JPH0459121A - Method for hot forming large square tube - Google Patents

Abstract

PURPOSE:To prevent the deterioration of the quality of material following the cold rapid bending of the thick wall steel sheet and to improve the working efficiency by heating the steel sheet in the heating furnace, and forming it with the forming press before its heating temperature is reduced under the prescribed temperature. CONSTITUTION:The steel sheet is heated uniformly to about 800 deg.C-1,000 deg.C during after fed in the heating furnace till carried out to its outlet. As soon as the steel sheet 1 is pulled out from the heating furnace 6, the steel plate 1 is transferred rapidly to the forming press machine 7, and the part corresponding to the corner part of the square steel tube is bent with the bending die 71. The similar shape 12 of a pentagonal square steel tube opened with the thick wall steel sheet 1 of flat sectional area, i.e. the semi formed square steel tube is formed. This is fed with the carrying roller 4 in the roll forming machine 8 composed of plural stages, on this place the circumferential side of the semi formed tube 12 is formed successively so that it approachs the square shape and also the opened width of the semi formed steel tube is made nallow and the above part is butt-welded at the same time with the high frequency resistance welder 9 and then the large steel tube 13 of square sectional area is manufactured.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for hot forming one-seam large diameter rectangular steel pipes used for construction and other purposes.

[Conventional technology]

Large-diameter rectangular steel pipes have recently attracted attention for their structural features as described below, and have recently been used as column materials for low- and medium-rise steel buildings.
Demand is definitely increasing, and the supply amount is also increasing year by year.

Since a large-diameter square steel pipe has a box-shaped cross section, from a structural standpoint (1) it has a large moment of inertia per weight and a large section modulus.

(2) Good balance of cross-sectional characteristics in the X and Y directions.

(3) Large secondary radius of cross section, strong against buckling.

(4) Extremely high torsional rigidity.

From a construction perspective.

(5) Since the cross section is box-shaped, the material can be used exposed without spoiling its aesthetic appearance.

(6) Fireproof coating, painting, and other decorations and construction around the column are easy and economical.

(7) The cross-sectional area of the column can be made relatively small;
It is possible to increase the usable area ratio for the same building floor area.

By the way, as a manufacturing method of this kind of large diameter rectangular steel pipe, the following two methods are conventionally practiced in Japan.

One method is to unwind the rolled coil using an uncoiler, perform width determination and bevel processing, and then pass it through preforming rolls, fin pass rolls, continuous forming rolls, etc. to change the cross section of the steel strip from an arc to a circle. After forming a round steel pipe, weld the steel pipe with electricity or gas to prevent misalignment of the gap edges on the pipe peripheral wall and restrain the steel pipe so that the abutting surfaces are in close contact with each other. This is a method of manufacturing large-diameter square steel pipes by passing them through sizing rolls and task head rolls to finally shape the cross section into a square shape.

Side length (diameter) 350I11o to 400II by the above construction method
It is possible to form large diameter rectangular steel pipes with a plate thickness of about 12 mm.

This method is characterized by high productivity and stable quality.

However, this method is not suitable for producing a wide variety of steel pipes in small quantities because it requires a large amount of money to set up production equipment and it takes time to adjust the size of the formed steel pipe.

In addition, a vast site is required to deploy the equipment.

The second method is a combination of press and roll forming, in which two thick steel plates are bent and formed using a forming press into a shape that approximates the cross section of a large-diameter rectangular steel pipe, and then passed through forming rolls. After shaping the steel pipe into a square cross section, and temporarily welding the butt surfaces, the relevant parts are welded from the front and back to form a single welded joint, producing a one-seam large-diameter square steel pipe (for example, (Refer to Japanese Patent Publication No. 58-13245) However, when welding the butt end faces around the steel plates in the above method, electric arc welding is applied to the butt line from the inside and outside, high-frequency resistance welding is performed on the part at once, or single-sided welding is performed. Large diameter rectangular steel pipes were also formed from chisels by submerged arc welding.

Although this method allows production of a wide variety of products using relatively small-scale equipment, productivity is considered to be inferior to the Sagi method.

According to the above construction method, the side length of the steel pipe is 350m+~600mm
It is possible to manufacture rectangular steel pipes with a thickness of approximately 9 to 32 mm (standard dimensions listed in the catalog).

By the way, in any of the above-mentioned construction methods, the thick steel plate at the corner portion of the square steel pipe is cold bent approximately at right angles, so the steel plate at the corner portion is bent.
With the neutral plane as the boundary, a tensile force is applied to the outer side member of the bend, and a compressive force is applied to the inner side member, so the corner of the steel pipe corresponding to the bending point of the steel plate is
A large radius is given to save the stress generated by bending.

The larger the wall thickness of the steel plate and the smaller the above-mentioned R, the greater stress is generated and cracks occur at the bent part of the steel plate, and the residual stress due to bending distortion increases the durability of the corner part of the square steel pipe. This is because there is a risk of speeding up the time.

In order to avoid this, there is a certain degree of functional relationship between the wall thickness of the steel plate of the square steel pipe and the size of R (outer radius) of the corner.

For example, according to the values listed in a certain catalog, the plate thickness t
If the plate thickness is 16 mm, R is 48 countries, if the plate thickness is 19 mm, R is 64 mm, and if plate thickness t is 22 nm, R is 7
When there are 7 strokes and the plate thickness t is 25 mm, R is 100 mm.

As can be easily understood from the above data, when the plate thickness t is relatively thin (in many cases, the diameter of the steel pipe is small), R/l is approximately 3, whereas when the plate thickness is thick As time goes on, the ratio gradually increases.

Demand for this type of large-diameter rectangular steel pipes has been increasing recently for many medium- and high-rise steel-framed buildings, such as buildings with 10 or 30 floors. The thickness of the square steel pipe column is at least 40 mm.
Since 50 square meters is required, the R of the corner portion of the square steel pipe must be made larger than expected, and the advantage of using the square steel pipe will be diminished.

Furthermore, in the current manufacturing method for one-seam large-diameter square steel pipes described above, no matter how rounded the corners are, it is impossible to bend a thick steel plate by approximately 90° by cold plastic working. (1) Even though the plastic deformation is within the elastic limit, it causes deterioration of the material of the part.

(2) Because the steel sheet is subjected to sudden bending, stress remains at the corners even after the steel sheet is plastically deformed.

(3) Depending on the selection of the material of the steel plate and the way in which the R/ ratio is given, there is a possibility that latent cracking may occur locally due to bending.

For example, it is impossible to avoid changes in the material of the corner parts of the square steel pipe that occur due to cold plastic working of the steel plate.

However, to date, no technical analysis or study has been conducted regarding the degree of material change.

In addition, once a building's columns have been constructed, they must not only support the weight of the structure for a long period of time, but also be able to withstand earthquakes.
There are also problems with the construction method, as it is required to withstand severe repeated loads due to external forces such as typhoons, and in principle cannot be replaced or repaired.

As such, there has been a demand for improving or confirming (identifying) the reliability of the material properties (rather than shape specifications) of this type of steel pipe material.

In response, even before this application was filed, as a measure to repair the deterioration of the material at the relevant point due to cold plastic working of thick-walled steel plates, efforts were being made to eliminate the residual stress at the corners of the steel pipe, homogenize the structure, and improve the material quality. In order to stabilize molded products, methods of annealing molded products are often used.

However, after plastic working, large-diameter rectangular steel pipes have an extremely large product capacity compared to the weight of the material, and the internal volume of an annealing furnace that can accommodate multiple steel pipes is also extremely large. In order to maintain quality, it is necessary to hold the pipe at high temperatures and heat for a considerable period of time, which increases both the equipment cost and operating system, resulting in an increase in costs that exceeds the benefits of annealed steel pipe. is expected.

In addition, even if heat treatment is applied to the entire steel pipe in spite of the above-mentioned disadvantages, the material at the corner of the steel pipe, which had deteriorated due to plastic processing due to rapid cold bending, will be completely restored to its former state. There is no guarantee that it will.

Therefore, it has been considered to perform hot bending instead of cold plastic working of thick steel plates during forming of square steel pipes.

In that case, the area corresponding to the corner of the steel material should be locally heated in advance by gas combustion or electric/magnetic means to raise the temperature to about 700°C to 900°C, and then before the temperature of the relevant area decreases. Then, hot working is carried out by applying the same part to the bending die of a forming press or by bending rolls. (1) In either case, local heating is performed in the longitudinal axis direction of the thick steel strip; If strain due to thermal expansion is applied unevenly to the steel plate, the material before forming may be bent.

In some cases, high quality square steel pipes cannot be obtained.

(2) When bending a steel plate using a press die, the entire length of the steel plate in the longitudinal axis direction is bent at once each time the steel plate is bent, so it is preferable that the steel plate should maintain the same heating temperature over its entire length. However, this temperature control is difficult.

(3) It is necessary to heat the relevant part to a predetermined temperature immediately before bending the steel plate with the mold, but the time required for this reduces the working efficiency of press working.

(4) In the bending process of steel plate by roll forming, the above (
In addition to having the same defects as described in section 3), it is necessary to install local heating devices immediately before each forming roll, which inevitably increases the number of heating devices installed.

There are quite a few problems, such as:

[Problem to be solved by the invention]

The construction method of the present invention was developed in order to satisfy the demands of many users for the large-diameter square steel pipes mentioned above. To provide a method for forming a large diameter rectangular steel pipe that prevents material deterioration due to rapid cold bending of thick steel plates and does not reduce processing efficiency.

In addition, in the method of the present invention, in addition to the manufacturing process of large-diameter square steel pipes based on bending thick steel strips using a forming press, a step of heating the entire steel plate to a required temperature is added as a pre-process. First, the feeding speed of the heated steel plate from the furnace can be matched to the working speed of the bending process using a press, thereby improving processing efficiency.

Second, by using an inexpensive heat source for the steel plate heating furnace and storing and heating multiple thick steel plates at the same time, the apparent time required to heat each unit of steel plate can be shortened.
Minimize product cost increases due to steel sheet heating.

Third, by heating the steel plate, the mechanical processing energy required for plastic deformation is reduced, reducing the capacity of forming presses and other equipment, while making it possible to manufacture square steel pipes using thicker steel plates. do.

Fourthly, by hot bending, the radius given to the corners of square steel pipes without residual stress can be made smaller relative to the steel plate wall thickness, thereby increasing the product's second moment of area, section modulus, etc. The purpose of this study is to develop a method for hot forming square steel pipes to form high-quality, large-diameter square steel pipes that are larger and have a better appearance.

[Means to solve the problem]

In order to achieve the above object, the construction method of the present invention has the following constituent requirements.

(1) After a thick steel strip of unit length is transferred in the longitudinal direction and subjected to width determining processing, the steel strip is charged into a heating furnace and the whole is uniformly heated to the required temperature, and the heating temperature is Transport the maintained steel plate to the forming press, and before the heating temperature drops below a predetermined temperature, for the steel plate portion corresponding to the corner of the square steel pipe obtained by forming the steel plate, A cross section of the steel plate is formed into a shape similar to a rectangular steel pipe that is at least close to a pentagon by sequentially bending the steel plate at nearly right angles, and a single welded joint is provided at a position avoiding a corner. Hot forming method for large diameter rectangular steel pipes.

[For production]

Regarding the operation of the molding method and device according to the present invention, the first
The principle is explained as follows according to the diagram.

The hot-rolled coil is unwound and conveyed in the longitudinal direction, passed through a leveler, and then passed through a width determining machine to be cut at right angles to required lengths in the longitudinal direction. The shape of the shear blade is usually straight, but
If necessary, it may have a convex shape.

The shape of the end face perpendicular to the longitudinal axis of the single steel plate material is
When each of them has an uneven shape, in the butt welding process of the peripheral wall edge of each unit of half-formed square steel pipe after bending and forming the sheet steel plate, the uneven shape of the end face is engaged with each other to separate the material. By bringing the forming rolls into the welding process, it becomes possible to perform the same work as butt welding the peripheral wall edges of continuous semi-formed rectangular steel pipes, thereby increasing the effective use of materials and welding efficiency. This will be explained in detail in the [Example] section.

It may be a thick steel strip cut to the required length, or it may be a material that has been formed into a single thick steel plate from the beginning, but after the width has been determined, the material is heated in a tunnel-type combustion furnace (directly heated). As a result of charging the steel plate from the entrance of the furnace (or indirect heating furnace) and pressing and transferring the steel plate in the heating furnace with the newly imported steel plate, the temperature of the steel plate is almost uniform by the time it reaches the exit of the combustion furnace. 7
It is heated to a range of 00°C to 1000°C.

Handling and transporting the heated steel plate is dangerous, so it is operated mostly by robot equipment, carried to the required position of the forming press machine, and the copper plate is bent against the mold, at least in the width direction. After precise positioning, operate the press to press the first plate along the longitudinal axis of the heated steel plate.
A hot bending process is applied to the corner portion, that is, the corner portion near the side edge of the thick steel strip.

Precise positioning of the heating steel plate in the width direction with respect to the die position can be achieved, for example, by using at least a pair of pins as stoppers that are raised and lowered from the bending die at a predetermined distance apart, and positioning the heating steel plate in the major semi-axial direction with respect to the pins. This is possible by moving the steel plate in the width direction so as to press the side edges, thereby allowing precisely positioned plastic working with respect to the folding die.

At this time, the size of R' (inner radius) given to the corner is automatically given by the curved surface formed at the tip of the bent male mold. Since the construction method of the present invention employs hot plastic working, the size of R' does not need to be restricted by conventional standards.

The above-mentioned bending process of the heated steel plate is carried out because the area and opportunities that the processing tools and other supporting members of the steel plate come into contact with the heated steel plate are relatively small, and the amount of processing per bend is relatively large. Compared to the amount of processing, there is relatively little reduction in the heating temperature of the thick steel plate due to processing tools, etc., and
In addition to processing tools, the amount of thermal energy conducted from the heated steel plate is also small.

However, the above-mentioned tools should be equipped with water holes, etc., without reducing their strength, so that a refrigerant is constantly allowed to flow through the holes, thereby carrying out the thermal energy leaking from the heated steel plate to the tools. It goes without saying.

As explained above, plastic working is performed while the strength of the heated steel plate is reduced to several minutes, so the mechanical energy input for bending a thick steel plate is comparatively low. In addition, there is no risk of deterioration of the material due to processing even if the radius given to the corner part is small compared to the plate thickness, and after bending and forming, there is no plasticity in the cooled state. There is little residual stress at the corners due to deformation.

Thus, before the heating temperature of the thick steel plate decreases.

It is necessary to complete the bending process at the locations corresponding to the second to fourth corners of the steel plate material, but in any case,
The widthwise positioning of the thick steel plate relative to the bending die of the press is performed precisely.

If the range of error is large, a rectangular steel tube will be formed that is twisted in the longitudinal direction, resulting in a decrease in quality.

However, if you spend a lot of time positioning the steel plate, by the time of the final bending process, the heating temperature of the steel plate will reach the specified processing temperature, that is, even if plastic deformation is applied, the steel plate will not reach the desired temperature after cooling. There is a risk that the heating temperature may drop below a level at which residual stress does not occur in some locations or the material does not deteriorate.

Therefore, in the construction method of the present invention, if the steel plate positioning means used in the first bending process is adopted as the steel plate positioning means used in all the steel sheet bending processes from the second bending process to the fourth bending process, the simple structure can be reduced. It has high accuracy and can reach the goal in a short time.

It is preferable that a plurality of positioning pins are installed for this purpose, and the position of the positioning pins can be moved and adjusted, and that the positioning pins can be slid in the vertical direction by applying fluid pressure or the like as necessary.

``The key point is to complete the four-fold bending process of the steel plate as quickly and accurately as possible before the heating temperature of the thick steel plate drops.

For this purpose, it is thought that selecting the folding press method rather than the folding roll method will achieve the objective more easily and in a shorter time.

Further, in handling the thick steel plate in this process, since the material is heated to nearly 1000°C, a robot-controlled conveying means is always used.

Thus, at the stage where the hot bending process of the thick steel plate using the press machine is completed, the cross section perpendicular to the longitudinal axis of the steel plate is approximately a cross section of a square steel pipe.

That is, while it is formed into a pentagonal shape with an opening, the heating temperature is also lowered to 600° C. or less due to heat radiation during processing.

The above-mentioned gap width of the opening of the pentagonal cross section of the steel material is the minimum dimension that allows the upper die of the bending die to be pulled out from the gap after the final bending process by the forming press.

Semi-formed steel pipes that have undergone hot bending using a forming press are
Shape the cross section into a square through a forming roll, fin pass roll, squeeze roll, etc., narrow the opening gap, make the light surface a metal touch, and adjust the abutting part so that there is no misalignment at the relevant point. Tacking is welding (spot or continuous) to fix the shape of the steel pipe cross section.

Thereafter, the tack welded parts are permanently welded from the inside of the steel pipe and the outer periphery to form a one-seam rectangular steel pipe.

Alternatively, a one-seam rectangular steel pipe may be formed by welding the abutting surfaces with metal by high-frequency resistance or induction heating (in this case, the grooves on the side edges of the thick steel strip (no processing required).

Alternatively, submerged arc welding is performed with the abutting surfaces facing each other to produce a one-seam square steel pipe.

Although various methods other than those described above can be used to form a welded joint on the butting surfaces, the method of the present invention does not require any method for forming a one-seam welded joint.

In addition, post-processing such as flaw detection inspection of welded joints, correction of distortion in the longitudinal axis direction of square steel pipes, and removal of build-up of welded joints if necessary is the same as the conventional manufacturing method for large-diameter square steel pipes. It is.

〔Example〕

Below, a general explanation will be given of an embodiment of the method of the present invention and a hot forming apparatus for thick-walled square steel pipes for carrying out the method. There may be various partial deformations within the
Unless there is a special reason, the constituent elements of the construction method of the present invention should not be interpreted in a limited manner based only on the explanations described in this example.

Figure 1 schematically shows a hot-forming apparatus for large-diameter rectangular steel pipes in which the method of the present invention is carried out. The steel plate 1 is unwound and passed through a leveler 2 to straighten the bending of the steel plate 1, and then passed through a shearer 3 to cut the continuous steel plate 1 into predetermined lengths in the longitudinal axis direction.

At this time, if necessary, a convex shearing blade is used to form the end surface of the steel strip 1 into an uneven shape.

While the steel plate 1 is transferred in the longitudinal axis direction by the transfer rollers 4..., the width direction dimension of the steel plate is determined by the cutting machine 5 (at this time, a beveling process is also performed if necessary). It is pushed and charged into a type heating furnace 6.

The heating furnace 6 is a slightly low-temperature artificial heating furnace that uses fuel such as gas or heavy oil as a heat source. After being charged into the heating furnace, the steel sheet is uniformly ( (both longitudinally and transversely) to 800°C to 1000°C. Also, adjust the temperature control in the heating furnace to the conditions on the right.

As soon as the steel plate 1 is pulled out from the heating furnace 6, the steel plate 1 is quickly transferred to the forming press machine 7, the widthwise position of the heated steel plate 1 is determined with respect to the bending die 71, and the thick steel plate 1 is A portion 14 corresponding to the first corner portion parallel to the side edge in the longitudinal axis direction (second corner portion 14)
(see figure) is positioned directly below the bending mold, and bend the part corresponding to the corner of the square steel pipe.

This forming is shown in FIG.
The second corner portion 15 is bent against the bending mold 71 by bending it to ZA (approximately 92″) and then applying the bent corner portion 14 or the side edge of the steel plate to a stopper pin. After positioning and bending at approximately 92°, positioning the portion 16 or 17 corresponding to the third corner with respect to the bending die 71, and bending the corner /B at approximately 115°.

Finally, the portion corresponding to the fourth corner 17 or 16 is bent in the same manner as described above, and finally, under the press forming machine 7, the thick steel plate 1 with a flat cross section is formed as shown in the second figure. A shape 12 approximating an open pentagonal square steel pipe, that is, a semi-formed square steel pipe is formed.

The forming of the thick steel plate 1 using the press machine 7 needs to be completed at least before the heating temperature of the steel plate cools down to about 750°C or less, so the positioning and bending of the steel plate must be done in an orderly and quick manner. Must be.

The radius given to each corner is extremely small compared to the thickness of the steel plate (not covered by conventional standards).

゛The size of the above-mentioned R is given by the size of the tip R' of the bending upper die. Further, the size of the opening width is the minimum width from which the bent upper mold after the final bending process can be pulled out without being supported by the opening of the steel plate 1.

Through this forming process, the steel strip is eventually formed into a shape approximating a square steel pipe with a pentagonal cross section and an opening as shown by the solid line in FIG. The semi-formed steel pipe 12 is fed into a roll forming machine 8 consisting of stages, where the circumferential side of the semi-formed steel pipe 12 is pressed from the left, right, upper and lower directions by forming rolls, and its cross section is successively shaped so that it approaches a square. , while narrowing the opening width of the semi-formed steel pipe and shaping it with forming rolls so that there is no misalignment on both butt edges (/A to 92
(This is why the pipe was bent to about 100 degrees)) The high-frequency resistance welding device 9 is used to butt-weld the parts at once to a large-diameter steel pipe 13 with a rectangular cross section (see Figure 2, indicated by the two-dot chain line).
Manufacture.

When using the above-mentioned welding apparatus, the welded joint can be completed in one go without requiring beveling on the abutting surfaces, which is efficient. However, if the steel plate is too thick, conventional methods will be used.

Note that the process of the high-frequency resistance welding device 9 described above may be replaced with a high-frequency induction welding device.

In addition, the above-mentioned welded joint can be formed by first welding the butt-welded part with a groove, and then temporarily welding (spot or continuous) to fix the cross-sectional shape of the steel pipe. For example, a method of performing main welding using methods such as submerged arc welding, a method of reinforcing a high-frequency resistance welded joint from one side with submerged arc welding, etc., and a method of reinforcing a high-frequency resistance welded joint from one side using submerged arc welding. Various known welding methods can be used depending on the characteristics required for the welded joint of the jointed steel pipe.

Among the above, shaping the cross section of a square steel pipe using forming rolls is as follows:
As the semi-formed steel pipe 12 is transported by the transport rollers 4, its cross section becomes closer to a square, and the widths of the opening surfaces of the steel pipe groove become narrower.

Therefore, a step is provided on the side with the opening of the rear end face of the preceding semi-formed steel pipe and the front end face of the following semi-formed steel pipe,
If this is finished into an in-rotation tube 19 and conveyed into the roll forming device while the front and rear semi-formed steel tubes are brought into contact with both end surfaces, the opening of the preceding semi-formed steel tube will change as the cross section of the preceding semi-formed steel tube approaches a square. Since the distance between the surfaces becomes narrower, the step interval at the rear end of the semi-formed steel pipe also becomes narrower.

The shaping force by the forming rolls is also transmitted through the step to the front end surface of the succeeding semi-formed steel pipe 12 which is engaged with the rear end surface of the preceding steel pipe by the in-rotating tube 19, and the opening width of the succeeding steel pipe 12 is also transmitted through the step. It acts to narrow it down.

In short, by arranging semi-formed square steel pipes 12 of unit length in the longitudinal direction, it is possible to perform continuous forming and butt welding of the opening surfaces, and to provide a connecting portion between both end surfaces that is sufficient to pass the required high frequency current. This has the advantage of eliminating the need for preliminary work such as welding tabs in advance to remove defective welding points that occur at both ends in the longitudinal axis direction when butt welding the opening of the unit long and half-formed square steel pipe 12. There is.

FIG. 3 is a perspective view of a main part of the steel pipe roll forming process when the end face of the steel plate perpendicular to the longitudinal axis direction is cut by an uneven shearing blade.

The thus formed large diameter rectangular steel pipe is smoothed by scraping the excess thickness of the welded joint on the outer periphery as necessary, and then sent to the strain straightening machine 10 by the transfer roller 4, and is straightened by a straightening machine using a press or rollers. 1o mainly corrects distortion in the longitudinal axis direction.

Next, the welded joints of the steel pipes are inspected through an ultrasonic flaw detector 18, and only those that pass are displayed as finished products.

Molded products that fail to pass the flaw detector 18 are returned to manual repair and remanufacturing processes depending on the degree of defect.

〔Effect of the invention〕

The construction method of the present invention is as described above, and (1) deterioration of the material due to cold plastic deformation of each corner portion, which was conventionally thought to be natural in thick-walled, large-diameter rectangular steel pipes, does not occur. In addition, there is almost no residual stress caused by corner molding.

(2) Even though a thick steel plate is used, the corner radius of the square steel pipe can be formed to be small. Therefore, even the corners can be incorporated into the strength calculation of the column, and the section modulus per weight, the second moment of inertia, etc. can be made relatively large.

This makes it possible to form larger diameter rectangular steel pipes that are more durable per weight.

(3) It is possible to form large diameter rectangular steel pipes using extremely thick steel plates, which is at its limit with conventional technology.

(4) The cross section can be made into a sharp box shape, which is advantageous in terms of construction and aesthetics.

(5) Since the bending process of thick steel plates in the method of the present invention is hot plastic working, the power required is small compared to the thickness of the workpiece and the degree of processing, so the equipment can be relatively small. It is possible to increase the capacity and reduce equipment costs.

(6) However, since hot plastic working is performed, it is inevitable that the cost will increase due to the processing required to heat the steel plate.

This method provides exceptional functions and effects that cannot be expected from conventional methods of forming thick-walled, large-diameter rectangular steel pipes.

[Brief explanation of drawings]

Fig. 1 is a schematic diagram of an embodiment of a production line for carrying out the hot forming method for thick-walled, large-diameter rectangular steel pipes according to the present invention, Fig. 2 is a cross-sectional view showing the bending process order of steel plates, and Fig. 3 is a It is a perspective view of the process of conveying a semi-formed rectangular steel pipe to a forming roll device. 1... Thick steel plate, 2... Leveler 3...
・Shearing machine, 4...transfer roller, 4...
(bevel) cutting machine, 6... continuous heating furnace, 7...
Forming press, 8... Roll forming device, 9...
・High frequency resistance welding equipment, IO...distortion straightening machine, 12...
・Semi-formed steel pipe 13... Square steel pipe molded product, 1
4.15.16 and 17... Square steel pipe corner, 18
...Ultrasonic flaw detector, 19...In-low type.

Claims (1)

[Claims] (1) After transferring a unit length thick cold steel strip in the longitudinal direction and performing width determining processing, the same steel plate is charged into a heating furnace and the whole is uniformly heated to the required temperature, and the heating The steel plate while maintaining its temperature is transported to the forming press, and before the heating temperature drops below a predetermined temperature, the portion of the steel plate that corresponds to the corner of the square steel pipe obtained by forming the steel plate is , characterized in that the cross section of the steel plate is formed into a shape approximating a rectangular steel pipe that is at least close to a pentagon by sequentially bending the steel plate at nearly right angles, and a single welded joint is provided at a position avoiding a corner. A hot forming method for large-diameter square steel pipes.