Coating agent, and method for producing steel material using same

Abstract

A coating agent according to the present embodiment contains a glass frit. The glass frit contains, by mass% in terms of oxides, 60.0-70.0% of SiO2, 10.0-24.0% of Al2O3, greater than 10.0% and up to 20.0% of Na2O, 0-5.0% of K2O, 0-6.0% of CaO, and 0-5.0% of MgO.

Description

Coating agent and method for manufacturing steel material using the same

[0001] The present disclosure relates to a coating agent and a method for manufacturing a steel product using the same.

[0002] For example, steel is hot-worked to produce a product. Before hot-working the steel, it is heated in a heating furnace. When the steel is carbon steel, the heating temperature may be approximately 1150 to 1300°C. In the heating furnace, the steel is heated at the above-mentioned high temperature for a long period of time. The heating furnace usually has an oxidizing atmosphere. Therefore, during heating, oxygen in the furnace atmosphere reacts with Fe in the surface layer of the steel, forming an oxide scale. The oxygen in the furnace atmosphere further reacts with carbon in the surface layer of the steel. In this case, a decarburized layer is formed in the surface layer of the steel. The decarburized layer reduces the strength of the surface layer of the steel. Therefore, even when heating at high temperatures, it is preferable to suppress decarburization of the steel.

[0003] Techniques for suppressing decarburization of the surface layer of a steel material during heating have been proposed in Japanese Patent Laid-Open Nos. 8-53709 (Patent Document 1) and 5-86415 (Patent Document 2).

[0004] The decarburization prevention material disclosed in Patent Document 1 contains as its active ingredients a mixture of 100 parts by weight of frit having a shoulder melting temperature of 1000°C or higher and 1350°C or lower, and 0.5 to 100 parts by weight of a metal powder having a melting point of 600°C or higher, dispersed in water and / or an organic solvent. Patent Document 1 describes that decarburization of steel is suppressed by applying the above-mentioned decarburization prevention agent to the surface of the steel and then heating the steel.

[0005] The composition for preventing oxidation decarburization disclosed in Patent Document 2 comprises 5 to 90 wt % of a flux component having a softening point or melting point of 1300° C. or less, 5 to 90 wt % of SiC, and Cr. ２ O ３ and 0 to 75% by weight of a refractory filler having a melting point of 1,300° C. or higher. Patent Document 2 describes that by applying the above-mentioned composition for preventing oxidation and decarburization to the surface of a steel material and then heating the steel material, oxidation and decarburization of the steel material surface can be suppressed.

[0006] JP-A-8-53709 JP-A-5-86415

[0007] However, decarburization of steel materials during heating at high temperatures of 1150 to 1300°C may be suppressed by using means other than those described in Patent Documents 1 and 2.

[0008] An object of the present disclosure is to provide a coating agent that can suppress decarburization of the surface layer of a steel material even when the steel material is heated at high temperatures, and a method for manufacturing a steel material using the coating agent.

[0009] The coating agent of the present disclosure contains glass frit. The glass frit contains, in mass % when converted into oxide, SiO ２ :60.0~70.0%, Al ２ O ３ :10.0~24.0%, Na ２ O: more than 10.0 to 20.0%, K ２ Contains O: 0 to 5.0%, CaO: 0 to 6.0%, and MgO: 0 to 5.0%.

[0010] The method for manufacturing steel material of the present disclosure includes a step of applying the coating agent of the present disclosure to the surface of steel material, and a step of heating the steel material to which the coating agent has been applied at 1150 to 1300°C.

[0011] The coating agent according to the present disclosure can suppress decarburization of the surface layer of the steel material even when the steel material is heated at high temperatures. The method for producing a steel material according to the present disclosure can produce a steel material in which decarburization is suppressed.

[0012] FIG. 1 is a schematic diagram of an EPMA spectrum for explaining how to determine the C concentration ratio ΔC in the examples.

[0013] The present inventors first investigated and studied a coating agent that can suppress decarburization of the surface layer of a steel material even when the steel material is heated at a high temperature of 1150 to 1300°C in a heating furnace. In this specification, high temperature means a temperature range of 1150 to 1300°C.

[0014] When steel is heated at high temperatures in a heating furnace with an oxidizing atmosphere, the surface layer of the steel is decarburized for the following reasons: Oxygen in the oxidizing atmosphere in the heating furnace comes into contact with the surface of the steel being heated at high temperatures. At this time, C in the surface layer of the steel reacts with the oxygen in contact with the steel surface to generate CO gas. The C in the surface layer of the steel is released from the surface layer as CO gas, resulting in decarburization of the steel surface.

[0015] CO gas is also used as a carburizing gas in carburizing treatment. Therefore, when CO gas comes into contact with the surface of a steel material, the carbon in the CO gas penetrates the surface of the steel material, carburizing the surface layer of the steel material. However, the CO gas generated by the decarburization of the surface layer of the steel material leaves the surface of the steel material and is released into the oxidizing atmosphere in the heating furnace.

[0016] Here, the inventors considered that if the CO gas generated by decarburization could be utilized for carburizing the surface layer of a steel material, decarburization of the surface layer of the steel material could be suppressed. If the CO gas generated by decarburization could be suppressed from diffusing into an oxidizing atmosphere and the CO gas could be retained on the surface of the steel material, the CO gas would promote carburization of the surface layer of the steel material. As a result, it is believed that decarburization of the surface layer of the steel material could be suppressed. As a specific means for suppressing the diffusion of CO gas into an oxidizing atmosphere, if a coating that suppresses the diffusion of CO gas during high-temperature heating could be formed on the surface of the steel material, the CO gas could be retained on the surface of the steel material. Hereinafter, a coating that suppresses the diffusion of CO gas will also be referred to as a "CO gas diffusion-suppressing film."

[0017] Based on the above considerations, the present inventors investigated the components of a coating agent capable of forming a CO gas emission-suppressing film when heated at high temperatures. If the coating agent contains a component that vitrifies on the steel surface when heated at high temperatures, a CO gas emission-suppressing film may be formed. The present inventors considered that it would be effective to use glass frit as a coating agent that vitrifies at high temperatures. Therefore, the present inventors further investigated the components of the glass frit.

[0018] As a component of glass frit, SiO, which is a component that increases the softening point, ２ and Al ２ O ３ and Na, a component that lowers the softening point. ２ O.K. ２ By appropriately adjusting the contents of O, CaO, and MgO, it is possible to form a glass coating that can sufficiently wet and spread over the surface of a steel material at high temperatures and sufficiently cover the surface of the steel material. Therefore, the inventors adjusted the contents of the above components and investigated the relationship between the contents of these components and the effect of suppressing decarburization of the surface layer of a steel material during high-temperature heating. As a result, the inventors obtained the following new findings.

[0019] In glass frit, K is used as a component to lower the softening point. ２ When the content of O, CaO and MgO is increased, the oxide scale (iron oxide scale) on the surface of the steel material becomes K. ２ It may react with O, CaO, and MgO. In this case, part of the oxide scale is destroyed, and decarburization is promoted in the destroyed part. On the other hand, Na, which is also a component that lowers the softening point, ２ O does not easily react with the oxide scale on the surface of steel. Therefore, Na is used as a component to lower the softening point in glass frit. ２ If the O content is increased, the oxide scale on the surface of the steel material is more likely to be maintained without being destroyed when heated at high temperatures, and in this case, decarburization of the steel material is suppressed.

[0020] Based on the above findings, the present inventors further investigated the effect of SiO ２、 Al ２ O ３ , Na ２ O.K. ２ The appropriate contents of O, CaO and MgO were investigated. As a result, it was found that the glass frit contains, in mass % converted into oxides, SiO ２ :60.0~70.0%, Al ２ O ３ :10.0~24.0%, Na ２ O: more than 10.0 to 20.0%, K ２ It has been found that if the steel contains 0 to 5.0% O, 0 to 6.0% CaO, and 0 to 5.0% MgO, decarburization of the surface layer of the steel material during high-temperature heating can be sufficiently suppressed.

[0021] The coating agent of this embodiment, which was completed based on the above findings, and the method for manufacturing a steel material using the same, are as follows.

[0022] The coating agent of the first embodiment contains glass frit. The glass frit contains, in terms of oxide, SiO. ２ :60.0~70.0%, Al ２ O ３ :10.0~24.0%, Na ２ O: more than 10.0 to 20.0%, K ２Contains O: 0 to 5.0%, CaO: 0 to 6.0%, and MgO: 0 to 5.0%.

[0023] The coating material of the second embodiment is the coating material of the first embodiment, and the glass frit satisfies the formula (1). ２ O] + [CaO] + [MgO]) / [Na ２ O]≦1.20 (1) where [K ２ O] is the mass % of K converted into oxide in the glass frit. ２ The O content is substituted for [CaO], the CaO content in mass % when converted to oxide in the glass frit is substituted for [CaO], the MgO content in mass % when converted to oxide in the glass frit is substituted for [MgO], and the Na ２ O] is the mass % of Na when converted into oxide in the glass frit. ２ The O content is substituted.

[0024] A third embodiment of the coating agent is the first or second embodiment of the coating agent, further comprising carbon powder, wherein the ratio of the mass (g) of the carbon powder to the mass (g) of the glass frit in the coating agent is 0.10 to 0.30.

[0025] The fourth form of the method for manufacturing steel material includes a step of applying any one of the first to third forms of coating agent to the surface of the steel material, and a step of heating the steel material to which the coating agent has been applied at 1150 to 1300°C.

[0026] The coating agent of this embodiment and the method for manufacturing a steel material using the coating agent will be described below.

[0027] [Regarding the Coating Agent] The coating agent of this embodiment contains glass frit. The glass frit is glass obtained by melting a glass raw material, solidifying it, and then pulverizing it. The glass frit is, for example, in the form of particles or flakes.

[0028] [Components of Glass Frit] The glass frit contained in the coating agent of the present embodiment contains, in mass % converted to oxide, SiO ２ :60.0~70.0%, Al ２ O ３:10.0~24.0%, Na ２ O: more than 10.0 to 20.0%, K ２ It contains 0 to 5.0% of O, 0 to 6.0% of CaO, and 0 to 5.0% of MgO. Each component will be explained below.

[0029] SiO ２ : 60.0 to 70.0% Silica (SiO ２ ) forms a CO gas emission suppression film on the steel surface together with other components during heating at high temperatures. ２ becomes the matrix of the CO gas diffusion-inhibitory film. The CO gas diffusion-inhibitory film allows oxygen to pass through. Therefore, oxygen outside the CO gas diffusion-inhibitory film penetrates into the interior of the CO gas diffusion-inhibitory film, that is, between the CO gas diffusion-inhibitory film and the steel surface. In this case, oxygen reacts with C in the steel surface layer to generate CO gas. On the other hand, CO gas does not easily penetrate the CO gas diffusion-inhibitory film. Therefore, CO gas does not easily diffuse from the CO gas diffusion-inhibitory film into the oxidizing atmosphere in the external heating furnace. Therefore, CO gas generated by C that has escaped from the steel surface layer remains between the CO gas diffusion-inhibitory film and the steel surface. As a result, C that has escaped from the steel surface layer penetrates back into the steel surface layer. Therefore, decarburization of the steel surface layer due to heating at high temperatures is suppressed. SiO ２ If the content is less than 60.0%, the above effects cannot be sufficiently obtained.

[0030] On the other hand, SiO ２ If the content exceeds 70.0%, the softening point of the glass frit becomes too high. In this case, the coating agent does not wet and spread easily when heated at high temperatures. Therefore, it becomes difficult for the CO gas emission suppression film formed by the coating agent to sufficiently cover the steel surface. As a result, decarburization of the steel surface layer is not sufficiently suppressed. SiO ２ If the content exceeds 70.0%, the raw material of the glass frit mixed with other components may not melt. In this case, the glass frit cannot be produced. Therefore, SiO ２ The content is 60.0 to 70.0%.

[0031] SiO ２ The lower limit of the SiO content is preferably 62.5%, and more preferably 65.0%. ２The upper limit of the content is preferably 68.0%, more preferably 66.0%, and even more preferably 65.5%.

[0032] Al ２ O ３ : 10.0 to 24.0% alumina (Al ２ O ３ ) is SiO ２ Similarly to the above, a CO gas emission suppression film is formed on the surface of the steel material during heating at high temperatures. ２ O ３ Furthermore, Al increases the softening point of the glass frit. Therefore, the coating agent is prevented from dripping off the steel material during heating at high temperatures, and the coating agent is more likely to remain on the steel material surface. As a result, the CO gas emission suppression film formed by the coating agent can sufficiently cover the steel material surface during heating at high temperatures, and decarburization of the steel material surface is sufficiently suppressed. ２ O ３ If the content is less than 10.0%, the above effects cannot be sufficiently obtained.

[0033] On the other hand, Al ２ O ３ If the content exceeds 24.0%, Al ２ O ３ SiO content ２ In this case, the softening point of the glass frit is lowered, and the coating agent tends to drip off the surface of the steel material during heating at high temperatures. ２ O ３ If the content exceeds 24.0%, the raw material of the glass frit mixed with other components may not melt, and in this case, the glass frit cannot be produced. ２ O ３ The content is 10.0 to 24.0%.

[0034] Al ２ O ３ The lower limit of the Al content is preferably 12.0%, and more preferably 14.0%. ２ O ３ The upper limit of the content is preferably 23.0%, more preferably 21.0%, and even more preferably 20.0%.

[0035] Na ２ O: Over 10.0 to 20.0% sodium oxide (Na ２ O) lowers the softening point of the glass frit. Furthermore, it lowers the viscosity of the coating agent during heating at high temperatures. Therefore, the fluidity of the coating agent increases during heating at high temperatures. As a result, the CO gas emission suppression film can sufficiently cover the steel surface. ２ O further contains other alkaline components (K ２ Compared with other oxides (Fe oxides, CaO, and MgO), Na is less likely to react with oxide scale, which is mainly composed of Fe oxides. Therefore, when carbon steel is heated at high temperatures, destruction of the scale due to reaction with the oxide scale is suppressed. As a result, decarburization due to destruction of the oxide scale during heating at high temperatures can be suppressed. ２ If the O content is 10.0% or less, the above effects cannot be sufficiently obtained. ２ If the O content is 10.0% or less, the glass frit raw material mixed with other components may not melt, making it impossible to produce glass frit.

[0036] On the other hand, Na ２ If the O content exceeds 20.0%, the softening point of the glass frit is excessively lowered. Furthermore, the viscosity of the coating agent is excessively lowered during heating at high temperatures. In this case, the coating agent drips off the steel surface during heating at high temperatures. As a result, the CO gas emission suppression film cannot sufficiently cover the steel surface, and the steel surface layer is decarburized. Therefore, Na ２ The O content is more than 10.0 to 20.0%.

[0037] Na ２ The lower limit of the O content is preferably 11.0%, and more preferably 12.0%. ２ The upper limit of the O content is preferably 18.0%, more preferably 16.0%, and even more preferably 14.0%.

[0038] K ２ O: 0 to 5.0% potassium oxide (K ２ O) may not be contained. ２ O is an optional component, and K ２The O content may be 0%. ２ O lowers the softening point of the glass frit, so that the coating agent can wet and spread during heating at high temperatures, and can sufficiently cover the steel surface. ２ Even if only a small amount of O is contained, the above effects can be obtained to some extent.

[0039] However, K ２ O is Na ２ Compared to O, it reacts more easily with oxide scale, which is mainly composed of Fe oxide. ２ If the O content exceeds 5.0%, K ２ O reacts with oxide scale and easily destroys it. ２ The O content is 0 to 5.0%.

[0040] K ２ The lower limit of the O content is preferably 0.1%, and more preferably 0.5%. ２ The upper limit of the O content is preferably 4.0%, and more preferably 3.0%.

[0041] CaO: 0 to 6.0% Calcium oxide (CaO) does not have to be contained. In other words, CaO is an optional component, and the CaO content may be 0%. When contained, CaO lowers the softening point of the glass frit. Therefore, during heating at high temperatures, the coating agent wets and spreads, and can sufficiently cover the steel surface. Even if even a small amount of CaO is contained, the above effects can be obtained to some extent.

[0042] However, CaO is ２ Compared with O, CaO reacts more easily with oxide scale mainly composed of Fe oxides. Therefore, if the CaO content exceeds 6.0%, CaO reacts with oxide scale during heating at high temperatures, making the oxide scale more likely to be destroyed.

[0043] CaO also promotes the permeation of oxygen into the CO gas emission suppression film. As described above, C in the surface layer of the steel reacts with oxygen that comes into contact with the steel surface to generate CO gas. However, if excessive oxygen in the oxidizing atmosphere in the furnace permeates the CO gas emission suppression film during high-temperature heating, the generated CO gas will further react with oxygen to form CO ２ It becomes a gas. ２ Gas does not contribute to carburizing the steel surface, so CO ２ If the CO gas is reduced due to the generation of gas, the decarburization of the steel material cannot be sufficiently suppressed. ２ Excessive gas is produced. Therefore, the CaO content is 0-6.0%.

[0044] The lower limit of the CaO content is preferably 0.1%, more preferably 0.3%, and the upper limit of the CaO content is preferably 5.5%, more preferably 5.0%, even more preferably 4.5%, and still more preferably 4.0%.

[0045] MgO: 0 to 5.0% Magnesia (MgO) does not have to be contained. In other words, MgO is an optional component, and the MgO content may be 0%. When contained, MgO lowers the softening point of the glass frit. Therefore, if MgO is contained, the coating agent can wet and spread during heating at high temperatures, and can sufficiently cover the steel surface. Even if even a small amount of MgO is contained, the above effect can be obtained to some extent.

[0046] However, MgO is ２ Compared to O, MgO reacts more easily with oxide scale, which is mainly composed of Fe oxide. Therefore, if the MgO content exceeds 5.0%, MgO reacts with oxide scale during heating at high temperatures, making it more likely to destroy the oxide scale. Therefore, the MgO content is 0 to 5.0%.

[0047] The lower limit of the MgO content is preferably 0.1%, more preferably 0.5%, and the upper limit of the MgO content is preferably 4.5%, more preferably 4.0%.

[0048] The glass frit of this embodiment contains the above-mentioned components. The glass frit may contain impurities in an amount of 0 to 1.0% by mass in terms of oxides as the remainder other than the above-mentioned components. The impurities are, for example, those mixed in from inorganic materials used as raw materials or the manufacturing environment when industrially manufacturing the glass frit. The impurities include, for example, Fe. ２ O ３ , B ２ O ３ , ZrO ２ , TiO ２ , Sb ２ O ３ , SrO, BaO, ZnO, PbO, Li ２ O and P ２ O ５ etc.

[0049] [Method for Analyzing Glass Frit Components] The composition of the glass frit is analyzed by the following method. The composition of the glass frit is obtained by performing inductively coupled plasma (ICP) analysis on the glass frit in a dry state. Specifically, the content of each element, such as Si, Al, Na, K, Ca, and Mg, is determined by ICP analysis. Assuming that each element forms an oxide, the content of the oxide of each element is converted from the content of each element obtained by ICP analysis. The content of each oxide when the total of the oxides obtained by conversion is taken as 100% is taken as the content (mass %) of each component in the glass frit.

[0050] [Effects of the Coating Agent of the Present Embodiment] When the coating agent of the present embodiment containing the glass frit having the above-described configuration is applied to the surface of a steel material and the steel material is heated at a high temperature of 1150 to 1300°C, the coating agent wets and spreads over the steel material surface during heating at high temperatures, forming a CO gas emission-suppressing film. The CO gas emission-suppressing film retains CO gas containing C that has escaped from the steel material surface between the coating agent and the steel material surface, suppressing the CO gas from escaping into an oxidizing atmosphere. Therefore, the C that has escaped from the steel material surface penetrates back into the steel material surface. As a result, decarburization of the steel material surface can be suppressed even when the steel material is heated at high temperatures.

[0051] [Preferable component ratio in the coating agent] Preferably, the coating agent of the present embodiment further satisfies the following formula (1): 0.40≦([K ２ O] + [CaO] + [MgO]) / [Na ２ O]≦1.20 (1) where [K ２ O] is the mass % of K converted into oxide in the glass frit. ２ The O content is substituted for [CaO], the CaO content in mass % when converted to oxide in the glass frit is substituted for [CaO], the MgO content in mass % when converted to oxide in the glass frit is substituted for [MgO], and the Na ２ O] is the mass % of Na when converted into oxide in the glass frit. ２ The O content is substituted.

[0052] As mentioned above, Na ２ O.K. ２ O, CaO, and MgO all lower the softening point of the glass frit and improve the wettability of the coating material on the steel surface during heating at high temperatures. ２ O, CaO, and MgO react with the oxide scale (iron oxide scale) formed on the surface of the steel material, partially destroying the oxide scale. In the area where the oxide scale is destroyed, decarburization is promoted. On the other hand, Na ２ O is K ２ Compared with O, CaO, and MgO, Na is less likely to react with oxide scale during heating at high temperatures. ２ With respect to the content of O, ２ The ratio of the contents of O, CaO and MgO is reduced.

[0053] Here, Na ２ O is K ２ Compared with O, CaO, and MgO, Na is more likely to lower the viscosity of the coating material during heating at high temperatures. ２ O content is K ２If the content is too large compared to the total content of O, CaO, and MgO, the viscosity of the coating agent will decrease too much during heating at high temperatures. If the viscosity of the coating agent decreases too much, the coating agent may drip off the steel material. If the coating agent drips off the steel material, decarburization of the surface layer of the steel material cannot be suppressed. Therefore, in the coating agent of this embodiment, preferably, Na ２ With respect to the O content, K ２ The ratio of the total content of O, CaO and MgO is increased to some extent.

[0054] FA is defined as follows: FA = ([K ２ O] + [CaO] + [MgO]) / [Na ２ O]

[0055] FA is an index relating to oxide scale maintenance and viscosity of the coating agent during heating at high temperatures. If the FA is 1.20 or less, the reaction between the coating agent and oxide scale is sufficiently suppressed. Therefore, during heating at high temperatures, the reaction between the coating agent and oxide scale is sufficiently suppressed, and the CO gas emission suppression film can sufficiently cover the steel surface. As a result, decarburization of the steel surface layer can be further suppressed.

[0056] If the FA is 0.40 or more, the viscosity of the coating agent becomes appropriate. Therefore, the coating agent is less likely to drip from the steel material during heating at high temperatures. Therefore, the coating agent can sufficiently cover the steel material surface, and the CO gas emission suppression film can sufficiently cover the steel material surface. As a result, decarburization of the steel material surface can be further suppressed.

[0057] A more preferred lower limit of FA is 0.45, even more preferably 0.55, and even more preferably 0.65. A more preferred upper limit of FA is 1.10, even more preferably 1.00, and even more preferably 0.90.

[0058] [Regarding the Preferred Softening Point of the Glass Frit] Preferably, the softening point of the glass frit is 900°C or higher. The softening point of the glass frit is determined by the composition of the glass frit. When the glass frit of this embodiment satisfies the above-mentioned components, the softening point is 900°C or higher. If the softening point of the glass frit is 900°C or higher, when the steel material is heated at a high temperature, the coating agent wets and spreads over the steel material surface, sufficiently covering the steel material surface. As a result, decarburization of the steel material surface layer is sufficiently suppressed. A more preferred lower limit of the softening point of the glass frit is 930°C, and even more preferred is 950°C. The upper limit of the softening point of the glass frit is not particularly limited, but a preferred upper limit is, for example, 1200°C, more preferably 1100°C, and even more preferably 1000°C.

[0059] [Method for Measuring Softening Point] The softening point of the coating agent is measured using a simultaneous thermogravimetry-differential thermal analyzer (TG-DTA). Specifically, 10 mg of the coating agent is placed in the sample holder of the TG-DTA. The coating agent is heated from room temperature to 1300°C at a temperature increase rate of 10°C / min. The atmosphere during heating is Ar. The differential heat at this time is measured, and the fourth inflection point on the DTA (differential thermal) chart is taken as the softening point of the coating agent.

[0060] [Coating Agent Containing Carbon Powder] Preferably, the coating agent of this embodiment further contains carbon powder. When the coating agent contains carbon powder, the ratio of the mass (g) of the carbon powder to the mass (g) of the glass frit is 0.10 to 0.30. These characteristics are described below.

[0061] [Regarding carbon powder] As described above, during heating at high temperatures, the coating agent applied to the steel surface melts and forms a CO gas emission-inhibiting film. The CO gas emission-inhibiting film covers the steel surface. The CO gas emission-inhibiting film allows oxygen in an oxidizing atmosphere to pass through. When the coating agent contains carbon powder, the carbon powder in the CO gas emission-inhibiting film reacts with oxygen to generate CO gas. The generated CO gas remains between the CO gas emission-inhibiting film and the steel surface. Therefore, the CO gas carburizes the surface layer of the steel.

[0062] As described above, when the coating agent contains carbon powder, it is possible to not only suppress decarburization of the surface layer of the steel material when heated at high temperatures in an oxidizing atmosphere, but also to carburize the surface layer of the steel material.

[0063] The carbon powder is a powder made of carbon. For example, the carbon powder is made of one or more selected from the group consisting of graphite powder, carbon black powder, charcoal powder, bamboo charcoal powder, and coke powder. A preferred carbon powder is graphite powder.

[0064] [Carbon Powder Ratio RA] When carbon powder is contained, the ratio of the mass (g) of carbon powder to the mass (g) of glass frit in the coating agent is defined as the "carbon powder ratio RA." Preferably, the carbon powder ratio RA is 0.10 to 0.30. Here, the mass (g) of glass frit means the mass (g) of glass frit in a dry state, and the mass (g) of carbon powder means the mass (g) of carbon powder in a dry state.

[0065] If the carbon powder ratio RA is 0.10 or more, during heating at high temperatures, the carbon in the CO gas emission-inhibiting film reacts with oxygen permeating from the outside of the CO gas emission-inhibiting film, generating a sufficient amount of CO gas between the CO gas emission-inhibiting film and the steel surface. This CO gas carburizes the steel surface. Therefore, when heated at high temperatures using a heating furnace in an oxidizing atmosphere, decarburization of the steel surface can be further suppressed, and the carbon concentration of the steel surface can be further increased. On the other hand, if the carbon powder ratio RA is 0.30 or less, the amount of carbon powder relative to the amount of glass frit in the coating agent is sufficient. In this case, the CO gas emission-inhibiting film can sufficiently cover the steel surface during heating at high temperatures. Therefore, the carbon powder ratio RA is 0.10 to 0.30.

[0066] A more preferable lower limit of the carbon powder ratio RA is 0.11, more preferably 0.12, even more preferably 0.13, even more preferably 0.14, and even more preferably 0.15. A more preferable upper limit of the carbon powder ratio RA is 0.29, even more preferably 0.28, even more preferably 0.27, even more preferably 0.26, even more preferably 0.25, and even more preferably 0.24.

[0067] [Method for Measuring Carbon Powder Ratio RA] The carbon powder ratio RA in the coating agent is measured using the following method. A thermogravimetry (TG) device is used to measure the mass of the coating agent, which changes due to heat. The mass of the coating agent is not particularly limited, but is, for example, 1 g. Specifically, the coating agent is heated from room temperature (25°C) to 100°C and held at 100°C. The moisture contained in the coating agent evaporates, and the mass remains constant for 30 seconds, at which point the mass X0 of the coating agent is measured. After measuring the mass, the coating agent is heated to 600°C and held at 600°C. This oxidizes the carbon powder in the coating agent, removing it from the coating agent. The mass X1 of the coating agent is measured after the carbon powder contained in the coating agent is removed, and the mass remains constant for 30 seconds. Mass X1 corresponds to the mass of the glass frit. Mass X0 - Mass X1 corresponds to the mass of the carbon powder. Using the obtained masses X0 and X1, the carbon powder ratio RA is calculated by the following formula: Carbon powder ratio RA = (X0 - X1) / X1

[0068] [Regarding Components of the Coating Agent Other Than Glass Frit and Carbon Powder] The coating agent of the present embodiment may further contain, in addition to the glass frit and carbon powder, one or more components selected from the group consisting of water, a suspending agent, and a dispersing agent. These components may not be contained. Water, a suspending agent, and a dispersing agent will be described below.

[0069] [Water] When the coating agent is in the form of a slurry, the coating agent may contain, for example, water. In the case of a slurry coating agent, for example, the preferred water content is 70 to 100 parts by mass per 100 parts by mass of glass frit. If the water content is too low or too high, it is difficult to apply the coating agent to the steel surface. By adjusting the water content, the viscosity of the slurry coating agent can be adjusted to a level that allows it to be applied to the steel surface at room temperature.

[0070] [Suspension Agent] The suspension agent thoroughly disperses the glass frit in the solution (water). The suspension agent contains, for example, gairome clay and bentonite and / or sepiolite. When the coating agent of this embodiment contains gairome clay and bentonite and / or sepiolite, the coating agent is less likely to drip from the steel surface. Furthermore, when the coating agent dries and solidifies, the coating agent is less likely to peel off from the steel surface.

[0071] The gairome clay contains kaolin clay and a plurality of quartz particles. Specifically, the gairome clay contains kaolinite, halloysite, and quartz.

[0072] Gairome clay improves the sagging resistance of the slurry coating material. A slurry coating material containing Gairome clay is less likely to drip after being applied to the surface of steel at room temperature. Therefore, if Gairome clay is included, the coating material will easily cover the entire surface of the steel at room temperature.

[0073] When the coating agent contains gairome clay, the preferred gairome clay content in the coating agent is 4 parts by mass or more per 100 parts by mass of glass frit. In this case, the coating agent is less likely to drip from the steel surface. A more preferred gairome clay content is 5 parts by mass or more, and even more preferably 6 parts by mass or more. On the other hand, the preferred upper limit of the gairome clay content is 30 parts by mass. In this case, the glass frit in the coating agent is sufficiently dispersed on the surface of the steel. Therefore, a CO gas emission suppression film is more sufficiently formed during heating at high temperatures.

[0074] Bentonite is a clay primarily composed of montmorillonite, and may further contain silicate minerals such as quartz and opal, silicate minerals such as feldspar and zeolite, carbonate minerals and sulfate minerals such as dolomite, and sulfide minerals such as pyrite.

[0075] Sepiolite is a hydrous magnesium silicate, e.g., Mg ８ Si １２ O ３０ (OH) ４ (OH ２ ) ４ ・8H ２ It is represented by the chemical formula O.

[0076] Both bentonite and sepiolite inhibit peeling of the coating agent. Specifically, the coating agent is applied to the steel surface at room temperature. Then, the coating agent is solidified by heating or drying. When the coating agent contains bentonite and / or sepiolite, the bentonite and sepiolite inhibit the solidified coating agent from peeling from the steel surface. A coating agent containing bentonite and / or sepiolite is unlikely to peel from the steel surface even when subjected to an external force. The coating agent may contain at least one of bentonite and sepiolite.

[0077] The content of bentonite and / or sepiolite is preferably 4 parts by mass or more relative to 100 parts by mass of glass frit. In this case, the peel resistance of the coating agent is further improved. The content of bentonite and / or sepiolite is more preferably 5 parts by mass or more.

[0078] The total content of bentonite and sepiolite is preferably less than 9 parts by mass relative to 100 parts by mass of glass frit, in which case the glass frit is sufficiently dispersed in the coating agent. The total content of bentonite and sepiolite is more preferably less than 8 parts by mass.

[0079] The suspending agent may contain clays other than the above-mentioned frog clay, bentonite and sepiolite. Clays include, for example, iron oxide (Fe ２ O ３ ) may be contained.

[0080] [Dispersant] The coating agent of this embodiment does not necessarily contain a dispersant. If a dispersant is contained, the amount of water in which the glass frit and carbon powder are dispersed can be reduced, resulting in improved adhesion of the coating agent to the steel surface. Examples of dispersants include inorganic salts such as sodium tripolyphosphate, sodium hexametaphosphate, sodium ultrapolyphosphate, acidic sodium hexametaphosphate, sodium borate, sodium carbonate, and polymetaphosphate; sodium citrate, sodium tartrate, polyacrylic acid, sodium polyacrylate, sodium sulfonate, polycarboxylate, β-naphthalenesulfonates, melamine sulfonates, and naphthalenesulfonic acid. In the coating agent, the total amount of dispersant preferably contained is less than 4.5 parts by mass per 100 parts by mass of glass frit.

[0081] [Method for Manufacturing Coating Agent] The coating agent of this embodiment is manufactured, for example, by the following method. Each component that serves as the raw material for glass frit satisfying the above-mentioned components is prepared. The prepared components are mixed. The mixed components are melted. The molten components are quenched in water or air to vitrify them. The melting temperature is, for example, 1400 to 1600°C. The vitrified intermediate product after quenching is pulverized. For example, a ball mill is used for pulverization. Through the above steps, glass frit is manufactured. The softening point of the glass frit can be adjusted to, for example, 900°C or higher by adjusting the components before mixing. A slurry coating agent is manufactured by adding, for example, water to the prepared glass frit.

[0082] When carbon powder is contained in the coating agent, carbon powder is further mixed into the above-mentioned slurry coating agent. At this time, the amount of carbon powder is adjusted so that the carbon powder ratio RA is 0.10 to 0.30. By the above manufacturing process, a carbon-containing coating agent is manufactured. Furthermore, if necessary, a suspending agent and / or a dispersing agent is added to the slurry coating agent. By the above process, the coating agent is manufactured.

[0083] [Method for manufacturing steel material using coating agent] The method for manufacturing steel material according to this embodiment includes a step of applying a coating agent to the surface of the steel material (coating step) and a step of heating the steel material to which the coating agent has been applied at 1150 to 1300° C. (heating step). Each step will be described below.

[0084] [Coating step] In the coating step, first, the coating agent of this embodiment manufactured by the above-mentioned [Manufacturing method of coating agent] is prepared. The prepared slurry-like coating agent is applied to the surface (upper surface, lower surface, and side surface) of the steel material before heating. The coating agent is applied to the surface of the steel material at room temperature, for example.

[0085] The method for applying the coating agent is not particularly limited. An operator may apply the coating agent to the surface of the steel material using a brush. Alternatively, the coating agent may be applied to the surface of the steel material by a spray or the like. Alternatively, the steel material may be immersed in a bathtub containing the coating agent (so-called "dipping in the hot water"). By these application methods, a slurry-like coating agent is applied to the surface of the steel material. After the slurry-like coating agent has been applied to the surface of the steel material, the coating agent may be dried before the steel material is heated at a high temperature.

[0086] The amount of coating agent applied to the steel surface is not particularly limited. For example, the amount of coating agent applied to the steel surface is 0.1 to 0.3 g / cm ２ is.

[0087] [Target Steel Material] In the manufacturing method of this embodiment, the type of target steel material is not particularly limited. The target steel material is preferably a steel material made of carbon steel. Here, the steel material made of carbon steel includes carbon steel materials for machine structures specified in JIS G 4051:2023 and carbon steel pipes for general machine structures specified in JIS G 3478:2021. When these steel materials are heated at high temperatures in a heating furnace in an oxidizing atmosphere, an oxide scale made of Fe oxides forms on the surface of the steel material.

[0088] [Heating step] The steel material coated with the coating agent is heated (heating step). In the heating step, the steel material coated with the coating agent is heated at a high temperature of 1150 to 1300°C. A preferred upper limit of the heating temperature is 1250°C, and more preferably 1200°C. The heating time is not particularly limited. For example, the heating time is 1.0 to 8.0 hours. A preferred upper limit of the heating time is 5.0 hours, and more preferably 2.5 hours.

[0089] The coating agent of this embodiment contains glass frit satisfying the above-mentioned components. When heated at high temperatures, the coating agent wets and spreads on the steel surface, forming a CO gas emission-inhibiting film that sufficiently covers the steel surface. This CO gas emission-inhibiting film is permeable to oxygen from outside. Between the CO gas emission-inhibiting film and the steel surface, oxygen that has permeated from outside combines with C that has escaped from the steel surface to generate CO gas. The CO gas emission-inhibiting film allows oxygen to permeate but does not easily allow CO gas to permeate. Therefore, CO gas remains between the CO gas emission-inhibiting film and the steel surface. The C that has escaped from the steel surface due to this CO gas penetrates back into the steel surface. As a result, decarburization of the steel surface can be suppressed even when heated at high temperatures of 1150 to 1300°C.

[0090] If the coating agent contains carbon powder and the carbon powder ratio RA is set to 0.10 to 0.30, oxygen that has permeated from the outside reacts with the carbon powder between the CO gas emission-suppressing film and the steel surface to generate CO gas. A sufficient amount of CO gas is generated by the carbon powder. Therefore, when the generated CO gas comes into contact with the steel surface, not only is decarburization of the steel surface suppressed, but the carbon concentration of the steel surface layer can also be increased.

[0091] The effects of the coating agent according to the embodiment of the present disclosure will be described in more detail below using examples. The conditions in the following examples are one example of conditions adopted to confirm the feasibility and effects of the coating agent according to the embodiment of the present disclosure. Therefore, the coating agent according to the embodiment of the present disclosure is not limited to this one example of conditions.

[0092] In this example, a coating agent was prepared by mixing multiple glass frits with different compositions and graphite powder in various mass ratios. Steel materials coated with the coating agent were heated at high temperatures, and the change in carbon concentration in the surface layer of the steel materials before and after heating was investigated. This example is described below.

[0093] The glass frits shown in Table 1 were prepared (glass frit symbols A to O). The compositions of the glass frits in Table 1 were determined by the method described in the above-mentioned [Method for analyzing glass frit components]. Note that glass frit symbol H indicates SiO ２ Since the content exceeded the upper limit, the mixed components could not be melted, and the glass frit could not be produced. ２ O ３ Since the content exceeded the upper limit, the mixed components could not be melted, and the glass frit could not be produced. ２ Since the O content was below the lower limit, the mixed components could not be melted, and glass frit could not be produced.

[0094]

[0095] Glass frits having the glass frit codes in Table 1, water, and a suspending agent were mixed to prepare coating agents having respective test numbers in Table 2. Each coating agent was produced by mixing 70 parts by mass of water, 6 parts by mass of frog clay, and 5 parts by mass of bentonite with 100 parts by mass of glass frit.

[0096]

[0097] In test numbers 5 to 15, the coating agent further contained carbon powder (graphite powder). The carbon powder ratio RA was as shown in "Carbon powder ratio RA" in Table 2. Note that the coating agents in test numbers 1 to 4 and 16 to 21 did not contain carbon powder.

[0098] Carbon steel equivalent to S45C of the JIS standard was used as the plate-shaped test specimen. The size of the plate-shaped test specimen was 10 mm thick, 20 mm long, and 20 mm wide. The C content of each plate-shaped test specimen was 0.45% by mass. Therefore, the C concentration [C] on the surface (steel surface) of each plate-shaped test specimen was Ｂ was set to 0.45% by mass.

[0099] The coating agent was applied to the 20 x 20 mm surface of the plate-shaped test piece using a brush, and then dried. The coating amount for each test number is as shown in Table 2. The coating amount was measured as follows: the mass of the plate-shaped test piece before coating and the mass of the entire test piece after coating with the coating agent were measured, and the coating amount was calculated from the difference between the two.

[0100] The plate-shaped test specimens coated with the coating material were heated for 1.0 hour at the heating temperatures (700 to 1200°C) shown in Table 2. The atmosphere was set to simulate LNG combustion. Specifically, the composition of the atmospheric gas was 2 to 3% O. ２ , 8-10% CO ２ , 15-20%H ２ O and the remainder is N ２ The atmospheric gas was adjusted so that

[0101] The carbon concentration (mass%) on the surface (steel surface) of the heated plate test specimen was measured by the following method. The observation surface was a cross section perpendicular to the longitudinal direction (cross section parallel to the width and thickness directions) at the longitudinal center of the plate test specimen. Five measurement points were identified in a row at 1 mm intervals in the width direction at the surface position of the width center of the plate test specimen on the observation surface. Of the five measurement points arranged in a row, the middle measurement point was positioned at the width center of the observation surface. Line analysis was performed using an electron probe microanalyzer (EPMA) on a line segment 2000 μm in the thickness direction from the measurement point. In the line analysis using the EPMA, the acceleration voltage was 15 kV, the irradiation current was 500 nA, the electron beam diameter was 3 μm, and the measurement pitch was 4 μm. The above-mentioned line analysis was performed on the five measurement line segments on the observation surface. In each measurement line, spectra based on Fe, O, and C were obtained from the surface of the plate-shaped test piece to a depth of 2000 μm.

[0102] Figure 1 shows a schematic diagram of spectra based on Fe, O, and C obtained by EPMA. In Figure 1, the horizontal axis indicates the distance (µm) in the thickness direction from the surface, which is the measurement point of the plate-shaped test piece. In Figure 1, the vertical axis of the spectra of Fe, O, and C indicates the concentration in mass %. Referring to Figure 1, the concentration of Fe is measured from the surface to the thickness D Ｃ The O concentration has a downward convex peak up to a thickness D Ｃ In other words, the thickness D based on these Fe and O Ｃ It was determined that the peaks up to were due to the coating material and oxide film attached to the surface of the plate-shaped test piece. Therefore, the thickness D of the spectrum based on Fe, O, and C obtained by EPMA Ｃ The peaks up to thickness D Ｃ is defined as follows: In the spectrum based on O, the upward convex peak due to the coating material and the oxide film decreases and the O concentration when it reaches a minimum point is [O] Ｓ Here, the minimum point is the transition point where the O concentration changes from decreasing to increasing, and is the position where the first derivative becomes zero. The distance in the thickness direction from the surface to the minimum point is called the thickness D Ｃ The thickness D Ｃ The thickness D is 500 μm from the thickness direction Ｒ The thickness D in the carbon (C) spectrum was determined. Ｃ From thickness D Ｒ The arithmetic average of the C concentration was calculated from the thickness D. It should be noted that, referring to FIG. 1, an extremely low and sharp peak of the C concentration such as peak LP is considered to be caused by impurities or the like. Ｃ From thickness D Ｒ The peaks up to 10 ...

[0103] The C concentration at the start of the peak decay is defined as the C concentration [C] Ｓ The C concentration at the end of the peak decay is the C concentration [C] ＭPeaks with a peak decay rate (%) of 30% or more calculated using the following formula were determined to be peak LPs and were excluded from the arithmetic mean of the C concentration. Peak decay rate = ([C] Ｓ -[C] Ｍ ) / [C] Ｓ ×100

[0104] The thickness D obtained by EPMA at five measurement lines Ｃ From thickness D Ｒ The arithmetic mean value of the C concentration up to the point where the surface of the steel material after heating is Ａ (% by mass).

[0105] Obtained C concentration [C] Ａ and the C concentration [C] on the surface (steel surface) of the plate-shaped test piece before heating Ｂ Based on this, the C concentration ratio ΔC was calculated using the following formula: ΔC = C concentration [C] Ａ / C concentration [C] Ｂ The obtained C concentration ratio ΔC is shown in the “ΔC” column of Table 2.

[0106] [Test Results] Referring to Tables 1 and 2, the coating agents of test numbers 1 to 8 satisfied the ranges for each component of the glass frit. Therefore, in the coating agents of these examples, the C concentration ratio ΔC was 1.00 or more, and decarburization of the steel surface layer during heating at high temperatures could be suppressed.

[0107] In particular, the coating agents of Test Nos. 7 and 8 had an FA of 0.40 or more and 1.20 or less, and a carbon powder ratio RA of 0.10 to 0.30. Therefore, in these examples, compared to the coating agents of Test Nos. 1 to 5, the C concentration ratio ΔC was increased, and excellent decarburization suppression effects of the steel surface layer during heating at high temperatures were obtained. Furthermore, the coating agent of Test No. 6 also had an FA of 0.40 or more and 1.20 or less, and a carbon powder ratio RA of 0.10 to 0.30. Therefore, compared to Test No. 5, which was manufactured under the same conditions as the coating agent of Test No. 6, the C concentration ratio ΔC was increased, and excellent decarburization suppression effects of the steel surface layer during heating at high temperatures were obtained. The coating agents of Test Nos. 1 and 3 had an FA of 0.40 or more and 1.20 or less. Therefore, in these examples, compared to the coating agents of Test Nos. 2 and 4, the C concentration ratio ΔC was increased, and excellent decarburization suppression effects of the steel surface layer during heating at high temperatures were obtained.

[0108] On the other hand, in the coating material of test number 9, SiO ２ is less than the lower limit, and Na ２ O is less than the lower limit, and K ２ The amount of O exceeded the upper limit, and the amount of CaO exceeded the upper limit. Therefore, the C concentration ratio ΔC was less than 1.00, and decarburization of the surface layer of the steel material could not be suppressed when heated at a high temperature.

[0109] In the coating material of test number 10, Na was added to each component of the glass frit. ２ O was below the lower limit and CaO was above the upper limit. Therefore, the C concentration ratio ΔC was less than 1.00, and decarburization of the surface layer of the steel material could not be suppressed during heating at high temperatures.

[0110] In test numbers 11 to 15, the heating temperature of the plate-shaped test specimen after application of the coating agent was less than 1150° C. Therefore, the C concentration ratio ΔC was less than 1.00, and decarburization of the steel surface layer could not be suppressed during heating at high temperatures.

[0111] In the coating material of test number 16, SiO ２ Therefore, the C concentration ratio ΔC was less than 1.00, and decarburization of the surface layer of the steel material could not be suppressed when heated at a high temperature.

[0112] In the coating material of test number 17, Al was used in each component of the glass frit. ２ O ３ Therefore, the C concentration ratio ΔC was less than 1.00, and decarburization of the surface layer of the steel material could not be suppressed when heated at a high temperature.

[0113] In the coating material of test number 18, Na was added to each component of the glass frit. ２ Therefore, the C concentration ratio ΔC was less than 1.00, and decarburization of the surface layer of the steel material could not be suppressed when heated at a high temperature.

[0114] In the coating material of test number 19, K was added to each component of the glass frit. ２ Therefore, the C concentration ratio ΔC was less than 1.00, and decarburization of the surface layer of the steel material could not be suppressed when heated at a high temperature.

[0115] In the coating agent of test number 20, the CaO content exceeded the upper limit of each component of the glass frit. Therefore, the C concentration ratio ΔC was less than 1.00, and decarburization of the steel surface layer could not be suppressed when heated at high temperatures.

[0116] In the coating material of test number 21, the MgO content exceeded the upper limit of each component of the glass frit, and therefore the C concentration ratio ΔC was less than 1.00, and decarburization of the steel surface layer could not be suppressed when heated at high temperatures.

[0117] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure.

Claims

1. A glass frit is contained, and the glass frit contains, in mass % when converted to oxide, SiO ２ :60.0~70.0%, Al ２ O ３ :10.0~24.0%, Na ２ O: more than 10.0 to 20.0%, K ２ A coating agent comprising: O: 0 to 5.0%, CaO: 0 to 6.0%, and MgO: 0 to 5.0%.

2. The coating material according to claim 1, wherein the glass frit satisfies the formula (1). ２ O] + [CaO] + [MgO]) / [Na ２ O]≦1.20 (1) where [K ２ O] is the mass % of K in terms of oxide in the glass frit. ２ The O content is substituted for [CaO], the CaO content in mass % when converted to oxide in the glass frit is substituted for [CaO], the MgO content in mass % when converted to oxide in the glass frit is substituted for [MgO], and the Na ２ O] is the mass % of Na in terms of oxide in the glass frit. ２ The O content is substituted.

3. The coating agent according to claim 1 or 2, further comprising carbon powder, wherein the ratio of the mass (g) of the carbon powder to the mass (g) of the glass frit in the coating agent is 0.10 to 0.

30.

4. A method for manufacturing steel material, comprising the steps of: applying the coating agent according to claim 1 to the surface of steel material; and heating the steel material to which the coating agent has been applied at 1150 to 1300°C.

Images