JP2010084171A - High toughness welded steel pipe having high crushing strength and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a welded steel pipe having excellent crushing strength, which can be produced in high productivity at a low cost without deteriorating its circularity and deformability, and has excellent brittle crack propagation arresting performance, and to provide a method for producing the same. <P>SOLUTION: The steel pipe is obtained by subjecting a thick steel plate having a composition comprising specified amounts of C, Si, Mn, P, S, Al, Nb, Ti and N, and further comprising one or more kinds selected from Cu, Ni, Cr, Mo and V to bending into a pipe shape and welding the butted parts so as to be a steel pipe, and thereafter expending the steel pipe, wherein the total of the volume fractions of a ferrite phase and a bainite phase in the steel pipe is ≥80%, the average hardness difference of the two phases is 50 to 150, the volume fraction of an insular martensite phase included in the balance is ≤2%, and the gathering degree of the (100) plane in the rolling face at the central position of the pipe thickness obtained by X-ray diffraction is ≥1.5. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a steel pipe for a high strength and high toughness line pipe used for transportation of oil and natural gas and a method for producing the same, and in particular, a high toughness with excellent crushing strength produced by cold forming and welding a thick steel plate. Welded steel pipe and manufacturing method thereof.

  Line pipes used in high external pressure environments such as submarine pipelines have a risk of being crushed when subjected to high compressive stress due to external pressure. Therefore, the line pipe used in the submarine pipeline needs to have a sufficient compressive strength in a piped state. However, in the case of a steel pipe that is made by cold-working a thick steel plate like a UOE steel pipe and then expanding it, the pipe is subjected to a large tensile load in the final process. Therefore, the compressive yield stress of these steel pipes is lower than the tensile yield stress of the steel pipes due to the back stress generated during the tensile load.

  Therefore, in order to ensure the compressive strength of the steel pipe, it is necessary to set the design strength of the thick steel plate high. When strength is supplemented with strengthening elements, there is concern about increased alloy costs and toughness deterioration of the base metal and weld heat-affected zone, so the development of a thick steel plate or pipe making method with less reduction in compressive yield stress after pipe making Is required. In addition, the line pipe preform laid on the seabed is required to have excellent brittle crack propagation stopping performance at low temperatures. For this reason, development of a line pipe that achieves both crushing strength and brittle crack propagation stopping performance is required.

In response to such a request, in Patent Document 1 and Patent Document 2, the O-press compression ratio and pipe expansion ratio during pipe making are used as parameters, and the compression ratio / pipe expansion ratio is reduced to the optimum range, so that A method for suppressing a decrease in compressive yield stress of a steel pipe is disclosed. For example, Patent Document 2 discloses a technique in which the ratio of the upset rate (that is, the compression rate) α during O molding to the tube expansion rate β during tube expansion is α / β ≧ 0.35. Patent Document 2 also discloses a method for suppressing a reduction in the compressive yield stress of a steel pipe after pipe making by increasing the pipe expansion rate extremely. Patent Document 3 discloses a method for improving the crushing strength of a steel pipe by external pressure by optimizing the order and degree of contraction and expansion. Patent Documents 4 to 7 disclose a method for suppressing a decrease in compressive yield stress of a steel pipe by reducing a back stress applied to the steel pipe in a pipe making process by low-temperature strain aging by heat treatment or coating heating after pipe forming. Is disclosed.
JP 2002-102931 A JP 2003-340518 A Japanese Patent Laid-Open No. 9-1233 JP-A-9-3545 JP 2002-295736 A JP 2003-342639 A JP 2004-35925 A

    However, in order to set the pipe making conditions to the optimum compression ratio / pipe expansion ratio as shown in Patent Document 1 and Patent Document 2, it is necessary to make the O-press compression ratio much larger than usual. Increasing the compression rate of the O press requires an increase in the press capability of the O press machine, and increases the cost due to the introduction of new equipment and the repair of equipment.

  Furthermore, line pipes for submarine pipelines, where securing compressive strength is a problem, are often designed with a thick wall from the viewpoint of securing buckling resistance, which increases the compressibility of the O-press. . In addition, the pipe expansion ratio can be reduced to the optimum range, but the roundness of the steel pipe will be lowered, and this also requires severe roundness from the viewpoint of buckling resistance. There are limitations in manufacturing line pipes for submarine pipelines.

  In addition, as described in Patent Documents 2 and 3, increasing the tube expansion rate or performing tube contraction and tube expansion is an increase in surface hardness due to excessive work hardening or brittleness due to an increase in residual stress. There is concern about the degradation of crack propagation stopping performance.

  Moreover, as described in Patent Documents 4 to 7, performing the low-temperature strain aging treatment by optimizing the coating heating conditions after pipe forming has a tremendous effect in terms of suppressing the decrease in compressive yield stress. However, the tensile stress-strain curve of the steel pipe changes from the round house type after pipe making to the Luders type, which lowers the deformability of the steel pipe such as bending buckling performance.

  Furthermore, the coating heating conditions vary depending on the coating material used, and may not necessarily match the target coating heating conditions. When performing low-temperature strain aging treatment by heat treatment instead of coating heating, As a result, the productivity will be significantly impaired.

  As described above, in the conventional technology, producing a welded steel pipe excellent in crushing strength without causing degradation in weldability, degradation in deformation performance, degradation in productivity and degradation in brittle crack propagation stopping performance, It was difficult.

  Therefore, in this study, a welded steel pipe that can be manufactured at high productivity and low cost without reducing roundness and deformation performance, has excellent brittle crack propagation stopping performance, and a manufacturing method thereof. The purpose is to provide.

  In order to solve the above-mentioned problems, the inventors diligently studied the microstructure of the steel sheet and the manufacturing method for achieving the microstructure, particularly the manufacturing process of controlled rolling, accelerated cooling and subsequent reheating, and the following. Obtained knowledge.

  First, in order to obtain excellent brittle crack propagation stopping performance, ferrite is processed by rolling in a two-phase region, a (100) plane texture is developed on the rolled surface, and separation is performed during brittle crack propagation. It turns out that it is essential to generate. Further, the degree of integration of the (100) plane of the rolled surface at the tube thickness center position is set to 1.5 or more, and the hardness difference between the ferrite phase and the bainite phase, which is the main metal structure, is set to 50 or more. It was found that when the DWTT test (Drop Weight Tear Test), which is an evaluation method of the fracture propagation stopping performance, was performed, separation occurred on the fracture surface, and a high ductile fracture surface ratio could be secured even at a lower temperature.

    Next, in order to solve the above problems, the inventors of the present invention relate to a steel pipe structure that has a high compressive yield stress and a steel pipe material that improves the compressive yield stress, and a method for manufacturing the steel pipe, which are parameters that serve as indices for the crushing strength of the steel pipe. investigated.

  First, welded steel pipes were produced by producing steel pipe materials that were formed into various structural forms by the manufacturing processes of controlled rolling and accelerated cooling, which are common methods for producing steel pipe materials, under the same conditions.

  Next, full thickness tensile specimens in the L direction and compression specimens in the L direction were taken from the welded steel pipe, and the ratio of compressive yield stress to tensile yield stress (compressive yield stress / tensile yield stress) was compared. It was found that this ratio was larger in the bainite single-phase steel pipe than in the double-phase steel pipe represented by the microstructure steel.

  Furthermore, by reducing island-like martensite (hereinafter referred to as MA), which is the hard second phase in the microstructure of bainite single-phase steel pipe, local strain generated around the hard phase in the pipe making stage It was found that the gradient can be relaxed and the decrease in compressive yield stress due to the Bauschinger effect can be suppressed.

  However, as described above, in order to ensure excellent brittle crack propagation stopping performance, it is necessary to form a multiphase structure. Therefore, in the present invention, the yield stress due to the Bauschinger effect of the multiphase structure steel is used. The manufacturing method for reducing the degree of decrease was examined and the following findings were obtained.

That is,
(1) It has been found that the compressive yield stress / tensile yield stress can be improved by reducing the hardness difference between the ferrite phase and the bainite phase constituting the multiphase steel.
(2) It has been found that even in the case of a multiphase steel, the compressive yield stress / tensile yield stress can be improved by reducing the hard second phase such as MA.

  With regard to (1), it is possible to obtain a desired structure by increasing the stop temperature of accelerated cooling. However, with respect to (2), a part of untransformed austenite remaining at the time of cooling stop is M- Since it transformed to A, a sufficient effect was not obtained. However, the inventors reduced the cooling stop temperature to a lower temperature, and rapidly reheated immediately after stopping the cooling, thereby tempering the bainite phase and decomposing the MA, thereby yield stress due to the Bausinger effect. It was found that there was less decrease in. In addition, rapid heating after cooling is stopped, while tempering of the bainite phase and decomposition of MA are suppressed while suppressing a decrease in the degree of texture accumulation due to tempering of the ferrite phase, rather than reheating by furnace heating after air cooling. I found out that I can do it.

The present invention is a further study of the above findings,
1st invention is the mass%, C: 0.03-0.08%, Si: 0.01-0.50%, Mn: 1.0-2.0%, P: 0.015% or less S: 0.005% or less, Al: 0.08% or less, Nb: 0.005-0.060%, Ti: 0.005-0.040%, N: 0.001-0.010% Further, Cu: 0.1 to 0.6%, Ni: 0.1 to 1.2%, Cr: 0.05 to 0.40%, Mo: 0.05 to 0.40%, V : One or more selected from 0.005 to 0.070%, and a thick steel plate composed of the remaining Fe and inevitable impurities is bent into a tubular shape, and the butt portion is welded to form a steel pipe Thereafter, the steel pipe is further expanded, and the metal structure of the steel pipe is a multiphase structure mainly composed of a ferrite phase and a bainite phase, The sum of the volume fractions of the ferrite phase and the bainite phase is 80% or more, the volume fraction of the island martensite phase contained in the balance is 2% or less, and the average hardness difference between the ferrite phase and the bainite phase Is high toughness welding excellent in crushing strength, characterized in that the integration degree of the (100) plane of the rolled surface at the tube thickness center position obtained by X-ray diffraction is 1.5 or more It is a steel pipe.

  The second invention further includes, in mass%, Ca: 0.0005 to 0.0100%, Mg: 0.0005 to 0.0100%, REM: 0.0005 to 0.0200%, Zr: 0.0005. The high toughness welded steel pipe having excellent crushing strength according to the first invention, characterized by containing one or more selected from -0.0300%.

  The third invention, after heating the steel having the component composition according to any one of the first or second invention to 1000 to 1200 ° C, the cumulative rolling reduction in the temperature range of 900 ° C or less is 50% or more, Immediately after hot rolling at a temperature of 660 ° C. or higher at a cumulative rolling reduction in the two-phase temperature range of 10 to 50%, cooling is performed to 200 to 420 ° C. at a cooling rate of 5 to 50 ° C./s. Immediately after stopping, the thick steel plate reheated to a temperature range of 320 to 500 ° C. at a temperature higher than the cooling stop temperature by 30 ° C. or more at a temperature rising rate of 4 ° C./s or more is cooled to room temperature, and then bent into a tube And it is the manufacturing method of the high toughness welded steel pipe excellent in the crushing strength characterized by welding a butt | matching part and making it a steel pipe, and expanding further.

  A fourth invention is a method for producing a high toughness welded steel pipe having excellent crushing strength according to the third invention, wherein the pipe expansion is performed at a pipe expansion ratio of 0.5 to 1.25%.

  According to a fifth invention, in the third or fourth aspect, the compression ratio of the compression processing applied to the steel sheet when the steel sheet is bent into a tubular shape is 1/4 or more of the tube expansion ratio. Is a method for producing a high toughness welded steel pipe having excellent crushing strength.

According to the present invention, a high-temperature steel plate manufactured by cold forming and welding a thick steel plate suitable for transportation of oil and natural gas having excellent crushing strength and roundness, particularly for thick-walled high-strength line pipes used for submarine pipelines. It is possible to manufacture tough welded steel pipes and is extremely effective in industry.

The composition of the high toughness welded steel pipe excellent in crush strength according to the present invention, the microstructure, and the form of the texture of the rolled surface at the center of the pipe thickness will be described.
Component composition Reasons for limiting the component composition will be described below. In addition, the unit which shows a component composition shall be all mass%.

C: 0.03-0.08%
C is an element that enhances the hardenability and is important for securing the strength, but if it is less than 0.03%, sufficient strength cannot be secured. Moreover, when it adds exceeding 0.08%, the volume fraction of the martensite and cementite in a structure | tissue will be increased, and the Bausinger effect will be enlarged. Therefore, the C content is in the range of 0.03 to 0.08%.

Si: 0.01 to 0.50%
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 martensite volume fraction increases and weldability deteriorates. The range is 0.01 to 0.50%. More preferably, it is 0.01 to 0.20% of range.

Mn: 1.0-2.0%
Mn is an element effective for improving strength and toughness. However, if it is less than 1.0%, the effect is not sufficient, and if it exceeds 2.0%, the hardenability increases and the martensite volume fraction increases, the surface hardness increases. In order to cause an increase and weldability deterioration, the Mn content is set to a range of 1.0 to 2.0%.

P: 0.015% or less P is an impurity element, and it is desirable to reduce it as much as possible in order to degrade toughness. However, excessive P reduction causes an increase in cost, so the P content is 0.015% or less. To do.

S: 0.005% or less S is an impurity element, and it is desirable to reduce as much as possible in order to degrade toughness. However, excessive S reduction causes an increase in cost, so the S content is 0.005% or less. To do.

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 should be 0.08% or less. . Preferably, it is 0.01 to 0.05% of range.

Nb: 0.005 to 0.060%
Nb is an element that enhances the effect of controlled rolling and improves strength and toughness by refining the structure. However, if it is less than 0.005%, there is no effect, and if it exceeds 0.060%, the toughness of the weld heat-affected zone deteriorates.

Ti: 0.005-0.040%
Ti is an effective element for suppressing the austenite coarsening during heating due to the pinning effect of TiN and improving the toughness of the base metal and the weld heat affected zone. However, if it is less than 0.005%, there is no effect, and if it exceeds 0.040%, TiN becomes coarse and conversely causes deterioration of the weld heat affected zone toughness, so the Ti content is 0.005 to 0.040. % Range. Furthermore, when the Ti content is 0.005 to 0.02%, more excellent toughness is exhibited.

N: 0.001 to 0.010%
N is an element effective for suppressing the austenite coarsening during heating by the pinning effect of TiN and improving the toughness of the base metal and the weld heat affected zone. However, if the content is 0.001% or less, there is no effect. If the content exceeds 0.010%, the addition causes the deterioration of the toughness of the weld heat-affected zone due to the coarsening of TiN and the increase in solute N. , N content is 0.001 to 0.010%. Furthermore, excellent toughness is exhibited by setting N to 0.001 to 0.006% and Ti / N to 1 to 5, more preferably 2 to 4 as a ratio by mass.

  Furthermore, in order to improve the intensity | strength and toughness of a steel plate, it is necessary to contain the 1 type (s) or 2 or more types chosen from Cu, Ni, Cr, Mo, and V shown below.

Cu: 0.1 to 0.6%
Cu is an element effective for improving toughness and increasing strength. In order to obtain the effect, it is preferable to add 0.1% or more, but adding over 0.6% causes deterioration of weldability and an increase in martensite volume fraction, so when adding Cu The content is in the range of 0.1 to 0.6%.

Ni: 0.1-1.2%
Ni is an element effective for improving toughness and increasing strength. In order to obtain the effect, it is preferable to add 0.1% or more, but adding over 1.2% is disadvantageous in terms of cost, and the weld heat affected zone toughness deteriorates. When added, the content is in the range of 0.1 to 1.2%.

Cr: 0.05-0.40%
Cr, like Mn, is an element effective for obtaining sufficient strength even at low C. In order to obtain the effect, it is preferable to add 0.05% or more, but adding over 0.40% causes deterioration of weldability and an increase in martensite volume fraction. Has a content of 0.05 to 0.40%.

Mo: 0.05-0.40%
Mo is an element that improves hardenability and greatly contributes to an increase in strength. However, if less than 0.05%, the effect cannot be obtained, and addition exceeding 0.40% leads to an increase in martensite volume fraction and deterioration of weld heat-affected zone toughness. The content is in the range of 0.05 to 0.40%. More preferably, the content is 0.05 to 0.3%.

V: 0.005-0.070%
V is an element contributing to an increase in strength. However, if it is less than 0.005%, there is no effect, and if it exceeds 0.070%, the toughness of the weld heat-affected zone deteriorates. Therefore, when V is added, its content is 0.005 to 0.070%. .

  Furthermore, when improving the toughness of a steel plate defect generation | occurrence | production or a welding heat affected zone, you may contain 1 type (s) or 2 or more types chosen from Ca, Mg, REM, and Zr shown below.

Ca: 0.0005 to 0.0100%
Ca is an element effective for controlling the morphology of MnS and contributes to improvement of the base material toughness. In order to obtain the effect, it is preferable to add 0.0005% or more. However, if it exceeds 0.0100%, Ca oxysulfide is excessively generated and becomes coarse or clustered to form a base material. In order to deteriorate toughness, when Ca is added, its content is set in the range of 0.0005 to 0.0100%.

Mg: 0.0005 to 0.0100%
Mg is an element that contributes to improving the toughness of the base material by finely dispersing alumina clusters (Al ₂ O ₃ ) as an Al—Mg-based oxide. In order to obtain the effect, it is preferable to add 0.0005% or more. However, if the addition exceeds 0.0100%, the toughness of the base metal decreases due to an increase in oxide. The amount is in the range of 0.0005 to 0.0100%.

REM: 0.0005 to 0.0200%
REM (Rare Earth Metals), like Ca, is an element that is effective in controlling the morphology of MnS, and contributes to the improvement of base metal toughness. In order to obtain the effect, it is preferable to add 0.0005% or more. However, addition of 0.0200% or more causes excessive generation of REM oxysulfide and deteriorates the base material toughness. When adding, the content shall be 0.0005 to 0.0200% of range.

Zr: 0.0005 to 0.0300%
Zr, like Ca, is an element effective for controlling the morphology of MnS and contributes to the improvement of the base metal toughness. In order to obtain the effect, it is preferable to add 0.0005% or more. However, if it exceeds 0.0300%, Zr oxysulfide is excessively generated, the base material toughness is deteriorated, and TiN is further added. Therefore, when Zr is added, the content is made 0.0005 to 0.0300%.

The balance other than the above is Fe and inevitable impurities.
In addition, since the formation of the ferrite phase during hot rolling is suppressed by containing B, and the development of the texture by the processing of the ferrite phase becomes difficult, in the present invention, B is treated as an inevitable impurity, 0.0005% or less.

Microstructure In the present invention, the form and volume fraction of the metal structure are defined. The metal structure is mainly composed of a ferrite phase and a bainite phase. The ferrite phase is an indispensable structure for developing the texture by processing during rolling and improving the brittle crack propagation stopping performance. On the other hand, in order to ensure strength, it is necessary to introduce a hard phase such as a bainite phase or a martensite phase. However, in the martensite phase, the hardness difference from the ferrite phase cannot be brought into a desired range, and the bausinger Since the decrease in yield stress due to the effect cannot be sufficiently suppressed and excellent compressive strength cannot be obtained, the total volume fraction of the ferrite phase and the bainite phase is set to 80% or more. The balance is MA, martensite, pearlite, cementite, etc., but these are preferably as small as possible.

  In particular, in order to prevent the occurrence of back stress due to local strain gradient generated around the hard second phase and the parent phase, and to suppress the decrease in compressive yield stress due to the Bauschinger effect, The volume fraction is 2% or less.

  As the remaining structure, when accelerated cooling is started from the two-phase region, about several percent of pearlite is observed, and cementite is observed as a decomposition product of MA. Further, as will be described later, martensite is mixed when the cooling rate after the end of rolling is excessive.

Hardness difference between microstructures: 50-150
In the present invention, the hardness difference between the ferrite phase and the bainite phase defined as the main metal structure is defined. The local strain gradient in the metal structure at the time of loading is not only between the hard second phase and the parent phase described above, but also between the soft phase and the hard phase of the multiphase steel that is the parent phase. It is generated and promotes a decrease in yield stress due to the Bauschinger effect, so that the compressive stress of the steel pipe is reduced.

  Therefore, by reducing the average hardness difference between the ferrite phase, which is the soft phase of the parent phase, and the bainite phase, which is the hard phase of the parent phase, to 150 or less, it is possible to suppress a decrease in yield stress due to the Bauschinger effect. On the other hand, the lower limit is set to 50 in order to ensure strength and facilitate the generation of separation.

  In addition, about the measuring method of average hardness, it is preferable to measure 20 or more arbitrary points with a micro Vickers tester with a load of 0.98 N or less, and take the average value.

Texture at the center of the plate thickness In order to obtain excellent brittle crack propagation stopping performance, it is necessary to generate a so-called separation of crack growth when a brittle crack is generated. It is known that separation tends to occur when a (100) plane and a (111) plane are generally developed on a rolled surface. In the present invention, the DWTT test was adopted as a method for evaluating the brittle crack propagation stopping performance, and various steel sheets were subjected to the DWTT test.

  As a result, it was found that there is a good correlation between the ductile fracture surface ratio of the DWTT test and the degree of integration of the (100) plane of the rolled surface at the tube thickness center position obtained by X-ray diffraction, and the ferrite phase and the bainite phase When the hardness difference is 50 or more, it was found that by setting the degree of integration of the (100) plane to 1.5 or more, excellent brittle crack propagation stopping performance can be obtained with the welded steel pipe in the range of the present invention. The lower limit of the degree of integration of the (100) plane was 1.5.

  Here, the degree of integration of the (100) plane is a plate plane taken in parallel with the steel sheet rolling plane from the tube thickness center position with respect to the X-ray diffraction intensity from the (200) plane in a random standard sample without a texture. The ratio of the X-ray diffraction intensity from the (200) plane.

Next, the suitable manufacturing method of the thick steel plate used as the raw material of the steel pipe concerning this invention is demonstrated. In the manufacturing method, slab heating temperature, hot rolling, accelerated cooling, and reheating conditions after accelerated cooling are defined.
The temperature defined by the heating temperature, rolling end temperature, cooling stop temperature, and reheating temperature is the average temperature of the entire steel sheet. 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 between the cooling start temperature and the cooling stop temperature (400 to 600 ° C.) by the time required for the cooling.

Slab heating temperature: 1000-1200 ° C
The minimum temperature is 1000 ° C. in order to obtain a minimum amount of Nb solid solution while austenizing the slab. On the other hand, when the slab is heated to a temperature exceeding 1200 ° C., the pinning effect due to TiN is weakened, austenite grains grow significantly, and the base material toughness deteriorates. For this reason, slab heating temperature shall be 1000-1200 degreeC.

Cumulative rolling reduction in a temperature range of 900 ° C. or less: 50% or more In the steel according to the present invention, 900 ° C. or less is an austenite non-recrystallization temperature region due to Nb addition. Austenite grains are extended by rolling under a large cumulative pressure below this temperature range, and in particular, the base material toughness is improved by making fine grains in the plate thickness direction. When the cumulative rolling reduction is less than 50%, the fine reduction is not sufficient and the toughness deteriorates, so the cumulative rolling reduction in the temperature range of 900 ° C. or lower is set to 50% or more.

Cumulative rolling reduction in two-phase temperature range: 10-50%
Austenite refined by austenite non-recrystallization zone rolling is further refined by performing hot rolling in the ferrite-austenite two-phase temperature range of Ar _{3 to} Ar ₁ .

  In addition, by applying processing to ferrite, strengthening by ferrite strengthening and brittle crack propagation stopping performance evaluation test such as DWTT, realize the texture form necessary to generate separation on the fracture surface of the test piece, It is possible to achieve excellent brittle crack propagation stopping performance.

  If the cumulative reduction in the two-phase temperature range is less than 10%, the texture development is small and the occurrence of separation is not sufficient, and the improvement of brittle crack propagation stopping characteristics cannot be obtained. On the other hand, if the cumulative rolling reduction exceeds 50%, the ferrite becomes brittle due to excessive processing to ferrite, and the base metal toughness deteriorates. For this reason, the cumulative rolling reduction in the two-phase temperature range is set to a range of 10 to 50%.

Rolling end temperature: 660 ° C. or higher When the rolling end temperature is lower than 660 ° C., the ferrite transformation proceeds and the effect of accelerated cooling is reduced, and the base metal toughness is deteriorated due to the coarsening of the ferrite. Is 660 ° C. or higher.

Cooling rate: 5-50 ° C./s
Since ferrite produced after rolling is not processed, it is harmful from the viewpoint of securing strength and toughness. Therefore, immediately after the rolling is completed, accelerated cooling is performed at a cooling rate of 5 ° C./s or more, and untransformed austenite is transformed into a bainite structure to prevent the generation of ferrite, and the strength is improved without impairing the base material toughness. On the other hand, when the cooling stop temperature is lowered as in the present invention, if the cooling rate is excessive, the martensite structure is mixed in the bainite structure. In the case of a cooling rate exceeding 50 ° C./s, the tendency is remarkable and a desired tissue form cannot be obtained. Therefore, the upper limit is set to 50 ° C./s.

Cooling stop temperature: 200-420 ° C
In order to set the tensile strength after reheating to 600 MPa or more, the cooling stop temperature is set to 420 ° C. or lower, and the portion that was untransformed austenite before the accelerated cooling of the steel sheet is made a bainite structure. If the cooling stop temperature exceeds 420 ° C, the transformation temperature is high and the steel sheet cannot be sufficiently strengthened, so the upper limit is set to 420 ° C. Also, if the cooling stop temperature is below 200 ° C., it is unavoidable that martensite is mixed, and because the strain in the plate of the steel pipe material is large, it is necessary to give excessive press compression or expansion for correction at the time of steel pipe production. The lower limit is 200 ° C. As a cooling method, any cooling equipment can be used depending on the manufacturing process, and for example, a water-cooled accelerated cooling equipment can be used.

Reheating treatment: 320 to 500 ° C
In the present invention, the reheating treatment is an important heat treatment, tempering the bainite structure of the steel sheet having accelerated cooling and having a multiphase structure to reduce the hardness difference between the ferrite phase and the bainite phase, and decompose MA. To do.

  In order to reduce the hardness difference between the ferrite phase and the bainite phase and achieve the MA decomposition, it is necessary to reheat to 320 ° C. or higher. Further, since the effect of reheating cannot be obtained unless the temperature is raised to 30 ° C. or higher than the cooling stop temperature, the lower limit is set to 30 ° C. or higher and 320 ° C. or higher than the cooling stop temperature.

  On the other hand, when heated to 500 ° C. or higher, not only the tempering effect is remarkable and the tensile strength is remarkably lowered, but also the hardness difference between the ferrite phase and the bainite phase is too small, and the accumulation degree of the texture is lowered. Since the phenomenon that the generation amount of separation decreases due to superposition occurs and the brittle crack propagation stopping performance decreases, the upper limit is set to 500 ° C.

Heating rate during reheating: 4 ° C / s or more Rapid heating after stopping cooling suppresses a decrease in texture accumulation due to tempering of the ferrite phase, rather than reheating by furnace heating after air cooling. However, the bainite phase can be tempered and the MA can be decomposed, which is advantageous from the viewpoint of productivity. Immediately after the cooling is stopped, it is rapidly heated to 4 ° C./s or more, preferably 6 ° C./s or more. It shall be reheated at a rate of temperature increase. The cooling process after reheating is not particularly defined, but air cooling is preferable because it can prevent the regeneration of MA.

  As equipment for performing reheating after accelerated cooling, a heating device is installed on the downstream side of the cooling equipment. As the heating device, it is preferable to use an induction heating device that can easily generate a temperature difference between the surface of the steel plate and the central portion of the plate thickness.

As equipment for performing the above-described manufacturing method, for example, a hot rolling mill, a cooling device, an induction heating device, and a hot leveler are sequentially arranged from the upstream side to the downstream side of the rolling line.
By installing an induction heating device or other heat treatment device on the same line as the hot rolling mill that is the rolling equipment and the cooling device arranged on the outlet side, the reheating treatment can be performed quickly after the completion of rolling and accelerated cooling. Since it can be performed, it is possible to heat the steel sheet after accelerated cooling without excessively reducing the steel sheet temperature.
Then, the suitable manufacturing method of the steel pipe concerning this invention is demonstrated. In the present invention, a steel pipe production method suitable for the present invention is based on a conventional method, but defines the pipe expansion ratio and the compression ratio of the compression process applied during cold bending.

Tube expansion rate Generally, a thick high-strength UOE steel pipe is piped at a tube expansion rate of about 0.9 to 1.2%. On the other hand, in order to secure the compressive strength in the circumferential direction of the steel pipe, it is effective to reduce the pipe expansion rate, and the UOE steel pipe that needs to secure the compressive strength is the lower limit of the normal range or smaller than that. The pipe is formed in a range of the expansion ratio (for example, 0.6 to 1.0%).

  In the present invention, the lower limit is set to 0.5% from the viewpoint of securing roundness, but the compression strength can be improved by the manufacturing method of the base material, so that the tube expansion rate ( For example, it can be manufactured at 1.1% or more. By using the steel sheet of the present invention, desired compression strength characteristics can be obtained with a tube expansion rate of up to 1.25%.

Compression rate of compression applied during cold bending Generally, thick high-strength UOE steel pipes are compressed in the range of about 0.3 to 0.5% (compression ratio / expansion ratio is 1/3 or more). Pipe making is performed by cold bending to which a compression process is applied. In order to ensure the compressive strength in the circumferential direction of the steel pipe, it is effective to increase the compression rate of the compression process applied to the steel sheet during cold bending.

  In the present invention, since the compressive strength of the base material, that is, the thick steel plate, which is a steel pipe material, is improved, it is possible to manufacture at a lower compressibility than the conventional method, and the compressibility is 1/4 or more of the expansion ratio. If so, desired compression strength characteristics can be obtained.

  Steel of the chemical composition shown in Table 1 (steel types A to H) is made into a slab by a continuous casting method, and the heated slab is rolled by hot rolling, and then immediately accelerated and cooled using a water-cooled cooling facility, and induction heating is performed. Reheating was performed using an apparatus to produce thick steel plates (No. 1 to 19) having a thickness of 8 mm and 26 mm. The induction heating device was installed on the same line as the cooling facility. For comparison, reheating was performed using a general heat treatment furnace (atmosphere furnace) instead of an induction heating apparatus.

  Table 2 shows the production conditions of each steel plate (No. 1 to 19).

  The heating temperature, rolling end temperature, cooling start and stop temperature, and reheating temperature were the average temperature of the entire steel sheet. The average temperature was obtained from the surface temperature of the slab or steel plate by calculation from parameters such as plate thickness and thermal conductivity.

  The accelerated cooling rate was an average cooling rate obtained by dividing the temperature difference between the accelerated cooling start temperature and the accelerated cooling stop temperature by the time required for the cooling.

  The fraction of the microstructure of the steel pipe is obtained by averaging the total area fraction of the ferrite phase and the bainite phase from image analysis of 10 optical micrographs observed at 400 times the center of the tube thickness. Assuming that the tissues were uniformly distributed, the area fraction value was considered equal to the volume fraction value.

  Similarly, the MA volume fraction of the steel pipe is obtained by averaging the area fraction from image analysis of five SEM (scanning electron microscope) photographs observed at 2000 times the structure at the center of the pipe thickness. Assuming that the second phase is uniformly distributed, the area fraction value is considered to be equal to the volume fraction value.

  The hardness difference between the ferrite phase and the bainite phase is obtained by measuring 40 points or more of each phase at a thickness of 1/4 t with a micro Vickers tester with a load of 0.98 N, and calculating the difference in the average value of the hardness of each phase. Obtained.

  The degree of integration of the (100) plane of the rolled surface at the tube thickness center position was collected parallel to the rolled surface from the tube thickness center position with respect to the X-ray diffraction intensity from the (200) plane in a random standard sample without texture. The ratio of the X-ray diffraction intensity from the (200) plane on the plate surface was used.

  Regarding the tensile yield stress in the circumferential direction of the steel pipe, a full-thickness specimen was prepared from a full-thickness sample taken from a tubular body of 90 ° or 270 ° in the circumferential direction from the seam weld and smoothed by a press. Two tensile yield stresses were measured under each condition and evaluated by an average value.

  Compressive yield stress in the circumferential direction of the steel pipe is 1 mm or more from the inner surface of the pipe body at 90 ° or 270 ° in the circumferential direction from the seam welded portion, and a round bar test piece having a diameter of 20 mm and a length of 60 mm in accordance with ASTM E9. The compressive yield stress was evaluated based on an average value of 0.2% proof stress measured two by two under each condition.

  The brittle crack propagation stopping property was evaluated by the DWTT test. The ductile fracture surface ratio of the DWTT test was determined by measuring two DWTT specimens (total thickness of steel pipes with a pipe thickness of 8 mm) sampled from the pipe thickness of 1 / 2t at 19 mm at -47 ° C. The average of the rate was obtained. The ductile fracture surface ratio was set to 85% or more as a necessary value in the present invention.

  Table 3 shows the test results obtained.

  Steel pipe No. Since 1, 2, 12, 13, and 14 all satisfy the component range, structure morphology range, and production condition range of the present invention, desired tensile strength characteristics, compressive yield stress characteristics, and DWTT characteristics are obtained. On the other hand, other steel pipes are outside the scope of the present invention, and therefore do not satisfy any of these characteristics.

  Steel pipe No. 3 and 4 have a ratio of compressive yield stress / tensile yield stress that is small because the ratio of tube expansion or the ratio of compressibility / expansion ratio is outside the scope of the present invention.

  Steel pipe No. In No. 5, since the heating temperature is high, the coarsening of the structure before rolling is inherited even after rolling, the toughness is deteriorated, and the DWTT characteristics are not satisfied. Steel pipe No. No. 6 has a high rolling end temperature and has a bainite single-phase structure, so that the texture is not developed and the DWTT characteristics are deteriorated. Steel pipe No. In No. 7, since rolling in the two-phase region is not performed, the strength is insufficient due to the fact that the ferrite is not processed, and the DWTT characteristic is deteriorated due to the fact that the texture is not developed.

  Steel pipe No. No. 8, because the cooling rate is too high, the part that was untransformed austenite before accelerated cooling was transformed into a mixed structure of bainite phase and martensite phase, and the ferrite phase and this mixed structure (bainite phase + martensite phase) And the hardness difference is too large, and the compressive yield stress / tensile yield stress is small. Steel pipe No. In No. 9, since the cooling stop temperature and the reheating temperature are too high, a decrease in strength and a deterioration in DWTT characteristics due to coarsening of cementite are observed.

  Steel pipe No. No. 10 shows no reduction in compressive / stress yield stress due to a large hardness difference between the ferrite phase and the bainite phase and a large MA volume fraction because no reheating was performed after cooling was stopped. . Steel pipe No. No. 11 has a high cooling stop temperature and no reheating, and therefore a decrease in compressive yield stress / tensile yield stress due to a large MA volume fraction is observed.

  Steel pipe No. In Nos. 15 and 16, since C, Mn, and Nb are larger than the claims of the present invention, the MA volume fraction is large, and the compressive yield stress / tensile yield stress is reduced. Steel plate No. In No. 17, since the Nb content is larger than the claimed range of the present invention, the DWTT characteristic is lowered due to toughness deterioration accompanying precipitation strengthening. Steel pipe No. No. 18, since Ti which is an essential additive element of the present invention is not added, the structure at the time of heating becomes coarse, and it is inherited by the structure after rolling, so that the toughness is deteriorated and the DWTT characteristic is lowered. Yes. Steel plate No. No. 19, after performing accelerated cooling under the conditions of the present invention, air cooling to room temperature, and then reheating by furnace heating, bainite is excessively tempered, and tensile strength and DWTT characteristics are degraded. .

  According to the present invention, a high-temperature steel plate manufactured by cold forming and welding a thick steel plate suitable for transportation of oil and natural gas having excellent crushing strength and roundness, particularly for thick-walled high-strength line pipes used for submarine pipelines. This makes it possible to manufacture tough welded steel pipes and is extremely effective in the industry.

Claims (5)

  In mass%, C: 0.03 to 0.08%, Si: 0.01 to 0.50%, Mn: 1.0 to 2.0%, P: 0.015% or less, S: 0.005 %: Al: 0.08% or less, Nb: 0.005 to 0.060%, Ti: 0.005 to 0.040%, N: 0.001 to 0.010%, and Cu : 0.1-0.6%, Ni: 0.1-1.2%, Cr: 0.05-0.40%, Mo: 0.05-0.40%, V: 0.005-0 A steel plate containing one or more selected from 0.070% and the balance Fe and unavoidable impurities is bent into a tube, the butt portion is welded to form a steel pipe, and further expanded The steel pipe is a multiphase structure in which the metal structure of the steel pipe is mainly composed of a ferrite phase and a bainite phase, and the ferrite phase and the The total volume fraction of the innite phase is 80% or more, the volume fraction of the island-like martensite phase contained in the balance is 2% or less, and the average hardness difference between the ferrite phase and the bainite phase is 50 or more and 150 A high toughness welded steel pipe having excellent crushing strength, characterized in that the degree of integration of the (100) plane of the rolled surface at the tube thickness center position obtained by X-ray diffraction is 1.5 or more.   Furthermore, by mass%, Ca: 0.0005-0.0100%, Mg: 0.0005-0.0100%, REM: 0.0005-0.0200%, Zr: 0.0005-0.0300% 2. The high toughness welded steel pipe having excellent crushing strength according to claim 1, comprising one or more selected from among them.   The steel having the component composition according to claim 1 or 2 is heated to 1000 to 1200 ° C, and the cumulative reduction rate in a temperature range of 900 ° C or less is 50% or more, and the cumulative reduction in a two-phase temperature range. After finishing the hot rolling at a temperature of 660 ° C. or higher at a rate of 10 to 50%, immediately cool down to 200 to 420 ° C. at a cooling rate of 5 to 50 ° C./s. After cooling the thick steel plate reheated to a temperature range of 320 ° C to 500 ° C at a temperature higher than the cooling stop temperature at a temperature increase rate of s or more by 30 ° C or more, it is bent into a tube, and the butt portion is welded. A method for producing a high toughness welded steel pipe having excellent crushing strength, wherein the pipe is further expanded after being formed into a steel pipe.   4. The method for producing a high toughness welded steel pipe having excellent crushing strength according to claim 3, wherein the pipe expansion is performed at a pipe expansion ratio of 0.5 to 1.25%.   5. The crushing strength according to claim 3, wherein when the steel plate is bent into a tubular shape, the compression ratio of compression processing applied to the steel plate is ¼ or more of the tube expansion ratio. An excellent high toughness welded steel pipe manufacturing method.