JP4510680B2 - High-strength steel pipe for pipelines with excellent deformation characteristics after aging and method for producing the same - Google Patents

Description

  The present invention is suitable as an API standard 70 to 100 grade pipeline for transporting natural gas or crude oil, and has a large deformation tolerance due to ground deformation after coating treatment for anticorrosion purposes. The present invention relates to an excellent steel pipe for pipelines and a method for manufacturing the same.

  In recent years, the importance of pipelines as a long-distance transportation method for crude oil or natural gas has increased. However, the environment in which pipelines are laid has been diversifying year by year, and in particular, pipeline deformation due to seasonal ground changes in the frozen land zone, strata changes due to earthquakes, ocean currents at the seabed, etc. cannot be ignored in laying design. It has become a problem. Therefore, there has been a demand for a high-strength steel pipe excellent in deformation performance that is not only excellent in internal pressure resistance but also difficult to buckle even if bending deformation occurs.

  As a steel pipe satisfying such requirements, Patent Document 1 discloses a manufacturing method of high-strength steel having excellent deformation performance and a low ratio of yield strength to tensile strength, and Patent Document 2 discloses a steel pipe and manufacturing method having a large work hardening index. Is disclosed. Patent Document 3 proposes a steel plate and a steel pipe having a low yield ratio and a large uniform elongation. Patent Document 4 proposes a steel pipe having a ratio of yield strength to stress at a nominal strain of 1.5% in a tensile test of 1.05 or more. Furthermore, the pipeline actually welds steel pipes circumferentially, but in that case, there may be minute defects in the welded part, and the longitudinal direction is set so that it can always buckle and deform from the steel pipe without breaking from there. Patent Document 5 discloses a steel pipe and a manufacturing method in which the ratio of yield strength in the circumferential direction is optimized.

  By the way, although the steel pipe for pipelines is subjected to a coating treatment for anticorrosion before laying, in recent years, the number of cases where fusion bonding is performed is increasing. In the case of this treatment, the steel pipe is heated to 150 ° C to 300 ° C. As described in Non-Patent Document 1, since a steel pipe formed in a cold state is aged by this heating, the stress-strain curve changes greatly as compared with the time of manufacture.

  The stress strain curve in the longitudinal direction in the state of the steel pipe has a round house shape due to the influence of cold forming, and approximates the stress strain curve with the n-th power hardening law as disclosed in Patent Document 2. There is a correlation between the n value and the deformation performance of the steel pipe.

  However, when the steel pipe is heated to 150 ° C. to 300 ° C., the stress-strain curve changes greatly and becomes complicated especially in a region where the strain is small. A stress-strain curve with a strain between 0.5% and 5% cannot be approximated by the n-th power hardening rule, and the n value cannot be evaluated. Moreover, a clear yield elongation may appear depending on a component, a manufacturing method, and a heating temperature. Furthermore, although the stress-strain curve differs between the longitudinal direction and the circumferential direction of the steel pipe, there has been no discussion about this difference from the viewpoint of deformability.

Japanese Patent Publication No. 6-15689 JP 11-279700 A Japanese Patent Application No. 2002-106536 JP 2002-194503 A Japanese Patent Application No. 2003-179865 OMAE 2004-51569

  As described above, it is difficult to predict the deformation performance of the steel pipe after heating because of the relationship between the mechanical properties and the deformation performance as manufactured in the conventional steel pipe production. A situation has been reached in which the influence of mechanical properties on deformation performance has to be investigated.

  Therefore, the present inventors investigated the shape change of the SS curve when heated to a temperature range of 150 ° C. to 300 ° C. in detail, and after clarifying the change in deformation performance of the steel pipe, We clarified the mechanical properties of the steel pipes that should be provided so that the deformation characteristics are not impaired by coating heating, and studied the steel pipe manufacturing method.

  The present invention is suitable for pipelines, and has high strength equivalent to API standards X70 to X100 grades, and has sufficient deformation performance even after aging by heating at the time of coating treatment, and the like, The manufacturing method is provided.

  In the present invention, in order to ensure the deformation performance of a line pipe that has been heat-treated using a high-strength steel pipe, the stress-strain curve in which the amount of strain in the circumferential direction and the longitudinal direction that changes after heating is up to 2% is controlled. Is based on the knowledge that clarifies the optimum microstructure obtained by appropriate chemical components and rolling conditions for the purpose, and the gist thereof is as follows.

(1) In mass%,
C: 0.02% to 0.09%,
Si: 0.001% to 0.8%,
Mn: 0.5% to 2.5%
P: 0.02% or less,
S: 0.005% or less,
Ti: 0.005 to 0.03%,
Nb: 0.005 to 0.1%,
Al: 0.001% to 0.1%,
N: 0.001% to 0.008%,
Containing, and, 2 <(3.5Ti + 8Nb) / (C + N) satisfies the condition of ≦ 5.66, further,
Ni: 0.1% to 1.0%
Cu: 0.1% to 1.0%,
Mo: 0.05% to 0.6%
Among them, a steel plate containing two or all of Ni and Cu and satisfying (Ni + Cu) -Mo> 0.5, the balance being iron and inevitable impurities, is cold-formed into a cylindrical shape, and the end faces Steel pipe welded with seam, the upper yield point is lower than the tensile strength in the circumferential tensile test after heating to 150 ° C to 300 ° C, and the yield ratio is 0.93 or less in the tensile test in the pipe axis direction A high-strength steel pipe for pipelines with excellent deformation characteristics after aging.

(2) A pipeline excellent in deformation characteristics after aging as described in (1), wherein the tensile elongation in the tube axis direction after heating to 150 ° C. to 300 ° C. is 1% or less. High strength steel pipe.

(3) Excellent elongation characteristics after aging according to (1) or (2), wherein the uniform elongation in the tube axis direction after being heated to 150 ° C. to 300 ° C. is 5% or more High strength steel pipe for pipelines.

(4) The structure of the steel pipe is a mixed structure of ferrite having an area ratio of 50% or less and an average crystal grain size of 10 μm or less and the balance martensite and / or bainite. 3) A high-strength steel pipe for pipelines having excellent deformation characteristics after aging according to any one of 3).

(5) In mass%,
Cr: 1% or less,
V: 0.1% or less ,
R EM: 0.02% or less,
Mg: 0.006% or less,
The high strength steel pipe for pipelines excellent in deformation characteristics after aging according to any one of (1) to (4), characterized by containing one or more of the above.

(6) After heating the steel slab comprising the component according to any one of (1) to (5) to 1000 ° C or higher, rough rolling is performed at a temperature of 900 ° C or higher and a cumulative reduction amount of 50% or higher, and then finish rolling. Is started at 870 ° C. or lower and finishes at 650 ° C. or higher, and accelerated cooling is started at a cooling rate of 5 ° C./s to 50 ° C./s from a temperature range of 600 ° C. or higher, at 200 ° C. to 600 ° C. The cooling is stopped, and the steel sheet is then rolled with a cumulative reduction of 1% to 5% in the cold, then heated to 100 to 250 ° C., then the steel sheet is formed into a cylindrical shape, and the butt portion is welded. A method for producing a high-strength steel pipe for pipelines, which is excellent in deformation characteristics after aging, characterized by heating the steel pipe to 150 ° C to 300 ° C.

  According to the present invention, it has excellent strength equivalent to API standard X70 to X100 grade suitable for pipelines, and even after being aged by being heated by coating treatment, It is possible to provide a high-strength steel pipe for pipelines that has sufficient deformation performance without breaking.

  The inventors of the present invention have made changes in the stress strain curves in the circumferential direction and the longitudinal direction of steel pipes of API standard X70 to X100 grade manufactured under various conditions after being heated to 150 ° C. to 300 ° C., and the deformation performance of the steel pipes associated therewith. We investigated in detail about the changes.

  First, a steel plate obtained by rolling steel pieces having various chemical components under different conditions is formed cold, and the butt portion is seam welded to obtain a steel pipe (outer diameter: 610 to 1320 cm (24 to 52 inches), wall thickness: 15 The steel pipe was heated to an arbitrary temperature of 150 ° C. to 300 ° C. for 60 seconds to investigate the tensile test and the bending deformation characteristics of the steel pipe. The tensile test piece in the longitudinal direction was a JIS No. 12 arc-shaped tensile test piece, and the tensile test piece in the circumferential direction was a round bar tensile test piece. In the bending test, a steel pipe cut to a length 20 times the outer diameter is used as a test piece, and an arbitrary internal pressure of 10 to 20 atmospheres is applied to the steel pipe as a water pressure to be bent at four points (the length of the bent portion: the outer diameter 6 times), the steel pipe was bent and deformed, and the bending angle and load load of the steel pipe were measured.

  As shown in FIG. 1, a diagram showing the relationship between the load applied to the steel pipe and the bending angle was prepared, and the bending angle (θ1) at the maximum load was adopted as the deformation characteristic.

  Test results show that the bending angle at the maximum load (θ1) correlates with Y / T in the longitudinal direction, but if the upper yield point in the circumferential direction is higher than the tensile strength, the Y / T in the longitudinal direction of the steel pipe is bad. Became clear. FIG. 2 shows the relationship between the yield ratio (Y / T) in the longitudinal direction and θ1. When the upper yield point in the circumferential direction is higher than the tensile strength, it has become clear that even if Y / T changes, θ1 is an extremely low value of 3 ° or less. On the other hand, when the upper yield point in the circumferential direction is lower than the tensile strength, θ1 shows a correlation with Y / T, and when Y / T is 0.93 or less, it is clear that θ1 shows a good value of 4 ° or more. became. Further, when the yield elongation was 1% or less in the tensile test in the longitudinal direction, higher θ1 was obtained even with the same Y / T.

  Therefore, in order to ensure the deformation characteristics of the high-strength line pipe after aging, it is essential that the upper yield point in the circumferential direction is lower than the tensile strength, and the Y / T in the longitudinal direction is limited to 0.93 or less. To do.

  Further, when Y / T is 0.93 and the yield elongation is 1% or less in the tensile test in the longitudinal direction, a higher θ1 is obtained. Therefore, the tensile elongation in the longitudinal direction is limited to 1% or less. It is desirable to further improve the deformability.

  Moreover, it became clear that (theta) 1 had a correlation with uniform elongation, and (theta) 1 fell rapidly when uniform elongation was 5% or less. Therefore, it is desirable that the uniform elongation in the longitudinal direction of the steel pipe after heating is 5% or more.

  Next, the reasons for limiting the component elements will be described.

The amount of C is limited to 0.02 to 0.09%. Carbon is an extremely effective element for improving the strength of steel. In order to obtain the target strength, a minimum of 0.02% is necessary. However, if the amount of C is more than 0.09%, the low temperature toughness and on-site weldability of the base metal and the weld heat affected zone will be deteriorated, so the upper limit was made 0.09% .

  Si is an element added for deoxidation and strength improvement. In order to exhibit the effect, 0.001% is necessary. However, if adding more than 0.8%, the local weldability deteriorates remarkably, so the upper limit of Si content was set to 0.8%.

  Mn is an essential element for improving the balance between strength and low temperature toughness, and its lower limit is 0.5%. However, if it exceeds 2.5%, the center segregation becomes prominent and the low temperature toughness is greatly deteriorated, so the upper limit was made 2.5%.

  In the present invention, the amounts of impurity elements P and S are 0.02% and 0.005% or less, respectively. The main reason for this is to improve the low temperature toughness of the base metal and the weld heat affected zone. Reduction of the P content prevents grain boundary fracture and improves low temperature toughness. On the other hand, the reduction of the amount of S has the effect of reducing the MnS stretched by hot rolling and improving the toughness. Although both elements are preferably as small as possible, P and S usually contain 0.001% or more and 0.0001% or more, respectively, from the balance between characteristics and cost.

Nb not only suppresses the recrystallization of austenite during controlled rolling and refines the structure, but also contributes to an increase in hardenability and strengthens the steel. Since this effect is small at less than 0.005%, the lower limit is set. However, if the Nb amount is more than 0.1%, the toughness of the weld heat affected zone is adversely affected, so the upper limit was made 0.1% .

  Addition of Ti forms fine TiN, refines the structure of the base material and the weld heat affected zone, and contributes to improvement of toughness. This effect becomes very remarkable by the combined addition with Nb. In order to fully exhibit this effect, it is necessary to add at least 0.005% of Ti. However, if the amount of Ti added is more than 0.03%, TiN coarsening or precipitation hardening due to TiC occurs, resulting in a decrease in low temperature toughness. Therefore, the upper limit is limited to 0.03%.

  Al is an element usually contained in steel as a deoxidizing material, and has an effect on refinement of the structure. However, if the Al content exceeds 0.1%, Al-based non-metallic inclusions increase to impair the cleanliness of the steel, so the upper limit was made 0.1%. Moreover, it plays the role which fixes the solid solution N which influences age hardening as AlN precipitation, and the addition of 0.001% or more is required as the effect.

  N forms TiN and suppresses the coarsening of the austenite grains in the slab reheating and in the weld heat affected zone, thereby improving the low temperature toughness of the base material and the weld heat affected zone. The minimum amount required for this is 0.001% or more. However, if N is added in excess of 0.008%, the amount of TiN produced increases, which causes a problem of low temperature toughness being lowered. Therefore, the upper limit of N content is set to 0.008%.

  Furthermore, the present inventors have found that it is effective to add C, N, Ti, and Nb appropriately in order to make the upper yield point in the circumferential direction lower than the tensile strength. When heated to 150 ° C. to 300 ° C., C and Nb in a solid solution move onto the dislocation, so that the dislocation is fixed and the yield point is greatly increased. Since the strain due to pipe making enters the circumferential direction, the yield point in the circumferential direction increases significantly compared to the longitudinal direction. Therefore, in order to suppress the upper yield point in the circumferential direction, it is necessary to make C and N in a solid solution state as low as possible. Therefore, the present inventor decided to precipitate C and N with Ti and Nb as much as possible during the manufacture of the steel sheet. As a result, when (3.5Ti + 8Nb) / (C + N) was 2 or more, the upper yield point in the circumferential direction was lower than the tensile strength, and deterioration in low temperature toughness of 8 or more was recognized.

Therefore, it is necessary to satisfy the condition of 2 <(3.5Ti + 8Nb) / (C + N) <8 between Ti, Nb, C, and N. The upper limit of (3.5Ti + 8Nb) / (C + N) is 5.66 or less based on the example.

  Ni is an element that improves the strength without deteriorating the low-temperature toughness, and it is preferable to add 0.1% or more. However, if the addition amount exceeds 1%, the HAZ toughness is lowered, so the upper limit was made 1.0%.

  Cu is an element that improves the strength of the base material and the weld heat affected zone, and is preferably added in an amount of 0.1% or more. However, the on-site weldability exceeding 1.0% significantly decreases the weldability, so the upper limit was made 1.0%.

  Mo is an element that can improve the hardenability and achieve high strength. In addition, the combined effect with Nb suppresses recrystallization of austenite during controlled rolling, and is effective in refining austenite crystal grains. Addition of 0.05% or more is preferable. However, if it exceeds 0.6%, the HAZ toughness is deteriorated, so the upper limit was made 0.6%.

  Furthermore, the present inventors have found that Ni, Cu, and Mo are elements that greatly influence the change in the stress strain curve in the longitudinal direction of the steel pipe after 150 ° C. to 300 ° C. When {(Ni + Cu) -Mo} is smaller than 0.5, Y / T in the longitudinal direction after being heated to 150 ° C. to 300 ° C. is larger than 0.93, and the deformability is deteriorated. On the other hand, it was found that when {(Ni + Cu) -Mo} exceeds 0.5, Y / T in the longitudinal direction can be secured to 0.93 or less. Most of Ni and Cu exist in a solid solution state in ferrite grains, and a stress field is generated between these elements and Fe atoms due to a difference in lattice constant, which contributes to aging, such as solid solution C and solid solution. Set to N. Since this stress field does not have a great influence on the subsequent plastic deformation behavior, it does not age even if solid solution elements gather in this stress field. On the other hand, Mo collects on dislocations in the state of C and clusters. C assembled as a cluster interacts with dislocations and contributes to aging. Thus, Cu and Ni work as an action to collect solute C and solute N in the stress field of the parent phase that does not contribute to aging, and reduce the amount of solute C and solute N that will collect on dislocations to suppress aging. To do. On the other hand, Mo works as an action of collecting solid solution C on the dislocation as a cluster, and as a result, promotes aging.

  Cr is an element that increases the strength of the base material and the welded portion, and it is preferable to add 0.1% or more selectively. However, if the Cr content exceeds 1%, the toughness of the heat affected zone and the on-site weldability may be significantly degraded. For this reason, it is preferable to make an upper limit into 1.0%.

  V has almost the same effect as Nb, but the effect is weaker than Nb. It also has the effect of suppressing softening of the weld. The upper limit of the V amount is preferably 0.1% from the viewpoint of the toughness of the weld heat affected zone. A preferable range of the V amount is 0.03 to 0.08%.

  Ca and REM control the form of sulfide (MnS) and improve low temperature toughness. It is preferable to add 0.001% or more and 0.002% or more, respectively. When the Ca content exceeds 0.01% and the REM content exceeds 0.02%, the formation of large inclusions becomes remarkable, not only detracting from the cleanliness of the steel, but also greatly deteriorates the field weldability. For this reason, it is preferable that the upper limit of the C amount is 0.01% and the upper limit of the REM amount is 0.02%.

  Mg forms a finely dispersed oxide and improves low temperature toughness by suppressing grain coarsening in the weld heat affected zone. In order to exert the effect, it is preferable to add 0.001% or more. On the other hand, if it exceeds 0.006%, a coarse oxide is produced and the low-temperature toughness deteriorates conversely, so the upper limit is preferably made 0.006%.

  In order to improve the deformation performance of the steel pipe, it is preferable to have a composite structure of soft ferrite and hard martensite and / or bainite. If the area ratio of ferrite exceeds 50%, the target strength is not reached, so the upper limit is made 50%. Preferably, it is 10% to 30%. When the average crystal grain size of ferrite exceeds 15 μm, the low temperature toughness of the base material is significantly reduced, so the upper limit is made 15 μm. Preferably it is 10 micrometers or less.

  In order to make the uniform elongation 5% or more, it is necessary to produce 5% or more of soft ferrite. The upper limit of the uniform elongation is inevitably about 17% because the ferrite fraction is within 50%.

  The area ratio of ferrite is an average value measured by a point count method with an interval of 5 μm using an optical microscope structure photograph. The crystal grain size is an average value measured by a cutting method at intervals of 10 μm. A sample for observation with an optical microscope is obtained by cutting a steel pipe in the longitudinal direction and mirror polishing and nital corrosion. An arbitrary position of 1/4, 1/2 of the plate thickness of the sample is observed with an optical microscope at 500 times and photographed.

  Further, since the softer the ferrite, the better the deformation performance, so the Vickers hardness of the ferrite phase measured in accordance with JIS Z 2244 using a micro Vickers hardness tester is preferably 170 Hv or less.

Next, the manufacturing method of this invention is demonstrated.
The method for producing a steel pipe of the present invention is as follows. After the steel is melted, it is cast into a steel slab, the steel slab is heated and hot-rolled, cooled to a steel plate, and the steel plate is cold-formed into a cylindrical shape. Then, the ends are welded together to form a steel pipe, and then the manufacturing process is reheated to 200 ° C to 300 ° C.

The heating temperature of the steel slab at the time of hot rolling is set to 1000 ° C. or higher. This is because the Ac ₃ point of the steel comprising the components of the present invention does not drop below 1000 ° C., and the steel can be heated to the austenite region if the heating temperature is 1000 ° C. or higher. Reheating is preferably in the range of 1050 ° C. to 1200 ° C. in which Nb is dissolved and crystal grains are not coarsened.

  After reheating, rough rolling is performed in a temperature range of 900 ° C. or higher, and finish rolling is subsequently performed at 870 ° C. or lower. The rough rolling is performed at 900 ° C. or higher because the temperature range in which austenite can be sufficiently recrystallized is 900 ° C. or higher. To reduce the crystal grains by recrystallization, the rolling reduction of 50% or more is required. is necessary. The starting temperature of finish rolling is 870 ° C. or lower in order to perform finish rolling at a sufficient non-recrystallization temperature. When the finish temperature of finish rolling is lower than 700 ° C., in the steel composed of the component of the present invention, ferrite processed in excessive rolling is generated and the deformation performance of the steel pipe is impaired, so the finish temperature of finish rolling is 700 ° C. That's it. The cumulative rolling reduction of finish rolling is determined by the ratio of the thickness at the time of rough rolling and the product sheet thickness, but from the viewpoint of securing low temperature toughness, the cumulative rolling reduction of finish rolling is preferably 50% or more.

  After finish rolling, cooling is started from a temperature range of 600 ° C. or higher. If the start temperature is less than 600 ° C., ferrite with an area ratio of 50% or more is generated before the start of cooling, so the start of cooling is set to 600 ° C. or more. The cooling stop temperature is limited to 300 ° C to 550 ° C. If it is lower than 200 ° C., there is a problem that the shape of the steel sheet is unstable or cracks in the cooling process, so the lower limit of the stop temperature is set to 200 ° C. In order to satisfy the target strength, the upper limit of the stop temperature needs to be 600 ° C. or less, and the upper limit is set to 600 ° C.

  The cooling rate is limited to 10 to 50 ° C./s in order to disperse fine ferrite and improve the balance between deformability and low temperature toughness. If it is less than 10 ° C / s, the ferrite grains become coarse and low temperature toughness cannot be ensured. On the other hand, the hardness of martensite or bainite exceeding 50 ° C / s becomes excessively hard, ensuring a uniform elongation of 5% or more. It is not possible and promotes excessive aging. Therefore, the cooling rate is preferably 10 to 50 ° C./s.

  Furthermore, in order for the upper yield point in the circumferential direction of the steel pipe after being heated to 150 ° C. to 300 ° C. not to exceed the tensile strength, it is effective to perform cold rolling and low-temperature heat treatment before forming the steel pipe. It became clear. The purpose of this is to make C and N in a solid solution as low as possible before making the steel pipe. Dislocations are introduced under light pressure at the steel plate stage, and C and N in the solid solution state are fixed to the dislocations by subsequent low temperature rolling to suppress C and N in the solid solution state. In order to exert the effect, a light reduction amount of 1% or more is necessary. Further, when the rolling amount is 5% or more, the upper yield point exceeds the tensile strength at this stage, so the rolling amount is limited to 1% to 5%. In order to fix C and N in a solid solution state to dislocations, the heat treatment temperature needs to be 100 ° C. or higher. Precipitation becomes prominent at 250 ° C. or higher, and low temperature toughness is deteriorated, so the heat treatment temperature is limited to 100 ° C. to 250 ° C.

  The steel plate thus manufactured is used as a steel pipe as it is by the UOE process, the electric sewing process, or the bend process. The steel tube is then heated to 150-300 ° C. This heating is performed before or during the coating process mainly for anticorrosion. Of course, there is no problem heating the steel pipe for other purposes such as removal of residual stress in the steel pipe. Since the target effect cannot be sufficiently exhibited at 150 ° C. or lower, the lower limit is set to 150 ° C. When the temperature exceeds 300 ° C., the precipitation phenomenon is noticeable and further aging occurs, so the upper limit of heating of the steel pipe needs to be 300 ° C. The heating holding time varies depending on the purpose of heating and is not particularly defined.

  Steel pieces having chemical components shown in Table 1 were melted, and continuously cast steel pieces were rolled under the conditions shown in Table 2 to obtain steel pieces. Furthermore, these steel plates were made into steel pipes by the UOE process, ERW process and bend pipe process. The steel pipe had an outer diameter of 762 mm and a wall thickness of 16 mm. The steel pipe was heated using a high-frequency induction heating device, the heating rate was 10 ° C./s, the maximum heating temperature was 160 to 300 ° C. (holding time 60 seconds), and the cooling was allowed to cool.

  The mechanical properties in the longitudinal direction of the steel pipe were measured by an arc-shaped full thickness tensile test piece according to API 5L, and the mechanical properties in the circumferential direction were measured by a round bar tensile test piece according to API 5L.

  In addition, specimens for microstructural observation were collected from steel pipes, polished and corroded, and each 1/4, 1/2, and 3/4 of the thickness was observed at 500 times, and an optical micrograph was taken. I took a picture. Using the obtained 15-view optical microscopic microstructure photographs, the area ratio of ferrite was measured by a point counting method at intervals of 5 μm, and the particle size of ferrite was measured by a cutting method at intervals of 15 μm, and obtained as an average value.

  The steel pipe deformability was evaluated by a 4-point bending test with internal pressure applied to the steel pipe. A steel pipe cut to a length of 12 m is used as a test piece, and an arbitrary internal pressure of 10 to 20 atmospheres is loaded with Ar gas into the steel pipe, and the steel pipe is bent and deformed by four-point bending (bending span length: 6 m). The bending angle and the applied load were measured. As shown in FIG. 1, a diagram showing the relationship between the load applied to the steel pipe and the bending angle was prepared, and the bending angle (θ1) at the maximum load was adopted as the deformation characteristic.

The results are shown in Table 3. Production No. which is an example of the present invention. The steel pipes 1 to 3, 5 to 9, 11 to 15, 17 and 18 have an upper yield point in the circumferential direction after heating lower than the tensile strength and the Y / T in the longitudinal direction is 0.93 or less. Good results were obtained with θ1 being an index of deformation performance of 4 ° or more. On the other hand, Production No. In 19 and 21, the upper yield point in the circumferential direction was high in tensile strength, so θ1 was 1 ° or less and the deformability was poor. In production No20, the upper yield point in the circumferential direction is higher than the tensile strength, and the Y / T and uniform elongation in the longitudinal direction are outside the scope of the present invention, so θ1 is remarkably low. No. 22 and no. 24, in addition to the fact that the upper yield point in the circumferential direction is higher than the tensile strength, not only the Y / T in the longitudinal direction is outside the scope of the present invention, but also the ferrite fraction is outside the scope of the present invention. No. and steel pipe deformation performance is low. No. 23 has a large ferrite grain size because the upper end point of the circumferential direction is higher than the tensile strength and the cooling end temperature is outside the range of the present invention.

The figure which shows the relationship between the load loaded on the steel pipe, and the bending angle. The figure which shows the relationship between the yield ratio (Y / T) of a steel pipe longitudinal direction, and (theta) 1.

Claims (6)

% By mass
C: 0.02% to 0.09%,
Si: 0.001% to 0.8%,
Mn: 0.5% to 2.5%
P: 0.02% or less,
S: 0.005% or less,
Ti: 0.005 to 0.03%,
Nb: 0.005 to 0.1%,
Al: 0.001% to 0.1%,
N: 0.001% to 0.008%,
And satisfying the condition of 2 <(3.5Ti + 8Nb) / (C + N) ≦ 5.66 ,
Ni: 0.1% to 1.0%
Cu: 0.1% to 1.0%,
Mo: 0.05% to 0.6%
Among them, a steel plate containing two or all of Ni and Cu and satisfying (Ni + Cu) -Mo> 0.5, the balance being iron and inevitable impurities, is cold-formed into a cylindrical shape, and the end faces A steel pipe welded with seam, the upper yield point is lower than the tensile strength in the circumferential tensile test after heating to 150 ° C to 300 ° C, and the yield ratio is 0.93 or less in the tensile test in the pipe axis direction A high-strength steel pipe for pipelines with excellent deformation characteristics after aging.   2. High strength for pipelines with excellent deformation characteristics after aging according to claim 1, wherein the yield elongation is 1% or less in a tensile test in the tube axis direction after being heated to 150 ° C. to 300 ° C. Steel pipe.   3. Pipeline having excellent deformation characteristics after aging according to claim 1 or 2, wherein the uniform elongation in the tube axis direction after being heated to 150 ° C to 300 ° C is 5% or more. High strength steel pipe.   The structure of the steel pipe is a mixed structure of ferrite having an area ratio of 50% or less and an average crystal grain size of 10 µm or less and the balance martensite and / or bainite. High-strength steel pipe for pipelines with excellent deformation characteristics after aging as described in 1. In mass%,
Cr: 1% or less,
V: 0.1% or less ,
R EM: 0.02% or less,
Mg: 0.006% or less,
The high strength steel pipe for pipelines excellent in deformation characteristics after aging according to any one of claims 1 to 4, characterized by containing one or more of the following. After heating the steel slab comprising the component according to any one of claims 1 to 5 to 1000 ° C or higher, rough rolling is performed at a temperature of 900 ° C or higher and a cumulative reduction amount of 50% or higher, and then finish rolling is performed at 870 ° C. Finishing rolling is started at the following and finished at 650 ° C. or higher, accelerated cooling is started at a cooling rate of 5-50 ° C./s from a temperature range of 600 ° C. or higher, cooling is stopped at 200 ° C.-600 ° C., and then continued. After cold rolling to a cumulative reduction of 1% to 5% to obtain a steel plate, then heating to 100 ° C. to 250 ° C., then forming the steel plate into a cylindrical shape, and welding the butt to obtain a steel pipe The manufacturing method of the high strength steel pipe for pipelines which was excellent in the deformation characteristic after aging characterized by heating to 150 to 300 degreeC.

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