JP2001162305A - Manufacturing method of steel tube - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of a steel tube which is high in strength and ductility, excellent in tubular shape, high in productivity, and adaptable to production of many kinds in small quantities. SOLUTION: In the manufacturing method of the steel tube in which a steel strip is heated above the Ac3 transformation point, and continuously formed by a forming roll to make an open tube, both edges thereof are heated, and butted to and joined with each other by a squeeze roll, and internal and external beads are removed to from a base tube, and further drawn, the drawing is achieved in the temperature range between 400 deg.C and Ar3 transformation point +50 deg.C and the drawing rolling is carried out with the draft of not less than 30%, and/or the steel tube is cooled at the cooling speed of >=30 deg.C/s within 0.5 second after the drawing is completed. The base tube is cooled after the internal and external beads are removed, and re-heated to the temperature range between 400 deg.C and Ar3 transformation point +50 deg.C, and the drawing maybe implemented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for manufacturing a steel pipe, and more particularly to a method for manufacturing an electric resistance welded steel pipe.

[0002]

2. Description of the Related Art In recent years, for the purpose of reducing the weight of automobiles, steel pipes are increasingly used as suspension parts for automobiles such as stabilizers and drive shafts. Generally, a welded steel pipe is used as such a steel pipe. Welded steel pipes are formed by forming a steel plate or a steel strip into a tube and welding its joints, and are manufactured by various manufacturing methods from small diameter to large diameter. The main manufacturing method is electric resistance welding (ERW) , Forging, and arc welding.

For small to medium diameter steel pipes, an electric resistance welding method using a high frequency current (electric resistance welded steel pipe, electric resistance welded pipe)
Is mainly used. In this method, a steel strip is continuously supplied and formed into an open pipe by forming into a tube with a forming roll, and then the end faces of both edges of the open pipe are heated by a high-frequency current to a temperature equal to or higher than the melting point of the steel. This is a method of manufacturing a product pipe by butt-welding the end faces of both edges (for example, 3rd Edition Iron and Steel Handbook, Volume III (2) 1056-1092).
page).

[0004] In the above-described method for manufacturing an electric resistance welded tube, since the strip is usually formed into an open tube, the forming load at the time of forming the open tube is high, and it is difficult to improve the productivity. However, there is a problem that the ductility and workability thereafter are reduced due to the large work hardening of the steel. Furthermore, since the vicinity of the edge of the open pipe is heated to a high temperature above the melting point and welded, a wide heat-affected zone is formed in the seam, and the characteristics of each part in the pipe circumferential direction become non-uniform. there were. In particular, this inhomogeneity has been a serious problem in high-strength steel pipes widely used as reinforcing materials for automobile bodies.

Further, in the above-described method for manufacturing an electric resistance welded pipe, a roll must be used in accordance with the product dimensions of the steel pipe.
There was a problem that it could not cope with small lot multi-product production. To solve this problem, for example, JP-A-63-33105 and JP-A-2-187214 propose a method of cold-rolling ERW steel pipes. However, in this method, since the cold rolling is performed, the rolling load is large and the size of the mill needs to be increased. Further, in order to prevent seizure between the material to be rolled and the roll, it is necessary to install a lubricating rolling device. There was a problem.

In order to solve these problems, Japanese Patent Application Laid-Open No. 58-97
No. 14, JP-A-60-15082 proposes a method of hot-rolling an ERW steel pipe. JP-A-58-97
No. 14, JP-A-60-15082 discloses the problem that the molding load is high at the time of forming an open tube, and the workability and ductility of the product tube are reduced due to large work hardening of the product tube. There is no problem that the quality of the seam is degraded, and that it is not possible to cope with the production of small lots of many kinds. However, simple reheating, pipe making, and rolling as described in these prior art documents do not provide the required workability with a stabilizer or a drive shaft.
These problems are particularly noticeable in high-strength steel pipes that are widely used as automobile suspension parts.

[0007]

DISCLOSURE OF THE INVENTION The present invention advantageously solves the above-mentioned problems, and provides a method of manufacturing a steel pipe capable of producing a high-strength, high-ductility steel pipe with high productivity, and capable of coping with small-lot, multi-product production. The purpose is to propose.

[0008]

Means for Solving the Problems In order to achieve the above-mentioned object, the present inventors have made various studies to improve the workability of a steel pipe manufactured through a heating-pipe-forming-draw-rolling process. Was done. As a result, in the steel pipe, by setting the diameter reduction rolling temperature at 400 ° C. to less than the Ar ₃ transformation point + 50 ° C., a high r-value rolling texture develops, and fine dispersion of the second phase such as cementite is promoted. It was found that the workability was significantly improved. This effect is described in the aforementioned JP-A-58-9714,
It cannot be obtained by the diameter reduction rolling disclosed in JP-A-60-15082. Further, for example, Japanese Patent Application Laid-Open No. 9-279233
The rolling temperature is set to Ar by plate rolling as disclosed in
_{If the} transformation temperature is lower than + 50 ° C., the rolling direction is low, and conversely, the rolling texture becomes low and the workability is reduced.

Also, by performing diameter reduction rolling on a normal ERW pipe at a temperature of 400 ° C. to less than the Ar ₃ transformation point + 50 ° C., the workability of the base material portion other than the seam portion is improved. However, good workability cannot always be obtained in the seam portion. this is,
It is considered that the seam portion has a hardened dislocation density and an extremely high dislocation density, so that a good rolling texture is unlikely to be generated, and that the seam portion is uneven in thickness due to a difference in deformation resistance. The present inventors studied a method for preventing this, manufactured the mother tube hot, and solved the above-mentioned problem by forming a seam portion before diameter reduction rolling into a structure having a low dislocation density similar to the base material portion. The inventors have found out that they can be solved, and have reached the present invention. Note that, as can be easily inferred from the above, the same effect can be expected even when the heating temperature of the pipe making is equal to or higher than the temperature at which martensitic transformation or bainite transformation occurs. However, in a continuous line of pipe making and diameter reduction rolling, in this method, the temperature difference between the seam portion and the base material portion becomes large, and the thickness tends to be uneven.

That is, the present invention is a method for manufacturing a steel pipe described below. [1] After heating the steel strip to the Ac ₃ transformation point or higher, it is continuously formed by a forming roll to form an open pipe, and both edges of the open pipe are heated and joined by squeeze rolls to join the inner and outer beads. In the method for producing a steel pipe, which is further subjected to reduction rolling after removing the mother pipe, the reduction rolling is performed at 400 ° C. to Ar.
_{(3) A} method for producing a steel pipe, which comprises rolling in a temperature range of less than the transformation point + 50 ° C and a rolling reduction of 30% or more.

[2] Within 0.5 seconds after the completion of the drawing rolling
The method for producing a steel pipe according to [1], wherein the steel pipe is cooled at a cooling rate of 30 ° C./s or more. [3] After removing the inner and outer beads, the mother pipe is cooled to 400 ° C.
The method for producing a steel pipe according to [1] or [2], wherein the drawing is performed by reheating to a temperature range of less than the Ar ₃ transformation point + 50 ° C.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, prior to forming a steel strip, the steel strip discharged from the uncoiler is heated to a temperature higher than the Ac ₃ transformation point. As the heating, any of a method using a heating furnace, an induction heating method using an induction coil, and a resistance heating method by energization can be suitably applied.

The heating of the steel strip is performed at a temperature higher than the transformation point of Ac ₃ , preferably 850 ° C. to 1050 ° C. Heating above 1050 ° C causes coarsening of the microstructure, resulting in reduced workability,
A large amount of scale is formed on the surface of the steel strip, and the surface skin of the steel pipe becomes defective. If the heating temperature is lower than the Ac ₃ transformation point, the workability of the seam portion is lower than that of the base material portion, and it is difficult to maintain uniform quality in the circumferential direction. If the heating temperature is higher than the Ac ₃ transformation point but lower than 850 ° C., the hot deformation resistance is not sufficiently reduced and the molding load is not reduced. For this reason, the heating temperature is not less than the Ac ₃ transformation point, preferably 850 ° C.
It is preferable that the temperature is not less than 1050 ° C.

The heated steel strip is continuously formed by a plurality of forming rolls to form an open pipe. For forming, a known processing method using a plurality of forming rolls can be suitably applied. Instead of heating the steel strip, the entire pipe may be heated after the pipe is opened. The heating method and the heating temperature may be the same as in the case of the steel strip. In the case of open tube heating, there is an advantage that the width of the heat-affected zone in the seam portion does not increase.

Next, both edges of the open tube are heated to a temperature equal to or higher than the melting point with a high-frequency current. The edge heating may be performed in the air or in an atmosphere in which the oxygen concentration is lower than that in the air (in a shield atmosphere), but is preferably in a shield atmosphere from the viewpoint of seam quality. If the temperature of the edge heating is lower than the melting point, the joining of the end face of the edge portion becomes insufficient and the seam quality is deteriorated.

The open pipe whose both edges are heated to the melting point or higher is abutted and pressed against both edges by a squeeze roll and joined (welded). Since the temperature of the end face of the edge portion exceeds the melting point of the pipe material, the molten steel forms beads (surplus) inside and outside the pipe at the time of abutting joining. For this reason, bead cutting is required after joining. The edge heating and the welding may be performed in the air or in an atmosphere in which the oxygen concentration is lower than that in the air (in a shield atmosphere), but preferably in a shield atmosphere from the viewpoint of seam quality.

After the seam welding, the formed inner and outer beads are removed. The removal of the inner and outer beads is preferably performed by a bead removing device provided with processing equipment such as cutting and polishing. Next, in the present invention, the steel pipe heated to a high temperature and subjected to electric resistance welding is subjected to bead removal after welding, immediately or after being subjected to soaking treatment, and then drawn and rolled to obtain a product pipe.

From the viewpoint of ensuring the dimensional accuracy of the product pipe after drawing and rolling the steel pipe, the temperature difference in the circumferential direction of the pipe is reduced to 20 ° C. or less by heating, cooling, etc. before the drawing and rolling of the steel pipe after welding. It is preferable to perform a soaking heat treatment. This soaking treatment is preferably performed by a steel tube soaking device provided with a heating means and a cooling means. The heating means is preferably a heating furnace, induction heating or the like, and the cooling means is preferably a fluid jet of water, gas or the like.

The drawing rolling temperature is from 400 ° C. to the Ar ₃ transformation point +50.
It should be less than ° C. If the reduction rolling temperature is less than 400 ° C., the rolling load becomes too high, while the reduction rolling temperature is reduced to the Ar ₃ transformation point +50.
If the temperature is higher than or equal to ℃, even if a large pressure is applied, the refinement of precipitates and microstructure is not promoted, and the texture of high r value is hard to develop, so that the workability is reduced, and The generation of scale is remarkable and the surface quality is deteriorated. The reduction rolling temperature is more preferably 650 ° C. to the Ar ₃ transformation point.

The reduction rolling is carried out at the above-described reduction rolling temperature and at a rolling reduction of 30% or more. Here, the rolling reduction and the strain rate are defined by the following equations. Rolling reduction _{= (1-d 1 / d} 0) × 100 (%) d 0: outer diameter before diameter reduction rolling, d _1: reduction ratio of the outer diameter of more than 30% after diameter reduction rolling is earned in one pass But you may earn with multiple passes. However, in the case of a plurality of passes, the rolling reduction per pass is preferably 10% or more. The strain rate is more preferably 1 s ^-1 or more.

Further, in the present invention, the steel pipe after the reduction rolling is
It is preferable to cool at a cooling rate of 30 ° C./s or more within 0.5 seconds from the end of rolling. The cooling end temperature is appropriately determined according to the chemical composition of the steel pipe, the required strength, and the like.
The temperature is preferably about 700 ° C. for ferrite-pearlite steel and about 400 ° C. for bainite steel. More preferred cooling rates are:
50 ° C / s or more.

The obtained product tube is cut to a predetermined size by a cutting machine and is corrected by a tube straightening device, or is straightened by a tube straightening device and then wound into a coil. Further, in the present invention, instead of performing the drawing rolling subsequent to the bead removal, the mother pipe after the bead removal is once set to a temperature lower than the target temperature provided within the temperature range of 400 ° C. to the Ar ₃ transformation point + less than 50 ° C. After being cooled and reheated to a temperature higher than the target temperature, drawing rolling may be performed. In this case, reduction rolling and subsequent cooling follow [1] and [2]. The cooling method after the bead is removed is not particularly limited, and may be natural cooling (air cooling) or forced cooling using wind, water, mist, or the like.

FIG. 1 shows a steel pipe production line suitable for carrying out the present invention [1] and [2]. In FIG. 1 (a), 1 is a steel strip, 11 is an uncoiler for dispensing a steel strip, and 12 is a joining device (relay welding machine) for connecting a rear end of a preceding steel strip and a front end of a following steel strip. , 13 is a looper (accumulator) for storing the steel strip, 2 is a steel strip heating apparatus for heating the steel strip, 3 is a forming apparatus including a group of forming rolls, 5 is an edge heating apparatus,
6 is a squeeze roll, 7 is an open pipe, 8 is a steel pipe, 16 is a product pipe, 21 is a drawing device, 18 is a cutting machine, 19 is a pipe straightening device, 20 is a thermometer, 23 is a recent quenching device, and 14 is a bead. It is a removal device.

FIG. 1B shows a row of steel pipe production equipment in which a steel pipe heating / heating unit 22 is provided on the exit side of the squeeze roll 6 and on the entrance side of the reduction mill 21 in addition to FIG. FIG.
The same or corresponding parts as those in FIG. The uncoiler 11 is a device for supplying the coiled steel strip 1 while rewinding it, and includes a mandrel, a guide, and the like.

In order to continuously supply the strip 1 discharged in a coil unit to the line, the relay welding machine 12 has a rear end of the discharged preceding coil and a front end of the succeeding coil being discharged. It is a device that welds and joins. This includes electrodes,
A flash butt welding machine including a clamp device or the like is suitable. The accumulator 13 is a device for storing an amount of strip necessary for continuous operation without stopping the line when joining the strip 1 with the relay welder 15.

The heating device 2 is a device for heating the steel strip 1 to a temperature higher than the transformation point of Ac ₃ , and it is preferable to use either a gas-fired continuous heating furnace or an induction heater for steel strip. When the ranges of the sheet thickness and the sheet passing speed are wide, it is preferable to install both of them because the temperature of the steel strip 1 can be controlled with higher accuracy. The forming apparatus 3 is a facility for continuously forming the steel strip 1 into a cylindrical shape, and forming an open pipe 7 with the width end faces of the steel strip 1 facing each other. This equipment has multiple molding stands,
A breakdown type or a cage type is typically formed of a forming roll or the like, but other types may be used.

The edge heating device 5 is a facility for heating the edge of the open tube 7 to a temperature range higher than the melting point by high frequency current. The edge of the open pipe 7 is preferably maintained in a shielded atmosphere (non-oxidizing atmosphere) by a shield device 13 for adjusting the atmosphere. The squeeze roll 6 is a facility which is supported by a housing or the like and abuts both edges of the open pipe 7 which are edge-heated to a melting point or higher, applies a compressive force in a circumferential direction, and presses and welds.

The bead removing device 14 is a facility for removing a bead generated during welding by grinding or cutting.
It is preferably made of a grinding wheel or a cutting tool.
Steel tube heating heat equalizer 22, the heating gas combustion type continuous heating furnace consists of induction heater and the pipe cooler for the tube, the target temperature provided in the temperature range below the stop rolling temperature suitably 400 ° C. to Ar ₃ transformation point + 50 ℃ In order to make uniform the circumferential temperature distribution of the steel pipe 8 after welding, it is necessary to cool or cool the high-temperature portion around the seam from the viewpoint of the purpose of performing the heat treatment and avoiding the occurrence of uneven wall thickness when drawing downstream. It is equipment installed for the purpose of heating. The pipe induction heater section and the pipe cooler section of this facility can be configured by combining, for example, a water spray cooling device including a pipe for cooling water, a header, a spray nozzle, and the like, and an induction heating device.

The drawing apparatus 21 continuously reduces (diameter-rolls) the outer diameter of the steel pipe 8 by using multiple stands in an appropriate temperature range.
A facility for obtaining a steel pipe having a predetermined product outer diameter, in which a plurality of stands (reducing rolling mills) in which a plurality of rolls (groove rolling rolls) are arranged in a circumferential direction in a housing are arranged in a plurality of tandem units. It is composed. For this purpose, a three-roll type stretch reducer, a two-roll type sizer, or the like is applicable.

The immediate quenching device 23 is a water cooling device including a cooling water pipe, a header, a spray nozzle, and the like. It is desirable to equip this device with a draining device such as a gas jet so that the cooling water does not flow forward, and an exhaust device so that the generated water vapor does not affect the measurement by the front and rear thermometers. The nearest cooling device 23 is installed immediately after the last stand of the drawing device.

The cutting machine 18 is a facility for cutting the drawn and rolled steel pipe 8 to a predetermined length between runs.
A rotary hot saw composed of a disc saw or the like can be applied. The pipe straightening device 19 is a facility for straightening the bent steel pipe 8 after cutting. For this, for example, a vertical inclined roller type straightening machine including a plurality of upper and lower opposed rollers and the like can be applied. Although the tube straightening device 19 is provided online in FIG. 1, it may be provided offline.

FIG. 2 is an explanatory view showing one example of a steel pipe manufacturing equipment line suitable for carrying out the present invention [3] in which the mother pipe after bead removal is once cooled, reheated and drawn and rolled. . FIG.
In the figure, reference numeral 24 denotes a steel pipe heating device composed of, for example, a gas-fired continuous heating furnace, and the same or corresponding members as in FIG. In this example, the mother pipe (steel pipe 8) after the bead removal is cut by the cutting machine 18 and then at room temperature to 40 ° C.
Air-cooled to a temperature range of less than 0 ° C, stored in a storage location (not shown),
After being reheated to a temperature range from 400 ° C. to the Ar ₃ transformation point + less than 50 ° C., it is rolled.

[0033]

EXAMPLES (Example 1) A product pipe was manufactured using the manufacturing equipment row shown in FIG. 1 under the following conditions. After the steel strip 1 having the composition shown in Table 1 was heated by the steel strip heating device 2 under the conditions shown in Table 2, it was formed into the open pipe 7 by the forming device 3.
Edge heating was performed on both edges of the open pipe to heat it to a melting point or higher by an edge heating device 5, roll-welded with a squeeze roll 6, and beads inside and outside the pipe were removed by a bead removing device 14 to obtain a steel pipe 8. This steel pipe is drawn 21
The product was drawn and rolled under the conditions shown in Table 2 to obtain a product tube 16. Further, the seam portion and the base material portion of the product pipe 16 are arranged in the longitudinal direction.
JIS No. 12 A tensile test pieces were cut out and examined for strength, elongation, and r value. The results are also shown in Table 2.

[0034]

[Table 1]

[0035]

[Table 2]

[0036]

[Table 3]

The examples of the present invention were excellent in ductility and r value. Example 2 A product pipe was manufactured using the manufacturing equipment shown in FIG. 2 under the following conditions. After the steel strip 1 having the composition shown in Table 1 was heated by the steel strip heating device 2 under the conditions shown in Table 3, it was formed by the forming device 3 to obtain an open pipe 7. The edge of the open pipe is heated to a temperature equal to or higher than the melting point by the edge heating device 5 and rolled and welded with the squeeze roll 6.
Further, the beads inside and outside the pipe were removed by a bead removing device 14 to obtain a steel pipe 8, which was cut into a predetermined length and cooled. The steel pipe was reheated by the steel pipe heating device 24 (the reheating temperature is the same as the drawing rolling entry side temperature in Table 3), and then drawn by the drawing device 21 under the conditions shown in Table 3 to obtain a product tube 16. Further, JIS No. 12A tensile test pieces in the longitudinal direction were cut out of the seam portion and the base material portion of the product tube 16, and the strength, elongation, and r value were investigated. The results are also shown in Table 3.

[0038]

[Table 4]

The examples of the present invention were excellent in ductility and r value.

[0040]

According to the present invention, a steel pipe excellent in workability can be manufactured. Further, according to the present invention, it is possible to obtain excellent characteristics which cannot be obtained by the conventional method for manufacturing an electric resistance welded steel pipe, and to use the pipe as a high strength steel pipe for a vehicle body undercarriage, which has been in demand in recent years. Can be. Furthermore, according to the present invention, the reheating reduces the forming load of the open tube, and also enables high-speed drawing and rolling, thereby improving productivity. It can be used for production.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing an example of a steel pipe manufacturing facility line suitable for carrying out the present invention [1] and [2].

FIG. 2 is an explanatory view showing one example of a steel pipe production facility line suitable for carrying out the present invention [3].

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Strip steel 2 Strip steel heating device 3 Forming processing device 5 Edge heating device 6 Squeeze roll (pressure welding device) 7 Open pipe 8 Steel pipe 11 Uncoiler 12 Strip steel joining device (relay welding machine) 14 Bead removal device 16 Product pipe 17 Looper (Accumulator) 18 Cutting machine 19 Tube straightening device 20 Thermometer 21 Drawing device 22 Steel tube heating and soaking device 23 Latest cooling device 24 Steel tube heating device

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yoshikazu Kawabata 1-1, Kawasaki-cho, Handa-shi, Aichi Prefecture Inside the Chita Works of Kawasaki Steel Works (72) Inventor Masanori Nishimori 1-1-1, Kawasaki-cho, Handa-shi, Aichi Prefecture Kawasaki (72) Inventor Motoaki Itaya 1-1-1, Kawasaki-cho, Handa-shi, Aichi Prefecture In-house Chita Works, Kawasaki Steel (72) Inventor Nochika Okabe 1-1-1, Kawasaki-cho, Handa-city, Aichi Prefecture Address: Chita Works, Kawasaki Steel Co., Ltd. Address F-term in Kawasaki Iron Works Chita Works (reference) 4E028 CA02 CA13 CA18 LA03

Claims (3)

[Claims] 1. After heating a steel strip to an Ac ₃ transformation point or higher, the steel strip is continuously formed by a forming roll into an open pipe, both edges of the open pipe are heated, and abutted and joined by a squeeze roll. After the bead is removed to form a mother pipe, in a method for producing a steel pipe which is further subjected to reduction rolling, the reduction rolling is carried out by 40%.
30% reduction in the temperature range from 0 ℃ to Ar ₃ transformation point + less than 50 ℃
%. 2. After completion of the reduction rolling, the temperature is reduced to 30 ° C. within 0.5 seconds.
2. The cooling method according to claim 1, wherein cooling is performed at a cooling rate of at least / s.
3. The method for producing a steel pipe according to claim 1. 3. The mother pipe is cooled after removing the inner and outer beads,
_{3. The} method according to claim 1, wherein the drawing is performed by reheating to a temperature range of less than 400.degree.
3. The method for producing a steel pipe according to claim 1.