JP7436826B2 - Nitrided parts and manufacturing method of nitrided parts - Google Patents

Description

The present invention relates to a nitrided part subjected to nitriding treatment and a method for manufacturing the nitrided part.

Mechanical parts used in automobiles, ships, industrial machinery, etc. are sometimes subjected to nitriding treatment to improve fatigue strength. Generally, the higher the hardness of the nitrided layer formed by nitriding treatment, the higher the fatigue strength of the nitrided part. Some nitrided parts are used after being partially plastically deformed after nitriding in order to correct deformation during nitriding or to improve fitting precision with other parts. However, since the nitrided layer is very hard, cracks may occur due to partial plastic deformation after the nitriding process. When cracks occur, the fatigue strength of nitrided parts decreases. The lower the hardness near the surface layer, the better the crack initiation resistance (hereinafter sometimes referred to as ductility) against such plastic deformation. Therefore, fatigue strength and ductility are in a trade-off relationship, and it is not easy to achieve both of these properties when plastic deformation is applied after nitriding treatment.

By the way, some nitrided parts include parts that require fatigue strength and parts that do not require fatigue strength but require ductility. If the hardness and ductility of such parts can be made to vary depending on the part after nitriding, it is possible to increase the fatigue strength of the part itself.

A technique for achieving both fatigue strength and ductility of nitrided parts is described in Patent Document 1, for example. Patent Document 1 describes a technique for increasing ductility by mechanically removing a compound layer generated by nitriding.

Further, Patent Document 2 describes a technique for increasing the maximum hardness or hardening depth after nitriding by partially applying strong working before nitriding.

The technique described in Patent Document 1 utilizes the fact that ductility increases when the compound layer is removed, thereby achieving both fatigue strength and ductility of the component. However, in order to accurately remove the compound layer of parts with complex shapes through processing, it is necessary to use specialized processing equipment with extremely high precision, or to use electrolytic polishing that includes many pre-treatments, which makes manufacturing difficult. There are major restrictions.

Further, the technique described in Patent Document 2 achieves both strength and toughness by partially applying strong processing before performing nitriding treatment to increase the amount of reinforcement of the processed portion. However, the equivalent strain in the example shown in the example of Patent Document 2 is large, 1 or more, and special equipment that applies large strain to the steel material is required, or the shape of the part where strain can be applied is limited. , there are significant manufacturing constraints.

Japanese Patent Application Publication No. 2018-053962 Japanese Patent Application Publication No. 2015-151562

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a nitrided component excellent in both fatigue strength and ductility, and a method for manufacturing the same. Another object of the present invention is to provide a method for manufacturing a nitrided part that has excellent fatigue strength and ductility without using special equipment or restricting the shape of the nitrided part.

The above problem is solved by the following means.
(1) The composition at a depth of 1.0 mm from the surface is in mass%,
C: 0.05-0.60%,
Si: 0.01 to 0.50%,
Mn: 1.50-3.00%,
P: 0.05% or less,
S: 0.005-0.100%,
Cr: 0.03-0.29%,
Al: 0.001-0.040%,
Contains N: 0.0030 to 0.0250%,
A steel composition in which the balance consists of Fe and impurities,
Has a nitride layer on the outermost layer,
Including cases where the structure at a depth of 1.0 mm from the surface is 80% or more of the total area ratio of ferrite and pearlite, the remainder consists of martensite and bainite, and the remainder is 0%,
The surface has a first region and a second region,
When the Vickers hardness is measured at a depth of 50 μm from the surface in each of the first region and the second region, the difference between the Vickers hardness H1 of the first region and the Vickers hardness H2 of the second region ( Nitrided parts with H1-H2) of 50HV or more.
(2) The steel composition is replaced by a part of the Fe, in mass %,
Ti: 0.05% or less,
The nitrided component according to (1), containing one or two types of Nb: 0.05% or less.
(3) The steel composition is replaced by a part of the Fe in mass %,
Mo: 0.50% or less,
Cu: 0.50% or less,
The nitrided component according to (1) or (2), containing one or more kinds of Ni: 0.50% or less.
(4) The steel composition is replaced by a part of the Fe in mass %,
Ca: 0.005% or less,
The nitrided component according to any one of (1) to (3), containing one or both of Bi: 0.30% or less.
(5) The steel composition, in mass %, in place of a part of the Fe,
The nitrided component according to any one of (1) to (4), wherein V: 0.05% or less.
(6) A method for manufacturing a nitrided component according to any one of (1) to (5) above, comprising:
At a temperature of 300°C or less, an equivalent strain of 0.05 or more to 1.00 mm is applied to a selected part of the surface layer of the formed material within a depth of 1.0 mm from the surface. A method for manufacturing nitrided parts, in which nitriding treatment is performed after applying plastic deformation to a value less than or equal to the amount of plastic deformation.

Note that in this specification, the term "surface" simply refers to the surface of the nitride layer formed on the outermost layer of the component.

According to the present invention, a nitrided component excellent in both fatigue strength and ductility can be provided. Further, according to the present invention, it is possible to provide a method for manufacturing a nitrided part that has excellent fatigue strength and ductility without using special equipment and without restricting the shape of the nitrided part.

FIG. 1 shows a rotating bending fatigue test piece prepared in an example. FIG. 2 shows a test piece for ductility evaluation prepared in an example.

The nitrided component and the method for manufacturing the nitrided component according to the present embodiment were discovered based on the following knowledge. The present inventors variously changed the chemical composition of steel and the conditions of the manufacturing process of nitrided parts, and investigated the conditions under which the amount of hardening after nitriding treatment would differ. As a result, the following knowledge (a) was obtained.

(a) When cold forging is used to mold a part, the hardness after nitriding may become higher than when molding is performed only by cutting. Generally, the equivalent strain introduced into steel by cold forging is less than 1 in many cases, so the present inventors created an equivalent strain of 0.05 or more and less than 1.00 in the steel material by plastic working. We investigated the conditions under which the hardness increases after nitriding by applying nitriding treatment and plastic working. As a result, the following findings (b) to (e) were obtained.

(b) In general, steel containing large amounts of Cr, V, Ti, Nb, and Al, which are alloying elements that improve hardness during nitriding, can be used regardless of whether cold plastic working is performed before nitriding. , the hardness after nitriding is constant.

(c) If structural carbon steel such as S20C, which has a low content of Cr, V, Ti, Nb, and Al, is cold plastic worked and then nitrided, the amount of hardening will be greater than that without plastic working. , the amount of increase is small.

(d) In order to increase the amount of hardening during nitriding by cold plastic working, in addition to keeping the amounts of Cr, V, Ti, Nb, and Al in the steel below a certain amount, a sufficient amount of Mn It is necessary to make the steel composition with added.

(e) Even if the steel satisfies the condition (d) above, if the structure contains a large amount of martensite or bainite, the difference in hardening amount depending on the presence or absence of cold plastic working will be small.

The present inventors considered that the above knowledge could be effectively utilized for nitrided parts in which parts requiring fatigue strength and parts requiring ductility are different, and conducted studies. As a result, the following knowledge (f) was obtained.

(f) Among nitrided parts, for nitrided parts where the parts that require fatigue strength and the parts that require ductility are different, use steel that satisfies the conditions of (d) and (e) above, and It was confirmed that by forming only the necessary parts using cold plastic working, it is possible to achieve both extremely high levels of fatigue strength and ductility as a part. The nitrided parts obtained by the present invention are suitable for use as machine parts for automobiles, industrial machines, construction machines, and the like.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a nitrided component and a method for manufacturing the same, which are embodiments of the present invention, will be described in detail. "%" in the content of each element means "mass%". Further, the content of each element in a chemical composition is sometimes referred to as "element amount". For example, the content of C may be expressed as C amount. Furthermore, a numerical range expressed using "~" means a range that includes the numerical values written before and after "~" as lower and upper limits. Furthermore, a numerical range in which "greater than" or "less than" is attached to a numerical value written before or after "~" means a range that does not include these numerical values as the lower limit or upper limit.

[Steel composition]
The steel composition of the nitrided part according to the invention contains the following elements: However, the steel composition described below is the composition at a depth of 1.0 mm from the surface.

C: 0.05-0.60%
Carbon (C) increases the hardness and fatigue strength of steel materials. If the C content is too low, the above effects cannot be obtained. On the other hand, if the C content is too high, cutting resistance will increase and machinability will decrease. Therefore, the C content is 0.05-0.60%. The C content is preferably 0.08% or more, more preferably 0.10% or more, and still more preferably 0.15% or more. Further, the C content is preferably 0.50% or less, more preferably 0.40% or less, and still more preferably 0.35% or less.

Si: 0.01~0.50%
Silicon (Si) strengthens steel materials by forming a solid solution in ferrite (solid solution strengthening). If the Si content is too low, the above effects cannot be obtained. On the other hand, if the Si content is too high, machinability deteriorates. Therefore, the Si content is 0.01 to 0.50%. The Si content is preferably 0.05% or more, more preferably 0.08% or more. Further, the Si content is preferably 0.40% or less, more preferably 0.35% or less.

Mn: 1.50-3.00%
Manganese (Mn) forms nitride and contributes to hardening of the diffusion layer. Mn also has the effect of forming MnS in the steel material and improving the machinability of the steel material. If the Mn content is too low, even if nitriding is performed after cold plastic working, a sufficient amount of nitrides will not be formed. On the other hand, if the Mn content is too high, a sufficient amount of nitrides will be formed even without plastic working, so that portions not subjected to plastic working will be excessively hardened, resulting in deterioration of ductility. Therefore, the Mn content is 1.50-3.00%. The Mn content is preferably 1.60% or more, more preferably 1.70% or more. The Mn content is preferably 2.80% or less, more preferably 2.60% or less.

P: 0.05% or less Phosphorus (P) is an impurity. P segregates at grain boundaries and causes grain boundary embrittlement cracking. Therefore, it is preferable that the P content is as low as possible. The P content is 0.05% or less. The preferred P content is 0.02% or less.

S: 0.005-0.100%
Sulfur (S) combines with Mn in the steel material to form MnS, thereby improving the machinability of the steel material. If the S content is too low, the above effects cannot be obtained. On the other hand, if the S content is too high, coarse MnS will be formed, reducing the fatigue strength of the steel material. Therefore, the S content is 0.005-0.100%. The S content is preferably 0.010% or more, more preferably 0.015% or more, and still more preferably 0.020% or more. The upper limit of the S content is preferably 0.080% or less, more preferably 0.070% or less, and still more preferably 0.060% or less.

Cr: 0.03-0.29%
Chromium (Cr) combines with N introduced into the steel material through nitriding treatment to form CrN in the nitrided layer, thereby strengthening the nitrided layer. If the Cr content is too low, the above effects cannot be obtained. On the other hand, if the Cr content is too high, Cr induces the precipitation of Mn nitrides even without cold plastic working, resulting in excessive hardening in areas that have not been subjected to plastic working, resulting in a decrease in ductility. . Therefore, the Cr content is 0.03-0.29%. The Cr content is preferably 0.04% or more, more preferably 0.05% or more. The Cr content is preferably 0.25% or less, more preferably 0.20% or less, and still more preferably 0.16% or less.

Al: 0.001-0.040%
Aluminum (Al) combines with N introduced into the steel material through nitriding treatment to form AlN in the nitrided layer, thereby strengthening the nitrided layer. Additionally, Al is used for deoxidation during steel manufacturing. If the Al content is too high, Al induces the precipitation of Mn nitrides even without cold plastic working, so that the parts not subjected to plastic working become excessively hardened and the ductility deteriorates. Therefore, the Al content is 0.001 to 0.040%. The Al content is preferably 0.005% or more, more preferably 0.010% or more. The Al content is preferably 0.035% or less, more preferably 0.030% or less.

N: 0.003-0.025%
Nitrogen (N) forms a solid solution in steel and increases the strength of the steel. If the N content is too low, the above effects cannot be obtained. On the other hand, if the N content is too high, bubbles will be generated in the steel material. Since air bubbles become defects, it is preferable to suppress the generation of air bubbles. Therefore, the N content is 0.003-0.025%. The N content is preferably 0.005% or more. The N content is preferably 0.020% or less, more preferably 0.018% or less, and still more preferably 0.017% or less.

The remainder of the nitrided component according to the invention consists of Fe and impurities. Here, impurities are those that are mixed in from ores used as raw materials, scraps, or the manufacturing environment during industrial manufacturing of steel materials, and to the extent that they do not adversely affect the steel materials for nitriding of the present invention. means permissible.

The nitrided component of this embodiment may contain an arbitrary element described below in place of a part of Fe, or may not contain an arbitrary element explained below.

Among the optional elements in this embodiment, the group consisting of Ti and Nb has an effect of preventing coarsening of austenite crystal grains, and may contain one or two types.

Ti: 0.05% or less Titanium (Ti) combines with N to form TiN and suppresses coarsening of crystal grains during hot forging and quenching and tempering. However, if the Ti content is too high, TiC will be generated and the variation in hardness of the steel material will increase. Therefore, the Ti content is 0.05% or less. When Ti is included, the Ti content is preferably 0.005% or more, more preferably 0.010% or more. The Ti content is preferably 0.04% or less, more preferably 0.03% or less.

Nb: 0.05% or less Niobium (Nb) combines with N to form NbN, and suppresses coarsening of crystal grains during hot forging and quenching and tempering. Furthermore, Nb delays recrystallization during hot forging and quenching and tempering, and suppresses coarsening of crystal grains. However, if the Nb content is too high, NbC will be generated, increasing the variation in hardness of the steel material. Therefore, the Nb content is 0.05% or less. When Nb is contained, it is preferably 0.005% or more, more preferably 0.010% or more. The Nb content is preferably 0.04% or less, more preferably 0.03% or less.

Among the optional elements in this embodiment, the group consisting of Mo, Cu, and Ni has the effect of increasing the strength of the nitrided component, and may contain one or more of them.

Mo: 0.50% or less When contained, molybdenum (Mo) increases the strength of the steel material by increasing the hardenability of the steel. As a result, the fatigue strength of the steel material increases. However, if the Mo content increases excessively, the effect will be saturated and the cost of the steel material will increase. Therefore, the Mo content is 0.50% or less. The Mo content is preferably 0.03% or more, more preferably 0.05% or more. The Mo content is preferably 0.40% or less, more preferably 0.30% or less, and still more preferably 0.20% or less.

Cu: 0.50% or less When copper (Cu) is contained, it dissolves in the ferrite and increases the strength of the steel material. Therefore, the fatigue strength of the steel material increases. However, when the Cu content increases excessively, it segregates at the grain boundaries of steel during hot forging and induces hot cracking. Therefore, the Cu content is 0.50% or less. The Cu content is preferably 0.05% or more, more preferably 0.10% or more. The Cu content is preferably 0.30% or less, more preferably 0.20% or less.

Ni: 0.50% or less When contained, nickel (Ni) dissolves in the ferrite and increases the strength of the steel material. Therefore, the fatigue strength of the steel material increases. Ni further suppresses hot cracking caused by Cu when the steel material contains Cu. However, if the Ni content is too large, the effect will be saturated and the manufacturing cost will increase. Therefore, the Ni content is 0.50% or less. The Ni content is preferably 0.05% or more, more preferably 0.10% or more. The Ni content is preferably 0.30% or less, more preferably 0.20% or less.

Among the optional elements in the present invention, the group consisting of Ca and Bi has the effect of improving the machinability of nitrided parts, and may contain one or two types.

Ca: 0.005% or less Calcium (Ca), when contained, improves the machinability of the steel material. However, if the Ca content is too high, coarse Ca oxides will be generated and the fatigue strength of the steel material will be reduced. Therefore, the Ca content is 0.005% or less. The Ca content for stably obtaining the above effects is preferably 0.0001% or more, more preferably 0.0003% or more. The Ca content is preferably 0.003% or less, more preferably 0.002% or less.

Bi: 0.30% or less When contained, bismuth (Bi) improves the machinability of the steel material. However, if the Bi content is too high, hot workability will deteriorate. Therefore, the Bi content is 0.30% or less. The Bi content for stably obtaining the above effects is preferably 0.05% or more, more preferably 0.10% or more. Bi content is preferably 0.25% or less, more preferably 0.20% or less.

In addition, in the present invention, it is necessary to reduce the content of V, which is often contained in practical nitriding steel.

V: 0.05% or less Vanadium (V) is an element that combines with N introduced into the steel material through nitriding treatment to form VN in the nitrided layer and strengthen the nitrided layer. However, since V induces the precipitation of Mn nitrides, steel containing V is excessively hardened even in areas that have not been subjected to cold plastic working, resulting in deterioration of ductility. Therefore, it is necessary to limit the V content to 0.05% or less. The upper limit of the V content is preferably 0.03% or more, and more preferably 0.02% or less.

Further, in the nitrided component of this embodiment, a nitrided layer is formed on the outermost surface.

[Nitrided parts structure]
In order to control the generation of Mn nitrides by the presence or absence of cold plastic working, it is desirable that the number of nucleation sites in the structure before plastic working is not too large. Therefore, the structure of the steel needs to be a structure consisting mainly of ferrite and pearlite. If the structure of the steel is martensite or bainite, portions that have not been subjected to cold plastic working may become excessively hardened and the ductility may deteriorate. Specifically, the area ratio of the structure at a depth of 1.0 mm from the surface is such that the total of ferrite and pearlite is 80% or more, and the remainder is martensite and bainite, so that Mn nitride You can control the generation of The total amount of ferrite and pearlite is preferably 85% or more, more preferably 90% or more. The remaining structure other than ferrite and pearlite may be 0%.

[Vickers hardness of nitrided parts]
The nitrided component of this embodiment has a first region and a second region on the surface. When the Vickers hardness is measured at a depth of 50 μm from the surface in the first region and the second region, the difference between the Vickers hardness H1 in the first region and the Vickers hardness H2 in the second region (H1-H2) It is preferable that the voltage is 50 HV or more. Note that a position 50 μm deep from the surface is included in the nitrided layer.

Nitrided parts are required to have excellent fatigue strength. Further, in nitrided parts that are subjected to straightening after nitriding treatment or plastic deformation to improve fitting accuracy with other parts, excellent ductility is also required to prevent cracking. In order to obtain a part that has both fatigue strength and ductility, the hardness after nitriding must be different between the part where fatigue strength is required and the part where ductility is required. Specifically, the Vickers hardness at a depth of 50 μm after nitriding is determined between a region that has undergone cold plastic deformation (first region) and a region that has not undergone cold plastic deformation (second region). It is desirable that the difference (H1-H2) is 50 HV or more. More preferably, the difference in Vickers hardness ( H1-H2) is preferably 60 HV or more, more preferably 70 HV or more.

The first region is provided in a nitrided part particularly at a location where fatigue strength is required. Further, the second area is an area other than the first area. Preferably, the second region is provided at a location where ductility is required. The Vickers hardness H1 of the first region is the average value obtained by measuring the Vickers hardness at a depth of 50 μm from the surface of the first region after nitriding at five measurement points arbitrarily selected within the first region. shall be. The Vickers hardness H2 of the second region is determined by measuring the Vickers hardness at a depth of 50 μm from the surface of the second region after nitriding at five measurement points arbitrarily selected within the second region. Take the average value. Then, (H1-H2) is the difference between the Vickers hardness H1 of the first region and the Vickers hardness H2 of the second region. Vickers hardness is measured according to JIS Z 2244:2009. The measurement load is 2.94N.

[Production method]
An example of a method for manufacturing a nitrided component according to this embodiment will be described.
The method for manufacturing a nitrided component of this embodiment includes a steel material preparation process, a molding process, a machining process, and a nitriding process, and also includes a heat treatment process for adjusting the structure as necessary. Each process will be explained below.

[Steel material preparation process]
In the steel material preparation step, first, molten steel that satisfies the chemical composition of the steel of this embodiment is manufactured. Next, using the produced molten steel, it is made into slabs (slabs, blooms) by a general continuous casting method. Alternatively, using molten steel, it is made into an ingot by an ingot-forming method. Billets are produced by hot working slabs or ingots. The hot working may be hot rolling or hot forging. Furthermore, the billet is heated, rolled, and cooled under standard conditions to produce a steel bar, which is used as a material for nitrided parts.

[Molding process]
In the forming process, the manufactured steel bar is hot forged to form a forged material. If the heating temperature during hot forging is too low, an excessive load will be placed on the forging equipment. On the other hand, if the heating temperature is too high, scale loss will be large. Therefore, the preferred heating temperature for hot forging is 1000 to 1300°C.

Further, the preferable finishing temperature for hot forging is 900° C. or higher. This is because if the finishing temperature is too low, the burden on the mold will increase. On the other hand, the preferable upper limit of the finishing temperature is 1250°C. After hot forging, it is best to let it cool. With the steel composition of this embodiment, by cooling from the finishing temperature during hot forging to room temperature, a structure containing ferrite and pearlite in a total area ratio of 80% or more is obtained.

[Machining process]
The above-mentioned forged material is machined into a predetermined part shape to obtain a formed material. Examples of machining to form a predetermined part shape include cutting or grinding.

Furthermore, cold plastic working is applied to appropriate parts of the above-mentioned material, including parts where fatigue strength is required. Plastic working may be performed by any method, such as roll working or cold forging, but in order to sufficiently increase the amount of change in hardness after nitriding depending on the presence or absence of cold working. It is necessary to apply an equivalent strain of 0.05 or more to a depth of 1.0 mm from the surface. In order to easily apply cold equivalent strain to the raw material, it is necessary that the cold equivalent strain is less than 1.00. The cold equivalent strain is preferably 0.07 or more, more preferably 0.10 or more. The cold equivalent strain is preferably less than 0.80, more preferably less than 0.60, and even more preferably less than 0.50. The area subjected to cold plastic working ultimately becomes the first area of the nitrided part, and the area not subjected to cold plastic working becomes the second area.

If the temperature of the formed material during cold plastic working is too high, introduced dislocations are likely to disappear. Therefore, the temperature at which the plastic working is performed needs to be 300° C. or lower. The upper limit of the temperature at which plastic working is performed is preferably 200°C or lower, more preferably 100°C or lower.

[Nitriding process]
A nitriding treatment is performed on the material that has been partially subjected to cold plastic working to obtain a nitrided part. In this embodiment, a well-known nitriding process is employed. Examples of the nitriding treatment include gas nitriding, salt bath nitriding, and ion nitriding. The gas introduced into the furnace during nitriding may be only _NH3 . Alternatively, a mixture containing NH ₃ and N ₂ and/or H ₂ may be used. Further, carburizing gas may be contained in these gases to perform soft nitriding treatment. Therefore, "nitriding" as used herein also includes "soft nitriding".

When performing gas nitrocarburizing treatment, for example, soaking is carried out for 1 to 3 hours at a soaking temperature of 550 to 630°C in an atmosphere containing a 1:1 mixture of endothermic modified gas (RX gas) and ammonia gas. do it.

The nitrided parts manufactured by the above manufacturing process have excellent fatigue strength and excellent ductility.

Using a vacuum melting furnace, 50 kg ingots of steels A to S having the chemical compositions shown in Table 1 were manufactured. The hyphen (-) in Table 1 means not included.

Each ingot was heated to 1250°C. The heated ingot was hot forged into a steel bar having a diameter of 50 mm, and then allowed to cool to room temperature. A portion of the steel bar of Steel A (Test No. 19 in Table 1) was heated to 870°C, water-cooled, and tempered at 180°C for 120 minutes. A steel bar was produced in this way.

Next, by cutting the steel bar, a plurality of tensile test pieces each having a parallel portion with a diameter of 20 mm and a length of 85 mm were created. The tensile test piece was prepared so that the test piece and the steel bar were parallel to each other, and the center of the cross section of the test piece was aligned with the center of the cross section of the steel bar. This test piece was not subjected to plastic working to introduce strain. Hereinafter, this test piece will be referred to as a test piece base material without plastic working.

In addition, a plurality of other tensile test pieces each having a parallel portion having a diameter of 20 mm and a length of 85 mm were created by cutting the steel bar. The tensile test piece was prepared so that the test piece and the steel bar were parallel to each other, and the center of the cross section of the test piece was aligned with the center of the cross section of the steel bar. This test piece was subjected to plastic working on the parallel parts by being stretched using a tensile tester. Specifically, after applying 10% deformation at a stroke speed of 8.0 mm/min at room temperature, the load was unloaded. Hereinafter, this test piece will be referred to as a test piece base material with plastic working. The average equivalent strain in the cross section of the parallel portion of the specimen was 0.10.

Next, a test piece for hardness measurement, a test piece for nitrogen concentration measurement, and a test piece for ductility evaluation were created from the test piece base material without plastic working. In addition, a test piece for hardness measurement and a rotary bending fatigue test piece were created from the test piece base material subjected to plastic working.

The size of the test piece for hardness measurement was 10 x 6 x 65 mm. The test piece for measuring nitrogen concentration had a diameter of 12 mm and a length of 65 mm. As shown in Figure 1, the rotary bending fatigue test specimen has a diameter of 12 mm and a length of 80 mm, with a parallel part of 10 mm in diameter at the center of the length, a radius of 1 mm at the center of the parallel part, and a depth of 1 mm. A test piece was prepared with a 1 mm semicircular notch. As shown in FIG. 2, the test piece for ductility evaluation had a diameter of 12 mm, a length of 180 mm, and a semicircular notch with a radius of 3 mm and a depth of 2 mm at the center in the length direction. Each test piece was created so that its radial center and longitudinal center coincided with the radial center and longitudinal center of the tensile test piece, respectively.

Each of the prepared test pieces was subjected to soft nitriding treatment at 570°C for 5 hours. During soft nitriding, ammonia and RX gas were introduced into the furnace at a flow rate ratio of 1:1, and after the treatment, they were taken out of the furnace and cooled with oil at 100°C.

[Evaluation test]
The following tests were conducted using various test pieces with each test number.

[Hardness measurement and tissue area ratio measurement]
The test piece for hardness measurement was embedded in a resin and polished so that the center of the test piece in the length direction was crossed and the cross section was the test surface. After polishing, Vickers hardness (HV) based on JIS Z 2244:2009 was measured at five arbitrary points located at a depth of 50 μm from the surface of the test piece. The test force was 2.94N. The average value of the five points obtained was defined as surface hardness.

After measuring the hardness, the sample was corroded with 3% nital for 10 seconds to reveal the structure. Thereafter, an optical microscope photograph was taken at a magnification of 200 times centered at a position 1.0 mm deep from the surface of the test piece, and the tissue fraction was determined from image analysis. No. In samples other than Nos. 15 and 19, a ferrite-pearlite structure was observed, and the remaining structures were martensite and bainite.

[Ono type rotating bending fatigue test]
An Ono-type rotating bending fatigue test was conducted using the above-mentioned nitrided rotating bending fatigue test piece (obtained from a test piece base material subjected to plastic working). A rotating bending fatigue test in accordance with JIS Z2274 (1978) was conducted at a rotational speed of 3000 rpm in an air atmosphere at room temperature (25° C.). Among the tests in which no breakage occurred up to 1.0×10 ⁷ repetitions, the highest stress was defined as the fatigue strength (MPa) of that test number. When the fatigue strength was 350 MPa or more, it was judged that the fatigue strength was excellent.

[Ductility evaluation test]
A four-point bending test was conducted at room temperature in the atmosphere using the above-mentioned nitrided test piece for ductility evaluation (obtained from a test piece base material without plastic working). The distance between fulcrums (distance in the axial direction of the test piece between the fulcrum closest to the end of the test piece and the fulcrum closest to that fulcrum) was 51 mm. The pushing speed was 0.5 mm/min. In order to measure the amount of strain at the notch bottom of the test piece, a strain gauge was attached to the center of the notch bottom parallel to the axial direction of the test piece. When the indentation stroke is increased at the above indentation speed and the increase in the strain gauge value when the indentation stroke increases by 0.01 mm is 0.24% or more, a crack has occurred in the test piece. The amount was defined as ductility. When the ductility was 1.5% or more, it was judged that the ductility was excellent. If the amount of strain exceeded 7%, the evaluation was discontinued at that point.

[Test results]
Table 2 shows the test results. In Test No. 1 to Test No. 13, the chemical composition and structure of the steel are within the scope of the present invention. For these test numbers, the difference in surface hardness between plastic working and non-plastic working is 93 points or more in HV, the fatigue strength of the test piece with plastic working (corresponding to the first area) is 360 MPa or more, and the fatigue strength of the test piece without plastic working is 360 MPa or more. Since the ductility of the test piece (corresponding to the second region) is 3.56% or more, it can be seen that both excellent fatigue strength and ductility can be achieved.

On the other hand, in the case of "comparative examples" with test numbers 14 to 20, which deviate from the provisions of the present invention, the chemical composition or microstructure of the steel is outside the scope of the present invention, and the target performance cannot be obtained. Not yet.

Regarding the organization, No. In samples other than Nos. 15 and 19, a ferrite-pearlite structure was observed, and the remaining structures were martensite and bainite. In test number 15, the Mn content was too high, resulting in a structure mainly composed of martensite and bainite. Test No. 19 was heated to 870°C, water-cooled, and tempered at 180°C for 120 minutes, resulting in a structure mainly composed of martensite and bainite. As a result, in test numbers 15 and 19, the Vickers hardness increased even in the test pieces without plastic working, and ΔHV became less than 50.

The embodiments of the present invention have been described above. However, the embodiments described above are merely examples for implementing the present invention. Therefore, the present invention is not limited to the embodiments described above, and can be implemented by appropriately modifying the embodiments described above without departing from the spirit thereof.

Claims (6)

The composition at a depth of 1.0 mm from the surface is in mass%,
C: 0.05-0.60%,
Si: 0.01 to 0.50%,
Mn: 1.50-3.00%,
P: 0.05% or less,
S: 0.005-0.100%,
Cr: 0.03-0.29%,
Al: 0.001-0.040%,
Contains N: 0.0030 to 0.0250%,
A steel composition in which the balance consists of Fe and impurities,
Has a nitride layer on the outermost layer,
Including cases where the structure at a depth of 1.0 mm from the surface is 80% or more of the total area ratio of ferrite and pearlite, the remainder consists of martensite and bainite, and the remainder is 0%,
The surface has a first region and a second region,
When the Vickers hardness is measured at a depth of 50 μm from the surface in each of the first region and the second region, the difference between the Vickers hardness H1 of the first region and the Vickers hardness H2 of the second region ( Nitrided parts with H1-H2) of 50HV or more. The steel composition is replaced by a part of the Fe in mass%,
Ti: 0.05% or less,
The nitrided component according to claim 1, containing one or two selected from the group consisting of Nb: 0.05% or less. The steel composition is replaced by a part of the Fe in mass%,
Mo: 0.50% or less,
Cu: 0.50% or less,
The nitrided component according to claim 1 or 2, containing one or more selected from the group consisting of Ni: 0.50% or less. The steel composition is replaced by a part of the Fe in mass%,
Ca: 0.005% or less,
The nitrided component according to any one of claims 1 to 3, containing one or two selected from the group consisting of Bi: 0.30% or less. The steel composition is replaced by a part of the Fe in mass%,
The nitrided component according to any one of claims 1 to 4, wherein V: 0.05% or less. A method for manufacturing a nitrided part according to any one of claims 1 to 5, comprising:
At a temperature of 300°C or less, an equivalent strain of 0.05 or more to 1.00 mm is applied to a selected part of the surface layer of the formed material within a depth of 1.0 mm from the surface. A method for manufacturing nitrided parts, in which nitriding treatment is performed after applying plastic deformation to a value less than or equal to the amount of plastic deformation.

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