JP2001252747A - Method for treating molten steel excellent in quality characteristic - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a treating method for molten steel excellent in a quality characteristic by which the additional quantity of Ti is reduced and the clogging of a nozzle and the unstable operation are prevented and the solidified structure can be fined by generating effective solidified nuclei. SOLUTION: Mg is added into a molten steel 11 having a Ti concentration and a N concentration satisfying the product of solubility for crystallizing TiN at not lower than the liquidus temperature of the molten steel. It is particuliarly effective in the case the molten steel is a ferritic stainless steel containing 10-23 wt.% Cr.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating molten steel having excellent quality characteristics and capable of refining a solidified structure when the molten steel is solidified.

[0002]

2. Description of the Related Art Conventionally, molten steel is subjected to decarburization scouring using a scouring furnace such as a top-bottom blow converter or an electric furnace to remove carbon, phosphorus, sulfur, etc., and then to remove Fe-Mn, Fe-Si, etc. It is manufactured by adding alloy iron and adjusting the components. Then, the molten steel is solidified by continuous casting or the like to produce a slab, and further,
A steel material is manufactured by heating and casting a slab. However, when the molten steel is solidified, the solidified structure of the slab becomes coarse, and internal defects such as cracks, center segregation, and center porosity occur inside the slab, resulting in a decrease in the yield of a good slab. Furthermore, the quality of the steel material processed using this is also reduced. In addition, surface defects such as barge flaws, edge seam flaws, and roping occur on steel materials that have been subjected to processing such as rolling using a slab having a coarse solidification structure, resulting in poor appearance, an increase in the amount of trim at the ends, and the like. Therefore, there is a problem that the quality and the yield of good products are reduced. As a countermeasure against this, Japanese Patent Laid-Open
As described in Japanese Patent Application Laid-Open No. 2-47522, an electromagnetic stirrer is provided at a position of 1.5 to 3.0 m from the surface of the molten metal in the mold during continuous casting, and is stirred with a thrust of 60 mmHg to obtain fine particles. Casting is performed by producing a slab having a solidified structure, or as described in JP-A-52-60231, by setting the degree of superheat of molten steel to 10 to 50 ° C. By performing electromagnetic stirring on the unsolidified layer of the slab, the solidified structure of the slab is made into a fine structure composed of equiaxed crystals, and steel materials without internal defects such as center segregation and center porosity are manufactured. I have. Further, JP-A-63
As described in -1460061, when a molten steel is cast into a mold, a slab having a fine solidified structure is produced by adding fine particles of TiN.

[0003]

However, in Japanese Patent Application Laid-Open No. 52-47522, the molten steel being solidified in a mold is electromagnetically stirred to suppress the growing columnar crystals (dendritic structure), Although the solidification structure in the vicinity of the part where stirring was given can be made fine to some extent, in order to make the entire slab fine, a multi-stage electromagnetic stirring device is required,
Equipment costs increase. Moreover, it is difficult to install a large number of electromagnetic stirring devices in the continuous casting machine in terms of installation space. As a result, there is a limit in manufacturing a slab in which the entire slab is miniaturized. Further, in Japanese Patent Application Laid-Open No. 52-60231, since low-temperature casting is performed by lowering the temperature of molten steel, inclusions may adhere to the inner surface of the immersion nozzle to cause nozzle clogging, or the surface of molten steel in the mold may be covered with molten metal. Operation becomes unstable. Also, JP-A-63-14600
In Japanese Patent No. 61, it is necessary to add 0.15% by weight of Ti in order to make the solidification structure of molten steel fine, so that the alloy cost becomes high, and nozzle clogging and scorching caused by TiN occur. There is a problem.

The present invention has been made in view of the above circumstances, and reduces the amount of Ti added to prevent clogging of a nozzle and instability of operation, to form an effective solidification nucleus and to make a solidified structure fine. It is an object of the present invention to provide a method for treating molten steel having excellent quality characteristics.

[0005]

According to the present invention, there is provided a method for treating molten steel having excellent quality characteristics according to the present invention, comprising: a Ti concentration and an N which satisfy a solubility product at which TiN is crystallized at a temperature not lower than a liquidus temperature of molten steel;
Mg is added to the molten steel of the concentration. By this method, Ti
At a high temperature at which N does not crystallize, MgO or M
gO.Al ₂ O ₃ Mg oxide is produced, and as the temperature of the molten steel is lowered, TiN is crystallized on the Mg oxide and dispersed in the molten steel to produce a large number of solidification nuclei. Thus, the solidified structure of the slab can be made fine. In addition, the addition of Mg is performed by adding M such as metallic Mg, Fe-SI-Mg,
This is performed by charging the g alloy.

Here, the Ti concentration and the N concentration are expressed by (1)
It is preferable to satisfy the formula. [% Ti] × [% N] ≧ ([% Cr] ^2.5 +150) × 10 ⁻⁶ (1) where [% Ti] is the Ti weight% contained in the molten steel, [%
[N] is the N weight% contained in the molten steel, and [% Cr] is the Cr weight% contained in the molten steel. Thereby, Mg is added within a predetermined range of the concentration of Ti and N contained in the molten steel, so that the generated TiN accompanies the highly dispersible Mg oxide and is stably dispersed in the molten steel. When the molten steel is solidified, it can act as a solidification nucleus to further refine the solidified structure of the slab.

Further, the molten steel is preferably a ferritic stainless steel molten steel containing 10 to 23% by weight of Cr. For ferritic stainless steel containing Cr whose solidification structure tends to coarsen, the effect of refining the solidification structure is exhibited,
Surface defects and internal defects occurring in the slab and steel material can be prevented. If the content of Cr is less than 10% by weight,
Corrosion resistance of the steel material manufactured from the molten steel is reduced, and the effect of refinement is reduced because the solidification structure is small in coarseness. On the other hand, C
When the content of r exceeds 23% by weight, the cost of ferroalloys increases because the corrosion resistance of steel materials manufactured from molten steel does not improve even if Cr alloy iron is added.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention will be described with reference to the accompanying drawings to provide an understanding of the present invention. TiN has a lattice matching degree with δ ferrite (which is a value obtained by dividing the difference between the lattice constant of TiN and the lattice constant of δ ferrite by the lattice constant of δ ferrite).
%, But this TiN is liable to agglomerate, and has a problem that coarse TiN causes clogging of the immersion nozzle or causes defects such as sliver flaws of steel. The feature of the present invention is that, based on the fact that TiN effectively acts as a solidification nucleus when molten steel is solidified, Mg oxide added by adding Mg to molten steel has a very good dispersibility. And the fact that TiN is preferentially crystallized on Mg oxide.
An object of the present invention is to enhance the dispersibility of N to disperse a large number of coagulation nuclei effective for making the structure fine. Hereinafter, the method for treating molten steel having excellent quality characteristics of the present invention will be described in detail. FIG. 1 is an overall view of an apparatus for treating molten steel applied to a method for treating molten steel having excellent quality characteristics according to an embodiment of the present invention.
(A) and (B) are each a schematic diagram of a solidified structure of a slab.

As shown in FIG. 1, a molten steel processing apparatus 10 used in a method for processing molten steel having excellent quality characteristics according to an embodiment of the present invention includes a ladle 12 containing molten steel 11, Sponge Ti, Fe-T provided above pan 12
hopper 13 for storing Ti (titanium) alloy such as i and F
a hopper 14 for storing N (nitrogen) alloys such as e-N, N-Mn, and N-Cr; and a hopper 13 for adding Ti alloy or N-containing alloy cut out from the hopper 14 to the molten steel 11. While being guided by a chute 15 and a guide pipe 16, a metal Mg wire 17 whose surface is covered with a thin steel plate
Supply device 1 which penetrates slag 18 and adds it to molten steel 11
9 is provided. Reference numeral 20 is a porous plug for supplying argon gas, which is an example of an inert gas, to the molten steel 11 in the ladle 12.

Next, a method for treating molten steel having excellent quality characteristics according to the present embodiment will be described. Using a refining furnace such as a top-bottom blow converter or an electric furnace, 150 tons of molten steel 11 having been subjected to decarburization and removal of impurities such as phosphorus and sulfur was placed in a ladle 12.
This molten steel 11 is a ferritic stainless steel molten steel,
Cr is contained by 10 to 23% by weight. Thereafter, while blowing argon gas at 3 to 5 NL / min from the porous plug 20, Fe—
150 kg of Ti and 30 kg of N-Mn, which is an example of an N-containing alloy, were added from the hopper 14, and the molten steel 11 was uniformly mixed with stirring.

When Ti and N are added to molten steel, the product of Ti concentration and N concentration, so-called solubility product [% Ti] × [% N]
Determines the crystallization temperature of TiN. At temperatures higher than the crystallization temperature of TiN, Ti and N added to the molten steel remain in solid solution in the molten steel, and when cooled below the crystallization temperature, T
Start crystallization as iN. TiN has a low degree of lattice matching with δ ferrite of 4%, and is likely to become an inoculation nucleus (solidification nucleus) of δ ferrite. And, when the molten steel is solidified, it is excellent in the effect of easily forming equiaxed crystals to make the solidified structure fine. In order for TiN to act as a solidification nucleus, it is necessary that TiN starts to crystallize at a liquidus temperature (eg, about 1500 ° C.) or higher of molten steel at which solidification starts. The effect of making the structure finer cannot be obtained. Therefore, it is necessary to add Ti and N to the molten steel 11 in such amounts that the solubility product at which TiN is crystallized at a temperature equal to or higher than the liquidus temperature. Specifically, it is preferable to add Ti and N so that the Ti concentration and the N concentration satisfy the solubility product obtained from the following equation (1). [% Ti] × [% N] ≧ ([% Cr] ^2.5 +150) × 10 ⁻⁶ (1) where [% Ti] is the weight percent of Ti contained in the molten steel, [%
[N] represents N weight% contained in the molten steel, and [% Cr] represents Cr weight% contained in the molten steel. In the present embodiment,
As a concentration satisfying the expression (1), for example, Ti is 0.02
Ti and N are added to the molten steel 11 so that 0% concentration and N become 0.024% concentration.

In order to further enhance the effect of miniaturization by TiN, it is conceivable to increase the amounts of TiN and N added to increase the amount of TiN crystallized at the same temperature. However, the amount of Ti and N is limited by steel. Even if the amounts of Ti and N are increased, T increases with time after crystallization.
A phenomenon is observed in which iN is aggregated and coarsened, and the number of solidified nuclei does not always increase. Rather, adverse effects such as clogging of nozzles by coarse TiN and generation of barbed flaws of steel material occur. In order to prevent such a phenomenon, Mg is added at a temperature higher than the crystallization temperature of TiN before adding Ti and N or after adding Ti and N to generate Mg oxide. Thereby, when TiN crystallizes as the temperature of the molten steel decreases, since the lattice matching between Mg oxide and TiN is close, TiN crystallizes preferentially on finely dispersed Mg oxide, and Mg TiN is more efficiently dispersed and crystallized in large numbers than in the case of no addition. Further, in order to maintain the yield of Mg added to the molten steel at a high level, after adding Ti and N, M
Preferred results are obtained by adding g and reducing the time to casting.

Accordingly, while supplying the metal Mg wire 17 through the guide pipe 16, 75 kg of Mg is supplied into the molten steel 11 while the supply device 19 is operated and 0.0005 to 0.0005.
By generating Mg oxide at a concentration of 0.010% by weight, the crystallized TiN is dispersed in a fine state. As a result, unstable operations such as nozzle clogging due to coarse TiN caused when Ti and N are added (no Mg added) can be prevented, and the molten steel 11 solidifies with a small amount of Ti and N added. The solidified structure of the cast slab could be made fine as shown in FIG. By making the solidification structure fine, shrinkage during solidification and internal cracks caused by coarse structure,
Internal defects such as center segregation and center porosity can be prevented. As described above, since the steel material obtained by processing a slab having a fine solidification structure has a fine solidification structure, it is possible to stably prevent the occurrence of surface defects such as barbed flaws, edge seam flaws, and roping.

[0014]

Next, an embodiment of a method for treating molten steel having excellent quality characteristics will be described. Using a top-bottom blow converter, 150 tons of molten steel from which decarburized and impurities such as phosphorus and sulfur have been removed are received in a ladle, and 70 NL / min of argon gas is blown from a porous plug while Fe-Ti and N -Mn is added to increase the Ti concentration of the molten steel to 0.013 to 0.125% by weight,
After the N concentration was adjusted to 0.012 to 0.024% by weight, metal Mg was added and continuous casting was performed to produce a slab. And the stability of casting operation, the fineness of the solidification structure of the slab, the presence of internal defects in the slab and the presence of surface defects in the steel,
The comprehensive evaluation was investigated. Table 1 shows the results. Example 1
Means that the Ti concentration of molten steel is 0.013% by weight and the N concentration is 0.1%.
This is a case in which 0.0035% by weight of metallic Mg is added after 012% by weight, the operation at the time of casting is stable, the solidification structure of the slab is fine, and there is no defect in the slab and the steel material. Good (）) was obtained. Example 2 is a case where the Cr concentration of molten steel was 10% by weight, the Ti concentration was 0.020% by weight, the N concentration was 0.024% by weight, and then 0.0015% by weight of metallic Mg was added. The operation at the time was stable, the solidification structure of the slab was fine, the slab and the steel material were free from defects, and good (○) was obtained as a comprehensive evaluation.
Example 3 was a case where the molten steel had a Cr concentration of 10% by weight, a Ti concentration of 0.125% by weight, an N concentration of 0.022% by weight, and then added Mg of 0.0025% by weight. The operation at the time was stable, the solidification structure of the slab was fine, the slab and the steel material were free from defects, and good (○) was obtained as a comprehensive evaluation.

[0015]

[Table 1]

On the other hand, in Comparative Example 1, the molten steel had a Cr concentration of 10% by weight, a Ti concentration of 0.021% by weight, an N concentration of 0.023% by weight, and no metallic Mg was added. Yes, operation becomes unstable due to nozzle clogging and the like during casting, and the solidified structure of the slab becomes coarse as shown in FIG. 2 (B), causing defects in the slab and steel material. Bad (x) result. Comparative Example 2 shows the C
When the r concentration was 23% by weight, the Ti concentration of the molten steel was 0.198% by weight, and the N concentration was 0.038% by weight, the solubility product of both elements was within the range where TiN was not precipitated, and no metallic Mg was added. In addition, nozzle clogging and the like occurred at the time of casting, the operation became unstable, and defects caused by coarse TiN occurred on the surface of the steel material, resulting in a poor (×) result as an overall evaluation.

The embodiment of the present invention has been described above.
The present invention is not limited to the above-described embodiment, and all changes in conditions that do not depart from the gist are within the scope of the present invention. For example, as a refining furnace, in addition to molten steel produced by primary refining such as decarburization, it can be applied to molten steel produced using a secondary refining device such as vacuum refining or general ladle refining. . Further, when N is added to the molten steel, nitrogen gas can be blown into the molten steel to increase the nitrogen concentration in the molten steel, or the nitrogen gas can be used in combination with the blowing of the nitrogen gas.

[0018]

The method for treating molten steel having excellent quality characteristics according to the first to third aspects is characterized in that molten steel having a Ti concentration and an N concentration satisfying a solubility product in which TiN is crystallized at a temperature higher than a liquidus temperature of the molten steel is added to the molten steel.
Is added, so that TiN can be easily dispersed in MgO or Mg.
It can be crystallized on the Mg oxide of O.Al ₂ O ₃ and dispersed in molten steel, which makes it possible to stabilize the work such as casting, makes the solidification structure of the slab fine, Defects and surface defects of steel materials can be prevented, and the yield of good products can be improved.

In the method for treating molten steel having excellent quality characteristics, the Mg alloy is added with the concentrations of Ti and N contained in the molten steel within a predetermined range.
N can be stably dispersed in the molten steel by the Mg oxide, and when the molten steel is solidified, it can act as a solidification nucleus to further refine the solidified structure of the slab.

According to the third aspect of the present invention, since the molten steel is a ferritic stainless steel molten steel containing 10 to 23% by weight of Cr, the effect of refining the solidification structure is sufficiently exhibited. In addition, surface defects and internal defects occurring in slabs and steel materials can be prevented.

[Brief description of the drawings]

FIG. 1 is an overall view of an apparatus for treating molten steel applied to a method for treating molten steel having excellent quality characteristics according to an embodiment of the present invention.

FIGS. 2A and 2B are schematic diagrams of a solidified structure of a cast slab, respectively.

[Explanation of symbols]

10: molten steel processing device, 11: molten steel, 12: ladle, 1
3: hopper, 14: hopper, 15: chute, 16: guide pipe, 17: metal Mg wire, 18: slag, 1
9: supply device, 20: porous plug

Continuation of the front page (51) Int.Cl. ⁷ Identification code FI Theme coat II (reference) B22D 11/108 B22D 11/108 CD 11/114 11/114 C21C 7/00 C21C 7/00 H (72) Inventor Ryusuke Miura 1-1-1, Hibata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka New Nippon Steel Corporation Yawata Works (72) Inventor Shintaro Kusunoki 1-1, Hibata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka New Nippon Steel Corporation Yawata Inside the steelworks (72) Inventor Yuji Koyama 1-1, Tobata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka New Nippon Steel Corporation Yawata Works F-term (reference) 4E004 MB14 NC02 4K013 AA02 BA14 CA02 CB01 CC02 CC06 CF13 DA03 DA05 DA09 DA13 EA23 EA32

Claims (3)

[Claims] 1. A method for treating molten steel having excellent quality characteristics, comprising adding Mg to molten steel having a Ti concentration and an N concentration satisfying a solubility product in which TiN is crystallized at a temperature not lower than a liquidus temperature of the molten steel. 2. The molten steel having excellent quality characteristics according to claim 1.
In the treatment method, the Ti concentration and the N concentration satisfy the following equation.
How to process molten steel with excellent quality characteristics
Law. [% Ti] × [% N] ≧ ([% Cr]^2.5 +150) ×
10^-6  However, [% Ti] is the Ti weight% contained in the molten steel, [%
[N] is N wt% contained in molten steel, and [% Cr] is in molten steel.
% Of Cr contained in the steel. 3. The method for treating molten steel having excellent quality characteristics according to claim 1, wherein the molten steel contains 10 to 2% of Cr.
A method for treating molten steel having excellent quality characteristics, characterized by being a ferritic stainless steel molten steel containing 3% by weight.