JPH06218431A - Method for straightening steel tube at worm state - Google Patents

Abstract

PURPOSE:To effectively straighten a bend as a cruck is prevented from occurring in a steel tube by heating the steel tube at the entrance of a rotary straightener to a specific temperature through an induction heating device. CONSTITUTION:A cold worked steel tube 2 of eutectoid steel or hyper-eutectoid steel containing 0.4wt.% C is fed from an inserting table 1 to an induction heating device 4 through an entrance trough 3. The steel tube 2 is heated at 70-200 deg.C and the bend is straightened by a straightener body 5 of rotary straightener method. Thereafter, the tube is air-cooled by a cooling table 6 and receives non-destructive inspection. When the temperature of the heated material exceeds 200 deg.C, it is heattreated actually and the metallic structure can not hold a state of cold working. The impact fracture transition temperature is 70 deg.C and since the steel tube at 70 deg.C is controlled by a condition similar to the normal temperature working, crucking by straightener correction is apt to occur.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of straightening a steel pipe using a rotary straightener.

[0002]

2. Description of the Related Art Some steel pipes for machine structures are required to be cold-worked with good material rigidity in order to ensure machinability. Eutectoid steel is often used. However, since these steels have a high carbon content, they have high strength and low toughness when they are cold-worked, and brittle cracks are likely to occur during bending straightening after cold-working. Therefore, it is necessary to take measures to prevent cracks in the bending correction process.

As a measure for preventing the occurrence of cracks when straightening the bend of a steel pipe, the steel pipe is heated under external heating,
Alternatively, it is effective to correct the steel pipe after it has been heated in the previous step and has not yet been sufficiently cooled. The methods include (1) a method that utilizes the processing heat during cold drawing, (2) a method that utilizes the residual heat immediately after heat treatment, and (3) a method that involves heating the steel pipe by immersing it in a heating liquid. .

On the other hand, after straightening, a non-destructive inspection is often carried out in the line, and it is necessary to cool the steel pipe after straightening in order to prevent damage to the inspection equipment and deterioration of inspection accuracy due to the heat of the steel pipe. .

In the above methods (1) and (2), since the entire steel pipe is in a considerably heated state, a cooling device having a large cooling capacity is required during the subsequent cooling, or the cycle time is shortened. become longer.

The method (2) can be used only when a heat-treated material is used. For this reason, the methods (1) and (2) are both difficult to apply as appropriate heating methods before correction.

In the method (3), it is possible to heat only the vicinity of the surface of the pipe by controlling the liquid temperature and the dipping time, which is effective as a crack prevention measure when using the straightener. In addition, the heat retained by the steel pipe is small compared to methods (1) and (2), so cooling after straightening is also easy. However, there is a concern that the steel pipe may be corroded by the liquid, and even if a liquid having an anticorrosive effect is used, it is necessary to dry it. Especially, when it is applied to a long-diameter material having a small diameter, there are many problems from this drying point.

Japanese Unexamined Patent Publication (Kokai) No. 58-116932 discloses a method for straightening a steel pipe in which a large-diameter steel pipe is conveyed in a spiral shape, heated in a induction heating furnace to a specific temperature, and then straightened by rollers. An apparatus is shown, which is for increasing the roundness and residual stress relief annealing by heating below the Ac ₁ point.

In Japanese Patent Laid-Open No. 58-202915, when straight and straight bending of a tube rod material after cold drawing is performed, this straightening is performed at a brittle transition temperature of the material or above, and annealing and re-annealing are performed. A method of manufacturing a tube rod material which omits a lubrication process is shown. However, in this method, even if a line is drawn between the completion of the drawing process and the straightening of the bend, the material temperature will drop during cooling,
It is not always possible to straighten a bend above the brittle transition temperature. Moreover, it is necessary to line up the drawing process and the bending straightening machine, and the equipment cost is high. In addition, when the material to be processed having a large amount of bending after the drawing process cannot be loaded into the straightening machine, there is a problem that the material temperature is high and thus the hand processing property is poor.

In Japanese Patent Laid-Open No. 4-111926, warm air is sent to the inside of a steel pipe from the pipe end to heat the vicinity of the inner surface to 400 ° C. or below, thereby warming the steel pipe with a straightener. The law is shown. However, the steel pipe
Since heating up to near ℃ is equivalent to heat treatment in the case of a material that should be used as it is in cold working, this method cannot be applied to a material that has a risk of blue-heat embrittlement. Furthermore, this method requires a warm air generator and a suction device for environmental protection.

[0011]

DISCLOSURE OF THE INVENTION An object of the present invention is to provide a simple, warm-working warm pipe for cold-worked steel pipe produced from eutectoid or hypereutectoid steel. To provide a correction method.

[0012]

The gist of the present invention lies in the following method.

When a steel pipe of eutectoid or hypereutectoid steel having a C content of 0.4% or more is warm-corrected by using a rotary straightener, an induction heating device is used to remove the steel pipe at the inlet of the rotary straightener. Heating temperature 70 ℃ or more, 2
A warm straightening method for steel pipes, which is characterized by keeping the temperature below 00 ° C.

The inventor of the present invention is intended for a material to be used as it is as cold-worked, and although the above steel is a low brittle material, its ductile brittle transition temperature is 100 ° C. at most.
Since it is before and after, we focused on induction heating devices that do not require large-scale equipment installation, and examined appropriate crack prevention solutions.

FIG. 1 is a schematic plan view showing the structure of an apparatus for carrying out the method of the present invention. In the drawing, 1 is a charging table, 2 is a steel pipe (as cold working), 3 is an inlet trough, 4 is an induction heating device, 5 is a rotary straightener type straightener main body, and 6 is a cooling table. .

[0016]

As shown in FIG. 1, the cold-worked steel pipe 2 is heated by the induction heating device 4 from the charging table 1 through the inlet trough 3.
And is then heated to the above-mentioned predetermined temperature, and the bending straightening is performed by the rotary straightener type straightening machine main body 5. Then, it is air-cooled on the cooling table 6 and undergoes a nondestructive inspection.

The induction heating device 4 is used in the method of the present invention for the following reasons.

Rapid heating is possible, and heating can be performed without impairing the efficiency of the conditioning equipment.

There is little variation in heating temperature due to current control or the like.

On-line processing is possible, and there is no setup change such as coil change.

The reason why the material heating temperature of the eutectoid or hyper-eutectoid steel having a C content of 0.4% or more is set to 200 ° C. or less is the same as the fact that heat treatment is practically performed when this temperature is exceeded. This is because the material strength after warming and the metal structure cannot be maintained in the same state as they were during cold working. Further, this is to prevent an increase in equipment cost and an increase in equipment space of the induction heating device.

The lower limit of heating temperature is set to 70 ° C. because the impact fracture transition temperature in the above eutectoid or hypereutectoid steel is 70 ° C.
This is because cracking is likely to occur due to straightener straightening.

[0023]

[Example] A steel pipe having a chemical composition corresponding to SUJ 2 specified in JIS G 4805 shown in Table 1 was cold drawn (drawing reduction ratio 36
%), Diameter 35.85mm, wall thickness 3.85mm, length 25000
mm tube. These steel pipes were warm-corrected under the conditions shown in Table 2.

The induction heating device has an output frequency of 500 Hz, an output power of 260 kW, an inverter output of 200 kW, and a coil inner diameter of 1
A coil with a length of 00 mm and a coil length of 400 mm was used.

The bending straightening machine is of the opposed type (2-2-2
The rotary straightener of 1) was used. Straightener crash setup is constant for No. 1 roll and No. 3 roll (No. 1 crash: 0
mm, No. 3 crush: 1.6 mm), and No. 2 roll has a crush of 1.9 mm as shown in Table 2.
(Normal setup) and 2.5mm. The No. 2 roll has an offset of 16.0 mm, and the feed speed of the processed material is 30 m / mi.
A constant condition of n was set. The material temperature was measured using a contact thermometer.

With respect to the steel pipe thus obtained, the presence or absence of cracks on the inner and outer surfaces of the steel pipe and the hardness of the steel pipe before and after correction were examined by eddy current flaw detection and ultrasonic flaw detection. A sample cut from the rear end of the tube was used as a sample for hardness measurement, and the hardness was measured by a four-point method at the center of the cross-section wall thickness by HRB. These results are shown in Table 2.
Are also shown.

As is apparent from Table 2, in the example of the present invention,
In contrast to the comparative example in which the heating conditions are outside the range defined by the present invention,
No cracking or low cracking rate.

[0028]

[Table 1]

[0029]

[Table 2]

[0030]

According to the method of the present invention, the C content is 0.4
% Of the eutectoid or hyper-eutectoid steel is cold-worked, and when straightening the bend with a rotary straightener, it is possible to prevent the occurrence of cracks in the pipe and perform effective straightening. it can.

[Brief description of drawings]

FIG. 1 is a schematic plan view showing the structure of an apparatus for carrying out the method of the present invention.

[Explanation of symbols]

1: Charging table, 2: Steel pipe, 3: Entrance trough, 4:
Induction heating device, 5: Rotary straightener type straightener main body, 6: Cooling table

Claims (1)

[Claims] 1. When using a rotary straightener to warm-correct a cold-worked steel tube of eutectoid or hypereutectoid steel having a C content of 0.4% or more, an induction heating device is used. The heating temperature of the steel pipe at the inlet of the rotary straightener
A warm straightening method for steel pipes characterized in that the temperature is 70 ° C or higher and 200 ° C or lower.