JP7106995B2 - Method for removing oxide scale - Google Patents

Description

The present invention removes oxide scale generated during the heating process of steel materials containing 0.3 to 1.5% Ni and 0.001 to 0.5% Si, especially brake disc materials, before hot working. It has to do with how descaling removes it.

Ni is widely used as an alloy additive element because it has effects such as increasing the strength of steel materials, improving toughness, increasing corrosion resistance, and improving plating adhesion. For example, Ni-containing steel is widely used for liquid tanks and the like as a steel material for low-temperature welded structures, and is also used as a material for brake discs.

By the way, iron oxides (scale) containing alloying elements are generated in the heating process and rolling process of alloy steel. If the steel is hot-worked while the scale remains on the surface of the steel, the scale is pushed into the steel and becomes scale defects, degrading the surface quality of the product.

In particular, in steel materials containing Ni, there is a problem that scales are difficult to remove even if descaling with high-pressure water is performed because Ni has the effect of increasing the adhesion between the scales generated during heating and the steel. When hot forging a brake disc material containing about 1% Ni, if the scale is not removed before forging, it causes scale flaws after forging.

In relation to this problem, in Patent Document 1, after mechanically grinding the surface of Ni-containing steel, which is difficult to descal, by applying an antioxidant, heating, and rolling, the formation of oxide scale itself proposed a method to suppress However, although Patent Document 1 reduces oxide scale, it does not solve the problem of difficulty in removing the generated oxide scale.

Patent Document 2 relates to the problem that steel containing Si has low descalability. We believe that the cause of the low descaling property is the Fe—Si oxide generated in the scale of the steel containing Si. proposed to form a new FeO layer (easily descaling) between the oxide of the steel and the steel and descaling again. Patent Document 2 requires a second descaling and reheating for that purpose, so the operation is complicated and the cost tends to increase, and it is not described whether descaling is possible with Ni-containing steel. .

Patent Literature 3 proposes coating Ni-containing steel with an anti-oxidation paint to prevent generation of oxide scale in a high-temperature oxidizing atmosphere (1100° C. or lower) such as in a soaking furnace. In Patent Document 3, as a matter of course, the anti-oxidation effect can be obtained only at the places where the anti-oxidation paint is applied. Patent Document 3 also fails to solve the problem of the difficulty in removing the generated oxide scale.

Patent document 4 teaches that descalability is significantly improved over the entire surface of the Ni-containing steel by placing a predetermined Si-containing material on a portion of the surface of the Ni-containing steel and heating the steel.

Japanese Patent Application Laid-Open No. 2006-212671 JP-A-6-269841 JP-A-11-222564 JP 2017-019013 A

In view of the above, an object of the present invention is to propose a novel method for improving the descaling property of Ni-containing steel materials and suppressing the occurrence of surface defects. In particular, the work environment in which descaling is performed is generally harsh, and another object is to propose a method that is practically applicable to such an environment. Specifically, the inventors have found that there are the following problems. In the prior art document mentioned above, it is proposed to apply an antioxidant to the surface of the steel material or to place a Si-containing material on the surface of the steel material. However, depending on the shape of the steel material to which these agents are applied and the properties of the agents (viscosity, volatility, handling, etc.), it is not possible to sufficiently apply them, and as a result, their effects cannot be obtained sufficiently. sometimes not. In addition, in an actual production line, it is common for steel materials that have been coated with chemicals to be transported in order to undergo processing in each process. may come into contact with Such contact may cause peeling of the applied drug or the like, and the effect of the drug or the like may not be obtained sufficiently.

As a result of extensive studies, the present inventors have found that by placing a mixture of a predetermined Si-containing material and a polysaccharide on the surface of the Ni-containing steel and heating the surface, the descalability of the entire surface of the Ni-containing steel is remarkably improved. The inventors have found that the method is extremely practical, and completed the present invention.

The present invention provides the following means.
[1]
A process of heating a steel material containing 0.3 to 1.5% by mass of Ni and 0.001 to 0.5% by mass of Si at a temperature of 1170° C. or more and 1300° C. or less for 15 minutes or more and 10 hours or less. A method for removing oxide scale produced in
a Si-containing material of 85 to 2000 g/m ² as an amount of Si per surface area of the steel material;
A polysaccharide of 1 to 10% of the Si amount in terms of mass % as the C amount is mixed,
Arranging the mixture on the steel material surface of the steel material,
performing the heating on the steel material,
A method for removing oxide scale, characterized by performing descaling thereafter.
[2]
In mixing the Si-containing material and the polysaccharide,
An Fe content of 0.3 to 20% of the Si amount in mass% as the Fe amount,
B content of 4 to 40 g/m ² as a B amount per surface area of the steel material,
P content of 8 to 80 g/m ² as a P amount per surface area of the steel material,
The method according to [1], wherein at least one S-containing material is further mixed in an S amount of 8 to 80 g/m ² per surface area of the steel material.
[3]
The polysaccharides comprise carboxymethylcellulose, hydroxyethylcellulose, hydroxypropylmethylcellulose, xanthan gum, tamarind seed gum, guar gum, locust bean gum, carrageenan, pectin, galactomannan, propylene glycol, starch, dextrin, or mixtures thereof. The method according to [1] or [2].
[4]
The method according to any one of [1] to [3], wherein the steel material is a brake disc material.

According to the present invention, descaling of Ni-containing steel having scale of Ni-containing steel, which was conventionally difficult, can be easily performed. Therefore, scale defects can be suppressed in subsequent processing. The present invention also provides a method of performing descaling that is practically applicable in generally harsh work environments. Furthermore, according to the present invention, descalability is significantly improved.

The schematic diagram of the oxide scale produced|generated in Ni containing steel. FIG. 4 is a schematic diagram showing how oxide scale is removed when normal descaling is applied to Ni-containing steel. FIG. 4 is a schematic diagram showing how oxide scale is removed when descaling is applied to Ni-containing steel using the present invention. FIG. 4 is a schematic diagram showing how oxide scale is removed when descaling is performed using a substance other than the mixture of the present invention. Schematic diagrams showing conditions during experiments in Examples and Comparative Examples.

According to the present invention, a steel material containing 0.3 to 1.5% by mass Ni and 0.001 to 0.5% by mass Si is heated at a temperature of 1170° C. or more and 1300° C. or less for 15 minutes or more and 10 hours or less. A method for removing oxide scale generated in the process of
a Si-containing material of 85 to 2000 g/m ² as an amount of Si per surface area of the steel material;
A polysaccharide of 1 to 10% of the Si amount in terms of mass % as the C amount is mixed,
Arranging the mixture on the steel material surface of the steel material,
performing the heating on the steel material,
A method for removing oxide scale is provided, characterized by subsequent descaling.

Without being bound by any particular theory, the reason why descaling of Ni-containing steel is difficult is believed to be as follows.
When steel containing Ni is heated in an oxygen atmosphere, scales are formed as shown in FIG. The most distinctive feature is that steel and scale form an intricate internal oxide layer. Outer layer scale and inner layer scale are generated on the upper part. The outer layer scale is a scale grown mainly by the outward diffusion of iron and has a dense structure, whereas the inner layer scale is a scale grown mainly by the inward diffusion of oxygen. It has many voids. The internal oxide layer is a layer in which oxygen diffuses into the steel (bare metal) and iron oxide precipitates.

In the internal oxide layer, since Ni is nobler than Fe, Fe is selectively oxidized, Ni is left behind and gradually becomes concentrated. Since Fe, which is more easily oxidized than Ni, is oxidized in the Ni-concentrated layer, an internal oxide layer is formed in which Fe is dispersed as an oxide in the Ni-concentrated layer. Since the internal oxide layer remains in a form in which the oxide of Fe, which is internally oxidized, bites into the steel in which Ni is concentrated, the scale bites into the base iron in the direction of search, and this internal oxide layer (especially the concentrated portion) ) produces an action (anchor effect) that anchors the scale to the steel.
As a result, removal of the scale becomes difficult, and even if descaling with high-pressure water is applied, it is difficult to descal the entire scale including the outer layer scale and the inner layer scale (Fig. 2).

According to the present invention, the steel material surface contains Si in an amount of 85 to 2000 g/m ² as a Si amount per surface area of the steel material,
A polysaccharide of 1 to 10% by mass of the Si amount as the C amount,
After disposing the mixture, the steel material is heated at a temperature of 1170° C. or more and 1300° C. or less for 15 minutes or more and 10 hours or less. This makes it possible to change the scale to a structure that facilitates descaling. Referring to schematic FIG. 3, the Si-containing and polysaccharide mixture is heated to produce a sufficient amount of liquid phase components within the scale. The liquid phase component permeates into the scale and weakens the bond of the scale at the grain boundary, so that the scale consisting of the outer layer scale and the inner layer scale can be easily peeled off. (Fig. 3) The liquid phase component in a sufficient amount to obtain good descaling property is defined as a volume fraction of 50% or more with respect to the total amount of inner layer scale.

The liquid phase component is a Si—Fe-based oxide derived from Si in the Si-containing material and Fe in the scale or steel material. The Si-containing material arranged on the surface of the steel material is oxidized to generate silica (SiO ₂ ) while the steel material is heated. Silica (SiO ₂ ) reacts with wustite (FeO) to produce Si—Fe-based oxides. And wustite (FeO) is generated by oxidation of Fe in the steel material. Even when the Si-containing material and Fe in the steel material react, Fe diffusing from the steel material to the outside within the scale forms a new scale (FeO) on the scale surface, and the FeO takes in the Si-containing material. As they form, they react steadily to form Si--Fe-based oxides. Since the eutectic point of FeO and Fe ₂ SiO ₄ is 1170° C., the Si—Fe oxide becomes a liquid phase component at 1170° C. or higher. Therefore, the lower limit of the heating temperature is set to 1170°C. The upper limit of the heating temperature is not particularly limited as long as the Si--Fe-based oxide is in liquid phase, but is set to 1300.degree. The reason for this is that if the temperature exceeds 1300° C., the internal oxide layer (especially the thickened portion) is melted, and although descaling is easier, the scale loss increases and the heat energy is also lost, which is not preferable. It is from.

The polysaccharide contained in the mixture is generally represented by the molecular formula of (C ₆ H ₁₀ O ₅ ) _n , that is, it is composed of C (carbon), H (hydrogen), and O (oxygen), so the above heating At temperature, CO ₂ gas and H ₂ O gas can be generated. Since the generated gas increases the gas pressure in the scale, it has the effect of facilitating the exfoliation of the scale itself. In addition, the gas generally moves to a well-ventilated place, that is, to a place where the Si—Fe liquid phase component exists, and the movement of the liquid phase component is also promoted. This further improves the effect of the liquid phase component to exfoliate the scale.
In general, the scale generated on the surface of the steel material has a three-layer structure of hematite (Fe ₂ O ₃ ), magnetite (Fe ₃ O ₄ ), and wustite (FeO) from the atmosphere side toward the steel material interface. At this time, hematite has a low gas permeability, and causes CO ₂ gas, H ₂ O gas, and other gases generated from the steel material to remain in the scale, and these gases reduce the adhesion between the scale and the steel material. This also has the effect of facilitating descaling.
On the other hand, the polysaccharide does not affect the reactivity itself of the generation of the Si—Fe liquid phase component.

Furthermore, in mixtures containing Si-containing materials and polysaccharides, the polysaccharides act as thickening agents that increase the viscosity of the mixture. For this reason, the mixture can be easily placed on the steel material, and even if the steel materials come into contact with each other when the steel materials are conveyed later, or when they come into contact with other equipment, etc., the mixture does not easily peel off. Therefore, a sufficient amount of the mixture can be placed, and as a result, the effect of improving the descalability can be sufficiently obtained.

When mixing the Si-containing material and the polysaccharide,
An Fe content of 0.3 to 20% of the Si amount in mass% as the Fe amount,
B content of 4 to 40 g/m ² as a B amount per surface area of the steel material,
P content of 8 to 80 g/m ² as a P amount per surface area of the steel material,
At least one S content of 8 to 80 g/m ² as the amount of S per surface area of the steel material may be further mixed.

Fe in the Fe-containing material promotes formation of a liquid phase component of Si in the Si-containing material and Si—Fe-based oxide. As described above, Si—Fe-based oxides are produced by reacting silica (SiO ₂ ) with wustite (FeO). Therefore, the Fe-containing material preferably contains wustite (FeO). However, even if the Fe-containing substance is an iron oxide other than wustite (FeO), it is reduced to form wustite (FeO) in the above heating temperature range. Moreover, even if the Fe-containing material is iron hydroxide, it is dehydrated and reduced to form wustite (FeO) in the above heating temperature range. When Si-containing materials and Fe-containing materials are mixed, they readily react to form Si--Fe-based oxides. In other words, since the Si-containing material and the Fe-containing material are mixed, an effect is obtained in which the liquid phase generation reaction proceeds rapidly. In addition, as described above, the polysaccharide does not affect the reactivity itself of the generation of the Si—Fe liquid phase component.

When at least one of the B-containing material, the P-containing material, and the S-containing material is mixed with the Si-containing material, the B-containing material, the P-containing material, and the S-containing material lower the eutectic point temperature, and the liquid phase formation reaction occurs. An effect that progresses from an early stage during heating is obtained. In other words, even at the same heating temperature (for example, 1170 ° C.), when at least one of the B-containing, P-containing, and S-containing substances is included, the liquid phase is higher than when they are not included. Higher yield and easier descaling. Moreover, the polysaccharide does not affect the effect of lowering the eutectic point temperature by at least one of the B-containing material, the P-containing material, and the S-containing material.

In the present invention, at least one of the Fe-containing material, the B-containing material, the P-containing material, and the S-containing material may be mixed with the Si-containing material. and at least one of S-containing substances. Due to the fact that the mixture contains Fe-containing material, the liquid phase generation reaction proceeds quickly, and the mixture contains at least one of B-containing material, P-containing material, and S-containing material, and the eutectic point temperature is reduced. It is possible to synergistically obtain the effect of lowering the temperature and progressing the liquid phase generation reaction from an early stage during heating.

When a mixture containing Si-containing substances and polysaccharides, and optionally at least one of Fe-containing substances, B-containing substances, P-containing substances, and S-containing substances is placed on the surface of the steel before heating, the temperature rises to 1170 ° C. or higher. In the state of , the reaction readily occurs to form a Si--Fe liquid phase oxide. On the other hand, Fe that diffuses outward within the scale from the steel material forms a new scale on the surface of the scale. Liquid phase oxides can be infiltrated. This Si—Fe-based liquid-phase oxide can easily move within the scale, and penetrates the scale/steel (internal oxide layer) interface as well. The time taken to penetrate to the scale/steel (internal oxide layer) interface is 15 minutes at 1170°C. The heating time is not particularly limited as long as it is 15 minutes or longer, but the upper limit is 10 hours. Even if it is longer than that, the effect is saturated, and it is considered that the heat energy is lost. Further, when heating is performed at 1170° C. or more and 1300° C. or less for 15 minutes or more and 10 hours or less, scale may grow and a scale having a thickness of 3 mm or more may be formed.

A mixture containing a Si-containing material, a polysaccharide, and optionally at least one of an Fe-containing material, a B-containing material, a P-containing material, and an S-containing material should be arranged as widely as possible on the surface of the steel material. It is desirable that the entire surface be treated, since the formation reaction of the -Fe-based liquid-phase oxide tends to proceed.
However, since the Si—Fe-based liquid phase oxide has the property of easily moving within the scale, it can also move over a distance of several tens of centimeters. Therefore, the mixture containing Si-containing substances and polysaccharides to be penetrated into the scale, and optionally containing at least one of Fe-containing substances, B-containing substances, P-containing substances, and S-containing substances, is not necessarily the entire surface of the steel material surface It is not necessary to evenly apply or stick it to the surface of the steel material, and if it is placed on a part of the surface of the steel material, even if it is unevenly distributed, the descaling property of the entire surface of the steel material can be improved. Therefore, the degree of placement of the mixture may be 99% or less in terms of area ratio with respect to the surface area of the steel plate. On the other hand, as described above, the mixture containing Si-containing material and polysaccharide and optionally at least one of Fe-containing material, B-containing material, P-containing material and S-containing material is formed on the surface of the steel material The formation reaction of a sufficient amount of Si--Fe-based liquid-phase oxide proceeds more easily when it is arranged as widely as possible. Therefore, the degree of arrangement of the mixture is preferably 50% or more in area ratio to the surface area of the steel plate. Unless otherwise specified, in the present invention, the steel material surface refers to the upper surface (front surface) of the steel material when the steel material is placed in a horizontal position, and does not include the side surface or the lower surface (rear surface) of the steel material.

In the present invention, as shown in FIG. 3, when the mixture is placed on the surface of the steel material before heating, a sufficient amount of Si—Fe in the scale is liquid phase oxidized during heating at a high temperature of 1170 ° C. or higher for 15 minutes or longer. The material penetrates through the grain boundaries of the scale and distributes throughout the scale and down to the scale/steel (internal oxide layer) interface. Furthermore, the effect of improving scale removability is also obtained by polysaccharides. Therefore, the outer layer scale and the inner layer scale can be easily removed by subsequent descaling. In addition, descaling in the present invention refers to a method of exfoliating and removing scales on the steel material surface by spraying water from a nozzle onto the surface of the steel material at a temperature of 900° C. or higher. The water pressure is desirably 3 MPa or more and 100 MPa or less.

On the other hand, when a substance other than a mixture of a Si-containing material and a polysaccharide (which may optionally contain at least one of Fe-containing material, B-containing material, P-containing material, and S-containing material) is used, That is, when using a substance (for example, Al ₂ O ₃ ) that does not generate a sufficient amount of liquid phase oxide and does not have the property that the liquid phase oxide infiltrates into the scale, as shown in FIG. , the material placed before heating remains between the outer and inner scales. This is because the outer layer scale has the property of growing outward due to the diffusion of Fe ions, and the inner layer scale has the property of growing due to the inward movement of oxygen, and the molten liquid phase oxide cannot move within the scale. be. When descaling is performed in such a state, the liquid phase oxide and the outer layer scale thereon can be removed, but it is difficult to remove the inner layer scale.

Conventional antioxidants are used to prevent the surface of steel materials from oxidizing during heating, and their basic principle is to block oxygen from the surface of steel materials. Therefore, it is necessary to uniformly apply the antioxidant to the entire surface of the portion where oxidation is not desired. In general, antioxidants are applied not only to the top surface (front surface) of the steel material, but also to the side and bottom surfaces (back surface) of the steel material. Also, the antioxidant needs to have high adhesion. This is because oxygen can enter and reach the steel surface even from a small crack, so it is necessary to prevent fine peeling after application and foaming of the antioxidant during heating ( For example, see Patent Document 3). In this way, since the antioxidant is used in close contact with the steel material, it is not assumed that the antioxidant is made into a liquid phase and moved within the scale.

The amount of Si contained in the mixture placed on the surface of the steel material is 85 to 2000 g/m ² as the amount of Si per surface area of the steel material. Here, the steel material surface area is the area of the upper surface (front surface) of the steel material when the steel material is placed in a horizontal position. The amount of Si is defined as the amount of Si content (g/m ² )×(total atomic weight of Si per molecule of Si-containing substance/molecular weight of Si-containing substance). If the amount to be placed is less than 85 g/m ² , the penetration of the Si—Fe-based liquid phase oxide into the entire scale and the scale/steel (internal oxide layer) interface is insufficient, and the descale by polysaccharides containing C and the like is insufficient. - The descalability may not be improved even considering the ring promoting effect. From the viewpoint of improving the descalability, the larger the Si content, the better, and the upper limit of the amount to be arranged is not particularly limited. However, if the amount to be placed exceeds 2000 g/m ² , even if a polysaccharide is added as a thickener, it will be difficult to place it on the surface of the steel material, or the Si-containing material will not come into contact with the atmosphere sufficiently, resulting in the formation of a liquid phase. It may be difficult for the reaction to proceed. Therefore, the upper limit of the amount of the Si-containing material is 2000 g/m ² .

The Si-containing material is not particularly limited as long as it contains Si, and any one of metal Si, SiC, SiN, and SiO ₂ or a combination thereof can be used. Silica (SiO ₂ ) may be used because it is stable, easy to handle, readily available, and inexpensive. Moreover, it is preferable that the Si-containing material is in the form of a paste. Being in the form of a paste, it has an appropriate viscosity, so that the Si-containing material and the mixture containing it can be easily increased in amount without spilling from the steel material, and can be easily applied to the surface of the steel material.

The mixing ratio of polysaccharides in the mixture placed on the surface of the steel material is generally represented by (C ₆ H ₁₀ O ₅ ) _n , so the amount of polysaccharides is controlled by the amount of C contained, and the amount of C is The amount of Si is 1 to 10%. Generally, as the amount of C increases, the thickening effect is considered to improve, so the amount of polysaccharides mixed as a thickening agent is defined by the amount of C. If the amount of C is less than 1%, the adhesion of the mixture to the steel material is low, and the mixture may peel off due to contact between the steel materials or contact with other equipment during transportation of the steel material. Also, from the viewpoint of descalability, descalability improves as the amount of C increases as described above. This tendency is remarkable when the scale is thick, for example, when the scale thickness is 3 mm or more. However, if the amount of C exceeds 10%, the amount of CO ₂ gas and H ₂ O gas generated increases, the ratio of voids occupying the scale increases, and the liquid phase generation reaction is hindered and the descaling property is reduced. In order to reduce the content, the upper limit of the amount of C is set to 10%.

The polysaccharide is not particularly limited as long as it contains an element capable of generating gas such as C, H, and O and has an action to adjust the viscosity appropriately as a thickening agent. Those which are easy to handle and readily available are preferred. Carboxymethylcellulose, hydroxyethylcellulose, hydroxypropylmethylcellulose, xanthan gum, tamarind seed gum, guar gum, locust bean gum, carrageenan, pectin, galactomannan, propylene glycol, starch, dextrin, or mixtures thereof may be used as polysaccharides.

The Fe inclusions may be iron oxides or iron hydroxides. Iron oxides or iron hydroxides (containing Fe) include wustite, magnetite, hematite, margemite, iron (II) hydroxide, iron (III) hydroxide, goethite, akaganite, lepidocrocite, ferroxyhyte, Or it may be a mixture thereof. Although wustite is preferable because it easily forms a liquid phase, other iron oxides can also be reduced to wustite, and the same effect can be obtained. Iron hydroxide is also dehydrated by heating at a high temperature and reduced to wustite. Moreover, the iron oxide or iron hydroxide (Fe-containing material) is desirably powdered so as to be easily mixed with the Si-containing material and the polysaccharide.

The amount of Fe in the material containing Fe (iron oxide or hydroxide) is suitably 0.3 to 20% by mass of the amount of Si in the material containing Si. The amount of Fe is defined as the amount of Fe content (g/m ² )×(sum of atomic weights of Fe per molecule of Fe-containing substance/molecular weight of Fe-containing substance). If the amount of Fe is less than 0.3%, the liquid phase generation reaction may not be promoted and the descaling property may not be improved. On the other hand, if the amount of Fe is more than 20%, the viscosity may increase to such an extent that it becomes difficult to mix with the Si-containing material and the polysaccharide and to dispose the mixture. In addition, if the upper limit of either the Si-containing material or the Fe-containing material contained in the mixture is exceeded, it will easily spill from the surface of the steel material into the heating furnace. Oxides can damage refractories in furnaces.

When B-containing, P-containing, and S-containing materials are mixed, the amount of each element per surface area of the steel material is 4 to 40 g/m ² as the amount of B per surface area of the steel material, and 8 to 8 g/m as the amount of P per surface area of the steel material. 80 g/m ² , and 8 to 80 g/m ² as the amount of S per surface area of the steel material.
The lower limits of the mixed amounts of the B-containing material, the P-containing material, and the S-containing material are set based on the following considerations.
B, P and S have the effect of lowering the eutectic point between _FeO and _Fe2SiO4 . In the case of B, the inventors have found that the concentration of B in the range of 1 cm deep from the surface of the steel material must be 0.005% by mass or more in order to obtain the effect. In addition, it has been found that the concentration of P and S needs to be 0.01% by mass or more. Based on this knowledge, B, S, and P are supplied from the B-containing material, the P-containing material, and the S-containing material within a depth range of 1 cm from the surface of the steel material. In that case, based on the fact that the density of Fe, which is the main component of the steel material, is 7.9 g/cm ³ , the lower limit of the mixed amount of each content is set as follows. (For B, 7.9 g/cm ³ ×1 cm×0.005%=3.95×10 ⁻⁴ g/cm ² ≈4 g/m ² , for P and S, 7.9 g/cm ³ ×1 cm ×0.01%=7.9×10 ⁻⁴ g/cm ² ≈8 g/m ² )
On the other hand, the upper limit is set based on the following idea. In general, the greater the amount of each of these elements added, the greater the effect of lowering the eutectic point. However, if the amount added is too large, the quality of the steel material may be affected. B is said to be an effective element for strengthening grain boundaries and increasing the strength of steel. there is As for S, if the content is too large, the workability may deteriorate, so 0.1% by mass is set as the upper limit concentration. As for P, grain boundary embrittlement tends to occur and workability may deteriorate, so the upper limit concentration is set at 0.1% by mass. This effect is thought to extend to a depth of ¹ cm from the surface of the steel material. The upper limit of the mixed amount of substances is set as follows. (For B, 7.9 g/cm ³ ×1 cm×0.05%=39.5×10 ⁻⁴ g/cm ² ≈40 g/m ² , for P and S, 7.9 g/cm ³ ×1 cm ×0.1%=79×10 ⁻⁴ g/cm ² ≈80 g/m ² )

The B-containing material is not particularly limited as long as it is a compound containing B, but B ₂ O ₃ (boron oxide) may be used in consideration of availability and handling. The S-containing material is not particularly limited as long as it is a compound containing S, but in consideration of availability and handling, CuS (copper sulfide), FeS (iron sulfide), and MnS (manganese sulfide) are used. may be used. The P-containing material is not particularly limited as long as it is a compound containing P, but P ₂ O ₅ (diphosphorus pentoxide) may be used in consideration of availability and handling.

A method of mixing a mixture containing a Si-containing material and a polysaccharide and optionally containing at least one of an Fe-containing material, a B-containing material, a P-containing material, and an S-containing material is capable of homogeneously mixing these materials. It is not particularly limited as long as it is used. For example, it may be mixed batchwise using a mixer, shooter, shovel, or the like, or may be continuously mixed by putting it in a hopper or the like.

The steel material handled in the present invention contains 0.3 to 1.5% by mass of Ni. The higher the Ni content, the more difficult the descaling. However, when the Ni content is less than 0.3%, descaling is possible by a general descaling method without using the present invention. . When the Ni content is 0.3% or more, descaling becomes difficult, and the effect of the present invention appears clearly. When the Ni content exceeds 1.5%, descaling is difficult even with the present invention.

The steel material handled in the present invention contains 0.001 to 0.5% by mass of Si. Si is an element that is added to steel materials from the viewpoint of ensuring strength and improving weldability, and may be added as appropriate according to the desired performance. However, if the Si content is less than 0.001%, the cost will increase, so this is the lower limit. In addition, when the Si content exceeds 0.5%, the descalability is good at 1170° C. or higher without using the present invention. This is because when the steel material is heated to form scales, the Si content in the steel material forms Si—Fe-based liquid-phase oxides, thereby improving the descaling property. Therefore, the upper limit of the Si content of the steel material handled in the present invention is set to 0.5% by mass.

The chemical components contained in the steel other than these are, in mass %, 0.001 to 1.0% for C, 0.001 to 2% for Mn, 0.001 to 0.1% for P, and 0.001 for S. 001% to 0.1% and 0.001% to 0.05% for B, the effects of the present invention can be enjoyed without affecting the descalability within the scope of the present invention.
C is an effective element for ensuring strength at a low cost, but if the amount added increases, the workability decreases.
Mn is also an element added to improve strength. The strength improvement effect of Mn increases as the Mn content increases. However, even if the Mn content exceeds 2%, the workability deteriorates although the strength increases with the amount added. Therefore, the Mn content is set to 0.001 to 2%.
P is also an alloying element that is effective in improving strength, and P-based oxides lower the eutectic temperature and increase the liquid phase, so that scale removability is improved. However, even if the content exceeds 0.1%, the effect on scale exfoliation is saturated, and intergranular embrittlement is likely to occur, resulting in deterioration of workability, so the upper limit is made 0.1%.
S is originally an impurity, and if it is contained in a large amount, it impairs cold or hot workability, so it is preferable that it is as small as possible. No problem.
B is an element that has the effect of improving the hardenability of steel, and since 0.001% or more is necessary to obtain this effect, the B content may be 0.001% or more. If the B content exceeds 0.05%, the above effect is saturated and not only does the cost rise, but the manufacturability during hot rolling is hindered, so the B content is 0.05% or less. .

The heating in the present invention may also serve as heating for hot working. Steel materials are often subjected to hot working so that the steel materials have desired shapes and properties. When heating is performed at 1170° C. or higher and 1300° C. or lower for 15 minutes or longer and 10 hours or shorter, scale may grow to a thickness of 3 mm or more. According to the present invention, after disposing a mixture comprising a Si-containing material and a polysaccharide and optionally at least one of an Fe-containing material, a B-containing material, a P-containing material and an S-containing material on the surface of the steel sheet, After the steel sheet is heated and descaled, scale is removed, especially scale having a thickness of 3 mm or more. In addition, the temperature of the steel material after descaling can often be maintained at the temperature necessary for hot working. Therefore, after descaling, hot working is possible without separate heating, and since the scale is sufficiently removed, no scale flaws occur during working. The type of hot working is not particularly limited, and includes hot rolling, hot forging, hot extrusion, and the like.

The steel material may be brake disc material. Typically, brake discs are formed by hot forging Ni-containing steel. In hot forging, a steel material is heated and softened, and then pressure is applied using a press machine or the like to form a mold. The heating temperature range for softening the steel material may overlap with or be lower than the temperature range of 1170° C. or higher and 1300° C. or lower. Therefore, by applying the present invention to hot forging of brake disc materials, it is possible to easily and efficiently obtain brake discs free of oxide scales.

Depending on the heating time of the steel material, the amount of the mixture containing Si-containing material, polysaccharide, and optionally at least one of Fe-containing material, B-containing material, P-containing material, and S-containing material may be changed. good. This is in order to secure a certain amount of liquid-phase Si--Fe-based oxides in the scale and to ensure a higher descaling property, and to prevent generation of liquid-phase Si--Fe-based oxides more than necessary. This is because it leads to cost reduction by not using excessive polysaccharides. That is, the amount of Si, polysaccharide, and at least one of the optionally added Fe-containing material, B-containing material, P-containing material, and S-containing material are controlled according to the expected amount of scale formation. In general, the amount of scale produced depends on the heating time at 1170° C. or higher. That is, the longer the heating time, the greater the amount of scale produced. Therefore, the amount of the mixture may be varied depending on the heating time. In order to secure a good descaling property, it is a standard to secure a liquid phase Si—Fe oxide amount of 50% or more in volume fraction with respect to the total amount of inner layer scale. Note that the properties of the scale and the state of the Ni-enriched layer may vary depending on the composition of the steel material and the oxidation conditions, and the amount of the mixture may be adjusted according to these conditions.

A mixture of Si-containing materials, polysaccharides, and optionally additional ingredients such as Fe, B, P, and S-containing materials was placed on the upper surface of the steel material having the chemical components shown in Table 1 below, and transported to a heating furnace. Assuming a brake disc, we adopted a cylindrical steel material. In the line used for transportation, the cylindrical surface of the steel material is continuously transported in a vertical state, so the surface on which the mixture is arranged and the bottom surface of the other steel material are in contact with each other. Fig. 5 shows the situation during the experiment. After that, in an LPG combustion gas atmosphere with an air ratio set to 1.1, it is heated at a predetermined temperature and for a predetermined time shown in Table 2, taken out from the furnace, and descaled at a steel temperature of 1200 ° C. (descaling water pressure: 150 kg / cm ² ), followed by hot forging (press pressure 2000 kg/cm ² ).
As an example, Si-containing materials and polysaccharides, and optionally Fe, B, P, and S-containing materials were mixed, and their mass ratios were changed (see Table 2), and placed on the steel material before heating. , heating, descaling, and hot forging to determine the presence or absence of surface flaws (scale flaws). In the comparative example, after forging in the same procedure as in the example, except that the Si-containing material and the polysaccharide, and optionally the Fe, B, P, and S-containing materials were mixed so as to be outside the scope of the present invention. The presence or absence of surface flaws (scale flaws) was determined.
Determination of surface flaws: × when surface flaws that pose a problem with the surface quality of the product are visually observed; A case where was confirmed by visual observation and cross-sectional observation was evaluated as ◯.
Table 2 shows the mixing conditions and surface flaw results.

(Regarding the amount of Si content)
In Examples 1 to 14 of the present invention, a mixture of a Si-containing material and a polysaccharide is placed in an amount within the range of the present invention, and heated at a temperature and time within the range of the present invention. The scale was sufficiently removed, and the subsequent forging did not cause unacceptable defects in terms of surface quality (○).
In Comparative Example 1, the amount of Si content was less than the lower limit of the present invention (85 g/m ² ), and even if descaling was performed, the scale was not sufficiently removed, and the subsequent forging caused a problem in terms of surface quality. A flaw was produced (x). It is suggested that the amount of Si—Fe-based liquid phase oxide produced was not sufficient.
In Comparative Example 7, the amount of Si content was greater than the upper limit of the present invention (2000 g/m ² ), and even if descaling was performed, the scale was not sufficiently removed, and the subsequent forging caused a problem in terms of surface quality. A flaw was produced (×). It is suggested that when the amount of the Si-containing material is large, the surface of the steel material is not sufficiently in contact with the atmosphere, and the reaction to generate the liquid phase is difficult to proceed.

(Regarding the amount of C content)
Here, the ratio of the amount of C contained in the polysaccharide to the amount of Si contained in the Si-containing material is considered.
In Comparative Examples 2, 3, and 4, the ratio of C was less than the lower limit value (1%) of the present invention, and the mixture partly separated during transportation of the steel material before heating. After descaling, flaws (Δ) were observed in the cross section although there was no quality problem, suggesting that the Si content was not sufficient.
In Comparative Examples 5 and 6, the ratio of C was higher than the upper limit value (10%) of the present invention, and even if descaling was performed, the scale was not sufficiently removed, and in subsequent forging, defects that became a problem in terms of surface quality occurred. (×). It is suggested that a large ratio of substances other than Si-containing substances hinders the formation reaction of the liquid phase.

Aside from the mixing conditions in Table 2 above, a test was conducted to confirm the effect on the descalability when only the amount of C in the mixture was changed. Specifically, in the mixture, the Si content is fixed at 1200 g/m ² as the Si content, and the Fe content is fixed at 1% of the Si content in terms of Fe, and only the C content is changed in the range of 0% to 15%. let me Carboxymethylcellulose was used as a C-containing polysaccharide (thickener). This mixture is placed on the steel material before heating, heated at 1250 ° C. for 5 hours in an LPG combustion gas atmosphere with the air ratio set to 1.1, and taken out from the furnace. A scaling water pressure of 150 kg/cm ² ) was applied. Table 3 shows the area ratio of peeled scale. It is suggested that the higher the ratio of C, the higher the scale peeling area ratio and the better the descalability. However, when the ratio of _C is 15%, the _{descalability} is lowered. It is thought that the descaling property was lowered by impeding the formation reaction of the phase.

(About heating temperature and heating time)
In Inventive Example 13, the heating temperature was 1300° C. and the heating time was 10 hours. Even in this case, the scale was sufficiently removed by descaling, and the subsequent forging did not cause unacceptable defects in terms of surface quality. (○). That is, it was found that the present invention is effective even when heated at high temperature for a long time. In Example 14 of the present invention, the heating temperature was 1180° C. and the heating time was 15 minutes. I didn't. That is, it was confirmed that the effects of the present invention can be achieved even at a heating temperature and a heating time near the lower limits of the range of the present invention.

Claims (4)

A process of heating a steel material containing 0.3 to 1.5% by mass of Ni and 0.001 to 0.5% by mass of Si at a temperature of 1170° C. or more and 1300° C. or less for 15 minutes or more and 10 hours or less. A method for removing oxide scale produced in
a Si-containing material of 85 to 2000 g/m ² as an amount of Si per surface area of the steel material;
The amount of C is 1.0% of the amount of Si in terms of % by mass . 0 to 10% polysaccharide, and
Arranging the mixture on the steel material surface of the steel material,
performing the heating on the steel material,
A method for removing oxide scale, characterized by performing descaling thereafter. In mixing the Si-containing material and the polysaccharide,
An Fe content of 0.3 to 20% of the Si amount in mass% as the Fe amount,
B content of 4 to 40 g/m ² as a B amount per surface area of the steel material,
P content of 8 to 80 g/m ² as a P amount per surface area of the steel material,
2. The method for removing oxide scale according to claim 1, further comprising mixing at least one S-containing substance with an S amount of 8 to 80 g/m ² per surface area of said steel material. The polysaccharides comprise carboxymethylcellulose, hydroxyethylcellulose, hydroxypropylmethylcellulose, xanthan gum, tamarind seed gum, guar gum, locust bean gum, carrageenan, pectin, galactomannan, propylene glycol, starch, dextrin, or mixtures thereof. 3. A method according to claim 1 or claim 2. A method according to any one of claims 1 to 3, wherein the steel material is brake disc material.

Images