JP7168059B2 - Steel for nitriding and quenching treatment, nitriding and quenching parts, and manufacturing method thereof - Google Patents

Description

TECHNICAL FIELD The present invention relates to a steel for nitriding and quenching treatment, a part for nitriding and quenching, and a method for manufacturing the same.

Mechanical structural parts such as automobile parts are generally manufactured by processing steel, which is the raw material of mechanical structural parts, into a predetermined shape and then performing surface hardening such as carburizing and quenching and nitriding.

In recent years, with the aim of improving the quality and performance of automobiles, the requirements for dimensional accuracy and surface strength of automobile parts have become stricter. In general, dimensional accuracy can be enhanced by reducing heat treatment strain, and surface strength can be enhanced by thickening the surface hardening treatment layer.
However, although the carburizing and quenching treatment can easily increase the surface strength, there is a problem that since the treatment temperature is high, the heat treatment strain is large and the dimensional accuracy cannot be improved.
On the other hand, since the nitriding treatment is performed at a relatively low temperature, the heat treatment distortion is small and the dimensional accuracy can be improved. However, since the nitriding treatment cannot thicken the surface-hardened layer, there is a problem that it is difficult to increase the strength.

Therefore, as a technique for improving both dimensional accuracy and surface hardness, Patent Document 1 discloses that the article to be treated is nitrided at the temperature of the mixed phase region of austenite and iron nitride (Fe ₄ N) in the iron-nitrogen system equilibrium diagram. A nitriding and quenching treatment is proposed in which the article to be treated is then rapidly cooled and reheated to form a two-layer structure of Fe ₁₆ N ₂ and nitrogen martensite on the surface of the article to be treated.
In addition, nitriding and quenching treatment generally refers to quenching (quenching) after nitrogen is diffused and permeated from the steel surface in _a temperature range of A1 or higher in the iron-nitrogen equilibrium diagram to form nitrogen austenite on the surface layer. It is a surface hardening method that generates hard nitrogen martensite by The nitriding and quenching treatment is performed at a lower temperature than the carburizing and quenching treatment, so that thermal strain is small, and compared to the nitriding treatment, the treatment time is shorter because the treatment is performed at a higher temperature. Hereinafter, the layer formed on the surface of the steel by nitrogen diffusion (nitriding treatment) is called the "nitrided layer", and the part where the nitrified layer is not formed (the part inside the nitrified layer) is called the "mother layer". The layer formed by quenching the nitriding layer is called the "hardened layer."

JP 2015-25161 A

In recent years, demands for not only dimensional accuracy and surface strength but also toughness and fatigue strength are increasing for mechanical structural parts such as automobile parts.
However, Patent Document 1 does not sufficiently examine the toughness and fatigue strength of mechanical structural parts (nitriding and quenching parts).

The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a nitriding and quenching steel capable of producing nitriding and quenching parts having excellent toughness and fatigue strength.
Another object of the present invention is to provide a nitriding hardened part excellent in toughness and fatigue strength, and a method for manufacturing the same.

The inventors of the present invention conducted intensive research focusing on the alloy composition of the steel for nitriding and quenching treatment, and found that the reason why the toughness and fatigue properties of the steel for nitriding and quenching treatment are reduced after nitriding and quenching treatment is immersion. It was found that this is mainly due to coarsening of crystal grains in the nitride layer and the hardened layer and ferrite between the crystal grains. The present inventors have found that by containing Mn and / or Ni in a predetermined ratio, the austenitization of the nitriding layer is promoted to reduce the amount of ferrite present between austenite grains, and as a result, the hardened layer It was found that ferrite between martensite grains can also be reduced. In addition, the present inventors have found that by containing an element that forms nitrides in a predetermined ratio during nitriding treatment, coarsening of austenite grains in the nitriding layer is suppressed, and as a result, martensite in the hardened layer It was found that coarsening of crystal grains can also be suppressed. The present invention has been completed based on these findings.

That is, the present invention
C: 0.3% by mass or less,
Mn: 0.3 to 2.8% by mass,
Ni: 0.05 to 1.0% by mass,
P: 0.03% by mass or less,
S: 0.03% by mass or less, and 1 selected from Cr: 0.1 to 4.5% by mass, Si: 0.1 to 1.0% by mass, and Al: 0.1 to 1.0% by mass including more than seeds, the total amount of Mn and Ni is 0.4 to 3.0% by mass, and the balance consists of Fe and unavoidable impurities,
Formula (1) below:
A1=0.30Cr+0.45Si+0.34Al-0.2(Mn+Ni) (1)
The steel for nitriding and quenching treatment has a nitriding index A1 represented by (in the formula, each element symbol represents mass % of each element) of 0.7 or less.

In addition, the present invention
C: 0.3% by mass or less,
Mn: 0.3 to 2.8% by mass,
Ni: 0.05 to 1.0% by mass,
P: 0.03% by mass or less,
S: 0.03% by mass or less,
Cr: 0.1 to 4.5% by mass, Si: 0.1 to 1.0% by mass and Al: one or more selected from 0.1 to 1.0% by mass, and V: 0.01 to 1.5% by mass, Nb: 0.01 to 3.0% by mass, and Ti: one or more selected from 0.01 to 1.5% by mass, and the total amount of Mn and Ni is 0.4 to 3 .0% by mass, the balance being Fe and unavoidable impurities,
Formula (3) below:
A3=0.30Cr+0.45Si+0.34Al+0.36V+0.35Nb+0.35Ti-0.20(Mn+Ni) (3)
The steel for nitriding and quenching treatment has a nitriding index A3 represented by (in the formula, each element symbol represents mass % of each element) of 0.7 or less.

The present invention also provides a nitriding and quenching component in which a hardened layer containing nitrogen martensite is formed by nitriding and quenching on the surface layer of the steel for nitriding and quenching.
Further, the present invention is a method for manufacturing a nitriding and quenching part, wherein the steel for nitriding and quenching treatment is worked into a part shape and the part is subjected to nitriding and quenching treatment.

ADVANTAGE OF THE INVENTION According to this invention, the steel for nitriding hardening process which can manufacture the nitriding hardening components excellent in toughness and fatigue strength can be provided.
Further, according to the present invention, it is possible to provide a nitriding hardened part excellent in toughness and fatigue strength and a method for manufacturing the same.

Hereinafter, preferred embodiments of the present invention will be specifically described, but the present invention should not be construed as being limited to these, and as long as it does not depart from the gist of the present invention, , various modifications, improvements, etc. may be made. A plurality of constituent elements disclosed in each embodiment can be appropriately combined to form various inventions. For example, some components may be omitted from all components shown in one embodiment, or components of different embodiments may be combined as appropriate.

(Embodiment 1)
The steel for nitriding and quenching treatment according to the present embodiment contains C, Mn and/or Ni, P, S, and one or more selected from Cr, Si and Al, and the balance is Fe and unavoidable consists of organic impurities. In addition, this nitriding and quenching steel may further contain one or more selected from Mo and B.
Here, the term "inevitable impurities" as used herein means components such as O that are difficult to remove. This component is inevitably mixed in at the stage of smelting the raw material.

<C: 0.6% by mass or less>
C is an element necessary for ensuring the refining hardness and strength of the steel for nitriding and quenching. However, if the C content is too high, the workability and weldability of the steel for nitriding and quenching treatment will deteriorate. Further, when the base material of the nitriding and hardening part does not require much hardness, the upper limit of the C content may be 0.3% by mass. Although the lower limit of the C content is not particularly limited, it is preferably 0.005% by mass, more preferably 0.010% by mass.

<Mn and/or Ni: 0.4 to 3.0% by mass in total>
Mn and Ni have the effect of increasing the hardenability of the steel for nitriding and quenching and increasing the austenite growth rate, so they are elements that reduce ferrite in the nitriding layer and suppress a decrease in fatigue strength. From the viewpoint of improving hardenability, the lower limit of the total amount of Mn and/or Ni is set to 0.4% by mass, preferably 0.42% by mass. On the other hand, if the content of Mn and/or Ni is too high, the hardness of the hardened layer decreases due to an increase in the amount of retained austenite during quenching. Therefore, the upper limit of the total amount of Mn and/or Ni is set at 3.0% by mass, preferably 2.8% by mass.
The contents of each of Mn and Ni are not particularly limited. Since Ni is expensive, it is economically disadvantageous. Therefore, the Mn content is preferably 3.0% by mass or less, more preferably 0.3 to 2.8% by mass. Also, the Ni content is preferably 3.0% by mass or less, more preferably 0.05 to 1.0% by mass.

<P: 0.03% by mass or less>
P is an element that easily causes grain boundary segregation to embrittle the grain boundary. If the P content is too high, grain boundary embrittlement lowers the toughness. Therefore, the upper limit of the P content is set to 0.03% by mass, preferably 0.025% by mass. On the other hand, the lower limit of the P content is not particularly limited, but is preferably 0.0001% by mass, more preferably 0.0005% by mass.

<S: 0.03% by mass or less>
S is a sulfide-forming element. If the S content is too high, coarse MnS is formed and the fatigue strength is lowered. Therefore, the upper limit of the S content is set to 0.03% by mass, preferably 0.025% by mass. On the other hand, the lower limit of the S content is not particularly limited, but is preferably 0.0001% by mass, more preferably 0.0005% by mass.

<One or more selected from Cr: 0.1 to 4.5% by mass, Si: 0.1 to 1.0% by mass, and Al: 0.1 to 1.0% by mass>
Cr, Si and Al are elements that form nitrides during nitriding and quenching. That is, these elements form nitrides during the nitriding and quenching treatment to refine the crystal structure of nitrogen austenite in the nitriding layer, and as a result, refine the crystal structure of nitrogen martensite in the hardened layer. This makes it possible to improve the fatigue strength and toughness of the nitriding and quenching part.
Cr is also an element that enhances hardenability. In order to obtain such an effect of Cr, the lower limit of the Cr content is set to 0.1% by mass, preferably 0.15% by mass. On the other hand, this effect saturates as the Cr content increases, so the upper limit of the Cr content is set to 4.5% by mass, preferably 4.0% by mass, and more preferably 1.5% by mass.

In addition to the above effects, Si is an element necessary for ensuring the strength of the base material of the nitriding and quenching part. In order to obtain such effects of Si, the lower limit of the Si content is set to 0.1% by mass, preferably 0.15% by mass, and more preferably 0.2% by mass. On the other hand, since this effect is saturated with an increase in the Si content, the upper limit of the Si content is set to 1.0% by mass, preferably 0.9% by mass, and more preferably 0.7% by mass.

In order to obtain the above effect of Al, the lower limit of the Al content is set to 0.1% by mass, preferably 0.15% by mass, more preferably 0.2% by mass. On the other hand, since this effect is saturated with an increase in the Al content, the upper limit of the Al content is set to 1.0% by mass, preferably 0.9% by mass, and more preferably 0.7% by mass.

Although the total amount of Cr, Si and Al is not particularly limited, it is preferably 0.1 to 4.5% by mass. If the total amount is less than 0.1% by mass, the refinement of the crystal structure of the nitrogen austenite in the nitriding layer and the nitrogen austenite in the hardened layer is insufficient, and the fatigue strength and toughness of the nitriding and quenching part are sufficiently improved. Sometimes I don't. On the other hand, if the total amount exceeds 4.5% by mass, it is uneconomical because the effects commensurate with the amount cannot be obtained.

<Mo: 0.1 to 2.0% by mass>
Mo is an element that enhances the hardenability of steel for nitriding and quenching. In order to obtain this effect, the lower limit of the Mo content is preferably 0.1% by mass, more preferably 0.15% by mass. On the other hand, Mo is expensive and excessive use is uneconomical, so the upper limit of the Mo content is preferably 2.0% by mass, more preferably 1.5% by mass.

<B: 0.0001 to 0.01% by mass>
B, like Mo, is an element that enhances the hardenability of the steel for nitriding and quenching treatment. To obtain this effect, the lower limit of the B content is preferably 0.0001% by mass, more preferably 0.0003% by mass. On the other hand, since this effect is saturated with an increase in the B content, the upper limit of the B content is preferably 0.01% by mass, more preferably 0.005% by mass.

In the steel for nitriding and quenching treatment according to the present embodiment, in order to efficiently exhibit fatigue properties and toughness, it is necessary to control the thickness of the nitriding layer that becomes a hardened layer after the quenching treatment.
Therefore, the steel for nitriding and quenching according to the present embodiment has a nitriding index A1 represented by the following formula (1) of 0.7 or less, preferably -0.10 to 0.68, more preferably 0 to It is controlled at 0.5.
A1=0.30Cr+0.45Si+0.34Al-0.2(Mn+Ni) (1)
In the formula, each element symbol is mass % of each element.
The nitriding index A1 is an index of the thickness of the nitrogen austenite metal structure grown from the surface by the nitriding treatment, and the smaller the nitriding index A1, the greater the thickness of the nitriding layer (hardened layer after quenching treatment). means that If the nitriding index A1 exceeds 0.7, the thickness of the nitriding layer (hardened layer after quenching treatment) becomes too small, and the fatigue properties cannot be fully exhibited by the quenching treatment. The nitriding index A1 is experimentally calculated from the relationship between the alloy composition and the thickness of the nitriding layer.

The metal structure of the steel for nitriding and quenching treatment according to the present embodiment is not particularly limited, but is preferably ferrite. Steel for nitriding and quenching treatment having such a metal structure forms a nitriding layer containing nitrogen austenite on the surface layer by nitriding in _a temperature range of A1 or higher, and then converts nitrogen austenite to nitrogen by quenching. A hardened layer can be formed by transforming to martensite.

(Embodiment 2)
The steel for nitriding and quenching according to the present embodiment contains C, Mn and/or Ni, P, S, and one or more selected from V, Nb and Ti, and the balance is Fe and unavoidable consists of organic impurities. In addition, this nitriding and quenching steel may further contain one or more selected from Mo and B.
The steel for nitriding and quenching according to the present embodiment contains at least one selected from V, Nb and Ti instead of at least one selected from Cr, Si and Al. It is different from the steel for nitriding and quenching treatment according to No. 1. Since the basic features of the nitriding and quenching steel according to the present embodiment are the same as those of the nitriding and quenching steel according to the first embodiment, only the differences will be described.

<One or more selected from V: 0.01 to 1.5% by mass, Nb: 0.01 to 3.0% by mass, and Ti: 0.01 to 1.5% by mass>
V, Nb and Ti are elements that form nitrides during nitriding and quenching. That is, these elements form nitrides during the nitriding and quenching treatment to refine the crystal structure of nitrogen austenite in the nitriding layer, and as a result, refine the crystal structure of nitrogen martensite in the hardened layer. This makes it possible to improve the fatigue strength and toughness of the nitriding and quenching part.
Moreover, both V and Ti can form carbides and refine the crystal structure of the steel for nitriding and quenching. In order to obtain such effects of V and Ti, the lower limits of the contents of V and Ti are each set to 0.01% by mass, preferably 0.05% by mass, and more preferably 0.2% by mass. On the other hand, this effect is saturated by increasing the content of V and Ti, so the upper limit of the content of V and Ti is set to 1.5% by mass, preferably 1.45% by mass, more preferably 0.5% by mass. % by mass.

Like V and Ti, Nb also forms carbides and refines the crystal structure of the steel for nitriding and quenching. In order to obtain such an effect of Nb, the lower limit of the Nb content is set to 0.01% by mass, preferably 0.05% by mass. On the other hand, since this effect is saturated with an increase in the Nb content, the upper limit of the Nb content is set to 3.0% by mass, preferably 2.8% by mass, more preferably 0.6% by mass.

Although the total amount of V, Nb and Ti is not particularly limited, it is preferably 0.01 to 3.0% by mass. If the total amount is less than 0.01% by mass, the refinement of the crystal structure of nitrogen austenite in the nitriding layer and nitrogen martensite in the hardened layer is insufficient, and the fatigue strength and toughness of the nitriding and quenching part are sufficiently improved. may not be able to be increased. On the other hand, if the total amount exceeds 3.0% by mass, it is uneconomical because the effect corresponding to the amount cannot be obtained.

The steel for nitriding and quenching treatment according to the present embodiment has a nitriding index A2 represented by the following formula (2) of 0.7 or less, preferably -0.10 to 0.68, more preferably 0 to 0.5. 5 is controlled.
A2=0.36V+0.35Nb+0.35Ti-0.20(Mn+Ni) (2)
In the formula, each element symbol is mass % of each element.
The nitriding index A2 is an index of the thickness of the nitrogen austenite metal structure grown from the surface by the nitriding treatment, and the smaller the nitriding index A2, the larger the thickness of the nitriding layer. When the nitriding index A2 exceeds 0.7, the thickness of the nitriding layer becomes too small, and the fatigue properties cannot be fully exhibited by the quenching treatment. The nitriding index A2 is experimentally calculated from the relationship between the alloy composition and the thickness of the nitriding layer.

(Embodiment 3)
The steel for nitriding and quenching according to the present embodiment is selected from C, Mn and/or Ni, P, S, one or more selected from Cr, Si and Al, and V, Nb and Ti. and the balance consists of Fe and unavoidable impurities. In addition, this nitriding and quenching steel may further contain one or more selected from Mo and B.
The nitriding and quenching steel according to the present embodiment differs from the nitriding and quenching steel according to the first embodiment in that it further contains one or more selected from V, Nb and Ti. Since the basic characteristics of the steel for nitriding and quenching treatment according to this embodiment are the same as those of the steel for nitriding and quenching treatment according to Embodiments 1 and 2, only the differences will be described.

The steel for nitriding and quenching treatment according to the present embodiment has a nitriding index A3 represented by the following formula (3) of 0.7 or less, preferably -0.10 to 0.68, more preferably 0 to 0.5. 5 is controlled.
A3=0.30Cr+0.45Si+0.34Al+0.36V+0.35Nb+0.35Ti-0.20(Mn+Ni) (3)
In the formula, each element symbol is mass % of each element.
The nitriding index A3 is an index of the thickness of the nitrogen austenite metal structure grown from the surface by the nitriding treatment, and the smaller the nitriding index A3, the larger the thickness of the nitriding layer. When the nitriding index A3 exceeds 0.7, the thickness of the nitriding layer becomes too small, and the fatigue properties cannot be fully exhibited by the quenching treatment. The nitriding index A3 is experimentally calculated from the relationship between the alloy composition and the thickness of the nitriding layer.

(Embodiment 4)
In the nitriding and quenching part according to the present embodiment, a hardened layer containing nitrogen martensite is formed on the surface layer of the nitriding and quenching steel according to the first to third embodiments by nitriding and quenching.
The smaller the average crystal grain size of the nitrogen martensite contained in the hardened layer, the better the toughness and fatigue characteristics of the nitriding and quenching part. is. On the other hand, the lower limit of the average crystal grain size of nitrogen martensite is not particularly limited, but is preferably 1 μm, more preferably 10 μm, and still more preferably 20 μm.

The thickness of the hardened layer is not particularly limited and may be appropriately adjusted according to the type of the nitriding and quenching part, but is preferably 105 to 500 μm, more preferably 108 to 450 μm, further preferably 110 to 400 μm. When the thickness of the hardened layer is within the above range, the fatigue properties can be stably ensured.

The Vickers hardness of the hardened layer is not particularly limited and may be appropriately adjusted according to the type of the nitriding and quenching part, but is preferably 500 HV or more, more preferably 600 to 1000 HV, and still more preferably 700 to 900 HV.

Properties such as toughness and fatigue strength in nitriding and hardening parts can also be affected by the average grain size of the base metal structure. For example, the smaller the average crystal grain size of the metal structure of the base material, the better the toughness and fatigue strength of the nitriding and quenching part. Regardless of whether the steel for nitriding and quenching is a hot-rolled material, a cold-rolled material, or an annealed material, there is a high possibility that the average grain size of the metal structure will change due to the nitriding and quenching treatment. It is desirable that the average grain size of the metallographic structure is small. Therefore, the average crystal grain size of the metal structure of the base material is preferably 100 μm or less, more preferably 95 μm or less, still more preferably 90 μm or less. On the other hand, if the average crystal grain size of the metal structure of the base material is too small, the hardness increases and workability decreases.
Here, "average crystal grain size" as used herein means that measured by a cutting method in accordance with JIS G0551:2013.

Nitriding and quenching parts having the characteristics described above are manufactured by working the steel for nitriding and quenching treatment into a part shape and performing nitriding and quenching treatment.
The shape of the part is not particularly limited, and any shape suitable for the type of the part may be used. For example, if the nitriding and quenching part is a bearing part, the required shape of the bearing part may be used. Also, the processing method is not particularly limited, and a method known in the art can be used.

The nitriding and quenching treatment is not particularly limited, and can be performed according to a method known in the technical field. Specifically, the steel for nitriding and quenching processed into a part shape is quenched (rapidly cooled) after nitrogen is diffused and permeated from the steel surface in a temperature range of A1 or higher in the iron _- nitrogen equilibrium diagram. good.

The nitriding and quenching part according to the present embodiment manufactured in this way is excellent not only in dimensional accuracy and surface strength but also in toughness and fatigue strength, and is suitable for use as an automobile part.

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

A hot-rolled steel sheet having an alloy composition shown in Table 1 was cold-rolled to a thickness of 2 mm, and then annealed at 700 to 960° C. to obtain a steel sheet (steel for nitriding and quenching treatment).

The steel plate obtained above was subjected to nitriding and quenching treatment. In the nitriding and quenching treatment, the steel sheet was subjected to nitriding treatment by contacting it with a mixed gas containing nitrogen gas, hydrogen gas and ammonia gas, followed by oil quenching to form a hardened layer on the surface layer of the steel sheet. Detailed conditions were as follows.
Nitriding temperature: 830°C
Nitriding time: 3 hours Nitrogen gas concentration in mixed gas: 79.95 to 99% by volume
Hydrogen gas concentration in mixed gas: 0.95 to 20% by volume
Ammonia gas concentration in mixed gas: 0.05 to 0.3% by volume
Quenching temperature: 60-70°C

The steel sheets obtained above were evaluated as follows.
(Average grain size)
Based on JIS G0551:2013, the crystal grain size of the metal structure (ferrite) of the base material portion and the metal structure of the hardened layer (nitrogen martensite) was measured using a cutting method. Specifically, the number of measurements was set to 10, and the crystal grain size was measured in 10 fields of view at a magnification of 100 to 500 using an optical microscope, and the average was taken.

(Thickness of hardened layer)
The thickness of the cured layer was measured using an optical microscope. Specifically, the number of measurements was set to 10, the thickness of the nitriding layer was measured in 10 fields of view at 100 times magnification using an optical microscope, and the average was taken.

(Vickers hardness of hardened layer)
Using a Vickers hardness tester, the test load was 980 mN, the number of tests was 5, the Vickers hardness was measured at a depth of 25 μm from the surface, and the average was taken.

(Nitrogen concentration in hardened layer)
The nitrogen concentration of the cured layer was measured using glow discharge optical emission spectroscopy (GD-OES). The measurement conditions were as follows.
Measuring machine: Marcus type high-frequency glow discharge luminescence surface analyzer Sputtering method: Normal sputtering Measuring depth: 10 μm depth from the surface
Measuring range: Φ4mm

(Fatigue properties)
Evaluation was performed in accordance with JIS Z2273:1978 and JIS Z2275:1978. The test piece has the shape of No. 1 test piece (d = 20, R = 42.5) described in JIS Z2275: 1978, and uses a PWOG type plane bending fatigue tester, stress ratio = -1, frequency 1250 rpm, maximum Under conditions of bending stress of 500 N/mm ² and 650 N/mm ² and 5 tests, the number of repetitions of stress until fatigue fracture occurred was measured. When the maximum bending stress is 500 N/mm ² , the majority of the test pieces that did not cause fatigue failure even when the number of cycles exceeded 2.0 × 10 ⁶ passed (○), and the maximum bending stress was 650 N. In the case of /mm ² , the majority of the test pieces that did not cause fatigue failure even when the number of repetitions exceeded 1.0 × 10 ⁵ was evaluated as a pass (○), and the other test specimens were evaluated as a failure (×). . This evaluation standard is a standard that assumes the case of using it as a general mechanical structural part such as a transmission part of an automobile.

(Toughness)
Evaluation was made in accordance with JIS Z2242:2005. The test piece was a V-notch test piece (notch depth 2 mm) described in JIS Z2242:2005, and the test was performed at room temperature using a Charpy impact tester. Also, the number of tests was three. In this evaluation, Charpy impact values of 30 J/cm ² or more were evaluated as ⊚, 15 J/cm ² or more and less than 30 J/cm ² as ◯, and less than 15 J/cm ² as X. . It should be noted that this evaluation standard is a standard that assumes use as a general mechanical structural component such as a transmission component for automobiles.

Table 2 shows the above results.

As shown in Table 2, in Examples 1 to 23, since the steel sheets satisfying the predetermined alloy composition and nitriding index were used, the fatigue properties and toughness were good after the nitriding and quenching treatment.
On the other hand, in Comparative Examples 1 to 13, since the steel sheets that did not satisfy the predetermined alloy composition were used, the fatigue properties and toughness were not sufficient after the nitriding and quenching treatment.
In particular, Comparative Example 1 does not contain an element that forms nitrides during the nitriding and quenching treatment, so the effect of suppressing the coarsening of nitrogen austenite due to the formation of nitrides cannot be obtained, and the grains of nitrogen martensite are large. became. Therefore, fatigue properties and toughness were lowered after nitriding and quenching.
In addition, in Comparative Example 2, the total amount of Mn and Ni was too small, so that quenching was insufficient and the fatigue characteristics and toughness were lowered. Similarly, in Comparative Example 3, since the total amount of Mn and Ni was too small, the austenitization of the nitriding layer was insufficient, and ferrite existing between austenite grains could not be sufficiently reduced. Therefore, the fatigue properties and toughness decreased after the nitriding and quenching treatment.

In addition, in Comparative Examples 4 to 7 and 10 to 13, the number of elements that form nitrides during nitriding and quenching treatment was too small, so the effect of suppressing coarsening of nitrogen austenite due to the formation of nitrides was not obtained, and nitrogen martensite was not obtained. grain size also increased. Therefore, fatigue properties and toughness were lowered after nitriding and quenching.
In addition, in Comparative Examples 8 and 9, since the amounts of Si and Al were too large, the nitrides were coarsened and tended to become starting points of fatigue fracture. Therefore, fatigue properties and toughness were lowered after nitriding and quenching.
Further, in Comparative Examples 14 to 16, since the nitriding index was 0.7 or more, the austenite growth rate was slowed down during the nitriding treatment, and the thickness of the nitriding layer was reduced. As a result, the thickness of the hardened layer was also reduced, resulting in deterioration in fatigue properties and toughness.

As can be seen from the above results, according to the present invention, it is possible to provide a nitriding and quenching steel capable of producing nitriding and quenching parts having excellent toughness and fatigue strength. Further, according to the present invention, it is possible to provide a nitriding hardened part excellent in toughness and fatigue strength and a method for manufacturing the same.

Claims (9)

C: 0.3% by mass or less,
Mn: 0.3 to 2.8% by mass,
Ni: 0.05 to 1.0% by mass,
P: 0.03% by mass or less,
S: 0.03% by mass or less, and 1 selected from Cr: 0.1 to 4.5% by mass, Si: 0.1 to 1.0% by mass, and Al: 0.1 to 1.0% by mass including more than seeds, the total amount of Mn and Ni is 0.4 to 3.0% by mass, and the balance consists of Fe and unavoidable impurities,
Formula (1) below:
A1=0.30Cr+0.45Si+0.34Al-0.2(Mn+Ni) (1)
A steel for nitriding and quenching treatment having a nitriding index A1 represented by (in the formula, each element symbol represents mass % of each element) of 0.7 or less. C: 0.3% by mass or less,
Mn: 0.3 to 2.8% by mass,
Ni: 0.05 to 1.0% by mass,
P: 0.03% by mass or less,
S: 0.03% by mass or less,
Cr: 0.1 to 4.5% by mass, Si: 0.1 to 1.0% by mass and Al: one or more selected from 0.1 to 1.0% by mass, and V: 0.01 to 1.5% by mass, Nb: 0.01 to 3.0% by mass, and Ti: one or more selected from 0.01 to 1.5% by mass, and the total amount of Mn and Ni is 0.4 to 3 .0% by mass, the balance being Fe and unavoidable impurities,
Formula (3) below:
A3=0.30Cr+0.45Si+0.34Al+0.36V+0.35Nb+0.35Ti-0.20(Mn+Ni) (3)
A steel for nitriding and quenching treatment having a nitriding index A3 represented by (in the formula, each element symbol represents mass % of each element) of 0.7 or less. 3. The steel for nitriding and quenching according to claim 1, further comprising one or more selected from Mo: 0.1 to 2.0% by mass and B: 0.0001 to 0.01% by mass. The steel for nitriding and quenching treatment according to any one of claims 1 to 3, wherein the total amount of Cr, Si and Al is 0.1 to 4.5% by mass. The steel for nitriding and quenching treatment according to any one of claims 2 to 4, wherein the total amount of V, Nb and Ti is 0.01 to 3.0% by mass. A nitriding and quenching part in which a hardened layer containing nitrogen martensite is formed by nitriding and quenching on the surface layer of the steel for nitriding and quenching according to any one of claims 1 to 5. 7. The nitriding and quenching part according to claim 6, wherein said nitrogen martensite has an average crystal grain size of 80 [mu]m or less. 8. The nitriding and quenching part according to claim 6, wherein the hardened layer has a Vickers hardness of 500 HV or more. A method of manufacturing a nitriding and quenching part, comprising processing the steel for nitriding and quenching according to any one of claims 1 to 5 into a part shape and performing nitriding and quenching treatment.