Bearing steel pipe

Abstract

To provide a bearing steel pipe having excellent machinability.SOLUTION: A bearing steel pipe has a chemical composition described in the specification, where Y-value defined by [Y=-0.521+0.630C-0.2645Si+0.132Mn+0.0528(Ni+Cu)+0.036 P-3.349S-0.0003Cr] satisfies 0.150 or less, a decarburized layer region is present within a range to the depth position of 0.10 mm at least from a surface of steel toward a thick center part, and the depth of the decarburized layer region is 0.10 mm or less, in the decarburized layer region an average crystal grain size of ferrite is 10 μm or more.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a steel pipe for a shaft.

Background Art

[0002] In a raceway ring used for a bearing, wear resistance is required because it repeatedly receives a large surface pressure locally. For such a raceway ring of a bearing, for example, high-carbon chromium bearing steel defined in JIS G 4805 is used.

[0003] In such bearing steel, since the carbon content is generally high, decarburization occurs on the surface of the steel when it is heated to a high temperature in an oxidizing atmosphere. Further, since such steel is usually heated to a high temperature in an oxidizing atmosphere in the processes of casting, block rolling, hot pipe making, and spheroidizing heat treatment, a decarburized layer is formed on the steel surface.

[0004] Bearing steel is generally used as a component after being cut, machined, and quenched. However, if a decarburized layer still exists after machining, a predetermined strength and structure cannot be obtained in the decarburized layer portion, so the slidability deteriorates. Therefore, when the decarburized layer is thick, it is necessary to remove the decarburized layer by grinding or other means before cutting. However, there is a problem that the manufacturing cost of the component increases significantly due to an increase in the number of working hours and a decrease in the yield.

[0005] Therefore, some techniques related to a heat treatment method for re-carburizing by atmosphere control have been disclosed so far.

[0006] Patent Document 1 describes carburizing in a high-temperature austenite region. Specifically, for bearing steel or hypereutectoid steel, a spheroidizing annealing method is disclosed in which carburizing treatment is performed in an austenite (γ) temperature range above the Acm point at the initial stage of the heat pattern of spheroidizing annealing, and decarburization occurring in conventional spheroidizing annealing can be prevented.

[0007] Patent Document 2 describes carburization in the ferrite (α) temperature range. It is clarified that carbon diffusion is faster in the lower-temperature α phase than in the higher-temperature γ phase, and re-carburization occurs in the α temperature range. During spheroidizing annealing, a heat treatment method for bearing steel is disclosed in which the carburizing atmosphere is set in the temperature range below the Ac 1 point, and re-carburization and spheroidization are carried out simultaneously.

[0008] Patent Document 3 discloses a method of re-carburizing by controlling the extraction temperature of steel, the flow rate of the atmospheric gas in the extraction section, the furnace internal pressure, the cooling rate, and the (CO) / CO 1 value of the atmosphere in the temperature range of 500 °C or more below the Ac point using a furnace with controllable atmosphere. 2 / CO 2 Patent Document 4 discloses a method in which spheroidizing annealing is carried out in a carburizing atmosphere during spheroidizing annealing, and then spheroidizing annealing and scale removal of the carburized part are carried out simultaneously in a direct-fired annealing furnace.

[0009] Patent Document 5 discloses a method of re-carburizing by annealing treatment in a heat treatment furnace with a carburizing atmosphere. Patent Document 6 discloses a method in which the CO concentration of the atmosphere is increased before final heating, then the CO concentration is once decreased, and then heating is carried out.

[0010] Patent Document 7 discloses a method of preventing decarburization of the inner surface of a steel pipe by removing the surface scale of the steel pipe before furnace heating and then annealing with both pipe ends sealed. Patent Document 8 discloses a method of preventing decarburization by applying graphite to the inner surface side of a steel pipe when heat-treating the steel pipe in an atmosphere furnace under an oxidizing atmosphere. Patent Document 9 discloses a method of reducing the thickness of a decarburized layer by heating steel in a carburizing atmosphere and then performing heat treatment according to a specific heat pattern.

[0011] Patent Document 7 discloses a method of preventing decarburization of the inner surface of a steel pipe by removing the surface scale of the steel pipe before furnace heating and then annealing with both pipe ends sealed. Patent Document 8 discloses a method of preventing decarburization by applying graphite to the inner surface side of a steel pipe when heat-treating the steel pipe in an atmosphere furnace under an oxidizing atmosphere. Patent Document 9 discloses a method of reducing the thickness of a decarburized layer by heating steel in a carburizing atmosphere and then performing heat treatment according to a specific heat pattern.

Prior Art Documents

Patent Documents

[0012]

Patent Document 1

[0013] In the method described in Patent Document 1, when steel is heated in the γ region for a long time, there is a problem that austenite grain size becomes coarse and uniformly dispersed spherical carbides cannot be obtained. In addition, it is necessary to set an atmosphere having a very high carbon potential. Usually, in such a case, there is also a big problem that soot is likely to occur on the surface of the steel material, which easily causes operation troubles.

[0014] In the method described in Patent Document 2, a mixed gas of CO, H 2 , CO 2 and N 2 is used as the carburizing gas. However, since it is necessary to perform carburization in a temperature range where the ferrite phase is stable, it is necessary to increase the CO concentration considerably. Therefore, the control of its gas composition is difficult and costly.

[0015] In the method described in Patent Document 3, when the C concentration of the steel is as high as 0.6% or more, the carbon potential of the steel becomes higher than the carbon potential on the atmosphere side, so that sufficient carburization cannot be achieved when the decarburized layer is thick. In the method described in Patent Document 4, when the C concentration is low, that is, the upper limit value of the C concentration in the decarburized layer is 0.5% or less, carburization in the carburizing atmosphere annealing becomes insufficient, and decarburization is not improved even by the diffusion of C in the oxidizing atmosphere annealing.

[0016] In the method described in Patent Document 5, when the decarburized layer before carburization is thin, an overcarburized layer is likely to become thick, and the cutting efficiency during processing decreases. In the method described in Patent Document 6, when the scale before heat treatment is thick, the oxygen content in the scale cannot be removed during heat treatment, and when the decarburized layer before heat treatment is thick, there is a problem that the decarburized layer tends to remain after heat treatment even if the oxygen content in the scale is removed.

[0017] In the method described in Patent Document 7, there is a problem that the man-hour for sealing both pipe ends of the steel pipe increases. Further, when the decarburized layer is thick, long-time annealing is required to carburize by the diffusion of C, and the heat treatment time becomes very long. In the method described in Patent Document 8, since a mixture of graphite and a liquid that is difficult to burn at high temperature is applied to the inner surface of the steel pipe, the man-hour and cost increase significantly. In addition, it is difficult to apply such a mixture to the inner surface of a steel pipe with a small inner diameter.

[0018] In the method described in Patent Document 9, since it is possible to control the depth of the decarburized layer or the overcarburized layer within the range of 0 to 0.2 mm from the surface layer, bearing steel excellent in cutting performance and having low cutting cost can be obtained. However, since either a decarburized layer or an overcarburized layer is formed, there is a problem that the properties of the surface layer portion are not stable. Therefore, as a result of the study by the present inventors, it has been found that there is room for further improvement in terms of machinability.

[0019] An object of the present invention is to provide a steel pipe for bearings excellent in machinability.

Means for Solving the Problems

[0020] The present invention has been made to solve the above problems, and the gist thereof is a steel pipe for a shaft as shown below.

[0021] (1) The chemical composition of the steel is, by mass%, C: 0.60 to 1.60%, Si: 0.10 to 2.00%, Mn: 0.01 to 2.00%, P: 0.040% or less, S: 0.040% or less, Al: 0.001 to 0.500%, N: 0.0200% or less, O: 0.0100% or less, Cr: 0.01 to 5.00%, Ni: 0.01 to 0.50%, Cu: 0.01 to 0.50%, The balance: Fe and impurities, The Y value defined by the following formula (i) satisfies 0.150 or less, A region where the carbon content is 95% or less of the average carbon content of the steel exists within at least the range from the surface of the steel to a depth position of 0.10 mm toward the center of the wall thickness, and the thickness of the region is 0.10 mm or less, In the region, the average crystal grain size of ferrite is 10 μm or more, Steel pipe for a shaft. Y = -0.521 + 0.630C - 0.2645Si + 0.132Mn + 0.0528(Ni + Cu) + 0.036P - 3.349S - 0.0003Cr ···(i) However, the element symbols in the above formula mean the content (mass%) of each element in the steel.

[0022] (2) The chemical composition of the steel is, by mass%, C: 0.60 to 1.60%, Si: 0.10 to 2.00%, Mn: 0.01 to 2.00%, P: 0.040% or less, S: 0.040% or less, Al: 0.001 to 0.500%, N: Below 0.0200%, O: Below 0.0100%, Cr: 0.01 - 5.00%, Ni: 0.01 - 0.50%, Cu: 0.01 - 0.50%, and contains one or more selected from the group consisting of the following Group A, Group B, and Group C, the balance: Fe and impurities, the Y value defined by the following formula (i) satisfies Y ≤ 0.150, a region where the carbon content is 95% or less of the average carbon content of the steel exists within at least the range from the surface of the steel to a depth position of 0.10 mm toward the center of the wall thickness, and the thickness of the region is 0.10 mm or less, in the region, the average crystal grain size of ferrite is 10 μm or more, Steel pipe for shaft use. Y = -0.521 + 0.630C - 0.2645Si + 0.132Mn + 0.0528(Ni + Cu) + 0.036P - 3.349S - 0.0003Cr ···(i) However, the element symbols in the above formula mean the content (mass%) of each element in the steel. [Group A] One or more selected from the group consisting of Sn: 0.100% or less, Sb: 0.100% or less, and Bi: 0.1000% or less [Group B] One or more selected from the group consisting of Mo: 1.00% or less, V: 1.00% or less, Ti: 1.000% or less, Nb: 1.000% or less, W: 1.000% or less, Ta: 0.500% or less, Zr: 0.500% or less, Hf: 0.500% or less, Co: 0.50% or less, and B: 0.0100% or less [Group C] One or more selected from the group consisting of Ca: 0.0050% or less, Mg: 0.0050% or less, and REM: 0.0050% or less

[0023] (3) The chemical composition contains one or more elements selected from Group A above. The steel pipe for shaft use according to (2) above.

[0024] (4) The chemical composition contains one or more elements selected from the group B. The steel pipe for a shaft according to (2) above.

[0025] (5) The chemical composition contains one or more elements selected from the group C. The steel pipe for a shaft according to (2) above.

Advantages of the Invention

[0026] According to the present invention, it is possible to stably obtain a steel pipe for a shaft excellent in machinability.

Embodiments for Carrying Out the Invention

[0027] Hereinafter, each requirement of the present invention will be described in detail.

[0028] Unless otherwise specified, the definitions of the terms in this specification are as follows. "Carburizing": It means that carbon (C) penetrates from the surface of the steel by heat treatment with atmosphere control. "Decarburization": It means that C desorbs from the surface layer part of the steel to the atmosphere side by heat treatment, and the C concentration decreases below the carbon concentration of the base material. "Recarburization": It means a reaction in which the C concentration in the part where the C concentration has decreased due to decarburization returns to the C concentration of the base material. "Overcarburization": It means that the carbon concentration on the surface of the steel exceeds the carbon concentration of the base material due to excessive progress of recarburization. "Spheroidization": It means making the form of carbides (mainly cementite) contained in the steel spherical. "Descaling": It means removing oxides (scale) generated on the surface of the steel by heat treatment at a high temperature such as hot pipe making or furnace heating.

[0029] (A) Chemical Composition The reasons for limiting the chemical composition of the steel pipe for a shaft according to an embodiment of the present invention are as follows. In the following description, "%" for the content of each element means "mass%".

[0030] C: 0.60 - 1.60% Carbon (C) is an element effective in improving the strength, hardness, and wear resistance of bearing steel and enhancing its lifespan. To obtain such effects, the C content should be 0.60% or more. On the other hand, as the C content increases, the carbon potential of the steel becomes higher, so decarburization progresses strongly in the process leading to hot rolling, and it becomes difficult to re-carburize during heat treatment under subsequent atmosphere control. Also, as the C content increases, carbides in the steel tend to coarsen, and coarse cementite-based (M 3 C (M: Fe, Cr, Mn)-based) carbides precipitate, reducing the rolling fatigue characteristics of the bearing steel. Therefore, the C content should be 1.60% or less. The C content is preferably 0.80% or more, more preferably 0.95% or more. Also, the C content is preferably 1.20% or less, more preferably 1.10% or less.

[0031] Si: 0.10 - 2.00% Silicon (Si) is necessary for deoxidizing the steel and also has the effects of enhancing the hardenability of the steel and suppressing decarburization. To obtain such effects, the Si content should be 0.10% or more. However, when its content exceeds 2.00%, the effects become saturated, and the toughness of the steel also deteriorates. Therefore, the Si content should be 2.00% or less. The Si content is preferably 0.15% or more, more preferably 0.20% or more. Also, the Si content is preferably 1.80% or less, more preferably 1.50% or less.

[0032] Mn: 0.01 - 2.00% Manganese (Mn) is effective for deoxidizing the steel and also has the effect of enhancing the hardenability of the steel. To obtain such effects, the Mn content should be 0.01% or more. However, when its content exceeds 2.00%, the effects become saturated and decarburization of the steel is promoted. Therefore, the upper limit of the Mn content is 2.00%. The Mn content is preferably 0.02% or more, more preferably 0.05% or more. Also, the Mn content is preferably 1.80% or less, more preferably 1.60% or less.

[0033] P: Below 0.040% Phosphorus (P) is an impurity inevitably contained. That is, the P content is over 0%. P segregates at grain boundaries and reduces the grain boundary strength. When the P content exceeds 0.040%, P segregates excessively at grain boundaries and reduces the grain boundary strength. In this case, cracks and flaws are likely to occur during hot working in the steel manufacturing process. Therefore, the P content should be 0.040% or less. It is preferably 0.030% or less. Although it is preferable that the P content is as low as possible, excessive reduction of the P content increases the manufacturing cost. Therefore, considering normal industrial production, the preferable lower limit of the P content is 0.001%, more preferably 0.002%.

[0034] S: 0.040% or less Sulfur (S) is an impurity inevitably contained. That is, the S content is over 0%. S forms sulfide-based inclusions. Coarse sulfide-based inclusions are likely to be the starting points of cracks or flaws during hot working in the steel manufacturing process. When the S content exceeds 0.040%, the sulfide-based inclusions become coarse, and cracks and flaws are likely to occur during hot working in the steel manufacturing process. Therefore, the S content should be 0.040% or less. The preferable upper limit of the S content is 0.030%, more preferably 0.020%. Although it is preferable that the S content is as low as possible, excessive reduction of the S content increases the manufacturing cost. Therefore, considering normal industrial production, the preferable lower limit of the S content is 0.001%, more preferably 0.002%.

[0035] Al: 0.001 - 0.500% Aluminum (Al) has the effect of deoxidizing steel. If the Al content is less than 0.001%, the above effect cannot be obtained sufficiently. Therefore, the Al content should be 0.001% or more. On the other hand, when the Al content exceeds 0.500%, clustered coarse oxides are generated. The clustered coarse oxides reduce the fatigue strength of bearing parts. Therefore, the Al content should be 0.500% or less. The preferable lower limit of the Al content is 0.005%. The preferable upper limit of the Al content is 0.400%, more preferably 0.300%. The Al content referred to in this specification means the content of total Al (Total Al).

[0036] N: 0.0200% or less Nitrogen (N) is an impurity that is inevitably contained. That is, the N content is more than 0%. N dissolves in the steel and reduces the hot workability of the steel. When the N content exceeds 0.0200%, the hot workability of the steel decreases. Therefore, the N content should be 0.0200% or less. The preferable upper limit of the N content is 0.0150%, more preferably 0.0100%. Although it is preferable that the N content is as low as possible, excessive reduction of the N content increases the manufacturing cost. Therefore, considering normal industrial production, the preferable lower limit of the N content is 0.0010%, more preferably 0.0020%.

[0037] O: 0.0100% or less Oxygen (O) is an impurity that is inevitably contained. If it is in a very small amount, O segregates at the grain boundaries of the ferrite phase and austenite phase respectively or at the interface between ferrite and austenite, and has the effect of strengthening the grain boundaries and interfaces. However, if the O content is too high, the interface is weakened and the toughness and hot workability of the steel decrease. Therefore, the O content should be 0.0100% or less. The preferable upper limit of the O content is 0.0040%, more preferably 0.0030%. Excessive reduction of the O content increases the manufacturing cost. Therefore, considering normal industrial production, the preferable lower limit of the O content is 0.0001%, more preferably 0.0003%.

[0038] Cr: 0.01~5.00% Chromium (Cr) enhances the hardenability of steel, thereby increasing the strength of bearing parts. If the Cr content is less than 0.01%, the above effects cannot be fully obtained. Therefore, the Cr content should be 0.01% or more. On the other hand, when the Cr content exceeds 5.00%, coarse Cr-based carbides are formed. In this case, the fatigue strength of bearing parts decreases. Therefore, the Cr content should be 5.00% or less. The preferable lower limit of the Cr content is 0.10%, more preferably 0.50%, and even more preferably 1.00%. The preferable upper limit of the Cr content is 3.00%, more preferably 2.00%, and even more preferably 1.65%.

[0039] Ni: 0.01~0.50% Nickel (Ni) enhances the hardenability of steel, thereby increasing the strength of bearing parts. In addition, it has the effect of suppressing decarburization. If the Ni content is less than 0.01%, the above effects cannot be fully obtained. Also, when the Ni content is less than 0.01%, decarburization will be promoted. Therefore, the Ni content should be 0.01% or more. On the other hand, Ni is a high-cost alloying element. When the Ni content exceeds 0.50%, it will cause an unnecessary increase in cost. Also, conversely, decarburization is promoted. Therefore, the Ni content should be 0.50% or less. The preferable lower limit of the Ni content is 0.02%, more preferably 0.03%, and even more preferably 0.04%. The preferable upper limit of the Ni content is 0.45%, more preferably 0.35%, and even more preferably 0.25%.

[0040] Cu: 0.01~0.50% Copper (Cu) enhances the hardenability of steel, thereby increasing the strength of bearing parts. In addition, it has the effect of suppressing decarburization. If the Cu content is less than 0.01%, the above effects cannot be fully obtained. Also, if the Cu content is less than 0.01%, decarburization will be promoted. Therefore, the Cu content should be 0.01% or more. On the other hand, if the Cu content is too high, the hot workability of the steel will decrease. Moreover, conversely, decarburization will be promoted. Therefore, the Cu content should be 0.50% or less. The preferred lower limit of the Cu content is 0.02%, more preferably 0.03%, and even more preferably 0.04%. The preferred upper limit of the Cu content is 0.45%, more preferably 0.35%, and even more preferably 0.25%.

[0041] [Optional additive element] [Any additive element in Group A] The above chemical composition may further contain one or more selected from Sn, Sb, and Bi in place of a part of Fe. All of these elements enhance the hardenability of steel and have the effect of suppressing decarburization to some extent.

[0042] Sn: 0.100% or less Sb: 0.100% or less Bi: 0.1000% or less Tin (Sn), antimony (Sb), and bismuth (Bi) are optional additive elements and may not be contained. That is, their contents may be 0%. When contained, they enhance the hardenability of the steel and slightly suppress decarburization. If even a small amount of these is contained, the above effects can be obtained to a certain extent. However, if their contents are too high, the strength of the steel becomes too high, the toughness of the steel decreases, or the hot workability decreases. Therefore, the content of Sn and Sb is 0.100% or less, and the Bi content is 0.1000% or less. The preferable lower limits of the contents of Sn and Sb are each more than 0%, more preferably 0.001%, and even more preferably 0.002%. Also, the preferable lower limit of the Bi content is more than 0%, more preferably 0.0001%, and even more preferably 0.0010%. The preferable upper limits of the contents of Sn and Sb are each 0.080%, more preferably 0.070%. Also, the preferable upper limit of the Bi content is 0.0800%, more preferably 0.0700%.

[0043] [Group B Optional Additive Elements] The above chemical composition may further contain one or more selected from Mo, V, Ti, Nb, W, Ta, Zr, Hf, Co, and B in place of a part of Fe. All of these elements increase the strength of the steel pipe for shafts.

[0044] Mo: 1.00% or less V: 1.00% or less Ti: 1.000% or less Nb: 1.000% or less W: 1.000% or less Molybdenum (Mo), vanadium (V), titanium (Ti), niobium (Nb), and tungsten (W) are optional additive elements and may not be contained. That is, their contents may be 0%. When contained, they form carbonitrides in the steel and increase the strength of the steel. If even a small amount of these elements is contained, the above effects can be obtained to a certain extent. However, if their contents are too high, the strength of the steel becomes too high and the toughness of the steel decreases.

[0045] Therefore, the contents of Mo and V are 1.00% or less, and the contents of Ti, Nb, and W are 1.000% or less. The preferable lower limits of Mo and V are each more than 0%, more preferably 0.01%, and even more preferably 0.02%. Also, the preferable lower limits of Ti, Nb, and W are each more than 0%, more preferably 0.001%, and even more preferably 0.010%. The preferable upper limits of Mo and V are each 0.80%, more preferably 0.70%. Also, the preferable upper limits of Ti, Nb, and W are each 0.800%, more preferably 0.700%.

[0046] Ta: 0.500% or less Zr: 0.500% or less Hf: 0.500% or less Co: 0.50% or less Tantalum (Ta), zirconium (Zr), hafnium (Hf), and cobalt (Co) are optional additive elements and may not be contained. That is, their contents may be 0%. When contained, they form carbonitrides in the steel and increase the strength of the steel. If even a small amount of these elements is contained, the above effects can be obtained to some extent. However, if their contents are too high, the strength of the steel becomes too high and the toughness of the steel decreases.

[0047] Therefore, the contents of Ta, Zr, and Hf are 0.500% or less, and the content of Co is 0.50% or less. The preferable lower limits of Ta, Zr, and Hf are each more than 0%, more preferably 0.001%, and even more preferably 0.010%. Also, the preferable lower limit of Co is more than 0%, more preferably 0.01%, and even more preferably 0.02%. The preferable upper limits of Ta, Zr, and Hf are each 0.400%, more preferably 0.300%. Also, the preferable upper limit of Co is 0.40%, more preferably 0.30%.

[0048] B: 0.0100% or less Boron (B) is an optional additive element and may not be contained. That is, the B content may be 0%. When contained, it segregates at the grain boundaries in the steel and increases the strength of the steel. However, if the B content is too high, boron carbonitrides are formed, reducing the toughness of the steel. Therefore, the B content is 0.0100% or less. The preferable lower limit of the B content is more than 0%, more preferably 0.0001%, still more preferably 0.0010%, and even more preferably 0.0020%. The preferable upper limit of the B content is 0.0050%, more preferably 0.0040%.

[0049] [Optional additive elements in Group C] The above chemical composition may further contain one or more selected from Ca, Mg, and REM in place of a part of Fe. All of these elements fix S in the steel as sulfides and improve the hot workability of the steel.

[0050] Ca: 0.0050% or less Calcium (Ca) is an optional additive element and may not be contained. That is, the Ca content may be 0%. When contained, it combines with O or S in the steel to form Ca-based oxides or Ca-based sulfides, thereby neutralizing the adverse effects of S and the like and strengthening the grain boundaries. If even a small amount of Ca is contained, the above effects can be obtained to some extent. However, if the Ca content is too high, the oxides in the steel become excessive, reducing the toughness and hot workability of the steel. Therefore, the Ca content is 0.0050% or less. The preferable lower limit of the Ca content is 0.0001%, more preferably 0.0010%. The preferable upper limit of the Ca content is 0.0040%, more preferably 0.0030%.

[0051] Mg: 0.0050% or less Magnesium (Mg) is an optional additive element and may not be contained. That is, the Mg content may be 0%. When contained, Mg immobilizes S in the steel as sulfide to render it harmless, strengthens the grain boundaries, and improves the hot workability of the steel. Even if a small amount of Mg is contained, the above effects can be obtained to a certain extent. However, if the Mg content is too high, the oxides in the steel will coarsen. Coarse oxides reduce the toughness of the steel. Therefore, the Mg content is 0.0050% or less. The preferable lower limit of the Mg content is 0.0001%, more preferably 0.0010%. The preferable upper limit of the Mg content is 0.0040%, more preferably 0.0030%.

[0052] REM: 0.0050% or less Rare earth elements (REM) are optional additive elements and may not be contained. That is, the REM content may be 0%. When contained, REM immobilizes S in the steel as sulfide to render S harmless and strengthens the grain boundaries. As a result, the hot workability of the steel is improved. Even if a small amount of REM is contained, the above effects can be obtained to a certain extent. However, if the REM content is too high, the oxides in the steel will coarsen. Coarse oxides reduce the toughness of the steel. Therefore, the REM content is 0.0050% or less. The preferable lower limit of the REM content is 0.0001%, more preferably 0.0010%. The preferable upper limit of the REM content is 0.0040%, more preferably 0.0030%.

[0053] In this embodiment, "REM" refers to the total 17 elements of Sc, Y, and lanthanoids, and "REM content" refers to the content when REM is one type, or the total content when there are two or more types. Also, REM is generally supplied as mischmetal, which is an alloy of multiple types of REM. Therefore, one or two or more individual elements may be added and contained, or for example, it may be added in the form of mischmetal.

[0054] The steel pipe for a shaft according to this embodiment contains each of the above elements, with the balance being Fe and impurities. Here, "impurities" means components that are mixed in due to raw materials such as ore and scrap, and various factors in the manufacturing process when steel is industrially manufactured, and are allowed within a range that does not adversely affect the present invention.

[0055] The chemical composition of the steel pipe for a shaft according to this embodiment can be measured by using various known measurement methods. Specifically, it is measured using an ICP emission spectrometry (conforming to JIS G 1258). Further, C and S are measured using a combustion-infrared absorption method (conforming to JIS G 1211 and 1215), N is measured using an inert gas combustion - thermal conductivity method (conforming to JIS G 1228), and O is measured using an inert gas fusion - non-dispersive infrared absorption method (conforming to JIS G 1239).

[0056] The chemical composition of the steel pipe for a shaft according to this embodiment satisfies that the Y value defined by the following formula (i) is 0.150 or less, on the premise that the content of each element is within the above range. The following formula (i) is a formula derived based on the investigation results of the thickness of the decarburized layer formed on the surface layer of steel having various chemical compositions. As a result of research conducted by the inventors, although the mechanism has not been clarified, it has been found that by reducing the Y value, the thickness of the decarburized layer can be controlled to be small. The Y value is preferably 0.130 or less, and more preferably 0.100 or less. Y = -0.521 + 0.630C - 0.2645Si + 0.132Mn + 0.0528(Ni + Cu) + 0.036P - 3.349S - 0.0003Cr ···(i) However, the element symbols in the above formula mean the content (mass%) of each element in the steel.

[0057] (B) Structure of the surface layer part In the steel pipe for a shaft according to an embodiment of the present invention, in the surface layer part of the steel, there exists a region where the carbon content is 95% or less of the average carbon content of the steel (hereinafter, also referred to as the "decarburized layer region"). Specifically, the decarburized layer region exists within a range of at least up to a depth position of 0.10 mm from the surface of the steel toward the center of the wall thickness.

[0058] In the steel pipe for a shaft in the present embodiment, as will be described later, it is decarburized by controlling the manufacturing conditions, but the decarburized layer is intentionally left to remain in the surface layer portion to improve the machinability. That is, since the decarburized layer exists, the thickness of the decarburized layer region is substantially more than 0 mm. Further, from the viewpoint of surely obtaining the effect of improving the machinability, the thickness of the decarburized layer region is preferably 0.01 mm or more, and more preferably 0.02 mm or more. As a result, it becomes easier to form the constituent cutting edges on the tool surface during cutting, and the tool life is extended.

[0059] However, if the decarburized layer remains excessively, the wear resistance required for the steel pipe for a shaft may deteriorate. In addition, if it exceeds 0.10 mm, the constituent cutting edges are likely to detach from the tool surface, and it becomes difficult to stably protect the tool surface, so that the machinability deteriorates instead. Therefore, the thickness of the decarburized layer region is set to 0.10 mm or less. By stably forming such a thin decarburized layer, it is possible to improve the machinability while maintaining the wear resistance. The wear resistance mentioned here is generally the performance required for parts manufactured by performing cutting processing using a steel pipe and then quenching and tempering.

[0060] Note that an overcarburized layer region may exist on the surface side from the decarburized layer region. In the present embodiment, the overcarburized layer region means a region where the carbon content is 0.05% or more higher in mass% than the average carbon content of the steel. However, if the thickness of the overcarburized layer region becomes excessive, it becomes difficult to obtain the effect of improving the machinability by forming the decarburized layer region. Therefore, the thickness of the overcarburized layer region is preferably 0.02 mm or less.

[0061] The measurement of the carbon content in the surface layer is carried out by a field emission type electron probe microanalyzer (FE-EPMA, using a wavelength dispersive detector, apparatus model JXA-8530F, manufactured by JEOL Ltd.). Specifically, in a cross-section parallel to the axial direction of the steel pipe and passing through the central axis, line analysis by FE-EPMA (acceleration voltage 15 kV) is performed at a pitch of 1 μm from the surface toward the center of the wall thickness, and the moving average of the measured values at nine points centered on a predetermined depth position is taken as the carbon content at that depth position. Then, a region where the carbon content is 95% or less of the average carbon content of the steel is defined as the decarburized layer region, and a region where the carbon content is 0.05% or more higher in mass% than the average carbon content of the steel is defined as the over-carburized layer region. And based on the results, the thicknesses of the decarburized layer region and the over-carburized layer region are determined.

[0062] Furthermore, in the steel pipe for shafts in the present embodiment, the average crystal grain size of ferrite in the decarburized layer region is 10 μm or more. By not only leaving the decarburized layer in the surface layer but also coarsening the ferrite in the decarburized layer region, it becomes possible to further improve the machinability. The average crystal grain size of ferrite is preferably 15 μm or more, and more preferably 20 μm or more. Note that the decarburized layer gradually changes from a ferrite-based structure toward the center of the wall thickness from the surface to a mixed structure of ferrite and carbide.

[0063] The average crystal grain size of ferrite in the decarburized layer region is measured in accordance with JIS G 0551:2020. Specifically, after nitriding and corroding a cross-section parallel to the axial direction of the steel pipe and passing through the central axis to reveal the metal structure, in the region identified as the decarburized layer region by the above FE-EPMA measurement, observation is carried out using an optical microscope, and the average crystal grain size of ferrite is determined by the cutting method for a total visual field range of 40000 μm 2 or more.

[0064] (C) Manufacturing method The steel pipe for shafts according to an embodiment of the present invention can be manufactured by the following method. The manufacturing method of the steel pipe for shafts in the present embodiment includes a pipe manufacturing process and a spheroidizing annealing process.

[0065] The pipe manufacturing process may be carried out under known conditions, and there are no particular restrictions on the manufacturing conditions. For example, after melting, refining, and casting the steel having the above-described chemical composition, it may be formed into a raw pipe by various hot pipe manufacturing methods (such as extrusion pipe manufacturing, punching pipe manufacturing, Mannesmann pipe manufacturing, etc.), and then cooled to room temperature. As methods for melting, refining, and casting the steel, not only the converter process but also the electric furnace process and the like may be used.

[0066] The heating temperature during hot pipe manufacturing is preferably higher in order to ensure hot workability. However, since the steel is usually heated in an oxidizing atmosphere furnace, a decarburized layer is likely to form on the surface layer. In order to suppress the formation of such a decarburized layer as much as possible, the heating temperature is preferably 1250 °C or lower, more preferably 1200 °C or lower.

[0067] In addition, in the cooling process after hot pipe manufacturing, since it is preferable to pass through the temperature range in which the steel decarburizes as quickly as possible, the cooling rate is preferably as fast as possible. Therefore, at the temperature of the steel pipe surface, the cooling rate is preferably 10 °C / s or higher, more preferably 15 °C / s or higher. Note that as the cooling method, any method such as water cooling, oil cooling, or mist cooling may be used.

[0068] Subsequently, spheroidizing annealing is performed on the raw pipe obtained by the pipe manufacturing process. In the manufacturing method of the present embodiment, by performing spheroidizing annealing in a carburizing atmosphere, carburization of the decarburized layer generated in the hot pipe manufacturing process and the like is promoted and re-carbonized.

[0069] Here, for example, as described in Patent Document 2 above, considering the diffusion rate of C, it has been extremely advantageous for carburization to perform the carburization treatment in a temperature range where the ferrite phase below the Ac 1 point stably exists. However, as a result of further studies by the present inventors, it has been found that, conversely, performing the carburization treatment at a higher temperature enables more precise control of the thickness of the decarburized layer. In addition, it has also been found that by performing the carburization treatment in a high-temperature range, the ferrite in the remaining decarburized layer can be coarsened, and the machinability can be further improved.

[0070] From the above viewpoints, the heating temperature during spheroidizing annealing shall be a temperature above the Ac 1 point and exceeding 800°C. The heating temperature is preferably 810°C or higher. Note that if the heating temperature is excessive, the carbide precipitated during cooling after heating tends to be layered, making it difficult to obtain a spheroidized structure. In addition, if the heating temperature is excessive, as described above, a high carbon potential atmosphere is required for carburizing, which is likely to cause operational troubles. Therefore, the upper limit temperature is preferably set at 950°C.

[0071] In order to ensure sufficient time for spheroidization and carburizing, the holding time during spheroidizing annealing in a carburizing atmosphere shall be 10 minutes or longer in the temperature range above the Ac 1 point and exceeding 800°C. The holding time is more preferably 15 minutes or longer, and even more preferably 20 minutes or longer.

[0072] Regarding the cooling after holding during spheroidizing annealing, in order to promote spheroidization and carburizing until the temperature reaches below the Ar 1 point, the cooling rate shall be 10°C / hour or higher and 100°C / hour or lower. More preferably, it is 15°C / hour or higher and 80°C / hour or lower, and even more preferably 20°C / hour or higher and 70°C / hour or lower. Note that the Ar 1 point approximately coincides with the Ac 1 point as long as the cooling rate is within the above order.

[0073] Ar 1 Regarding the cooling rate in the temperature range below the Ar 1 point, it is preferably the above cooling rate until 650°C. Also, there is no particular problem even if the furnace atmosphere is changed from a carburizing atmosphere to an oxidizing atmosphere below 650°C.

[0074] The total time (time in the furnace) staying in the furnace including heating, holding, and cooling in the spheroidizing annealing process shall be 2 hours or longer and 15 hours or shorter from the viewpoints of promoting spheroidization and carburizing and manufacturing efficiency. The time in the furnace is more preferably 3 hours or longer and 12 hours or shorter, and even more preferably 4 hours or longer and 10 hours or shorter.

[0075] There are no particular restrictions on the method for controlling the atmosphere gas to form a carburizing atmosphere. Examples of the atmosphere gas include a method using only endothermic reformed gas (RX gas), a method using a CO-CO 2 -H 2 -H 2 O-N 2 system gas, etc.

[0076] When using only RX gas to form a carburizing atmosphere, the fuel cost for generating the required RX gas becomes high. On the other hand, when using a mixed gas of RX gas and NX gas, the carbon potential is lower than that of an atmosphere of only RX gas, but the fuel cost for generating NX gas is lower than that of RX gas, so it is relatively inexpensive. The carbon potential of the atmosphere gas can be controlled by the input ratio of both gases. Regarding the carbon potential, a higher value works more advantageously for carburizing, but when it exceeds 0.70, overcarburization tends to progress, and soot is likely to occur on the surface of the steel material, easily causing operational troubles. It is desirable to keep the carbon potential below 0.70.

[0077] Note that descaling, other heat treatments, and cold working may be performed in the processes before and after the spheroidizing annealing process.

[0078] Hereinafter, the present invention will be described more specifically by way of examples, but the present invention is not limited to these examples.

Examples

[0079] Steel having the chemical compositions shown in Tables 1-1 to 1-3 was melted in a high-frequency vacuum melting furnace to obtain a 50 kg ingot with an outer diameter of 150 mm. The obtained ingot was processed to an outer diameter of 110 mm by hot forging, and then processed into a billet with an outer diameter of 100 mm by machining. After heating at 1200 °C, it was subjected to Mannesmann-type piercing hot pipe making (Φ86 mm, wall thickness 8 mm).

[0080]

Table 1-1

[0081]

Table 1-2

[0082]

Table 1-3

[0083]

Table 2

[0084] For the raw pipe obtained by hot pipe making, spheroidizing annealing was carried out under the conditions shown in Table 2 in a carburizing atmosphere. In the temperature range lower than 500°C, it was air-cooled in an air atmosphere. The Ac 1 point and Ac m point were calculated using Thermo-Calc (version 2020a, TCFE8 database).

[0085] As the carburizing atmosphere gas, a mixed gas of RX gas and NX gas (CO: 13 vol%, CO 2 : 0.5 vol%, O 2 : 0.1 vol%, H 2 O: 0.7%, H 2 : 26 vol%, the balance N 2 ) was used.

[0086] For each of the steel pipes before and after spheroidizing annealing, the carbon content in the surface layer was measured. Specifically, in a cross-section parallel to the axial direction of the steel pipe and passing through the central axis, line analysis was performed by FE-EPMA (equipment model JXA-8530F, manufactured by JEOL Ltd.) at a pitch of 1 μm from the surface toward the center of the wall thickness. The moving average of the measured values at nine points centered on a predetermined depth position was taken as the carbon content at that depth position. The acceleration voltage in FE-EPMA was set to 15 kV. The region where the carbon content was 95% or less of the average carbon content of the steel was defined as the decarburized layer region. Also, the region where the carbon content was 0.05% or more higher in mass% than the average carbon content of the steel was defined as the overcarburized layer region. And based on the results, the thicknesses of the decarburized layer region and the overcarburized layer region were determined.

[0087] Subsequently, for the steel pipe after spheroidizing annealing, the average crystal grain size of ferrite in the decarburized layer region was measured. Specifically, after nitriding corrosion of a cross-section parallel to the axial direction of the steel pipe and passing through the central axis to reveal the microstructure, in the region identified as the decarburized layer region by the above FE-EPMA measurement, observation was carried out using an optical microscope, and the average crystal grain size of ferrite was determined by the intercept method.

[0088] Furthermore, an evaluation test of machinability was conducted using the steel pipes of test numbers 1 and 27. The length of each steel pipe was 135 mm. The evaluation test environment and test conditions for machinability are as follows. Also, for machinability, it was evaluated by the maximum flank wear width on the tool side (representing the degree of tool wear).

[0089] <Test environment> Machine used NL2500 / 750 (DMG MORI SEIKI) Holder DCLNR2525M-12 (KYOCERA) Insert CNMG120408-LP_MC6115 (MITSUBISHI MATERIALS) Lubrication Dry Tool protrusion 40 mm Chuck pressure 1.5 MPa Grip length 1.5 MPa Grip method Internal expansion

[0090] <Test conditions> Cutting speed: 250 m / min Feed rate: 0.2 mm / rev Depth of cut: 1.0 mm on one side Amount of machining: Machining from an outer diameter of 86 mm to 77 mm Cumulative cutting distance (after one test): Approximately 500 m in 4-pass machining Number of test pieces: 10 pieces

[0091] In Invention Examples Nos. 1 to 26, the conditions of the present invention were satisfied, and it was confirmed that the decarburized layer thickness was within the range of 0.10 mm in the direction from the surface layer toward the center of the wall thickness.

[0092] On the other hand, in Comparative Examples Nos. 27 to 37, since they deviated from the conditions of the present invention, the desired decarburized layer thickness could not be obtained.

[0093] Also, as a result of the machinability evaluation test, the maximum flank wear width on the tool side was 0.20 mm for the steel pipe of test number 27, which is a comparative example, whereas it was 0.16 mm for the steel pipe of test number 1. Thus, it can be seen that, compared with the comparative examples, the wear width is reduced by about 20% in the present invention examples, and the tool life is greatly improved.

Industrial applicability

[0094] According to the present invention, it is possible to stably obtain a steel pipe for shafts having excellent machinability.

Claims

1. The chemical composition of the steel is, by mass%, C: 0.60 to 1.60%, Si: 0.10 to 2.00%, Mn: 0.01 to 2.00%, P: 0.040% or less, S: 0.040% or less, Al: 0.001 to 0.500%, N: 0.0200% or less, O: 0.0100% or less, Cr: 0.01 to 5.00%, Ni: 0.01 to 0.50%, Cu: 0.01 to 0.50%, the balance being Fe and impurities, the Y value defined by the following formula (i) satisfies 0.150 or less, a region where the carbon content is 95% or less of the average carbon content of the steel exists within at least the range from the surface of the steel to a depth position of 0.10 mm toward the center of the wall thickness, and the thickness of the region is 0.10 mm or less, in the region, the average crystal grain size of ferrite is 10 μm or more, a steel pipe for a shaft. Y = -0.521 + 0.630C - 0.2645Si + 0.132Mn + 0.0528(Ni + Cu) + 0.036P - 3.349S - 0.0003Cr... (i) However, the element symbols in the above formula mean the content (mass%) of each element in the steel.

2. The chemical composition of the steel is, by mass%, C: 0.60 to 1.60%, Si: 0.10 to 2.00%, Mn: 0.01 to 2.00%, P: 0.040% or less, S: 0.040% or less, Al: 0.001 to 0.500%, N: 0.0200% or less, O: 0.0100% or less, Cr: 0.01 to 5.00%, Ni: 0.01 to 0.50%, Cu: 0.01 to 0.50%, and further contains one or more selected from the group consisting of the following Group A, Group B, and Group C, the balance being Fe and impurities, the Y value defined by the following formula (i) satisfies 0.150 or less, a region where the carbon content is 95% or less of the average carbon content of the steel exists within at least the range from the surface of the steel to a depth position of 0.10 mm toward the center of the wall thickness, and the thickness of the region is 0.10 mm or less, in the region, the average crystal grain size of ferrite is 10 μm or more, a steel pipe for a shaft. Y = -0.521 + 0.630C - 0.2645Si + 0.132Mn + 0.0528(Ni + Cu) + 0.036P - 3.349S - 0.0003Cr... (i) However, the element symbols in the above formula mean the content (mass%) of each element in the steel. [Group A] One or more selected from the group consisting of Sn: 0.100% or less, Sb: 0.100% or less, and Bi: 0.1000% or less [Group B] One or more selected from the group consisting of Mo: 1.00% or less, V: 1.00% or less, Ti: 1.000% or less, Nb: 1.000% or less, W: 1.000% or less, Ta: 0.500% or less, Zr: 0.500% or less, Hf: 0.500% or less, Co: 0.50% or less, and B: 0.0100% or less [Group C] One or more selected from the group consisting of Ca: 0.0050% or less, Mg: 0.0050% or less, and REM: 0.0050% or less

3. The chemical composition contains one or more elements selected from the group A The steel pipe for a shaft according to claim 2

4. The chemical composition contains one or more elements selected from the group B The steel pipe for a shaft according to claim 2

5. The chemical composition contains one or more elements selected from the group C The steel pipe for a shaft according to claim 2

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