CA2594631C - Steel tubing with enhanced slot-ability characteristics for warm temperature service in casing liner application and method of manufacturing the same - Google Patents

Abstract

A post-yield hardened steel tube, particularly useful for creating slotted liners, for use in various applications in the oil and gas industries. The steel specifications meet the broad API 5CT standard, but the resulting slotted tube exhibits both enhanced slot-ability characteristics and superior thermo- mechanical characteristics in buckling resistance and localization resistance. A method of manufacturing a steel tube with substantial post-yield hardening behavior across a temperature range between room temperature and 350 °C while providing good slot- ability, comprising using a steel meeting the broad API 5CT standard but with very small quantities of sulfur, performing a standard hot rolling process followed by a specifically defined heat treatment cycle, so as to create a microstructure characterized either ferrite plus pearlite or a ferrite plus bainite-pearlite.

Description

Steel Tubing with Enhanced Slot-ability Characteristics for Warm Temperature Service in Casing Liner Applications and Method of Manufacturing the Same BACKGROUND OF THE INVENTION
Field of The Invention [0001] The present invention relates to controlling the mechanical properties of buckling resistance (or buckling "stability") and localization resistance, while also improving a characteristic called "slot-ability", that permits faster and less expensive slotting of steel tube, and particularly casing liners, using cutting blades.
The present invention therefore relates to both a steel tube with enhanced slot-ability characteristics that will hold up to high temperature/high stress service in casing and tubing applications as well as a method of manufacturing such a steel tube.

[0002] As used herein "slot-ability" should be understood as a very specific subset of machinability, and is referred to the level of difficulty required to plunge a thin saw blade, with an approximate thickness the range of 0.010" and 0.028", through the side wall of a steel tubular. Slot-ability can be quantified as to how many times, or how well, a given saw blade may cut a slot in the tubular wall material before breaking.
The importance of not confusing machinability with slot-ability can be appreciated from the general observation that K-55 materials are generally more "machinable"
but at the same time less "slottable" than L-80 materials.

[0003] The steel tubing is particularly suited for use in the oil and gas industries for applications in which slotted liners are needed. Examples of such applications include sand control, horizontal wells and heavy oil recovery through steam injection, to name a few. The term SAGD service is used to reference steam-assisted gravity drainage, which is a particularly high temperature and high stress oilfield tubular procedure used for thermal oil recovery, and which particularly benefits from use of slotted liners as taught herein.
Brief Description of Prior Art

[0004] Slotted tubes are obtained by cutting longitudinal slots through the wall of a steel tube using very thin saw blades and machines specially designed for slotting steel. Once slotted, the tubes, in many applications, are installed in wells where they will serve in warm temperature environments. In one typical example, a slotted tube is installed in a well where steam will be injected to stimulate heavy or extra-heavy oil production. In this and other applications, installation and service loads can be quite high and during service it is common for the material to go into its plastic region. Thus, in order to satisfactorily withstand all of the various load requirements, the slotted steel tubing must consistently maintain minimum yield strength (YS) and minimum ultimate tensile strength (UTS) at room temperature as well as exhibit good and consistent YS and UTS behavior at temperatures up to about 350 C and good thermal fatigue (TF) behavior. In addition, the steel tubing must have good and consistent post-yield hardening modulus (PYHM) and limited post-yield relaxation (PYR) in the plastic region across all the temperature and strain ranges, as well as limited yield plateau (YP) at all service temperatures.

[0005] Conventionally, steel tubing used for thermal well applications is standard steel tubing as defined in the American Petroleum Institute (API) "Specification 5CT
for Casing and Tubing" (API 5CT) ¨ International Standard ISO 11960 "Petroleum and natural gas industries ¨ Steel pipes for uses as casing or tubing for wells" (ISO
11960). While such steel tubing, may have a sufficient slot-ability it will not necessarily also have the very important thermo-mechanical properties desirable for that subset of casing that is employed as a slotted liner.

[0006] The two most common casing grades used for slotted liner are Grades K55 and L80. L80 has been shown to exhibit reasonably good slot-ability but most steels have demonstrated unfavorable slot-ability. The unacceptable K55 materials are those that demonstrate a yield strength close to the lower end of the specified API
range. Thus a critical feature of the invention is not exclusively improved slot-ability but that in combination with good thermo-mechanical properties, which simply are not found in L80 grades that do offer good slot-ability. As a consequence, slotting K55 materials to form slotted tubing has created problems such as low productivity rates, high tooling consumption, and significant operative delays, each of which increases the cost associated with slotting tubing. Accordingly, the present invention is directed toward remedying these problems, and in particular enables improved cutting mechanics and thermo-mechanical material properties for slotted casing liners used commonly in SAGD service. The modified steels illustrated herein have both surprisingly well-controlled mechanical properties and a much improved "slot-ability" that permits faster and less expensive slotting of casing liners using cutting blades.

[0007] Waid et al. (US Pat. No. 4,256,517) discusses the control of certain material mechanical properties (specifically YjT ratio) by alloying a plain carbon-manganese steel solely with chromium, and then performing a specific heat treatment.

[0008] Watari (US Pat. No. 5,922,145) discusses steels comprising S between 0.005% and 0.030% ; Mo max 0.50% and Mn up to 0.50% , but significantly lack of any understanding of advantage if Ti max were less than 0.02%, or even O.035%.

Watari emphasized Ti between 0.04% and 1.0%.

[0009] Okada et al. (US Pat. No. 5,948,183) discusses steels having a "low yield ratio", useful for oilfield tubular goods, comprising S less than 0.015% and Ti that only need to be less than about 0.20%. Only a single specification of a low yield ratio appears to be recognized in Okada et al. In contrast, the present invention specifically characterizes post-yield material properties by specifying hardening moduli at specific strain values as well as yield and ultimate strengths. The present application also teaches molybdenum as an agent able to improve needed mechanical properties.
Okada et al. acknowledges the possibility of molybdenum but specifies that the low yield ratio they seek is achieved using other alloying elements.

[00010] Kimura (US Pat. No. 6,869,489 B2) teaches molybdenum in an amount between 0.25% and 2.0% that also contains a carbide that has been precipitated using a heat treatment for spheroidizing to a particle size not larger than 1 11M., in order to achieve a higher machinability.

[00011] Ishida et al. (US Pat. No. 6,761,853 B2) includes 27 Tables comparing various alloys of steels with and without various identified "machinabilty improving elements". The present invention in contrast is concerned with the different problem of achieving enhanced slot-ability, while not compromising or degrading the critical performance attributes of good buckling resistance and good localization resistance, that are required for slotted tubing after it is placed into a difficult application, such as SAGD service.

[00012] The role of sulfur as to an expected effect on the "machinability" of steel is generally known. Most known prior art teaches away from using very small quantities of sulfur (i.e. 0.005 wt% to 0.030 wt%) where the problem is machinability and instead recommends a range generally greater than 0.03% and less than O.50%.
See, e.g. Riekels (US Pat. No. 4,255,188). Subsequent prior art suggests that lower amounts of sulfur (i.e. 0.01 wt% to 0.03 wt%) also may offer some improvement in machinability See, e.g., Yaguchi et al. (US Pat. No. 6,579,385 B2).
SUMMARY OF THE INVENTION

[00013] The present invention relates to steel tubing specifically manufactured to be slotted, yet to still retain or achieve high buckling resistance (or buckling "stability") and high localization resistance. The steel tubing is capable of maintaining a minimum YS and a minimum UTS at room temperature as well as exhibit good and consistent YS and UTS behavior at temperatures up to about 350 C.
In addition, the steel tubing has good TF behavior as well as a good and consistent PYHM and limited PYR in the plastic region all across the temperature and strain ranges, as well as limited YP all across the service temperature range. The present invention also relates to a process of manufacturing such steel tubing. A
preferred staring material is a steel within the API classification of K55 steel, but that that has been heat treated so as to exhibit refined thermo-mechanical properties that make it more suitable for use as a thermal well tubular product. While slot-ability is important for the efficiency of the manufacturing process, adequate post-yield strain hardening is equally important to prevent buckling of a liner structure that has been placed in SAGD service, in the event each joint of liner is not exactly the same.

[00014] The prior art generally has looked at steel compositions with alloys of the type taught herein, but has not appreciated an unexpected result if the specific compositions as recited and claimed were subjected to a particular heat treatment cycle. The prior art failed to appreciate how to achieve surprisingly enhanced slot-ability in combination with high buckling resistance and high localization resistance, which according to the present invention appears to be a result of a microstructure having minimized fractions of bainite while also having a minimum fraction of 80%
ferrite-perlite. Likewise, Research conducted in support of the present invention indicates that the presence of sulfur in the range of 0.005 % to 0.03 % by weight surprisingly improves the slot-ability response of K55 steels subjected to high post-yield hardening, without encroaching on the risk of any expectable corrosion cracking mechanism.

[00015] The two key aspects of good slotted liner performance in the oilfield, and especially SAGD service, are buckling resistance and localization resistance.

[00016] Thermo-mechanical properties desired preferably are evaluated by testing against a preferred specification, using Stress parameters of 0.5 %, 1.35% and 5 Strain parameters of 3.5%. A Zone 1 stress ratio (a 1.35 % / G o.5%) at all temperatures should be >1.3 at static conditions. A Zone 2 stress ratio (G
3.5% / G
1.35% ) at all temperatures should be >1.2 at static conditions. Hardening modulus at 3.5% strain, at all temperatures, should be? 800 MPa at static conditions. A
Yield Plateau, at all temperatures above yield, should have a length < 0.5% strain, at static conditions. Room temperature mechanical response should meet Yield Stress and Ultimate Stress requirements of the API K55 specification. For liners in SAGD
service, the range of Yield Strengths is typically between 379 MPa and 551 MPa.

[00017] It is believed that buckling resistance for SAGD liners is largely associated with post-yield hardening behavior in the strain range of approximately 1.35% to 3.5% ("Zone 2"), whereas localization resistance is generally associated with hardening from yield to about 1.35% ("Zone 1"). Localization resistance is an indication of the liner's capacity to effectively accommodate axial variability in parameters associated with material properties, pipe section properties, and loading conditions, and should be considered a requirement for thermal liner.

[00018] A goal for a "zone 1 stress ratio" of 1.3%, as defined through liner analysis, is considered a target value. Prominent post-yield hardening behavior generally exhibited by the compared baseline Grade K55 samples at temperatures above 180 C (generally 1.2% and greater at static conditions) is now believed likely to result in relatively good localization resistance.

[00019] Materials with substantial yield plateau are not believed to exhibit high zone 1 stress ratios. While plateaus were observed in baseline Grade K55 samples at room temperature, it appears that the present teachings about a need for localization resistance (and hence zone 1 hardening) is most important at temperatures near to or higher than the point of initial yielding. Hence, localization resistance is viewed as an important aspect of thermal liner design and it is expected that SAGD
operators will look at localization resistance values as a key design feature

[00020] The steel tubing of the present invention is designed for use in various applications in the oil and gas industries, and in particular for steel tubing that first is slotted and then is used in the high temperature and high stress environment of SAGD
service.
BRIEF DESCRIPTION OF THE DRAWINGS

[00021] Figure 1 A is a photograph showing the microstructure of the steel of Comparative Example 2 at 200x magnification.

[00022] Figure 1B is a photograph showing the microstructure of the steel of Comparative Example 2 at 500x magnification.

[00023] Figure 2A is a photograph showing the microstructure of the steel of Comparative Example 1 at 200x magnification.

[00024] Figure 2B is a photograph showing the microstructure of the steel of Comparative Example 1 at 500x magnification.
[0010] Figure 3A is a photograph showing the microstructure of the steel of Comparative Example 3 at 200x magnification.
[0011] Figure 3B is a photograph showing the microstructure of the steel of Comparative Example 3 at 500x magnification.
[0012] Figure 4A is a photograph showing the microstructure of the steel of Example 1 at 200x magnification.
[0013] Figure 48 is a photograph showing the microstructure of the steel of Example 1 at 500x magnification.
[0014] Figure 5 is a photograph showing the microstructure of the steel of Comparative Example 5 at 2000x magnification.

[0015] Figure 6 is a photograph showing the microstructure of the steel of Comparative Example 6 at 2000x magnification.
[0016] Figure 7 is a photograph showing the microstructure of the steel of Example 2 at 1500x magnification.
[0017] Figure 8 is a photograph showing the microstructure of the steel of Comparative Example 4 at 1500x magnification.
[0018] Figure 9 is a photograph showing the microstructure of the steel of Comparative Example 7 at 1500x magnification.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0019] Although the present invention is susceptible to embodiment in various forms, a presently preferred embodiment will be described hereinafter with the understanding that the present disclosure is to be considered an exemplification of the invention and is not intended to limit the invention to the specific embodiment illustrated.
[0020] The present invention relates to steel tubing with enhanced slot-ability for use in various applications in the oil and gas industries. More particularly, the invention relates to steel tubing having a specific chemical composition and a ferrite plus pearlite or a ferrite plus bainite-pearlite microstructure and a process for manufacturing the steel tubing.
[0021] As noted above, while some steel tubing under the broad API 5CT
standard material may not be difficult to slot, the most common liner Grades K55 and present disadvantages. As a result, slotting Grade K55 liners involves long machining times per piece, high tooling consumption and operative delays. All of these effectively increase the cost of slotting a steel tube. To help reduce the cost of slotting steel tubing, the present invention is directed to steel tubing, and a method of manufacturing the steel tubing, having both enhanced slot-ability characteristics and superior thermo-mechanical properties.

[0022] In addition to reducing the cost associated with slotting tubing, production of a steel tube with enhanced slot-ability characteristics for use in oil and gas wells also has the benefit of helping to give well operators peace of mind. Use of such steel can help assure well operators that the steel tubing used in a particular well will cope with all of the loading conditions expected to occur during the life of the well, including operative and shut down conditions. Moreover, thermal well designs and operations frequently require materials to operate in their plastic regions in temperatures ranging from room temperature to about 350 C. The steel tubing of the present invention has been designed in light of the operative conditions of the steel tubing in such applications, and has more restricted and stable behavior for key variables such as YS, UTS, PYHM, PYR, TF and YP.
[0023] The steel tubing of the present invention is a result of intensive research by the inventors. During the course of the research, the inventors realized that the addition of small quantities of sulfur, combined with the standard hot rolling process and a specifically defined heat treatment cycle, produced steel tubing with enhanced slot-ability characteristics. Additionally, small additions of Mo have shown to be beneficial to enhancing and making more stable properties such as YS, UTS, PYHM, PYR, TF and YP over the material service operative range.
[0024] The process for making the steel tubing of the present invention consists in making billets of acceptable steel, as by cutting steel bars into billets, and hot rolling the billets into tubes. The tubes are then air cooled to room temperature.
Then, in a final heat treatment process, the tubes are heated to approximately 40 C above the corresponding AC3 temperature and soaked at that temperature for a predetermined period of time, after which the tubes are air cooled back to room temperature.

[0025] The preferred method for conducting the final heat treatment cycle involves linearly heating the tubes from room temperature to approximately 40 C above the corresponding AC3 temperature over the course of about 30 minutes. Once at 40 C
above the corresponding AC3 temperature, the tubes are soaked for about 10 minutes.
Finally, the tubes are air cooled back down to room temperature, a process that takes approximately 80 minutes.

[0026] In a preferred embodiment, a K55 steel having a steel chemistry as taught herein first is heated to above the eutectoid temperature in order to effectively create a uniform austenite structure. However, that austenite structure is unstable at lower temperatures, so as the steel is cooled the microstructure will change. The resultant microstructure is dependent upon the rate at which the steel cools after the soaking period. In the preferred embodiment the desired microstructure was achieved using 40 C above the corresponding AC3 temperature, a 10 minute soak, and a target cooling time of 80 minutes. Workers of ordinary skill could employ isothermal transformation diagrams to decide on a range of useful cooling paths that will achieve the desired microstructure (minimized fractions of bainite and a minimum fraction of 80% ferrite-perlite. Microstructure) as steel chemistry, AC3 temperatures, soaking time and tubing sizes vary.

[0027] The hardening behavior of a material correlates to the slot-ability behavior and hardening behavior also determines effectiveness of a material's thermo-mechanical behavior in SAGD service. Concerning desired thermo-mechanical material properties, there is a distinction between the broad definition of the K55 grade steel defined in API SIT and the typical range of properties exhibited by the K55 grade steels in the preferred embodiment. API SIT defines K55 grade steel mechanically by setting a range for yield strengths between 55 ksi and 80 ksi and a minimum ultimate tensile strength of 95 ksi. The preferred embodiment uses a K55 steel with a static yield strength at room temperature that is typically lower than 65 ksi. Given that the yield strength of preferred material is at the low end of the API range, there will be significantly more hardening.

[0028] With the above-described process for producing steel tubing having the chemical composition described below, the resulting steel tubing has a ferrite plus pearlite or a ferrite plus bainte-pearlite microstructure. It is this combination of a specific chemical composition along with the above-noted microstructures that renders the steel tubing having enhanced slot-ability characteristics of the present invention.

[0029] As noted-above, the defined steel chemistry allows for the production of the desired steel tubing after the hot rolling and heat treatment operations. In particular, the carbon content helps achieve a minimum specified strength level controlled by a 5 minimum YS and also minimum TS. The addition of micro-alloying elements, such as titanium, contributes to the strength level of the steel tubing and helps give the steel tubing a minimum desired toughness. Molybdenum contributes to achieving the desired strength level of the steel tubing and helps give the steel better and more stable mechanical behavior at warm temperatures. Further, a controlled range of 10 sulfur causes enhanced slot-ability performance without a compromising risk of environmental cracking in the expected service environment. That is, small additions of sulfur can, on the one hand, help to improve steel machinability. On the other hand, too much sulfur may result in steel tubing that is more prone to cracking due to hydrogen embrittlement when hydrogen is present in the environment, something that typically happens in casing and tubing applications for oil and gas wells.
Thus, the range for sulfur listed below represents a compromise between the desire to enhance machinability and the desire to prevent cracking and hydrogen embrittlement.
Note that something similar would happen if Ti levels in the steel tubing are too high.

[0030] The preferred ranges (in weight %) of the elements making up the chemical composition of the steel tubing of the present invention are as follows:
Carbon: 0.05-0.40 Manganese: 0.50-1.60 Phosphorus: maximum of about 0.020 Sulfur: 0.005-0.030 Silicon: maximum of about 0.40 Chromium: maximum of about 0.50 Molybdenum: maximum of about 0.50 Niobium: maximum of about 0.050 Titanium: maximum of about 0.035 Vanadium: maximum of about 0.090 Copper: maximum of about 0.300 Aluminum: maximum of about 0.040

[0031] More preferred for the ranges (in weight %) of the elements making up the chemical composition of the steel tubing of the present invention are as follows:
Carbon: 0.28-0.40 Manganese: 1.20-1.45 Phosphorus: maximum of about 0.020 Sulfur: 0.015-0.030 Silicon: maximum of about 0.40 Chromium: maximum of about 0.50 Molybdenum: maximum of about 0.20 Niobium: maximum of about 0.010 Titanium: maximum of about 0.020 Vanadium: maximum of about 0.020 Copper: maximum of about 0.250 Aluminum: maximum of about 0.035

[0032] Even more preferred for the ranges (in weight %) of the elements making up the chemical composition of the steel tubing of the present invention are as follows:
Carbon: 0.31-0.34 Manganese: 1.25-1.40 Phosphorus: maximum of about 0.020 Sulfur: 0.015-0.025 Silicon: maximum of about 0.40 Chromium: maximum of about 0.50 Molybdenum: maximum of about 0.11 Niobium: maximum of about 0.005 Titanium: maximum of about 0.015 Vanadium: maximum of about 0.010 Copper: maximum of about 0.250 Aluminum: maximum of about 0.025

[0033] Steel tubing having a chemical composition as described above and which as been subjected to the above-described heat treatment process preferably will preferably have the following properties:
Minimum yield strength at room temperature of 55 ksi (379.2 MPa);
Maximum yield strength at room temperature of 80 ksi (551.6 MPa);
Minimum ultimate tensile strength at room temperature of 95 ksi (655 MPa);
Minimum elongation at room temperature of 20 %; and Minimum impact toughness at room temperature of 30 J (on a longitudinal full-sized sample).

[0034] The steel tubing also preferably exhibits reduced/controlled yield strength derating at temperatures up to 350 C. Specifically, the ratio of actual material yield strength at a given temperature versus original material yield strength at room temperature is preferably greater than 0.75 at 350 C and greater than 0.80 at 180 C.
Further, the steel tubing preferably exhibits reduced/controlled tensile strength derating at temperatures up to 350 C. Specifically, the ratio of actual material tensile strength at the given temperature versus original material tensile strength at room temperature is preferably greater than 0.92 at 350 C, greater than 1.06 at 180 C, and greater than 1.1 at 230 C and 280 C. Additionally, the steel tubing preferably exhibits reduced/controlled post-yield material relaxation at temperatures up to 350 C.
Specifically, the ratio of material static yield strength versus material yield strength is preferably greater than 0.83 at any strain level up to 4% and temperature up to 350 C.
Further, the steel tubing preferably exhibits a minimum post-yield hardening modulus at different temperatures and strain levels up to 350 C, exhibits hardening modulus greater than 7,500 MPa at 1.5% strain at any temperature up to 350 C, and exhibits a hardening modulus greater than 3,500 MPa at 4% strain at any temperature up to 350 C.

[0035] Other chemical compositions and/or manufacturing routes would be able to give steel tubing having a ferrite-pearlite microstructure, but such methods would also result in higher fractions of bainite and other secondary structures, as well as no (or less) homogeneous distributions. These are factors that would work against the slot-ability of the steel tubing.

[0036] To demonstrate the enhanced slot-ability characteristics of the steel tubing of the present invention, the inventors performed the below-described comparative tests.

[0037] Four steel tubes having the chemical compositions and microstructures shown in TABLE 1, below, were prepared. All four steel tubes had an outer diameter of 244.50 mm and a wall thickness of 10.03 mm. Example 1 (the steel produced according to the present invention) and Comparative Example 1 were normalized.

That is, the steel tubes of Example 1 and Comparative Example 1 were subjected to the heat treatment cycle described above. The steel tubes of Comparative Examples 2 and 3 were left in their as rolled states. The microstructure of the steel tubes of Example 1 may be seen in Figures 4A and 4B. The microstructure of the steel tubes of Comparative Example 1 may be seen in Figures 2A and 2B. The microstructure of the steel tubes of Comparative Example 2 may be seen in Figures in lA and 1B.
The microstructure of steel tubes of Comparative Example 3 may be seen in Figures and 3B.

Comparative Comparative Comparative Example 1 Example 1 Example 2 Example 3 Ferrite plus Ferrite plus Microstructure bainite- Ferrite pearlite Ferrite pearlite mainly bainite pearlite %C x100 29 32 32 29 %Mn x100 130 134 135 134 %S x1000 13 2 1 13 %P x1000 12 13 14 14 %Si x100 29 37 37 30 %Ni x100 5 5 6 5 %Cr x100 22 4 4 22 %Mo x100 15 9 9 15 %V x1000 3 2 0 0 %Cu x100 9 8 8 9 %Sn x1000 11 = 9 10 12 %As x1000 6 4 0 0 %Al x1000 20 21 17 21 %Ca ppm 19 18 13 14 %Nb x1000 1 1 0 2 %Ti x1000 2 12 12 2 %B ppm 1 1 0 1 %Ceq x100 59.06 57.84 58.03 59.67 %Pcm x100 39.13 41.24 41.28 39.33

[0038] The following tests were performed to characterize the steel tubing of Example 1 and Comparative Examples 1-3:
= Tensile tests ¨ API longitudinal full-size standard (38 mm) specimens were machined from each sample and tested.
= Hardness Rockwell C tests ¨ The harness tests were performed in four different positions: at 00, 900, 180 and 270 , with nine indentations in each position, which correspond to three indentations (external, internal and mid-wall) per position.
= Impact transition curves ¨ From each sample, five sets of three Charpy specimens, CL and LC 10x7.5 mm, were performed and tested at ¨60 , -40 , -20 , 0 and 21 C. The shear area was determined using the direct measurement method (ASTM E 23).
= Machinability tests ¨ Two different tests were carried out. Both tests were performed using GLOBUS HSS 2' 3/4" x 0.0018" x 1" /
72T model saws. The first test consisted of cutting slots of 500 mm of length while the second test consisted of cutting slots with 5 mm of depth and 500 mm of length. In both cases, the relative machinability was measured by the total length of slotting made with each saw. Each saw was used until it was unusable or until it was broken. In the case of Comparative Examples 1 and 3, the feed rate was reduced from the 250 mm/min used in the other samples to a rate of 180 mm/min to prevent the saws from continually breaking.

[0039] The results of the above-described tests are depicted in Table 2, below.

Example 1 Comparative Comparative Comparative Example 1 Example 2 Example 3 YS (MPa) 461 455 444 536 Mechanical UTS (MPa) 705 691 709 738 Properties YS/UTS 0.65 0.66 0.63 0.73 Elongation % 27.4 30.3 36.3 23.0 Hardness HRC 17 13 15 19 Energy (J) at 29.7 59.0 26.7 14.0 C, LC
Shear Area (%) 41.7 53.3 27.3 15.7 at 20 C, LC
Toughness Energy (J) 20 C, CL at 24.3 47.7 36.0 12.7 Shear Area (%) 46 49.7 35.3 10.7 at 20 C, CL
Slotting 8.00 5.50 7.00 5.72 Distance (m) Machinability Cutting Speed (First Trial) (m/min) Feed Rate (mm/min) Slotting 28.00 14.20 15.00 14.30 Distance (m) Machinability Cutting Speed (Second Trial) (m/min) Feed Rate (mm/min)

[0040] Note that the HRC hardness values listed in TABLE 2 are a general average, 5 which was calculated as follows. First, the average of the individual external, internal and mid-wall measurements for each quadrant was calculated. Next, the average external, internal and mid-wall measurements for each quadrant were averaged to generate a general external, internal and mid-wall HRC value. Then, the general external, internal and mid-wall HRC values were averaged together to generate the 10 HRC value listed in TABLE 2.

[0041] The toughness values listed in TABLE 2 represent the toughness values at the transition temperature of the steel tubing of Example 1 and Comparative Examples 1-3. The transition temperature was determined by examining the values of the energy and shear area at each of the four measured temperatures. TABLE 3 represents the Charpy transition curves for the 10x7.5-LC specimens. TABLE 4 represents the Charpy transition curves for the 10x7.5-CL specimens.

Temp Energy (Joules) Shear Area ( /0) Example (0C) 1 2 3 Average 1 2 3 Average -60 8 8 7 7.7 0 0 0 0.0 -40 10 11 9 10.0 8 9 8 8.3 Example 1 -20 14 11 10 11.7 18 16 13 15.7 0 25 17 22 21.3 31 22 29 27.3 20 29 30 30 29.7 38 43 44 41.7 -60 4 10 8 7.3 0 0 0 0.0 -40 16 8 16 13.3 7 0 7 4.7 Comparative -20 31 22 25 26.0 23 18 19 20.0 Example 1 0 39 40 41 40.0 35 33 37 35.0 20 62 57 58 59.0 52 50 58 53.3 -60 3 2 4 3.0 0 0 0 0.0 -40 3 3 3 3.0 0 0 0 0.0 Comparative -20 6 23 6 11.7 7 16 5 9.3 Example 2 0 32 29 8 23.0 28 22 15 21.7 20 25 17 38 26.7 26 16 40 27.3 -60 4 6 6 5.3 0 0 0 0.0 -40 6 4 6 5.3 0 0 0 0.0 Comparative -20 9 7 11 9.0 0 0 0 0.0 Example 3 0 9 9 13 10.3 8 7 12 9.0 20 15 13 14 14.0 17 15 15 15.7 Temp Energy (Joules) Shear Area (%) Example 0 -( C) 1 2 3 Average 1 2 3 Average -60 7 7 7 7.0 0 0 0 0.0 -40 10 12 15 12.3 8 8 11 9.0 , Example 1 -20 13 10 14 12.3 20 17 22 19.7 0 18 18 15 17.0 31 33 29 31.0 20 27 23 23 24.3 50 45 43 46.0 -60 11 4 4 6.3 0 0 0 0.0 -40 18 19 19 18.7 10 10 10 10.0 Comparative -20 26 19 25 23.3 16 14 18 16.0 Example 1 0 33 32 37 34.0 35 30 = 32 32.3 20 47 45 51 47.7 50 44 55 49.7 Comparative -60 3 3 2 2.7 0 0 0 0.0 Example 2 -40 4 3 3 3.3 0 0 0 0.0 -20 8 9 9 8.7 8 9 8 8.3 0 13 23 19 18.3 13 23 20 18.7 20 33 39 36 36.0 35 37 34 35.3 -60 4 3 5 4.0 0 0 0 0.0 -40 5 4 5 4.7 0 0 0 0.0 Comparative -20 7 9 8 8.0 0 0 0 0.0 Example 3 0 12 8 8 9.3 10 8 7 8.3 20 13 11 14 12.7 11 9 12 10.7

[0042] From TABLE 3 and TABLE 4, it is apparent that no specimen had 100% of shear area at room temperature. The maximum value for an LC specimen was 53%
and the maximum value for a CL specimen was 50%, both of which correspond to Example 1. Thus, the transition temperature (as measured by 50% of the shear area) was about 20 C for Example 1.

[0043] Also, as shown in TABLE 2, Example 1 had the best results for both the first trial (8.0 m) and the second trial (28.0 m). In other words, the steel tubing of Example 1 had enhanced slot-ability as compared to the steel tubing of Comparative Examples 1-3.

[0044] A comparative test was performed using steel tubing have the below described composition:

E 2 Comparative Comparative Comparative Comparative xample Example 4 Example 5 Example 6 Example 7 Microstructure %C x100 33 32 21 26 14 %Mn x100 130 134 135 54 125 %S x1000 16 1.2 1.2 1 1 %P x1000 17 11 13 7 12 %Si x100 33 35 30 26 29 %Ni x100 5 4 4 4.5 5 %Cr x100 4 3 25 91 20 %Mo x100 10 10 13 39 7 %V x1000 2 3 3 4 44 %Cu x100 10 8 10 8.5 20 %Sn x1000 9 9 12 7 7 %As x1000 6 4 5 6 4 %Al x1000 20 17 24 28 22 %Ca ppm 11 9 11 12 %Nb x1000 1 1 28 34 15 %Ti x1000 11 12 4 2 1 %B x1000

[0045] Tests were performed at varying temperature (25 C, 180 C, 230 C, 280 C
and 330 C) and at a strain rate of 1.67 x 10-5 sec-1 (10-3 min-1). Two tests were performed in each condition. The strain measurement was performed with a longitudinal LVDT
gauge within the reduced section. The stress relaxation response was measured at three (3) hold points (1 hour per hold point) at approximately 1%, 3% and 5%
strain.
At each hold step the applied strain was held as constant as possible and the stress and strain were monitored with time. Tests were continued up to specimen necking.

[0046] The tests results are summarized in TABLES 6-10 below. The tables include yield stress (a), quasi static yield (aqs), delta yield strain (Aa) and modulus of strain hardening (da/dE) at strains of 0.5%, 1.5% and 4%. In addition, the yield strength at 0.2% offset (YSo20/), the ultimate tensile strength (UTS) and the hardening index (n) were calculated. With the exception of Comparative Example 7 at 330 C, the results presented below represent the average of two tests performed in each condition. In that regard, the estimated errors in the duplicated tests are + 0.1 for n, lOMPa for YS, 10 MPa for UTS, and + 20 MPa for A. The microstructure of the steel of Example 2 may be seen in Figure 7. The microstructure of the steel of Comparative Example 4 may be seen in Figure 8. The microstructure of the steel of Comparative Example 5 may be seen in Figure 5. The microstructure of the steel of Comparative Example 6 may be seen in Figure 6. The microstructure of the steel of Comparative Example 7 may be seen in Figure 9.

cY aq, Au du/de Steel Temp Strain (MPa) (MPa) (u-uqs) (MPa) n YS0 2% UTS
0.5% 450 390 60 25 C 1.5% 515 450 65 7550 0.22 460 683 4% 650 565 85 3575 0.5% 400 350 50 22400 180 C 1.5% 520 455 65 9700 0.28 395 729 4% 680 595 85 4750 0.5% 405 340 65 22675 Example 230 C 1.5% 550 460 90 10250 0.28 380 780 4% 720 600 120 5050 0.5% 375 315 60 24000 280 C 1.5% 500 425 75 10675 0.32 350 772 4% 690 580 110 5525 0.5% 370 315 55 21450 330 C 1.5% 490 435 55 9475 0.29 350 704 4% 660 580 80 4775 6s Au du/de Steel Temp Strain (MPa) 041:1,0 (a_aqs) (mpo n YS02% UTS
0.5% 475 425 50 25 C 1.5% 570 515 55 8350 0.22 465 729 4% 705 635 70 3875 0.5% 465 405 60 23250 180 C 1.5% 580 520 60 9675 0.25 405 750 4% 740 660 80 4625 0.5% 470 400 70 22550 Comparative 230 C 1.5% 595 520 75 9525 0.24 435 812 Example 4 4% 765 660 105 4600 0.5% 445 380 65 23150 280 C 1.5% 550 475 75 9525 0.26 415 793 4% 745 640 105 4850 0.5% 415 360 55 21575 330 C 1.5% 525 450 75 9100 0.26 390 717 4% 685 585 100 4450 Steel Temp Strain cy Go AG cirri& n YS02% UTS
0.5% 655 585 70 C 1.5% 645 565 80 5600 0.13 660 726 4% 730 640 90 2375 0.5% 590 510 80 _ 180 C 1.5% 645 560 85 5600 0.13 575 736 4% 735 635 100 2400 0.5% 585 490 95 17550 Comparative 230 C 1.5% 665 560 105 6650 0.15 575 774 Example 5 4% 760 645 115 2850 0.5% 590 485 105 16525 280 C 1.5% 650 545 105 6075 0.14 580 760 4% 745 620 125 2600 0.5% 550 450 100 13200 330 C 1.5% 630 500 130 5050 0.12 560 693 4% 700 560 140 2100 G Gqs AG daide Steel Temp Strain n YS0 2 0A, UTS
(MPa) (MPa) (G-G) (MPa) 0.5% 650 600 50 25 C 1.5% 645 590 55 5600 0.13 645 727 4% 740 670 70 2400 0.5% 585 525 60 180 C 1.5% 655 585 70 6100 0.14 570 748 4% 750 665 85 2625 =
0.5% 575 500 75 17250 Comparative 230 C 1.5% 670 580 90 6700 0.15 575 776 Example 6 4% 765 675 90 2875 0.5% 545 465 80 17450 280 C 1.5% 630 530 100 6725 0.16 550 744 4% 735 620 115 2950 0.5% 545 455 90 14175 330 C 1.5% 620 520 100 5375 0.13 530 712 4% 710 590 120 2300 a aqs Aa da/dc Steel Temp Strain n YS0 2% UTS
(MPa) (MPa) (a-aqs) (MPa) 0.5% 400 345 55 25 C 1.5% 405 340 65 6475 0.24 405 550 4% 500 430 70 3000 0.5% 335 295 40 180 C 1.5% 375 325 50 7000 0.28 340 560 4% 490 425 65 3425 0.5% 315 265 50 15750 Comparative 230 C 1.5% 410 340 70 7075 0.27 315 575 Example 7 4% 530 440 90 3375 0.5% 315 255 60 17000 280 C 1.5% 420 345 75 7550 0.27 295 607 4% 540 440 100 3650 0.5% 320 260 60 16650 330 C 1.5% 410 345 65 7100 0.26 300 597 4% 535 445 90 3475

[0047] The results of the comparative test show that the strain and hardening effect is higher in Example 2 and Comparative Example 4, and particularly Example 2, than in the other Comparative Examples. This is explained by considering the lower dislocation levels in Example 2 and Comparative Example 4 (ferrite-pearlite structure vs. tempered martensite).

[0048] Additionally, Example 2 and Comparative Example 4 have nearly the same chemical composition and yield strength values, but Comparative Example 4 has coarser ferritic grain size and some acicular shaped grains. Because yield strength depends directly on the square root of the dislocation density, and inversely on the square root of the ferritic grain size, dislocation density should be higher in Comparative Example 4 than in Example 2. This dislocation density is the reason why the aging effect is lower in Comparative Example 4.

[0049] Further, the stress relaxation is more pronounced at higher strains and temperatures. This is reasonable since higher strains imply a high level of dislocations to be recovered and the mobility of the dislocations increases with , .

temperature. There is not a clear difference between the six analyzed steels in stress relaxation behavior.

[0050] While preferred embodiments of the invention have been shown and described, it is to be understood that the scope of the claims should not be limited by the preferred embodiments set forth in the examples above, but should be given the broadest interpretation consistent with the description as a whole.

Claims (30)

WE CLAIM:

1. A steel tubing adapted to enable post-yield hardening behavior across a temperature range between room temperature and 350 °C while providing slot-ability, buckling resistance and localization resistance, wherein the steel has a chemical composition as follows:
0.05 to 0.40 wt. % carbon;
0.50 to 1.60 wt. % manganese;
a maximum of 0.020 wt. % phosphorous;
0.005 to 0.030 wt. % sulfur;
a maximum of 0.40 wt.% silicon;
0.04 to 0.50 wt. % chromium;
a maximum of 0.50 wt. % molybdenum;
a maximum of 0.050 wt. % niobium;
a maximum of 0.035 wt. % titanium;
a maximum of 0.090 wt. % vanadium;
a maximum 010.30 wt. % copper;
a maximum of 0.040 wt. % aluminum; and balance substantially iron, wherein the steel tubing has a post-yield hardening microstructure consisting essentially of either ferrite plus pearlite or ferrite plus bainite-pearlite.

2. The steel tubing according to claim 1, wherein the steel has a chemical composition as follows:
about 0.28 to about 0.40 wt. % carbon;
about 1.20 to about 1.45 wt. % manganese;
about 0.015 to about 0.030 wt. % sulfur;
about 0.10 to about 0.20 wt. % molybdenum;
a maximum of 0.010 wt. % niobium;
a maximum of 0.020 wt. % titanium;
a maximum of 0.020 wt. % vanadium:
a maximum of 0.25 wt. % copper; and a maximum of 0.035 wt. % aluminum.

3. The steel tubing according to claim 1, wherein the steel has a chemical composition as follows:
about 0.31 to about 0.34 wt. % carbon;
about 1.25 to about 1.40 wt. % manganese;
about 0.015 to about 0.025 wt. % sulfur;
about 0.10 to about 0.20 wt. % molybdenum;
a maximum of 0.005 wt. % niobium;
a maximum of 0.015 wt. % titanium;
a maximum of 0.010 wt. % vanadium;
a maximum of 0.25 wt. % copper; and a maximum of 0.025 wt. % aluminum.

4. The steel tubing according to claim 1, wherein the steel has a chemical composition as follows:
about 0.29 wt. % carbon;
about 1.30 wt. % manganese;
about 0.013 wt. % sulfur;
about 0.012 wt. % phosphorus;
about 0.29 wt. % silicon:
about 0.22 wt. % chromium;
about 0.15 wt. % molybdenum;
about 0.001 wt. % niobium;
about 0.002 wt. % titanium;
about 0.003 wt. % vanadium;
about 0.09 wt. % copper; and about 0.020 wt. % aluminum.

5. The steel tubing according to any one of claims 1 to 4, having at least one of the following properties:
minimum yield strength at room temperature of 55 ksi (379.2 MPa);
maximum yield strength at room temperature of 80 ksi (551.6 MPa);
minimum ultimate tensile strength at room temperature of 95 ksi (655 MPa);
minimum elongation at room temperature of 20 %; and minimum impact toughness at room temperature of 30 J.

6. The steel tubing according to any one of claims 1 to 4, having at least one of the following properties:
a ratio of actual material yield strength at a given temperature versus original material yield strength at room temperature of greater than 0.75 at 350 °C, and greater than 0.80 at 180 °C;
a ratio of actual material tensile strength at a given temperature versus original material tensile strength at room temperature of greater than 0.92 at 350 °C, greater than 1.06 at 180 °C, and greater than 1.1 at 230 °C and 280 °C;
a ratio of material static yield strength versus material yield strength of greater than 0.83 at any strain level up to 4% and temperature up to 350 °C;
a hardening modulus greater than 7,500 MPa at 1.5% strain at any temperature up to 350 °C; and a hardening modulus greater than 3,500 MPa at 4% strain at any temperature up to 350 °C.

7. The steel tubing according to any one of claims 1 to 4, having the following properties:
minimum yield strength at room temperature of 55 ksi (379.2 MPa);
maximum yield strength at room temperature of 80 ksi (551.6 MPa);
minimum ultimate tensile strength at room temperature of 95 ksi (655 MPa);
minimum elongation at room temperature of 20 %; and minimum impact toughness at room temperature of 30 J.

8. The steel tubing according to any one of claims 1 to 4, having the following properties:
a ratio of actual material yield strength at a given temperature versus original material yield strength at room temperature of greater than 0.75 at 350 °C, and greater than 0.80 at 180 °C;
a ratio of actual material tensile strength at a given temperature versus original material tensile strength at room temperature of greater than 0.92 at 350 °C, greater than 1.06 at 180 °C, and greater than 1.1 at 230 °C and 280 °C;

a ratio of material static yield strength versus material yield strength of greater than 0.83 at any strain level up to 4% and temperature up to 350 °C;
a hardening modulus greater than 7,500 MPa at 1.5% strain at any temperature up to 350 °C; and a hardening modulus greater than 3,500 MPa at 4% strain at any temperature up to 350 °C.

9. A method of treating a steel tube to enable post-yield hardening behavior across a temperature range between room temperature and 350 °C while providing slot-ability, buckling resistance and localization resistance, comprising the steps of:
creating a billet from steel, wherein the steel has a chemical composition as follows:
0.05 to 0.40 wt. % carbon;
0.50 to 1.60 wt. % manganese;
a maximum of 0.020 wt. % phosphorous;
0.005 to 0.030 wt. % sulfur;
a maximum of 0.40 wt.% silicon;
0.04 to 0.50 wt. % chromium;
a maximum of 0.50 wt. % molybdenum;
a maximum of 0.050 wt. % niobium;
a maximum of 0.035 wt. % titanium;
a maximum of 0.090 wt. % vanadium;
a maximum of 0.30 wt. % copper;
a maximum of 0.040 wt. % aluminium; and balance substantially iron;
hot rolling the billet into a tube and cooling the tube to room temperature;
heating the tube to a first temperature above the corresponding AC3 temperature, and soaking the tube at approximately that first temperature for a first predetermined period of time; and air cooling the tube from that first temperature to room temperature over a second predetermined period of time sufficient to create a post-yield hardened steel tube characterized by a microstructure consisting essentially of either ferrite plus pearlite or ferrite plus bainite-pearlite.

10. The method according to claim 9, wherein the steel has a chemical composition as follows:
about 0.28 to about 0.40 wt. % carbon;
about 1.20 to about 1.45 wt. % manganese;
about 0.015 to about 0.030 wt. % sulfur;
about 0.10 to about 0.20 wt. % molybdenum;
a maximum of 0.010 wt. % niobium;
a maximum of 0.020 wt. % titanium;
a maximum of 0.020 wt. % vanadium;
a maximum of 0.25 wt. % copper; and a maximum of 0.035 wt. % aluminum.

11. The method according to claim 9, wherein the steel has a chemical composition as follows:
about 0.31 to about 0.34 wt. % carbon;
about 1.25 to about 1.40 wt. % manganese;
about 0.015 to about 0.025 wt. % sulfur;
about 0.10 to about 0.20 wt. % molybdenum;
a maximum of 0.005 wt. % niobium;
a maximum of 0.015 wt. % titanium;
a maximum of 0.010 wt. % vanadium;
a maximum of 0.25 wt. % copper; and a maximum of 0.025 wt. % aluminum.

12. The method according to claim 9, wherein the steel has a chemical composition as follows:
about 0.29 wt. % carbon;
about 1.30 wt. % manganese;
about 0.013 wt. % sulfur;
about 0.012 wt. % phosphorus;

about 0.29 wt. % silicon;
about 0.22 wt. % chromium;
about 0.15 wt. % molybdenum;
about 0.001 wt. % niobium;
about 0.002 wt. % titanium;
about 0.003 wt. % vanadium;
about 0.09 wt. % copper; and about 0.020 wt. % aluminum.

13. The method according to any one of claims 9 to 12, wherein the first temperature is approximately 40 °C above the corresponding AC3 temperature, the first predetermined period of time is about 30 minutes, the second predetermined period of time is approximately 80 minutes and the post-yield hardened steel tube has at least one of the following properties:
minimum yield strength at room temperature of 55 ksi (379.2 MPa);
maximum yield strength at room temperature or 80 ksi (551.6 MPa);
minimum ultimate tensile strength at room temperature of 95 ksi (655 MPa);
minimum elongation at room temperature of 20 %; and minimum impact toughness at room temperature of 30 J.

14. The method according to any one of claims 9 to 12, wherein the first temperature is approximately 40 °C above the corresponding AC3 temperature, the first predetermined period of time is about 30 minutes, the second predetermined period of time is approximately 80 minutes and the post-yield hardened steel tube has at least one of the following properties:
a ratio of actual material yield strength at a given temperature versus original material yield strength at room temperature of greater than 0.75 at 350 °C, and greater than 0.80 at 180 °C;
a ratio of actual material tensile strength at a given temperature versus original material tensile strength at room temperature of greater than 0.92 at 350 °C, greater than 1.06 at 180 °C, and greater than 1.1 at 230 °C and 280 °C;
a ratio of material static yield strength versus material yield strength of greater than 0.83 at any strain level up 10 4% and temperature up to 350 °C;

a hardening modulus greater than 7,500 MPa at 1.5% strain at any temperature up to 350 °C; and a hardening modulus greater than 3,500 MPa at 4% strain at any temperature up to 350 °C.

15. The method according to any one of claims 9 to 12, wherein the first temperature is approximately 40 °C above the corresponding AC3 temperature, the first predetermined period of time is about 30 minutes, the second predetermined period of time is approximately 80 minutes and the post-yield hardened steel tube has the following properties:
minimum yield strength at room temperature of 55 ksi (379.2 MPa);
maximum yield strength at room temperature of 80 ksi (551.6 MPa);
minimum ultimate tensile strength at room temperature of 95 ksi (655 MPa);
minimum elongation at room temperature of 20 %; and minimum impact toughness at room temperature of 30 J.

16. The method according to any one of claims 9 to 12, wherein the first temperature is approximately 40 °C above the corresponding AC3 temperature, the first predetermined period of time is about 30 minutes, the second predetermined period of time is approximately 80 minutes and the post-yield hardened steel tube has the following properties:
a ratio of actual material yield strength at a given temperature versus original material yield strength at room temperature of greater than 0.75 at 350 °C, and greater than 0.80 at 180 °C;
a ratio of actual material tensile strength at a given temperature versus original material tensile strength at room temperature of greater than 0.92 at 350 °C, greater than 1.06 at 180 °C, and greater than 1.1 at 230 °C and 280 °C;
a ratio of material static yield strength versus material yield strength of greater than 0.83 at any strain level up to 4% and temperature up to 350 °C;
a hardening modulus greater than 7,500 MPa at 1.5% strain at any temperature up to 350 °C; and a hardening modulus greater than 3,500 MPa at 4% strain at any temperature up to 350 °C.

17. A method of producing steel tubing with slot-ability, buckling resistance and localization resistance, comprising the steps of:
producing a solid bar front a steel. wherein the steel has a chemical composition as follows:
0.05 to 0.40 wt. % carbon;
0.50 to 1.60 wt, % manganese;
a maximum of 0.020 wt. % phosphorous;
0.005 to 0.030 wt. % sulfur;
a maximum of 0.40 wt.% silicon;
0.04 to 0.50 wt. % chromium;
a maximum of 0.50 wt. % molybdenum;
a maximum of 0.050 wt. % niobium;
a maximum of 0.035 wt. % titanium;
a maximum of 0.090 wt. % vanadium;
a maximum of 0.30 wt. % copper;
a maximum of 0.040 wt. % aluminum; and balance substantially iron;
cutting the bar into billets;
hot rolling the billets into tubing;
cooling the tubing to room temperature;
heating the tubing to approximately 40 °C above the corresponding AC3 temperature;
soaking the tubing at approximately 40 °C above the corresponding AC3 temperature for about 10 minutes; and cooling the tubing to room temperature to create resulting steel tubing which is post-yield hardened and exhibits a microstructure consisting essentially of either ferrite plus pearlite or ferrite plus bainite-pearlite.

18. The method according to claim 17, wherein the steel has a chemical composition as follows:
about 0.28 to about 0.40 wt. % carbon;
about 1.20 to about 1.45 wt. % manganese;

about 0.015 to about 0.030 wt. % sulfur;
about 0.10 to about 0.20 wt. % molybdenum;
a maximum of 0.010 wt. % niobium;
a maximum of 0.020 wt. % titanium;
a maximum of 0.020 wt. % vanadium;
a maximum of 0.25 wt. % copper; and a maximum of 0.035 wt. % aluminum.

19. The method according to claim 17, wherein the steel has a chemical composition as follows:
about 0.31 to about 0.34 wt. % carbon;
about 1.25 to about 1.40 wt. % manganese;
about 0.015 to about 0.025 wt. % sulfur;
about 0.10 to about 0.20 wt. % molybdenum;
a maximum of 0.005 wt. % niobium;
a maximum of 0.015 wt. % titanium;
a maximum of 0.010 wt. % vanadium;
a maximum of 0.25 wt. % copper; and a maximum of 0.025 wt. % aluminum.

20. The method according to claim 17, wherein the steel has a chemical composition as follows:
about 0.29 wt. % carbon;
about 1.30 wt. % manganese;
about 0.013 wt. % sulfur;
about 0.012 wt. % phosphorus;
about 0.29 wt. % silicon;
about 0.22 wt. % chromium;
about 0.15 wt. % molybdenum;
about 0.001 wt. % niobium;
about 0.002 wt. % titanium;
about 0.003 wt. % vanadium;
about 0.09 wt. % copper; and about 0.020 wt. % aluminum.

21. The method according to any one of claims 17 to 20, wherein the step of cooling to room temperature to create the resulting tubing is by air over approximately 80 minutes and the resulting steel tubing has at least one of the following properties:
minimum yield strength at room temperature of 55 ksi (379.2 MPa);
maximum yield strength at room temperature of 80 ksi (551.6 MPa);
minimum ultimate tensile strength at room temperature of 95 ksi (655 MPa);
minimum elongation at room temperature of 20 %; and minimum impact toughness at room temperature of 30 J.

22. The method according to any one of claims 17 to 20, wherein the resulting steel tubing has at least one of the following properties:
a ratio of actual material yield strength at a given temperature versus original material yield strength at room temperature of greater than 0.75 at 350 °C, and greater than 0.80 at 180 °C;
a ratio of actual material tensile strength at a given temperature versus original material tensile strength at room temperature of greater than 0.92 at 350 °C, greater than 1.06 at 180 °C, and greater than 1.1 at 230 °C and 280 °C;
a ratio of material static yield strength versus material yield strength of greater than 0.83 at any strain level up to 4% and temperature up to 350 °C;
a hardening modulus greater than 7,500 MPa at 1.5% strain at any temperature up to 350 °C; and a hardening modulus greater than 3,500 MPa at 4% strain at any temperature up to 350 °C.

23. The method according to any one of claims 17 to 20, wherein the step of cooling to room temperature to create the resulting tubing is by air over approximately 80 minutes and the resulting steel tubing has the following properties:
minimum yield strength at room temperature of 55 ksi (379.2 MPa);
maximum yield strength at room temperature of 80 ksi (551.6 MPa);
minimum ultimate tensile strength at room temperature of 95 ksi (655 MPa);

minimum elongation at room temperature of 20 %; and minimum impact toughness at room temperature of 30 J.

24. The method according to any one of claims 17 to 20, wherein the resulting steel tubing has the following properties:
a ratio of actual material yield strength at a given temperature versus original material yield strength at room temperature of greater than 0.75 at 350 °C, and greater than 0.80 at 180 °C;
a ratio of actual material tensile strength at a given temperature versus original material tensile strength at room temperature of greater than 0.92 at 350 °C, greater than 1.06 at 180 °C, and greater than 1.1 at 230 °C and 280 °C;
a ratio of material static yield strength versus material yield strength of greater than 0.83 at any strain level up to 4% and temperature up to 350 °C;
a hardening modulus greater than 7,500 MPa at 1.5% strain at any temperature up to 350 °C; and a hardening modulus greater than 3,500 MPa at 4% strain at any temperature up to 350 °C.

25. The steel tubing according to any one of claims 1 to 8, wherein the steel comprises:
a maximum of 0.05 wt. % nickel;
a maximum of 0.011 wt. % tin;
a maximum of 0.006 wt. % arsenic;
a maximum of 19 ppm calcium; and a maximum of 1 ppm boron.

26. The steel tubing according to any one of claims 1 to 8, wherein the steel comprises:
about 0.05 wt. % nickel;
about 0.009 to about 0.011 wt. % tin;
about 0.006 wt. % arsenic;
about 11 to about 19 ppm calcium; and a maximum of 1 ppm boron.

27. The method according to any one of claims 9 to 16, wherein the steel comprises:
a maximum of 0.05 wt. % nickel;
a maximum of 0.011 wt. % tin;
a maximum of 0.006 wt. % arsenic;
a maximum of 19 ppm calcium; and a maximum of 1 ppm boron.

28. The method according to any one of claims 9 to 16, wherein the steel comprises:
about 0.05 wt. % nickel;
about 0.009 to about 0.011 wt. % tin;
about 0.006 wt. % arsenic;
about 11 to about 19 ppm calcium: and a maximum of 1 ppm boron.

29. The method according to any one of claims 17 to 24, wherein the steel comprises:
a maximum of 0.05 wt. % nickel;
a maximum of 0.011 wt. % tin;
a maximum of 0.006 wt. % arsenic;
a maximum of 19 ppm calcium; and a maximum of 1 ppm boron.

30. The method according to any one of claims 17 to 24, wherein the steel comprises:
about 0.05 wt. % nickel;
about 0.009 to about 0.011 wt. % tin:
about 0.006 wt. % arsenic;
about 11 to about 19 ppm calcium; and a maximum of 1 ppm boron.