JP2012241267A - High compressive strength steel pipe and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel pipe, in which the decrease in yield stress caused by the Bauschinger effect is inhibited by optimizing chemical components and metal structure of a steel sheet, and which has high compressive strength and superior weld HAZ toughness and shows an API-X80 grade or higher.SOLUTION: The steel pipe for a sour resistant line pipe of high compressive strength of a tensile strength of ≥630 MPa includes, by mass, C, Si, Mn, P, S, Al, 0.003-0.070% of Nb, 0.005-0.035% of Ti, and 0.01-0.5% of Mo, wherein C(%)-0.065Nb(%)-0.025Mo(%) is 0.025 to 0.060, C(%)+0.67Nb(%) is ≤0.10, and Pcm value is ≤0.20. The steel pipe is characterized in that in its metal structure, the total area fraction of bainite is ≥95%, fine precipitates containing Nb are precipitated in a dispersed state in bainite, and the area fraction of island martensite is ≤3%.

Description

  The present invention relates to steel pipes of API-X80 grade or higher that have excellent sour resistance for oil and natural gas transportation, and particularly to deep sea line pipes, risers, conductor casings, and the like that require high collapse resistance. The present invention relates to a high compressive strength steel pipe suitable for use.

  With the increasing energy demand in recent years, oil and natural gas pipelines have been actively developed, and many pipelines across the ocean have been developed in order to remote gas fields and oil fields and diversify transportation routes. ing. Line pipes used in submarine pipelines are thicker than onshore pipelines to prevent collapse due to water pressure, and high roundness is required. As a material, high compressive strength is required to resist compressive stress generated in the pipe circumferential direction by external pressure.

  On the other hand, API-X65 grade is widely used as the strength grade of steel pipes used for submarine pipelines, but the application of high-strength steel pipes of X70 grade or higher has spread due to demands for reducing pipeline construction costs. In addition, the demand for high-strength steel pipes for high-strength line pipes of X80 grade or higher is increasing.

The DNV standard (OS F-101) is often applied to the design of submarine pipelines. In this standard, pipe diameter D, pipe thickness t, and roundness are factors that determine the collapse pressure due to external pressure. The collapse pressure is determined using f ₀ and the tensile yield strength fy of the material. However, even if the size and strength of the pipe are the same, the collapse pressure varies depending on the pipe manufacturing method, so the yield strength is multiplied by a different coefficient (αfab) depending on the manufacturing method. As for this coefficient, 1.0 for the seamless pipe, that is, the tensile yield strength can be applied as it is, but 0.85 is given as a coefficient for the pipe manufactured by the UOE process.

  This is because the compressive strength of the pipe manufactured by the UOE process is lower than the tensile strength. However, UOE steel pipe has a pipe expansion process at the final stage of pipe making and is compressed after tensile deformation is given in the pipe circumferential direction. As a result, the yield strength decreases due to the Bauschinger effect.

  From the above, it is necessary to increase the compressive strength of the pipe in order to increase the anti-collapse performance, but in the case of a steel pipe manufactured through a tube expansion process by cold forming, the strength is reduced due to the Bauschinger effect. It was a problem.

  Many studies have been made on improving the collapse resistance of UOE steel pipe, and Patent Document 1 discloses a method of maintaining a temperature for a certain time or more after heating and expanding a steel pipe by energization heating. According to this method, the dislocation introduced by the pipe expansion is recovered and the yield strength is increased, but the productivity is inferior because it is necessary to continue the electric heating for 5 minutes or more after the pipe expansion.

Similarly, as a method of recovering the decrease in yield strength due to the Bauschinger effect by heating after tube expansion, in Patent Document 2, the outer surface of the steel tube is subjected to tensile deformation by heating to a temperature higher than the inner surface. The method of recovering the bausinger effect and maintaining the work hardening of compression on the inner surface side is disclosed in Patent Document 3 in which the accelerated cooling after hot rolling in the steel plate manufacturing process of steel added with Nb and Ti is performed at Ar ₃ temperature. There have been proposed methods of performing heating from the above to 300 ° C. or less, and heating the steel pipe by the UOE process.

However, in the method of Patent Document 2, it is extremely difficult to manage the heating temperature and the heating time of the outer surface and the inner surface of the steel pipe separately in actual production, particularly in the mass production process, The method of Patent Document 3 requires that the accelerated cooling stop temperature in steel plate production be a low temperature of 300 ° C. or lower, so that the distortion of the steel plate increases and the roundness in the case of using a steel pipe in the UOE process decreases. In order to perform accelerated cooling from the Ar ₃ temperature or higher, it is necessary to perform rolling at a relatively high temperature, which causes a problem that the toughness deteriorates.

  On the other hand, as a method for increasing the compressive strength by a method of forming a steel pipe without heating after the pipe expansion, Patent Document 4 discloses a method in which the compression ratio during O-molding is made larger than the subsequent pipe expansion ratio. According to this method, since there is substantially no tensile pre-strain in the pipe circumferential direction, the Bauschinger effect is not exhibited and a high compressive strength is obtained. However, if the expansion ratio is low, it is difficult to maintain the roundness of the steel pipe, and the collapse resistance performance of the steel pipe may be deteriorated.

  Further, Patent Document 5 discloses a method for improving the anti-collapse performance by making the diameter near the seam welded portion having a low compressive strength and the diameter at a position 180 ° from the welded portion the maximum diameter of the steel pipe. However, when actual pipelines are laid, collapse is a problem where the pipe that reaches the seabed undergoes bending deformation (sag bend), and is welded circumferentially regardless of the position of the seam weld on the steel pipe. Since it is laid on the seabed, there is no practical effect even if the seam weld has a long diameter.

  Further, Patent Document 6 proposes a steel plate in which the yield stress reduction due to the Bauschinger effect is small by performing reheating after accelerated cooling to reduce the hard second phase fraction of the steel plate surface layer portion.

  Patent Document 7 discloses a method for manufacturing a steel sheet for a high-strength sour line pipe having a thickness of 30 mm or more, in which the surface layer of the steel sheet is heated while suppressing the temperature rise at the center of the steel sheet in the reheating treatment after accelerated cooling. Proposed. According to this, since the hard second phase fraction of the steel plate surface layer portion is reduced while suppressing a drop in DWTT (Drop Weight Tear Test) performance, the hardness of the steel plate surface layer portion is reduced and the material variation is reduced. In addition to obtaining a small steel plate, it is expected that the Bausinger effect will be reduced by reducing the hard second phase.

  However, it is difficult for the techniques described in Patent Documents 6 and 7 to stably obtain a strength of X70 grade or higher, and the Bausinger effect is affected by various tissue factors such as crystal grain size and solute carbon content. Therefore, a steel pipe with high compressive strength cannot be obtained simply by reducing the hard second phase.

JP 9-49025 A JP 2003-342639 A JP 2004-35925 A JP 2002-102931 A JP 2003-340519 A JP 2008-56962 A JP 2009-52137 A

  The present invention has been made in view of the above circumstances, and is a steel pipe having high strength and excellent toughness required for application to API-X80 grade or higher submarine pipelines, risers or conductor casings. It does not require special forming conditions or heat treatment after pipe making, and optimizes the chemical composition and metal structure of the steel sheet to suppress yield stress reduction due to the Bauschinger effect and provide a steel pipe with high compressive strength. Objective.

  The present inventors first tried various experiments in order to achieve both compression strength improvement by suppressing the Bauschinger effect, strength toughness, and sour resistance performance, and as a result, the following knowledge was obtained.

  In the present invention, the compressive strength means the compressive yield strength, and if the compressive yield strength is 530 MPa or more, the compressive strength is good. More preferably, it is 570 MPa or more.

  (1) The strength reduction due to the Bauschinger effect is caused by the occurrence of reverse stress due to dislocation accumulation at the heterogeneous interface or the hard second phase. To prevent this, first of all, island martensite (the location of dislocation accumulation) It is most effective to reduce the hard phase such as MA).

  (2) However, high-strength steel materials of API-X80 grade or higher have many alloy components and have very high hardenability, so that MA is easily generated by accelerated cooling at the time of steel plate production. This is because concentration of carbon to untransformed austenite occurs at the time of bainite transformation after accelerated cooling, and the region enriched with carbon transforms to MA. In order to suppress such MA formation, it is effective to suppress the concentration of carbon to untransformed austenite by precipitation as a carbide after accelerated cooling.

  (3) The precipitation of carbides as described above is effective for increasing the strength, but by adding Nb and Mo, which are carbide forming elements, a very fine composite carbide composed of Nb and Mo. Since generation is obtained, high strength can be ensured, and further, as described above, reduction in compression strength due to the Bauschinger effect can be suppressed by reducing MA.

  (4) By optimizing the amount of C in steel and the amount of carbide-forming elements such as Nb and securing a sufficient amount of solute C, the interaction between dislocation and solute C is promoted. This prevents the movement of the steel and prevents a decrease in strength due to reverse stress. However, an excessive amount of solute C promotes the formation of MA and causes a decrease in compressive strength due to the Bauschinger effect. For this reason, it is necessary to control the amount of dissolved C very strictly. By strictly limiting the relationship between C added to the steel material and carbide forming elements to a certain range, the effect of the dissolved C can be effectively utilized to generate MA. Can be suppressed.

The present invention has been made based on the above findings,
1st invention is the mass%, C: 0.03-0.08%, Si: 0.01-0.30%, Mn: 1.2-2.0%, P: 0.020% or less , S: 0.0030% or less, Al: 0.01-0.08%, Nb: 0.003-0.070%, Ti: 0.005-0.035%, Mo: 0.01-0. 5%, N: 0.0020 to 0.0060%,
C (%)-0.065Nb (%)-0.025Mo (%) is 0.025-0.060,
C (%) + 0.67Nb (%) is 0.10 or less,
The Pcm value represented by the following formula (1) is 0.20 or less, and the balance is a steel pipe composed of Fe and inevitable impurities,
The metal structure has an area fraction of bainite of 95% or more, fine precipitates containing Nb are dispersed in bainite, and the area fraction of island martensite (MA) is 3% or less. A high compressive strength steel pipe having a tensile strength of 630 MPa or more.
Pcm = C (%) + Si (%) / 30 + Mn (%) / 20 + Cu (%) / 20 + Ni (%) / 60 + Cr (%) / 20 + Mo (%) / 15 + V (%) / 10 + 5B (%) (1) )
However, each element is content (mass%), and the element which does not contain is set to 0.

The second invention further includes, in mass%, Cu: 0.5% or less, Ni: 1.0% or less, Cr: 1.0% or less, V: 0.07% or less, Ca: 0.0005- Containing one or more selected from 0.0035%,
C (%)-0.065Nb (%)-0.025 Mo (%)-0.057 V (%) is 0.025-0.060, The high compressive strength as described in 1st invention characterized by the above-mentioned Steel pipe.
However, each element is content (mass%), and the element which does not contain is set to 0.

The third invention, the steel slab was heated to 1000 to 1200 ° C., pre-recrystallization temperature region rolling reduction of 60% or more, the finish rolling temperature is subjected to hot rolling of _{Ar 3 ~ (Ar 3 + 70} ℃) , _{subsequently,} in _(Ar 3 -30 ℃) temperatures above the 10 ° C. / sec or more cooling rate, cooling the steel sheet to 300 to 600 ° C., by performing the reheating the steel sheet to 550 to 700 ° C. subsequently A steel sheet is manufactured by the following, and then the steel sheet is formed in a cold shape to form a steel pipe shape, the butt portion is welded, and then the pipe expansion ratio is 0.4% to 1.2%. A method for producing a high compressive strength steel pipe according to the first or second invention.

ADVANTAGE OF THE INVENTION According to this invention, the high strength and high compressive strength steel pipe required in order to apply to a submarine pipeline, a riser, a conductor casing, etc. is obtained.

The form for implementing this invention is demonstrated below.
First, the reason for limitation of each component requirement of this invention is demonstrated.

1. About a chemical component, the reason for limitation of the chemical component which the high intensity | strength high toughness steel plate of this invention contains is demonstrated first. In addition, all the component% description means the mass%.

C: 0.03-0.08%
C is the most effective element for increasing the strength of the steel sheet produced by accelerated cooling. However, if it is less than 0.03%, sufficient strength cannot be secured. On the other hand, if it exceeds 0.08%, MA is generated and the compressive strength is lowered, and the weld heat affected zone toughness (hereinafter also referred to as HAZ toughness) is deteriorated. Therefore, the C content is set in the range of 0.03 to 0.08%.

Si: 0.01-0.30%
Si is an element added for deoxidation, and this effect is exhibited at 0.01% or more, but if it exceeds 0.3%, the toughness and weldability are deteriorated, and further, the formation of MA is promoted. Therefore, the compressive strength is reduced. Accordingly, the Si content is in the range of 0.01 to 0.30%.

Mn: 1.2 to 2.0%
Mn is added to improve the strength and toughness of the steel, but if it is less than 1.2%, the effect is not sufficient, and if it exceeds 2.0%, the weldability deteriorates. Therefore, the Mn content is in the range of 1.2 to 2.0%.

P: 0.020% or less P is an unavoidable impurity element and does not have a great influence on the strength of the steel material, but is an element that deteriorates the HAZ toughness, so the P content is 0.020% or less. Preferably, it is 0.015% or less.

S: 0.0030% or less S is an unavoidable impurity element, and generally becomes an MnS-based inclusion in steel, leading to deterioration of toughness, particularly a decrease in Charpy absorbed energy, so the amount of S is 0.0030% or less. And When higher performance is required, it is effective to further reduce the amount of S, preferably 0.0020% or less.

Al: 0.01 to 0.08%
Al is added as a deoxidizer, and this effect is exhibited at 0.01% or more. However, if it exceeds 0.08%, ductility is deteriorated due to a decrease in cleanliness. Therefore, the Al amount is set to 0.01 to 0.08%.

Nb: 0.003 to 0.070%
Nb is an important element in the present invention. Nb is an element that precipitates as NbC and is extremely effective for increasing the strength, suppresses grain growth during rolling, and improves toughness through fine graining. However, if the Nb content is less than 0.003%, the effect is small. Even if the Nb content exceeds 0.070%, the solid solution Nb content during slab heating required for precipitation strengthening does not increase, and the strength increase is saturated. Moreover, since it is also an element which has a bad influence on HAZ toughness, Nb amount shall be 0.003-0.070% of range. When more severe HAZ toughness is required, 0.03 to 0.05% is desirable.

Ti: 0.005-0.035%
Ti is an important element in the present invention. Ti forms a fine composite carbide together with Nb, V, and Mo, but the composite carbide mainly composed of NbC is further refined by containing a certain amount or more, and greatly contributes to an increase in strength. However, if the content is less than 0.005%, the effect is not sufficient. On the other hand, if the content exceeds 0.035%, the HAZ toughness deteriorates. From the viewpoint of fully utilizing precipitation strengthening and suppressing HAZ toughness deterioration, the Ti amount is set to 0.005 to 0.035%.

Mo: 0.01 to 0.5%
Mo, like Nb and Ti, produces a composite carbide and is an extremely effective element for increasing the strength by precipitation strengthening, and its effect can be obtained by adding 0.01% or more. However, if added over 0.5%, the HAZ toughness of the weld zone deteriorates. Therefore, the Mo content is set to 0.01 to 0.5%.

N: 0.0020 to 0.0060%
N is contained as an impurity in the steel, but if it exists as a solid solution element in the steel as in C, it promotes strain aging and contributes to the prevention of a decrease in compressive strength due to the Bauschinger effect. However, if it is less than 0.0020%, the effect is small, and if it exceeds 0.0060%, the toughness deteriorates. Therefore, the N amount is in the range of 0.0020 to 0.0060%.

C (%)-0.065 Nb (%)-0.025 Mo (%): 0.025-0.060
This requirement is the most important component in the present invention. In this formula, each element symbol is a content (% by mass). In the present invention, by suppressing the occurrence of reverse stress by the interaction between the solid solution C and the dislocation, the Bausinger effect is reduced and the compressive strength of the steel pipe is increased, so that an effective amount of solid solution C in the steel is ensured. It is important to do. In general, C in steel precipitates as cementite or MA, and also combines with carbide-forming elements such as Nb and Mo to precipitate as carbide, so that the amount of dissolved C decreases. At this time, if there is too much Nb and Mo content with respect to C content, since the precipitation amount of Nb and Mo carbide will increase, sufficient solute C amount cannot be obtained.

  For that purpose, C (%)-0.065Nb (%)-0.025Mo (%) needs to be 0.025 or more. Further, if the amount of dissolved C is too large, MA is generated and the compressive strength is lowered, so the upper limit of C (%)-0.065Nb (%)-0.025Mo (%) needs to be 0.060. There is.

C (%) + 0.67 Nb (%): 0.10 or less This requirement is necessary for obtaining sufficient strength by precipitation strengthening of carbides. In order to obtain a sufficient amount of carbide precipitation, it is necessary to obtain a sufficient amount of solid solution Nb in the slab heating stage before rolling the steel sheet, but the melting temperature of NbC varies depending on the amounts of C and Nb. For this reason, when the amount of C and Nb added is large, the melting temperature of NbC increases and a sufficient amount of Nb solid solution cannot be obtained. In the general slab heating temperature range, if C (%) + 0.67Nb (%) exceeds 0.10, the melting temperature of NbC increases, resulting in insufficient strength due to insufficient amount of solid solution Nb. In the present invention, C (%) + 0.67 Nb (%) is specified to be 0.10 or less. In consideration of variations in the slab heating temperature, it is preferable to set C (%) + 0.67Nb (%) to 0.08 or less in order to obtain a sufficient amount of solid solution Nb more reliably.

  In the present invention, in addition to the above chemical components, the following elements can be contained as selective elements.

Cu: 0.5% or less Cu is an element effective for improving toughness and increasing strength, but if contained over 0.5%, the HAZ toughness of the welded portion deteriorates. Therefore, when it contains Cu, it is preferable to make the content into 0.5% or less.

Ni: 1.0% or less Ni is an element effective for improving toughness and increasing strength, but if it exceeds 1.0%, the HAZ toughness of the welded portion deteriorates. Therefore, when it contains Ni, it is preferable to make the content into 1.0% or less.

Cr: 1.0% or less Cr is an element effective for increasing the strength by increasing the hardenability. However, if it exceeds 1.0%, the HAZ toughness of the welded portion is deteriorated. Therefore, when it contains Cr, it is preferable to make the content into 1.0% or less.

V: 0.07% or less V is a very effective element for generating a composite carbide as in Nb and Ti and increasing the strength by precipitation strengthening. However, if it exceeds 0.07%, the HAZ toughness of the welded portion Deteriorates. Therefore, when it contains V, it is preferable to make the content into 0.07% or less. In addition, in a portion that receives a thermal history of multiple cycles such as a meeting part HAZ of a welded part, since it precipitates as VC and hardens the HAZ part to cause remarkable toughness deterioration, More preferably, the content of is less than 0.04%.

Ca: 0.0005 to 0.0035%
Ca is an element effective for controlling the form of sulfide inclusions and improving ductility. However, if it is less than 0.0005%, there is no effect, and even if it exceeds 0.0035%, it is effective. Saturates, but rather deteriorates toughness due to reduced cleanliness. Therefore, when it contains Ca, it is preferable to make Ca amount into the range of 0.0005 to 0.0035%.
C (%)-0.065 Nb (%)-0.025 Mo (%)-0.057 V (%): 0.025 to 0.060
V, which is a selective element of the present invention, is an element that forms a carbide similarly to Nb, and these elements are also preferably added within a range where a sufficient amount of solute C can be obtained. However, if the value of the relational expression represented by C (%) − 0.065Nb (%) − 0.025Mo (%) − 0.057V (%) is less than 0.025, the amount of dissolved C is insufficient. It is preferable to set C (%)-0.065Nb (%)-0.025Mo (%)-0.057V (%) to 0.025 to 0.060.
However, each element is content (mass%), and the element which does not contain is set to 0.

Pcm value represented by the following formula (1) is 0.20 or less Pcm = C (%) + Si (%) / 30 + Mn (%) / 20 + Cu (%) / 20 + Ni (%) / 60 + Cr (%) / 20 + Mo (% ) / 15 + V (%) / 10 + 5B (%) (1)
However, each element is a content (% by mass), and an element not added is 0.
The Pcm value defined here is an index representing weldability, and the higher the Pcm value, the worse the welded HAZ toughness. Especially in high strength steels of API-X80 grade or higher, the effect becomes significant, so the Pcm value must be strictly limited. However, if the Pcm value is 0.20 or less, good toughness of the welded HAZ part can be secured, so the upper limit is made 0.20. If there is a more stringent HAZ toughness requirement, the upper limit is desirably set to 0.18.

  The balance of the steel of the present invention is Fe and inevitable impurities, but elements other than the above and inevitable impurities can be contained unless the effects of the present invention are impaired.

2. About metal structure The reason for limitation of the metal structure in the present invention is shown below.

Bainite area fraction: 95% or more In order to suppress the Bausinger effect and obtain a high compressive strength, a uniform structure without a soft ferrite phase or a hard second phase is used, and a local structure generated inside the structure at the time of deformation. It is necessary to suppress the accumulation of dislocations. Therefore, it is a bainite-based structure. In order to obtain the effect, the area fraction of bainite needs to be 95% or more.

  As will be described later, after accelerated cooling subsequent to hot rolling, immediately after reheating, fine precipitates containing Nb are dispersed and precipitated in bainite. It is. The particle size of the fine precipitate containing Nb is preferably 10 nm or less in order to ensure a sufficient precipitation strengthening amount.

Island-like martensite (MA) area fraction: 3% or less Island-like martensite (MA) is a very hard phase, which promotes the accumulation of local dislocations during deformation and has a compressive strength due to the Bauschinger effect. In order to reduce, it is necessary to restrict | limit the area fraction severely. However, when the area fraction of MA is 3% or less, the influence is small and the compressive strength does not decrease. Therefore, the area fraction of island-like martensite (MA) is specified to 3% or less.

  In the present invention, by having the above-described metallographic features, a decrease in compressive strength due to the Bauschinger effect is suppressed and a high compressive strength is achieved, but in order to obtain a greater effect, the size of the MA is fine. It is desirable that As the average particle size of MA is smaller, the local strain is dispersed without concentrating, so that the amount of strain concentration is reduced and the occurrence of the Bausinger effect is further suppressed. For that purpose, it is preferable that the average particle diameter of MA is 1 μm or less.

  In addition, the bainite phase generated by accelerated cooling after hot rolling, particularly in the steel sheet surface layer portion, has a cooling stop temperature lowering and becomes a structure containing MA, but when the grain size of bainite is small, MA becomes fine, Since it becomes easy to decompose into cementite by subsequent reheating, the particle size of bainite is preferably 5 μm or less.

  Metal structures other than the above may include structures such as ferrite, cementite, and martensite. If the total of these structures is less than 5% in terms of area fraction, it is particularly effective for bauschinger characteristics and other performances. Does not affect. Therefore, it is preferable that the total area fraction of the structures other than bainite and MA is less than 5%.

  Generally, the metal structure of a steel plate manufactured by applying accelerated cooling is different in the plate thickness direction of the steel plate and may not be uniform. The collapse of a steel pipe that is subjected to external pressure occurs because the plastic deformation of the inner surface of the steel pipe with a small circumference first occurs, so the characteristics of the inner surface of the steel pipe are important for compressive strength. Collect from. Therefore, the above-mentioned metal structure defines the structure on the inner surface side of the steel pipe, and the structure representing the performance of the steel pipe is the structure at the position of the plate thickness ¼ on the inner surface side.

3. Manufacturing conditions The third invention of the present invention is a manufacturing method in which the steel slab containing the above-described chemical components is heated and hot-rolled, subjected to accelerated cooling, and subsequently tempered by induction heating. Below, the reason for limitation of the manufacturing conditions of a steel plate is demonstrated.

  In the present invention, the temperatures in the production conditions are all steel plate average temperatures. The average steel plate temperature is obtained by simulation calculation or the like from the plate thickness, surface temperature, cooling conditions, and the like. For example, the average temperature of a steel plate is calculated | required by calculating the temperature distribution of a plate | board thickness direction using the difference method. In addition, in the case of a slow cooling rate such as air cooling, the steel plate average temperature has almost no temperature difference between the steel plate surface and the steel plate center portion, so that the steel plate surface temperature can be set as the steel plate average temperature. However, in the case of rapid cooling or rapid heating, such as immediately after reheating by accelerated cooling or induction heating, a temperature difference occurs between the steel sheet surface and the steel sheet center. In such a case, since the temperature difference inside the steel sheet is almost eliminated by cooling after cooling is stopped or after heating, the steel sheet surface temperature at that time may be used.

Steel slab heating temperature: 1000-1200 ° C
If the steel slab heating temperature is less than 1000 ° C., the solid solution of NbC is insufficient, and strengthening by subsequent precipitation cannot be obtained, and if it exceeds 1200 ° C., the toughness and DWTT characteristics deteriorate. Therefore, the slab heating temperature is in the range of 1000 to 1200 ° C. When further superior DWTT performance is required, the upper limit of the slab heating temperature is desirably 1150 ° C.

Reduction ratio of non-recrystallized region: 60% or more In order to obtain a fine bainite structure and high base metal toughness for reducing the Bausinger effect, sufficient reduction is performed at a temperature not higher than the non-recrystallization temperature in the hot rolling process. There is a need. However, since the effect is insufficient when the rolling reduction is less than 60%, the rolling reduction is set to 60% or more in the non-recrystallized region. Preferably it is 70% or more. Note that the rolling reduction is the cumulative rolling reduction when rolling is performed in a plurality of rolling passes. Further, although the non-recrystallization temperature varies depending on the alloying elements such as Nb and Ti, the upper limit temperature of the non-recrystallization temperature region may be set to 950 ° C. with the addition amount of Nb and Ti of the present invention.

Rolling end temperature: Ar ₃ to (Ar ₃ + 70 ° C.)
In order to suppress the strength reduction due to the Bauschinger effect, it is necessary to make the metal structure a bainite-based structure and suppress the formation of soft structures such as ferrite. For this reason, the hot rolling needs to be performed at an Ar ₃ temperature or higher, which is a ferrite formation temperature. Moreover, in order to obtain a finer bainite structure, the lower the end temperature of rolling, the better. When the end temperature of rolling is too high, the bainite grain size becomes too large. For this reason, the upper limit of the rolling end temperature is (Ar ₃ + 70 ° C.).

Incidentally, Ar ₃ temperature is a function of the alloy components of the steel, may be determined by measuring the transformation temperature by experiment for each steel, but can also be calculated by the following equation from the component (2).
Ar ₃ (° C.) = 910-310C (%)-80Mn (%)-20Cu (%)-15Cr (%)-55Ni (%)-80Mo (%) (2)
Here, each element is content (mass%), and the element which is not added is set to 0.

  Following the hot rolling, accelerated cooling is performed. The conditions for accelerated cooling are as follows.

Cooling start temperature: (Ar ₃ −30 ° C.) or more The metal structure is made to be a bainite-based structure by accelerated cooling after hot rolling. When the cooling start temperature is lower than the Ar ₃ temperature, which is the ferrite formation temperature, ferrite and bainite Thus, the strength is greatly reduced by the Bauschinger effect and the compressive strength is reduced.

However, when the accelerated cooling method is employed, if the accelerated cooling start temperature is (Ar ₃ −30 ° C.) or higher, the ferrite area fraction is low and the strength reduction due to the Bauschinger effect is small. Therefore, the cooling start temperature is set to (Ar ₃ −30 ° C.) or higher.

Cooling rate: 10 ° C / second or more The accelerated cooling method, which is performed at a cooling rate of 10 ° C / second or more, is an indispensable process for obtaining a high-strength and high-toughness steel sheet. The strength increase effect by is obtained. However, if the cooling rate is less than 10 ° C./second, not only a sufficient strength cannot be obtained, but also C diffusion occurs, so that C is concentrated to untransformed austenite, and the amount of MA produced increases. As described above, the Bausinger effect is promoted by the hard second phase such as MA, which causes a decrease in compressive strength. However, if the cooling rate is 10 ° C./second or more, the diffusion of C during cooling is small, and the production of MA is also suppressed. Therefore, the lower limit of the cooling rate during accelerated cooling is set to 10 ° C./second.

Cooling stop temperature: 300-600 ° C
By accelerating cooling after the end of rolling, the average temperature of the steel sheet is cooled to 300 to 600 ° C., whereby a bainite phase can be generated. When the cooling stop temperature is less than 300 ° C., the island-shaped martensite (MA) is excessively generated, so that the compressive strength and the HIC resistance deteriorate. On the other hand, when the cooling stop temperature exceeds 600 ° C., pearlite is generated, the compression strength and the HIC resistance are similarly deteriorated, and the effect of transformation strengthening by bainite transformation is not sufficient and the strength is lowered. The cooling stop temperature is more preferably 400 to 600 ° C. from the viewpoint of increasing the driving force of the ferrite transformation at the time of reheating and sufficiently obtaining the effect of precipitation strengthening by precipitates at the time of ferrite transformation.

  After the accelerated cooling described above, reheating is performed to a temperature equal to or higher than the cooling stop temperature and to a temperature of 550 to 700 ° C. This process is an important manufacturing condition in the present invention. The reasons for limiting the manufacturing conditions will be described below.

Temperature of steel plate during reheating: 550 to 700 ° C
After the accelerated cooling described above, reheating is performed to a temperature equal to or higher than the cooling stop temperature and to a temperature of 550 to 700 ° C. This temperature is the average temperature of the steel sheet. This process is an important manufacturing condition in the present invention.

  In order to obtain a structure in which fine precipitates are dispersed in the bainite phase, it is necessary to reheat to a temperature of 550 to 700 ° C. immediately after the accelerated cooling and above the cooling stop temperature.

  In order to make maximum use of precipitation strengthening of the composite carbide containing Nb, the reheating temperature is preferably set to 600 to 680 ° C. as a temperature range in which precipitation is most likely to occur. In this reheating, it is preferable to raise the temperature to 50 ° C. or higher than the cooling stop temperature.

  The means for reheating is not particularly limited, but rapid heating is possible without reducing productivity by using an induction heating apparatus installed on the same line as the hot rolling and accelerated cooling apparatus. Further, if the reheating start temperature can be maintained at the bainite transformation stop temperature or higher, an off-line heat treatment facility such as a gas combustion furnace can be used.

  In addition, if the heating rate at the time of reheating is less than 0.5 ° C./sec, it takes a long time to reach the target reheating temperature, so that sufficient strength cannot be obtained due to coarsening of precipitates. In addition, the production efficiency deteriorates. In order to suppress the deterioration of toughness, it is effective to finely and uniformly disperse precipitates by suppressing the coarsening of precipitates during the temperature rise. From this viewpoint, the rate of temperature rise is 3 ° C / It is preferable to set it as sec or more. Moreover, the cooling after reheating is not specifically limited, For example, it can be allowed to cool.

  The present invention forms a steel pipe by using the steel plate manufactured by the above-described method. The steel pipe is formed into a steel pipe shape by cold forming such as UOE process or press bend. Thereafter, welding is performed. Any welding method may be used as long as sufficient joint strength and joint toughness can be obtained, but it is preferable to use submerged arc welding in terms of excellent welding quality and production efficiency. . After welding the butt, pipe expansion is performed to remove residual welding stress and improve the roundness of the steel pipe. The expansion ratio at this time needs to be 0.4% or more as a condition for obtaining a predetermined roundness of the steel pipe and removing the residual stress. Moreover, since the fall of the compressive strength by a Bauschinger effect will become large when a pipe expansion rate is too high, the upper limit shall be 1.2%.

  The steel pipe of the present invention is intended for application to high-strength steel pipes of API-X80 grade or higher, and the tensile strength is specified to be 630 MPa or higher. This is because a steel pipe having a relatively low tensile strength with a tensile strength of less than 630 MPa is sufficient without the addition of an alloying element that deteriorates the HAZ toughness of the welded portion without applying precipitation strengthening as in the present invention. This is because strength can be obtained. In the present invention, the HAZ toughness of the welded portion preferably has an absorbed energy at −10 ° C. of 70 J or more, more preferably 100 J or more.

  Steel of chemical composition (steel types A to I) shown in Table 1 was made into a slab by a continuous casting method, and a thick steel plate (No. 1 to 15) having a plate thickness of 20 mm was produced using this. Table 2 shows steel plate manufacturing conditions, metal structure and mechanical properties of the steel pipe. The reheating process at the time of steel plate manufacture performed reheating using the induction heating furnace installed on the same line as the accelerated cooling equipment. The steel plate average temperature at the time of reheating was the surface temperature of the steel plate at the time when the surface temperature after heating and the center temperature became substantially equal. Using these steel plates, steel pipes with an outer diameter of 610 mm were manufactured by the UOE process.

  As for the tensile characteristics of the steel pipe manufactured as described above, a tensile test was performed using a full thickness test piece in the pipe circumferential direction as a tensile test piece, and the tensile strength was measured. In the compression test, a test piece having a diameter of 20 mm and a length of 60 mm was taken in the pipe circumferential direction from a position on the inner surface of the steel pipe, and the compression test was performed to measure the yield strength of compression. For evaluation of HAZ toughness, the absorbed energy at −10 ° C. was obtained by a Charpy test piece taken from the welded heat affected zone of the seam welded portion of the steel pipe from the pipe circumferential direction. For the metal structure, a sample was taken from the position of the plate thickness ¼ on the inner surface side of the steel pipe, and after polishing, etched with nital and observed with an optical microscope. And the bainite area fraction was calculated | required by image analysis using the 3-5 photograph image | photographed by 200 time. For the observation of MA, electrolytic etching (two-stage etching) was performed after nital etching, followed by observation with a scanning electron microscope (SEM). And the area fraction of MA was calculated | required by image analysis from the photograph image | photographed 1000 times.

  In Table 2, No. 1 as an example of the present invention. In all of Nos. 1 to 6, the chemical composition, production method and microstructure were within the scope of the present invention, the compressive yield strength was high compressive strength of 530 MPa or more, and the tensile strength and HAZ toughness were also good.

  On the other hand, no. 7-10, although a chemical component is in the range of this invention, since a manufacturing method is outside the range of this invention, tensile strength or compressive strength is inferior. No. Nos. 11 to 15 are insufficient in tensile strength, compressive strength, or HAZ toughness because the chemical components are outside the scope of the present invention.

  According to the present invention, a steel pipe having a high compressive strength and high strength equal to or higher than API-X80 grade can be obtained, so that it can be applied to a deep sea line pipe, a riser, a conductor casing, or the like that requires high collapse resistance. Can do.

Claims (3)

In mass%, C: 0.03 to 0.08%, Si: 0.01 to 0.30%, Mn: 1.2 to 2.0%, P: 0.020% or less, S: 0.0030 % Or less, Al: 0.01 to 0.08%, Nb: 0.003 to 0.070%, Ti: 0.005 to 0.035%, Mo: 0.01 to 0.5%, N: 0 0020 to 0.0060%,
C (%)-0.065Nb (%)-0.025Mo (%) is 0.025-0.060,
C (%) + 0.67Nb (%) is 0.10 or less,
The Pcm value represented by the following formula (1) is 0.20 or less, and the balance is a steel pipe composed of Fe and inevitable impurities,
The metal structure has an area fraction of bainite of 95% or more, fine precipitates containing Nb are dispersed in bainite, and the area fraction of island martensite (MA) is 3% or less. A high compressive strength steel pipe having a tensile strength of 630 MPa or more.
Pcm = C (%) + Si (%) / 30 + Mn (%) / 20 + Cu (%) / 20 + Ni (%) / 60 + Cr (%) / 20 + Mo (%) / 15 + V (%) / 10 + 5B (%) (1) )
However, each element is content (mass%), and the element which does not contain is set to 0. Further, in mass%, Cu: 0.5% or less, Ni: 1.0% or less, Cr: 1.0% or less, V: 0.07% or less, Ca: 0.0005 to 0.0035% Containing one or more selected from
The high compression strength steel pipe according to claim 1, wherein C (%)-0.065Nb (%)-0.025Mo (%)-0.057V (%) is 0.025-0.060. .
However, each element is content (mass%), and the element which does not contain is set to 0. The steel slab was heated to 1000 to 1200 ° C., and hot rolling was performed at a rolling reduction temperature in the non-recrystallization temperature range of 60% or more and a rolling end temperature of Ar ₃ to (Ar ₃ + 70 ° C.), followed by (Ar ₃ The steel plate is cooled to 300 to 600 ° C. at a cooling rate of 10 ° C./second or more from a temperature of −30 ° C. or higher, and then the steel plate is reheated to 550 to 700 ° C. to produce a steel plate, Thereafter, the steel sheet is formed in a cold shape to form a steel pipe shape, the butt portion is welded, and then, the pipe expansion is performed at a pipe expansion ratio of 0.4% to 1.2%. Or the manufacturing method of the high compressive strength steel pipe of 2.