Title of Invention: ERW Steel Pipe for Hollow Stabilizer, Hollow Stabilizer, and Manufacturing Method Therefor
Technical field
[0001]
　TECHNICAL FIELD The present invention relates to an electric resistance welded steel pipe (electric resistance welded steel pipe) for a hollow stabilizer, a hollow stabilizer, and a method for producing them.
　This application claims priority based on Japanese Patent Application No. 2019-069150 filed in Japan on March 29, 2019 and Japanese Patent Application No. 2020-019189 filed in Japan on February 06, 2020, The contents of which are incorporated herein.
Background technology
[0002]
　2. Description of the Related Art In recent years, from the viewpoint of protecting the global environment, automobile exhaust gas regulations have been strengthened, and weight reduction of automobile bodies has been promoted in order to improve fuel efficiency. As one method for reducing the weight of the car body, it is being considered to replace solid parts using steel bars with hollow parts using steel pipes.
　For example, even in the stabilizer that suppresses rolling of the car body during cornering and improves running stability at high speeds, by changing from solid parts using steel bars to hollow parts using steel pipes (hollow stabilizer), Consideration is being given to reducing the weight of the vehicle.
[0003]
　Electric resistance welded steel pipes (electric resistance welded steel pipes) are widely used as materials for hollow stabilizers because they are relatively inexpensive and have excellent dimensional accuracy.
[0004]
　For example, in Patent Document 1, C: 0.35% or less, Si: 0.25% or less, Mn: 0.30 to 1.20%, Cr: less than 0.50%, N + O: 0.0200% or less , Ti: 4 to 12 times the (N + O) in steel, B: 0.0005 to 0.009%, or further Ca: 200 ppm or less and / or Nb: C × 4/10 or less, DI value C, Si, Mn, Cr content is adjusted so that the is 1.0 in. or more, and the C, Si, Mn, Cr content is adjusted so that the Ceq is 0.60% or less. A steel for electric resistance welded steel pipes for hollow stabilizers is disclosed.
[0005]
　Further, in Patent Document 2, C: 0.35% or less, Si: 0.25% or less, Mn: 0.30 to 1.20%, Cr: less than 0.50%, N + O: 0.0200% or less , Ti: 4 to 12 times the (N + O) in steel, B: 0.0005 to 0.009%, or further Ca: 200 ppm or less, DI value is 1.0 in. or more, A steel slab in which the C, Si, Mn, and Cr contents are adjusted and the C, Si, Mn, and Cr contents are adjusted so that the Ceq is 0.60% or less is subjected to hot rolling and rolled. A manufacturing method of steel for electric resistance welded steel pipes for hollow stabilizers is disclosed, which is wound while controlling the winding temperature at 570 to 690°C.
[0006]
　Further, Patent Document 3 proposes a method for manufacturing a high-strength, high-ductility electric resistance welded steel pipe that can be used for stabilizers and the like. In Patent Document 3, C: 0.18 to 0.28%, Si: 0.10 to 0.50%, Mn: 0.60 to 1.80%, Ti: 0.020 to 0.050%, B: 0.0005 to 0.0050%, Cr: 0.20 to 0.50%, Mo: 0.5% or less, Nb: 0.015 to 0.050%. It is disclosed that an electric resistance welded steel pipe made of steel containing Ca: 0.0050% or less is normalized at 850 to 950° C. and then quenched.
[0007]
　The hollow stabilizer is made by further stretching the electric resistance welded steel pipe, or by 1) forming the thick electric resistance welded steel pipe with the required thickness/outer diameter ratio into the desired shape by cold forming such as bending. 2) A thick electric resistance welded steel pipe is heated and formed into a desired shape by hot forming such as pressing, followed by heating, water cooling, and quenching. It is manufactured by cooling with water, quenching, and then tempering. In particular, the latter hot forming method is advantageous as a manufacturing process because it is easier to process and mold than the former cold forming method and can handle complicated shapes. be.
　However, in the electric resistance welded steel pipes described in Patent Documents 1 to 3, when such hot forming is performed, the fatigue strength of the steel pipe may decrease due to insufficient hardenability of the welded portion.
[0008]
　In response to such problems, Patent Document 4 discloses, in mass%, C: 0.15 to 0.40%, Si: 0.05 to 0.50%, Mn: 0.30 to 2.00%, Al: 0.01 to 0.10%, Ti: 0.001 to 0.04%, B: 0.0005 to 0.0050%, N: 0.0010 to 0.0100%, and Ti and N has a composition that satisfies (N / 14) < (Ti / 47.9), the balance is Fe and inevitable impurities, and the bond width of the electric resistance weld is 25 μm or less. A resistance welded steel pipe is disclosed.
　Patent Document 4 describes that by setting the width of the decarburized layer formed during welding to 25 μm or less, the decrease in quenching hardness of the electric resistance welded portion can be suppressed, and the durability against fatigue is improved. .
[0009]
　However, as a result of studies by the present inventors, although a certain fatigue strength improvement effect can be obtained in the electric resistance welded steel pipe of Patent Document 4, the shape of the welded part, the roughness of the inner surface of the steel pipe, and the It was found that sufficient durability (fatigue strength) may not be obtained due to the influence of inclusions and the like.
prior art documents
patent literature
[0010]
Patent Document 1: Japanese Patent Publication No. 1-58264
Patent Document 2: Japanese Patent Publication No. 61-45688
Patent Document 3: Japanese Patent Laid-Open Publication No. 6-93339
Patent Document 4: Japanese Patent No. 5516780
SUMMARY OF THE INVENTION
Problems to be Solved by the Invention
[0011]
　The present invention has been made in view of the above problems. An object of the present invention is to provide an electric resistance welded steel pipe for a hollow stabilizer that has sufficient workability and excellent fatigue strength after quenching and tempering, a hollow stabilizer that has excellent fatigue strength, and a method for producing the same.
Means to solve problems
[0012]
　The present inventors paid attention to the relationship between the fatigue strength and the bead cut and inner surface roughness of the inner surface of the electric resistance welded steel pipe, and conducted earnest investigations. As a result, the following things were found.
(i) If the maximum value of either the bead cut depth or the inner surface roughness (depth) is large, the fatigue strength deteriorates.
(ii) Fatigue fracture due to bead cut or inner surface roughness is a crack from the inner surface.
　On the other hand, as a result of examination by the present inventors, it was found that the following fatigue fracture occurs even when the depth of the bead cut and the inner surface roughness are suppressed.
(I) Elongated inclusions such as MnS promote fatigue fracture.
(II) Fracture originating from inclusions is not from the steel surface but from the inside.
[0013]
　Based on these facts, the present inventors found that, in electric resistance welded steel pipes suitably used as hollow stabilizers, in addition to the composition of the steel plate to be subjected to electric resistance welding, the inner surface roughness of the steel pipe, the shape of the welded portion (bead cut ), and found that the size of inclusions greatly affects the fatigue strength.
[0014]
　In addition, the present inventors have found that the characteristics of the electric resistance welded steel pipe described above also affect the fatigue strength of hollow stabilizers obtained by subjecting the electric resistance welded steel pipe to processing and heat treatment.
[0015]
　The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] An electric resistance welded steel pipe for hollow stabilizers according to one aspect of the present invention is an electric resistance welded steel pipe for hollow stabilizers, comprising a base material portion and a welded portion, wherein the base material portion has a chemical composition of , C: 0.30 to 0.38%, Si: 0.15 to 0.30%, Mn: 1.20 to 1.50%, Al: 0.020 to 0.060%, Ti: 0.020 ~0.050%, B: 0.0010-0.0050%, Cr: 0.10-0.25%, Mo: 0-0.20%, Ca: 0.0005-0.0050%, Cu: 0-0.25%, Ni: 0-0.25%, V: 0-0.05%, N: 0.0060% or less, P: 0.020% or less, S: 0.0100% or less, O : 0.0050% or less, the balance being Fe and impurities, the thickness of the base material portion is 2.0 to 6.0 mm, the outer diameter of the electric resistance welded steel pipe is 10 to 40 mm, and the In the C-direction cross section of the electric resistance welded steel pipe, there is a concave bead cut in the region including the weld on the inner surface side of the electric resistance welded steel pipe, and the shortest distance from one opening edge of the bead cut to the other opening edge When a virtual line is drawn with a distance, the maximum depth from the virtual line to the bottom of the bead cut is 300 μm or less, the maximum diameter of inclusions contained in the base material portion is 300 μm or less, and the electric resistance welded steel pipe The surface roughness of the inner surface of the base metal portion of is 300 μm or less in maximum valley depth Rv, and the maximum hardness of the electric resistance welded steel pipe including the welded portion is 300 Hv or less.
[2] In the electric resistance welded steel pipe for a hollow stabilizer described in [1], the chemical composition of the base material portion may satisfy the following formula (1).
　Ca×(1−124×O)/(1.25×S)≧0.20. . . (1)
[3] In the electric resistance welded steel pipe for hollow stabilizers according to [1] or [2], the chemical components of the base material portion are Cu: 0.05-0.25%, Ni: 0.05-0. It may contain one or more selected from the group consisting of 25% and V: 0.01 to 0.05%.
[4] In the electric resistance welded steel pipe for a hollow stabilizer according to any one of [1] to [3], the metal structure of the base material portion has an area ratio of 40% to 60% ferrite and 60% to 40% % perlite.
[5] A method for manufacturing an electric resistance welded steel pipe for a hollow stabilizer according to another aspect of the present invention is a method for manufacturing an electric resistance welded steel pipe for a hollow stabilizer according to any one of [1] to [4], comprising: An electric resistance welded steel pipe formed by resistance welding is subjected to heat treatment in which it is heated to 800° C. or higher and 1000° C. or lower and then allowed to cool.
[6] A hollow stabilizer according to another aspect of the present invention is a hollow stabilizer comprising a base material portion and a welded portion, wherein the base material portion has, as a chemical component, C: 0.30 to 0.30% by mass. 0.38%, Si: 0.15-0.30%, Mn: 1.20-1.50%, Al: 0.020-0.060%, Ti: 0.020-0.050%, B : 0.0010-0.0050%, Cr: 0.10-0.25%, Mo: 0-0.20%, Ca: 0.0005-0.0050%, Cu: 0-0.25%, Ni: 0 to 0.25%, V: 0 to 0.05%, N: 0.0060% or less, P: 0.020% or less, S: 0.0100% or less, O: 0.0050% or less, The balance is composed of Fe and impurities, the thickness of the base material portion is 2.0 to 6.0 mm, the outer diameter of the base material portion is 10 to 40 mm, and in the C direction cross section, the hollow stabilizer On the inner surface side of the hollow portion, there is a recess in the area including the welded portion, and when a virtual line is drawn at the shortest distance from one end of the inner surface side of the welded portion to the other end, the virtual The maximum depth from the line to the bottom of the recess is 300 μm or less, the maximum diameter of inclusions contained in the base material portion is 300 μm or less, and the surface roughness of the inner surface side of the base material portion is the maximum The valley depth Rv is 300 μm or less, and the surface hardness is 455 Hv or more.
[7] In the hollow stabilizer described in [6], the chemical component of the base material may satisfy the following formula (2).
　Ca×(1−124×O)/(1.25×S)≧0.20. . . (2)
In the hollow stabilizer described in [8], [6] or [7], the chemical components of the base material portion are Cu: 0.05 to 0.25%, Ni: 0.05 to 0.25 % and V: 0.01 to 0.05%.
[9] A method for manufacturing a hollow stabilizer according to another aspect of the present invention is the method for manufacturing a hollow stabilizer according to any one of [6] to [8], comprising: A pipe drawing step of drawing the electric resistance welded steel pipe for a hollow stabilizer according to 1, a heat treatment step of heating the electric resistance welded steel pipe after the drawing step to 800 ° C. to 1000 ° C., and the electric resistance welded steel pipe after the heat treatment step a processing step of processing the welded steel pipe into a hollow stabilizer shape; a quenching step of heating the electric resistance welded steel pipe after the processing step to 900° C. to 1200° C. and then cooling it at a cooling rate of 30° C./s or more; and a tempering step of heating the electric resistance welded steel pipe after the quenching step to 200 to 400°C.
[10] A method for manufacturing a hollow stabilizer according to another aspect of the present invention is the method for manufacturing a hollow stabilizer according to any one of [6] to [8], comprising: A processing step of processing the electric resistance welded steel pipe for a hollow stabilizer into a hollow stabilizer shape according to , and after heating the electric resistance welded steel pipe after the processing step to 900 ° C. to 1200 ° C., at a cooling rate of 30 ° C./s or more. It includes a quenching step of cooling and a tempering step of heating the electric resistance welded steel pipe after the quenching step to 200 to 400°C.
Effect of the invention
[0016]
　According to the above aspect of the present invention, it is possible to provide an electric resistance welded steel pipe excellent in workability and fatigue strength after quenching and tempering, a hollow stabilizer excellent in fatigue strength, and a method for producing them.
　The electric resistance welded steel pipe of the present invention is also excellent in fatigue strength after heat treatment such as working, quenching and tempering. In addition, the electric resistance welded steel pipe of the present invention ensures cold workability equal to or higher than that of the conventional steel pipe. Therefore, the electric resistance welded steel pipe of the present invention is suitable as an electric resistance welded steel pipe for hollow stabilizers. In addition, the hollow stabilizer of the present invention is excellent in fatigue strength, and if applied as an automobile part, it can contribute to weight reduction of automobile bodies.
Brief description of the drawing
[0017]
1 is a diagram showing an example of an electric resistance welded steel pipe according to the present embodiment; FIG.
2A] A cross-sectional view of the electric resistance welded steel pipe according to the present embodiment shown in FIG. 1 taken along line AA. [FIG.
2B is an enlarged view of the range of B in FIG. 2A. FIG.
3 is a diagram schematically showing bead cut depths before and after pipe drawing. FIG.
4 is a diagram showing the relationship between defect depth (largest value among inner surface roughness, bead cut depth, or maximum inclusion diameter) and fatigue life. FIG.
5 is a schematic diagram showing an example of a hollow stabilizer according to the present embodiment; FIG.
[Fig. 6] Fig. 6 is a schematic diagram showing the shape of a stabilizer when performing a fatigue test. [Fig.
MODE FOR CARRYING OUT THE INVENTION
[0018]
　An electric resistance welded steel pipe for a hollow stabilizer according to an embodiment of the present invention (an electric resistance welded steel pipe according to the present embodiment) and a hollow stabilizer according to an embodiment of the present invention (the hollow stabilizer according to the present embodiment) will be described below.
　First, an electric resistance welded steel pipe according to this embodiment will be described with reference to the drawings.
　Referring to FIG. 1, an electric resistance welded steel pipe 1 according to the present embodiment comprises a base metal portion 2 having a predetermined chemical composition and a weld portion 3. As shown in FIG.
　In addition, the electric resistance welded steel pipe 1 according to the present embodiment is assumed to be applied to a hollow stabilizer, and the thickness of the base material portion 2 (steel plate) of the electric resistance welded steel pipe is 2.0 to 6.0 mm, and the outer diameter is 10 to 40 mm.
[0019]
　The electric resistance welded steel pipe 1 according to the present embodiment is obtained by forming a steel plate such as a hot-rolled steel plate or a cold-rolled steel plate, which is a raw material of the steel pipe, into a tubular shape with a forming roll or the like, and welding the butt portions by electric resistance welding. . By electric resistance welding, the end of the steel pipe is melted, and the solid-liquid coexisting phase is crushed and joined while the liquid phase is discharged. The joint portion and the welding heat affected zone are the welded portion 3, and the base material portion 2 is the portion that is not affected by the heat of the electric resistance welding.
　In addition, the electric resistance welded steel pipe 1 according to the present embodiment has a region including the welded portion 3 (welded portion 3 and its surrounding area), there is a concave bead cut 4 in the area containing the weld 3, as shown in FIG. 2B.
[0020]

　The base material of the steel pipe contains the following chemical compositions. In the present embodiment, % relating to chemical components is % by mass unless otherwise specified.
[0021]
C: 0.30 to 0.38%
　C is an element that dissolves in steel or precipitates as carbide in steel to increase the strength of steel. In order to secure the strength generally required when used as a hollow stabilizer, the C content is made 0.30% or more. On the other hand, when the C content exceeds 0.38%, workability and weldability deteriorate. Therefore, the C content is made 0.38% or less. Preferably, it is between 0.33% and 0.37%.
[0022]
Si: 0.15 to 0.30%
　Si is an element that contributes to solid solution strengthening. In order to obtain this effect, the Si content is set to 0.15% or more. On the other hand, if the Si content exceeds 0.30%, Si—Mn inclusions, which cause welding defects during electric resistance welding, are likely to form. Therefore, the Si content is set to 0.30% or less. Preferably, it is 0.20% to 0.30%.
[0023]
Mn: 1.20 to 1.50%
　Mn is an element that improves the hardenability of steel and contributes to strength improvement. To obtain this effect, the Mn content is set to 1.20% or more. On the other hand, when the Mn content exceeds 1.50%, the weldability deteriorates, and coarse inclusions such as MnS are formed in the weld zone, resulting in a reduction in fatigue strength. Therefore, the Mn content is set to 1.50% or less. Preferably, it is 1.20% to 1.40%.
[0024]
Al: 0.020 to 0.060%
　Al is an element that acts as a deoxidizing agent and also has the effect of fixing N and ensuring the amount of solid solution B effective for improving hardenability. In order to obtain such effects, the Al content is made 0.020% or more. On the other hand, if the Al content exceeds 0.060%, the formation of inclusions increases and fatigue strength may decrease. Therefore, the Al content is set to 0.060% or less. Preferably, it is 0.020% to 0.050%.
[0025]
Ti: 0.020 to 0.050%
　Ti is an effective element for fixing N and securing a solid solution B amount effective for improving hardenability. In addition, Ti is an element that precipitates as fine carbides, suppresses coarsening of crystal grains during welding and heat treatment, and contributes to the improvement of toughness. In order to obtain such effects, the Ti content is made 0.020% or more. On the other hand, when the Ti content exceeds 0.050%, coarse inclusions are formed and the toughness and fatigue strength are lowered. Therefore, the Ti content is made 0.050% or less. It is preferably 0.020 to 0.040%.
[0026]
B: 0.0010 to 0.0050%
　B is an element that greatly improves the hardenability of steel materials even when contained in a very small amount. B is also an element having an effect of grain boundary strengthening. In order to obtain these effects, the B content is made 0.0010% or more. On the other hand, when the B content exceeds 0.0050%, coarse B-containing precipitates are formed, which may reduce the toughness. Therefore, the B content is set to 0.0050% or less. Preferably, it is 0.0020 to 0.0035%.
[0027]
Cr: 0.10 to 0.25%
　Cr is an element that improves the hardenability of steel. In order to obtain this effect, the Cr content is set to 0.10% or more. On the other hand, when the Cr content exceeds 0.25%, inclusions are formed in the weld, degrading the soundness of the weld. Therefore, the Cr content is set to 0.25% or less. Preferably, it is 0.10 to 0.20%.
[0028]
Ca: 0.0005 to 0.0050%
　Ca is an element that refines inclusions in steel pipes and contributes to improvement in fatigue strength. If the Ca content is less than 0.0005%, this effect cannot be sufficiently obtained. Therefore, Ca content is made 0.0005% or more. On the other hand, when the Ca content exceeds 0.0050%, coarse inclusions are formed and the toughness and fatigue strength are lowered. Therefore, the Ca content is made 0.0050% or less. Preferably, it is 0.0010 to 0.0030%.
[0029]
Mo: 0 to 0.20%
　Since Mo does not necessarily need to be contained, its lower limit is 0%. However, Mo is an element that improves the hardenability of steel and also improves the strength of steel through solid-solution strengthening. To obtain the above effects, the Mo content is preferably 0.05% or more. On the other hand, when the Mo content exceeds 0.20%, coarse carbides are formed and the toughness deteriorates. Therefore, Mo content shall be 0.20% or less. Preferably, it is 0.05 to 0.10%.
[0030]
Cu: 0 to 0.25%
　Cu is not necessarily included, so the lower limit is 0%. However, Cu is an element having the effect of increasing the resistance to delayed fracture in addition to improving the hardenability, and may be contained. In order to obtain the above effects, the Cu content is preferably 0.05% or more. On the other hand, when the Cu content exceeds 0.25%, there is a concern that workability may deteriorate. Therefore, even when Cu is contained, the content of Cu is set to 0.25% or less. Preferably, it is 0.05 to 0.10%.
[0031]
Ni: 0 to 0.25%
　Ni is not necessarily included, so the lower limit is 0%. However, Ni is an element that has the effect of increasing the resistance to delayed fracture in addition to improving hardenability, and may be contained. In order to obtain the above effects, the Ni content is preferably 0.05% or more. On the other hand, Ni is an effective element, and even if the Ni content exceeds 0.25%, the effect is saturated. Therefore, even when Ni is contained, the Ni content is set to 0.25% or less. Preferably, it is 0.05 to 0.10%.
[0032]
V: 0 to 0.05%
　Since V does not necessarily have to be contained, its lower limit is 0%. However, V is an effective element for improving the strength of steel and may be contained. In order to obtain the above effects, the V content is preferably 0.01% or more. On the other hand, if the V content exceeds 0.05%, there is a concern that workability will deteriorate. Therefore, even when it is contained, the V content is set to 0.05% or less.
[0033]
N: 0.0060% or less
　N is contained as an impurity. If the N content exceeds 0.0060%, precipitation as BN may reduce the hardenability improvement effect of solute N, or deteriorate toughness due to coarsening of nitrides and age hardening. . Therefore, the N content is made 0.0060% or less. On the other hand, N is an element that contributes to strength improvement by nitrides and carbonitrides. Therefore, the N content may be 0.0010% or more.
[0034]
P: 0.020% or less
　P is an impurity element that adversely affects weld crack resistance and toughness. Although the P content is preferably as small as possible, a P content of 0.020% or less is acceptable. Therefore, the P content is made 0.020% or less. Preferably, it is 0.015% or less.
[0035]
S: 0.0100% or less
　S is an impurity element that affects the formation of non-metallic inclusions such as MnS in steel materials and reduces fatigue strength and workability. Also, S is an element that adversely affects toughness, anisotropy, and reheat cracking susceptibility. The lower the S content, the better, but 0.0100% or less is acceptable. Therefore, the S content is made 0.0100% or less. Preferably, it is 0.0050% or less.
[0036]
O: 0.0050% or less
　O is present in steel mainly as oxide inclusions and is an element that lowers the workability, toughness and fatigue strength of steel pipes. Although the smaller the O content, the better, but 0.0050% or less is acceptable. Therefore, the O content is made 0.0050% or less. Preferably, it is 0.0040% or less.
[0037]
Balance: Fe and impurities
　The electric resistance welded steel pipe according to the present embodiment contains the above elements, and the balance is basically Fe and impurities. Impurities include those contained in raw materials such as ores and scraps, and those mixed in during the manufacturing process. Impurities include, for example, H in addition to P, S, N, and O described above.
[0038]
Ca×(1−124×O)/(1.25×S)≧0.20
　In the electric resistance welded steel pipe 1 according to the present embodiment, the content of each element is limited to the above range, and the following The ESSP value represented by formula (1) is preferably 0.20 or more.
　ESSP value=Ca×(1−124×O)/(1.25×S) . . . (1)
　The ESSP value is an index for the remaining Ca (available Ca) after subtracting the Ca bound to oxygen. Specifically, it is an index indicating whether or not there is an effective Ca amount required according to the S content, on the premise that Ca bonds with S at an atomic weight ratio.
　When the ESSP is less than 0.20, the Ca content is insufficient with respect to the O content and S content, and MnS tends to be generated. Since MnS stretched by rolling becomes a starting point of deterioration of fatigue strength and hydrogen cracking, ESSP is preferably 0.20 or more.
[0039]

　Usually, an electric resistance welded steel pipe is formed into an open pipe by forming a steel plate into a cylindrical shape while running it with a group of forming rolls, and the open pipe is made of steel plates. Both ends in the width direction of the steel plate that are butted against each other are melted by heating by induction heating or the like, and in this state, the both ends in the width direction of the steel plate are pressed by a pair of squeeze rolls, thereby welding the both ends to create an electric resistance. It is obtained by making it a welded part. At this time, a part of the molten metal formed between both ends in the width direction of the steel plate is pushed out to form a weld bead protruding from the surface of the electric resistance welded steel pipe at the welded portion. This weld bead causes processing defects in the post-process, and also causes deterioration in the appearance of the electric resistance welded steel pipe used as the final product. Therefore, this weld bead is cut by a cutting jig or the like (bead cut).
　Therefore, as shown in FIGS. 2A and 2B, in the electric resistance welded steel pipe 1 according to the present embodiment, when the electric resistance welded steel pipe 1 is viewed in a cross section (C section) perpendicular to the pipe axis direction, the electric resistance welded steel pipe 1 There is a concave bead cut 4 in the area containing the weld 3 on the inner surface side of the .
[0040]
　As a result of investigations by the present inventors, as shown in FIG. 3, the shape of the weld after the weld bead has been cut, particularly the bead cut depth on the inner surface side of the electric resistance welded steel pipe, is affected not only by workability but also by fatigue. It was newly found that it greatly affects the strength. For example, if the welded portion is excessively shaved by bead cutting, the bead cut will break after the tube is drawn, and cracks will occur from the bent portion, resulting in early rupture.
　As a result of further studies by the present inventors, when a virtual line 10 is drawn at the shortest distance from one opening edge 5 of the bead cut 4 to the other opening edge 5, the bead cut 4 The bead cut depth d, which is the maximum depth to the bottom (in other words, the maximum value of the distance from the imaginary line 10 to the bottom of the concave bead cut 4 in the direction perpendicular to the imaginary line 10), is 300 μm. exceeds the standard depth after drawing, fatigue cracks are generated and grow at an early stage from the bent portion, and the fatigue strength is reduced, and after quenching and tempering, the fatigue strength of the present embodiment is reduced. It was found that the standard value of 80,000 repetitions of load stress until fracture in the fatigue test was not stably reached. Therefore, in the electric resistance welded steel pipe 1 according to the present embodiment, when a virtual line 10 is drawn at the shortest distance from one opening edge 5 of the bead cut 4 to the other opening edge 5, the bottom of the bead cut 4 from the virtual line 10 The maximum depth (bead cut depth d) to 300 μm or less. It is preferably 100 μm or less.
　Since the bead cut depth is improved by drawing, the upper limit of the bead cut depth d of the steel pipe before drawing was set at 300 μm, which is the standard for bead cut folding after drawing. If the bead cut depth before drawing is 300 μm or less, the bead cut depth after drawing will be 300 μm or less.
　In the electric resistance welded steel pipe 1 according to this embodiment, it is not necessary to set the lower limit of the bead cut depth d, and the smaller the better.
[0041]
　The bead cut depth d as described above can be controlled by appropriately setting the position of the cutting bit when cutting the weld bead formed by electric resistance welding.
　In addition, since fatigue fracture occurs mainly from the inner surface side, the shape of the welded portion 3 on the outer surface side of the electric resistance welded steel pipe 1 is not particularly limited. may
[0042]
　The above bead cut depth d can be measured by the following method. That is, the maximum thickness of the lateral side of the welded portion and the thickness of the welded portion (butt portion) are measured with a micrometer, and the thickness of the thinnest portion of the welded portion is subtracted from the maximum thickness.
[0043]

　By controlling the shape of the welded portion described above, It is possible to suppress the fatigue fracture that occurs. However, since fatigue fracture also occurs from parts other than the welded part, the fatigue strength cannot be sufficiently improved only by controlling the shape of the welded part.
　As a result of investigation by the present inventors, as shown in FIG. 4, it was found that the fatigue strength decreases as the surface roughness of the base material portion 2 on the inner surface side of the electric resistance welded steel pipe 1 increases.
　Therefore, in the electric resistance welded steel pipe 1 according to the present embodiment, the surface roughness of the inner surface of the base material portion 2 is set to 300 μm or less in terms of the maximum valley depth Rv specified in JISB0601:2013. When the maximum valley depth Rv of the surface on the inner side (inner surface) exceeds 300 μm, fatigue cracks are generated and grow at an early stage, resulting in a decrease in fatigue strength, and after quenching and tempering, the reference value of fatigue strength in the present embodiment. , which is the number of repetitions of 80,000 times cannot be stably satisfied. The maximum valley depth Rv of the surface is preferably 100 μm or less.
　The maximum valley depth Rv as described above is obtained by appropriately performing rolling, pickling, etc. for scale control, etc. is obtained by suppressing
[0044]
　The surface roughness of the inner surface of the base material portion 2 of the steel pipe described above can be measured by the following method. That is, the height distribution is measured using a contact-type or non-contact shape measuring device, and the surface roughness is calculated according to JISB0601:2013.
[0045]

　As a result of investigations by the present inventors, even if the above-described bead cut depth and inner surface roughness are controlled within a predetermined range, 4, it was found that the fatigue strength decreased as the maximum diameter of inclusions contained in the base material portion 2 increased. Therefore, in order to improve the fatigue strength of an electric resistance welded steel pipe, it is necessary to suppress not only fatigue fracture originating from the surface but also fatigue fracture originating from inclusions present inside the steel plate used for the steel pipe.
　In the electric resistance welded steel pipe 1 according to this embodiment, the maximum diameter of inclusions contained in the base material portion 2 is set to 300 μm or less. When the maximum inclusion diameter exceeds 300 μm, fatigue cracks occur and grow at an early stage and the fatigue strength decreases, and after quenching and tempering, the number of repetitions is 80,000 times, which is the reference value for fatigue strength in this embodiment. not reach. Preferably, the maximum inclusion diameter is 100 μm or less.
[0046]
　The maximum diameter of inclusions contained in the base material portion 2 can be measured by the following method.
　That is, the cross section of the steel pipe was observed with an optical microscope and SEM at a pitch of 45°, the area of ​​the largest inclusion found was measured, and the diameter of the circle having the same area as the measured maximum inclusion area was taken as the maximum inclusion diameter. do.
[0047]
When
　electric resistance welded steel pipe is used as a hollow stabilizer, for example, it is subjected to cold working and heat treatment. Therefore, cold workability is also important.
　If the hardness of the welded portion 3 or the like becomes excessively high, cracks may occur during processing. Therefore, in the electric resistance welded steel pipe 1 according to this embodiment, the maximum hardness of the welded portion 3 and the base material portion 2 is set to 300 Hv or less.
　On the other hand, if the hardness of the welded portion 3 is too low, the fatigue strength may be lowered and the tube may be poorly drawn. Therefore, the maximum hardness of the welded portion 3 is preferably equal to or higher than the hardness of the base material portion 2 .
　The maximum hardness of the welded portion 3 can be controlled by performing heat treatment such as normalizing after welding.
[0048]
　The maximum hardness of the welded portion 3 and the base material portion 2 can be measured by the following method. That is, the hardness of the welded portion 3 is measured up to ±2.5 mm at intervals of 0.5 mm to the left and right of the butted portion. Further, the hardness of the base material portion 2 is 1/4 thickness and 1/2 thickness in the thickness direction at positions of 90°, 180°, and 270° clockwise from the welded portion (electric resistance welded portion) 3. , 1 point for 3/4 wall thickness. The hardness (Vickers hardness) is measured according to JIS Z 2244:2009, and the measurement load is 1 kgf or less.
[0049]
　In the electric resistance welded steel pipe 1 according to the present embodiment, the metal structure of the base material portion 2 is not limited, but in order to satisfy the characteristics generally required when used as a hollow stabilizer and the appropriate hardness that does not cause poor drawing. In addition, it is preferable that the metallographic structure is composed of 40% to 60% ferrite and 60% to 40% pearlite in terms of area ratio.
[0050]
　The area ratio of the metal structure of the base material portion 2 can be measured by the following method. That is, the cross section of the steel pipe was distinguished into ferrite and pearlite by nital etching, the electric resistance weld was set to 0°, and the metallographic structures of the inner and outer surfaces at 90°, 180°, and 270° clockwise were examined with an optical microscope. Photographs are taken at a magnification of 100 times to determine the fractions of ferrite and pearlite in the metal structure. The area ratio of the metal structure of the base material portion 2 may be obtained by averaging the area ratios measured at each position.
[0051]
　Next, the hollow stabilizer according to this embodiment will be described.
　The hollow stabilizer according to this embodiment is produced by stretching and heat-treating the above-described electric resistance welded steel pipe for a hollow stabilizer according to this embodiment, processing it into the shape of the hollow stabilizer, and then performing quenching and tempering. Obtainable.
　Therefore, the hollow stabilizer according to this embodiment is composed of the base material portion and the welded portion.
　Assuming the shape of the stabilizer to be used, the hollow stabilizer according to this embodiment has a base material portion with a wall thickness of 2.0 to 6.0 mm and a base material portion with an outer diameter of 10 to 40 mm.
　The hollow stabilizer has a shape as shown in FIG. 5, for example. In FIG. 5 , the hollow stabilizer 10 is arranged in the suspension mechanism of the vehicle 11 . The hollow stabilizer 10 includes a torsion portion 20 extending in the width direction of the vehicle body 12 , a pair of bent portions 21 connected to both ends of the torsion portion 20 , and a pair of arm portions 22 connected to the bent portions 21 . The torsion portion 20 is supported by, for example, a portion of the vehicle body 12 via a pair of support portions 30 having rubber bushes or the like. The pair of arm portions 22 are connected to, for example, suspension arms (not shown) of a suspension mechanism portion via connection members 31 such as stabilizer links.
[0052]

　As described above, the hollow stabilizer according to the present embodiment is formed by drawing and heat-treating the hollow stabilizer-use electric resistance welded steel pipe according to the present embodiment as necessary, processing it into the shape of the hollow stabilizer, and then quenching it. and can be obtained by tempering. In these steps, the chemical composition does not substantially change. It is the same as the composition range and reason for limitation.
[0053]

　As described above, the electric resistance welded steel pipe usually has a recessed weld bead in the area including the welded part on the inner surface side. There is a bead cut formed by cutting by Therefore, on the inner surface side of a hollow stabilizer formed by drawing an electric resistance welded steel pipe or not, there are concave portions caused by bead cuts, or bead cuts are caused by drawing. There may be recesses formed by folding. These recesses reduce the fatigue strength of the hollow stabilizer.
　Since the hollow stabilizer according to the present embodiment is manufactured using an electric resistance welded steel pipe with a small maximum depth of the bead cut, the depth of the concave portion caused by the bead cut is small, and the bead cut is prevented from folding. be. Therefore, the maximum depth of the concave portion is small.
　Specifically, in the hollow stabilizer according to the present embodiment, when viewed in a cross section (C-direction cross section) in a direction perpendicular to the central axis direction of the hollow portion, the inner surface side of the hollow portion of the hollow stabilizer includes a welded portion. In the region, when a virtual line is drawn at the shortest distance from one end of the inner surface side of the welded portion to the other end, the maximum depth from the virtual line to the bottom of the recess is 300 μm or less. Preferably, the maximum depth is 100 μm or less.
　In the hollow stabilizer according to the present embodiment, since the maximum depth is 300 μm or less, the fatigue strength is excellent.
　The depth of the above recess is measured by measuring the maximum thickness of the sides of the recess and the thickness of the recess, and subtracting the thickness of the thinnest portion of the recess from the maximum thickness. The thickness may be measured, for example, with a micrometer.
[0054]

　By controlling the shape of the concave portion of the welding portion described above, welding of the inner surface of the hollow portion of the hollow stabilizer Fatigue strength is improved because it is possible to suppress fracture when a repeated load is applied, such as fatigue fracture originating from the part. However, since fatigue fracture also occurs from parts other than the welded part, the fatigue strength cannot be sufficiently improved only by controlling the shape of the welded part.
　Therefore, in the hollow stabilizer according to the present embodiment, the surface roughness of the inner surface of the hollow portion of the hollow stabilizer is set to 300 μm or less in terms of the maximum valley depth Rv specified in JISB0601:2013. When the maximum valley depth Rv of the inner surface (inner surface) exceeds 300 μm, fatigue cracks are generated and grow early, resulting in a decrease in fatigue strength. The maximum valley depth Rv of the surface is preferably 100 μm or less.
　The maximum valley depth Rv as described above can be obtained by appropriately drawing an electric resistance welded steel pipe having an inner maximum valley depth of 300 μm or less and drawing the pipe.
[0055]
　The surface roughness of the inner surface of the hollow portion is calculated by measuring the height distribution using a contact or non-contact shape measuring device and calculating the surface roughness according to JISB0601:2013.
[0056]
As the maximum inclusion diameter increases, the fatigue strength decreases. Therefore, in the hollow stabilizer according to this embodiment, the maximum diameter of inclusions contained in the base material portion is set to 300 μm or less. When the maximum inclusion diameter exceeds 300 μm, fatigue cracks are generated and grow early, resulting in a decrease in fatigue strength. Preferably, the maximum inclusion diameter is 100 μm or less.
　In order to set the maximum inclusion diameter of the hollow stabilizer within the above range, an electric resistance welded steel pipe in which the base material portion (steel plate) has a maximum inclusion diameter of 300 μm or less may be used as the raw material.
[0057]
　The maximum diameter of inclusions contained in the base material portion can be measured by the following method.
　That is, the cross section of the steel pipe was observed with an optical microscope and SEM at a pitch of 45°, the area of ​​the largest inclusion found was measured, and the diameter of the circle having the same area as the measured maximum inclusion area was taken as the maximum inclusion diameter. do.
[0058]

　The hollow stabilizer according to the present embodiment has a surface hardness of 455 Hv or more. If the surface hardness is less than 455Hv, the fatigue strength is lowered. The upper limit of surface hardness is not particularly limited.
[0059]
　Regarding the surface hardness (Vickers hardness), for example, the surface hardness of 8 points shifted by 45° in the circumferential direction of the bending portion (shoulder portion) 21 of the hollow stabilizer in FIG. 6 is measured, and the average hardness is calculated. calculate. The measurement conditions at that time are based on JIS-Z2244:2009, and the measurement load is 1 kgf or less.
[0060]
　Next, a preferred method for manufacturing the electric resistance welded steel pipe 1 according to this embodiment will be described.
　If the electric resistance welded steel pipe 1 according to the present embodiment has the above configuration, the effect can be obtained regardless of the manufacturing method. However, the following method is preferable because it can be produced stably.
[0061]
　In the method of manufacturing an electric resistance welded steel pipe according to the present embodiment, first, a steel slab having a predetermined chemical composition is heated, hot-rolled, and wound. At that time, it is preferable to control the heating temperature to 1000° C. or higher and the winding temperature to 800° C. or lower. If the heating temperature is less than 1000°C, rolling itself becomes difficult. Alternatively, if the winding temperature is higher than 800° C., a large amount of scale remains, and scratches are likely to occur in the subsequent pipe-making process. In addition, as for the pickling treatment of the scale, stricter treatment conditions are required than for thin scales.
　After hot rolling, pickling may be performed, or hot rolling may be performed without pickling. However, when a steel sheet as hot-rolled is subjected to pipe-making, which will be described later, it is preferable to appropriately select the shape of a group of forming rolls. In addition, depending on the state of scale under the rolling conditions, if the pickling treatment is not performed appropriately (selection of an inhibitor, etc.), the surface roughness of the inner surface of the base material portion 2 of the electric resistance welded steel pipe 1 exceeds 300 μm in terms of Rv. It is feared that it will become
[0062]
　Next, the steel plate obtained by hot rolling is formed into a cylindrical shape while being run by a group of forming rolls to form an open pipe, and both ends in the width direction of the steel plate that are butted against each other in the open pipe are heated by induction heating or the like. Then, both ends of the steel plate in the width direction are pressed by a pair of squeeze rolls in this state to weld the ends to form an electric resistance weld. The conditions in this case are not limited, and may be conditions that are commonly used.
[0063]
　A bead cut is performed on an electric resistance welded steel pipe obtained by electric resistance welding. Bead cut is important. For the bead cut, select the cutting tool shape and cutting position so that the bead that rises on the inner surface after welding is smooth after cutting and the bead cut depth is a predetermined value or less, and cuts the inner surface bead. do. The bead on the outer surface side may also be cut.
[0064]
　After bead cutting, the electric resistance welded steel pipe is subjected to a heat treatment (so-called normalization treatment) for heating to 800° C. or more and 1000° C. or less. This heat treatment can reduce the maximum hardness, including the weld zone. The heating temperature is desirably equal to or higher than the austenite transformation temperature of 800° C. or higher, but if the hardness and structure can be controlled, it does not necessarily need to exceed the transformation temperature. A hardened structure generated by welding or the like causes scratches during cold working, so the heat treatment can suppress the occurrence of scratches due to cold working.
　If the heating temperature is less than 800°C, a hardened structure remains and the cold workability deteriorates. Therefore, it is preferable to set the heating temperature to 800° C. or higher. On the other hand, if the heating temperature exceeds 1000° C., the structure becomes coarse austenite (γ) grains and a hardened structure or carbides are generated. In this case, workability deteriorates. Therefore, it is preferable to set the heating temperature to 1000° C. or lower. More preferably, it is 950° C. or less.
　Regarding the cooling rate after heating, it is allowed to cool in order to generate ferrite and pearlite structures. In the case of standing cooling, the average cooling rate from 800 to 500° C. is preferably 3° C./s or less.
[0065]
　The electric resistance welded steel pipe 1 according to the present embodiment is obtained by the manufacturing method described above. The electric resistance welded steel pipe 1 according to the present embodiment is further subjected as it is, drawn or diameter-reduced and rolled, formed by cold working such as bending and pressing, and subjected to heat treatment (quenching and tempering, etc.) to form a hollow stabilizer. can be
[0066]
　The hollow stabilizer according to this embodiment can be obtained by using the electric resistance welded steel pipe according to this embodiment as a raw material and by a manufacturing method including the following steps.
(I) A processing step of processing the electric resistance welded steel pipe into a hollow stabilizer shape
. Quenching step for cooling
(III) Tempering step for heating the electric resistance welded steel pipe after the quenching step to 200 to 350° C.
　However, if necessary, the following steps may be further performed before the working step. .
(IV) A step of drawing an electric resistance welded steel pipe
(V) A heat treatment step of heating the electric resistance welded steel pipe after the drawing step to 800° C. to 1000° C.
[0067]
(Pipe drawing process)
　Based on the desired shape of the stabilizer, the electric resistance welded steel pipe is drawn as necessary. For example, the stretched pipe has a reduction in area of ​​10 to 40% (value obtained by dividing the difference between the cross-sectional area before stretched and the cross-sectional area after stretched by the cross-sectional area before stretched×100).
[0068]
(Heat treatment process)
　The workability of the electric resistance welded steel pipe that has been drawn is reduced. Therefore, heat treatment may be performed in order to ensure workability in the subsequent processing step.
　When the heat treatment is performed, it is preferable to heat the electric resistance welded steel pipe after the drawing process to 800° C. to 1000° C. and air-cool it.
[0069]
(Processing step)
　In the processing step, the electric resistance welded steel pipe is processed into a hollow stabilizer shape. Processing is not limited as long as the desired shape is obtained, and may be, for example, hot processing, warm processing, or cold processing.
[0070]
(Quenching process)
(Tempering process)
　In the quenching process and the tempering process, the electric resistance welded steel pipe having a hollow stabilizer shape after the working process is heated to 900°C to 1200°C, and then cooled at a cooling rate of 30°C/s or more. (quenching), heating to 200° C. to 400° C., and cooling (tempering) by water cooling or air cooling. The structure and surface hardness can be controlled by controlling the quenching temperature, cooling rate, and tempering temperature.
Example
[0071]
(Example 1)
　A slab having the chemical composition (% by mass, balance Fe and impurities) shown in Table 1 was heated to 1000°C or higher, hot rolled, coiled at a temperature of 800°C or lower, pickled, and heated. Rolled steel plate. These hot-rolled steel sheets were formed into steel pipe shapes, and steel pipes 1 to 19, X, and Y were obtained by electric resistance welding.
[0072]
[table 1]

[0073]
　A bead cut was performed on the inner surface of this steel pipe. Except for the steel pipe 18 and Y, bead cutting was performed while controlling the bead cutting depth in the cross section in the C direction to be 300 μm or less. In addition, all steel pipes had a surface roughness Rv of 300 μm or less, but the steel pipe 19 was made to have a surface roughness Rv of more than 300 μm by shot blasting.
[0074]
　The electric resistance welded steel pipe after bead cutting was subjected to heat treatment under the heat treatment temperature and cooling conditions shown in Table 2.
　The heat-treated electric resistance welded steel pipe was subjected to maximum hardness measurement and metallographic observation by the methods described above. Table 2 shows the results. Except for steel pipes 11 to 14, the maximum inclusion diameter was 300 μm or less.
[0075]
　Fatigue strength was evaluated under the following conditions in order to confirm whether the electric resistance welded steel pipe is suitable for stabilizers.
　Electric resistance welded steel pipes 1 to 19 with an outer diameter of 34 mm and a wall thickness of 5.5 mm were drawn and quenched and tempered to obtain an electric resistance welded steel pipe with an outer diameter of 24 mm and a wall thickness of 4.9 mm. The tube drawing was carried out at a rate of area reduction of 40%. After that, the cold workability was evaluated by judging from scratches on the surface and the like. However, steel pipe X and steel pipe Y were not drawn. Quenching and tempering (QT) was carried out by holding at 1000° C. for 1 hour and tempering at a temperature range of 150° C. to 250° C. for 10 minutes. Since the fatigue strength is affected by the hardness, the tempering temperature was controlled so that the hardness was between 530 and 570 Hv.
[0076]
　A fatigue test piece was taken from this quenched and tempered material, and a torsional fatigue test was performed. The test method complied with JIS Z 2273:1978. The load stress was 350 MPa, and the stress ratio R (minimum stress/maximum stress) was set to -1.
　After the test piece fractured, the starting point of the fatigue crack was investigated, and the fracture starting point was classified by the bead cut break, the inner surface dent, and the inclusion. If the part was the starting point, the depth was measured, and if the inclusion was the starting point, the diameter of the inclusion was measured. As for the evaluation of the fatigue strength, as a reference value for this study, it was judged that the fatigue strength was excellent if the number of repetitions (the number of ruptures) was 80,000 times (8.0×10 4 times) or more. When the number of repetitions was 100,000 or more, it was determined that no breakage occurred, and the test was completed.

The scope of the claims
[Claim 1]
　An electric resistance welded steel pipe for a hollow stabilizer, comprising a base material portion and a welded portion,
　wherein the base material portion contains, in terms of chemical components,
　C: 0.30 to 0.38%,
　Si: 0.15 to 0.15 % by mass. 0.30%,
　Mn: 1.20-1.50%,
　Al: 0.020-0.060%,
　Ti: 0.020-0.050%,
　B: 0.0010-0.0050%,
　Cr : 0.10-0.25%,
　Mo: 0-0.20%,
　Ca: 0.0005-0.0050%,
　Cu: 0-0.25%,
　Ni: 0-0.25%,
　V: 0 to 0.05%,
　N: 0.0060% or less,
　P: 0.020% or less,
　S: 0.0100% or less,
　O: 0.0050% or less,
the balance being Fe and impurities ,
　the wall thickness of the base material portion is 2.0 to 6.0 mm, the outer diameter of the electric resistance welded steel pipe is 10 to 40 mm, and in
　the C-direction cross section of the
　　electric resistance welded steel pipe, the inner surface side of the electric resistance welded steel pipe, A concave bead cut exists in a region containing the weld,
　　When a virtual line is drawn at the shortest distance from one opening edge of the bead cut to the other opening edge, the maximum depth from the virtual line to the bottom of the bead cut is 300 μm or less, and the
　base material portion The maximum diameter of inclusions included is 300 μm or less,
　the surface roughness of the inner surface side of the base material portion of the electric resistance welded steel pipe is 300 μm or less in maximum valley depth Rv, and the electric resistance
　including the welded portion
An electric resistance welded steel pipe for a hollow stabilizer, the welded steel pipe having a maximum hardness of 300 Hv or less .
[Claim 2]

The electric resistance welded steel pipe for a hollow stabilizer according to claim 1 　, wherein the chemical composition of the base material portion satisfies the following formula (1) .
　Ca×(1−124×O)/(1.25×S)≧0.20. . . (1)
[Claim 3]
　The chemical composition of the base material portion is selected from the group consisting of
　Cu: 0.05 to 0.25%,
　Ni: 0.05 to 0.25%, and
　V: 0.01 to 0.05%. The electric resistance welded steel pipe for a hollow stabilizer according to claim 1 or 2
, containing one or more .
[Claim 4]

The electric resistance welding for a hollow stabilizer according to any one of claims 1 to 3, 　wherein the metal structure of the base material portion consists of 40% to 60% ferrite and 60% to 40% pearlite in area ratio. steel pipe.
[Claim 5]
　A method for manufacturing an electric resistance welded steel pipe for a hollow stabilizer according to any one of claims 1 to 4, wherein the electric resistance welded steel pipe formed by electric resistance welding is heated to 800° C. or more and 1000° C. or less. A method for manufacturing an electric resistance welded steel pipe for a hollow stabilizer, wherein the heat treatment is performed by standing to cool.
[Claim 6]
　A hollow stabilizer consisting of a base material part and a welded part,
　wherein the base material part contains, in terms of chemical composition,
　C: 0.30 to 0.38%,
　Si: 0.15 to 0.30% by mass%. ,
　Mn: 1.20 to 1.50%,
　Al: 0.020 to 0.060%,
　Ti: 0.020 to 0.050%,
　B: 0.0010 to 0.0050%,
　Cr: 0.10 ~0.25%,
　Mo: 0-0.20%,
　Ca: 0.0005-0.0050%,
　Cu: 0-0.25%,
　Ni: 0-0.25%,
　V: 0-0. 05%,
　N: 0.0060% or less,
　P: 0.020% or less,
　S: 0.0100% or less,
　O: 0.0050% or less,
the balance being Fe and impurities, the
　base material The wall thickness of the base material portion is 2.0 to 6.0 mm, the outer diameter of the base material portion is 10 to 40 mm, and in the
　C-direction cross section,
　　on the inner surface side of the hollow portion of the hollow stabilizer, in the region including the weld portion there is a recess,
　　When a virtual line is drawn at the shortest distance from one end of the inner surface side of the weld to the other end, the maximum depth from the virtual line to the bottom of the recess is 300 μm or less, and the
　base material the maximum diameter of inclusions contained in the base material portion is 300 μm or less,
　the surface roughness on the inner surface side of the base material portion is 300 μm or less in maximum valley depth Rv, and the
　surface hardness is 455 Hv or more
. stabilizer.
[Claim 7]

7. The hollow stabilizer according to claim 6, 　wherein the chemical composition of the base material portion satisfies the following formula (2) .
　Ca×(1−124×O)/(1.25×S)≧0.20. . . (2)
[Claim 8]
　The chemical composition of the base material portion is selected from the group consisting of
　Cu: 0.05 to 0.25%,
　Ni: 0.05 to 0.25%, and
　V: 0.01 to 0.05%. 8. Hollow stabilizer according to claim 6 or 7
, containing one or more .
[Claim 9]
　A method for manufacturing the hollow stabilizer according to any one of claims 6 to 8, wherein a
　pipe drawing step of drawing the electric resistance welded steel pipe for a hollow stabilizer according to any one of claims 1 to 4;
　A heat treatment step of heating the electric resistance welded steel pipe after the pipe drawing step to 800° C. to 1000° C.,
　a processing step of processing the electric resistance welded steel pipe after the heat treatment step into a hollow stabilizer shape, and a
　processing step after the processing step. A quenching step of heating the electric resistance welded steel pipe to 900° C. to 1200° C. and then cooling it at a cooling rate of 30° C./s or more, and a quenching
　step of heating the electric resistance welded steel pipe after the quenching step to 200° C. to 400° C. and a returning step
.
[Claim 10]
　A method for manufacturing the hollow stabilizer according to any one of claims 6 to 8, which
　comprises a processing step of processing the electric resistance welded steel pipe for hollow stabilizer according to any one of claims 1 to 4 into a hollow stabilizer shape. a
　quenching step of heating the electric resistance welded steel pipe after the working step to 900° C. to 1200° C. and then cooling it at a cooling rate of 30° C./s or more
　; and a tempering step of heating to ~400°C
.