JP7545949B2 - Steel material, steel part, and manufacturing method of steel part - Google Patents

Description

This disclosure relates to steel materials, steel parts, and methods for manufacturing steel parts.

One method for manufacturing machine structural parts such as gears is to harden the surface layer of low carbon steel (case hardened steel) by surface hardening treatment such as carburizing or carbonitriding. Carbonitriding causes nitrides such as CrN and _MnSiN2 to precipitate in the surface layer where nitrogen has penetrated, improving the hardness and fatigue properties of the surface layer.

For example, Patent Document 1 shows a steel material and a carburized steel part with excellent stability of rolling fatigue life, and a manufacturing method thereof. The steel material has a predetermined composition and a Cr segregation ratio of 2.0 or less measured under predetermined conditions. In other words, the Cr segregation ratio is the average of 8 measurement points, which are determined by performing a line analysis of the Cr concentration using an EPMA to determine the minimum Cr concentration [Cr]min and maximum Cr concentration [Cr]max, and calculating [Cr]max/[Cr]min, and is 2.0 or less.

Patent Document 2 shows a carbonitrided bearing steel that has excellent surface fatigue properties such as wear resistance, spalling, and pitting. The carbonitrided bearing steel has a specified composition, with a Si+Cr content in the range of 1.8-2.8%, and has a surface hardened layer formed by carbonitriding or quenching and tempering after carbonitriding, with a C+N content in the range of 1.0-2.0% up to 0.1 mm from the surface.

Furthermore, Patent Document 3 discloses a case-hardening steel that has excellent resistance to grain coarsening and cold workability, and does not require softening annealing. The case-hardening steel has a specified composition, and is shown to have an average Vickers hardness of 180 or less in a cross section, and a maximum standard deviation of the Vickers hardness variation of 5 or less.

JP 2017-150066 A Japanese Patent Application Publication No. 10-060586 JP 2006-265704 A

Conventionally, there have been technologies that have been proposed to achieve steel parts with improved fatigue properties and steel materials with improved workability by specifying the respective component compositions. However, it has not been possible to achieve a steel material that can be easily processed into part shapes in the manufacturing process and that can reliably ensure high surface hardness after carbonitriding, and a steel part formed into the part shape from such a steel material. The present invention has been made in consideration of the above circumstances, and aims to achieve a steel material that is excellent in workability into part shapes and that can reliably ensure high surface hardness after carbonitriding, a steel part having high surface hardness formed from such a steel material into the part shape, and a manufacturing method for such a part.

Aspect 1 of the present invention is
C: 0.15% by mass or more and 0.30% by mass or less,
Si: 0.36% by mass or more and 1.00% by mass or less,
Mn: 0.40% by mass or more and 1.50% by mass or less,
Cr: 1.20% by mass or more and 1.90% by mass or less,
Mo: 0.30% by mass or more and 0.80% by mass or less,
P: more than 0% by mass and not more than 0.05% by mass,
S: more than 0% by mass and not more than 0.05% by mass,
Al: 0.005% by mass or more and 0.2% by mass or less,
A steel material containing N: more than 0 mass% and 0.050 mass% or less, O: more than 0 mass% and 0.005 mass% or less, with the balance being Fe and unavoidable impurities,
In the range from the outermost surface to a depth of 200 μm, the structure is composed of tempered martensite and retained austenite,
In a range from the outermost surface to a position having a depth of 100 μm or more and 200 μm or less, the ratio of the retained austenite structure is 10 vol% or more and 70 vol% or less, and the half width of the X-ray diffraction peak in the (211) plane is 5.5° or more,
The steel material has, at a position 150 μm deep from the outermost surface, a Vickers hardness of 600 HV or more, a C concentration of 0.5 mass% or more and 0.9 mass% or less, and an N concentration of 0.4 mass% or more and 1.20 mass% or less, and further has an average circular equivalent diameter of carbides, nitrides and carbonitrides of 0.1 μm or more and 1.0 μm or less, a total area ratio of carbides, nitrides and carbonitrides having a circular equivalent diameter of 0.1 μm or more and 1.0 μm or less of 5% or more, and a number density of 0.1 pieces/ ^μm2 or more and 10 pieces/ ^μm2 or less.

Aspect 2 of the present invention is
C: 0.15% by mass or more and 0.30% by mass or less,
Si: 0.36% by mass or more and 1.00% by mass or less,
Mn: 0.40% by mass or more and 1.50% by mass or less,
Cr: 1.20% by mass or more and 1.90% by mass or less,
Mo: 0.30% by mass or more and 0.80% by mass or less,
P: more than 0% by mass and not more than 0.05% by mass,
S: more than 0% by mass and not more than 0.05% by mass,
Al: 0.005% by mass or more and 0.2% by mass or less,
A steel material containing N: more than 0 mass% and 0.050 mass% or less, O: more than 0 mass% and 0.005 mass% or less, with the balance being Fe and unavoidable impurities,
The steel has a mixed structure of ferrite and pearlite and has an internal hardness of 200 HV or less in Vickers hardness.

Aspect 3 of the present invention is
The steel material according to aspect 1 or 2, further comprising one or more selected from the group consisting of Cu: more than 0 mass% and 1.0 mass% or less, Ni: more than 0 mass% and 1.0 mass% or less, and B: more than 0 mass% and 0.0050 mass% or less.

Aspect 4 of the present invention is
The steel material according to any one of aspects 1 to 3, further containing one or more selected from the group consisting of V: more than 0 mass% and 0.50 mass% or less, and Nb: more than 0 mass% and 0.10 mass% or less.

Aspect 5 of the present invention is
C: 0.15% by mass or more and 0.30% by mass or less,
Si: 0.36% by mass or more and 1.00% by mass or less,
Mn: 0.40% by mass or more and 1.50% by mass or less,
Cr: 1.20% by mass or more and 1.90% by mass or less,
Mo: 0.30% by mass or more and 0.80% by mass or less,
P: more than 0% by mass and not more than 0.05% by mass,
S: more than 0% by mass and not more than 0.05% by mass,
Al: 0.005% by mass or more and 0.2% by mass or less,
A steel part containing N: more than 0 mass% and not more than 0.050 mass%, O: more than 0 mass% and not more than 0.005 mass%, with the balance being Fe and unavoidable impurities,
In the range from the outermost surface to a depth of 100 μm, the structure is composed of tempered martensite and retained austenite,
At the outermost surface, the proportion of the retained austenite structure is 10 volume % or more and 70 volume % or less, and the half width of the X-ray diffraction peak in the (211) plane is 5.5° or more,
The steel part has, at its outermost surface, a Vickers hardness of 600 HV or more, a C concentration of 0.5 mass% or more and 0.9 mass% or less, an N concentration of 0.4 mass% or more and 1.20 mass% or less, an average equivalent circle diameter of the carbides, nitrides and carbonitrides of 0.1 μm or more and 1.0 μm or less, a total area ratio of the carbides, nitrides and carbonitrides of 0.1 μm or more and 1.0 μm or less in equivalent circle diameter is 5% or more, and a number density of 0.1 pieces/ ^μm2 or more and 10 pieces/μm2 or ^less .

Aspect 6 of the present invention is
The steel part according to aspect 5, further comprising one or more selected from the group consisting of Cu: more than 0 mass% and 1.0 mass% or less, Ni: more than 0 mass% and 1.0 mass% or less, and B: more than 0 mass% and 0.0050 mass% or less.

Aspect 7 of the present invention is
The steel part according to aspect 5 or 6, further containing one or more selected from the group consisting of V: more than 0 mass% and 0.50 mass% or less, and Nb: more than 0 mass% and 0.10 mass% or less.

Aspect 8 of the present invention is
A method for producing a steel part according to aspect 5, comprising the steps of:
C: 0.15% by mass or more and 0.30% by mass or less,
Si: 0.36% by mass or more and 1.00% by mass or less,
Mn: 0.40% by mass or more and 1.50% by mass or less,
Cr: 1.20% by mass or more and 1.90% by mass or less,
Mo: 0.30% by mass or more and 0.80% by mass or less,
P: more than 0% by mass and not more than 0.05% by mass,
S: more than 0% by mass and not more than 0.05% by mass,
Al: 0.005% by mass or more and 0.2% by mass or less,
A steel slab containing N: more than 0 mass% and 0.050 mass% or less, O: more than 0 mass% and 0.005 mass% or less, with the balance being Fe and unavoidable impurities,
As a cooling rate measurement condition, a step of roughly forming the material into a shape such that when the material is kept at a heating temperature of 900° C. or less and 800° C. or more for 30 minutes to 1 hour, and then air-cooled from the heating temperature to 300° C. under atmospheric conditions, the material has an average cooling rate of 0.39° C./s or more;
a two-stage normalizing process of heating to a first temperature of 850° C. to 950° C., holding the first temperature for 0.5 hours to 2 hours, then lowering the temperature to a second temperature of 650° C. to 700° C., holding the second temperature for 3.5 hours to 5 hours, and then air-cooling or furnace-cooling to room temperature;
A cutting process;
a carbonitriding process including a first step of holding the material after cutting at a temperature of 900° C. to 950° C. and in an atmosphere having a carbon equivalent Cp of 0.5% to 1.2% for 2 hours to 7 hours, a second step of holding the material at the same temperature as in the first step and in an atmosphere having a carbon equivalent Cp of 0.5% to 0.7% for 2 hours to 4 hours, and a third step of holding the material at a temperature of 800° C. to 900° C. and in an atmosphere having the same carbon equivalent Cp as in the second step and an _NH3 content of 6% to 12% for 4 hours to 6 hours;
This is a method for manufacturing a steel part, which includes the steps of performing oil quenching after the carbonitriding treatment and then performing tempering.

Aspect 9 of the present invention is
A method for producing a steel part according to claim 6, comprising the steps of:
9. The method for producing a steel part according to claim 8, wherein the steel billet further contains one or more selected from the group consisting of Cu: more than 0 mass% and 1.0 mass% or less, Ni: more than 0 mass% and 1.0 mass% or less, and B: more than 0 mass% and 0.0050 mass% or less.

Aspect 10 of the present invention is
A method for producing a steel part according to aspect 7, comprising the steps of:
10. The method for producing a steel part according to claim 8 or 9, wherein the steel billet further contains one or more elements selected from the group consisting of V: more than 0 mass% and 0.50 mass% or less, and Nb: more than 0 mass% and 0.10 mass% or less.

The present invention provides a steel material that is excellent in workability into part shapes and can reliably ensure high surface hardness after carbonitriding, a steel part having high surface hardness formed by forming the steel material into part shapes, and a method for manufacturing the part.

FIG. 2 is a graph showing the relationship between the half-value width and the Vickers hardness at a position 150 μm deep from the outermost surface of a carbonitrided steel material. FIG. 1 is a diagram showing the relationship between the size of a steel material (diameter of a cylindrical test piece) and the average cooling rate. FIG. 1 is a diagram showing the relationship between the holding time of the second stage of two-stage normalizing and the internal hardness of No. 2 in the examples. FIG. 1 is a diagram showing the relationship between the holding time of the second stage of two-stage normalizing and the internal hardness of No. 1 in the examples. FIG. 2 is a diagram showing the relationship between the Mn content and the surface hardness of a carbonitrided steel material.

Conventionally, it has not been possible to produce steel materials that can be easily processed into part shapes during manufacturing and that can reliably ensure high surface hardness after carbonitriding, and steel parts formed from such steel materials into part shapes. The reasons for this are, for example, that the size of the steel materials and steel parts is large, and the amount of Cr and Mn in the parent phase is reduced by the carbonitriding process, which leads to poor quenching during quenching after carbonitriding and makes it difficult to obtain high surface hardness. In order to ensure hardenability, it is possible to increase the amount of alloy components, but this makes it easier for bainite to form in the steel structure, and also causes grain coarsening due to mixed grains, which increases the internal hardness of the steel material, which can lead to reduced manufacturability, such as reduced machinability and longer normalizing times.

The inventors therefore conducted extensive research to obtain steel materials and steel parts with high surface hardness without compromising workability or manufacturability, for example, to realize steel materials for carbonitriding (also called "steel materials for case hardening" or "steel materials before carbonitriding") with excellent workability and surface hardenability, and carbonitriding steel materials (also called "case hardened steel materials") and steel parts (case hardened steel parts) with high surface hardness, by suppressing the occurrence of poor hardening (reduction in hardness) in the surface layer of steel materials or steel parts after carbonitriding even if the cooling rate in quenching after carbonitriding is slow.

As a result, it was found that while keeping the predetermined composition, particularly the Si and Cr contents within a certain range, and increasing the amount of Mn, which does not precipitate as a nitride by itself, is effective in ensuring the hardenability of the surface structure after carbonitriding, and that, since an excessive amount of Mn increases the amount of bainite structure formed and leads to a longer normalizing time, it is sufficient to adjust the amount of Mn within a manufacturable range. Furthermore, it was found that by adjusting the chemical composition in this way and performing, in particular, two-stage normalizing in the manufacturing process, a steel material (steel material for carbonitriding) having a desired structure, an internal hardness of 200 HV or less in Vickers hardness, excellent workability into part shapes, and capable of reliably ensuring high surface hardness after carbonitriding can be obtained, and that by performing at least carbonitriding on the steel material, a steel material after carbonitriding (carbonitrided steel material) with high surface hardness and a steel part with high surface hardness obtained by processing the steel material after carbonitriding into a part shape can be obtained. Furthermore, they also discovered a manufacturing method that can easily produce these steel parts.

Below, we will explain the steel material, steel parts, and manufacturing method for steel parts according to this embodiment.

[Steel]
The chemical composition of the steel material will be explained below, but this chemical composition also applies to the chemical composition of the steel part and the steel billet used in the production of the steel part and the steel material.
(Chemical composition)
C: 0.15% by mass or more and 0.30% by mass or less C is an effective element for ensuring the core hardness of steel parts. For this reason, the C content is set to 0.15% by mass or more. The C content is preferably 0.16% by mass or more, more preferably 0.17% by mass or more. However, if the C content exceeds 0.30% by mass, the machinability and cold forgeability of the steel material deteriorate, and further the toughness of the steel part deteriorates. For this reason, the C content is set to 0.30% by mass or less. The C content is preferably 0.24% by mass or less, more preferably 0.23% by mass or less.

Si: 0.36% by mass or more and 1.00% by mass or less Si is an element effective in improving the solid solution strengthening, hardenability, and temper softening resistance of the matrix. For this reason, the Si content is set to 0.36% by mass or more. The Si content is preferably 0.38% by mass or more, more preferably 0.40% by mass or more. However, if the Si content is too high, the machinability and cold forgeability of the steel material are significantly reduced. For this reason, the Si content is set to 1.00% by mass or less. The Si content is preferably 0.70% by mass or less, more preferably 0.60% by mass or less.

Mn: 0.40% by mass or more and 1.50% by mass or less Mn is effective for solid solution strengthening of the matrix, and also contributes greatly to improving hardenability. Although it forms a composite nitride with Si during carbonitriding, it is an element that has a large degree of influence on hardenability because a larger proportion of it remains in the matrix than Cr. For this reason, the Mn content is set to 0.40% by mass or more. The Mn content is preferably 0.70% by mass or more, more preferably 1.00% by mass or more. However, if the Mn content is too large, the machinability and cold forgeability of the steel material are reduced due to the generation of bainite structure and the coarsening of crystal grains. In addition, residual austenite is generated excessively after carburization, and the strength of the parts is likely to decrease. For this reason, the Mn content is set to 1.50% by mass or less.

Cr: 1.20% by mass or more and 1.90% by mass or less Cr is an element that improves hardenability, forms precipitates such as carbides, nitrides, and carbonitrides in the surface hardened layer by surface hardening treatment such as carbonitriding, and contributes to improving the rolling fatigue life. Furthermore, Cr is also an element that greatly contributes to the stability of the rolling fatigue life. For this reason, the Cr amount is set to 1.20% by mass or more. The Cr amount is preferably 1.30% by mass or more, more preferably 1.35% by mass or more. However, if the Cr amount becomes too large, the machinability and cold forgeability of the steel material decrease, and further coarse precipitates are precipitated, which decreases the rolling fatigue life and the stability of the rolling fatigue life. In addition, since an oxide layer is formed on the surface layer during carburization, there is a risk of inhibiting the penetration of carbon and nitrogen. Furthermore, even if a large amount is added to improve the hardenability, most of it becomes nitride, so it is difficult to contribute to the improvement of the hardenability required for quenching after carbonitriding. For this reason, the Cr amount is set to 1.90% by mass or less. The Cr amount can be, for example, 1.70 mass % or less, or further 1.60 mass % or less.

Mo: 0.30% by mass or more and 0.80% by mass or less Mo is an element that significantly improves hardenability and is effective in improving impact strength. For this reason, the Mo content is set to 0.30% by mass or more. The Mo content is preferably 0.35% by mass or more, more preferably 0.40% by mass or more. However, if the Mo content is too high, the machinability decreases and the cost increases. For this reason, the Mo content is set to 0.80% by mass or less. The Mo content is preferably 0.55% by mass or less, more preferably 0.50% by mass or less.

P: more than 0 mass% and 0.05 mass% or less P is an element that is inevitably contained as an impurity, and it segregates at grain boundaries and reduces workability. For this reason, the P content is set to 0.05 mass% or less. The P content is preferably suppressed to 0.04 mass% or less, and more preferably to 0.03 mass% or less. However, it is practically difficult to reduce the P content to 0 mass%, and excessive reduction leads to an increase in steelmaking costs. Therefore, the lower limit of the P content may be about 0.001 mass%.

S: more than 0 mass% and 0.05 mass% or less S is an element that is inevitably contained as an impurity, and if the amount of S is too large, it precipitates as MnS, which becomes the starting point of fine cracks and reduces wear resistance. For this reason, the amount of S is set to 0.05 mass% or less. The amount of S is preferably kept to 0.04 mass% or less, more preferably 0.03 mass% or less. However, it is practically difficult to make the amount of S 0 mass%, and excessive reduction leads to an increase in steelmaking costs. Therefore, the lower limit of the amount of S can be about 0.001 mass%.

Al: 0.005% by mass or more and 0.2% by mass or less Al is an element that has a deoxidizing effect on steel material, and also combines with N to form nitrides, thereby refining crystal grains and contributing to improving rolling fatigue life. For this reason, the Al content is set to 0.005% by mass or more. The Al content is preferably 0.010% by mass or more, more preferably 0.015% by mass or more. However, even if the Al content exceeds 0.2% by mass, this effect is saturated, so the Al content is set to 0.2% by mass or less. The Al content is preferably 0.1% by mass or less, more preferably 0.050% by mass or less.

N: more than 0 mass% and 0.050 mass% or less N is an element that forms nitrides with Al, which inhibit the growth of austenite grains and refine the grains, thereby contributing to improving the rolling fatigue life. For this reason, the N content is more than 0 mass%, preferably 0.0010 mass% or more, more preferably 0.0015 mass% or more, and even more preferably 0.0020 mass% or more. However, if the N content is too high, coarse nitrides of Al and Ti are generated and become the starting points of fine cracks. For this reason, the N content is 0.050 mass% or less. The N content is preferably 0.040 mass% or less, more preferably 0.020 mass% or less.

O: more than 0 mass% and 0.005 mass% or less O is an element that combines with Al and Si to form oxide-based inclusions, adversely affecting the rolling fatigue life and further the cold workability. For this reason, the O content is set to 0.005 mass% or less. The O content is preferably 0.004 mass% or less, more preferably 0.003 mass% or less. However, it is practically difficult to reduce the O content to 0 mass%, and excessive reduction leads to an increase in steelmaking costs. For this reason, the lower limit of the O content may be about 0.0001 mass%.

The balance is Fe and unavoidable impurities. As unavoidable impurities, the inclusion of trace elements (e.g., As, Sb, Sn, etc.) that are brought in depending on the conditions of raw materials, materials, manufacturing equipment, etc. is permitted. Note that, for example, there are elements such as P, S, and O, which are generally preferable to have in small amounts and are therefore unavoidable impurities, but whose composition ranges are separately specified as above. For this reason, in this specification, when referring to the "unavoidable impurities" that make up the balance, this is a concept that excludes elements whose composition ranges are separately specified.

The steel material, steel parts, and steel pieces used in the manufacture of these according to this embodiment may contain the elements described above in their chemical composition. The optional elements described below do not have to be included, but including them as necessary together with the above elements will contribute to further improvement of hardenability, etc. The optional elements are described below.

At least one selected from the group consisting of Cu: more than 0 mass% and not more than 1.0 mass%, Ni: more than 0 mass% and not more than 1.0 mass%, and B: more than 0 mass% and not more than 0.0050 mass%. Cu, Ni, and B all act as elements that improve the hardenability of the parent phase, and contribute to further improvement of rolling fatigue properties by increasing hardness. In order to effectively exert these effects, Cu and Ni are each preferably contained in an amount of more than 0 mass%, more preferably 0.01 mass% or more, even more preferably 0.02 mass% or more, and even more preferably 0.03 mass% or more. The amount of B is preferably more than 0 mass%, more preferably 0.0001 mass% or more, even more preferably 0.0005 mass% or more, and even more preferably 0.0010 mass% or more. On the other hand, from the viewpoint of suppressing deterioration of the manufacturability of the steel material, the contents of Cu and Ni are each preferably 1.0 mass% or less, more preferably 0.20 mass% or less, and even more preferably 0.15 mass% or less. From the above viewpoints, the content of B is preferably 0.0050 mass% or less, more preferably 0.0040 mass% or less, and further preferably 0.0030 mass% or less. Any of these elements may be contained alone, or two or more kinds may be contained.

At least one selected from the group consisting of V: more than 0 mass% and 0.50 mass% or less, and Nb: more than 0 mass% and 0.10 mass% or less V and Nb are elements that bond with one or more of C and N to form carbonitrides, thereby exerting effects such as dispersion strengthening and grain refinement, and improving the rolling fatigue life. In order to effectively exert these effects, it is preferable that V and Nb are each contained in an amount of more than 0 mass%. However, if contained in excess, the precipitated carbonitrides become coarse and become the starting point of fracture, which rather worsens the rolling fatigue life. Therefore, the upper limit of the V amount is 0.50 mass%, and the upper limit of the Nb amount is 0.10 mass%. Any one of these elements may be contained alone, or two types may be contained.

(Steel structure and properties)
In the manufacture of steel parts, processing such as cutting, rolling, and forging is carried out one or more times, but in this specification, the steel before the final processing is referred to as a steel material, and the steel material after the final processing to obtain the shape of the product is referred to as a steel part. The steel material includes a steel material before carbonitriding treatment (steel material for carbonitriding treatment) obtained by performing a two-stage normalizing process described below, and a carbonitrided steel material obtained by performing a carbonitriding process on the steel material, and these steel materials have different structures and properties as described in detail below. The steel part is obtained by processing the carbonitrided steel material, and its structure and properties are substantially the same as those of the carbonitrided steel material, except for the measurement position, as described below.

The following will explain the structure of the steel material before carbonitriding. Note that in the following explanation of the steel structure, the mechanism by which having such a structure can improve various properties may be explained. These are mechanisms that the inventors have devised based on the knowledge currently available, but please note that they do not limit the technical scope of the present invention.

[Structure and properties of steel before carbonitriding]
The steel material before carbonitriding (steel material for carbonitriding) has a mixed structure of ferrite and pearlite, and has an internal hardness of 200 HV or less in Vickers hardness.

In this embodiment, from the viewpoint of improving workability, the content of Mn and Mo is kept below a certain level, and the normalizing time is ensured to be at least a certain level, thereby reducing the proportion of bainite in the structure of the steel material before carbonitriding, forming a mixed structure of ferrite and pearlite, and suppressing the internal hardness to 200 HV or less in Vickers hardness. This improves machinability in, for example, a cutting process to form the part shape, and suppresses increases in cutting processing time and tool wear. The internal hardness is preferably 180 HV or less. The internal hardness is the Vickers hardness at a diameter D/4 part of the steel material.

[Structure and properties of carbonitrided steel]
Next, the steel structure and characteristics of the carbonitrided steel material obtained by subjecting the above-mentioned steel material for carbonitriding to carbonitriding will be described. The carbonitrided steel material has a structure consisting of tempered martensite and retained austenite in a range from the outermost surface to a depth of 200 μm, a ratio of the retained austenite structure is 10 volume % or more and 70 volume % or less in a range from a depth of 100 μm to a depth of 200 μm from the outermost surface, a half width of an X-ray diffraction peak in a (211) plane is 5.5° or more, and at a depth of 150 μm from the outermost surface, a Vickers hardness is 600 HV or more, a C concentration is 0.5 mass % or more and 0.9 mass % or less, and an N concentration is 0.4 mass % or more and 1.20 mass % or less, an average circle equivalent diameter of the carbides, nitrides and carbonitrides is 0.1 μm or more and 1.0 μm or less, a total area ratio of the carbides, nitrides and carbonitrides having circle equivalent diameters of 0.1 μm or more and 1.0 μm or less is 5% or more and a number density is 0.1 particles/μm The density is ² or more and ¹⁰ or less per μm2. Each requirement will be described below.

(The structure is tempered martensite and retained austenite in the range from the outermost surface to a depth of 200 μm.)
(In the range from the outermost surface to a position having a depth of 100 μm or more and 200 μm or less, the ratio of the retained austenite structure is 10 vol% or more and 70 vol% or less, and the half-width of the X-ray diffraction peak in the (211) plane is 5.5° or more)
(At a depth of 150 μm from the outermost surface, the Vickers hardness is 600 HV or more)
In order to extend the rolling fatigue life of parts, it is necessary to suppress the occurrence of indentations caused by foreign matter getting caught, which is the starting point of flaking, and for this purpose, it is important to make the structure of the carbonitrided steel material a tempered martensite structure and to improve the hardness at the position that will become the outermost surface of the manufactured steel part. Note that in this embodiment, it is assumed that the surface polishing amount of the steel material is about 150 μm when the carbonitrided steel material is processed into a steel part, that is, the position that will become the outermost surface of the steel part is assumed to be a position about 150 μm deep from the outermost surface of the steel material, and the structure etc. at a certain depth from the outermost surface are specified.

In this embodiment, the half-width of the X-ray diffraction peak in the (211) plane, which is an index of the amount of martensite structure, is set to 5.5° or more. Figure 1 shows the results of the inventors using the results of the examples described later to organize the relationship between the above half-width and the Vickers hardness at a position 150 μm deep from the outermost surface of the carbonitrided steel material. The dotted line in Figure 1 indicates an approximation line calculated by the least squares method. From this Figure 1, it can be seen that in order to achieve a Vickers hardness of 600 HV or more at a position 150 μm deep from the outermost surface, the above half-width needs to be 5.5° or more. The half-width is preferably, for example, 6.0° or more. On the other hand, the upper limit of the half-width can be, for example, about 7.5°. In this embodiment, it is preferable that the structure other than the retained austenite is tempered martensite, and the area ratio of tempered martensite to the entire structure is preferably 30% or more, more preferably 50% or more. The area ratio of tempered martensite can be determined, for example, by subtracting the proportion (volume %) of retained austenite from the entire structure. The Vickers hardness at a depth of 150 μm from the outermost surface is preferably 700 HV or more. In this embodiment, a hardness of 600 HV or more at a depth of 150 μm from the outermost surface is evaluated as a high surface hardness of the carbonitrided steel material. In addition, when the above surface hardness is achieved by performing a quenching process, it can also be said to have "excellent surface hardenability."

In the structure of carbonitrided steel, the more retained austenite, the more processing-induced transformation occurs during fatigue progression, increasing the proportion of martensite structure and improving life, so it is necessary to maintain a certain amount or more. Therefore, in the range from the outermost surface to a depth of 100 μm to 200 μm, the proportion of retained austenite structure is 10 volume % or more. The proportion of the retained austenite structure is preferably 20 volume % or more. However, since an excessive increase in retained austenite leads to a decrease in hardness, it is set to 70 volume % or less. The proportion of the retained austenite structure is preferably 50 volume % or less.

(At a depth of 150 μm from the outermost surface, the C concentration is 0.5 to 0.9 mass % and the N concentration is 0.4 to 1.20 mass %)
In order to improve the hardness of the martensite structure after quenching and to improve fatigue properties by precipitation of precipitates, the surface of the steel part requires a certain level of C and N concentration by carburizing and carbonitriding. Therefore, the C concentration is set to 0.5 mass% or more. The C concentration is preferably 0.6 mass% or more. The N concentration is also set to 0.4 mass% or more. The N concentration is preferably 0.5 mass% or more. On the other hand, if the C concentration or N concentration is excessive, the retained austenite structure becomes excessive, which leads to a decrease in hardness. Therefore, the C concentration of the steel material at a depth of 150 μm from the outermost surface is set to 0.9 mass% or less, and the N concentration is set to 1.20 mass% or less. The C concentration is preferably 0.8 mass% or less, and the N concentration is preferably 1.0 mass% or less.

(At a position 150 μm deep from the outermost surface, the average equivalent circle diameter of the carbides, nitrides and carbonitrides is 0.1 μm or more and 1.0 μm or less, the total area ratio of the carbides, nitrides and carbonitrides having an equivalent circle diameter of 0.1 μm or more and 1.0 μm or less is 5% or more, and the number density is 0.1 pieces/ ^μm2 or more and 10 pieces/ ^μm2 or less)
The above carbides, nitrides, and carbonitrides refer to all carbides in which a carbide-forming element is bonded to carbon, all nitrides in which a nitride-forming element is bonded to nitrogen, and carbonitrides in which these are combined, as shown below. Hereinafter, these will be collectively referred to as "precipitates."
Carbide [(Fe, Cr) ₃ C, (Fe, Cr) ₇ C ₃ , Mo ₂ C, VC, etc.]
Nitrides [(Cr, V, Al, Mn, Si)N, etc.]
Carbonitrides [(Fe,Cr) ₃ (C,N), (Fe,Cr) ₇ (C,N) ₃ , Mo ₂ (C,N), V(C,N), etc.]

From the viewpoint of improving rolling fatigue properties, it is necessary to precipitate a certain amount of precipitates in the surface layer of the steel material. However, if coarse precipitates are generated, they may become the starting point of crack generation, or if alloy components such as Cr and Mn are excessively precipitated, the alloy amount of the parent phase may decrease, leading to a decrease in hardenability and a decrease in hardness. Therefore, the average circle equivalent diameter of carbides, nitrides, and carbonitrides is 0.1 μm or more and 1.0 μm or less, the total area ratio of carbides, nitrides, and carbonitrides having circle equivalent diameters of 0.1 μm or more and 1.0 μm or less is 5% or more, and the number density is 0.1 pieces/μm2 ^or more and 10 pieces/ ^μm2 or less. The total area ratio of the precipitates is preferably 10% or more. The upper limit of the total area ratio of the precipitates is, for example, about 15% from the viewpoint of suppressing a decrease in hardenability due to excessive precipitation.

[Structure and properties of steel parts]
The steel structure and properties of steel parts are
In the range from the outermost surface to a depth of 100 μm, the structure is composed of tempered martensite and retained austenite,
At the outermost surface, the proportion of the retained austenite structure is 10 volume % or more and 70 volume % or less, and the half width of the X-ray diffraction peak in the (211) plane is 5.5° or more,
At the outermost surface, the hardness is 600 HV or more in Vickers hardness, the C concentration is 0.5 mass% or more and 0.9 mass% or less, the N concentration is 0.4 mass% or more and 1.20 mass% or less, the average circular equivalent diameter of the carbides, nitrides and carbonitrides is 0.1 μm or more and 1.0 μm or less, the area ratio of the carbides, nitrides and carbonitrides having a circular equivalent diameter of 0.1 μm or more and 1.0 μm or less is 5% or more, and the number density is 0.1 pieces/ ^μm2 or more and 10 pieces/ ^μm2 or less.

In this embodiment, as described above, it is assumed that the position that will be the outermost surface of the steel part is a position approximately 150 μm deep from the outermost surface of the steel material. Carbonitrided steel material and steel parts differ in the manufacturing process in terms of whether or not the part shape is processed, such as by cutting, but the effect of such processing is small on the steel structure and characteristics. Therefore, the steel structure and characteristics of steel parts differ from those of carbonitrided steel material only in the locations where the steel structure and characteristics are measured, and the specifications and preferred ranges of the steel structure and characteristics are the same as those of the above-mentioned carbonitrided steel material.

In the present invention, even when manufacturing large steel parts, it is possible to achieve both high surface hardness and excellent workability during manufacturing. The term "large" refers to a diameter of 20 mm or more in the case of cylindrical steel material such as steel bars. In addition, in the present invention, steel parts differ from carbonitrided steel materials in that they are machined into the shape of parts such as gears. When machined into the shape of parts, the surface of the carbonitrided steel material is cut, and for example, a position about 150 μm deep from the outermost surface of the carbonitrided steel material can become the outermost surface of the steel part.

[Method of manufacturing steel parts]
The method for producing a steel part according to the present invention comprises the steps of:
As a cooling rate measurement condition, a step of roughly forming the material into a shape such that when the material is kept at a heating temperature of 900° C. or less and 800° C. or more for 30 minutes to 1 hour, and then air-cooled from the heating temperature to 300° C. under atmospheric conditions, the material has an average cooling rate of 0.39° C./s or more;
a two-stage normalizing process of heating to a first temperature of 850° C. to 950° C., holding the first temperature for 0.5 hours to 2 hours, then lowering the temperature to a second temperature of 650° C. to 700° C., holding the second temperature for 3.5 hours to 5 hours, and then air-cooling or furnace-cooling to room temperature;
A cutting process;
a carbonitriding process including a first step of holding the material after cutting at a temperature of 900° C. to 950° C. in an atmosphere of Cp: 0.5% to 1.2% for 2 hours to 7 hours, a second step of holding the material at the same temperature as in the first step in an atmosphere of Cp: 0.5% to 0.7% for 2 hours to 4 hours, and a third step of holding the material at a temperature of 800° C. to 900° C. in an atmosphere of Cp the same as in the second step and with an _NH3 content of 6% to 12% for 4 hours to 6 hours;
The method includes a process of performing oil quenching after the carbonitriding process, and then performing tempering. Each process will be described below.

[Rough molding process]
A steel slab having the above-mentioned chemical composition is held at a heating temperature of 900° C. or lower and 800° C. or higher for 30 minutes to 1 hour as cooling rate measurement conditions, and then roughly formed into a shape such that the average cooling rate is 0.39° C./s or higher when air-cooled in the atmosphere from the heating temperature to 300° C. In this embodiment, by using a roughly formed material roughly formed into a shape such that the average cooling rate determined under the above measurement conditions is 0.39° C./s or higher, it is possible to suppress a decrease in surface hardness when carbonitriding and oil quenching, which will be described later, are performed.

Rough forming into a shape that achieves the average cooling rate refers to adjusting the thickness and diameter of the roughly formed material and processing it, for example, hot or cold rolling or forging, so that the average cooling rate is 0.39°C/s or more. When processing, it is necessary to understand in advance the relationship between the size of the steel material and the average cooling rate under the above measurement conditions, for example, as shown in Table 3 and Figure 2 in the examples described later. The dotted line in Figure 2 indicates an approximation line calculated by the least squares method.

The average cooling rate is determined by averaging the degree of temperature drop from the temperature at the time of removal from the heating furnace to 300°C. It may be measured by the method described in the examples below, or it may be determined from the formula: average cooling rate = (cooling start temperature - cooling end temperature) / cooling time (where the cooling start temperature is a heating temperature of 900°C or less and 800°C or more, and the cooling end temperature is 300°C).

If the average cooling rate under the above cooling rate measurement conditions is below 0.39°C/s, the surface hardness after carbonitriding and oil quenching may decrease, even if the component composition is controlled as described above. In the manufacturing method of this embodiment, it is necessary to subject a roughly formed material of a size that can ensure the above average cooling rate to carbonitriding or the like.

There are no particular restrictions on the manufacturing conditions for the steel slab, and it may be manufactured under conditions that are normally used. For example, it can be obtained by melting and casting in a converter or the like to obtain a slab having the above-mentioned chemical composition, and then performing rolling such as blooming and bar rolling.

If necessary, after the above-mentioned rough forming process and before the two-stage normalizing process described below, a solution treatment may be performed in which the material is heated at 1200 to 1300°C for 1 to 2 hours and then air-cooled, and annealing may be performed in which the material is heated at 900 to 1000°C for 1 to 2 hours and then air-cooled.

[Two-stage baking process]
The roughly formed material is subjected to a two-stage normalization in which the material is heated at a temperature (first temperature) of 850° C. or more and 950° C. or less for 0.5 hours or less and 2 hours or less as a first stage, then cooled to a temperature (second temperature) of 650° C. or more and 700° C. or less, and then heated at the second temperature for 3.5 hours or more and 5 hours or less as a second stage, and then air-cooled or furnace-cooled to room temperature.

In the first heating stage, the steel must be held at a temperature and time such that the entire steel is in the austenite region (A3 point) or higher and the crystal grains do not become coarse. From this viewpoint, the temperature is set to 850°C or higher and 950°C or lower, and the holding time is set to 0.5 hours or higher and 2 hours or lower. In the second heating stage, the steel must be held in a temperature range just below the A1 point in order to homogenize the structure while converting it to ferrite + pearlite. From this viewpoint, the steel is held at a temperature of 650°C or higher and 700°C or lower for 3.5 hours or higher and 5 hours or lower. If the normalizing time in the second heating stage is too short, the proportion of bainite structure increases and the hardness increases, leading to increased cutting time and increased tool wear. The holding time in the second heating stage is preferably 4.0 hours or higher.

In this embodiment, the two-stage normalizing process reduces the bainite structure in the structure of the roughly formed material, lowering hardness and suppressing deterioration in forgeability and machinability, making it easier to perform processing after this process. By performing this normalizing process after rough forming for processing into part formation, the hardness can be reduced, making it easier to perform cutting processing, for example. From the viewpoint of manufacturing costs, it is desirable to have the holding time for each stage as short as possible within the above range. In this specification, "holding" includes cases where the temperature and atmosphere conditions are constant, as well as cases where at least one of the temperature and atmosphere conditions varies within the specified range.

The steel material for carbonitriding treatment according to this embodiment is obtained by carrying out this two-stage normalizing process.

If necessary, after the two-stage normalizing and before the cutting process described below, a forging process such as cold forging, warm forging, or hot forging may be performed. The steel material for carbonitriding according to this embodiment has excellent workability, so it can be forged well in this forging process.

[Cutting process]
After the two-stage normalizing, cutting is performed to obtain the part shape. The cutting can be performed by a commonly used method. According to the manufacturing method of this embodiment, the two-stage normalizing reduces the bainite structure in the structure of the roughly formed material, and a desired mixed structure of ferrite and pearlite is obtained, suppressing the internal hardness, allowing the material to be cut well. In other words, the steel material for carbonitriding obtained by carrying out the two-stage normalizing process can be cut well in this cutting process.

[Carbonitriding process]
After cutting, a carbonitriding treatment is performed including a first step of holding the material at a temperature of 900° C. to 950° C. and in an atmosphere with a carbon equivalent Cp of 0.5% to 1.2% for 2 hours to 7 hours, a second step of holding the material at the same temperature as in the first step and in an atmosphere with a carbon equivalent Cp of 0.5% to 0.7% for 2 hours to 4 hours, and a third step of holding the material at a temperature of 800° C. to 900° C. and in an atmosphere with the same carbon equivalent Cp as in the second step and an _NH3 content of 6% to 12% for 4 hours to 6 hours.

The carbon equivalent Cp (%) can be calculated by the following formula.
Cp = (proportion of carbon monoxide in the heat treatment furnace) ² × (amount of carbon on the surface of the steel piece)/{(equilibrium constant at each temperature) × (proportion of carbon dioxide in the heat treatment furnace)}
In the above formula, the units of the proportions of carbon monoxide and carbon dioxide in the heat treatment furnace are volume %, and the unit of the surface carbon amount of the steel slab is mass %.

By performing carbonitriding under the above conditions and quenching as described below, it is possible to realize both the amount of retained austenite (residual γ) and the form of precipitates specified for the steel part, and as a result, it is possible to provide a steel part that has excellent rolling fatigue properties while maintaining hardness. Note that the above " _NH3 amount" indicates the ratio to the amount of base gas (RX gas) introduced into the furnace, and can be adjusted with a flow meter.

[Quenching and tempering process]
After the carbonitriding treatment, oil quenching is performed at, for example, 100 to 140° C., and then tempering is performed. The tempering may be performed at, for example, 150 to 200° C. By performing the oil quenching and tempering, the carbonitrided steel material according to this embodiment is obtained.

In addition to the above steps, the manufacturing method of the steel part of this embodiment may include one or more of a finishing process and cold forging and hot forging for part molding. The above finishing process, etc., process the steel part into the shape of the product.

The present invention will be described in more detail below with reference to examples. The present invention is not limited to the following examples, and can be modified as appropriate within the scope of the above and below-described aims, and all such modifications are within the technical scope of the present invention.

[Steel manufacturing]
Steels having the chemical compositions shown in Table 1 were melted in a VIF furnace or a converter, and ingots were obtained by the steel ingot process. In Tables 1 and 2, underlined values indicate values outside the range of the present invention.

The obtained ingot was heated to 1200°C to 1300°C for 60 minutes or more, then hot-rolled or hot-forged, and then allowed to cool. It was then heated to 800°C to 1000°C, and hot-rolled or hot-forged to produce a steel bar with a diameter of 45 mm.

The steel bar was cut lengthwise to a thickness of approximately 10 mm to obtain a disk-shaped sample. The disk-shaped sample was heated and held in an electric furnace at a temperature range of 850°C to 950°C for 0.5 to 2 hours (first heating and holding stage), then cooled to a temperature range of 650°C to 700°C and held at that temperature range for 3.5 to 5 hours (second heating and holding stage), in a two-stage normalizing process, to obtain a disk-shaped steel material before carbonitriding.

[Evaluation of steel before carbonitriding treatment]
(Observation of the structure of steel before carbonitriding treatment)
The disk-shaped steel material was embedded and polished, and then etched with a nital solution. The outermost surface of the side (curved surface) of the disk-shaped steel material and the structure of the diameter/4 part of the disk-shaped steel material were observed at magnifications of 100 times and 400 times, respectively, to determine whether the structure was a bainite structure or a ferrite + pearlite structure. As a result, the outermost surface of the side (curved surface) of the disk-shaped steel material and the structure of the diameter/4 part of the disk-shaped steel material of No. 1 and 2 were both ferrite + pearlite structures. Note that, since the Mn content and the amount of other alloy elements of No. 4 and No. 7 are equal to or less than the same, they are considered to be ferrite + pearlite structures like No. 1 to 2. On the other hand, since the Mo or Cr content of No. 3 and No. 5 to 6 is higher than that of No. 1 and 2, it is possible that the second heating holding time in the two-stage normalizing process must be longer than that of No. 1 and 2, within the specified range, in order to obtain a ferrite + pearlite structure.

(Measurement of the hardness (internal hardness) of steel before carbonitriding treatment)
After embedding and polishing the steel material, the Vickers hardness of the steel material at the diameter (D)/4 portion was measured as the internal hardness in accordance with JIS Z 2244. The internal hardness of 200 HV or less was evaluated as excellent in workability. As a result, the Vickers hardness of the steel material at the diameter (D)/4 portion of No. 1 and No. 2 was both 176 HV.

The effect of the two-stage normalizing conditions and Mn content on the internal hardness after two-stage normalizing was evaluated. Using each of the steel pieces No. 1 and No. 2, the holding time at 650°C in the second stage of the above-mentioned two-stage normalizing process was changed, and the structure and internal hardness of the resulting steel material before carbonitriding treatment were determined as described above. The results are shown in Figure 3 for No. 2 (Mn content 1.20%) and Figure 4 for No. 1 (Mn content 1.45%). From the results in Figures 3 and 4, it can be seen that the minimum holding time in the second stage of two-stage normalizing to achieve 200HV or less in No. 2 (Mn content 1.20%) is 2.5 hours, and that the minimum holding time in the second stage of two-stage normalizing to achieve 200HV or less in No. 1 (Mn content 1.45%) is 3.5 hours. From the results of Fig. 3 and Fig. 4, it can be seen that the more the Mn content, the easier it is for bainite to form, and therefore the holding time during normalizing must be longer. When the Mn content is in the range of 0.40 mass% to 1.50 mass%, and the other components are equivalent to those of No. 1-2 or No. 7, in order to reliably obtain a steel material with excellent workability and an internal hardness of 200 HV or less in Vickers hardness, it is preferable to hold the second stage of the two-stage normalizing for 3.5 hours (hr) or more. Note that, since the Mn content of steels No. 4 and No. 7 is less than that of No. 1 and 2, and the other alloy components are equivalent or less, it is estimated that the holding time of the second stage of the two-stage normalizing to suppress the internal hardness is sufficient for 2 hours (hr) or less. On the other hand, in the case of steels No. 3 and No. 4, the holding time of the second stage of the two-stage normalizing to suppress the internal hardness is sufficient for 2 hours (hr) or less. For Nos. 5 and 6, the Mo or Cr content is higher than Nos. 1 and 2, so in order to suppress the internal hardness, it may be necessary to make the holding time for the second stage of the two-stage normalizing longer than Nos. 1 and 2 above, within the specified range.

[Manufacturing of carbonitrided steel]
From the above steel bar, cylindrical test pieces with a diameter of 12 mm and a length of 36 mm and a length of 26 mm and a length of 52 mm were obtained by machining. In this example, in order to simulate a state in which the cooling rate slows down due to the increase in size of the part and the hardenability deteriorates, cylindrical test pieces with a diameter of 12 mm and a cylindrical test piece with a diameter of 26 mm, which is larger than the diameter of the test pieces, were prepared.

Using each of the cylindrical test pieces, in the first step, the specimens were held at a temperature of 900°C to 950°C and an atmosphere of carbon equivalent Cp: 0.5% to 1.2% for 2 to 7 hours, and in the second step, the specimens were held at the same temperature as in the first step and an atmosphere of carbon equivalent Cp: 0.5% to 0.7% for 2 to 4 hours, and then in the third step, the specimens were held at a temperature of 800°C to 880°C and an atmosphere of the same carbon equivalent Cp concentration as in the second step and an NH ₃ content of 6% to 12% for 4 to 6 hours to perform carbonitriding. Thereafter, the specimens were oil quenched at 140°C and tempered at 160°C for 2 hours to obtain a carbonitrided steel material. The Cp value can be derived by the above-mentioned formula, but in actual operation, the carbon monoxide partial pressure was always adjusted to be constant, and the carbon dioxide partial pressure was varied to control the carbon equivalent Cp value to a target value. The amount of NH ₃ mentioned above indicates the ratio to the amount of base gas (RX gas) introduced into the furnace, and was adjusted by a flow meter.

[Evaluation of carbonitrided steel]
(Measurement of carbon and nitrogen content in the surface layer)
Using an electron beam microprobe X-ray analyzer (Electron Probe X-ray Micro Analyzer: EPMA, product name "JXA-8500F") manufactured by JEOL Datum, a line analysis was performed at a pitch of 10 μm from the outermost surface of the side surface of the cylindrical test piece to a depth of about 3.0 mm in the direction of the central axis, to measure the carbon content and nitrogen content. The average value of the concentrations at a total of five points from the position of 130 μm to the position of 170 μm deep from the outermost surface was taken as the concentration at a position of 150 μm deep from the outermost surface. As described above, in this embodiment, it is assumed that the surface polishing amount of the steel is about 150 μm when the carbonitrided steel material is processed into a steel part. Therefore, the carbon content and nitrogen content measured above are also the carbon content and nitrogen content of the outermost surface of the steel part.

(Evaluation of the morphology of precipitates in the surface region)
In order to observe a position 150 μm deep from the outermost surface of the side surface of a cylindrical test piece having a diameter of 12 mm in the direction of the central axis, the embedding material in which the steel material is embedded in resin was corroded with picral liquid, and the above position was observed with a scanning electron microscope (SEM) at a magnification of 10,000 times. Observation was performed in three fields of view (one field size was 12.0 μm × 8.6 μm). Then, one micrograph of one of the visual fields was used to measure the average equivalent circle diameter (also referred to as "average particle size") (μm) of the carbides, nitrides and carbonitrides, the total area ratio (%) of carbides, nitrides and carbonitrides having an equivalent circle diameter of 0.1 to 1.0 μm, which is the ratio of the total structure to the total area ratio, and the number density (pieces/μm 2 ) of carbides, nitrides and carbonitrides having an equivalent circle diameter of 0.1 to 1.0 μm, using particle analysis software (manufactured by SUMITOMO METAL TECHNOLOGY, product name: "Particle Analysis III for Windows. Version ^3.00 "). In determining the average equivalent circle diameter, carbides, nitrides and carbonitrides having an equivalent circle diameter of 0.01 μm or more were targeted.

Under the specified carbonitriding conditions, carbides are hardly precipitated and nitrides are predominant, so the amount of precipitates is proportional to the amount of N. There is no significant difference in the amount of N between steels of different sizes, and the size of the precipitates is determined at the carbonitriding stage. Since it is not thought that differences in cooling rate, i.e., the size of the steel, will cause significant differences in the measured values of the precipitates, data was only collected for test pieces with a diameter of 12 mm regarding the morphology of the precipitates. As mentioned above, it is assumed that the surface polishing amount of the steel is approximately 150 μm when the carbonitrided steel is processed into a steel part. Therefore, the average circle equivalent diameter, total area ratio, and number density of the precipitates measured above are also the average circle equivalent diameter, total area ratio, and number density of the outermost surface of the steel part, respectively.

(Observation of the structure of the surface region)
The cylindrical test piece was embedded and polished so that the diameter direction of the test piece could be observed, and then etched with a nital solution. The structures of the outermost surface and diameter/4 part of the steel were observed at magnifications of 100x and 400x, respectively. The results are shown in Table 2, with tempered martensite as M, retained austenite as γ, and ferrite as F. As mentioned above, it is assumed that the surface polishing amount of the steel is about 150 μm when the carbonitrided steel is processed into a steel part. Therefore, the observation results of the above structure are also the observation results of the structure in the range from the outermost surface to a depth of 100 μm of the steel part. Since the structures of the outermost surface and diameter/4 part were both as shown in Table 2, it can be said that the structure shown in Table 2 is also in the range from the outermost surface to a depth of 200 μm.

(Measurement of the amount of retained austenite in the surface layer and the half-value width)
Electrolytic polishing was performed on five points at depths of 0 μm (outermost surface), 10 μm, 25 μm, 50 μm, and 100 μm from the outermost surface of the side surface of the cylindrical test piece toward the central axis, and the amount of retained austenite and the half-value width were measured using an X-ray measuring device (manufactured by Rigaku Corporation, product name "AutoMATE"). The measurement conditions were a slit diameter of φ1.0, a voltage of 40 kV, a current of 40 mA, a measurement angle of 119 to 138°, and an irradiation time of 180 seconds, and the average value of each of the five points was calculated as the value from the outermost surface of the carbonitrided steel material to a depth of 100 μm. As described above, when the carbonitrided steel is processed into a steel part, the amount of surface polishing of the steel is assumed to be about 150 μm, and the amount of retained austenite and the half width are obtained in the range from the outermost surface to a position having a depth of 100 μm to 200 μm. In this embodiment, the amount of retained austenite and the half width are obtained in the range from the outermost surface of the carbonitrided steel to a depth of 100 μm. Therefore, the amount of retained austenite and the half width measured above are also the amount of retained austenite and the half width of the outermost surface of the steel part.

(Measurement of surface hardness)
The carbonitrided steel was cut into test pieces so that the surface carbonitrided layer could be observed, and the test pieces were embedded in resin and polished. Then, in accordance with JIS Z 2244, the Vickers hardness at a depth of 150 μm from the outermost surface of the test pieces and the hardness distribution at 0.1 mm intervals up to a depth of 3.0 mm from the outermost surface were measured. As described above, it is assumed that the surface polishing amount of the steel is about 150 μm when the carbonitrided steel is processed into a steel part, and the hardness value measured above is also the hardness of the outermost surface of the steel part.

In the present invention, steel materials for practical use in parts are deemed to pass if they have a surface hardness of 600HV or more and a half-value width of 5.5° or more. These characteristics are particularly affected by the carbonitriding conditions, especially the C and N amounts in carbonitriding, and are therefore hardly affected by the surface properties after carbonitriding and subsequent quenching and tempering. In this example, the measurement results of both a cylindrical test piece with a diameter of 12 mm and a cylindrical test piece with a diameter of 26 mm were compared, and the cylindrical test piece with a diameter of 26 mm was evaluated as having a high surface hardness when it achieved the desired surface hardness, as an example of a steel material with a large size and a slower cooling rate. These results are shown in Table 2.

These results are considered. Nos. 1 to 4 satisfy the chemical composition including Mn according to this embodiment, and even in the case of a test piece with a large size and a relatively slow cooling rate (a cylindrical test piece with a diameter of 26 mm, with an average cooling rate of 0.39°C/s during cooling under the specified cooling rate measurement conditions), a surface hardness of 600 HV or more and a half-width of 5.5° or more were achieved. In contrast, No. 5 had an Mn content within the specified range, but did not achieve the target surface hardness and half-width because of an excessive Cr content. Also, Nos. 6 and 7 did not achieve the target surface hardness and half-width because of an insufficient Mn content.

[Measurement of average cooling rate]
From the above steel bars, cylindrical test pieces of 6 mm diameter x 18 mm diameter, 12 mm diameter x 36 mm diameter, 18 mm diameter x 45 mm diameter, and 26 mm diameter x 52 mm diameter were obtained. In each cylindrical test piece, a hole of 5 mm diameter for inserting a thermocouple was drilled in the center of the test piece, and the test piece was silver-plated. After the tip of the thermocouple was crimped and inserted into the processed hole and fixed, the test piece was placed in an atmospheric furnace and held at 840 ° C x 30 min as the cooling rate measurement condition. After holding, the test piece was removed from the furnace and cooled (air-cooled) in the air to 300 ° C. The temperature of the test piece was measured every 0.1 seconds, and the average cooling rate when the temperature was lowered from 840 ° C. to 300 ° C. was calculated. The results are shown in Figure 2 and Table 3.

As shown in Table 3 and Figure 2 above, the average cooling rate decreases as the size of the steel increases. This indicates that larger steel has a slow average cooling rate and is more susceptible to poor hardening.

In this embodiment, it was found that if a steel piece satisfying the specified composition is roughly molded into a shape in which the average cooling rate is 0.39°C/s or more when the steel piece is cooled in air from the heating temperature to 300°C under the cooling rate measurement conditions of 900°C or less, the decrease in surface hardness after carbonitriding, oil quenching and tempering can be suppressed. In addition, the inventors conducted a separate study and measured the surface hardness after carbonitriding and quenching using a steel material with a diameter of 40 mm x 80 mm (estimated cooling rate: 0.09°C/s) that satisfies the composition according to this embodiment, but the target surface hardness could not be achieved. From this, in the manufacturing of steel parts in this embodiment, it can be said that the subsequent process needs to be carried out on the roughly molded material that satisfies the composition according to this embodiment and can achieve an average cooling rate of 0.39°C/s or more as determined under the above cooling rate measurement conditions. In the above examples, cylindrical test pieces with diameters of 12 mm and 26 mm were used, and the average cooling rate during cooling was within the specified range under the cooling rate measurement conditions as shown in Table 3.

Using No. 1, 2, 4 and No. 7, which have almost the same composition range of components other than Mn, the relationship between the Mn content and the surface hardness of cylindrical test pieces with a diameter of 26 mm after carbonitriding treatment was summarized. The results are shown in Figure 5. The dotted line in Figure 5 shows an approximation line calculated by the least squares method. From the results in Figure 5, the higher the Mn content, the more the decrease in surface hardness after carbonitriding, that is, the poor quenching, which occurs in steel materials with large sizes and relatively slow cooling rates, is improved. For example, the Vickers hardness of the No. 7 sample was 780 HV for a test piece with a diameter of 12 mm, and the surface hardness was high, but in the case of a steel material enlarged to a diameter of 26 mm, the Vickers hardness decreased to 542 HV. On the other hand, in No. 7, which has the specified composition, especially the Mn content, which is above a certain level, the Vickers hardness of the test piece was 780 HV for a test piece with a diameter of 12 mm, and the surface hardness was improved. For all samples 1 to 4, the Vickers hardness was 600 HV or more for the 12 mm diameter test piece as well as the 26 mm diameter test piece, and high surface hardness was achieved regardless of the size of the steel. In addition to the amount of Mn, the amount of N that penetrates after carbonitriding and the amount of residual γ are thought to affect the surface hardness, but at the level of 600 HV or more required in this embodiment, within the range of this embodiment, these effects are thought to be smaller than that of Mn.

The steel material and steel parts according to this embodiment can be used, for example, as rolling parts such as gear parts and shafts used in various industrial machines and the steel material for said parts, particularly as machine structural parts including rolling contact type gears and the like that are used after carbonitriding treatment and the steel material for said parts, but are not limited to this.

Claims (10)

C: 0.15% by mass or more and 0.30% by mass or less,
Si: 0.36% by mass or more and 1.00% by mass or less,
Mn: 0.40% by mass or more and 1.50% by mass or less,
Cr: 1.20% by mass or more and 1.90% by mass or less,
Mo: 0.30% by mass or more and 0.80% by mass or less,
P: more than 0% by mass and not more than 0.05% by mass,
S: more than 0% by mass and not more than 0.05% by mass,
Al: 0.005% by mass or more and 0.2% by mass or less,
A steel material containing N: more than 0 mass% and 0.050 mass% or less, O: more than 0 mass% and 0.005 mass% or less, with the balance being Fe and unavoidable impurities,
In the range from the outermost surface to a depth of 200 μm, the structure is composed of tempered martensite and retained austenite,
In the range from the outermost surface to a depth of 100 μm or more and 200 μm or less, the ratio of the retained austenite structure is 10 vol % or more and 70 vol % or less, and the half width of the X-ray diffraction peak in the (211) plane is 5.5 ° or more,
At a position 150 μm deep from the outermost surface, the Vickers hardness is 600 HV or more, the C concentration is 0.5 mass % or more and 0.9 mass % or less, and the N concentration is 0.4 mass % or more.1. 20 mass% or less, and further, the average equivalent circle diameter of the carbides, nitrides and carbonitrides is 0.1 μm or more and 1.0 μm or less, and the average equivalent circle diameter of the carbides, nitrides and carbonitrides is 0.1 μm or more and 1.0 μm or less. A steel material having a total area ratio of nitrides, carbides, and carbonitrides of 5% or more and a number density of 0.1 particles/ ^μm2 or more and 10 particles/ ^μm2 or less. C: 0.15% by mass or more and 0.30% by mass or less,
Si: 0.36% by mass or more and 1.00% by mass or less,
Mn: 0.40% by mass or more and 1.50% by mass or less,
Cr: 1.20% by mass or more and 1.90% by mass or less,
Mo: 0.30% by mass or more and 0.80% by mass or less,
P: more than 0% by mass and not more than 0.05% by mass,
S: more than 0% by mass and not more than 0.05% by mass,
Al: 0.005% by mass or more and 0.2% by mass or less,
A steel material containing N: more than 0 mass% and 0.050 mass% or less, O: more than 0 mass% and 0.005 mass% or less, with the balance being Fe and unavoidable impurities,
A steel material whose structure is a mixture of ferrite and pearlite and whose internal hardness is 200HV or less in Vickers hardness. The steel material according to claim 1 or 2 further contains one or more selected from the group consisting of Cu: more than 0 mass% and not more than 1.0 mass%, Ni: more than 0 mass% and not more than 1.0 mass%, and B: more than 0 mass% and not more than 0.0050 mass%. The steel material according to any one of claims 1 to 3 further contains one or more selected from the group consisting of V: more than 0 mass% and 0.50 mass% or less, and Nb: more than 0 mass% and 0.10 mass% or less. C: 0.15% by mass or more and 0.30% by mass or less,
Si: 0.36% by mass or more and 1.00% by mass or less,
Mn: 0.40% by mass or more and 1.50% by mass or less,
Cr: 1.20% by mass or more and 1.90% by mass or less,
Mo: 0.30% by mass or more and 0.80% by mass or less,
P: more than 0% by mass and not more than 0.05% by mass,
S: more than 0% by mass and not more than 0.05% by mass,
Al: 0.005% by mass or more and 0.2% by mass or less,
A steel part containing N: more than 0 mass% and not more than 0.050 mass%, O: more than 0 mass% and not more than 0.005 mass%, with the balance being Fe and unavoidable impurities,
In the range from the outermost surface to a depth of 100 μm, the structure is composed of tempered martensite and retained austenite,
At the outermost surface, the proportion of the retained austenite structure is 10 volume % or more and 70 volume % or less, and the half width of the X-ray diffraction peak in the (211) plane is 5.5° or more,
At the outermost surface, the hardness is 600 HV or more in Vickers hardness, the C concentration is 0.5 mass% or more and 0.9 mass% or less, and the N concentration is 0.4 mass% or more and 1.20 mass% or less, Furthermore, the average equivalent circle diameter of the carbides, nitrides and carbonitrides is 0.1 μm or more and 1.0 μm or less, and the average equivalent circle diameter of the carbides, nitrides and carbonitrides is 0.1 μm or more and 1.0 μm or less. A steel part having a total area ratio of 5% or more and a number density of 0.1 pieces/ ^μm2 or more and 10 pieces/ ^μm2 or less. The steel part according to claim 5, further comprising one or more selected from the group consisting of Cu: more than 0 mass% and not more than 1.0 mass%, Ni: more than 0 mass% and not more than 1.0 mass%, and B: more than 0 mass% and not more than 0.0050 mass%. The steel part according to claim 5 or 6, further comprising one or more selected from the group consisting of V: more than 0 mass% and 0.50 mass% or less, and Nb: more than 0 mass% and 0.10 mass% or less. A method for manufacturing a steel part according to claim 5, comprising the steps of:
C: 0.15% by mass or more and 0.30% by mass or less,
Si: 0.36% by mass or more and 1.00% by mass or less,
Mn: 0.40% by mass or more and 1.50% by mass or less,
Cr: 1.20% by mass or more and 1.90% by mass or less,
Mo: 0.30% by mass or more and 0.80% by mass or less,
P: more than 0% by mass and not more than 0.05% by mass,
S: more than 0% by mass and not more than 0.05% by mass,
Al: 0.005% by mass or more and 0.2% by mass or less,
A steel slab containing N: more than 0 mass% and 0.050 mass% or less, O: more than 0 mass% and 0.005 mass% or less, with the balance being Fe and unavoidable impurities,
As a cooling rate measurement condition, a step of roughly forming the material into a shape such that when the material is kept at a heating temperature of 900° C. or less and 800° C. or more for 30 minutes to 1 hour, and then air-cooled from the heating temperature to 300° C. under atmospheric conditions, the material has an average cooling rate of 0.39° C./s or more;
a two-stage normalizing process in which the steel sheet is heated to a first temperature of 850° C. or more and 950° C. or less, held at the first temperature for 0.5 hours or more and 2 hours or less, then cooled to a second temperature of 650° C. or more and 700° C. or less, held at the second temperature for 3.5 hours or more and 5 hours or less, and then air-cooled or furnace-cooled to room temperature;
A cutting process;
a carbonitriding process including a first step of holding the material after cutting at a temperature of 900° C. to 950° C. and in an atmosphere having a carbon equivalent Cp of 0.5% to 1.2% for 2 hours to 7 hours, a second step of holding the material at the same temperature as in the first step and in an atmosphere having a carbon equivalent Cp of 0.5% to 0.7% for 2 hours to 4 hours, and a third step of holding the material at a temperature of 800° C. to 900° C. and in an atmosphere having the same carbon equivalent Cp as in the second step and an _NH3 content of 6% to 12% for 4 hours to 6 hours;
A method for manufacturing a steel part, comprising the steps of performing oil quenching after carbonitriding and then performing tempering. A method for manufacturing a steel part according to claim 6, comprising the steps of:
9. The method for producing a steel part according to claim 8, wherein the steel billet further contains one or more selected from the group consisting of Cu: more than 0 mass% and 1.0 mass% or less, Ni: more than 0 mass% and 1.0 mass% or less, and B: more than 0 mass% and 0.0050 mass% or less. A method for manufacturing a steel part according to claim 7, comprising the steps of:
The method for producing a steel part according to claim 8 or 9, wherein the steel slab further contains one or more elements selected from the group consisting of V: more than 0 mass% and 0.50 mass% or less, and Nb: more than 0 mass% and 0.10 mass% or less.

Images