JP2023036442A - Electroseamed steel pipe and its manufacturing method - Google Patents

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 The present invention relates to an electric resistance welded steel pipe excellent in workability suitable for automobile underbody parts, particularly for torsion bars, and a method for producing the same.

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

There are various methods for forming a hollow torsion bar. For example, as shown in FIG. There is a method of performing refining treatment by quenching and tempering.

The electric resistance welded steel pipe used as a material for such hollow torsion bars (electric resistance welded steel pipe for hollow torsion bars) has workability that can withstand the widening process, that is, no cracks occur at a widening rate of 20%. (Also referred to as being excellent in processing in the present invention) is required, but the welded part of the electric resistance welded steel pipe has a thinner part at the weld bead cut part on the inner surface compared to the base material part, or is hardened. Because of this, cracks may occur at the welded portion during the flaring process.

In addition, in the quenching and tempering processes that are performed after the flaring process, it is necessary to ensure the strength of the hollow torsion bar. However, there is a problem that a hollow torsion bar using a component material with a high carbon content cannot be expected to have high workability.

In order to solve this problem, various proposals have been made for hollow torsion bars that are made of steel materials having a relatively high carbon content and yet have excellent workability.
For example, in the technique proposed in Patent Document 1, when applying an electric resistance welded steel pipe to a hollow torsion bar, shot peening is applied to the welded seam to strengthen the electric resistance welded steel pipe, and the welded seam is used as a fatigue starting point to achieve early fatigue resistance. avoiding damage.

Further, according to the technique proposed in Patent Document 2, a steel pipe made of carbon steel with a C content of 0.3 to 0.8% is processed in a temperature range from (Ac ₁ transformation point -50 ° C.) to Ac ₁ transformation point. By performing reduction rolling with a cumulative diameter reduction rate of 30% or more at , a cementite grain size of 1.0 μm or less is obtained, and an electric resistance welded steel pipe with improved cold workability and induction hardenability can be obtained.

JP-A-6-50370 Japanese Patent Application Laid-Open No. 2001-355047

However, the technique described in Patent Document 1 has a problem that cracks occur due to insufficient workability of the weld seam and the shape of the weld bead cut in a torsion bar in which the pipe end is usually widened. was there.

Further, in the technique described in Patent Document 2, since the reduction rolling temperature is low, work strain remains in the material, and the uniform elongation u-EL and the work hardening coefficient n value become small. As a result, there remains a problem that processing cracks occur in the welded portion and the base material during the pipe end flaring process.

To provide an electric resistance welded steel pipe suitable for automobile underbody parts, particularly for torsion bars, which is excellent in cold workability, especially widening workability, even when a material having a relatively high carbon content is used. for the purpose.

As mentioned above, automobile suspension parts with excellent workability, especially steel pipes for hollow torsion bars manufactured by cold widening and subsequent quenching and tempering, have a relatively high carbon content. There is a demand for steel pipes that are excellent in workability, especially in widening workability. At the same time, in order to avoid the above-mentioned problems that ordinary electric resistance welded steel pipes have, it is necessary that the welded seam portion has material properties equivalent to those of the base material portion.

The inventors have repeatedly conducted intensive research to obtain such material properties, utilized a hot diameter reduction rolling process that can avoid the peculiarities of the material of the weld seam, and set the diameter reduction rolling end temperature within the optimum range. By doing so, it is possible to control the existence ratio (area ratio) of cementite per unit area in the metal microstructure, and to obtain high uniform elongation u-EL and work hardening coefficient n value, which are important for flaring workability. Therefore, it was found that even an electric resistance welded steel pipe made of a steel material having a relatively high carbon content can provide a steel pipe excellent in flaring workability.

The present invention has been completed based on these findings and further studies. 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: 0.0001-0.0100%, Al: 0.01-0.10%, Cr: 0.01-0.50%, Ti: 0.010-0.050%, B: 0.0005-0. 0050%, N: 0.0005 to 0.0100%, Ca: 0.0001 to 0.0050%, the balance being Fe and unavoidable impurities, and the unit area of cementite at least in the widened portion An electric resistance welded steel pipe characterized by having a per existence ratio (area ratio) S _θ of 2.0 μm ⁻¹ or less.

2. 2. The electric resistance welded steel pipe according to 1 above, wherein the flaring portion has a uniform elongation U-EL of 15% or more and a work hardening coefficient n value defined by the following formula (1) of 0.150 or more.
n = (lnσ _10% - lnσ _5% ) / (lne _10% -lne _5% ) (1)
where σ _10% : True stress when 10% tension is applied,
σ _5% : True stress when 5% tension is applied
e _10% : true strain at 10% tension
e _5% : true strain at 5% tension

3. In mass %, further, as selective 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. 3. The electric resistance welded steel pipe according to 1 or 2 above.

4. A steel plate having the composition described in 1 above is formed into a substantially cylindrical shape by cold forming to form an open pipe, and the ends of the open pipe in the width direction are butted against each other and electric resistance welded to form an electric resistance welded steel pipe, The electric resistance welded steel pipe is heated to a heating temperature of 850 to 1000° C., and the heated electric resistance welded steel pipe is subjected to hot diameter reduction under the conditions of a rolling end temperature of 850° C. or less and a cumulative diameter reduction rate of 30 to 90%. A method for manufacturing an electric resistance welded steel pipe by rolling.

5. Further, in mass %, the steel plate further contains, as selective elements, Nb: 0.0010 to 0.0500%, Mo: 0.05 to 0.30%, Cu: 0.05 to 1.00%, Ni: 0.00%. 05 to 1.00%, W: 0.001 to 0.100%, V: 0.005 to 0.500%, and REM: 0.020% or less 4. The method for producing an electric resistance welded steel pipe according to 4 above.

According to the present invention, even when a material having a relatively high carbon content is used, an electric resistance welded steel pipe suitable for automobile suspension parts, especially for torsion bars, which is excellent in cold workability, especially flaring workability. Obtainable.

FIG. 10 is a schematic diagram of flaring process. It is a figure which shows the calculation method of the existence ratio (area ratio) of a cementite.

Embodiments of the present invention will be described below.
[Component composition]
The electric resistance welded steel pipe of the present invention (hereinafter also simply referred to as electric resistance welded steel pipe) has a predetermined composition. The reasons for limiting the content of each of these compositions will be described below. Unless otherwise specified, "%" in the composition of the electric resistance welded steel pipe indicates "% by mass".

C: 0.20-0.40%
C is an element that promotes the formation of martensite by improving the hardenability and has the effect of increasing the strength (hardness) of steel by forming a solid solution. In order to secure the strength (hardness) required for the hollow torsion bar, the content of 0.20% or more is required. Therefore, the C content is made 0.20% or more. On the other hand, if the C content exceeds 0.40%, the risk of quench cracking increases and the toughness after quenching decreases. Therefore, the C content should be 0.40% or less, preferably 0.38% or less.

Si: 0.01-1.00%
Si is an element that acts as a deoxidizing agent and also as a solid-solution strengthening element. A content of 0.01% or more is required to obtain the above effect. Therefore, the Si content is set to 0.01% or more. On the other hand, when the content exceeds 1.00%, the electric resistance weldability deteriorates. Therefore, the Si content should be 1.00% or less, preferably 0.50% or less.

Mn: 0.10-2.00%
Mn is an element that forms a solid solution and contributes to improving the strength of steel and improves the hardenability of steel. In order to secure the strength (hardness) required for the hollow torsion bar, the content of 0.10% or more is required. Therefore, the Mn content should be 0.10% or more, preferably 0.50% or more. On the other hand, if the content exceeds 2.00%, the toughness decreases and the possibility of quench cracking increases. Therefore, the Mn content should be 2.00% or less, preferably 1.80% or less.

P: 0.005 to 0.100%
P is an element contained in steel as an impurity, segregates at grain boundaries and the like, increases weld cracking, and lowers toughness. Therefore, in order to use it as a hollow torsion bar, it is necessary to reduce the P content to 0.100% or less. Therefore, the P content should be 0.100% or less, preferably 0.050% or less. On the other hand, P is an element that acts as a solid-solution strengthening element, and if it is excessively reduced, the steelmaking cost will increase, so the lower limit is made 0.005%.

S: 0.0001 to 0.0100%
S is an element that exists as sulfide inclusions in steel and lowers hot workability, toughness, and fatigue resistance. In order to use it 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 made 0.0001%.

Al: 0.01-0.10%
Al is an element that acts as a deoxidizing agent, bonds with N, and has the effect of ensuring the amount of solid solution B that is effective in improving hardenability. In addition, Al precipitates as AlN and has the effect of preventing coarsening of austenite grains during heating for quenching. In order to obtain the above effect, the content of 0.01% or more is required. Therefore, the Al content is set to 0.01% or more. On the other hand, if the content exceeds 0.10% and is large, the amount of oxide inclusions increases and the fatigue life decreases. Therefore, the Al content should be 0.10% or less, preferably 0.05% or less.

Cr: 0.01 to 0.50%,
Cr is an element that has the effect of improving hardenability. In order to obtain the above effects, the Cr content should be 0.01% or more, preferably 0.05% or more. On the other hand, when the Cr content exceeds 0.50%, oxides are likely to be formed, and Cr oxides remain in the electric resistance welded portion, degrading the electric resistance welded quality. Therefore, the Cr content should be 0.50% or less, preferably 0.25% or less.

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 effects cannot be obtained sufficiently. Therefore, the Ti content is set to 0.010% or more. On the other hand, when the Ti content exceeds 0.050%, the workability and toughness of the steel deteriorate. Therefore, the Ti content should be 0.050% or less, preferably 0.040% or less.

B: 0.0005 to 0.0050%
B is an element that can improve the hardenability of steel when added in a very small amount. Moreover, B has the effect of strengthening grain boundaries, and suppresses grain boundary embrittlement due to P segregation. In order to obtain the above effect, the content of 0.0005% or more is required. Therefore, the B content should be 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 is economically disadvantageous. Therefore, the B content should be 0.0050% or less, preferably 0.0030% or less.

Ca: 0.0001-0.0050%
Ca is an element that has the effect of controlling the morphology of sulfide-based inclusions into fine, substantially spherical inclusions. By adding Ca, it is possible to reduce the number of coarse MnS particles having a particle size of 10 μm or more and coarse TiS particles having a particle size of 10 μm or more, which are the starting points of corrosion pits. In order 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% and is large, the number of coarse CaS-based clusters becomes too large, and rather causes fatigue cracks, resulting in deterioration of corrosion fatigue resistance. Therefore, Ca should be 0.0050% or less, preferably 0.0030% or less.

N: 0.0005 to 0.0100%
Although N is inevitably contained as an impurity, it is an element that acts as a solid-solution strengthening element, and it is necessary to add 0.0005% or more to obtain this effect. In addition, N binds to nitride-forming elements in the steel, and contributes to suppression of coarsening of crystal grains and increase in strength after tempering. On the other hand, a content exceeding 0.0100% reduces the toughness of the weld zone. Therefore, N should be 0.0100% or less, preferably 0.0050% or less.

Furthermore, in another embodiment of the present invention, the above component composition is optionally selected from the group consisting of Nb, Mo, Cu, Ni, W, V, and REM, and the amounts described below are 1 or 2 or more. can be included in

Nb: 0.0010-0.0500%
Nb is an element that forms fine carbides and contributes to an increase in strength (hardness). On the other hand, if the Nb content exceeds 0.0500%, the effect of addition is saturated and the effect corresponding to 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.

Mo: 0.05-0.30%
Mo is an element that improves hardenability, and is effective in increasing strength of steel and improving fatigue strength. In order to obtain the effect, addition of 0.05% or more is preferable. On the other hand, when Mo is added in excess of 0.30%, workability is remarkably lowered. More preferably, the lower limit is 0.10% and the upper limit is 0.20%.

Cu: 0.05-1.00%
Cu is an element that acts as a solid-solution strengthening element and an element that has the effect of improving corrosion resistance. To exhibit these effects, addition of 0.05% or more is preferable. On the other hand, since Cu is an expensive alloying element, if the Cu content exceeds 1.00%, the material cost rises. Therefore, the Cu content is preferably 1.00% or less, more preferably 0.50% or less.

Ni: 0.05-1.00%
Ni is an element that acts as a solid-solution strengthening element and an element that has the effect of improving corrosion resistance. To exhibit these effects, addition of 0.05% or more is preferable. On the other hand, Ni is an expensive alloying element, so if the Ni content exceeds 1.00%, the material cost rises. Therefore, the Ni content is preferably 1.00% or less, more preferably 0.50% or less.

W: 0.001 to 0.100%
Like Nb, W is an element that forms fine carbides and contributes to an increase in strength (hardness), and from the viewpoint of enhancing this effect, addition of 0.001% or more is preferable. On the other hand, if the W content exceeds 0.100%, the effect of addition is saturated and the effect corresponding to 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.

V: 0.005-0.500%
V, like Nb and W, is an element that forms fine carbides and contributes to an increase in strength (hardness), and from the viewpoint of enhancing this effect, addition of 0.005% or more is preferable. On the other hand, if the V content exceeds 0.500%, the effect of addition is saturated and the effect corresponding to 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.

REM: 0.020% or less REM (rare earth metal), like Ca, is an element that has the effect of controlling the morphology of sulfide-based inclusions into fine, substantially spherical inclusions. Optionally, REM can be added to complement the action of Ca. On the other hand, if the REM content exceeds 0.020%, the amount of inclusions that initiate fatigue cracks becomes excessive, so that the corrosion fatigue resistance deteriorates. Therefore, the REM content is preferably 0.020% or less, more preferably 0.010% or less. Although the lower limit of the REM content is not particularly limited, the REM content is preferably 0.001% or more when adding REM from the viewpoint of increasing the effect of adding REM.

[Metal microstructure]
Existence ratio (area ratio) S _θ per unit area of cementite at least in the flared portion: 2.0 μm ⁻¹ or less [mechanical properties]
Uniform elongation u-EL: 15% or more Work hardening coefficient n value: 0.150 or more It is also characterized by the fact that working cracks do not occur. It is known that it is important to increase the uniform elongation u-EL and the work hardening coefficient n value of the material in order to improve the workability of opening, but it is a carbon content that cannot be expected to have high workability in the first place. This is not easy for electric resistance welded steel pipes for torsion bars, which require the use of steel materials with a relatively high carbon content.
Therefore, the inventors conducted intensive research and found that the existence ratio (area ratio) _Sθ of cementite, which is one of the basic constituent microstructures of the ERW steel pipe of the present invention, is large in the uniform elongation u-EL and the work hardening coefficient n value. revealed to have an impact.

That is, in order to correspond to the degree of flaring applied to the torsion bar (in the present invention, the flaring rate (=outer diameter ratio before and after flaring) is 20%), at least flaring It has been found that it is necessary for the portion to have an existence ratio (area ratio) S _θ of cementite per unit area of 2.0 μm ⁻¹ or less. At the same time, it has been found that the uniform elongation u-EL of the widened portion is preferably 15% or more and the work hardening coefficient n value is 0.150 or more.
If the above S _θ does not satisfy the required value, cracking will occur in processing with a widening rate of 20%, and if the above u-EL and n values do not satisfy the preferable ranges, the widening rate will % processing, cracks are likely to occur.

In the present invention, the existence ratio (area ratio) S _θ of cementite per unit area is determined by the following formula (2).
S _θ = 2N/L (2)
N: Total number of cementite grain boundaries that intersect the straight lines marked on the metal microstructure photograph L: Total length of the straight lines marked on the metal microstructure photograph (μm)
The above N is 1 in the case of complete intersection, 0.5 in the case of contact with the cementite grain boundary, and 1.5 in the case where the straight line is closed inside the cementite crystal (inside the grain). .
That is, N is 8 at the maximum for each crystal grain, and N=0 is possible for small crystal grains that are all contained in the lattice. can be 0.5 or even 2 or more. In addition, closing the straight line means the edge of the field of view.

The uniform elongation u-EL was determined according to JISZ2241, and the tensile test piece shape was determined according to JIS No. 11 (tubular test piece) to determine the plastic elongation value up to the maximum load point. Also, the work hardening coefficient n value was determined by the following equation (1), i.e., the change in true strain with respect to the change in true stress of a tube with a strain of 5 to 10% by performing the same tensile test. σ is the true stress and e is the true strain.

n = (lnσ _10% - lnσ _5% ) / (lne _10% -lne _5% ) (1)
where σ _10% : True stress when 10% tension is applied,
σ _5% : True stress when 5% tension is applied
e _10% : true strain at 10% tension
e _5% : true strain at 5% tension

In the present invention, the main metallic microstructure of the steel pipe is preferably composed of ferrite, pearlite and cementite. The term "mainly" means that ferrite, pearlite, and cementite account for 90% or more of an arbitrary observation surface. The remainder is not particularly limited as long as it has a structure normally found in electric resistance welded steel pipes, but for example, bainite in an area ratio of 10% or less is acceptable.
Moreover, the metal microstructure defined in the present invention may be at least the flared portion. Of course, it may be the entire electric resistance welded steel pipe.

[Production method]
A method for manufacturing an electric resistance welded steel pipe for a hollow torsion bar according to the present invention will be described below.
The electric resistance welded steel pipe for a hollow torsion bar is produced by electric resistance welding a steel plate having the chemical composition described above to form an electric resistance welded steel pipe, reheating the electric resistance welded steel pipe, and then subjecting it to hot diameter reduction rolling. can be done. As the steel sheet, any steel sheet can be used as long as it has the chemical composition described above. The steel sheet is preferably a hot-rolled steel sheet.
The electric resistance welded pipe is not particularly limited and may be formed according to a conventional method. For example, the steel plate is continuously cold-formed with a plurality of rolls to form a substantially cylindrical open pipe, and then the ends of the open pipe in the width direction are abutted with each other with squeeze rolls and electric-resistance welded. It can be a sewn steel pipe. The electric resistance welding can be performed by, for example, high-frequency resistance welding, induction heating, or the like.

Heating temperature for reheating (reheating temperature): 850 to 1000 ° C
If the reheating temperature is less than 850°C, the desired weld zone toughness cannot be ensured. On the other hand, when the reheating temperature exceeds 1000° C., surface decarburization becomes significant, and sufficient quenching strength cannot be obtained. Therefore, the heating temperature is set to 850 to 1000.degree.

Rolling end temperature in hot diameter reduction rolling: 850°C or less If the diameter reduction rolling end temperature exceeds 850°C, the desired metal microstructure cannot be obtained. That is, the splitting of layered cementite in pearlite is insufficient, and the existence ratio (area ratio) S _θ of cementite per unit area becomes larger than 2.0 μm ⁻¹ , and cracks are likely to occur during the flaring process. On the other hand, if the diameter-reduction 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. , it becomes difficult to form the desired torsion bar shape, especially to widen the pipe ends.
Therefore, it is essential that the diameter reduction rolling finish temperature be 850° C. or lower, and preferably 650° C. or higher. In addition, the upper limit of the diameter reduction rolling end temperature is preferably 800°C.

Cumulative diameter reduction rate in hot diameter reduction rolling: 30 to 90%
When the cumulative diameter reduction rate is less than 30%, the division of the layered cementite in the pearlite is insufficient, and the existence ratio (area ratio) S _θ of cementite per unit area becomes larger than 2.0 μm ⁻¹ . Cracks are more likely to occur during processing. On the other hand, if the cumulative diameter reduction ratio exceeds 90%, it becomes difficult to ensure dimensional accuracy, particularly the outer diameter and wall thickness accuracy, so that the product yield decreases and cracks may occur during the flaring process. Therefore, the cumulative diameter reduction ratio should be in the range of 30 to 90%. The cumulative diameter reduction rate in the present invention is obtained by {(outer diameter of raw pipe-outer diameter of product)/outer diameter of raw pipe}×100.

Molten steel having the composition shown in Table 1 was melted in a converter and made into a slab (steel material) by a continuous casting method. These slabs (steel materials) were hot-rolled to form hot-rolled sheets having a thickness of 12 mm, and then formed into substantially cylindrical open tubes by roll forming. Then, while pressing the butted portion with a squeeze roll, the butted portion was electric resistance welded by high frequency resistance welding to form an electric resistance welded steel pipe (size: outer diameter 89.1 mm x wall thickness 4.3 mm). After that, these electric resistance welded steel pipes were used as raw steel pipes, and were subjected to diameter reduction rolling under the conditions shown in Table 2 to obtain product steel pipes (outer diameter: 24.2 mm x wall thickness: 4 mm).
For some of the product steel pipes, the outer diameter of the product steel pipe was changed in order to confirm the influence of the cumulative diameter reduction rate.

(1) Existence ratio of cementite per unit area (area ratio) S _θ
The measurement of S _θ was carried out by collecting test specimens for structural observation (observation of cross sections in the pipe circumference (C) direction) from 10 locations in the longitudinal direction of the obtained steel pipe, polishing the cross sections, and corroding them with a nital solution. Then, using a scanning electron microscope (magnification: 3000 times), the structure of the secondary electron image was observed under an acceleration voltage of 10 to 15 kV.
Furthermore, as shown in FIG. 2, vertical and horizontal grid lines were marked on the photograph, and then the value of S _θ defined by the following equation (2) was determined. The above observations and measurements were made on 10 specimens collected for each test level, at three locations in the pipe thickness direction (1/4 thick part from the outer surface, the center of the pipe thickness (1/2 thick part), and from the inner surface 1/4 thick part), and the S _θ values of 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 that intersect the straight lines marked on the microstructure photograph of the metal L: Total length of the straight lines marked on the microstructure photograph of the metal It was set to 0.5 when they were in contact with each other, and 1.5 when the straight line was closed inside the cementite crystal (intragranular).

(2) Tensile test A tensile test piece (JIS No. 11 test piece) was taken from the product steel pipe according to JISZ2241, and the tensile properties of the product pipe (0.2% proof stress YS (MPa), tensile strength TS (MPa) , elongation EL (%), uniform elongation u-EL, and work hardening coefficient n value) were determined. The n value was obtained from the change in true strain with respect to the change in true stress of a pipe with a strain of 5 to 10%, that is, by the following equation (1). σ is the true stress and e is the true strain.

n = (lnσ _10% - lnσ _5% ) / (lne _10% -lne _5% ) (1)
where σ _10% : True stress when 10% tension is applied,
σ _5% : True stress when 5% tension is applied
e _10% : true strain at 10% tension
e _5% : true strain at 5% tension

(3) Flaring process The product steel pipe described above was cut to a length of 150 mm to obtain a specimen for flaring process. Lubricating oil (machining oil) was applied to the contact portion between the punch and the specimen. In addition, the burrs generated at the time of cutting were lightly removed from the pipe ends with a chamfering sander. A universal testing machine was used for the flaring test.
As shown in Fig. 1, one tube end of the test piece is fixed, and a tapered portion (angle of 60 degrees) has a tip length of 10 mm from the opposite tube end, and the outer diameter of the test piece is widened. A punch having an outer diameter ratio of 1.1 to 1.6 times (10 to 60% widening rate) was pushed in to a length of 100 mm. The flaring rate of the present invention is obtained by {(diameter after flaring-diameter before flaring)/diameter before flaring}×100.
After that, the flaring processed part was visually observed, and if there was no cracking or local necking, it was determined that the workability was good.
Table 2 shows the results of the investigations (1) to (3) above.

As shown in Table 2, all the steel pipes according to the present invention are excellent in flaring workability. On the other hand, steel pipes manufactured with steel components or manufacturing conditions outside the range of the present invention are all inferior in widening workability, and the widening workability usually performed with a torsion bar is less than 20%. It can be seen that cracks have occurred in the

According to the present invention, even when a material with a relatively high carbon content is used, it is suitable for cold working, especially for automotive suspension parts, especially for torsion bars. It becomes possible to manufacture and provide steel pipes.

Claims (5)

% 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: 0.0001-0.0100%, Al: 0.01-0.10%, Cr: 0.01-0.50%, Ti: 0.010-0.050%, B: 0.0005-0. 0050%, N: 0.0005 to 0.0100%, and Ca: 0.0001 to 0.0050%, with the balance being Fe and unavoidable impurities,
1. An electric resistance welded steel pipe, characterized in that an existence ratio (area ratio) S _θ of cementite per unit area in at least the flared portion is 2.0 μm ⁻¹ or less. 2. The electric resistance welded steel pipe according to claim 1, wherein the uniform elongation U-EL in the flared portion 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)
where σ _10% : True stress when 10% tension is applied,
σ _5% : True stress when 5% tension is applied
e _10% : true strain at 10% tension
e _5% : true strain at 5% tension % by mass, and
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 A composition containing one or more selected from 0.020% or less The electric resistance welded steel pipe described. A steel plate having the composition according to claim 1 is formed into a substantially cylindrical shape by cold forming to form an open pipe, and the ends of the open pipe in the width direction are butted against each other and electric resistance welded to form an electric resistance welded steel pipe. , the electric resistance welded steel pipe is heated to a heating temperature of 850 to 1000 ° C., and the electric resistance welded steel pipe after heating is subjected to hot shrinkage under the conditions of a rolling end temperature of 850 ° C. or less and a cumulative diameter reduction rate of 30 to 90%. A method for manufacturing an electric resistance welded steel pipe by diameter rolling. Further, in mass %, the steel plate further contains, as selective elements, Nb: 0.0010 to 0.0500%, Mo: 0.05 to 0.30%, Cu: 0.05 to 1.00%, Ni: 0.00%. 05 to 1.00%, W: 0.001 to 0.100%, V: 0.005 to 0.500%, and REM: 0.020% or less The manufacturing method of the electric resistance welded steel pipe according to claim 4.

Images