JPH0331523B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for manufacturing high-frequency bent pipes used in piping systems for power generation and chemical plants. (Conventional technology) 1.0 conventionally used for power generation and chemical plants
Welded pipe fittings (hereinafter referred to as elbows) specified in Japanese Industrial Standards JIS B2311, 2312, etc. are used as carbon steel bent pipes with a small bending radius of ~2.0 x (outer diameter of the pipe).
is used. The processing of the elbow is a high-temperature pipe expansion bending process known as Hamburg processing, which is published in the 3rd edition of the Steel Handbook compiled by the Japan Iron and Steel Institute, page 179.In terms of materials, the raw pipe and the bent pipe have almost the same properties. has. However, the product shape is
Since the product only has a bent part with a bending angle of up to 180 degrees, the number of welding steps required during piping construction increases.In addition, the number of steps required for inspecting the welded portion also increases, lengthening the construction period and increasing construction costs. There is also the problem that the cost is also high. 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 method for manufacturing elbow braces is cold bending, but when trying to obtain an elbow brace with a small bending radius, it is not suitable for practical use because it does not satisfy the dimensional tolerance of the aspect ratio among the cross-sectional shape and dimensional tolerances of the bent part. becomes. 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, in the case of a small bending radius of 1.0 x 3.0 x (outer diameter of the tube) as required, forced cooling is required to further narrow the heated bending area to prevent buckling. Therefore, the cooling rate of the outer surface layer of the bent portion (up to 1 to 2 mm below the surface) increases, and hardening of the outer surface layer of the bent portion is unavoidable. Normally, in pipe welding of carbon steel pipes, Hv248 is the highest hardness that can be used practically from the viewpoint of safety against fracture.
Although the following is desired, there is a problem in that within the above-mentioned component range, this hardness regulation cannot be satisfied by high-frequency bending as it is. One effective solution to this problem is softening annealing, but it requires a large furnace due to the shape of the elbow, and has low productivity and high cost. On the other hand, as shown in Japanese Patent Application No. 62-7862 previously filed by the present applicants, carbon steel that satisfies C+Mn/6≦0.33% is used as a raw tube, and the outer surface of the raw tube is 850% There is a method to obtain a low-hardness elbow brace with high-frequency bending by heating it to ~950°C and bending it while cooling with water. However, even with this method, if the wall thickness of the raw tube is large, even if the temperature of the outer surface of the tube is heated to the upper limit of 950℃, the temperature of the inner surface of the tube is low and the structure after bending cannot be stabilized. However, if you try to stabilize the structure, the temperature of the outer surface of the tube will exceed 950℃, which increases the hardness of the outer surface of the tube after bending. It is difficult to obtain such quality. (Problems to be Solved by the Invention) That is, the present invention overcomes these conventional drawbacks, and uses high-frequency bending to produce a thick-walled steel pipe with mechanical properties on the order of ASTM A106B class with a small bending radius and The purpose of this invention is to provide a method for manufacturing an inexpensive elbow brace that is Hv248 or less in the as-processed state. (Means for solving the problem) As a result of investigating the influence of various components and the influence of high frequency bending processing conditions on improving the hardness of the outer surface layer, the inventors found that the outer surface temperature of the raw pipe exceeded 950℃. ℃
Even at the following heating temperatures, the steel pipe of the present invention has the formula C+Mn/6+(Cr+Mo+
V)/5+(Ni+Cu)/15 (hereinafter referred to as C equivalent)
It was found that there is a good correlation between the C equivalent given by and the maximum hardness after bending. The present invention has been made based on the above knowledge, and its gist is as follows: (1) In a method for manufacturing a high-frequency bent pipe whose radius of curvature is three times or less the outside diameter of the pipe, in weight%, C0 .05～0.30% Si0.10～0.50% Mn0.30～1.50% P≦0.03% S≦0.03% Al0.005～0.05% N≦0.015% The remainder consists of Fe and inevitable impurities, C
+Mn/6+(Cr+Mo+V)/5+(Ni+
Carbon steel that satisfies Cu)/5≦0.38% is used as a raw tube, heated until the outer surface of the raw tube reaches 850 to 1080℃, and then bent while cooling with water. Manufacturing method of hard high frequency bent pipe. (2) In the manufacturing method of high-frequency bent pipes with a radius of curvature less than three times the outer diameter of the pipe, in weight%: C0.05-0.30% Si0.10-0.50% Mn0.30-1.50% P≦0.03% S≦ 0.03% Al0.005~0.05% N≦0.015% and Cr≦0.50% Ni≦0.50% Mo≦0.50% V≦0.10% Ti≦0.05% Nb≦0.05% B≦0.003% Cu≦0.50% Zr≦0.05% One or more of Ca≦0.005%, the remainder consisting of Fe and unavoidable impurities, C+Mn/6+(Cr+Mo+V)/5+(Ni+
Carbon steel that satisfies Cu)/15≦0.38% is used as a raw tube, heated until the outer surface of the raw tube reaches 850 to 1050℃, and then bent while cooling with water. The hardness lies in the manufacturing method of high-frequency bent pipes. In the present invention, the carbon steel pipe used as the base pipe of the bent pipe is preferably a seamless steel pipe, but electric resistance welded steel pipes, UO steel pipes, etc. can also be used. (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 stand; ,
The arrow 7 indicates the cooling water sprayed from the heating device 3, and the shaded area 8 indicates the heating processing area. The device tightens the tip of the steel pipe 1 with a clamp 5, heats the steel pipe 1 locally at a high temperature with a heating device 3, and propels the steel pipe 1 in the direction of the arrow by an appropriate means.
It is now possible to bend. 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. Therefore, 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. When the wall thickness of the pipe is 12.5mm to 50.4mm,
In order to stabilize the structure, it is necessary to operate the heating temperature range of the outer surface from 950 to 1050°C. That is,
The minimum heating temperature is to set the heating temperature of the inner surface to A _c3 or higher. If the wall thickness of the tube is less than 12.2 mm, the heating temperature range of the outer surface can be operated at 850-950°C. When manufacturing a high-frequency bent pipe with a small bending radius, it is necessary to forcibly apply a sufficient axial compressive force to reduce the wall thickness of the bent portion subjected to tensile processing. At this time, if the width of the heating process becomes wide, wrinkles will occur on the inside after the bending process starts, and furthermore, buckling will occur, making it impossible to process. Therefore, in order to narrow the heating processing width as much as possible, spray water cooling, which has a cooling effect rather than forced air cooling, is performed immediately after heating processing, and this water cooling makes the deformation resistance in the water cooling area much larger than the deformation resistance in the heating processing area. , which attempts to solve the above problems. This spray water cooling has a slower cooling rate than the conventional forced water cooling for the purpose of hardening. 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 the strength of steel pipes, and the amount necessary to perform its function is 0.05% or more. However, if the C content exceeds 0.30%, the welding workability of the steel pipe will be significantly impaired and the hardness after top bending will become significantly high, so the upper limit was set at 0.30%. Si is an element that is used as a deoxidizing element and is effective in securing external high temperature strength, and at least 0.10% or more is required to exhibit the deoxidizing function. but,
Since a large amount of content impairs high frequency bending property, the upper limit was set at 0.50%. Mn is the most effective reinforcing element next to C, and the amount required to exhibit its function is 0.30% or more. However, excessive addition increases the maximum hardness after bending and impairs welding workability, so the upper limit is set at 1.50%.
regulated. P and S are elements mixed as impurities,
To prevent cracking during high-temperature bending, the upper limit of each content was regulated to 0.03% or less. Al is an effective element as a grain refining element,
By this addition, it was possible to increase the upper limit contents of C, Si, and Mn. In order to exhibit this function, it is necessary to contain 0.005% or more, but
Since a large amount of content impairs the toughness of the steel pipe, the content was set at 0.050%. The upper limit of N was set at 0.015%, which does not cause any problem in weldability. In addition to the above basic ingredients, the following regulations have been established for selectively used ingredients. Nb, Ti, and Zr are all elements that have a crystal grain refining effect, but the upper limit was set to 0.05% of each addition amount so that the effect would not be saturated. V has a grain refining and reinforcing effect, but if it exceeds 0.10%, the effect is saturated, so 0.10% is set as the upper limit. Cr, Mo, Ni, and Cu are effective as reinforcing elements, but adding a large amount of any of them increases the hardness after bending, so the upper limit of each was regulated to 0.50%. Ca contributes to improving toughness by controlling the form of sulfides, but excessive addition causes deterioration of weldability, so the upper limit was set at 0.005%. B has the effect of increasing the strength of steel pipes when added in a small amount, but the effect is saturated when added in a large amount exceeding 0.003%, so the upper limit was set at 0.003%. The upper limit of C equivalent is the maximum hardness of the outer surface side after bending under the conditions of high frequency bending and C
Since it shows a good correlation with the equivalent weight, it was set to 0.38% or less in order to keep Hv248 or less. Next, the characteristics of the steel pipe of the present invention will be shown in Examples in comparison with comparative materials. (Example) Table 1 shows the chemical composition of the steel pipes subjected to bending. A1 to A8 steel pipes are comparison steel pipes, which are as-rolled carbon steel pipes with a C equivalent of over 0.38%, and B1 to B12 steel pipes are steel pipes of the present invention, which have a C equivalent of more than 0.38%.
It is as-rolled steel pipe with a content of 0.38% or less.

【table】

[Table] Table 2 shows the mechanical properties and bending conditions of the raw steel pipes in Table 1, and the maximum hardness of the bent portion after bending.

【table】

[Table] For mechanical properties, JIS No. 12 test pieces were used.
Tensile strength and elongation were measured. As shown in Table 2, the maximum heating temperature of all test tubes was 850 to 1050°C, and high-frequency bending was performed using spray water cooling. The relationship between the C equivalent and the maximum hardness when bending is performed under these conditions is
As shown in the figure. As is clear from Figure 1, the C equivalent is
It is clear that the comparison steel pipes of A1 to A8 steel pipes with a hardness of more than 0.38% all have a maximum hardness of Hv248 or more and cannot be put to practical use. In contrast, the steel pipe used in the present invention
By setting the C equivalent to 0.38% or less, B1 to B12 steel pipes make it possible to manufacture high-frequency bent pipes that satisfy a maximum hardness of Hv248 or less. (Effects of the Invention) According to the present invention, bent pipes with a small bending radius for power generation and chemical plants conventionally configured with elbows can be bent.
It is possible to manufacture elbowless pipes by high-frequency bending using seamless steel pipes or steel pipes manufactured by other manufacturing methods as raw pipes, 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 the C equivalent and the maximum hardness Hv after bending, and the bending conditions at this time were that the maximum heating temperature of the outer surface was 850 to 1050°C, and spray water cooling. FIG. 2 is a plan view of an example of a bending machine used in an embodiment of the present invention. DESCRIPTION OF SYMBOLS 1... Steel pipe, 2... Guide roller, 3... Heating device, 4... Bending arm, 5... Clamp, 6...
...Pipe end support stand, 7...Spray cooling water, 8...
Heat processing area.

Claims (1)

[Claims] 1. A method for manufacturing a high-frequency bent pipe whose radius of curvature is three times or less the outside diameter of the pipe, in terms of weight %: C0.05-0.30% Si0.10-0.50% Mn0.30-1.50% P≦ 0.03% S≦0.03% Al0.005~0.05% N≦0.015% The remainder consists of Fe and inevitable impurities, C+
Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15
≦0.38% carbon steel is used as the raw tube, and the outer surface of the raw tube is heated to 850 to 1050℃, and then bent while cooling with water. Method for manufacturing bent pipes. 2. In the method of manufacturing high-frequency bent pipes with a radius of curvature less than three times the outer diameter of the pipe, in weight%: C0.05-0.30% Si0.10-0.50% Mn0.30-1.50% P≦0.03% S≦0.03% Al0.005～0.05% N≦0.015% and Cr≦0.50% Ni≦0.50% Mo≦0.50% V≦0.10% Ti≦0.05% Nb≦0.05% B≦0.003% Cu≦0.50% Zr≦0.05% Ca≦ 0.005% of one or more of the following, the remainder consisting of Fe and unavoidable impurities, C+Mn/6+(Cr+Mo+V)/5+(Ni+
The raw pipe is made of carbon steel that satisfies Cu)/15≦0.38%,
Heat until the outer surface of the raw tube reaches 850-1050℃,
A method for manufacturing a high-frequency bent pipe with low hardness as it is processed, characterized by bending it while cooling with water.