REDUCED IRON POWDER AND METHOD FOR PREPARING SAME AND BEARING

- JFE STEEL CORPORATION

Reduced iron powder that has fewer coarse inclusions, has excellent formability, has high porosity after sintering, has excellent reactivity per unit mass, and can be effectively used as reaction material even to the particle inside is provided. Reduced iron powder has an apparent density of 1.00 Mg/m3 to 1.40 Mg/m3.

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Description
TECHNICAL FIELD

The disclosure relates to reduced iron powder, a method for preparing the same, and a bearing produced from the reduced iron powder.

BACKGROUND

Two main types of iron powder based on its preparation method are typically known: reduced iron powder; and atomized iron powder. The apparent density of iron powder currently known is 2.3 Mg/m3 or more in reduced iron powder, and 2.5 Mg/m3 or more in atomized iron powder. The specific surface area of iron powder is 0.10 m2/g or less in reduced iron powder, and 0.07 m2/g or less in atomized iron powder.

Iron powder having such characteristics has many uses, and particularly its uses in chemical reaction material, sintered machine parts, etc. make up a high proportion. In chemical reaction material, large specific surface area is required for efficient reaction. In sintered machine parts, high porosity is required as oilless bearings (which is also referred to as “oil retaining bearings”).

The specific surface area is larger when the apparent density is lower. Iron powder with low apparent density is needed to produce sintered machine parts with high porosity.

As an example of sintered machine parts, an oilless bearing is described below. It is important that the oilless bearing maintains appropriate oil content. If the oil content is low, adequate lubricity and durability cannot be obtained. To maintain appropriate oil content, the sintered body needs to be increased in porosity. JP 2001-132755 A (PTL 1) describes a relevant technique.

With reduction in size of machine parts, oilless bearings of approximately 2 mm in outer diameter and 0.6 mm in inner diameter have been produced in recent years. However, the use of conventional reduced iron powder for smaller parts causes poor formability and poor yield rate because conventional reduced iron powder has coarse pores and iron portions, making production difficult. This has increased demand for iron powder that is finer in microstructure, is more porous, and has fewer inclusions than conventional iron powder.

If a part requires contacting with another part as in the case of a bearing, the presence of inclusions in the part damages the other part and shortens the life of the product. Besides, in the case where the inclusions do not sinter with the surrounding iron powder, the inclusions cause structural defects. This significantly decreases the yield rate or the strength, particularly when producing small machine parts.

The “inclusions” mentioned here has the following meaning. Reduced iron powder is produced from iron ore or mill scale. The purity of the reduced iron powder as the product is determined by the purity of the iron oxide as the raw material. The most common impurity is oxygen. Oxygen mostly appears as a thin film of surface oxide. Basic impurities include carbon, magnesium, aluminum, silicon, phosphorus, sulfur, chromium, manganese, nickel, and copper. Many of these impurities are present as oxides, and are called inclusions.

For use in chemical reaction material, iron powder with large specific surface area, i.e. low apparent density, is known to be useful as described in JP 4667835 B2 (PTL 2) and JP 4667937 B2 (PTL 3), given that larger specific surface area of powder contributes to more efficient reaction.

CITATION LIST Patent Literatures

PTL 1: JP 2001-132755 A

PTL 2: JP 4667835 B2

PTL 3: JP 4667937 B2

SUMMARY Technical Problem

In the case of using conventional reduced iron powder to produce a bearing, the shaft is damaged or the bearing develops structural defects because the reduced iron powder contains inclusions exceeding 200 μm.

Besides, in the production of bearings, there is a possibility that the circulation performance of lubricating oil cannot be obtained because, with reduction in size of bearings, pores or iron microstructure becomes large relative to a bearing as mentioned above. In other words, although conventional reduced iron powder has fine pores, its many inclusions cause the product to fail. Bearings with inner diameter of 0.6 mm and outer diameter of 2.0 mm can be produced at a relatively high yield rate even when conventional reduced iron powder is used. In the case of producing smaller bearings, for example, with inner diameter of 0.4 mm and outer diameter of 1.4 mm using conventional reduced iron powder, however, formability is insufficient and the yield rate drops significantly, making mass production difficult.

Atomized iron powder is not suitable for use in the aforementioned small bearings, as its smooth surface causes insufficient bonding power between iron powder particles during forming and leads to a significantly lower rattler value. Moreover, in the production of oilless bearings, atomized iron powder has a major drawback of having few pores and hindering sufficient circulation of oil. Atomized iron powder is also problematic in that it has few fine pores, although its inclusions are few.

From the perspective of using reduced iron powder as chemical reaction material, the powder is required to have large specific surface area so that the powder has excellent reactivity per unit mass and even the particle inside can be effectively used as reaction material.

As described above, reduced iron powder whose apparent density is much lower than 2.0 Mg/m3 and whose specific surface area is 0.2 m3/g or more, which is much higher than 0.1 m3/g, is needed in order to produce bearings with inner diameter of less than 0.6 mm and outer diameter of less than 2.0 mm at a high yield rate. Such reduced iron powder, however, cannot be prepared by conventional production methods.

It could therefore be helpful to provide reduced iron powder that has fewer coarse inclusions, has excellent formability, has high porosity after sintering, has excellent reactivity per unit mass, and can be effectively used as reaction material even to the particle inside, a method for preparing the same, and a bearing produced from the reduced iron powder.

Solution to Problem

We Thus Provide:

1. Reduced iron powder having an apparent density of 1.00 Mg/m3 to 1.40 Mg/m3.

2. The reduced iron powder according to 1., having an amount of oxygen of 0.38 mass % or less.

3. The reduced iron powder according to 1. or 2., having a specific surface area of 0.20 m2/g or more.

4. A method for preparing reduced iron powder, for use in preparing the reduced iron powder according to any one of 1. to 3., comprising: agglomerating precursor iron oxide powder whose mean particle size measured by a laser diffraction method is 3.0 μm or less to obtain iron oxide powder; and thereafter reducing the iron oxide powder at 800° C. to 1000° C. with hydrogen to obtain the reduced iron powder.

5. The method for preparing reduced iron powder according to 4., comprising classifying and selecting the iron oxide powder so that its mean particle size measured by the laser diffraction method is 50 μm to 200 μm, before the reduction of the iron oxide powder.

6. The method for preparing reduced iron powder according to 4. or 5., wherein the iron oxide powder has an iron content of 68.8 mass % or more.

7. A bearing produced from the reduced iron powder according to any one of 1. to 3. as a raw material.

Advantageous Effect

It is thus possible to obtain reduced iron powder that has fewer coarse inclusions, has excellent formability, has high porosity after sintering, has excellent reactivity per unit mass, and can be effectively used as reaction material even to the particle inside.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a flow diagram illustrating a process of preparing reduced iron powder according to one of the disclosed embodiments; and

FIG. 2 is a diagram illustrating an appearance image and cross-sectional image of each of a conventional example and Examples 1 and 2.

 DETAILED DESCRIPTION

We succeeded in producing new reduced iron powder having an apparent density of 1.00 Mg/m3 to 1.40 Mg/m3 and a specific surface area of 0.20 m 2/g or more by a new preparation method. The reduced iron powder according to the disclosure has sufficiently low apparent density, and therefore has excellent formability, has excellent reactivity per unit mass, and can be effectively used as reaction material even to the particle inside. The reduced iron powder according to the disclosure also has fine iron microstructure (see the white portions in the cross-sectional images in FIG. 2), as a result of which inclusions are finely dispersed. Hence, the reduced iron powder can be used as raw material to produce high-strength bearings at a high yield rate. For example, bearings with inner diameter of 0.4 mm and outer diameter of 1.4 mm can be mass-produced at a high yield rate.

The following describes a method for preparing reduced iron powder according to one of the disclosed embodiments, with reference to FIG. 1. First, iron oxide powder (precursor iron oxide powder) having a predetermined mean particle size is agglomerated to obtain iron oxide powder. The obtained iron oxide powder is then classified and selected so that its mean particle size is set to a predetermined range. After this, the iron oxide powder is reduced with hydrogen gas (Fe2O3+3H2=2Fe+3H2O) and crushed as appropriate to obtain reduced iron powder (porous iron powder).

It is important to refine the precursor iron oxide powder as starting material so that its mean particle size (D50) measured by a laser diffraction method is 3.0 μm or less, in order to set the apparent density of the reduced iron powder to 1.40 Mg/m3 or less to thus make the inclusions in the reduced iron powder finer. The refinement makes pores smaller, which contributes to finer inclusions. The mean particle size of the precursor iron oxide powder is preferably 2.0 μm or less. No lower limit is placed on the mean particle size of the precursor iron oxide powder, yet in industrial terms the lower limit is approximately 0.5 μm.

An example of the method for preparing the precursor iron oxide powder is a method of neutralizing and extracting waste acid after pickling steel sheets in a steelworks. For example, a method using a spray roasting furnace by the Ruthner process and a fluidized roasting method by the Lurgi process are available.

It is essential to agglomerate the precursor iron oxide powder to obtain iron oxide powder formed by the coagulation of the precursor iron oxide powder. Effective methods of agglomerating the precursor iron oxide powder include a method of mixing a binder and water into the precursor iron oxide powder using a Henschel mixer and drying the mixture, and a method of dissolving the precursor iron oxide powder in water together with a binder to form slurry and then drying the droplets with hot air (spray dryer). In both methods, the binder may be PVA, starch, or the like.

When a vessel or a reducing furnace is charged with the iron oxide powder to reduce the iron oxide powder, voids formed between coagulated particles ensure appropriate air permeability, thus facilitating the reduction. To achieve this, the mean particle size of the iron oxide powder after the agglomeration is important. Moreover, the particle size of the iron oxide powder to be reduced correlates with the particle size of the reduced iron powder. It is therefore preferable to classify and select the iron oxide powder after the agglomeration to control its mean particle size, before reducing the iron oxide powder.

The mean particle size of the iron oxide powder after the agglomeration is important, as mentioned above. However, not all particles necessarily maintain their shape. For example, a plurality of particles may bond with each other, or one particle may be broken. Accordingly, we made careful examination, and discovered that the mean particle size of the reduced iron powder effective in practical terms is 50 μm to 100 μm and, to achieve this, the mean particle size of the iron oxide powder is preferably 50 μm to 200 μm. Thus, it is preferable to appropriately classify and select the iron oxide powder after the agglomeration so that its mean particle size is set to 50 μm to 200 μm.

It is also preferable that the iron content in the iron oxide powder is 68.8 mass % or more. This sufficiently reduces the amount of oxygen in the reduced iron powder, and further enhances the effect of improving chemical reactivity and the effect of producing high-strength bearings at a high yield rate. No upper limit is placed on the iron content in the iron oxide powder, yet the upper limit is approximately 77 mass %.

The iron oxide powder after the agglomeration is reduced to obtain reduced iron powder (also simply referred to as iron powder). We discovered the conditions for preparing iron powder that has low apparent density, i.e. approximately half that of conventional reduced iron powder or atomized iron powder, and in which inclusions are finely dispersed, by appropriately managing the reduction temperature in this reduction step which is hydrogen reduction of iron oxide. It is important to set the reduction temperature during the reduction to 800° C. or more and 1000° C. or less. If the reduction temperature is less than 800° C., it is difficult to remove oxygen in the reduced iron powder by reduction reaction. As a result, a large amount of oxygen remains in the iron powder. This causes insufficient chemical reactivity and decreases formability, and leads to a lower yield rate in the production of bearings. If the reduction temperature is more than 1000° C., the sintering of the iron powder progresses and the apparent density exceeds 1.40 Mg/m3. This causes insufficient chemical reactivity, and leads to a lower yield rate in the production of bearings.

The reduction time is preferably 120 min or more, to sufficiently reduce iron oxide powder yielded from fine precursor iron oxide powder of 3.0 μm or less in mean particle size to obtain reduced iron powder of 1.00 Mg/m3 to 1.40 Mg/m3 in apparent density. No upper limit is placed on the reduction time, yet the upper limit may be approximately 240 min in terms of process efficiency.

The conditions other than the reduced iron powder preparation conditions described above may be well-known reduced iron powder preparation conditions. An example of the reduction method is a method of heating iron powder at atmospheric pressure using a belt furnace or the like in a reducing atmosphere such as hydrogen.

The following describes reduced iron powder according to one of the disclosed embodiments. The reduced iron powder has an apparent density of 1.00 Mg/m3 to 1.40 Mg/m3, and can be prepared by the preparation method described above for the first time. If the apparent density of the reduced iron powder is less than 1.00 Mg/m3, the specific surface area is excessively large, which increases the risk of a dust explosion, i.e. rapidly progressing reaction with oxygen in the air. If the apparent density of the reduced iron powder is more than 1.40 Mg/m3, chemical reactivity is insufficient. Besides, the strength of the green compact decreases. This facilitates failures in subsequent steps, and leads to a lower yield rate in the production of bearings.

When the apparent density of the reduced iron powder is in the range of 1.00 Mg/m3 to 1.40 Mg/m3, the green strength increases, and bearings can be produced at a high yield rate. Moreover, by limiting the apparent density to this range, coarse inclusions are effectively reduced, and the strength after sintering is improved, thus contributing to higher bearing quality. Further, the reduced iron powder has excellent reactivity per unit mass, and can be effectively used as reaction material even to the particle inside. The apparent density is measured according to JIS-Z-2504.

The amount of oxygen in the reduced iron powder is preferably 0.38 mass % or less. This further enhances the effect of improving chemical reactivity and the effect of producing high-strength bearings at a high yield rate. No lower limit is placed on the amount of oxygen in the reduced iron powder, yet the lower limit is approximately 0.10 mass %.

If the specific surface area of the reduced iron powder is less than 0.20 m2/g, iron powder particles characteristic of the disclosure are not formed sufficiently, leading to insufficient chemical reactivity. The specific surface area of the reduced iron powder is therefore preferably 0.20 m2/g or more. No upper limit is placed on the specific surface area of the iron powder, yet the upper limit is preferably approximately 0.4 m2/g in terms of handling and the like. The specific surface area is measured by a BET method using nitrogen gas.

A bearing can be produced from the reduced iron powder as raw material. The bearing has an excellent yield rate in bearing production and excellent strength and porosity, and has high chemical reactivity, as described in the following examples. The method for producing the bearing from the reduced iron powder as raw material may be a conventional method except that the reduced iron powder is used as raw material.

EXAMPLES

Table 1 compares conventional reduced iron powder (reduced iron powder obtained through two reduction steps), conventional atomized iron powder, and reduced iron powders (Comparative Examples 1 to 5, Examples 1 to 4) obtained through the preparation process illustrated in FIG. 1. In Comparative Examples 1 to 5 and Examples 1 to 4, hydrogen was used as reducing gas. The conventional reduced iron powder was prepared as follows: Using iron ore or mill scale as raw material, coke powder was added and primary reduction using a tunnel furnace was performed without an agglomeration step and a classification step in FIG. 1, and then reduction in the thick-line box was performed.

The iron powder evaluation items listed in Table 1 were evaluated as follows.

The mean particle size of precursor iron oxide powder was measured by a volume-based laser diffraction method.

The iron content in iron oxide powder was measured according to JIS-M-8212.

The mean particle size of iron oxide powder after agglomeration was measured by a laser diffraction method, and set as 50% particle size.

The apparent density of reduced iron powder was measured according to JIS-Z-2504.

The mean particle size of reduced iron powder was measured by a volume-based laser diffraction method, and set as 50% particle size.

The specific surface area of reduced iron powder was measured by a BET method using nitrogen gas.

The amount of oxygen in reduced iron powder was measured by an inert gas fusion infrared absorption method (GFA).

The yield rate in bearing production was evaluated as pass when the failure rate from the green compacting in the shape of a cylinder with an inner diameter of 0.4 mm, an outer diameter of 1.4 mm, and a height of 2 mm to 2.5 mm to the completion of sintering was 5% or less (a yield rate of 95% or more). The strength was evaluated as pass when the strength upon compressing the cylinder in a lying state was 17 N/mm2 or more, and fail when the strength was less than 17 N/mm2.

The porosity is a factor determining the performance of an oilless bearing, and its appropriate value is 18% to 22%.

The porosity was measured by mercury porosimetry.

The reaction rate in chemical reaction was evaluated based on the reaction in which sulfur content in soil adsorbed to iron (Fe+S=FeS). Adsorptivity by this reaction is required to be a predetermined level or more in practical terms. Accordingly, in Table 1, chemical reactivity is set as an index represented by a ratio to 1 as the minimum required level.

FIG. 2 illustrates an appearance image and cross-sectional image of the reduced iron powder of each of Examples 1 and 2, in comparison with the conventional reduced iron powder. The appearance image was taken using a scanning electron microscope (SEM), and the cross-sectional image was taken using an optical microscope. Many pores were contained inside particles in Examples 1 and 2, as compared with the conventional reduced iron powder.

TABLE 1 Preparation conditions for reduced iron powder Characteristics of reduced iron powder Mean particle size Iron content Mean particle Mean Specific of precursor iron in iron oxide size of iron oxide Reduction Reduction Apparent particle surface Amount of oxide powder powder powder time temperature density size area oxygen (μm) (mass %) (μm) (min) (° C.) (Mg/m3) (μm) (m2/g) (mass %) — — — — — 2.2 to 55 to 0.07 to <0.40 2.7 105 0.10 — — — — — 2.5 to 50 to 0.04 to <0.30 3.1 90 0.08 2.8 68.8 120 240 1050 1.48 100 0.20 0.22 2.8 68.8 120 240 780 0.98 80 0.31 0.40 3.2 68.8 150 240 850 0.95 90 0.28 0.45 2.8 68.8 45 240 850 1.49 60 0.22 0.21 2.8 68.8 220 240 850 0.95 150 0.33 0.55 2.8 68.8 50 240 1000 1.38 80 0.22 0.25 2.8 68.8 120 240 1000 1.32 80 0.25 0.27 2.8 68.8 120 240 800 1.03 60 0.28 0.36 2.8 68.2 110 240 850 1.12 80 0.25 0.43 0.7 69.0 90 240 850 1.05 75 0.30 0.37 Characteristics in bearing production Chemical Yield rate Strength Porosity reactivity (%) (N/mm2) (%) (—) Remarks 84 15 18 0.7 Conventional reduced iron powder 62 9 24 0.5 Conventional atomized iron powder 92 18 20 0.8 Comparative Example 1 92 19 21 0.9 Comparative Example 2 92 19 20 0.9 Comparative Example 3 88 16 16 0.8 Comparative Example 4 92 19 26 0.8 Comparative Example 5 96 21 22 1.4 Example 1 98 25 21 1.3 Example 2 98 24 20 1.3 Example 3 95 17 19 1.2 Example 4 97 23 22 1.3 Example 5

Comparative Example 1 is iron powder obtained by reducing iron oxide powder at 1050° C. Its apparent density was 1.48 Mg/m3, which is outside the range according to the disclosure. While the degree of reduction was relatively favorable, the yield rate in bearing production was evaluated as fail. The chemical reactivity was also evaluated as fail.

Comparative Example 2 is iron powder obtained by reducing iron oxide powder at 780° C. Its apparent density was 0.98 Mg/m3, which is outside the range according to the disclosure. The yield rate in bearing production and the chemical reactivity were evaluated as fail.

Comparative Example 3 is iron powder obtained using precursor iron oxide powder of 3.2 μm in mean particle size and by reducing iron oxide powder after agglomeration at 850° C. Its apparent density was 0.95 Mg/m3, which is outside the range according to the disclosure. The degree of reduction was relatively low. The yield rate in bearing production was poor, and the chemical reactivity was evaluated as fail.

Comparative Example 4 is reduced iron powder obtained using iron oxide powder after agglomeration of 45 μm in mean particle size. Its apparent density was 1.49 Mg/m3, which is outside the range according to the disclosure. The degree of reduction was high, and the strength of the bearing was evaluated as pass. Meanwhile, the yield rate in bearing production was evaluated as fail. The chemical reactivity was also evaluated as fail.

Comparative Example 5 is reduced iron powder obtained using iron oxide powder after agglomeration of 220 μm in mean particle size. Its apparent density was 0.95 Mg/m3, which is outside the range according to the disclosure. The strength of the bearing was evaluated as pass, but the yield rate in bearing production was evaluated as fail. The porosity was excessive, and the chemical reactivity was evaluated as fail.

Example 1 is iron powder obtained using iron oxide powder after agglomeration of 50 μm in mean particle size and by reducing the iron oxide powder after agglomeration at 1000° C. Its apparent density was 1.38 Mg/m3. The degree of reduction was high, and the yield rate in bearing production and the strength and porosity of the bearing were all evaluated as pass. The chemical reactivity was also favorable.

Example 2 is iron powder obtained using iron oxide powder after agglomeration of 120 μm in mean particle size and by reducing the iron oxide powder after agglomeration at 1000° C. Its apparent density was 1.32 Mg/m3. The degree of reduction was favorable, and the yield rate in bearing production and the strength and porosity of the bearing were all evaluated as pass. The chemical reactivity was also favorable.

Example 3 is iron powder obtained using iron oxide powder after agglomeration of 120 μm in mean particle size and by reducing the iron oxide powder after agglomeration at 800° C. Its apparent density was 1.03 Mg/m3. The degree of reduction was favorable, and the yield rate in bearing production and the strength and porosity of the bearing were all evaluated as pass. The chemical reactivity was also favorable.

Example 4 is iron powder obtained using iron oxide powder after agglomeration having an iron content of 68.2 mass %. Its apparent density was 1.12 Mg/m3, while the amount of oxygen in the reduced iron powder was 0.43 mass %. The chemical reactivity was favorable, and the yield rate in bearing production and the strength and porosity of the bearing were all evaluated as pass.

Example 5 is iron powder obtained using precursor iron oxide powder of 0.7 μm in mean particle size, with the mean particle size of iron oxide powder being 90 μm. Its apparent density was 1.05 Mg/m3. The chemical reactivity was favorable, and the yield rate in bearing production and the strength and porosity of the bearing were all evaluated as pass.

Claims

1. Reduced iron powder having an apparent density of 1.00 Mg/m3 to 1.40 Mg/m3.

2. The reduced iron powder according to claim 1, having an amount of oxygen of 0.38 mass % or less.

3. The reduced iron powder according to claim 1, having a specific surface area of 0.20 m2/g or more.

4. A method for preparing reduced iron powder, for use in preparing the reduced iron powder according to claim 1, comprising:

agglomerating precursor iron oxide powder whose mean particle size measured by a laser diffraction method is 3.0 μm or less to obtain iron oxide powder; and
thereafter reducing the iron oxide powder at 800° C. to 1000° C. with hydrogen to obtain the reduced iron powder.

5. The method for preparing reduced iron powder according to claim 4, comprising

classifying and selecting the iron oxide powder so that its mean particle size measured by the laser diffraction method is 50 μm to 200 μm, before the reduction of the iron oxide powder.

6. The method for preparing reduced iron powder according to claim 4,

wherein the iron oxide powder has an iron content of 68.8 mass % or more.

7. A bearing produced from the reduced iron powder according to claim 1 as a raw material.

8. A bearing produced from the reduced iron powder according to claim 2 as a raw material.

9. A bearing produced from the reduced iron powder according to claim 3 as a raw material.

Patent History
Publication number: 20180221960
Type: Application
Filed: Jan 14, 2016
Publication Date: Aug 9, 2018
Applicant: JFE STEEL CORPORATION (Chiyoda-ku, Tokyo)
Inventors: Satoshi MACHIDA (Chiyoda-ku, Tokyo), Naomichi NAKAMURA (Chiyoda-ku, Tokyo)
Application Number: 15/542,459
Classifications
International Classification: B22F 9/22 (20060101); B22F 1/00 (20060101);