JPH084876B2 - Initial casting method in continuous casting of stainless steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention improves the quality of a slab obtained in the initial stage of continuous casting of stainless steel by the use of a specific initial mold powder and its charging method. The present invention relates to an initial casting method in continuous casting of possible stainless steel.

[Conventional technology]

In continuous casting, while injecting molten steel from a dipping nozzle inserted in a cylindrical mold (mold) whose inner wall surface forms the mold surface, mold powder is poured onto the upper surface of molten steel in the mold to cast the slab of the mold. It is a method of pulling out from below. In this case, the mold powder has various effects such as the following: lubrication between the inner wall surface of the mold and the solidification shell of the molten steel, heat retention of the upper surface of the molten steel in the mold, and non-metallic inclusions mixed in the molten steel. It is used for various functions such as absorption / adsorption of substances, and cooling assist to prevent a decrease in cooling by filling the gap created between the molten steel and the inner wall surface of the mold due to cooling contraction.

The slab within a few meters from the tip of the slab obtained in the initial stage of casting in such continuous casting is a portion produced by first injecting molten steel into a cooled mold,
It is well known that internal quality and appearance are inferior to those of subsequent slabs, and the tendency is remarkable especially in stainless steel. In other words, in the initial stage of continuous casting, the molten steel temperature in the mold is extremely lower than that in the stable period, so that dequel (integrated solidification of impurities and metal) may occur or the floating of the entrapped mold powder may be hindered. To do. Also, due to unmolding (semi-molten state) of mold powder, non-uniform cooling, and other factors, mold powder flows in unevenly and discontinuously between the mold and solidification shell, causing powder entrapment, and ) Defects are likely to occur.

The poor quality part of the slab in the initial stage of continuous casting is usually cut and scraped in the state of a slab, but especially in the case of stainless steel with high manufacturing cost, the poor quality part is shortened as much as possible to be scrap. There is a strong demand to reduce the amount of carbonization and improve the manufacturing yield.

Therefore, conventionally, in order to improve the quality defect of the slab in the early stage of continuous casting of stainless steel, the following measures to be implemented in the initial stage of continuous casting have been proposed and implemented.

(1) A method in which an initial mold powder having a lower melting point and a lower viscosity value is used.

(2) A method in which the mold powder is melted in a crucible or the like, and is poured into the mold in a molten state.

According to the measures (1) and (2), the molten layer of the mold powder in the initial stage of continuous casting is kept thick to some extent even at a low temperature, so that the mold powder flows between the inner wall surface of the mold and the solidified shell. Was increased, and the effect of improving the casting surface was visually observed. However, the quality of the inside of the slab remains strongly affected by the low temperature, and for example, even in the investigation of large non-metallic inclusions in the slab, almost no improvement effect was recognized.

(3) A method of sealing the molten steel surface in casting with an inert gas.

This measure is a measure to prevent the progress of oxidation of molten steel before continuous casting, but it is difficult to seal due to the opening of the upper part of the mold and complicated work, and it is almost useful in actual operation. Absent.

(4) A method of preventing entrainment of mold powder by reversibly delaying the timing of introducing the initial mold powder.

One of the causes of poor quality of the continuous casting initial slab is the presence of large non-metallic inclusions inside the slab. And the main cause of this non-metallic large inclusion formation is the injection of the first mold powder before the molten metal surface has not risen sufficiently after injecting the molten steel into the mold from the immersion nozzle, and this melts and floats on the surface. It is about to be caught up soon and mixed into the slab and pulled out. Therefore, the higher the level of the molten metal in the mold when the first mold powder is charged, the larger the amount of non-metallic large inclusions and the remarkable improvement effect. However, this injection timing is lost after the molten steel level reaches a steady position set above the molten steel jet of the immersion nozzle inserted in the mold, and it is lost not only inside the mold but also in appearance. Also, since the quality will be poor, it is desirable to input at a timing before that and as late as possible. However, it is currently very difficult to always carry out this pouring work at an appropriate timing due to the fact that the rising speed of the molten metal level is fast and varies, and there are individual differences among workers.

(5) A method of using an exothermic mold powder.

This measure has come to be implemented as an improvement means instead of the measures (1) to (4), in which none of the measures (1) to (4) achieves a satisfactory improvement effect.

In order to compensate for the insufficient temperature of the molten steel injected into the mold at the start of continuous casting and the insufficient melting of the mold powder initially charged by it, namely Ca as the mold powder.
By using an exothermic material containing a metal component such as -Si, the quality of the initial cast slab can be improved both internally and externally. However, if it is only exothermic, it is not effective, and since it is exothermic, the melting speed of the mold powder is fast, so it often becomes molten slag while the molten steel is at a low level. May flow into the slab and mold unevenly and discontinuously to form slag bite or uneven shell with a large amount of depletion, or sometimes it may develop abnormally and cause shell breakage. It has certain drawbacks, and the amount of heat generated by the initial molding powder and the method of using it have not been clarified at all.

[Problems to be Solved by the Invention]

The present invention eliminates the above-mentioned drawbacks of the prior art, and when a continuous casting of stainless steel is started by using a heat-generating mold powder as an initial mold powder, it is always stable and the internal appearance of the cast piece In order to obtain a good quality initial casting slab and shorten the defective part as much as possible to reduce the amount of scrap and improve the product yield, An object is to determine an appropriate amount of use and configure an initial casting method.

[Means for solving the problem]

The present inventors have representatively used Al ₂ O ₃ as a mold powder.
-SiO ₂ combination -CaO based inorganic powder exothermic agent and oxidizing agent, further by adjusting a wide exothermic mold powder of a combination of low melting point modifiers, which initial mold powders of the continuous casting of stainless steel As a result, we examined the effect of the heat-generating powder on its properties (components, calorific value, melting point, etc.), and the relationship between the quantity and the timing. As a result, we obtained the following findings.

That is, if a large amount of exothermic mold powder is added in order to secure the heat generation amount, the thickness of the powder molten layer becomes thicker, and the initial excess of the mold powder due to the partial excessive inflow of the mold powder into the molten steel. Is deteriorated in appearance, and a similar state occurs even if the melting rate is too high. On the contrary, if the input amount is too small, the upper surface of the molten steel cannot be covered and the heat retaining effect cannot be expected. It is to make the initial mold powder have the original functions such as the lubrication effect with the mold surface and the heat retention effect on the molten metal surface, and to perform the stable initial casting without the above-mentioned drawbacks. The amount of heat and the melting point that are closely related to the melting rate are possible within a specific range that is well-balanced with each other, and the basicity of the powder (CaO / SiO ₂ ) is also specified. Need to be in the range. As for the injection timing, as long as the initial mold powder having a property within the above-mentioned specific range is used, the injection timing is excluding extremely early injection, for example, injection before the molten metal reaches the immersion nozzle discharge port. However, there is almost no difference in the quality of the initial slab. And
It was found that the difference in the quality of the initial slab can be roughly predicted from the stability by visual observation such as the quality of the molten state of the mold powder sprayed on the upper surface of the molten steel in the mold.

The inventors of the present invention have further completed trials based on such findings, and have confirmed the specific range of the above characteristics to complete the present invention.

The initial casting method in continuous casting of stainless steel according to the present invention will be described in detail below.

The initial mold powder used in the present invention is
Al ₂ O ₃ —SiO ₂ —CaO type inorganic powder is used as the main component, and a low melting point modifier, a heat generating agent and an oxidizer are added to the inorganic powder to obtain the following properties.

(A) Melting point: 900-1,100 ° C (b) Calorific value: 400 Kcal / Kg or more (c) Basicity (CaO / SiO ₂ : 0.7-1.1.

The Al ₂ O ₃ —SiO ₂ —CaO-based inorganic powder is obtained from Portland cement, fly ash, etc. and is also used as a normal mold powder. As an example of its composition, Al ₂ O ₃ is 10% (weight%, the same applies hereinafter), SiO ₂ is 43%, CaO
Is shown to be 47%.

Since this initial mold powder has a high melting temperature (1400 ° C. in the case of the above composition example), Na ₂ O, F, MgO, Fe ₂ O ₃ and B _{2 are} used as low melting point adjusters so as to reach the above melting temperature. At least one of O ₅ and BaO is added.

Further, C and at least Ca-Si of Ca-Si (calcium silicon) are contained as an exothermic agent so as to have exothermicity of the exothermic amount, and KMnO ₄ , Fe as an oxidant for causing the exothermic agent to generate heat. _{At least one of 2} O ₃ and MnO is added.

Since basicity mainly determined by the inorganic powder Al ₂ O ₃ -SiO ₂ -CaO-based, Ca content and Si in Ca-Si to be contained as an exothermic agent
The inorganic powder may be selected in consideration of the amount in advance.

An example of the initial molding powder prepared in this way is shown below.

The value of the basicity shown in this table indicates the CaO / SiO ₂ (weight ratio) of the inorganic powder, but CaO and SiO ₂ produced by the oxidation of the Ca and Si contents of Ca-Si contained as the exothermic agent. It changes with the amount. However, the basicity of the initial mold powder is 0.7 including the change in CaO and SiO _2.
It is in the range of ~ 1.1.

When such an initial mold powder is obtained, stainless molten steel is injected into the mold from the dipping nozzle according to the normal condition, and when the molten metal surface rises to a certain degree (for example, above the discharge port of the dipping nozzle), The above-mentioned initial mold powder was added to the molten metal surface, that is, ² per 1 cm ² of surface area of the molten steel upper surface in the mold.
Dispense as uniformly as possible at a rate of ~ 4g, and when the level of the molten metal reaches a predetermined level, pull out the dummy bar from the lower end of the mold to start continuous casting. This is the initial casting. As the mold powder to be added in the stable period following this initial casting, a commercially available mold powder may be used.

The calorific value of the initial mold powder used in the present invention, the meaning of the limiting value of each characteristic of the melting point and the basicity does not exist independently for each characteristic, and the mutual mutual requisites for achieving the object of the present invention. It is established as an equilibrium, but one characteristic is explained as follows while fixing the other characteristics. If the calorific value is less than 400 Kcal / Kg, the insufficient melting of the initial mold powder that comes into contact with the low temperature molten steel that is first injected into the mold and cooled cannot be compensated. When the melting point exceeds 1100 ° C, the initial mold powder remains in an insufficiently melted state, and when it is less than 900 ° C, conversely, it melts immediately after being charged and mixes into the molten steel, or there is a gap between the inner walls of the mold. Since it is easy to flow into the rules, the strictness of the injection timing becomes necessary. If the basicity (CaO / SiO ₂ ) is less than 0.7 or more than 1.1, the major components of the mold powder (CaO wt%, SiO ₂ wt%) change significantly, so the composition changes greatly and the melting point of the mold powder changes. And its viscosity is greatly changed, which is not preferable. Moreover, lubrication as the initial mold powder, absorption of inclusions,
Adsorption and cooling assistance will not be sufficient. Also, if the input amount is less than 2 g / cm ² , the initial surface of the mold powder cannot be completely covered by the mold powder, and the original functions of the mold powder such as heat retention of the melt surface and absorption / adsorption of non-metallic inclusions If it exceeds 4 g / cm ² , partial excess inflow of molten mold powder into molten steel easily occurs.

The casting object of the present invention is stainless steel, but among them, SUS304 and Ti-containing stainless steel, for example, SUS409, SUS
The effect is remarkable for 321, SUS631, etc.

[Examples and comparative examples]

The initial casting method in continuous casting of stainless steel according to the present invention will be described more specifically with reference to Examples and Comparative Examples.

Similar to the above example, various initial mold powders were prepared for the Al ₂ O ₃ —SiO ₂ —CaO based inorganic powder with respect to the low melting point modifier, the heat generating agent and the basicity. In addition, a conventional initial mold powder was also prepared. These are shown in Table 2.

When performing continuous casting of stainless steel, initial casting was performed by using the above-mentioned initial mold powders at the start. Stainless steel grades to be cast are SUS304 and SUS430.
And SUS410. In addition, the molten metal level when pouring molten steel into the mold from the immersing nozzle inserted from above the mold rises up to about 150 cm after passing the nozzle discharge port to achieve a steady molten metal level. . The amount (g / cm ² ) of the initial mold powder used in the initial casting was set to various amounts, and various positions of the rising metal surface level were taken as the injection timing. When the initial mold powder is added and the slab is pulled out from the mold and the molten metal level reaches a steady state, a low melting point regulator, exothermic agent, and oxidizer are added as normal mold powder. A commercially available product having a basicity in the range of 0.7 to 1.2 was added.

The quality of the inside and the appearance of each of the obtained initial cast pieces (about 5 cm from the tip) was examined. The quality items, survey methods, and evaluation criteria are as follows.

(I) Depletion Investigation method: The appearance of the front and back surfaces of the cast slab is visually observed from the tip of the cast initial slab.

Evaluation Criteria: The length (m) in which depletion is generated from the tip of the initial cast piece is displayed.

 The boundary of quality is generally about 1.5 to 2.0 m.

(Ii) Large inclusions Survey method: 700 mm from the tip of the initial slab, 700 mm
At the position of 1200 mm and the slab (thickness x width) x
A 100 mm long slab is cut out and cut into 1/2 width to obtain a slab sample. After subjecting the slab surface and cross section of the slab sample of this 1/2 width to shear polishing, it is further buffed to a mirror surface to complete a slab sample for investigating large inclusions. This mirror surface is inspected with the naked eye or a magnifying glass at a magnification of about 100 times, and the number of large non-metallic inclusions of 25 μm or more is determined according to a predetermined procedure (position, field size, number of fields,
Count by averaging). Then, the rank is divided into 10 ranks according to the counted number, and the most desirable minimum is evaluated as 1 rank and the maximum is evaluated as 10 ranks.

Evaluation criteria: 1 to 2 ranks are good for the boundary of pass / fail, and 3
~ 10 ranks are scrapped off as defective parts.

(Iii) Stability of molten state of input mold powder Investigating method: The molten state of the initially injected mold powder on the upper surface of the molten steel in the mold is visually observed.

Evaluation criteria: This molten state is good, somewhat unstable, unstable,
Very unstable ranks are classified, and only the good condition is of course the good / bad boundary.

Table 3 shows the results obtained based on the quality items, survey methods, and tables or criteria described above.

From Table 3, when performing the initial casting in the continuous casting of stainless steel according to the method of the present invention, compared to the case where the conventional initial mold powder is used, the depletion defect that occurs on the surface of the initial slab is generated. It can be seen that the length is shortened, large non-metallic inclusions are dramatically reduced, and the quality of the inside of the slab is significantly improved. When any of the three characteristics of the initial mold powder used in the method of the present invention, namely the melting point, the calorific value, the basicity, and the input amount, does not satisfy the conditions specified in the present invention, the improvement effect is recognized. However, when the above conditions are satisfied, it can be seen that there is a considerable allowable range in the closing timing and it is difficult for work mistakes to occur.

〔The invention's effect〕

As described above in detail, the initial casting method in the continuous casting of stainless steel according to the present invention has an initial mold powder used in the continuous casting of stainless steel as a main component.
By adding a low melting point modifier, exothermic agent and oxidizer to the Al ₂ O ₃ -SiO ₂ -CaO system inorganic powder, the calorific value, melting point and basicity are adjusted to be in a specific range in which they are balanced with each other. By adjusting and pouring it into the molten metal at a rate of ² to 4 g / cm2, even if the pouring is done without strict timing, it will always be a stable and stable slab. It has become possible to obtain casting slabs of almost the same quality. Further, in order to stably obtain a slab in a steady state, removal of the deckle generated on the molten metal surface at the initial stage of casting was also extremely reduced.

 Therefore, the following benefits can be obtained.

(1) Yield improvement due to the shorter cutting length of the defective part of the initial cast piece.

(2) Improvement of flaw removal yield in flaw removal carried out to improve the appearance quality of the initial slab.

(3) Reduction of defective coils due to poor quality of the initial slab.

(4) Stabilization of operation in the early stage of casting.

Claims (1)

[Claims] 1. Continuous casting in which a mold powder is introduced into the upper part of a mold while injecting stainless molten steel from a dipping nozzle inserted in a cylindrical mold whose inner wall surface forms a mold surface, and a cast piece is drawn out from below the mold. first upon start using the initial mold powder and an initial mold _{powder, Al 2 O 3 -SiO 2 Na} 2 O to -CaO based inorganic powder as the low melting point modifiers, F, MgO, Fe _{At least one of 2} O ₃ , B ₂ O ₅ and BaO, and at least Ca-Si of C and Ca-Si as an exothermic agent,
KMnO ₄ , Fe ₂ O ₃ and Mn are used as oxidizers that cause the exothermic agent to generate heat.
The following characteristics by adding one or more types of O (a) Melting point: 900-1100 ° C (b) Calorific value: 400 Kcal / Kg or more (c) Powder having basicity (CaO / SiO ₂ : 0.7-1.1) An initial casting method in continuous casting of stainless steel, characterized in that the initial molding powder is added and ^{2 to 4} g per 1 cm ² of the surface area of the molten steel upper surface in the mold is charged.