Method and apparatus for producing steel pipes having particular properties

The invention relates to a method and to an apparatus for producing pipes made of steel. According to the invention, within a period of time of no more than 20 seconds after the last deformation at a temperature greater than 700° C., but less than 1050° C., during passage a cooling medium is applied with elevated pressure onto the outside circumference of the pipe over a length of greater than 400 times the pipe wall thickness in a quantity which during rapid cooling provides an equivalent cooling speed of greater than 1° C./second of the pipe wall over the pipe length to a temperature in the range of 500° C. to 250° C., whereupon further cooling of the pipe down to room temperature is carried out by exposure to air.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a National Stage of International Application No. PCT/AT2009/000439 filed Nov. 16, 2009, and which claims the benefit of Austrian Patent Application No. A 1814/2008, filed Nov. 20, 2008, the disclosures of which are incorporated herein by reference.

The invention relates to a method for producing pipes made of steel having improved strength and improved toughness of the material.

In addition, the invention relates to a device for producing pipes having a special profile of properties, consisting of a device for applying a cooling medium to the surface of the pipe.

In manufacturing seamless pipes, the properties of the material of the pipe wall may exhibit substantial variations locally and from one lot to the next. These differences in properties are usually based on an irregular microstructure and an unfavorable steel composition and/or an increased proportion of contaminants and accompanying elements.

For pipes that are subjected to high stresses, a microstructure that meets these requirements and is uniform within narrow limits over the length of the pipe as well as coaxially in the pipe wall while also having a material composition that is free of harmful elements should be obtained for the reasons given above.

Pipes that are 7 meters or more long and have an outside diameter of less than 200 mm with a wall thickness of less than 25 mm can be subjected to a heat treatment only with a great deal of complexity, but such a heat treatment produces a uniformly fine structure with the desired microstructure over the entire volume of the pipe while minimizing bending at a right angle to the longitudinal direction.

There are known methods in which a pipe is rotated about its axis and is cooled on the outside surface and/or on the inside surface. However, such heat treatment methods presuppose an approximately uniformly high temperature of the material over the length of the pipe in order to achieve a homogeneous microstructure in the wall.

Using pipe precursor material from the same parent melt having a chemical composition in wt % according to Table 1:

DesignationCSiMnPSCrNiCuAlMoFePipe blank0.18190.29101.42310.01460.00650.04150.02750.02110.02740.0126remainderdiameter
ultimately pipes having the following dimensions were produced:

After the last step and/or after the final shaping in the discharge station of the stretch reducing plant, the pipe was introduced into a cooling through-zone at a temperature of 880° C. after a period of 12 sec.

Assuming the defined conversion behavior of the steel, the cooling medium flow was directed only at the outside surface of the pipe in investigations on individual lots in pipe production, such that a cooling rate of approx. 6° C./sec was measured by adjusting the cooling medium flow at the following final temperatures:

Temperature Identification of the Sample

After achieving these specified final cooling temperatures, the cooling medium supply was shut down and the pipe was cooled further to room temperature at a low intensity essentially in stationary air.

Samples were taken of the pipes that had been heat treated in various ways and labeled as P1through P4and then tests of materials were performed on these samples.

A determination of the microstructure revealed that there was an advantageously directional structure in each case, essentially without texture but with a grain size and structure distribution which depend on the final cooling temperature.

FIG. 1shows the structure of sample P1, where the grain size is 20 μm to 30 μm with a high ferrite content. The remaining component of the structure was mainly perlite.

FIG. 2shows a much smaller average grain size of sample P2of approx. 5 μm to 8 μm, which correlates with a low final cooling medium temperature of T2=480° C. In addition, the perlite content in the ferrite has a finer structure and the amount is slightly greater.

FIG. 3shows that the material of sample P3has a fine grain due to a high seed count conversion and recrystallization of the structure at a final cooling temperature of T3=380° C. and also has largely homogeneously distributed ferrite regions which increase strength. Perlite and the structure of the upper intermediate stage and/or upper bainite were the other constituents of the refined structure.

FIG. 4shows the structure of pipe wall P4, which was formed in rapid cooling after shaping to a final cooling temperature T4=300° C. Extremely fine-grained ferrite phases, which are globulitic due to end limitation with fine lamellar perlite and intermediate stage components in the lower bainite range, result in high strength values with improved strain results for the material.

In cooling of the pipe wall at a rate of greater than 1° C./sec immediately after hot shaping of the basic iron material, an austenite structure shaped in this way can be largely undercooled with respect to the equilibrium resulting in a conversion of the structure as a function of the extent of the undercooling and the seed state. The desired uniform microstructure can be established advantageously by means of the inventive method over the entire length of a pipe and surprisingly also over its cross section and this microstructure also determines the properties of the material. In other words, if fundamental material properties are required of a pipe, choice of an alloy is indicated. An advantageous and favorable profile of properties of the material which is provided can be achieved through an inventive method in the device according to the invention.

FIG. 5shows in a bar graph the measured values for strain limit (Rp) (0.2) [MPa], tensile strength (Rm) [MPa], necking (Ac) [%] and toughness (KV450) [J] of the samples P1through P4, i.e., as a function of the mechanical properties of the material which are achieved through the different cooling parameters in the refining technology.

With the same steel composition, the strain limit of the material of the pipe wall can be increased from 424 [MPa] to 819 [MPa] while at the same time the decline in strain values from 26 [%] to 10 [%] can be minimized, which causes the toughness of the material to decline from 170 [J] to 160 [J].

At high final cooling temperatures as is the case for sample material P1for example, there is a great deal of recrystallization and formation of large grains, which imparts high toughness and necking to the material but causes a comparatively low level of strength.

Cooling to lower ambient temperatures increases the strength of the pipe wall and naturally also slightly reduces the necking and toughness of the material, as illustrated on the basis of samples P2, P3and P4.

With the inventive method, microstructures can also be adjusted in the material in a targeted manner, yielding the profile of properties of the pipe wall. For example, a high measure of conversion to a lower bainite structure can be achieved in sample pipe P4by means of a low conversion temperature, so an increased toughness of the material could be achieved.

FIG. 6shows the measured hardness values over the length of the pipe of experimental pipes P1and P4. It has been found that a scattering S of the material hardness over the length of the pipe is also reduced with an increase in hardness [HRB] and strength levels of the material due to intensified application of cooling medium.

FIG. 7shows the hardness curve of the material in the quadrants as a function of the thickness of the pipe wall of experimental pipe P2.

The measurement results of the four quadrants Q1to Q4are averages of four measurements spaced a distance apart in each quadrant in the external, central and internal areas of the pipe wall.

As also shown by the comparison of the respective hardness values over the cross section of the pipe wall in the quadrants, there are only extremely minor differences in material strength, so the achievable quality of the product is represented by using the inventive method and such a device.