JP2000273585A - Ferritic stainless steel and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technique for melting a Ti-containing ferritic stainless steel without clogging of immersion nozzle and obtaining a cast slab free from surface defects. SOLUTION: In a tundish, a slag composition is regulated so that it satisfies FeO+MnO+Cr2O3<=3% and CaO/(SiO2+Al2O3)=1.0 to 3.0, and also a steel composition is regulated so that it contains 0.01-0.3% Ti and 10-30% Cr and satisfies T [O] concentration <=70 ppm and log [Ti (ppm)×N (ppm)×C (ppm)] <=8.5. Further, a Ca-containing metal wire is added to the molten steel in the tundish under the following condition: log[C×13.3+118.6×B/A]]=0.5 to 2.5 (where A is the diameter (mm) of the metal wire; B is the coating thickness (mm) of the metal wire; and C is the feed speed (m/min) of the metal wire).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a ferritic stainless steel and a method for producing the same, and more particularly, to a Ti-containing ferritic stainless steel having no surface defects of cast slabs and excellent cleanliness, and no nozzle clogging during continuous casting. And a method for producing a Ti-containing ferritic stainless steel.

[0002]

2. Description of the Related Art Various inventions and proposals have been made on a method for producing stainless steel, that is, a refining method, but they are insufficient in terms of prevention of clogging of a submerged nozzle and reduction of surface defects of a slab.

[0003] For example, Japanese Patent No. 2749695,
No. 7708, 69th Special Steel Subcommittee Material “Improvement of VOD Refining Technology”, Japanese Patent Publication No. 2-50965, Japanese Patent Publication No. 62-1444, Japanese Patent Publication No. 8-19455, etc. Although there are inventions and documents discussing the reduction of nonmetallic inclusions and prevention of immersion nozzle clogging, there is no document discussing not only refining but also prevention of immersion nozzle blockage due to secondary oxidation in the continuous casting process.

Further, with respect to the method of adding a Ca-containing metal wire to molten steel in a tundish in a continuous casting process for the purpose of preventing clogging of a submerged nozzle or reducing non-metallic inclusions, Japanese Patent Publication Nos. 39366, JP-A-54-86433, JP-B-60-32686, JP-A-58-9036
0, JP-A-58-154747, JP-A-3-165595, JP-A-5-237613, JP-A-8-257713, JP-B-63-4167
There are various inventions such as JP-A No. 1, 207565/1990 and JP-A-1-99761. However, none of them can simultaneously satisfy the prevention of immersion nozzle blockage and the reduction of non-metallic inclusions, and the occurrence of surface defects on the slab is inevitable.
In particular, the strict specifications required today are not acceptable.

[0005]

As described above, the ferrite stainless steel containing Ti is liable to block the immersion nozzle due to the presence of deoxidation products and Ti oxides / carbonitrides. There is a problem that defects are likely to occur, and such a problem cannot be solved effectively yet. In addition, in a situation where the specifications for steel sheets are strict as in today, it is important to prevent the occurrence of surface defects caused by inclusions.

Accordingly, an object of the present invention is to develop a technology capable of producing a Ti-containing ferritic stainless steel as a high cleanness slab having no clogging of an immersion nozzle during continuous casting and having no surface defects.

[0007]

Means for Solving the Problems In the refining process of a Ti-containing ferritic stainless steel, the present inventors adjusted the slag composition and product components and then added the Ca-containing metal wire to the molten steel in the tundish in the continuous casting process. Was added.

[0008] Even in the prior art, as described above, it has been known to add a Ca-containing metal wire to molten steel in a tundish. However, the slag composition and the steel composition in the finish refining or in the ladle are simultaneously adjusted. That, furthermore, when adding the Ca-containing metal wire to the molten steel in the tundish prepared as above, the metal wire diameter, adjusting the supply speed of the appropriate metal wire according to the coating thickness of the metal wire, and Their significance has not been revealed in the prior art.

Therefore, the present inventors add Ca-containing metal wires to molten steel in a tundish in a continuous casting process, the effect of the slag composition and the molten steel composition on the immersion nozzle blockage in the refining process of ferritic stainless steel. The effects of the diameter of the metal wire, the coating thickness of the metal wire, the feeding speed of the metal wire, and the degree of superheat of the molten steel on the clogging of the immersion nozzle and the occurrence of surface defects of the slab were investigated.

Here, the metal wire used was a sheath made of mild steel and filled with Ca-containing particles such as a Ca-Si alloy and a Ca-Al alloy. In addition, such a metal wire itself is already known.

In a tundish, a non-oxidizing gas atmosphere is usually maintained to prevent oxidation of the steel. For example, FeO, MnO, and Cr ₂ O ₃ in slag in a ladle are mixed with molten steel. Since it serves as an oxygen supply source, the number of inclusions that cause clogging of the immersion nozzle in molten steel is increased. Therefore, in the present _{invention, FeO + MnO + Cr 2 O} 3 content in the slag was investigated the effect on the immersion nozzle clogging performed ladle adjusting slag composition before the start of casting.

FIG. 1 shows the relationship between the content of FeO + MnO + Cr ₂ O ₃ in the slag and the ratio of inclusions to the inner diameter of the immersion nozzle after casting. As is clear from the results shown in FIG.
To ensure stable casting without clogging of the immersion nozzle,
It can be seen that the slag composition should be controlled to FeO + MnO + Cr ₂ O ₃ ≦ 3%.

Next, CaO / (SiO _{2 in} the slag before continuous casting
+ Al ₂ O ₃ ) was found to increase the product T [O] concentration and increase inclusion defects in the product. That is, FIG. 2 shows the relationship between the product T [O] concentration and the inclusion defect.
As is clear from the results shown in FIG. 2, it can be seen that the product T [O] concentration should be ≦ 70 ppm in order to reduce inclusion defects.

FIG. 3 shows the slag in the slag before continuous casting.
The relationship between CaO / (SiO ₂ + Al ₂ O ₃ ) and the product T [O] concentration is shown. In order to make the product T [O] concentration ≤ 70 ppm, CaO / (SiO ₂ + Al
₂ O ₃ ) ≧ 1.0. However, CaO / (S
When iO ₂ + Al ₂ O ₃ )> 3.0, there is no significant difference in the product T [O] concentration.
In other words, to suppress the amount of inclusions, CaO / (Si
O ₂ + Al ₂ O ₃ ) ≦ 3.0. Therefore, CaO / (SiO ₂
+ Al ₂ O ₃ ) = 1.0 to 3.0.

[0015] Ti combines with C and N in steel to form oxides and carbonitrides, which causes nozzle clogging and surface defects in products. Therefore, the effects of Ti, C, and N on nozzle blockage and slab surface defects were investigated.

That is, FIG. 4 shows the relationship between Ti, C, and N and nozzle blockage and slab surface defects. To prevent nozzle clogging and reduce slab surface defects, Log [Ti (pp
m) × N (ppm) × C (ppm)] ≦ 8.5.

When a metal wire containing Ca is added to a continuous casting tundish to prevent clogging of the immersion nozzle, the metal wire supply speed is adjusted according to the metal wire diameter and the coating thickness of the metal wire. is important. Therefore, the effects of the metal wire diameter, the coating thickness of the metal wire, and the metal wire supply speed on the immersion nozzle blockage were investigated. As a result, as shown in FIG. 5, the metal wire diameter A (mm),
When the coating thickness B (mm) of the metal wire and the feeding speed C (m / min) of the metal wire are added in a relation of Log [C × (13.3 + 18.6 × B / A)] = 0.5 to 2.5, It has been found that it is possible to cast a Ti-containing ferritic stainless steel without nozzle clogging.

Log [C × (13.3 + 11.8 × B / A)] <0.5
In the case of (1), the metal wire melts on the surface of the molten steel in the tundish, so that the nozzle blockage preventing effect cannot be obtained. On the other hand, if the value of this equation exceeds 2.5, the coating of the metal wire does not dissolve in the molten steel in the tundish, so that the effect of preventing nozzle clogging cannot be obtained. Therefore, Log [C × (13.3
+ 118.6 × B / A)] = 0.5 to 2.5, the Ca-containing metal wire is melted in the molten steel in the tundish, and the effect of preventing the immersion nozzle from clogging by lowering the melting point of inclusions is obtained.
Furthermore, the effect of the degree of superheating of the molten steel in the tundish (hereinafter simply referred to as the degree of superheating) on the operation and quality of ferritic stainless steel was investigated.

As shown in the graphs of FIGS. 6 and 7, when the superheat degree of the molten steel is less than 15 ° C., the unslagged powder in the mold is easily captured by the solidified shell, and the superheat degree in the tundish and the mold is reduced. It was found that the inclusion flotation in the interior was inhibited, and inclusion defects occurred in the product. On the other hand, when the superheat degree of molten steel was higher than 80 ° C., it was found that the initial solidified shell in the mold was redissolved by the discharge flow from the immersion nozzle, and the frequency of breakout was increased. Therefore, in a preferred embodiment of the present invention, the degree of superheat of molten steel is 15 to 80 ° C.

Therefore, the present invention is as follows. (1) Contains Ti = 0.01 to 0.3%, Cr = 10 to 30%,
[Ti (ppm) × N (ppm) × C (ppm)] A ferritic stainless steel excellent in cleanliness and having a steel composition satisfying ≦ 8.5.

(2) Ti = 0.01 to 0.3%, Cr = 10 to 30%, T [O] concentration ≦ 70 ppm, Log [Ti (ppm) × N (ppm) × C (pp
m)] A ferritic stainless steel excellent in cleanliness, having a steel composition satisfying ≦ 8.5.

(3) FeO + MnO + Cr ₂ O ₃ ≦ 3%, CaO / (SiO ₂ + Al
₂ O ₃ ) = 1.0 to 3.0 When the slag composition is satisfied, the molten steel of ferritic stainless steel containing Ti = 0.01 to 0.3% and Cr = 10 to 30% is continuously cast. A method for producing a ferritic stainless steel, comprising adding a Ca-containing metal wire to the molten steel in a casting tundish under the following conditions.

Log [C × (13.3 + 118.6 × B / A)] = 0.5 to 2.5 where A: metal wire diameter (mm), B: metal wire coating thickness (mm), C: metal wire supply speed (4) The method for producing a ferritic stainless steel according to the above (3), wherein the molten steel in the tundish is continuously cast with a degree of superheat of 15 to 80 ° C.

[0024]

Next, embodiments and specific examples of the present invention will be further described. In the present invention, the refining process up to continuous casting may be performed in the same manner as in the prior art. For example, tapping steel from the converter to the ladle and performing out-of-pile refining as needed, then Ti: 0.01-0.
After adjusting the composition to 3% and Cr: 10-30%, it is poured into a tundish.

If Ti is less than 0.01%, the oxidation resistance and corrosion resistance of the product deteriorate, and if Ti exceeds 0.3%, scum is generated in the mold during continuous casting, and the slag surface properties are significantly impaired. Cr contains 10 to 30% as ferritic stainless steel.

In the present invention, the slag composition in the ladle before casting is FeO + MnO + Cr ₂ O ₃ ≦ 3%, CaO / (SiO ₂ + Al
₂ O ₃ ) = adjusted to 1.0 to 3.0. As described above, when the slag composition is outside these ranges, the generation of inclusions is excessive and the cleanliness is reduced.

After adjusting the molten steel in this way, the molten steel is injected into the immersion nozzle through a tundish, and continuous casting is performed. Ca-containing metal wire is added to the molten steel in the tundish to separate the inclusions by flotation to further improve the cleanliness of the steel. The amount of Ca added at this time may be the same as in the conventional art, and is not particularly limited in the present invention, but the supply of the metal wire is as follows.

Log [C × (13.3 + 118.6 × B / A)] = 0.5 to 2.5 where A: metal wire diameter (mm), B: metal wire coating thickness (mm), C: metal wire supply speed (m / min) This is because the metal wire is rapidly dissolved in the molten steel in the tundish so that the inclusion Ca can effectively exert the effect of removing inclusions.

As described above, although not particularly limited in the present invention, the amount of Ca added to molten steel is approximately 5 to 620 g per ton of molten steel (provided that Ca is pure). In this way, since the amount of inclusions of the molten steel in the tundish is reduced as much as possible, the molten steel is continuously cast preferably at a superheat degree of 15 to 80 ° C.

The steel composition of the molten steel obtained at this time, that is, the slab (product) is as described above with respect to Ti and Cr, but preferably satisfies one or both of the following relational expressions.

T [O] concentration ≦ 70 ppm Log [Ti (ppm) × N (ppm) × C (ppm)] ≦ 8.5 If any of these ranges are not satisfied, as described above, a large amount of inclusions Is inevitable, and the cleanliness of steel is inevitably reduced.

The casting operation itself, such as injection of molten steel from the tundish into the immersion nozzle, may be a conventional operation, and is not limited in the present invention. Next, the present invention will be described more specifically with reference to examples.

Example Ti-containing ferritic stainless steel (Composition: C = 0.01%,
Si = 0.46%, Mn = 0.25%, Cr = 11.33%, S = 0.008
%, P = 0.021%, Ti = 0.172%), and then poured into a tundish, and the slag composition at the time of finish refining was adjusted as shown in Table 1.

Thereafter, a Ca-containing metal wire (diameter 10 mm, coating thickness 0.2 mm) was added under the same conditions as shown in Table 1. This corresponds to approximately 3 to 1200 g / ton of molten steel in terms of the amount of Ca added. The results are summarized in Table 1.

A to H in Table 1 are examples according to the present invention. There was no clogging of the immersion nozzle, and the number of product inclusion defects was at a level without any problem. I to P in Table 1 are comparative examples out of the range of the present invention. Comparative Example I is an example where FeO + MnO + Cr ₂ O ₃ was high, Comparative Example J was an example where CaO / (SiO ₂ + Al ₂ O ₃ ) was low and Comparative Example K was high. In each case, the immersion nozzle tended to close, and it was difficult to continue casting.

Comparative Example L is an example in which the contents of Ti, N, and C were high, and clogging of the immersion nozzle or inclusion defect in the product was observed. In Comparative Example M, since the supply rate was small with respect to the diameter of the metal wire and the coating thickness, the metal wire melted on the surface of the molten steel in the tundish, and violent smoke was generated.
The immersion nozzle was prone to blockage. On the other hand, in Comparative Example N, since the supply rate was excessively large with respect to the diameter and the coating thickness of the metal wire, the metal wire was not completely melted in the molten steel in the tundish, and the metal wire jumped out of the molten steel in the tundish again. Was.

In Comparative Example O, since the degree of superheating of the molten steel was low, the floating separation of inclusions in the tundish and the mold was insufficient, and large inclusions of 120 microns were found in the slab. Breakout occurred during casting of Comparative Example P.
Due to the high degree of superheat of the molten steel, it was considered that the cause was re-melting of the solidified shell.

[0038]

[Table 1]

[0039]

As described above, according to the present invention, it is possible to stably produce a ferritic stainless steel having no clogging of the immersion nozzle and no slab surface defects.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between FeO + MnO + Cr ₂ O _{3 in} slag and the ratio of inclusions to the immersion nozzle inner diameter area after casting.

FIG. 2 is a graph showing a relationship between a product T [O] and the number of inclusion defects per meter of slab.

FIG. 3 is a graph showing a relationship between FeO + MnO + Cr ₂ O _{3 in} slag and slab T [O] concentration.

FIG. 4 is a graph showing the relationship between Log [Ti (ppm) × C (ppm) × N (ppm)] and the number of inclusion defects per meter of slab.

FIG. 5 is a graph showing the relationship between Log [C × (13.3 + 118.6 × B / A)] and the ratio of inclusions to the immersion nozzle inner diameter area after casting.

FIG. 6 is a graph showing the relationship between the degree of superheat of molten steel and the number of inclusion defects per meter of cast slab.

FIG. 7 is a graph showing the relationship between the degree of superheating of molten steel in a tundish and the frequency of breakdown occurring due to remelting of a solidified shell.

Claims (4)

[Claims] 1. Cleanliness having a steel composition containing Ti = 0.01 to 0.3% and Cr = 10 to 30% and satisfying Log [Ti (ppm) × N (ppm) × C (ppm)] ≦ 8.5. Excellent ferritic stainless steel. 2. Ti = 0.01-0.3%, Cr = 10-30%, T [O] concentration ≦ 70 ppm, Log [Ti (ppm) × N (ppm) × C (ppm)]
A ferritic stainless steel excellent in cleanliness, having a steel composition satisfying ≦ 8.5. 3. FeO + MnO + Cr ₂ O ₃ ≦ 3%, CaO / (SiO ₂ + Al ₂ O
₃ ) For continuous casting of molten steel of ferritic stainless steel containing Ti = 0.01-0.3% and Cr = 10-30% produced by making the slag composition satisfying = 1.0-3.0. A method for producing a ferritic stainless steel, comprising adding a Ca-containing metal wire to the molten steel in a tundish under the following conditions. Log [C x (13.3 + 118.6 x B / A)] = 0.5 to 2.5 where A: metal wire diameter (mm), B: metal wire coating thickness (mm), C: metal wire supply speed (m / min) 4. The superheat degree of the molten steel in the tundish is reduced.
The method for producing a ferritic stainless steel according to claim 3, wherein the continuous casting is performed at a temperature in the range of 15 to 80C.