shaft component

Abstract

To provide a shaft member that is excellent in torsional fatigue strength and suitable for shafts of a reduction gear or a speed change gear of automobiles and construction machines.SOLUTION: A shaft member has an approximately cylindrical outer shape and includes a carburized layer on a surface and a core inside, and the core has, in mass%, C: 0.10 to 0.30%, Si: 0.03 to 1.50%, Mn: 0.30 to 1.00%, P: 0.035% or less, S: 0.035% or less, Cr: 0.02 to 1.80%, Mo: 0.02 to 0.50%, Al: 0.005 to 0.100%, N: 0.002 to 0.0250%, balance of Fe and impurities, and satisfies formula (1), and in at least a part of a surface layer region in a longitudinal direction, an average compressive residual stress in a longitudinal direction is 1700 MPa or more, an average volume of residual austenite is 5.0% or less, the average Vickers hardness is 770 HV or more. 0.98<0.25Si+Mn+0.40Cr+0.25Mo<1.33 (1).SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present disclosure relates to a shaft member.

Background Art

[0002] In speed reducers and transmissions of automobiles, construction machinery, etc., shafts provided with oil holes for supplying lubricating oil are used. For example, in order to miniaturize the power train of an automobile, it is necessary to miniaturize the shaft, which is a component thereof. To achieve this, it is necessary to increase the strength of the shaft.

[0003] When the shaft is twisted, stress concentrates on the oil hole, resulting in fatigue failure. Therefore, an improvement in torsional fatigue strength is required for the shaft. For the purpose of improving torsional fatigue strength, for example, in Patent Document 1, it has been proposed to improve torsional fatigue strength by including a predetermined chemical composition, having a deep case hardening depth, a large compressive residual stress, and reducing the incomplete quenching layer. Also, in Patent Document 2, it has been proposed to improve torsional fatigue strength by including C, Si, Mn, Cr, Mo, B, etc., having a case hardening layer depth within a predetermined range, and improving the toughness of the case hardened layer.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] Although various carburized members and the like for the purpose of improving torsional fatigue strength have been proposed as in Patent Documents 1 and 2, a shaft member having higher torsional fatigue strength is desirable.

[0006] The objective of this disclosure is to provide a shaft member that exhibits excellent torsional fatigue strength and is suitable for use in the shafts of speed reducers and transmissions in automobiles, construction machinery, and the like. [Means for solving the problem]

[0007] The means for solving the above problems include the following embodiments. <1> A shaft member having a roughly cylindrical outer shape, comprising a carburized layer on the surface and a core portion located inside the carburized layer, The core portion is by mass % C: 0.10~0.30%, Si: 0.03~1.50%, Mn: 0.30~1.00%, P: 0.035% or less, S: 0.035% or less, Cr: 0.02~1.80%, Mo: 0.02~0.50%, Al: 0.005~0.100%, N: 0.0020~0.0250%, and O: 0.0015% or less It contains, with the remainder being Fe and impurities, and has a chemical composition that satisfies the following formula (1): When the region from the surface of the shaft member to a depth of 0.30 mm is defined as the surface region, In at least a portion of the longitudinal direction of the shaft member, there is a high compressive residual stress area in the surface region where the average compressive residual stress in the longitudinal direction is 1700 MPa or more. The aforementioned high-compression residual stress portion is a shaft member in which the average volume fraction of retained austenite is 5.0% or less and the average Vickers hardness is 770HV0.2 or higher. 0.98<0.25Si+Mn+0.40Cr+0.25Mo<1.33 (1) However, the element symbols in formula (1) indicate the mass percentage content of the element in the core portion. <2> A shaft member having a roughly cylindrical outer shape, comprising a carburized layer on the surface and a core portion located inside the carburized layer, The core part is, by mass% C: 0.10 to 0.30%, Si: 0.03 to 1.50%, Mn: 0.30 to 1.00%, P: 0.035% or less, S: 0.035% or less, Cr: 0.02 to 1.80%, Mo: 0.02 to 0.50%, Al: 0.005 to 0.100%, N: 0.0020 to 0.0250%, and O: 0.0015% or less contains, and further contains one or more selected from the group consisting of the following Group A, Group B, Group C, and Group D, (Group A) Cu: 0.40% or less, Mg: 0.0050% or less, V: 0.50% or less, Nb: 0.100% or less, Ti: 0.100% or less, Ca: 0.0050% or less, and one or more selected from the group consisting of B: 0.0050% or less (Group B) Ni: 0.30% or less (Group C) Sn: 0.100% or less (Group D) Bi: 0.020% or less, and one or two selected from the group consisting of Pb: 0.09% or less The balance consists of Fe and impurities, and has a chemical composition satisfying the following formula (1), When the region from the surface of the shaft member to a depth of 0.30 mm is defined as the surface layer region, In at least a part of the longitudinal direction of the shaft member, there is a high compressive residual stress portion where the average compressive residual stress in the longitudinal direction in the surface layer region is 1700 MPa or more, The high compressive residual stress portion has a shaft member in which the average volume ratio of retained austenite is 5.0% or less and the average Vickers hardness is 770 HV0.2 or more. 0.98 < 0.25Si + Mn + 0.40Cr + 0.25Mo < 1.33 ···(1) However, the element symbols in the formula (1) indicate the content of the element in mass % in the core part. <3> The shaft member according to <2>, wherein the chemical composition contains Group A. <4> The shaft member according to <2>, wherein the chemical composition contains Group B. <5> The shaft member according to <2>, wherein the chemical composition contains Group C. <6> The shaft member according to <2>, wherein the chemical composition contains Group D. <7> The shaft member according to any one of <1> to <6>, wherein at least one hole is formed on the outer peripheral surface, and at least the region where the hole is formed in the longitudinal direction of the shaft member is the high compressive residual stress part.

Advantages of the Invention

[0008] According to the present disclosure, there is provided a shaft member having excellent torsional fatigue strength and suitable for shafts of speed reducers and transmissions in automobiles, construction machines, etc.

Brief Description of the Drawings

[0009] [Figure 1] It is a (A) front view and (B) side view showing an example of a tool used for cold roller burnishing. [Figure 2] It is a schematic view showing a method of performing cold roller burnishing on a workpiece using the tool shown in FIG. 1. [Figure 3] It is a perspective view showing the sample shape used in the examples. [Figure 4] It is a diagram showing the process of manufacturing a sample of the shaft member in the examples. [Figure 5] It is a diagram showing the heat pattern of the gas carburizing treatment in the examples. [Figure 6] It is a diagram showing an example of the torsional fatigue strength when cold roller burnishing is performed and when it is not performed after gas carburizing. [Figure 7]This is a schematic diagram illustrating an example of a method for measuring longitudinal residual stress in the surface region of a shaft member. [Modes for carrying out the invention]

[0010] An example of an embodiment of this disclosure will be described. In this specification, a numerical range indicated by "~" means a range that includes the numbers before and after "~" as the lower and upper limits, respectively. Furthermore, if the numbers before and after "~" are preceded by "greater than" or "less than," the numerical range means a range that does not include those numbers as the lower or upper limit. In the numerical ranges described stepwise in this specification, the upper or lower limit of one stepwise numerical range may be replaced with the upper or lower limit of another stepwise numerical range, or with the values ​​shown in the examples. Furthermore, when referring to the elemental content in chemical composition, "%" means "mass%". Furthermore, the term "process" includes not only independent processes, but also any process that cannot be clearly distinguished from other processes, as long as its intended purpose is achieved.

[0011] The inventors of this disclosure have diligently studied how to improve the torsional strength characteristics of carburized shaft members and have found that applying cold roller burnishing (sometimes abbreviated as "RB" in this disclosure) using a specific tool after carburizing improves the hardness of the surface layer of the shaft member and also improves the torsional fatigue strength of the shaft member by introducing compressive residual stress.

[0012] <Shaft component> The shaft member according to this disclosure has a substantially cylindrical outer shape and includes a carburized layer on the surface and a core portion inside the carburized layer, the core portion being by mass % C: 0.10~0.30%, Si: 0.03~1.50%, Mn: 0.30~1.00%, P: 0.035% or less, S: 0.035% or less, Cr: 0.02~1.80%, Mo: 0.02~0.50%, Al: 0.005~0.100%, N: 0.0020~0.0250%, and O: 0.0015% or less It contains [a certain compound], with the remainder being Fe and impurities, and has a chemical composition that satisfies formula (1) described later. Furthermore, the shaft member according to this disclosure has a high compressive residual stress region in at least a portion of the longitudinal direction of the shaft member, where the average compressive residual stress in the longitudinal direction of the surface region is 1700 MPa or more, with the average volume fraction of retained austenite being 5.0% or less and the average Vickers hardness being 770HV0.2 or more.

[0013] Furthermore, the shaft members relating to this disclosure include carburized shaft components, and for example, shaft components used in automobiles and industrial machinery, such as transmission shafts. Furthermore, the shaft members relating to this disclosure include shaft members of any shape having a substantially cylindrical outer shape, and may, for example, be hollow or solid cylindrical or rod-shaped shaft members, or have a stepped shape on the surface. In addition, the shaft members relating to this disclosure are not limited to shaft parts as final products, but also include shaft members before the final product is manufactured. Furthermore, while the following description mainly focuses on shaft members having at least one hole, such as an oil hole, on their outer surface, shaft members without such holes are also included in the shaft members of this disclosure.

[0014] (Chemical composition of the core) The chemical composition of the shaft member relating to this disclosure will be explained below. Note that, because carbon is introduced into the surface layer of the shaft member through carburizing treatment, the chemical composition differs between the surface layer and the interior (core) of the shaft member. The chemical composition shown below (Cs: excluding the C content of the surface layer) is the chemical composition of the core located inside the carburized layer of the shaft member, specifically the chemical composition of the interior of the shaft member at a depth of 3 mm or more from the outer surface.

[0015] C: 0.10~0.30% Carbon (C) increases the strength of the shaft member (especially the core). On the other hand, if the C content is too high, the strength of the steel material used to process the shaft member becomes too high, reducing the machinability of the steel material. Therefore, the C content should be 0.10 to 0.30%. Preferably, the C content is 0.13 to 0.28%, and more preferably 0.15 to 0.26%.

[0016] Si: 0.03~1.50% Silicon (Si) increases the strength of the shaft member (especially the core). On the other hand, if the Si content is too high, the strength of the steel material used to process the shaft member becomes too high, reducing the machinability of the steel material. Therefore, the Si content should be 0.03 to 1.50%. Preferably, the Si content is 0.10 to 1.20%, and more preferably 0.20 to 1.00%.

[0017] Mn: 0.30~1.00%, Manganese (Mn) is an element that enhances the hardenability of steel, thereby increasing the strength of shaft members (especially the core). On the other hand, if the Mn content is too high, the strength of the steel material used to process the shaft members becomes too high, reducing the machinability of the steel material. Therefore, the Mn content should be 0.30 to 1.00%. Preferably, the Mn content is 0.40 to 0.95%, and more preferably 0.50 to 0.90%.

[0018] P:0.035% or less Phosphorus (P) is an impurity. P segregates at grain boundaries, reducing grain boundary strength. As a result, the torsional fatigue strength of the shaft member decreases. Therefore, the P content should be 0.035% or less. The preferred upper limit for the P content is 0.030%, and more preferably 0.025%. Although a lower P content is preferable, from the viewpoint of suppressing the cost of removing P, the P content may be 0.0020% or more.

[0019] S: 0.035% or less Sulfur (S) combines with manganese (Mn) to form MnS, which improves machinability. If the S content is too low, this effect cannot be obtained. On the other hand, if the S content is too high, coarse MnS is formed, reducing the torsional fatigue strength of the shaft member. Therefore, the upper limit of the S content should be 0.035%. The preferred upper limit of the S content is 0.025%, and more preferably 0.020%. From the viewpoint of suppressing the cost of removing S, the lower limit of the S content may be 0.004% or higher.

[0020] Cr: 0.02~1.80% Chromium (Cr) enhances the hardenability of steel and increases the strength of shaft members (especially the core). However, if the Cr content is too high, the strength of the steel used to process the shaft members becomes too high, reducing the machinability of the steel. Therefore, the Cr content should be 0.02 to 1.80%. Preferably, the Cr content is 0.80 to 1.40%, and more preferably 0.90 to 1.30%.

[0021] Mo: 0.02~0.50% Molybdenum (Mo) enhances the hardenability of steel and increases the strength of shaft members (especially the core). However, if the Mo content is too high, the strength of the steel material used to process the shaft member becomes too high, reducing the machinability of the steel material. Therefore, the Mo content should be 0.02 to 0.50%. Preferably, the Mo content is 0.10 to 0.40%, and more preferably 0.15 to 0.35%.

[0022] Al: 0.005~0.100% Aluminum (Al) deoxidizes steel. Al further combines with nitrogen to form AlN, which refines the crystal grains. As a result, the torsional fatigue strength of the shaft member increases. On the other hand, if the Al content is too high, hard and coarse Al2O3 is formed, reducing the machinability of the steel and also decreasing the torsional fatigue strength. Therefore, the Al content should be 0.005 to 0.100%. Preferably, the Al content is 0.010 to 0.070%, and more preferably 0.012 to 0.050%.

[0023] N: 0.0020~0.0250% Nitrogen (N) forms nitrides, which refine the crystal grains and increase the torsional fatigue strength of the shaft member. On the other hand, if the N content is too high, coarse nitrides are formed, reducing the forgeability of the steel. Therefore, the N content should be 0.0020 to 0.0250%. Preferably, the N content is 0.0030 to 0.0230%, and more preferably 0.0050 to 0.0200%.

[0024] O: 0.0015% or less Oxygen (O) is an impurity. O combines with other elements in the steel to form coarse oxide inclusions. Coarse oxide inclusions reduce the torsional fatigue strength of the shaft member. O also combines with Al to form hard oxide inclusions. Oxide inclusions reduce the torsional fatigue strength of the shaft member. Therefore, the O content should be 0.0015% or less. The lower the O content, the better. The preferred upper limit for O content is 0.0013%, and the more preferred upper limit is 0.0012%.

[0025] Remainder: Fe and impurities The remainder of the chemical composition of the shaft member described above consists of iron (Fe) and impurities. Impurities are components that are introduced from ore or scrap used as raw materials for steel, or from the environment during the manufacturing process, and are not intended to be included in the shaft member.

[0026] The chemical composition of the core further satisfies the following formula (1). 0.98<0.25Si+Mn+0.40Cr+0.25Mo<1.33 (1) However, the element symbols in formula (1) indicate the mass percentage content of each element in the core. The fact that the chemical composition of the core satisfies formula (1) makes it easier to secure retained austenite before the roller burnishing process described later when manufacturing the shaft member according to this disclosure.

[0027] The shaft member may further contain, in place of some of the Fe, one or more elements selected from the group consisting of Cu, Mg, V, Nb, Ti, Ca, B, Ni, Sn, Bi, and Pb. These elements are optional and do not necessarily have to be included in the shaft member. The optional elements are described below.

[0028] Cu:0.40% or less Copper (Cu) enhances the hardenability of steel, thereby increasing its fatigue strength. To obtain the above effect from Cu, a Cu content of 0.05% or more is preferable. However, if the Cu content is too high, it will segregate at the grain boundaries of the steel during hot forging, inducing hot cracking. Therefore, the upper limit of the Cu content should be 0.40%. A Cu content of 0.30% or less is preferable, and 0.25% or less is more preferable.

[0029] Mg: 0.0050% or less Magnesium (Mg) controls the morphology of nonmetallic inclusions that initiate torsional fatigue fracture, thereby improving torsional fatigue strength. If the Mg content is less than 0.0005%, the above effect is not sufficiently obtained. However, if the Mg content exceeds 0.0050%, the above effect saturates. Therefore, the Mg content should be 0.0050% or less.

[0030] V: 0.50% or less Vanadium (V) combines with C and N to form precipitates. These V precipitates complement the grain refinement of the AlN-quenched portion. The V precipitates increase the torsional fatigue strength of the shaft member. To obtain the above effects of V, a content of 0.01% or more is preferable. However, if the V content exceeds 0.50%, the precipitates become coarser, and the torsional fatigue strength decreases. Therefore, the upper limit of the V content is 0.50%. The V content is preferably 0.40% or less.

[0031] Nb: 0.100% or less Niobium (Nb) combines with carbon (C) and nitrogen (N) to form precipitates. These Nb precipitates complement the grain refinement achieved by AlN in the quenched areas. The Nb precipitates increase the torsional fatigue strength of the shaft member. To obtain the above effects from Nb, a content of 0.010% or more is preferable. However, if the Nb content exceeds 0.100%, the precipitates become coarser, and the torsional fatigue strength decreases. Therefore, the upper limit of the Nb content is 0.100%. The Nb content is preferably 0.050% or less.

[0032] Ti:0.100% or less Titanium (Ti) combines with carbon (C) and nitrogen (N) to form precipitates. These Ti precipitates complement the grain refinement achieved by AlN in the quenched areas. The Ti precipitates increase the torsional fatigue strength of the shaft member. To obtain the above effects from Ti, a content of 0.01% or more is preferable. However, if the Ti content exceeds 0.100%, the precipitates become coarser, and the torsional fatigue strength decreases. Therefore, the upper limit of the Ti content is 0.100%. The Ti content is preferably 0.080% or less.

[0033] Ca: 0.0050% or less Calcium (Ca) controls the morphology of nonmetallic inclusions that serve as the starting point for torsional fatigue fracture, thereby improving torsional fatigue strength. If the Ca content is less than 0.0002%, the above effect is not sufficiently obtained. However, if the Ca content exceeds 0.0050%, coarse oxide inclusions are formed in the steel. Therefore, the Ca content should be 0.0050% or less. Preferably, the Ca content is 0.0020% or less, and more preferably 0.0015% or less.

[0034] B: 0.0050% or less Boron (B) has the effect of suppressing grain boundary segregation of P and increasing torsional fatigue strength. To obtain the above effect from B, a content of 0.0005% or more is preferable. However, the effect saturates even if the B content exceeds 0.0050%. Therefore, the B content is 0.0050% or less. The B content is preferably 0.0030% or less, and more preferably 0.0020% or less.

[0035] Ni: 0.30% or less Nickel (Ni) enhances the hardenability and toughness of steel. To obtain the above effects from Ni, a content of 0.05% or more is preferable. However, if the Ni content is too high, there will be an excess of retained austenite after carburizing and quenching. In this case, sufficient work-induced martensitic transformation will not occur during machining after tempering. As a result, the torsional fatigue strength of the shaft member will decrease. Therefore, the upper limit of the Ni content is 0.30%. The Ni content is preferably 0.20% or less.

[0036] Sn: 0.100% or less Tin (Sn) is an element that enhances the corrosion resistance of steel. However, a content exceeding 0.100% causes embrittlement, leading to a decrease in torsional fatigue strength. Therefore, the Sn content should be 0.100% or less.

[0037] Bi:0.020% or less Bismuth (Bi) is an element that enhances the machinability of steel. To obtain the above effect, a content of 0.001% or more is preferable. On the other hand, if the content exceeds 0.020%, the hot ductility decreases. Therefore, the Bi content should be 0.020% or less, and the preferred content is 0.001 to 0.020%.

[0038] Pb: 0.09% or less Lead (Pb) is an element that enhances the machinability of steel. While it is possible to include a larger amount from the standpoint of machinability, it is an environmentally harmful substance, so the Pb content in the shaft members according to this disclosure is limited to 0.09% or less.

[0039] The above optional elements can be divided into the following groups based on the effects of including them within the ranges described above. Group A (improvement in torsional fatigue strength): Cu, Mg, V, Nb, Ti, Ca, B Group B (Improved toughness): Ni Group C (Improved corrosion resistance): Sn Group D (Improved machinability): Bi, Pb The shaft member relating to this disclosure may contain one or more types selected from the groups consisting of Group A, Group B, Group C, and Group D.

[0040] C content (Cs) in the surface layer The shaft member according to this disclosure has a carburized layer on its surface, so the carbon content in the surface layer is higher than that in the core. The carbon contained in the surface layer of the shaft member increases the torsional fatigue strength of the shaft member. The carbon (Cs) content in the surface layer is not particularly limited as long as it is higher than the carbon content in the core layer. However, if the carbon (Cs) content in the surface layer is low, the hardness of the carburized layer will decrease. As a result, the torsional fatigue strength of the shaft member will decrease. On the other hand, if the Cs content is high, hard protereminate cementite will form in the surface layer of the shaft member, and this cementite will become the starting point for fracture, further reducing the torsional fatigue strength. In addition, tool wear during cutting will increase, and machinability will decrease. Therefore, the carbon (Cs) content in the surface layer is preferably 0.70 to 1.10%, and more preferably 0.75 to 0.95%.

[0041] (Average compressive residual stress in the longitudinal direction in the surface region: 1700 MPa or more) The shaft member according to this disclosure has a high compressive residual stress section in which, when the area from the surface to a depth of 0.30 mm is defined as the surface region, at least a portion of the shaft member in the longitudinal direction has a high compressive residual stress section in which the average compressive residual stress in the longitudinal direction of the shaft member in the surface region (which may be referred to in this disclosure as "average compressive residual stress in the longitudinal direction of the surface region" or "average compressive residual stress in the surface region") is 1700 MPa or more. The shaft member according to this disclosure has a high compressive residual stress section in which the average compressive residual stress in the longitudinal direction of the surface region is 1700 MPa or more, thereby exhibiting high torsional fatigue strength.

[0042] A high-compressive-residual-stress area may be formed along the entire length of the shaft member, or it may be formed only in a part of the shaft member, particularly in a part where stress is concentrated. For example, in a shaft member in which oil holes are formed, stress concentrates in the oil holes when the shaft is twisted. Therefore, if the high-compressive-residual-stress area has an average compressive-residual-stress of 1700 MPa or more in the surface region at least where the oil holes are formed, it can exhibit high torsional fatigue resistance.

[0043] Furthermore, even in shaft members where oil holes are not formed, stress from the twisting of the shaft tends to concentrate in areas where the cross-sectional area perpendicular to the axial direction is relatively small, such as areas where the diameter is smaller than at other locations. Thus, regardless of the presence or absence of holes, it is preferable that the average compressive residual stress in the surface region is 1700 MPa or higher throughout the circumferential direction of areas where stress tends to concentrate.

[0044] The average compressive residual stress in the longitudinal direction in the surface region (high compressive residual stress area) is measured as follows. The longitudinal compressive residual stress is determined by X-ray diffraction. If holes are present, it is preferable to measure the longitudinal compressive residual stress in the surface region near the holes and at a sufficiently distant position in the circumferential direction from the holes. For example, if the shaft member has through holes in the radial direction, it is preferable to measure the longitudinal compressive residual stress in the surface region at an intermediate position in the circumferential direction between the two holes (i.e., at a position 90° circumferentially from each hole). A Rigaku AutoMATE micro-X-ray stress analyzer is used to measure the longitudinal compressive residual stress. A Cr tube is used as the light source. The tube voltage is 40kV, the tube current is 40mA, and the collimator diameter is 1.0mm. Figure 7 schematically shows an example of a method for measuring longitudinal residual stress in the surface region of a shaft member. The longitudinal compressive residual stress is measured at each depth position from the surface of the shaft member to a depth of 0.30 mm at 0.05 mm intervals. The longitudinal compressive residual stress on the surface of the shaft member is determined by X-ray diffraction by irradiating the shaft member with X-rays in an in-plane direction parallel to the longitudinal direction, without electropolishing. The longitudinal compressive residual stresses at depths of 0.05 mm, 0.10 mm, 0.15 mm, 0.20 mm, 0.25 mm, and 0.30 mm from the surface are determined by X-ray diffraction after electropolishing. Specifically, as shown in Figure 7, the measurement surface at each depth position is exposed by electropolishing. X-rays are irradiated onto the exposed measurement surface in an in-plane direction parallel to the longitudinal direction of the shaft member 30, and the longitudinal compressive residual stress at each depth position is measured by X-ray diffraction. The arithmetic mean of the longitudinal compressive residual stresses at each depth position obtained, including the longitudinal compressive residual stress of the surface, is defined as the average longitudinal compressive residual stress (average compressive residual stress).

[0045] (Average volume fraction of retained austenite in areas with high compressive residual stress: 5.0% or less) The shaft member according to this disclosure has an average volume fraction of retained austenite in the high-compression residual stress area of ​​5.0% or less. The shaft member according to this disclosure can more reliably increase torsional fatigue strength because the average volume fraction of retained austenite in the high-compression residual stress area in the surface region is 5.0% or less. The main metal structure in the high-compression residual stress area is, for example, martensite. The microstructure in the surface region is measured as follows:

[0046] The retained austenite volume fraction is determined by X-ray diffraction. If holes are present, it is preferable to measure the retained austenite volume fraction in the surface region near the holes and at a sufficiently distant position circumferentially from the holes. For example, if the shaft member has through holes in the radial direction, it is preferable to measure the retained austenite volume fraction in the surface region at an intermediate position circumferentially between the two holes (i.e., at a 90° circumferential angle from each hole). A Rigaku AutoMATE micro-X-ray stress analyzer is used for the measurements. A Cr tube is used as the light source. The tube voltage is 40kV, the tube current is 40mA, and the collimator diameter is 1.0mm. The retained austenite volume fraction is measured based on the integrated intensity ratio of the diffraction peaks of the (221) plane of the bcc structure and the (220) plane of the fcc structure obtained by X-ray diffraction. The retained austenite volume fraction is measured at each depth position at 0.05mm intervals, up to a depth of 0.30mm from the surface of the shaft member. The retained austenite volume fraction on the surface of the shaft member is determined by X-ray diffraction without electropolishing. The retained austenite volume fractions at depths of 0.05 mm, 0.10 mm, 0.15 mm, 0.20 mm, 0.25 mm, and 0.30 mm from the surface are determined by electrolytic polishing and X-ray diffraction. Specifically, the measurement surface at each depth is exposed by electrolytic polishing. X-rays are irradiated onto the exposed measurement surface, and the retained austenite volume fraction at each depth is measured by X-ray diffraction. The arithmetic mean of the retained austenite volume fractions obtained at each depth, including the compressive residual stress in the longitudinal direction of the surface, is defined as the average retained austenite volume fraction (average volume fraction of retained austenite). The volume fraction of retained austenite can be measured at the same location as the residual stress measurement described above.

[0047] Other microstructures in the surface region besides retained austenite include martensite, as well as ferrite, pearlite, and protereminate cementite. However, low-strength microstructures such as ferrite and pearlite are prone to crack formation, reducing the torsional fatigue strength of the shaft member. Furthermore, the presence of protereminate cementite can cause cracks to originate from it, reducing the torsional fatigue strength of the shaft member. Therefore, it is preferable that the total volume fraction of ferrite, pearlite, and protereminate cementite in the surface region be 10% or less.

[0048] (Average Vickers hardness of the highly compressive residual stressed area: 770HV0.2 or higher) The shaft member according to this disclosure has an average Vickers hardness of 770HV0.2 or higher in the high-compression residual stress area. Having an average Vickers hardness of 770HV0.2 or higher in the high-compression residual stress area in the surface region makes it possible to more reliably increase the torsional fatigue strength. The average Vickers hardness in the surface region (high compressive residual stress area) is measured as follows. If a hole is formed, it is preferable to measure the Vickers hardness in the surface region near the hole and in the surface region at a position sufficiently far from the hole in the circumferential direction. For example, if the shaft member has through holes in the radial direction, it is preferable to measure the Vickers hardness in the surface region at an intermediate position in the circumferential direction between the two holes (i.e., at a position 90° in the circumferential direction from each hole). A shaft member is cut radially (perpendicular to the longitudinal direction), and a sample is taken including the cut surface up to a depth of 0.30 mm from the surface. The area up to a depth of 0.30 mm from the surface is designated as the test surface. A Vickers hardness test (HV0.2) is performed on the collected sample in accordance with JIS Z 2244-1:2020. In the Vickers hardness test, the Vickers hardness is determined at 0.05 mm intervals up to a depth of 0.30 mm from the surface. The test force is 1.961 N. The Vickers hardness is determined at depths of 0.05 mm, 0.10 mm, 0.15 mm, 0.20 mm, 0.25 mm, and 0.30 mm from the surface. The arithmetic mean of the Vickers hardness at each depth is defined as the average Vickers hardness.

[0049] (Hole in the shaft component) The shaft member according to this disclosure may have at least one hole formed on its outer circumferential surface. The hole may be perpendicular to the longitudinal (axial) direction of the shaft member or at a predetermined angle, and may be one or more holes formed from the outer circumferential surface of the shaft member. The hole may be a through hole or a non-through hole. For example, it may be provided as a hole for passing oil in a shaft component of an automobile or the like. The diameter of the hole is not particularly limited and depends on the diameter of the shaft member and the application, but for example, it is between 1.0 mm and 6.0 mm.

[0050] <Method for manufacturing shaft components> Next, an example of a method for manufacturing a shaft member according to this disclosure will be described. While the method for manufacturing the shaft member according to this disclosure is not particularly limited, one example involves applying a carburizing treatment to a shaft member having the aforementioned chemical composition to form a carburized layer on the surface, and then applying roller burnishing under specific conditions to the region to be designated as a high-compression residual stress area. For example, a method may include a step of processing steel material to obtain a rough member (steel material processing step), a step of applying a carburizing treatment to the rough member (carburizing treatment step), and a step of applying cold roller burnishing to the outer surface of the carburized rough member to introduce strain into the surface layer (cold roller burnishing step). Other steps may also be included; for example, constant temperature annealing may be performed before the steel material processing step to make the steel material easier to process.

[0051] [Steel processing process] A steel material having the aforementioned chemical composition is prepared, and the steel material is processed to manufacture a rough member having a desired shape close to that of the shaft member. A known method can be used for processing the steel material. Examples of processing methods include hot working, cold working, and cutting. The rough member will have the same shape as the shaft member.

[0052] [Carburizing process] Next, the rough material is subjected to carburizing, constant temperature holding, and quenching treatments.

[0053] (Carburizing treatment) Carburizing temperature (T1): 900~1050℃ If the carburizing temperature (T1) is too low, the surface layer of the coarse material will not be sufficiently carburized. On the other hand, if the carburizing temperature (T1) is too high, the austenite grains will coarseen, reducing the torsional fatigue strength of the shaft material. Therefore, the carburizing temperature (T1) is preferably 900 to 1050°C.

[0054] Carbon potential (Cp1) during carburizing treatment: 0.8~1.1% If the carbon potential (Cp1) is too low, sufficient carburization will not occur. On the other hand, if the carbon potential (Cp1) is too high, more than 3% of the hard protereminate cementite precipitated during carburizing will remain even after carburizing and quenching. In this case, cracks will occur starting from the protereminate cementite, reducing the torsional fatigue strength of the shaft member. In addition, tool wear during machining will increase, and the machinability of the carburized material will decrease. Therefore, the carbon potential (Cp1) is preferably 0.8 to 1.1%. The carbon potential (Cp1) may be varied within the above range during the carburizing treatment.

[0055] Carburizing time (t1): 60 minutes or more If the carburizing time (t1) is too short, sufficient carburizing will not occur. Therefore, it is preferable to set the carburizing time (t1) to 60 minutes or longer. On the other hand, if the carburizing time (t1) is too long, productivity will decrease. Therefore, it is preferable to set the upper limit of the carburizing time (t1) to 240 minutes.

[0056] (Constant temperature maintenance treatment) After carburizing, a constant temperature holding treatment is performed. The constant temperature holding treatment is carried out under the following conditions, for example:

[0057] Constant temperature holding temperature (T2): 820~870℃ If the constant temperature holding temperature (T2) is too low, it becomes difficult to control the atmosphere, such as the carbon potential. In this case, it becomes difficult to adjust the volume fraction of retained austenite. On the other hand, if the constant temperature holding temperature (T2) is too high, the strain generated during quenching increases, which may cause quench cracks. Therefore, the constant temperature holding temperature (T2) is preferably 820 to 870°C.

[0058] Carbon potential (Cp2) during constant temperature holding treatment: 0.7~1.0% If the carbon potential (Cp2) during constant temperature holding treatment is too low, carbon that entered during carburizing will be released back into the environment. On the other hand, if the carbon potential (Cp2) is too high, hard proteroprecipitation cementite will precipitate. In this case, cracks will start from the proteroprecipitation cementite, reducing the torsional fatigue strength of the shaft member. In addition, tool wear during machining will increase, and the machinability of the carburized material will decrease. Therefore, the carbon potential (Cp2) is preferably 0.7 to 1.0%.

[0059] Constant temperature holding time (t2): 20~60 minutes If the constant temperature holding time (t2) is too short, the temperature of the rough material will not be uniform, and the distortion that occurs during quenching will increase. In this case, quench cracks may occur in the carburized material. On the other hand, if the constant temperature holding time (t2) is too long, productivity will decrease. Therefore, the constant temperature holding time (t2) should be between 20 and 60 minutes.

[0060] (Heat treatment) After constant temperature holding treatment, the material is subjected to quenching by a known method. The quenching treatment can be, for example, oil quenching or water quenching.

[0061] (Tempering treatment) To increase the toughness of the shaft members, they undergo carburizing and quenching followed by tempering. The tempering temperature is, for example, 150-200°C. The holding time at the tempering temperature is, for example, 60-150 minutes.

[0062] [Cold roller burnishing process] Next, cold roller burnishing is performed on at least a portion of the outer surface of the carburized material. At this time, the processing is carried out so that the average compressive residual stress of the surface layer treated with cold roller burnishing is 1700 MPa or more. Figure 1 shows an example of a tool that can be suitably used when performing roller burnishing in the manufacture of a shaft member according to the present disclosure. (A) is a front view, and (B) is a side view. This disc-shaped tool 10 is made of cemented carbide and has a tapered shape near its outer edge, as shown in Figure 1(A).

[0063] When performing roller burnishing on a carburized shaft member using such a tool, as shown in Figure 2, the workpiece (shaft member) 20 is set on a lathe (not shown), cutting oil is applied, and then the shaft is rotated at high speed. The contact portion 12 of the disc-shaped tool 10, which is rotatably supported by the support 14, is pressed perpendicularly to the outer surface of the rapidly rotating shaft member 20 with pressure F, and moved in a direction X parallel to the axis of the shaft member 20. For example, the following processing conditions can be used. Contact part: R0.5~1.5 Feed rate: 0.01~0.15mm / rev Peripheral speed: 200~1000rpm Cutting depth: 0.15~0.60mm

[0064] By performing roller burnishing using a disc-shaped tool 10 as shown in Figures 1 and 2, a processing-induced transformation occurs in the surface region of the shaft member 20, improving the hardness of the surface portion of the shaft member 20 and imparting a large compressive residual stress (average compressive residual stress: 1700 MPa or more). This improves the torsional fatigue strength of the shaft member 20. [Examples]

[0065] The following describes embodiments of the shaft member relating to this disclosure. However, the shaft member relating to this disclosure is not limited to the embodiments described below.

[0066] <Sample Manufacturing> A sample of the shaft component was manufactured using the process shown in Figure 4.

[0067] (Steel processing) Steel materials having the chemical composition shown in Table 1 were used as the raw material. These materials were held at 925°C for 60 minutes, then at 650°C for another 60 minutes, and then air-cooled. From these isothermal annealed materials, a hollow sample 30 with an oil hole, simulating a shaft, was processed, having the shape and size shown in Figure 3. Sample 30 has a through hole 42 provided in the axial direction, as well as an oil hole 46 that penetrates radially in the center of the longitudinal direction. Both ends of sample 30 are fixed parts, and the cylindrical body is made up of a large diameter section 34 and a small diameter section 36 located in the center where the oil hole 46 is formed. The diameter of the large diameter section 34 is 22.4 mm, and the diameter of the oil hole 46 is 4 mm. The remainder of the chemical composition shown in Table 1 consists of Fe and impurities. Underlined elements are outside the scope of this disclosure, and blank spaces indicate that the element is not included (not intentionally added).

[0068] [Table 1]

[0069] (Gas carburizing treatment) Each sample was subjected to gas carburizing and tempering using the heat pattern shown in Figure 5. "Cp1" and "Cp2" represent the carbon potential, respectively, while oil quenching (oil temperature: 60°C) refers to cooling the sample in oil at 60°C. Specifically, each sample was heated to 930°C, then maintained at 930°C for 80 minutes under an atmospheric carbon potential of Cp11.0%, followed by a change in carbon potential to Cp20.8% and maintaining at 930°C for 60 minutes, then cooled to 830°C and heated for 30 minutes before being cooled in 60°C oil. Finally, it was heated to 180°C, maintained for 120 minutes, and then allowed to cool to room temperature in the atmosphere.

[0070] (Roller burnishing process) After gas carburizing treatment, a disc-shaped cemented carbide tool (diameter: 30 mm, maximum wall thickness: 11 mm) as shown in Figure 1 was prepared. Cutting oil was applied to the sample, and cold roller burnishing was performed on the outer surface of the sample under the following conditions. Feed rate: 0.05mm / rev Peripheral speed: 1000rpm Cut: 0.5mm on each side Note that "cutting depth: 0.5mm on one side" means that if the position where the sample (shaft member) and the tool make contact is considered as 0mm, the tool is pushed in 0.5mm from that position.

[0071] <Measurement> For the surface region of each sample that underwent cold roller burnishing, the average Vickers hardness, average compressive residual stress, and average volume fraction of retained austenite were measured using the method described above.

[0072] (Mean compressive residual stress) When the average compressive residual stress in the longitudinal direction of the sample was measured in the surface region near the hole of the shaft member sample, it was found that when gas carburizing was followed by roller burnishing, the average compressive residual stress up to 0.30 mm from the surface increased to over 1700 MPa.

[0073] (Average volume fraction of retained austenite) When the average volume fraction of retained austenite in the surface region near the hole of a shaft member sample was measured, it was found that after gas carburizing followed by roller burnishing, the average volume fraction of retained austenite decreased to 5.0% or less due to processing-induced transformation up to 0.30 mm from the surface.

[0074] (Average Vickers stiffness) When the average Vickers hardness in the surface region was measured in the C-section (a section perpendicular to the longitudinal direction of the shaft member) near the hole of the shaft member sample, the average Vickers hardness up to 0.30 mm from the surface improved to 770 HV 0.2 or higher after gas carburizing followed by roller burnishing.

[0075] <Rating> [Torsion fatigue test - Measurement of torsional fatigue strength] Torsional strength was evaluated using a resonant torsional fatigue testing machine. Torsional fatigue tests were conducted at room temperature in air, with a stress ratio R=-1 and a speed of 373-439 cmp. The number of test terminations was 1.0 × 10⁻⁶. 6 The number of times was 1.0 × 10. 6 After repeated cycles, the highest torque at which fatigue failure did not occur was defined as the torsional fatigue strength. Figure 6 shows the torsional fatigue strength with and without cold roller burnishing after gas carburizing. Applying cold roller burnishing after gas carburizing improves the torsional fatigue strength by approximately 1.5 times compared to the case without cold roller burnishing.

[0076] In this test, the torsional fatigue strength of Comparative Example 101, which did not undergo cold roller burnishing (RB), was used as the baseline. A torsional fatigue strength of 1.30 times or more the baseline was considered to indicate excellent torsional fatigue strength (pass). The results are shown in Table 2.

[0077] [Table 2]

[0078] In Table 2, under "Cold RB," "Yes*" for No. 107 means that cold roller burnishing was performed using a spherical tool, and "SP*" for No. 108 means that shot peening was performed.

[0079] The sample in this disclosure had a chemical composition, average compressive residual stress in the surface region, average volume fraction of retained austenite, and average Vickers hardness all within the scope of this disclosure, and its torsional fatigue strength was satisfactory. On the other hand, the sample in the comparative example had a chemical composition and / or average compressive residual stress in the surface region outside the scope of this disclosure, and its torsional fatigue strength was less than 1.30 times the standard. [Explanation of Symbols]

[0080] 10 Roller burnishing tools 20 Workpiece 30 Shaft component samples 46 Oil hole

Claims

1. A shaft member having a roughly cylindrical outer shape, comprising a carburized layer on the surface and a core portion located inside the carburized layer, The core portion is by mass % C: 0.10-0.30%, Si: 0.03 to 1.50%, Mn: 0.30-1.00%, P: 0.035% or less, S: 0.035% or less, Cr: 0.02-1.80%, Mo: 0.02-0.50%, Al: 0.005-0.100%, N: 0.0020 to 0.0250%, and O: 0.0015% or less It contains, with the remainder being Fe and impurities, and has a chemical composition that satisfies the following formula (1): When the region from the surface of the shaft member to a depth of 0.30 mm is defined as the surface region, In at least a portion of the longitudinal direction of the shaft member, there is a high compressive residual stress area in the surface region where the average compressive residual stress in the longitudinal direction is 1700 MPa or more. The shaft member wherein the high compressive residual stress portion has an average volume fraction of retained austenite of 5.0% or less and an average Vickers hardness of 770HV0.2 or higher. 0.98<0.25Si+Mn+0.40Cr+0.25Mo<1.33...(1) However, the element symbols in formula (1) indicate the mass percentage content of the element in the core portion.

2. A shaft member having a roughly cylindrical outer shape, comprising a carburized layer on the surface and a core portion located inside the carburized layer, The core portion is by mass % C: 0.10-0.30%, Si: 0.03 to 1.50%, Mn: 0.30-1.00%, P: 0.035% or less, S: 0.035% or less, Cr: 0.02-1.80%, Mo: 0.02-0.50%, Al: 0.005-0.100%, N: 0.0020 to 0.0250%, and O: 0.0015% or less It contains, and further contains one or more selected from the following groups A, B, C, and D, (Group A) Cu: 0.40% or less, Mg: 0.0050% or less, V: 0.50% or less, Nb: 0.100% or less, Ti: 0.100% or less, Ca: 0.0050% or less, B: One or more types selected from the group consisting of 0.0050% or less. (Group B) Ni: 0.30% or less (Group C) Sn: 0.100% or less (Group D) Bi: 0.020% or less, One or two types selected from the group consisting of Pb: 0.09% or less. The remainder consists of Fe and impurities, and has a chemical composition that satisfies the following formula (1). When the region from the surface of the shaft member to a depth of 0.30 mm is defined as the surface region, In at least a portion of the longitudinal direction of the shaft member, there is a high compressive residual stress area in the surface region where the average compressive residual stress in the longitudinal direction is 1700 MPa or more. The shaft member wherein the high compressive residual stress portion has an average volume fraction of retained austenite of 5.0% or less and an average Vickers hardness of 770HV0.2 or higher. 0.98<0.25Si+Mn+0.40Cr+0.25Mo<1.33...(1) However, the element symbols in formula (1) indicate the mass percentage content of the element in the core portion.

3. The shaft member according to claim 2, wherein the chemical composition includes group A.

4. The shaft member according to claim 2, wherein the chemical composition includes group B.

5. The shaft member according to claim 2, wherein the chemical composition includes group C.

6. The shaft member according to claim 2, wherein the chemical composition includes group D.

7. The shaft member according to any one of claims 1 to 6, wherein at least one hole is formed on the outer circumferential surface, and at least the region in the longitudinal direction of the shaft member in which the hole is formed is the high compressive residual stress portion.

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