JP2024137532A - Manufacturing method of soft-nitrided parts - Google Patents

Abstract

The present invention provides a method for producing a soft-nitrided part with excellent bending fatigue properties at low cost. The method includes a cold working step of cold working a raw steel material to obtain a precursor having a maximum strength value of 1.5 or more in the inverse pole figure intensity distribution, and a nitriding step of nitriding the precursor. [Selected Figure] None

Description

The present invention relates to a method for manufacturing soft nitrided parts.

In order to address environmental issues such as reducing carbon dioxide emissions, there is a demand for reducing the weight of various industrial product parts. Nitriding is a heat treatment that improves the fatigue properties of steel parts while having the advantage of less heat treatment distortion compared to carburizing and quenching. Nitriding is applied to various steel parts, including vehicle gears. Improving fatigue properties makes it possible to reduce the size of steel parts, thereby realizing weight reduction in industrial products such as vehicles. For this reason, improving the fatigue properties of steel parts is desirable.

Patent Document 1 discloses a structural steel for nitrocarburizing. This structural steel for nitrocarburizing contains, by weight, 0.20-0.50% C, 0.03-0.50% Si, 0.30-3.00% Mn, 0.10-1.00% Cr, 0.03-1.00% Mo, 0.01-0.10% Al, 0.03-0.50% V, 0.015-0.070% S, 0-0.040% Pb, and 15 ppm or less O, with the balance being Fe and unavoidable impurities. In this structural steel for nitrocarburizing, a predetermined relationship is specified between the contents of S, Pb, and oxygen and the target material hardness and the target core hardness. This structural steel for soft nitriding is said to have excellent fatigue strength and machinability, so that it can be used after being machined in the as-rolled, as-forged, or normalized state, and then soft-nitrided and shot peened.

Patent document 2 describes a soft nitriding steel with excellent bending fatigue strength. This soft nitriding steel contains, in mass %, the following alloy elements: C: 0.01% to 0.15%, Si: 0.01% to 1.5%, Mn: 0.15% to 2%, Cu: 0.5% to 2%, N: limited to less than 0.005%, and contains one or more of Ti: 0.01% to 0.5%, Nb: 0.005% to 0.5%, and V: 0.05% to 0.5%, with C+N≦Ti/4.0+Nb/7.7+V/4.3, and may further contain, as necessary, one or more of Ni: 0.5% to 2% or less, Cr: 0.1% to 2%, Al: 0.05% to 0.5% or less, S: 0.03% to 0.1%, and Pb: 0.005% to 0.3%, with the balance consisting of Fe and unavoidable impurities. The area ratio of ferrite is 90% or more, with the remainder being carbide and pearlite structures. The average size of the pearlite is 20 μm or less. In this soft nitriding steel, Cu is an element that contributes to the age hardening of the core hardness during soft nitriding, and is considered essential for obtaining high fatigue strength.

Japanese Patent Application Publication No. 09-227992 JP 2002-069572 A

For example, if the addition of a large amount of Cu is required, as in the case of the nitrocarburizing steel disclosed in Patent Document 2, not only will the manufacturing cost of the steel increase, but it may also lead to high-temperature embrittlement of the steel, resulting in a decrease in yield due to an increase in cracks after continuous casting, and an increase in additional costs due to maintenance. If the addition of a rare metal such as V is required, as in the case of the nitrocarburizing steel disclosed in Patent Document 1, it may lead to an increase in the manufacturing cost of the steel and an increase in the environmental load. Therefore, it is desirable to propose a manufacturing method for nitrocarburizing parts that are low cost and have excellent bending fatigue properties.

The present invention was made in consideration of the above-mentioned circumstances, and its purpose is to provide a low-cost method for manufacturing soft-nitrided parts with excellent bending fatigue properties.

To achieve the above objective, the manufacturing method of the present invention for soft-nitrided parts is as follows.

[1] A cold working process in which a raw steel material is cold worked to obtain a precursor having a maximum intensity value of 1.5 or more in an inverse pole figure intensity distribution;
and a nitriding step of nitriding the precursor.

[2] The method for manufacturing a nitrided part described in [1] above, in which the cold working includes at least one of cold stretching, drawing, and forging.

[3] The method for manufacturing a nitrided part according to [1] or [2] above, in which the cold working step forms a portion in the precursor where the maximum value is 1.5 or more.

[4] A method for manufacturing a nitrocarburized part according to any one of [1] to [3], in which the cold working step is performed two or more times.

[5] A method for manufacturing a nitrided part according to any one of [1] to [4], in which in the cold working step, the cold working is repeated until the maximum value is 1.5 or more.

The present invention provides a low-cost method for manufacturing soft-nitrided parts with excellent bending fatigue properties.

FIG. 2 is a schematic diagram showing the shape of a test piece.

The manufacturing method for the nitrocarburized parts according to this embodiment is described below.

First, we will provide an overview of the manufacturing method for soft-nitrided parts according to this embodiment.

The manufacturing method for soft-nitrided parts according to this embodiment includes a cold working process in which raw steel is cold worked to obtain a precursor having a maximum intensity value of 1.5 or more in the inverse pole figure intensity distribution, and a nitriding process in which the precursor is nitrided.

The manufacturing method for soft-nitrided parts according to this embodiment makes it possible to provide soft-nitrided parts with excellent bending fatigue properties at low cost.

The method for manufacturing a nitrocarburized part according to this embodiment and the nitrocarburized part realized by this manufacturing method are described in detail below.

One example of a nitrocarburized part realized by the manufacturing method for a nitrocarburized part according to this embodiment (hereinafter referred to as a nitrocarburized part according to this embodiment) is a part that forms a vehicle such as an automobile. Examples of parts in the automotive field include engine crankshafts, timing gears, etc., transmission gears, ring gears, sun gears, planetary gears, etc., steering pinions, worms, etc., for the suspension, and worms for power windows in the interior.

Nitriding includes both nitriding, in which only nitrogen penetrates steel, and soft nitriding, in which nitrogen and carbon penetrate steel simultaneously, and both are treatments that do not cause the steel to transform into martensitic. Of these, nitriding in this embodiment refers to soft nitriding.

In the manufacturing method of the nitrocarburized part according to this embodiment, as described above, the raw steel material is nitrided to manufacture the nitrocarburized part. That is, the main body of the nitrocarburized part according to this embodiment has a surface layer in which the steel is nitrided. As an example, this surface layer is about 0.5 mm from the surface of the main body. This embodiment also includes cases in which the thickness of this surface layer is greater than 0.5 mm. Hereinafter, the raw steel material may be simply referred to as steel.

The soft-nitrided part according to this embodiment may be a combination of the main body with another structure that is made of a metal or metal alloy and has a non-nitrided surface. The soft-nitrided part according to this embodiment may be a combination of the main body with another structure that is not made of metal.

The steel used in the manufacturing method of the soft-nitrided parts according to this embodiment may contain Fe (iron) as the main component, and other components such as C (carbon), Si (silicon), Mn (manganese), P (phosphorus), S (sulfur) and impurities. It is not excluded that this steel may further contain optional components described below.

This steel may contain one or more optional components selected from Al (aluminum), N (nitrogen), Cr (chromium), Ti (titanium), Nb (niobium), V (vanadium), Hf (hafnium), Ta (tantalum), Sn (tin), Sb (antimony), Se (selenium), Ca (calcium), Pb (lead), and Bi (bismuth).

One example of a steel that can be used in the manufacturing method of the soft-nitrided parts according to this embodiment is carbon steel for machine construction (S45C) as specified in JIS G4051.

The following describes in detail the content and effects of each component in particularly suitable steels among the steels that can be used in the manufacturing method for soft-nitrided parts according to this embodiment. In the following description, when simply referring to the content, it refers to the content (mass%) in the steel. Note that the steels that can be used in the manufacturing method for soft-nitrided parts according to this embodiment are not limited to the steels described below.

The steel that can be used in the manufacturing method of the soft-nitrided part according to this embodiment may preferably contain, as a component composition (chemical composition), C: 0.04% by mass or more and 0.35% by mass or less, Si: 0.01% by mass or more and 1.20% by mass or less, Mn: 0.30% by mass or more and 1.80% by mass or less, P: 0.1% by mass or less, S: 0.5% by mass or less, Al: 0.010% by mass or more and 0.300% by mass or less, and N: 0.0250% by mass or less, with the remainder being Fe (iron) and impurities.

This steel may contain one or more optional components selected from the following composition: Cr (chromium): 2.0 mass% or less, Mo (molybdenum): 1.0 mass% or less, Cu (copper): 1.0 mass% or less, Ni (nickel): 1.0 mass% or less, and B (boron): 0.01 mass% or less.

In addition, this steel may contain one or more optional components selected from the following composition: Ti (titanium): 0.1 mass% or less, Nb (niobium): 0.1 mass% or less, V (vanadium): 0.2 mass% or less, Hf (hafnium): 0.1 mass% or less, and Ta (tantalum): 0.1 mass% or less.

In addition, this steel may contain one or more optional components selected from the following composition: Sn (tin): 0.1 mass% or less and Sb (antimony): 0.1 mass% or less.

In addition, this steel may contain one or more optional components selected from the following composition: Se (selenium): 0.3 mass% or less, Ca (calcium): 0.1 mass% or less, Pb (lead): 0.3 mass% or less, and Bi (bismuth): 0.3 mass% or less.

The C content may be 0.04% by mass or more and 0.35% by mass or less. In order to increase the central hardness after nitriding, the C content should be 0.04% by mass or more. If the C content exceeds 0.35% by mass, the load during cold working increases, which may shorten the life of the mold. The C content is preferably 0.05% by mass or more and 0.27% by mass or less, more preferably 0.10% by mass or more and 0.25% by mass or less.

The Si content may be 0.01% by mass or more and 1.20% by mass or less. In nitriding steel, Si serves as a deoxidizer. If the steel contains excessive Si, it may reduce the cold workability of the steel material and may reduce the fatigue strength through a decrease in the toughness of the steel. The Si content is preferably 0.05% by mass or more and 0.70% by mass or less, more preferably 0.10% by mass or more and 0.50% by mass or less.

The Mn content may be 0.30 mass% or more and 1.80 mass% or less. Mn improves hardenability and strengthens the pre-nitrided structure of the steel, thereby strengthening the post-nitrided structure (nitrided steel). In order to obtain sufficient fatigue strength, the Mn content should be 0.30 mass% or more. If the steel contains excessive Mn, it may lead to an increase in deformation resistance. The Mn content is preferably 0.40 mass% or more and 1.70 mass% or less, and more preferably 0.50 mass% or more and 1.30 mass% or less.

The P content may be 0.1 mass% or less. Since P segregates at the grain boundaries of nitriding steel and reduces toughness, the lower the P content, the more desirable it is. The P content is permitted up to 0.1 mass%. The P content is preferably 0.02 mass% or less. There is no problem if there is no particular lower limit for the P content, but since the inclusion of P is usually unavoidable, unnecessary reduction in P content may increase refining time and refining costs. Therefore, it is reasonable and preferable to set the P content to 0.003 mass% or more.

The S content may be 0.5% by mass or less. S exists as a sulfide-based inclusion and is an element that is effective in improving machinability. Excessive S inclusion in steel may lead to a decrease in cold workability. There is no particular lower limit for the S content, but since the inclusion of S is usually unavoidable, excessive reduction in S content may increase refining costs. Therefore, it is reasonable to set the S content to 0.003% by mass or more. The S content is preferably 0.004% by mass or more and 0.3% by mass or less, and more preferably 0.005% by mass or more and 0.09% by mass or less.

The Al content may be 0.010% by mass or more and 0.300% by mass or less. Al forms oxides and is an element effective in deoxidizing nitrided steel. Furthermore, Al has the effect of suppressing the formation of coarse oxide-based inclusions in nitrided steel. If the Al content is less than 0.010% by mass, these effects may not be obtained. Excessive Al inclusion in steel may lead to an increase in inclusions (Al oxides), which may increase the starting points of fatigue fracture and cause low fatigue strength.

The N content may be 0.0250% by mass or less. N combines with Al to form nitride (AlN). Finely precipitated AlN increases the hardness of the steel after nitriding (nitrided steel). Excessive N inclusion in steel may cause surface cracks in the steel billet after casting. There is no particular lower limit for the N content, but since the inclusion of N is usually unavoidable, excessive reduction in N content may increase refining costs. The N content is 0.0010% by mass or more, preferably 0.0015% by mass or more. The N content is preferably 0.0180% by mass or less, more preferably 0.0020% by mass or more and 0.0150% by mass or less.

The Cr content may be 2.0 mass% or less. Cr contributes to improving hardenability and temper softening resistance, and is also a useful element for promoting spheroidization of carbides. If the Cr content exceeds 2.0 mass%, the Cr nitride layer formed on the surface after soft nitriding becomes thick, which may hinder the penetration of N into the interior and lead to insufficient hardness. The Cr content is preferably 0.40 mass% or more and 1.90 mass% or less, and more preferably 0.70 mass% or more and 1.75 mass% or less.

The Mo content may be 1.0 mass% or less. Mo improves hardenability and strengthens the pre-nitriding structure, thereby strengthening the post-nitriding structure. However, if the Mo content is 1.0 mass% or more, the hardenability becomes excessive, the hardness after rolling increases, and the workability and machinability may decrease. In order to realize the effect of Mo in improving the strength of the steel material, it is preferable to contain Mo in an amount of 0.01 mass% or more in the steel. The Mo content is more preferably 0.03 mass% or more and 0.50 mass% or less, and even more preferably 0.05 mass% or more and 0.25 mass% or less.

The Cu content may be 1.0 mass% or less. Cu improves hardenability and strengthens the pre-nitriding structure, thereby strengthening the post-nitriding structure. To achieve this effect, it is preferable to contain Cu in the steel at 0.01 mass% or more. If the Cu content exceeds 1.0 mass%, the surface of the rolled material will become rough, and there is a concern that these will remain as scratches. The Cu content is more preferably 0.015 mass% or more and 0.5 mass% or less, and even more preferably 0.03 mass% or more and 0.3 mass% or less.

The Ni content may be 1.0 mass% or less. Ni is an element useful for improving toughness. In order to obtain these effects, it is preferable to contain Ni in the steel at 0.01 mass% or more. If the Ni content exceeds 1.0 mass% in the steel, the above effects will saturate. The Ni content is more preferably 0.015 mass% or more and 0.5 mass% or less, and even more preferably 0.03 mass% or more and 0.3 mass% or less.

The B content may be 0.01 mass% or less. B segregates at grain boundaries and suppresses diffusion-type transformation, which is effective in improving hardenability, and also has the effect of strengthening grain boundaries, suppressing the occurrence and progression of fatigue cracks, and improving fatigue strength. In order to obtain this effect of B, it is preferable to contain B in steel at 0.0003 mass% or more. If the B content exceeds 0.01 mass%, the toughness of the steel decreases, so the B content is preferably 0.01 mass% or less. The B content is more preferably 0.0005 mass% or more and 0.005 mass% or less, and even more preferably 0.0007 mass% or more and 0.002 mass% or less.

The Ti content may be 0.1 mass% or less. Ti improves the strength of the steel by combining with carbon and nitrogen to form fine precipitates during soft nitriding. However, even if the Ti content in the steel exceeds 0.1 mass%, the effect is saturated. The Ti content is preferably 0.005 mass% or more and 0.08 mass% or less, and more preferably 0.01 mass% or more and 0.06 mass% or less.

The Nb content may be 0.1 mass% or less. Nb combines with carbon and nitrogen to form fine precipitates during soft nitriding, improving the strength of the steel. However, even if the Nb content exceeds 0.1 mass%, the effect saturates. The Nb content is preferably 0.005 mass% or more and 0.08 mass% or less, and more preferably 0.01 mass% or more and 0.06 mass% or less.

The V content may be 0.2 mass% or less. V combines with carbon and nitrogen to form fine precipitates during soft nitriding, improving the strength of the steel. To obtain this effect of V, it is preferable to contain at least 0.003 mass% or more of V in the steel. If the V content in the steel exceeds 0.2 mass%, the alloy cost will increase and the effect will saturate. The V content is more preferably 0.005 mass% or more and 0.15 mass% or less, and even more preferably 0.01 mass% or more and 0.10 mass% or less.

The Hf content may be 0.1 mass% or less. Hf improves the strength of the steel by combining with carbon and nitrogen to form fine precipitates during soft nitriding. To obtain this effect of Hf, it is preferable to contain at least 0.003 mass% or more of Hf in the steel. If the Hf content in the steel exceeds 0.1 mass%, coarse precipitates may be generated during casting and solidification, which may lead to a deterioration of fatigue strength. The Hf content is more preferably 0.005 mass% to 0.06 mass%, and even more preferably 0.01 mass% to 0.05 mass%.

The Ta content may be 0.1 mass% or less. Ta combines with carbon and nitrogen to form fine precipitates during soft nitriding, improving the strength of the steel. To obtain this effect of Ta, it is preferable to contain at least 0.003 mass% or more of Ta in the steel. On the other hand, if the Ta content in the steel exceeds 0.1 mass%, cracks are more likely to occur during casting solidification, and defects may remain even after rolling and forging. The Ta content is more preferably 0.005 mass% or more and 0.06 mass% or less, and even more preferably 0.01 mass% or more and 0.05 mass% or less.

The Sb content may be 0.1 mass% or less. Sb is an effective element for suppressing decarburization of the steel surface and preventing a decrease in surface hardness. To achieve this effect, it is preferable to contain 0.0003 mass% or more of Sb in the steel. If the steel contains an excessive amount of Sb, the workability of the steel decreases. The Sb content is more preferably 0.001 mass% or more and 0.05 mass% or less, and even more preferably 0.0015 mass% or more and 0.035 mass% or less.

The Sn content may be 0.1 mass% or less. Sn is an effective element for improving the corrosion resistance of the steel surface. From the viewpoint of improving corrosion resistance, it is preferable to contain 0.003 mass% or more of Sn in the steel. If excessive Sn is contained in the steel, the workability decreases. The Sn content is more preferably 0.0010 mass% or more and 0.050 mass% or less, and even more preferably 0.0015 mass% or more and 0.035 mass% or less.

The Se content may be 0.3 mass% or less. Se improves machinability by combining with Mn and Cu and dispersing as precipitates in the steel. To obtain this effect, it is preferable to contain at least 0.001 mass% Se in the steel. If the Se content in the steel exceeds 0.3 mass%, the effect will saturate. The Se content is more preferably 0.005 mass% to 0.1 mass%, and even more preferably 0.008 mass% to 0.09 mass%.

The Ca content may be 0.1 mass% or less. Ca improves machinability by combining with S and dispersing in the steel as sulfides. To obtain this effect, it is preferable to contain at least 0.0005 mass% or more of Ca in the steel. If the Ca content in the steel exceeds 0.1 mass%, the effect will saturate. The Ca content is more preferably 0.0010 mass% or more and 0.0500 mass% or less, and even more preferably 0.0015 mass% or more and 0.0300 mass% or less.

The Pb content may be 0.3 mass% or less. Pb has the effect of refining chips during cutting. Adding Pb is effective when improving chip disposal. To obtain this effect, it is preferable to contain 0.01 mass% or more of Pb in the steel. Even if excessive Pb is contained in the steel, the effect of improving chip disposal becomes saturated. The Pb content is preferably 0.01 mass% or more and 0.2 mass% or less, and more preferably 0.01 mass% or more and 0.1 mass% or less.

The Bi content may be 0.3 mass% or less. Bi has the effect of refining chips during cutting. Adding Bi is effective when improving chip disposal. To obtain this effect, it is preferable to contain Bi at 0.01 mass% or more. Even if the steel contains excessive Bi, the effect of improving chip disposal becomes saturated. The Bi content is preferably 0.01 mass% or more and 0.2 mass% or less, and more preferably 0.01 mass% or more and 0.1 mass% or less.

The remainder other than the elements described above is Fe and impurities. Impurities are substances that are mixed in from raw materials such as ore and scrap, or from the manufacturing environment, during industrial production of steel, and are permitted to the extent that they do not adversely affect the characteristics of this embodiment.

When steel is cold worked, crystal orientation rotation occurs according to the working style. As the amount of working increases, crystal orientation rotation becomes more pronounced, and the accumulation of crystal orientation progresses. The inverse pole figure intensity distribution obtained by EBSD (electron backscatter diffraction) is a representative index showing the degree of accumulation of crystal orientation. When the maximum value of strength in the inverse pole figure intensity distribution is 1.5 or more, the fatigue strength of the steel is improved. To sufficiently improve the fatigue strength of steel, the maximum value of strength in the inverse pole figure intensity distribution is preferably 1.7 or more, and more preferably 2.0 or more.

In this embodiment, the maximum strength value in the inverse pole figure intensity distribution of the soft nitrided part measured in a direction parallel to the part surface at a depth of 0.5 mm from the surface of the body is 1.5 or more.

In this embodiment, cold working includes processing that includes at least one element of cold tensile processing (cold tensile processing), cold drawing, and cold forging (cold forging). Specifically, for example, cold forging gear forming, spline forming, drawing (diameter reduction) forming, forward extrusion forming, backward extrusion forming, and cold rolling gear forming and spline forming are also included in the cold working in this embodiment.

There is no limit to the number of times the steel is cold worked. For example, the steel may be cold worked only once to achieve a maximum strength value of 1.5 or greater in the inverse pole figure intensity distribution of the precursor.

For example, the steel may be cold worked multiple times (two or more times) to make the maximum strength value in the inverse pole figure intensity distribution of the precursor 1.5 or more.

In addition, the maximum strength value in the inverse pole figure intensity distribution of the precursor can be used as a guideline, and cold working can be repeated until this maximum value becomes 1.5 or more.

When cold working steel, the amount of deformation per cold working step is arbitrary.

Crystal orientation rotation due to cold working occurs regardless of the steel's composition and structure, and the orientation accumulation behavior depends on the processing method. For this reason, the manufacturing method for nitrocarburized parts according to this embodiment can be applied to all steel materials as raw steel materials, and is not limited to the examples given in this embodiment.

The steel according to this embodiment will be described below based on an example. Note that the steel according to this embodiment is not limited to this example.

Example 1
Carbon steel for machine construction (S45C) specified in JIS G4051 was formed into a round bar (an example of raw steel material) having a diameter of 20 mm by hot rolling.

Next, this round bar was cold drawn (an example of cold working) to produce a round bar-shaped precursor. As shown in Table 1, precursors were produced by changing the number of cold drawing processes from one to three, and by changing the diameter of the cold drawing die (precursor No. 2 to 6). As a comparison for the precursors produced by cold drawing, a round bar that was not cold drawn was secured (precursor No. 1).

15 pieces of each of precursors No. 1 to 6 were prepared.

The diameters of precursors No. 2 to 6 and the cumulative reduction in area (%) from repeated cold drawing are shown in Table 1. The cumulative reduction in area is the reduction in the area of the cross section intersecting the axial direction when a round bar with a diameter of 20 mm is processed into a precursor.

Next, parts of these round bars or precursors (precursor Nos. 1 to 6) were cut to obtain Ono-type rotating bending fatigue test pieces (hereinafter simply referred to as test pieces) with a parallel section diameter of 10 mm.

In addition, FIG. 1 shows a schematic diagram showing the shape of the test piece. The test piece is linear along the axis G, and the cross section perpendicular to the axis G is a circular rod shape whose center overlaps with the axis G. Both ends of the test piece in the direction along the axis G are gripping parts 1, 1 described later. A parallel part 2 is disposed between the gripping parts 1, 1 in the direction along the axis G. In this embodiment, the diameter of the parallel part 2 shown in FIG. 1 is 10 mm. Also, the diameter of the gripping parts 1, 1 is 12 mm. The length between the gripping parts 1, 1 including the parallel part 2 is 30 mm. Both ends of the parallel part 2 in the direction along the axis G are narrowed into a curved shape of R15 from the end of the gripping part 1 to the center in the axial direction.

Next, five randomly selected test pieces from the 15 test pieces corresponding to each precursor were measured using the EBSD method to obtain the inverse pole figure strength distribution and the maximum strength value of each test piece. The average value of the maximum strength values of the five test pieces corresponding to each precursor (hereinafter sometimes referred to as the average strength value) was then obtained. The inverse pole figure strength distribution was obtained for a cross section overlapping with the central axis (axis center G shown in Figure 1) of the cross section perpendicular to the longitudinal direction at the gripping portion of the test piece.

The cross-section was measured by cutting the gripping portion in the longitudinal direction to expose the cross-section.

The calculation of the inverse pole figure intensity distribution based on the measurement results by the EBSD method was performed using the crystal orientation analysis software OIM manufactured by TSL. Note that a scanning electron microscope (manufactured by JEOL Ltd., model: JSM-7001F) was used for the measurement by the EBSD method.

The measurement field of view for the EBSD method was a square area with sides of 0.5 mm, and the inverse pole figure intensity distribution was analyzed in the direction along the central axis (axis center G) (the same as the tensile axis direction in the fatigue test described below) using the spherical harmonics method under the conditions of Series Rank: 13, Gaussian Smoothing: 5.0°, and Sample Symmetry: Triclinic.

Next, of the 15 test pieces corresponding to each precursor, the remaining 10 test pieces were subjected to a gas nitrocarburizing heat treatment (nitriding) at 570°C for 3 hours to obtain post-treatment test pieces (an example of a nitrocarburized part) (an example of a nitriding process). The nitriding treatment was carried out in a mixed gas containing ammonia and carbon monoxide.

Next, the treated test pieces corresponding to each precursor were subjected to a rotating bending fatigue test at a stress amplitude of 500 MPa, and the fatigue fracture life (times) was measured as the fatigue strength for each, and the average fatigue fracture life (average fatigue strength, times) for the 10 treated test pieces corresponding to each precursor was calculated. Note that the larger the average fatigue fracture life, the better the bending fatigue properties, which is preferable. The average fatigue fracture life for each precursor is also shown in Table 1.

Furthermore, the fatigue fracture life improvement rate due to cold drawing was calculated for each post-treatment test piece corresponding to precursor No. 2 to 6. The fatigue fracture life improvement rate due to cold drawing is the average value of the fatigue fracture life of precursor No. 2 to 6 divided by the average value of the fatigue fracture life of the round bar (precursor No. 1). The larger the fatigue fracture life improvement rate, the more excellent the bending fatigue properties, which is preferable. The fatigue fracture life improvement rates corresponding to each precursor are also shown in Table 1.

When the fatigue fracture life of the treated test piece (nitrocarburized part) is extended (i.e., the fatigue fracture life improvement rate exceeds 1) compared to the fatigue fracture life of the treated test piece (precursor No. 1) that was nitrided without cold drawing, it can be said that the bending fatigue properties of the treated test piece are superior.

As shown in Table 1, the greater the average strength value of the test piece (precursor), the greater the improvement rate in fatigue fracture life. Therefore, it can be seen that the greater the average strength value, the better the bending fatigue properties of the post-treatment test piece (nitrocarburized part).

Among these, when the fatigue fracture life of the treated test piece (nitrocarburized part) is at least twice as long (i.e., the fatigue fracture life improvement rate is 2 or more) as compared with the fatigue fracture life of the treated test piece (precursor No. 1) that was nitrided without cold drawing, it can be said that the bending fatigue properties of the treated test piece are particularly excellent.

As shown in Table 1, when the average strength value of the test piece (precursor) is 1.5 or more, the fatigue fracture life improvement rate is 2 or more. Therefore, it can be seen that when the average strength value is 1.5 or more, the bending fatigue properties of the treated test piece (nitrocarburized part) are excellent. It can also be seen that the greater the average strength value of the test piece (precursor), the greater the fatigue fracture life improvement rate, and the greater the average strength value of the test piece (precursor), the better the bending fatigue properties of the treated test piece (nitrocarburized part).

Example 2
First, steels (steels No. 1 to 31) having the chemical compositions shown in Table 2 were melted and formed into round bars having a diameter of 20 mm (an example of a raw steel material) by hot rolling.

Then, this round bar was subjected to cold tensile processing (an example of cold processing) to produce a round bar-shaped precursor with a diameter of 16 mm (an example of a cold processing process), and then a portion of this precursor was cut to obtain an Ono-type rotating bending fatigue test specimen (hereinafter simply referred to as the test specimen) with a parallel portion diameter of 10 mm.

In this example, the test piece has the same shape as the test piece in Example 1 (see Figure 1).

Then, the test piece was measured by the EBSD method to obtain the inverse pole figure strength distribution and the maximum strength value. The inverse pole figure strength distribution was obtained for a cross section overlapping with the central axis (axis center G shown in Figure 1) of a cross section perpendicular to the longitudinal direction at the gripping portion of the test piece.

The cross-section was measured by cutting the gripping portion in the longitudinal direction to expose the cross-section.

The calculation of the inverse pole figure intensity distribution based on the measurement results using the EBSD method was performed in the same manner as in Example 1.

The measurement field of view range and the analysis conditions for the inverse pole figure intensity distribution using the EBSD method were the same as in Example 1.

Next, the test pieces were subjected to a gas nitrocarburizing heat treatment (nitriding) at 570°C for 3 hours to obtain post-treatment test pieces (an example of a nitrocarburized part) (an example of a nitriding process). The nitriding treatment was carried out in a mixed gas containing ammonia and carbon monoxide.

Next, the treated test pieces of each component composition were separated into those having a maximum strength value of 1.5 or more in the inverse pole figure strength distribution and those having other maximum strength values. Then, the treated test pieces having a maximum strength value of 1.5 or more in the pole figure strength distribution and those having other maximum strength values were subjected to a rotating bending fatigue test to evaluate the fatigue limit (stress equivalent to 1× ¹⁰⁷ times) of each.

As shown in Table 2, it can be seen that the treated test pieces (nitrocarburized parts, with a maximum strength value of 1.5 or more in the pole figure intensity distribution) manufactured by the manufacturing method of the nitrocarburized parts according to this embodiment have significantly superior bending fatigue properties compared to those manufactured by a manufacturing method not according to this embodiment.

Thus, the steel forming the post-treatment test piece (main body of the soft-nitrided part) according to this embodiment has excellent fatigue properties despite being low-cost and not requiring a large amount of a relatively expensive metal such as Cu.

In addition, the steel used to form the post-treatment test specimen (main body of the soft-nitrided part) according to this embodiment does not require the addition of rare metals such as V, is low cost, and yet has excellent fatigue properties.

After the treatment, the test pieces were measured using the EBSD method to determine the inverse pole figure strength distribution and the maximum strength value. The maximum strength value of the inverse pole figure strength distribution for each test piece after the treatment was the same as the maximum strength value of the inverse pole figure strength distribution for the corresponding test piece before nitriding (the state of each test piece before nitriding after the treatment).

The inverse pole figure strength distribution was determined for a portion of the gripped portion of the test piece at a depth of 0.5 mm from the surface after treatment. The measurement for the portion of the gripped portion of the test piece at a depth of 0.5 mm from the surface after treatment was performed by cutting a portion of the surface of the gripped portion (position 3 shown in Figure 1 as an example) 0.5 mm below the surface to expose that portion.

In other words, the maximum intensity value of the inverse pole figure intensity distribution inside the test piece (in this embodiment, the part 0.5 mm deep from the surface) was the same before and after nitriding the test piece.

In this way, a low-cost method for manufacturing soft-nitrided parts with excellent bending fatigue properties can be provided.

The embodiments disclosed in this specification are merely examples, and the present invention is not limited to these embodiments. They may be modified as appropriate without departing from the scope of the present invention.

The present invention can be applied to the manufacturing method of soft-nitrided parts.

1: Grip part 2: Parallel part G: Shaft center

Claims (7)

a cold working step of cold working a raw steel material to obtain a precursor having a maximum intensity value of 1.5 or more in an inverse pole figure intensity distribution;
and a nitriding step of nitriding the precursor. The method for manufacturing a nitrided part according to claim 1, wherein the cold working includes at least one of cold stretching, drawing, and forging. The method for manufacturing a nitrocarburized part according to claim 1, wherein the cold working step forms a portion in the precursor in which the maximum value is 1.5 or more. The method for manufacturing a nitrocarburized part according to claim 2, wherein the cold working step forms a portion in the precursor in which the maximum value is 1.5 or more. The method for manufacturing a nitrocarburized part according to claims 1 to 4, wherein the cold working step is performed two or more times. The method for manufacturing a nitrocarburized part according to claims 1 to 4, wherein in the cold working step, the cold working is repeated until the maximum value is 1.5 or more. The method for manufacturing a nitrided part according to claim 5, wherein in the cold working step, the cold working is repeated until the maximum value is 1.5 or more.

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