JPH08141713A - Mold powder for continuous casting of steel - Google Patents

Abstract

(57) [Summary] [Object] The present invention is directed to a mold powder for continuous casting of steel, particularly a mold powder for continuous casting of medium carbon steel. [Structure] Main components are CaO and SiO ₂ , CaO / Si
O ₂ (wt% basicity) is 1.2 to 1.6, and MgO
It is a powder for continuous casting of steel having a content of 1.5 wt% or less.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a continuous casting mold powder (hereinafter simply referred to as a mold powder for continuous casting, which is added to molten steel in a mold during continuous casting of steel, particularly medium carbon steel (carbon content 0.08 to 0.16 wt%). The present invention relates to a powder that stabilizes a continuous casting operation and can continuously cast a slab having excellent slab surface quality.

[0002]

2. Description of the Related Art In continuous casting of steel, powder is added to the molten steel surface in a mold to prevent oxidation of the molten steel, to keep the molten metal surface warm, and to absorb inclusions floating on the molten metal surface. . Further, the powder melted on the steel bath surface flows between the mold and the slab to form a film layer, which acts as a lubricant,
Furthermore, heat removal is controlled.

The chemical composition of the conventional powder has the following composition. _{Si 2 O, CaO; 20~40wt%} , Al 2 O 3; 1~10wt%, Na; 1~30wt%, F; 1~20wt%, Li; 0~20wt%, MgO; 2~10wt%, ZrO ₂ ; 0 to ₂₀ wt%, B; 0 to 10 wt%

On the other hand, the carbon concentration in steel is 0.08 to 0.16.
In continuous casting of steel in the medium carbon region of wt% (hereinafter referred to as medium carbon steel), the solidification shrinkage rate is large when the steel changes from the liquid phase to the solid phase, so crack defects are likely to occur on the surface of the slab. However, high-speed casting has been difficult.

On the other hand, Materials and Processes-Vol. 4, No. 4 (1991, p. 1247) reduce the heat flux between the mold and the surface of the slab as a countermeasure against the crack defects of the medium carbon steel. , Basicity (wt% CaO / SiO ₂ ) 1.
2 or more to increase the contact resistance due to volumetric shrinkage during crystallization, and to add ZrO ₂ in an amount of 3.0 wt% or more to make the powder film opaque and reduce the radiant heat transfer, and clean slabs It is disclosed that can be manufactured (prior art 1).

In addition, Shinagawa Technical Report No. 32 (1989, 1
Page 47) investigates the heat removal rate in the mold and the characteristics of the powder in order to prevent vertical cracking of the slab, which is a problem when casting the medium carbon steel, and crystallizes the molten powder during the solidification process. It has been reported that it was found that by raising the temperature, heat removal in the mold can be relaxed and the occurrence of vertical cracks can be reduced.

Further, it is assumed that the higher the basicity powder, the higher the crystallization temperature tends to be, and Na ₂ O, F, Li
_Since components such as ₂ O, B ₂ O ₃ , BaO, MgO, and Al ₂ O ₃ influence the crystallization temperature, it is necessary to pay attention to the composition of these components when setting the powder quality. It is described (prior art 2). In the above, crystallization means that melted powder flows between the mold and the surface of the slab, and when it solidifies, part of the melt becomes a glass phase (supercooled body).
However, this is a phenomenon in which other parts are precipitated as crystals.

[0008]

In the prior art 1, although the crystallinity of the powder melt is increased by increasing the basicity to 1.2 or more, the powder melt has a crystallinity higher than that of the recently-proposed high-speed casting powder. When a low melting point promoting element such as Na, F or Li and a low viscosity promoting element coexist, the crystallinity decreases,
Crystallinity that suppresses surface defects cannot be obtained only by the effect of increasing the basicity.

Further, in Prior Art 1, the crystallization promoting effect of ZrO ₂ is disclosed, but when ZrO ₂ is added to the powder, the melting point of the powder rises and the powder molten layer on the surface of the molten steel becomes thin, There is a problem that the slab surface defect occurs because the absorption capacity of non-metallic inclusions floating on the surface decreases.

Although the prior art 2 also discloses that the high basicity is effective, the effect on the crystallization of other flux components is only qualitatively discussed, and the effective crystal is effective. It does not quantitatively refer to the accelerating elements.

The present invention solves the above-mentioned problems and sufficiently achieves high crystallinity of the molten powder while maintaining the rapid melting and high lubrication characteristics originally required for the powder, thereby achieving the surface of the slab. The present invention provides a powder capable of obtaining a high-quality product having no crack defects.

[0012]

Means for Solving the Problems As a result of various experiments and studies, the inventors of the present invention have found that in the range where the rapid melting and high lubrication characteristics originally required by the powder are maintained, the mold powder for continuous casting is mainly used. CaO is a component, SiO ₂ basicity of _{(wt%) CaO / SiO 2} 1.2~1.6 (w
By setting the content of MgO in the range of (t% ratio) to 1.5 wt% or less, crystallization of the powder melt can be promoted as compared with the conventional high basicity powder.
Based on the finding that continuous casting of medium carbon steel with clean surface quality becomes possible, the inventors have made the following inventions.

(1) In the invention of claim 1, the main component is Ca.
O, SiO ₂ , CaO / SiO ₂ (wt% basicity) is 1.2 to 1.6, and MgO content is 1.5 wt%
The following are powders for continuous casting of steel. (2) The invention of claim 2 provides a powder for continuous casting of steel having the following component composition. (A) MgO content is 1.5 wt% or less, (b) Content of at least one of cryolite, NaF, fluorite, soda ash, Li ₂ O is 4 to 25 wt%, (c) carbon The content of at least one of black and scaly graphite is 1 to 8 wt.
%, (D) The balance is CaO, SiO ₂ , CaO / SiO
₂ (wt% basicity) is 1.2 to 1.6.

[0014]

[Function] Generally, the melt formed by the powder for continuous casting is a silicate melt (molten silicate), and various ions,
It is an ionic melt composed of complex ions. Alkali metal oxides with high ionicity in slag and metals in alkaline earth metal oxides have a strong ionization tendency, while Si
A substance having a high covalent bond such as O ₂ does not simply ionize but forms a complex ion such as SiO ₄ ⁴⁻ or Si ₂ O ₇ ⁶⁻ .

The latter is a network forming oxide (hereinafter referred to as NWF) which greatly contributes to so-called glass formation, and the former is SiO ₂ as represented by CaO.
O is added to the network structure of SiO ₂ to break the intermolecular bond of SiO ₂ and reduce the degree of polymerization.
(Hereinafter referred to as NWM). In this, Al ₂
O ₃ and Fe ₂ O ₃ are generally NWF, but become NWM in the molten slag having a high oxygen potential.

Further, MgO, which the present inventors have noticed, is Ca
In the O / SiO _{2 type} melt, although the effect is slightly small,
It behaves as an NWM and has been evaluated as a crystallization promoting element. Further, conventionally, MgO was inevitably contained in the powder in an amount of 1.7 wt% or more as an impurity in the raw material for producing mold powder.

On the other hand, the crystallization of the powder is evaluated by measuring the temperature at which crystals crystallize from the melt of the powder during the cooling process of the molten powder, by using the differential thermal analysis method to measure the exothermic peak associated with the crystallization. . The higher the crystal formation temperature, the easier the crystallization will proceed.

The present inventors have studied the method of controlling the crystal formation temperature by increasing and decreasing the constituent elements of the powder, and as a result, C
MgO in the region of aO / SiO ₂ of 1.2 to 1.6
Is in the range of 0 to 10 wt% (including 0 wt%), MgO
I made a new discovery that the crystal formation temperature rises as the amount of components decreases.

That is, by reducing the amount of MgO in the powder melt, the crystal formation temperature rises, and as a result,
It has been found that not only the crystal formation is accelerated, but also the crystal growth in a temperature range where the viscosity is relatively low is promoted, and the crystallization can be further promoted.

Furthermore, since MgO has a high melting point and is a refractory substance, reducing the amount of MgO also improves the meltability of the mold powder. That is, the reduction of the MgO content promotes the crystallization effect and enables the meltability to be improved.

Regarding Numerical Limits The inventors have studied the crystallization of the powder melt in order to obtain good casting of medium carbon steel and clean slab, and as a result, the crystal formation temperature of the powder melt was set to 1130 ° C. or higher. By doing so, it was found that the effect of reducing surface defects of the slab becomes large. In the case of a mold powder having a viscosity value of 0.1 Pa · S or less at 1300 ° C., it is necessary to select the following component composition.

Main component: The main component of the powder of the present invention is Ca
O and SiO ₂ , which are pre-melted materials that are pre-melted are used. The mixing ratio can be determined by the basicity. If the basicity is less than 1.2, the melting point of the powder is too low, which is not desirable. On the other hand, if the basicity exceeds 1.6, the melting point of the powder is too high, which is not desirable.

MgO: When MgO exceeds 1.5 wt%, the crystal formation temperature becomes less than 1130 ° C., which is not desirable as described above.

Physical property adjusting agent; powder of cryolite (including artificial cryolite), NaF, fluorspar, soda ash, Li ₂ O, etc.
It is desirable that the total of four or more species be 4 to 25 wt%. This is because the solubility of the powder on the molten steel surface is undesired if it is less or more than this range.

Melt rate adjusting material: Carbon black and scaly graphite are preferable. The melting rate adjusting material has a function of adjusting the melting rate of the powder added to the molten steel surface. Carbon black is 0.5-5 wt%, scaly graphite is 0.5-3 wt%
% Or less is desirable. The specific blending amount may be determined by casting conditions. More preferably, carbon black is 1.2 to 2.0 wt% and scaly graphite is 0.5 to 1.5 wt.
% Is good.

[0026]

EXAMPLES The present invention will be described with reference to examples and comparison with comparative examples. Melt silica stone, lime, fluorite, glass powder, alkali metal carbonate, alkaline earth metal carbonate having extremely low MgO content and an industrial reagent having a mold powder composition substantially free of MgO in an electric furnace Then, it was rapidly cooled and then pulverized to prepare an adjusted mold powder main material.

To this main raw material, one or more powders of artificial cryolite, NaF, fluorspar, soda ash, Li ₂ O and the like are added as a physical property modifier in a total amount of 4 to 25 wt%, and the range of the present invention is further added. Specimens (hereinafter, referred to as invention bodies) 1 to 4 having the amount of MgO component in each of them were adjusted. For comparison, powder samples (hereinafter, referred to as comparative samples) 1 to 6 in which the amount of MgO component was outside the range of the present invention were prepared. Table 1 shows the amounts of MgO, basicity (CaO / SiO ₂ ), and crystal formation temperature of Inventions 1-4 and Comparatives 1-6.

Continuous casting of medium carbon steel having high cracking sensitivity at a casting speed of 1.6 m / min or 1.8 m / min using each of the above inventive bodies 1 to 4 and comparative bodies 1 to 6 The carbon concentration in steel, the amount of mold powder consumed, and the index of vertical crack initiation of slab were examined, and the results are shown in Table 2.

[0029]

[Table 1]

[0030]

[Table 2]

As is clear from Tables 1 and 2, MgO
Inventors 1 to 4 whose amount and basicity (CaO / SiO ₂ ) are within the range of the present invention have high crystal formation temperatures of 1130.
C., vertical cracking does not occur even in the casting results using the invention bodies 1 to 4, and a slab having a good surface can be produced,
High-speed casting of medium carbon steel, which was difficult in the past, could be easily performed.

On the other hand, the basicity (CaO / SiO ₂ )
Within the scope of the present invention from 1.2 to 1.6
In Comparatives 1 to 6 whose amount exceeds the range of the present invention, the crystal formation temperature is 1125 ° C. or lower, the vertical cracking generation index becomes high, and the tendency becomes remarkable as the amount of MgO increases.

[0033]

As described above, according to the present invention, it is possible to obtain a mold powder for continuous casting having a large crystallinity within a range that does not impair the meltability, and to crack even medium carbon steel having a high cracking susceptibility. It is possible to continuously cast a product of excellent quality with no result.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Takashi Mori, 1-2 Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Yukei Kondo 1-2-1-2, Marunouchi, Chiyoda-ku, Tokyo Main Steel Pipe Co., Ltd. (72) Inventor Ryuzo Nishimachi Marunouchi 1-2-2, Chiyoda-ku, Tokyo Japan Steel Pipe Co., Ltd. (72) Inventor Keiji Watanabe 1-2, Marunouchi, Chiyoda-ku, Tokyo Japan Steel Pipe Co., Ltd. (72) Inventor Akira Miyamoto 1-2 1-2 Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Pipe Co., Ltd. (72) Inori Hidenori Sakai 2-3-2 Marunouchi, Chiyoda-ku, Tokyo Steel Pipe Mining Co., Ltd. (72) Inventor Masaaki Shimizu 2-3-2 Marunouchi, Chiyoda-ku, Tokyo Steel Pipe Mining Co., Ltd. (72) Inventor Hiroshi Izumi Maru, Chiyoda-ku, Tokyo The inner-chome No. 3 No. 2 steel pipe mining within Co., Ltd. (72) inventor Ken Kawashima, Chiyoda-ku, Tokyo Marunouchi 2-chome No. 3 No. 2 steel pipe Mining Co., Ltd. in

Claims (2)

[Claims] 1. Main components are CaO and SiO ₂ , and CaO /
SiO ₂ (wt% basicity) is 1.2 to 1.6, M
Powder for continuous casting of steel having a gO content of 1.5 wt% or less. 2. A powder for continuous casting of steel having the following composition of components. (A) MgO content is 1.5 wt% or less, (b) Content of at least one of cryolite, NaF, fluorite, soda ash, Li ₂ O is 4 to 25 wt%, (c) carbon The content of at least one of black and scaly graphite is 1 to 8 wt.
%, (D) The balance is CaO, SiO ₂ , CaO / SiO
₂ (wt% basicity) is 1.2 to 1.6.