Carburizing coating agent and method for manufacturing steel using the same

Abstract

A carburizing coating agent according to an embodiment of the present disclosure contains glass frit and carbon powder. The glass frit contains, in mass % oxide equivalents, 20.0-40.0% SiO2, 10.0-30.0% Al2O3, 9.0-40.0% BO3, 0-30.0% Na2O, 0-5.0% KO, 0-7.0% CaO, and 0-5.0% MgO, and the ratio of the mass (g) of the glass frit to the mass (g) of the carbon powder in the carburizing coating agent is 0.10-0.49.

Description

[Technical Field]

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

[0002] For steel materials that require fatigue strength or wear resistance, carburizing is sometimes performed, in which carbon (C) is impregnated into the surface of the steel material and then quenched to harden the surface. Gas carburizing is the main method used for carburizing.

[0003] Gas carburizing involves heating and holding steel in an atmosphere containing a carburizing gas such as natural gas, propane gas, or butane gas. Gas carburizing requires that the steel surface come into contact with a high concentration of carburizing gas. Therefore, equipment is used that fills the furnace with carburizing gas. If carburizing could be performed in a heat treatment furnace that heats in an oxidizing atmosphere without using such dedicated gas carburizing equipment, manufacturing costs could be reduced.

[0004] Therefore, as an alternative to the above-mentioned gas carburizing, methods of applying a specific paint to the surface of a steel material to perform the carburizing treatment have been proposed in JP 2000-119838 A (Patent Document 1), JP 2001-115249 A (Patent Document 2), and JP 2016-223011 A (Patent Document 3).

[0005] The carburizing treatment methods of Patent Documents 1 and 2 use a coat-type carburizing composition. The coat-type carburizing composition contains an olefin polymer resin and an aqueous medium. The coat-type carburizing composition further contains 10 to 100 parts by mass of charcoal powder per 100 parts by mass of the resin as a carburizing agent, and 5 to 80 parts by mass of sodium carbonate per 100 parts by mass of the resin as a carburizing accelerator. In the carburizing treatment methods of Patent Documents 1 and 2, the coat-type carburizing composition is applied to the portion of a steel material to be carburized and dried to form a coating. The steel material is then heated in a heat treatment furnace in an oxidizing atmosphere to carry out the carburizing treatment.

[0006] The carburizing method of Patent Document 3 involves preparing a surface treatment agent containing a sodium silicate solution and one or more metal compounds selected from the group consisting of metal oxides and carbonates. In this carburizing method, the surface treatment agent is applied to the surface of a steel material, and then the carburizing treatment is carried out in a heat treatment furnace in an oxidizing atmosphere. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-119838 [Patent Document 2] Japanese Patent Application Laid-Open No. 2001-115249 [Patent Document 3] Japanese Patent Application Laid-Open No. 2016-223011 Summary of the Invention [Problem to be solved by the invention]

[0008] However, the carburizing treatment may be carried out in a heat treatment furnace with an oxidizing atmosphere by means other than those described in Patent Documents 1 to 3.

[0009] An object of the present disclosure is to provide a carburizing coating material that enables carburizing treatment using a heat treatment furnace in an oxidizing atmosphere, and a method for manufacturing steel using the carburizing coating material. [Means for solving the problem]

[0010] The carburizing coating material according to the present disclosure contains glass frit and carbon powder. The glass frit contains, in mass % oxide equivalents, 20.0-40.0% SiO2, 10.0-30.0% Al2O3, 9.0-40.0% BO3, 0-30.0% Na2O, 0-5.0% KO, 0-7.0% CaO, and 0-5.0% MgO, and the ratio of the mass (g) of the glass frit to the mass (g) of the carbon powder in the carburizing coating material is 0.10-0.49.

[0011] The method for manufacturing a steel material according to the present disclosure includes a step of applying the carburizing coating agent of the present disclosure to the surface of a steel material, and a step of heating the steel material to which the carburizing coating agent has been applied at 950 to 1300°C. [Effects of the Invention]

[0012] The carburizing coating material according to this embodiment enables carburizing treatment using a heat treatment furnace in an oxidizing atmosphere. The method for manufacturing a steel material according to this embodiment can manufacture a steel material with a carburized surface layer. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a schematic diagram of an EPMA spectrum for explaining how to determine the C concentration ratio ΔC in the examples. DETAILED DESCRIPTION OF THE INVENTION

[0014] The present inventors have investigated a carburizing method using a heat treatment furnace with an oxidizing atmosphere. First, the inventors have investigated the principle by which a carburized layer is formed on the surface of a steel material by gas carburizing.

[0015] In gas carburizing, the carburized layer is formed according to the following principle: Steel is heated in the temperature range of 950 to 1300°C. A carburizing gas such as carbon monoxide (CO) gas comes into contact with the surface of the steel in this temperature range. At this time, the carbon in the carburizing gas penetrates and diffuses from the surface of the steel into the surface layer of the steel. As a result, a carburized layer is formed on the surface layer of the steel. According to the above principle, the carburizing gas functions as a carbon source, forming a carburized layer on the surface layer of the steel.

[0016] However, in a heat treatment furnace with an oxidizing atmosphere, it is difficult to fill the atmosphere with carburizing gas. Therefore, the inventors investigated the use of a new carbon source instead of carburizing gas to perform carburizing treatment. Here, the inventors considered using carbon powder as the carbon source. Carbon powder reacts with oxygen in an oxidizing atmosphere to generate CO gas. If this CO gas comes into contact with the surface of a steel material, a carburized layer may be formed on the surface of the steel material.

[0017] However, even if CO gas is generated by the carbon powder, if the CO gas does not remain near the surface of the steel material, a sufficient carburized layer will not be formed.

[0018] The inventors of the present invention believed that if a coating could be formed on the surface of a steel material that inhibits the diffusion of CO gas generated from the carbon powder applied to the surface of the steel material into the furnace, carburizing would be possible even in a heat treatment furnace with an oxidizing atmosphere. Specifically, a carburizing coating agent containing carbon powder and a composition that vitrifies on the steel surface in the temperature range of 950 to 1300°C is used. In this case, a vitrified coating containing carbon powder is formed on the steel surface in the high-temperature range. If this coating can confine the CO gas generated by the carbon powder between the steel surface and the coating, the CO gas will remain on the steel surface. As a result, a carburized layer can be formed on the surface of the steel material even in a heat treatment furnace with an oxidizing atmosphere. In the following description, the above-mentioned coating that inhibits CO gas emission and keeps it on the steel surface will be referred to as a "CO gas emission-inhibiting film."

[0019] Based on the above considerations, the inventors investigated the composition of a carburizing coating agent capable of forming a CO gas emission suppression film in the high temperature range. As a result, the inventors concluded that it would be effective to use glass frit as a composition that vitrifies in the high temperature range. Therefore, the inventors further investigated the composition of the glass frit.

[0020] By adjusting the content of SiO2 and Al2O3, which are components that increase the softening point, and B2O3, Na2O, KO, CaO, and MgO, which are components that decrease the softening point, in the composition of the glass frit, it is possible to form a CO gas emission-suppressing film that can sufficiently wet and spread over the steel surface and sufficiently cover the steel surface in the temperature range of 950 to 1300°C. Therefore, the present inventors adjusted the content of the above components to investigate the relationship between the composition of the glass frit and the carburizing effect on the steel surface at high temperatures. As a result, it was found that if the glass frit has a composition, calculated as oxide mass %, of SiO2: 20.0-40.0%, Al2O3: 10.0-30.0%, B2O3: 9.0-40.0%, Na2O: 0-30.0%, K2O: 0-5.0%, CaO: 0-7.0%, and MgO: 0-5.0%, it may be possible to carburize the surface of steel when the steel is heated in the temperature range of 950-1300°C.

[0021] However, even when using a carburizing coating agent containing the above-mentioned glass frit and carbon powder, there were cases where a sufficient carburized layer was not formed on the surface of the steel material. As a result of investigation, it was found that the reason for the insufficient formation of the carburized layer was due to the mixing ratio of the glass frit and the carbon powder. This point will be explained below.

[0022] In a carburizing coating, if the mass (g) of carbon powder is too small relative to the mass (g) of glass frit, CO gas, a carburizing gas, is not generated sufficiently. In this case, an adequate carburized layer is not formed on the surface of the steel material. On the other hand, if the mass of carbon powder is too large relative to the mass of glass frit, the CO gas diffusion-inhibiting film is not stably formed in the high-temperature range. As a result, cracks may occur in the CO gas diffusion-inhibiting film or parts of the CO gas diffusion-inhibiting film may peel off in the temperature range of 950 to 1300°C. This causes CO gas to diffuse into the heat treatment furnace.

[0023] Therefore, the present inventors considered that there is an appropriate mixing ratio between the mass (g) of glass frit and the mass (g) of carbon powder. As a result of extensive investigations, the present inventors found that in a carburizing coating agent containing glass frit of the above-mentioned composition, if the ratio of the mass (g) of glass frit to the mass (g) of carbon powder is 0.10 to 0.49, sufficient CO gas can be generated from the carbon powder, which is a carbon source, and further, a CO gas emission-suppressing film can be stably formed in a high temperature range.

[0024] The gist of the carburizing coating agent according to the embodiment of the present disclosure, which was completed based on the above findings, and the method of manufacturing a steel product using the same, is as follows.

[0025] The carburizing coating material of the first embodiment contains glass frit and carbon powder. The glass frit contains, in mass % converted to oxides, 20.0-40.0% SiO2, 10.0-30.0% Al2O3, 9.0-40.0% B2O3, 0-30.0% Na2O, 0-5.0% KO, 0-7.0% CaO, and 0-5.0% MgO, and the ratio of the mass (g) of the glass frit to the mass (g) of the carbon powder in the carburizing coating material is 0.10-0.49.

[0026] The method for manufacturing a steel material of this embodiment includes a step of applying the carburizing coating agent described in the first embodiment to the surface of a steel material, and a step of heating the steel material to which the carburizing coating agent has been applied at 950 to 1300°C.

[0027] The carburizing coating agent of this embodiment and the method for manufacturing a steel material using the carburizing coating agent will be described below.

[0028] [About carburizing coating agent] The carburizing coating material of this embodiment contains glass frit and carbon powder. The glass frit and carbon powder will be described below.

[0029] [About glass frit] Glass frit is glass obtained by melting glass raw materials, solidifying them, and then pulverizing them. The glass frit is, for example, in the form of particles or flakes. The glass frit contained in the carburizing coating agent of this embodiment contains, in mass % converted to oxides, 20.0 to 40.0% SiO2, 10.0 to 30.0% Al2O3, 9.0 to 40.0% B2O3, 0 to 30.0% Na2O, 0 to 5.0% KO, 0 to 7.0% CaO, and 0 to 5.0% MgO. Each component will be described below.

[0030] SiO2: 20.0~40.0% Silica (SiO2), together with other components, forms a CO gas diffusion-inhibitory film on the surface of steel in a heat treatment furnace with an oxidizing atmosphere at temperatures between 950 and 1300°C. SiO2 serves as the matrix of the CO gas diffusion-inhibitory film. The CO gas diffusion-inhibitory film is oxygen-permeable. Therefore, oxygen outside the CO gas diffusion-inhibitory film penetrates into the interior of the film, i.e., between the film and the steel surface. In this case, oxygen reacts with carbon on the steel surface to generate CO gas. However, the generated CO gas does not easily permeate the CO gas diffusion-inhibitory film. Therefore, CO gas is not easily diffused from the CO gas diffusion-inhibitory film into the external oxidizing atmosphere. Therefore, CO gas generated from the carbon powder, which is the carbon source, remains between the CO gas diffusion-inhibitory film and the steel surface. As a result, carburization by CO gas is promoted. If the SiO2 content is less than 20.0%, the above effects are not fully achieved.

[0031] On the other hand, if the SiO2 content exceeds 40.0%, the softening point of the carburizing coating becomes too high. In this case, the carburizing coating becomes difficult to wet and spread in the temperature range of 950 to 1300°C. Therefore, it becomes difficult for the CO gas emission suppression film formed by the carburizing coating to adequately cover the steel surface. As a result, the steel surface layer is not sufficiently carburized. Therefore, the SiO2 content is 20.0 to 40.0%.

[0032] The lower limit of the SiO2 content is preferably 22.0%, and more preferably 25.0%. The upper limit of the SiO2 content is preferably 37.0%, and more preferably 35.0%.

[0033] Al2O3: 10.0~30.0% Alumina (Al2O3), like SiO2, forms a CO gas emission suppression film on the steel surface together with other components in the temperature range of 950 to 1300°C in a heat treatment furnace with an oxidizing atmosphere. Al2O3, together with SiO2 and B2O3, forms the matrix of the CO gas emission suppression film. Al2O3 also increases the softening point of the glass frit. Therefore, in the above temperature range, it prevents the carburizing coating from dripping from the steel surface, making it more likely to remain on the steel surface. As a result, in the above temperature range, the CO gas emission suppression film formed by the carburizing coating can sufficiently cover the steel surface, promoting carburization by CO gas. If the Al2O3 content is less than 10.0%, the above effects cannot be fully achieved.

[0034] On the other hand, if the Al2O3 content exceeds 30.0%, the SiO2 content relative to the Al2O3 content decreases. In this case, the softening point of the glass frit decreases. As a result, the carburizing coating material tends to drip off the steel surface in the high temperature range. Therefore, the Al2O3 content is 10.0 to 30.0%.

[0035] The lower limit of the Al2O3 content is preferably 12.0%, more preferably 14.0%, and even more preferably 18.0%. The upper limit of the Al2O3 content is preferably 28.0%, more preferably 25.0%, and even more preferably 23.0%.

[0036] B2O3: 9.0~40.0% Boron oxide (B2O3), like SiO2, forms a CO gas emission suppression film on the steel surface together with other components in the temperature range of 950 to 1300°C in a heat treatment furnace with an oxidizing atmosphere. B2O3, together with SiO2 and Al2O3, forms the matrix of the CO gas emission suppression film. B2O3 also lowers the softening point of the carburizing coating material. Therefore, in the above temperature range, the carburizing coating material becomes more easily wetted and spreads, and as a result, the CO gas emission suppression film formed by the carburizing coating material can adequately cover the steel surface. If the B2O3 content is less than 9.0%, the above effect cannot be fully achieved.

[0037] On the other hand, if the B2O3 content exceeds 40.0%, the softening point of the carburizing coating becomes too low. In this case, the carburizing coating tends to drip from the steel surface in the above temperature range. As a result, the steel surface layer is not sufficiently carburized. Therefore, the B2O3 content is 9.0 to 40.0%.

[0038] The lower limit of the B2O3 content is preferably 9.5%, more preferably 10.0%, even more preferably 20.0%, and still more preferably 25.0%. The upper limit of the B2O3 content is preferably 37.0%, more preferably 35.0%, and even more preferably 33.0%.

[0039] Na2O: 0-30.0% Sodium oxide (Na2O) may not be contained. In other words, Na2O is an optional component, and the Na2O content may be 0%. When Na2O is contained, that is, when the Na2O content exceeds 0%, Na2O lowers the softening point of the glass frit. This increases the fluidity of the carburizing coating agent in the temperature range of 950 to 1300°C. As a result, the CO gas emission suppression film can sufficiently cover the steel surface in the above temperature range.

[0040] Furthermore, compared with other alkaline components (KO, CaO, and MgO), NaO is less likely to react with oxide scale, which is mainly composed of Fe oxides. Therefore, when a steel material made of carbon steel (hereinafter also referred to as carbon steel material) is heated in the temperature range of 950 to 1300°C, destruction of the scale due to reaction with the oxide scale may be suppressed. As a result, decarburization due to destruction of the oxide scale can be suppressed in the high temperature range. Even if the content of NaO is even a small amount, the above effects can be obtained to some extent.

[0041] However, if the Na2O content exceeds 30.0%, the softening point of the glass frit will be excessively lowered. In this case, the carburizing coating agent will drip from the steel surface in the above temperature range. As a result, the CO gas emission suppression film will not be able to sufficiently cover the steel surface, and the steel surface layer will not be sufficiently carburized. Therefore, the Na2O content is 0 to 30.0%.

[0042] The lower limit of the Na2O content is preferably 0.1%, more preferably 0.5%, even more preferably 3.0%, even more preferably 5.0%, even more preferably 6.0%, and even more preferably 10.0%. The upper limit of the Na2O content is preferably 28.0%, more preferably 26.0%, even more preferably 25.0%, even more preferably 23.0%, even more preferably 20.0%, and even more preferably 19.0%.

[0043] K2O: 0-5.0% Potassium oxide (K2O) may not be contained. In other words, K2O is an optional component, and the K2O content may be 0%. When K2O is contained, that is, when the K2O content exceeds 0%, K2O lowers the softening point of the glass frit. This increases the fluidity of the carburizing coating agent in the temperature range of 950 to 1300°C. As a result, the CO gas emission suppression film can sufficiently cover the steel surface in the above temperature range.

[0044] However, compared to Na2O, KO reacts more easily with oxide scale, which is mainly composed of Fe oxide. Therefore, when carbon steel is heated in the temperature range of 950 to 1300°C, KO reacts with and easily destroys oxide scale. For these reasons, in the carburizing coating material of this embodiment, a low KO content is preferable. Therefore, the KO content is 0 to 5.0%.

[0045] The lower limit of the K2O content is preferably 0.1%, more preferably 0.5%, and even more preferably 1.0%. The upper limit of the K2O content is preferably 4.0%, more preferably 3.0%, and even more preferably 2.0%.

[0046] CaO: 0 to 7.0% Calcium oxide (CaO) may not be contained. In other words, CaO is an optional component, and the CaO content may be 0%. When contained, that is, when the CaO content exceeds 0%, CaO lowers the softening point of the glass frit. Therefore, the fluidity of the carburizing coating material increases in the temperature range of 950 to 1300°C. As a result, the CO gas emission suppression film can sufficiently cover the steel surface in the above temperature range. However, compared to Na2O, CaO reacts more easily with oxide scale, which is mainly composed of Fe oxide. Therefore, when the above-mentioned carbon steel is heated at high temperatures, CaO reacts with the oxide scale and easily destroys it.

[0047] CaO also promotes oxygen permeation through the CO gas emission suppression film. As mentioned above, the carbon powder in the carburizing coating reacts with oxygen to generate CO gas. However, if excessive oxygen from 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 CO2 gas. Since CO2 gas does not contribute to the carburization of the steel surface, if CO gas is reduced due to the generation of CO2 gas, the steel surface will not be sufficiently carburized. If the CaO content exceeds 7.0%, excessive CO2 gas will be generated. Therefore, the CaO content should be 0 to 7.0%.

[0048] The lower limit of the CaO content is preferably 0.1%, more preferably 0.3%, even more preferably 1.0%, even more preferably 2.0%, and even more preferably 3.0%. The upper limit of the CaO content is preferably 6.0%, more preferably 5.0%, and even more preferably 4.0%.

[0049] MgO: 0-5.0% Magnesia (MgO) may not 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, which allows the carburizing coating agent to wet and spread and sufficiently cover the steel surface during heating in the temperature range of 950 to 1300°C. Even if only a small amount of MgO is contained, the above effects can be obtained to some extent.

[0050] However, MgO reacts more readily with oxide scale, which is primarily composed of Fe oxides, than NaO, although not as readily as KO and CaO. Therefore, if the MgO content exceeds 5.0%, MgO reacts with oxide scale during high-temperature heating, making it more likely to be destroyed. Therefore, the MgO content is limited to 0 to 5.0%.

[0051] The lower limit of the MgO content is preferably 0.1%, and more preferably 0.5%. The upper limit of the MgO content is preferably 4.5%, more preferably 4.0%, even more preferably 3.0%, even more preferably 2.0%, and even more preferably 1.0%.

[0052] The glass frit of this embodiment contains the above-mentioned components. The glass frit may contain 0 to 1.0% by mass of impurities, calculated as oxides, as the remainder of the above-mentioned components. The impurities are, for example, those mixed in from inorganic raw materials or the manufacturing environment during industrial production of the glass frit. Examples of the impurities include Fe2O3, BO3, ZrO2, TiO2, Sb2O3, SrO, BaO, ZnO, PbO, Li2O, and PO5.

[0053] [Method for analyzing glass frit components] The composition of the glass frit is analyzed as follows. The composition of the glass frit is obtained by performing high-frequency inductively coupled plasma (ICP) analysis on the dried glass frit. Specifically, the content of each element, such as Si, Al, B, 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.

[0054] [About carbon powder] The carburizing coating agent further contains carbon powder. The carbon powder is a powder made of carbon. For example, the carbon powder is 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.

[0055] As described above, in an oxidizing atmosphere at temperatures between 950 and 1300°C, the carburizing coating agent forms a CO gas emission-inhibiting film on the surface of the steel material. At this time, the carbon in the carbon powder is contained in the CO gas emission-inhibiting film and functions as a carbon source in the above temperature range. Specifically, the carbon in the CO gas emission-inhibiting film reacts with oxygen that permeates the film to generate CO gas. The CO gas emission-inhibiting film is permeable to oxygen but not to CO gas. Therefore, the generated CO gas remains between the CO gas emission-inhibiting film and the steel surface. Therefore, when CO gas comes into contact with the steel surface, the surface layer of the steel material is carburized.

[0056] [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 carburizing coating material is defined as the “carbon powder ratio RA.” In the carburizing coating material of this embodiment, the carbon powder ratio RA is 0.10 to 0.49. Here, the ratio of the mass (g) of the carbon powder to the mass (g) of the glass frit means the ratio of the mass (g) of the carbon powder in a dry state to the mass (g) of the glass frit in a dry state.

[0057] If the carbon powder ratio RA is less than 0.10, the carbon supply source is too small. In this case, in the above temperature range, the carbon in the CO gas emission-inhibitory film reacts with oxygen that has permeated through the film from the outside, and a sufficient amount of CO gas cannot be generated. As a result, the surface layer of the steel material is not sufficiently carburized in the above temperature range. On the other hand, if the carbon powder ratio RA exceeds 0.49, the carbon supply source is excessively high. In this case, the CO gas emission-inhibiting film is not stably formed in the above temperature range. Specifically, in the above temperature range, cracks may occur in the CO gas emission-inhibiting film or parts of the CO gas emission-inhibiting film may peel off. As a result, the CO gas formed between the CO gas emission-inhibiting film and the steel surface diffuses outside the CO gas emission-inhibiting film. As a result, the steel surface layer is not sufficiently carburized.

[0058] If the carbon powder ratio RA is 0.10 to 0.49, the carbon source in the CO gas emission-inhibitory film is in an appropriate amount. Therefore, in the above temperature range, the CO gas emission-inhibitory film can stably cover the steel surface layer while generating sufficient CO gas for carburization. As a result, the steel surface layer is sufficiently carburized. The lower limit of the carbon powder ratio RA is preferably 0.11, more preferably 0.12, even more preferably 0.13, even more preferably 0.14, and even more preferably 0.15. The upper limit of the carbon powder ratio RA is preferably 0.47, more preferably 0.45, even more preferably 0.40, even more preferably 0.38, and even more preferably 0.36.

[0059] [Method for measuring carbon powder ratio RA] The carbon powder ratio RA in the carburizing coating is measured by the following method. The mass of the carburizing coating material, which changes due to heat, is measured using a thermogravimetry (TG) device. The mass of the carburizing coating material is not particularly limited, but is, for example, 1 g. Specifically, the carburizing coating is heated from room temperature (25°C) to 100°C and held at 100°C. The water contained in the carburizing coating evaporates, and the mass remains constant for 30 seconds, at which point the mass of the carburizing coating, X0, is measured. After measuring the mass, the carburizing coating is heated to 600°C and held at 600°C. This oxidizes the carbon powder in the carburizing coating, removing it from the carburizing coating. The carbon powder contained in the carburizing coating is removed, and the mass remains constant for 30 seconds, at which point the mass, X1, of the carburizing coating is measured. 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

[0060] [Effects of the carburizing coating agent of this embodiment] The carburizing coating material of this embodiment contains glass frit and carbon powder. When a steel material coated with the carburizing coating material of this embodiment is heated at 950 to 1300°C, the carburizing coating material spreads over the surface of the steel material and forms a CO gas emission suppression film.

[0061] The carbon powder contained in the CO gas emission-inhibiting film functions as a carbon source in the above temperature range. Specifically, the carbon powder in the CO gas emission-inhibiting film reacts with oxygen that permeates the film to generate CO gas. The CO gas emission-inhibiting film holds CO gas between the coating material and the steel surface, preventing it from escaping into an oxidizing atmosphere. As a result, the CO gas comes into contact with the steel surface, carburizing the surface layer of the steel.

[0062] [Ingredients other than glass frit and carbon powder in carburizing coating] The carburizing coating agent of this embodiment may further contain, in addition to the glass frit and carbon powder, one or more 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.

[0063] [water] When the carburizing coating agent is made into a slurry, the carburizing coating agent may contain, for example, water. In the case of a slurry carburizing coating agent, the preferred water content is, for example, 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 carburizing coating agent to the surface of a steel material. By adjusting the water content, the viscosity of the slurry carburizing coating agent can be adjusted to a level that allows it to be applied to the surface of a steel material at room temperature.

[0064] [Suspension agent] The suspending agent disperses the glass frit sufficiently in the solution (water). The suspending agent contains, for example, Gaime clay and bentonite and / or sepiolite. When the carburizing coating agent of this embodiment contains Gaime clay and bentonite and / or sepiolite, the carburizing coating agent is less likely to drip from the steel surface. Furthermore, when the carburizing coating agent dries and solidifies, it is less likely to peel off from the steel surface.

[0065] The gairome clay contains kaolin clay and a plurality of quartz particles. Specifically, the gairome clay contains kaolinite, halloysite, and quartz.

[0066] Gairome clay improves the sagging resistance of the slurry-type carburizing coating. A slurry-type carburizing coating 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 carburizing coating will easily cover the entire surface of the steel at room temperature.

[0067] When the carburizing coating agent contains gairome clay, the gairome clay content in the carburizing coating agent is preferably 4 parts by mass or more per 100 parts by mass of glass frit. In this case, the carburizing 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 carburizing coating agent is sufficiently dispersed on the surface of the steel. Therefore, a CO gas diffusion suppression film is more sufficiently formed in the high temperature range.

[0068] 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.

[0069] Sepiolite is a hydrous magnesium silicate, e.g., Mg8Si 12 O 30 It has the chemical formula (OH)4(OH2)4·8H2O.

[0070] Both bentonite and sepiolite prevent the carburizing coating from peeling off. Specifically, the carburizing coating is applied to the surface of a steel material at room temperature. The carburizing coating is then solidified by heating or drying. When the carburizing coating contains bentonite and / or sepiolite, the bentonite and sepiolite prevent the solidified carburizing coating from peeling off from the steel surface. A carburizing coating containing bentonite and / or sepiolite is unlikely to peel off from the steel surface even when subjected to an external force. The carburizing coating may contain at least one of bentonite and sepiolite.

[0071] 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 peeling resistance of the carburizing coating agent is further improved. The content of bentonite and / or sepiolite is more preferably 5 parts by mass or more.

[0072] 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 this case, the glass frit is sufficiently dispersed in the carburizing coating agent. The total content of bentonite and sepiolite is more preferably less than 8 parts by mass.

[0073] The suspending agent may contain clays other than the above-mentioned frog's eye clay, bentonite and sepiolite. The clays may contain, for example, iron oxide (Fe2O3).

[0074] [Dispersant] The carburizing coating agent of this embodiment does not need to contain a dispersant. If it does contain a dispersant, the amount of water in which the glass frit and carbon powder are dispersed can be reduced, resulting in improved adhesion of the carburizing coating agent to the steel surface. The dispersant is, for example, one or more selected from the group consisting of inorganic salts such as sodium tripolyphosphate, sodium hexametaphosphate, sodium ultrapolyphosphate, acidic sodium hexametaphosphate, sodium borate, sodium carbonate, polymetaphosphate, sodium citrate, sodium tartrate, polyacrylic acid, sodium polyacrylate, sodium sulfonate, polycarboxylate, β-naphthalenesulfonates, melamine sulfonates, and naphthalenesulfonic acid. In the carburizing coating agent, the total content of the dispersant is preferably less than 4.5 parts by mass per 100 parts by mass of glass frit.

[0075] [Preferable softening point of glass frit] The softening point of the glass frit contained in the carburizing coating agent is preferably 600°C or higher. The softening point of the glass frit is determined by the composition of the glass frit. With the composition of the glass frit of this embodiment, the softening point is 600°C or higher. If the softening point of the glass frit is 600°C or higher, the carburizing coating agent will wet and spread over the steel surface in the high temperature range of 950 to 1300°C, and will sufficiently cover the steel surface. As a result, the carburizing coating agent will promote the formation of a carburized layer on the steel surface. Therefore, the softening point of the glass frit is preferably 600°C or higher. A more preferable lower limit of the softening point of the glass frit is 630°C, and a further more preferable lower limit is 650°C. The upper limit of the softening point of the glass frit is not particularly limited, but is preferably less than 900°C, and more preferably 890°C.

[0076] [Softening point measurement method] The softening point of the carburizing coating is measured using a simultaneous thermogravimetry-differential thermal analyzer (TG-DTA). Specifically, 10 mg of carburizing coating is placed in the TG-DTA sample holder. The carburizing coating is heated from room temperature to 1300°C at a heating rate of 10°C / min. The atmosphere during heating is Ar. The differential heat is measured at this time, and the fourth inflection point on the DTA (differential thermal) chart is taken as the softening point of the carburizing coating.

[0077] [Manufacturing method of carburizing coating agent] The carburizing coating material of this embodiment is produced, for example, by the following method. Specifically, the components that serve as raw materials for the glass frit having the above-mentioned chemical composition are prepared. The prepared components are mixed. The mixed components are melted. The melted components are vitrified by quenching in water or air. 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. Glass frit is produced by the above steps. The softening point of the glass frit can be adjusted to, for example, 600°C or higher by adjusting the components before mixing. A carburizing coating agent in a slurry form is produced by adding, for example, water to the prepared glass frit.

[0078] Carbon powder is further mixed into the slurry carburizing coating agent. At this time, the amount of carbon powder is adjusted so that the carbon powder ratio RA is 0.10 to 0.49. Furthermore, a suspending agent and / or a dispersing agent is added to the slurry carburizing coating agent as needed. Through the above steps, the carburizing coating agent is produced.

[0079] [Method of manufacturing steel using carburizing coating] The method for manufacturing a steel material according to this embodiment includes a step of applying a carburizing coating agent to the surface of the steel material (application step), and a step of heating the steel material to which the carburizing coating agent has been applied at 950 to 1300° C. (heating step). Each step will be described below.

[0080] [Coating process] In the coating process, first, the carburizing coating of this embodiment manufactured by the above-mentioned [Manufacturing Method of Carburizing Coating] is prepared. The prepared carburizing coating in a slurry form is applied to the surface (top surface, bottom surface, and side surface) of the steel material before heating. The carburizing coating is applied to the surface of the steel material at room temperature, for example.

[0081] The method for applying the carburizing coating agent is not particularly limited. An operator may apply the carburizing coating agent to the surface of the steel material using a brush. Alternatively, the carburizing coating agent may be applied to the surface of the steel material by a spray or the like. The steel material may be immersed in a bath containing the carburizing coating agent (so-called "dipping in the hot water"). By these application methods, a slurry-like carburizing coating agent is applied to the surface of the steel material. After the slurry-like carburizing coating agent has been applied to the surface of the steel material, the carburizing coating agent may be dried before the steel material is heated in the above temperature range.

[0082] The amount of carburizing coating agent applied to the steel surface is not particularly limited. For example, the amount of the slurry-like carburizing coating agent applied to the steel surface is 0.1 to 0.3 g / cm. 2 is.

[0083] [Target steel materials] In the manufacturing method of this embodiment, the type of steel material to be used is not particularly limited. The steel material to be used is preferably a carbon steel material. Here, carbon steel materials include 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.

[0084] [Heating process] The steel material coated with the carburizing coating agent is heated (heating step). In the heating step, the steel material coated with the carburizing coating agent is heated in a temperature range of 950 to 1300°C. The heating time is not particularly limited. A preferred upper limit of the heating temperature is 1250°C, and more preferably 1200°C. The heating time is, for example, 1.0 to 8.0 hours. A preferred upper limit of the heating time is 5.0 hours, and more preferably 2.5 hours.

[0085] The carburizing coating agent of this embodiment contains glass frit having the above-described composition and carbon powder, and further has a carbon powder ratio RA of 0.10 to 0.49. When steel is maintained in a heat treatment furnace with an oxidizing atmosphere at a temperature range of 950 to 1300°C, the carburizing coating agent of this embodiment spreads over the steel surface, wetting it and forming a CO gas emission-inhibiting film that sufficiently covers the steel surface. This CO gas emission-inhibiting film allows oxygen to pass through to a certain extent. Oxygen that has permeated from the outside between the CO gas emission-inhibiting film and the steel surface combines with carbon in the CO gas emission-inhibiting film to generate CO gas. The CO gas emission-inhibiting film allows oxygen to pass through but inhibits CO gas permeation. Therefore, CO gas remains between the CO gas emission-inhibiting film and the steel surface. This allows the steel surface to be sufficiently carburized. [Example]

[0086] The effects of the carburizing coating according to the embodiment of the present disclosure will be explained 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 carburizing coating according to the embodiment of the present disclosure. Therefore, the carburizing coating according to the embodiment of the present disclosure is not limited to this one example of conditions.

[0087] In this example, a carburizing coating agent was prepared by mixing multiple glass frits with different compositions and graphite powder in various mass ratios. Steel materials coated with the carburizing coating agent were heated at high temperatures, and the change in carbon concentration in the surface layer of the steel material before and after heating was investigated. This example is described in detail below.

[0088] 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 for glass frit symbol F, the SiO2 content exceeded the upper limit, so the mixed components could not be melted, and glass frit could not be produced. Furthermore, for glass frit symbol H, the Al2O3 content exceeded the upper limit, so the mixed components could not be melted, and glass frit could not be produced. Note that the impurity in glass frit symbol A was Fe2O3.

[0089] [Table 1]

[0090] Glass frits with the glass frit codes in Table 1, water, and a suspending agent were mixed to prepare carburizing coating agents with the respective test numbers in Table 2. Each carburizing coating agent was produced by mixing 70 parts by mass of water, 6 parts by mass of gairome clay, and 5 parts by mass of bentonite with 100 parts by mass of glass frit.

[0091] [Table 2]

[0092] Carbon steel equivalent to JIS S45C was used as the plate specimen. The size of the plate specimen was 10 mm thick, 20 mm long, and 20 mm wide. The C content of each plate specimen was 0.45% by mass. Therefore, the C concentration [C] on the surface (steel surface) of each plate specimen was B was set to 0.45% by mass.

[0093] The above carburizing coating was applied to the 20 x 20 mm surface of the plate-shaped test specimen using a brush, and then dried. The amount of coating for each test number is shown in Table 2. The amount of coating was measured as follows: the mass of the plate-shaped test specimen before coating and the mass of the entire test specimen after coating with the carburizing coating were measured, and the amount of carburizing coating applied was calculated from the difference between the two.

[0094] After applying the carburizing coating, the plate-shaped test specimens were heated for 1.0 hour at the heating temperatures (900-1300°C) listed in Table 2. The atmosphere was adjusted to simulate LNG combustion. Specifically, the atmospheric gas composition was adjusted to 2-3% O, 8-10% CO, 15-20% H, and the balance N.

[0095] The carbon concentration (mass %) on the surface (steel surface) of the plate-shaped test piece after heating was measured by the following method. The observation surface was a cross section perpendicular to the longitudinal direction (a cross section parallel to the width and thickness directions) at the longitudinal center of the plate-shaped test piece. 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-shaped test piece 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. For the line analysis using the EPMA, the acceleration voltage was 15 kV, the probe 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 five measurement lines on the observation surface. For each measurement line, spectra based on Fe, O, and C were obtained from the surface of the plate-shaped test piece to a thickness position of 2000 μm.

[0096] Figure 1 shows a schematic diagram of the 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 C The O concentration has a convex peak downwards 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 10 ... C The peaks up to thickness D C is defined as follows: In the spectrum based on O, the upward convex peaks caused by the carburizing coating and oxide film decrease and reach a minimum point, which is the O concentration [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 And the thickness D CThe thickness D is 500 μm from the R The thickness D in the carbon (C) spectrum was determined. C From thickness D R The arithmetic mean of the C concentration was calculated up to the thickness D. Referring to Figure 1, the extremely low and sharp peak of the C concentration, such as the peak LP, is thought to be caused by impurities. C From thickness D R The peaks up to 10 ...

[0097] The C concentration at the start of the peak decay is the C concentration [C] S The C concentration at the end of the peak decay is the C concentration [C] M Peaks with a peak decay rate (%) of 30% or more calculated using the following formula were determined to be peak LP and were excluded from the arithmetic mean of the C concentration. Peak decay rate = ([C] S -[C] M ) / [C] S ×100

[0098] 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 is heated is calculated as the C concentration [C] A (mass%).

[0099] Obtained C concentration [C] A and the carbon concentration [C] on the surface of the plate-shaped test piece (steel surface) 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.

[0100] [Test Results] Referring to Tables 1 and 2, the carburizing coating materials of Test Nos. 1 to 7 satisfied the ranges for each component of the glass frit and had a carbon powder ratio RA of 0.10 to 0.49. Therefore, the carburizing coating materials of these examples had a C concentration ratio ΔC of 1.30 or more, and were capable of carburizing using a heat treatment furnace in an oxidizing atmosphere.

[0101] On the other hand, the carbon powder ratio RA of the carburizing coating materials of test numbers 8 and 9 was less than 0.10. Therefore, the C concentration ratio ΔC was less than 1.30, and carburizing treatment using a heat treatment furnace with an oxidizing atmosphere was not possible.

[0102] For the carburizing coating material of test number 10, the carbon powder ratio RA exceeded 0.49. As a result, the C concentration ratio ΔC was less than 1.30, and carburizing treatment using a heat treatment furnace with an oxidizing atmosphere was not possible.

[0103] For the carburizing coating material of test number 11, the heating temperature of the plate test piece after coating was less than 950°C. As a result, the C concentration ratio ΔC was less than 1.30, and carburizing treatment using a heat treatment furnace with an oxidizing atmosphere was not possible.

[0104] In the carburizing coating material of test number 12, the SiO2 content of each component of the glass frit was below the lower limit. As a result, the C concentration ratio ΔC was less than 1.30, and carburizing treatment using a heat treatment furnace with an oxidizing atmosphere was not possible.

[0105] In the carburizing coating material of test number 13, the Al2O3 content was below the lower limit of each component of the glass frit. As a result, the C concentration ratio ΔC was less than 1.30, and carburizing treatment using a heat treatment furnace in an oxidizing atmosphere was not possible.

[0106] In the carburizing coating material of test number 14, the B2O3 content exceeded the upper limit of each component of the glass frit. As a result, the C concentration ratio ΔC was less than 1.30, and carburizing treatment using a heat treatment furnace with an oxidizing atmosphere was not possible.

[0107] In the carburizing coating materials of test numbers 15 and 16, the KO content of each component of the glass frit exceeded the upper limit. As a result, the C concentration ratio ΔC was less than 1.30, and carburizing treatment using a heat treatment furnace in an oxidizing atmosphere was not possible.

[0108] In the carburizing coating materials of test numbers 17 and 18, the CaO content exceeded the upper limit of each component of the glass frit. As a result, the C concentration ratio ΔC was less than 1.30, and carburizing treatment using a heat treatment furnace in an oxidizing atmosphere was not possible.

[0109] In the carburizing coating materials of test numbers 19 and 20, the MgO content of each component of the glass frit exceeded the upper limit. As a result, the C concentration ratio ΔC was less than 1.30, and carburizing treatment using a heat treatment furnace in an oxidizing atmosphere was not possible.

[0110] 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 carburizing coating agent, Glass frit and Carbon powder, The glass frit is The mass % when converted to oxide is: Yes ２ :20.~40.0%� Al ２ O ３ : 1000~3000 B ２ O ３ :9.0~40.0%、 Na ２ O:0~30.0%、 K ２ O: 0~5.0%, CaO: 0 to 7.0%, and MgO: 0 to 5.0%; the ratio of the mass (g) of the carbon powder to the mass (g) of the glass frit in the carburizing coating agent is 0.10 to 0.49; Carburizing coating agent.

2. A step of applying the carburizing coating agent according to claim 1 to a surface of a steel material; and heating the steel material coated with the carburizing coating agent at 950 to 1300°C. Steel manufacturing method.

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