Manufacturing methods for metal products

Abstract

To provide a metal product having excellent corrosion resistance under severe conditions, and a method for producing the same.SOLUTION: A metal product has: a sintered body containing an iron alloy with a chromium content of 3 mass%-9.5 mass%; and a coating containing phosphorus, disposed on the surface of the sintered body.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing a metal product. [Background Art]

[0002] As a method for producing a metal product, there is known a method (powder metallurgy) in which a compact produced by filling a mold with raw material metal powder is heat-treated, and the metal powder is sintered. Powder metallurgy is suitable for mass-producing products of the same shape, and is employed as a method for producing metal products of various shapes and materials.

[0003] Powder metallurgy is also used for producing metal products made of stainless steel (an alloy obtained by adding chromium to iron to improve corrosion resistance). For example, Patent Document 1 describes a method of producing a sintered body by press-molding a composition containing alloy powder of ferritic stainless steel and then heat-treating the molded product. [Prior Art Documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2020-37735 [Summary of the Invention] [Problems to be Solved by the Invention]

[0005] Metal products made of stainless steel have excellent corrosion resistance because chromium added to iron forms an oxide film on the surface. However, with the recent expansion of applications of metal products, corrosion resistance at a level that can cope with harsher conditions than before has been demanded. For example, metal products used as components for vehicles, machine tools and the like are assumed to be placed under an extremely wide temperature range.

[0006] One method for improving the corrosion resistance of metal products is to chemically treat the surface of the metal product. However, metal products made of stainless steel have an oxide film formed on the surface by chromium, which makes chemical surface treatment difficult and limits further improvement in corrosion resistance. In view of the above circumstances, the present invention aims to provide a metal product with excellent corrosion resistance under harsh conditions, and a method for manufacturing the same. [Means for solving the problem]

[0007] The following embodiments are included as specific means for solving the above problems. <1> A metal product comprising a sintered body containing an iron alloy with a chromium content of 3% to 9.5% by mass, and a phosphorus-containing coating disposed on the surface of the sintered body. <2> The aforementioned coating contains at least one selected from the group consisting of manganese, zinc, and calcium. <1> The metal products listed above. <3> A method for manufacturing a metal product, comprising the steps of: heat-treating a molded body containing iron alloy powder having a chromium content of 3% to 9.5% by mass to obtain a sintered body; and forming a phosphorus-containing film on the surface of the sintered body. <4> The aforementioned coating contains at least one selected from the group consisting of manganese, zinc, and calcium. <3> A method for manufacturing metal products as described above. [Effects of the Invention]

[0008] The present invention provides a metal product with excellent corrosion resistance under harsh conditions, and a method for manufacturing the same. [Modes for carrying out the invention]

[0009] The embodiments for carrying out the present invention will be described in detail below. However, the present invention is not limited to the following embodiments. In the following embodiments, the components (including elemental steps, etc.) are not essential unless otherwise specified. The same applies to numerical values ​​and their ranges, and they do not limit the present invention.

[0010] In this specification, the term "process" includes not only processes that are independent of other processes, but also processes that cannot be clearly distinguished from other processes, provided that the purpose of the process is achieved.

[0011] In this specification, numerical ranges indicated using "~" include the numbers before and after "~" as the minimum and maximum values, respectively. In numerical ranges described stepwise within this specification, the upper or lower limit of one numerical range may be replaced with the upper or lower limit of another numerical range described stepwise. Furthermore, in numerical ranges described within this specification, the upper or lower limit of that range may be replaced with the values ​​shown in the examples.

[0012] <Metal products> The metal product of this disclosure is a metal product having a sintered body containing an iron alloy with a chromium content of 3% to 9.5% by mass, and a phosphorus-containing coating disposed on the surface of the sintered body. The above metal products have a lower chromium content compared to typical stainless steel (ISO standards define stainless steel as an iron alloy containing 10.5% by mass or more chromium). Because the chromium content is kept low, the formation of an oxide film on the surface is suppressed, resulting in lower corrosion resistance due to oxide film formation compared to typical stainless steel. However, suppressing oxide film formation allows for the formation of a phosphorus-containing film on the surface of the sintered body, providing strong corrosion resistance. As a result, a level of corrosion resistance that can withstand harsh conditions is achieved compared to metal products made from conventional stainless steel.

[0013] (Sintered body) The sintered body is not particularly limited as long as it contains an iron alloy with a chromium content of 3% to 9.5% by mass. The iron content in the iron alloy is preferably 80% by mass or more, more preferably 85% by mass or more, and even more preferably 90% by mass or more. Chromium contained in iron alloys contributes to improving the electrical resistance of metal products and also contributes to improving the corrosion resistance of components by forming a strong oxide film on the surface of metal products. By setting the chromium content to 3% by mass or more, corrosion resistance is imparted through the formation of an oxide film. On the other hand, by setting the chromium content to 9.5% by mass or less, the formation of an oxide film on the surface of the sintered body is suppressed, and the formation of a phosphorus-containing film becomes possible. The chromium content may be 4% by mass or more, 5% by mass or more, or 9% by mass or more of the total iron alloy. It may also be less than 9.5% by mass of the total iron alloy, 9.4% by mass or less, or 9.2% by mass or less.

[0014] The iron alloy may further contain silicon. The inclusion of silicon can be expected to improve the electrical resistance of the metal product, reduce iron loss by decreasing eddy current losses, increase magnetic permeability by coarsening the crystal grains, and suppress changes in magnetic properties due to ambient temperature. Furthermore, it can strengthen the Fe matrix, improving the strength of the component against repeated impacts. From the viewpoint of fully obtaining these effects, it is preferable that the silicon content be 1.5% by mass or more of the total iron alloy.

[0015] When an iron alloy contains silicon, it is preferable that the silicon is dissolved in the raw iron alloy powder or partially diffused and attached to it, from the viewpoint of uniform distribution of alloy components and handling. When silicon is dissolved in iron alloy powder and blended, it is preferable that the silicon content be 3.5% by mass or less of the total iron alloy powder, from the viewpoint of preventing the powder from hardening and losing its compressibility.

[0016] Iron alloys may further contain elements other than iron (Fe), chromium (Cr), and silicon (Si). For example, they may contain Al, Ni, Pb, Al, Cu, Ti, Au, Ag, Co, Mg, Wo, Mo, Ta, Nd, P, etc. When the Fe alloy further contains elements other than Fe, Cr and Si, the total content thereof is preferably not more than 15% by mass of the entire Fe alloy, more preferably not more than 10% by mass, and still more preferably not more than 5% by mass.

[0017] (Phosphorus-containing film) The metal product of the present disclosure has a phosphorus-containing film disposed on the surface of the sintered body described above. This film improves the corrosion resistance of the metal product under severe conditions.

[0018] Whether a phosphorus-containing film exists on the surface of the sintered body can be determined by whether phosphorus exists on the surface of the sintered body. Specifically, it can be determined by a known method such as elemental analysis. The phosphorus-containing film refers to a region containing phosphorus that exists on the surface of the sintered body, and the boundary between the film and the sintered body may be clear or unclear (for example, the phosphorus content changes in the thickness direction). When the sintered body contains phosphorus, it is determined that a phosphorus-containing film exists on the surface of the sintered body when phosphorus is present on the surface of the sintered body at a higher concentration than inside the sintered body.

[0019] The method for forming a phosphorus-containing film on the surface of the sintered body is not particularly limited. For example, the formation may be performed by a phosphate coating method, which is one of chemical conversion treatments for metal products. In the phosphate coating method, by immersing the sintered body in a liquid containing phosphate and heating as necessary, a film can be formed on the surface of the sintered body by the phosphate coating method.

[0020] When forming a film on the surface of the sintered body by the phosphate coating method, the type of phosphate used is not particularly limited, and one or more types selected from manganese phosphate, zinc phosphate, calcium phosphate, and the like can be used.

[0021] A phosphorus-containing coating may also contain elements found in the phosphate used to form the coating. For example, it may contain at least one element selected from the group consisting of manganese, zinc, and calcium.

[0022] <Method of manufacturing metal products> The present disclosure is a method for manufacturing a metal product, comprising the steps of: heat-treating a molded body containing iron alloy powder having a chromium content of 3% to 9.5% by mass to obtain a sintered body (sintering step); and forming a phosphorus-containing film on the surface of the sintered body (film formation step).

[0023] According to the above method, metal products with excellent corrosion resistance under harsh conditions can be manufactured.

[0024] (Sintering process) In the sintering process, a molded body containing iron alloy powder with a chromium content of 3% to 9.5% by mass is heat-treated to obtain a sintered body. The iron alloy powder used in the above process is not particularly limited as long as it is an iron alloy powder with a chromium content of 3 to 9.5% by mass. The chromium contained in iron alloy powder contributes to improving the electrical resistance of metal products and also contributes to improving the corrosion resistance of components by forming a strong oxide film on the surface of metal products. By setting the chromium content to 3% by mass or more, corrosion resistance is imparted through the formation of an oxide film. On the other hand, by setting the chromium content to 7% by mass or less, the formation of an oxide film on the surface of the sintered body is suppressed, and the formation of a phosphorus-containing film becomes possible. The chromium content may be 4% by mass or more, 5% by mass or more, or 9% by mass or more of the total iron alloy powder. It may also be less than 9.5% by mass of the total iron alloy, 9.4% by mass or less, or 9.2% by mass or less.

[0025] The iron alloy powder may further contain silicon. The inclusion of silicon in the iron alloy powder is expected to improve the electrical resistance of metal products, reduce iron loss by decreasing eddy current losses, increase magnetic permeability by coarsening the crystal grains, and suppress changes in magnetic properties due to ambient temperature. Furthermore, it is expected to strengthen the Fe matrix, improving the strength of the component against repeated impacts. From the viewpoint of fully obtaining these effects, it is preferable that the silicon content be 1.5% by mass or more of the total iron alloy powder.

[0026] When iron alloy powder contains silicon, it is preferable that the silicon is solid-dissolved or partially diffused and attached to the iron alloy powder, from the viewpoint of uniform distribution of alloy components and handling. When silicon is solid-dissolved and blended with iron alloy powder, it is preferable that the silicon content be 3.5% by mass or less of the total iron alloy powder, from the viewpoint of preventing the powder from hardening and impairing its compressibility.

[0027] The iron alloy powder may further contain elements other than iron (Fe), chromium (Cr), and silicon (Si). For example, it may contain Al, Ni, Pb, Al, Cu, Ti, Au, Ag, Co, Mg, Wo, Mo, Ta, Nd, P, etc. If the iron alloy powder further contains elements other than Fe, Cr, and Si, the total content of these elements is preferably 5% by mass or less of the total iron alloy powder, more preferably 3% by mass or less, and even more preferably 2% by mass or less.

[0028] The particle size of the iron alloy powder is not particularly limited. For example, the volume-average particle size (D50) measured by laser diffraction-scattering may be in the range of 8 μm to 150 μm. If the iron alloy powder is a mixture of fine and coarse powder, only the coarse powder may be used after removing the fine powder. For example, coarse powder with a volume-average particle size (D50) of 75 μm to 150 μm may be used as the iron alloy powder. A molded body containing iron alloy powder may also contain both iron alloy powder and silicon powder. If at least a portion of the silicon is in powder form (i.e., not included in the iron alloy powder), the dispersion of silicon in the sintered body can be made more uniform. If the molded body contains silicon powder, the amount is preferably 0.1 to 3.5 parts by mass per 100 parts by mass of iron alloy powder.

[0029] When the molded body contains iron alloy powder and silicon powder, the volume-average particle size (D50) of the silicon powder, as measured by laser diffraction / scattering, is preferably 1 μm to 45 μm. When the particle size of the silicon powder is within the above range, the silicon powder tends to be thinly and uniformly adsorbed around the iron alloy powder by van der Waals forces. Furthermore, when a molded body containing iron alloy powder and silicon powder is sintered, the silicon powder adsorbed around the iron alloy powder rapidly diffuses into the iron alloy. As a result, the alloy components are uniformly distributed in the resulting sintered body. In addition, problems such as residual pores in the areas where the silicon powder was present do not occur.

[0030] The method for obtaining a molded body containing iron alloy powder is not particularly limited. For example, a common method such as filling a mold with raw material powder and applying pressure can be employed.

[0031] The method for obtaining a sintered body by heat-treating a molded body is not particularly limited and can be carried out using known apparatus such as a pusher furnace or a vacuum sintering furnace. The configuration of the heat treatment apparatus is not particularly limited, and a general configuration can be used.

[0032] The heat treatment temperature is not particularly limited and can be set according to the size of the molded body, the type of raw material powder, etc. For example, the maximum temperature reached inside the heat treatment apparatus may be in the range of 1100°C to 1300°C. The heat treatment time is not particularly limited and can be set according to the size of the molded body, the type of raw material powder, etc. For example, the time it takes for the temperature inside the heat treatment apparatus to reach 1100°C or higher, preferably 1200°C or higher, may be in the range of 10 to 300 minutes.

[0033] (Film formation process) In the coating formation process, a phosphorus-containing coating is formed on the surface of the sintered body. The method for forming a phosphorus-containing film on the surface of the sintered body is not particularly limited. For example, it may be carried out by a phosphate coating method, which is one of the chemical conversion treatments. A phosphate coating can be formed on the surface of a sintered body by immersing it in a phosphate-containing liquid and heating it as needed. For example, a coating can be formed on the surface of a sintered body by immersing it in a phosphate-containing liquid (phosphate concentration: 30-50 (total acidity), 80°C-100°C).

[0034] When forming a coating on the surface of a sintered body using a phosphate coating method, there are no particular restrictions on the type of phosphate used, and one or more types can be selected from manganese phosphate, zinc phosphate, calcium phosphate, etc.

[0035] (Other processes) The above method may include steps other than the sintering step and the film formation step. For example, the process may include a step for post-processing such as polishing or cutting between the sintering step and the film formation step, or after the film formation step. [Examples]

[0036] The embodiments described above will be further explained below based on the following examples. However, this disclosure is not limited to the following examples.

[0037] <Example 1> A cylindrical molded body with a diameter of 11.3 mm and a height of 10 mm was produced by filling a mold with iron alloy powder (D50: 80 μm) having the composition shown below and applying pressure. (Composition of iron alloy powder) Fe...90.5% by mass Cr...6.5% by mass Si...3 mass%

[0038] The obtained molded body was heat-treated using a vacuum sintering furnace (maximum temperature reached: 1250°C, heat treatment time at 1200°C or higher: 240 minutes) to obtain a sintered body. Next, the sintered body was immersed in an aqueous solution of manganese phosphate and dried to form a phosphorus-containing film on the surface of the sintered body, thereby obtaining the sample of Example 1.

[0039] <Comparative Example 1> A cylindrical molded body with a diameter of 11.3 mm and a height of 10 mm was produced by filling a mold with iron alloy powder (D50: 80 μm) having the composition shown below and applying pressure. (Composition of iron alloy powder) Fe...87% by mass Cr...13% by mass

[0040] The resulting molded body was heat-treated in a vacuum sintering furnace (maximum temperature reached: 1250°C, heat treatment time at 1200°C or higher: 240 minutes). This sintered body was used as the sample for Comparative Example 1.

[0041] Furthermore, when the above sample was immersed in an aqueous solution of manganese phosphate, no phosphorus-containing film was formed on the surface.

[0042] <Comparative Example 2> The sintered body prepared in Example 1, without a phosphorus-containing film formed on its surface, was used as the sample for Comparative Example 2.

[0043] <Corrosion resistance evaluation test> The prepared samples were placed in a chamber adjusted to a temperature of 65°C and a relative humidity of 90% for 24 hours. After the test, the samples were removed from the chamber and examined for their appearance.

[0044] In "Example 1," the chromium content of the iron alloy was in the range of 3% to 9.5% by mass, and a phosphorus-containing film was formed on the surface. The sample maintained its metallic luster and good appearance even after the corrosion resistance evaluation test. In "Comparative Example 1," a sample of iron alloy with a chromium content exceeding 9.5% by mass and no phosphorus-containing film formed on the surface, the metallic luster was lost and rust developed in some areas after the corrosion resistance evaluation test. In "Comparative Example 2," the iron alloy sample, which had a chromium content in the range of 3% to 9.5% by mass and did not have a phosphorus-containing film formed on its surface, lost its metallic luster and rusted overall after the corrosion resistance evaluation test.

Claims

1. A metal product comprising a sintered body containing chromium and silicon, with a chromium content of 3% to 9.5% by mass of an iron alloy (excluding those with a carbon content of 1% to 3.5% by mass), and a phosphorus-containing coating disposed on the surface of the sintered body.

2. The metal product according to claim 1, wherein the coating comprises at least one selected from the group consisting of manganese, zinc, and calcium.

3. A method for manufacturing a metal product, comprising the steps of: heat-treating a molded body containing iron alloy powder (excluding those with a carbon content of 1% to 3.5% by mass) which contains chromium and silicon, and in which the chromium content is 3% to 9.5% by mass, to obtain a sintered body; and forming a phosphorus-containing film on the surface of the sintered body.

4. The method for producing a metal product according to claim 3, wherein the coating comprises at least one selected from the group consisting of manganese, zinc, and calcium.