JP2016190261A - Mold flux for continuous casting - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method that effectively suppresses crystallization of a cast piece (solidifying shell) side of a flux film flowing and formed between a mold and the cast piece (solidifying shell).SOLUTION: Mold flux for continuous casting is characterized in that main crystal crystallized or precipitated in the mold flux solidified after being melted once is melilite solid solution between gehlenite and akermanite, and a portion of calcium contained in the melilite is replaced by strontium, and SrO/(CaO+SrO) defined in atomic%, namely, the SrO substitution ratio is in a range of 0.3-0.7, AlO/(AlO+MgO) is in a range of 0.4-0.8 and (CaO+SrO)/SiOis 1.1-2.0, and a solidification temperature is decreased by 70°C or more in the case of 10°C per minute in comparison with the case when a cooling speed is 2°C per minute.SELECTED DRAWING: None

Description

  The present invention relates to a mold flux that has the characteristics of being difficult to be caught in molten metal in continuous casting of steel or the like and the lubricity in the mold, and particularly relates to a mold flux having excellent lubricity in the mold. is there.

  The mold flux added in the mold is melted to form a molten layer having a thickness of several mm to several tens of mm. The melted mold flux flows between the mold and the slab (solidified shell) to form a flux film. The mold side of the flux film is cooled and solidified to crystallize or precipitate crystals (hereinafter abbreviated as crystal / precipitate).

  The crystal layer thus formed exhibits a necessary slow cooling effect by an action such as shielding radiant heat transfer. On the other hand, the region in which the glass film is kept without crystallization on the slab (solidified shell) side of the flux film exhibits the effect of reducing the frictional resistance between the slab and the mold.

  That is, in order to maintain good lubricity in the mold, a certain amount of glass phase is required to remain in the flux film.

Conventionally, the most common caspidine (Cuspidine: 3CaO · 2SiO ₂ · CaF ₂ ), akermanite (Akermanite: 2CaO · MgO · 2SiO ₂ ), akermanite and gehlenite (Gehlenite: 2CaO · Mellilite, which is a solid solution of Al ₂ O ₃ · SiO ₂ ), is known.

  The inventor discloses a mold flux using akermanite or melilite as a main crystal phase in Patent Documents 1 and 2.

  Further, the inventor discloses in Patent Document 3 that a melilite crystal mold flux that simultaneously satisfies the three requirements of low fluorine concentration, high basicity, and lubricity.

JP 2003-225744 A Japanese Patent Laid-Open No. 2005-40835 JP 2010-125457 A

  As a result of the inventor's research and development that maximizes the lubricity of the mold flux that uses the merilite containing SrO disclosed in Patent Document 3 as the main crystal and that can be applied to steel types that require high lubricity. , SrO-containing melilite specifically suppressed crystallization, and found a composition region where a lot of glass remained in the flux film.

The present invention has been made by the above discovery by the inventor, and is a mold flux disclosed in Patent Document 3 whose lubricity is high in a mold flux that is difficult to be caught in molten steel because of high viscosity and low SiO ₂ activity. The purpose is to dramatically increase compared to flux and to facilitate application to a wide range of casting conditions (such as molten steel composition and mold size).

  The inventor has found that the main crystal which is crystallized / precipitated in the mold flux which has been once melted and solidified is melilite which is a total solid solution of gelenite and akermanite, and a part of calcium contained in melilite is placed in strontium. In the alternative mold flux, we found a composition region where crystallization was specifically suppressed and a lot of glass remained after solidification.

  Specifically, in the composition area, melilite, which is a solid solution of all akermanite and gehlenite, the proportion of gehlenite in the composition is more than 40%, and the proportion of SrO in the total concentration of CaO and SrO (atomic It is a composition region having a composition ratio (greater than 30%).

The present invention has been made based on the above discovery of the inventor,
Continuous casting in which the main crystal that is crystallized / precipitated in the mold flux that has been once melted and solidified is the total solid solution melilite of gelenite and akermanite, and a part of the calcium contained in the melilite is replaced by strontium For mold flux
The most important feature is that the composition ratio of crystallizing / precipitating a crystal having a composition in which the ratio of galenite in the composition of melilite is high and the ratio of CaO and SrO is rich in SrO is used.

The total solid solution melilite of gehlenite and akermanite can take any composition between gehlenite and akermanite. The fact that the proportion of gehlenite in the composition of melilite is higher than a certain value expresses that the proportion of gehlenite in the composition of melilite is higher. Specifically, it means that Al ₂ O ₃ / (Al ₂ O ₃ + MgO) defined by atomic% is in the range of 0.4 to 0.8.

Al ₂ O ₃ / (Al ₂ O ₃ + MgO) defined by atomic% is in the range of 0.4 to 0.8. The composition of melilite, which is a total solid solution of akermanite and gehlenite, is first galenite. It is shown that the ratio is more than 40%, and further, the upper limit value of the ratio of gehlenite is 80%. This is because if the ratio of gehlenite in the composition of melilite, which is a solid solution of all percentages, exceeds 80%, which is the upper limit, the melting point of melilite increases and the lubricity deteriorates.

The concentration ratio of MgO and Al ₂ O ₃ is an indicator of the composition ratio of akermanite and gehlenite, whereas akermanite has a composition ratio of 2CaO · MgO · 2SiO ₂ and contains MgO but does not contain Al ₂ O _3. There is because not including MgO include Al ₂ O ₃ composition ratio of _{2CaO · Al 2 O 3 · SiO} 2.

  In addition, the composition region in which a crystal having a composition in which the ratio of CaO and SrO is rich in SrO is crystallized / precipitated is specifically SrO / (CaO + SrO) defined by atomic%, that is, the SrO substitution ratio is 0.3. It is in the range of -0.7.

  SrO / (CaO + SrO) defined by atomic%, that is, the SrO substitution ratio is in the range of 0.3 to 0.7, the proportion of SrO in the total concentration of CaO and SrO (atomic composition ratio) is from 30% The upper limit that can be implemented is 70%. This is because if the SrO substitution ratio exceeds 70% of the upper limit, the mold flux becomes expensive.

Furthermore, in addition to the regulation of the concentration ratio of MgO and Al ₂ O ₃ described above, when (CaO + SrO) / SiO ₂ defined by atomic% is 1.1 to 2.0, crystallization is specifically suppressed and solidified. It is a discovery that forms the basis of the present invention that a lot of glass remains later.

When (CaO + SrO) / SiO ₂ defined by atomic% is smaller than 1.1, in addition to this, the tension at the interface with the molten steel decreases due to the increase in the SiO ₂ activity, and it becomes easy to get caught in the molten steel. Because it ends up. On the other hand, if (CaO + SrO) / SiO ₂ defined by atomic% is larger than 2.0, the crystallization inhibiting action is lost.

  The characteristic that crystallization is specifically suppressed appears when the solidification temperature decreases by 70 ° C. or more when the cooling rate is 10 ° C./min, compared to when the cooling rate is 2 ° C./min. When the crystallization inhibitory action appears most strongly, it may solidify in a glassy state at a cooling rate of 10 ° C. per minute, and the solidification temperature may not be defined. In that case, the solidification temperature difference between 2 ° C. and 10 ° C. per minute is infinite. For these reasons, the upper limit value of the solidification temperature difference at 2 ° C./min and 10 ° C./min is infinite.

  In the case of mold flux that promotes crystallization, the solidification temperature drop is about 20 to 40 ° C. when the cooling rate is 2 ° C. and 10 ° C. per minute. On the other hand, the mold flux in which the solidification temperature is reduced by 70 ° C. or more when the cooling rate is 2 ° C. per minute is higher than the case where the solidification temperature is 10 ° C./min. That is, during casting, a lot of glass remains in the flux film, and the lubricity in the mold is kept good.

  The present invention can effectively suppress crystallization on the slab (solidified shell) side of the flux film formed by flowing between the mold and the slab (solidified shell).

  Although not specified in the present invention, it is desirable from the viewpoint of improving lubricity that the absolute value of the solidification temperature when the cooling rate is 2 ° C. per minute is 1180 ° C. or less.

  In the present invention, the absolute value of the solidification temperature is not particularly specified when the cooling rate is 10 ° C./min compared to the case where the cooling rate is 2 ° C./min. This is because the absolute value of the solidification temperature when the cooling rate is 2 ° C. per minute naturally satisfies the condition of 1180 ° C. or less.

In the present invention, the TiO ₂ concentration is desirably ₂ atomic% or less. This is because, when the TiO ₂ concentration exceeds ₂ atomic%, high melting point perovskite (CaO · TiO ₂ ) crystallizes / precipitates as a secondary crystal, and the effect of suppressing crystallization is impaired. A more desirable upper limit of the TiO ₂ concentration is 1 atomic% or less.

  In the present invention, since crystallization on the slab (solidified shell) side of the flux film formed by flowing between the mold and the slab (solidified shell) can be effectively suppressed. Lubricity is dramatically improved with respect to the disclosed mold flux, and application to a wide range of casting conditions (such as molten steel composition and mold size) is facilitated.

  In the present invention, a composition in which the ratio of gehlenite in the composition of melilite is increased and the ratio of CaO and SrO is rich in SrO for the purpose of dramatically improving the lubricity with respect to the mold flux disclosed in Patent Document 3. This was realized by making the crystal of the crystal / crystallized composition region.

Hereinafter, the results of tests conducted to confirm the effects of the present invention will be described.
The test was performed using a test continuous casting machine having a mold with an inner surface of 85 mm × 500 mm, and the composition was mass%, 0.05% C, 0.01% Si, 0.3% Mn, 0.01 The low carbon steel of% P, 0.01% S, 0.01% sol.Al was cast at a drawing speed of 2.0 m / min.

Examples and comparative examples of the present invention are shown in Table 1 below.
The difference in solidification temperature in Table 1 below indicates the difference between the solidification temperature when the cooling rate is 10 ° C. per minute and the solidification temperature when the cooling rate is 2 ° C. per minute.

  In addition, the lubricity in Table 1 below is determined by measuring the frictional resistance in the mold at the time of the test casting, and when the value increases by less than 10% with respect to Example A, the upper is 10% or more and 20%. If it increased by less than%, it was determined to be B, and if it increased by 20% or more, it was determined to be 丙.

  Moreover, the entrainment defect of the flux in the following Table 1 investigates the residue after the surface layer 10 mm of the long side of the slab obtained by the test casting is cut out 100 mm in the full width 500 mm × drawing direction and dissolved in the acid, An index representing the result of counting the number of spherical inclusions having a diameter of 80 μm or more. In Example A, the case of less than 2 times the case A, the case of 2 times or more and less than 5 times the case of B, the case of 5 times or more Judged as 丙.

  A to D in Table 1 are examples of the present invention, Examples A to C are examples that satisfy the requirements of claims 1 and 2, and Example D is an example that satisfies only the requirements of claim 1. .

  Examples A to C satisfying the requirements of claims 1 and 2 have characteristics that are difficult to be caught in molten steel when used for continuous casting, and are excellent in lubricity because crystallization of the flux film is suppressed.

  Further, Example D is an example in which the effect of the present invention was exhibited although the perovskite crystallized / precipitated and the lubricity was slightly lowered because the requirement of claim 2 was not satisfied.

  On the other hand, E to H in Table 1 are comparative examples that do not satisfy the requirements defined in claim 1 of the present invention.

In Comparative Example E, SrO / (CaO + SrO) defined in atomic%, that is, the SrO substitution ratio is small, and in Comparative Example F, Al ₂ O ₃ / (Al ₂ O ₃ + MgO) defined in atomic% is small and close to akermanite. Since it is a composition and does not satisfy the requirements defined in claim 1 of the present invention, crystallization of the flux film is excessively promoted and the lubricity in the mold is inferior. In these comparative examples E and F, the solidification temperature did not decrease so much when the cooling rate was 10 ° C. per minute compared with the case where the cooling rate was 2 ° C. per minute, and the effect of suppressing crystallization was insufficient. .

In Comparative Example G, (CaO + SrO) / SiO ₂ defined in atomic% is lower than the requirement defined in claim 1 of the present invention. Although excellent, the SiO ₂ activity was high, and many entanglements in the molten steel occurred.

Further, Comparative Example H is an example that does not satisfy the requirement defined in claim 1 of the present invention because the main crystal that is crystallized / precipitated in the mold flux that has been once melted and then solidified is caspodyne. When caspidine is the main crystal, crystallization cannot be suppressed even if the other requirements specified in the present invention are satisfied. Therefore, in Comparative Example H, B ₂ O ₃ which is a lower oxide is added to crystal. Suppression was suppressed. As a result, the tension at the interface with the molten steel decreased, and the mold flux was likely to occur.

An object of the present invention is to sufficiently enhance the lubricity in a mold in a mold flux that has high interfacial tension and viscosity and is difficult to be wound into molten steel. Therefore, it goes without saying that it is preferable that the concentration of lower oxides such as FeO, MnO, and B ₂ O ₃ that cause a decrease in interfacial tension is low. Specifically, the concentration of these lower oxides is desirably 1.5 atomic% or less.

  The present invention is not limited to the above example, and it goes without saying that the embodiments may be changed as appropriate within the scope of the technical idea described in each claim.

Claims (2)

The main crystal that crystallizes or precipitates in the mold flux that has solidified after being melted is the total solid solution melilite of Gehlenite and Akermanite, and one of the calcium contained in the melilite. In mold flux for continuous casting where the part is replaced by strontium,
SrO / (CaO + SrO) defined by atomic%, that is, SrO substitution ratio is in the range of 0.3 to 0.7, Al ₂ O ₃ / (Al ₂ O ₃ + MgO) defined by atomic% is 0.4 to 0.8 (CaO + SrO) / SiO ₂ defined by atomic%, 1.1 to 2.0,
A mold flux for continuous casting, wherein the solidification temperature decreases by 70 ° C. or more when the cooling rate is 10 ° C./min, compared to 2 ° C./min. The mold flux for continuous casting according to claim 1, wherein the TiO ₂ concentration is ₂ atomic% or less.