Patent Application
20070267110
While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will herein be described in detail, several preferred embodiments, with the understanding that the present disclosure should be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the embodiments so illustrated.
When making reference to percentages of compositional components, it is to be understood that, unless otherwise specified, the percentage given is by weight of the total alloy.
As indicated hereinabove, the present invention is directed to a steel composition and processing method used in producing an as-welded tubular steel product, and particularly a casing product for the OCTG market, having a minimum yield strength in the range of 80 ksi (552 MPa) to 125 ksi (862 MPa). Preferably, the invention is directed to a steel composition and processing method used in producing an as-welded tubular steel product having a minimum yield strength in excess of 100 ksi (689 MPa), and a minimum ultimate tensile strength in excess of 110 ksi (758 MPa).
This invention employs a controlled steel composition and steel processing procedure that produces a skelp having the desired bainitic microstructure and properties, such that the subsequent pipe formation process of forming, welding and sizing hardens the material to the desired strength level. By producing a skelp having a bainitic microstructure, the invention provides a steel pipe which is not only stronger than known as-welded pipe products having a predominantly ferrite+pearlite microstructure, but also exhibits increased weldability and formability when compared to other such products.
While a preferred use of the steel pipe made according to the invention is for oil and gas well casings used in the OCTG market, the scope of the invention is not limited to that particular use. Rather, the steel pipe of the invention is suitable for use in any application for which a steel pipe having a yield strength in excess of 80 ksi (552 MPa) is desirable.
An important feature of the present invention is the composition of the steel used in forming the skelp. Use of the proper steel composition is believed to be important in achieving the benefits of the invention, namely, an as-welded pipe having a bainitic microstructure which provides a minimum yield strength in the range of 80 ksi (552 MPa) to 125 ksi (862 MPa). The key compositional parameters considered to be important to the invention are: (i) a low carbon content of less than 0.10%, to maintain a good level of formability and weldability in the skelp; (ii) a Mn content in the range of 1.5% to 2.5%, to help produce the desired as-rolled bainitic microstructure; (iii) the addition of Mo (0.10% to 0.50%), Cr (up to 0.50%), Ni (up to 0.50%) and/or B (0.0003% to 0.0030%) to help produce the desired as-rolled bainitic microstructure, and (iv) the addition of Nb, V and Ti, singly or in combination, to increase the strength of the as-rolled skelp.
The component percentages of the preferred steel composition can be adjusted upwardly or downwardly, depending upon the pipe strength level that is desired in a given application. For example, an increase in the pipe strength will be expected as carbon, manganese, molybdenum, chromium and/or nickel levels are increased. This is due to the effect of these elements in reducing the transformation temperature range. Additionally, increasing the niobium (columbium) content will be expected to increase the strength of the steel due to increased precipitation strengthening.
The use of each of these elements as an additive in a steel composition, in order to achieve these desired properties, is well-known in the art of steel processing; however, the use of the particular combinations of those elements described herein, in combination with the specific processing steps described hereinbelow, is believed to be novel and non-obvious.
While the processing of the skelp utilized in forming the casing products of the invention is similar to conventional steel processing procedures used in manufacturing steel skelp for forming as-welded pipe products for the OCTG market, as described above, certain departures from the conventional procedures are required in order to achieve the benefits of the invention. Specifically, it is believed that the temperature of the skelp must be carefully controlled during the coiling process, within the range of 850° F. to 950° F., in order to achieve the desired bainitic microstructure in the skelp, which enables the manufacture of an as-welded pipe having the desired minimum yield strength in excess of 80 ksi (552 MPa).
The processing procedure utilized in connection with the invention is illustrated in
The slab heating temperature must be controlled to completely dissolve the Nb precipitates, and, depending on the Nb, C and N levels in the steel, is chosen to ensure complete solution of all alloy carbides and nitrides. The minimum reheating temperature required to achieve this goal depends in large part on the Nb content of the steel, as further discussed in the Examples below. Preferably, a reheating temperature of approximately 2300° F., which is well above the minimum required reheating temperature for compositions made according to the invention (as shown below), is utilized.
The rolling temperature must likewise be controlled to ensure that rolling is completed in the fully austenitic phase field. It is well-known that the austenite phase starts to decompose to ferrite+pearlite and lower temperature transformation products below the transformation start (Ar3) temperature, which, for steel compositions according to the invention, is in the range of from about 1250° F. to about 1350° F. Thus, it is preferable to maintain the rolling temperature at well in excess of the Ar3 temperature, to ensure that the skelp is fully austenitic during rolling. Thus, a rolling temperature of approximately 1500° F., well above the Ar3 temperature, is preferably utilized.
Following the rolling step, the skelp is then passed through a runout table 7, where the skelp is cooled with water to a coiling temperature in the range of about 850° F. to about 950° F.—well below the 1100° F. to 1200° F. coiling temperature which is typically used in forming the skelp for other as-welded casing products, as described above. The skelp is then passed to a coiler 8, where it is formed into a hot-rolled steel coil 9. The purpose of coiling at a lower temperature is to produce a largely bainitic microstructure in the skelp, which will strengthen during the subsequent pipe forming operation.
Particularly, it has been found that the use of a coiling temperature of about 850° F. to about 950° F. is critical to obtaining a skelp having the desired bainitic microstructure, which is suitable for forming a casing product of this invention having the desired minimum yield strength. The use of the preferred composition, together with the steel processing procedure described above, has been found to produce a skelp having a yield strength of approximately 80 ksi (552 MPa). Upon formation of the skelp so obtained into pipe, using the pipe-formation method described hereinbelow, an increase in yield strength of approximately 25 to 30 ksi (172 to 207 MPa) has been observed.
Once processing of the skelp has been completed, the coiled skelp is then transferred to a pipe mill for pipe formation. The preferred pipe formation process, facets of which are known in the art, utilizes electric resistance welding (ERW) to produce a forged weld along the pipe seam. The use of ERW in the manufacturing of steel pipe is well known in the pipe manufacturing industry. While the use of ERW is preferred in forming pipe from skelp made according to the invention, other welding methods known in the art of pipe formation could likewise be utilized in practicing the invention. Such methods include TIG (tungsten inert gas) welding, electron beam welding, and laser welding, to name but a few.
The preferred pipe formation process utilizes a coiled skelp made according to the invention as described hereinabove, which has a thickness that is approximately the ultimate pipe wall thickness and a width that is a multiple of the ultimate pipe circumference. For example, if the intent is to make 4.5″ (outside diameter)×0.250″ (wall thickness) pipe, the skelp would be rolled to a nominal 0.250″ thick and 71.2″ wide (i.e. 5 multiples of the desired pipe circumference), for slitting into 5 strips of equal width.
The as-rolled coil is first slit into multiples of the desired input width corresponding to the desired pipe size. The dimensions may vary somewhat depending on the specific pipe mill used. For example, at one mill, to make 4.5 ″×0.250″ pipe, the nominal slit width may be 13.90″; while at another mill, to make 5.5 ″×0.304″ pipe, the nominal slit width may be 16.90″ and for 7″×0.362″ pipe, the slit width may be 21.61″; while at still another mill, the slit widths for 4.5″×0.250 ″, 5.5″×0.304″ and 7 ″×0.362″ may be 13.96 ″, 17.08″ and 21.58″ respectively. The slit strips are then fed into the pipe mill for forming, welding and sizing, to produce a casing product having the desired final dimensions.
While in the preferred embodiment of the invention the coiled skelp is slit into multiple strips for pipe formation, this slitting procedure is not necessary to obtain the benefits of the invention. For example, a hot-rolled skelp could be produced according to the invention in which the width of the skelp is approximately equal to the desired pipe circumference, thereby eliminating the need for slitting of the skelp into strips prior to pipe formation.
The stages in the pipe formation process are shown in
Several steel compositions have successfully been prepared according to the invention, and have been utilized to form as-welded casing products having the desired minimum yield strength in the range of 80 ksi (552 MPa) to 125 ksi (862 MPa).
Two highly preferred steel compositions which have been successfully prepared and formed into pipe, using the methods described hereinabove, and shown to yield as-welded casing products having the highly preferred minimum yield strength in excess of 100 ksi (689 MPa), are listed in Table 1.
A total of twelve heats (batches) corresponding to Composition A were used to prepare skelps as described hereinabove, namely, by (i) heating the cast steel slab to a temperature of approximately 2300° F.; (ii) hot-rolling the heated slab in a steckel mill at a temperature of approximately 1500° F., to obtain a skelp having a thickness of approximately 0.25 inches; (iii) cooling the skelp with water to a coiling temperature in the range of 850° F. to 950° F.; and (iv) forming the skelp into a coil using conventional coiling methods. It was found that the minimum slab heating temperature for Composition A must be above about 2170° F. in order to completely dissolve the Nb precipitates.
The precise composition of each of the twelve heats prepared according to Composition A is listed in Table 2.
The skelps obtained from each of Heats A1-A12 were tested for yield strength and ultimate tensile strength, with the yield strength of the skelps ranging from 75.4 ksi (520 MPa) to 91.4 ksi (640 MPa), and the ultimate tensile strength of the skelps ranging from 100.9 ksi (696 MPa) to 111.9 ksi (772 MPa). Those skelps were then formed into pipes having an outer diameter of 4.5 inches and a wall thickness of 0.25 inches, using the pipe formation method described hereinabove, and tested to determine the minimum yield strength and ultimate tensile strength for each pipe. The results of that strength testing are listed in Table 3.
Additionally, skelps from each of heats A10 and A12 were formed into pipes having an outer diameter of 5.5 inches and a wall thickness of 0.29 inches, using the pipe formation method described hereinabove, and tested to determine the minimum yield strength and ultimate tensile strength for each pipe. The results of that strength testing are listed in Table 4.
As can be seen from Tables 3 and 4, of the 67 representative pipes made according to the preferred composition (Composition A) and preferred processing method of the invention, 66 of those pipes obtained the highly preferred minimum yield strength of at least 100 ksi (689 MPa). Similarly, each of the 67 pipes made according to the composition and method of the invention obtained the highly preferred minimum ultimate tensile strength of at least 110 ksi (758 MPa). Thus, it is clear from the above data that the methods of the invention were successful in obtaining as-welded pipe for use in casing products for the OCTG market, having a minimum yield strength of about 100 ksi and a minimum ultimate tensile strength of about 110 ksi.
A total of 2 heats corresponding to Composition B were likewise used to prepare skelps as described hereinabove, namely, by (i) heating the cast steel slab to a temperature of approximately 2300° F.; (ii) hot-rolling the heated slab in a steckel mill at a temperature of approximately 1500° F., to obtain a skelp having a thickness of approximately 0.25 inches; (iii) cooling the skelp with water to a coiling temperature in the range of 850° F. to 950° F.; and (iv) forming the skelp into a coil using conventional coiling methods. In contrast to the slabs prepared according to Composition A, a lower minimum slab heating temperature of above about 2060° F. was required for Composition B, as a result of the lower Nb content of Composition B.
The precise composition of each of the 2 heats prepared according to Composition B is listed in Table 5.
The skelps obtained from Heats B1 and B2 were tested for yield strength and ultimate tensile strength, and were found to have yield strengths of 74.2 ksi (512 MPa) (B1) and 78.0 ksi (538 MPa) (B2), and ultimate tensile strengths of 102.5 ksi (707 MPa) (B1) and 104.7 ksi (722 MPa) (B2). Those skelps were then formed into pipes having an outer diameter of 4.5 inches and a wall thickness of 0.25 inches, using the pipe formation method described hereinabove, and tested to determine the minimum yield strength and ultimate tensile strength for each pipe. The results of that strength testing are listed in Table 6.
As can be seen from Table 6, each of the ten representative pipes made according to the preferred composition (Composition B) and preferred processing method of the invention obtained the highly preferred minimum yield strength of at least 100 ksi (689 MPa) and the highly preferred minimum ultimate tensile strength of at least 110 ksi (758 MPa). Thus, it is clear from the above data that the methods of the invention were again successful in obtaining as-welded pipe for use in casing products for the OCTG market, having a minimum yield strength of about 100 ksi and a minimum ultimate tensile strength of about 110 ksi.
In addition to the foregoing examples of as-welded pipes made according to the composition and method of the invention, a number of additional as-welded pipes were made using the identical steel processing and pipe formation methods described above, but using different preferred steel compositions—which differed from those of Compositions A and B primarily in the fact that they contained higher amounts of carbon and lower amounts of Nb and V than Compositions A and B of Examples I and II. Three such compositions are listed in Table 7.
Skelps were prepared from one heat corresponding to each of Compositions C, D and E as described hereinabove, namely, by (i) heating the cast steel slab to a temperature of approximately 2300° F.; (ii) hot-rolling the heated slab in a steckel mill at a temperature of approximately 1500° F., to obtain a skelp having a thickness of 0.25 inches; (iii) cooling the skelp with water to a coiling temperature in the range of 850° F. to 950° F., and (iv) forming the skelp into a coil using conventional coiling methods.
The skelps obtained from heats C and E were tested for yield strength and ultimate tensile strength, with the yield strength of the skelps ranging from 77.4 ksi (534 MPa) to 86.5 ksi (596 MPa), and the ultimate tensile strength of the skelps ranging from 92.8 ksi (640 MPa) to 106.6 ksi (735 MPa).
The skelps of Heats C, D and E were then formed into pipes having an outer diameter of 4.5 inches and a wall thickness of 0.25 inches, using the pipe formation method described hereinabove, and tested to determine the minimum yield strength and ultimate tensile strength for each pipe. The results of that strength testing are listed in Table 8.
Thus, while the as-welded pipes made according to Compositions C, D and E did not always achieve the highly desired minimum yield strength of 100 ksi (689 MPa) or the highly desired minimum ultimate tensile strength of at least 110 ksi (758 MPa), each of those pipes did achieve the desired minimum yield strength well in excess of 80 ksi (552 MPa). It is believed that the slightly lower yield strength and tensile strength of pipes made from Compositions C, D and E, relative to those made from Compositions A and B, is attributable to the lower levels of Nb which are present in Compositions C, D and E.
Four additional heats corresponding to Composition A were used to prepare skelps as described hereinabove, namely, by (i) heating the cast steel slab to a temperature of approximately 2300° F.; (ii) hot-rolling the heated slab in a steckel mill at a temperature of approximately 1500° F., to obtain a skelp having a thickness of 0.25 inches; (iii) cooling the skelp with water to a coiling temperature in the range of 850° F. to 950° F., and (iv) forming the skelp into a coil using conventional coiling methods. The precise composition of each of those heats is listed in Table 9.
The skelps obtained from each of Heats A13-A16 were formed into pipes having an outer diameter of 5.5 inches and a wall thickness of 0.304 inches, using the pipe formation method described hereinabove, and tested to determine the minimum yield strength and ultimate tensile strength for each pipe. The results of that strength testing are listed in Table 10.
As can be seen from Table 10, while each of the 39 representative pipes made from Heats A13-A16 and the preferred processing method of the invention obtained a minimum yield strength well in excess of the 80 ksi (552 MPa) preferred minimum yield strength, a number of those pipes did not achieve the highly preferred minimum yield strength of at least 100 ksi (689 MPa) or the highly preferred minimum ultimate tensile strength of at least 110 ksi (758 MPa).
In order to consistently obtain pipe having the highly preferred minimum yield strength of at least 100 ksi (689 MPa) for this size pipe at this particular pipe mill, it was theorized that a higher carbon content would increase the yield strength and ultimate tensile strength of the pipe, while still retaining the benefits of the invention. Thus, two additional heats generally corresponding to Composition A in other parameters, but having a higher carbon content of at least about 0.075%, were prepared. This compositions for those two heats are listed in Table 11.
The skelps obtained from each of Heats F1 and F2 were likewise formed into pipes having an outer diameter of 5.5 inches and a wall thickness of 0.304 inches, using the pipe formation method described hereinabove, and tested to determine the minimum yield strength and ultimate tensile strength for each pipe. The results of that strength testing are listed in Table 12.
As can be seen from Table 12, each of the 17 representative pipes made according to the preferred composition (Composition F) and preferred processing method of the invention obtained the highly preferred minimum yield strength of at least 100 ksi (689 MPa) and the highly preferred minimum ultimate tensile strength of at least 110 ksi (758 MPa). Thus, it is clear from the above data that the methods of the invention were again successful in obtaining as-welded pipe for use in casing products for the OCTG market, having a minimum yield strength of about 100 ksi and a minimum ultimate tensile strength of about 110 ksi.
The foregoing description and drawings merely explain and illustrate the invention, and the invention is not so limited as those skilled in the art who have the disclosure before them will be able to make modifications and variations therein without departing from the scope of the invention.
