JP5786351B2 - Steel pipe for line pipes with excellent anti-collapse performance - Google Patents

Description

  The present invention relates to a steel pipe for a line pipe excellent in sour resistance for oil and natural gas transportation, and is particularly suitable for use in a thick-walled deep-sea line pipe that requires high collapse resistance. The present invention relates to a steel pipe for line pipes with excellent collapse performance.

  As energy demand has increased in recent years, oil and natural gas pipelines have been actively developed, and many pipelines across the ocean have been developed for the remote location of gas and oil fields and the diversification of 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.

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 obtained using 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 (α _fab ) 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 is lower than the tensile strength. This phenomenon occurs because in the UOE steel pipe manufacturing process, there is a pipe expansion process at the final stage of pipe making, and after this process is subjected to tensile deformation in the pipe circumferential direction, compression is applied.

  That is, the yield strength is reduced by the Bausinger effect. Therefore, it is necessary to increase the compressive strength of the pipe in order to improve the anti-collapse performance, but in the case of a steel pipe manufactured through a pipe expansion process by cold forming, the problem of strength reduction due to the Bausinger effect is solved. It has not been.

  Many studies have been made on improving the collapse resistance of UOE steel pipes. For example, Patent Document 1 discloses a method in which a steel pipe is heated by energization heating and the temperature is maintained for a predetermined time or longer after pipe expansion. Although this method recovers the dislocations introduced by the pipe expansion and increases the yield strength, it is necessary to continue the electric heating for 5 minutes or more after the pipe expansion, and there is a problem that the productivity is inferior.

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 has been proposed a method in which heating is performed after the above process is performed up to 300 ° C. and formed into a 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 terms of actual manufacturing, particularly in mass production. Moreover, since the method of patent document 3 needs to make the accelerated cooling stop temperature in steel plate manufacture into the low temperature of 300 degrees C or less, the distortion of a steel plate becomes large and the roundness at the time of using a steel pipe by a UOE process falls. Furthermore, 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 steel pipe forming method without performing 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 expressed and a high compressive strength is obtained. However, when the pipe expansion rate is low, it becomes 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.

JP 9-49025 A JP 2003-34639 A JP 2004-35925 A JP 2002-102931 A JP 2003-340519 A JP 2008-56962 A

  By controlling the metal structure as in the technique described in Patent Document 6, the compressive strength of the steel pipe is improved, and a certain effect is obtained for improving the collapse resistance performance. However, depending on the laying conditions of the pipeline, the compressive strength Improvement alone is not enough to prevent collapse.

  In laying a submarine pipeline, the pipe that reaches the seabed receives a bending moment. Therefore, according to the DNV standard, it is determined that a steel pipe is laid within a strain range in which plastic deformation does not occur, that is, a bending strain is within 0.5%. However, it is difficult to keep the position of the laying ship constant due to a sudden change in ocean current or the influence of wind, and there is a risk of receiving an excessive bending moment. When subjected to such a large bending deformation, the collapse pressure of the pipe is greatly reduced, and even if it has a high compressive stress due to heat treatment, a decrease in the collapse pressure is inevitable.

  The present invention was made in view of the above circumstances, and has a steel pipe for line pipes having excellent anti-collapse performance necessary for application to a thick-walled submarine pipeline, especially when subjected to large bending deformation at the time of laying, An object of the present invention is to provide a steel pipe for a line pipe that maintains a high resistance to collapse.

  In order to elucidate the collapse behavior due to the external pressure of a steel pipe subjected to bending deformation, the present inventors tried various experiments and analyzes, and as a result, obtained the following knowledge.

A) In order to prevent the collapse of the submarine pipe, as shown in the DNV standard (OS F-101), the improvement of the fabrication factor (α _fab ), that is, the compressive strength in the pipe circumferential direction of the steel pipe is high. It is essential that the ratio of the yield strength in the compression test to the yield strength in the tensile test be a certain value or more.

  B) Even in the case of the steel pipe as described above, the cross-sectional shape becomes flat due to bending deformation, and therefore, collapse is likely to occur due to external pressure. Such flattening of the steel pipe can be prevented by improving the tensile characteristics in the pipe axis direction of the steel pipe. Specifically, the ratio between the yield strength and the tensile strength in the pipe axis direction tensile test, that is, the pipe axis direction. It is effective to make the yield ratio of a certain value or more. FIG. 1 shows changes in flatness ratio due to bending moments of steel pipes having various tensile properties.

  FIG. 1 shows the result of FEM analysis of bending deformation of a steel pipe having an outer diameter of 762 mm and a pipe thickness of 34.6 mm. The flatness (flat part of the cross section of the steel pipe when a bending moment is applied to both ends of the pipe length of 8 m. The outer diameter (major axis-minor axis) / initial outer diameter) was determined. Here, the yield ratio (YR) in the tube axis direction, that is, the ratio of the yield strength to the tensile strength in the tube axis direction tensile test is 85% and 92%. Moreover, C-TS in a figure means the pipe circumferential direction tensile strength.

  It is clear that the steel pipe with a higher yield ratio in the tube axis direction (ratio between the yield strength and the tensile strength in the tube axis direction tensile test) has a smaller flatness. The flatness referred to here has the same meaning as the roundness defined in DNV OS-F101, and the collapse pressure decreases as the flatness increases. That is, when subjected to bending deformation such as laying on the sea floor, it can be said that as the yield ratio in the tube axis direction increases, the flatness of the pipe decreases and the collapse pressure increases.

  C) Even if the yield ratio in the tube axis direction is high, if the yield strength in the tube axis direction tension is low, the steel pipe will be flattened due to bending deformation at an early stage. It must be higher than a certain percentage compared to the tensile yield strength.

The present invention has been made on the basis of the above findings, and the first invention relates to the yield strength and tensile strength in the pipe axial direction tensile test in the steel pipe for line pipe, respectively, L-YS (T) and L-TS. (T), when the yield strength in the pipe circumferential direction tensile test is C-YS (T) and the yield strength in the pipe circumferential direction compression test is C-YS (C), L-YS (T) / L- TS (T) is 0.9 or more, L-YS (T) / C-YS (T) is 0.95 or more, and C-YS (C) / C-YS (T) is 0.9 or more. This is a steel pipe for line pipes with excellent anti-collapse performance.
Here, (C) and (T) mean a test value in the compression direction and a test value in the tensile direction, respectively.

  The second invention is the steel pipe for line pipes, wherein the chemical component is mass%, C: 0.03 to 0.1%, Si: 0.5% or less, Mn: 1.0 to 2.0%, P: 0.015% or less, S: 0.003% or less, Al: 0.08% or less, Nb: 0.005-0.05%, Ti: 0.005-0.03%, Cu: 0.5% or less, Ni: 1.0% or less, Cr: 0.5% or less, Mo: 0.5% or less, V: 0.1% or less, Ca: 0.0005 to 0.0035% 1 or more selected from the group consisting of Fe and inevitable impurities, and the metal structure has a bainite structure of 80% or more in area fraction. It is a steel pipe for line pipes, with excellent collapse resistance performance.

  According to the first invention of the present invention, a steel pipe for a line pipe having excellent collapse resistance can be obtained even when it is subjected to excessive bending deformation when laying a submarine pipeline. Further, according to the second invention, the strength and toughness of the steel pipe and the toughness of the welded portion can be further improved.

It is a figure which shows the relationship between a bending moment and flatness.

  The reasons for limiting the respective constituent requirements of the present invention will be described below.

1. Regarding the mechanical characteristics of steel pipes The greatest feature of the present invention is defined by the DNV standard, etc., by controlling the characteristics of the steel pipe in the axial direction, which was not considered at all as a countermeasure for preventing the collapse of conventional submarine pipes. Even if an excessive bending exceeding the stress range is applied, collapse is less likely to occur. For that purpose, the yield strength and tensile strength in the pipe axial direction tensile test are L-YS (T) and L-TS (T), respectively, the yield strength in the pipe circumferential direction tensile test is C-YS (T), and the pipe The following characteristics are required when the yield strength C-YS (C) in the circumferential compression test is used.

L-YS (T) / L-TS (T): 0.9 or more This relational expression indicates that the yield strength [L-YS (T)] and the tensile strength [L-TS ( T)] and may be abbreviated as “yield ratio of tensile in the tube axis direction” in the following description. When the yield ratio of the tensile in the tube axis direction is less than 0.9, the flatness of the cross section when the steel pipe is subjected to bending deformation becomes large, and collapse easily occurs. However, if the yield ratio of the tensile in the tube axis direction is 0.9 or more, such flattening is sufficiently suppressed. Therefore, the yield ratio in the tensile in the tube axis direction is specified to be 0.9 or more. Preferably, it is 0.92 or more.

L-YS (T) / C-YS (T): 0.95 or more This relational expression is the yield strength [L-YS (T)] in the pipe axial direction tensile test and the yield in the pipe circumferential direction tensile test. It is a ratio to the strength [C-YS (T)], and may be abbreviated as “ratio of the yield strength in the tube axis direction and the yield strength in the tube circumferential direction” in the following description. The flatness of the steel pipe due to bending deformation is not only determined by the yield ratio in the pipe axis direction, but is greatly influenced by the balance between the tensile properties in the pipe circumferential direction and the pipe axis direction. If the yield stress in the pipe axial direction tension is too low relative to the yield stress in the pipe circumferential tension, the amount of deformation of the pipe and the flattening amount when the bending moment is applied increase, and the collapse performance deteriorates. However, there is no problem if the ratio between the yield strength in the tube axis direction and the yield strength in the tube circumferential direction is 0.95 or more. Therefore, the ratio of the yield strength in the tube axis direction tension to the yield strength in the tube direction tension is 0. .95 or more. Preferably, it is 0.97 or more.

C-YS (C) / C-YS (T): 0.9 or more This relational expression is the yield strength [C-YS (C)] in the pipe circumferential direction compression test and the pipe circumferential direction tensile test of the steel pipe. It is a ratio to the yield strength [C-YS (T)], and may be abbreviated as “ratio of the yield strength in pipe circumferential compression and the yield strength in pipe circumferential tension” in the following description. This relational expression has the same meaning as the fabrication factor defined by DNV. As a basic measure for preventing collapse, the yield strength in pipe circumferential direction compression is the same as the yield strength in pipe circumferential tension. It means that it needs to be raised above a certain value. When this value is smaller than 0.9, the compressive strength is low, so that it is necessary to increase the thickness of the steel pipe in order to prevent collapse, which increases the construction cost of the pipeline. However, if it is 0.9 or more, a higher compressive strength can be obtained than a steel pipe made by a general UOE process, and a high collapse resistance performance can be obtained without increasing the pipe thickness. Therefore, the ratio of the yield strength in the pipe circumferential direction compression and the yield strength in the pipe circumferential direction tension is specified to be 0.9 or more. Preferably, it is 0.92 or more.

  In addition, the present invention is intended for laying a submarine pipeline, and the effect is exhibited particularly in a steel pipe having a pipe thickness of 20 mm or more. Therefore, it is preferably applied to a steel pipe having a pipe thickness of 20 mm or more. It is not limited to the plate thickness.

  In the present invention, high collapse resistance can be obtained by controlling the characteristics of the above-mentioned steel pipe in the axial direction and the pipe circumferential direction, but the strength, toughness, welding workability, etc. preferable as a steel pipe applied to a submarine pipeline can be obtained. In order to improve performance, the chemical composition and metal structure of the material are defined as follows.

2. Regarding Chemical Components First, the reasons for limiting the chemical components of the present invention will be described. In addition, all component% means the mass%.

C: 0.03-0.1%
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, and if it exceeds 0.1%, not only the toughness is degraded, but also the formation of MA (island martensite) is promoted, so the compression strength decreases. Also invite. Therefore, the C content is set within a range of 0.03 to 0.1%. In order to obtain higher toughness and compressive strength, the content is preferably in the range of 0.03 to 0.08%.

Si: 0.5% or less Si is contained for deoxidation, but if it exceeds 0.5%, the toughness and weldability are deteriorated and the formation of MA is also promoted. Accordingly, the Si amount is set to a range of 0.5% or less.

Mn: 1.0-2.0%
Mn is contained for improving the strength and toughness of the steel, but if it is less than 1.0%, its effect is not sufficient, and if it exceeds 2.0%, the weldability and the HIC resistance are deteriorated. Therefore, the Mn content is in the range of 1.0 to 2.0%.

P: 0.015% or less P is an inevitable impurity element and degrades the toughness of the steel material. In particular, since the hardness of the weld heat affected zone is increased, the toughness of the weld heat affected zone is significantly deteriorated. Therefore, the P content is 0.015% or less. Preferably, it is 0.008% or less.

S: 0.003% or less S is an MnS-based inclusion in the steel and acts as a void generation starting point at the time of impact fracture, which causes a decrease in absorbed energy in the Charpy impact test. Therefore, the S content is 0.003% or less. When higher absorbed energy is required, it is effective to further reduce the amount of S, preferably 0.0015% or less.

Al: 0.08% or less Al is contained as a deoxidizer, but if it exceeds 0.08%, ductility is deteriorated due to a decrease in cleanliness. Therefore, the Al content is 0.08% or less.

Nb: 0.005 to 0.05%
Nb suppresses grain growth during rolling, and improves toughness by making fine grains. However, when the Nb content is less than 0.005%, the effect is not obtained, and when it exceeds 0.05%, the toughness of the weld heat affected zone is lowered. Therefore, the Nb content is in the range of 0.005 to 0.05%.

Ti: 0.005 to 0.03%
Ti not only suppresses crystal grain growth during slab heating by forming TiN, but also suppresses crystal grain growth in the weld heat affected zone, and refines the microstructure of the base material and the weld heat affected zone to improve toughness. Improve. However, if the amount of Ti is less than 0.005%, the effect is not obtained, and if it exceeds 0.03%, the toughness is deteriorated. Therefore, the Ti content is in the range of 0.005 to 0.03%.

  In the present invention, in addition to the above chemical components, when further improving the strength, toughness or welded portion properties of the steel sheet, one or more of Cu, Ni, Cr, Mo, V, and Ca shown below are selectively contained. It is preferable to make it.

Cu: 0.5% or less Cu is an element effective for improving toughness and increasing strength. To obtain this effect, 0.05% or more is preferable, but more than 0.5% is contained. Then, the toughness of the weld heat affected zone deteriorates. Therefore, when it contains Cu, it is preferable to set it as 0.5% or less.

Ni: 1.0% or less Ni is an element effective for improving toughness and increasing strength. To obtain this effect, 0.05% or more is preferable, but more than 1.0% is contained. Then, the toughness of the weld heat affected zone deteriorates. Therefore, when it contains Ni, it is preferable to set it as 1.0% or less.

Cr: 0.5% or less Cr is an element effective for increasing the strength by enhancing the hardenability. To obtain this effect, it is preferable to contain 0.05% or more, but 0.5% If contained in excess, the toughness of the weld heat affected zone may be deteriorated. Therefore, when it contains Cr, it is preferable to set it as 0.5% or less.

Mo: 0.5% or less Mo is an element effective for improving toughness and increasing strength. To obtain this effect, 0.05% or more is preferable, but more than 0.5% is contained. Then, the toughness of the weld heat affected zone deteriorates. Therefore, when it contains Mo, it is preferable to set it as 0.5% or less.

V: 0.1% or less V is an element that increases the strength without deteriorating toughness. To obtain this effect, it is preferably contained in an amount of 0.01% or more. In order to reduce the toughness of the weld heat affected zone, when V is contained, the content is preferably 0.1% or less.

Ca: 0.0005 to 0.0035%
Ca is an element effective for controlling the form of sulfide inclusions and improving ductility, but if it is less than 0.0005%, there is no effect, and even if added over 0.0035%, it is effective. Saturates, but rather deteriorates toughness due to reduced cleanliness. Therefore, when it contains Ca, it is preferable to set it as 0.0005 to 0.0035% of range.
In the present invention, the strength of the steel pipe is not particularly specified, but in order to obtain a high strength of about API grade X65 or higher in a thick steel pipe for a submarine pipe, the Ceq value represented by the following formula (1) is 0.28 or higher. Preferably there is. Here, each element symbol means content (mass%), and is 0 when not contained.
Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 (1)
The balance of the steel of the present invention is Fe and unavoidable impurities. Elements other than the above and unavoidable impurities can be contained as long as the effects of the present invention are not impaired.

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

Bainite fraction: 80% or more In order to suppress the Bausinger effect and obtain a high compressive strength, the structure should have a soft ferrite phase and a hard second phase, and the accumulation of local dislocations generated inside the structure during deformation It is necessary to suppress. Therefore, a bainite-based structure is preferable. In order to obtain the effect, the area fraction of bainite needs to be 80% or more. Furthermore, when high compressive strength is required, the bainite area fraction is desirably 90% or more.

  The steel pipe of the present invention has a high compressive strength by having the above characteristics as a metal structure, but other than the above, structures such as cementite, pearlite, martensite, etc. have a total of 5% or less of their fractions. If there is no adverse effect, it can be contained.

  It should be noted that island martensite (MA) is a very hard phase, promotes the accumulation of local dislocations during deformation, and causes a reduction in compressive strength due to the Bauschinger effect. There is. However, when the MA fraction is 3% or less, the influence is small and the compressive strength does not decrease. Therefore, the island-like martensite (MA) fraction is preferably 3% or less. The MA fraction can be determined by performing electrolytic etching (two-stage etching) after nital etching and then observing with a scanning electron microscope (SEM).

  Generally, the metal structure of a steel sheet manufactured by applying accelerated cooling may differ in the thickness direction of the steel sheet. Since the collapse of a steel pipe subject to external pressure occurs because the plastic deformation on the inner surface of the steel pipe with a small circumference first occurs, the characteristics of the inner surface of the steel pipe are important for compressive strength. Collect. 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 1/4 on the inner surface side.

  Since the steel pipe of the present invention exhibits excellent anti-collapse performance as long as the above-described characteristics in the pipe axis direction and the pipe circumferential direction are obtained, the steel pipe manufacturing method is not particularly defined. However, in order to obtain appropriate pipe axial and pipe circumferential characteristics and strength / toughness and other performance suitable for application to submarine pipelines and excellent roundness, A steel pipe manufactured by a press bend process is suitable. Further, in a manufacturing process of a steel plate that is a material of the steel pipe, it is preferable to manufacture a steel plate that is a material of the steel pipe by a TMCP process to which controlled rolling and accelerated cooling are applied. Below, the preferable manufacturing conditions of the steel plate and steel pipe which are the raw materials of a steel pipe are demonstrated.

  The steel plate as the material of the steel pipe of the present invention is to heat-roll and hot-roll the steel slab containing the chemical components described above, and then perform accelerated cooling, or perform tempering by induction heating following accelerated cooling. It is preferable to manufacture by. Below, the suitable conditions of the manufacturing conditions of a steel plate are demonstrated. In addition, the following temperature represents the average temperature of the steel plate thickness direction unless otherwise indicated. The average temperature in the plate thickness direction is obtained by simulation calculation or the like from the plate thickness, surface temperature, cooling conditions, and the like. For example, the average temperature in the plate thickness direction can be obtained by calculating the temperature distribution in the plate thickness direction using the difference method.

Slab heating temperature: 1000-1200 ° C
If the slab heating temperature is less than 1000 ° C., sufficient strength cannot be obtained, and if it exceeds 1200 ° C., toughness and DWTT characteristics deteriorate. Therefore, it is preferable to set it as the range of slab heating temperature 1000-1200 degreeC. When more excellent DWTT characteristics are required, it is more desirable to set the upper limit of the slab heating temperature to 1100 ° C.

Reduction ratio in non-recrystallization temperature range: 60% or more In order to obtain a fine bainite structure and high base metal toughness, it is preferable to perform sufficient reduction in the non-recrystallization temperature range in the hot rolling process. However, since the effect is insufficient when the rolling reduction in this temperature range is less than 60%, it is preferable to set the rolling reduction to 60% or more in the non-recrystallization temperature range. Note that the rolling reduction is the cumulative rolling reduction when rolling is performed in a plurality of rolling passes. Moreover, although the non-recrystallization temperature region varies depending on the alloying elements such as Nb and Ti, the upper limit temperature of the non-recrystallization temperature region may be regarded as 950 ° C. with the addition amount of Nb and Ti of the present invention.

Rolling end temperature: In order to make the metal structure of Ar ₃ or higher a structure having a bainite fraction of 80% or higher, it is preferable to finish rolling at Ar ₃ temperature or higher, which is a ferrite formation temperature, and immediately cool the steel sheet. Moreover, if it rolls after producing | generating a ferrite, it will become a structure | tissue where the metal structure was extended | stretched in the longitudinal direction of the steel plate, and the yield ratio of a pipe-axis direction when it becomes a steel pipe will fall. Therefore, the rolling end temperature is preferably set to Ar ₃ temperature or higher.

Incidentally, Ar ₃ temperature is a function of the alloy components of the steel, it may be determined by measuring the transformation temperature by experiment for each steel, but can also be calculated by the following equation (2) from the chemical components.
Ar ₃ (° C.) = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo (2)
Here, each element symbol means content (mass%), and is 0 when not contained.
It is preferable to perform accelerated cooling subsequent to hot rolling. Preferred conditions for accelerated cooling are as follows.

Cooling rate: 10 ° C./second or more The cooling rate is preferably controlled in order to obtain a high-strength and high-toughness steel sheet in particular. By cooling at a high cooling rate, the strength is increased by transformation strengthening and the structure is refined. Toughness is obtained. However, when the cooling rate is less than 10 ° C./second, sufficient strength and toughness cannot be obtained. Therefore, the cooling rate during accelerated cooling is preferably 10 ° C./second or more.

Accelerated cooling stop temperature: 500 ° C. or less In order to obtain a fine bainite structure by accelerated cooling, it is necessary to cool to a temperature range where the bainite transformation sufficiently proceeds. When the cooling stop temperature exceeds 500 ° C., not only the bainite transformation is insufficient and sufficient strength cannot be obtained, but a hard second phase such as cementite and MA is generated in the subsequent air cooling process, and the toughness is increased. to degrade. Therefore, the cooling stop temperature is preferably 500 ° C. or lower. Moreover, in order to obtain higher strength and toughness with a thick material, it is more preferable to set the temperature to 400 ° C. or lower.

  The steel sheet after accelerated cooling can be used for steel pipe production by the UOE process or press bend process as it is. However, in order to fully satisfy the characteristics of the pipe axis direction and the pipe circumferential direction of the present invention, heat treatment is performed after accelerated cooling. It is preferable to apply. The heating temperature of the steel sheet surface layer at this time is preferably 450 to 700 ° C. This improves the yield strength by recovering the dislocations introduced in large quantities by the bainite transformation during accelerated cooling, and further utilizing the interaction with carbon dissolved in the steel. This is because the difference in yield strength between the tube axis direction and the tube circumferential direction after the process can be reduced.

  In addition, hard second phases such as cementite and MA generated during accelerated cooling are decomposed by heat treatment, the base material toughness is improved, and the Bausinger effect of the steel sheet is reduced, so that a high compressive strength in the steel pipe is obtained. . Such an effect cannot be obtained at a heat treatment temperature lower than 450 ° C., and when the temperature exceeds 700 ° C., the strength of the steel sheet is lowered. Therefore, the heat treatment temperature is preferably 450 to 700 ° C.

  Such heat treatment may be performed in an off-line heat treatment furnace such as a gas combustion furnace after the accelerated cooling of the steel sheet. Moreover, in order to improve the manufacturing efficiency of a steel plate, you may use the induction heating type online heating equipment installed in the same line as the rolling and cooling line of a steel plate.

  In the present invention, the steel plate manufactured by the above-described method is formed into a steel pipe by using the steel plate as a raw material, and the method of forming the steel pipe includes a method of forming into a steel pipe shape by cold forming such as UOE process or press bend. Then, seam welding is performed. Any method can be used as the welding method in the seam welding process as long as sufficient joint strength and joint toughness can be obtained, but it is preferable to use submerged arc welding from the viewpoint of excellent welding quality and production efficiency. After welding the butt portion, pipe expansion is performed for the purpose of removing welding residual stress and improving the roundness of the steel pipe. This pipe expansion rate is preferably 0.4% or more as a condition for obtaining a predetermined roundness of the steel pipe and removing the residual stress.

  Further, if the tube expansion rate is too high, the yield strength in the tube axis direction tensile test due to the Bauschinger effect decreases, so the upper limit is preferably made 1.0%. More preferably, the upper limit of the pipe expansion rate is 0.8%.

  Steels (steel types A to D) having chemical components shown in Table 1 were made into slabs by a continuous casting method, and steel plates (Nos. 1 to 6) having a plate thickness of 30 mm were manufactured using the slabs. The steel sheet production conditions are shown in Table 2. The reheating process at the time of steel plate manufacture performed heat processing using the induction heating furnace installed on the same line as the accelerated cooling equipment. The heat treatment temperature is the average temperature of the steel plate, but in induction heating, a temperature difference occurs between the steel plate surface layer and the central portion immediately after heating, and therefore the steel plate temperature at the time when the surface layer temperature and the center temperature are almost equal to each other in induction heating. Using these steel plates, steel pipes having various outer diameters were produced by the UOE process. Table 2 also shows the tube expansion ratio when manufacturing the steel pipe.

  The tensile properties of the steel pipes manufactured as described above were as follows: the yield strength and tensile strength were determined by using a rectangular test piece with a full thickness in the pipe axis direction and a round bar tensile test piece with a parallel part diameter of 10 mm taken from the inner surface of the pipe in the pipe circumferential direction. The 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 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 area fraction of bainite was calculated | required using the image analysis apparatus using the 3-5 photograph image | photographed by 200 time.

  Table 2 shows the metal structure of the steel pipe, and Table 3 shows the mechanical properties. The deformation behavior of the steel pipe when a bending moment was applied to these steel pipes was analyzed by FEM, and the flatness of the steel pipe when the bending moment was 11.5 MN · m was obtained. Further, the collapse pressure of the steel pipe flattened by bending as described above was determined based on the DNV standard (OS F-101). However, the compressive yield strength in the circumferential direction of the steel pipe was not the value obtained by multiplying the yield strength in the tensile test by the fabrication factor, but the compressive yield strength obtained in the actual compressive test. Table 3 shows the flatness and collapse pressure of the steel pipe.

No. which is an example of the present invention. 1 to 4 all have mechanical properties within the scope of the present invention, high compressive strength, and low flatness during bending deformation. As a result, a high collapse pressure is obtained.
On the other hand, no. Nos. 5 and 6 have a low collapse pressure because the mechanical properties are outside the scope of the present invention, and the compressive strength is low and the flatness during bending deformation is also high.

  According to the present invention, it is possible to obtain a steel pipe that has a high compressive strength and is suppressed from being flattened by bending deformation when laying a pipeline. Therefore, it can be applied to a deep sea line pipe that requires high collapse resistance.

Claims (2)

In steel pipes for line pipes,
In mass%, C: 0.03-0.1%, Si: 0.5% or less, Mn: 1.0-2.0%, P: 0.015% or less, S: 0.003% or less, Al: 0.08% or less, Nb: 0.005 to 0.05%, Ti: 0.005 to 0.03%, Cu: 0.5% or less, Ni: 1.0% or less , Cr: 0.5% or less, Mo: 0.5% or less, V: 0.1% or less, Ca: 0.0005% to 0.0035%, and the balance is Fe And having a chemical component consisting of inevitable impurities,
L-YS (T) and L-TS (T) are the yield strength and tensile strength in the pipe axial direction tensile test, C-YS (T) are the yield strength in the pipe circumferential direction tensile test, and pipe circumferential direction compression test L-YS (T) / L-TS (T) is 0.9 or more and L-YS (T) / C-YS (T) is 0. 95 or more and C-YS (C) / C-YS (T) is 0.9 or more, The steel pipe for line pipes excellent in the collapse-proof performance characterized by the above-mentioned.   The steel pipe for a line pipe according to claim 1, wherein the metal pipe has a bainite structure having an area fraction of 80% or more.

Images