Electric welded steel pipe and method for manufacturing the same

Abstract

To provide an electroseamed steel pipe suitable for cold processability, automobile undercarriage components excellent mouth widening workability in particular, and in particular, a torsion bar even when relatively high carbon content is used in the material.SOLUTION: An electroseamed steel pipe includes, in mass%, C: 0.20 to 0.40% composition, and at least in a mouth widened portion, a main metal microstructure is composed of ferrite, pearlite and cementite, and an abundance ratio of cementite per unit area (Area ratio) Sθ is set to 2.0 μm-1 or less.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a processed electric resistance welded steel pipe excellent in workability suitable for automotive underbody parts, particularly for torsion bars, and a method for manufacturing the same.

Background Art

[0002] Many automobiles are equipped with torsion bars for the purpose of suppressing the rolling of the vehicle body during cornering and improving the running stability during high-speed driving. Conventionally, a solid torsion bar using a bar steel has been used as such a torsion bar. In recent years, however, a hollow torsion bar using a steel pipe has been generally adopted for weight reduction. Note that, as such a steel pipe, an electric resistance welded steel pipe (also referred to as an electric resistance steel pipe in the present invention) is used.

[0003] There are various methods for forming a hollow torsion bar. For example, as shown in FIG. 1, a so-called expanding process in which a punch is pushed into the inner surface of the pipe to expand the pipe is performed cold on the electric resistance welded steel pipe, and then a heat treatment process including quenching and tempering is performed.

[0004] For an electric resistance steel pipe (electric resistance steel pipe for a hollow torsion bar) used as a material for such a hollow torsion bar, workability that can withstand the expanding process, that is, no cracking occurs at an expanding rate of 20% (also referred to as excellent in workability in the present invention) is required. However, in the welded portion of the electric resistance steel pipe, there is a portion where the wall thickness is thinner or a portion where hardening has occurred in the welded bead cut portion on the inner surface compared to the base material portion. Therefore, cracking may occur in the welded portion during the expanding process.

[0005] In addition, in the quenching and tempering processes performed after the expanding process, it is necessary to ensure the strength of the hollow torsion bar. Therefore, it is necessary to select a component system having a relatively high carbon content as the steel component of the material. However, there is a problem that a hollow torsion bar using a material having a relatively high carbon content cannot be expected to have high workability.

[0006] In order to solve this problem, various hollow torsion bars made of steel materials with a relatively high carbon content and excellent workability have been proposed. For example, in the technology proposed in Patent Document 1, when applying an electric resistance welded steel pipe to a hollow torsion bar, shot peening is performed on the welded seam portion to strengthen the electric resistance welded steel pipe and avoid premature failure starting from the welded seam portion.

[0007] Also, according to the technology proposed in Patent Document 2, by performing ironing rolling with a cumulative reduction rate of 30% or more in the temperature range of (Ac1 transformation point - 50°C) to the Ac1 transformation point on a steel pipe made of carbon steel with a C content of 0.3 to 0.8% as a raw material, a structure with a cementite grain size of 1.0 μm or less is obtained, and an electric resistance welded steel pipe with improved cold workability and high-frequency hardenability is obtained.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0009] However, the technology described in Patent Document 1 has a problem that cracks occur due to insufficient workability of the welded seam portion and the influence of the welded bead cut shape in a torsion bar where flaring processing is performed on a normal pipe end.

[0010] Also, in the technology described in Patent Document 2, since the ironing rolling temperature is low, processing strain remains in the raw material, and the uniform elongation u-EL and the work hardening coefficient n value become small. As a result, in the flaring processing of the pipe end, there remains a problem that processing cracks occur in the welded portion and the base material portion.

[0011] An object of the present invention is to provide an electric resistance welded steel pipe that is excellent in cold working, particularly in flaring workability, and is suitable for automotive underbody parts, particularly torsion bars, even when a material having a relatively high carbon content is used.

Means for Solving the Problems

[0012] As described above, in the case of a steel pipe for a hollow torsion bar manufactured by cold flaring and subsequent quenching and tempering, which is an automotive underbody member excellent in workability, particularly a steel pipe with a relatively high carbon content, a steel pipe excellent in workability, particularly flaring workability, is required. At the same time, in order to avoid the above-mentioned problems of ordinary electric resistance welded steel pipes, it is necessary to have material properties equivalent to those of the base material part in the welded seam part.

[0013] The inventors repeatedly conducted intensive research to obtain such material properties, utilized a hot sizing rolling process that can avoid the specificity of the material in the welded seam part, and performed the sizing rolling end temperature within an optimal range to control the area ratio (area rate) of cementite per unit area in the metal microstructure, and obtained a high uniform elongation u-EL and a work hardening coefficient n value important for flaring workability, and thus found that it is possible to provide a steel pipe excellent in flaring workability even for an electric resistance welded steel pipe made of a steel material with a relatively high carbon content.

[0014] The present invention has been completed through further consideration based on such findings. That is, the gist of the present invention is as follows. 1. By mass%, C: 0.20 to 0.40%, Si: 0.01 to 1.00%, Mn: 0.10 to 2.00%, P: 0.005 to 0.100%, S: 0.0001 to 0.0100%, Al: 0.01 to 0.10%, Cr: 0.01 to 0.50%, Ti: 0.010 to 0.050%, B: 0.0005 to 0.0050%, N: 0.0005 to 0.0100% and Ca: 0.0001 to 0.0050% are included, and the balance is Fe and inevitable impurities, and has a composition, and at least the area ratio (area rate) S of cementite per unit area in the flaring work part θ is 2.0 μm -1Electric resistance welded steel pipe characterized by the following.

[0015] 2. The electric resistance welded steel pipe according to 1 above, wherein in the mouth widening processing section, the uniform elongation U-EL is 15% or more, and the work hardening coefficient n value defined by the following formula (1) is 0.150 or more. n=(lnσ 10% -lnσ 5% ) / (lne 10% -lne 5% )·····(1) Here, σ 10% : True stress when 10% tension is applied, σ 5% : True stress when 5% tension is applied e 10% : True strain when 10% tension is applied e 5% : True strain when 5% tension is applied

[0016] 3. By mass%, further, as selected elements, Nb: 0.0010 to 0.0500%, Mo: 0.05 to 0.30%, Cu: 0.05 to 1.00%, Ni: 0.05 to 1.00%, W: 0.001 to 0.100%, V: 0.005 to 0.500% and REM: 0.020% or less, the electric resistance welded steel pipe according to 1 or 2 above, having a composition containing one or more selected therefrom.

[0017] 4. A method for manufacturing an electric resistance welded steel pipe, comprising forming a steel plate having the composition according to 1 above into a substantially cylindrical shape by cold forming to obtain an open pipe, butt-welding the widthwise end portions of the open pipe together, and then performing electric resistance welding to obtain an electric resistance welded steel pipe, heating the electric resistance welded steel pipe to a heating temperature of 850 to 1000°C, and subjecting the heated electric resistance welded steel pipe to hot sizing rolling under the conditions of a rolling finish temperature of 850°C or lower and a cumulative diameter reduction rate of 30 to 90%.

[0018] 5. The manufacturing method of the electric resistance welded steel pipe according to item 4 above, wherein the steel sheet further contains, by mass%, as selected elements, one or more selected from Nb: 0.0010 to 0.0500%, Mo: 0.05 to 0.30%, Cu: 0.05 to 1.00%, Ni: 0.05 to 1.00%, W: 0.001 to 0.100%, V: 0.005 to 0.500%, and REM: 0.020% or less.

Advantages of the Invention

[0019] According to the present invention, even when using a material with a relatively high carbon content, it is possible to obtain an electric resistance welded steel pipe that is excellent in cold working, particularly in the flaring workability, and is suitable for automotive underbody parts, especially torsion bars.

Brief Description of the Drawings

[0020]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0021] Hereinafter, embodiments of the present invention will be described. [Component Composition] The electric resistance welded steel pipe of the present invention (hereinafter, also simply referred to as an electric resistance welded steel pipe) has a predetermined composition. Hereinafter, the reasons for limiting the respective contents of such a composition will be described. Unless otherwise specified, “%” regarding the composition of the electric resistance welded steel pipe refers to “mass%”.

[0022] C: 0.20 to 0.40% C promotes the formation of martensite through improving hardenability, and has the effect of increasing the strength (hardness) of steel by solid solution. To ensure the required strength (hardness) of the hollow torsion bar, a content of 0.20% or more is required. Therefore, the C content is set to 0.20% or more. On the other hand, when the C content exceeds 0.40%, in addition to the high risk of burning cracks, the toughness after hardening decreases. Therefore, the C content is 0.40% or less, preferably 0.38% or less.

[0023] Si: 0.01 - 1.00% Si acts as a deoxidizer and also as a solid solution strengthening element. To obtain the above effects, a content of 0.01% or more is required. Therefore, the Si content is set to 0.01% or more. On the other hand, when the content exceeds 1.00%, the electric seam weldability decreases. Therefore, the Si content is 1.00% or less, preferably 0.50% or less.

[0024] Mn: 0.10 - 2.00% Mn is an element that contributes to the improvement of the strength of steel by solid solution and also improves the hardenability of steel. To ensure the required strength (hardness) of the hollow torsion bar, a content of 0.10% or more is required. Therefore, the Mn content is 0.10% or more, preferably 0.50% or more. On the other hand, when the content exceeds 2.00%, in addition to the decrease in toughness, the possibility of burning cracks increases. Therefore, the Mn content is 2.00% or less, preferably 1.80% or less.

[0025] P: 0.005 - 0.100% P is an element contained in steel as an impurity, segregates at grain boundaries, etc., increases the weld cracking susceptibility and decreases the toughness. Therefore, to be used as a hollow torsion bar, it is necessary to reduce the P content to 0.100% or less. Therefore, the P content is 0.100% or less, preferably 0.050% or less. On the other hand, P acts as a solid solution strengthening element, and if it is reduced excessively, the steelmaking cost will increase. Therefore, the lower limit is 0.005%.

[0026] S: 0.0001~0.0100% S exists as sulfide inclusions in steel and is an element that reduces hot workability, toughness, and fatigue resistance respectively. In order to be used as a hollow torsion bar, it is necessary to reduce the S content to 0.0100% or less. Preferably, it is 0.0050% or less. On the other hand, if the amount of S is excessively reduced, the steelmaking cost will increase significantly, so the lower limit is set at 0.0001%.

[0027] Al: 0.01~0.10% Al acts as a deoxidizer and has the effect of combining with N to ensure a solid solution B amount effective for improving hardenability. Also, Al precipitates as AlN and has the effect of preventing the coarsening of austenite grains during hardening heating. In order to obtain the above effects, a content of 0.01% or more is required. Therefore, the Al content is 0.01% or more. On the other hand, if the content exceeds 0.10%, the amount of oxide inclusions increases and the fatigue life decreases. Therefore, the Al content is 0.10% or less, preferably 0.05% or less.

[0028] Cr: 0.01~0.50%, Cr is an element that has the effect of improving hardenability. In order to obtain the above effect, the Cr content is 0.01% or more, preferably 0.05% or more. On the other hand, if the Cr content exceeds 0.50%, oxides are likely to be formed, and Cr oxides remain in the electric resistance welded part, resulting in a decrease in the quality of the electric resistance welded product. Therefore, the Cr content is 0.50% or less, preferably 0.25% or less.

[0029] Ti: 0.010~0.050% Ti is an element that has the effect of fixing N in steel as TiN. If the Ti content is less than 0.010%, the above effect cannot be obtained sufficiently. Therefore, the Ti content is 0.010% or more. On the other hand, if the Ti content exceeds 0.050%, the workability and toughness of the steel decrease. Therefore, the Ti content is 0.050% or less, preferably 0.040% or less.

[0030] B: 0.0005 to 0.0050% B is an element that can improve the hardenability of steel by adding a small amount. Also, B has the effect of strengthening the grain boundaries and suppressing grain boundary embrittlement due to P segregation. To obtain the above effects, a content of 0.0005% or more is required. Therefore, the B content is 0.0005% or more, preferably 0.0010% or more. On the other hand, even if the content exceeds 0.0050%, the effect is saturated and it becomes economically disadvantageous. Therefore, the B content is 0.0050% or less, preferably 0.0030% or less.

[0031] Ca: 0.0001 to 0.0050% Ca is an element that has the effect of controlling the morphology of sulfide-based inclusions into fine substantially spherical inclusions. By adding Ca, the number of coarse MnS particles with a particle size of 10 μm or more and coarse TiS particles with a particle size of 10 μm or more, which are the starting points of corrosion pits, can be reduced. To obtain the above effects, the Ca content is set to 0.0001% or more. On the other hand, if the content exceeds 0.0050% in a large amount, too many coarse CaS-based clusters are formed, which instead become the starting points of fatigue cracks and the corrosion fatigue resistance characteristics deteriorate. Therefore, Ca is 0.0050% or less, preferably 0.0030% or less.

[0032] N: 0.0005 to 0.0100% N is inevitably contained as an impurity, but it is an element that acts as a solid solution strengthening element, and an addition of 0.0005% or more is required to obtain this effect. Also, N combines with nitride-forming elements in the steel, contributing to the suppression of grain coarsening and further increasing the strength after tempering. On the other hand, a content exceeding 0.0100% reduces the toughness of the welded part. Therefore, N is 0.0100% or less, preferably 0.0050% or less.

[0033] Furthermore, in other embodiments of the present invention, the above component composition can further optionally contain one or more selected from the group consisting of Nb, Mo, Cu, Ni, W, V, and REM in the amounts described below.

[0034] Nb: 0.0010 - 0.0500% Nb is an element that forms fine carbides and contributes to an increase in strength (hardness). To exhibit this effect, an addition of 0.0010% or more is preferred. On the other hand, when the Nb content exceeds 0.0500%, the addition effect saturates and an effect commensurate with the content cannot be obtained, which is economically disadvantageous. Therefore, the Nb content is preferably 0.0500% or less, more preferably 0.0300% or less.

[0035] Mo: 0.05 - 0.30% Mo is an element that improves hardenability, increases the strength of steel, and is effective in improving fatigue strength. To obtain this effect, an addition of 0.05% or more is preferred. On the other hand, when Mo is added in excess of 0.30%, the workability significantly decreases. More preferably, the lower limit is 0.10% and the upper limit is 0.20%.

[0036] Cu: 0.05 - 1.00% Cu is an element that acts as a solid-solution strengthening element and has the effect of improving corrosion resistance. To exhibit these effects, an addition of 0.05% or more is preferred. On the other hand, since Cu is an expensive alloying element, when the Cu content exceeds 1.00%, it causes an increase in material cost. Therefore, the Cu content is preferably 1.00% or less, more preferably 0.50% or less.

[0037] Ni: 0.05 - 1.00% Ni is an element that acts as a solid-solution strengthening element and has the effect of improving corrosion resistance. To exhibit these effects, an addition of 0.05% or more is preferred. On the other hand, since Ni is an expensive alloying element, when the Ni content exceeds 1.00%, it causes an increase in material cost. Therefore, the Ni content is preferably 1.00% or less, more preferably 0.50% or less.

[0038] W: 0.001 - 0.100% Like Nb, W is an element that forms fine carbides and contributes to an increase in strength (hardness). From the viewpoint of enhancing such an effect, an addition of 0.001% or more is preferable. On the other hand, when the W content exceeds 0.100%, the addition effect saturates and an effect commensurate with the content cannot be obtained, which is economically disadvantageous. Therefore, the W content is preferably 0.100% or less, more preferably 0.080% or less.

[0039] V: 0.005 - 0.500% Like Nb and W, V is an element that forms fine carbides and contributes to an increase in strength (hardness). From the viewpoint of enhancing such an effect, an addition of 0.005% or more is preferable. On the other hand, when the V content exceeds 0.500%, the addition effect saturates and an effect commensurate with the content cannot be obtained, which is economically disadvantageous. Therefore, the V content is preferably 0.500% or less, more preferably 0.300% or less.

[0040] REM: 0.020% or less REM (rare earth metal) is an element that, like Ca, has an effect of controlling the form of sulfide-based inclusions into fine substantially spherical inclusions. REM can be optionally added to complement the effect of Ca. On the other hand, when the REM content exceeds 0.020%, the amount of inclusions serving as fatigue crack initiation points becomes excessive, and thus the corrosion fatigue resistance characteristics deteriorate instead. Therefore, the REM content is preferably 0.020% or less, more preferably 0.010% or less. Note that the lower limit of the REM content is not particularly limited, but from the viewpoint of enhancing the addition effect of REM, when REM is added, the REM content is preferably 0.001% or more.

[0041] [Metallic microstructure] At least the area ratio (area fraction) S of cementite per unit area in the flared portion θ : 2.0 μm -1 or less [Mechanical properties] Uniform elongation u-EL: 15% or more Work hardening coefficient n value: 0.150 or more The electric resistance welded steel pipe of the present invention is characterized in that no processing cracks occur in either the electric resistance welded part or the base material part in the hollow torsion bar subjected to end flaring processing. Although it is known that it is important to increase the uniform elongation u-EL and the work hardening coefficient n value of the material to improve the flaring processability, it is not easy in the case of an electric resistance welded steel pipe for a torsion bar that requires the use of a steel material with a relatively high carbon content, which is a carbon content that cannot originally be expected to have high processability. Therefore, the inventors conducted intensive research and found that the presence ratio (area ratio) S of cementite, which is one of the basic constituent microstructures of the electric resistance welded steel pipe of the present invention θ greatly affects the uniform elongation u-EL and the work hardening coefficient n value.

[0042] That is, in order to correspond to the degree of flaring processing applied to the torsion bar (in the present invention, the flaring processing rate (= outer diameter ratio before and after flaring processing) is 20%), at least in the flaring processing part, the presence ratio (area ratio) S per unit area of cementite θ should be 2.0 μm -1 or less. It was also found that it is preferable that the uniform elongation u-EL of such a flaring processing part is 15% or more and the work hardening coefficient n value is 0.150 or more. When the above S θ does not satisfy the required value, cracks occur in the processing with a flaring processing rate of 20%. When the above u-EL and n values do not satisfy the preferable range, cracks are likely to occur in the processing with a flaring processing rate of 20%.

[0043] Note that the presence ratio (area ratio) S per unit area of cementite in the present invention θ is obtained by the following formula (2). S θ = 2N / L ······ (2) N: Total number of cementite grain boundaries intersecting the straight line drawn on the metal microstructure photograph L: Total length of the straight line drawn on the metal microstructure photograph (μm) Regarding the above N, it is set to 1 in the case of complete intersection, 0.5 when in contact with the grain boundaries of cementite, and 1.5 when the straight line closes inside the crystal of cementite (intragranular). That is, for each grain, N can be at most 8. For small grains that fit entirely within the lattice, N can be 0, but if such small grains are slightly displaced, N can be 0.5 or 2 or more. Also, the straight line closing means the edge of the field of view.

[0044] The uniform elongation u-EL was determined according to JIS Z2241. The tensile test piece was in the shape of JIS No. 11 (tubular test piece), and the tensile test was performed to obtain the plastic elongation value up to the maximum load point. Also, the work hardening coefficient n value was determined by performing the above tensile test and calculating the change in true strain with respect to the change in true stress in the pipe with a strain of 5 to 10%, that is, by the following formula (1). Here, σ is the true stress and e is the true strain. n=(lnσ 10% -lnσ 5% ) / (lne 10% -lne 5% )·····(1) Here, σ 10% : The true stress when a 10% tensile test is performed, σ 5% : The true stress when a 5% tensile test is performed e 10% : The true strain when a 10% tensile test is performed e 5% : The true strain when a 5% tensile test is performed

[0045] In the present invention, it is preferable that the main metal microstructure of the steel pipe is composed of ferrite, pearlite, and cementite. Here, the term "main" means that ferrite, pearlite, and cementite account for 90% or more in terms of the area ratio on any observation surface. The remainder is not particularly limited as long as it is a structure normally found in electric welded steel pipes. For example, bainite with an area ratio of 10% or less is acceptable. Also, the metal microstructure defined in the present invention may be at least the beveled portion. Of course, it may also be the entire electric welded steel pipe.

[0046] [Manufacturing Method] Hereinafter, the manufacturing method of the electric resistance welded steel pipe for a hollow torsion bar of the present invention will be described. The electric resistance welded steel pipe for a hollow torsion bar can be manufactured by electric resistance welding a steel plate having the above-described component composition to form an electric resistance welded steel pipe, reheating such an electric resistance welded steel pipe, and then performing hot sizing rolling. As the steel plate, any steel plate having the above-described component composition can be used. The steel plate is preferably a hot rolled steel plate. The electric resistance welding can be performed according to a conventional method without particular limitation. For example, the steel plate can be continuously cold formed by a plurality of rolls to form a substantially cylindrical open pipe, and then the widthwise end portions of the open pipe can be butted by squeeze rolls and subjected to electric resistance welding to obtain an electric resistance welded steel pipe. The electric resistance welding can be performed, for example, by high-frequency resistance welding, induction heating, or the like.

[0047] Heating temperature during reheating (reheating temperature): 850 to 1000 °C If the reheating temperature is less than 850 °C, the desired toughness of the welded portion cannot be ensured. On the other hand, when the reheating temperature exceeds 1000 °C, surface decarburization becomes remarkable, and sufficient hardening strength cannot be obtained. Therefore, the heating temperature is set to 850 to 1000 °C.

[0048] Rolling finish temperature in hot sizing rolling: 850 °C or lower If the sizing rolling finish temperature exceeds 850 °C, a desired metal microstructure cannot be obtained. That is, the lamination of cementite in pearlite is insufficiently divided, and the area ratio (area rate) S of cementite per unit area θ becomes larger than 2.0 μm -1 and cracks are likely to occur during flaring. On the other hand, if the sizing rolling finish temperature is less than 650 °C, the workability deteriorates, and the uniform elongation u-EL and the work hardening coefficient n value, which are important elements of the present invention, do not satisfy the desired values, making it difficult to form the torsion bar into the desired shape, particularly the flaring process applied to the pipe end. Therefore, it is important that the finishing temperature of the diameter-reducing rolling is 850°C or lower, and it is preferably 650°C or higher. Note that the upper limit of the finishing temperature of the diameter-reducing rolling is preferably 800°C.

[0049] Cumulative diameter reduction rate in hot diameter-reducing rolling: 30 - 90% When the cumulative diameter reduction rate is less than 30%, the lamination of the lamellar cementite in the pearlite is insufficiently broken, and the area ratio (area rate) S of the cementite per unit area θ becomes larger than 2.0 μm -1 and cracking is likely to occur during the flaring process. On the other hand, when the cumulative diameter reduction rate exceeds 90%, it becomes difficult to ensure dimensional accuracy, particularly the outer diameter and wall thickness accuracy, resulting in a decrease in product yield, and cracking may occur during the flaring process. Therefore, the cumulative diameter reduction rate is in the range of 30 - 90%. Note that the cumulative diameter reduction rate in the present invention is obtained by {(outer diameter of the raw pipe - outer diameter of the product) / outer diameter of the raw pipe} × 100.

Examples

[0050] The molten steel having the composition shown in Table 1 was melted in a converter and made into slabs (steel materials) by the continuous casting method. These slabs (steel materials) were hot-rolled to obtain hot-rolled sheets with a thickness of 12 mm, and then formed by roll forming to obtain substantially cylindrical open pipes. Subsequently, while pressing the butted portion with squeeze rolls, the butted portion was electric resistance welded by high-frequency resistance welding to obtain electric resistance welded steel pipes (size: outer diameter 89.1 mm × wall thickness 4.3 mm). Thereafter, these electric resistance welded steel pipes were used as raw material steel pipes and subjected to diameter-reducing rolling under the conditions shown in Table 2 to obtain product steel pipes (outer diameter 24.2 mm × wall thickness 4 mm). Note that for some of the product steel pipes, the outer diameter of the product steel pipes was changed in order to confirm the influence of the cumulative diameter reduction rate.

[0051]

Table 1

[0052] (1) Area ratio (area rate) S of cementite per unit area θ S θThe measurement was carried out by collecting test pieces for microstructure observation (cross-sectional observation in the circumferential (C) direction of the pipe) from the obtained steel pipes at 10 locations in the pipe longitudinal direction, polishing the cross-sections, corroding them using nital solution, and observing the microstructure of the secondary electron image under an accelerating voltage of 10 - 15 kV using a scanning electron microscope (magnification: 3000 times). Furthermore, as shown in Figure 2, after drawing vertical and horizontal grid lines on the photograph, the value of S θ defined by the following formula (2) was obtained. The above observations and measurements were carried out for 10 observation pieces collected for each test level at 3 locations in the pipe thickness direction (1 / 4 thickness part from the outer surface, pipe thickness center (1 / 2 thickness part), 1 / 4 thickness part from the inner surface), and the S θ values at all such locations (30 locations) were averaged to obtain S θ for each test level. S θ = 2N / L······(2) N: Total number of cementite grain boundaries intersecting the straight line drawn on the metal microstructure photograph L: Total length of the straight line drawn on the metal microstructure photograph In addition, in the case of complete intersection, it was set to 1, in the case of contacting the cementite grain boundary, it was set to 0.5, and in the case of the straight line closing inside the cementite crystal (intragranular), it was set to 1.5.

[0053] (2) Tensile test Tensile test pieces (JIS No. 11 test pieces) were collected from the product steel pipes in accordance with JIS Z2241, and the tensile properties of the product pipes (0.2% proof stress YS (MPa), tensile strength TS (MPa), elongation EL (%), uniform elongation u - EL, and work hardening coefficient n value) were obtained. The n value was obtained from the change in true strain with respect to the change in true stress of the pipe with a strain of 5 - 10%, that is, calculated by the following formula (1). Here, σ is the true stress and e is the true strain. n=(lnσ 10% -lnσ 5% ) / (lne 10% -lne 5% )·····(1) Here, σ 10% : True stress when 10% tensile is applied, σ 5% : True stress when 5% tensile is applied e 10% : True strain when 10% tension is applied e 5% : True strain when 5% tension is applied

[0054] (3) Flaring process The above-mentioned product steel pipe was cut into a length of 150 mm to serve as a test piece for the flaring process. Lubricating oil (machining oil) was applied to the contact part between the punch and the test piece. Also, the tube end was lightly deburred with a chamfering sander at the time of cutting. A universal testing machine was used for the flaring process test. As shown in Fig. 1, one end of the test piece was fixed, and the other end had a tapered part (angle 60 degrees) for a tip length of 10 mm. A punch was pushed in up to a length of 100 mm such that the outer diameter ratio before and after the flaring process was 1.1 to 1.6 times (flaring process rate 10 to 60%) with respect to the outer diameter of the test piece. Note that the flaring process rate of the present invention is obtained by {(diameter after flaring process - diameter before flaring process) / diameter before flaring process} × 100. Thereafter, the flared part was visually observed, and if there were no cracks or local necking, it was judged that the workability was good. The results obtained from the above investigations (1) to (3) are shown in Table 2.

[0055]

Table 2

[0056] As shown in Table 2, all the steel pipes according to the present invention are excellent in flaring processability. On the other hand, it can be seen that the steel pipes produced with steel components outside the scope of the present invention or under manufacturing conditions are all inferior in flaring processability, and usually, the flaring process rate carried out with a torsion bar is less than 20% and cracks have occurred.

Industrial applicability

[0057] According to the present invention, even when a material with a relatively high carbon content is used, it becomes possible to manufacture and provide an electric resistance welded steel pipe that is excellent in cold working, particularly in flaring processability, and is suitable for automotive underbody parts, especially for torsion bars.

Claims

1. By mass%, C: 0.20 to 0.40%, Si: 0.01 to 1.00%, Mn: 0.10 to 2.00%, P: 0.005 to 0.100%, S: 0.0001 to 0.0100%, Al: 0.01 to 0.10%, Cr: 0.01 to 0.50%, Ti: 0.010 to 0.050%, B: 0.0005 to 0.0050%, N: 0.0005 to 0.0100%, Mo: 0.05 to 0.30% and Ca: 0.0001 to 0.0050%, having a composition consisting of the balance Fe and unavoidable impurities, The mouth widening processing rate exceeds 60% and cracks do not occur, and at least the existence ratio (area ratio) S per unit area of cementite in the mouth widening processed part θ is 2.0 μm -1 or less, and in the mouth widening processed part, the uniform elongation U-EL is 17% or more, and the work hardening coefficient n value defined by the following formula (1) is 0.173 or more. A electric welded steel pipe characterized by this. n = (lnσ 10% - lnσ 5% ) / (lne 10% - lne 5% ) ······ (1) Here, σ 10% : True stress when 10% tension is applied, σ 5% : True stress when 5% tension is applied e 10% : True strain when 10% tension is applied e 5% : True strain when 5% tension is applied

2. By mass%, further, As a selected element, a composition containing one or more selected from Nb: 0.0010 to 0.0500%, Cu: 0.05 to 1.00%, Ni: 0.05 to 1.00%, W: 0.001 to 0.100%, V: 0.005 to 0.500% and REM: 0.020% or less, the electric resistance welded steel pipe according to Claim 1.

3. A method for manufacturing the electric resistance welded steel pipe according to Claim 1, comprising forming a steel plate having the composition according to Claim 1 into a substantially cylindrical shape by cold forming to obtain an open pipe, butt-welding the widthwise end portions of the open pipe together, and performing electric resistance welding to obtain an electric resistance welded steel pipe, heating the electric resistance welded steel pipe to a heating temperature of 850 to 1000°C, and subjecting the heated electric resistance welded steel pipe to hot sizing rolling under the conditions of a final rolling temperature of 850°C or lower and a cumulative diameter reduction rate of 72.8 to 90%.

4. Further, the steel plate has a composition containing, by mass%, as a selected element, one or more selected from Nb: 0.0010 to 0.0500%, Cu: 0.05 to 1.00%, Ni: 0.05 to 1.00%, W: 0.001 to 0.100%, V: 0.005 to 0.500% and REM: 0.020% or less, the method for manufacturing an electric resistance welded steel pipe according to Claim 3.

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