JP2017205793A - Continuous casting mold flux and continuous casting method of steel using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a mold flux capable of eliminating a stripe-shaped pattern generated in a hot rolled coil.SOLUTION: The present invention controls a content percentage of POin a flux component of a conventionally unconscious continuous casting mold flux in 0.50% or less in mass%. A desirable flux component is 3-25% and C:1-10% in total in one kind or more of components among SiO:20-45%, CaO:30-50%, F:2-15%, LiO, NaO, KO, MgO, SrO, BaO, BO, AlO, TiO, ZrO, FeO, and MnO in mass%, and a residual part is POand an inevitable impurity.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a mold flux for continuous casting and a steel continuous casting method using the same.

  In continuous casting of steel, a mold flux for continuous casting is supplied to the mold surface to lubricate and keep the heat inside the mold. The component of the mold flux for continuous casting is variously set according to the steel type to be cast.

For example, in continuous casting of low carbon steel, the casting speed is higher than other steel types. Therefore, lubrication in the mold is the first, and the basicity of the mold flux (CaO / SiO ₂ ) is set to around 1.0. Al ₂ O ₃ , MgO, Na ₂ O, F, etc. are added to adjust the viscosity and freezing point appropriately. In the case of mold flux for tin steel contained in low-carbon steel, the basicity is set lower than 1.0 in order to prevent inclusion in the molten steel in the mold. , Increase the viscosity.

On the other hand, in the mold flux for continuous casting of medium carbon steel, in addition to lubrication in the mold, it is important to prevent non-uniform solidification. The basicity and F concentration are increased to promote crystallization and prevent non-uniform solidification. ing. These flux components contain a trace amount of P ₂ O ₅ derived from impurities in the raw material. However, the content is not consciously controlled.

On the other hand, Patent Document 1 discloses a mold flux for high Al steel in which a large amount of P ₂ O ₅ of 1.0 to 8.0% is intentionally added to enhance the peelability of the scale. . This mold flux has a basicity of 1.0 to 2.0, and by adding a large amount of P ₂ O ₅ , the melting point of the firelite layer formed between the slab and the scale is lowered. It has improved peelability. However, outside this patent document 1, there is hardly any patent document in which the amount of P ₂ O ₅ added to the mold flux is conscious.

As described above, the conventional continuous casting mold flux excluding Patent Document 1 generally contains about 0.5 to 1.0% of P ₂ O ₅ . Among such conventional continuous casting mold fluxes, those for low carbon steel are also used for continuous casting of low carbon steel to very low carbon steel.

Japanese Patent No. 5083143

  However, when an ultra-low carbon steel having a C content of 0.005% or less is continuously cast and the obtained slab is hot-rolled into a coil, a streak pattern may occur in the rolling direction. This streak pattern is not preferable because it causes spots in hot dip galvanizing applied after cold rolling, and causes deterioration in coil quality and yield. Accordingly, an object of the present invention is to provide a technique capable of eliminating a streak pattern generated in a hot-rolled coil.

In order to solve the above-mentioned problems, the present inventor has examined the streaky pattern generated in the hot-rolled coil of extremely low carbon steel from various angles. As a result of analysis based on the EPMA results, it was confirmed that the P concentration was higher in the streaky pattern portion than in the other portions. Further, the present inventor presumed that this P was derived from the mold flux. That is, P ₂ O ₅ in the mold flux adhering to the slab surface moves through the scale layer generated on the slab surface and is reduced when it comes into contact with Al in the slab, and is concentrated as P on the slab surface. Presumed to be.

The present invention has been completed on the basis of the above-described knowledge. In the present invention, the content of P ₂ O ₅ in the flux component of the continuous casting mold flux, which has not been considered in the past, is set to 0. Control to 50% or less.

Note that flux component of the continuous casting mold flux, in _{mass%, SiO 2: 20~45%,} CaO: 30~50%, F: 2~15%, Li 2 O, Na 2 O, K 2 O MgO, SrO, BaO, B ₂ O ₃ , Al ₂ O ₃ , TiO ₂ , ZrO ₂ , FeO, MnO in a total of 3 to 25%, C: 1 to 10%, the balance P ₂ O ₅ and inevitable impurities are preferable.

  The steel continuous casting method of the present invention is characterized by using the mold flux for continuous casting according to claim 1 or 2.

According to the present invention, since the content of P ₂ O ₅ in the flux component of the mold flux for continuous casting is controlled to 0.50% or less by mass%, even if the mold flux adheres to the surface of the slab, P Concentration can be suppressed. As a result, it is possible to effectively suppress the occurrence of a streak pattern as in the prior art. Therefore, it is possible to improve the quality of the hot rolled coil surface and the hot dip galvanized coil surface and improve the production efficiency.

Is a graph showing the relationship between the P ₂ O ₅ concentration and number of streaky pattern in mold flux.

Embodiments of the present invention will be described below.
The mold flux for continuous casting contains SiO ₂ and CaO as main components, and additionally contains F, Na ₂ O, Al ₂ O _{3 and the} like, and further contains P ₂ O ₅ and inevitable impurities. In the present invention, the content of P ₂ O ₅ in the flux component was controlled to 0.50% or less by mass by selecting a raw material having a small amount of P ₂ O ₅ or purifying the raw material. As shown in the examples described later, when the content ratio of P ₂ O ₅ is 0.50% or less, the generation rate of the streak pattern can be made substantially zero. The lower the content of P ₂ O _5, the better. However, since it takes a purification cost to make it completely zero, it may be set to 0.01 to 0.5%. Hereinafter, other components of the mold flux will be described.

SiO ₂ and CaO are basic components of the mold flux. In a preferred embodiment, they are mass%, and SiO ₂ : 20 to 45% and CaO: 30 to 50%. The basicity is preferably in the range of 1.0 to 1.6. If the basicity exceeds this range, the freezing point increases, which is not preferable.

  Moreover, it is preferable to contain 2 to 15% of F. F is a component added to reduce the solidification temperature and viscosity of the mold flux. If the content is less than 2%, the effect is not obtained. % Range is preferred.

Furthermore, Li ₂ O, Na ₂ O, K ₂ O, MgO, SrO, BaO, B ₂ O ₃ , Al ₂ O ₃ , TiO ₂ , ZrO ₂ , FeO, MnO are combined in a total of 3 components. -25% is contained. These components are also components for adjusting the solidification temperature and viscosity of the mold flux. By adjusting each of the above components, the viscosity at the time of melting is 0.3 to 6.0 poise (1300 ° C.), and the solidification point is 1050. It is preferable to set it as the range of -1250 degreeC. In addition, when the viscosity is lower than 0.3 poise, it becomes easy to be caught in molten steel in the mold, and when it is higher than 6 poise, the lubricity when used at a casting speed of 2.0 m / min or more is stabilized. Disappear. When the freezing point is lower than 1050 ° C., it is difficult to adjust the melting rate, and when it is higher than 1250 ° C., the lubricity in the mold becomes unstable.

C is an aggregate and contains 1 to 10%. Others are P ₂ O ₅ and unavoidable impurities, but as described above, the content of P ₂ O ₅ is 0.50% or less.

  The mold flux described above is suitable for continuous casting of ultra-low carbon steel having a C content of 0.005% or less, and is used by being put into a mold as in the conventional case. By using the mold flux of the present invention, the streak pattern can be eliminated as shown in the following examples.

  A very low carbon steel having the composition shown in Table 1 was cast by a vertical bending type continuous casting machine (hereinafter referred to as a continuous casting machine) to produce a slab as a material for hot rolling. The continuous casting machine was composed of two strands. The mold of each strand was 250 mm in thickness, 1600 mm in width, and the casting speed was 1.5 m / min.

  A single ladle of 250 ton of molten steel was used per casting, and five slabs of 7000 mm in length were cast from each strand, a total of ten. During casting, two different mold fluxes were used for the two strands, and the slab quality was compared. A total of four castings were performed, and a total of eight types of mold fluxes were compared.

Table 2 shows the specifications of the mold flux. SiO ₂ and CaO were combined in a range of 71 to 75% by mass, and the basicity as a concentration ratio thereof was made constant at 1.1. In addition, Al ₂ O ₃ , MgO, Na ₂ O, B ₂ O ₃ , ZrO ₂ and F are added, the viscosity at the time of melting is 2.1 to 2.3 poise (1300 ° C.), and the freezing point is 1080 to 1110. The temperature was adjusted so as to be in the range of ° C. As other impurity components, P ₂ O ₅ and FeO, S were contained in a small amount, but the concentration of P ₂ O ₅ varied in the range of 0.1 to 3.0% among the eight mold fluxes. The concentration of FeO or S was constant at 0.1%.

  The cast slab was not subjected to maintenance such as cutting, but was carried directly into a heating furnace and heated at 1200 ° C. for 2 hours. The heated slab was rolled to a plate thickness of 3 mm by rough rolling followed by finish rolling, and wound into a coil. The coil was pickled, passed through an inspection line, and the number of streak patterns was counted visually. Then, the coil of a galvanized steel plate was finished through each process of cold rolling and galvanization. In the coil of the galvanized steel sheet, whether or not shipment was possible was evaluated.

Table 3 shows the number of streak patterns per coil. In all of the examples of the present invention, no streak pattern was generated by visual inspection, and it was evaluated that it was possible to ship as a steel plate of good quality. On the other hand, in the comparative example, the number of streak patterns increases as the concentration of P ₂ O ₅ in the mold flux increases, and it is necessary to take care to cut out the pattern portion or to downgrade and ship for quality degradation. occured. Further, Comparative Example 5 was evaluated as being temporarily suspended and difficult to ship because there were too many streaky patterns.

These results are shown in FIG. As shown in the figure, it was confirmed that the generation of streak patterns can be prevented by controlling the concentration of P ₂ O ₅ in the mold flux to 5% or less.

Claims (3)

A mold flux for continuous casting, wherein the content of P ₂ O ₅ in the flux component is controlled to 0.50% or less by mass%. In mass% flux _{component, SiO 2: 20~45%, CaO} : 30~50%, F: 2~15%, Li 2 O, Na 2 O, K 2 O, MgO, SrO, BaO, B 2 O ₃ , Al ₂ O ₃ , TiO ₂ , ZrO ₂ , FeO, MnO, 3 to 25% in total, C: 1 to 10% in total, the balance being P ₂ O ₅ and inevitable impurities The mold flux for continuous casting according to claim 1.   A continuous casting method for steel using the mold flux for continuous casting according to claim 1 or 2.

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