JP2001286991A - Method for continuously casting stainless steel thin cast slab excellent in surface characteristic and its apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To continuously cast a stainless steel thin cast slab excellent in the surface characteristics without developing surface flaws and the uneven pickling. SOLUTION: In a method for continuously casting the stainless steel thin cast slab while cooling and solidifying molten steel on the peripheral surface of cooling rolls by pouring the moten stainless steel into a molten steel pooling part formed with a pair of cooling rolls having parallel axes and rotated in mutually reverse directions and side weirs, the viscosity at 1400 deg.C of scum floated up on the molten steel surface of the molten steel pooling part is regulated to <=30 Pa.s (desirably, <=5 Pa.s) and this solidified phase is regulated to <=20%. Further, suitably, in the molten steel pooling part, scum weirs having a width of >=90% of the width of the cooling roll are dipped and disposed in parallel with the cooling rolls to flow down the scum into the edge parts of the thin cast slab.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of injecting molten steel of stainless steel into a pool formed by a pair of cooling drums and side dams which rotate in opposite directions and are parallel to each other. The present invention relates to a method for continuously casting a thin slab of stainless steel having good surface properties while cooling and solidifying the molten steel on the surface.

[0002]

2. Description of the Related Art Heretofore, a twin-drum continuous casting method has been described, that is, molten steel is poured into a pool formed by a pair of cooling drums and side dams which are parallel to each other and rotate in opposite directions. There have been many attempts to continuously cast thin stainless steel slabs having good surface properties by using a method of cooling and solidifying the molten steel on the peripheral surface of the steel and continuously casting the slab while generating a solidified shell. ing.

[0003] However, during continuous casting, the molten steel surface in the pool is oxidized by the oxygen in the casting atmosphere,
Scum mainly composed of oxides (hereinafter referred to as "scum") floats due to the floating of inclusions in molten steel, and this scum is
It flows along with the molten steel that flows with the rotation of the cooling drum, flows along the surface of the cooling drum under the pool, and enters between the peripheral surface of the cooling drum and the solidified shell being formed, It causes surface defects such as rough surface and cracks on the surface of the thin steel slab.

The occurrence of surface defects caused by this scum is as follows:
The same applies to the case where stainless steel thin slabs are continuously cast by the twin belt continuous casting method. For example,
If the melting point of the scum floating on the molten steel surface is high,
Non-uniform spots float stably on the surface of the molten steel, and the scum flows along with the flowing molten steel, and when it reaches the periphery of the cooling drum, the scum is formed before a solidified shell is formed. It is caught on the peripheral surface and eventually remains between the peripheral surface of the cooling drum and the solidified shell in the process of being formed, and adheres and remains as uneven spots on the surface of the continuously cast slab. Become.

If a thin steel slab having this surface defect is rolled as it is, a fatal product defect (slab slab crack) will be induced. Therefore, it is necessary to suppress the occurrence of this surface defect during continuous casting as much as possible. However, for this purpose, it is important to minimize the intrusion of scum between the peripheral surface of the cooling drum and the solidified shell being formed, and to stably grow the solidified shell on the peripheral surfaces of both cooling drums. is there.

[0006] Since scum is a floating substance mainly composed of oxide, a technique for suppressing the generation of scum by setting the atmosphere surrounding the pool in a non-oxidizing atmosphere or an inert atmosphere has been proposed (particularly, Japanese Patent Laid-Open Publication No. HEI 10-163556). See JP-A-62-130749 and JP-A-3-27848). However, the outside air is drawn in with the rotation of the cooling drum from the gap of the cooling drum, or the oxide floats from the molten steel depending on the type and cleanliness of the molten steel, or due to a refractory such as a molten steel injection nozzle. It is difficult to completely prevent the generation of scum with these techniques, for example, oxides are generated and floated.

Therefore, according to Japanese Patent Application Laid-Open No. 4-197560, an inert gas is blown from the roll side toward the center of the pool in the direction of the roll axis on the surface of the pool and the scum floating on the surface is removed from the side dam side. It was suggested to collect. In the technique according to this proposal, scum is usually led to the molten steel flow and tends to gather mainly in the center of the pool, so this is collected on the side weir side by an inert gas flow, and the The aim is to minimize scum entrainment in the above.

According to this technique, scum entrainment in the center of the slab is certainly suppressed, and the quality of the center of the slab is improved. However, as apparent from FIG. It can be seen that the scum is entangled at both ends of the slab more frequently than in the past, since it is almost limited to the center of the slab. After all, in the case of a product, both ends of the slab must have a considerable width range (for example, 50 mm or more according to FIG. 3) as a product yield drop portion (trim portion). The yield is extremely poor and cannot be applied to the actual continuous casting technology.

[0009] Based on this technology, the present applicant has disclosed in
In Japanese Patent Application Laid-Open No. 106304/1994, a pair of barriers extending in the axial direction of the cooling drum are provided in the well pool portion so as to prevent the scum from being caught and caught in the solidified shell and ensuring castability. A technique of immersing and arranging in molten steel at a distance from the steel was proposed. According to this technology, the scum flows away from the cooling drum on the surface of the molten steel in accordance with the flow of the molten steel discharged from the molten steel nozzle. Although it decreases, when the scum amount increases, it adheres to the barrier, and the flow becomes slow or stays near the side weir, so that the required effect by the scum flow (scum entrainment suppression effect) cannot be obtained.

[0010] Therefore, the above technique requires that the amount of one charge be 6
It cannot be applied to continuous casting for a long time ranging from 0 to 360 ton. On the other hand, the present applicant has disclosed Japanese Patent Application Laid-Open No. 4-2849.
No. 48, in the continuous casting of thin cast slab of austenitic stainless steel, not from the viewpoint of suppression of scum formation and scum entrainment, the properties of the scum so that the presence of scum does not lead to the occurrence of surface defects From the viewpoint of improvement, a technique for adjusting the viscosity of the scum to 2 to 50 poise was proposed.

However, the above technique limits the viscosity of the scum and further limits the calculated δ ferrite content in the molten steel to 4 to 10% so that coarse γ grains are not formed in the scum wrapped portion. Even when the scum is involved, the scum is uniformly stretched on the surface of the thin slab, the thickness is made uniform, and the difference in the solidification structure inside the slab, that is, the difference in the γ grain size expressed in the surface structure is eliminated. Therefore, it is intended to eliminate uneven gloss of the product.

That is, the above-described technique is a technique for suppressing uneven gloss of a product due to the presence of coarse γ grains, and is different from a surface defect targeted by the present invention. Further, in the above technique, it is necessary to maintain the viscosity of the scum almost constant within the range of 2 to 50 poise until the end of the continuous casting. However, when the continuous casting is performed for a long time, the viscosity of the scum is inevitably changed. However, the above technique is not practical because a difference occurs in the scum thickness on the surface of the thin slab.

In addition, the applicant of the present invention, in the twin-belt continuous casting method of ordinary steel, similarly sets the melting point of scum floating on the surface of molten steel in a mold to 1400 from the viewpoint of improving the properties of scum. A technique for lowering the temperature to below ℃ has been proposed (see JP-A-8-33951). This technology is
If the scum is made into a low-melting scum with a low viscosity and made to have a smooth property at the temperature of the molten steel, the scum becomes difficult to be caught by the solidified shell, and even if the scum is caught and caught by the solidified shell, It is based on the idea that it does not cause the occurrence of surface defect flaws in thin slabs because it is thinned and miniaturized and dispersed on the slab surface, and it is actually effective in suppressing the occurrence of surface defects in ordinary steel. There is something.

However, when this technique is applied to continuous casting of austenitic stainless steel thin slabs, a scum having a low melting point and a low viscosity can be obtained even if no defect is formed on the surface of the thin slab. A metastable γ phase (white structure) free of δ ferrite, which is the second phase, is formed inside the thin slab where 存在 exists, and appears on the slab surface as pickling unevenness due to pickling after casting. It turned out to be.

As described above, in continuous casting of austenitic stainless steel thin slabs, there are two types of surface defects caused by scum: pickling unevenness and surface defects. And
These surface defects are in a trade-off relationship in terms of the properties of the scum that causes them, and the surface defects of austenitic stainless steel thin slabs simply suppress scum entrainment, or In fact, it cannot be solved simply by improving the properties of

[0016]

However, in particular,
When manufacturing stainless steel for which the quality of product surface quality is strictly required, it is essential to produce thin cast slabs with excellent surface properties at the stage of continuous casting. In order to continuously cast slabs, it is necessary to approach scum that greatly affects the quality of the surface properties of the thin slab from a viewpoint different from the conventional viewpoint (generation, properties, and the like).

Accordingly, the present invention is based on this premise, and in addition to the generation and improvement of properties of scum, takes some measures from another viewpoint, and simultaneously solves the occurrence of surface defects and pickling unevenness which are in a trade-off relationship. The task (objective) is to do this.

[0018]

As described above, the reason that the technique proposed by the present applicant in Japanese Patent Application Laid-Open No. 8-33951 cannot be applied to continuous casting of austenitic stainless steel thin slabs is as follows. A metastable γ phase (white structure) that reaches the surface of the slab is generated inside the thin slab having low viscosity scum on the surface. This is because it has been found to appear.

However, the present inventor has concluded from the above that if low-viscosity scum does not exist on the surface, no metastable γ phase (white void structure) is formed inside the thin slab. The idea was reached that the problem of uneven pickling could be solved if it was engulfed at a place other than the surface of the thin cast slab corresponding to the product. Then, for example, in the case of a stainless steel thin slab having a plate width of 1000 to 1400 mm, usually, a portion within a range of about 2 cm from both ends of the thin slab is cut off as a product yield drop portion (trim portion). The idea was that if the scum could be concentrated into the falling part (trim part), the problem of uneven pickling could be solved.

Based on the above idea, the inventor of the present invention has made intensive studies and research and has obtained the following findings. (A) When the viscosity of the scum at 1400 ° C. (corresponding to the solidus of austenitic stainless steel) is 30 Pa · s or less, the fluidity is improved and the scum is continuously connected.

(B) When the solid phase ratio of the scum is 20% or less, no surface flaw is formed even when the scum is caught in the solidified shell. (C) Since the scum is continuously connected and flows, the scum should be continuously and intensively flown into the area near both ends of the thin slab (the end area that becomes a product yield dropping portion (trim portion)). Can be.

(D) 90 mm of the width of the cooling drum is
% Of the scum weir having a width of at least
By immersing and arranging, the scum can flow intensively into the region near both ends of the thin slab. The present invention is based on the above findings (a) to (d), and the gist is as follows. (1) Pour molten stainless steel into a pool formed by a pair of cooling drums and side weirs whose axes are parallel and rotate in opposite directions, and cool and solidify the molten steel on the peripheral surface of the cooling drum while the stainless steel is cooled. In a method of continuously casting a thin steel slab, a method of producing a scum floating on a molten steel surface of the pool in the above-mentioned pool is provided.
A thin stainless steel slab having excellent surface properties, wherein the viscosity at 0 ° C. is 30 Pa · s or less, the solid phase ratio is 20% or less, and the scum is poured into the end of the thin slab. Continuous casting method. (2) Inject molten stainless steel into a pool formed by a pair of cooling drums and side weirs whose axes are parallel to each other and rotate in opposite directions, and cool and solidify the molten steel on the peripheral surface of the cooling drum to form a stainless steel. In a method of continuously casting a thin steel slab, a method of producing a scum floating on a molten steel surface of the pool in the above-mentioned pool is provided.
A scum weir having a viscosity at 0 ° C. of 30 Pa · s or less, a solid fraction of 20% or less, and a width of 90% or more of the cooling drum width is provided in the pool. Wherein the scum is poured into the end of the thin slab and the stainless steel thin slab having excellent surface properties is cast. (3) The continuous casting method of a thin stainless steel slab having excellent surface properties according to the above (1) or (2), wherein the viscosity of the scum is 5 Pa · s or less. (4) The continuous casting method for a thin stainless steel slab having excellent surface properties according to the above (1), (2) or (3), wherein the solid phase ratio is 10% or less. (5) A pair of cooling drums whose axes are parallel to each other and rotating in opposite directions is provided, and a pool is formed by a side dam, and the stainless steel molten steel injected into the pool is cooled by the peripheral surface of the cooling drum. In an apparatus for continuously casting a thin stainless steel slab while solidifying, a scum dam having a width of 90% or more of the cooling drum width is immersed and arranged in the pool portion in parallel with the cooling drum. Continuous casting equipment for stainless steel thin slabs with excellent surface properties.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, scum is made to have good fluidity. The scum property is specifically defined by viscosity and solid fraction. 1400 of scum
The viscosity at ° C (corresponding to the solidus of austenitic stainless steel melt) is specified as 30 Pa · s or less. If the viscosity of the scum exceeds 30 Pa · s, the fluidity can be secured to some extent, but the scum may not be continuously connected due to the influence of the molten steel flow. Therefore, the upper limit of the scum viscosity (1400 ° C.) is set to 30 Pa · s.

When the scum viscosity (1400 ° C.) is 30 Pa · s or less, sufficient fluidity can be secured at the temperature of the molten steel to achieve the object of the present invention. The preferred range of the scum viscosity is 5 Pa · s or less, but the most preferred range is 1 Pa · s or less. The scum viscosity is
Since the viscosity varies depending on the scum composition, a low-melting agent such as a compound containing an alkali metal element and / or an alkaline earth metal element may be added to reduce the viscosity to 30 Pa · s or less. Scum reacts with a low melting point agent to lower the melting point,
It becomes a low viscosity, low melting point scum.

The low melting point agent may be directly applied to the scum floating on the surface of the molten steel, or may be supplied through a coating of a cooling drum, a side weir, a scum weir, a nozzle or the like. Alternatively, the melting point of the scum may be reduced by gasifying the melting point reducing agent and bringing the gas into contact with the scum through the atmosphere surrounded by the pool.

In the present invention, in addition to the scum viscosity,
The solid fraction is defined as 20% or less. This is to prevent a surface flaw from being formed even when the scum is caught in the solidified shell and fixed to the surface of the thin slab. That is, if the solid phase ratio of the scum exceeds 20%, a surface flaw that cannot be ignored in the subsequent process is formed, and excellent surface properties cannot be obtained in the cold-rolled product.

Therefore, the upper limit of the solid fraction of the scum is set to 20%, but the preferable range is 10% or less. The most preferred range is 5% or less. In the basin,
The generation and ascent of a certain amount of scum is inevitable,
If the atmosphere in the basin is a non-oxidizing or inert atmosphere, the amount of floating scum can be suppressed, which is more effective.

In the present invention, the scum according to the above definition is intentionally continuously and intensively poured into a region near both ends of the thin slab (an end region that becomes a product yield drop portion (trim portion)). It is characterized by doing so. This means that, in the vicinity of both side weirs, the scum is intentionally continuously and intensively caught between the molten steel and the cooling drum, but not between the molten steel and the cooling drum other than near the side weir. That's why.

As described above, the metastable γ phase (white structure) which causes uneven pickling, except in the vicinity of both ends of the thin slab.
Can be suppressed from being generated. Japanese Patent Application Laid-Open No. 4-197560 discloses a technique in which an inert gas is blown onto a scum floating on the surface of molten steel to collect the scum on a side weir side, thereby minimizing scum entrainment in a central portion of a slab. In this technique, the quality improvement region is almost limited to the center of the slab, and scum entrainment at both ends of the slab occurs more widely than in the past (see FIG. 3 in the above publication). No consideration is given to the treatment of scum unevenly distributed on the side weir side. As described above, the technique described in the above publication is fundamentally different from the technical idea of the present invention.

In order to continuously and intensively flow the scum into the vicinity of both ends of the thin slab, the scum may be unevenly distributed near the side weir. Is not limited to For example, gas may be blown to the scum and pushed to the side weir side, or it may be made to flow to the side weir side using molten steel flow.

In particular, 90% of the width of the cooling drum is
If the scum weir having the above width is immersed and arranged in parallel with the cooling drum axis, the scum can be efficiently unevenly distributed near the side weir, and as a result, the scum is in the area near both ends of the thin slab. Can be poured more intensively. When the scum weir is immersed and arranged in the pool, the scum weir may be vertical or inclined, and the interval between the cooling drum and the scum weir is not particularly specified. The interval can be appropriately set in consideration of the flow of molten steel and the flow of scum in the pool.

In order to allow the scum generated and floated between the scum weir and the cooling drum to flow between the pair of scum weirs, a hole or a slit may be appropriately formed near the surface of the molten steel of the scum weir. Good. However, in the present invention,
The width of the scum weir is defined as 90% or more of the width of the cooling drum. If the width is less than 90%, the scum unevenly distributed in the vicinity of the side weir may flow from the gap between the side weir and the end of the scum weir to the cooling drum side. This is because the area becomes wider than the part (trim part).

The width of the scum weir is preferably at least 95% of the width of the cooling drum. Hereinafter, examples of the present invention will be described.

[0034]

EXAMPLE In the twin drum type continuous casting machine, the width of the scum weir was changed (85%, 93% and 97% of the cooling drum width).
The austenitic stainless steel having the chemical composition shown in Table 1 was continuously cast into a strip-shaped slab having a thickness of 3.5 mm. In addition,
As the cooling drum, a cooling drum having a width of 1330 mm with dimples randomly provided on the peripheral surface was used. After the continuous casting, the composition of the scum in the portion corresponding to the flaw portion and the uneven pickling portion of the slab and the composition of the scum attached to the scum weir were analyzed. Table 2 shows the results.

From the scum composition shown in Table 2, 1400 ° C.
The viscosity and solid fraction at (solidification shell formation temperature, corresponding to the solidus temperature of the molten steel) were calculated and evaluated. Table 3 shows the results together with the width of the scum weir. Viscosity (μ)
Was calculated according to the following equation. μ = A · T exp (B / T) A = exp [−19.81 + 1.73 (NCaO + NMnO + NMgO + NFeO) +5.82
・ NCaF ₂ +7.02 (NNa ₂ O + NK ₂ O) -35.76 ・ NAl ₂ O ₃ ] B = 31140-23896 (NCaO + NMnO + NMgO + NFeO) -46356 ・ N
CaF ₂ -39159 (NNa ₂ O + NK ₂ O) + 68833 · NAl ₂ O ₃ N: mole fraction T: temperature (K) The solid phase ratio is calculated by the general program “SO
LGASMIX "(Nippon Steel Technical Report No. 342 (1991) P38-
43).

FIG. 1 shows the relationship between the above-mentioned viscosity and solid fraction, and uneven pickling and surface flaws caused by scum. As shown in FIG. 1, a scum having a viscosity of 30 Pa · s or less and a solid fraction of 20% or less generates uneven pickling. From Table 3, this scum is poured into the end of the thin slab. As a result, it is understood that a thin slab having no pickling unevenness or scratches was obtained, and as a result, a cold-rolled product having no gloss unevenness or scratches was obtained.

[0037]

[Table 1]

[0038]

[Table 2]

[0039]

[Table 3]

[0040]

According to the present invention, the scum that inevitably floats on the surface of the molten steel in the pool is converted into scum with high fluidity at the end of the thin slab that becomes the product yield (trim). Therefore, a stainless steel thin slab having excellent surface properties, free from surface flaws and free from pickling unevenness, can be stably continuously cast.

Therefore, the present invention makes a great contribution to the production technology of high quality stainless steel.

[Brief description of the drawings]

FIG. 1 is a view showing the relationship between the viscosity and solid phase ratio of scum and the pickling unevenness and surface flaws caused by the scum.

Claims (5)

[Claims] 1. A stainless steel molten steel is poured into a pool formed by a pair of cooling drums and side weirs whose axes are parallel and rotate in opposite directions, and the molten steel is cooled and solidified on the peripheral surface of the cooling drum. In the method of continuously casting a thin stainless steel slab, while the viscosity at 1400 ° C. of the scum floating on the molten steel surface of the pool is 30 Pa · s or less,
A continuous casting method for a thin stainless steel slab having excellent surface properties, wherein the solid phase ratio is 20% or less and the scum is poured into an end of the thin slab. 2. A stainless steel molten steel is poured into a pool formed by a pair of cooling drums and side weirs, whose axes are parallel to each other and rotate in opposite directions, and the molten steel is cooled and solidified on the peripheral surface of the cooling drum. In the method of continuously casting a thin stainless steel slab, while the viscosity at 1400 ° C. of the scum floating on the molten steel surface of the pool is 30 Pa · s or less,
A scum dam having a solid phase ratio of not more than 20% and a width of not less than 90% of the width of the cooling drum is immersed and arranged in the pool in parallel with the cooling drum. A continuous casting method of a thin stainless steel slab having excellent surface properties, characterized by being poured into an end of the piece. 3. The continuous casting method for a thin stainless steel slab having excellent surface properties according to claim 1, wherein the viscosity of the scum is 5 Pa · s or less. 4. The continuous casting method for a thin stainless steel slab having excellent surface properties according to claim 1, wherein the solid phase ratio is 10% or less. 5. A basin formed by a pair of cooling drums and side dams whose axes are parallel and rotate in opposite directions to each other,
In a device for continuously casting a thin stainless steel slab while cooling and solidifying the molten stainless steel injected into the basin on the peripheral surface of the cooling drum, at least 90% of the width of the cooling drum is added to the basin. A continuous casting apparatus for stainless steel thin slabs having excellent surface properties, wherein a scum weir having a width of 3 mm is immersed and arranged in parallel with the cooling drum.