JP2014184463A - Mold flux for continuous casting of steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a longitudinal crack caused on a cast metal surface, when continuously casting sub-peritectic steel.SOLUTION: A mold flux used for continuously casting the hypoperitectic steel is provided. The mold flux contains CaO, SiO, alkali metal oxide and a fluorine compound as a basic component, satisfies 1.1≤(CaO)/(SiO)≤1.9, 0.10≤(CaF)/((CaO)+(SiO)+(CaF))≤0.40, and 0≤(fluoride of alkali metal)/((CaO)+(SiO)+(fluoride of alkali metal))≤0.10, and has a coagulation point of 1,300°C or higher, and a viscosity of 1 poise or less at 1,450°C. When the mass concentration of a mold flux inside component (i) is represented as W, the mold flux satisfies (CaO)=(W-(CaF)×0.718), (SiO)=W,(CaF)=(W-W×1.27-W×0.613-W×0.403)×2.05 and (fluoride of alkali metal)=W×1.74+W×1.35+W×1.23. Since both soft cooling effect and maintenance of casting mold inner lubricity can be made compatible, even when an overheat degree of molten steel is large when continuously casting the hypoperitectic steel, the longitudinal crack of the cast metal surface can be prevented.

Description

  The present invention relates to a mold flux for continuous casting of steel capable of preventing longitudinal cracks generated on the surface of a cast slab, particularly when continuously casting hypoperitectic steel having a C concentration of 0.08 to 0.18% by mass. .

  In the continuous casting of steel, hypoperitectic steel with a C concentration of 0.08 to 0.18% by mass is due to the fact that the thickness of the solidified shell formed by solidification of the molten steel in the mold tends to be uneven. Longitudinal cracks are likely to occur on the surface.

  In continuous casting, in order to make the thickness of the solidified shell in the mold uniform, it is effective to gently cool the tip of the solidified shell (hereinafter referred to as slow cooling). It is relatively simple to use.

  The mold flux is supplied to the surface of the molten steel injected into the mold, melts by supplying heat from the molten steel, flows into the gap with the solidified shell along the mold, and forms a film. This film is solidified into a glass shape immediately after the start of casting by cooling from the mold, but crystals precipitate out of the glass over time. When the crystallization of the film is promoted, the roughness of the surface of the film on the mold side increases, so that the interfacial thermal resistance between the mold and the film increases. Moreover, since the radiation heat transfer in a film is also suppressed, the molten steel and solidified shell which touched the film are cooled slowly by these effects.

By the way, the composition of a general crystal precipitated in the film is caspidine (Capidine: Ca ₄ Si ₂ O ₇ F ₂ ).

In order to promote the crystallization of the film, the following methods have been considered so far.
First, when controlling the melt physical properties of the mold flux, increasing the freezing point is an effective method for promoting crystallization. In Patent Document 1, the solidification point is increased to 1150 to 1250 ° C. to enhance the crystallinity. It is disclosed.

  This Patent Document 1 describes that when the freezing point of the mold flux is higher than 1250 ° C., the lubricity between the mold and the solidified shell is hindered and breakout (breakage of the solidified shell and breakage of steel) cannot be prevented. ing.

Moreover, when controlling the chemical component in the mold flux, it is effective to increase the mass concentration ratio of CaO and SiO ₂ (hereinafter referred to as basicity). It is also effective to reduce the MgO concentration.

  For example, Patent Document 2 describes that setting the basicity to 1.2 to 1.6 and setting the MgO concentration to 1.5% by mass or less is effective for film crystallization. However, in the embodiment of the mold flux described in Patent Document 2, the highest crystal formation temperature is 1145 ° C., so that a corresponding slow cooling effect can only be obtained.

  On the other hand, Patent Document 3 discloses a method for suppressing radiant heat transfer in a film by adding iron or an oxide of a transition metal to a mold flux.

However, when these oxides are added, CaO, SiO ₂ and CaF ₂ in the mold flux are diluted. In particular, in the invention described in Patent Document 3, in order to obtain a sufficient effect of suppressing radiant heat transfer, as shown in the examples, a total of 10% by mass or more of iron and transition metal oxides are added. There is a need to. In that case, in the composition having a basicity of around 1.0 shown in the examples, the precipitation of caspidyne becomes difficult and the solidification temperature of the mold flux decreases.

  In addition, considering that the freezing point of the example described in Patent Document 3 is about 1050 ° C. and the freezing point of the mold flux for hypoperitectic steel is about 1150 to 1250 ° C. as described above, the freezing point is 100 It is also lower than ℃. Therefore, as a result, since the crystallization of the film is inhibited, the slow cooling effect due to the crystallization, such as an increase in interfacial thermal resistance, is impaired.

Moreover, in patent document 4 which the inventor proposed previously, the composition range of the mold flux in which caspodyne is easy to precipitate is disclosed in the quaternary system of CaO—SiO ₂ —CaF ₂ —NaF. This compositional range substantially coincides with the primary region of caspodyne according to subsequent reports (ISIJ International, 42 (2002), p489).

  In addition, the inventor added a transition metal oxide to the basic composition adjusted in Patent Document 5 within the scope of the invention described in Patent Document 4, so that the solidification temperature is not impaired without impairing the slow cooling effect. Has proposed a method to reduce

  The invention proposed in Patent Document 5 has a slow cooling effect of a mold flux having a freezing point of 1250 ° C. or higher, which has been conventionally considered difficult to use because the lubricity is hindered. In the general freezing point range of 1209 to 1239 ° C.

  However, even in the casting of hypoperitectic steel, the degree of superheat of the molten steel (molten steel temperature and liquid temperature), as in the case of casting a steel type containing alloy elements such as Cu, Ni, Cr, Mo, Nb, V, Ti, B, etc. When the difference between the phase line temperature and the phase line temperature is large, vertical cracks are more likely to occur.

  When the superheat degree of molten steel is large at the time of casting such subperitectic steel, even the inventions described in Patent Documents 4 and 5 proposed by the inventor are sufficient for preventing or suppressing vertical cracks. May not be effective.

JP-A-8-197214 JP-A-8-141713 JP-A-7-185755 JP 2001-179408 A JP 2006-289383 A

  The problem to be solved by the present invention is an alloy such as Cu, Ni, Cr, Mo, Nb, V, Ti, B, etc., even with a mold flux in which transition metal oxides are added to the composition range to facilitate precipitation of caspidyne. When the subperitectic steel containing the element is continuously cast, a sufficient effect may not be obtained.

The mold flux for continuous casting of steel of the present invention is:
When continuously casting hypoperitectic steel with a C concentration of 0.08-0.18% by mass, like steel types containing alloy elements such as Cu, Ni, Cr, Mo, Nb, V, Ti, B, etc. In order to prevent vertical cracks occurring on the surface of the slab even when the degree of superheat is large,
Using CaO, SiO ₂ , alkali metal oxide, and fluorine compound as basic components, satisfy the following (1), (2), and (3), and have a freezing point of 1300 ° C or higher and a viscosity at 1450 ° C of 1poise or lower Is the most important feature.
1.1 ≦ f (1) ≦ 1.9… (1)
0.10 ≦ f (2) ≦ 0.40… (2)
0 ≦ f (3) ≦ 0.10… (3)
However, f (1) = (CaO) _h / (SiO ₂ ) _h (i)
f (2) = (CaF ₂ ) _h / ((CaO) _h + (SiO ₂ ) _h + (CaF ₂ ) _h ) (b)
f (3) = (alkali metal fluoride) _h / ((CaO) _h + (SiO ₂ ) _h
+ (Alkali metal fluoride) _h )… (c)
(CaO) _h = (W _CaO − (CaF ₂ ) _h × 0.718)… (A)
(SiO ₂ ) _h ＝ W _SiO2 … (B)
(CaF ₂ ) _h = (W _F −W _Li2O × 1.27−W _Na2O × 0.613−W _K2O × 0.403) × 2.05… (C)
(Alkali metal fluoride) _h = W _Li2O x 1.74 + W _Na2O x 1.35 + W _K2O x 1.23… (D)
Here, W _i is the mass concentration (mass%) of the component i in the mold flux.

  In the present invention, by further increasing the solidification point of the mold flux to a range of 1300 ° C. or higher, which is not considered so far, further crystallization is promoted to obtain a slow cooling effect. In addition, the viscosity at 1450 ° C. is sufficiently reduced to 1 poise or less to maintain the lubricity in the mold.

  In the present invention, further crystallization of the film to be formed is promoted to obtain a slow cooling effect, and it is possible to simultaneously maintain lubricity in the mold, so Cu, Ni, Cr, Mo, Nb, V, Ti, Even when hypoperitectic steel containing an alloy element such as B is continuously cast, it is possible to prevent vertical cracks occurring on the surface of the slab.

In the present invention, when continuously casting hypoperitectic steel containing alloy elements such as Cu, Ni, Cr, Mo, Nb, V, Ti, B, etc., it prevents vertical cracks that occur on the surface of the slab. The purpose is to adjust CaO, SiO ₂ , alkali metal oxides, and fluorine compounds to the optimal range and maintain the composition within the primary crystal range of caspodyne, while setting the freezing point to 1300 ° C or higher and the viscosity at 1450 ° C to 1poise or lower It was realized by doing.

Hereinafter, the mold flux for continuous casting of steel of the present invention will be described.
Patent Document 1 described above describes that when the freezing point is higher than 1250 ° C., the lubricity is hindered and breakout cannot be prevented. In the invention described in Patent Document 1, the proper viscosity of the mold flux is 0.6 to 2.5 poise at 1300 ° C., but many of the examples described in Patent Document 1 are 1 poise or more.

  By the way, in order to maintain the lubricity in the mold at the time of continuous casting, it is necessary to reduce the resistance force (friction force in the mold) when the solidified shell is pulled out below the mold. Since the mold flux is interposed between the inner wall of the mold and the solidified shell, the frictional force can be reduced by reducing the viscosity of the mold flux.

  When the viscosity of the mold flux is lowered, the mold flux is easily caught in the molten steel in the mold, and the entrained mold flux droplets become non-metallic inclusions near the surface of the slab, deteriorating its cleanliness. There is a problem.

  However, even if the viscosity at 1300 ° C is lowered to 1 poise or less, it is possible to prevent entrainment if the composition of the mold flux is kept basic. Have been described.

  The present invention is based on the above concept in a mold flux used for continuous casting of hypoperitectic steel having a C concentration of 0.08 to 0.18% by mass.

In the present invention, the basic components are CaO, SiO ₂ , and a fluorine compound, which are constituents of caspidine. In addition, an alkali metal oxide is added so that the freezing point can be adjusted relatively easily.

Then, the respective concentrations of CaO, SiO ₂ , fluorine compound, and alkali metal oxide are adjusted to the ranges of the following formulas (1), (2), and (3) that facilitate crystallization of caspodyne.

1.1 ≦ f (1) ≦ 1.9… (1)
0.10 ≦ f (2) ≦ 0.40… (2)
0 ≦ f (3) ≦ 0.10… (3)
However, f (1) = (CaO) _h / (SiO ₂ ) _h (i)
f (2) = (CaF ₂ ) _h / ((CaO) _h + (SiO ₂ ) _h + (CaF ₂ ) _h ) (b)
f (3) = (alkali metal fluoride) _h / ((CaO) _h + (SiO ₂ ) _h
+ (Alkali metal fluoride) _h )… (c)
(CaO) _h = (W _CaO − (CaF ₂ ) _h × 0.718)… (A)
(SiO ₂ ) _h ＝ W _SiO2 … (B)
(CaF ₂ ) _h = (W _F −W _Li2O × 1.27−W _Na2O × 0.613−W _K2O × 0.403) × 2.05… (C)
(Alkali metal fluoride) _h = W _Li2O x 1.74 + W _Na2O x 1.35 + W _K2O x 1.23… (D)
Here, W _i is the mass concentration (mass%) of the component i in the mold flux.

By adjusting the concentrations of CaO, SiO ₂ , fluorine compound, and alkali metal oxide to satisfy the conditions of the above formulas (1), (2), and (3), the composition of the mold flux is changed to the primary crystal of caspodyne. It becomes possible to maintain the range.

  According to the inventor's investigation results, a more desirable range of f (1) is 1.2 to 1.8, and further 1.3 to 1.7. Further, a more desirable range of f (2) is 0.12 to 0.35, and further 0.15 to 0.30. A more desirable range of f (3) is 0.08 or less.

  The viscosity of the mold flux is usually based on 1300 ° C. However, in the present invention in which the composition was maintained in the primary crystal range of caspidine with a melting point of 1410 to 1420 ° C., the mold flux was already solidified by crystallization at 1300 ° C., and the value at 1300 ° C. cannot be obtained. The viscosity at ℃ is 1 poise or less.

  With such a viscosity, it is possible to maintain lubricity even in a state where the freezing point is raised to 1300 ° C. or higher, which has been considered difficult in the past. As the freezing point increases, the effect of slow cooling in the mold increases accordingly. However, in the present invention, it is impossible to raise the freezing point of the mold flux above the melting point of cuspidine.

  In addition, in order to increase the strength when it becomes a steel material, not only a little Mn is added to the subperitectic steel that is put into practical use, so MnO generated by oxidation of Mn in the molten steel during casting is contained in the mold flux. Migrate to

  Since this MnO is a component that inhibits crystallization of caspidyne, the freezing point is raised to 1300 ° C or higher after blending MnO in advance in an amount that matches the MnO concentration that rises due to MnO migrating into the mold flux. It is desirable to keep it. According to the inventor's investigation, the desired concentration of MnO is 0.1 to 10% by mass, and should be set according to the Mn concentration in the molten steel.

In some cases, MgO, Al ₂ O ₃ , BaO, B ₂ O ₃ or the like may be added in order to adjust physical properties such as the freezing point, viscosity, and surface tension of the mold flux. However, in order to promote crystallization of caspodyne, these concentrations should be low, and it is desirable not to exceed 7 mass% in total concentration. When ordinary raw materials are used, the total concentration of these unavoidably contained components is about 2 to 5% by mass, but it can also be reduced by using artificial raw materials such as a premelt base material.

  Next, the results of experiments conducted to confirm the effects of the present invention will be described.

Example 1
Various invention examples and comparative examples of the mold flux of the present invention shown in Tables 1 and 2 below were prepared. Here, f (1), f (2) and f (3) shown in Table 2 below are indices calculated by the above-mentioned formulas (a), (b) and (c), By adjusting these indices within the ranges of the formulas (1), (2), and (3), crystallization of caspodyne in the mold flux can be promoted.

  The compositions of the mold fluxes of Invention Examples A to H all satisfy the above formulas (1) to (3), the freezing point is 1300 ° C. or higher, and the viscosity at 1450 ° C. is 1 poise or less. On the other hand, as for the composition of the mold flux of Comparative Examples a to c, as a result of the composition of the mold flux not satisfying any of the formulas (1) to (3), at least one of the freezing point and the viscosity at 1450 ° C. is This is outside the scope of the present invention. In Tables 1 and 2, those marked with * are outside the scope of the present invention.

  The inventive examples A to H and the comparative examples a to c were used for continuous casting of a subperitectic steel having a composition shown in Table 3 below and containing Nb and Ti and having a high degree of superheat, and the results were compared. Here, 2.5 tons of molten steel is used, a slab having a width of 500 mm and a thickness of 85 mm is used using a vertical bending type continuous casting machine, the casting speed is 1.0 m / min, and the specific water amount of the secondary cooling water is 1.1 liters. / Kg.

  The mold flux added to the mold is properly used, and for the slow cooling effect, the local heat flow rate in the mold, the surface temperature of the slab, the thickness of the solidified shell and the growth rate are obtained, and the lubricity is the friction force in the mold. And compared the obtained results. The results are summarized in Table 4 below.

  The local heat flow rate in the mold was evaluated based on the temperature measured with a thermocouple embedded at a position 35 mm below the meniscus at the center of the width of the long side surface. The frictional force in the mold was obtained from the pressure difference of the hydraulic cylinder used for the mold oscillation. The temperature of the slab surface was measured at the center of the inner curved side width of the roll segment immediately before the bending portion of the vertical bending die continuous casting machine. In addition, the thickness and growth rate of the solidified shell were determined by adding the FeS alloy to the molten steel in the mold immediately before the end of casting, cutting the slab at that part in the casting direction, and the concentration distribution of S on the cut surface. Was evaluated by a method of transferring to a photographic paper.

As a result of evaluating the local heat flow rate in the mold, it was 1.48 MW / m ² or more in the comparative example, whereas it decreased to 1.42 MW / m ² or less in the inventive example, and a slow cooling effect could be confirmed. .

In addition, the friction force in the mold is 2.09 × 10 ⁻² (N / mm ² ) or less for both the invention example and the comparative example, and there is no problem in lubricity, and a normal oscillation mark is present on the slab surface. Formed at regular intervals.

  Moreover, the measurement result of the slab surface temperature showed that the temperature when using the mold flux of the invention example was higher than that when using the mold flux of the comparative example, and the slow cooling effect could be confirmed.

Furthermore, the results of evaluation of the thickness of solidified shell and the growth rate, the solidification coefficient whereas Comparative Example is 18.1～24.8Mm / min ^2, invention examples 11.2～13.8Mm / min ² A clear slow cooling effect on the growth of the solidified shell was confirmed.

  In the inventive examples, the obtained slab had good surface properties, and there were no surface defects such as vertical cracks or depletion. On the other hand, in Comparative Example c, vertical cracks having a length of about 100 mm were present at two locations in the center of the width.

(Example 2)
Of the mold fluxes tested in Example 1, continuous casting on a larger scale than Example 1 was performed using Invention Example A and Comparative Example a.

  For each casting using each mold flux, 300 tons of molten steel having the composition shown in Table 5 below was provided, and 11 slabs having a width of 2300 mm, a thickness of 300 mm, and a length of 6 m were cast at a speed of 0.70 m / min. The result of the surface of the obtained slab was as follows.

  In Invention Example A, 12 slabs having a good surface with no vertical cracks were obtained and could be supplied to the rolling process as they were.

  On the other hand, in Comparative Example a, vertical cracks occurred on the surfaces of the first, second, eleventh, and twelfth slabs after the start of casting. And all the slabs in which vertical cracks occurred required maintenance.

  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)

A mold flux used for continuous casting of hypoperitectic steel having a C concentration of 0.08 to 0.18% by mass,
Using CaO, SiO ₂ , alkali metal oxide, and fluorine compound as basic components, satisfy the following (1), (2), and (3), and have a freezing point of 1300 ° C or higher and a viscosity at 1450 ° C of 1poise or lower A mold flux for continuous casting of steel.
1.1 ≦ f (1) ≦ 1.9… (1)
0.10 ≦ f (2) ≦ 0.40… (2)
0 ≦ f (3) ≦ 0.10… (3)
However, f (1) = (CaO) _h / (SiO ₂ ) _h (i)
f (2) = (CaF ₂ ) _h / ((CaO) _h + (SiO ₂ ) _h + (CaF ₂ ) _h ) (b)
f (3) = (alkali metal fluoride) _h / ((CaO) _h + (SiO ₂ ) _h
+ (Alkali metal fluoride) _h )… (c)
(CaO) _h = (W _CaO − (CaF ₂ ) _h × 0.718)… (A)
(SiO ₂ ) _h ＝ W _SiO2 … (B)
(CaF ₂ ) _h = (W _F −W _Li2O × 1.27−W _Na2O × 0.613−W _K2O × 0.403) × 2.05… (C)
(Alkali metal fluoride) _h = W _Li2O x 1.74 + W _Na2O x 1.35 + W _K2O x 1.23… (D)
Here, W _i is the mass concentration (mass%) of the component i in the mold flux.   The mold flux for continuous casting of steel according to claim 1, further comprising 0.1 to 10% by mass of MnO.