Nitriding steel with excellent core hardness

Abstract

To provide a steel material that has excellent cold forgeability while suppressing reduction in core part hardness after nitridation.SOLUTION: Provided is a nitridation steel that comprises, in mass%, C: 0.20 to 0.45%, Si: 0.1 to 0.4%, Mn: 0.2 to 1.0%, Cr: 1.50 to 2.80%, Mo: 0.03 to 0.30%, Al: 0.005 to 0.300%, N: 0.004 to 0.030, V: 0.08 to 0.30%, the balance: Fe and unavoidable impurities, and, furthermore, among the unavoidable impurities, P: 0.030% or less, S: 0.030% or less, and the steel structure is a structure made of ferrite and carbide, the area ratio of carbide in the structure is 8 to 20%, and the percentage of carbide having an aspect ratio of 3.0 or more is 50% or more in area ratio.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a nitriding steel suitable for materials of parts such as gears used in, for example, automobiles, construction machines, machine tools, etc., that is, suitable for materials of parts used after surface hardening by subjecting to surface hardening treatments such as gas nitriding or gas soft nitriding after cold forging to allow N to penetrate into the surface layer of steel.

Background Art

[0002] As nitriding steel for cold forging, hitherto, for example, in mass%, C: 0.01 to 0.15%, Si < 0.10%, Mn: 0.10 to 0.50%, P ≦ 0.030%, S ≦ 0.050%, 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% and O ≦ 0.0030% are contained, the balance being composed of Fe and impurities, [399×C + 26×Si + 123×Mn + 30×Cr + 32×Mo + 19×V ≦ 160], [20 ≦ (669.3×log e C - 1959.6×log e N - 6983.3)×(0.067×Mo + 0.147×V) ≦ 80] and [140×Cr + 125×Al + 235×V ≧ 160], and a cold forging nitriding steel having such a chemical composition has been proposed (see Patent Document 1). This proposal ensures cold forging properties by reducing the Si content, assuming that if the Si content is too high, it becomes hard and the cold forging property deteriorates.

[0003] Furthermore, a nitrided steel component has been proposed that, as a component made of nitrided steel, contains, by mass%, C: 0.05~0.20%, Si: less than 0.30%, Mn: 1.00% or less, Cr: 0.50~1.50%, Al: 0.040% or less, N: 0.0100% or less, and Ti: 0.50~1.50%, satisfying Ti-4×C-3.4N≧0.20, with the remainder being Fe and impurity elements, and the structure after nitriding treatment following quenching treatment is a tempered martensitic structure, and has a surface hardness of Hv650 or higher and an internal hardness of Hv150 or higher, thereby achieving high surface hardness and deep hardening depth through a short nitriding treatment (see Patent Document 2). However, this proposal requires a large amount of Ti to obtain a deep hardening depth after nitriding. Also, although Cr improves surface hardness, its inclusion reduces the diffusion rate of nitrogen, making it difficult to obtain a deep hardening depth, so the amount of Cr is reduced. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2013-185186 [Patent Document 2] Japanese Patent Publication No. 2004-300472 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] The proposals in the aforementioned Patent Documents 1 and 2 improve cold forging properties by reducing the amount of alloying components added during the nitriding process, which contributes to improving surface hardness. However, it is presumed that in order to achieve core hardness with such steel, it will be necessary to further appropriately control the cold forging conditions and age hardening. Therefore, while ensuring cold forging properties is important, it is not easy to suppress the reduction in core hardness after nitriding.

[0006] Furthermore, Patent Document 2 raises concerns about increased production costs in order to properly control manufacturing conditions, such as the high temperature required for the precipitation treatment.

[0007] Therefore, taking into account the background of nitriding machine structural steel as described above, the problem that the present invention aims to solve is to provide a steel material that has excellent cold forging properties while suppressing the reduction in core hardness after nitriding.

[0008] However, since nitriding does not involve quenching from the austenite region, it is not possible to utilize strengthening through martensitic transformation. Therefore, in order to ensure the desired core hardness in nitrided parts, it is necessary to include a large amount of alloying elements, but on the other hand, this will worsen cold forgeability.

[0009] Furthermore, cold forging materials containing a large amount of alloy components requires prolonged heat treatment, hindering manufacturability. If the content of alloy components that contribute to hardness, such as carbon, is reduced to ensure cold forgeability, the amount of nitride formed during nitriding may be insufficient, potentially resulting in insufficient surface hardness and hardened layer depth. Moreover, recrystallization can occur during the nitriding treatment after cold forging, making it easy to lose the work hardening achieved through cold forging. [Means for solving the problem]

[0010] Therefore, after diligent research, the inventors optimized the balance of Al, V, and Cr components and appropriately controlled the carbides, resulting in the development of a steel that exhibits excellent workability while also possessing superior hardness after nitriding. Specifically, they found that by creating a microstructure before cold forging in which carbides are densely present within the ferrite, it is possible to achieve a hardness suitable for cold forging while suppressing recrystallization of the core due to the subsequent nitriding treatment.

[0011] In other words, the first means for solving the problems of the present invention is a nitride steel comprising, by mass%, C: 0.20~0.45%, Si: 0.1~0.4%, Mn: 0.2~1.0%, Cr: 1.50~2.80%, Mo: 0.03~0.30%, Al: 0.005~0.300%, N: 0.004~0.030%, V: 0.08~0.30%, with the remainder being Fe and unavoidable impurities, further comprising P: 0.030% or less and S: 0.030% or less, and having a microstructure consisting of ferrite and carbides, with a carbide area ratio to ferrite of 8~20%, and carbides with an aspect ratio of 3.0 or more accounting for 50% or more of the carbides by area ratio.

[0012] The second method, in addition to the chemical components described in the first method, contains, as a selective additional component, one or more of the following in mass%, in the form of Nb: 0.10% or less, Ti: 0.020 to 0.200%, and B: 0.0030% or less, with the remainder being Fe and unavoidable impurities. Furthermore, among the unavoidable impurities, P: 0.030% or less, S: 0.030% or less, the steel structure consists of ferrite and carbides, the area ratio of carbides to ferrite is 8-20%, and the proportion of carbides with an aspect ratio of 3.0 or more in the carbides is 50% or more in terms of area ratio, making it a steel for nitriding.

[0013] In other words, the nitriding steel described in the first and second methods has a structure consisting of ferrite + carbides before cold forging into machine parts and before nitriding treatment, with a dense distribution of many carbides with a high aspect ratio.

[0014] Furthermore, the nitriding steel described in the first and second methods may also be characterized in that, when the nitriding steel is cold forged and its surface layer is further nitrided, the value of the relationship (X0-X1) / X0 between the hardness X0 after cold forging (before nitriding) and the surface hardness X1 after nitriding becomes 0.25 or less.

[0015] In other words, the third method consists of, by mass%, C: 0.20-0.45%, Si: 0.1-0.4%, Mn: 0.2-1.0%, Cr: 1.50-2.80%, Mo: 0.03-0.30%, Al: 0.005-0.30%, N: 0.004-0.03%, V: 0.08-0.30%, with the remainder being Fe and unavoidable impurities, and furthermore, among the unavoidable impurities, P: 0.030% or less, S: 0.030% or less, with a steel structure of ferrite and This nitriding steel has a structure consisting of carbides, where the area ratio of carbides to ferrite is 8-20%, and the proportion of carbides with an aspect ratio of 3.0 or more in the carbides is 50% or more in terms of area ratio. When this nitriding steel is cold forged and the surface layer is further nitrided, the value of the relationship (X0-X1) / X0 between the hardness X0 after cold forging and the surface hardness X1 after nitriding is 0.25 or less.

[0016] Furthermore, the fourth method is a nitriding steel characterized in that, in addition to the chemical components described in the first method, it contains, as a selective additional component, one or more of the following in mass%, Nb: 0.10% or less, Ti: 0.020~0.200, and B: 0.0030% or less, with the remainder being Fe and unavoidable impurities, wherein of the unavoidable impurities, P: 0.030% or less and S: 0.030% or less, and the steel structure is composed of ferrite and carbides, the area ratio of carbides to ferrite is 8~20%, and the proportion of carbides with an aspect ratio of 3.0 or more in the carbides is 50% or more in area ratio, and when this nitriding steel is cold forged and the surface layer is further nitrided, the value of the relationship (X0-X1) / X0 between the hardness X0 after cold forging (before nitriding) and the surface hardness X1 after nitriding is 0.25 or less.

[0017] Furthermore, other means include machine structural parts made from nitriding steel described in the first and second means, which are cold-forged and have their surface layers further nitrided, wherein the value of the relationship (X0-X1) / X0 between the hardness X0 after cold forging (before nitriding) and the surface hardness X1 after nitriding is 0.25 or less.

[0018] In other words, the first of the other methods is a steel structure consisting of, by mass%, C: 0.20-0.45%, Si: 0.1-0.4%, Mn: 0.2-1.0%, Cr: 1.50-2.80%, Mo: 0.03-0.30%, Al: 0.005-0.30%, N: 0.004-0.03%, V: 0.08-0.30%, with the remainder being Fe and unavoidable impurities, and furthermore, among the unavoidable impurities, P: 0.030% or less, S: 0.030% or less, and the steel structure is A nitriding steel having a structure consisting of ferrite and carbides, where the area ratio of carbides in the structure is 8-20%, and the proportion of carbides with an aspect ratio of 3.0 or more in the carbides is 50% or more in area ratio, is cold-forged and the surface layer is further nitrided, and the mechanical structural part is such that the value of the relationship between the hardness X0 after cold forging (before nitriding) and the surface hardness X1 (after nitriding) is 0.25 or less.

[0019] Furthermore, the second of the other methods consists of, by mass%, C: 0.20-0.45%, Si: 0.1-0.4%, Mn: 0.2-1.0%, Cr: 1.50-2.80%, Mo: 0.03-0.30%, Al: 0.005-0.300%, N: 0.004-0.030%, V: 0.08-0.30%, and further containing one or more of the following as selective additional components: Nb: 0.10% or less, Ti: 0.020-0.200%, B: 0.0030% or less, with the remainder being Fe and unavoidable impurities, and further unavoidable The nitriding steel is cold-forged and its surface layer is further nitrided, and the steel structure consists of ferrite and carbides, with a carbide area ratio of 8-20% and carbides with an aspect ratio of 3.0 or more accounting for 50% or more of the carbides by area ratio. The nitriding steel is cold-forged and its surface layer is further nitrided, and the value of the relationship (X0-X1) / X0 between the hardness X0 after cold forging and the surface hardness X1 after nitriding is 0.25 or less. [Effects of the Invention]

[0020] In the means of the present invention described above, while having a hardness suitable for cold forging property, recrystallization of the core part due to subsequent nitriding treatment can be suppressed, so it is difficult for the core part hardness to be reduced by nitriding. That is, in the means of the present invention, by making the structure before cold forging a structure in which carbides are densely present in ferrite, without using the strengthening by martensitic transformation, the cold forging property is not deteriorated by the addition of alloying elements, and while having excellent cold forging workability, it is possible to provide a nitriding steel that can secure the core part hardness as desired for machine structural parts after nitriding, and further suppress the reduction of the core part hardness due to nitriding. And since an excessively long heat treatment process is not required for cold forging, the productivity is not deteriorated. Also, a sufficient case depth and surface hardness can be obtained in the surface layer by nitriding treatment.

Brief Description of the Drawings

[0021] [Figure 1] It is a secondary electron image of the steel of the present invention taken with a scanning electron microscope (SEM) of ×10000. The gray part is the ferrite matrix, and the white and bright needle-like parts are carbides. [Figure 2] It is a SEM image of a reference steel composed of ferrite matrix and carbides shown for comparison. The carbides are spherical with a low aspect ratio, and the proportion occupied by the carbides is also small.

Embodiments for Carrying Out the Invention

[0022] Prior to the description of the embodiments for carrying out the invention of the present application, the reasons for defining the chemical composition of the steel of the present invention and the reasons for defining the structure of the nitriding steel, the proportion of carbides, and carbides with an aspect ratio of 3.0 or more will be explained. In addition, % in the chemical composition is mass %.

[0023] C: 0.20 to 0.45% Carbon (C) is a component that increases the hardness of the material. If the C content is less than 0.20%, the hardness of the core after nitriding will decrease, leading to insufficient strength. If the C content exceeds 0.45%, the material hardness will increase too much, reducing workability (machinability, cold workability). Also, if there is too much C, nitrogen diffusion will be inhibited, reducing the depth of the hardened layer. Therefore, the C content should be between 0.20% and 0.45%.

[0024] Si: 0.1~0.4% Si is a necessary component for deoxidation during manufacturing. Insufficient Si can easily lead to inadequate deoxidation during manufacturing, resulting in a decrease in the position of intervening materials. From this perspective, Si should be 0.1% or higher. On the other hand, excessive Si increases the hardness of the material and reduces processability. From this perspective, Si should be 0.4% or higher.

[0025] Mn: 0.2~1.0% Mn is a component that increases the hardness of the material. If the amount of Mn is too low, the core hardness will be insufficient. From this perspective, the amount of Mn should be 0.2% or more. If the amount of Mn is too high, the processability will decrease. From this perspective, the amount of Mn should be 1.0% or less.

[0026] Cr: 1.50~2.80% Cr is a component that increases both surface hardness and material hardness. If Cr is too low, the hardness after nitriding will be insufficient, and the core hardness will also be insufficient. From these perspectives, the Cr content should be 1.50% or more. On the other hand, if Cr is too high, the increased material hardness will actually decrease the workability. Also, the diffusion of nitrogen will be inhibited, resulting in a reduction in the hardened layer depth. From these perspectives, the Cr content should be 2.80% or less.

[0027] Mo: 0.03~0.30% Mo is a component that increases the hardness of the material. If the amount of Mo is too low, the hardness of the core after nitriding will decrease, leading to insufficient strength. Therefore, the amount of Mo should be 0.03% or more. On the other hand, if the amount of Mo is too high, the workability (machinability, cold workability) will decrease due to the increase in material hardness. Therefore, the amount of Mo should be 0.30% or less.

[0028] Al: 0.005~0.300% Al is a useful component for deoxidation during steel manufacturing and also contributes to surface hardness and hardened layer depth after nitriding. If Al is too low, it is likely to lead to insufficient deoxidation during manufacturing, which in turn reduces the position of intervening materials and results in insufficient surface hardness and hardened layer depth after nitriding. From these perspectives, Al should be 0.005% or more. On the other hand, if Al is too high, the formation of coarse nitrides (AlN) will reduce fatigue properties and workability. Therefore, Al should be 0.300% or less.

[0029] N: 0.004~0.030% Nitrogen (N) is a component that provides a pinning effect through the formation of carbonitrides, and in appropriate amounts, it improves toughness and fatigue properties. If N is too low, there will be insufficient fine carbonitrides, causing the crystal grains to coarseen and reducing toughness and fatigue properties. From this perspective, N should be 0.004% or higher. On the other hand, if N is too high, coarse carbonitrides will be formed, reducing fatigue properties and workability. This is because the reduction in nitrides that provide the pinning effect causes the crystal grains to coarseen. From these perspectives, N should be 0.030% or less.

[0030] V: 0.08~0.30% V is a component that contributes to ensuring sufficient hardened layer depth. If V is too low, the hardened layer depth will be insufficient. Therefore, V should be 0.08% or higher. On the other hand, if V is too high, processability will deteriorate and costs will increase. Therefore, V should be 0.30% or lower.

[0031] The remainder of the chemical components defined in this invention consists of Fe and unavoidable impurities. Of the unavoidable impurities, the upper limits for P and S are defined as follows.

[0032] P:0.030% or less P is an unavoidable impurity. Since P promotes grain boundary segregation, it reduces toughness. Therefore, the amount of P, an unavoidable impurity, should be kept below 0.030%.

[0033] S: 0.030% or less S is an unavoidable impurity. If the amount of S exceeds 0.030%, a large amount of coarse MnS will be formed, leading to a decrease in toughness and fatigue strength. Therefore, the amount of S, an unavoidable impurity, should be kept below 0.030%.

[0034] Furthermore, in the present invention, one or more of the following Nb, Ti, and B may be selectively added.

[0035] Nb: 0.10% or less Nb is a useful component for generating carbonitrides that provide a pinning effect, preventing grain coarsening. Excessive Nb increases the amount of coarse carbonitrides and decreases the amount of carbonitrides with the pinning effect, making grain coarsening more likely. Therefore, when adding Nb, the concentration should be 0.10% or less.

[0036] Ti: 0.020~0.200% Ti is a useful component for generating carbonitrides, which provide a pinning effect that prevents grain coarsening. If the amount of Ti is too low, there will be insufficient fine nitrides, making grain coarseness more likely. Also, nitrogen will not be fixed, forming BN and reducing hardenability. From these perspectives, when adding Ti, the amount should be 0.020% or more. On the other hand, if the amount of Ti is too high, the amount of coarse carbonitrides will increase and the amount of carbonitrides with a pinning effect will decrease, making grain coarseness more likely. From this perspective, when adding Ti, the amount should be 0.200% or less.

[0037] B: 0.0030% or less B is an ingredient that increases the hardness of the material. If B is present in excess, the increased hardness of the material will reduce its workability. Therefore, when adding B, the amount should be 0.0030% or less.

[0038] Microstructure of nitriding steel: A structure consisting of ferrite and carbide. The nitriding steel of the present invention has a microstructure consisting of ferrite and carbides before cold forging or nitriding. In the case of nitriding, since quenching from the austenite region is not performed, strengthening by martensitic transformation is not used. However, in the present invention, by normalizing at 730 to 760°C, for example, carbides are densely distributed in the microstructure derived from the lamellar structure of ferrite and carbides, so the movement of the transformation is suppressed. Therefore, recrystallization is delayed when nitriding is performed, and the core hardness can be maintained.

[0039] Area percentage of carbides in the tissue: 8-20% Percentage of carbides with an aspect ratio of 3.0 or higher in the carbide material: 50% or more by area ratio This method compares the area ratios of carbides and ferrites in images captured by a microscope. Specifically, for example, using a scanning electron microscope with x10000, the area ratios of carbides and ferrites within the captured region of a secondary electron image are calculated using image analysis software to determine the area ratio of carbides in the total tissue (ferrite + carbides). In this invention, the area ratio of carbides in the structure is high at 8-20%, and there are many carbides with a high aspect ratio. Therefore, when normalized at point A3 or below, a large number of needle-shaped carbides with a high aspect ratio, derived from the lamellar structure, are densely distributed, and the core hardness is maintained even after nitriding treatment.

[0040] Regarding the hardness X0 before nitriding and the hardness X1 after nitriding, (X0-X1) / X0 ≤ 0.25 This index indicates the degree to which core hardness is maintained after nitriding. When the change in hardness before and after nitriding is small, and the core hardness is maintained after nitriding, the value of (X0-X1) / X0 will be small when comparing the hardness of the steel for nitriding before nitriding with the hardness of the core after nitriding, because the change is small. Therefore, by setting (X0-X1) / X0 ≤ 0.25, it is possible to determine whether the core hardness is maintained by the nitriding treatment.

[0041] Next, embodiments for carrying out the invention will be described using the steel of the present invention as an example. Each of the steels No. 1 to 17 and comparative steels No. 18 to 23 of the present invention, as shown in Table 1, with their respective chemical compositions, and the remainder consisting of Fe and unavoidable impurities, which together make up 100% of the chemical composition, was melted in a 100 kg vacuum induction melting furnace (VIM). Next, 40 mm diameter steel bars were produced by hot forging, and then heat-treated at a temperature of 730-760°C for 4-8 hours as described in Table 2, followed by air cooling. The heat treatment was carried out using a Kanthal furnace in the following procedure: The test material was placed in the furnace set to the predetermined holding temperature, the heating time of the test material was allowed to proceed for 30 minutes, and then it was held for an arbitrary period of time, followed by air cooling or water cooling. The selection of the holding time should take into consideration the amount and dimensions of the steel material being charged into the furnace. Furthermore, after heat treatment, the specimens were processed into predetermined test pieces by cold forging, and then subjected to nitriding treatment at 520°C for 30 hours.

[0042] [Table 1]

[0043] [Table 2]

[0044] For microstructural observation of nitriding steel, the specimen was cut parallel to the rolling direction through its center before cold forging. The cut surface was polished, and the polished surface was etched with Nital solution. Subsequently, the microstructure was observed using an optical microscope to identify ferrite and carbides. Figure 1 shows the secondary electron image captured with a 10,000x scanning electron microscope (SEM).

[0045] Specifically, the area ratio and aspect ratio of carbides were calculated by, for example, using image analysis software to determine the area of ​​carbides and ferrites within the target region of a scanning electron microscope image acquired with a field of view of ×10000, and then determining the area ratio based on the calculation results.

[0046] Furthermore, the aspect ratio of the carbides was determined by calculating the aspect ratio from the length-to-width ratio of the shape of the carbides identified based on images captured with a scanning electron microscope. Then, the proportion of "carbides with an aspect ratio greater than 3" among all the identified carbides was calculated as an area ratio based on the area calculated using image analysis software.

[0047] Furthermore, the microstructure of the inventive steels No. 1 to 17, before cold forging and nitriding, consisted of ferrite and carbides. The area ratio of carbides in the microstructure was 9-20%, and the proportion of carbides with an aspect ratio of 3 or higher within the carbides was 53-80%. The ferrite was densely mixed with numerous needle-shaped carbides with high aspect ratios.

[0048] Furthermore, to evaluate the cold forgeability, the hardness (HRB) of each of the steels No. 1 to 17 of the present invention and No. 18 to 23 of the comparative steels shown in Table 1 was measured using a Rockwell hardness tester after softening heat treatment and before cold forging. Steels with a hardness of less than 90 HRB were evaluated as having excellent cold forgeability. As shown in Table 2, all of the inventive steels had a hardness of 87 HRB or less.

[0049] Next, the surface hardness of the nitriding steel after cold forging and nitriding treatment, the effective hardened layer depth of the nitrided layer after nitriding treatment, and the core hardness after nitriding treatment were evaluated using the following methods. The results are shown in Table 3.

[0050] <Method for evaluating nitriding properties> (1) Surface hardness The hardness distribution from the surface of each of the steels No. 1 to 17 and comparative steels No. 18 to 23 shown in Table 1 was measured using a Vickers hardness tester in accordance with the Japanese Industrial Standard JIS Z2244. In Table 3, surface hardness is defined as the hardness at a depth of 0.05 mm from the surface. This allowed us to measure the hardness after cold forging and the surface hardness after nitriding. Furthermore, the change in hardness due to nitriding was evaluated by determining whether the change was 0.25 or less using the formula (X0-X1) / X0, where X0 is the hardness after cold forging and X1 is the surface hardness after nitriding.

[0051] (2) Curing depth The hardness distribution from the surface of each of the inventive steels No. 1 to 17 and comparative steels No. 18 to 23 shown in Table 1 was measured using a Vickers hardness tester in accordance with the Japanese Industrial Standard JIS Z2244, and the depth of the effective hardened layer is shown in Table 3. In this invention, the depth of the effective hardened layer is defined as the depth to which the hardness is 550 Hv.

[0052] (3) Core hardness after nitriding Test specimens of each of the inventive steels No. 1-17 and comparative steels No. 18-23 shown in Table 1 were cut after nitriding, and the core hardness of the cross-section was measured using a Vickers hardness tester in accordance with JIS Z2244.

[0053] [Table 3]

[0054] As shown in Table 2, the steels No. 1 to 17 of the invention in Table 1 all have a hardness of 87 HRB or less after softening heat treatment, ensuring cold forgeability. Furthermore, the area ratio of carbides is in the range of 8-20%, and the proportion of carbides with an aspect ratio of 3.0 or higher is 53-80% or more in terms of area ratio. Additionally, the distribution of carbides, which appear whitish in the SEM image in Figure 1, confirms that numerous needle-shaped carbides are densely mixed and distributed within the ferrite. As shown in Table 3, all of the inventive steels exhibited a surface hardness of 720-805 Hv after nitriding, and a sufficient hardened layer depth was observed. Furthermore, the change in hardness before and after nitriding (X0-X1) / X0 was 0.21 or less, indicating that the reduction in core hardness compared to before nitriding was suppressed.

[0055] It has been confirmed that when the nitriding steel of the present invention is cold-forged and applied to mechanical structural parts such as gears, the cold forging process can be performed appropriately, and when nitriding is performed, a nitrided layer with sufficient hardness and surface hardness can be obtained with sufficient hardness depth, and the core hardness does not decrease easily after nitriding, thus making it possible to obtain suitable mechanical structural parts.

[0056] Comparative steel No. 18 had an excess of carbon and manganese, resulting in inferior hardness after softening heat treatment and poor cold forgeability. Comparative steel No. 19 has an insufficient V value, resulting in insufficient hardening layer depth due to nitriding. Comparative steel No. 20 has insufficient chromium content, resulting in inadequate surface hardness after nitriding. Comparative steel No. 21 had insufficient carbon content, resulting in insufficient core hardness after nitriding. The core hardness decreased significantly compared to before nitriding due to the effects of the nitriding treatment. Comparative steel No. 22 has poor cold forgeability because its normalizing temperature was low and its carbide area ratio was excessive. Comparative steel No. 23 had a high normalizing temperature and long holding time, resulting in a low proportion of carbides with a high aspect ratio. This led to poor core hardness after nitriding, and the core hardness was significantly reduced due to the nitriding treatment.

Claims

1. Nitriding steel comprising, by mass%, C: 0.20-0.45%, Si: 0.1-0.4%, Mn: 0.2-1.0%, Cr: 1.50-2.80%, Mo: 0.03-0.30%, Al: 0.005-0.300%, N: 0.004-0.030%, V: 0.08-0.30%, with the remainder being Fe and unavoidable impurities, wherein of the unavoidable impurities, P: 0.030% or less, S: 0.030% or less, and the steel structure having a structure consisting of ferrite and carbides, with a carbide area ratio of 8-20%, and carbides with an aspect ratio of 3.0 or more accounting for 50% or more of the carbides by area ratio.

2. In addition to the chemical components described in claim 1, the material contains, as a selective additional component, one or more of the following in mass%, in the form of Nb: 0.10% or less, Ti: 0.020 to 0.200%, and B: 0.0030% or less, with the remainder being Fe and unavoidable impurities. Furthermore, among the unavoidable impurities, P: 0.030% or less, S: 0.030% or less, the steel structure is composed of ferrite and carbides, the area ratio of carbides in the structure is 8 to 20%, and the proportion of carbides with an aspect ratio of 3.0 or more in the carbides is 50% or more in terms of area ratio, for nitriding steel.

3. A nitrided steel having a structure consisting of ferrite and carbides, wherein the steel is composed of, in mass percent, C: 0.20-0.45%, Si: 0.1-0.4%, Mn: 0.2-1.0%, Cr: 1.50-2.80%, Mo: 0.03-0.30%, Al: 0.005-0.30%, N: 0.004-0.030%, V: 0.08-0.30%, with the remainder being Fe and unavoidable impurities, and further, of the unavoidable impurities, P: 0.030% or less, S: 0.030% or less, and the steel structure is composed of ferrite and carbides, with a carbide area ratio of 8-20% in the structure, and carbides with an aspect ratio of 3.0 or more accounting for 50% or more in area ratio within the carbides, and when this nitrided steel is cold forged and the surface layer is further nitrided, the hardness X after cold forging before nitriding is 0 and core hardness X after nitriding 1 The relationship (X 0 -X 1 ) / X 0 Nitriding steel characterized by having a value of 0.25 or less.

4. In addition to the chemical components described in claim 1, the material contains, as a selective additional component, one or more of the following in mass%, in the form of Nb: 0.10% or less, Ti: 0.020 to 0.200%, and B: 0.0030% or less, with the remainder being Fe and unavoidable impurities. Furthermore, among inevitable impurities, P: 0.030% or less, S: 0.030% or less, its steel structure consists of ferrite and carbide, the area ratio of carbide in the structure is 8 - 20%, and the ratio of carbide with an aspect ratio of 3.0 or more in the carbide occupies 50% or more in terms of area ratio. It is a steel for nitriding. When this steel for nitriding is cold forged and then the surface layer is nitrided, the hardness X after cold forging before nitriding 0 and the core hardness X after nitriding 1 satisfy the relational expression (X 0 - X 1 ) / X 0 is 0.25 or less. The steel for nitriding is characterized by this.

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