Patent Application
20100193087
This disclosure relates to a martensitic stainless steel seamless pipe for oil country tubular goods, more particularly to a seamless steel pipe for oil country tubular goods which has a high strength, such as a yield strength of 110 ksi (758 MPa) or more, and a superior low temperature toughness and to a method for manufacturing the martensitic stainless steel seamless pipe.
In consideration of a steep rise in crude oil prices and depletion of petroleum resources to be expected in near future, in recent years, for example, deep oil wells; oil wells and gas wells with a severe corrosive environment containing carbon dioxide, chlorine ions, and the like; and oil wells with a severe drilling environment, such as in a cold district or on a sea bed, to which attention has not been paid in the past, have been aggressively developed. Oil country tubular goods used in the environments as described above are required to include a material which simultaneously has a high strength, a superior corrosion resistance, and also a superior toughness.
Heretofore, in oil wells and gas wells with an environment containing carbon dioxide CO2, chlorine ions Cl−, and the like, as oil country tubular goods used for drilling operation, a 13% Cr martensitic stainless steel pipe has been frequently used.
For example, in Japanese Unexamined Patent Application Publication No. 2002-363708, martensitic stainless steel suitably used for oil country tubular goods has been proposed which contains 0.01% to 0.1% of C, 9% to 15% of Cr, and 0.1% or less of N, and which has a high toughness even though having a relatively high C content and a high strength. According to a technique disclosed in Japanese Unexamined Patent Application Publication No. 2002-363708, when the amount of carbides present in prior-austenite grain boundaries is decreased to 0.5 volume percent or less, the maximum minor axis of the carbides is set to 10 and 200 nm, the ratio between an average Cr concentration and an average Fe concentration in the carbides is set to 0.4 or less, a M23C6 type carbide is suppressed from being precipitated, and a M3C type carbide is positively precipitated, the toughness can be significantly improved. To control the structure and the composition of the carbides described above in a desired range, according to the technique disclosed in Japanese Unexamined Patent Application Publication No. 2002-363708, air cooling (spontaneous cooling) is performed after hot working, air cooling (spontaneous cooling) is performed after a solution treatment, or following air cooling (spontaneous cooling) performed after a solution treatment, tempering is performed at a low temperature of 450° C. or less.
However, according to the technique disclosed in Japanese Unexamined Patent Application Publication No. 2002-363708, when only air cooling (spontaneous cooling) is performed after hot rolling, or when only air cooling (spontaneous cooling) is performed after a solution treatment, there has been a problem in that a desired strength of a 110 ksi grade of yield strength (758˜862 MPa), and a superior low temperature toughness cannot be simultaneously obtained. In addition, to ensure a strength of a 110 ksi grade of yield strength by the technique disclosed in Japanese Unexamined Patent Application Publication No. 2002-363708, the C content must be set to 0.01 mass percent or more. However, when the C content is set to 0.01 mass percent or more, the low temperature toughness is degraded, and a superior low temperature toughness having a fracture transition temperature of −60° C. or less cannot be disadvantageously ensured. In addition, when the technique disclosed in Japanese Unexamined Patent Application Publication No. 2002-363708 is applied to a steel pipe to perform low temperature tempering at 450° C. or less, a working stress is generated by correction performed immediately after the finish of heating of the tempering treatment, and there has been a problem in that variation of steel pipe characteristics is increased.
It could therefore be helpful to provide a seamless steel pipe for oil country tubular goods which simultaneously has a high strength of a 110 ksi grade of yield strength and a superior low temperature toughness and a stable method for manufacturing the seamless steel pipe.
We thus provide:
A seamless steel pipe for oil country tubular goods which simultaneously has a high strength of a 110 ksi grade of yield strength and a superior low temperature toughness having a fracture transition temperature vTrs of −60° C. or less can be easily and also stably manufactured, and significant industrial advantages can be obtained.
“Superior low temperature toughness” indicates the case in which the fracture transition temperature vTrs in a Charpy impact test is −60° C. or less.
We carried out intensive research on the influence of component compositions and heat treatment conditions upon the change in toughness with an increase in strength of a 13 Cr martensitic stainless steel pipe. As a result, we discovered that, in a component system in which the C content is controlled to be less than 0.010 mass percent, the Cr content is set to a relatively low content, such as approximately 11% of Cr, and the Ni content is also set to a relatively low content, such as 4.0% or less, after a quenching treatment is performed, when an appropriate tempering treatment is performed in which heating is performed to a tempering temperature in the range of more than 450° C. to 550° C., and cooling is then performed, even if Mo is not added, a high strength of a 110 ksi grade of yield strength can be ensured, and a high toughness having a vTrs of −60° C. or less can also be obtained. First, the results of fundamental experiments performed us will be described.
After a quenching treatment (810° C.×15 minutes) was performed on a seamless steel pipe having a composition containing on a mass percent basis, 0.008% of C, 0.12% of Si, 1.14% of Mn, 0.019% of P, 0.001% of S, 0.04% of Al, 10.9% of Cr, 2.3% of Ni, 0.5% of Cu, 0.01% of N, and the balance being Fe, a tempering treatment was then performed in such a way that heating was performed to a temperature in the range of 425° C. to 575° C., and spontaneous cooling was then performed. In addition, in the cooling of the tempering treatment, a correctional treatment was performed. A tensile test and a Charpy impact test were performed on the obtained seamless steel pipe, so that tensile characteristics (yield strength YS, and tensile strength TS) and the low temperature toughness (fracture transition temperature vTrs) were measured. The obtained results are shown in
First, a method for manufacturing a seamless steel pipe for oil country tubular goods will be described. As a starring material, a stainless steel seamless pipe is used which has a composition containing less than 0.010% of C, 1.0% or less of Si, 0.1% to 2.0% of Mn, 0.020% or less of P, 0.010% or less of S, 0.10% or less of Al, 10% to 14% of Cr, 0.1% to 4.0% of Ni, 0.05% or less of N, and the balance being Fe and inevitable impurities. In addition, hereinafter, “mass percent” is simply represented by “%.” First, the reasons for limiting the composition of the starting material will be described.
C: Less than 0.010%
C is an important element relating to the strength of martensitic stainless steel and, to ensure a desired strength, the content is preferably 0.003% or more; however, when the content is 0.010% or more, the toughness and also the corrosion resistance are liable to be degraded. Hence, the C content is limited to less than 0.010%. In addition, to stably ensure the strength and the toughness, the content is preferably in the range of 0.003% to 0.008%.
Si is an element functioning as a deoxidizing agent in a normal steelmaking process, and the content is preferably 0.1% or more; however, when the content is more than 1.0%, the toughness is degraded, and hot workability is also degraded. Hence, the Si content is limited to 1.0% or less. In addition, the content is preferably in the range of 0.1% to 0.3%.
Mn is an element to increase the strength and, to ensure a strength necessary as a steel pipe for oil country tubular goods, the content must be 0.1% or more. However, when the content is more than 2.0%, the toughness is adversely influenced. Hence, the Mn content is limited in the range of 0.1% to 2.0%. In addition, the content is preferably in the range of 0.5% to 1.5%.
P is an element to degrade the corrosion resistance, such as CO2 corrosion resistance, and is preferably decreased as much as possible. However, an excessive decrease may cause an increase in cost. As the range in which the corrosion resistance, such as CO2 corrosion resistance, is not degraded and in which the decrease can be industrially performed at a relatively low cost, the P content is limited to 0.020% or less. In addition, the content is preferably 0.015% or less.
Al is an element having a strong deoxidizing function and, to obtain this effect, the content is preferably 0.001% or more. However, when the content is more than 0.10%, the toughness is adversely influenced. Hence the Al content is limited to 0.10% or less. In addition, the content is preferably 0.05% or less.
Cr is an element to improve the corrosion resistance by forming a passivation film and is also an element to particularly contribute to an effective improvement in CO2 corrosion resistance and resistance to CO2 stress corrosion cracking. When the content is 10% or more, corrosion resistance required for oil country tubular goods can be ensured, and hence the lower limit is set to 10%. On the other hand, when the content is large, such as more than 14%, since ferrite is easily generated, a large amount of an expensive austenite generation element must be added to stably ensure a martensitic phase or to prevent degradation of the hot workability, so that economical problems may arise. Hence, the Cr content is limited in the range of 10% to 14%. In addition, to ensure more stable microstructure and hot workability, the content is preferably in the range of 10.5% to 11.5%.
Ni has a function to strengthen a passivation film and is an element to improve the corrosion resistance, such as CO2 corrosion resistance. To obtain the effect as described above, the content must be 0.1% or more. On the other hand, when the content is more than 4.0%, the improvement effect is saturated and, as a result, a manufacturing cost is inevitably increased. Hence, the Ni content is limited in the range of 0.1% to 4.0%. In addition, the content is preferably in the range of 1.5% to 3.0%.
N is an element to significantly improve pitting resistance and, when the content is 0.003% or more, the effect described above becomes significant. On the other hand, when the content is more than 0.05%, various nitrides are formed and, as a result, the toughness is degraded. Hence, the N content is limited to 0.05% or less. In addition, the content is preferably in the range of 0.01% to 0.02%.
Although the components described above are basic components of the starting material, besides those basic components described above, at least one selected from the group consisting of 2.0% or less of Cu and 2.0% or less of Mo and/or at least one selected from the group consisting of 0.10% or less of V, 0.10% or less of Nb, and 0.10% or less of Ti may also be contained.
At least one selected from the group consisting of 2.0% or less of Cu and 2.0% or less of Mo
Cu and Mo are elements each having a function to improve the corrosion resistance and, whenever necessary, at least one of them may be selected and contained.
Cu is an element having a function to improve the pitting resistance by strengthening a passivation film, and to obtain the effect as described above, the content is preferably 0.2% or more. On the other hand, when the content is more than 2.0%, Cu is partly precipitated and, as a result, the toughness is degraded. Hence, when Cu is contained, the content thereof is preferably limited to 2.0% or less. In addition, more preferably, the content is in the range of 0.2% to 1.0%.
In addition, Mo is an element having a function to increase the resistance against pitting caused by Cl− and, to obtain the above effect, the content is preferably 0.2% or more. On the other hand, when the content is more than 2.0%, the strength is not only decreased, but material cost is also increased. Hence, the Mo content is preferably limited to 2.0% or less. In addition, more preferably, the content is in the range of 0.2% to 1.0%.
At Least One Selected from the Group Consisting of V: 0.10% or Less, Nb: 0.10% or Less, and Ti: 0.10% or Less
V, Nb, and Ti are components to increase the strength and, whenever necessary, at least one of them may be selected and contained.
To obtain the effect as described above, at least one of 0.02% or more of V, 0.01% or more of Nb, and 0.02% or more of Ti is preferably contained. On the other hand, when at least one of more than 0.10% of V, more than 0.10% of Nb, and more than 0.10% of Ti is contained, the toughness is degraded. Hence, when being contained, the contents of V, Nb, and Ti are each preferably limited to 0.10% or less. In addition, more preferably, the V content is 0.02% to 0.05%, the Nb content is 0.01% to 0.05%, and the Ti content is 0.02% to 0.05%.
The balance other than those components described above contains Fe and inevitable impurities. In addition, as the inevitable impurities, 0.010% or less of O may be contained.
Although a method for manufacturing a starting material having the above composition is not particularly limited, it is preferable that after molten steel having the above composition is formed by a commonly known steelmaking method, for example, using a converter, an electrical furnace, a vacuum melting furnace, and the like, a steel pipe material, such as a billet, be formed by a common method, such as a continuous casting method, or an ingot-making and blooming method. Subsequently, the steel pipe material is heated and is processed by hot working using a common Mannesmann-plug mill type or Mannesmann-mandrel mill type manufacturing process to form a seamless steel pipe having a desired dimension, and this seamless steel pipe is preferably used as the starting material. In addition, a seamless steel pipe may also be manufactured by press type hot extrusion. In addition, after the pipe is formed, the seamless steel pipe is preferably cooled to room temperature at a cooling rate equivalent to or more than that of air cooling.
The starting material (seamless steel pipe) is first processed by a quenching treatment.
The quenching treatment is a treatment in which after re-heating is performed to a heating temperature for quenching equivalent to or more than the Ac3 transformation point, cooling is performed from the heating temperature for quenching to a temperature range of 100° C. or less at a cooling rate equivalent to or more than that of air cooling. As a result, a fine martensitic microstructure can be obtained. When a heating temperature for quenching is less than the Ac3 transformation point, since heating cannot be performed to the austenite single phase region, and a sufficient martensitic microstructure cannot be obtained by subsequent cooling, a desired strength cannot be ensured. Hence, the heating temperature for quenching of the quenching treatment is limited to be equivalent to or more than the Ac3 transformation point. In addition, the heating temperature is preferably 950° C. or less. The cooling from the quenching heating temperature is performed to a temperature range of 100° C. or less at a cooling rate equivalent to or more than that of air cooling. Since the starting material has high hardenability, when the cooling is performed to a temperature range of 100° C. or less at a cooling rate approximately equivalent to that of air cooling, a sufficiently quenched microstructure (martensitic microstructure) can be obtained. In addition, a holding time at the heating temperature for quenching is preferably set to 10 minutes or more in view of uniform heating.
The seamless steel pipe processed by the quenching treatment is subsequently processed by a tempering treatment. The tempering treatment is an important treatment to ensure a superior low temperature toughness. The tempering treatment is defined as a treatment in which after heating is performed to a tempering temperature in the range of more than 450° C. to 550° C. and is maintained preferably for 30 minutes or more, cooling is performed preferably to room temperature preferably at a cooling rate equivalent to or more than that of air cooling. As a result, a seamless steel pipe which, simultaneously has a high strength of YS 110 ksi or more and a superior low temperature toughness having a vTrs of −60° C. or less can be obtained. When the tempering temperature is 450° C. or less, since the tempering is insufficient, the toughness is degraded, and as a result, a high strength and a high toughness cannot be simultaneously obtained. On the other hand, when the tempering temperature is more than 550° C., besides a decrease in strength, since the grain boundaries become brittle, the intergranular fracture is liable to occur, and the toughness is also degraded. Hence, a high strength and a high toughness cannot be simultaneously obtained. The tempering temperature is preferably in the range of 500° C. to 550° C. In addition, to stably maintain the properties, the holding time at the tempering temperature is preferably set to 30 minutes or more. In addition, the cooling from the tempering temperature is preferably performed at a cooling rate equivalent to or more than that of air cooling.
Whenever necessary, a correction treatment for correcting defect in pipe shape may be performed in the cooling of the tempering treatment. The correction treatment is preferably performed in a temperature range of 400° C. or more. When the temperature of the correction treatment is less than 400° C., a working strain is locally applied to the steel pipe when the correction treatment is performed. Hence, variation in mechanical characteristics is liable to be generated. Thus, the correction treatment is performed in a temperature range of 400° C. or, more.
A seamless steel pipe manufactured by the above-described manufacturing method is a martensitic stainless steel seamless pipe which has the composition described above and which simultaneously has a high strength of a 110 ksi grade of yield strength and a superior low temperature toughness having a fracture transition temperature vTrs of −60° C. or less in a Charpy impact test. In addition, this martensitic stainless steel seamless pipe has a microstructure including a tempered martensitic phase as a primary phase. Hence, a steel pipe can be obtained which simultaneously has a desired high strength and a desired high toughness and which also has a sufficient corrosion resistance as oil country tubular goods.
After various types of molten steel having the compositions shown in Table 1 were degassed, slabs were formed by a continuous casting method, and billets (size: 207 mm in diameter) were obtained by billet rolling of the slabs processed by re-heating, so that steel-pipe materials were prepared. After the steel pipe materials were heated and formed into pipes by hot working using a Mannesmann-type manufacturing process, cooling was performed, so that seamless steel pipes (outside diameter: 177.8 mm, and wall thickness: 12.7 mm) were obtained.
The seamless steel pipes thus obtained were subjected to a quenching treatment and a tempering treatment, and were further subjected to a correction treatment whenever necessary.
After API strip tensile specimens were obtained from the seamless steel pipes which were subjected to the quenching treatment and the tempering treatment and were further subjected to the correction treatment whenever necessary, a tensile test was performed, so that the tensile characteristics (yield strength YS, and tensile strength TS) were obtained.
In addition, V-notch test pieces (10 mm thick) in accordance with JIS Z 2242 standard were obtained from the seamless steel pipes which were subjected to the quenching treatment and the tempering treatment and were further subjected to the correction treatment whenever necessary, a Charpy impact test was carried out to obtain the fracture transition temperature vTrs and absorption energy vE−60 at a temperature of −60° C., so that the toughness was evaluated. In addition, after test pieces were obtained from 12 points along the circumference of each steel pipe subjected to the correction treatment, a Charpy impact test was performed at a temperature of −60° C., and the variation was evaluated from the average value (ave) and the minimum value (min) of the absorption energy vE−60.
In addition, corrosion test pieces having a thickness of 3 mm, a width of 25 mm, and a length of 50 mm were formed from the steel pipes by machining, and a corrosion test was performed.
The corrosion test was performed in such a way that the corrosion test pieces were immersed for one week (168 hours) in a test solution, a 20%-NaCl aqueous solution (solution temperature: 80° C., and a CO2 gas environment at 30 bar pressure), which was placed in an autoclave. The weights of the test pieces subjected to the corrosion test were measured, and corrosion rates were obtained by calculating the weight loss before and after the corrosion test. In addition, the surfaces of the test pieces subjected to the corrosion test were observed with a loupe having a magnification of 10 to confirm the pitting generation. As for the pitting, in the case in which at least one pit was observed, it was regarded that pitting occurred, and in the other cases, it was regarded that no pitting occurred. The obtained results are shown in Table 3.
According to our examples, a martensitic stainless steel seamless pipe could be obtained which had a sufficient corrosion resistance as oil country tubular goods and which simultaneously had a high strength of a 110 ksi grade of YS and a superior low temperature toughness having a vTrs of −60° C. or less. On the other hand, according to comparative examples out of our range, since the strength was not sufficient, or the low temperature toughness was degraded, desired high strength and high toughness could not be ensured.
H
0.012
I
0.012
J
K
14.5
H
H
I
I
J
K
385
575
−50
703
−30
H
889
−55
H
−40
I
902
−50
I
−35
J
687
−30
K
706
−25
| Number | Date | Country | Kind |
|---|---|---|---|
| 2007-172560 | Jun 2007 | JP | national |
This is a §371 of International Application No. PCT/JP2007/070209, with an international filing date of Oct. 10, 2007 (WO 2009/004741 A1, published Jan. 8, 2009), which is based on Japanese Patent Application No. 2007-172560, filed Jun. 29, 2007, the subject matter of which is incorporated by reference.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2007/070209 | 10/10/2007 | WO | 00 | 12/17/2009 |
