The invention relates to a method for producing a non-destructively tested elongated hollow body consisting of steel and having a polygonal, in particular square or rectangular, cross-section, in which an intermediate hollow body having a round cross-section is produced in each case from a flat pre-material or an intermediate hollow body having a round cross-section is produced from a block-shaped pre-material, whereby the intermediate hollow body is cooled or quenched with partial or full phase transformation.

It is generally known that for decades hot-finished circular, square or rectangular and elongate hollow profiles consisting of steel have been used in the steel industry. The fields of application include the modern steel architecture in structural engineering, bridge construction, industrial construction, sports facilities construction, mechanical engineering, construction of agricultural devices and conveyor systems, shipbuilding and fairground ride construction for fairs. The hot-finished hollow profiles pass through a heating process in a final production step e.g. in normalising temperature range at approximately 850 to 1050°C. Square or rectangular hollow profiles achieve wall thicknesses of up to approximately 30 mm and outer dimensions in the range of 40 x 40 mm to 400 x 400 mm or 50 x 30 mm to 500 x 300 mm respectively. Typical lengths of the profiles are 12 m or 16 m. The materials used include general structural steels, high-strength fine-grained structural steels and special grades as well as non-weldable grades corresponding to the desired intended purposes. Hot-finished, square or rectangular hollow profiles are characterised by smaller corner radii than in the case of cold-finished profiles and thus have larger cross-sectional areas. Therefore, a higher load can be accommodated with identical profile dimensions.

German laid open document DE 2 348 152 discloses a method for producing elongate hollow bodies consisting of steel and having a polygonal cross-section. In this method, a hollow steel pipe having a substantially round cross-section is hot-rolled and is then austenitised in a gas-heated oven with an excess of air and above an Ac3 temperature of the steel in question. The austenitising temperature is preferably between 871 and 954°C. Then, quenching occurs in water to a temperature of less than 93°C and then heating occurs to a tempering temperature which is above a stress-relief annealing temperature and below an Ac1 temperature of the steel in question. Preferably, the tempering temperature is between 621 and 663°C. Then, the steel pipe is tempered at this temperature and is rolled out within the tempering temperature range to the desired polygonal cross-sectional shape, in particular to that of a rounded rectangle and is then cooled in air. The hollow bodies produced according to this method are intended to be free of surface defects, such as e.g. buckling, and to have high yield stresses, distinctive notch impact strength and strain properties as a result of the quenching and tempering steps. A suitable pre-material for the method is an Si-AI-killed steel having approximately 0.2% carbon, 1.45% Mn and 0.06% V steel, which is martensitic and has good weldability. Further German laid open document DE 197 03 586 shows a method for producing elongated hollow steel bodies having a polygonal cross-section. Starting from a flat pre- material an intermediate hollow body having a polygonal cross-section is obtained by squeeze-moulding and welding. The intermediate hollow body has a curvature radius in its edge region being larger than the respective curvature radius of the final elongate hollow steel body having a polygonal cross-section. Afterwards the intermediate hollow body is heated in an oven and is finally rolled. During final-rolling the curvature radii are decreased to reach the desired shape of the final elongate hollow steel body having a polygonal cross-section. Moreover the European patent EP 0 485 572 discloses a method for producing a seamless steel pipe having a round cross-section. This method describes producing an intermediate hollow body having a round cross-section from a round block-shaped pre- material. A testing apparatus is arranged at a suitable position, especially between a rolling mill and a cooling bed, within the production line in order to test the pipe for dimensional changes or flaws. The achieved testing results are used to provide control information for the production method. The testing apparatus has a source/detector apparatus using a penetrative source of radiation, e.g. X-ray or gamma ray beam, for nondestructive testing. In addition German laid open document DE 10 2012 006 472 teaches the production of welded steel pipes having a round cross-section from a flat pre-material. This flat pre- material undergoes a non-destructive control via ultrasound or eddy current testing before U-/0-forming and welding. In particular homogeneity of material properties is continuously or discontinuously obtained to provide control information for the production method. In general, non-destructive testing, in particular ultrasonic testing for defects, in particular laminar imperfections and inclusions, of hollow bodies consisting of steel and having a square or rectangular cross-section is not possible. Nowadays, hollow bodies consisting of steel and having a polygonal, in particular square or rectangular, cross-section are rolled and heat-treated in various steps. In addition, concavities, convexities, twisting and deviations in squareness and straightness occur owing to a separate heat-treatment step.

The object of the invention is to provide a method for producing an elongated hollow body consisting of steel and having a polygonal, in particular square or rectangular, cross- section which is characterised by produced hollow bodies having increased dimensional stability, in particular narrow dimension tolerances. The hollow bodies are heat-treated and tested in a non-destructive manner.

The object is achieved by a method having the features of claim 1. Advantageous embodiments of the invention are described in dependent claims 2 to 14 and a use described in claim 15.

In accordance with the invention, in the case of a method for producing an elongated hollow body consisting of steel and having a polygonal, in particular square or rectangular, cross-section, in which an intermediate hollow body having a round cross-section is produced from a flat pre-material or an intermediate hollow body having a round cross- section is produced from a block-shaped pre-material whereby the intermediate hollow body is cooled or quenched with partial or full phase transformation, the intermediate hollow body is tested in a non-destructive manner preferably at lower temperatures, especially at room temperature, and the intermediate hollow body is final-shaped without intended change (intended reduction or intended increase) of the wall thickness of the intermediate hollow body to form a final hollow body having a polygonal, in particular square or rectangular, cross-section, in particular having rounded corners, is final-heat- treated immediately prior to the final-shaping of the intermediate hollow bodies or the hollow body produced by final-shaping is final-heat-treated during the final-shaping, a simplification in the non-destructive testing is achieved by virtue of the fact that the intermediate hollow body having the round cross-section, is tested in a non-destructive manner prior to the final-shaping. During the final-shaping, a logarithmic reduction ln(C0/C1 ) of a starting periphery CO of the intermediate hollow body occurs during an in particular multi-stage shaping to a final periphery of the hollow body in a range between 0 and 0.3, wherein an extension of the hollow body outweighs an increase in the wall thickness. The shaping speed is between about 0.2 and 5 m/s, preferably between about 0.5 and 5 m/s. The final-shaping of the intermediate hollow body is more or less a thermo- mechanical shaping step, in particular a drawing or stretching or pressing, especially a roll-drawing or roll-stretching or roll-pressing.

In conjunction with the present invention the term "without intended change" in the feature "the intermediate hollow body is final-shaped without intended change of the wall thickness of the intermediate hollow body to form a final hollow body" is to be understood that the average wall thickness of the intermediate hollow body is amended (reduced or increased) by less than 10%, preferably less than 5%, compared with the wall thickness of the hollow body having a polygonal, in particular square or rectangular, cross-section. The wall thickness is to be understood as an average wall thickness. Also in conjunction with the present invention, non-destructive testing is understood to mean at least one of the following testing methods or combinations thereof. The nondestructive testing occurs prior to or after a heat treatment (e.g. quenching, normalising, soft annealing) of the hollow body.

- eddy current testing of surfaces, typical minimum defect depth for reliable recognition approximately 0.2 mm or 5% of the wall thickness of the hollow body, whichever value is higher; for wall thicknesses of less than or equal to 15 mm, defects on the outer surface and also on the inner surface of the hollow body are recognised whereas for wall thicknesses of greater than 15 mm only defects on the outer surface of the hollow body are recognised;

- ultrasonic testing of surfaces (inner and outer), typical minimum defect depth for reliable recognition approximately 0.3 mm or 5% of the wall thickness of the hollow body, whichever value is higher in the longitudinal direction, tangential direction and oblique direction. Typically, a test frequency for ultrasonic testing methods is in the range of 2-25 MHz;

- ultrasonic testing for laminar imperfections and other defects in a wall volume using reflection/reference defects (e.g. flat bottom bores) having a minimum size of the reference surface of approximately at least 1 mm;

- magnetic particle testing for local testing of the surface of a hollow body, a pipe inside and outside surface as well as end faces, edges of a ploygonal structure and its corners and end faces. Local magnetic particle testing can also take place on the final hollow product. Preferably the non-destructive testing of the intermediate hollow body is an eddy current testing or ultrasonic testing or a combination hereof.

Also in conjunction with the present invention, the term "immediately" in the feature "under final-heat-treating of the intermediate hollow body immediately prior to the final-shaping" is understood to mean that the time frame between the final-heat-treating and the final- shaping is less than 5 minutes, preferably less than 60 seconds.

Further in conjunction with the present invention the term "room temperature" in the feature "the intermediate hollow body is at room temperature for the non-destructive testing" is understood to mean a temperature of the intermediate hollow body that is between 5 °C to 60 °C.

In a particularly advantageous manner, provision is made in the method in accordance with the invention that the non-destructive testing of the intermediate hollow body is carried out after the intermediate heat treatment of the intermediate hollow body or prior to the final-shaping of the intermediate hollow body to control if requirements of customers for the final hollow body-product are met. Accordingly the final hollow body having a polygonal, in particular square or rectangular, cross-section, will not be tested any more. The core idea of the invention is to rely on non-destructive testing results before final- shaping in order to meet the customers' demands. Additionally, non-destructive testing takes place on the seamless or the welded intermediate hollow body prior to the intermediate or final heat treatment of the intermediate hollow body or on the flat pre- material for a welded intermediate body. Optionally additionally non-destructive testing takes place on the hollow body after final-shaping externally in the region of its corners and/or ends. The non-destructive testing for defects in the wall of the intermediate hollow body, e.g. for laminar imperfections, can be effected with conventional ultrasonic testing facilities using typical testing standards for hollow bodies having a circular cross-section. When testing flat pre-material, testing is effected in this initial state by means of conventional ultrasonic testing facilities more typically used for this semi-finished product. Since the ultimate shaping of the intermediate hollow body having a round cross-section to form a hollow body having a polygonal cross-section is effected within a provided heat treatment step, very narrow dimension tolerances can be maintained for the hollow body. Depending upon the desired properties of the hollow body having a polygonal cross- section, heat treatments such as cooling with water, oil or polymers also take place after austenitising the intermediate hollow body prior to ultrasonic testing. The final-heat treatment (e.g. tempering, normalising, age hardening or dual-phase annealing) is effected during or immediately prior to the final-shaping to form a hollow body consisting of steel and having a polygonal, in particular square or rectangular, cross-section. As a basic condition for all final-heat treatments, it is applicable that reheating the intermediate hollow body to the predefined temperature range, in order to achieve the desired mechanical properties, lasts for at least 1 minute per mm of wall thickness and at the most 6 minutes per mm of wall thickness if heated in an oven using inductive heating lower times can be used.

With reference to an additional and optional intermediate heat treatment of the

intermediate hollow body in terms of a pre-normalising step, provision is preferably made that the intermediate hollow body is intermediately heat treated prior to the final-shaping, in particular by heating above an Ac3 temperature of the corresponding steel in order to austenitise and subsequently cool the intermediate hollow body prior to the final-shaping. Preferably, heating to an Ac3 temperature of the corresponding steel + 20 Kelvin, preferably to 870 to 980°C, maintaining said temperature for at least 5 minutes, and cooling with water, oil or polymers prior to the final-shaping are provided. By way of this cooling step, diffusion processes in the intermediate hollow body are minimised or avoided. In a particularly advantageous manner, a subsequent final-heat treatment is in the form of tempering at a temperature between 580°C and Ac1 - 20 Kelvin with this temperature being maintained for between 5 and 60 minutes, which is performed immediately prior to the final-shaping or with the final-shaping of the intermediate hollow body in a common step.

The temperatures Ac1 and Ac3 are defined as follows on the basis of generally known equations adapted by appropriate trials:

Ac1 = 734.2 - 13.9 ^* %Mn + 22.2 ^* %Si + 23.3 ^* %Cr - 14.4 ^* %Ni

Ac3 = 960.3 - 254.4 ^* %C ^1/2 - 14.2 ^* %Ni + 51.7 ^* %Si

Preferably, as an alternative final-heat treatment, provision is made that the hollow body is final-heat treated in the form of normalising at a temperature of at least Ac3 + 20 Kelvin with this temperature being maintained for at least 5 minutes. With respect to age hardening of the hollow bodies, provision is preferably made that the hollow body is final-heat-treated in the form of age hardening at a temperature of less than Ac1 and with a tolerance of +/- 30 Kelvin with this temperature being maintained for 10 to 60 minutes. This particular age hardening temperature is dependent upon the type of age hardening.

With respect to dual-phase annealing of the hollow bodies, provision is preferably made that the hollow body is final-heat treated in the form of dual-phase annealing at a temperature between Ac1 and Ac3 and with this temperature being maintained for 5 to 60 minutes. The obtained dual-phase microstructure can consist of combinations of ferrite, perlite, bainite, residual austenite and martensite.

In an advantageous manner, provision is made that the non-destructive testing is effected as eddy current testing or ultrasonic testing or as a combination thereof. Defects in the surface of the seamless or welded intermediate hollow body or of the flat pre-material can be detected by the eddy current testing and defects in the wall of the seamless or welded intermediate hollow body or of the flat pre-material can be detected by the ultrasonic testing.

Provision is preferably made that the final hollow body having rounded corners is tested externally in a non-destructive manner, in particular by means of magnetic particle testing, after the final production step in the region of its corners and/or ends. In a particularly advantageous manner, the method can begin with shaping, in particular bending and/or rolling, the flat pre-material to form a slotted hollow body having adjoining abutment ends, and welding the abutment ends to form a welded intermediate hollow body having a round cross-section, or with shaping, in particular rolling, the block-shaped pre-material to form a seamless intermediate hollow body having a round cross-section. Depending upon the subsequent field of application, known steels are used in this case. The flat pre-material in the form of strips or sheets can be cold-shaped or hot-shaped. With reference to the known shaping of seamless intermediate hollow bodies from a block-shaped pre-material, a combination of separate or integrated shaping steps is used, such as piercing, reducing the wall thickness and stretching, optionally flattening the surface and optionally rolling to size to a defined outer diameter prior to or in combination with a heat treatment. Internal tools are used for piercing, reducing the wall thickness and flattening. Rolling to size occurs without an internal tool. A logarithmic surface reduction ln(D0/(2 ^* S1 )) over the entire shaping of the block-shaped pre-material with a diameter DO to form intermediate hollow bodies having a round cross-section with a wall thickness S1 is preferably between 0.6 and 4.0.

Provision is preferably made that the intermediate hollow body is final-shaped immediately after the final-heat treatment at a temperature between 550°C and Ac1 - 20 Kelvin or at a temperature of at least Ac3 + 20 Kelvin and thus in the temperature ranges of hardening or normalising.

The present invention is particularly suitable for producing hollow bodies, in particular hollow profiles, having a polygonal, in particular square or rectangular, cross-section having rounded corners which are then used in the steel industry, in particular for cranes, in mechanical engineering, offshore applications, deep sea applications and wind turbines as well as for components subjected to high levels of vibration.

By using the present method in accordance with the invention, a rectangular final hollow body was produced experimentally from steel grade API 5L X70Q having the dimensions 200 mm x 140 mm and a wall thickness of 6.3 mm. After rolling an intermediate hollow body having a circular cross-section, austenitising (heating to and maintaining at a temperature of greater than Ac3) was performed, followed by quenching. Then, ultrasonic testing of the intermediate hollow body having a round cross-section was performed. Then, simultaneous final-shaping and tempering were performed at a tempering temperature of 550 to 750°C. The then measured tolerances met the specifications from standard EN 10210-2:2006. Also, no cracks occurred in the corners of the hollow body. Corresponding magnetic particle testing was performed. The following mechanical properties were achieved: yield point Rt0.5 > 485 MPa, tensile strength > 570 MPa, notch impact strength > 150 J/cm ² at -40°C and hardness < 240HV10. Typical lengths of the hollow bodies, in particular hollow profiles, are 12 m or 16 m.

Also, in accordance with the present method in accordance with the invention, a square hollow body, in particular a hollow profile, was produced experimentally from steel grade S355G15+N pursuant to EN 10225:2009 having the dimensions 160 mm x 160 mm and a wall thickness of 10 mm. After producing the intermediate hollow body, ultrasonic testing was performed. Then, the intermediate hollow body was final-shaped into its ultimate dimensions at a normalising temperature between 880 and 960°C. The then measured tolerances met the specifications from standard EN 10210-2:2006. Also, no cracks occurred in the corners of the hollow profile. Corresponding magnetic particle testing was performed. The mechanical properties met the specification from standard EN

10225:2009.

In order to be able to perform the final-shaping, in particular final-rolling, during the heat treatment, the device for final-shaping, in particular final-rolling, is to be arranged immediately downstream of the heat treatment oven in the normal product sequence.

The production method in accordance with the invention will be described in more detail hereinafter with the aid of exemplified embodiments illustrated in a drawing in which: Figure 1 shows a process flow chart of an exemplified embodiment.

In a first process sub-variation hereof, initially a flat pre-material 1 a is tested in a nondestructive manner by means of a non-destructive testing device 3a. Then, a slotted hollow body having adjoining abutment ends is produced from the flat pre-material 1 a by shaping, in particular bending and/or rolling. Then, the abutment ends are welded to form a welded intermediate hollow body having a round cross-section 2b. Alternatively to the non-destructive testing device 3a for the flat pre-material 1 a or in addition thereto this intermediate hollow body 2b can be tested in a non-destructive manner by means of an alternative or additional non-destructive testing device 3a. In a second process sub-variation hereof, starting from a block-shaped pre-material 1 b, a seamless intermediate hollow body having a round cross-section 2c is produced from a block-shaped pre-material 1 b. Shaping, in particular rolling, is used as the production method. This seamless intermediate hollow body having a round cross-section 2c is tested in a non-destructive manner by means of a non-destructive testing device 3a.

The then available intermediate hollow bodies having a round cross-section 2b, 2c are subjected to an intermediate heat treatment depending upon the respective material. This intermediate heat treatment consists of intermediate heating of the intermediate hollow body 2b, 2c in an intermediate heating oven 4a and subsequent cooling of the intermediate hollow body 2b, 2c in an intermediate cooling path 4b prior to the final- shaping to form an intermediate hollow body having a round cross-section with intermediate heat treatment 2b', 2c'. Alternatively to the above-described non-destructive testing devices 3a or in addition thereto, the intermediate hollow body having a round cross-section 2b, 2c can be tested in a non-destructive manner before the intermediate heat treatment 2b', 2c' by means of an alternative or additional non-destructive testing device 3a. Therefore, the non-destructive testing takes place after the intermediate shaping and prior to the final-shaping by means of one or a plurality of non-destructive testing devices 3a at different positions during the production process. Then, the intermediate hollow bodies 2b' or 2c' are subjected to a final-heat treatment in a final-heating oven 5a and a final-shaping device 5b in order to obtain a hollow body 6 having a polygonal, in particular square or rectangular, cross-section. Alternatively, the final-heat treatment and final-shaping can be performed in a common step. Finally, the final hollow body 6 is externally tested in the region of its corners, edges and/or ends using a non-destructive final testing device 7, in particular a magnetic particle testing device.

List of reference numerals

1 a Flat pre-material

1 b Block-shaped pre-material

2b Welded intermediate hollow body having a round cross-section

2c Seamless intermediate hollow body having a round cross-section

2b' Welded intermediate hollow body having a round cross-section with intermediate heat treatment

2c' Seamless intermediate hollow body having a round cross-section with intermediate heat treatment

3a Non-destructive testing device

4a Intermediate heating oven

4b Intermediate cooling path

5a Final-heating oven

5b Final-shaping device

6 Hollow body having a polygonal cross-section

7 Non-destructive final testing device