JP2020169371A - Nitriding steel sheet - Google Patents

Abstract

To provide a nitriding steel sheet having excellent hole expandability.SOLUTION: A nitriding steel sheet according to the disclosure has a chemical composition of, by mass%, C: 0.03-0.15%, Si: 0.01-0.50%, Mn: 0.20-0.64%, P: 0.050% or less, S: 0.030% or less, Al: 0.01-0.10%, Cr: 0.5-3.0%, V: 0.02-0.30%, N: 0.008% or less, Mo: 0-0.30%, Cu: 0-0.30%, and Ni: 0-0.30%, with a balance comprising Fe and impurities, the ratio of the solid solution V content (mass%) in the nitriding steel sheet to the V content (mass%) in the chemical composition being 50.0% or more, and the total area ratio of ferrite and pearlite in the microstructure of the nitriding steel sheet being 80.0% or more.SELECTED DRAWING: None

Description

The present disclosure relates to a steel sheet for nitriding. In the present specification, "nitriding" includes not only "nitriding" in which nitrogen (N) is invaded and diffused, but also "soft nitriding" in which N and carbon (C) are invaded and diffused. In the following description, "nitriding" and "soft nitriding" are included and simply referred to as "nitriding".

Mechanical structural parts used in automobiles, motorcycle transmissions, etc. are usually subjected to surface hardening treatment from the viewpoints of improving bending fatigue strength, pitching strength, and wear resistance. Typical surface hardening treatments include carburizing quenching, induction hardening, and nitriding.

Carburizing and quenching is a process in which a low-carbon steel material or a low-alloy steel material is generally used, and C is invaded and diffused in an austenite region having ₃ or more points of Ac, and then quenched. Carburizing and quenching provides high surface hardness and a deep effective hardening layer. However, in carburizing and quenching, the structure of the steel material is deformed, so that there is a problem that the shape of the steel material after carburizing and quenching is deformed. Therefore, when high dimensional accuracy is required for mechanical structural parts, it is essential to perform straightening and tempering (for example, press tempering) of the steel material after carburizing and quenching, which increases the number of manufacturing steps and the manufacturing cost.

Induction hardening is a process in which quenching is performed by rapidly heating to an austenite region of ₃ points or more, and then cooling and quenching. Induction hardening is relatively easy to adjust the depth of the effective hardened layer, but it is not a surface hardening treatment that penetrates and diffuses C like carburizing hardening. Therefore, in order to obtain the required surface hardness, effective hardened layer depth, and core hardness, a medium carbon steel material having a higher C content than the carburized steel material is used. However, the medium carbon steel material has a problem that the hardness of the material is higher than that of the low carbon steel material and it is difficult to process.

In contrast, nitride, at a temperature of 400 to 650 ° C. below A _c1 point is a process of obtaining a high surface hardness and moderate effective case depth by intrusion and diffusion of N. Nitriding has a lower processing temperature than carburizing and induction hardening. Therefore, the heat treatment deformation of the steel material is small. Furthermore, nitrocarburizing Among the nitride at temperatures below 500 to 650 ° C. A _c1 point, a process to obtain a high surface hardness by penetration and diffusion of N and C, the number processing time hours and short Therefore, it is suitable for mass production. Therefore, nitriding is an effective surface hardening treatment for increasing the surface hardness of mechanical structural parts.

Nitriding steel materials used as materials for nitriding parts manufactured by performing nitriding have been proposed in Japanese Patent Application Laid-Open No. 2013-194301 (Patent Document 1) and Japanese Patent Application Laid-Open No. 2013-185186 (Patent Document 2). There is.

The bainite steel material disclosed in Patent Document 1 has a mass% of C: more than 0.15% and 0.35% or less, Si: 0.20% or less, Mn: 0.10 to 2.0%, P: 0.030% or less, S: 0.050% or less, Cr: 0.80 to 2.0%, V: 0.10 to 0.50%, Al: 0.01 to 0.06%, N : 0.0080% or less and O: 0.0030% or less, the balance is composed of Fe and impurities, and further has a chemical composition in which Fn1 is 20 to 80 and Fn2 is 160 or more, and the structure is as follows. It has a ferrite pearlite structure, a ferrite bainite structure, or a ferrite pearlite bainite structure, the area ratio of ferrite in the structure is 20% or more, and the V content in the precipitate by extraction residue analysis is 0.10%. It is as follows. Here, Fn1 = (669.3 × log _e C-1959.6 × log _e N-6983.3) × (0.067 × Mo + 0.147 × V), and Fn2 = 140 × Cr + 125 × Al + 235 × V. Is. It is described in Patent Document 1 that the above-mentioned steel material for nitriding has excellent machinability and can obtain high core hardness, high surface hardness, and deep effective hardened layer depth in nitrided parts. ..

The cold forging steel disclosed in Patent Document 2 has a mass% of C: 0.01 to 0.15%, Si: less than 0.10%, Mn: 0.10 to 0.50%, P: 0.030% or less, S: 0.050% or less, Cr: 0.80 to 2.0%, V: 0.03% or more and less than 0.10%, Al: 0.01 to 0.10%, N : 0.0080% or less and O: 0.0030% or less, the balance is composed of Fe and impurities, and the chemical composition is Fn1 of 160 or less, Fn2 of 20 to 80, and Fn3 of 160 or more. Have. Here, Fn1 = 399 × C + 26 × Si + 123 × Mn + 30 × Cr + 32 × Mo + 19 × V, and Fn2 = (669.3 × log _e C-1959.6 × log _e N-6983.3) × (0.067 ×). Mo + 0.147 × V), and Fn3 = 140 × Cr + 125 × Al + 235 × V. The above-mentioned steel for cold forging is excellent in cold forging property and machinability after cold forging, and in the cold forged part, high core hardness, high surface hardness and deep effective hardened layer depth are obtained. It is described in Patent Document 2 that it can be obtained.

Japanese Unexamined Patent Publication No. 2013-194301 Japanese Unexamined Patent Publication No. 2013-185186

Among the conventional mechanical structural parts, the mechanical structural parts, which are nitrided parts, are mainly manufactured by hot forging steel bars, forming them into a desired shape, and then nitriding them. That is, the conventional mechanical structural parts (nitriding parts) are manufactured by integral molding using one steel material. However, instead of integral molding, a plurality of parts may be combined to manufacture a mechanical structural part. For example, a pulley of a continuously variable transmission (CVT) used in an automobile or the like includes a fixed sheave and a movable sheave. FIG. 1 shows a cross-sectional view including the central axis of the fixed sheave. With reference to FIG. 1, the fixed sheave 1 includes a disc-shaped sheave portion 10 and a rod-shaped shaft portion 20. As shown in FIG. 1, the fixed sheave 1 is integrally formed by hot forging one steel bar. That is, in FIG. 1, the sheave portion 10 and the shaft portion 20 are integrated.

By the way, the required characteristics of the sheave portion 10 and the shaft portion 20 of the fixed sheave 1 are different. For example, the sheave portion 10 is required to have surface fatigue strength, while the shaft portion 20 is required to have bending fatigue strength. Therefore, it is conceivable that different surface hardening treatments are performed on the sheave portion 10 and the shaft portion 20. However, when the sheave portion 10 and the shaft portion 20 are integrally molded as shown in FIG. 1, different surface hardening treatments cannot be performed. Therefore, the inventors of the present application manufacture the sheave portion 11 and the shaft portion 21 as separate parts as shown in FIG. 2, and then attach the sheave portion 11 to the shaft portion 21 as shown in FIG. , It was considered to manufacture the fixed sheave 50. Hereinafter, the fixed sheave 50 will be referred to as a split fixed sheave 50.

When the sheave portion 11 and the shaft portion 21 are separate parts as in the split fixed sheave 50, the sheave portion 11 and the shaft portion 21 can be made of different materials, or different surface hardening treatments can be performed. To do. Therefore, the degree of freedom in designing the fixed sheave is increased. As described above, when a plurality of parts are combined into one machine structural part as represented by the split and fixed sheave 50, the degree of freedom in designing the mechanical structural part is increased.

By the way, when a plurality of parts are combined into one machine structural part, the part may be drilled. FIG. 4 is a schematic view showing an example of hole expanding processing. With reference to FIG. 4, in the hole expanding process, the steel plate 30 which is the material of the component and has the through hole formed in the center is sandwiched between the die 31 and the plate retainer 32, and the through hole of the steel plate 30 is expanded by the conical punch 33. .. When such a hole expanding process is performed, cracks may occur at the edge of the through hole of the steel plate 30 after the hole expanding process. Here, in the present specification, the resistance to cracking of the edge of the steel sheet 30 after the hole expanding process is referred to as "hole expanding property". Perforation property is a property different from ductility. That is, even a steel material having high ductility may have low hole expandability. When the sheave portion 11 of the split fixing sheave 50 is manufactured from a steel plate, a through hole is formed in the central portion of the steel plate, and then the through hole of the steel plate is widened by a hole expanding process. After that, a sheave-shaped intermediate product is manufactured by press working. Then, the intermediate product is nitrided to manufacture the sheave portion 11. A shaft portion 21 manufactured from steel bar is fitted into a through hole (hole) of the manufactured sheave portion 11 to manufacture a split fixing sheave 50.

When a plurality of parts are combined to manufacture one machine structural part as represented by the above-mentioned divided and fixed sheave 50, it is conceivable that some parts are manufactured by using a steel plate instead of a steel bar. When the hole expanding process is performed in the manufacturing process of the nitrided part using the steel sheet, the steel sheet is required to have excellent hole expanding property.

In the above-mentioned Patent Documents 1 and 2, the material of the mechanical structural parts is steel bar, and no study on hole expandability has been made.

An object of the present disclosure is to provide a nitriding steel sheet having excellent hole expandability.

The steel sheet for nitrided according to the present disclosure has a chemical composition of mass%, C: 0.03 to 0.15%, Si: 0.01 to 0.50%, Mn: 0.25 to 0.64%, P. : 0.050% or less, S: 0.030% or less, Al: 0.01 to 0.10%, Cr: 0.5 to 3.0%, V: 0.02 to 0.30%, N: 0.008% or less, Mo: 0 to 0.30%, Cu: 0 to 0.30%, Ni: 0 to 0.30%, and the balance consists of Fe and impurities.
The ratio of the solid solution V content (mass%) in the nitriding steel sheet to the V content (mass%) in the chemical composition is 50.0% or more.
In the microstructure of the nitriding steel sheet, the total area ratio of ferrite and pearlite is 80.0% or more.

The nitriding steel sheet according to the present disclosure is excellent in hole expandability.

FIG. 1 is a cross-sectional view including the central axis of the fixed sheave. FIG. 2 is a cross-sectional view of a sheave portion and a shaft portion of the split and fixed sheave. FIG. 3 is a cross-sectional view of the split and fixed sheave. FIG. 4 is a schematic view of the hole expanding process. FIG. 5 is a schematic view after completion of the hole expanding process of FIG.

As described above, the present inventors have studied the hole expandability of the nitriding steel plate in order to use the nitriding steel plate as a part of the mechanical structural parts represented by the split-fixed sheave. As a result, the following findings were obtained.

First, the chemical composition and microstructure of the steel sheet in which the surface hardness and the hardness of the core portion became appropriate after the nitriding treatment and the hole expanding property was improved were examined. As a result, the present inventors considered that the hole widening property was enhanced by suppressing the Mn content to 0.64% or less. Therefore, as a result of further examination, the chemical composition was C: 0.03 to 0.15%, Si: 0.01 to 0.50%, Mn: 0.25 to 0.64%, P: in mass%. 0.050% or less, S: 0.030% or less, Al: 0.01 to 0.10%, Cr: 0.5 to 3.0%, V: 0.02 to 0.30%, N: 0 008% or less, Mo: 0 to 0.30%, Cu: 0 to 0.30%, Ni: 0 to 0.30%, and the balance consists of Fe and impurities. In the microstructure, ferrite and pearlite The present inventors considered that if the nitriding steel plate has a total area ratio of 80.0% or more, the surface hardness and the hardness of the core portion become appropriate after the nitriding treatment, and the hole expanding property is improved.

However, even if the nitriding steel sheet has a chemical composition in which the content of each element satisfies the above range and the total area ratio of ferrite and pearlite in the microstructure is 80.0% or more, the hole expandability is low. A case has occurred. Therefore, the present inventors further conducted a study to improve the hole-expandability. Here, the present inventors paid attention to the solid solution V content in the nitriding steel sheet. In a steel sheet for nitriding, the V content (mass%) in the chemical composition of the entire steel sheet is defined as [V] _B. Further, the solid solution V content (mass%) in the steel sheet is defined as [V] _S. Here, the solid solution V amount ratio RV (%) is defined by the following formula.
RV = [V] _S / [V] _B x 100

V-carbide, V-nitride, and V-carbonitride can be produced in the nitriding steel sheet having the above-mentioned chemical composition. Hereinafter, in the present specification, V-carbide, V-nitride, and V-carbonitride are collectively referred to as "V-carbide and the like". Even if the Mn content in the chemical composition of the nitriding steel sheet is 0.64% or less, if a large amount of V-carbide or the like is present in the steel sheet, the hole expandability is lowered due to the V-carbide or the like. If the solid solution V content ratio RV is less than 50.0%, more than half of the V content in the steel sheet is not solid solution, and more than half of the V is precipitated in the steel sheet as V carbide or the like. .. In this case, the hole expanding property is reduced. On the other hand, when the solid solution V content ratio RV is 50.0% or more, more than half of the V content in the steel sheet is solid solution, and the formation of V carbides and the like is suppressed. Therefore, excellent hole expandability can be obtained on the premise that the chemical composition has a chemical composition in which the content of each element is within the above range and the total area ratio of ferrite and pearlite in the microstructure is 80.0% or more. ..

The steel sheet for nitrided product of [1] completed based on the above findings has a chemical composition of% by mass, C: 0.03 to 0.15%, Si: 0.01 to 0.50%, Mn: 0. .20 to 0.64%, P: 0.050% or less, S: 0.030% or less, Al: 0.01 to 0.10%, Cr: 0.5 to 3.0%, V: 0. 02 to 0.30%, N: 0.008% or less, Mo: 0 to 0.30%, Cu: 0 to 0.30%, Ni: 0 to 0.30%, and the balance is from Fe and impurities. Naru,
The ratio of the solid solution V content (mass%) in the nitriding steel sheet to the V content (mass%) in the chemical composition is 50.0% or more.
In the microstructure of the nitriding steel sheet, the total area ratio of ferrite and pearlite is 80.0% or more.

The steel sheet for nitriding according to [2] is the steel sheet for nitriding according to [1], and has a chemical composition of Mo: 0.01 to 0.30%, Cu: 0.01 to 0.30%, and. Ni: Contains one element or two or more elements selected from the group consisting of 0.01 to 0.30%.

Hereinafter, the nitriding steel sheet of the present embodiment will be described in detail. Unless otherwise specified, "%" for an element means mass%.

[Chemical composition]
The chemical composition of the nitriding steel sheet of the present embodiment contains the following elements.

C: 0.03 to 0.15%
Carbon (C) increases the strength of the steel sheet and increases the hardness of the core of the steel sheet after the nitriding treatment. If the C content is less than 0.03%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the C content exceeds 0.15%, the hole expandability of the steel sheet is lowered even if the content of other elements is within the range of the present embodiment. Further, the Vickers hardness at a depth of 0.02 mm from the surface of the steel sheet after the nitriding treatment is less than HV650, and sufficient surface fatigue strength cannot be obtained. Therefore, the C content is 0.03 to 0.15%. The lower limit of the C content is preferably 0.04%, more preferably 0.05%. The preferable upper limit of the C content is 0.13%, more preferably 0.11%, still more preferably 0.10%.

Si: 0.01 to 0.50%
Silicon (Si) dissolves in the steel sheet to increase the hardness of the core of the steel sheet and increase the surface fatigue strength. Si further enhances the hardenability of the steel sheet. If the Si content is less than 0.01%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Si content exceeds 0.50%, the total area ratio of ferrite and pearlite in the microstructure becomes less than 80.0% even if the content of other elements is within the range of this embodiment, and the steel sheet The ability to widen holes is reduced. Therefore, the Si content is 0.01 to 0.50%. The lower limit of the Si content is preferably 0.02%, more preferably 0.03%, still more preferably 0.05%. The preferred upper limit of the Si content is 0.45%, more preferably 0.40%, still more preferably 0.30%.

Mn: 0.25 to 0.64%
Manganese (Mn) dissolves in the steel sheet to increase the hardness of the core of the steel sheet and increase the surface fatigue strength. Furthermore, Mn has a high affinity for N. Therefore, Mn combines with N invading the steel material during the nitriding treatment to form a nitride or a cluster which is a precursor stage of the nitride. As a result, the hardness of the nitrided layer is increased, and the surface fatigue strength is increased. If the Mn content is less than 0.20%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Mn content exceeds 0.64%, the total area ratio of ferrite and pearlite in the microstructure is less than 80.0% even if the content of other elements is within the range of this embodiment, and the steel sheet The ability to widen holes is reduced. Therefore, the Mn content is 0.25 to 0.64%. The preferable lower limit of the Mn content is 0.22%, more preferably 0.24%, still more preferably 0.26%. The preferred upper limit of the Mn content is 0.62%, more preferably 0.60%, still more preferably 0.58%.

P: 0.050% or less Phosphorus (P) is an impurity that is inevitably contained. That is, the P content is more than 0%. P segregates at the grain boundaries and causes grain boundary cracks, which lowers the hole expanding property of the steel sheet. When the P content exceeds 0.050%, the hole expanding property of the steel sheet is remarkably lowered even if the content of other elements is within the range of the present embodiment. Therefore, the P content is 0.050% or less. The preferred upper limit of the P content is 0.030%, more preferably 0.020%. It is preferable that the P content is as low as possible. However, excessive reduction of P content raises manufacturing costs. Therefore, when considering normal industrial production, the preferable lower limit of the P content is 0.001%, more preferably 0.002%, still more preferably 0.005%.

S: 0.030% or less Sulfur (S) is an impurity that is inevitably contained. That is, the S content is more than 0%. S segregates at the grain boundaries and causes grain boundary cracks, which lowers the hole expanding property of the steel sheet. When the S content exceeds 0.030%, the hole expanding property of the steel sheet is remarkably lowered even if the other element content is within the range of the present embodiment. Therefore, the S content is 0.030% or less. The preferred upper limit of the S content is 0.020%, more preferably 0.015%. It is preferable that the S content is as low as possible. However, excessive reduction of S content raises manufacturing costs. Therefore, when considering normal industrial production, the preferable lower limit of the S content is 0.001%, and more preferably 0.002%.

Al: 0.01 to 0.10%
Aluminum (Al) deoxidizes steel. Al further combines with N invading from the surface of the steel sheet to form AlN during the nitriding treatment, thereby increasing the hardness of the nitrided layer. If the Al content is less than 0.01%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Al content exceeds 0.10%, coarse Al-based inclusions are generated even if the other element content is within the range of the present embodiment, and the strength of the steel sheet is lowered. Therefore, the Al content is 0.01 to 0.10%. The lower limit of the Al content is preferably 0.02%, more preferably 0.03%, still more preferably 0.04%. The preferred upper limit of the Al content is 0.09%, more preferably 0.08%, still more preferably 0.06%, still more preferably 0.05%.

Cr: 0.5-3.0%
Chromium (Cr) dissolves in the steel sheet to increase the hardness of the core of the steel sheet and increase the surface fatigue strength. During the nitriding process, Cr further combines with N that penetrates the steel material to form a nitride or a cluster that is a precursor stage of the nitride. As a result, the hardness of the nitrided layer is increased, and the surface fatigue strength is increased. If the Cr content is less than 0.5%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Cr content exceeds 3.0%, the hardness of the core portion is saturated. Therefore, the Cr content is 0.5 to 3.0%. The preferred lower limit of the Cr content is 0.6%, more preferably 0.7%, and even more preferably 0.8%. The preferred upper limit of the Cr content is 2.8%, more preferably 2.5%, still more preferably 2.3%.

V: 0.02 to 0.30%
Vanadium (V) combines with N invading from the surface of the steel sheet to form a nitride during the nitriding treatment, and increases the hardness of the nitrided layer. V further increases the fatigue strength of the steel sheet by forming V carbides or the like in a region (core portion or the like) where the amount of nitrogen invaded is relatively small. If the V content is less than 0.02%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the V content exceeds 0.30%, the strength of the steel sheet is saturated even if the content of other elements is within the range of this embodiment. Therefore, the V content is 0.02 to 0.30%. The lower limit of the V content is preferably 0.03%, more preferably 0.05%, still more preferably 0.08%. The preferred upper limit of the V content is 0.25%, more preferably 0.20%, still more preferably 0.15%.

N: 0.008% or less Nitrogen (N) is an impurity that is inevitably contained. That is, the N content is more than 0%. N combines with V and Al before the nitriding treatment to form a nitride. In this case, since the solid solution V content is low, the formation of the nitrided layer is inhibited during the nitriding treatment, and the hardness of the nitrided layer cannot be sufficiently increased. When the N content exceeds 0.008%, the above effect is significantly reduced even if the content of other elements is within the range of the present embodiment. Therefore, the N content is 0.008% or less. The upper limit of the N content is 0.006%, more preferably 0.005%, still more preferably 0.003%. The N content is preferably as low as possible. However, excessive reduction of N content raises manufacturing costs. Therefore, the preferred lower limit of the N content is 0.001%, more preferably 0.002%.

The balance of the chemical composition of the nitriding steel sheet according to the present embodiment consists of Fe and impurities. Here, the impurities are mixed from ore, scrap, manufacturing environment, etc. as a raw material when the nitriding steel sheet is industrially manufactured, and adversely affect the nitriding steel sheet of the present embodiment. Means what is acceptable to the extent that it does not exist.

[About Optional Elements]
The nitriding steel sheet of the present embodiment may further contain one element or two or more elements selected from the group consisting of Mo, Cu and Ni, instead of a part of Fe. Mo, Cu, and Ni are all optional elements, and all of them increase the hardness of the core of the nitriding steel sheet.

Mo: 0-0.30%
Molybdenum (Mo) is an optional element and may not be contained. That is, the Mo content may be 0%. When Mo is contained, Mo increases the hardness of the core portion of the steel sheet after the nitriding treatment. If the Mo content is even a little, the above effect can be obtained to some extent. However, if the Mo content exceeds 0.30%, bainite or martensite is generated even if the content of other elements is within the range of the present embodiment, and the hole expanding property of the steel sheet is lowered. Therefore, the Mo content is 0 to 0.30%. The lower limit of the Mo content is preferably 0.01%, more preferably 0.02%, still more preferably 0.03%. The preferred upper limit of the Mo content is 0.25%, more preferably 0.20%, still more preferably 0.16%.

Cu: 0-0.30%
Copper (Cu) is an optional element and may not be contained. That is, the Cu content may be 0%. When Cu is contained, Cu increases the hardness of the core of the steel sheet after the nitriding treatment. If the Cu content is contained even in a small amount, the above effect can be obtained to some extent. However, if the Cu content exceeds 0.30%, the hot workability of the steel sheet is lowered even if the content of other elements is within the range of this embodiment. Therefore, the Cu content is 0 to 0.30%. The lower limit of the Cu content is preferably 0.01%, more preferably 0.05%, still more preferably 0.10%. The preferred upper limit of the Cu content is 0.28%, more preferably 0.25%, still more preferably 0.20%.

Ni: 0 to 0.30%
Nickel (Ni) is an optional element and may not be contained. That is, the Ni content may be 0%. When Ni is contained, Ni increases the hardness of the core portion of the steel sheet after the nitriding treatment. If even a small amount of Ni is contained, the above effect can be obtained to some extent. However, if the Ni content exceeds 0.30%, the effect is saturated. Therefore, the Ni content is 0 to 0.30%. The lower limit of the Ni content is preferably 0.01%, more preferably 0.05%, still more preferably 0.10%. The preferred upper limit of the Ni content is 0.28%, more preferably 0.25%, still more preferably 0.20%.

[Microstructure of nitriding steel sheet]
In the microstructure of the nitriding steel sheet of the present embodiment, the total area ratio of ferrite and pearlite is 80.0% or more. That is, the microstructure of the nitriding steel sheet of the present embodiment is mainly composed of ferrite and pearlite. In the microstructure, if the total area ratio of ferrite and pearlite is less than 80.0%, the content of each element is within the range of this embodiment, and the solid solution V amount ratio RV is 50.0% or more. Even if there is, the hole expanding property of the nitriding steel plate is lowered. If the total area ratio of ferrite and pearlite is 80.0% or more, it is assumed that the content of each element is within the range of this embodiment and the solid solution V amount ratio RV is 50.0% or more. As a result, the hole expandability of the nitriding steel plate is improved. The preferable lower limit of the total area ratio of ferrite and pearlite in the microstructure of the nitriding steel sheet is 85.0%, more preferably 90.0%, still more preferably 95.0%. In the microstructure of the nitriding steel sheet of the present embodiment, the regions other than ferrite and pearlite are, for example, bainite, martensite, retained austenite, precipitates, and inclusions.

[Measurement method of total area ratio of ferrite and pearlite]
The total area ratio (%) of ferrite and pearlite in the microstructure of the nitriding steel sheet of the present embodiment is measured by the following method. When the thickness of the nitriding steel sheet is defined as t (mm), a sample is taken from the surface of the nitriding steel sheet at a depth of t / 4. Of the surfaces of the collected samples, the surface perpendicular to the rolling direction is used as the observation surface. After mirror polishing the observation surface, the observation surface is etched with 2% alcohol nitrate (Nital corrosive liquid). The etched observation surface is observed using a 400x optical microscope to generate a photographic image of any five fields of view. The size of each field of view is 100 μm × 100 μm.

In each field of view, each phase of ferrite, pearlite, bainite, etc. has a different contrast for each phase. Therefore, each phase is identified based on the contrast. Of the identified phase, the total area ([mu] m ²⁾ of the ferrite in each field, and determines the total area of perlite (μm ^2). The ratio of the total area of the total area of ferrite and the total area of pearlite in all fields of view to the total area of all fields of view is defined as the total area ratio (%) of ferrite and pearlite. The total area ratio (%) of ferrite and pearlite shall be the value obtained by rounding off the second decimal place.

[Solid solution V amount ratio RV]
In the nitriding steel sheet of the present embodiment, the ratio of the solid solution V content (mass%) in the nitriding steel sheet to the V content (mass%) in the chemical composition is defined as the solid solution V content ratio RV (%). Define. At this time, the solid solution V amount ratio RV is 50.0% or more.

In the nitriding steel sheet of the present embodiment, the amount of V-carbide and the like in the steel sheet is small in order to suppress the formation of V-carbide and the like as much as possible. Specifically, the solid solution V amount ratio RV is 50.0% or more. In this case, when the hole expanding process is performed on the nitriding steel sheet, processing defects due to V-carbide or the like in the steel sheet are less likely to occur, and the hole expanding property is improved. If the solid solution V amount ratio RV is less than 50.0%, there are many V carbides and the like present in the steel sheet. In this case, the strength of the steel sheet is high, and further, V-carbide or the like hinders uniform forming, so that the hole expanding property is lowered. Further, during the nitriding process, the solid solution V combines with N invading from the surface of the steel sheet to form a nitride to increase the hardness of the nitrided layer, or a region in which the amount of nitrogen invaded is relatively small (core portion, etc.). In, V carbide or the like is formed to increase the strength inside the steel sheet. Therefore, in the nitriding steel sheet of the present embodiment, the solid solution V amount ratio RV is 50.0% or more. The preferable lower limit of the solid solution V amount ratio RV is 55.0%, more preferably 60.0%, still more preferably 70.0%.

[Method of determining solid solution V amount ratio RV]
The solid solution V amount ratio RV of the nitriding steel sheet of the present embodiment can be obtained by the following method. First, the precipitates and inclusions in the nitriding steel sheet are captured as residues. Specifically, a test piece of 10 mm × 10 mm × thickness 5 mm including the t / 2 depth position is collected from the surface of the nitriding steel sheet. Of the test pieces, the thickness direction is the same as the thickness direction of the nitriding steel sheet. A constant current electrolysis is carried out on the collected test piece using a 10% AA solution (a liquid obtained by mixing tetramethylammonium chloride, acetylacetone, and methanol at a volume fraction of 1:10: 100).

More specifically, in order to remove the deposits on the surface of the test piece, first, the above-mentioned 10% AA solution was applied under the conditions of current: 1000 mA, time: 28 minutes, and room temperature (15 to 30 ° C.). Perform the preliminary electrolysis used. After pre-electrolysis, the test piece is immersed in an alcohol solution, and then ultrasonic cleaning is performed to remove deposits on the surface of the test piece. The mass of the test piece from which the deposits have been removed, that is, the mass W1 of the test piece before constant current electrolysis is measured.

Next, a new 10% AA solution is prepared. Then, using a new 10% AA solution, constant current electrolysis is performed on the test piece under the conditions of current: 173 mA, time: 142 minutes, and room temperature. After constant current electrolysis, the test piece is immersed in an alcohol solution, and then ultrasonic cleaning is performed to remove deposits on the surface of the test piece.

The 10% AA solution used for constant current electrolysis and the alcohol solution used for subsequent ultrasonic cleaning are suction-filtered with a filter having a mesh size of 0.2 μm to collect the residue. Further, the mass of the test piece after ultrasonic cleaning (that is, the test piece from which the residue has been removed), that is, the mass W2 of the test piece after constant current electrolysis is measured. Then, the mass W3 of the test piece electrolyzed with a constant current is obtained from the following equation.
Mass W3 = Mass W1-Mass W2

Next, the residue collected on the filter is transferred to a petri dish and dried. Then, to measure the residual mass W _R. Then, the residue is analyzed by an ICP emission spectrometer (high frequency inductively coupled plasma emission spectrophotometer) in accordance with JIS G1258 (2014) to determine the V content [V] _R (mass%) in the residue. Based on the obtained V content [V] _R , the V mass W _V1 in the residue is determined by the following formula.
V weight of residue _{W V1 = W R × [V} ] R

V mass contained in the constant current electrolyzed test piece based on the mass W3 of the constant current electrolyzed test piece and the V content [V] _B (mass%) in the chemical composition of the steel sheet for nitriding. _Find W _V0 .
V mass contained in the constant current electrolyzed test piece W _V0 = W3 × [V] _B

In the constant current electrolyzed test piece based on the mass W3 of the constant current electrolyzed test piece, the V mass W _{V1 in} the residue, and the V mass W _V0 contained in the constant current electrolyzed test piece. The solid-dissolved V content (mass%) of
Solid solution V content (mass%) = (W _V0 −W _V1 ) / W3 × 100

The solid solution V content ratio RV (%), which is the ratio of the solid solution V content (mass%) to the V content [V] _B (mass%) in the chemical composition, is calculated by the following formula.
Solid solution V content ratio RV = solid solution V content (mass%) / [V] _B (mass%) x 100

The nitriding steel sheet having the above configuration has an element content within the range of the present embodiment, a total area ratio of ferrite and pearlite of 80.0% or more in the microstructure, and further, a nitriding steel sheet. The ratio of the V content dissolved in the steel sheet for nitriding (solid solution V content ratio RV) to the V content in the chemical composition of the above is 50.0% or more. Therefore, the formation of V-carbide and the like in the steel sheet is suppressed, and the hole expandability of the nitriding steel sheet is improved. Further, when the nitriding treatment is performed to manufacture the nitriding component, the hardness of the nitrided layer of the nitriding component and the hardness of the core portion of the nitriding component become sufficiently high.

[Manufacturing method of nitriding steel sheet]
An example of the method for manufacturing the nitriding steel sheet of the present embodiment will be described. The method for manufacturing a nitriding steel sheet described below is an example for manufacturing the nitriding steel sheet of the present embodiment. Therefore, the nitriding steel sheet having the above-mentioned structure may be manufactured by a manufacturing method other than the manufacturing methods described below. However, the manufacturing method described below is a preferable example of the manufacturing method of the nitriding steel sheet of the present embodiment.

An example of the method for manufacturing a nitriding steel sheet of the present embodiment includes a material preparation step of preparing a material which is a slab or an ingot, and a hot working step of hot-working the material to manufacture a nitriding steel sheet. Hereinafter, each step will be described.

[Material preparation process]
In the material preparation process, the material for the nitriding steel sheet is prepared. The material referred to here is, for example, a slab or an ingot. When manufacturing a material, for example, the material is manufactured by the following method. First, molten steel having a chemical composition in which the content of each element is within the range of the present embodiment is produced. The refining method of molten steel is not particularly limited, and a well-known method may be used. For example, smelting in a converter (primary smelting) is performed on hot metal produced by a well-known method. Well-known secondary refining is carried out on the molten steel discharged from the converter. In the secondary refining, the addition of alloying elements for adjusting the components is carried out to produce molten steel having a chemical composition in which the content of each element is within the range of the present embodiment.

The material is produced by a well-known casting method using the molten steel produced by the above-mentioned refining method. For example, an ingot is manufactured by an ingot method using molten steel. Further, a slab may be manufactured by a continuous casting method using molten steel. A material (slab or ingot) is manufactured by the above method.

[Hot working process]
In the hot working process, the slab is hot-rolled to produce a steel sheet for nitriding. First, the slab is heated in a heating furnace. The heated slab is hot-rolled using a hot-rolling machine to produce a steel sheet for nitriding. The hot rolling mill includes, for example, a rough rolling mill and a finishing rolling mill located downstream of the rough rolling mill. The rough rolling mill includes one or a plurality of rough rolling stands arranged in a row. Each rough rolling stand contains a plurality of rolls arranged one above the other. The finish rolling mill includes a plurality of finish rolling stands arranged in a row. Each finish rolling stand contains a plurality of rolls arranged one above the other. The heated slab is rolled by a rough rolling mill and then further rolled by a finishing rolling mill to produce a steel sheet. After cooling the steel sheet after hot rolling, the steel sheet is wound into a coil using a winder.

The heating temperature of the slab in the heating furnace in the hot working process is defined as the heating temperature T1 (° C.). Of the finish rolling mills, the temperature of the steel sheet on the outlet side of the finish rolling stand that finally rolls the steel sheet is defined as the finish temperature T2 (° C.). The average cooling rate CR (° C./sec) is defined as the average cooling rate of the finish rolling mills from the time when the finish rolling stand for rolling down the steel sheet is finally rolled out to the time when the steel sheet is rolled up by the winder. The temperature of the steel sheet at the start of winding by the winding machine is defined as the winding temperature T3 (° C.). In this case, the heating temperature T1, the finishing temperature T2, the average cooling rate CR, and the winding temperature T3 are the following conditions.

Heating temperature T1: 1000-1250 ° C
The heating temperature T1 of the slab in the heating furnace is 1000 to 1250 ° C. If the heating temperature T1 is less than 1000 ° C., V carbides and the like in the slab are not sufficiently solid-solved and remain. Therefore, even in the nitriding steel sheet after hot rolling, V carbides and the like remain, and the solid solution V amount ratio RV is less than 50.0%. On the other hand, if the heating temperature T1 exceeds 1250 ° C., the fuel intensity becomes high. Therefore, the heating temperature T1 is 1000 to 1250 ° C. The preferred lower limit of the heating temperature T1 is 1020 ° C, more preferably 1050 ° C. The preferred upper limit of the heating temperature T1 is 1230 ° C, more preferably 1200 ° C. The heating temperature T1 is the temperature (° C.) in the furnace near the extraction port in the heating furnace.

Finishing temperature T2: 780-900 ° C
The finishing temperature T2 is 780 to 900 ° C. If the finishing temperature T2 is less than 780 ° C., V carbides and the like in the steel sheet are generated during rolling, and the solid solution V amount ratio RV is less than 50.0%. On the other hand, if the finishing temperature exceeds 900 ° C., the winding temperature T3 often increases even if the rolled steel sheet is cooled at the average cooling rate CR described later. In this case, a ferrite transformation accompanied by phase interface precipitation of a large number of V carbides or the like occurs, and the solid solution V content is reduced. As a result, the solid solution V amount ratio RV becomes less than 50.0%. Therefore, the finishing temperature T2 is 780 to 900 ° C. The finishing temperature T2 means the temperature of the steel sheet on the outlet side of the finishing rolling stand that finally rolls the steel sheet in the finishing rolling mill. The finishing temperature T2 can be measured by a thermometer arranged on the outlet side of the finishing rolling stand that finally rolls the steel sheet in the finishing rolling mill. The preferred lower limit of the finishing temperature T2 is 790 ° C, more preferably 800 ° C. The preferred upper limit of the finishing temperature T2 is 890 ° C, more preferably 880 ° C.

The finishing temperature T2 further satisfies the following equation (1).
T2- (ST + 300 × V + 100 × Mn-115) ≧ 0 (1)
ST = (9500 / (6.72-LOG ₁₀ (V × C))-273.15) (2)
Here, T2 is the finishing temperature (° C.), and ST is the solid solution temperature of the V carbide or the like represented by the formula (2). The V content (mass%) in the steel sheet is substituted for "V" in the formulas (1) and (2), and the C content (mass%) in the steel sheet is substituted for "C". To.

It is defined as FT2 = T2- (ST + 300 × V + 100 × Mn-115). FT2 is an index showing the solid solution status of V carbide in the steel sheet in finish rolling. As shown in FT2, the precipitation state of V carbides and the like in the steel sheet at the finishing temperature T2 affects the V content, Mn content, and C content in the steel sheet. If the V content, C content and Mn content are high, the V carbide or the like cannot be sufficiently solid-solved unless the finishing temperature T2 is also raised. Specifically, if the finishing temperature T2 is in the range of 780 to 900 ° C. and the FT2 satisfies the formula (1), the V carbide in the steel sheet is sufficiently solid-solved. Therefore, the solid solution V amount ratio RV is 50.0% or more.

Average cooling rate CR: 30 to 80 ° C./sec The average cooling rate CR is 30 to 80 ° C./sec. If the average cooling rate CR is less than 30 ° C./sec, ferrite is formed in a high temperature range, and V carbides and the like are formed due to phase interface precipitation. In this case, the solid solution V amount ratio RV is less than 50.0%. On the other hand, if the average cooling rate CR exceeds 80 ° C./sec, bainite is formed in the microstructure, and the total area ratio of ferrite and pearlite in the microstructure may be less than 80.0%. Therefore, the average cooling rate CR is set to 30 to 80 ° C./sec. The preferred lower limit of the average cooling rate CR is 35 ° C./sec, more preferably 40 ° C./sec. The preferred upper limit of the average cooling rate CR is 75 ° C./sec, more preferably 70 ° C./sec. The average cooling rate CR is obtained by the following method. The time TCR (seconds) from the finishing temperature T2 to the winding temperature T3 is measured. Based on the obtained time, the average cooling rate CR (° C./sec) is calculated by the following formula.
Average cooling rate CR = (T2-T3) / TCR

Winding temperature T3: 450-600 ° C
The take-up temperature T3 is 450 to 600 ° C. If the take-up temperature T3 is less than 450 ° C., bainite is formed in the microstructure, and the total area ratio of ferrite and pearlite in the microstructure may be less than 80.0%. On the other hand, if the take-up temperature T3 exceeds 600 ° C., ferrite is formed in a high temperature range, and V carbides and the like are formed due to phase interface precipitation. In this case, the solid solution V amount ratio RV is less than 50.0%. Therefore, the take-up temperature T3 is 450 to 600 ° C. The preferred lower limit of the take-up temperature T3 is 460 ° C, more preferably 470 ° C. The preferred upper limit of the take-up temperature T3 is 580 ° C, more preferably 560 ° C. As described above, the take-up temperature T3 is the steel plate temperature (° C.) at the start of take-up. The temperature of the steel sheet at the start of winding can be measured by a temperature gauge arranged in the winding machine.

The take-up temperature T3 further satisfies the equation (3).
T3- (300 × Mn + 290) ≧ 0 (3)
Here, the Mn content (mass%) in the steel sheet is substituted for "Mn" in the formula (3).

FT3 = T3- (300 × Mn + 290) is an index showing the state of the microstructure at the time of winding. The higher the Mn content in the steel sheet, the easier it is for bainite to be formed in the high temperature region, and specifically, the total area ratio of ferrite and pearlite in the microstructure tends to be less than 80.0%. If the take-up temperature T3 is in the range of 450 to 600 ° C. and the FT3 satisfies the formula (3), the take-up temperature T3 is appropriate and the total area ratio of ferrite and pearlite in the microstructure is 80. It becomes 0% or more.

The nitriding steel sheet of the present embodiment is manufactured by the above manufacturing method. The above-mentioned method for manufacturing a nitriding steel sheet is an example for manufacturing the nitriding steel sheet of the present embodiment. Therefore, the nitriding steel sheet having the above-mentioned structure may be manufactured by a manufacturing method other than the above-mentioned manufacturing method. However, the above-mentioned manufacturing method is a preferable example of the manufacturing method of the nitriding steel sheet of the present embodiment.

[About nitrided parts]
The nitriding steel sheet of the present embodiment is used as a material for nitriding parts to be drilled. The nitrided component is, for example, a mechanical structural component, for example, as shown in FIGS. 2 and 3, the sheave portion 11 of the split and fixed sheave 50.

The nitriding component includes a nitriding layer formed by nitriding treatment and a core portion inside the nitriding layer. The depth of the nitrided layer is not particularly limited, but the depth from the surface of the nitrided layer is, for example, 0.2 mm to 1.0 mm. The chemical composition of the core portion is the same as the chemical composition of the nitriding steel sheet of the present embodiment.

The nitriding component of the present embodiment is subjected to a well-known nitriding treatment using the nitriding steel plate of the present embodiment so that the Vickers hardness of the nitrided layer and the Vickers hardness of the core portion are within the ranges described below. Can be done. Hereinafter, a method for manufacturing a nitrided part will be described.

A method for manufacturing a nitrided part using a nitriding steel plate includes a machining process, a drilling process, a hole expanding process, a forging (pressing) process, and a nitriding process.

[Machining process]
In the machining process, a coarse steel sheet having a predetermined shape is collected from the nitriding steel sheet. When the nitrided part is the sheave portion 11, a disc-shaped coarse steel plate is collected in the machining process.

[Drilling process]
In the drilling process, a hole is drilled in the rough steel sheet to form a through hole in the rough steel sheet.

[Drilling process]
In the hole expanding process, as shown in FIG. 4, the coarse steel plate 30 having the through holes formed is sandwiched between the die 31 and the plate retainer 32, and the through holes are expanded by the conical punch 33.

[Forging (pressing) process]
In the forging process, a rough-shaped steel sheet that has been subjected to hole expansion processing is forged to produce an intermediate product having a predetermined shape.

[Nitriding process]
In the nitriding step, nitriding is performed on the intermediate product. In the present specification, "nitriding" includes not only "nitriding" in which nitrogen (N) is invaded and diffused, but also "soft nitriding" in which N and carbon (C) are invaded and diffused. In the present specification, "nitriding" and "soft nitriding" are included and simply referred to as "nitriding". When distinguishing between soft nitriding and nitriding, it is called "nitriding that is not soft nitriding".

Nitriding includes, for example, gas nitriding, salt bath nitriding, ion nitriding and the like. Preferably, gas nitriding is performed. In the case of gas nitriding, gas nitriding is performed in an atmosphere containing NH ₃ , H ₂ , and N ₂ . The total time of the nitriding process, that is, the time from the start to the end of the nitriding process is not particularly limited. The processing time of the entire nitriding process is, for example, 1.5 to 10 hours. Nitriding treatment temperature equal to or less than the A _C1 transformation point, for example, 400 to 650 ° C.. More specifically, the nitriding treatment temperature in the case of nitriding other than soft nitriding is, for example, 400 to 550 ° C. In the case of soft nitriding, the nitriding treatment temperature is, for example, 500 to 650 ° C.

The atmosphere of the gas nitriding treatment inevitably contains impurities such as oxygen and carbon dioxide in addition to NH ₃ , H ₂ and N ₂ . A preferable atmosphere contains NH ₃ , H ₂ and N ₂ in a total of 99.5% (volume%) or more.

When soft nitriding is carried out, the atmosphere of the soft nitriding treatment is, for example, a gas atmosphere in which RX gas and ammonia gas are mixed 1: 1. The RX gas is a gas obtained by mixing a hydrocarbon gas such as butane and propane with air and passing it through a heated Ni catalyst to react, and is a mixed gas containing CO, H ₂ , N _{2, and the} like.

[Hardness of nitrided layer of nitrided parts and hardness of core]
In the nitriding parts manufactured by the above-mentioned manufacturing process using the nitriding steel sheet of the present embodiment, the hardness of the nitrided layer and the hardness of the core portion can be in the following ranges.

Vickers hardness HV _{0.02 of the} nitrided layer: 650 or more Among the nitrided layers, the Vickers hardness HV _0.02 at a depth of 0.02 mm from the surface is 650 or more. In this case, the wear resistance of the surface of the nitrided component is increased, and the surface fatigue strength of the nitrided component is further increased. The preferred lower limit of the Vickers hardness HV _0.02 of the nitrided layer is 660, and more preferably 680. The upper limit of the Vickers hardness HV _0.02 of the nitrided layer is not particularly limited, but is, for example, 820, more preferably 800.

Vickers hardness HV _C of the core portion: the Vickers hardness HV _C of the core portion at the center position in the thickness direction of more than 180 nitride component is 180 or more. In this case, the surface fatigue strength of the nitrided component is increased. A preferred lower limit of the Vickers hardness HV _C of the core portion is 182, more preferably from 185.

The Vickers hardness HV _0.02 of the nitrided layer of the nitrided part and the Vickers hardness HV _{C of the} core can be measured by the following methods. The Vickers hardness at any 10 positions of the nitrided layer at a depth of 0.02 mm from the surface is determined by a Vickers hardness test based on JIS Z 2244 (2009). The test force is 0.49N. The average of the obtained 10 Vickers hardnesses is defined as the Vickers hardness HV _{0.02 of the} nitrided layer. Further, the Vickers hardness at any 10 positions at the center position in the thickness direction of the nitrided part is determined by a Vickers hardness test based on JIS Z 2244 (2009). The test force is 0.49N. Average of 10 Vickers hardness obtained is defined as Vickers hardness HV _C of the core portion.

As described above, in the nitriding component manufactured by using the nitriding steel sheet of the present embodiment, sufficient hardness is obtained for both the nitrided layer and the core portion, and excellent surface fatigue strength is expected.

Hereinafter, the effects of one aspect of the present invention will be described in more detail with reference to Examples. The conditions in the following examples are one condition example adopted for confirming the feasibility and effect of the nitriding steel sheet of the present embodiment. Therefore, the present invention is not limited to this one-condition example. The present invention may adopt various conditions as long as the gist of the present invention is not deviated and the object of the present invention is achieved.

A slab having the chemical composition shown in Table 1 was prepared.

"-" In Table 1 means that the content of the corresponding element was below the detection limit. A hot working process was carried out on each slab to produce a nitriding steel sheet having a plate thickness of 5.0 mm. Table 2 shows the heating temperature T1, finishing temperature T2, FT2 value, average cooling rate CR, winding temperature T3, and FT3 value in the hot working process in each test number.

The following test was carried out on the nitriding steel sheet manufactured by the above manufacturing process.

[Microstructure observation test]
When the thickness of the nitriding steel plate of each test number was defined as t (mm), a sample was taken from the surface of the nitriding steel plate at a depth of t / 4. Of the surfaces of the collected samples, the surface perpendicular to the rolling direction was used as the observation surface. After mirror polishing the observation surface, the observation surface was etched with 2% alcohol nitrate (Nital corrosive liquid). The etched observation surface was observed using a 400x optical microscope to generate a photographic image of any five fields of view. The size of each field of view was 100 μm × 100 μm.

In each field of view, each phase of ferrite, pearlite, bainite, etc. has a different contrast for each phase. Therefore, each phase was identified based on the contrast. Of the identified phase, the total area of the ferrite in the field of view ([mu] m ^2), and to determine the total area of perlite (μm ^2). The ratio of the total area of the total area of ferrite and the total area of pearlite in all fields of view to the total area of all fields of view was defined as the total area ratio (%) of ferrite and pearlite. The total area ratio (%) of ferrite and pearlite was a value obtained by rounding off the second decimal place.

[Solid solution V amount ratio RV measurement test]
A test piece of 10 mm × 10 mm × thickness 5 mm including the t / 2 depth position was collected from the surface of the nitriding steel sheet of each test number. Among the test pieces, the thickness direction was the same as the thickness direction of the nitriding steel sheet. A constant current electrolysis was carried out on the collected test pieces using a 10% AA solution (a liquid in which tetramethylammonium chloride, acetylacetone, and methanol were mixed at a volume fraction of 1:10: 100). More specifically, in order to remove the deposits on the surface of the test piece, first, the above-mentioned 10% AA solution was applied under the conditions of current: 1000 mA, time: 28 minutes, and room temperature (15 to 30 ° C.). The preliminary electrolysis used was carried out. After the preliminary electrolysis, the test piece was immersed in an alcohol solution, and then ultrasonic cleaning was performed to remove deposits on the surface of the test piece. The mass of the test piece from which the deposits were removed, that is, the mass W1 of the test piece before constant current electrolysis was measured. Next, a new 10% AA solution was prepared. Then, using a new 10% AA solution, constant current electrolysis was performed on the test piece under the conditions of current: 173 mA, time: 142 minutes, and room temperature. After constant current electrolysis, the test piece was immersed in an alcohol solution, and then ultrasonic cleaning was performed to remove deposits on the surface of the test piece.

The 10% AA solution used for constant current electrolysis and the alcohol solution used for subsequent ultrasonic cleaning were suction-filtered with a filter having a mesh size of 0.2 μm to collect a residue. Further, the mass of the test piece after ultrasonic cleaning (that is, the test piece from which the residue was removed), that is, the mass W2 of the test piece after constant current electrolysis was measured. Then, the mass W3 of the test piece electrolyzed with a constant current was obtained from the following equation.
Mass W3 = Mass W1-Mass W2

Next, the residue collected on the filter was transferred to a petri dish and dried. Then, to measure the residual mass W _R. Then, in accordance with JIS G1258 (2014), the residue was analyzed by an ICP emission spectrometer (high frequency inductively coupled plasma emission spectrophotometer) to determine the V content [V] _R (mass%) in the residue. .. Based on the obtained V content [V] _R , the V mass W _V1 in the residue was determined by the following formula.
V weight of residue _{W V1 = W R × [V} ] R

V mass contained in the constant current electrolyzed test piece based on the mass W3 of the constant current electrolyzed test piece and the V content [V] _B (mass%) in the chemical composition of the steel for nitrided metal. W _V0 was _calculated .
V mass contained in the constant current electrolyzed test piece W _V0 = W3 × [V] _B

In the constant current electrolyzed test piece based on the mass W3 of the constant current electrolyzed test piece, the V mass W _{V1 in} the residue, and the V mass W _V0 contained in the constant current electrolyzed test piece. The solid solution V content (mass%) of the above was determined.
Solid solution V content (mass%) = (W _V0 −W _V1 ) / W3 × 100

The solid solution V content ratio RV (%), which is the ratio of the solid solution V content (mass%) to the V content [V] _B (mass%) in the chemical composition, was calculated by the following formula.
Solid solution V content ratio RV = solid solution V content (mass%) / [V] _B (mass%) x 100
Table 2 shows the obtained solid solution V content (mass%) and the solid solution V amount ratio RV (%).

[Hole openness evaluation test]
A 150 mm × 150 mm plate-shaped test piece was collected from the nitriding steel plate of each test number. One side of the plate-shaped test piece was parallel to the rolling direction, and the other side was parallel to the plate width direction. A through hole having a diameter of Do = 10 mm at the center position of the plate-shaped test piece was prepared by punching with a punch having a diameter of 10 mm and a die having an inner diameter of 11.2 mm. A hole expansion test conforming to JIS Z 2256 (2010) was performed on a plate-shaped test piece having a through hole to evaluate the hole expansion property.

Specifically, the test apparatus shown in FIG. 4 was prepared. The plate-shaped test piece 30 having the through hole formed was sandwiched between the die 31 and the plate retainer 32 and fixed. The inner diameter Dd of the die 31 was set to 40 mm. The punch 33 was a conical punch, and the angle of the tip of the punch 33 was 60 °. A punch 33 was arranged on the opposite side of the through hole of the plate-shaped test piece 30 to the burr 34, and the hole was widened. The hole expanding process was advanced while moving the punch 33, and the punch 33 was stopped when a crack occurred at the edge of the through hole of the plate-shaped test piece 30. As shown in FIG. 5, among the inner diameters of the through holes of the plate-shaped test piece 30 in which the punch 33 was stopped, the inner diameter in the rolling direction and the inner diameter in the plate width direction were measured, and the average value was taken as the inner diameter Dh.

The hole expansion rate (HEL) (%) was calculated by the following formula.
HEL = (Dh-Do) / Do × 100

When the hole expansion rate was 60% or more, it was judged that the hole expansion property was excellent (indicated by "○" in the "HEL" column in Table 2). On the other hand, when the hole expansion rate is less than 60%, it is determined that the hole expansion property is low (indicated by "x" in the "HEL" column in Table 2).

[Nitriding layer and core hardness evaluation test for nitrided parts]
The following nitriding treatment was carried out on the nitriding steel sheets of each test number to manufacture simulated nitriding parts simulating the nitriding parts. Specifically, the nitriding steel sheets of each test number were machined, and test pieces having a size of 10 mm × 30 mm × thickness of 5 mm were collected. The test piece was subjected to gas nitrocarburizing treatment. The atmosphere in the furnace in gas nitrocarburizing was a 1: 1 mixture of a mixed gas of ammonia gas + RX gas. The treatment temperature was 580 ° C. and the temperature was maintained for 120 minutes. After holding, it was immersed in an oil tank at 60 ° C. and oil-cooled. Through the above steps, a simulated nitrided part was manufactured.

The hardness of the nitrided layer and the core of the manufactured simulated nitrided part was measured by the following method. In the simulated nitrided parts of each test number, the Vickers hardness at any 10 points of the nitrided layer at a depth of 0.02 mm from the surface was determined by the Vickers hardness test in accordance with JIS Z 2244 (2009). The test force was 0.49 N. The average of the obtained 10 Vickers hardnesses was defined as the Vickers hardness HV _{0.02 of the} nitrided layer. Further, the Vickers hardness of the core portion at any 10 positions at the center position in the plate thickness direction of the simulated nitrided part was determined by a Vickers hardness test based on JIS Z 2244 (2009). The test force was 0.49 N. Average of 10 Vickers hardness obtained was defined as Vickers hardness HV _C of the core portion. Table 2 shows the obtained Vickers hardness HV _0.02 and HV _C.

[Test results]
With reference to Table 2, in Test Nos. 1-25, the chemical composition was appropriate and the production conditions were also appropriate. Therefore, the total area ratio of ferrite and pearlite was 80.0% or more, and the solid solution V amount ratio RV was 50.0% or more. As a result, the hole expanding rate HEL was 60% or more, and excellent hole expanding property was exhibited. Also, nitride layer hardness HV _0.02 after the nitriding treatment is 650 or more, core hardness HV _C is 180 or more, in the nitride layer and the core portion, sufficient hardness was obtained.

On the other hand, in test number 26, the C content was too low. Therefore, core hardness HV _C after nitriding is less than 180.

In test number 27, the C content was too high. Therefore, the nitriding layer hardness HV _0.02 after nitriding was less than 650. Further, the solid solution V amount ratio RV was less than 50.0%, and the hole expansion ratio HEL was less than 60%.

In test number 28, the Si content was too high. Therefore, the total area ratio of ferrite and pearlite was less than 80.0%, and the hole expansion ratio HEL was less than 60%.

In test number 29, the Mn content was too high. Therefore, the total area ratio of ferrite and pearlite was less than 80.0%, and the hole expansion ratio HEL was less than 60%.

In test number 30, the N content was too high. Therefore, the solid solution V amount ratio RV was less than 50.0%, and the nitriding layer hardness HV _0.02 after nitriding was less than 650.

In test number 31, the S content was too high. Therefore, the hole expansion rate HEL was less than 60%.

In test number 32, the Mn content and the P content were too high. Therefore, the hole expansion rate HEL was less than 60%.

In test number 33, the P content was too high. Therefore, the hole expansion rate HEL was less than 60%.

In test number 34, the finishing temperature T2 was too low. Therefore, the solid solution V amount ratio RV was less than 50.0%. As a result, the hole expansion rate HEL was less than 60%. Further, the nitriding layer hardness HV _0.02 after nitriding was less than 650, and the core hardness HV _C was less than 180.

In test number 35, the finishing temperature T2 was too high. Therefore, the solid solution V amount ratio RV was less than 50.0%. As a result, the hole expansion rate HEL was less than 60%. Further, the nitriding layer hardness HV _0.02 after nitriding was less than 650, and the core hardness HV _C was less than 180.

In test number 36, the take-up temperature T3 was too high. Therefore, the solid solution V amount ratio RV was less than 50.0%. As a result, the hole expansion rate HEL was less than 60%. Further, the nitriding layer hardness HV _0.02 after nitriding was less than 650, and the core hardness HV _C was less than 180.

In test number 37, the take-up temperature T3 was too low. Therefore, the total area ratio of ferrite and pearlite was less than 80.0%, and the hole expansion ratio HEL was less than 60%.

In test number 38, FT2 did not satisfy formula (1). Therefore, the solid solution V amount ratio RV was less than 50.0%. As a result, the hole expansion rate HEL was less than 60%. Furthermore, core hardness HV _C after nitriding is less than 180.

In test number 39, FT3 did not satisfy formula (3). Therefore, the total area ratio of ferrite and pearlite was less than 80.0%. As a result, the hole expansion rate HEL was less than 60%.

The embodiments of the present invention have been described above. However, the embodiments described above are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-described embodiment can be appropriately modified and implemented without departing from the spirit of the present invention.

1 Fixed sheaves 10, 11 Sheaves 20, 21 Shafts 50 Split fixed sheaves

Claims (2)

Nitriding steel plate
The chemical composition is mass%,
C: 0.03 to 0.15%,
Si: 0.01-0.50%,
Mn: 0.25 to 0.64%,
P: 0.050% or less,
S: 0.030% or less,
Al: 0.01 to 0.10%,
Cr: 0.5-3.0%,
V: 0.02 to 0.30%,
N: 0.008% or less,
Mo: 0-0.30%,
Cu: 0-0.30%,
Ni: 0 to 0.30% and
The rest consists of Fe and impurities
The ratio of the solid solution V content (mass%) in the nitriding steel sheet to the V content (mass%) in the chemical composition is 50.0% or more.
In the microstructure of the nitriding steel sheet, the total area ratio of ferrite and pearlite is 80.0% or more.
Nitriding steel plate. The nitriding steel sheet according to claim 1.
The chemical composition is
Mo: 0.01-0.30%,
Cu: 0.01 to 0.30%, and
Ni: Contains one element or two or more elements selected from the group consisting of 0.01 to 0.30%.
Nitriding steel plate.

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