JP2002327245A - High tension steel pipe superior in workability, and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a welded steel pipe with high strength and superior workability, without needing an expensive strengthening element. SOLUTION: The high tension steel pipe employs a steel strip for the material, which is rolled in one pass at a reduction rate of 10-50% after hot rolling, and is made by means of welding both ends in the width direction of the strip. The above material may be pickled, for instance, after being rolled in one pass at a reduction rate of 10-50%. The method for manufacturing the high tension steel pipe comprises rolling a hot-rolled steel strip in one pass at a reduction rate of 10-50%, removing the hot rolling scale with brushing and pickling, slitting the obtained steel strip so as to give required width, forming it into an open pipe, and welding both the ends in the width direction. The steel strip which has been rolled in one pass at a reduction rate of 10-50% after pickled, can be also used for the material.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength steel pipe having improved workability and mechanical strength without requiring addition of a special alloy component effective for improving strength, and a method for producing the same.

[0002]

2. Description of the Related Art A steel pipe used as a structural material is required to have high mechanical strength in order to reduce the weight of a structure, and to sufficiently satisfy the required strength even if the thickness is reduced. The strength of the steel material is improved by adding a strengthening element such as Mo, W, Ti, Nb, etc. However, since the workability decreases with the increase in strength, it is devised in a pipe forming process or a processing process after pipe forming. Cost. In addition, since a reinforcing element is required, the cost of steel material inevitably increases. By the way, the present applicant has developed a method of mechanically descaling a hot-rolled steel strip prior to pickling,
Used on the actual line. In this method, the hot-rolled steel strip sent from the hot-rolling process is strongly reduced at a rolling reduction of 10% or more to weaken the adhesive force of the hot-rolled scale to the base steel, and then most of the hot-rolled scale is subjected to heat treatment by brushing. Removed from the surface of the strip. Therefore, the load in the subsequent pickling step is greatly reduced, and in some cases, the pickling step can be omitted.

[0003]

The steel strip rolled under high pressure before pickling has a significantly improved thickness accuracy and the thickness variation in the longitudinal direction of the coil is within a range of ± 0.025 mm. The number of seat crowns starting from the coil edge is small, and it is expected to be applied to applications that require good shape accuracy. However, when the steel strip is used as a sheet material, toughness is slightly inferior, and improvement is required for applications where mechanical strength is required to be reliable. The present invention was discovered in the process of investigating and studying the physical properties of a steel strip that was rolled under high pressure before pickling, and required an expensive strengthening element by effectively utilizing the physical properties of the steel strip. It is an object of the present invention to provide a steel pipe having high strength and excellent workability without causing any problem.

[0004]

The high-strength steel pipe of the present invention comprises:
It is characterized in that a steel strip which has been subjected to one-pass rolling at a reduction rate of 10 to 50% after hot rolling is used as a material, and both ends in the width direction are welded. As the material, for example, a steel strip which has been pickled after one-pass rolling at a rolling reduction of 10 to 50% is used. In this case, after the hot-rolled steel strip is rolled in one pass at a rolling reduction of 10 to 50%, the hot-rolled scale is removed by brushing and pickling, and the obtained steel strip is slit into a required width and formed into an open pipe. ,
A high-strength steel pipe is manufactured by welding both ends in the width direction. A steel strip from which hot-rolled scale has been removed by ordinary pickling is also used. In this case, the hot-rolled steel strip from which the hot-rolled scale has been removed by pickling is subjected to one-pass rolling at a rolling reduction of 10 to 50%, and then the obtained steel strip is slit into a required width and formed into an open pipe. Weld both ends in the direction. Depending on the application, a plated steel strip can also be used.

[0005]

When the present inventors use a steel strip which has been strongly rolled before pickling as a material for a steel pipe having a small number of stress concentration sites, the present inventors utilize the high tensile strength imparted by the strongly rolling.
It was found that a high strength steel pipe was obtained. For example, STLM
When a 11A class steel is rolled down with a rolling reduction of about 25%, a tensile strength of 450N class is obtained, and a welded steel pipe formed by roll forming exhibits a high strength of STKM14A class. Further, in the case of the plate shape, the expansion is only about 5 to 9%, but after the pipe is formed, the expandability is improved to about 60%.
Such high strength and high workability of the welded steel pipe cannot be predicted from the plate-shaped steel strip, and it is suitable for reinforcement, bicycle frames, etc. used in various parts of automobiles utilizing its excellent physical properties. Welded steel pipe.

[0006] As a result of investigating the causes of the excellent strength and workability of a welded steel pipe made of a steel strip rolled under high pressure before pickling, the rolling history of the steel strip affects the mechanical strength and ductility. And got the following inference. In a normal cold-rolled steel strip, cold rolling is performed at a rolling rate of at least 50%, usually about 70%, at least from the requirement of guaranteeing the thickness accuracy. The rolling reduction of 50% or more is usually a cumulative rolling reduction obtained by repeating 4 to 5 passes of cold rolling at a rolling reduction of about 8 to 15%. In normal cold rolling in which cold working is repeated in a short time, the material temperature increases for each pass and dynamic strain aging progresses, so that the material deteriorates.

[0007] On the other hand, in the steel strip which has been strongly rolled before pickling, it is cooled immediately after being rolled in a single pass by a heavy rolling in which the rolling reduction is set to 10 to 50%, and is then brushed (de-rolled). (Scaling) → Transfer to pickling. Therefore, progress of dynamic strain aging, which is a cause of embrittlement, is avoided, and a steel strip having high strength is obtained. In addition, the absence of the developed texture seen in ordinary cold-rolled materials is also considered to be one of the causes of good ductility and toughness. The effect of the 1-pass rolling at a rolling reduction of 10 to 50% on the physical properties of the welded steel pipe is not limited to the strong rolling before pickling.
The same occurs when pass rolling is performed. Even from this fact, the strain, stress, etc., introduced into the steel strip by one-pass rolling at a rolling reduction of 10 to 50% are different from those of a normal cold-rolled steel strip, and are high when the steel pipe shape has few stress concentration sites. It is considered that strength and high workability can be obtained. However, in one-pass rolling, a rolling reduction of at least 15% is required to secure the target plate thickness, and the upper limit of the rolling reduction is set to 50% in consideration of the load of the rolling mill.

[0008]

Example 1 A hot-rolled steel strip of structural carbon steel STKM11A was rolled in one pass at a rolling reduction of 25%, and then hot-rolled scale was removed by brushing and pickling to produce a steel strip having a thickness of 1.2 mm. . The obtained steel strip has reached the strength level of STKM16A as seen in the mechanical properties shown in Table 1. Further, the yield strength ratio (0.2%
The yield strength / tensile strength was over 95% at any position of the coil.

[0009]

[0010] The obtained steel strip is 158 mm in width and 79 mm in width.
, And formed into an open pipe by a roll forming method. Under the welding conditions shown in Table 2, the diameter was 50.8 mm and the width was 25.4.
mm welded steel pipe was manufactured. Edge failure was a concern during roll forming due to the use of a material with a significantly high yield strength ratio. However, when formed at 1.1 times the product radius, no edge failure occurred and variations in welding conditions, etc. No adverse effects were observed.

[0011]

[0012] The mechanical properties of the welded steel pipe were compared with those of a welded steel pipe manufactured under the same welding conditions using STKM11A manufactured by a normal cold rolling process. As can be seen from the test results in Table 3, although the welded steel pipe according to the present invention has a slightly lower strength than the strength level of the material steel strip, the elongation as an index of ductility is 50.8 mm in diameter. 25 with welded steel pipe
%, A high value of 20% for a welded steel pipe having a diameter of 25.4 mm. Further, in a pipe expansion test in which a pipe end is pushed and spread using a conical cone, 1.41D (D:
Radius of welded steel pipe), 1.6 for welded steel pipe with a diameter of 25.4 mm.
OD showed extremely good tube expandability. It is a physical property that cannot be predicted from a conventional cold-rolled welded steel pipe that exhibits excellent workability despite being subjected to strong rolling at a rolling reduction of 25% and plastic working during pipe forming. . Moreover, it has the required characteristics of STKM14 class in terms of strength.

[0013]

Next, the bending workability was examined by a bending test in which the welded steel pipe was bent 2D and 4D. As can be seen from the investigation results in Table 4, any pipe diameter could be bent without any defects such as cracks, breaks, and defective shapes at the inner bending radius of 4D. The welded steel pipe having a diameter of 25.4 mm was fractured and flattened about 3% inside the bend. However, the bendability itself was not much different from that of a welded steel pipe manufactured from STKM11A cold-rolled material.

[0015]

Further, the deformation of the pipe end caused by the cutting was investigated as follows. For a welded steel pipe having an outer diameter of 50.8 mm, a test piece was sampled from a portion where the outer diameter dimension was stable, and for a welded steel pipe having an outer diameter of 25.4 mm, a test piece was sampled from an arbitrary position, and the outer diameter of the test piece was measured before cutting. The test piece was cut by a wet micro cutter at the outer diameter measurement position, and the outer diameter of the cut tube end was measured at the same position. FIG. 1 shows the measurement results. Outer diameter 50.8
At the end of the tube formed by cutting a welded steel tube of mm, the outer diameter at the bead position was reduced, and the tube was deformed into a lateral oval shape having a larger 90 ° position, and the amount of deformation was 0.30 mm. A pipe end formed by cutting a welded steel pipe having an outer diameter of 25.4 mm is deformed into a vertical oval shape having a slightly larger outer diameter at a bead position, and the deformation amount is only 0.03.
mm. From these results, it was confirmed that the dimensional accuracy of the inner and outer diameters was within a range that can be sufficiently guaranteed in any of the welded steel pipes.

[0017]

Example 2 A hot rolled steel strip of structural carbon steel STKM11A was pickled and then rolled in one pass at a rolling reduction of 30% to produce a steel strip having a thickness of 1.2 mm. The obtained steel strip has reached the strength level of STKM16A as seen in the mechanical properties shown in Table 1. Further, the yield strength ratio, which is an index of the roll formability during pipe making, exceeded 95% at any position of the coil.

[0018]

From the obtained steel strip, a diameter of 3 was obtained in the same manner as in Example 1.
Welded steel pipes of 4.0 mm and 22.2 mm were manufactured. Also in this case, no edge failure occurs during roll forming,
No adverse effects such as changes in welding conditions were observed. As shown in Table 6, the manufactured welded steel pipe has a slightly lower strength than the strength level of the material steel strip, but the elongation, which is an index of ductility, is 21% for a 34.0 mm diameter welded steel pipe. ,
The value was as high as 22% for a welded steel pipe having a diameter of 22.2 mm.
Further, in a pipe expansion test in which the pipe end is spread using a conical cone, a welded steel pipe having a diameter of 34.0 mm has a diameter of 1.49D and a diameter of 22.0 mm.
With a 2 mm welded steel pipe, extremely good pipe expandability of 1.41D was exhibited. Moreover, in terms of strength, it had the required characteristics of the STKM14A class. The bending workability itself was not much different from that of the welded steel pipe manufactured from STKM11A cold rolled material, and the pipe end formed by cutting did not show any significant shape collapse.

[0020]

[0021]

As described above, in the present invention, by using a steel strip that has been rolled in one pass at a rolling reduction of 10 to 50% as a material, even a STKM11A class steel material is comparable to STKM14A. High strength steel pipe with high strength. Moreover, it does not require an expensive reinforcing element and is excellent in workability such as spreading, bending, flattening and the like, so that it is used as an inexpensive high-strength structural material.

[Brief description of the drawings]

FIG. 1 is a chart showing a shape change of a pipe end formed by cutting a welded steel pipe.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Takakatsu Adachi 1000-7, Oki, Shimozuma, Ibaraki Pref. Inside the Nisshin Steel Pipe Co., Ltd. No. 1 F-term in Shin Steel Co., Ltd. (reference) 4K032 BA03 CG01

Claims (4)

[Claims] 1. A high-strength steel pipe obtained by using a steel strip which has been subjected to one-pass rolling at a reduction rate of 10 to 50% after hot rolling as a raw material and welding both ends in the width direction. 2. The high-strength steel pipe according to claim 1, wherein the steel strip is made of a steel strip pickled after one-pass rolling at a rolling reduction of 10 to 50%. 3. The hot-rolled steel strip is subjected to one-pass rolling at a rolling reduction of 10 to 50%, the hot-rolled scale is removed by brushing and pickling, and the obtained steel strip is slit into a required width to form an open pipe. A method for manufacturing a high-strength steel pipe, comprising forming and welding both ends in the width direction. 4. A hot-rolled steel strip from which hot-rolled scale has been removed by pickling is rolled in one pass at a rolling reduction of 10 to 50%, and the obtained steel strip is slit into a required width and formed into an open pipe. A method for manufacturing a high-strength steel pipe, comprising welding both ends in the width direction.