JP2000319763A - HIGH Cr STEEL PIPE FOR LINE PIPE - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the low temp. toughness of a line pipe by allowing it to have a compsn. contg. C, Si, Mn, P, Cr, Ni and N, and the balance Fe with inevitable impurities. SOLUTION: This steel pipe is composed of, by mass, <=0.02% C, <=0.5% Si, 0.2 to 3.0% Mn, <0.010% P, 10.0 to 14.0% Cr, 0.2 to 3.0% Ni and <=0.02% N, and the balance Fe with inevitable impurities. Among the componential elements, P exerts remarkable influence on the toughness of high Cr steel of about >=600 MPa yield strength. For example, by reducing the content of P from 0.010 to <0.010%, the absorbed energy (vE-40) at -40 deg.C is remarkably improved from about 100 J to about 200 to 250 J. Moreover, the amt. of P to be concentrated in the boundary is stepwise reduced in accordance with the change of the P content from 0.010 to <0.010%. Moreover, by reducing the P content preferably to <0.005%, vE-40 is exceedingly improved and reaches about 300 J.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-temperature line pipe having excellent low-temperature toughness suitable for transportation of oil and natural gas.
Cr steel pipe. In the present invention, “%” related to the composition means “mass%” unless otherwise specified.

[0002]

2. Description of the Related Art In recent years, oil and natural gas, which are easily drilled, have been exhausted, and well-drilled environments such as severely corroded, deep, cold regions and the sea floor have to be modified. Is gone. Oil and natural gas produced from such wells often contain a large amount of carbon dioxide gas. In such an environment, carbon steel or low alloy steel is significantly corroded. Inhibitors have been added as anticorrosion measures. However, since the use of inhibitors is expensive and the effect is insufficient at high temperatures, there is a tendency in recent years to use corrosion-resistant materials that do not require the use of inhibitors. As such a corrosion-resistant material, a martensitic stainless steel containing 13% of Cr is widely used in oil country tubular goods.

[0003] On the other hand, for line pipes, 12% Cr martensitic stainless steel with a reduced C content is specified in the API standard. This steel is hardly generally used as a line pipe because it requires preheating and post-heating for girth welding, resulting in high cost and poor toughness of a welded portion. For this reason, as a material for corrosion-resistant line pipe, duplex stainless steel containing Cr and containing Ni and Mo has been used because of its excellent weldability and corrosion resistance. However, the duplex stainless steel has a problem that the quality is excessive and the cost is high in some wells.

In order to solve this problem, Japanese Patent Application Laid-Open No. 8-295939
Japanese Patent Application Publication No. JP-A-2005-115122 discloses a line pipe for 10 to 14% Cr steel with C and N reduced to 0.03% or less and 0.02% or less and Cu adjusted to 0.2 to 1.0%, and then heat-treated under specific conditions. High
A method for producing a Cr martensitic steel pipe has been proposed, and it is disclosed that the toughness is improved by heat treatment in the two-phase region.
It is described that a steel pipe having excellent corrosion resistance, weldability, and heat affected zone (HAZ) toughness in a carbon dioxide gas environment is thereby obtained.

However, in the above-mentioned method, since the toughness is mainly improved by heat treatment, the low-temperature toughness is naturally limited, and there is a problem that it is not possible to cope with a higher toughness requirement.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a high Cr steel pipe for line pipes in which the low temperature toughness is further improved by changing the component system in view of the problems of the prior art.

[0007]

Means for Solving the Problems The present inventors have conducted intensive studies to achieve the above object, and as a result, the fracture surface of the conventional material after the Charpy impact test shows the appearance of grain boundary fracture. Confirmed that P was concentrated. From this, it is considered that the low temperature toughness of the conventional material is insufficient because the grain boundary strength is low, and the cause of the decrease in the grain boundary strength is P enrichment at the grain boundary. Based on this idea, the effect of the P content on the low-temperature toughness of high-Cr steels was examined by experiments. As shown in FIG. 2, for example, at the same strength level, in a high-Cr steel with extremely low P, The quenching (Q) -tempering (T) treatment also improves the toughness obtained by quenching (Q) -two-phase region heat treatment (Q ')-tempering (T) treatment of a conventional material having a normal level of P. It has been found that a much greater improvement in toughness can be obtained. FIG. 2 is a sample obtained by heating a steel pipe material having the composition shown in the figure, forming the pipe into a seamless steel pipe having a diameter of 273 mm × t 13 mm, air cooling to room temperature, and performing QT or QQ′-T processing. Strength (YS) obtained by organizing the tensile test and Charpy impact test results of
It shows the relationship between the absorbed energy at 0 ° C. (vE _-40 ).

The present invention has been further studied based on such knowledge, and the gist of the present invention is that the present invention, which has been further studied based on this knowledge, has a high Cr content for line pipes. A steel pipe having the following composition: C: 0.02% or less (0.015% or less), Si: 0.5% or less (0.3% or less), Mn: 0.2 to 3.0% (1.0 to 2.0%), P: less than 0.010% ( The line is characterized by being Cr: 10.0-14.0%, Ni: 0.2-3.0% (2.0-3.0%), N: 0.02% or less (0.015% or less), the balance being Fe and inevitable impurities. High Cr steel pipe for pipe.

In the present invention, Cu: 1.0% or less (0.2 to 1.0%) may be added to the composition. In the present invention, V: 0.3% or less (0.03 to 0.15%) may be added to the composition. In addition, the inside of () shows a further preferable range.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting the composition of the steel pipe of the present invention will be described below. C: 0.02% or less C can be reduced as much as possible from the viewpoints of HAZ hardness reduction, toughness improvement, weld cracking resistance improvement, general corrosion resistance in an environment containing carbon dioxide and chloride, and pitting corrosion resistance. It is desirable to reduce it. In particular, in order to enable welding without preheating, the C content needs to be 0.02% or less, and therefore the upper limit of the C content is set to 0.02%. The content is preferably 0.015% or less from the viewpoint of ensuring better weldability.

Si: 0.5% or less Si is added as a deoxidizing agent. However, since it is a ferrite-forming element, if it is contained in a large amount, ferrite is likely to be formed, and the toughness of the base material and HAZ is deteriorated. Also,
If ferrite is present, hot workability may be reduced, which may hinder production. For this reason, the amount of Si was limited to 0.5% or less. It is preferably at most 0.3%.

Mn: 0.2-3.0% Mn is an element that acts as a deoxidizing agent and further increases the strength. Further, since it is an austenite forming element, it also has the function of suppressing ferrite formation and improving the toughness of the base material and HAZ. In order to obtain such an effect, 0.
The content of Mn is limited to 0.2 to 3.0% because the effect is saturated even if added over 3.0%. Preferably it is 1.0 to 2.0%.

P: less than 0.010% P greatly affects the toughness of a high Cr steel having a strength level of about YS600 MPa or more as in the present invention. For example, as shown in FIG. 1, a high Cr steel (Q
-T material) to reduce the amount of P from 0.010% or more to less than 0.010%, so that vE- ₄₀ is reduced from about 100 J to 200 to 25.
Dramatically improved to about 0J. The amount of P enrichment at the grain boundaries decreases stepwise as the amount of P changes from 0.010% or more to less than 0.010%. Therefore, in the present invention, P is set to 0.
Limited to less than 010%. From FIG. 2, P is 0.005%.
If it is reduced to less than 30, vE _-40 will be further improved and 30
It reaches about 0J. For this reason, it is more preferable that P is less than 0.005%.

Cr: 10.0 to 14.0% Cr is a basic element required to secure a martensitic structure and to enhance the overall corrosion resistance and pitting corrosion resistance in a corrosive environment containing carbon dioxide gas. In order to obtain these effects, it is necessary to add 10.0% or more. On the other hand, when the content exceeds 14.0%, the formation of ferrite becomes easy, and a large amount of austenite-forming element must be added in order to secure a stable martensite structure or to prevent a decrease in hot workability, resulting in an increase in cost. Therefore, the Cr content is set to 10.0 to 14.0%.

Ni: 0.2-3.0% Ni is an austenite-forming element, and functions to suppress the formation of ferrite, improve the toughness of the base material and HAZ, and suppress the decrease in hot workability. Further, the corrosion resistance and the pitting corrosion resistance in a corrosive environment containing carbon dioxide are improved. This effect is evident with additions above 0.2%. However, the addition of more than 3.0% is disadvantageous in that the effects of improving toughness, hot workability and corrosion resistance are saturated, resulting in unnecessarily high costs. Therefore, the amount of Ni is set to 0.2 to 3.0%. To obtain more stable characteristics, 2.0 to 3.
0% is preferred.

N: 0.02% or less N is the same as C, avoiding welding cracks, improving the toughness of HAZ,
In order to reduce the hardness of HAZ and HAZ, it is desirable to reduce the hardness as much as possible. If it exceeds 0.02%, these effects cannot be sufficiently obtained. Incidentally, the content is preferably 0.015% or less.

Cu: 1.0% or less Cu, like Ni and Mn, is an austenite forming element, suppresses the formation of ferrite, has an effect on improving the toughness of HAZ, improving the overall corrosion resistance, and has the hot workability. Can be added as appropriate because it has the effect of suppressing the decrease in the amount of carbon dioxide and the effect of stabilizing the passive film and improving the pitting corrosion resistance in an environment containing carbon dioxide and chloride. Is not dissolved and precipitates, which adversely affects the toughness of the HAZ. Therefore, the Cu content is preferably set to 1.0% or less. The preferred range in terms of the various effects is 0.2 to 1.0%.
It is.

V: not more than 0.3% V is an element useful for improving the high-temperature strength, and may be added as appropriate. However, if added more than 0.3%, the strength increases with deterioration in toughness. It is better to stay in the following range. In addition, from a viewpoint of high temperature strength improvement, 0.03 to 0.15% is preferable. Other elements are inevitably contained, but are desirably reduced as much as possible from the viewpoint of ensuring base material toughness. In addition, both S and O can be allowed up to 0.01%.

Next, a preferred manufacturing process of the steel pipe of the present invention will be described. The steel having the above composition is melted in a converter or an electric furnace, and solidified by a continuous casting method or an ingot forming method to obtain a steel pipe material. In the process, ladle refining of molten steel, vacuum degassing, etc. are performed as necessary. Ac ₃
It is heated to a point or more, and a seamless steel pipe is formed by hot rolling using a plug mill method, a mandrel mill method, or the like, or further formed into a steel pipe having a desired size while being hot using a sizer and a stretch reducer.

After pipe formation, heat treatment is performed to obtain a desired strength-toughness balance. This heat treatment includes quenching-tempering (QT), quenching-two-phase region heat treatment-tempering (QQ ').
-T), quenching-two-phase region heat treatment (QQ '), and two-phase region heat treatment-tempering (Q'-T) may be employed that suits the target mechanical properties. The quenching (Q) is immediately below the Ms point (about 200 ° C or less) from the hot state after pipe making.
Direct quenching (DQ) for cooling to a temperature below the Ms point (about 200 ° C. or less) after reheating to the γ region after pipe formation may be performed. RQ may be performed after DQ. In the composition according to the present invention, a martensite structure can be obtained even if Q is performed by ordinary air cooling, but cooling faster than air cooling by blast cooling, water cooling, etc. suppresses the growth of austenite grains until the start of transformation. After the transformation, the structure becomes finer and the toughness is improved.

The two-phase region heat treatment (Q ') is performed from the point of Ac _{1 to the} point of (Ac ₁
Point + 50 ° C). By heating at _one or more points of Ac, a fine two-phase structure of martensite and austenite is formed. C and N diffuse and concentrate from the martensite phase to the austenite phase because the solubility in the martensite phase is lower than the solubility in the austenite phase.
Therefore, in Q ′, an austenite phase in which C and N are concentrated and a tempered martensite phase in which C and N are diluted are formed, and the tempered martensite containing a large amount of carbonitride is formed by tempering (T) after Q ′. A site phase and a tempered martensite phase having a very small grain boundary strength of carbonitride are formed, and the formation of the tempered martensite phase having a high grain boundary strength results in a steel pipe having high toughness.

However, when the Q ′ temperature exceeds (Ac ₁ point + 50 ° C.), the ratio of the tempered martensite phase diluted with C and N, which eventually becomes a tempered martensite phase having a high grain boundary strength, decreases. , The effect of improving toughness is reduced. Further, coarsening of the grains also leads to a decrease in toughness. The holding time of Q ′ is preferably 10 to 60 min. The cooling after the holding is preferably performed at a cooling rate higher than the air cooling.

The tempering (T) is less than Ac ₁ point (preferably
(550 ° C or higher). After maintaining the temperature at this temperature, cooling is performed at a cooling rate higher than air cooling. This results in a structure including a tempered martensite phase with a low carbonitride and high grain boundary strength, and thus a steel pipe having high toughness. The retention time of T is 10 ~
Preferably, it is 60 min.

[0024]

EXAMPLES Steel having the composition shown in Table 1 was melted in a converter, subjected to vacuum degassing, solidified by a continuous casting method, and billets were rolled into cast pieces to obtain steel pipe blanks. These steel pipe materials are converted to φ by the Mannesmann-Plug Mill type manufacturing equipment.
A 273 mm × t 13 mm seamless steel pipe was piped, the occurrence of pipe flaws was investigated, and the pipe was heat-treated under the conditions shown in Table 1 and YS was adjusted to about 600 MPa from a steel pipe base material. Test specimens were collected and investigated for tensile properties, low-temperature toughness, and corrosion resistance (pitting corrosion resistance). In addition, TIG welding using a duplex stainless steel as a welding material, using a steel pipe base material (voltage 15 V, current 200
A, a steel pipe joint was produced at a welding speed of 10 cm / min and a heat input of 18 kJ / cm 2, and the low temperature toughness of the HAZ (1 mm from the bond) was investigated. In Table 1, RQ is indicated as Q.

The tensile test was performed according to ASTM A370. The low temperature toughness was determined by conducting a Charpy impact test,
Evaluation was made based on the absorption energy (vE _-40 ) at 40 ° C. The corrosion test was performed in a carbon dioxide gas corrosion test method (3.0 mm × 25 mm in a 20% NaCl aqueous solution saturated with 3.0 MPa of carbon dioxide in an autoclave).
A test piece of × 50 mm was immersed and kept at 80 ° C. for 7 days). The corrosion rate was determined from the weight measurement before and after the test, and the corrosion rate of 0.1 mm / year or more was evaluated as x, and the corrosion rate of less than 0.1 mm / year was evaluated as o.

The results are shown in Table 1.

[0027]

[Table 1]

The examples of the present invention show no pitting corrosion, are excellent in pitting corrosion resistance, and are at a practically usable level. They are excellent not only in the low-temperature toughness of the base material but also in the low-temperature toughness of HAZ. Sufficient properties. In the comparative example having a high P amount,
In particular, the low-temperature toughness of the base material and HAZ is inferior to those of the present invention. Further, the corrosion rate was high in the comparative example having a low Ni content.

[0029]

As described above, the steel pipe of the present invention exhibits excellent corrosion resistance in an environment containing carbon dioxide gas and chloride, and is excellent in base metal toughness and HAZ toughness, so that it is inexpensive as a line pipe material for oil and natural gas transportation. To contribute to industrial development.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the amount of P and vE- ₄₀ .

FIG. 2 is a graph showing an effect of improving low-temperature toughness due to extremely low P.

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Takaaki Toyooka 1-1-1 Kawasaki-cho, Handa-shi, Aichi Prefecture Kawasaki Steel Corporation Chita Works

Claims (3)

[Claims] 1. A high Cr steel pipe for a line pipe, wherein the composition is as follows: C: 0.02% or less, Si: 0.5% or less, Mn: 0.2 to 3.0%, P: less than 0.010%, Cr: 10.0 to 14.0% , Ni: 0.2 to 3.0%, N: 0.02% or less, with the balance being Fe and unavoidable impurities. 2. The high Cr steel pipe for a line pipe according to claim 1, wherein Cu: 1.0% or less is added to the composition. 3. The high Cr steel pipe for a line pipe according to claim 1, wherein V: 0.3% or less is added to the composition.