JPS63313615A - Production of high frequency bent pipe - Google Patents

Abstract

PURPOSE:To reduce piping cost by heating and bending a base steel pipe at a specific temperature zone and immediately after that, cooling the outer surface of the steel pipe by water from a fixed upper limit to a special lower limit of temperature. CONSTITUTION:The point of a steel pipe 1 to be bent is tightened to a clamp 5 and the steel pipe 1 is heated by a heater 3 at a temperature area from the AC3 transformation point or over to the austenite grain coarsening temperature or below to carry out a high frequency bending. Immediately after the bending is performed, the steel pipe 1 begins to be cooled in sprayed cooling water 7. The upper limit of temperature is given by a temperature by 100 deg.C lower than the bending temperature and the lower limit of temperature is a temperature given by a formula on thickness (mm) or shown by Tc=450-30Xt<1/20> deg.C. In this manner, bending operation is simplified and buckling is prevented in bending a steel pipe to a small radius. Accordingly, since elbowless piping is performed, welding operation is decreased and piping cost can be reduced.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a method for manufacturing high-frequency bent pipes with excellent dimensional accuracy and material properties for use in piping systems for power generation, chemical plants, etc. Involved.

[Prior art] Japanese Industrial Standard JIS B 231 is used as a bending pipe with a small bending radius required for conventional power generation, chemical plants, etc.
A welded pipe joint (hereinafter referred to as an elbow) specified in 1.2312 etc. is used. Manufacture of elbows is usually carried out by a high temperature tube expansion bending process known as the hanging process described in the 3rd edition Steel Handbook VIP, 179 compiled by the Japan Iron and Steel Institute. The elbow manufactured by this method not only has good dimensional accuracy, but also has material properties that are almost the same as the raw pipe, making it excellent in terms of usability. However, when piping is performed using elbows, there are serious drawbacks in piping construction.

In other words, since an elbow is a steel pipe with only a bent portion up to a maximum bending angle of 120°, it is necessary to weld two parts of each elbow to adjacent steel pipes during piping construction. As a result, the number of welding steps required during piping construction increases, and the number of steps required for inspecting the welded parts also increases, resulting in a disadvantage in terms of construction time and construction costs, which is a major drawback.

As a method to improve the above-mentioned drawbacks of elbows, aiming at reducing the number of welding locations, bent pipes with a straight pipe section at one or both ends of the bent part (hereinafter referred to as elbowless) are proposed.
It was hoped that this would be realized.

A cold target method is considered as a method for manufacturing elbow braces, but in addition to those with a large bending radius, the bending pipe with a small radius, which is the subject of the present invention, has the aspect ratio of the dimensional tolerance of the cross-sectional shape of the bent part. The tolerances could not be satisfied and it was not possible to obtain a product suitable for practical use.

On the other hand, regarding the manufacture of elbow braces using the high-frequency bending method, for example, Japanese Patent Laid-Open No. 53-135870 and Japanese Patent Laid-open No. 5
A method according to Japanese Patent No. 3-135871 has been proposed.

These methods perform forced air cooling after bending to prevent buckling, and are effective for manufacturing elbowless pipes when the bending radius is more than three times the outer diameter of the raw pipe. However, with this method, the bending radius is 3
When attempting to manufacture small bent pipes of twice the size or less, buckling occurs, making it difficult to secure the shape, and it has not been put to practical use.
On the other hand, in the high-frequency bending method, it is possible to prevent buckling by water cooling immediately after the bending process, but in this case, the hardness after the bending process, especially at the surface, becomes extremely high, so it is necessary to Another problem occurs in that heat treatment such as returning or softening annealing is required, resulting in poor productivity and high costs.

Under the above circumstances, elbowless steel pipes that can be used as they are after being subjected to high-frequency bending with a small bending radius have not been put into practical use.

[Problems to be Solved by the Invention] It is an object of the invention to provide a manufacturing method for obtaining an elbow brace that is manufactured by a high-frequency bending method, has good dimensions and shape as it is bent, and does not require heat treatment after processing.

[Means for Solving the Problems] The present inventors ensured the dimensions and shape of the elbow brace after high-frequency bending, and also developed a high-frequency bending process that would not cause hardening to the extent that it would be a problem in use while still being processed. We considered the conditions.

First, the conditions necessary to obtain a bent pipe with good dimensions and shape without buckling when bending a small radius, especially less than three times the outside diameter of the raw pipe, by high-frequency bending are as follows: It is necessary to perform the bending process after heating to a completely austenitized state, and to perform water cooling immediately after the bending process. This water cooling limits the deformation area to only the minute area where the bending process is in progress by making the deformation resistance of the part where the bending process has been completed higher than that of the subsequent part that is currently being bent. For this purpose, it is necessary to water-cool the area immediately after bending as soon as possible to a temperature range that provides a sufficient deformation resistance difference. The present inventors conducted a detailed investigation on these points, and based on this fact, examined the other condition for preventing hardening after bending. The reason for hardening due to water cooling is that the material undergoes hardening during the cooling process (transforms into a martensitic structure), or transforms into a rapidly cooled structure such as bainite even if it does not become hardened. Therefore, in order to prevent hardening, it is necessary to prevent the transformation into quenched structures such as martensite and bainite, or, if these quenched structures are unavoidable, switch from water cooling to air cooling at an appropriate temperature or higher during the cooling process. It is effective to cause auto-tempering during cooling.

Based on the above ideas, we investigated the cooling conditions to minimize hardening and found that either the quenched structure is not generated, or the quenched structure that has been generated is kept at a low hardness by the auto-tempering effect during subsequent cooling. The steel pipe surface temperature T (tl:) at which water cooling must be stopped varies depending on the wall thickness t (mm) of the steel pipe. For thick walled materials, a low temperature range is allowed, but for thin walled materials, water cooling is particularly important. It was found that it was necessary to keep the stopping temperature high. Steel pipe surface temperature T and steel pipe wall thickness t at which this water cooling should be stopped
As a result of quantitatively examining the relationship between

The temperature drop of the steel pipe water-cooled after processing is qualitatively shown in Figure 1. That is, since the steel pipe is cooled from the outer surface while passing through the water cooling zone, the outer surface portion is cooled to a lower temperature than the inner surface side. When the air cools again after passing through the water cooling zone, the temperature on the outer surface side increases once due to the heat retained on the higher temperature inner surface side. This increase continues until the temperature difference between the inner and outer surfaces approaches an equilibrium state, and then eventually the entire wall thickness shifts to a process where it is naturally cooled at a cooling rate commensurate with the thickness. In order to prevent the rapid cooling structure from occurring during cooling, as mentioned above, it is necessary to suppress the cooling rate of the outer surface of the steel pipe while passing through the water cooling zone to a certain level or less on the slow cooling side. - Even if a quenched structure occurs temporarily, in order to cause auto-tempering during subsequent cooling and achieve hardness reduction,
It is necessary that the temperature reached by recuperation after passing through the water cooling zone enters a high temperature range above a certain value. In either case, the outer surface temperature (T) of the steel pipe after passing through the water-cooling zone is an important factor, so the relationship between the water-cooling stop temperature and the maximum hardness was investigated in detail through simulation experiments in the laboratory.

Figure 2 shows an example of the results, in which the influence of the water-cooling stop temperature was investigated using two types of steel pipes with a wall thickness of 10 wm. It can be seen that the maximum hardness measured at a point 1 mm below the surface changes depending on the water cooling stop temperature, and increases significantly as the water cooling stop temperature becomes lower. Although the change in hardness is continuous, one criterion is the water cooling stop temperature (TC
), it has been found that this Tc changes particularly depending on the wall thickness of the vibrator.

The results of investigating the wall thickness dependence of Tc are approximately as follows (1
) could be expressed by the formula.

T c-450-30x FT(t: ) -(1)t
:■ From the above, when manufacturing high-frequency bent pipes with a bending radius of 3 times or less of the diameter of the raw pipe using a raw pipe with a wall thickness of t (IIm), the raw pipe should be austenitized and then bent. The outer surface is cooled with water immediately after processing, and the upper limit is set at a temperature where the outer surface temperature is 100℃ lower than the bending temperature (
It has become clear that a high-frequency bent tube with excellent dimensions and shape and low hardness can be obtained by selecting conditions that allow the tube to exit the water cooling zone within the temperature range with the lower limit of Tc given by formula 1).

The present invention has been made based on the above-mentioned new knowledge, and the gist thereof is to provide a method for manufacturing a high-frequency bent pipe, in which a base steel pipe Ac3
Bending is performed after heating in a temperature range above the transformation point and below the austenite grain coarsening temperature, and immediately after processing, the outer surface temperature of the steel pipe is set to a temperature 100°C5 lower than the bending temperature, and the temperature is determined from equation (1). The present invention provides a method for manufacturing a high frequency bent pipe, characterized by water cooling until the temperature falls within a temperature range having a lower limit of TC.

In addition, in the method of the present invention, seamless steel pipes,
An electric resistance welded steel pipe, a Uo steel pipe, etc. can be used.

[Function] Fig. 3 shows an example of equipment for carrying out bending according to 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 roller that supports the above-mentioned steel pipe. steel pipe 1
4 is a rotatable bending arm equipped with a clamp 5 at the tip; 6 is a tube; On the end support stand, the arrow 7 indicates the cooling water sprayed from the heating coil 3, and the shaded area 8 indicates the heating processing area. The apparatus is capable of bending the steel pipe 1 by tightening the distal end of the steel pipe 1 with a clamp 5, locally heating the steel pipe 1 to a high temperature with a heating device 3, and propelling it in the direction of the arrow with an appropriate means.

Next, the reasons for the limitations of the present invention will be explained.

First, regarding the bending conditions, in order to ensure the working temperature, it is necessary to keep the heating within the temperature range that maintains the austenite phase and does not cause coarsening of crystal grains. The heating temperature that can prevent coarsening of crystal grains varies depending on the steel type and composition, but generally the upper limit is 950 to 1100°C.

It is desirable that water cooling after bending be performed immediately after bending as much as possible; any time delay before water cooling may cause shape defects. However, if water cooling is continued for too long, the hardness will increase, so it is necessary to adjust the feed rate or the length of the water cooling zone so that the material passes through the water cooling zone within an appropriate range. The most important point of the present invention is to ensure that the outer surface temperature falls within a certain temperature range during water cooling after bending. The upper limit of the temperature range is the limit necessary to not damage the dimensions and shape, and is 100% higher than the bending temperature.
℃ lower temperature. The lower limit of the temperature range is Tc-45, which is the limit necessary to keep the hardness after bending low.
By 0-30X FT, the temperature is limited according to the wall thickness t of the raw pipe.

Next, to apply the present invention! The I1 component range will be described. The method of the present invention basically does not require absolute regulations on the applicable steel type components, and is a widely versatile technology.
Here are some examples of appropriate component ranges from the perspective of fields of use that require steel pipes with a special shape: elbowless.

First, C is an element necessary to ensure the strength of steel pipes, but if its content exceeds 0.30%, the hardness after top bending will significantly increase, impairing welding workability, so the upper limit should be set at 0.2%.
5%.

In addition to being used as a deoxidizing element, Si is also an effective element for ensuring high-temperature strength, but since a large amount of Si impairs high-frequency bending properties, the upper limit is set at 0.50%.

Mn is an effective reinforcing element next to C, but excessive addition increases the maximum hardness after bending and also poses a problem in welding, so the upper limit is regulated to 1.50%.

P and S are elements mixed as impurities, but each is regulated to 0.03% or less to prevent cracking during high temperature bending.

The upper limit is set at 0.05% in order to limit the use of A or as a deoxidizing element within the necessary range.

The upper limit of N is 0.015%, which is within a range that does not affect weldability.

In addition to the basic ingredients listed above, the following regulations have been established for selectively used ingredients.

Nb, Ti, and Zr are all elements that have the effect of making crystal grains finer, but the upper limit of the amount of each added is set at 0.05% so that the effect is not saturated.

(2) has a grain refining and reinforcing effect, but if it exceeds 0.10%, the effect is saturated, so it should be kept at 0.1θ% or less.

Cr, Mo, Ni, and (:u) are effective as reinforcing elements, but since adding a large amount of any of them increases the hardness after bending, the upper limit of each was set at 0.50%.

Ca contributes to improving toughness by controlling the form of sulfides, but excessive addition causes deterioration of weldability, so 5
The upper limit is 0 ppm.

B has the effect of increasing the strength of the base material when added in a small amount, but the effect is saturated when added in a large amount exceeding 30 ppm, so the upper limit is set at 30 ppm.

[Example] Table 1 shows the components and bending conditions of the steel pipes subjected to the high frequency bending test. Elbow restraints were manufactured using seven types of test steel pipes under the bending conditions shown in Table 1. As a result of changing the water-cooling stop temperature in three types, we created conditions that fell within the range of the method of the present invention, conditions in which the water-cooling stop temperature deviated to the higher side, and conditions in which the water-cooling stop temperature deviated to the lower side.

These results indicate the conditions that satisfy the method of the present invention with symbols such as O△, those where the water-cooling stop temperature deviates from the high side are indicated by double symbols such as ◎, and those where the water cooling stop temperature deviates from the low side are indicated by black symbols such as M■. Organized by representing them with colored symbols. Figure 4 shows the tensile strength (TS) of the main tube on the horizontal axis and the Vickers hardness after bending on the vertical axis.
(Hv) is taken to illustrate the above results.

Of these, all of the elbow braces in which the water-cooling stop temperature indicated by the double symbol was on the high side had irregular shape defects on the inner surface of the steel pipe. Therefore, under conditions where the water cooling stop temperature remains within 100°C of the bending temperature, although the hardness is sufficiently reduced, the shape is rejected and it has been proven that it cannot be put to practical use.

Among the conditions under which an elbow break with no problems in shape was obtained, the results obtained when the conditions according to the method of the present invention were satisfied were significantly lower than those with the same base material strength.
It is clearly recognized from FIG. 4 that it has desirable characteristics in terms of usage characteristics. In other words, if the strength of the main tube is high (there is an unavoidable tendency for the hardness after bending to be high, but by adopting the method of the present invention, bending can be performed for a given strength of the main tube. After that, it became possible to manufacture elbow braces with low hardness.

[Effects of the Invention] According to the method of the present invention, it is possible to replace bent pipes with small bending radii for use in power generation and chemical plants, which were conventionally constructed with elbows, with elbowless pipes using high-frequency bending, resulting in a significant reduction in the number of welding locations. Cost reductions can be achieved and a significant contribution can be made to the construction of safe plants with low hardness and short weld lines.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram of the temperature transition of a steel pipe during the high-frequency bending process, and Figure 2 shows the change in hardness of a portion 1 mm below the outer surface with respect to the temperature at which water cooling was stopped immediately after high-frequency bending. FIG. FIG. 3 is a plan view of an example of a bending machine used in carrying out the present invention, and FIG. 4 is a diagram showing the relationship between the maximum hardness of elbow braces manufactured by the method of the present invention and the conventional method and the strength of the main tube. 1... Steel pipe 2... Guide roller 3.
... Heating device 4 ... Bending arm 5 ... Clamp 6 ... Pipe end support stand 7 ... Spray cooling water 8 ... Hardness of 1 mm of heating processing area surface Hv Fig. 3 1 : Steel pipe 2: Guide roller 3: Heating device 4: Bending arm 5: Clamp 6: Pipe end support 7: Spray cooling water 8: Maximum hardness in heating processing area Hv

Claims (1)

[Claims] In the production of high-frequency bent pipes, the base steel pipe is heated to a temperature range above the A_c_3 transformation point and below the austenite grain coarsening temperature and then bent, and the outer surface temperature of the steel pipe is immediately after the bending process temperature. The upper limit is 100°C lower than T, which is determined by the following formula (1) depending on the wall thickness t (mm) of the base steel pipe.
A method for manufacturing a high-frequency bent pipe, characterized by water-cooling the pipe to a temperature range having a lower limit of _c. T_c=450-30×√t(℃)…(1)