.

In the present invention, the stirring time after adding REM4 is set to a short time of 0.10 to 0.9 times the uniform mixing time τ. As described above, this uniform mixing time τ is defined as the time when the concentration of the tracer reaches ± 5% of the equilibrium concentration, with S added to the molten steel as the tracer.

In the present invention, the stirring time is set to 0.10 to 0.9 times the uniform mixing time τ. If the stirring time is shorter than this, the stirring becomes insufficient and the REM concentration in the molten steel becomes non-uniform. This is because if it is longer than that, the reaction with the slag 5 proceeds and the same problem as before occurs. After the above stirring is completed, the molten steel pan 1 is allowed to stand. The stirring time is preferably 0.20 times or more and 0.8 times or less the uniform mixing time τ.

FIG. 2 is a graph in which the horizontal axis represents the stirring time / uniform mixing time τ, and the vertical axis represents the concentration / equilibrium concentration value. In this case, stirring was continued until the uniform mixing time τ reached 1.5 times. As shown in the graph of FIG. 2, S, which is usually used as a tracer, reaches an equilibrium concentration when the stirring time reaches the uniform mixing time τ. However, REM approaches equilibrium concentration much earlier than S.

FIG. 3 is a graph when stirring is performed for a time of 0.10 times the uniform mixing time τ after addition and then allowed to stand, and both the horizontal axis and the vertical axis are the same as the graph of FIG. From this graph, it can be seen that S does not reach the equilibrium concentration when the stirring time is short, but REM rapidly reaches the equilibrium concentration.

In this way, REM is more likely to be uniformly dispersed in molten steel than S, which is usually used as a tracer, because the specific densities of REM or REM alloy (Fe-Si-REM) are 6.7 and 6.0, respectively. It is considered that it is much heavier than 2.3 of S and is close to the specific density of molten steel.

That is, it is presumed that the REM penetrates deeply into the molten steel due to the inertial force applied upward, and then gently spreads over the entire pan by the inertial convection even after the stirring treatment of the molten steel in the pan. Therefore, the REM can be sufficiently homogenized in the molten steel pan after that without stirring for the uniform mixing time τ or more in the stirring step.

FIG. 4 is a graph in which the stirring time / uniform mixing time τ is taken on the horizontal axis and the REM yield is taken on the vertical axis. As shown in the graph of FIG. 4, the REM yield gradually decreases as the stirring time becomes longer, and the REM yield decreases to about 30% when the stirring is performed for a uniform mixing time of τ or more as in the conventional case.

However, if the value of the stirring time / uniform mixing time τ is set to 0.10 to 0.9 as in the present invention, the REM yield can be increased to about 40 to 60%. Therefore, according to the present invention, the amount of REM added can be reduced as compared with the conventional case.

FIG. 5 is a graph in which the horizontal axis represents the stirring time / uniform mixing time τ, and the vertical axis represents the time required for the REM concentration to become uniform / uniform mixing time τ. As shown in this graph, when the stirring time is equal to the uniform mixing time τ, the time required for the REM concentration in the molten steel to become uniform is minimized.

On the other hand, when the value of stirring time / uniform mixing time τ is set to 0.10 to 0.9 as in the present invention, the time required for the REM concentration in the molten steel to become uniform is 3 to 9. Double. This indicates that although the stirring itself may be short, it is necessary to take a long standing time thereafter. However, since the molten steel can be allowed to stand for a sufficient time in the post-stirring process, this point does not pose a practical problem.

As shown above, the stirring time is preferably short from the viewpoints of REM yield, prevention of temperature decrease of molten steel, prevention of melting of refractories, cost reduction of stirring utility, etc., and the REM concentration in molten steel is increased. Longer ones are preferable for homogenization. Taking these into consideration comprehensively, in the present invention, it was decided to stir for a time of 0.10 to 0.9 times the uniform mixing time τ.

As the stirring method for molten steel, in addition to the RH (vacuum degassing device) shown in this embodiment, a method such as a CAS method (component fine adjustment equipment) or bottom bubbling (gas blowing from the bottom of the furnace) shall be adopted. Can be done.

The REM may be a pure metal such as Ce or La, an alloy of the REM metal, or an alloy with another metal. The shape of the REM may be lumpy, granular, wire or the like. Since the amount of REM added is extremely small, in order to make the REM concentration in the molten steel uniform, as described above, the addition to the reflux molten steel in the RH vacuum degassing tank or the Ar gas after the addition of the ladle is used. Stirring is desirable. 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 The chemical composition of 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 20 ppm, T.I. O: It is preferably carbon steel or alloy steel having a chemical composition of 5 to 50 ppm and the balance being Fe and impurities. By adding the necessary processing to steel, it can be applied to thin plates, thick plates, steel pipes, shaped steel, steel bars, etc. 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. The C content is preferably 0.0005% or more in order to secure the strength or hardness of the steel. However, if the C content exceeds 1.5%, the toughness is impaired. Therefore, the C content is preferably adjusted in the range of 0.0005 to 1.5% depending on the strength of the desired 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 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 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 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 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 processability deteriorate. Therefore, T.I. The Al content is preferably 0.005 to 1.5%.

In the above exemplified steel, the above are essential elements, but in the present invention, in addition to these, one kind selected from (i) Cu, Ni, Cr and Mo as an optional element according to each application. As described above, (ii) one or more selected from Nb, V and Ti, and (iii) B may be contained.

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 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 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. It is 05% or more.

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 is impaired. Therefore, Nb: 0.1% or less, V: 0.3% or less, Ti: 0.25% or less are preferable. In order to surely obtain the effect of improving the strength of the 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 steel and enhances its strength. However, if it is contained in excess of 0.005%, the precipitate of B may be increased and the toughness of the steel 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 steel, the B content is preferably 0.0005% or more.

REM: 0.1-20ppm
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 20 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 20 ppm. The REM content is preferably 15 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 clusters 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.

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 ladle, and then the discharged molten steel is deoxidized with Al or Al—Si in an RH vacuum degassing device. Killed steel or Al-Si killed steel was melted.

At this time, the components containing oxygen in the molten steel having the amount of dissolved oxygen before deoxidation by Al or Al—Si and the molten steel after deoxidation during or after the steel is discharged to the ladle are adjusted. The alloy for use was put in. 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. 6 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 indicate the charging conditions for which the amount of oxygen brought in is larger in comparison between 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) in Tables 2 to 4 is the amount of oxygen brought in from each component adjusting alloy (mass ppm) and the amount of input of the component adjusting alloy (kg). × Oxygen concentration in the component adjusting alloy (%) / 100 / molten steel amount (kg) × ¹⁰⁶ was calculated, and the total amount of oxygen brought in from all the component adjusting alloys was calculated.

Further, after deoxidizing Al or Al—Si and adding the alloy for adjusting the composition, REM was added. "REM" indicates the total content of each REM element such as Ce, La, Pr and Nd. The REM is Ce, La, mischmetal (45% Ce-35% La-6% Pr-9% Nd), or an alloy of mischmetal, Si, and Fe (Fe-30% Si-30% REM alloy). Was added as. The results are also shown in Tables 5 to 7 described later. The stirring time / uniform mixing time τ after charging the REM was as shown in Tables 5 to 7.

The molten Al killed steel or the molten Al—Si killed steel was continuously cast by a vertical bending type continuous casting machine 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 nozzle blockage status, etc. of the sample collected from the slab.

The "maximum cluster diameter" in Tables 5 to 7 is the long diameter of the inclusions photographed by taking a photograph (40 times) of the inclusions extracted from a slab with a mass of 1 kg (using a minimum mesh of 20 μm) with a stereomicroscope. The average value of the minor axis 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 obtained by electrolyzing slime (using a minimum mesh of 20 μm) from a slab having a mass of 1 kg, and all inclusions having a mass of 20 μm or more observed with an optical microscope (100 times). The number was measured by converting the number into 1 kg units.

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. UST defect generation rate or separation generation rate (= number of defect generation plates / total number of inspection plates x 100,%) in the plate, and in the case of steel pipe, UST defect generation rate (= defect generation) in the welded part of the oil pipe Number of pipes / total number of inspection pipes x 100,%).

In the case of a thick plate, the presence or absence of separation was confirmed by observing the fracture surface after the Charpy test. 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 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 5 to 7 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,%). “T.Al” and “TO” in Tables 2 to 4 indicate “total Al” and “total O”, respectively.

Further, the "nozzle blockage status" in Tables 5 to 7 was evaluated by measuring the adhesion thickness of inclusions on the inner wall of the immersion nozzle after casting. From the average value of 10 points in the circumferential direction, the nozzle blockage status was classified into ◯: adhesion thickness less than 1 mm, Δ: adhesion thickness 1 to 3 mm, and ×: adhesion thickness 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 C14 are comparative examples that do not satisfy the stirring time: 0.1 to 0.9 times the uniform mixing time τ.

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 the simultaneous generation of liquid FeO and the generation of alumina clusters can be prevented, 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.

1 Molten steel pan 2 Molten steel 3 RH (vacuum degassing device)
4 REM
5 slag

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.
One or more REMs are added to the molten steel deoxidized by the Al or Al—Si, and then the alloy for adjusting the composition is added, and the molten steel to which the REM is added is mixed with the molten steel having a uniform mixing time of 0. A method for producing steel, which is agitated for 10 to 0.9 times as long. 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: 0.1-20ppm,
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 S added to the molten steel is used as a tracer, and the 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 molten steel is agitated by any one of a CAS method, an RH vacuum degassing method, and a bottom bubbling method.

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