JPS63317217A - Production of high frequency bending pipe of low hardness as it is worked - Google Patents

Abstract

PURPOSE:To manufacture the elbowless of low hardness as it is worked with small radius by locally heating the bending part of the carbon steel blank pipe contg. C, Si and Mn at specified weight within the specified temp. range and executing high frequency bending with cooling it slowly. CONSTITUTION:The carbon steel blank pipe 1 whose contg. component wt.% are 0.05-0.25% C, 0.10-0.35% Si and 0.30-1.00% Mn and whose radius of curvature is less than three times the pipe outer diameter is supported by a pipe end supporting base 6, passed through a high frequency heating coil 3 with the guidance of a guide roller 2, the tip is bent and held by the clamp of a bending arm 4. The external surface of the heating part of a advancing blank pipe 1 is heated up to 850-950 deg.C by the heating coil 3 and the inner surface is made at the temp. capable of plastic working of more than Ac3 and not coarse grained. This local heating part is subjected to bending by the bending arm 4 with slowly cooling it at about <=50 deg.C/sec cooling speed by a water jet 7.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for manufacturing high-frequency bent pipes used in piping systems for power generation, chemical plants, and the like.

[Conventional technology] Conventionally, the percentage by weight used for power generation, chemical plants, etc.
Below, Mn 1. ASTM below OH
'06B class 1.0~2. Welded pipe joints (hereinafter referred to as elbows) specified in Japanese Industrial Standards JIS B 2311 and 2312 are used as carbon steel bent pipes having a small bending radius of OX (outer diameter of the pipe). For elbow processing, refer to the 3rd edition of the Steel Handbook compiled by the Japan Iron and Steel Institute, VI P.
179 is a tube expansion bending process at a high temperature, commonly known as hangrug process, and in terms of materials, the raw tube and the bent tube have almost the same properties. However, since the product shape is only a bent part with a zero bending angle up to 180 degrees, the number of welding steps increases during piping construction. There is a problem that the length of time is longer and construction costs are higher.

Therefore, there is a demand for a bent pipe (hereinafter referred to as "elbowless") with fewer welded parts, that is, a bent pipe having straight pipe parts at both ends of the bent part.

One of the methods for manufacturing elbow braces is cold cracking, but when trying to obtain an elbow brace with a small bending radius, the dimensional tolerance of the aspect ratio among the cross-sectional shape dimensional tolerances of the bent part is not satisfied, so it is not suitable for practical use. It becomes something that does not exist.

Another method is high-frequency bending known from, for example, Japanese Patent Laid-Open No. 53-135870 and Japanese Patent Laid-Open No. 53-135871. With this processing method, if the bending radius exceeds 3 x (outer diameter of the pipe), the heated bending area is narrowed by forced air cooling, which allows processing without buckling, so there are no problems with the material of the bent part. do not have. However, as desired 1.0-3. In the case of a small bending radius of OX (outer diameter of the pipe), cooling is required to further narrow the heated bending area to prevent buckling. Therefore, if the cooling rate of the outer surface layer of the bent portion (up to 1 to 2 mm below the surface) becomes too fast, hardening of the outer surface layer of the bent portion is unavoidable. Usually, in pipe welding of carbon steel pipes, Hv is the highest hardness that can be used in practice from the viewpoint of safety against fracture.
Although a hardness of 248 or less is desired, there is a problem that this hardness regulation cannot be satisfied if high-frequency bending is performed in the above-mentioned component range. Softening annealing is an effective way to solve this problem, but due to the shape of the elbowless, a large furnace is required, and the productivity is poor and the cost is high. Therefore, elbowless with high-frequency bending is not practical. It had not been done.

[Problems to be solved by the invention] That is, the present invention overcomes these conventional drawbacks, and A.
The purpose of this invention is to provide a method for manufacturing an inexpensive elbowless steel tube with a small bending radius by high-frequency bending, and Hv248 or less in the as-processed state, from a steel pipe having mechanical properties on the order of STM A106B class.

[Means for solving the problem] As a result of investigating the influence of high-frequency bending conditions on improving the hardness of the outer surface layer, the present inventors found that the cooling rate during bending and the bending process are as shown in Fig. 1. We obtained the knowledge that there is a good correlation with the maximum hardness.

The present invention has been made based on the above-mentioned findings, and the gist thereof is to provide a method for manufacturing a high-frequency bent pipe with a radius of curvature of 3 times or less the outside diameter of the pipe, in which the weight percentage is GO, 05 to 0. .25% St O, 10~0.35% Mn 0.30~1.0G% The balance consists of Fe and impurities as a raw tube made of carbon steel, and heated until the outer surface of the raw tube reaches 850~950°C. , a method for manufacturing a high-frequency bent pipe having low hardness as processed, characterized by bending the pipe while cooling it with a slowly cooling liquid.

[Function] Fig. 2 shows an example of a pipe bending device used to carry out the bending method of the present invention, in which 1 is a steel pipe to be bent, 2 is a guide roller that supports and guides the steel pipe, and 3 is an annular pipe. , a heating device consisting of a high-frequency inductor that locally heats the steel pipe 1 from its outer periphery in a narrow width and has a cooling tank; 4 is a rotatable bending arm equipped with a clamp 5 at the tip; 6 is a tube end support The arrow on the table 7 indicates the slow cooling liquid sprayed from the heating device 3, and the shaded area 8 indicates the heating processing area. The device is capable of bending the steel pipe 1 by tightening the distal end of the steel pipe 1 with a clamp 5, heating the steel pipe 1 locally to a high temperature with a heating device 3, and propelling it in the direction of the arrow with an appropriate means.

The reasons for the limitations of the present invention will be explained below.

First, regarding bending conditions, under normal bending conditions, when heating with a high-frequency heating coil, a coil as narrow as possible is used in order to narrow the heating bending area. For this reason, it is difficult to uniformly heat the entire wall thickness, and the temperature of the inner surface is lower than that of the outer surface.In order to stabilize the structure, it is necessary to operate within the heating temperature range of 850 to 950 degrees Celsius for the outer surface. . That is, the minimum heating temperature is to set the inner surface heating temperature AC3 or higher, and the maximum heating temperature is to prevent the structure of the outer surface from becoming coarse.

When manufacturing a high-frequency bent pipe with a small bending radius, if the heating processing width where the bending moment acts becomes wide, wrinkles and buckling will occur on the inside of the bent pipe, making processing impossible.

Therefore, if forced air cooling similar to normalizing is performed immediately after heating processing, an increase in hardness can be prevented, but a bent pipe with a smooth shape cannot be manufactured.

Therefore, in order to narrow the width of heating processing, the above-mentioned problems are solved by controlling the cooling rate from the viewpoint of shape and hardness by spraying a slowly cooling liquid.

In other words, when the cooling rate of the outer surface of the raw pipe is below 5°C, wrinkles will occur more often on the inside during bending, and near the upper limit of the component system, as the cooling rate increases, the hardness will increase. Preferably, the temperature is 50°C or higher.

The slow cooling liquid used in the present invention includes PVA (polyvinyl alcohol), PVP (polyvinyl pyrrolidone), PG^ (polyalkylene glycol),
Examples include water-soluble boria such as PEG (polyethylene glycol). The same effect can also be obtained by heating fats and oils such as mineral oil, vegetable oil, and animal oil, as well as by heating water.

Next, the reason for limiting the components of the raw pipe used in the present invention will be described. C is an element necessary to ensure mechanical properties, and the amount necessary to exhibit its function is 0.05 or more. However, since increasing the C content impairs the welding workability of steel pipes, the condition O:25* was set.

Mn is also an element necessary to ensure mechanical properties,
The amount required to perform its function is 0.30 or more. However, excessive content impairs welding workability and excessively increases hardenability, so the upper limit was set as t,oox.

St is a necessary element as a deoxidizing element, and must be contained at least 0.10* in order to exhibit at least the deoxidizing function. It is also an element that affects mechanical properties and hardenability, albeit to a small extent.

However, since a large amount of content impairs high frequency bending workability, the upper limit was set at 0.35 zero.

Next, the features of the present invention will be illustrated with examples in which the cooling rate is varied.

[Example] Table 1 shows three types of steel pipes subjected to high-frequency bending with a bending radius of 2D while changing the cooling rate to such conditions.

The cooling rate is 800~4 by fusing the thermocouple to the outer surface of the steel pipe.
Measurements were made between 00°C.

Among these, those whose cooling rate exceeds 50℃/S (C, F
, G) has a maximum hardness of lv24B or more, and it is clear that it cannot be put to practical use.

In contrast, the present invention (like D, E, H, I, and J) produces a high-frequency bent pipe that satisfies the maximum hardness of 1v248 or less by setting the cooling rate to 50°C/S or less. This is what made it possible.

[Effects of the Invention] Therefore, according to the present invention, bent pipes with a small bending radius for use in power generation and chemical plants, which are conventionally constructed with elbows, can be replaced with seamless steel pipes or steel pipes manufactured by other manufacturing methods. It becomes possible to manufacture elbowless pipes by high-frequency bending, which has the advantages of significant cost reduction and low hardness, which contributes to safety.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between cooling rate and maximum hardness Hv during high-frequency bending, and FIG. 2 is a plan view of an example of a bending machine used in carrying out the present invention. 1... Steel pipe 2... Guide roller 3...
・Heating device 4...Bending arm 5...Clamp 6...Pipe end support stand 7...Slow cooling liquid spray 8...Heating processing area

Claims (1)

[Claims] A method for manufacturing a high-frequency bent carbon steel pipe having a radius of curvature of 3 times or less the outer diameter of the pipe, comprising: C 0.05 to 0.25% Si 0.10 to 0.35% Mn in weight% 0.30~1.00% The balance is made of Fe and impurities. Carbon steel is used as a raw tube, heated until the outer surface of the raw tube reaches 850~950℃, and then bent while being cooled with a slow cooling liquid. A method for producing high-frequency bent pipes with low hardness as processed, characterized by: