RAW MATERIAL FOR SHOT-PEENING MATERIALS, FINISHED WIRE, METHOD OF MANUFACTURING SHOT-PEENING MATERIALS, AND SHOT-PEENING MATERIALS

The purposes of the present inventions are to provide a raw material for shot-peening materials wherein breaking a wire is prevented in obtaining a finished wire to improve the productivity, to provide a finished wire and a method of manufacturing shot-peening materials by which productivity is improved, and to provide shot-peening materials that are manufactured by that method. The finished wire, of which the area of carbides with a particle size of 2 μm or less is 80% or more of the total area, is manufactured by the steps of wiredrawing a raw material to obtain a wire, and repeatedly annealing and cold-drawing the wire. The raw material for the shot-peening materials comprises, by mass %, 0.95-1.10% carbon, 0.15-0.30% silicon, 0.40% or less manganese, 0.020% or less phosphorus, 0.010% or less sulfur, 1.40-1.60% chromium, 0.0015% or less oxygen, and the remaining materials of iron and unavoidable impurities. The method of manufacturing the shot-peening materials uses that raw material. The shot-peening materials are manufactured by that method.

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Description
TECHNICAL FIELD

The present inventions relate to a raw material for shot-peening materials, a finished wire, a method of manufacturing shot-peening materials, and shot-peening materials.

BACKGROUND ART

In an existing process of manufacturing shot-peening materials, a raw material, e.g., so-called bearing steel, is wiredrawn to obtain a wire. The wire is cut at the same length as its diameter. The cut wire is then thrown at a rigid wall to round off its edges. The rounded-off wire is ground to a predetermined degree of sphericity. Then, it is quenched and tempered to get a predetermined Vickers hardness. This process is well known (e.g., Japanese Laid-open Publication No. 2001-79766).

DISCLOSURE OF INVENTION

However, when a bearing steel that is not modified from a standard steel (e.g., a high carbon chromium bearing steel, SUJ2 of JIS (Japan Industrial Standards) G4805, of which the components are shown in Table 2) has been used as a raw material for shot-peening materials, there has been a problem of the wire breaking. This does not improve the productivity of the shot-peening materials. That is, once a wire breaks in a process for wiredrawing, a line for manufacturing a wire, which runs at 100 m/min, stops. Thus, the productivity is significantly affected. As a result, the productivity of shot-peening materials cannot increase. Especially, for a wire having a diameter of less than 0.7 mm (for example, a diameter of 0.3 mm), a wire often breaks in a wiredrawing process and the problem becomes significant. Thus, the most appropriate method of manufacturing shot-peening materials from a small diameter finished wire has not been established.

The purpose of the present inventions is to solve that problem and to provide a raw material for shot-peening materials that is used for preparing a finished wire by wiredrawing. Breaking a wire is thus prevented in wiredrawing, so as to improve the productivity of the shot-peening materials. Another purpose is to provide a finished wire. Other purposes are to provide a method of manufacturing shot-peening materials by which improved productivity is achieved and to provide shot-peening materials that are manufactured by that method. That is, in the method a specific raw material for shot-peening materials is used for maintaining its purity to prevent a wire from breaking. A finished wire that is appropriate for subsequent processes is obtained from the raw material. The good structure of the raw material is maintanined after quenching and tempering by retaining fine carbides in the structure before quenching. The present inventions also provide shot-peening materials that have long lives and that induce appropriate compressive residual stress in a work. The present inventions further provide a method of manufacturing shot-peening materials that are readily quenched, to be suitable for shot-peening.

As the result of the inventors' keen study to solve the problem, they have discovered that a raw material for shot-peening materials that contains by percentages by mass 0.95-1.10% carbon, 0.15-0.30% silicon, 0.40% or less manganese, 0.020% or less phosphorus, 0.010% or less sulfur, 1.40-1.60% chromium, 0.0015% or less oxygen, and the remaining materials of iron and unavoidable impurities, can achieve the purpose. The first feature of the present inventions has been made based on that finding. This raw material is selected to reduce non-metallic inclusions, which deteriorate the connections in a base metal, to prevent a wire from breaking by cracking. That is, there is a need to reduce oxides, compared to the method of manufacturing shot-peening materials disclosed in Japanese Patent Laid-open No. 2001-79766. Thus, the content of oxygen is significantly reduced to 0.0015% or less. The maximum content of silicon is reduced by 17%. The contents of manganese and phosphorus are also reduced to reduce sulfides, especially MnS. In general the maximum contents other than iron are reduced. The content of carbon is unchanged, to maintain the amount of carbides, which are essential to the material.

The finished wire for the shot-peening materials of the present inventions is manufactured by the steps of wiredrawing the raw material of the first feature of the present inventions to obtain a wire, and repeatedly annealing and cold-drawing the wire to obtain a finished wire. The area of carbides with sizes of 2 μm or less is 80% or more of the total area of the finished wire. That is, a wire is manufactured by “wiredrawing” an ingot. Next, the wire is repeatedly “annealed” and “cold-drawn” to become a finished wire. Then, the carbides of the finished wire are at a condition where the area of carbides with sizes of 2 μm or less is 80% or more of the total area of the finished wire. In the second feature of the inventions, the wire that is obtained by wiredrawing the raw material for the shot-peening materials of the first feature of the inventions is repeatedly annealed and drawn, because its ability to elongate decreases and its breaking becomes easier when the rate of reduction of the cross-sectional area of it is high. Thus, it is annealed to eliminate work hardening when its elongation decreases such that the cross section is not reduced as before. It is preferable to wiredraw it again under the condition where its elongation is recovered. It is cold-drawn, because the refinement of the crystalline grains and work hardening occur under cold-drawing. By hot-drawing it, crystalline grains are elongated, and no refinement occurs. The annealing is preferably done three, four, or five times. The reduction of the cross-sectional area of the wire is preferably 10% to 40%. The area of carbides with particle sizes of 2 μm or less is 80% or more of the total area of the finished wire, because then a structure appropriate for the shot-peening materials is maintained in a small diameter “finished wire.” By maintaining the structure of the finished wire to be one that is appropriate, the quality of the final product is improved and the loss of raw materials is avoided.

The term “finished wire” is defined as a small diameter wire that is finished before being cut. It is obtained by repeatedly annealing and cold-drawing a wire that is prepared by rolling or drawing.

The sizes of the carbides of the finished wire are maintained to be the same as those before quenching by annealing at 720° C. or lower, because the coarsening of carbides is prevented by the annealing. Therefore, a larger force can be applied to a work by shot-peening materials, because the breaking strength of them increases as a result of the refinement of the crystalline grains. Annealing is preferably performed at 700° C. by using a bright annealing furnace. The use of it eliminates an acid cleaning process, since no oxidized scales are formed on the surface of the wire.

The method of manufacturing the shot-peening materials of the present inventions is characterized in that it comprises the steps of cutting and plastic-forming the finished wire of the second feature to make raw shot-peening materials, and quenching and tempering them. The present inventions provide shot-peening materials that are efficiently manufactured and have a good quality, since they are made from the finished wire, which has a proper elongation and has a reliable quality.

The tempering parameters are defined by the following equation: T{(21.3 −5.8×[C])+log(t)}: where T denotes the tempering temperature (K), t the tempering time (hr), and [C] the carbon content (%). The tempering temperature T and the carbon content C are selected so that the parameters become 6200-7300. When the tempering temperature T, the carbon content C, and the tempering time t, are selected so that the parameters become 6200-7300, the coarsening of crystalline grains is prevented and the residual stress is relieved, to increase toughness. Thus, that selection is preferable. The method may further comprise the step of plastic-forming the raw shot-peening materials. This method has an advantage in that almost no breaking is generated during a shot-peening process, because the corners of the shot-peening materials are rounded off. Plastic-forming the raw shot-peening material means to make the raw shot-peening material in a spherical shape from a short piece that is made by cutting the finished wire by plastic-forming. That is, the raw shot-peening materials are not rounded by cutting or grinding.

The method of manufacturing the shot-peening materials of the present inventions is further characterized in that it comprises the steps of wiredrawing a raw material for the shot-peening materials of the first feature to obtain a wire, repeatedly annealing and cold-drawing the wire to obtain a finished wire, obtaining the raw shot-peening materials by cutting the finished wire and plastic-forming, and quenching and tempering the raw shot-peening materials. It has an advantage in that the crystalline grains are refined and the toughness is recovered after the heat treatment.

The method of manufacturing the shot-peening materials of the present inventions is further characterized in that the quenching temperature is 820-850° C. in the aforementioned method. It has an advantage in that almost no residual austenite is generated, and overall, martensite is uniformly obtained in the structure.

The shot-peening materials of the present inventions are those manufactured by the methods that are described above. They substantially have a structure of tempered martensite as a base metal in which fine carbides precipitate. The ratio of the area of the carbides to the total area is preferably 70-95%. When the metallic portion, which is a binder, becomes less, the binding function of the base metal become weaker. When the metallic portion is significantly reduced such that the ratio exceeds 95%, the carbides come close to each other, resulting in weakening the binding function of the base metal. If the ratio is less than 70%, the shot-peening materials will not have the required hardness, such as a Vickers hardness of 950 HV, which is appropriate for a material used for shot-peening materials, such as JIS SUJ2. The present inventions can provide materials that are suitable for shot-peening. The sizes of the carbides are preferably smaller than 2 μm in diameter, more preferably 1-0.1 μm. When they are 2 μm or larger, they often cause a crack in the shot-peening materials. When they are 1-0.1 μm in diameter their effect becomes less, because their exposure on the surface of the base metal is reduced. Thus, that is preferable. The raw material of the shot-peening materials of the present inventions comprises by percentages by mass 0.95-1.10% carbon, 0.15-0.30% silicon, 0.40% or less manganese, 0.020% or less phosphorus, 0.010% or less sulfur, 1.40-1.60% chromium, 0.0015% or less oxygen, and the remaining materials of iron and unavoidable impurities. The raw material is wiredrawn to be a wire. The wire is repeatedly annealed and cold-drawn to be a finished wire. The finished wire is cut and subjected to plastic-forming to be raw shot-peening materials. The shot-peening materials are obtained by quenching and tempering the raw shot-peening materials. Their structures consist of fine carbides and tempered martensite. The ratio of the area of carbides to the total area is 70-95%. The Vickers hardness of them is preferably adjusted to 950 HV to 1050 HV by plastic-forming. When their Vickers hardness is 950 HV to 1050 HV a work having high hardness can be properly processed by shot-peening.

Since the raw material of the present inventions is selected so as to reduce the possibility of breaking a wire, the productivity is improved. In addition, even a small diameter wire can be processed at a high speed. The finished wire of the present inventions is suitable for manufacturing shot-peening materials. The method of manufacturing the shot-peening materials of the present inventions is high in both productivity and quality. The shot-peening materials of the present inventions are suitable for a shot-peening process.

The basic Japanese patent applications, No. 2008-046967, filed Feb. 28, 2008, and No. 2008-169971, filed Jun. 30, 2008, are hereby incorporated in their entirety by reference in the present application. The present inventions will become more fully understood from the detailed description given below. However, the detailed description and the specific embodiment are illustrations of desired embodiments of the present inventions, and are described only for an explanation. Various possible changes and modifications will be apparent to those of ordinary skill in the art on the basis of the detailed description. The applicant has no intention to dedicate to the public any disclosed embodiment. Among the disclosed changes and modifications, those which may not literally fall within the scope of the present claims constitute, therefore, a part of the present inventions in the sense of the doctrine of equivalents. The use of the articles “a,” “an,” and “the” and similar referents in the specification and claims are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by the context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illustrate the inventions, and so does not limit their scope, unless otherwise claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flowchart of the method of manufacturing the shot-peening materials of the present inventions.

FIG. 2a shows a microstructure of the finished wire for the shot-peening materials of the present inventions photographed by a SEM (×1,000).

FIG. 2b shows a microstructure of the finished wire for the shot-peening materials of the present inventions photographed by a SEM (×5,000).

FIG. 3a shows a microstructure of the shot-peening materials of the present inventions photographed by a SEM (×1,000).

FIG. 3b shows a microstructure of the shot-peening materials of the present inventions photographed by a SEM (×3,000).

FIG. 4a shows a microstructure of the shot-peening materials of the present inventions photographed by a SEM (×1,000).

FIG. 4b shows a microstructure of the shot-peening materials of the present inventions photographed by a SEM (×3,000).

FIG. 5 is a graph showing the distribution of the sizes of the carbides of the shot-peening materials of the present inventions.

FIG. 6a is a graph showing the relationship between the ratio of the area of carbides and the hardness (a hardness derived from values obtained by the hardesses of two respective structures) of the shot-peening materials of the present inventions (for a structure having a higher hardness).

FIG. 6b is a graph showing the relationship between the ratio of the area of carbides and hardness (a hardness derived from values obtained by the hardesses of two respective structures) of the shot-peening materials of the present inventions (for a structure having a lower hardness).

 BEST MODE FOR CARRYING OUT THE INVENTION

The inventors provide shot-peening materials that are suitable for a shot-peening process and that are made to form a wire, by adjusting the components of a bearing steel, SUJ of JIS G4805, to the components that are suitable for a wire for shot-peening materials and that emphasize the features of the components. The components of the raw material of the shot-peening materials of the present inventions are shown in Table 1. The components of the existing material, SUJ2 (the Japanese Industrial Standards), are shown in Table 2 as a comparative example.

TABLE 1 (Percentages by mass) Iron and unavoid- able impu- Carbon Silicon Manganese Phosphorus Sulfur Chromium Oxygen Copper Nickel Molybdenum rities Working 0.95-1.10 0.15-0.30 ≦0.4 ≦0.02 ≦0.01 1.40-1.60 ≦0.0015 ≦0.15 ≦0.15 ≦0.06 Remaining Example

TABLE 2 (Percentages by mass) Iron and unavoid- able impu- Carbon Silicon Manganese Phosphorus Sulfur Chromium Oxygen Copper Nickel Molybdenum rities Comparative 0.95-1.10 0.15-0.35 ≦0.50 ≦0.025 ≦0.025 1.30-1.60 Not spec- Not spec- Not spec- Not spec- Remaining Example ified ified ified ified

As shown in Tables 1 and 2, the raw material of the present invention has 0.010% or less sulfur and 0.0015% or less oxygen, which are both lower than those of the bearing steel, SUJ, of JIS G4805. Thus, the precipitation of impurities such as sulfides and oxides is reduced and the purity of the material is maintained. So, a non-uniform structure, which may break a wire, is prevented. As a result, breaking a wire is prevented or minimized. Breaking a wire during wiredrawing is prevented even though the wire is small in diameter.

The contents of manganese and phosphorus are limited to lower values, compared to the percentage of the bearing steel, SUJ, of JIS G4805. These limitations are preferable to suppress the generation of residual austenite and an intergranular ternary compound.

The content of copper is preferably limited to 0.15% or less to avoid the deterioration of carburizing.

The content of nickel is preferably limited to 0.15% or less to avoid deterioration of carburizing.

Example 1

The flowchart of manufacturing shot-peening materials by using the raw material for the shot-peening materials of the present inventions is shown in FIG. 1. By reference to FIG. 1, manufacturing shot-peening materials is described below. As in the flowchart, at a first step a raw material having the components of Table 1 is prepared. At a second step the raw material is wiredrawn to be a wire. At a third step the wire is repeatedly annealed and cold-drawn. At a fourth step the wire is cut. At a fifth step the cut wire is subject to plastic formation. At a sixth step it is quenched and tempered. At a seventh step it is shot just to harden the shot-peening materials themselves.

Comparisons between example 1 and the bearing steel of Table 2, SUJ of JIS G4805, during the first three steps, are shown in Table 3. The wire after the wiredrawing of the second step has a diameter of 1.6 mm and a hardness of 320 HV.

TABLE 3 Working Example Comparative Example Containing 0.95-1.10% carbon, Containing 0.95-1.10% carbon, 0.15-0.30% silicon, 0.40% or less 0.15-0.35% silicon, 0.50% or less manganese, 0.020% or less manganese, 0.025% or less phosphorus, 0.010% or less sulfur, phosphorus, 0.025% or less sulfur, 1.40-1.60% chromium, 0.0015% or 1.30-1.60% chromium, and the less oxygen, 0.15% or less copper, remaining materials of iron and 0.15% or less nickel, 0.06% or less unavoidable impurities molybdenum, and the remaining materials of iron and unavoidable impurities Breaking a wire did not occur at the Breaking a wire occurred at the step of wiredrawing to obtain a step of wiredrawing to obtain a finished wire in manufacturing the finished wire in manufacturing shot-peening materials. the shot-peening materials.

As shown in FIG. 3, no breaking of a wire occurred at the step of wiredrawing to obtain a finished wire in manufacturing the shot-peening materials in Example 1. By contrast, breaking a wire occurred at that step in the comparative example.

TABLE 4 Working Example Comparative Example Containing 0.010% or less sulfur Containing 0.025% or less sulfur and 0.0015% or less oxygen Breaking a wire did not occur at the Breaking a wire occurred at the step of wiredrawing to obtain a step of wiredrawing to obtain a finished wire in manufacturing the finished wire in manufacturing shot-peening materials. the shot-peening materials.

The finished wire for the shot-peening materials is manufactured by repeatedly annealing and cold-drawing at the third step. The diameter of the wire for the finished wire is 1.6 mm and its hardness is 320 HV. Since the wire is not work-hardened and has the hardness of 320 HV, cold-drawing can be easily performed. Specifically, at the third step, where annealing and cold-drawing are repeated, a bright annealing furnace (BA) that is 8 m long is used to heat one portion, which is 4 m long, at 700° C. and to cool the other portion, which is 4 m long. An example of the changes of the sizes, which example includes annealing four times, is as follows:

Diameter: 1.6 mm-1.5 mm-1.4 mm→(Annealing in the BA)→1.3 mm-1.2 mm-1.1 mm-1.0 mm-→(Annealing in the BA)→0.9 mm-0.8 mm-0.75 mm-0.7 mm (Annealing in the BA)→0.6 mm-0.55 mm-0.5 mm-0.45 mm-0.4 mm→(Annealing in the BA)→0.35 mm-0.3 mm (End).

TABLE 5 Comparative Example 1 Working Example Comparative Example 2 Repeatedly annealing and cold- Repeatedly annealing and cold- Repeatedly annealing and cold- drawing to obtain a finished wire drawing to obtain a finished wire drawing to obtain a finished wire with a diameter of more than 0.6 mm with a diameter of 0.25-0.6 mm with a diameter less than 0.25 mm The annealing is performed at a The annealing is performed at a temperature of 720° C. or less. temperature over 720° C. When the diameter exceeds 1.0 mm, Breaking a wire occurs at the Cutting a wire with a diameter the wire is too thick for shot- step of wiredrawing to obtain of less than 0.25 mm is difficult. peening materials. the finished wire in manufacturing When the annealing temperature the shot-peening materials. exceeds 720° C., the It is necessary that wiredrawing temperature is over the be properly performed by transformation temperature, annealing, to relieve the residual and so the possibility increases stress and soften the steel. that the structure of a base metal will change.

The observation of the structure of the finished wire shows that the area of carbides with particle sizes of 2 μm or less is 80% or more of the total area of the finished wire. A microstructure of the finished wire (with a diameter of 0.3 mm and after being etched by nital) for the shot-peening materials of the present inventions photographed by a SEM is shown in FIG. 2. It shows that the structure of the finished wire that is good for shot-peening materials is obtained. The diameter of the finished wire is arranged to be 0.25-0.6 mm by repeatedly annealing and cold-drawing. When the final diameter is 0.3 mm, the Vickers hardness is 350 HV. Even though the Vickers hardness is 350 HV, cutting the small diameter wire is easy because the material has not been greatly work-hardened. In Example 1, manufacturing the shot-peening materials that have a diameter of 0.3-0.6 mm is described. However, shot-peening materials that have a diameter up to about 1.0 mm are usable. The shot-peening materials that have a diameter of 1.0 mm can be easily manufactured by a method other than that of the present inventions. The smaller the diameter of the shot-peening materials is, the more difficult manufacturing them is. From this point of view, the method of the present inventions is appropriate for manufacturing the shot-peening materials that have a diameter of 0.6 mm or less (the finished wire that has a diameter of 0.6 mm or less). The diameter of shot-peening materials can be measured, for example, by the method of measuring a particle under the Japanese Industrial Standards as specified by JIS G5904.

TABLE 6 Working Example Comparative Example 1 Comparative Example 2 The area of carbides The area of carbides The area of carbides with particle sizes with particle sizes with particle sizes of 2 μm or less is of 2 μm or less is of more than 2 μm is 80% or more of the less than 80% of the 80% or more of the total area. total area. total area Cutting the wire Cutting the wire is Cutting the wire is is easy. difficult because it is difficult and the wire considerably work- is easy to break. hardened, even though its hardness is 350HV.

At the fourth step the finished wire is cut. One end of it is abutted against a stopper. It is fixed by a wire-gripping mechanism for preventing buckling and maintaining a constant length without fluctuation. Then it is cold-sheared by a cutting die. A cold-cutting machine with a mechanical press driven by a cam of a crank shaft, or a press driven by fluid pressure, or an electrical press, is used. A dieing machine may also be used. The cut length is arranged to be from the same length to one and half times the diameter of the finished wire.

At the fifth step, plastic-forming is applied to the cut wire. For example, it is rounded by press-forming. Alternatively, it is shot against a wall at a high speed to round off the corners.

At the sixth step it is quenched and tempered. In Example 1 the structure and hardness are adapted to suit quenching and tempering. The tempering parameters, which are shown in Table 7, are adjusted to be 6200-7300. The effects are shown in Table 7.

TABLE 7 Comparative Example Working Example Comparative Example Tempering parameters <6200 Tempering parameters = Tempering parameters >7300 T((21.3 − 5.8 × [C %]) + log[t]): 6200-7300 The life of shot-peening Their toughness and Residual compressive materials is short because hardness are satisfactory. stress is not provided to they lack toughness. Their lives are extended. a work because its Residual compressive hardness is low. stress is provided to a work.

As shown in Table 7, the life of the shot-peening materials is improved by adjusting the tempering parameters. The non-uniform structure caused by the precipitation of impurities has an impact on the life. The finished wire in which the area of carbides with sizes of 2 μm or less is 80% or more of the total area is used in the embodiment (see FIG. 2). This has a good impact on the life. In addition, an appropriate condition of shot-peening (such as making the hardness of the shot-peening materials the same as, or more than, that of a work) has a good impact on the life, as described below. The Vickers hardness of the shot-peening materials that have a diameter of 0.6 mm is 940 HV. That of the materials in the embodiment that have a diameter of 0.3 mm is 960 HV. FIG. 2a shows a microstructure of the finished wire that has a diameter of 0.3 mm, as photographed by a SEM (×1,000). FIG. 2b shows a microstructure of the finished wire that has a diameter of 0.3 mm, as photographed by a SEM (×5,000).

Observations of the structures of the shot-peening materials of the working example and the comparative example are described below. The structure of the materials of the working example is composed of fine carbides and tempered martensite (see Table 8). This structure has an effect in that the materials are seldom broken by a break being initiated at the area of an impurity. This effect is apparent when the structures of the materials before and after shot-peening are compared. The microstructures of the shot-peening materials that have a diameter of 0.6 mm (before shot-peening) and a diameter of 0.3 mm (after shot-peening) as photographed by a SEM after being etched by nital are shown in FIGS. 3 and 4. FIG. 3a shows a microstructure of the shot-peening materials that have a diameter of 0.6 mm before shot-peening. It is photographed by a SEM (×1,000). FIG. 3b shows a microstructure of the shot-peening materials that have a diameter of 0.6 mm before shot-peening. It is photographed by a SEM (×3,000). FIG. 4a shows a microstructure of the shot-peening materials that have a diameter of 0.3 mm after shot-peening. It is photographed by a SEM (×1,000). FIG. 4b shows a microstructure of the shot-peening materials that have a diameter of 0.3 mm after shot-peening. It is photographed by a SEM (×3,000).

TABLE 8 Working Example Comparative Example The structure is composed of fine The structure is composed of fine carbides carbides with sizes of less than 2 μm with sizes of 2 μm or larger and and tempered martensite. imperfectly tempered martensite. The materials are seldom broken by When a broken shot-peening material is a break being initiated at the area of mixed into the shot-peening materials, it an impurity during shot-peening. possibly causes a scar on a work and initiates a break.

TABLE 9 Comparative Example 1 Working Example Comparative Example 2 The quenching The quenching The quenching temperature is lower temperature is temperature is higher than 820° C. 820-850° C. than 850° C. The solution of carbides is The solution of The solution of too little in quantity to get carbides is carbides is too fine carbides. appropriate. great in quantity.

The shot-peening materials that are obtained in Example 1 and the comparative examples are described below (see Table 9). As shown in Table 9, the carbides are appropriately dissolved when the quenching temperature is 820-850° C. FIG. 5 shows a graph of the distribution of the sizes of the carbides of the shot-peening materials of FIG. 3b. Each carbide is projected on a grid paper to measure its area. The size of each carbide is calculated as the square root of the area. Thus, the sizes of the carbides and their distribution are obtained. The average size is 0.8 μm. The sizes range between 0.5 and 2.0 μm. Since the carbides of less than 0.5 μm are substantially unmeasurable, they are excluded from the measurement. The average size is calculated as a numeric mean.

Example 2

The conditions and results of shot-peening in which the shot-peening materials of the present inventions are shot against a work are shown in Table 10. Example 2 shows that the shot-peening materials of the present inventions, of which the hardness is 950 HV, cause a smaller amount to be scraped off when they are shot against a hard work. They also cause work-hardening and larger residual stress in the work (see Experiment Nos. 11-17). The hardness of the materials is initially 950 HV, but possibly increases to 1050 HV after shot-peening.

The amount being scraped off is measured as follows:

A Method of Measuring an Amount Being Scraped Off

The diameter of a work before shot-peening and that after shot-peening are measured by a laser dimension-measuring device. The amount is calculated by the following equation. It is the mean value of ten measurements (n=10). The measurement is performed at the center of the target of the shot (where the maximum amount being scraped off would be measured). The amount being scraped off=(D1−D2)/2, where D1 denotes the diameter of the work before shot-peening, and D2 the diameter of the work after shot-peening.

The residual stress of a work is measured as follows:

A Method of Measuring a Residual Compressive Stress

The method of measuring the residual compressive stress of a work after shot-peening is an X-ray stress measuring method. It is commonly used as a non-destructive measurement, and provided in JIS B 2711. The method utilizes the diffraction of X-rays. Since the specimen of the measurement is a steel having a martensite structure, the measurement is performed using CrKα characteristic radiation and an X-ray stress constant of k=−318)(MPa/°. The measurement is performed at the center of the target of the shot. Before the measurement, an area that is approximately double the cross-sectional dimension of the incident X-ray beam is removed from the specimen by electrolytic grinding. The peak value of the residual compressive stress (the maximum value) is obtained based on the distribution of the measurement of the residual stress.

The hardness of the cross section of a work is measured as follows:

A Method of Measuring the Hardness of the Cross Section

HV0.3 in Table 10 means a Vickers hardness of a cross section measured at a depth of 50 μm from the surface with a load of 300 g. It is generally known that the hardness is very low near the surface of gas-carburized steel, because an oxidized and nonmartensitic layer is formed from the surface to a depth of about 25 μm. Thus, the measurement of such a portion is not useful for the evaluation of the material and the heat treatment. So, the measurement is performed on the cross section.

A Method of Measuring the Relative Hardness

The relative hardness in Table 10 means a value that is calculated by subtracting the hardness of the shot-peening materials from that of the surface of the work. The hardness of the surface of the work is measured at its surface. While the hardness at the cross section is used for evaluating the material and heat treatment, the hardness at the surface is important for selecting the shot materials. Thus, the hardness is measured by putting an indenter directly on the surface. The hardness is measured by a micro-Vickers hardness tester (a load of 500 g). Therefore, for a carburized steel the measured hardness includes a value of an oxidized and nonmartensitic layer. Though an oxidized and nonmartensitic layer is not formed in a vacuum-carburized steel, the hardness of a surface would decrease, depending on the characteristics of the quenching.

The ratio of the area of carbides is preferably 70-95%, more preferably 80-95%. FIGS. 6a and 6b are graphs showing the relationships between the ratios of the area of carbides and the hardness (a hardness derived from values obtained by the hardesses of two respective structures) of the shot-peening materials of the present inventions (for structures having a higher hardness and a lower hardness, respectively). These graphs illustrate that the Vickers hardness is 920 HV-1030 HV when the ratio of the area of carbides is 70-95%. For the hardness of 950 HV the ratio is 70-78%. The hardness HV(m) of the shot-peening materials can be calculated by the following equations, 1, 2, and 3.


HV(m)={f(C)−f(T,t)}(1−γR/100)+400×γR/100  (1)


f(C)=−660C2+1373C+278  (2)


f(T,t)=0.05T(log(t)+17)−318=0.052λ−318  (3)

where

    • C: Carbon content (mass %) at the surface after carburizing,
    • T: Tempering temperature (K),
    • t: Holding time for tempering (hr),
    • γR: Amount of residual austenite (volume %),
    • λ: Tempering parameters (6200-7300),
    • f(C): Maximum hardness of martensite (HV), and
    • f(T,t): Hardness decreased by tempering (HV).

The following equation is obtained by substituting equations 2 and 3 for equation 1.


HV(m)=(−660C2+1373C+596−0.05λ)(1−γR/100)+4γR  (4)

The carbon content is assumed to be 0.75%. This value is used as the allowable limit of the carbon content in the matrix, i.e., martensite. The carbides, which precipitate, are excluded by assuming that their carbon content is 0.75%. Thus, the equation is used for calculating the hardness of the matrix. The residual austenite γR that remains after the heat treatment is estimated as follows:


Ms=667−195C−44.9Mn−19.6Ni−21.4Cr−20.7Mo


γR=100·exp(−0.011(Ms−Tq)),

where Ms denotes the temperature of the initiation of martensite transformation, and Tq the lowest temperature achievable during quenching. The hardness derived from values obtained by the hardesses of two respective structures of the shot-peening materials, which are calculated by these equations, is shown in FIGS. 6a and 6b.

In comparative example 1, the shot-peening materials (CCW) (having the hardness of 700 HV and the compositions including, by mass %, 0.81% carbon, 0.48% manganese, 0.23% silicon, 0.012% phosphorus, 0.004% sulfur, and unavoidable impurities) are made by cutting a steel wire and rounding off the cut wire. They do not induce a high residual stress in a work having a high hardness, such as super carburizing steel. (It is a steel appropriate for carburizing by dispersing carbides, and has a Vickers hardness of 880 HV-990 HV.) (See Experiment No. 8.) The corresponding example of the present inventions is Experiment No. 15. In comparative example 2, the super-hard shot-peening materials (having a Vickers hardness of 1380 HV) are too hard, and so have a disadvantage in that a work (JIS SCM420H, a quenched steel) is scraped off too much. Thus, a shot-peening pressure is controlled to be low when they are used as the shot-peening materials (see Experiment No. 10). The corresponding example of the present inventions is Experiment No. 11.

TABLE 10 Experiment Heat Shot-Peening Shot-Peening No. Work Treatment Materials Conditions 1 SCM420H Gas Eutectoid Comparison Condition 1: Carburizing Example 1 CCW (700HV, 0.6 mm-dia.), 2 Vacuum Shot Pressure 0.3 MPa, 3 Eutectoid Coverage 300%, 4 Super Carburizing Arc-Height 0.498 mmA 5 Carburized  5 6 Steel 10 7 20 8 30 9   20S 10 SCM420H Vacuum Comparison Condition 2: Super-Hard (HV1380, Eutectoid Example 2 0.2 mm-dia.), Shot Pressure 0.5 MPa, Carburizing Coverage 300%, Arc-Height 0.2 mmA 11 Present Condition 3: 12 Super Invenitions Present Inventions 13 Carburized  5 (HV950, 0.3 mm-dia.), 14 Steel 20 Shot Pressure 0.5 Mpa, 15 30 Coverage 300%, 16   20S Arc-Height 0.372 mmA Residual Peak Stress at Residual Position of Experiment Measurement of Surface Relative Surface Stress Peak Residual No. Scraping off HV0.3 Hardness (MPa) (MPa) Stress (μm) 1 0.0 524 175.6 −431 −1323 80 2 0.0 734 69.5 −521 −1386 60 3 0.0 770 −9.8 −538 −1271 80 4 0.0 897 −64.7 −514 −1362 80 5 0.0 923 −92.4 −356 −1215 100 6 0.0 950 −9.2 −578 −1108 60 7 0.0 988 −172.6 −517 −1178 100 8 1.0 963 −213.8 −422 −1180 80 9 0.6 958 −255.5 −639 −1123 60 10 81.7 1178 437.9 −1269 −2088 30 11 0.0 1077 11.2 −1167 −2041 50 12 0.0 918 244 −1100 −1724 50 13 0.0 998 144.8 −951 −1625 60 14 0.0 1126 108 −932 −1590 60 15 0.0 1045 10.5 −675 −1532 70 16 0.0 1106 −1.3 −929 −1673 60 Note 1: A number in the “Heat Treatment” column indicates the ratio of area of carbides that precipitate in super-carburized steel. Note 2: A number with a suffix “s” in the “Heat Treatment” column (20S) indicates that the material is prepared by a subzero treatment at the ratio of area of carbides of 20%.

As described above, the present inventions provide a raw material for shot-peening materials that is used for preparing a finished wire by wiredrawing wherein breaking a wire is prevented in manufacturing the shot-peening materials. Thereby the productivity is improved. The present inventions also provide the finished wire, a method of manufacturing the shot-peening materials by which the productivity is improved, and shot-peening materials that are manufactured by that method. They provide shot-peening materials that have a long life and induce appropriate residual stress in a work. In addition, they provide a method of manufacturing shot-peening materials that are readily quenched, and thereby appropriate for shot-peening.

Claims

1. A raw material for shot-peening materials comprising, by percentages by mass:

0.95-1.10% carbon, 0.15-0.30% silicon, 0.40% or less manganese, 0.020% or less phosphorus, 0.10% or less sulfur, 1.40-1.60% chromium, 0.0015% or less oxygen, and remaining materials of iron and unavoidable impurities.

2. A finished wire for shot-peening materials prepared by the steps of:

wiredrawing the raw material for the shot-peening materials of claim 1 to obtain a wire; and
repeatedly annealing and cold-drawing the wire to obtain a finished wire, wherein an area of carbides with a particle size of 2 μm or less is 80% or more of a total area of the finished wire.

3. The finished wire for shot-peening materials of claim 2, wherein the annealing is performed at a temperature of 720° C. or lower.

4. A method of manufacturing shot-peening materials comprising the steps of:

cutting and plastic-forming the finished wire for the shot-peening materials of claim 2 to obtain raw shot-peening materials; and quenching and tempering the raw shot-peening materials.

5. The method of manufacturing the shot-peening materials of claim 4, wherein tempering parameters=T ((21.3−5.8×[C])+log(t), where T denotes tempering temperature (K), t tempering time (hr), and [C] carbon content (%), are 6200-7300.

6. The method of manufacturing shot-peening materials further comprising the step of:

plastic-forming the shot-peening materials that are manufactured by the method of claim 4 or 5.

7. A method of manufacturing shot-peening materials comprising the steps of:

wiredrawing the raw material for the shot-peening materials of claim 1 to obtain a wire;
repeatedly annealing and cold-drawing the wire to obtain a finished wire;
cutting and plastic-forming the finished wire to obtain raw shot-peening materials; and
quenching and tempering the raw shot-peening materials.

8. The method of manufacturing the shot-peening materials of claim 4, 5, or 7, wherein a temperature of the quenching is 820-850° C.

9. The shot-peening materials manufactured by the method of claim 4 or 7, wherein a matrix of a structure thereof is tempered martensite, and fine carbides precipitate therein, and wherein a ratio of an area of carbides to a total area is 70-95%.

10. Shot-peening materials manufactured by the steps of:

wiredrawing a raw material for shot-peening materials comprising, by percentages by mass, 0.95-1.10% carbon, 0.15-0.30% silicon, 0.40% or less manganese, 0.020% or less phosphorus, 0.010% or less sulfur, 1.40-1.60% chromium, 0.0015% or less oxygen, and remaining materials of iron and unavoidable impurities to obtain a wire;
annealing and cold-drawing the wire to obtain a finished wire;
cutting and plastic-forming the finished wire to obtain raw shot-peening materials; and
quenching and tempering the raw shot-peening materials,
wherein a matrix of a structure thereof is tempered martensite, and fine carbides precipitate therein, and wherein a ratio of an area of carbides to a total area is 70-95%.

11. The shot-peening materials of claim 10, wherein a Vickers hardness thereof is 950 HV-1050 HV.

Patent History
Publication number: 20100322816
Type: Application
Filed: Nov 14, 2008
Publication Date: Dec 23, 2010
Inventors: Yuji Kobayashi (Aichi), Toshiya Tsuji (Aichi)
Application Number: 12/866,384