JP2016002591A - Mold flux for continuous casting and continuous casting method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a mold flux for continuous casting and a continuous casting method capable of suppressing the occurrence of a surface flaw and the occurrence of breakout by optimally performing a control of rim produced along a wall surface of a casting mold on a meniscus position.SOLUTION: A mold flux for continuous casting primarily comprises CaO and SiO. Therein, CaO/SiOis 0.90 to 1.20, MgO is 2 to 7 mass%, NaO is 3 to 7 mass%, a solidification temperature is 1050 to 1150°C and a viscosity at 1300°C is 1.0 to 3.0 P. NaO/MgO is set less than 1.0, and an Fe source contained on a surface of the mold flux before fusion is set to be 0.2 mass% or more. A low-carbon steel slab having a C-concentration of 0.02 to 0.07 mass% is cast by using the mold flux.

Description

  The present invention relates to a mold flux that is supplied into a mold and covers the surface of molten steel during continuous casting, and a continuous casting method using the mold flux. In the present invention, in particular, when continuously casting low carbon steel having a C concentration of 0.02 to 0.07% by mass, a powder rim generated by sintering the mold flux along the mold wall surface (hereinafter simply referred to as a rim). The purpose is to suppress surface flaws and breakouts that occur due to the above.

  During continuous casting of steel, the mold flux keeps the molten steel in the mold, shields the molten steel from the atmosphere, melts and removes nonmetallic inclusions in the molten steel, lubricates between the mold and the solidified shell, and controls the growth of the solidified shell. Plays a big role.

With regard to such mold fluxes, conventionally, there have been many proposals for mold flux films that have been developed with a focus on CaO / SiO ₂ (hereinafter also referred to as basicity), solidification temperature, and viscosity, and formed between the mold and the solidified shell. There are many (for example, Patent Documents 1 and 2).

  However, the factors affecting the castability and slab quality are not only the mold flux film but also the mold wall surface in the molten steel surface position (hereinafter referred to as meniscus position) in the process of mold flux flowing between the solidified shell and the mold. The rim generated along the line is also a major factor.

Japanese Patent No. 3226829 JP 2012-66304 A

  The problem to be solved by the present invention is that most of the conventional techniques proposed for the mold flux are related to the mold flux film, and the rim has not been considered.

The present invention
In order to optimally control the rim generated along the mold wall surface at the meniscus position,
CaO, and SiO ₂ as the main component, CaO / SiO ₂ is 0.90 to 1.20, MgO is 2-7 wt%, with Na ₂ O is 3-7% by mass, the solidification temperature of 1050-1,150 ° C., a viscosity at 1300 ° C. 1.0 to 3.0 poise mold flux for continuous casting,
The main features are that Na ₂ O / MgO is less than 1.0 and the Fe source contained on the mold flux surface before melting is 0.2 mass% or more.

In the present invention, the Fe source refers to all compounds containing Fe, and is appropriately selected in consideration of molten steel contamination in the component range required for the product. For example, scales such as FeO and Fe ₂ O ₃ are examples.

  In the present invention, the fact that the Fe source is contained on the surface of the mold flux before melting refers to a state in which the Fe source is attached to the surface side portion of the granular mold flux in contact with the rim.

In the present invention, the strength of the rim generated in the meniscus is increased by setting the Na ₂ O / MgO, which is the concentration ratio of the low melting point compound MgO ₂ and Na ₂ O, to less than 1.0, and the slab due to rim collapse Suppresses biting of the rim into the Further, the solidification temperature of the mold flux is lowered by setting the Fe source contained on the mold flux surface before melting to 0.2 mass% or more.

  In the present invention, rim generation is promoted by mainly using MgO, which has a high surface tension in the liquid phase, as the low melting point compound that plays a role in binder formation. At the same time, the presence of an Fe source serving as a binder in rim generation on the surface of the mold flux promotes rim generation and secures strength. Therefore, it is possible to effectively suppress the occurrence of surface flaws and breakout due to the collapse of the rim.

It is a figure explaining the generation | occurrence | production mechanism of the surface wrinkles and breakout which generate | occur | produce by the biting of a rim, (a) is explanatory drawing of the growth process of a rim, (b) is explanatory drawing of the rim collapse and the winding process to the solidification shell, (C) is explanatory drawing of the process in which a rim fuse | melts and a surface flaw and a breakout generate | occur | produce. It is the figure which showed the influence on the surface tension by the raise of MgO source. FIG. 4 is a diagram for explaining the principle that the rim strength can be secured by increasing the MgO source, (a) is a diagram explaining that the rim is firmly held by the surface tension of the liquid phase, and (b) is the surface tension of the liquid phase. It is a figure explaining that a rim | limb is missing by disturbance and is weak. It is the figure which showed the surface flaw occurrence rate of the prior art example, the comparative examples 1 and 2, and the invention examples 1 and 2. It is a diagram showing the relationship of Na ₂ O / MgO and surface flaws incidence. It is the figure which showed the relationship between the Fe source concentration contained in the mold flux surface before melting, and the surface flaw occurrence rate.

  The inventors used SEM-EDS analysis (Scanning Electron Microscope: SEM), Si / Li semiconductor detector and multi-wave height analyzer attached to the surface of the continuous cast slab. As a result of the energy dispersion method (Energy Dispersive Spectroscopy: EDS) that identifies the element from the peak energy of the X-ray spectrum and quantitatively analyzes it from the peak height, Ca, Si, Mg, etc. were detected from the heel. . Since these are components contained in the mold flux slag, it was confirmed that surface defects occurred due to the mold flux.

  On the other hand, the form of the surface flaw is not a form in which mold flux is bitten, but is accompanied by leakage of molten steel in the mold, and the cause is that the mold flux follows the inner wall surface of the mold during continuous casting. It was confirmed that the rim generated by sintering was broken and caught in the solidified shell.

  That is, the rim 1 that causes surface flaws is generated when the mold flux 2 is sintered along the inner wall surface of the mold 3 during continuous casting (see FIG. 1A). When the rim 1 is fragile and collapses during continuous casting, the broken rim 1a bites into the solidified shell 4 (see FIG. 1B). As a result, the rim 1a bitten by the solidified shell 4 is melted by the heat held by the molten steel 5, and the molten steel 5 leaks from the melted portion to cause surface defects and breakout (see FIG. 1C).

The inventors conducted a study focusing on the distribution amount of Na ₂ O and MgO, which are low melting point compounds that affect the rim formation, in order to suppress surface defects and breakout caused by the rim. On the other hand, the presence state of the Fe source contained in the mold flux surface before melting, which becomes a binder for rim generation, was also considered and examined.

First, by increasing the amount of Fe source contained on the surface of the mold flux before melting, the solidification temperature of the mold flux mainly composed of CaO ₂ and SiO ₂ is lowered. As a result, the generation of the liquid phase that plays the role of a binder in the generation of the rim is promoted, and the rim is likely to grow.

On the other hand, in the low melting point compound, it is known that when MgO is contained, the surface tension becomes larger than when Na ₂ O is contained (see FIG. 2). Therefore, by increasing the content of MgO, the bonding strength between the high melting point compounds that are the main components in the generation of the rim is strengthened, and the generation of the rim is promoted (see FIG. 3A).

Further, in the mold flux mainly composed of CaO ₂ and SiO ₂ , when Na ₂ O / MgO is 1.0 or more, the rim to be produced is in a solid-liquid coexistence state at an ambient temperature of 1000 to 1100 ° C. near the meniscus. As a result, the generated rim becomes fragile and the rim collapses (see FIG. 3B).

Therefore, in the present invention, Na ₂ O / MgO is set to 1.0 for the purpose of ensuring the strength of the rim in the atmosphere near the meniscus while promoting the growth of the rim generated along the mold wall surface at the meniscus position. This was realized by setting the Fe source contained on the mold flux surface before melting to 0.2% by mass or more.

That is, the present invention
CaO, and SiO ₂ as the main component, CaO / SiO ₂ is 0.90 to 1.20, MgO is 2-7 wt%, with Na ₂ O is 3-7% by mass, the solidification temperature of 1050-1,150 ° C., a viscosity at 1300 ° C. 1.0 to 3.0 poise mold flux for continuous casting,
The Na ₂ O / MgO is less than 1.0, and the Fe source contained on the mold flux surface before melting is 0.2% by mass or more.

In the present invention, CaO / SiO ₂ is set to 0.90 to 1.20 and the solidification temperature is set to 1050 to 1150 ° C. during the continuous casting of the low carbon steel slab having a C concentration of 0.02 to 0.07 mass%, which is the object of the present invention. This is to generate components and crystals suitable for solidified shell generation control as a countermeasure against generated surface cracks (longitudinal cracks).

  Moreover, the viscosity at 1300 ° C is 1.0 to 3.0 poise because the flow rate of molten mold flux necessary for continuous casting of the low-carbon steel slab at a high speed is secured, and the breakout due to the restraint between the mold and the solidified shell is ensured. It is for suppressing.

The reason why MgO is 2 to 7% by mass, Na ₂ O is 3 to 7% by mass and Na ₂ O / MgO is less than 1.0 is to optimally control the generation of rims generated during continuous casting. In other words, when Na ₂ O / MgO exceeds 1.0, the rim in the meniscus region becomes a solid-liquid coexistence state and cannot retain its shape. As a result, the rim melts due to the vertical oscillation of the mold oscillation, which is the original role of the rim. This is because it becomes impossible to play the role of a pump for injecting the mold flux in a state, and inflow failure occurs. At the same time, the missing rim bites into the solidified shell and becomes a surface defect of the slab.

  The reason why the Fe flux of 0.2% by mass or more is contained in the mold flux surface before melting is to effectively serve as a binder during rim generation in the rim generation process. That is, when the Fe source contained on the mold flux surface before melting is less than 0.2% by mass, it is impossible to generate a stable rim, and the rim is lost. However, when the content of the Fe source on the surface of the mold powder is increased, the molten steel is contaminated. Therefore, the upper limit is desirably 5.0% by mass or less.

  When a low carbon steel slab having a C concentration of 0.02 to 0.07% by mass is continuously cast using the mold flux of the present invention, a slab having no surface defects can be produced without causing breakout. This is the continuous casting method of the present invention.

  The results of experiments conducted to confirm the effect of the present invention will be described below.

  The inventors have obtained a conventional mold flux (conventional example), a mold flux (comparative examples 1 and 2) to be compared and out of the range defined in the present invention, and a mold flux of the present invention (inventive examples 1 and 2). Was used to continuously cast low carbon steel slabs, and the effect was confirmed.

  Using a curved type and vertical bending type (VB type) continuous casting machine, low carbon steel with the composition shown in Table 1 below is continuously cast at a casting speed of 1500 mm / min. A slab having a thickness of 250 mm and a width of 1260 mm was obtained, and the presence or absence of surface defects was examined.

Table 2 below shows the components, basicity, Na ₂ O / MgO ₂ , solidification temperature, and viscosity at 1300 ° C. of the conventional examples, comparative examples 1 and 2, and invention examples 1 and 2. Moreover, the generation | occurrence | production rate of the surface flaw of the low carbon steel slab continuously cast using the prior art example, the comparative examples 1 and 2, and the invention examples 1 and 2 is shown in FIG. Here, the occurrence rate of surface flaws was determined by visually confirming the presence or absence of surface flaws for each slab cooled to room temperature, and the ratio of the number of slabs with surface flaws to the number of target slabs for each mold flux was derived.

Comparative Example 1 is an example in which only the Na ₂ O / MgO ₂ is reduced as compared with the conventional example without including an Fe source on the mold flux grain surface before melting. Comparative Example 2 is an example in which Na ₂ O / MgO is promoted to promote rim formation while maintaining the same level as in the conventional example, and an Fe source is added to the mold flux grain surface before melting.

In contrast, Invention Example 1 is an example in which the strength of the rim is increased by making Na ₂ O / MgO smaller than that of Comparative Example 2. Inventive Example 2 is an example in which the amount of Fe source added is reduced with respect to Inventive Example 1, in consideration of molten steel contamination by the Fe source contained on the mold flux grain surface before melting.

In the case of the slab cast using the comparative example 1 in which Na ₂ O / MgO is lower than that of the conventional example, the surface flaw occurrence rate is reduced to 0.18% from 0.42% when the conventional example is used, Although an improvement effect was observed, the surface flaw occurrence rate could not be reduced to 0%.

  This is because there is no opportunity for the rim to grow because there is no Fe source on the surface of the mold flux grains before melting.

  In addition, in the case of slab cast using Comparative Example 2 in which Fe source was added to the mold flux grain surface before melting, the occurrence rate of surface flaws was 0.20%, an improvement effect compared to the case of using the conventional example However, the incidence of surface flaws could not be reduced to 0%.

This is because the Fe source was present on the surface of the mold flux grain before melting, but the rim was easily formed, but the rim was in a solid-liquid coexistence state due to the high Na ₂ O / MgO, and the shape of the rim was maintained. This is because a sufficient strength was not obtained.

In contrast, Invention Example 1 in which Na ₂ O / MgO was made smaller than Comparative Example 2 produced a stable rim, resulting in a surface flaw occurrence rate of 0% and a marked improvement effect. It was. In addition, in Invention Example 2, in which the amount of Fe source added was reduced from that in Invention Example 1 in consideration of molten steel contamination by the Fe source on the surface of the mold flux grains before melting, the occurrence rate of surface flaws was 0%. Good results were obtained for.

FIG. 5 shows the relationship between the surface flaw generation rate shown in FIG. 4 in Comparative Example 2 and Invention Examples 1 and 2, and Na ₂ O / MgO which is a low melting point compound. From FIG. 5, it was confirmed that the occurrence of surface defects was low when Na ₂ O / MgO was less than 1.0.

  On the other hand, FIG. 6 shows the relationship between the surface flaw generation rate shown in FIG. 4 and the Fe source concentration on the mold flux grain surface before melting in the conventional example and Invention Examples 1 and 2. From FIG. 6, it was confirmed that the generation rate of surface defects can be reduced by setting the Fe source on the surface of the mold flux grains before melting to 0.2 mass% or more.

  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.

1 rim 1a broken rim 2 mold flux 3 mold 4 solidified shell

Claims (2)

CaO, and SiO ₂ as the main component, CaO / SiO ₂ is 0.90 to 1.20, MgO is 2-7 wt%, with Na ₂ O is 3-7% by mass, the solidification temperature of 1050-1,150 ° C., a viscosity at 1300 ° C. 1.0 to 3.0 poise mold flux for continuous casting,
A mold flux for continuous casting, characterized in that Na ₂ O / MgO is less than 1.0 and the Fe source contained on the mold flux surface before melting is 0.2 mass% or more.   A continuous casting method, wherein the mold flux for continuous casting according to claim 1 is used in casting a low carbon steel slab having a C concentration of 0.02 to 0.07 mass%.

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