JP5141073B2 - X70 grade or less low yield ratio high strength high toughness steel pipe and method for producing the same - Google Patents

Description

  The present invention relates to a large-diameter welded steel pipe (UOE steel pipe) with small material deterioration after coating treatment, which is suitable mainly for line pipes for transporting crude oil and natural gas, and a method for producing the same.

  In recent years, steel materials for welded structures are required to have a low yield ratio and high uniform elongation from the viewpoint of earthquake resistance in addition to high strength and high toughness. In general, by making the metal structure of steel a structure in which hard phases such as bainite and martensite are moderately dispersed in a soft phase like ferrite, low yield ratio and high uniform elongation of steel are achieved. It is known to be possible.

  As a production method for obtaining a structure in which a hard phase is appropriately dispersed in the soft phase as described above, quenching from a two-phase region of ferrite and austenite (Q ′) between quenching (Q) and tempering (T). There is known a heat treatment method for applying (see, for example, Patent Document 1).

  In this heat treatment method, a low yield ratio can be achieved by appropriately selecting the Q 'temperature, but the number of heat treatment steps increases, resulting in a decrease in productivity and an increase in manufacturing cost.

As a method that does not increase the number of manufacturing steps, a method is disclosed in which after the end of rolling at an Ar ₃ temperature or higher, the start of accelerated cooling is delayed until the temperature of the steel material becomes equal to or lower than the Ar ₃ transformation point at which ferrite forms (for example, , See Patent Document 2).

  However, since it is necessary to cool the temperature range from the end of rolling to the start of accelerated cooling at a cooling rate that is about the ability to cool, productivity is extremely reduced.

As a technique for achieving a low yield ratio without performing a complex heat treatment as disclosed in Patent Document 1 and Patent Document 2, the rolling of the steel material is completed at the Ar ₃ transformation point or higher, and the subsequent accelerated cooling rate and A method is known in which a two-phase structure of acicular ferrite and martensite is achieved by controlling the cooling stop temperature to achieve a low yield ratio (see, for example, Patent Document 3).

However, in the technique described in Patent Document 3, as shown in the examples, since the hot rolling finish temperature is low, the productivity is extremely lowered and the manufacturing cost is increased.
JP-A-55-97425 JP 55-41927 A Japanese Patent Laid-Open No. 1-176027

  By the way, in the UOE steel pipe used for the line pipe, the steel sheet is formed into a cold tube, the butt portion is welded, and then the coating treatment is usually applied to the outer surface of the steel pipe from the viewpoint of corrosion prevention. Strain aging occurs due to heating during the coating process, and the yield stress increases.

  As a steel material excellent in strain aging resistance and a manufacturing method thereof, for example, as described in JP-A-2002-220634, the content of C and N that cause strain aging is limited, and Nb, Ti Is added, and a method for suppressing strain aging is disclosed by combining with C and N. However, according to the embodiment, the yield ratio of the steel sheet according to the invention described in JP-A-2002-220634 is 80% or more. It is.

  As described above, even if a low yield ratio of the steel plate is achieved, it is difficult to achieve a low yield ratio in the steel pipe.

  Therefore, the present invention provides a method for producing a high uniform elongation, low yield ratio, high strength, high toughness steel pipe of API 5L X70 grade or lower having a low yield ratio even after coating treatment, with high production efficiency and low cost. For the purpose.

  In order to solve the above-mentioned problems, the present inventors diligently studied a manufacturing process for performing accelerated cooling after controlled rolling and subsequent reheating, and obtained the following findings (a) to (c).

  (A) In the accelerated cooling process, during the bainite transformation, that is, in the temperature region where untransformed austenite is present, cooling is stopped, and then reheating is performed from the bainite transformation finish temperature (hereinafter referred to as Bf point) or higher. As a result, a three-phase structure in which island-like martensite (hereinafter referred to as MA), which is a hard phase, is uniformly generated in the mixed phase of ferrite and bainite is obtained.

  Since the produced MA is stable even at the heating temperature (up to 300 ° C.) during coating, the yield ratio can be lowered even after coating.

  MA can be easily identified by, for example, etching with a 3% nital solution (nital: nitrate alcohol solution), followed by electrolytic etching and observing.

  (B) Since an untransformed austenite is stabilized by adding an appropriate amount of an austenite stabilizing element such as Cu or Ni, it is a hard phase without adding a large amount of a hardenability improving element such as C or Mn. MA can be generated.

  (C) By stopping accelerated cooling at a high temperature, the dislocation density in bainite is lowered, and the addition amount of Ti and N is controlled, so that an increase in yield ratio due to strain aging after coating can be suppressed.

The present invention has been made by further studying the above findings, that is, the present invention
1. In mass%, C: 0.03-0.1%, Si: 0.01-0.5%, Mn: 1.2-2.0%, Mo: 0.05-0.4%, Cu + Ni: 0.1% or more, Ti: 0.005 to 0.04%, Nb: 0.005 to 0.07%, Al: 0.08% or less, N: 0.005% or less, Ti / N Is 3 or more, satisfies the formula (1), and the remainder is made of Fe and inevitable impurities, heated to a temperature of 1000 to 1300 ° C., and hot-rolled at a rolling end temperature of Ar ₃ or higher Accelerated cooling to 500 to 650 ° C. at a cooling rate of 5 ° C./s or more, and then immediately reheating to 550 to 750 ° C. at a heating rate of 0.5 ° C./s or more. Is a three-phase structure of island martensite, and the volume fraction of island martensite is 3-15% A steel sheet having a volume fraction of retained austenite of 2% or more and a uniform elongation in the longitudinal direction of 12% or more is formed into a tubular shape in the cold, the butt portion is welded to form a steel pipe, and the heating temperature is The coating process is performed at 300 ° C. or lower,
A method for producing a low yield ratio high strength high toughness steel pipe of X70 grade or less .
Ceq1 ≦ 0.40 (1)
However, Ceq1 = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5
2. In addition to the component composition, by mass%, V: 0.005 to 0.1%, Cr: 0.5% or less, Ca: 0.0005 to 0.003%, B: 0.005% or less 2. A method for producing a low yield ratio, high strength, high toughness steel pipe of X70 grade or less according to 1, characterized by containing two or more.
3. The component composition described in 3.1 or 2, and a three-phase structure of ferrite, bainite, and island martensite, the volume fraction of island martensite is 3 to 15%, and the volume fraction of retained austenite is 2 % X70 grade or lower low yield ratio high strength high toughness steel pipe, characterized in that a steel sheet having a metal structure of at least% is used as a base material and a coating treatment is performed at a heating temperature of 300 ° C. or lower.

  According to the present invention, a bainite phase generated by accelerated cooling after rolling, a ferrite phase generated by subsequent reheating, and a high uniform elongation and low yield ratio having a three-phase structure in which MA, which is a hard phase, is uniformly generated. A high-strength, high-toughness steel pipe can be manufactured at a low cost without deteriorating the weld heat-affected zone toughness and without using a large amount of alloying elements.

  UOE steel pipes mainly used for line pipes can be manufactured stably in a large amount at a low cost, and the productivity and economic efficiency in laying pipelines can be remarkably improved, which is extremely useful in industry.

The low yield ratio, high strength, high toughness steel pipe according to the present invention is a steel pipe (base material) having a specific metal structure that is cold-formed into a tubular shape, welded to a butt portion to form a steel pipe, and a heating temperature of 300 ° C. or less. It is manufactured by applying the coating process. Furthermore, by reducing the dislocation density and solute N in bainite, which cause strain aging, a low yield ratio and a high uniform elongation are achieved in the steel pipe after the coating treatment.
[Metal structure]
In the present invention, the base material has a three-phase structure in which MA, which is a hard phase, is uniformly formed on ferrite and bainite.

  The proportion of MA in the three-phase structure is 3 to 15% in terms of the volume fraction of MA (ratio of the volume of MA in an arbitrary cross section of the steel pipe in the pipe length direction and the pipe circumferential direction, etc.).

  If the volume fraction of MA is less than 3%, it may be insufficient to achieve a low yield ratio, and if it exceeds 15%, the base material toughness may be deteriorated. From the viewpoint of low yield ratio, high uniform elongation, and base metal toughness, the volume fraction of MA is particularly preferably 5 to 15%.

  Austenite stabilized by Cu and Ni remains in MA, and a residual austenite volume fraction of 2% or more is necessary.

  Since the above-mentioned MA is stable without being decomposed even when heated during the coating process, it is possible to achieve a low yield ratio even after the coating process.

  The volume fraction of MA can be obtained, for example, by calculating from the area ratio occupied by MA by performing image processing on a microstructure photograph of at least four fields of view obtained by SEM observation.

  The average particle size of MA is desirably 10 μm or less. The average particle diameter of MA can be obtained as an average value of the diameters obtained by subjecting the microstructure obtained by SEM observation to image processing, obtaining the diameter of a circle having the same area as each MA, and obtaining the diameter of each MA. .

  The ferrite in the three-phase structure has a fraction of 5% or more in order to ensure strength that satisfies the APIX70 standard. In addition, it is desirable that the bainite in the three-phase structure has a fraction of 10% or more in order to ensure toughness that satisfies the APIX70 standard.

  In addition, as long as the metal structure is composed of a three-phase structure of ferrite, bainite, and MA, the one containing a structure other than ferrite, bainite, and MA is included in the scope of the present invention unless the effects of the present invention are lost. Means that

  When one or two or more different metal structures such as pearlite are mixed in the three-phase structure of ferrite, bainite, and MA, the strength decreases, so that the fraction of the structure other than ferrite, bainite, and MA is smaller. good.

  However, when the fraction of the structure other than ferrite, bainite and MA is low, the influence is negligible. Therefore, other metal structures of 3% or less in total fraction, that is, one or more kinds of pearlite, cementite, etc. You may contain.

  In the above-described three-phase structure of ferrite, bainite and hard MA, ferrite, which is a soft phase, is responsible for deformation, so that it is possible to achieve high uniform elongation of 10% or more, and to earthquake zones that undergo large deformation. In addition to the low yield ratio required when applied, it satisfies high uniform elongation performance.

[Chemical component] In the following description, all units shown in% are% by mass.
C: 0.03 to 0.1%
C is an important element for MA generation, but if it is less than 0.03%, it is insufficient for generation of MA, and sufficient strength cannot be secured. Addition over 0.1% leads to deterioration of HAZ toughness and an increase in yield ratio due to strain aging, so the C content is specified to be 0.03 to 0.1%. More preferably, it is 0.03 to 0.08%.

Si: 0.01-0.5%
Si is added for deoxidation, but if it is less than 0.01%, the deoxidation effect is not sufficient, and if it exceeds 0.5%, the toughness and weldability are deteriorated, so the Si content is 0.01 to 0.00. Specify 5%. More preferably, it is 0.01 to 0.3%.

Mn: 1.2 to 2.0%
Mn is added to improve strength and toughness, further improve hardenability and promote MA formation. However, if it is less than 1.2%, its effect is not sufficient, and if it exceeds 2.5%, toughness and weldability deteriorate. Therefore, the Mn content is specified to be 1.2 to 2.0%. Addition of 1.5% or more is desirable in order to stably produce MA regardless of changes in components and production conditions.

Mo: 0.05-0.4%
Mo is an element that improves the hardenability, and is an element that contributes to an increase in strength by strengthening the MA formation and the bainite phase. However, if it exceeds 0.4%, the weld heat-affected zone toughness is deteriorated, so the Mo content is specified to be 0.05 to 0.4%. Furthermore, it is preferable to make Mo content into 0.1 to 0.3% from a viewpoint of weld heat affected zone toughness.

Cu + Ni: 0.1% or more Cu and Ni are important elements in the present invention. In order to produce MA without adding a large amount of a hardenability improving element such as C and Mn, it is necessary to add 0.1% or more of Cu + Ni which is an element that stabilizes untransformed austenite. Furthermore, in order to produce MA more stably, addition of 0.3% or more is preferable.

  By using steel with a certain amount of Cu and Ni added, accelerating cooling is stopped during the bainite transformation, and then reheating is performed continuously, so that MA which is a hard phase can be generated without lowering the production efficiency. Is possible.

  Moreover, since untransformed austenite is stabilized, pearlite transformation and cementite formation during reheating and subsequent air cooling are suppressed.

Ti: 0.005-0.04%
Ti is an important element that suppresses austenite coarsening during slab heating, improves base metal toughness, reduces solid solution N, and suppresses the yield ratio increase due to strain aging due to the pinning effect of TiN.

  The effect is manifested by adding 0.005% or more. However, since addition exceeding 0.04% causes deterioration of the weld heat affected zone toughness, the Ti content is specified to be 0.005 to 0.04%. From the viewpoint of weld heat affected zone toughness, the Ti content is preferably 0.005% or more and less than 0.02%.

Nb: 0.005 to 0.07%
Nb is an element that improves toughness by refining the structure and contributes to an increase in strength by improving the hardenability of solid solution Nb. However, if it is less than 0.005%, there is no effect, and if it exceeds 0.07%, the toughness of the weld heat-affected zone deteriorates, so the Nb content is specified to be 0.005 to 0.07%.

Al: 0.08% or less Al is added as a deoxidizing agent, but if it exceeds 0.08%, the cleanliness of the steel decreases and the toughness deteriorates, so the Al content is specified to be 0.08% or less. To do. Preferably, the content is 0.01 to 0.08%.

N: 0.005% or less If N exceeds 0.005%, the weld heat-affected zone toughness deteriorates and there is a concern that the yield ratio will increase due to strain aging.

Ti / N: 3 or more By fixing solid solution N, which is the cause of strain aging, as TiN, it is possible to suppress an increase in the yield ratio after the coating treatment. Furthermore, by optimizing Ti / N, which is the ratio of Ti amount to N amount, the austenite coarsening of the weld heat affected zone can be suppressed by TiN particles, and good weld heat affected zone toughness can be obtained. . Preferably, Ti / N is 3 to 8, more preferably 3 to 5.

Ceq1 ≦ 0.40
However, Ceq1 = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5
The present invention is intended for UOE steel pipes of X70 grade or less. When Ceq1 represented by the above formula exceeds 0.40, it becomes difficult to satisfy the upper limit of strength of X70 grade. Therefore, Ceq1 ≦ 0.40.

  In the present invention, for the purpose of further improving the strength and toughness of the steel sheet and improving the hardenability and promoting the production of MA, it contains one or more of V, Cr, B, and Ca shown below. Also good.

V: 0.005-0.1%
It is an element that enhances hardenability and contributes to an increase in strength. However, if it is less than 0.005%, there is no effect, and if it exceeds 0.1%, the toughness of the weld heat-affected zone deteriorates. Therefore, when V is added, it is specified as 0.005 to 0.1%. .

Cr: 0.5% or less Cr is an effective element for obtaining sufficient strength even at low C as in Mn. In order to acquire the effect, it is preferable to add 0.1% or more. However, if it is added in a large amount, weldability deteriorates.

B: 0.005% or less B is an element contributing to strength increase and HAZ toughness improvement. In order to obtain the effect, it is preferable to add 0.0005% or more, but if added over 0.005%, the weldability is deteriorated, so when added, the content is made 0.005% or less.

Ca: 0.0005 to 0.003%
Ca improves the toughness by controlling the form of sulfide inclusions. The effect appears at 0.0005% or more, and when it exceeds 0.003%, the effect is saturated, and conversely, the cleanliness is lowered and the toughness is deteriorated. And

Next, the manufacturing method of the high strength steel pipe original plate which concerns on this invention is demonstrated.
[Production conditions]
In the description, temperatures such as heating temperature, rolling end temperature, cooling end temperature, and reheating temperature are the average temperatures of the steel plates. The average temperature is obtained by calculation based on the surface temperature of the slab or steel plate, taking into account parameters such as plate thickness and thermal conductivity. The cooling rate is an average cooling rate obtained by dividing the temperature difference required for cooling to the cooling end temperature (500 to 650 ° C.) by the time required for the cooling after the hot rolling is completed. The temperature increase rate is an average temperature increase rate obtained by dividing the temperature difference required for reheating up to the reheating temperature (550 to 750 ° C.) by the time required for reheating after cooling.

Heating temperature: 1000-1300 ° C
If the heating temperature is less than 1000 ° C., the solid solution of the carbide is insufficient and the required strength cannot be obtained, and if it exceeds 1300 ° C., the base material toughness deteriorates, so the temperature is set to 1000 to 1300 ° C.

Rolling end temperature: Ar3 temperature or higher If the rolling end temperature is lower than Ar3 temperature, the subsequent ferrite transformation rate decreases, so that the concentration of C into untransformed austenite at the time of reheating becomes insufficient and MA is not generated. Therefore, the rolling end temperature is set to Ar3 temperature or higher.

Cooling conditions after hot rolling Immediately after the rolling, accelerated cooling at a cooling stop temperature of 500 to 650 ° C. is performed at a cooling rate of 5 ° C./s or more. When the cooling rate is less than 5 ° C./s, pearlite is generated at the time of cooling, so that strengthening by bainite cannot be obtained, so that sufficient strength cannot be obtained. Therefore, the cooling rate after the end of rolling is specified to be 5 ° C./s or more.

  Further, when the cooling start temperature is lower than the Ar3 temperature and ferrite is generated, the strength is lowered and MA is not generated. Therefore, the cooling start temperature is set to the Ar3 temperature or higher.

  In the present invention, the ferrite transformation can be completed without maintaining the temperature during the subsequent reheating by supercooling to the bainite transformation region by accelerated cooling.

  The cooling stop temperature is an important production condition in the present invention. In the present invention, C-concentrated untransformed austenite present after reheating is transformed into MA upon subsequent air cooling.

  That is, it is necessary to stop the cooling in a temperature range where untransformed austenite during the bainite transformation exists. If the cooling stop temperature is less than 500 ° C., the bainite transformation is completed, so MA is not generated during air cooling, and a low yield ratio cannot be achieved.

  If it exceeds 650 ° C., C is consumed in the pearlite that precipitates during cooling and MA is not generated, so the accelerated cooling stop temperature is specified to be 500 to 650 ° C. From a viewpoint of MA production | generation, Preferably it is 530-650 degreeC.

Reheating after stopping accelerated cooling Immediately after stopping accelerated cooling, reheating is performed to a temperature of 550 to 750 ° C. at a temperature rising rate of 0.5 ° C./s or more. This process is an important production condition in the present invention. Untransformed austenite in which C is concentrated during air cooling after reheating due to ferrite transformation from untransformed austenite at the time of reheating and the accompanying discharge of C to untransformed austenite. Transforms into MA.

  Therefore, it reheats from the temperature more than Bf point to the temperature range of 550-750 degreeC after accelerated cooling. If the rate of temperature rise is less than 0.5 ° C./s, it takes a long time to reach the target reheating temperature, so that the production efficiency deteriorates, and pearlite transformation occurs, so MA cannot be obtained, and a sufficiently low yield ratio. Can't get.

  If the reheating temperature is less than 550 ° C., ferrite transformation does not occur sufficiently and C is not sufficiently discharged into untransformed austenite, MA is not generated, and a low yield ratio cannot be achieved. If the temperature exceeds 750 ° C., sufficient strength cannot be obtained due to softening of bainite, so the temperature range of reheating is specified to be 550 to 750 ° C.

  In the present invention, after accelerated cooling, it is important to perform reheating from a temperature range where untransformed austenite exists, and when the reheating start temperature becomes less than the Bf point, bainite transformation is completed and untransformed austenite does not exist. The reheating start needs to be higher than the Bf point.

  In order to reliably concentrate C that undergoes ferrite transformation into untransformed austenite, it is desirable to raise the temperature by 50 ° C. or more from the reheating start temperature. There is no need to set the temperature holding time at the reheating temperature.

  If the production method of the present invention is used, even if it is cooled immediately after reheating, sufficient MA can be obtained, so a low yield ratio and a high uniform elongation can be achieved. However, the diffusion of C is further promoted and the MA volume fraction is increased. In order to secure the temperature, the temperature can be kept within 30 minutes.

  When the holding time exceeds 30 minutes, the bainite phase dislocation may be recovered and the strength may be lowered. The cooling rate after reheating is preferably basically air cooling.

  Under the manufacturing conditions described above, the mechanism of MA generation is as follows. At the end of accelerated cooling, the microstructure is bainite and untransformed austenite. Reheating above the Bf point causes ferrite transformation from untransformed austenite, but since ferrite has a small amount of C solid solution, C is untransformed austenite. Is discharged.

  Therefore, the amount of C in untransformed austenite increases with the progress of ferrite transformation during reheating. At this time, if Cu, Ni or the like, which is an austenite stabilizing element, is contained in a certain amount or more, untransformed austenite in which C is concentrated remains even at the end of reheating, and is transformed into MA by cooling after reheating. Finally, a three-phase structure of bainite, ferrite, and MA is obtained.

  FIG. 1 shows a scanning electron of a steel sheet of the present invention (0.05 mass% C-1.5 mass% Mn-0.1 mass% Ni-0.1 mass% Mo-0.01 mass% Ti) manufactured using the above manufacturing method. The photograph observed with the microscope (SEM) is shown. Insular martensite (MA) is uniformly formed in the mixed structure of ferrite (F) and bainite (B).

  In the present invention, any apparatus can be used as a steel sheet manufacturing apparatus. However, an example of a preferable facility is a hot rolling mill, an accelerated cooling apparatus, induction heating in the rolling line from upstream to downstream. A device and a hot leveler are sequentially arranged.

  By installing an induction heating device or other heat treatment device on the same line as a hot rolling mill that is a rolling facility and an accelerated cooling device that is a subsequent cooling facility, reheating treatment can be performed quickly after completion of rolling and cooling. Therefore, it can heat, without reducing the steel plate temperature after rolling cooling too much.

[Method of manufacturing welded steel pipe]
The welded steel pipe according to the present invention is formed by cold forming a steel plate manufactured under the above-described manufacturing conditions into a tubular shape, welding the butt portion to form a steel pipe, and then performing a coating process in a temperature range of 300 ° C. or lower. . The method for forming into a tubular shape is not particularly specified.

  When the heating temperature of the steel pipe during coating exceeds 300 ° C., the yield stress in the pipe length direction after coating increases due to strain aging, leading to an increase in yield ratio. Furthermore, since MA decomposes into cementite and ferrite and the hardness of the second phase is lowered, the yield ratio is increased, so that it is defined as 300 ° C. or lower.

  Steels (steel types A to J) having chemical components shown in Table 1 were made into slabs by a continuous casting method, and UOE steel pipes (No. 1 to 18) having a pipe thickness of 18, 26 mm and an outer diameter of 36 inches were produced.

After rolling the heated slab by hot rolling, immediately cool it using a water-cooled accelerated cooling facility, reheat it using an induction heating furnace or gas combustion furnace, produce a steel plate, and use the steel plate. A welded steel pipe was manufactured by the UOE process, and then the outer surface of the steel pipe was coated.
The outer surface coating is performed using an epoxy resin or the like for the purpose of preventing corrosion of the outer surface.

  The coating procedure is as follows. First, after the roughness of the outer surface is made constant by shot blasting or the like, the pipe is heated to a predetermined temperature, a primer is applied, and coating is performed with a resin. The primer is used to increase the adhesion between the coating agent and the pipe, and it is necessary to apply the primer uniformly to the pipe and to heat the pipe in order to increase the bonding ability. The induction furnace was installed on the same line as the accelerated cooling equipment.

  Table 2 shows the production conditions of each steel plate (No. 1 to 18). The heating temperature, rolling end temperature, cooling stop (end) temperature, reheating temperature, and other temperatures were the average temperature of the steel sheet. The average temperature was determined from the surface temperature of the slab or steel plate by parameters and calculations such as plate thickness and thermal conductivity.

  Moreover, a cooling rate is an average cooling rate which divided the temperature difference required for cooling to the cooling stop (end) temperature (200-600 degreeC) after completion | finish of hot rolling by the time required to perform the cooling. The reheating rate (temperature increase rate) is the average temperature increase rate divided by the time required to reheat the temperature difference required for reheating up to the reheating temperature (540 to 780 ° C.) after cooling. is there.

  The tensile properties of the steel pipe manufactured as described above were measured. The measurement results are also shown in Table 2. Tensile strength was evaluated by taking two full thickness tensile test pieces in the vertical direction of rolling, conducting a tensile test, and evaluating the average value. Tensile strength of 517 MPa or more (API 5L X60 or more) and 680 MPa or less were determined as strengths necessary for the present invention.

  Yield ratio and uniform elongation were evaluated by the average value of two tensile test specimens taken in the rolling direction. The yield ratio before coating was 80% or less, the yield ratio after coating was 85% or less, and the uniform elongation was 12% or more.

  For base metal toughness, three full-size Charpy V-notch specimens in the pipe circumferential direction were collected, Charpy test was performed, the absorbed energy at -10 ° C was measured, and the average value was obtained. The absorption energy at −10 ° C. was determined to be 200 J or more. The amount of retained austenite was quantified by X-ray diffraction, and 2% or more was considered good.

  The weld heat-affected zone (HAZ) toughness consists of three full-size Charpy V-notch specimens so that the ratio of the notch length from the center of the thickness of the seam weld zone to the weld metal: HAZ = 1: 1. The samples were collected and tested, the Charpy absorbed energy at −10 ° C. was measured, and the average value was obtained.

  In Table 2, all of No. 1 to 7 as examples of the present invention have chemical components and production methods within the scope of the present invention, high tensile strength of 517 MPa or more, yield ratio before coating of 80% or less, coating The yield ratio was 85% or less and the uniform elongation was 12% or more, and the toughness of the base metal and the weld heat affected zone was good.

  The structure of the steel sheet is a three-phase structure of ferrite, bainite, and island martensite, the volume fraction of island martensite is in the range of 3 to 15%, and the volume fraction of retained austenite is 2% or more. It was. In addition, the volume fraction of island martensite was calculated | required by image processing from the microstructure observed with the scanning electron microscope (SEM).

  In Nos. 8 to 13, the chemical components are within the scope of the present invention, but the production method is outside the scope of the present invention, so any of the structure, strength, yield ratio, and uniform elongation was insufficient. . Nos. 14 to 18 had chemical components outside the scope of the present invention, so that sufficient strength could not be obtained, yield ratio was high, uniform elongation was low, or toughness was inferior.

The SEM photograph which shows an example of the metal structure of the preform | base_material of this invention steel pipe.

Claims (3)

In mass%, C: 0.03-0.1%, Si: 0.01-0.5%, Mn: 1.2-2.0%, Mo: 0.05-0.4%, Cu + Ni: 0.1% or more, Ti: 0.005 to 0.04%, Nb: 0.005 to 0.07%, Al: 0.08% or less, N: 0.005% or less, Ti / N Is 3 or more, satisfies the formula (1), and the remainder is made of Fe and inevitable impurities, heated to a temperature of 1000 to 1300 ° C., and hot-rolled at a rolling end temperature of Ar ₃ or higher Accelerated cooling to 500 to 650 ° C. at a cooling rate of 5 ° C./s or more, and then immediately reheating to 550 to 750 ° C. at a heating rate of 0.5 ° C./s or more. Is a three-phase structure of island martensite, and the volume fraction of island martensite is 3-15% A steel sheet having a volume fraction of retained austenite of 2% or more and a uniform elongation in the longitudinal direction of 12% or more is formed into a tubular shape in the cold, the butt portion is welded to form a steel pipe, and the heating temperature is The coating process is performed at 300 ° C. or lower,
A method for producing a low yield ratio high strength high toughness steel pipe of X70 grade or less .
Ceq1 ≦ 0.40 (1)
However, Ceq1 = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 In addition to the component composition, by mass%, V: 0.005 to 0.1%, Cr: 0.5% or less, Ca: 0.0005 to 0.003%, B: 0.005% or less The method for producing a low yield ratio, high strength, high toughness steel pipe of X70 grade or less according to claim 1, comprising two or more kinds. The component composition according to claim 1, a three-phase structure of ferrite, bainite, and island martensite, the volume fraction of island martensite is 3 to 15%, and the volume fraction of retained austenite is 2 % X70 grade or lower low yield ratio high strength high toughness steel pipe, characterized in that a steel sheet having a metal structure of at least% is used as a base material and a coating treatment is performed at a heating temperature of 300 ° C. or lower.