Method for producing a tempered seamlessly hot-fabricated steel pipe

Abstract

A method for producing a tempered, seamlessly hot-rolled steel pipe includes heating a hollow block to forming temperature and rolling the heated block in a rolling mill to form a pipe with a finished diameter after rolling. Subsequently, the pipe is tempered with appropriate tempering parameters after rolling whereby the diameter of the pipe increases during tempering. The finished diameter of the pipe to be tempered after rolling in the rolling mill is adjusted as a function of a value of the growth in diameter of the pipe during tempering.

Claims

1. A method for producing a tempered, seamlessly hot-rolled steel pipe, comprising: heating a hollow block to forming temperature and rolling the heated block in a rolling mill to form a pipe with a finished diameter after rolling; subsequently tempering the pipe after said rolling with appropriate tempering parameters whereby the diameter of the pipe increases during said tempering; and adjusting the finished diameter of the pipe to be tempered after said rolling as a function of a value of the growth in diameter of the pipe during said tempering, and wherein the tempering comprises heating in a furnace, subsequent continuous flow cooling in a cooling path and an annealing process, adjusting the tempering parameters on the basis of a bandwidth of previously determined connections between diameter, pipe wall thickness, material quality, tempering parameters and diameter growth, and that subsequently on the basis of the measured finished diameter of the pipe being rolled finely adjusting the tempering parameters with respect to a target diameter of the pipe to be achieved after tempering.

2. The method as claimed in claim 1 including adjusting the tempering parameters to produce a pipe with the target diameter which corresponds to a finished diameter after tempering in a preset tolerance range.

3. The method as claimed in claim 1 wherein the finished diameter is achieved after tempering without the assistance of sizing rolling.

4. The method as claimed in claim 1 including adjusting the finished diameter for the pipe to be tempered as a function of a value of growth in the diameter of the pipe during tempering and of a value of growth in diameter of a group of pipes with a same nominal diameter as the pipe to be tempered but with wall thicknesses, material qualities or specifications which differ from each other.

5. The method as claimed in claim 4 including adjusting the tempering parameters in such a way that starting from a single finished diameter for each pipes in the group, a pipe is produced with the target diameter.

6. The method as claimed in claim 1 including measuring a finished diameter of the pipe after rolling.

7. The method as claimed in claim 1 including changing the target diameter of the pipe after tempering by changing a cooling rate in a cooling path.

8. The method as claimed in claim 7 including changing the cooling rate of external cooling of the pipe by varying at least one chosen from the quantity of cooling water, the temperature of the cooling water and the transportation speed of the pipe in the cooling path.

9. The method as claimed in claim 8 including controllably adjusting the water quantity poured onto the pipe to be cooled between 50 and 300 m.sup.3/hr, controllably adjusting the cooling water temperature to below 40 C. and controllably adjusting the transportation speed of the pipe in the cooling path to values between 0.1 m/s and 1 m/s.

10. The method as claimed in claim 8 including internal cooling of the pipe in addition to the external cooling, wherein the quantity of cooling water is between 50 and 250 m.sup.3/hr.

11. The method as claimed in claim 8 including using two or more annular showers or annular sprays for the external cooling.

12. The method as claimed in claim 1 including heating for hardening purposes in a furnace which has at least two zones over the furnace length, of which the first of said zones serves for heating purposes and the second of said zones serves for temperature equalization in the pipe.

13. The method as claimed in claim 1 including heating for hardening of the pipe in a first furnace and providing temperature equalization in the pipe in a second furnace.

14. The method as claimed in claim 12 wherein the heating for hardening purposes takes place in three zones, wherein the first zone serves for preheating, the second zone for heating and the third zone for temperature equalization in the pipe.

15. The method as claimed in claim 12 wherein the time at which the temperature is held at hardening temperature is at least 3 minutes, wherein the holding time begins when the lowest temperature achieved in the pipe reaches the value of the target pipe temperature minus 20 C.

16. The method as claimed in claim 14 wherein the different zones are located within one furnace.

17. The method as claimed in claim 14 wherein the different zones are located within a plurality of furnaces.

18. The method as claimed in claim 2 wherein the finished diameter is achieved after tempering without the assistance of sizing rolling.

19. The method as claimed in claim 2 including adjusting the finished diameter for the pipe to be tempered as a function of a value of growth in the diameter of the pipe during tempering and of values of growth in diameter of a group of pipes with the same nominal diameter as the pipe to be tempered but with wall thicknesses, material qualities or specifications which differ from each other.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The method in accordance with the invention is explained in more detail with the aid of the appended figures in which:

(2) FIG. 1 is a schematic illustration of the factors influencing the target diameter after tempering;

(3) FIG. 2 illustrates the influence of the pipe diameter on the growth with internal cooling;

(4) FIG. 3 illustrates the influence of the pipe diameter on the growth without internal cooling;

(5) FIG. 4 illustrates the influence of the flow rate on the growth without internal cooling; and

(6) FIG. 5 illustrates the influence of the flow rate on the growth with internal cooling.

DESCRIPTION OF THE PREFERRED EMBODIMENT

(7) FIG. 1 schematically illustrates how the method in accordance with the invention is applied in order that a single finished diameter for the rolling mill is set for different target diameters to be achieved after tempering. Target diameter is understood to be a desired variable. The finished diameter after rolling or the finished diameter after tempering is understood as a specific actual variable. FIG. 1 shows diameter values or ranges of five exemplified pipe types which are qualitatively defined by the influencing factors of wall diameter W, material quality G and specification S. Material quality G is to be understood essentially to be the material properties, and specification S is to be understood essentially to be the dimensions and tolerances.

(8) Whether different pipe types can be rolled in one rolling mill with a single finished diameter in accordance with embodiments of the present invention, although the subsequent tempering leads to a different diameter growth, is explained with the aid of FIG. 1. For this purpose, the different target diameters after tempering (see the points marked with x in FIG. 1) are plotted for the five pipe types with the same nominal diameter in terms of a nominal width. These result from the specification S of the respective pipe type since all dimensions and tolerances are maintained. In a corresponding manner, the first and second or third and fourth pipe types with the same specification X or Y each have the same target diameter after tempering. Within the permitted tolerances of the specification, these could also easily be selected to be different from each other. By tests and production results, the minimum and maximum diameter growth in absolute values is now determined for each pipe type and, starting from the target diameter after tempering, is applied in terms of a reduction in diameter. The minimum diameter growth is plotted in the form of the region with a white background with the legend Minimum growth of the pipe diameter during tempering and results for this pipe type from the minimum required tempering parameters such as, e.g., a minimum cooling rate, in order to achieve the desired target structure during tempering. By changing the tempering parameters, it is possible at the start for the Minimum growth of the pipe diameter during tempering region to be enlarged with the minimum resulting diameter growth and a greater diameter growth to be achieved in a corresponding manner. This region of the additional diameter growth is plotted as a hatched region with the legend Region of influence of the diameter growth. A comparison of the Minimum growth of the pipe diameter during tempering and Region of influence of the diameter growth regions for the five pipe types shows that there is a type of intersection region which is plotted with the arrow symbol and the legend permitted region for the diameter prior to tempering. The diameter prior to tempering corresponds to the previously described finished diameter after rolling. The permitted region for the diameter prior to tempering is limited in the upwards direction by the smallest diameter of the five Minimum growth of the pipe diameter during tempering regions (see fourth pipe type from the left, value between the Minimum growth of the pipe diameter during tempering and Region of influence of the diameter growth regions). The permitted region for the diameter prior to tempering is defined in the downwards direction by the largest diameter of the respective lower limit value for the five Region of influence of the diameter growth regions (see first pipe type from the left, lowermost limit value of Region of influence of the diameter growth).

(9) On this basis, the finished diameter of the rolling mill is adjusted to a value within the permitted region for the diameter prior to tempering preferably in the middle of the permitted region for the diameter prior to tempering. All five pipe types can now be rolled on this rolling mill uniformly and the target diameters deviating from each other at the end after tempering are achieved by an appropriate adjustment of the tempering parameters. The permitted region for the diameter prior to tempering has a sufficient bandwidth to also allow for any production tolerances. For other groups of pipe types with the same nominal diameter it can be the case that the resulting permitted region for the diameter prior to tempering is very narrow or there is no corresponding region in the sense of an intersection region. For this case, the groups are then to be selected differently or sub-groups of pipe types are to be formed for which then a permitted region for the diameter prior to tempering with a sufficient bandwidth again results.

(10) FIGS. 2 to 5 show by way of example the dependency of the diameter growth of the pipe upon the tempering parameters, in particular the cooling parameters. With the aid of the adapted quenching parameters, in particular the pipe speed, the volume flow and with or without internal cooling, it is possible for an identical finishing diameter of the rolling mill, which is within preset tolerances of, e.g., +/0.5%, to achieve the desired target diameter after tempering depending on pipe type.

(11) Thus, FIG. 2 shows how the growth of the diameter during tempering increases in dependence upon the diameter size with constant pipe wall thickness for a material family A from the oil field pipe range (OCTG).

(12) The flow rate of the pipe through the cooling path is in this case kept constant at 35% of the maximum value, the quenching conditions on the outside, i.e., the water quantity, the number of annular showers and the water pressure. In addition, in this case the pipes were also quenched on the inside with a constant quantity of water over time.

(13) FIG. 3 shows the same dependency as FIG. 2, but without additional internal cooling and for a selected flow rate of 22% of the maximum value.

(14) FIGS. 4 and 5 show how the selected flow rate influences the diameter growth of the pipe for the nominal dimensions 406.414.6 mm from the material group B. In this case, also the cooling conditions on the outside are kept constant. In the tests in accordance with FIG. 4, work was carried out without additional internal cooling, but in the tests in accordance with FIG. 5, work was carried out with internal cooling.

(15) In the tables of values of FIGS. 4 and 5, the minimum and maximum growth are shown within practicable values for the tempering parameters, such as flow rate and with or without internal cooling. For the nominal dimensions 406.414.6 mm, FIG. 5 provides a minimum growth of the diameter of 0.9 mm and FIG. 4 a maximum growth of 1.46 mm.