Nitriding steel and nitrided parts

Abstract

To provide a nitriding steel and a nitriding component having excellent machinability and also high fatigue characteristics after nitriding.SOLUTION: A nitriding steel has a predetermined chemical composition and has a D value, an index of quenchability represented by formula (1), of 8.5-21.0. The metallographic structure has, in area ratio, bainite of 30% or more and less than 95%, proeutectoid ferrite of more than 5%, and perlite of less than 20%. The Vickers hardness is 200-260 HV. There is also provided a nitriding component. Formula (1): D=√C×(1+0.64Si)×(1+4.10Mn)×(1+2.83P)×(1-0.62S)×(1+2.33Cr)×(1+0.52Ni)×(1+3.14Mo)×(1+0.27Cu)×(1+2.5V), where each element symbol in formula (1) denotes the content of each element (mass%).SELECTED DRAWING: None

Description

[Technical field]

[0001] The present disclosure relates to steels for nitriding and nitrided parts. [Background technology]

[0002] Steel that has been nitrided to improve fatigue strength (i.e., nitrided parts) is often used as mechanical parts in automobiles, ships, industrial machinery, etc. When steel is nitrided, a hard nitride layer is formed on the surface, improving fatigue properties. Furthermore, because nitriding is performed at low temperatures, it can reduce distortion compared to other surface hardening processes.

[0003] To improve the fatigue properties of nitrided steel, the nitrided layer can be hardened. However, in parts that are subject to surface fatigue, such as gears, large shear stress is applied from the surface to deep inside, so it can be difficult to improve surface fatigue properties by only hardening the extreme surface layer. Therefore, there is a demand for nitriding steel with high hardness that can improve the hardness of the unnitrided layer (i.e., the unnitrided portion) in addition to the hardness of the nitrided layer.

[0004] For example, technology relating to nitriding steel having a Vickers hardness exceeding 200 HV has been studied, and such technology is disclosed in, for example, Patent Document 1. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2019-218633 A Summary of the Invention [Problem to be solved by the invention]

[0006] As described above, nitriding steel is required to have high hardness, but if it is too hard, the machinability may deteriorate. In addition, nitrided parts obtained from nitriding steel may be required to have not only surface fatigue properties but also rotating bending fatigue properties (hereinafter, surface fatigue properties and rotating bending fatigue properties may be collectively referred to as "fatigue properties").

[0007] Various techniques have been studied, including that described in Patent Document 1, but the current situation is that there are insufficient techniques for achieving both machinability and fatigue properties.

[0008] The present disclosure has been made in consideration of these circumstances, and a problem that one embodiment of the present disclosure aims to solve is to provide a nitriding steel that has excellent machinability and high fatigue properties after nitriding. A problem to be solved by another embodiment of the present disclosure is to provide a nitrided part using the above-mentioned nitriding steel. [Means for solving the problem]

[0009] The present disclosure includes the following aspects. <1> In mass%, C: 0.10-0.23%, Si: 0.10 to 0.50%, Mn: 1.50-2.15%, S: 0.005~0.050%, Cr: 0.50~0.90%, Al: 0.001 to 0.050%, V: 0.05~0.25%, N: 0.0030 to 0.0250%, and P: Contains 0.050% or less, The balance has a chemical composition consisting of Fe and impurities, The D value, which is an index of hardenability represented by the following formula (1), is 8.5 to 21.0, The metal structure is expressed as the area ratio, Bainite is 30% or more and less than 95%; Proeutectoid ferrite is more than 5%; Perlite is less than 20%; Nitriding steel with a Vickers hardness of 200 to 260 HV. D=√C×(1+0.64Si)×(1+4.10Mn)×(1+2.83P)×(1-0.62S)×(1+2.33Cr)×(1+0.52Ni)×(1+3.14Mo)×(1+0.27Cu)×(1+2.5V)...Formula (1) In formula (1), each element symbol indicates the content (mass %) of each element. <2> In mass%, Ti: 0.005% or less, Nb: 0.005% or less, Mo: 0.10% or less Cu: 0.30% or less, Ni: 0.30% or less, Ca: 0.0050% or less, Pb: 0.09% or less, and Bi:0.20% or less Contains one or more of the following: <1> The nitriding steel according to claim 1. <3> <1> or <2> and a surface layer having a higher N content than the chemical composition of the core. Effect of the Invention

[0010] According to one embodiment of the present disclosure, a nitriding steel is provided that has excellent machinability and high fatigue properties after nitriding. According to another embodiment of the present disclosure, there is provided a nitrided part using the above-mentioned nitriding steel. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic diagram showing a rotating bending fatigue test piece used in the examples. [Diagram 2] FIG. 2 is a schematic diagram showing a small roller test piece used in the examples. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, an embodiment that is an example of the present disclosure will be described in detail. The "%" for the content of each element means "mass %." The content of each element in a chemical composition may be expressed simply as "amount" or "concentration." For example, the content of C may be expressed as C amount or C concentration. A numerical range expressed using "~" means that the range includes the numerical values ​​before and after "~" as the lower and upper limits. The term "process" includes not only an independent process but also a process that cannot be clearly distinguished from other processes as long as the intended purpose of the process is achieved. "Nitriding steel" is sometimes called "steel" or "steel material." "Vickers hardness" is sometimes simply called "hardness."

[0013] <Steel for nitriding> The nitriding steel according to the present disclosure is In mass%, C: 0.10-0.23%, Si: 0.10 to 0.50%, Mn: 1.50-2.15%, S: 0.005~0.050%, Cr: 0.50~0.90%, Al: 0.001 to 0.050%, V: 0.05~0.25%, N: 0.0030 to 0.0250%, and P: Contains 0.050% or less, The balance has a chemical composition consisting of Fe and impurities, The D value, which is an index of hardenability represented by the following formula (1), is 8.5 to 21.0, The metal structure is expressed as the area ratio, Bainite is 30% or more and less than 95%; Proeutectoid ferrite is more than 5%; Perlite is less than 20%; Nitriding steel with a Vickers hardness of 200 to 260 HV. D=√C×(1+0.64Si)×(1+4.10Mn)×(1+2.83P)×(1-0.62S)×(1+2.33Cr)×(1+0.52Ni)×(1+3.14Mo)×(1+0.27Cu)×(1+2.5V) Formula (1) In formula (1), each element symbol indicates the content (mass %) of each element.

[0014] The nitriding steel according to the present disclosure has excellent machinability and high fatigue properties (rotating bending fatigue properties and surface fatigue properties) after nitriding. The nitriding steel according to the present disclosure was discovered based on the following findings.

[0015] The present inventors have investigated the conditions for producing a nitriding steel that exhibits excellent machinability even when the Vickers hardness is increased to 200 HV or more by changing the chemical composition and metal structure of the steel in various ways, and have obtained the following findings (a) and (b). (a) If the Vickers hardness is the same, a mixed structure of bainite and other metal structures has better machinability than pearlite and than a mixed structure of ferrite and pearlite. However, even in the case of a mixed structure containing bainite, if the Vickers hardness of the steel exceeds 260HV, sufficient machinability may not be obtained. (b) In order to obtain a highly machinable structure mainly composed of bainite and pro-eutectoid ferrite with a C concentration of 0.1 mass% or more and a steel with a Vickers hardness of 260 HV or less, it is sufficient to replace part of the bainite with pro-eutectoid ferrite. However, if the amount of pro-eutectoid ferrite becomes too large and a large amount of pearlite is mixed into the metal structure, the machinability improving effect of bainite cannot be obtained and only machinability commensurate with the Vickers hardness can be obtained.

[0016] Next, the present inventors investigated the conditions for changing the metal structure of steel containing 0.1 mass % or more of C into bainite mixed with pro-eutectoid ferrite, and obtained the following findings (c) to (f). (c) In order to convert the metal structure of steel in which the hardenability has been reduced by reducing the number of alloying elements into bainite mixed with pro-eutectoid ferrite, it is necessary to cool the steel from a heated state by oil cooling or accelerated cooling similar to oil cooling, or to perform a special heat treatment such as austempering, which limits the production facilities available. (d) When a steel with medium hardenability is used, the metal structure can be a mixed structure of bainite and pro-eutectoid ferrite even if the cooling after hot rolling or hot forging is performed at a cooling rate equivalent to natural cooling. However, in this case, precise cooling rate control is required because the metal structure changes significantly due to changes in the thickness or diameter of the steel material or changes in the outside air temperature during natural cooling. (e) In order to obtain the desired metal structure even if the cooling rate varies without using special production equipment, a steel with high hardenability that completely transforms into bainite or a metal structure close to bainite when cooled after hot rolling or hot forging is selected as A. 3 Simply normalize directly above the point. (f) If the hardenability becomes excessive, the metal structure will be pure bainite even if the normalizing conditions are optimized. Therefore, the hardenability of steel is A 3 The normalizing just above the point must be such that the metal structure becomes a mixed structure of bainite and proeutectoid ferrite.

[0017] From the above findings, it has been discovered that the nitriding steel according to the present disclosure has excellent machinability and further has high fatigue properties after nitriding. The nitriding steel according to the present disclosure can be suitably used for manufacturing nitrided parts. Specifically, the nitriding steel according to the present disclosure has, for example, excellent machinability when formed by machining, and excellent rotating bending fatigue properties and surface fatigue properties after nitriding, and is suitable for use as a steel material for machine parts such as automobiles, industrial machinery, and construction machinery.

[0018] Furthermore, the nitriding steel according to the present disclosure is also suitable from the standpoint of manufacturing costs. For example, in Patent Document 1, the metal structure of the steel is adjusted to be mainly bainite, and the balance of the contents of V, Cr, and Nb contained in the steel is adjusted to improve the core hardness and the surface hardness, thereby improving the surface fatigue properties. The steel in Patent Document 1 contains a large amount of Mn or Cr to turn the metal structure into bainite, and the C concentration is low, at a maximum of 0.098%, even in the examples, so that the hardness during cutting is not excessively high while turning the metal structure into bainite. However, in order to reduce the C concentration while containing a large amount of Mn or Cr in the steel, it is necessary to use an expensive alloy with a low C content as the raw material for Mn and Cr, which may increase the alloy cost. In contrast, as described above, the nitriding steel according to the present disclosure uses steel with a high C concentration of 0.1 mass % or more, yet has excellent machinability, which makes it possible to reduce the use of expensive alloys and facilitates inexpensive production.

[0019] The nitriding steel according to the present disclosure will be described in detail below.

[0020] [Chemical composition (essential elements)] The chemical composition of the nitriding steel according to the present disclosure contains the following elements:

[0021] C: 0.10-0.23% Carbon (C) increases the hardness and fatigue strength of steel. If the C content is too low, the hardness or fatigue strength will be insufficient. In addition, in order to reduce the C content, other alloy raw materials with a low C content are required, which increases the cost of the alloy. On the other hand, if the C content is too high, the cutting resistance increases and the machinability decreases even if the metal structure is optimized. Therefore, the C content is 0.10 to 0.23%. The lower limit of the C content is preferably 0.11%, and more preferably 0.12%. The upper limit of the C content is preferably 0.21%, more preferably 0.20%, and further preferably 0.19%.

[0022] Silicon: 0.10 to 0.50% Silicon (Si) dissolves in ferrite to strengthen steel (solid solution strengthening). Si is an element that promotes the formation of pro-eutectoid ferrite, and when combined with Cr or Mn, which enhances hardenability, it becomes easier to form a mixed structure of bainite and pro-eutectoid ferrite. If the Si content is too low, the above effects cannot be obtained. On the other hand, if the Si content is too high, machinability deteriorates. Therefore, the Si content is 0.10 to 0.50%. The lower limit of the Si content is preferably 0.15%, and more preferably 0.20%. The upper limit of the Si content is preferably 0.45%, and more preferably 0.35%.

[0023] Mn: 1.50-2.15% Manganese (Mn) has the effect of improving the machinability of steel by increasing hardenability and transforming the structure into bainite, and by forming MnS in the steel. Mn also forms nitrides and contributes to hardening the diffusion layer (surface layer). If the Mn content is too low, the above effects cannot be obtained. On the other hand, if the Mn content is too high, the hardenability increases excessively, making it difficult to make part of the metal structure into pro-eutectoid ferrite. Therefore, the Mn content is 1.50 to 2.15%. The lower limit of the Mn content is preferably 1.60%, and more preferably 1.70%. The upper limit of the Mn content is preferably 2.05%, and more preferably 2.00%.

[0024] S: 0.005~0.050% Sulfur (S) combines with Mn in steel to form MnS, which improves the machinability of the steel. S is particularly important in steels with increased internal hardness to improve fatigue properties, and if the S content is too low, the above effect cannot be obtained. On the other hand, if the S content is too high, coarse MnS is formed, reducing the fatigue strength of the steel. Therefore, the S content is 0.005 to 0.050%. The lower limit of the S content is preferably 0.010%, more preferably 0.015%, and further preferably 0.020%. The upper limit of the S content is preferably 0.040%, and more preferably 0.030%.

[0025] Cr: 0.50~0.90% Chromium (Cr) combines with N introduced into the steel material by nitriding to form CrN in the nitrided layer, strengthening the nitrided layer. It also has the effect of improving the hardenability of the steel and making the metal structure a metal structure mainly composed of bainite. If the Cr content is too low, the above effects cannot be obtained. On the other hand, if the Cr content is too high, the hardenability becomes excessively high, resulting in hardness and poor machinability. Therefore, the Cr content is 0.50 to 0.90%. The lower limit of the Cr content is preferably 0.55%, and more preferably 0.60%. The upper limit of the Cr content is preferably 0.85%, and more preferably 0.80%.

[0026] Al: 0.001 to 0.050% Aluminum (Al) combines with N introduced into the steel material by nitriding to form AlN in the nitrided layer, strengthening the nitrided layer. Al is also used for deoxidization during steel manufacturing. If the Al content is too low, the above effects cannot be obtained. On the other hand, if the Al content is too high, it may inhibit the diffusion of nitrogen during nitriding and reduce the thickness of the nitrided layer, thereby deteriorating the fatigue properties. Therefore, the Al content is 0.001 to 0.050%. The lower limit of the Al content is preferably 0.005%, and more preferably 0.010%. The upper limit of the Al content is preferably 0.045%, and more preferably 0.040%.

[0027] V: 0.05~0.25% Vanadium (V) is an element that combines with N introduced into the steel material by nitriding to form VN in the nitrided layer, strengthening the nitrided layer. If the V content is too low, the above effects cannot be obtained. On the other hand, if the V content is too high, the base material becomes too hard and the machinability deteriorates. Therefore, the V content is 0.05 to 0.25%. The lower limit of the V content is preferably 0.06%, and more preferably 0.10%. The upper limit of the V content is preferably 0.20%, more preferably 0.17%, and further preferably 0.15%.

[0028] N: 0.0030~0.0250% Nitrogen (N) dissolves in steel material to increase the strength of the steel material. It also has the effect of shortening the time until the N concentration in the nitrided layer reaches a concentration at which alloy nitrides can precipitate. If the N content is too low, the above effect cannot be obtained. On the other hand, if the N content is too high, bubbles are generated in the steel material. Since bubbles become defects, it is preferable to suppress the generation of bubbles. Therefore, the N content is 0.0030 to 0.0250%. The lower limit of the N content is preferably 0.0050%, and more preferably 0.0080%. The upper limit of the N content is preferably 0.0200%, more preferably 0.0180%, and further preferably 0.0150%.

[0029] P:0.050% or less Phosphorus (P) is an impurity. Since P segregates at the grain boundaries and causes grain boundary embrittlement cracking, it is preferable that the P content is as low as possible. Therefore, the P content is 0.050% or less. The preferred upper limit of the P content is 0.030%. The lower limit of the P content is preferably 0% (that is, it is preferable that the P content is not contained), but from the viewpoint of reducing the dephosphorization cost, it may be more than 0% (for example, 0.0001% or more).

[0030] The balance of the steel for nitriding according to the present disclosure is composed of Fe and impurities. Here, the term "impurities" refers to substances that are mixed in from raw materials such as ore and scrap, or the manufacturing environment, during industrial production of steel material, and that are acceptable within a range that does not adversely affect the steel for nitriding according to the present disclosure.

[0031] [Chemical composition (arbitrary elements)] The nitriding steel according to the present disclosure may contain any element in place of a portion of Fe.

[0032] Ti: 0.005% or less Nb: 0.005% or less Titanium (Ti) and niobium (Nb) combine with N to form nitrides, and when N is depleted, they combine with C to form carbides. In the present disclosure, in order to generate proeutectoid ferrite, slow cooling or normalizing is performed after hot rolling or hot forging. In either case, Ti and Nb carbides tend to precipitate coarsely at relatively high temperatures. These carbides precipitated at high temperatures do not contribute to strengthening, and on the contrary, they consume carbon in austenite when forming carbides, thereby reducing the hardness of the base material. On the other hand, depending on the slow cooling start temperature after hot rolling or hot forging, the amount of Ti and Nb precipitated at high temperatures may be small and the amount of Ti and Nb precipitated at low temperatures may be large. In that case, the hardness of the base material is increased. Thus, in manufacturing the nitriding steel of the present disclosure, it is necessary to reduce the content of Ti and Nb in order to suppress the variation in hardness. In addition, Ti and Nb are expensive elements, and limiting their content also suppresses the alloy cost. Therefore, the nitriding steel of the present disclosure is permitted to contain one or more of Ti and Nb, but if Ti is contained, the Ti content is 0.005% or less, and if Nb is contained, the Nb content is 0.005% or less. When Ti is contained, the upper limit of the Ti content is preferably 0.004%, and more preferably 0.003%. When Ti is contained, the lower limit of the Ti content is preferably 0% (i.e., it is preferable that Ti is not contained), but from the viewpoint of reducing the Ti removal cost, the Ti content may be more than 0% (for example, 0.001% or more). When Nb is contained, the upper limit of the Nb content is preferably 0.004%, and more preferably 0.003%. When Nb is contained, the lower limit of the Nb content is preferably 0% (that is, it is preferable that Nb is not contained), but from the viewpoint of reducing the cost of Nb removal, it may be more than 0% (for example, 0.001% or more).

[0033] In the nitriding steel according to the present disclosure, among the optional elements, the group consisting of Mo, Cu and Ni has the effect of increasing the strength of the nitrided part, and one or more kinds may be contained.

[0034] Mo: 0.10% or less When molybdenum (Mo) is contained, it increases the hardenability of steel, thereby increasing the strength of the steel. As a result, the fatigue strength of the steel increases. However, Mo is an expensive element, and if the content increases, the manufacturing cost of the parts increases. Therefore, if Mo is contained, the Mo content is 0.10% or less. When Mo is contained, the lower limit of the Mo content is preferably 0.02%. When Mo is contained, the upper limit of the Mo content is preferably 0.08%, and more preferably 0.05%.

[0035] Cu:0.30% or less When copper (Cu) is contained, it dissolves in ferrite to increase the strength of the steel. This increases the fatigue strength of the steel. However, if the Cu content is excessively high, it segregates at the grain boundaries of the steel during hot rolling or hot forging, inducing hot cracking. Therefore, when Cu is contained, the Cu content is 0.30% or less. When Cu is contained, the lower limit of the Cu content is preferably 0.05%. When Cu is contained, the upper limit of the Cu content is preferably 0.25%, and more preferably 0.20%.

[0036] Ni: 0.30% or less When nickel (Ni) is contained, it dissolves in ferrite to increase the strength of the steel. This increases the fatigue strength of the steel. Furthermore, when the steel contains Cu, Ni suppresses hot cracking caused by Cu. However, if the Ni content is too high, the effect saturates and the manufacturing cost increases. Therefore, when Ni is contained, the Ni content is 0.30% or less. When Ni is contained, the lower limit of the Ni content is preferably 0.05%. When Ni is contained, the upper limit of the Ni content is preferably 0.25%, and more preferably 0.20%.

[0037] In the nitriding steel according to the present disclosure, among the optional elements, the group consisting of Ca, Pb and Bi has the effect of improving the machinability of the nitriding steel, and one or more types may be contained.

[0038] Ca:0.0050% or less Calcium (Ca), when contained, improves the machinability of steel. However, if the Ca content is too high, coarse Ca oxides are generated, and the fatigue strength of the steel decreases. Therefore, when Ca is contained, the Ca content is 0.0050% or less. When Ca is contained, the lower limit of the Ca content for more stably obtaining the above-mentioned effects is preferably 0.0001%, and more preferably 0.0003%. When Ca is contained, the upper limit of the Ca content is preferably 0.0035%, and more preferably 0.0020%.

[0039] Pb: 0.09% or less Lead (Pb) improves the machinability of steel when it is contained. However, from an environmental perspective, it is desirable to use as little Pb as possible. Therefore, when Pb is contained, the Pb content is 0.09% or less. When Pb is contained, the lower limit of the Pb content is preferably 0.02%, and more preferably 0.03%. When Pb is contained, the upper limit of the Pb content is preferably 0.08%, and more preferably 0.07%.

[0040] Bi:0.20% or less Bismuth (Bi) improves the machinability of steel. However, if the Bi content is too high, the hot workability deteriorates. Therefore, if Bi is contained, the Bi content is 0.20% or less. When Bi is contained, the lower limit of the Bi content is preferably 0.03%, and more preferably 0.05%. When Bi is contained, the upper limit of the Bi content is preferably 0.18%, and more preferably 0.16%.

[0041] The nitriding steel according to the present disclosure may contain, in addition to the above-mentioned optional elements, one or more elements selected from the group consisting of Hf, Zr, Co, W, Mg and REM (rare earth elements). When Hf is contained, the Hf content is, for example, 0.0005 to 0.0050%. When Zr is contained, the Zr content is, for example, 0.0005 to 0.0050%. When Co is contained, the Co content is, for example, 0.01 to 0.10%. When W is contained, the W content is, for example, 0.01 to 0.20%. When Mg is contained, the Mg content is, for example, 0.0005 to 0.0100%. When REM is contained, the REM content is, for example, 0.0005 to 0.0100%.

[0042] [Hardenability] Hardenability: D value 8.5~21.0 The D value, which is an index of the hardenability of steel, is expressed by the following formula (1). D=√C×(1+0.64Si)×(1+4.10Mn)×(1+2.83P)×(1-0.62S)×(1+2.33Cr)×(1+0.52Ni)×(1+3.14Mo)×(1+0.27Cu)×(1+2.5V) Formula (1) In formula (1), each element symbol indicates the content (mass %) of each element.

[0043] If the D-value is too low, quenching is required to mix bainite into the metal structure, which places significant restrictions on equipment and raises concerns about the need for additional processes such as cleaning after quenching. On the other hand, if the D-value is too high, complicated heat treatment such as extremely long furnace cooling and two-stage cooling will be required to obtain the target metal structure. Therefore, the D-value, an index of the hardenability of steel, is 8.5 to 21.0. The lower limit of the D value is preferably 9.0, and more preferably 9.5. The upper limit of the D value is preferably 20.0, and more preferably 19.0.

[0044] [Metal structure] Area ratio of bainite: 30% or more and less than 95% When cutting, if the hardness is the same, a mixed structure of ferrite and pearlite mixed with bainite is more preferable than a mixed structure of ferrite and pearlite because it reduces cutting resistance. To obtain the effect of reducing cutting resistance, the area ratio of bainite must be 30% or more. If the area ratio of bainite is too high, the hardness will be too high and the required machinability will not be obtained. Therefore, the area ratio of bainite is less than 95%. The lower limit of the area fraction of bainite is preferably 35%, and more preferably 40%. The upper limit of the area ratio of bainite is preferably 85% or less, and more preferably 75% or less.

[0045] Area ratio of proeutectoid ferrite: over 5% If the metal structure of steel containing 0.1% or more C is made of bainite only, the hardness increases too much, and the machinability deteriorates even if the metal structure is bainite. It is preferable that the metal structure of steel containing 0.1% or more C contains bainite and pro-eutectoid ferrite is mixed to reduce the hardness. In the present disclosure, ferrite grains are regarded as pro-eutectoid ferrite, and this pro-eutectoid ferrite includes both equiaxed polygonal ferrite and plate-shaped Widmanstätten ferrite. In order to effectively reduce the hardness while obtaining the effect of reducing the cutting resistance by bainite, the area ratio of pro-eutectoid ferrite needs to be more than 5%. The lower limit of the area ratio of pro-eutectoid ferrite is preferably 10%, more preferably 20%, and further preferably 25%. The upper limit of the area ratio of pro-eutectoid ferrite is not particularly limited, but from the viewpoint of more easily achieving both hardness and machinability, it is preferably less than 70%. The upper limit of the area ratio of pro-eutectoid ferrite is more preferably 60%.

[0046] Perlite area ratio: less than 20% Although hardness decreases if part of the bainite becomes pearlite, in order to reduce cutting resistance, it is better to convert part of the bainite to pro-eutectoid ferrite rather than pearlite. Therefore, the area ratio of pearlite should be low, and should be less than 20%. The smaller the area ratio of pearlite, the better. For example, it is preferably 15% or less, more preferably 10% or less, and most preferably 0%.

[0047] As described above, the metal structure of the steel for nitriding according to the present disclosure includes bainite and pro-eutectoid ferrite, and may also include pearlite. The metal structure of the steel for nitriding according to the present disclosure may also include other metal structures than bainite, pro-eutectoid ferrite, and pearlite, such as austenite. When the other metal structures are included, the area ratio of the other metal structures is preferably as small as possible, for example, 10% or less, and more preferably 5% or less.

[0048] The area ratio of the metal structure is measured as follows. A sample containing a cross section perpendicular to the longitudinal direction of the nitriding steel is taken from the nitriding steel and the cross section is polished. The above cross section is corroded with nital solution to reveal the metal structure. The center of the thickness direction of the above cross section is observed with an optical microscope (200x) and optical microscope images of five randomly selected fields are obtained. The thickness direction of steel for nitriding means the direction of the shortest line segment that passes through the center (center of gravity) of the cross section and connects two points on the surface in a vertical cross section of the steel for nitriding. For example, in the case of a round steel bar, the thickness direction is the radial direction, and the thickness is considered to be the diameter. The metal structure of each optical microscope image is separated, and the area ratio of each metal structure is calculated using the point counting method. For each metal structure, the average of the area ratios obtained from five optical microscope images is calculated and this is used as the area ratio of the metal structure of the steel for nitriding.

[0049] [Hardness] Vickers hardness: 200~260HV In order to sufficiently improve the machinability of steel containing 0.1% or more C, it is necessary to control the hardness of the steel within a specified range by setting the chemical composition and metal structure of the steel within a specified range. In order to sufficiently improve the machinability of a mixed structure (mainly bainite) of bainite and pro-eutectoid ferrite containing 0.1% or more C, it is necessary to make the Vickers hardness of the steel 260HV or less. When the chemical composition and metal structure specified in this disclosure are satisfied, the Vickers hardness of the steel is 260HV or less. In addition, from the viewpoint of obtaining excellent fatigue properties (rotating bending fatigue properties, surface fatigue properties), the Vickers hardness of the steel is 210HV or more. The lower limit of the Vickers hardness is preferably 210 HV, and more preferably 215 HV. The upper limit of the Vickers hardness is preferably 250 HV, and more preferably 240 HV.

[0050] The Vickers hardness is measured as follows. A sample is taken from the steel for nitriding, the sample including a cross section perpendicular to the longitudinal direction of the steel for nitriding, the sample including a cross section including the center of the steel for nitriding in the thickness direction. The sample is embedded in resin and the cross section of the sample is polished. On the above cross section of the sample, measure the Vickers hardness at five randomly selected points in the center of the thickness direction of the nitriding steel in accordance with JIS Z2244-1:2020 with a test force of 2.94N. The average of the Vickers hardness values ​​obtained at the above five points is calculated and used as the Vickers hardness of the nitriding steel.

[0051] <Nitriding parts> The nitrided component according to the present disclosure includes a core portion having the metal structure and Vickers hardness of the nitriding steel according to the present disclosure, and a surface layer portion having a higher N content than the chemical composition of the core portion. That is, the nitrided part according to the present disclosure is obtained by processing the nitriding steel according to the present disclosure into a desired part shape and then performing a nitriding treatment. Therefore, the nitrided part has a core part located deep inside, which is not affected by the nitriding treatment, and has the chemical composition and Vickers hardness of the nitriding steel, and the surface layer part hardened by the nitriding treatment has a higher N content than the chemical composition of the core part.

[0052] The thickness of the surface layer (i.e., the distance from the surface of the nitrided part in the depth direction perpendicular to the surface) is, for example, about 100 to 500 μm, depending on the chemical composition of the nitriding steel, the conditions of the nitriding treatment, etc. The position deeper than this thickness becomes the core.

[0053] The Vickers hardness of the surface layer of the nitrided part varies depending on the chemical composition of the nitriding steel, the conditions of the nitriding treatment, and the like, but is, for example, about 650 to 800 HV.

[0054] The Vickers hardness of the surface layer of the nitrided part can be measured as follows. The Vickers hardness of the surface layer of the nitrided part can also be the Vickers hardness of the surface layer of the steel obtained by subjecting a nitriding treatment to a steel for nitriding that is not machined into a part shape. The steel to be used for nitriding is subjected to a nitriding treatment, and a sample including a cross section perpendicular to the longitudinal direction of the steel after nitriding is taken from the steel after nitriding. At that time, the sample should include a cross section including an area of ​​1.0 mm from the surface of the steel after nitriding in the depth direction perpendicular to the surface. The sample is embedded in resin and the cross section of the sample is polished. On the above cross section of the sample, measure the Vickers hardness at five randomly selected points 50μm from the surface of the steel after nitriding in the depth direction perpendicular to the surface, using a test force of 2.94N in accordance with JIS Z2244-1:2020. The average of the Vickers hardness values ​​obtained at the above five points is calculated and used as the Vickers hardness of the steel after nitriding (or the nitrided part, if the steel has been machined into a part shape).

[0055] The nitrided parts are not particularly limited and can be used as machine parts for automobiles, industrial machines, construction machines, etc. The nitrided parts can be particularly suitably used as gears, and can also be used as gears for geared shafts.

[0056] <Manufacturing method> An example of a method for producing the nitriding steel and nitrided parts according to the present disclosure will be described.

[0057] The method for manufacturing a nitrided component according to the present disclosure includes a nitriding steel preparation step according to the present disclosure, a machining step, and a nitriding treatment step. The nitriding steel preparation step also includes a heat treatment step for adjusting the metal structure as necessary. Each step will be described below.

[0058] [Steel preparation process for nitriding] Molten steel satisfying the chemical composition of the steel for nitriding according to the present disclosure is manufactured. The manufactured molten steel is made into a slab (slab, bloom) by a general continuous casting method. Alternatively, the molten steel is made into an ingot by an ingot casting method. The slab or ingot is hot worked to manufacture a billet. The hot working for obtaining the billet may be hot rolling or hot forging.

[0059] The billet obtained as described above can be used to perform hot rolling or hot forging as described below to obtain the nitriding steel according to the present disclosure.

[0060] (1) Manufacturing of steel for nitriding by hot rolling The billet is heated under general conditions and hot-rolled. The heating temperature is, for example, 1000 to 1300°C. The preferred finishing temperature for hot rolling is 900°C or higher. If the finishing temperature is too low, the load on the rolls increases. On the other hand, the preferred upper limit of the finishing temperature is 1250°C.

[0061] When the obtained rolled material is directly cut, the finishing temperature and cooling rate of the hot rolling are adjusted so that the metal structure after hot rolling becomes a predetermined structure. Specifically, the finishing temperature of the hot rolling is set to 1000°C or more, and the cooling rate from 1000°C to 500°C is slowly cooled to 0.1 to 0.5°C / sec. This cooling rate can be easily achieved by covering the steel material after hot rolling with a cover for the purpose of heat insulation when cooling the steel material, and does not have a significant effect on the production rate. However, the hot rolling conditions described here are only an example for adjusting the microstructure of the steel material, and these conditions are not necessary as long as the area ratio of the metal structure is within the range specified in the present disclosure.

[0062] (Normalizing) If the structure after hot rolling does not become the desired metal structure, heat treatment is performed after hot rolling to generate a sufficient amount of pro-eutectoid ferrite. Specifically, after heating at 860 to 940 ° C, cooling is performed so that the average cooling rate from the heating temperature to 500 ° C is 0.5 to 2.0 ° C / sec. This cooling rate can be achieved by cooling in the air. When the heating temperature exceeds 940 ° C., pro-eutectoid ferrite may not be sufficiently generated by simply cooling in the air, so it is desirable to slow the cooling rate to 0.1 to 0.5 ° C / sec by cover slow cooling or the like. The normalizing conditions described here are one example for adjusting the microstructure of the steel material, and as long as the area ratio of the metal structure is within the range specified in the present disclosure, these conditions are not necessary.

[0063] In this manner, the nitriding steel according to the present disclosure can be obtained as a rolled material (for example, a steel bar).

[0064] (2) Manufacturing of steel for nitriding by hot forging The billet is heated under typical conditions, and the rolled material obtained by hot rolling is then hot forged. Note that the rolled material may be any material that satisfies the requirements for the nitriding steel according to the present disclosure.

[0065] The rolled material is formed into a nitrided part raw material by hot forging. If the heating temperature of the hot forging is too low, an excessive load is applied to the forging device. On the other hand, if the heating temperature is too high, the scale loss is large. Therefore, the preferable heating temperature is 1000 to 1300°C. The preferred finishing temperature for hot forging is 900°C or higher. If the finishing temperature is too low, the burden on the die increases. On the other hand, the preferred upper limit of the finishing temperature is 1250°C.

[0066] When directly cutting the nitrided part rough material after hot forging, the finishing temperature and cooling rate of hot forging are adjusted so that the structure after hot forging becomes a predetermined structure. Specifically, the finishing temperature of hot forging is set to 1000°C or more, and the cooling rate from 1000°C to 500°C is slowly cooled to 0.1 to 0.5°C / sec. Such a cooling rate can be easily achieved by covering the material after hot forging with a cover for the purpose of heat insulation when cooling it, and does not have a significant effect on the production rate. However, the hot forging conditions described here are only an example for adjusting the microstructure of the steel material, and these conditions are not necessary as long as the area ratio of the metal structure is within the range specified in this disclosure.

[0067] (Normalizing) The normalizing is as described above in the production of steel for nitriding by hot rolling.

[0068] In this manner, the nitriding steel according to the present disclosure can be obtained as a forging material (for example, steel bar).

[0069] [Machining process] The steel for nitriding obtained as described above is machined to form a desired nitrided part shape, for example by cutting, grinding, or the like.

[0070] [Nitriding process] The machined steel for nitriding is subjected to nitriding. In the present disclosure, a well-known nitriding process may be adopted. The nitriding process may be, for example, gas nitriding, salt bath soft nitriding, ion nitriding, etc. The gas introduced into the furnace during nitriding is NH 3 It may be only NH 3 And, N 2 and / or H 2 The gas mixture may contain a carburizing gas to perform the soft nitriding treatment. Therefore, in the present disclosure, "nitriding" also includes "soft nitriding". When gas soft nitriding is performed, for example, the material is soaked at a temperature of 550 to 630° C. for 1 to 3 hours in an atmosphere in which endothermic transformation gas (RX gas) and ammonia gas are mixed in a ratio of 1:1.

[0071] In this manner, a nitrided component according to the present disclosure can be obtained. EXAMPLES

[0072] The present disclosure will be specifically described below with reference to examples, although the present disclosure is not limited to these examples.

[0073] Using a vacuum melting furnace, 100 kg ingots of steels A, B, D, Q and R, and 50 kg ingots of steels C, E to P, S and T, each having the chemical composition shown in Table 1, were produced. Each ingot was heated to 1250°C. The heated ingot was hot forged into a steel bar having a diameter of 35 mm, and then allowed to cool to room temperature. The hot forged steel bar having a diameter of 35 mm was subjected to the heat treatment shown in Table 2 for test numbers 1 to 17, 20, 22, and 24 (normalizing treatment in which the bar was heated to a temperature of 880 to 980°C for 1 hour and then allowed to cool). Test numbers 18, 19, 21, and 23 were left as forged.

[0074] From each steel bar, a rotating bending fatigue test specimen was prepared having a parallel section of 10 mm in diameter with a circular notch of R1 and a depth of 1 mm (Fig. 1, the values ​​in the figure are in mm), a square bar specimen of 13 mm x 13 mm x 50 mm, a small roller specimen for roller pitting tests having a test section of 26 mm in diameter and 28 mm in width (Fig. 2, the values ​​in the figure are in mm), and a machined test specimen of 30 mm in diameter and 300 mm in length. The rotating bending fatigue test pieces and square bar test pieces were prepared from the midpoint between the center and the outer periphery of the steel bar. The small roller test pieces and cutting test pieces were prepared from the center of the steel bar. Note that the roller pitting test was not performed on some test numbers that did not achieve a sufficiently high rotating bending fatigue strength in the rotating bending fatigue test described below, and were not expected to achieve sufficient surface fatigue strength either. As a mating material for the small roller, a large roller test piece was prepared with a diameter of 130 mm, a width of 18 mm, and a crowning R in the width direction of the outer periphery of 150. The material was commercially available SUJ2 steel, which was machined to the above dimensions after quenching and tempering.

[0075] Of the prepared test pieces, the test pieces excluding the cut test pieces and some of the square bar test pieces were subjected to gas soft-nitriding treatment at 590℃ for 2h. During soft-nitriding, ammonia and RX gas were introduced into the furnace at a flow rate ratio of 1:1, and after treatment, the pieces were removed from the furnace and quenched in oil at 100℃. After soft nitriding, the gripping portions of the rotating bending fatigue test specimens and small roller test specimens were machined so that the gripping portions at both ends were parallel.

[0076] The following tests were carried out using the various test pieces with the respective test numbers.

[0077] [Vickers hardness measurement] The Vickers hardness of the square bar test pieces before nitrocarburizing was measured as follows. A sample (13mm x 13mm x 10mm) was taken by cutting the square bar test piece 10mm from the end in the longitudinal direction perpendicular to the longitudinal direction before soft nitriding. The sample was embedded in resin so that the cut surface of the sample was the test surface, and the above cross section of the sample was polished. In the above cross section of the sample, the Vickers hardness was measured at five randomly selected points in the center of the square bar test piece in the thickness direction according to JIS Z2244-1:2020 with a test force of 2.94N. The average value of the Vickers hardness obtained at the above five points was calculated and used as the Vickers hardness of the square bar test piece before soft nitriding. The Vickers hardness before soft nitriding is sometimes called the "Vickers hardness of the core."

[0078] In addition, the Vickers hardness of the surface layer of the square bar test piece after soft nitriding was measured as follows. A sample (13 mm x 13 mm x 10 mm) was taken by cutting the square bar test piece vertically in the longitudinal direction at a position 10 mm from the end in the longitudinal direction after soft nitriding. The sample was embedded in resin so that the cut surface of the sample was the test surface, and the above cross section of the sample was polished. In the above cross section of the sample, five points were randomly selected at a position 50 μm from the surface of the square bar test piece in the depth direction perpendicular to the surface, and the Vickers hardness was measured according to JIS Z2244-1:2020 with a test force of 2.94 N. The average value of the Vickers hardness obtained at the above five points was calculated and used as the Vickers hardness of the surface layer of the square bar test piece after soft nitriding.

[0079] [Area ratio of metal structure] The area ratio of the metal structure after the Vickers hardness measurement was measured as follows. The cross section of the sample used to measure the Vickers hardness of the square bar test piece before nitrocarburizing was corroded with a nital solution to reveal the metal structure. Next, the center of the cross section in the thickness direction was observed with an optical microscope (200x), and optical microscope images of five randomly selected fields were obtained. The metal structure was then separated for each optical microscope image, and the area ratio of each metal structure was calculated by the point counting method. The average value of the area ratios obtained from the five optical microscope images for each metal structure was calculated, and this was used as the area ratio of the metal structure of the square bar test piece before nitrocarburizing.

[0080] [Rotary bending fatigue test] The Ono-type rotating bending fatigue test was carried out using the above-mentioned nitriding-treated rotating bending fatigue test piece. Specifically, the rotating bending fatigue test conforming to JIS Z2274:1978 was carried out at a rotation speed of 3000 rpm in an air atmosphere at room temperature (25°C). The number of repetitions was 1.0 × 10 7 The highest stress among the tests at which no fracture occurred up to 1000 cycles was defined as the rotating bending fatigue strength (MPa) of that test number. A rotating bending fatigue strength of 500 MPa or more was determined to be excellent.

[0081] [Roller pitching test] The small roller test piece that had been subjected to the above-mentioned nitriding treatment and the large roller were subjected to a roller pitting test with a slip ratio of 40%, while pressing the outer periphery against the large roller so that the rotation axes were parallel. The peripheral speed of the small roller was 1500 rpm. The test was performed while spraying oil on the contact area, and the rotation speed of the small roller was 2.0 x 10 7 Among the tests in which pitting did not occur up to the first 1000 cycles, the highest surface pressure was defined as the surface fatigue strength (MPa) of that test number. A state in which spalling of 1 mm or more occurred was considered to be pitting. A surface fatigue strength of 2000 MPa or more was determined to be excellent.

[0082] [Cutting test] The outer circumference of the cutting test piece was machined using an NC lathe. A PVD-coated carbide tool with a chip breaker was used, with a cutting speed of 200 m / min, feed rate of 0.5 mm / rev, and depth of cut of 1.5 mm. Lubricated with a water-soluble lubricant, machinability was evaluated based on the combined force of the principal, feed, and thrust forces of the cutting resistance during the turning process. A cutting resistance of 1200 N or less was deemed to have excellent machinability.

[0083] The test results are shown in Table 2. In Table 2, the Vickers hardness of the square bar test piece before soft nitriding is indicated as "core", and the Vickers hardness of the surface layer of the square bar test piece after soft nitriding is indicated as "surface layer". Additionally, "F" stands for pro-eutectoid ferrite, "B" for bainite, and "P" for pearlite.

[0084] In Tables 1 and 2, underlined values ​​mean values ​​outside the ranges specified in this disclosure. Each of the Cu, Ni, and Mo contents is an impurity when it is 0.01%, and each of the Ti, Nb, Pb, Bi, and Ca contents is an impurity when it is written as "<".

[0085] [Table 1]

[0086] [Table 2]

[0087] [Test Results] Test Nos. 1 to 8 are examples within the ranges specified in the present disclosure, with high bending fatigue strength of 500 MPa or more, high surface fatigue strength of 2000 MPa or more, and low cutting resistance of 1200 N or less. Thus, Test Nos. 1 to 8 had excellent machinability and high fatigue properties after nitriding.

[0088] In contrast, in test numbers 9 to 24, which are examples outside the range specified in the present disclosure, it is clear that at least one of the fatigue properties and the machinability is poor.

[0089] Specifically, Test No. 9 is an example using Steel Type I with a low Mn content, and has low hardenability, so the area ratio of bainite is low and the area ratio of pearlite is high, resulting in high cutting resistance. Test No. 9 also had low bending fatigue strength and surface fatigue strength.

[0090] Test No. 10 is an example in which steel type J, which contains a lot of Mn, was used. Since the hardenability was excessively high, the area ratio of bainite was high and the area ratio of pro-eutectoid ferrite was low. In addition, the Vickers hardness of the core was high, and the cutting resistance was high.

[0091] Test No. 11 is an example in which steel type K, which has a low Cr content, was used, and since the hardness of the surface layer after nitriding was low, the bending fatigue strength was low.

[0092] Test No. 12 is an example in which steel type L containing a large amount of Cr was used. Since the hardenability was excessively high, the area ratio of bainite was high and the area ratio of pro-eutectoid ferrite was low, resulting in low cutting resistance.

[0093] Test No. 13 is an example using steel type M, which contains a lot of C, and the Vickers hardness of the core was high. In addition, because steel type M has a large D value, the area ratio of bainite was 100% under the above heat treatment conditions, and it was not possible to obtain a pro-eutectoid ferrite structure. As a result, test No. 13 had a high cutting resistance.

[0094] Test No. 14 is an example in which steel type N with a small amount of V was used, and the Vickers hardness of the surface layer and core after nitriding was low, and the bending fatigue strength and surface fatigue strength were also low.

[0095] Test No. 15 is an example using steel type O with a low Si content, in which the area ratio of pro-eutectoid ferrite was low and the area ratio of bainite was high, resulting in high Vickers hardness at the core. As a result, Test No. 15 had high cutting resistance.

[0096] Test No. 16 is an example using steel type P with a large D value, and under the above heat treatment conditions, the area ratio of bainite was 100%, and pro-eutectoid ferrite structure could not be obtained, and the Vickers hardness of the core was high. As a result, Test No. 16 had high cutting resistance.

[0097] Test No. 17 is an example using steel type Q, which is low in Mn and Cr, and has low hardenability, resulting in a low area ratio of bainite. In addition, because steel type Q does not contain V, it is not possible to increase strength, and the Vickers hardness of the core was low. As a result, Test No. 17 had low bending fatigue strength and surface fatigue strength.

[0098] Test No. 18 is an example using steel type R, which contains little Mn and Cr, and has low hardenability, so bainite was not formed. In addition, steel type R has a small D value, so bainite could not be formed under the above heat treatment conditions. As a result, test No. 18 had high cutting resistance. In addition, test No. 18 also had low bending fatigue strength. Although steel type R contains a lot of C, which increases the Vickers hardness of the core, bainite was not formed, so the Vickers hardness of the core of test No. 18 was not excessively high, being 233HV.

[0099] Test No. 19 is an example using steel type S, which contains little Mn and Cr, and has low hardenability, so bainite was not formed. In addition, steel type S has a small D value, so bainite could not be formed under the above heat treatment conditions. Furthermore, steel type S contains a lot of Si. As a result, test No. 19 had high cutting resistance. In addition, test No. 19 also had low bending fatigue strength. Although steel type S contains a lot of C, which increases the Vickers hardness of the core, bainite was not formed, so the Vickers hardness of the core of test No. 19 was not excessively high, being 244HV.

[0100] Test No. 20 is an example using steel type T with a small D value, and under the above heat treatment conditions, the amount of bainite generated was small and the Vickers hardness of the core was low. As a result, Test No. 20 had low bending fatigue strength.

[0101] Test No. 21 is an example in which the heat treatment conditions were changed (as forged) using steel type B, and the area ratio of bainite was set to 100%. As a result, Test No. 21 had a high Vickers hardness in the core and a high cutting resistance.

[0102] Test No. 22 is an example in which the heat treatment conditions were changed (heated to 980°C, then normalized) using steel type B to increase the area ratio of bainite to 97%. As a result, Test No. 22 had a high Vickers hardness in the core and a high cutting resistance.

[0103] Test No. 23 is an example in which the heat treatment conditions were changed (as forged) using steel type D, and the area ratio of bainite was set to 100%. As a result, Test No. 23 had a high Vickers hardness in the core and a high cutting resistance.

[0104] Test No. 24 is an example in which the heat treatment conditions were changed (heated to 980°C, then normalized) using steel type B to set the area ratio of bainite to 100%. As a result, Test No. 24 had a high Vickers hardness in the core and a high cutting resistance. A comparison of test numbers 2, 21, and 22 using steel type B, and a comparison of test numbers 4, 23, and 24 using steel type D, showed that the cutting resistance can be reduced and the machinability can be improved by controlling the metal structure through heat treatment.

Claims

1. In mass percent, C: 0.10-0.23%, Si: 0.10 to 0.50%, Mn: 1.50 to 2.15%, S: 0.005-0.050%, Cr: 0.50-0.90%, Al: 0.001-0.050%, V: 0.05-0.25%, N: 0.0030 to 0.0250%, and P: 0.050% or less; The balance has a chemical composition consisting of Fe and impurities, The D value, which is an index of hardenability represented by the following formula (1), is 8.5 to 21.0, The metal structure is expressed as the area ratio, Bainite is 30% or more and less than 95%; Proeutectoid ferrite is more than 5%; Perlite is less than 20%; Nitriding steel with a Vickers hardness of 200 to 260 HV. D=√C×(1+0.64Si)×(1+4.10Mn)×(1+2.83P)×(1-0.62S)×(1+2.33Cr)×(1+0.52Ni)×(1+3.14Mo)×(1+0.27Cu)×(1+2.5V)...Formula (1) In formula (1), each element symbol indicates the content (mass %) of each element.

2. In mass percent, Ti: 0.005% or less, Nb: 0.005% or less, Mo: 0.10% or less, Cu: 0.30% or less, Ni: 0.30% or less, Ca: 0.0050% or less, Pb: 0.09% or less, and Bi: 0.20% or less 2. The nitriding steel according to claim 1, which contains one or more of the following:

3. 3. A nitrided component comprising: a core portion having the chemical composition, metal structure and Vickers hardness according to claim 1 or 2; and a surface layer portion having a higher N content than the chemical composition of the core portion.

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