METHOD FOR MANUFACTURING OF SQUARE PIPES FROM COATED STEEL STRIPS

Abstract: The invention concerns a method for manufacturing of square pipes from coated steel strips. Both sides of the strip are coated with metal, say zinc or aluminium. The strips also can be industrially painted e.g. with PVC-plastics directly upon the steel surface or upon the metal-coated surface. The shaping is carried out in a continuous roller shaping device by at first shaping a profile (6) with a bottom part (7) and side parts (8) equal to the finished pipe. The top parts (9) are made equal to half of the bottom part of the pipe and ended with foldings (5). The angle (11) between the bottom part (7) and the side parts (8) is made acute and the angle (12) between the side parts (8) and the top parts (9) is made obtuse. The facing coated surfaces (13) at the foldings (5) are heated by laser and compressed at the following roller shaping when the finished shape of the pipe is made.

Description: METHOD FOR MANUFACTURING OF SQUARE PIPES FROM COATED STEEL STRIPS BACKGROUND TECHNICS It has been proved that thin-walled square pipes have become more and more used in e.g. frames for buil¬ dings, furniture and automobiles. The pipes can be either square or more or less rectangular in cross-section. Thin-walled pipes are defined as pipes with wall thick- nesses up to 3 - t mm. Common lengths of course vary within wide limits, but regarding storing, transport and waste of course long initial pipes are preferred. Conventionelly square pipes are manufactured by bending steel strips or steel plates. In a first step the strip or the plate is shaped mainly into a round pipe with a gap between the bended sides in the longitudinal direction of the pipe of about 2 - 3 times the plate thickness. This gap is filled up by welding, mostly by TIG-welding. This welding, where proportionately a lot of material shall be added to the thin surfaces of the seam, is very time-consuming and determines fully the highest speed for the manufacturing of the pipes. Of course the weld must be of a very god quality as the width of the jointing surfaces is not greater than the thickness of material of the pipe. After welding the round pipe it is shaped into a square pipe in a subsequent roller shaping. The greatest drawback with the conventional manufacturing however is that when welding, so high temperatures will be reached close to the seams that eventual coating will be destroyed, either the coating consists of metals as e.g. zinc, aluminum, nickel or copper or blendings of these, or industrially painted surfaces. By industrially painted surfaces is usually ment surfaces coated by e.g. PVC-plastics, polyester or acrylate. The coating on the outside of the pipes natural¬ ly can be repaired with a result which is more or less acceptable. But the reparation is devastating for the economy. On the contrary it has shown impossible to re¬ store an acceptable preventing coating on the inner side of the pipe. Depending on the use of the pipes the inner sides can be exposed e.g. to air, more or less polluted and wet, or to agents flowing through, which destroy a pipe with a protection which is not satisfactory for the purpose. By that reson the application of the coating is postponed until after the welding when it frequently is carried out as dip-coating. This is an expensive and time-consuming process and the depth of the furnaces determine the maximum pipe length. Another point of view of the manufacturing of square pipes is that from a storing and economical point of view it is desirable to start with a steel strip with a desired width, thickness and coating by slitting from wider plate rolls with a thickness and a coating corre¬ sponding to the finished pipe and after that subsequently shape and join the pipe and cut it into desired lengths an d even carry out some after-treatment. THE INVENTION Surprisingly it has appeared that square pipes can be continuously manufactured from steel strips, e.g. slitted from wide coils with a desired thickness and a completely treated surface by broadening the surfaces of the joint, having engaging coatings to melt together af¬ ter a quick heating by help of a laser heater and on the same time providing the pipe with such stresses that a durable joint is reached and where the coating in the area of the joint keeps its protecting and visible effect without considerable affection. The invention will be described more precisely in the enclosed claims, in a more detailed description below and by help of drawings enclosed on which: figure 1 shows a square pipe in a perspective projection and figure 2 a cross-section of the continuously manufactured pipe section at the moment in which the laser heating is started. CLOSER DESCRIPTION OF THE INVENTION When manufacturing of a square pipe 1 , a start is made from a steel strip with a thickness equal to the thickness of the finished pipe 1. The width is equal to the periphery of the pipe 1 , to which an addition is made for the two foldings 5 (see below). After that the strip is bent in a continuous roller shaping path constituted by a number of facing roller couples arranged in groups after each other making the strip symmetrically shaped into a determined cross-section of the pipe 1. In order to be able to heat coated surfaces 13, which are to be melted together by laser heating and to build in stresses in the finished pipe 1 making the joint 4 to close at e.g. bending stresses in the pipe 1, a determined profile 6 is shaped. The width of the bottom part 7 of the pro¬ file 6 is made equal to the bottom 2 of the pipe 1 , the side parts 8 of the profile 6 equal to the sides 3 of the pipe 1 and the top parts 9 of the profile 6 equal to half the width of the bottom 2 of the pipe 1. The angles 11 of the bottom of the profile 6, i.e. the angles between the bottom part 7 of the profile 6 and the side parts 8, are made acute down to 70° and the angles 12 of the top parts of the profile 6, i.e. the angles between the side parts 8 of the profile 6 and top parts 9, are made obtuse and mainly as much over 90° as the bottom angles 11 are below 90 . However the lower edges 10 of the foldings 5 have to contact each other. The foldings 5 ends the top parts 9 of the profile 6 and are bended in a right angle (90°) from the same. The width of the foldings 5 is adjusted making a sufficiant jointing surface at melting of the two coatings of their each other facing, coated surfaces 13. When said profile 6 is reached during the continuous shaping in the roller shaping device said coated surfaces 13 are heated into melting or at least into a plastic consistency by help of a laser beam directed to the foldings 5. By help of laser the heating is very quickly made and the heating can be concentrated to exactly the areas which are to be heated. After a de¬ sired concistency succesively is reached on the heated coated surfaces 13 the shaping of the profile 6 into a square pipe 1 is carried on in the roller shaping device. This shaping mainly is to change the obtuse angles 12 of the upper parts of the profile 6 into right angles by de¬ pressing the upper parts 9 of the profile 6 to the center of the pipe and with an over-bending of the angles 12 of the upper parts of the profile 6 in a way that these af¬ ter springing back become right. If the method is unifor¬ med controlled the bottom angles 11 of the profile 6 also will be 90°. But at this method no over-bending is made of these angles but a certain spring back force is rea¬ ched striving to press the melted surfaces of the fol¬ dings 5 together, which strengthens the jointing force of the joint ^. This is especially useful if the square pipe 1 is subjected to such bending stresses where ten- sile forces arise in the upper side of the jointed square pipe 1.

Claims: PATENT CLAIMS 1. A method for manufacturing of thin-walled, square pipes ( 1 ) of coated steel strips in a continuous process, c h a r a c t e r i z e d in that the cross- section of the strip successively and symmetrically is shaped between roller couples into a flat bottom part (7) equal to the desired width of the pipe, which by lower corners with acute bottom angles (11), is leading into flat side parts (8) equal to the sides of the pipe (1) and these in its turn leading into top parts (9) by ob¬ tuse top part angles (12), the width of which is equal to half of the width of the pipe and each ending with flat foldings (5) in angles of 90° to the top parts (9) and that the bottom angles (11) and the top part angles (12) at the profile (6) are dimentioned that the lower edges (10) of the foldings (5) contacts, that the coated sur¬ faces (13) of the foldings (5) facing each other are suc¬ cessively heated by laser heating into a temperature at which their coatings are melting together at the follow- ing compression when at the same time the shape of the pipe (1) is completed. 2. A method for manufacturing of square pipes (1) according to claim 1, c h a r a c t e r i z e d in that the steel strips are coated with a metallic coating mainly consisting of zinc or aluminum or mixtures of these. 3. A method for manufacturing of square pipes (1) according to claim 1 or 2, c ha r a c t e r i z e d in that the steel strips are industrially painted.

Inventors: INGVARSSON LARS INGVAR, INGVARSSON, LARS, INGVAR
Applicants: ORTIC AB
Application year: 1990
Main classification: B21D5
IPC codes: B21C37, B21D5, B23K26, B23K33