Steel and steel parts

Abstract

The present invention provides a steel that ensures machinability and exhibits excellent fatigue strength. The steel has a specified chemical composition and a structure in which the area ratio of bainite exceeds 50%, the bainite contains precipitates of one or more of Cr, Mo, V, and Nb, and has a surface hardness of 700 HV or more, a core hardness of 250 HV or more, and a surface compressive residual stress of 750 MPa or more.

Description

[Technical Field]

[0001] The present invention relates to steel and steel parts. In particular, the present invention relates to a steel that has excellent fatigue properties due to a surface region (hereinafter sometimes referred to as a "hardened layer") with high surface hardness and compressive residual stress and a region with high core hardness, and further has a certain degree of machinability during processing, making it easily workable into desired shapes. The present invention also relates to steel parts when this steel is used, for example, as a desired machine structural part. The steel and steel parts of the present invention are particularly suitable for use in parts for automobiles and construction machinery. [Background technology]

[0002] Steel mechanical structural parts, such as automotive gears, require excellent fatigue properties, so they are usually subjected to surface hardening treatments, such as carburizing, induction hardening, and nitriding.

[0003] Of these, carburizing is effective in improving the fatigue properties of parts by penetrating and diffusing carbon into steel in the high-temperature austenite region, resulting in a deep hardening depth. However, because carburizing generates heat treatment distortion in the steel, it has been difficult to apply to parts that require strict dimensional accuracy from the perspective of quietness, etc.

[0004] Furthermore, because induction hardening is a process in which the surface layer of a part is hardened by high-frequency induction heating, heat treatment distortion occurs in the steel, and, like carburizing, there are problems with dimensional accuracy.

[0005] On the other hand, nitriding is a process in which nitrogen penetrates and diffuses into steel at a relatively low temperature below the Ac1 transformation point, forming a hardened layer near the surface and increasing surface hardness. Therefore, the heat treatment distortion of the steel mentioned above is small, and in recent years, this process has become widely used for machine structural parts and the like. Nitrogen softening, in which carbon penetrates and diffuses into the steel at the same time as nitrogen, is also widely used. Nitrogen softening has the characteristic of forming a hardened layer more quickly.

[0006] In recent years, the application of shot peening has been considered to meet the need for improving the fatigue strength of machine structural parts due to demands for further miniaturization, etc. However, even if shot peening introduces a hardened layer onto the surface of a part to suppress the occurrence of surface cracks, unless the hardness inside the part is also ensured at the same time, cracks will occur from within, and fatigue strength will not be improved. Therefore, to improve the fatigue strength of parts, it is necessary to ensure the hardness of the core of the part, i.e., the material part that makes up the part, which is not affected by the hardening caused by the above-mentioned nitriding treatment, nitrocarburizing treatment, and shot peening.

[0007] To solve the above problems, Patent Documents 1 and 2 propose soft-nitrided steels that contain about 0.20% C, Mn, Mo, and V together with Si to ensure core hardness, and Pb and / or Ca to ensure machinability. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Patent No. 3340016 [Patent Document 2] Patent No. 4752635 Summary of the Invention [Problem to be solved by the invention]

[0009] However, the addition of Pb described in Patent Document 1 not only places a heavy burden on the environment, but also raises concerns that it may have an adverse effect on fatigue properties. Furthermore, the steel described in Patent Document 2 has the problem of poor machinability when Pb and Ca are not added. On the other hand, when Pb or Ca is added, there is a concern that it may have an adverse effect on fatigue properties, as with the technology described in Patent Document 1. Another disadvantage is that the alloy cost increases.

[0010] The present invention advantageously solves the above problems, and aims to provide a steel that exhibits excellent fatigue properties by ensuring machinability by controlling the precipitation state of component elements, and by suppressing fatigue fracture at the surface by increasing the hardness of the near-surface region and imparting compressive residual stress, while also ensuring core hardness. Another object of the present invention is to provide a steel part that uses the above steel and exhibits excellent fatigue properties. [Means for solving the problem]

[0011] In order to solve the above problems, the inventors have conducted extensive research into the effects of the chemical composition and structure of steel on machinability and fatigue properties. As a result, they discovered that by including appropriate amounts of Cr, Mo, V, and Nb in the steel's composition, and by making the area ratio of bainite in the steel's structure more than 50%, excellent machinability can be obtained during the processing stage. Furthermore, the inventors have found that the fatigue strength of the steel surface can be increased by controlling the surface hardness and surface compressive residual stress in the region near the surface of the steel to a predetermined level or higher. Furthermore, the present inventors have newly discovered that, in the inner region of the steel, by dispersing and precipitating, preferably fine, precipitates containing at least one of Cr, Mo, V, and Nb in the bainite, the core hardness is increased, thereby obtaining excellent fatigue properties for the steel as a whole. The present inventors have also discovered that, similar to the above steel, excellent fatigue properties can be obtained for steel parts used in desired applications.

[0012] The present invention has been completed as a result of further investigation based on the above findings. That is, the gist and configuration of the present invention are as follows. 1. By mass%, C: 0.010 or more, 0.100% or less, Si: 1.00% or less, Mn: 0.50% or more, 3.00% or less, P: 0.020% or less, S: 0.060% or less, Cr: 0.30% or more, 3.00% or less, Mo: 0.005% or more, 0.400% or less, V: 0.02% or more, 0.50% or less, Nb: 0.003% or more, 0.150% or less, Al: 0.005% or more, 0.200% or less, N: 0.0200% or less and Sb: 0.0005% or more, 0.0200% or less and the balance being Fe and unavoidable impurities, The steel has a structure in which the area ratio of bainite is more than 50%, The bainite contains precipitates of one or more of Cr, Mo, V, and Nb, The surface hardness is 700HV or more and the core hardness is 250HV or more. Steel with a surface compressive residual stress of 750 MPa or more.

[0013] 2. The component composition is, in mass%, W: 0.3% or less, Co: 0.3% or less, Hf: 0.2% or less, Zr: 0.2% or less, B: 0.0100% or less, Cu: 0.3% or less, Ni: 0.3% or less, Pb: 0.2% or less, Bi: 0.2% or less, Zn: 0.2% or less, Sn: 0.2% or less and Ti: 0.1% or less The steel according to 1 above, further containing one or more selected from the following:

[0014] 3. A core portion having the chemical composition described in 1 or 2 above and a structure in which the area ratio of bainite is more than 50%; and a surface layer portion having a chemical composition in which the content of at least nitrogen or carbon is higher than that of the chemical composition of the core portion; The steel part has precipitates of one or more of Cr, Mo, V and Nb dispersed in the bainite. [Effects of the Invention]

[0015] According to the present invention, it is possible to provide a steel that has excellent machinability in the machining stage and also has improved fatigue properties, more specifically, bending fatigue strength. Furthermore, according to the present invention, it is possible to provide a steel part having improved fatigue properties, more specifically improved bending fatigue strength. Therefore, the steel of the present invention is extremely useful as a material for mechanical structural parts of automobiles, etc. Furthermore, the steel parts of the present invention are extremely useful when applied as mechanical structural parts of automobiles, etc. The steel and steel parts of the present invention can achieve these effects even when inexpensive chemical compositions are used. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a schematic diagram showing a rotating bending fatigue test piece. [Figure 2] 1 is a graph showing the effect of surface hardness on rotating bending fatigue strength. [Figure 3] 1 is a graph showing the effect of compressive residual stress on rotating bending fatigue strength. [Figure 4] 1 is a graph showing the effect of core hardness on rotating bending fatigue strength. [Figure 5] 1 is a flowchart showing a typical example of a manufacturing process for steel or steel parts as a product. DETAILED DESCRIPTION OF THE INVENTION

[0017] The present invention will be described in detail below. Note that the following description is of preferred embodiments of the present invention, and the present invention is not limited to the embodiments described below. Furthermore, in this specification, any numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits, respectively, unless otherwise specified. Furthermore, if a unit is attached to only one of the numerical values ​​written before and after "~", the same unit will be attached to the other value unless otherwise specified.

[0018] (steel) The steel of the present invention has a composition, by mass%, of C: 0.010 to 0.100%, Si: 1.00% or less (inclusive 0%), Mn: 0.50 to 3.00%, P: 0.020% or less (inclusive 0%), S: 0.060% or less (inclusive 0%), Cr: 0.30 to 3.00%, Mo: 0.005 to 0.400%, V: 0.02 to 0.50%, Nb: 0.003 to 0.150%, Al: 0.005 to 0.200%, N: 0.0200% or less (inclusive 0%), Sb: 0.0005 to 0.0200% or less, with the balance being Fe and impurities. The steel may further contain optional elements described below in place of the Fe. Furthermore, the steel of the present invention has a predetermined structure mainly composed of bainite and satisfies predetermined ranges of surface hardness, core hardness, and surface compressive residual stress. By satisfying these structural requirements, the steel of the present invention can achieve both excellent machinability in the processing stage and excellent fatigue properties for the steel as a whole.

[0019] Ingredient composition The reasons for limiting the content of each element in the composition to the above ranges will be explained below. Note that "%" in the composition below means "% by mass" unless otherwise specified. As will be described later, the steel of the present invention typically has different surface and core hardnesses, with the surface having a higher Vickers hardness than the core. In this case, the steel typically has a hardened layer with a relatively high hardness formed in the surface region extending from the surface toward the interior, and a portion excluding the hardened layer (hereinafter also referred to as the core of the steel) with a relatively low hardness toward the interior. When the steel thus has a core and a hardened layer, the following description of the chemical composition refers to the chemical composition of the core of the steel. The hardened layer and core of the steel can be confirmed according to the technique described below.

[0020] C: 0.010% or more, 0.100% or less C is necessary to ensure the hardness of the inner region (core) of the steel. If the C content is less than 0.010%, the hardness of the core decreases, resulting in poor fatigue properties for the steel as a whole. Therefore, the C content is set to 0.010% or more, and preferably 0.030% or more. On the other hand, if the C content exceeds 0.100%, machinability (machinability) decreases. Therefore, the C content is set to a range of 0.100% or less, and preferably 0.080% or less.

[0021] Si: 1.00% or less Silicon is effective in ensuring strength. To achieve this, it is preferable to include 0.01% or more of silicon. However, if the silicon content exceeds 1.00%, solid solution strengthening deteriorates machinability, so the silicon content is set to 1.00% or less. The preferred silicon content is 0.50% or less.

[0022] Mn: 0.50% or more, 3.00% or less Mn improves the hardenability of steel and stabilizes the formation of bainite. It also improves machinability by forming MnS together with S. If the Mn content is less than 0.50%, the amount of MnS formed is insufficient, resulting in reduced machinability. Therefore, the Mn content is set to 0.50% or more, and preferably 1.50% or more. On the other hand, if the Mn content exceeds 3.00%, the steel becomes excessively hard and the machinability deteriorates. Therefore, the Mn content is set to 3.00% or less, preferably 2.50% or less, and more preferably 2.00% or less.

[0023] P:0.020% or less P is an element that is mixed into steel as an impurity and is known to cause surface cracks in cast slabs. Therefore, it is desirable to minimize the P content, but up to 0.020% is acceptable. However, since it is costly to reduce the P content to less than 0.001%, the P content can be set to 0.001% or more, and industrially, it is sufficient to reduce it to 0.020%.

[0024] S: 0.060% or less S is an element that is mixed into steel as an impurity, but it also has the effect of improving machinability. Specifically, if the S content is less than 0.010%, the amount of MnS produced in the steel decreases, which can lead to a decrease in machinability. Therefore, the S content is preferably 0.010% or more, and more preferably 0.020% or more. On the other hand, if the S content exceeds 0.060%, not only does the above effect saturate, but the excess precipitated MnS also reduces the amount of solute Mn. Therefore, the S content is limited to 0.060% or less, and preferably 0.040% or less.

[0025] Cr: 0.30% or more, 3.00% or less When nitriding or nitrocarburizing is performed, Cr forms CrN precipitates together with nitrogen diffused from the steel surface during the treatment, thereby strengthening the hardened layer through precipitation. The hardened layer can be formed, for example, by nitriding or nitrocarburizing, as described below. These treatments increase the N concentration at least in the surface region of the steel obtained after the treatment compared to the composition of the steel before nitriding or nitrocarburizing, resulting in a hardened layer. The core, i.e., the portion other than the hardened layer, maintains the composition of the steel before nitriding or nitrocarburizing, except for N and C. Cr also improves the hardenability of the steel and facilitates the formation of bainite. If the Cr content is less than 0.30%, for example, the amount of CrN precipitated in the hardened layer during nitriding will be insufficient, making it difficult to ensure strength. Therefore, the Cr content should be 0.30% or more, preferably 0.50% or more. On the other hand, if the Cr content exceeds 3.00%, the steel will become excessively hard and its machinability will be impaired. Therefore, the Cr content is set to a range of 3.00% or less, and preferably to a range of 1.50% or less.

[0026] Mo: 0.005% or more, 0.400% or less When nitriding or nitrocarburizing is performed, Mo forms nitride precipitates together with nitrogen that diffuses from the steel surface during the treatment, contributing to an increase in hardness in the surface layer region. Mo also facilitates the formation of bainite, contributing to an increase in machinability and core hardness. To achieve these effects, it is necessary to add 0.005% or more of Mo. On the other hand, adding too much Mo excessively improves the hardenability of the steel, leading to the formation of martensite, which excessively increases hardness and reduces machinability. Therefore, the Mo content is limited to a range of 0.400% or less, preferably 0.150% or less.

[0027] V: 0.02% or more, 0.50% or less When nitriding or soft nitriding is performed, V forms nitride precipitates together with nitrogen diffused from the steel surface during the treatment, contributing to an increase in hardness in the surface region. Furthermore, V forms fine precipitates due to the temperature increase during nitriding or soft nitriding, for example, and increases core hardness. Since the above effect cannot be obtained when the V content is less than 0.02%, the V content is set to 0.02% or more, preferably 0.05% or more. On the other hand, excessive V addition causes the precipitates to coarsen, saturating the hardness improvement effect. Furthermore, hardness increases after hot forging, and machinability decreases. Therefore, the V content is set to 0.50% or less, preferably 0.40% or less.

[0028] Nb: 0.003% or more, 0.150% or less When nitriding or soft nitriding is performed, Nb forms nitride precipitates together with nitrogen diffused from the steel surface during the treatment, contributing to an increase in hardness in the surface layer region. Furthermore, Nb forms fine precipitates due to the temperature increase during nitriding or soft nitriding, increasing the core hardness. If the Nb content is less than 0.003%, the above effect cannot be obtained, so the Nb content is set to 0.003% or more, preferably 0.020% or more. On the other hand, if Nb is added in excess, the precipitates become coarse and the strength improvement effect saturates. Furthermore, the hardness after hot forging increases and the machinability decreases. Therefore, the Nb content is set to 0.150% or less, preferably 0.120% or less.

[0029] Al: 0.005% or more, 0.200% or less Al is an element useful for improving surface hardness, particularly after nitriding or nitrocarburizing. If the Al content is less than 0.005%, the above effect cannot be obtained, so the Al content is set to 0.005% or more, preferably 0.010% or more. On the other hand, if the Al content exceeds 0.200%, the depth of the hardened layer decreases. Therefore, the Al content is set to 0.200% or less, preferably 0.100% or less, and more preferably 0.050% or less.

[0030] N: 0.0200% or less N is an element that forms carbonitrides in steel and has the effect of improving strength. However, if the N content exceeds 0.0200%, the hardness after hot forging increases and machinability decreases. Therefore, the N content is set to 0.0200% or less. On the other hand, although there is no particular lower limit for the N content, from the viewpoint of improving strength, it is preferable that the N content be 0.0020% or more.

[0031] Sb: 0.0005% or more, 0.0200% or less Sb is an element that promotes the formation of bainite and increases hardness. To achieve this effect, the Sb content must be 0.0005% or more, and preferably 0.0010% or more. On the other hand, if Sb is added in an amount exceeding 0.0200%, the above effect saturates, leading not only to an increase in component costs but also to a decrease in base material toughness due to segregation. Therefore, the Sb content is set to 0.0200% or less, and preferably 0.0100% or less.

[0032] The composition of the alloy contains the basic elements described above, with the remainder being Fe and inevitable impurities. Inevitable impurities are impurities that are inevitably mixed in from raw materials, the manufacturing process, or manufacturing equipment, and are permitted to be present to the extent that they do not impair the objectives of the present invention. Examples of raw materials include iron ore, reduced iron, and scrap. Examples of impurities include O, H, and As. Furthermore, in addition to the above basic elements, the composition may contain one or more optional elements selected from W: 0.3% or less, Co: 0.3% or less, Hf: 0.2% or less, Zr: 0.2% or less, B: 0.0100% or less, Cu: 0.3% or less, Ni: 0.3% or less, Pb: 0.2% or less, Bi: 0.2% or less, Zn: 0.2% or less, Sn: 0.2% or less, and Ti: 0.1% or less. The following describes the optional elements that can be contained and their contents.

[0033] W: 0.3% or less W is an element effective in further improving the strength of steel. However, if the W content exceeds 0.3%, the toughness of the steel decreases. Therefore, when W is added, the W content is set to 0.3% or less, preferably 0.25% or less. On the other hand, although there is no particular lower limit for the W content, from the viewpoint of improving strength, it is preferable that the W content be 0.01% or more.

[0034] Co:0.3% or less Co is an element effective in further improving the strength of steel. However, if the Co content exceeds 0.3%, the toughness of the steel decreases. Therefore, when Co is added, the Co content is set to 0.3% or less, preferably 0.25% or less. On the other hand, although there is no particular lower limit for the Co content, from the viewpoint of improving strength, it is preferable that the Co content be 0.01% or more.

[0035] Hf: 0.2% or less Hf is an element effective in further improving the strength of steel. However, if the Hf content exceeds 0.2%, the toughness of the steel decreases. Therefore, when Hf is added, the Hf content is set to 0.2% or less, preferably 0.15% or less. On the other hand, although there is no particular lower limit for the Hf content, from the viewpoint of improving strength, it is preferable that the Hf content be 0.01% or more.

[0036] Zr: 0.2% or less Zr is an element effective in further improving the strength of steel. However, if the Zr content exceeds 0.2%, the toughness of the steel decreases. Therefore, when Zr is added, the Zr content is set to 0.2% or less, preferably 0.15% or less. On the other hand, although there is no particular lower limit for the Zr content, from the viewpoint of improving strength, it is preferable that the Zr content be 0.01% or more.

[0037] B: 0.0100% or less B is an element that has the effect of improving hardenability and promoting the formation of bainite structure. However, if the B content exceeds 0.0100%, B precipitates as BN, which not only saturates the effect of improving hardenability but also increases the cost of the component. Therefore, when B is added, the B content is set to 0.0100% or less, more preferably 0.0080% or less. On the other hand, although there is no particular lower limit for the B content, from the viewpoint of improving hardenability, the B content is preferably set to 0.0003% or more, and more preferably 0.0005% or more.

[0038] Cu:0.3% or less Cu is an element that forms intermetallic compounds with Fe and Ni during nitriding or soft nitriding, and by precipitation hardening, improves the strength of steel obtained through nitriding or soft nitriding. Cu also contributes to the formation of bainite. However, if the Cu content exceeds 0.3%, hot workability deteriorates. Therefore, when Cu is added, the Cu content should be 0.3% or less, and more preferably 0.25% or less. On the other hand, although there is no particular lower limit for the Cu content, it is preferable that the Cu content be 0.05% or more from the viewpoint of improving strength.

[0039] Ni: 0.3% or less Ni is an element that has the effect of increasing hardenability and suppressing low-temperature brittleness. However, if the Ni content exceeds 0.3%, the hardness increases, which not only adversely affects machinability but also becomes costly. Therefore, when Ni is added, the Ni content is set to 0.3% or less, more preferably 0.25% or less. On the other hand, although there is no particular lower limit for the Ni content, it is preferable that the Ni content be 0.05% or more from the viewpoint of suppressing low-temperature brittleness.

[0040] Pb: 0.2% or less Pb is an element that improves the machinability of steel. However, if the Pb content exceeds 0.2%, toughness decreases. Furthermore, excessive addition of Pb can impair the fatigue properties of steel. Pb is also an element that places a large burden on the environment and is unfavorable in terms of cost. Therefore, when Pb is added, the Pb content should be 0.2% or less, and more preferably 0.1% or less. From the perspectives of improving fatigue properties, reducing environmental impact, and cost, the Pb content can be set to 0% (not contained). On the other hand, although there is no particular lower limit for the Pb content, it is preferably 0.02% or more from the perspective of improving machinability.

[0041] Bi:0.2% or less Bi is an element that has the effect of improving the machinability of steel. However, if the Bi content exceeds 0.2%, toughness decreases. Therefore, when Bi is added, the Bi content is set to 0.2% or less, and more preferably 0.1% or less. On the other hand, although there is no particular lower limit for the Bi content, it is preferably set to 0.02% or more from the viewpoint of improving machinability.

[0042] Zn: 0.2% or less Zn is an element that has the effect of improving the machinability of steel. However, if the Zn content exceeds 0.2%, toughness decreases. Therefore, when Zn is added, the Zn content is set to 0.2% or less, and more preferably 0.1% or less. On the other hand, although there is no particular lower limit for the Zn content, it is preferably set to 0.02% or more from the viewpoint of improving machinability.

[0043] Sn: 0.2% or less Sn is an element that has the effect of improving the machinability of steel. However, if the Sn content exceeds 0.2%, toughness decreases. Therefore, when Sn is added, the Sn content is set to 0.2% or less, and more preferably 0.1% or less. On the other hand, although there is no particular lower limit for the Sn content, it is preferably set to 0.02% or more from the viewpoint of improving machinability.

[0044] Ti: 0.1% or less Ti is an element effective in further improving the strength of steel. However, if the Ti content exceeds 0.1%, the toughness of the steel decreases. Therefore, when Ti is added, the Ti content is set to 0.1% or less, and more preferably, 0.01% or less. On the other hand, although there is no particular lower limit for the Ti content, from the viewpoint of improving strength, it is preferable that the Ti content be 0.003% or more. The composition of the steel of the present invention may be such that the balance other than the elements described above is Fe and unavoidable impurities.

[0045] Here, the hardened layer that the steel may have will be described. The hardened layer can be effectively formed by, for example, subjecting a steel material to nitriding or soft nitriding, or by further subjecting the steel material to shot peening. First, in one example of nitriding or soft nitriding, nitrogen in the atmosphere of these treatments diffuses from the steel surface into the steel, forming a hardened layer in the surface region of the steel. More specifically, in the outermost surface layer of the steel, Fe, the main component of the steel material subjected to nitriding or nitrocarburizing, and some of the other components combine with nitrogen to form nitrides, forming a compound layer composed of these nitrides. In the case of nitrocarburizing, carbon from the nitrocarburizing atmosphere also diffuses into the compound layer. Nitrogen diffuses into the steel to form a diffusion layer inside the compound layer in the steel thickness direction, resulting in a diffusion layer with a higher nitrogen concentration than before the nitriding or nitrocarburizing treatment. In the case of nitrocarburizing, the diffusion layer has higher nitrogen and carbon concentrations. In other words, a nitrogen diffusion layer or a nitrogen and carbon diffusion layer is formed adjacent to the compound layer in the depth direction. These compound and diffusion layers have greater hardness than the steel material before the nitriding or nitrocarburizing treatment. In this way, a hardened layer composed of the compound and diffusion layers is formed in the surface region of the steel.

[0046] On the other hand, the core, which is located further inward in the steel thickness direction than the hardened layer, is a portion where nitrogen diffusion does not occur. This core has the chemical composition described above. The hardened layer has a chemical composition with a higher N content or a higher N and C content than the chemical composition of the core.

[0047] Here, the hardened layer and core of steel can be distinguished by hardness. When hardness is measured at regular intervals from the surface to the center of a cross section of steel in the thickness direction, the hardness decreases as the distance from the surface increases in the depth direction, eventually reaching a constant value. This constant hardness corresponds to the core, and the part where a significant increase in hardness is observed compared to the core corresponds to the hardened layer. The hardness distribution that distinguishes between this core and the hardened layer (surface region) varies somewhat depending on, for example, the measuring device used and the chemical composition of each steel type, so it is not appropriate to specify it using a specific numerical value.

[0048] After the nitriding or nitrocarburizing treatment, the hardened layer can be further strengthened by a shot peening treatment, which will be described later. The chemical composition of the steel according to the present invention has been described above.

[0049] organization Next, the reasons for limiting the microstructure of the steel according to the present invention to each range will be explained. [Bainite: area fraction of more than 50% and containing specified precipitates] In the present invention, it is extremely important that the area ratio of bainite to the entire structure is more than 50%. It is also important that this bainite ultimately contains precipitates of one or more of Cr, Mo, V, and Nb. Examples of the precipitates include precipitates consisting of the elements Cr, Mo, V, and Nb alone or their compounds, as well as composite precipitates of two or more selected from Cr, Mo, V, and Nb. The precipitates may also be mixtures of these (hereinafter, sometimes collectively referred to as "Cr, Mo, V, and Nb precipitates"). Among these, the composite precipitates are preferred from the viewpoint of increasing the core hardness and improving fatigue properties. Furthermore, in the case of steel that has been subjected to nitriding or nitrocarburizing, the "Cr, Mo, V, and Nb precipitates" may be dispersed precipitates such as (Cr, Mo, Nb, V)(C, N) in which nitrogen or nitrogen and carbon are dispersed. Here, the notation (Cr, Mo, Nb, V)(C, N) indicates carbides, nitrides, or carbonitrides of Cr, Mo, Nb, and / or V.

[0050] By dispersing and precipitating one or more of the above "Cr, Mo, V, and Nb precipitates" in the bainite that mainly constitutes the steel structure, it is possible to sufficiently increase the core hardness of the steel, and improve the fatigue properties of the steel. For example, even if steel has undergone nitriding or soft nitriding treatment and has a hardened layer in the surface region and a core portion inside, the hardness increases not only in the hardened layer but also in the core portion other than the hardened layer, thereby improving the fatigue properties of the steel as a whole.

[0051] Here, the presence of "Cr, Mo, V, and Nb precipitates" in a steel material prior to nitriding or nitrocarburizing is usually disadvantageous from the viewpoint of machinability during cutting performed before nitriding or nitrocarburizing. In this regard, the bainite transformation process makes it more difficult for "Cr, Mo, V, and Nb precipitates" to form in the matrix compared to the normal ferrite-pearlite transformation process. Therefore, to ensure good machinability during cutting, the steel material prior to nitriding and nitrocarburizing must have a bainite-based structure. This bainite-based structure is maintained in the steel after cutting and after subsequent nitriding or nitrocarburizing treatment. Specifically, the area ratio of bainite to the entire structure is greater than 50%, preferably greater than 60%, and more preferably greater than 80%. The area ratio of bainite may be 100%. While ferrite, pearlite, and other structures other than bainite are conceivable, it goes without saying that the less of these structures present, the better.

[0052] The area percentage of each structure can be determined as follows. That is, for example, a test piece is taken from a steel obtained by the manufacturing method described below, and a vertical cross section (L cross section) parallel to the rolling direction is polished and then etched with nital. Using an optical microscope or scanning electron microscope (SEM), the type of structure is identified by observing the structure of the cross section of this test piece (for example, observing the structure under an optical microscope at 200x magnification). Then, the area percentage (area %) of each structure relative to the entire observed area is determined. Furthermore, the presence of precipitates of one or more of Cr, Mo, V, and Nb in bainite can be confirmed by the method used in the examples.

[0053] Furthermore, by incorporating precipitates of one or more of Cr, Mo, V, and Nb into the bainite structure of the steel after cutting, the core hardness increases, and the fatigue properties of the steel as a whole can be significantly improved. Furthermore, even if the steel has a hardened layer (nitrided portion or soft-nitrided portion) in the surface region, for example, by dispersing and precipitating one or more of Cr, Mo, V, and Nb in the structure of the core other than the hardened layer, the core hardness increases, and the fatigue properties of the steel after nitriding or soft-nitriding can be significantly improved.

[0054] Here, it is preferable that the "Cr, Mo, V and Nb precipitates" in the bainite are present in a large amount and in a fine manner in order to increase the core hardness and further to contribute to precipitation strengthening after nitriding or soft nitriding. From this viewpoint, the particle size of one or more of the precipitates of Cr, Mo, V, and Nb is preferably less than 20 nm, more preferably 10 nm or less. The measurement limit of the particle size of the precipitates, i.e., the smallest particle size that can be measured, is 1 nm.

[0055] In addition, precipitates of one or more of Cr, Mo, V, and Nb are present in a unit area of ​​1 μm 2 Preferably, 100 or more particles are dispersed per particle, and more preferably, 200 or more particles are dispersed per particle. The more precipitates there are, the better, so there is no particular upper limit to the number density. The particle size and number density of precipitates can be measured by observation using a transmission electron microscope. The above values ​​can be obtained by observing a total of 1,000 or more precipitates in five or more fields of view. The particle size is the equivalent circle diameter.

[0056] The above-mentioned precipitates in bainite can be effectively formed, for example, by subjecting a steel material to nitriding or nitrocarburizing treatment. As a specific example, as described above, a steel material having a structure mainly composed of bainite with suppressed Cr, Mo, V, and Nb precipitates in the bainite can be subjected to nitriding or nitrocarburizing treatment maintained at a temperature of about 570°C, thereby precipitating nitrides, carbides, and / or carbonitrides of Cr, Mo, V, and / or Nb in the bainite. The N and C that form these precipitates usually penetrate the hardened layer of the resulting steel mainly from the surface, whereas in the core, they are solid-solved in the steel material prior to the treatment. In this way, it is possible to control the precipitation behavior in bainite to achieve both machinability and fatigue strength.

[0057] Surface hardness, core hardness, surface compressive residual stress The steel of the present invention has a surface hardness of 700 HV or more, a core hardness of 250 HV or more, and a surface compressive residual stress of 750 MPa or more. The experimental results that led to limiting the hardness and compressive residual stress of the steel of the present invention to the above ranges are described below.

[0058] That is, as described in the examples below, various component compositions and processing conditions were changed to prepare steels with different surface hardness, surface compressive residual stress, and core hardness as rotating bending fatigue test pieces as shown in Figure 1, and the fatigue properties of each were evaluated. The results of the rotating bending fatigue test (fatigue limit, MPa, indicating rotating bending fatigue strength) are summarized in relation to the surface hardness, surface compressive residual stress, and core hardness and are shown in Figures 2 to 4. The methods for measuring the surface hardness, surface compressive residual stress, and core hardness, as well as the method for measuring the rotating bending fatigue strength, were the same as those used in the examples below.

[0059] [Surface hardness: 700HV or more] First, Figure 2 shows the relationship between the fatigue limit (fatigue strength) and surface hardness in a rotating bending fatigue test. In the experiment whose results are shown in Figure 2, the effect of surface hardness on rotating bending fatigue strength was investigated under conditions of a surface compressive residual stress of 750 MPa or more and a core hardness of 250 HV or more. In Figure 2, high fatigue strengths with a fatigue limit of 700 MPa or more are indicated by black circles (invention examples), and low fatigue strengths with a fatigue limit of less than 700 MPa are indicated by white triangles (comparison examples).

[0060] Figure 2 shows that when the surface hardness is 700 HV or higher, the fatigue limit is significantly increased, and the rotating bending fatigue strength is particularly excellent. When the fracture surface of the steel after fatigue testing was observed, it was estimated that the fracture originated from the interior of the specimen in specimens with a surface hardness of 700 HV or higher, but from the surface in specimens with a surface hardness of less than 700 HV. In other words, when the surface hardness of the steel is sufficiently high, at 700 HV or higher, crack initiation and propagation from the surface are suppressed, thereby improving fatigue strength. Surface hardness is usually 1000 HV or less.

[0061] [Surface compressive residual stress: 750 MPa or more] Figure 3 shows the relationship between the fatigue limit (fatigue strength) and surface compressive residual stress in a rotating bending fatigue test. In the experiment shown in Figure 3, the surface hardness was set to 700 HV or more and the core hardness was set to 250 HV or more, and the effect of surface compressive residual stress on rotating bending fatigue strength was investigated. Also in Figure 3, high fatigue strengths with a fatigue limit of 700 MPa or more are indicated by black circles (invention examples), and low fatigue strengths with a fatigue limit of less than 700 MPa are indicated by white squares (comparison examples).

[0062] Figure 3 shows that when the surface compressive residual stress is 750 MPa or higher, the fatigue limit increases significantly, and the rotating bending fatigue strength is particularly excellent. When the fracture surface of the steel was observed after the fatigue test, it was estimated that the fracture originated inside the specimen in the case of specimens with a surface compressive residual stress of 750 MPa or higher, but that the fracture originated on the surface in the case of specimens with a surface compressive residual stress of less than 750 MPa. In other words, when the surface compressive residual stress of the steel is sufficiently high, at 750 MPa or higher, the initiation or propagation of cracks from the surface is suppressed, which is thought to have improved the fatigue strength. Surface compressive residual stress is usually 1800 MPa or lower.

[0063] [Core hardness: 250HV or more] Next, Figure 4 shows the relationship between the fatigue limit (fatigue strength) and core hardness in a rotating bending fatigue test. In the experiment whose results are shown in Figure 4, the surface hardness was set to 700 HV or more and the surface compressive residual stress was set to 750 MPa or more, and the effect of core hardness on rotating bending fatigue strength was investigated. In Figure 4, high fatigue strength with a fatigue limit of 700 MPa or more is indicated by black circles (invention examples), and low fatigue strength with a fatigue limit of less than 700 MPa is indicated by white circles (comparison examples).

[0064] Figure 4 shows that when the core hardness is 250 HV or higher, the fatigue limit increases significantly, and the rotating bending fatigue strength is particularly excellent. As mentioned above, when the surface hardness is 700 HV or higher and the compressive residual stress is 750 MPa or higher, the steel undergoes fatigue failure from the inside. Therefore, it is thought that the fatigue strength is improved by further increasing the core hardness while still satisfying the above surface hardness and compressive residual stress, as it becomes more difficult for cracks to initiate and propagate from within the steel. On the other hand, from the viewpoint of toughness when the steel is used as a part (steel part), the core hardness is preferably 325 HV or less, and more preferably 310 HV or less.

[0065] (Steel parts) The steel part of the present invention has a core portion having a predetermined chemical composition and a predetermined structure mainly composed of bainite, and a surface layer portion having a chemical composition with a higher content of nitrogen, carbon, or nitrogen and carbon than the chemical composition of the core portion. Furthermore, the bainite contains dispersed precipitates of one or more of Cr, Mo, V, and Nb. The steel part of the present invention has such a structure that it can exhibit excellent fatigue properties as a whole.

[0066] The steel part can be the steel of the present invention itself, or the steel can be further given the shape of various parts, preferably machine structural parts. Among these, the steel part is particularly preferably a toothed part such as a gear. The teeth of a gear-shaped toothed part are required to have excellent bending fatigue strength because bending stress occurs at the tooth root. Forming the above-described hardness distribution in this tooth part and imparting compressive residual stress to the surface helps ensure the durability of the toothed part. In this regard, the steel part of the present invention has a core with the above-described chemical composition and a structure mainly composed of bainite, with specified precipitates dispersed in the bainite. Therefore, even in a gear-shaped part, cracking from within can be effectively suppressed and excellent fatigue properties can be exhibited.

[0067] In addition, even for steel parts other than toothed parts that are subject to repeated bending stress, bending fatigue in this area is important for ensuring the durability of the steel part. Therefore, by forming a hardened layer consisting of the above-mentioned compound layer and diffusion layer in the area where such repeated bending stress occurs, it is possible to obtain an improved durability effect. Therefore, the steel part of the present invention is not limited to toothed parts.

[0068] Composition of steel parts The composition of the steel part in the core is the same as the composition of the steel described above, and the effects of each element are also the same. When the steel has a hardened layer (surface region) and a core, the composition of the steel part in the core can be the same as the composition of the steel described above, and the effects of each element can also be the same. Furthermore, the surface layer (portion other than the core) has a chemical composition with a higher content of nitrogen, carbon, or nitrogen and carbon than the chemical composition of the core. Steel parts are usually obtained through the above-mentioned nitriding or nitrocarburizing treatment, and therefore have a different content of nitrogen and carbon than the core. When the steel has a structure with a hardened layer and a core, the chemical composition of the surface layer of the steel part can be the same as the chemical composition of the hardened layer of the steel.

[0069] Steel part structure The structure of the steel part has a core with an area ratio of bainite of more than 50%, similar to that described above for the steel, and the effects thereof are similar. When the steel has a structure having a hardened layer and a core, the structure of the core of the steel part can be similar to the structure of the core of the above-mentioned steel, and the effects thereof can be similar. Furthermore, in the surface layer portion, as described above for steel, the area fraction of bainite is preferably greater than 50%, and the area fraction of bainite in the entire structure of the steel part, including the core and surface layer portions, is also preferably greater than 50%. However, the surface layer portion of a steel part usually corresponds to the hardened layer consisting of the compound layer and diffusion layer described above, and since the compound layer is composed of nitrides, the area fraction of bainite in the surface layer portion excluding the compound layer is preferably greater than 50%. Similarly, for the above-mentioned steel, the area fraction of bainite in the surface layer region (hardened layer) excluding the compound layer is preferably greater than 50%.

[0070] The bainite that constitutes the microstructure of steel parts must contain dispersed precipitates of one or more of Cr, Mo, V, and Nb. A composite precipitate containing Cr, V, and Nb precipitates is preferred. Furthermore, composite precipitates containing nitrogen or nitrogen and carbon, such as (Cr,Mo,Nb,V)(C,N), may be dispersed in the bainite containing Cr, Mo, V, and / or Nb. More preferred are composite precipitates containing Cr(C,N), V(C,N), and Nb(C,N). Here, Cr(C,N) represents Cr carbide, nitride, or carbonitride; V(C,N) represents V carbide, nitride, or carbonitride; and Nb(C,N) represents Nb carbide, nitride, or carbonitride. By dispersing and precipitating the above-mentioned precipitates in bainite, which mainly constitutes the structure at least in the core of the steel part, it is possible to sufficiently increase the hardness in the core of the steel part, and to improve the fatigue properties of the steel part.

[0071] Preferably, the bainite that primarily constitutes the surface layer of the steel part (excluding the compound layer) also contains dispersed precipitates of one or more of Cr, Mo, V, and Nb, and more preferably contains composite precipitates containing Cr, V, and Nb precipitates. The precipitates may be dispersed as composite precipitates of (Cr,Mo,Nb,V)(C,N), and even more preferably as composite precipitates containing Cr(C,N), V(C,N), and Nb(C,N). The notation (C,N) is as explained above. This can further improve the fatigue properties of the steel part as a whole. Other suitable conditions and measurement techniques for precipitates are the same as those described above for steel.

[0072] As an example, the steel part can be obtained by subjecting a steel material to nitriding or soft nitriding, similar to the steel described above. That is, the core of the steel part may correspond to the core of the steel described above, and the surface layer of the steel part may correspond to the surface layer region (hardened layer) of the steel described above.

[0073] Surface hardness, core hardness, and surface compressive residual stress of steel parts The surface hardness of the steel part is preferably 700 HV or more, and usually 1000 HV or less, for the same reasons as those for the steel described above. For the same reasons as those for the steels described above, the core hardness of the steel part is preferably 250 HV or more, and more preferably 325 HV or less, and even more preferably 310 HV or less. The surface compressive residual stress of the steel part is preferably 750 MPa or more, and usually 1800 MPa or less, for the same reasons as those for the steel described above. The methods for measuring the surface hardness, surface compressive residual stress, and core hardness, as well as the method for measuring the rotating bending fatigue strength, are the same as those used in the examples described below.

[0074] Here, the surface layer and core of a steel part can be distinguished by hardness in the same manner as described above for the hardened layer and core of steel. That is, when hardness is measured at regular intervals from the surface to the center of a cross section of a steel part in the thickness direction, the hardness decreases with increasing distance from the surface in the depth direction, and eventually reaches a constant hardness value. This constant hardness value corresponds to the core, and the part where a significant increase in hardness is observed compared to the core corresponds to the surface layer.

[0075] (Manufacturing method) Next, an example of a manufacturing process that can satisfactorily manufacture the steel and steel parts of the present invention will be described. Figure 5 shows a typical manufacturing process for producing steel and steel parts according to the present invention when the steel and steel parts are obtained from steel bars. Here, S1 is a manufacturing process for converting steel material into steel bars, S2 is a process for transporting the steel bars, and S3 is a finishing process for converting the steel bars into nitrided steel or soft-nitrided steel as steel, or nitrided parts or soft-nitrided parts as steel parts. Whether a final product is steel or a steel part depends on whether the final product is to be used as a machine structural part as is, and does not mean that there is any difference in its structural characteristics (composition, structure, mechanical properties, etc.).

[0076] First, in the steel bar manufacturing process (S1), steel ingots with a specified chemical composition are hot-rolled to form steel bars, which are then inspected for quality and shipped. Next, in process (S2), the steel bars are transported to the finishing process (S3) for producing finished steel and steel parts. In the finishing process (S3) for producing the steel (nitrided steel or nitrocarburized steel) or steel part (nitrided part or nitrocarburized part) as a product, the steel bar is cut to the desired dimensions and hot forged or cold forged as needed. Then, cutting (machining) such as drilling or turning is performed as needed to produce the desired shape (for example, a gear product or shaft part). The steel material processed into the desired shape is then subjected to nitriding or nitrocarburizing. Shot peening may also be performed. In this way, the steel or steel part (product) is produced.

[0077] In the case of hot forging, cold straightening may be performed after the hot forging. In addition, steel and steel parts as products may be coated with paint, plating, etc. Alternatively, instead of the manufacturing flow illustrated in FIG. 5, rolled material (steel bar) may be directly finished into the desired shape by cutting processes such as turning or drilling, and then subjected to nitriding or soft nitriding to produce steel or steel parts as products.

[0078] Although not shown in Fig. 5, by performing a hot working step before the nitriding or nitrocarburizing treatment and setting the heating temperature and working temperature during the hot working under specific conditions, the steel material can be effectively formed into the desired structure described above at the cutting stage. That is, the steel material can have a structure mainly composed of bainite, and the formation of Cr, Mo, V, and Nb precipitates can be effectively suppressed. This improves the machinability of the steel material to be cut.

[0079] The above-mentioned hot working mainly means hot rolling or hot forging, but hot forging may be further carried out after hot rolling. Needless to say, cold forging may also be carried out after hot rolling.

[0080] Here, when hot rolling is performed as a hot working step before nitriding or nitrocarburizing, it is preferable to satisfy the following conditions during the hot rolling. In particular, when the hot working step is a hot rolling step, i.e., when hot forging is not performed after the hot rolling, it is more preferable to satisfy the following conditions during the hot rolling step. [Heating temperature: 950~1250℃] In the hot rolling process, carbides remaining from the melting stage are dissolved to prevent the formation of fine precipitates in the rolled material (steel bar that will be used for cold forging and / or cutting to produce steel or steel parts), thereby impairing forgeability. Specifically, if the heating temperature during hot rolling is less than 950°C, the carbides remaining from the melting stage are less likely to dissolve. This results in poor fatigue properties for the resulting steel and steel parts. On the other hand, if the heating temperature exceeds 1250°C, the crystal grains become coarse, which tends to deteriorate forgeability. For this reason, the heating temperature during hot rolling is preferably in the range of 950°C to 1250°C.

[0081] [Finishing temperature: 800°C or higher] If the finishing temperature in hot rolling is less than 800°C, a ferrite phase will be formed, which is disadvantageous in forming a bainite phase with an area ratio of more than 50% relative to the entire structure before nitriding or nitrocarburizing. Furthermore, the rolling load will be high. Consequently, the fatigue properties of the resulting steel and steel parts will be poor. Therefore, it is preferable that the finishing temperature in hot rolling be 800°C or higher. The upper limit of the finishing temperature is preferably set to about 1100°C.

[0082] [Cooling rate in the temperature range of at least 700-550°C: more than 0.4°C / s] In cutting, it is desirable to prevent the precipitation of fine precipitates before the steel material is finished into the desired shape, thereby preventing deterioration of workability and machinability. Therefore, in the temperature range of at least 700 to 550°C, which is the precipitation temperature range of fine precipitates, it is preferable to set the cooling rate after hot rolling to a rate exceeding 0.4°C / s, which is the critical cooling rate at which the precipitation of fine precipitates can be suppressed. Furthermore, if the cooling rate is slow, it is difficult to obtain a bainite structure, and the core hardness and fatigue strength of the finished steel and steel parts are likely to decrease. The upper limit of the cooling rate is preferably set to about 200° C. / s.

[0083] On the other hand, when the hot working step immediately before the nitriding or nitrocarburizing treatment is a hot forging step, that is, when only hot forging is performed or when hot forging is performed after hot rolling, it is preferable to satisfy the following conditions in the hot forging step. Note that when hot rolling is performed before hot forging, the hot rolling conditions do not necessarily have to satisfy the above conditions.

[0084] [Hot forging conditions] In hot forging, in order to make bainite account for more than 50% of the area of ​​the entire structure, and from the viewpoint of machinability in cold straightening and cutting after hot forging, it is desirable to prevent the precipitation of fine precipitates before cold straightening and cutting. Therefore, the heating temperature during hot forging is preferably set to 950 to 1250°C. The finishing temperature in the hot forging is preferably 800°C or higher, and can be 1100°C or lower. Furthermore, it is preferable that the cooling rate after hot forging is greater than 0.4°C / s at least in the temperature range of 700 to 550°C. Any of the preferable conditions is set for the same reasons as in the case of hot rolling described above.

[0085] The resulting steel material (rolled or forged material) is then cut to form a part. The steel material at the cutting stage has a structure mainly composed of bainite and has a reduced amount of fine precipitates, making it excellent in machinability. Thereafter, nitriding or soft nitriding can be carried out satisfactorily under the following conditions. By carrying out this nitriding or soft nitriding, nitrogen in the atmosphere and the component elements in the steel combine to form a compound layer made of compounds on the steel surface. Furthermore, a nitrogen (N) diffusion layer is formed inside the compound layer, and a hardened layer made of the compound layer and the diffusion layer is formed. This hardened layer can function suitably as a surface layer region in steel or a surface layer portion in a steel part.

[0086] In order to suitably obtain the steel or steel part of the present invention through this series of manufacturing steps, a steel bar having the above-mentioned chemical composition is obtained in the steel bar manufacturing step S1 shown in Fig. 5. As a result, the steel having the above-mentioned chemical composition can also be used as a steel material to be subjected to hot working steps including hot rolling and / or hot forging in the subsequent finishing step S3 to obtain the steel or steel part.

[0087] In order to satisfy the above-mentioned surface hardness and core hardness, it is preferable to set the nitriding or soft-nitriding temperature in the nitriding or soft-nitriding step to 550 to 590°C. Maintaining this temperature forms fine precipitates, which contributes to an increase in hardness. It is also preferable to set the nitriding or soft-nitriding time to a long time of at least 3 hours (h). This allows for the formation of a good surface layer region (hardened layer) in steel or a good surface layer portion in steel parts.

[0088] On the other hand, if the nitriding or nitrocarburizing time is this long, excessive N2 gas may be generated in the outermost layer of the compound layer, causing the porous layer to grow excessively and resulting in a decrease in fatigue strength. Therefore, it is preferable to suppress the excessive generation of N2 gas. In this regard, the thickness of the porous layer can be reduced by lowering the nitriding potential of the atmosphere during nitriding or nitrocarburizing. Therefore, it is preferable to confirm in advance the relationship between the nitriding potential and the porous layer thickness for each standard or component composition of the steel material, and to adopt a nitriding potential that can achieve the desired porous layer thickness. From the perspective of fatigue strength, it is desirable that the region where pores occur be 5 μm or less from the surface of the steel.

[0089] In nitriding, N penetrates and diffuses into the steel, so nitriding can be carried out in a mixed atmosphere of nitrogenous gases such as NH3 and N2, for example, an atmosphere of NH3:N2=50:50. In soft nitriding, N and C penetrate into the steel simultaneously to form a compound layer in which C is solid-dissolved, and N is then diffused into the base steel, so soft nitriding can be carried out in a mixed atmosphere of nitrogenous gases such as NH3 and N2 and carburizing gases such as CO2 and CO, for example, an atmosphere of NH3:N2:CO2=50:45:5.

[0090] The resulting nitrided or nitrocarburized material can be further shot peened under the following suitable conditions. This shot peening process plastically deforms the steel surface, effectively generating compressive residual stress on the surface. That is, to obtain a predetermined compressive residual stress on the steel surface, shot material having a hardness sufficiently greater than that of the nitrided or nitrocarburized material surface can be bombarded at an appropriate injection pressure so that the surface of the nitrided or nitrocarburized material undergoes sufficient plastic deformation. For example, it is preferable to project shot material such as cast steel having a hardness of about 600 HV or more at an injection pressure of 0.2 MPa or more. If the shot material is too large, it will be difficult to impact it at the desired position, so it is recommended to use shot material with a diameter of 2 mm or less. Shot peening may be performed two or more times to improve the deterioration of surface roughness caused by the initial shot peening, for example.

[0091] The steel or steel part of the present invention can be suitably obtained by the above-mentioned manufacturing steps. In other words, the steel or steel part of the present invention can be suitably obtained, for example, according to the following manufacturing flow. In mass%, C: 0.010 or more, 0.100% or less, Si: 1.00% or less, Mn: 0.50% or more, 3.00% or less, P: 0.020% or less, S: 0.060% or less, Cr: 0.30% or more, 3.00% or less, Mo: 0.005% or more, 0.400% or less, V: 0.02% or more, 0.50% or less, Nb: 0.003% or more, 0.150% or less, Al: 0.005% or more, 0.200% or less, N: 0.0200% or less and Sb: 0.0005% or more, 0.0200% or less A method for manufacturing steel or a steel part, comprising: subjecting a steel material having a chemical composition comprising the above-mentioned components, with the balance being Fe and unavoidable impurities, to hot working at a heating temperature of 950 to 1250°C and a finishing temperature of 800°C or higher; cooling at a cooling rate of more than 0.4°C / s in a temperature range of at least 700 to 550°C after the hot working; subsequently subjecting the steel material to nitriding or nitrocarburizing; and then subjecting the steel material to shot peening.

[0092] The component composition is further expressed in mass% as follows: W: 0.3% or less, Co: 0.3% or less, Hf: 0.2% or less, Zr: 0.2% or less, B: 0.0100% or less, Cu: 0.3% or less, Ni: 0.3% or less, Pb: 0.2% or less, Bi: 0.2% or less, Zn: 0.2% or less, Sn: 0.2% or less and Ti: 0.1% or less A manufacturing method for steel or steel parts that contains one or more selected from the above.

[0093] The hot working may be hot rolling.

[0094] The hot working may be hot forging.

[0095] The hot working can be hot rolling followed by hot forging. [Example]

[0096] Examples of the present invention will be specifically described below. Molten steels (steel types 1 to 41) having the chemical compositions shown in Table 1 were cast into slabs with a cross section of 300 mm x 400 mm using a continuous casting machine. This slab was soaked at 1250°C for 30 minutes and then hot rolled into a slab with a rectangular cross section of 140 mm on a side. This slab was further hot rolled according to the conditions shown in Table 2 to produce an 80 mmφ steel bar (as-hot-rolled steel material). Furthermore, this steel bar was subjected to hot forging according to the conditions shown in Table 2 to produce a smaller diameter steel bar (hot-forged steel material) of 35 mm. Some of the as-hot-rolled steel material was not subjected to hot forging and was used as-hot-rolled material.

[0097] [Table 1] TIFF0007800789000002.tif233122

[0098] [Table 2] TIFF0007800789000004.tif208170

[0099] The resulting steel materials (hot-rolled or hot-forged) were subjected to peripheral turning tests to evaluate tool life as a measure of machinability. Test specimens for evaluation were prepared by cutting the hot-rolled or hot-forged materials to 200 mm lengths. Mitsubishi Materials Corporation's CSBNR 2020 folder and Mitsubishi Materials Corporation's SNGN 120408 UTi20 high-speed tool steel insert were used. The peripheral turning test conditions were a depth of cut of 1.0 mm, a feed rate of 0.25 mm / rev, and a cutting speed of 200 m / min. Yushiroken lubricant was used. Tool life was evaluated as the time (seconds) until tool wear (flank wear) reached 0.2 mm. The results are shown in Table 3. A tool life of 1200 s or longer is considered to be excellent machinability.

[0100] Furthermore, hardness measurements were performed on the above-mentioned hot-rolled and hot-forged materials. Test specimens for evaluation were taken from the center of the hot-rolled or hot-forged materials. Hardness measurements were performed using a Vickers hardness tester in accordance with JIS Z2244. Hardness was measured at five points at 1 / 4 radial positions with a test load of 2.94 N (300 gf), and the average value was taken as hardness HV. The results are shown in Table 3.

[0101] Furthermore, the structure of the as-hot-rolled and hot-forged materials was identified and the area fraction was measured. Specifically, test specimens were taken from the as-hot-rolled and hot-forged materials, and the cross sections parallel to the rolling direction (L cross sections) were polished and then etched with nital. The type of structure was identified by observing the cross sections of these test specimens using an optical microscope (optical microscope structure observation at 200x magnification), and the area fraction of the bainite structure was determined. The results are shown in Table 3.

[0102] [Table 3] TIFF0007800789000006.tif224170

[0103] Furthermore, the above-mentioned as-hot-rolled material and hot-forged material were cut parallel to the longitudinal direction to obtain the rotating bending fatigue test specimens shown in Fig. 1. These test specimens were then subjected to nitriding or soft nitriding under the conditions shown in Table 2. Here, in order to obtain the desired precipitation state and hardness distribution, the treatment temperature, treatment time, and nitriding potential (atmospheric gas composition) were appropriately adjusted to the conditions shown in Table 2. Next, the nitrided or nitrocarburized test pieces were subjected to shot peening under the conditions shown in Table 2. In order to obtain the desired compressive residual stress, the shot material and injection pressure were adjusted appropriately to the conditions shown in Table 2.

[0104] The test pieces thus obtained after shot peening (hereinafter collectively referred to as "nitrided materials") were used as steel products or steel parts. The surface and core hardness, effective case depth, and compressive residual stress of the surface of these nitrided materials were measured. Furthermore, the microstructure and precipitates were observed, and fatigue properties were evaluated.

[0105] The surface hardness was measured at five points 50 μm from the surface on the cross section of the nitrided material, and the average value was taken as the surface hardness (HV). Note that this position 50 μm from the surface was confirmed to be included in the hardened layer (surface layer) according to the above-mentioned confirmation method for the hardened layer (surface layer) and core. The hardness of the core was measured at two points 2 mm from the surface in the same cross section, and the average value was taken as the core hardness (HV). Note that this position 2 mm from the surface was confirmed to be included in the core according to the above-mentioned method for confirming the hardened layer (surface layer) and core. The effective hardened layer depth (HV550) was determined by measuring the hardness at 0.1 mm intervals from the surface toward the axial center and interpolating the depth at which HV550 was achieved. The hardness was measured using a Vickers hardness tester in accordance with JIS Z2244 under a test load of 2.94 N (300 gf).

[0106] The compressive residual stress (MPa) was measured by electrolytically polishing the notch bottom of the nitrided test piece to about 50 μm from the surface, and measuring the axial stress using AutoMATE manufactured by Rigaku Corporation. 2 It was measured using the ψ method. It was measured using α-Fe 211 diffraction (unstrained 2θ = 156.40°) and calculated using a stress constant of -318 MPa / deg (minus 318 MPa / deg).

[0107] For the observation of the structure and precipitates, a specimen for transmission electron microscope observation was prepared from the radial quarter position of the parallel part of the nitrided material by electrolytic polishing using the twin jet method. The obtained specimen was then subjected to a transmission electron microscope with an accelerating voltage of 200 V to identify the type of structure and determine the area ratio of the bainite structure. It was also confirmed that the precipitates shown in Table 4 were present dispersed in the bainite. Furthermore, the composition of the observed precipitates was determined by energy dispersive X-ray spectroscopy (EDX).

[0108] For the evaluation of fatigue properties, test pieces (see Figure 1) obtained from the above nitrided material, which had not undergone hardness measurement, microstructure observation, or precipitate observation, were used as test pieces for the rotating bending fatigue test. Then, using an Ono-type rotating bending fatigue tester, the fatigue test results were plotted in an SN diagram with the stress amplitude on the vertical axis and the number of repetitions N until fracture on the horizontal axis, and the fatigue limit strength (MPa) was calculated. The fatigue limit strength was 10 for N=2 or more. 7 The fatigue limit strength was 700 MPa or more, which is the maximum stress at which the test continued for more than 100 cycles. The rotation speed was 3500 rpm. If the fatigue limit strength is 700 MPa or more, it can be said that the fatigue properties are excellent.

[0109] The above measurement results, observation results and evaluation results are shown in Table 4. It was confirmed that the final steel and steel parts (nitrided materials) had the chemical composition shown in Table 1 in the core, and that the surface layer (hardened layer) had a higher content of N and / or C than the chemical composition shown in Table 1.

[0110] [Table 4] TIFF0007800789000008.tif205170

[0111] As is clear from Tables 3 and 4, all of Examples Nos. 1 to 26 exhibit excellent machinability and fatigue strength. On the other hand, Comparative Examples Nos. 27 to 60 are inferior in machinability, fatigue properties, or both, because at least one of the chemical composition, steel structure, precipitates, hardness, and compressive residual stress is outside the range of the present invention.

Claims

1. In mass%, C: 0.010 or more, 0.100% or less, Si: 1.00% or less, Mn: 0.50% or more, 3.00% or less, P: 0.020% or less, S: 0.060% or less, Cr: 0.30% or more, 3.00% or less, Mo: 0.005% or more, 0.400% or less, V: 0.02% or more, 0.50% or less, Nb: 0.003% or more, 0.150% or less, Al: 0.005% or more, 0.200% or less, N: 0.0200% or less and Sb: 0.0005% or more, 0.0200% or less and the balance being Fe and unavoidable impurities, The steel has a structure in which the area ratio of bainite is more than 50%, The bainite contains precipitates of one or more of Cr, Mo, V, and Nb, The surface hardness is 700 HV or more and the core hardness is 250 HV or more, A steel having a surface compressive residual stress of 750 MPa or more.

2. The component composition is, in mass%, W: 0.3% or less, Co: 0.3% or less, Hf: 0.2% or less, Zr: 0.2% or less, B: 0.0100% or less, Cu: 0.3% or less, Ni: 0.3% or less, Pb: 0.2% or less, Bi: 0.2% or less, Zn: 0.2% or less, Sn: 0.2% or less and Ti: 0.1% or less The steel according to claim 1, further comprising one or more selected from the group consisting of:

3. A steel sheet comprising: a core portion having the chemical composition according to claim 1 or 2 and a structure in which the area ratio of bainite is more than 50%; and a surface layer portion having a chemical composition in which the content of at least nitrogen or carbon is higher than that of the chemical composition of the core portion, The steel part has precipitates of one or more of Cr, Mo, V and Nb dispersed in the bainite.