Electric welded steel pipe for mechanical structural components and manufacturing method thereof

Abstract

An electric-steel welded steel pipe for a machine structural component, comprising a processed portion X that is at least one of a drawn portion and a steel pipe bent portion, the processed portion X comprising a base material portion and a welded portion, the base material portion comprising, in mass%, C: 0.150-0.230%, Si: 0-0.50%, Mn: 0.50-1.65%, Nb: 0.010-0.050%, Mo: 0.10-0.60%, and A l: 0.005-0.060%, and the remainder: Fe and impurities, the area ratio of tempered bainite in the base material portion relative to the entire microstructure is 80% or more, the tensile strength of the base material portion is 850-1000 MPa, in the tensile test of the base material portion, a yield elongation of 0.2% or more was observed, and the ratio of the Vickers hardness of the base material portion at a depth of 50 μm from the outer surface to the Vickers hardness at the center of the wall thickness of the base material portion is 95% or more.

Description

Technical Field

[0001] The present disclosure relates to an electric-welded steel pipe for mechanical structural components and a method for manufacturing the same. Background Art

[0002] Mechanical structural components made of electric welded steel pipes are being used.

[0003] For example, Patent Document 1 discloses a steel material for automotive chassis components, a type of mechanical structural component. Specifically, Patent Document 1 discloses a steel material for automotive chassis components that exhibits high fatigue properties, requires minimal heat treatment costs, and exhibits excellent formability. The steel material is characterized by being a steel material with composite additions of Nb and Mo, and having a Vickers hardness at the center of the plate thickness after bending to achieve a bend radius of 2 to 5 times the plate thickness, and a maximum Vickers hardness within 0.5 mm from the surface of the plate, of 50 to 150 points.

[0004] Patent Document 1: Japanese Patent Application Laid-Open No. 2008-63656 Summary of the Invention

[0005] Problems to be solved by the invention

[0006] In recent years, there has been a demand for lightweight machine structural components.

[0007] In order to reduce the weight of a machine structural component, a processed portion X, which is at least one of a drawn portion and a steel pipe bent portion, in the machine structural component is required to have excellent tensile strength and further, excellent fatigue limit.

[0008] In the present disclosure, the so-called "steel pipe bending portion" refers to a portion where the steel pipe bending process is performed, and the so-called "steel pipe bending process" refers to bending process performed on at least a portion of the steel pipe in the axial direction, and the steel pipe axis (i.e., the center axis) is bent.

[0009] In contrast, the "bending process in which the curvature R of the outer surface of the plate becomes 2 to 5 times the plate thickness" in Patent Document 1 is a bending process in which a portion of the steel plate in the circumferential direction of the steel pipe is bent strongly (i.e., under the condition that the curvature R of the outer surface of the plate becomes 5 times or less of the plate thickness), rather than a bending process in which the central axis of the steel pipe is bent (see FIG. 2 of the same document), and therefore does not belong to the "steel pipe bending process" in the present disclosure. The above-mentioned bending process in Patent Document 1 is a strong bending process (i.e., a bending process with a small curvature R) for forming the ear portion of an axle beam as a running part of an automobile (see above, see FIG. 2 of Patent Document 1). Figure 1(See Figure 6). Patent Document 1 utilizes the fact that during this strong bending process, the hardness of the surface layer of the steel plate becomes significantly higher than that of the central portion of the wall thickness (see paragraphs 0022 and 0023 of the document). In particular, paragraph 0022 states that if the bending radius (R) of the outer surface of the plate exceeds five times the plate thickness, the increase in hardness due to work hardening becomes insufficient. Therefore, it is necessary to set the bending process to a strong bending process in which the bending radius (R) of the outer surface of the plate is two to five times the plate thickness.

[0010] On the other hand, Patent Document 1 does not consider the drawn portion and the bent portion of the steel pipe at all.

[0011] An object of one aspect of the present disclosure is to provide an electric resistance welded steel pipe for machine structural parts that includes a processed portion X that is at least one of a drawn portion and a steel pipe bent portion, the processed portion X having excellent tensile strength and fatigue resistance, and a method for manufacturing the electric resistance welded steel pipe for machine structural parts.

[0012] Means for solving problems

[0013] Means for solving the above-mentioned problems include the following.

[0014] <1> An electric-steel welded steel pipe for a machine structural component, comprising a processed portion X that is at least one of a drawing portion and a steel pipe bending portion.

[0015] The processing part X includes the base material part and the welding part.

[0016] The chemical composition of the above-mentioned base material is expressed in mass %:

[0017] C: 0.150~0.230%,

[0018] Si: 0-0.50%,

[0019] Mn: 0.50-1.65%,

[0020] P: 0~0.030%

[0021] S: 0~0.010%,

[0022] Nb: 0.010~0.050%,

[0023] Mo: 0.10~0.60%,

[0024] Al: 0.005~0.060%,

[0025] N: 0~0.0060%

[0026] Ti: 0~0.030%,

[0027] V: 0~0.100%

[0028] Cr: 0-0.5%,

[0029] Cu: 0~0.500%,

[0030] Ni: 0~0.500%,

[0031] B: 0~0.0030%,

[0032] Ca: 0~0.0030%,

[0033] Mg: 0-0.0040%, and

[0034] The rest is composed of Fe and impurities.

[0035] The area ratio of the tempered bainite in the base material portion relative to the entire microstructure is 80% or more.

[0036] The tensile strength of the above-mentioned base material is 850-1000 MPa.

[0037] In the tensile test of the base material, a yield elongation of 0.2% or more was observed.

[0038] The ratio of the Vickers hardness of the base material portion at a depth of 50 μm from the outer surface to the Vickers hardness of the base material portion at a center portion of the wall thickness is 95% or more.

[0039] <2> according to <1> The electric welded steel pipe for mechanical structural components is an electric welded steel pipe for automobile running components.

[0040] <3> according to <1> or <2> The electric welded steel pipe for mechanical structural components has an outer circumference of the processed portion X of 50 to 500 mm and a maximum wall thickness of the processed portion X of 1.0 to 5.0 mm.

[0041] <4> A method for manufacturing electric welded steel pipes for mechanical structural components, which is to manufacture <1> ~ <3> The method for welding an electric-steel pipe for a mechanical structural component as described in any one of the preceding claims comprises the following steps:

[0042] A process for preparing a rolled electric-resistance welded steel pipe, wherein the rolled electric-resistance welded steel pipe comprises a base metal portion A and a weld portion A, wherein the chemical composition of the base metal portion A is as follows (in mass %):

[0043] C: 0.150~0.230%,

[0044] Si: 0-0.50%,

[0045] Mn: 0.50-1.65%,

[0046] P: 0~0.030%

[0047] S: 0~0.010%,

[0048] Nb: 0.010~0.050%,

[0049] Mo: 0.10~0.60%,

[0050] Al: 0.005~0.060%,

[0051] N: 0~0.0060%

[0052] Ti: 0~0.030%,

[0053] V: 0~0.100%

[0054] Cr: 0-0.5%,

[0055] Cu: 0~0.500%,

[0056] Ni: 0~0.500%,

[0057] B: 0~0.0030%,

[0058] Ca: 0~0.0030%,

[0059] Mg: 0-0.0040%, and

[0060] The rest is composed of Fe and impurities.

[0061] The area ratio of bainite in the base material portion A relative to the entire microstructure is 80% or more.

[0062] The tensile strength of the base material part A is 600-800 MPa.

[0063] In the tensile test of the base material portion A, a total elongation of 13.0% or more was observed;

[0064] a cold working step of subjecting at least a portion of the rolled electric-resistance welded steel pipe in the pipe axial direction to at least one of cold drawing and cold pipe bending, with the maximum reduction of area being 10 to 40%; and

[0065] The tempering step is to perform tempering at a temperature of 450 to 650° C. on the cold-worked as-rolled electric resistance welded steel pipe to obtain the electric resistance welded steel pipe for machine structural parts.

[0066] <5> according to <4> In the method for manufacturing electric welded steel pipes for mechanical structural components, the outer diameter of the rolled electric welded steel pipes is 50 to 150 mm, and the wall thickness of the rolled electric welded steel pipes is 2.0 to 4.0 mm.

[0067] Effects of the Invention

[0068] According to one aspect of the present disclosure, there is provided an electric resistance welded steel pipe for machine structural parts including a processed portion X that is at least one of a drawn portion and a steel pipe bent portion, the processed portion X having excellent tensile strength and fatigue limit, and a method for manufacturing the electric resistance welded steel pipe for machine structural parts. BRIEF DESCRIPTION OF THE DRAWINGS

[0069] Figure 1 This is a plan view of a fatigue test piece in Examples. DETAILED DESCRIPTION

[0070] In the present disclosure, a numerical range expressed using “to” means a range including the numerical values ​​described before and after “to” as the lower limit and the upper limit.

[0071] In the present disclosure, “%” indicating the content of a component (element) means “mass %”.

[0072] In this disclosure, the content of C (carbon) may be described as “C content.” The contents of other elements may be described similarly.

[0073] In the present disclosure, the term "process" includes not only independent processes but also processes that cannot be clearly distinguished from other processes as long as the intended purpose of the process can be achieved.

[0074] In this disclosure, "as-rolled electric resistance welded steel pipe" refers to electric resistance welded steel pipe that has not undergone any heat treatment other than weld heat treatment after pipemaking. Specifically, the "as-rolled" part of "as-rolled electric resistance welded steel pipe" means "in a state of being formed by roll forming."

[0075] "Pipe making" refers to a process in which a hot-rolled steel sheet unwound from a hot coil is roll-formed into an open pipe, and the butted portions of the resulting open pipe are electric-welded to form an electric-weld joint.

[0076] The term "hot coil" refers to a hot-rolled steel sheet produced using a hot strip mill and wound into a coil.

[0077] "Roll forming" means continuously bending a hot-rolled steel sheet unwound from a hot coil into an open tubular shape.

[0078] Hot-rolled steel sheets manufactured using a hot strip mill are different from thick steel plates manufactured using a plate mill in that they are continuous steel sheets.

[0079] Since thick steel plates are not continuous steel sheets, they cannot be used for continuous bending, i.e., roll forming.

[0080] Based on the above point, electric-resistance-welded steel pipes are clearly distinguished from welded steel pipes manufactured using thick steel plates (for example, UOE steel pipes).

[0081] In the processed portion X of the electric resistance welded steel pipe of the present disclosure or the as-rolled electric resistance welded steel pipe of the present disclosure, the base metal portion refers to a portion excluding the weld portion and the heat-affected zone.

[0082] The heat affected zone (hereinafter also referred to as "HAZ") is a portion affected by the heat generated by welding (or, when weld heat treatment is performed after welding, by the heat generated by both welding and weld heat treatment).

[0083] Electric-welded steel pipes for mechanical structural components

[0084] The electric-resistance welded steel pipe for machine structural parts disclosed herein (hereinafter also referred to as "electric-resistance welded steel pipe") is a welded steel pipe for machine structural parts that includes a processed portion X that is at least one of a drawn portion and a steel pipe bending portion.

[0085] The processing part X includes the base material part and the welding part.

[0086] Chemical composition of the base material in mass %:

[0087] C: 0.150~0.230%,

[0088] Si: 0-0.50%,

[0089] Mn: 0.50-1.65%,

[0090] P: 0~0.030%

[0091] S: 0~0.010%,

[0092] Nb: 0.010~0.050%,

[0093] Mo: 0.10~0.60%,

[0094] Al: 0.005~0.060%,

[0095] N: 0~0.0060%

[0096] Ti: 0~0.030%,

[0097] V: 0~0.100%

[0098] Cr: 0-0.5%,

[0099] Cu: 0~0.500%,

[0100] Ni: 0~0.500%,

[0101] B: 0~0.0030%,

[0102] Ca: 0~0.0030%,

[0103] Mg: 0-0.0040%, and

[0104] The rest is composed of Fe and impurities.

[0105] The area ratio of tempered bainite in the base metal part to the entire microstructure is 80% or more.

[0106] The tensile strength of the base material is 850-1000MPa.

[0107] In the tensile test of the base material, a yield elongation of 0.2% or more was observed.

[0108] The ratio of the Vickers hardness of the base material portion at a depth of 50 μm from the outer surface to the Vickers hardness of the base material portion at a center portion of the wall thickness is 95% or more.

[0109] The electric resistance welded steel pipe of the present disclosure includes a processed portion X, which has excellent tensile strength (specifically, the tensile strength of the base material portion at the processed portion X is 850 to 1000 MPa) and also has excellent fatigue resistance.

[0110] The above effect is achieved by combining the following technical features:

[0111] Chemical composition of the base material at the processed portion X,

[0112] The above microstructure of the base material part mainly composed of tempered bainite,

[0113] The yield elongation and

[0114] The above-mentioned Vickers hardness ratio is 95% or more.

[0115] Hereinafter, the electric resistance welded steel pipe of the present disclosure will be described in detail.

[0116] The electric resistance welded steel pipe of the present disclosure includes a processed portion X that is at least one of a drawn portion and a steel pipe bending portion.

[0117] Here, the "steel pipe bending" at the "steel pipe bending portion" is bending performed on at least a portion of the steel pipe in the pipe axial direction, as described above, and the pipe axis (ie, the center axis) of the steel pipe is bent.

[0118] That is, the processed portion X, which is at least one of the drawn portion and the steel pipe bent portion, is at least a portion of the electric resistance welded steel pipe in the pipe axial direction.

[0119] The processed portion X includes a base material portion and a welded portion.

[0120] The processed portion X is a portion in which at least a portion in the pipe axial direction of an as-rolled electric resistance welded steel pipe including a base material portion A and an electric resistance weld portion A (i.e., an electric resistance welded steel pipe that has not been subjected to heat treatment other than weld heat treatment after pipemaking) has been processed and tempered in sequence.

[0121] Here, the base metal portion A and the welded portion A refer to the base metal portion and the welded portion in the rolled electric-welded steel pipe, respectively.

[0122] The processing is at least one of drawing and steel pipe bending. The processed portion X may be a portion obtained by subjecting the same portion of the rolled electric resistance welded steel pipe to both drawing and steel pipe bending.

[0123] Tempering refers to a heat treatment in which the temperature is heated to the Ac1 point or lower. For an example of the tempering conditions, reference can be made to the production method X described later.

[0124] The processed portion X may be a portion of the electric resistance welded steel pipe of the present disclosure in the pipe axial direction, or may be the entire electric resistance welded steel pipe of the present disclosure.

[0125] As an example of an electric resistance welded steel pipe whose entirety is a processed portion X, there is an electric resistance welded steel pipe produced by subjecting a rolled electric resistance welded steel pipe to drawing over its entire length and then subjecting it to tempering.

[0126] Examples of electric resistance welded steel pipes in which a portion in the pipe axial direction serves as a processed portion X include those manufactured by subjecting a rolled electric resistance welded steel pipe to a portion in the pipe axial direction (i.e., a process in which the pipe axis is bent) and then tempering the entire steel pipe.

[0127] The processed portion X is distinguished from the non-processed portion in appearance. Specifically, the portion that is, in appearance, at least one of the drawn portion and the bent portion of the steel pipe is considered the processed portion X in this disclosure. Alternatively, the portion of the base material with a tensile strength of 850 to 1000 MPa may be considered the processed portion X.

[0128] The chemical composition of the base material portion at the processed portion X will be described in detail below. As one of the characteristics of the chemical composition of the base material portion, the base material portion contains both Nb and Mo. Nb and Mo contribute to precipitation strengthening described below.

[0129] The area ratio of the tempered bainite in the base material portion at the processed portion X relative to the entire microstructure is 80% or more.

[0130] This feature also contributes to precipitation strengthening described later.

[0131] This feature indicates that the electric resistance welded steel pipe disclosed herein is produced by processing and tempering an as-rolled electric resistance welded steel pipe in which the area ratio of bainite in the base material portion A relative to the entire microstructure is 80% or more.

[0132] The tensile strength of the base material portion at the processed portion X is 850 to 1000 MPa.

[0133] As described above, the electric resistance welded steel pipe of the present disclosure is obtained by sequentially subjecting a rolled electric resistance welded steel pipe to processing (that is, at least one of drawing and steel pipe bending) and tempering.

[0134] Generally speaking, tempered electric resistance welded steel pipes may have lower strength than untempered electric resistance welded steel pipes (i.e., as-rolled electric resistance welded steel pipes). This is presumably because the strain introduced into the steel structure is reduced by tempering.

[0135] However, the electric resistance welded steel pipe of the present disclosure, despite being tempered, can ensure excellent tensile strength (i.e., 850 to 1000 MPa) as the tensile strength of the base material portion of the processed portion X. The following reasons 1 and 2 are considered to be the reasons for achieving such excellent tensile strength (i.e., 850 to 1000 MPa).

[0136] Factor 1: This is because plastic strain is introduced into the steel structure by at least one of drawing and steel pipe bending, causing work hardening.

[0137] Factor 2 is that composite carbides containing Nb and Mo precipitate at dislocations generated in the steel structure due to the introduction of plastic strain during tempering, thereby exhibiting precipitation strengthening.

[0138] It is believed that the microstructure, in which the area ratio of tempered bainite in the base metal portion is 80% or greater, contributes to the effective exertion of the precipitation strengthening effect. Specifically, it is believed that by having an area ratio of bainite of 80% or greater in the base metal portion of the as-rolled electric resistance welded steel pipe, Nb and Mo can be dissolved in the steel before tempering. As a result, complex carbides containing Nb and Mo are precipitated during tempering, enabling effective precipitation strengthening.

[0139] In the electric resistance welded steel pipe of the present disclosure, a yield elongation of 0.2% or more was observed in a tensile test of the base material portion at the processed portion X. However, the yield elongation of 0.2% or more refers to a substantial yield elongation.

[0140] Observing a yield elongation of 0.2% or more in a tensile test of the base material portion at the processed portion X means that the residual strain is reduced. Therefore, by satisfying this characteristic, the introduction of strain due to repeated stress can be suppressed, and the fatigue properties of the processed portion X can be improved.

[0141] Furthermore, the above-mentioned characteristics mean that the electric resistance welded steel pipe of the present disclosure is manufactured by subjecting a rolled electric resistance welded steel pipe to processing and tempering.

[0142] In contrast to the electric resistance welded steel pipe of the present disclosure, substantial yield elongation (i.e., yield elongation of 0.2% or more) is not observed in as-rolled electric resistance welded steel pipe (i.e., electric resistance welded steel pipe not subjected to heat treatment other than weld heat treatment after pipemaking).

[0143] In the electric-resistance welded steel pipe of the present disclosure, in the processed portion X, the ratio of the Vickers hardness of the base material portion at a depth of 50 μm from the outer surface to the Vickers hardness at the center of the wall thickness of the base material portion (hereinafter also referred to as "hardness ratio [depth 50 μm / center of wall thickness]") is 95% or greater.

[0144] The electric-resistance welded steel pipe disclosed herein, while satisfying the aforementioned tensile strength and yield elongation, achieves a hardness ratio (depth 50 μm / center wall thickness) of 95% or higher, thereby suppressing the occurrence of fatigue cracks originating from the outer surface. This characteristic contributes to the fatigue strength of the processed portion X.

[0145] The above-mentioned characteristic of a hardness ratio [depth 50 μm / center thickness] of 95% or more means that the formation of a decarburized layer is suppressed in the region including the outer surface of the base material. The decarburized layer is generally formed by "quenching" by heating to a temperature exceeding the Ac1 point.

[0146] The above characteristics are achieved by manufacturing the as-rolled electric resistance welded steel pipe by subjecting it to tempering (heating to a temperature below the Ac1 point) rather than quenching (heating to a temperature exceeding the Ac1 point).

[0147] As described above, the tensile strength, microstructure, yield elongation, and hardness ratio (50 μm depth / center wall thickness) of the electric resistance welded steel pipe of the present disclosure, among other characteristics, are closely related not only to the chemical composition but also to the manufacturing conditions used to produce the electric resistance welded steel pipe of the present disclosure. An example of a manufacturing method for producing the electric resistance welded steel pipe of the present disclosure is described below as Manufacturing Method X.

[0148] As the application of the electric-resistance welded steel pipe for machine structural parts disclosed herein, that is, the machine structural parts, preferably, automobile chassis parts, bearings, or motor covers, more preferably, automobile chassis parts.

[0149] Examples of automobile chassis include axle beams, trailing arms, suspension members, link materials, and torsion beams.

[0150] <Chemical composition of base material>

[0151] Hereinafter, the chemical composition of the base material portion at the processed portion X of the electric resistance welded steel pipe of the present disclosure (that is, the chemical composition in the present disclosure) will be described.

[0152] C: 0.150~0.230%

[0153] C is an element that combines Nb and Mo to form fine composite carbides of Nb and Mo, thereby increasing the tensile strength and fatigue limit of the processed portion X. If the C content is less than 0.150%, this effect may not be achieved. If the C content is less than 0.150%, low-temperature coiling may be necessary during the hot-rolling process to further increase tensile strength. Low-temperature coiling can increase the unevenness of tensile strength. Therefore, the C content is set to 0.150% or more. The lower limit of the C content is preferably 0.160%, and more preferably 0.170%.

[0154] On the other hand, if the C content exceeds 0.230%, not only is there a risk of significant formation and growth of pearlite and martensite in the microstructure, reducing cold workability, but also a risk of reduced fatigue limit. Therefore, the C content is set to 0.230% or less. The upper limit of the C content is preferably 0.220%, and more preferably 0.210%.

[0155] Si: 0-0.50%

[0156] Si is an optional element. That is, the Si content may be 0% or may exceed 0%.

[0157] Si is an element that contributes to the deoxidation of steel. From the viewpoint of more effectively achieving the above-mentioned effect, the lower limit of the Si content is preferably 0.01%, and more preferably 0.05%.

[0158] On the other hand, if the Si content exceeds 0.50%, the toughness of the base material may be reduced, and excessive Si oxides may be generated during electric welding, thereby reducing the mechanical properties of the processed portion X. Therefore, the Si content is 0.50% or less. The upper limit of the Si content is preferably 0.48%, and more preferably 0.40%.

[0159] On the other hand, if the Si content is excessively reduced, the production cost increases. Therefore, in consideration of industrial productivity, the lower limit of the Si content is preferably 0.01%, more preferably 0.05%.

[0160] Mn: 0.50~1.65%

[0161] Mn is an element that increases the tensile strength and fatigue resistance of electric-resistance welded steel pipe. If the Mn content is less than 0.50%, these effects may not be achieved. Therefore, the Mn content should be at least 0.50%. The lower limit of the Mn content is preferably 0.60%, and more preferably 0.70%.

[0162] On the other hand, if the Mn content exceeds 1.65%, there is a risk that excessive Mn oxides will be generated during electric welding, deteriorating the mechanical properties of the processed portion X. Therefore, the Mn content is set to 1.65% or less. The upper limit of the Mn content is preferably 1.60%, more preferably 1.50%.

[0163] P: 0~0.030%

[0164] P is an element that may be included as an impurity. If the P content exceeds 0.030%, weldability may be reduced and / or toughness may be reduced. Therefore, the P content is 0.030% or less. The upper limit of the P content is preferably 0.015%, and more preferably 0.010%.

[0165] The P content may be 0% or may exceed 0%.

[0166] If the P content is excessively reduced, the production cost increases. Therefore, in consideration of industrial productivity, the lower limit of the P content is preferably 0.001%, more preferably 0.005%.

[0167] S: 0~0.010%

[0168] S is an element that may be included as an impurity. If the S content exceeds 0.010%, weldability may be reduced and / or toughness may be decreased. Therefore, the S content is 0.010% or less. The upper limit of the S content is preferably 0.005%, and more preferably 0.003%.

[0169] The S content may be 0% or may exceed 0%.

[0170] If the S content is excessively reduced, the production cost increases. Therefore, in consideration of industrial productivity, the lower limit of the S content is preferably 0.0001%, more preferably 0.0005%.

[0171] Nb: 0.010~0.050%

[0172] Nb is an element contained together with Mo and, through tempering (described later), combines with C to form fine composite carbides containing Nb and Mo. This allows the tensile strength of the processed portion X to be maintained at a high level while significantly improving the fatigue limit of the processed portion X. If the Nb content is less than 0.010%, this effect may not be achieved. Therefore, the Nb content is set to 0.010% or more. The preferred lower limit of the Nb content is 0.015%, and the more preferred lower limit is 0.020%.

[0173] On the other hand, if the Nb content exceeds 0.050%, the toughness of the base material and the toughness of the weld after electric welding will decrease. Therefore, the Nb content is 0.050% or less. The preferred upper limit of the Nb content is 0.045%, and the more preferred upper limit is 0.040%.

[0174] Mo: 0.10~0.60%

[0175] Mo is contained along with Nb and, during the tempering described below, combines with carbon to form fine composite carbides containing Nb and Mo. This maintains the tensile strength of the processed portion X at a high level while significantly improving the fatigue limit of the processed portion X. If the Mo content is less than 0.10%, this effect may not be achieved. Therefore, the Mo content is set to 0.10% or more. The lower limit of the Mo content is preferably 0.15%, and more preferably 0.20%.

[0176] On the other hand, if the Mo content exceeds 0.60%, workability is reduced and toughness is also reduced. Therefore, the Mo content is 0.60% or less.

[0177] The upper limit of the Mo content is preferably 0.55%, more preferably 0.50%.

[0178] Al: 0.005~0.060%

[0179] Al is an element that contributes to the deoxidation of steel. If the Al content is less than 0.005%, this effect may not be achieved. Therefore, the Al content is 0.005% or more. The lower limit of the Al content is preferably 0.010%, and more preferably 0.020%.

[0180] On the other hand, if the Al content exceeds 0.060%, excessive amounts of alumina-based oxides remain in the processed portion X, degrading the mechanical properties of the processed portion X. Therefore, the Al content is 0.060% or less. The upper limit of the Al content is preferably 0.045%, more preferably 0.040%.

[0181] N: 0~0.0060%

[0182] Nitrogen is an element that may be contained as an impurity. If the N content exceeds 0.0060%, N may dissolve in the steel, reducing workability. Therefore, the N content is 0.0060% or less. The upper limit of the N content is preferably 0.0055%, and more preferably 0.0050%.

[0183] The N content may be 0% or may exceed 0%.

[0184] If the N content is excessively reduced, the production cost increases. Therefore, in consideration of industrial productivity, the lower limit of the N content is preferably 0.0001%, more preferably 0.0005%.

[0185] Ti: 0~0.030%

[0186] Ti is an optional element. That is, the Ti content may be 0% or may exceed 0%.

[0187] Ti is an element that combines with C and / or N in steel to form nitrides or carbonitrides, thereby improving the toughness of steel materials by refining the structure. To more effectively exert these effects, the lower limit of the Ti content is preferably greater than 0%, more preferably 0.005%, and even more preferably 0.007%.

[0188] On the other hand, if the Ti content exceeds 0.030%, coarse Ti nitrides and / or coarse Ti carbonitrides may form, thereby reducing the fatigue limit and toughness of the processed portion X. Therefore, the Ti content is 0.030% or less. The upper limit of the Ti content is preferably 0.025%, and more preferably 0.020%.

[0189] V: 0~0.100%

[0190] V is an optional element. That is, the V content may be 0% or may exceed 0%.

[0191] V is an element that combines with C and / or N in steel to form at least one selected from V carbides, V nitrides, and V carbonitrides, thereby improving the toughness of the steel. To more effectively exert this effect, the lower limit of the V content is preferably greater than 0%, more preferably 0.005%, and even more preferably 0.010%.

[0192] On the other hand, if the V content exceeds 0.100%, at least one selected from coarse V carbides, coarse V nitrides, and coarse V carbonitrides may form, which may reduce the fatigue limit and toughness of the processed portion X. Therefore, the V content is 0.100% or less. The upper limit of the V content is preferably 0.090%, and more preferably 0.080%.

[0193] Cr: 0~0.5%

[0194] Cr is an optional element. That is, the Cr content may be 0% or may exceed 0%.

[0195] Cr is an element that improves the hardenability of steel and increases the strength of steel. From the viewpoint of more effectively exerting these effects, the lower limit of the Cr content is preferably more than 0%, more preferably 0.1%, and even more preferably 0.15%.

[0196] On the other hand, if the Cr content exceeds 0.5%, Cr oxides may form in the weld zone, reducing the toughness of the weld zone. Therefore, the Cr content is kept at 0.5% or less. The upper limit of the Cr content is preferably 0.4%, and more preferably 0.3%.

[0197] Cu: 0~0.500%

[0198] Cu is an optional element. That is, the Cu content may be 0% or may exceed 0%.

[0199] Cu is an element that can increase the strength of steel materials. From the viewpoint of more effectively exerting this effect, the lower limit of the Cu content is preferably more than 0%, more preferably 0.001%, and even more preferably 0.010%.

[0200] On the other hand, if the Cu content exceeds 0.500%, not only will the toughness of the steel material decrease due to excessive strengthening, but the liquid metal embrittlement effect of Cu will also make cracks more likely to occur on the slab surface during slab casting. Therefore, the Cu content is kept at 0.500% or less. The upper limit of the Cu content is preferably 0.400%, and more preferably 0.300%.

[0201] Ni: 0~0.500%

[0202] Ni is an optional element. That is, the Ni content may be 0% or may exceed 0%.

[0203] Nickel is an element that dissolves in steel, thereby increasing its strength and toughness. Furthermore, nickel is an element that suppresses the liquid metal embrittlement of Cu in Cu-containing steel. To more effectively exert these effects, the lower limit of the Ni content is preferably greater than 0%, more preferably 0.001%, and even more preferably 0.010%.

[0204] On the other hand, if the Ni content exceeds 0.500%, the weldability of the steel may be reduced. Therefore, the Ni content is 0.500% or less. The upper limit of the Ni content is preferably 0.450%, more preferably 0.400%.

[0205] B: 0~0.0030%

[0206] B is an optional element. That is, the B content may be 0% or may exceed 0%.

[0207] B is an element that dissolves in steel to improve its hardenability and strength. To more effectively exert these effects, the lower limit of the B content is preferably greater than 0%, more preferably 0.0001%, and even more preferably 0.0005%.

[0208] On the other hand, if the B content exceeds 0.0030%, coarse nitrides may be formed, which may reduce the fatigue limit of the steel material. Therefore, the B content is 0.0030% or less. The upper limit of the B content is preferably 0.0025%, and more preferably 0.0020%.

[0209] Ca: 0~0.0030%

[0210] Ca is an optional element. That is, the Ca content may be 0% or may exceed 0%.

[0211] Ca is an element that controls the morphology of sulfide inclusions and thereby improves the toughness of steel. To more effectively exhibit this effect, the lower limit of the Ca content is preferably greater than 0%, more preferably 0.0001%, and even more preferably 0.0010%.

[0212] On the other hand, if the Ca content exceeds 0.0030%, coarse Ca oxides may be formed, which may reduce the toughness of the steel. Therefore, the Ca content is 0.0030% or less. The upper limit of the Ca content is preferably 0.0025%, and more preferably 0.0020%.

[0213] Mg: 0~0.0040%

[0214] Mg is an optional element. That is, the Mg content may be 0% or may exceed 0%.

[0215] Mg is an element that forms fine oxides and improves the toughness of the weld heat-affected zone (HAZ). To more effectively exhibit this effect, the lower limit of the Mg content is preferably greater than 0%, more preferably 0.0001%, and even more preferably 0.0010%.

[0216] On the other hand, if the Mg content exceeds 0.0040%, coarse oxides may be formed, which may reduce the toughness of the steel. Therefore, the Mg content is 0.0040% or less. The upper limit of the Mg content is preferably 0.0035%, and more preferably 0.0030%.

[0217] The rest: Fe and impurities

[0218] In the chemical composition of the base material portion at the processed portion X, the remainder other than the above-mentioned elements is Fe and impurities.

[0219] Here, impurities refer to components contained in raw materials (such as ore and scrap) or components mixed in during the manufacturing process, and are not intentionally contained in steel.

[0220] Impurities include all elements other than the above-mentioned elements. The impurity elements may be one or more.

[0221] Examples of impurities include O, Sb, Sn, W, Co, As, Pb, Bi, H, and REM. "REM" refers to a rare earth element, that is, at least one element selected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

[0222] Among the above-mentioned elements, the content of O is preferably controlled to be 0.006% or less.

[0223] Generally, Sb, Sn, Co, and As may be mixed in an amount of, for example, 0.1% or less; Pb and Bi may be mixed in an amount of, for example, 0.005% or less; and H may be mixed in an amount of, for example, 0.0004% or less.

[0224] There is no need to particularly control the contents of other elements as long as they are within normal ranges.

[0225] <Microstructure of the base material>

[0226] The microstructure of the base material portion at the processed portion X of the electric resistance welded steel pipe of the present disclosure will be described.

[0227] Generally speaking, the microstructure of the base material is mainly composed of tempered bainite.

[0228] Specifically, the area ratio of the tempered bainite in the base material portion relative to the entire microstructure is 80% or more.

[0229] As described above, the above-mentioned microstructure effectively contributes to the development of precipitation strengthening by Mo and Nb during tempering, thereby contributing to the improvement of tensile strength and fatigue strength.

[0230] From the viewpoint of further improving the tensile strength and fatigue strength of the processed portion X, the area ratio of tempered bainite is preferably 85% or more, more preferably 90% or more.

[0231] The concepts of bainite and tempered bainite in the present disclosure each include granular bainite, upper bainite, lower bainite, and autotempered martensite.

[0232] Regarding these structures, the change in the structural morphology caused by tempering in the present disclosure is small. Therefore, in the present disclosure, it is not necessary to strictly distinguish bainite from tempered bainite.

[0233] The area ratio of the tempered bainite in the base material portion to the entire microstructure may be 100% or less than 100%.

[0234] When the area ratio of tempered bainite is less than 100%, the remainder preferably contains at least one selected from ferrite (for example, polygonal ferrite, acicular ferrite, etc.) and pearlite.

[0235] However, the concept of pearlite also includes degenerate pearlite.

[0236] The area ratio of the tempered bainite in the base material portion relative to the entire microstructure was confirmed as follows.

[0237] In the C section of the processing portion X (i.e., the section perpendicular to the tube axis direction of the processing portion X), a sample for microstructure observation is collected from the center of the wall thickness of the base material portion. The size of the observation surface in the sample is set to 3mm×3mm. Next, the observation surface of the sample is mirror-polished. Thereafter, the observation surface of the sample is etched with 3% nital (nitric acid ethanol etching solution). The etched observation surface is observed using a 500x optical microscope. Specify any 5 fields of view in the observation surface and generate a photographic image of each specified field of view. Each field of view area is set to 200μm×200μm.

[0238] The area of ​​tempered bainite in the photographic images of five viewing fields was determined, and the obtained area was divided by the total area of ​​the five viewing fields and multiplied by 100 to determine the area ratio (%) of the tempered bainite in the base material portion to the entire microstructure.

[0239] <Tensile Strength of Base Material>

[0240] The tensile strength of the base material portion at the processed portion X is 850 to 1000 MPa.

[0241] A tensile strength of 850 MPa or more of the base material portion at the processed portion X contributes to ensuring the strength of the electric resistance welded steel pipe for use as a machine structural component. The tensile strength is preferably 900 MPa or more.

[0242] A tensile strength of 1000 MPa or less in the base material portion at the processed portion X contributes to the ease of manufacture of electric-resistance-welded steel pipes for machine structural components (e.g., ease of manufacture of the hot-rolled steel sheet used as the raw material, ease of roll-forming when the hot-rolled steel sheet is roll-formed to produce the pipe, etc.). The tensile strength is preferably 950 MPa or less.

[0243] The tensile strength of the base material portion at the processed portion X was measured as follows.

[0244] A tensile test piece is collected from the base material at the processed portion X, from a region centered at the center of the wall thickness that is 70% of the total thickness. The type of tensile test piece collected is selected from the round bar test pieces specified in JIS Z 2241 (2011) according to the thickness of the base material. If JIS Z 2241 (2011) does not specify a test piece of appropriate size, a test piece proportionally scaled down from the specified test piece (proportional test piece) may be used.

[0245] The collected tensile test pieces were subjected to a tensile test at room temperature (20±15° C.) in the atmosphere in accordance with JIS Z 2241 (2011). The tensile strength (TS) was determined from the obtained stress-strain curve.

[0246] <Yield elongation of base material>

[0247] In the tensile test of the base material portion of the electric resistance welded steel pipe disclosed herein, a yield elongation of 0.2% or more was observed.

[0248] The tensile test of the base material portion referred to here is a tensile test for measuring the tensile strength of the base material portion. The test method is as described in the section on the tensile strength of the base material portion.

[0249] This feature (ie, a yield elongation of 0.2% or more is observed in a tensile test of the base material portion) contributes to suppressing the introduction of strain due to repeated stress as described above, and further contributes to improving the fatigue strength of the processed portion X.

[0250] <Hardness ratio〔Depth 50μm / Center wall thickness〕>

[0251] The electric resistance welded steel pipe disclosed herein has a hardness ratio [depth 50 μm / center thickness] (i.e., the ratio of the Vickers hardness of the base material portion at a depth of 50 μm from the outer surface to the Vickers hardness of the base material portion at the center thickness) of 95% or more.

[0252] This feature (ie, a hardness ratio [depth 50 μm / center thickness] of 95% or more) contributes to suppressing the occurrence of fatigue cracks from the outer surface side as described above, and further contributes to improving the fatigue strength of the processed portion X.

[0253] On the other hand, the hardness ratio [50 μm depth / center of wall thickness] is preferably 120% or less, and more preferably 115% or less. When the hardness ratio [50 μm depth / center of wall thickness] is 120% or less, stress can be borne by the entire wall thickness (particularly, stress concentration in the center of the wall thickness can be suppressed), resulting in further improvement in the fatigue strength of the processed portion X.

[0254] The Vickers hardness at a depth of 50 μm from the outer surface of the base material portion is determined as follows. In the C section of the processed portion X, five points at intervals of 0.5 mm on a line corresponding to a depth of 50 μm from the outer surface of the base material portion are designated as measurement points. At each of the five measurement points, the Vickers hardness is measured under a load of 100 gf in accordance with JIS Z 2244 (2009). The arithmetic mean of the measured values ​​at the five measurement points is set as the "Vickers hardness at a depth of 50 μm."

[0255] The Vickers hardness at the center of the wall thickness of the base material is determined as follows. In the C section of the processed portion X, five points spaced 0.5 mm apart on a line corresponding to the center of the wall thickness of the base material are designated as measurement points. At each of the five measurement points, the Vickers hardness is measured under a load of 100 gf in accordance with JIS Z 2244 (2009). The arithmetic mean of the values ​​measured at the five measurement points is designated as the "Vickers hardness at the center of the wall thickness."

[0256] The hardness ratio [depth 50 μm / center of wall thickness] was calculated by dividing the "Vickers hardness at a depth of 50 μm" by the "Vickers hardness at the center of wall thickness" and multiplying the result by 100.

[0257] <Outer Circumference of Processing Portion X>

[0258] The outer perimeter of the processed portion X is preferably 50 to 500 mm.

[0259] The outer circumference in the range of 50 to 500 mm is approximately equivalent to the outer circumference of a steel pipe having an outer diameter of 16 to 160 mm.

[0260] The outer perimeter of the processed portion X is more preferably 50 to 400 mm, and further preferably 100 to 300 mm.

[0261] <Maximum wall thickness of processed portion X>

[0262] The maximum thickness of the processed portion X (ie, the maximum value of the thickness of the processed portion X) is preferably 1.0 to 5.0 mm, more preferably 1.5 to 4.5 mm, and even more preferably 2.0 to 4.0 mm.

[0263] [An Example of a Method for Manufacturing Electric-Resistance-Welded Steel Pipes for Machine Structural Components (Manufacturing Method X)]

[0264] Hereinafter, an example of a production method for producing the electric resistance welded steel pipe of the present disclosure (hereinafter referred to as “production method X”) will be described.

[0265] The following manufacturing method X is a method for manufacturing electric resistance welded steel pipes in the examples described below.

[0266] Preparation method X includes the following steps:

[0267] A step of preparing a rolled electric-resistance welded steel pipe (hereinafter also referred to as "rolled electric-resistance welded steel pipe preparation step"), wherein the rolled electric-resistance welded steel pipe comprises a base metal portion A and a weld portion A, wherein the chemical composition of the base metal portion A is (in mass %):

[0268] C: 0.150~0.230%,

[0269] Si: 0-0.50%,

[0270] Mn: 0.50-1.65%,

[0271] P: 0~0.030%

[0272] S: 0~0.010%,

[0273] Nb: 0.010~0.050%,

[0274] Mo: 0.10~0.60%,

[0275] Al: 0.005~0.060%,

[0276] N: 0~0.0060%

[0277] Ti: 0~0.030%,

[0278] V: 0~0.100%

[0279] Cr: 0-0.5%,

[0280] Cu: 0~0.500%,

[0281] Ni: 0~0.500%,

[0282] B: 0~0.0030%,

[0283] Ca: 0~0.0030%,

[0284] Mg: 0-0.0040%, and

[0285] The rest is composed of Fe and impurities.

[0286] The area ratio of bainite in the base material part A to the entire microstructure is 80% or more.

[0287] The tensile strength of the base material part A is 600-800 MPa.

[0288] In the tensile test of the parent material part A, a total elongation of more than 13.0% was observed;

[0289] a cold working step of subjecting at least a portion of the rolled electric-resistance welded steel pipe in the pipe axial direction to at least one of cold drawing and cold pipe bending, with the maximum reduction of area being 10 to 40%; and

[0290] The tempering step is to temper the cold-worked as-rolled electric resistance welded steel pipe at a tempering temperature of 450 to 650° C. to obtain an electric resistance welded steel pipe for machine structural parts.

[0291] According to the manufacturing method X, the electric-resistance welded steel pipe of the present disclosure (that is, the electric-resistance welded steel pipe for machine structural components of the present disclosure) can be manufactured.

[0292] Hereinafter, each step in Production Method X will be described.

[0293] <Preparation process of rolled electric-welded steel pipe>

[0294] The as-rolled electric-resistance-welded steel pipe preparation step is a step of preparing the as-rolled electric-resistance-welded steel pipe.

[0295] This step may be a step of simply preparing the previously manufactured rolled electric-welded steel pipe, or a step of manufacturing the rolled electric-welded steel pipe.

[0296] In the manufacturing method X, the as-rolled electric resistance welded steel pipe corresponds to the raw material of the manufactured electric resistance welded steel pipe for machine structural parts.

[0297] In manufacturing method X, at least a portion of the rolled electric-resistance welded steel pipe in the pipe axial direction is sequentially subjected to prescribed cold working (specifically, at least one of cold drawing and cold pipe bending) and prescribed tempering to produce an electric-resistance welded steel pipe for machine structural components. At this time, at least a portion of the base metal portion A and at least a portion of the welded portion A of the rolled electric-resistance welded steel pipe are converted into the base metal portion and welded portion, respectively, of the processed portion X of the electric-resistance welded steel pipe for machine structural components.

[0298] The chemical composition of the base material portion A of the as-rolled electric resistance welded steel pipe is the same as the chemical composition of the base material portion at the processed portion X of the finally obtained electric resistance welded steel pipe for machine structural parts, and the preferred range is also the same.

[0299] Each step of manufacturing method X does not affect the chemical composition of the steel. Therefore, the chemical composition of the base material portion at the processed portion X of the electric resistance welded steel pipe manufactured by manufacturing method X can be considered to be the same as the chemical composition of the base material portion A of the as-rolled electric resistance welded steel pipe as a raw material.

[0300] In the microstructure of the base material portion A, the area ratio of bainite (ie, quenched bainite) is 80% or more.

[0301] In the manufacturing method X, the bainite in the as-rolled electric resistance welded steel pipe is converted into tempered bainite in the processed portion X of the electric resistance welded steel pipe for machine structural parts through a cold working step and a tempering step.

[0302] The area ratio of bainite in the base material portion A in the as-rolled electric resistance welded steel pipe is measured in the same manner as the area ratio of tempered bainite in the base material portion at the processed portion X.

[0303] From the viewpoint of further improving the tensile strength and fatigue strength of the processed portion X of the finally obtained electric resistance welded steel pipe for machine structural parts, the area ratio of bainite is preferably 85% or more, more preferably 90% or more.

[0304] The area ratio of bainite may be 100% or may be lower than 100%.

[0305] When the area ratio of bainite is less than 100%, the remainder preferably contains polygonal ferrite.

[0306] The tensile strength of the base material portion A in the rolled electric resistance welded steel pipe is 600 to 800 MPa.

[0307] At least a portion of the base material portion A having the aforementioned tensile strength is converted into a base material portion at the processed portion X through a cold working step and a tempering step. At this time, as described above, it is believed that the tensile strength of the base material portion A of 600 to 800 MPa is increased by the effects of work hardening during cold working and precipitation strengthening due to tempering, resulting in a tensile strength of 850 to 1000 MPa.

[0308] The tensile strength of the base material portion A in the rolled electric resistance welded steel pipe is measured in the same manner as the tensile strength of the base material portion at the processed portion X of the electric resistance welded steel pipe for machine parts.

[0309] In the tensile test of the base material portion A of the rolled electric resistance welded steel pipe, a total elongation (EL) of 13.0% or more was observed.

[0310] The tensile test of the base material portion A referred to here is a tensile test for measuring the tensile strength of the base material portion A.

[0311] By observing a total elongation of 13.0% or more in the tensile test of the base material portion A, workability can be ensured when the rolled electric resistance welded steel pipe is processed to obtain an electric resistance welded steel pipe for machine structural parts. The lower limit of the total elongation is preferably 14.0%, more preferably 15.0%.

[0312] The upper limit of the total elongation is preferably 25.0%, more preferably 23.0%.

[0313] <Cold working process>

[0314] The cold working step is a step of performing at least one of cold drawing and cold pipe bending on at least a portion of the rolled electric resistance welded steel pipe in the pipe axial direction, with a maximum reduction of area of ​​10 to 40%.

[0315] By cold working in the above manner, plastic strain is effectively introduced into the steel structure, and dislocations are effectively introduced.

[0316] Here, the maximum reduction of area refers to the reduction of area at a location where the reduction of area becomes maximum in the region subjected to cold working.

[0317] The cold-drawn portion obtained by cold drawing has the advantage of excellent uniformity of hardness in the C-section of the steel pipe (ie, the section perpendicular to the pipe axis).

[0318] Compared to the processed portion of a plate, for example, in Patent Document 1, where the outer surface of the plate is bent to a radius of 2 to 5 times the plate thickness, the cold-bent portion obtained by cold-bending a plate has the advantage of superior uniformity of hardness across the wall thickness. This may reduce the number of subsequent component processing steps.

[0319] As described above, the cold-drawn portion and the cold-bent steel pipe portion each have superior hardness uniformity compared to, for example, the processed portion described in Patent Document 1. Therefore, the processed portion as a whole can bear stress (in other words, the concentration of stress on the portion with low hardness can be suppressed), which is advantageous in terms of fatigue strength.

[0320] There are no particular restrictions on the cold drawing method, and a common method can be applied.

[0321] As a method of cold steel pipe bending, for example, rotary bending, bending using a press brake, press bending, hydroforming, etc. can be applied.

[0322] <Tempering process>

[0323] The tempering step is a step of tempering the cold-worked rolled electric resistance welded steel pipe at a tempering temperature of 450 to 650° C. to obtain an electric resistance welded steel pipe for machine structural parts.

[0324] Tempering is performed, for example, in a heat treatment furnace.

[0325] During tempering in this step, by setting the tempering temperature to 450°C or higher, fine complex carbides of Nb and Mo precipitate on dislocations introduced by cold working. This effectively achieves precipitation strengthening due to Nb and Mo, enabling the aforementioned tensile strength (850 to 1000 MPa) to be achieved at the processed portion X of the electric resistance welded steel pipe of the present disclosure.

[0326] Furthermore, in the tempering process, the tempering temperature is set to 450°C or higher, thereby reducing the residual strain in the steel structure. As a result, a yield elongation of 0.2% or higher can be achieved in a tensile test of the processed portion X of the electric resistance welded steel pipe of the present disclosure.

[0327] The tempering temperature is preferably 500°C or higher.

[0328] By setting the tempering temperature to 650°C or lower during this step, the formation of a decarburized layer near the outer surface can be suppressed compared to heat treatment (e.g., quenching) at temperatures exceeding 650°C. As a result, a hardness ratio (depth 50μm / center of wall thickness) of 95% or higher can be achieved.

[0329] The tempering temperature is preferably 600° C. or lower.

[0330] The tempering time (ie, the holding time at the tempering temperature) is not particularly limited, but is, for example, 20 to 60 minutes.

[0331] The cooling method after the holding time at the tempering temperature is not particularly limited, and may be slow cooling (eg, air cooling) or rapid cooling (eg, water cooling).

[0332] <Outer diameter of rolled electric-welded steel pipe>

[0333] The outer diameter of the as-rolled electric resistance welded steel pipe is preferably 50 to 150 mm, more preferably 50 to 130 mm, and even more preferably 50 to 100 mm.

[0334] <Wall Thickness of As-Rolled Electric-Welded Steel Pipe>

[0335] The wall thickness of the as-rolled electric-resistance-welded steel pipe is preferably 2.0 to 4.0 mm, more preferably 2.5 to 3.5 mm.

[0336] [An Example of a Method for Manufacturing Rolled Electric-Resistance-Welded Steel Pipe (Manufacturing Method A)]

[0337] The step of preparing the as-rolled electric resistance welded steel pipe in the above-mentioned manufacturing method X (ie, an example of the method for manufacturing an electric resistance welded steel pipe for machine structural components disclosed herein) may also be a step of manufacturing the as-rolled electric resistance welded steel pipe.

[0338] An example of a method for producing an as-rolled electric resistance welded steel pipe is shown below as Production Method A.

[0339] Preparation method A includes the following steps:

[0340] a slab preparation step of preparing a slab having the same chemical composition as that of the base material portion A of the as-rolled electric-resistance-welded steel pipe;

[0341] a hot rolling step of heating the prepared slab to a slab heating temperature of 1070 to 1300° C. and subjecting the heated slab to hot rolling at a finishing rolling temperature FT of 850 to 950° C. to obtain a hot-rolled steel sheet;

[0342] The cooling step is to cool the hot rolled steel sheet obtained in the hot rolling step at an average cooling rate CR from the finishing rolling temperature FT to 580°C. 580 Cooling at a rate of 20 to 90°C / second to a coiling temperature CT of 480 to 580°C;

[0343] a coiling step of coiling the cooled hot-rolled steel sheet at the coiling temperature CT to obtain a hot coil made of the hot-rolled steel sheet;

[0344] The pipe making process involves unwinding a hot-rolled steel sheet from a hot coil, roll-forming the unwinded hot-rolled steel sheet into an open pipe, and then welding the butted portions of the open pipe to form a welded portion, thereby producing an electric-welded steel pipe.

[0345] The above-mentioned hot rolling process, cooling process and coiling process are carried out using a hot strip mill.

[0346] Hereinafter, each step in Production Method A will be described.

[0347] <Slab preparation process>

[0348] The slab preparation process is a process of preparing a slab.

[0349] This process may be a process of simply preparing a prefabricated slab, or a process of manufacturing the slab.

[0350] The chemical composition of the prepared slab is the same as the chemical composition of the base material portion A of the as-rolled electric resistance welded steel pipe obtained by the manufacturing method A, and the preferred range is also the same.

[0351] Each step of the manufacturing method A does not affect the chemical composition of the steel. Therefore, the chemical composition of the base material portion A of the as-rolled electric resistance welded steel pipe manufactured by the manufacturing method A can be considered to be the same as the chemical composition of the raw material slab.

[0352] When manufacturing a slab in this process, first, molten steel having the above-mentioned chemical composition is manufactured, and the slab is manufactured using the molten steel. The chemical composition of the molten steel can be considered to be the same as that of the slab.

[0353] In this case, the slab may be produced by continuous casting, or a steel ingot may be produced by using molten steel and then bloom-rolling the steel ingot to produce the slab.

[0354] <Hot rolling process>

[0355] The hot rolling step is a step of heating a slab to a slab heating temperature of 1070 to 1300° C. and hot rolling the heated slab under conditions of a finish rolling temperature FT of 850 to 950° C. to obtain a hot-rolled steel sheet.

[0356] By heating the slab to a slab heating temperature of 1070-1300°C, carbides, nitrogen compounds, and carbonitrides precipitated during the solidification of the molten steel can be dissolved in the steel material, thereby increasing the strength.

[0357] If the slab heating temperature is 1070° C. or higher, carbides, nitrogen compounds, and carbonitrides precipitated during the solidification of the molten steel can be sufficiently dissolved in the steel. The slab heating temperature is preferably 1100° C. or higher.

[0358] When the slab heating temperature is 1300° C. or lower, coarsening of austenite grains can be suppressed.

[0359] In this process, the heated slab is hot-rolled to obtain a hot-rolled steel sheet.

[0360] Hot rolling is performed using, for example, a roughing mill and a finishing mill disposed downstream of the roughing mill.

[0361] The hot roughing mill comprises a single rolling mill stand or a plurality of rolling mill stands arranged in a row, each of which has at least one pair of rolls. The roughing mill may be of a reversing type or a tandem type.

[0362] The finishing mill is located downstream of the roughing mill. It comprises a plurality of rolling mill stands arranged in a row from upstream to downstream of the rolling line. Each rolling mill stand comprises at least one pair of rollers. The finishing mill can be either a reversing type or a tandem type.

[0363] Hot rolling is performed at a finishing rolling temperature FT of 850 to 950°C.

[0364] Here, the finishing rolling temperature FT refers to the surface temperature of the steel plate at the exit side of the final rolling stand of the finishing mill.

[0365] When the finishing temperature FT is lower than 850°C, the rolling resistance of the steel sheet increases, resulting in reduced productivity. Furthermore, the steel sheet is rolled in a dual-phase region of ferrite and austenite. In this case, the area ratio of bainite in the steel sheet microstructure falls below 80%. Therefore, the lower limit of the finishing temperature FT in Method A is 850°C. The lower limit of the finishing temperature FT is preferably 860°C, and more preferably 870°C.

[0366] On the other hand, if the finishing temperature FT exceeds 950°C, the temperature of the steel sheet becomes difficult to lower even after cooling (described later). As a result, the area ratio of bainite in the steel sheet microstructure falls below 80%. Therefore, the upper limit of the finishing temperature FT in Method A is 950°C. The upper limit of the finishing temperature FT is preferably 930°C, and more preferably 900°C.

[0367] <Cooling process>

[0368] The cooling step is a step of cooling the hot rolled steel sheet obtained in the hot rolling step at an average cooling rate CR from the finishing rolling temperature FT to 580°C. 580 The steel is cooled at a rate of 20 to 90°C / second to a coiling temperature CT of 480 to 580°C.

[0369] Under these conditions, the hot-rolled steel sheet obtained in the hot rolling process can be cooled to the coiling temperature CT while suppressing the passage of the ferrite nose in the CCT diagram (Continuous Cooling Transformation diagram). As a result, in the resulting as-rolled electric-resistance-welded steel pipe, an area ratio of bainite in the base material portion A relative to the entire microstructure of 80% or more can be achieved.

[0370] At the average cooling rate CR 580 If the average cooling rate CR is lower than 20°C / s, the hot rolled steel sheet is likely to pass through the ferrite nose in the CCT diagram, and ferrite is likely to be excessively formed. As a result, the area ratio of bainite may fall below 80%, and / or the tensile strength of the base material portion A of the rolled electric resistance welded steel pipe may fall below 600 MPa. Therefore, the average cooling rate CR 580 The lower limit of the average cooling rate CR is 20℃ / s. 580 The lower limit of is preferably 30°C / second, more preferably 40°C / second, and further preferably 50°C / second.

[0371] On the other hand, at the average cooling rate CR 580If the average cooling rate CR exceeds 90°C / s, excessive martensite is likely to form, resulting in the area ratio of bainite being less than 80% and / or the tensile strength of the base material portion A of the rolled electric resistance welded steel pipe being more than 800 MPa. 580 The upper limit is 90°C / second.

[0372] The hot rolled steel sheet is cooled from the finishing temperature FT to 580°C, for example, by a water cooling device arranged in a run-out table (for example, water cooling nozzles arranged above and / or below the run-out table). In this case, the hot rolled steel sheet leaving the final rolling mill stand of the finishing mill is cooled at an average cooling rate CR of 20 to 90°C / s from the finishing temperature FT to 580°C on the run-out table. 580 Being water-cooled.

[0373] In this case, the average cooling rate CR 580 The measurement can be performed by the following method.

[0374] The surface temperature of the hot-rolled steel sheet is measured by placing thermometers at multiple locations along the conveyor path, extending from upstream to downstream. The feed rate of the hot-rolled steel sheet is calculated from the roller rotation speed of the final stand of the finishing mill. Based on the temperature measurement results obtained by the thermometers and the feed rate of the hot-rolled steel sheet, the average cooling rate CR from the finishing temperature FT to 580°C is calculated. 580 .

[0375] When the coiling temperature CT is lower than 580° C., the cooling method and cooling rate from 580° C. to the coiling temperature CT are not particularly limited.

[0376] <Coiling process>

[0377] The coiling step is a step of obtaining a hot coil made of the hot-rolled steel sheet by coiling the cooled hot-rolled steel sheet at a coiling temperature CT.

[0378] The coiling temperature CT is 480-580°C.

[0379] When the coiling temperature CT is lower than 480°C, martensite tends to form excessively, resulting in the area ratio of bainite falling below 80% and / or the tensile strength of the base material portion A of the as-rolled electric resistance welded steel pipe exceeding 800 MPa. Therefore, the lower limit of the coiling temperature CT is 480°C.

[0380] On the other hand, when the coiling temperature CT exceeds 580°C, ferrite and / or pearlite tend to form excessively. As a result, the area ratio of bainite may fall below 80%, and / or the tensile strength of the base material portion A of the as-rolled electric resistance welded steel pipe may fall below 600 MPa. Therefore, the upper limit of the coiling temperature CT is 580°C.

[0381] <Pipe Making Process>

[0382] The pipe-making process involves unwinding a hot-rolled steel sheet from a hot coil, roll-forming the unwinded hot-rolled steel sheet into an open pipe, and then welding the butted portions of the resulting open pipe to form an electric-weld joint, thereby producing an electric-weld steel pipe.

[0383] The pipe making process can be performed according to a known method.

[0384] Production method A may include other steps as needed.

[0385] Other steps include the following: a step of performing weld heat treatment on the electric resistance welded portion after the pipe making step; and a step of reducing the outer diameter of the electric resistance welded steel pipe using a sizing mill after the pipe making step (if the step of performing weld heat treatment is included, after the step of performing weld heat treatment).

[0386] Each step of the above-mentioned production method A does not affect the chemical composition of the steel.

[0387] Therefore, the chemical composition of the base material portion A of the as-rolled electric resistance welded steel pipe produced by the production method A can be considered to be the same as the chemical composition of the raw material (molten steel or slab).

[0388] Example

[0389] Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

[0390] [Examples 1 to 12, Comparative Examples 1 to 20]

[0391] <Manufacture of Rolled Electric Welded Steel Pipe>

[0392] According to the above-mentioned manufacturing method A, the rolled electric-resistance welded steel pipes of Examples 1 to 12 were obtained respectively.

[0393] Furthermore, the chemical composition or manufacturing conditions of the electric resistance welded steel pipes of each example were changed to obtain the as-rolled electric resistance welded steel pipes of Comparative Examples 1 to 20.

[0394] The details are shown below.

[0395] Molten steel (steels A to R) having the chemical compositions shown in Table 1 was melted in a furnace and then cast to produce slabs having a thickness of 250 mm (slab preparation step).

[0396] In Table 1, the numerical values ​​shown in the columns of each element are mass % of each element.

[0397] The remainder other than the elements shown in Table 1 is Fe and impurities.

[0398] The underlined elements in Tables 1 to 3 are outside the scope of the present disclosure.

[0399] The slab obtained above was heated to a slab heating temperature of 1200° C. and hot rolled to obtain a hot-rolled steel sheet (hot rolling step). At this time, the hot rolling conditions were adjusted so that the finishing rolling temperature FT became the value shown in Table 2.

[0400] The hot rolled steel sheet obtained in the hot rolling process was cooled to the coiling temperature CT shown in Table 2 (cooling process). At this time, the average cooling rate CR from the finishing temperature FT to 580°C was used. 580 The cooling conditions were adjusted so as to obtain the values ​​shown in Table 2.

[0401] The cooled hot-rolled steel sheet was coiled at the coiling temperature CT shown in Table 2 to obtain a hot coil made of a hot-rolled steel sheet having a thickness of 3.0 mm (coiling step).

[0402] The above-mentioned hot rolling process, cooling process and coiling process are carried out using a hot strip mill.

[0403] A hot-rolled steel sheet was unwound from the hot coil, and the unwound hot-rolled steel sheet was roll-formed into an open pipe. The butted portions of the resulting open pipe were then electric-welded to form a welded joint. The pipe was then reduced in diameter using a sizing mill to obtain a rolled electric-welded steel pipe having an outer diameter of 75 mm and a wall thickness of 3.0 mm (pipemaking process).

[0404] For the above-mentioned rolled electric welded steel pipe, the following operations are performed by the above-mentioned method:

[0405] Measurement of the area ratio of bainite in the base material portion A relative to the entire microstructure (hereinafter also referred to as "bainite area ratio");

[0406] Identification of the type of the remainder (i.e., structures other than bainite);

[0407] Determination of the tensile strength (TS) of the parent material part A; and

[0408] Determination of total elongation (%) in tensile test.

[0409] The results are shown in Table 2.

[0410] In Table 2, in the "Remainder" column, "F" represents ferrite (that is, at least one of polygonal ferrite and acicular ferrite. The same applies hereinafter), and "M" represents martensite.

[0411] <Manufacture of Electric-Welded Steel Pipes for Mechanical Structural Components>

[0412] The as-rolled electric-resistance welded steel pipe is used in accordance with manufacturing method X to obtain an electric-resistance welded steel pipe for machine structural parts.

[0413] The electric-resistance welded steel pipe for machine structural parts of this embodiment is particularly intended to be used for automobile parts.

[0414] Specifically, the as-rolled electric resistance welded steel pipe was cold worked under the conditions (maximum reduction of area) shown in Table 2, and then tempered under the conditions shown in Table 2 to obtain an electric resistance welded steel pipe for machine structural parts.

[0415] In Examples 1 to 9, as cold working, cold drawing was performed over the entire length of the as-rolled electric resistance welded steel pipe.

[0416] In Examples 10 to 12, as the cold working, cold steel pipe bending was performed on a portion of the rolled electric resistance welded steel pipe in the pipe axial direction by rotary bending.

[0417] The tempering time (ie, the holding time at the tempering temperature) was set to 30 minutes, and the cooling method after the holding time at the tempering temperature was set to air cooling.

[0418] In Comparative Example 17, tempering was omitted.

[0419] In Comparative Example 19, after cold working and before tempering, “quenching” was performed under the conditions of heating to 950° C., holding at this temperature for 20 minutes, and then water cooling.

[0420] In Table 2, in the "Quenching" column, "Y" means that the "quenching" was performed, and "N" means that the "quenching" was not performed.

[0421] Through the above steps, an electric resistance welded steel pipe for machine structural parts including a processed portion X having the maximum outer diameter and the maximum wall thickness shown in Table 2 was obtained.

[0422] For the above-mentioned electric welded steel pipes for mechanical structural components, the following operations are performed respectively by the above-mentioned methods:

[0423] Measurement of the area ratio of tempered bainite in the base material portion at the processed portion X relative to the entire microstructure (hereinafter also referred to as "tempered bainite area ratio");

[0424] Confirmation of the type of the remainder (i.e., the structure other than tempered bainite);

[0425] Determination of the tensile strength (TS) of the base material at the processed portion X;

[0426] Confirmation of the presence or absence of yield elongation (i.e., yield elongation of 0.2% or more) in a tensile test; and

[0427] The hardness ratio [depth 50 μm / center thickness] (ie, the ratio of the Vickers hardness at a depth of 50 μm from the outer surface of the base material to the Vickers hardness at the center thickness of the base material) was measured.

[0428] The results are shown in Table 2.

[0429] In Table 2, in the "Yield Elongation" column, "Y" means that a yield elongation of 0.2% or more was observed, and "N" means that a yield elongation of 0.2% or more was not observed.

[0430] [Table 1]

[0431]

[0432] [Table 2]

[0433]

[0434] As shown in Tables 1 and 2, in the electric-resistance welded steel pipes for machine structural components of Examples 1 to 12, the chemical composition of the base material portion at the processed portion X is the chemical composition disclosed herein, the area ratio of tempered bainite in the base material portion at the processed portion X relative to the entire microstructure is 80% or more, the tensile strength of the base material portion at the processed portion X is 850 to 1000 MPa, a yield elongation of 0.2% or more is observed in a tensile test of the base material portion, and the hardness ratio [depth 50 μm / center of wall thickness] is 95% or more.

[0435] That is, in Examples 1 to 12, electric resistance welded steel pipes for machine structural parts having excellent tensile strength and fatigue strength were obtained.

[0436] The electric-welded steel pipes for machine structural components of Examples 1 to 12 are produced by sequentially subjecting as-rolled electric-welded steel pipes to cold working under conditions of a maximum reduction of area of ​​10 to 40% and tempering at a tempering temperature of 450 to 650°C. The chemical composition of the base material portion A is the chemical composition disclosed herein, the area ratio of bainite in the base material portion A relative to the entire microstructure is 80% or greater, the tensile strength of the base material portion A is 600 to 800 MPa, and a total elongation of 13.0% or greater is observed in a tensile test of the base material portion A.

[0437] In contrast, the results of the comparative examples are as follows.

[0438] The Mo content in the chemical composition of Comparative Example 1 was too low. Therefore, the precipitation strengthening effect during tempering was insufficient, and the tensile strength of the electric resistance welded steel pipe for machine structural parts was low.

[0439] The Mo content in the chemical composition of Comparative Example 2 was too high. Therefore, the tensile strength of the base material portion A of the rolled electric resistance welded steel pipe in Comparative Example 2 was too high, the total elongation was too low, and cracking occurred during cold working, making it impossible to manufacture electric resistance welded steel pipes for machine structural parts.

[0440] The Nb content in the chemical composition of Comparative Example 3 was too low. Therefore, the precipitation strengthening effect during tempering was insufficient, and the tensile strength of the electric resistance welded steel pipe for machine structural parts was low.

[0441] The C content in the chemical composition of Comparative Example 4 was too low. Therefore, the tensile strength of the electric resistance welded steel pipe for machine structural parts was low.

[0442] The chemical composition of Comparative Example 5 has an excessively high C content. Consequently, the base material portion A of the rolled electric resistance welded steel pipe has excessively high tensile strength and excessively low total elongation. As a result, cracking occurs during cold working, making it impossible to manufacture electric resistance welded steel pipes for machine structural parts.

[0443] The Mn content in the chemical composition of Comparative Example 6 was too high. Consequently, the base material portion A of the rolled electric resistance welded steel pipe had excessively high tensile strength and excessively low total elongation. As a result, cracking occurred during cold working, making it impossible to manufacture electric resistance welded steel pipes for machine structural parts.

[0444] The Mn content in the chemical composition of Comparative Example 7 is too low. Consequently, the bainite area ratio and tensile strength of the base metal portion A of the as-rolled electric resistance welded steel pipe are too low. As a result, the tempered bainite area ratio and tensile strength of the base metal portion at the processed portion X of the electric resistance welded steel pipe for machine structural components are also too low.

[0445] The Nb content in the chemical composition of Comparative Example 8 was too high. Therefore, the precipitation strengthening effect during tempering became excessive, and the tensile strength of the base material portion at the processed portion X of the electric resistance welded steel pipe for machine structural parts was too high.

[0446] The Al content in the chemical composition of Comparative Example 9 was too high. Therefore, cracking occurred during cold working, which was presumably caused by excessively generated alumina-based oxides, making it impossible to produce electric resistance welded steel pipes for machine structural parts.

[0447] In Comparative Example 10, the finishing temperature FT was too high. Therefore, the bainite area ratio of the base material portion A of the as-rolled electric resistance welded steel pipe was low, and the tempered bainite area ratio of the base material portion at the processed portion X of the electric resistance welded steel pipe for machine structural parts was low.

[0448] In Comparative Example 11, the finishing temperature FT was too low. Therefore, the bainite area ratio of the base material portion A of the as-rolled electric resistance welded steel pipe was low, and the tempered bainite area ratio of the base material portion at the processed portion X of the electric resistance welded steel pipe for machine structural parts was low.

[0449] In Comparative Example 12, the average cooling rate CR 580 As a result, the tensile strength of the base material portion A of the rolled electric resistance welded steel pipe exceeds the upper limit, and the total elongation is low. As a result, cracking occurs during cold working, making it impossible to produce electric resistance welded steel pipes for machine structural parts.

[0450] In Comparative Example 13, the average cooling rate CR 580 Therefore, the tensile strength of the base material portion A of the rolled electric resistance welded steel pipe is low, and the tensile strength of the base material portion at the processed portion X of the electric resistance welded steel pipe for machine structural parts is low.

[0451] In Comparative Example 14, the coiling temperature CT was too high. Therefore, the bainite area ratio of the base material portion A of the as-rolled electric resistance welded steel pipe was low, and the tempered bainite area ratio of the base material portion at the processed portion X of the electric resistance welded steel pipe for machine structural parts was low.

[0452] In Comparative Example 15, the coiling temperature CT was too low. Consequently, the tensile strength of the base material portion A of the rolled electric resistance welded steel pipe exceeded the upper limit, resulting in low total elongation. Consequently, cracking occurred during cold working, making it impossible to produce electric resistance welded steel pipe for machine structural components.

[0453] In Comparative Example 16, the reduction of area during cold working was too low. Therefore, the work hardening effect by cold working was insufficient, and the tensile strength of the base material portion at the worked portion X of the electric resistance welded steel pipe for machine structural parts was low.

[0454] In Comparative Example 17, the as-rolled electric resistance welded steel pipe was not heat treated. Therefore, a yield elongation of 0.2% or greater was not observed in the tensile test of the base material. Therefore, it is believed that since the electric resistance welded steel pipe of Comparative Example 17 (i.e., the as-rolled electric resistance welded steel pipe) was not tempered, the reduction in residual strain was insufficient, and the precipitation strengthening effect during tempering was not achieved, resulting in insufficient fatigue strength.

[0455] In Comparative Example 17, the measurement of the tempered bainite area ratio was omitted (the description in the "tempered bainite area ratio (%)" column in Tables 2 and 3 was set to "-").

[0456] In Comparative Example 18, the tempering temperature was too high. Therefore, the tempered bainite area ratio of the base material portion at the processed portion X of the electric resistance welded steel pipe for machine structural parts was low, and the tensile strength of the base material portion at the processed portion X was low.

[0457] In Comparative Example 19, the as-rolled electric resistance welded steel pipe was quenched and then tempered. As a result, the hardness ratio [depth 50 μm / center wall thickness] fell below 95%. Therefore, it is believed that the electric resistance welded steel pipe for machine structural components in Comparative Example 19 is susceptible to fatigue cracking from the outer surface, and the fatigue strength of the processed portion X is inferior. The reason for the hardness ratio [depth 50 μm / center wall thickness] falling below 95% is believed to be the formation of a decarburized layer in the region including the outer surface of the base material.

[0458] In Comparative Example 20, the tempering temperature was low. Consequently, a yield elongation of 0.2% or greater was not observed in the tensile test of the base material. Therefore, it is believed that the electric resistance welded steel pipe of Comparative Example 20 did not sufficiently reduce residual strain, resulting in insufficient fatigue strength.

[0459] [Verification of fatigue strength]

[0460] Since it is difficult to prepare fatigue test specimens for actual electric resistance welded steel pipes, fatigue strength was verified using the hot-rolled steel sheets (hot coils) of Examples 1 to 3 and Comparative Examples 17 to 20.

[0461] Hot-rolled steel sheets were unwound from the hot coils, and the unwound hot-rolled steel sheets were subjected to cold working (cold rolling) and tempering as shown in Table 3.

[0462] In Comparative Example 17, heat treatment after cold working was omitted.

[0463] In Comparative Example 19, after cold working and before tempering, tempering was performed under the conditions of heating to 950° C., holding at this temperature for 20 minutes, and then water cooling.

[0464] Then, the hot rolled steel sheet is cold rolled to obtain a cold rolled steel sheet, and the obtained cold rolled steel sheet is collected. Figure 1 Fatigue test piece described in.

[0465] Specifically, full-thickness fatigue test specimens of cold-rolled steel sheets were collected, with the longitudinal direction of the fatigue test specimens being parallel to the cold-rolling direction. Figure 1 The values ​​in represent the dimensions of the corresponding positions (in mm).

[0466] The obtained fatigue test piece was used to carry out a plane bending fatigue test at room temperature. The test conditions were set to an alternating load with a stress ratio of -1 and a frequency of 20 Hz. The fracture repetition number was set to 3 × 10 5 The stress amplitude at this time is set as the fatigue limit (MPa).

[0467] The results are shown in Table 3.

[0468] [Table 3]

[0469]

[0470] As shown in Table 3, the hot-rolled steel sheets of Examples 1 to 3, which had an area ratio of tempered bainite of 80% or more, a yield elongation of 0.2% or more, and a hardness ratio [depth 50 μm / center of wall thickness] of 95% or more, were confirmed to have superior fatigue strength compared to the hot-rolled steel sheets of Comparative Examples 17 to 20.

[0471] Among them, Comparative Examples 17 and 20 are hot-rolled steel sheets in which a yield elongation of 0.2% or more was not confirmed, Comparative Example 18 is a hot-rolled steel sheet in which the area ratio of tempered bainite is less than 80%, and Comparative Example 19 is a hot-rolled steel sheet in which the hardness ratio [depth 50 μm / center of wall thickness] is less than 95%.

[0472] From the above results, it can be expected that the electric resistance welded steel pipes for machine structural components of Examples 1 to 12 described above are excellent in fatigue strength.

Claims

1. An electric-resistance welded steel pipe for a machine structural component, comprising a processed portion X that is at least one of a drawn portion and a steel pipe bending portion. The processing part X includes a base material part and a welding part. The chemical composition of the base material is expressed in mass %: C：0.160～0.230％、 Si: 0-0.50%, Mn: 0.50-1.65%, P：0～0.030％、 S：0～0.010％、 Nb: 0.010~0.050%, Mo: 0.10~0.60%, Al：0.005～0.060％、 N：0～0.0060％、 Ti: 0~0.030%, V：0～0.100％、 Cr：0～0.5％、 Cu: 0~0.500%, Ni: 0~0.500%, B：0～0.0030％、 Ca: 0~0.0030%, Mg: 0-0.0040%, and The rest is composed of Fe and impurities. The area ratio of the tempered bainite in the base material portion relative to the entire microstructure is 80% or more. The tensile strength of the base material is 850-1000 MPa. In the tensile test of the base material, a yield elongation of 0.2% or more was observed. The ratio of the Vickers hardness of the base material portion at a depth of 50 μm from the outer surface to the Vickers hardness of the base material portion at a center portion of the wall thickness is 95% or more.

2. The electric-resistance welded steel pipe for machine structural parts according to claim 1, which is an electric-resistance welded steel pipe for automobile running parts.

3. The electric-welded steel pipe for mechanical structural components according to claim 1 or claim 2, wherein: The outer circumference of the processing portion X is 50 to 500 mm, and the maximum wall thickness of the processing portion X is 1.0 to 5.0 mm.

4. A method for manufacturing an electric-resistance welded steel pipe for a machine structural component, the method comprising the following steps: A process for preparing a rolled electric-resistance welded steel pipe, wherein the rolled electric-resistance welded steel pipe comprises a base metal portion A and a weld portion A, wherein the base metal portion A has a chemical composition (in mass %) of: C：0.150～0.230％、 Si: 0-0.50%, Mn: 0.50-1.65%, P：0～0.030％、 S：0～0.010％、 Nb: 0.010~0.050%, Mo: 0.10~0.60%, Al：0.005～0.060％、 N：0～0.0060％、 Ti: 0~0.030%, V：0～0.100％、 Cr：0～0.5％、 Cu: 0~0.500%, Ni: 0~0.500%, B：0～0.0030％、 Ca: 0~0.0030%, Mg: 0-0.0040%, and The rest is composed of Fe and impurities. The area ratio of bainite in the base material portion A relative to the entire microstructure is 80% or more. The tensile strength of the base material part A is 600-800 MPa. In the tensile test of the base material portion A, a total elongation of 13.0% or more was observed; a cold working step of performing at least one of cold drawing and cold pipe bending on at least a portion of the rolled electric-resistance welded steel pipe in the pipe axial direction, with the cold working being performed under conditions of a maximum reduction of area of ​​10 to 40%; and The tempering step is to perform tempering at a temperature of 450 to 650° C. on the rolled electric resistance welded steel pipe subjected to the cold working to obtain the electric resistance welded steel pipe for machine structural parts.

5. The method for manufacturing an electric-resistance welded steel pipe for a machine structural component according to claim 4, wherein: The outer diameter of the rolled electric welded steel pipe is 50-150 mm, and the wall thickness of the rolled electric welded steel pipe is 2.0-4.0 mm.

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