JPH08157944A - Decarburization preventive agent of slab - Google Patents

Abstract

PURPOSE: To obtain a decarburization preventive agent of a hot slab capable of effectively preventing only the decarburization without suppressing the oxidation of slab surfaces within a heating furnace in view of the fact that oxidized layers of the slab surfaces have an effect of removing the oscillation marks formed by a continuous casting machine and the surface layer part-thickened layers, etc., of the alloy components and trace components existing in molten steel. CONSTITUTION: This decarburization preventive agent is applied on the slab surfaces prior to insertion into the heating furnace and contains a colloidal inorg. material to be crystallized, a silicate maternal of a m.p. of <=1300 deg.C, at least one kind of borate material and phosphate material and a dispersant and prevents the crack defect of the slab surface layer parts formed at the time of hot rolling.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improvement of a decarburizing inhibitor for a slab, and in particular, it is applied to the surface of the slab before the hot rolling furnace of the slab and the surface layer of the slab in a high temperature atmosphere of the heating furnace. It prevents the formation of a decarburized layer and allows complete and easy peeling after extraction.

[0002]

2. Description of the Related Art Generally, a slab after continuous casting is heated in a heating furnace at a predetermined temperature for a predetermined time in order to obtain a temperature condition required for hot rolling. COG (coke oven gas) and BFG (blast furnace gas) + COG are the main fuels of the heating furnace, and the excess air ratio is 1.1 to 1.3. The components in the combustion gas are O ₂ ; 0.5-6%, CO ₂ ; 7-15%, H
₂ O; about 10 to 20% is present.

All of the above three components oxidize the surface layer of the slab by the following reaction at high temperature. Fe + O ₂ → FeO, Fe ₂ O ₃ Fe + CO ₂ → FeO, Fe ₂ O ₃ Fe + H ₂ O → FeO, Fe ₂ O ₃ These oxide layers are formed on the slab surface layer part by about 0.1 to 3 mm and heated. Yield loss, but on the other hand,
It has the function of removing (scale-off) the oscillation mark generated during continuous casting, the alloy component and the trace amount of concentrated component in the molten steel.

On the other hand, however, FeO and Fe ₂ O ₃ in the oxide layer produced by the above reaction react with C (carbon) contained in the steel inside the slab, and as shown in FIG. It also has a function of forming the decarburized layer 5 under the oxide layer (oxide scale) 3 in the surface layer portion. This decarburized layer 5
Grows to a thickness of about 1 to 10 mm in the heating furnace, and deepens as the C content decreases. Moreover, the mechanical properties such as workability are significantly different from those of the non-decarburized layer at high temperatures, and cracking of the decarburized layer occurs during processing in the roughing mill and finish rolling mill in the next process, and further in the cold rolling mill. This causes cracks and crystal grains to fall off, causing defects in cold-rolled products.

[0005]

Various means for preventing oxidation and means for preventing decarburization have heretofore been proposed as measures for eliminating the decarburized layer that causes such problems in the heating furnace. For example, in Japanese Patent Application Laid-Open No. 55-126313, after a powdery or granular flux is sprayed or applied on the surface of a steel material, the steel material surface and the flux are melted by induction heating to cover the slab surface, and oxidation and decarburization at high temperature are performed. It is proposed to protect it from damage such as. In addition, JP-A-55-
Japanese Patent No. 136502 proposes a hot rolling method in which a part or the whole of a hot-rolled material of a hot-rolled metal is covered with foam to prevent temperature drop and oxidation during the hot rolling process. . Further, Japanese Patent Laid-Open No. 61-204385 discloses an apparatus for applying an oxidation suppressing material to a steel material to be heated prior to hot working.
No. 8 discloses that, before heating the slab of Si-containing steel, an oxide simple substance consisting of CaO ₃ and MgO is applied to the surface of the slab to modify the oxidation product on the surface of the slab and suppress the generation of red scale. A method of manufacturing a hot-rolled steel sheet that can be used is shown. In addition, in JP-A-2-209420, in view of the fact that the conventional antioxidant does not effectively exhibit the antioxidant action in the heat treatment process at a high temperature of 1000 ° C. or higher,
A high-temperature antioxidant for steel products has been proposed in which an antioxidant made of a glass composition having a viscosity at 1200 ° C. of 10 ³ poise or more is applied to the surface of steel products. Further, Japanese Patent Application Laid-Open No. 3-232922 discloses a coating agent containing a metal powder that easily forms an iron oxide and a composite oxide in an oxidation process,
A scale modifier has been proposed which has good adhesion, is easy to form into a film, and easily burns completely in the range of 120 to 500 ° C.

[0006] In all of these, an agent mainly containing a refractory is applied to the surface of the slab before the heating furnace, the refractory is melted in the heating furnace, and a thick molten film is formed on the surface layer of the slab to form an oxide layer. Is to prevent. When these oxidation preventing means are adopted, the decarburized layer cannot theoretically be generated because the oxide layer is not generated at all. However, the fact that there is no oxide layer means that there is no scale-off in the heating furnace as described above, and the oscillation marks generated by the continuous casting machine, the alloy components present in the molten steel, and trace amounts In the heating furnace, it is not possible to remove a portion that locally changes the steel property, such as the surface layer concentrated layer of the component. Therefore, there is a problem that defects and patterns are generated in the steps of rough rolling, finish rolling and cold rolling.

Further, since these conventional antioxidants do not oxidize the surface layer of the slab, they firmly adhere to the surface layer of the slab on the outlet side of the heating furnace, and the descaling operation does not peel off the surface of the slab and the raw material is directly applied to the rough rolling machine. Many have the problem of being caught. Therefore, the present invention has been made by paying attention to such conventional problems, and decarburization of a hot rolled slab capable of effectively preventing only decarburization without suppressing the oxidation of the slab surface. The purpose is to provide an inhibitor.

[0008]

Means for Solving the Problems As a result of intensive investigations for achieving the above-mentioned object, the present inventors have found that colloidal inorganic substances, silicate substances, borate substances, phosphate substances, and fire resistance depending on the situation. By compounding the materials, a decarburizing inhibitor composition having an excellent effect was found, and the present invention was completed.

That is, the present invention uses, as an active ingredient, a colloidal inorganic substance that crystallizes, at least one of a silicate substance, a borate substance and a phosphate substance having a melting point of 1300 ° C. or lower, and a dispersant. It is contained. In addition, at least one refractory material can be included as an active ingredient. The colloidal inorganic substance of the present invention is preferably one that crystallizes to form a regular film, and one that does not crystallize does not facilitate peeling of the film after heating because it is close to the linear expansion coefficient of the slab, which is preferable. Absent.

The colloidal inorganic substance can be selected from the group consisting of alumina, silica, aluminum phosphate, zirconium silicate, zirconium borate, zirconia, titania and vanadium oxide. The silicate substance, borate substance, and phosphate substance of the present invention have a melting point of 1300 ° C. or less, and when incorporated into a colloidal inorganic substance, the “amorphous film forming temperature” of the colloidal inorganic substance is
And the “crystallization temperature” can be changed, and the thermal properties of the colloidal inorganic substance can be adjusted according to the slab by adjusting the blending amount. 1300 ° C
Those which exceed the range are not preferable because they do not significantly change the thermal properties of the colloidal inorganic substance, and a strong and adherent coating cannot be obtained in a heating furnace.

The dispersant of the present invention promotes the dispersion of particles of the colloidal inorganic substance and forms a uniform and smooth dry coating film on the slab surface to enhance the adhesion between the coating film and the slab surface. Dispersants are corn starch, tapioca starch, sodium alginate, guar gum, xanthan gum, casein, gelatin, α-starch, dextrin, methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, carboxymethyl cellulose, hydroxymethyl propyl cellulose, polyvinyl alcohol, polypropylene glycol, polyethylene oxide, polyvinyl butyral. , Pullulan, sodium polyacrylate, fluorocarbon.

Addition of a refractory material to the active ingredient effectively improves the prevention of decarburized layer formation when heat treatment is carried out at high temperature for a long time. Refractory materials include oxides of alumina, silica, magnesia, zirconia and titania, mullite, andalusite, chamotte, magnesite, spinel, dolomite, montmorillonite, kaolinite and sepiolite, and silicon carbide, titanium carbide,
It may be selected from the group consisting of carbides of tungsten carbide, boron carbide and molybdenum carbide.

The mixture of colloidal inorganic material, silicate material, borate material, phosphate material, and optionally refractory material of the present invention is dispersed in water or / in a suitable organic solvent and applied to the slab surface. Applied. In this case, the organic solvent preferably has a boiling point of 150 ° C or lower. 150
If the temperature exceeds ℃, the drying speed of the coating film becomes slow, and the coating film may swell or peel due to volatilization of the organic solvent in the furnace, whereby the decarburization preventing effect may not be obtained.

A mixed liquid containing an organic solvent in water is convenient for lowering the interfacial tension, promoting the spread of the coating film after coating, and obtaining a smooth coating film surface. Suitable organic solvents include alcohols such as methanol, ethanol, propanol, butanol, pentanol and 2-ethoxyethanol, ethers such as propylene glycol methyl ether, dimethylformamide, dimethyl sulfoxide and the like. .

The amount of the decarburizing inhibitor of the present invention applied to the slab may be such that the target slab surface is uniformly covered, and the amount is 20 to 600 g / cm ² (solid content conversion), preferably 50 to 400 g. / Cm ² is a standard. The decarburizing inhibitor of the present invention may be applied to the surface of the slab by any coating method such as a brush coating method, a mist spray method, and a dipping method as long as there is no practical problem.

[0016]

The function mechanism of the slab decarburization inhibitor of the present invention will be described with reference to FIG. The colloidal inorganic substance 1 contained in the decarburization inhibitor is composed of very small particles, and when it is applied to the surface of the slab 2 and the slab temperature rises, it becomes a thin film and the surface of the slab is oxidized by the continuous casting machine. Disperse in scale 3. This thin colloidal film does not have an antioxidant effect on the slab surface, but only prevents decarburization. That is, when the temperature is about 300 to 600 ° C, the colloidal particles are dehydrated, and the particles are condensed at 600 ° C or higher. Then, a strong and dense amorphous film 4 is formed between the oxide scale 3 and the slab 2 and fused on the surface of the slab.

Even if this amorphous film 4 is formed, O ₂ , CO _2, H ₂ O in the atmosphere gas of the heating furnace raises the degree of oxidation of the oxide scale 3 in the surface layer of the slab, and from the inside of the slab 2. Since iron diffuses into the surface layer portion, the slab 2 is oxidized even if there is this dense film 4 as in the case without it. On the other hand, regarding the decarburizing reaction in which CO is generated from the interface of the slab base iron by reacting the oxidized scale 3 and carbon in steel, the strong amorphous form formed at the interface between the base iron of the slab 2 and the oxide scale 3 is used. The decarburization reaction does not proceed because the coating 4 inhibits the production of CO.

As described above, according to the present invention, since the slab can be sufficiently oxidized in the heating furnace, there is no problem in scale-off after extraction. Moreover, decarburization can be easily prevented by forming a strong amorphous film. The slab decarburization inhibitor of the present invention comprises at least one of a silicate substance, a borate substance and a phosphate substance 1300.
A low melting point compound having a melting point of ℃ or less changes the "amorphous film forming temperature" and "crystallization temperature" of the colloidal inorganic substance. Therefore, in the present invention, the thermal properties of the colloidal inorganic substance can be adjusted according to the steel type by adjusting the blending amount of the low melting point compound.

The dispersant composed of an organic polymer as a decarburization inhibitor for a slab of the present invention promotes particle dispersion of the colloidal inorganic substance and forms a uniform and smooth dry film of the colloidal inorganic substance on the surface of the slab. To enhance the adhesion between the film and the slab. According to the experiments conducted by the present inventors, those dispersants selected from the group of organic polymers described in claim 5 showed good results.

The slab decarburization inhibitor refractory material of the present invention more effectively improves the formation of the decarburized layer, especially when heating at elevated temperatures for extended periods of time. Such refractory materials are
Non-metallic inorganic substances that are resistant to high temperatures and chemically stable are preferable, and those selected from the group of refractory materials according to claim 6 are preferable.

[0021]

EXAMPLES The present invention will be described below based on examples.
Using a slab heating furnace, stainless steel (SUS430,
The decarburizing inhibitor of the present invention and the decarburizing inhibitor of the comparative example were applied to a slab having a thickness of 250 mm), and a heating experiment was performed.

Both the product of the present invention and the comparative product are in water or water /
At the heating furnace inlet, what was dispersed in the organic solvent mixture,
It was applied to the slab surface by the mist spray method. The dried film thickness was 50 to 200 μm. The coating film of the coating agent after heating and the produced oxide scale were removed from the surface of the slab with a brush. Tables 1 to 3 show the composition (% by weight), water content, and viscosity of the decarburizing inhibitor used in this experiment. In the table, MC is methyl cellulose, PVA is polyvinyl alcohol, and PGME is propylene glycol methyl ether. The numbers are% by weight.

[0023]

[Table 1]

[0024]

[Table 2]

[0025]

[Table 3]

The test results are shown in Table 4.

[0027]

[Table 4]

In Table 4, the judgment criteria for dry film strength, decarburization prevention effect, and peeling performance are as follows. [Drying film strength] ○: 100% adhesion △: 90 to 99% adhesion ×: 90% or less adhesion [Decarburization prevention effect] ○: No decarburization layer △: Decarburization layer 15% or less ×: Decarburization layer 15% or more [Peeling Properties] ○: 100% peeling Δ: 90 to 99% peeling ×: 90% or less peeling

[0029]

As described above, according to the first aspect of the present invention.
According to the invention, the composition of the decarburization inhibitor of the slab has a colloidal inorganic substance that forms a dense amorphous film by heating, and a low melting point that adjusts the amorphous film formation temperature of the colloidal inorganic substance. Contain at least one of silicate, borate, and phosphate substances, and a dispersant that promotes particle dispersion of colloidal inorganic substances and enhances adhesion between the amorphous film and the slab, as active ingredients. Therefore, the decarburization reaction of the slab in the heating furnace can be suppressed by the amorphous coating of the colloidal inorganic substance to effectively prevent the formation of the decarburization layer. As a result, the rough rolling, finish rolling, It is possible to prevent cracks during rolling such as cold rolling, missing crystal grains, and defects such as patterns from occurring, and to significantly reduce the defective rate of cold-rolled products and improve product quality. To be

Further, according to the invention of claim 2 of the present invention,
Since at least one kind of the refractory material as described in claim 6 is further contained as the active ingredient, it is possible to effectively improve the generation of the decarburized layer particularly when heating at high temperature for a long time. Play. Further, according to the inventions of claims 3 and 4, at least one or more of alumina, silica, aluminum phosphate, zirconium silicate and zirconium borate having a melting point of 1300 ° C. or less is used as the colloidal inorganic substance. The alkali metal salt or alkaline earth metal salt having a melting point of 1300 ° C. or lower is used in order to adjust the amorphous film formation temperature. It is possible to obtain the effect that the thermal properties of the colloidal inorganic substance that prevents the formation of the decarburized layer can be adjusted according to the steel type by adjusting the blending amount of the metal salt.

Further, by using the dispersant according to claim 5 of the present invention, the dispersion of particles of the colloidal inorganic substance is promoted, and the adhesion between the slab surface and the dry film of the colloidal inorganic substance can be enhanced. Further, further decarburization prevention effect can be obtained.

[Brief description of drawings]

FIG. 1 is a schematic view of an oxide scale and a decarburized layer formed on a surface layer of a slab in a heating furnace.

FIG. 2 is a schematic view of a cross section of a slab surface layer portion for explaining the function of the decarburization inhibitor for the slab of the present invention.

[Explanation of symbols]

 1 Decarburization inhibitor 2 Slab 3 Oxide scale 4 Amorphous film (of colloidal inorganic substance)

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masaharu Ikeda 1 Kawasaki-cho, Chuo-ku, Chiba, Chiba Prefecture Inside the Chiba Works, Kawasaki Steel Co., Ltd. (72) Kuniaki Sato, 1 Kawasaki-cho, Chuo-ku, Chiba, Chiba Steel Works Co., Ltd. Chiba Steel Works (72) Inventor Akira Kawarada 1 Kawasaki-cho, Chuo-ku, Chiba, Chiba Prefecture Kawasaki Steel Works Chiba Works (72) Inventor Yuji Miki 1 Kawasaki-cho, Chuo-ku, Chiba City Kawasaki Steel Manufacturing Co., Ltd. Steel Research Laboratory (72) Inventor Hisaki Muraki Yokkaichi City, Mie Prefecture 6-6-9 Hakkato Co., Ltd. Yokkaichi Research Center

Claims (6)

[Claims] 1. A colloidal inorganic substance which crystallizes, 13
A decarburization inhibitor for a slab, which comprises at least one of a silicate substance, a borate substance and a phosphate substance having a melting point of 00 ° C. or less and a dispersant as an active ingredient. 2. The decarburization preventive agent for slabs according to claim 1, further comprising at least one kind of refractory material as the active ingredient. 3. The colloidal inorganic substance is alumina,
The decarburization inhibitor for a slab according to claim 1 or 2, which is composed of at least one of silica, aluminum phosphate, zirconium silicate, zirconium borate, zirconia, titania, and vanadium oxide. 4. The decarburization inhibitor for slabs according to claim 1, wherein the silicate substance, the borate substance and the phosphate substance are alkali metal salts or alkaline earth metal salts, respectively. 5. The dispersant is corn starch, tapioca starch, sodium alginate, guar gum, xanthan gum, casein, gelatin, alpha starch, dextrin,
Methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, carboxymethyl cellulose, hydroxymethyl propyl cellulose, polyvinyl alcohol,
Polypropylene glycol, polyethylene oxide,
The decarburization inhibitor for a slab according to claim 1 or 2, which is selected from the group of organic polymers consisting of polyvinyl butyral, pullulan, sodium polyacrylate, and fluorocarbon. 6. The refractory material comprises oxides of alumina, silica, magnesia, zirconia and titania, mullite,
The compound selected from the group consisting of complex oxides of andalusite, chamotte, magnesite, spinel, dolomite, montmorillonite, kaolinite and sepiolite, and carbides of silicon carbide, titanium carbide, tungsten carbide, boron carbide and molybdenum carbide. Decarburization inhibitor for the slabs listed.