JP2005146407A - Ultrahigh strength steel sheet and ultrahigh strength steel tube having excellent high speed ductile fracture property, and their production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an ultrahigh strength steel sheet and an ultrahigh strength steel tube having high speed ductile fracture properties and having a tensile strength of ≥800 MPa, and their production methods by establishing a simple test method capable of properly evaluating high speed ductile fracture properties, and based on the knowledge obtained thereby. <P>SOLUTION: The ultrahigh strength steel sheet having high speed ductile fracture properties has a microstructure composed of ferrite of 1 to 5% or >5 to 60% in an area ratio, and the balance bainite+martensite, and in which the accumulation degree of (100) in the cross-section rotated by 45° from the rolling face with the rolling direction as the axis is ≤3. The steel tube uses the steel sheet as a base metal. The average grain size of the ferrite is desirably controlled to ≤5 μm. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an ultra-high-strength line pipe excellent in high-speed ductile fracture characteristics having a tensile strength (TS) of 800 MPa or more, which is suitable for a natural gas / crude oil transportation line pipe or the like.

  In recent years, in pipelines for crude oil and natural gas, it has been required to reduce the outer diameter and weight of the line pipe for the purpose of increasing the internal pressure for the purpose of improving transportation efficiency and improving the efficiency of local construction. Development of high-strength steel pipes exceeding 760 MPa (for example, Patent Documents 1 and 2) is in progress.

  Also, in pipelines, ductile cracks that occur in the base material of steel pipes can propagate over a long distance ranging from 100 m to several km at a high speed of 100 m / s or more in the pipe axis direction, requiring arrest resistance. Is done. The arrest resistance is a property that stops the propagation of cracks, a property that a brittle crack propagates and stops the base material, that is, a brittle fracture resistance property, and a property that a ductile crack propagates and stops the base material, that is, a high speed. Classified as ductile fracture characteristics.

Brittle fracture characteristics, drop weight destructive test (D rop W eight T ear T est, referred DWTT test) performed, ductile fracture rate is evaluated at a temperature equal to or higher than 85% (referred DWTT transition temperature). Brittle cracks often occur from welds, and steel pipes with excellent brittle fracture resistance evaluated by performing a DWTT test by introducing a weld bead at the center of the specimen and introducing the brittle cracks have been proposed. (For example, patent document 3).

  On the other hand, the full crack burst test for determining whether or not the ductile cracks generated by explosion after the explosive is mounted on the surface of the steel pipe is optimal for the evaluation of the high-speed ductile fracture characteristics. However, since the cost required for the full crack burst test is very high, conventionally, the Charpy absorbed energy or the absorbed energy required by the DWTT test (referred to as DWTT absorbed energy) that is relatively well matched with the result of the full crack burst test. It was evaluated.

  However, a good correlation was observed between the full crack burst test and Charpy absorbed energy and DWTT absorbed energy, which is a steel material having a strength of about X65. For ultra-high strength steel exceeding X100, evaluation of high-speed ductile fracture characteristics The Charpy impact test and the DWTT test were not suitable. Therefore, a test method that can easily evaluate high-speed ductile fracture characteristics is required as an alternative to the full-crack burst test, which has a high test cost. Steel pipes that have excellent high-speed ductile fracture characteristics by utilizing the knowledge gained from these tests The development of was requested.

JP-A-9-41074 Japanese Patent Laid-Open No. 9-41080 JP 11-36042 A

  The present invention establishes a simple test method capable of appropriately evaluating high-speed ductile fracture characteristics, and based on the knowledge obtained thereby, has a tensile strength of 800 MPa or more (API standard X100 or more) excellent in high-speed ductile fracture characteristics. An ultra-high strength steel pipe and a method for producing the same are provided.

  The present inventor examines a simple test method capable of appropriately evaluating the high-speed ductile fracture characteristics of an ultra-high-strength steel pipe having a tensile strength of 800 MPa or more, and further obtains an ultra-high-strength steel pipe excellent in high-speed ductile fracture characteristics. To study the components and microstructure of the base material for the purpose, to obtain the knowledge that it is effective to optimize the microstructure and texture, to further study the production conditions, high-speed ductile ultra-high strength steel sheet and steel pipe, It came to invent those manufacturing methods. The gist of the present invention is as follows.

  (1) The microstructure has an area ratio of 1 to 5% of ferrite and the balance is bainite martensite, and the degree of integration of (100) in the section rotated 45 ° from the rolling surface with the rolling direction as the axis is 3 or less. An ultra-high-strength steel sheet with excellent high-speed ductile fracture characteristics.

  (2) The microstructure has an area ratio of more than 5% to 60% of ferrite and the balance of bainite / martensite, and the degree of integration of (100) in the cross section rotated 45 ° from the rolling surface with the rolling direction as the axis. An ultra-high-strength steel sheet excellent in high-speed ductile fracture characteristics, characterized by being 3 or less.

  (3) The ultra-high strength steel sheet having excellent high-speed ductile fracture characteristics according to (2), wherein the average grain size of ferrite is 5 μm or less.

(4) In mass%,
C: 0.03-0.10%,
Si: 0.01 to 0.6%,
Mn: 1.5 to 2.5%
P: 0.015% or less,
S: 0.003% or less,
Ni: 0.1 to 2.0%,
Mo: 0.15-0.60%,
Nb: 0.001 to 0.10%,
Ti: 0.005 to 0.030%,
Al: 0.06% or less, further,
B: 0.0001 to 0.005%,
N: 0.0001 to 0.006%,
V: 0.001 to 0.10%,
Cu: 0.01 to 1.0%,
Cr: 0.01 to 0.8%
Zr: 0.0001 to 0.005%,
Ta: 0.0001 to 0.005%,
Ca: 0.0001 to 0.01%,
REM: 0.0001 to 0.01%,
Mg: 0.0001 to 0.006%
The super high strength steel sheet having excellent high-speed ductile fracture characteristics according to (2) or (3), wherein one or more of the above are contained, and the balance is iron and inevitable impurities.

  (5) A method for producing a steel sheet according to any one of (1) to (4), wherein the steel comprising the component (4) is melted, continuously cast, and then reheated to 1100 to 1250 ° C. A method for producing an ultra-high-strength steel sheet having excellent high-speed ductile fracture characteristics.

  (6) The method for producing an ultra-high strength steel sheet having excellent high-speed ductile fracture characteristics according to (5), wherein hot rolling is performed in a recrystallization region at 900 ° C. or higher after reheating.

  (7) It is characterized by performing hot rolling with a cumulative reduction amount of 60% or more in an unrecrystallized region of 880 ° C. or less after reheating or subsequent recrystallization region rolling (5) Or the manufacturing method of the ultra high strength steel plate excellent in the high-speed ductile fracture characteristic as described in (6).

  (8) After reheating or subsequent recrystallization zone rolling, non-recrystallization zone rolling is started at 880 ° C. or less, and hot rolling is finished in the non-recrystallization zone at 600 to 800 ° C. It is excellent in the high-speed ductile fracture property according to any one of (5) to (7), characterized in that it is cooled in the range of 600 to 350 ° C at an average cold speed of the central portion at 0.5 to 10 ° C / s or less A method for manufacturing ultra-high strength steel sheets.

  (9) An ultra-high-strength steel pipe excellent in high-speed ductile fracture characteristics, wherein the base material is made of the ultra-high-strength steel sheet according to any one of (1) to (4).

(10) The component of the weld metal is mass%,
C: 0.04 to 0.14%,
Si: 0.05-0.4%
Mn: 1.2-2.2%,
P: 0.01% or less,
S: 0.010% or less,
Ni: 1.3-3.2%
Cr + Mo + V: 1.0 to 2.5%,
Ti: 0.003 to 0.050%,
Al: 0.02% or less,
B: 0.005% or less,
O: 0.01 to 0.03%
And the balance consists of iron and unavoidable impurities, and the super high strength steel pipe having excellent high-speed ductile fracture characteristics according to (9).

  (11) A method for manufacturing a steel pipe according to (9) or (10), wherein the ultra-high strength steel sheet according to any one of (1) to (4) is formed into a tubular shape in a UO process, and ends thereof A method for producing an ultra-high strength steel pipe excellent in high-speed ductile fracture characteristics, characterized in that submerged arc welding is performed using a welding wire and a firing-type flux or a melt-type flux, followed by pipe expansion.

  (12) When submerged arc welding is performed between end portions using a welding wire and a firing-type flux or a melt-type flux, the pipe is expanded after being welded with a heat input of 1.5 to 3.5 kJ / mm. The method for producing an ultra-high strength steel pipe having excellent high-speed ductile fracture characteristics according to (11), wherein:

(13) In mass%,
C: 0.01 to 0.12%,
Si: 0.3% or less,
Mn: 1.2-2.4%
Ni: 4.0 to 8.5%,
Cr + Mo + V: 3.0-5.0%,
Ti: 0.005 to 0.15%,
(11) or (12), characterized in that submerged arc welding is performed using a welding wire containing Al and 0.02% or less, the balance being iron and inevitable impurities. An excellent method for manufacturing ultra-high strength steel pipes.

  According to the present invention, it is possible to provide an ultra-high-strength steel pipe excellent in high-speed ductile fracture characteristics and having a tensile strength of 800 MPa or more (API standard X100 or more) and a manufacturing method thereof, and the industrial contribution is extremely remarkable.

  This inventor examined the method of evaluating the high-speed ductile fracture characteristic of a steel plate. The high-speed ductile fracture characteristic is a characteristic in which the propagating crack stops, and is considered to have a correlation with the crack propagation energy. Therefore, using various steel materials, load-displacement curves in the Charpy impact test were obtained, and crack generation energy and propagation energy were separated and evaluated. As a result, it was found that in the ultrahigh strength steel of 800 MPa or more, the energy of crack generation is much larger than the energy of propagation. In other words, the absorbed energy measured by the Charpy impact test is a test that evaluates the energy of crack initiation and propagation at the same time, and is not suitable for the evaluation of high-speed ductile fracture characteristics that have a large correlation with the energy of crack propagation. It was. The same applies to the DWTT test.

  Next, the present inventor examined a test method for properly evaluating the energy of crack propagation. Since the crack progressed in the 45 ° direction as the ratio of the plate width direction to the plate thickness direction increased, the DWTT test piece was used (in the full crack burst test, the crack progressed in the 45 ° direction). Further, as a method of reducing the energy of crack generation by sharpening the tip of the notch, a method of introducing a press notch that applies pressure to a wedge-shaped jig and further introducing a ductile crack by three-point bending was studied.

  As a result, when the press notch was introduced into the center of the test piece and the three-point bending was applied to the center part on the opposite side of the press notch and both ends on the press notch side, the maximum load was reached after reaching the maximum load. When the DWTT test (hereinafter referred to as the pre-crack DWTT test) is performed using a test piece that has been introduced with a ductile crack, it stops when the load falls within a range of up to 5%, and the obtained absorbed energy (pre-crack DWTT). It was found that the energy of propagation of cracks can be properly evaluated.

  Furthermore, the factor which improves the high-speed ductile fracture characteristic of a steel plate was examined by the precrack DWTT test. First, in order to clarify the relationship between the precrack DWTT energy and the microstructure of a 0.05C-2Mn-Ni-Cu-Cr-Mo-Ti-B steel sheet. The microstructure of the steel sheet was observed using an optical microscope, the area ratio of ferrite was measured, and the relationship between the precrack DWTT energy and the area ratio of ferrite was investigated. As a result, as shown in FIG. 1, it was found that the precrack DWTT energy has a correlation with the area ratio of ferrite in the microstructure.

  Furthermore, the inventor has investigated in detail the relationship between the precrack DWTT energy and the texture. As a result, as shown in FIG. 2, it was found that the precrack DWTT energy was remarkably reduced when the accumulation of (100) planes in the plate thickness section in the 45 ° direction from the rolling direction was 3 or more. The (100) plane accumulation is evaluated according to how many times the measured value of the intensity by X-ray diffraction is a standard sample having a random orientation. That is, the accumulation of the (100) plane of 3 means that the intensity of the (100) plane is three times that of the standard sample having a random orientation.

  The reason why the accumulation of the (100) plane of the cross section of the plane rotated 45 ° from the rolling plane with the rolling direction as the axis (the normal direction of this plane is called the D direction) and the precrack DWTT energy is recognized is that of ductile fracture It is considered that the crack propagates all at once when the crack propagates in the direction of 45 ° from the rolling direction and the (100) plane corresponding to the cleavage plane of steel accumulates in this direction. Moreover, as a result of observing the microstructure of the part where (100) planes were accumulated in the plate thickness cross section in the D direction, it was found that it was mainly bainite martensite. That is, austenite after hot rolling has (100) accumulated in the plate thickness section in the D direction. On the other hand, in the ferrite after hot rolling, (100) is accumulated on the plate thickness section in the rolling direction. Therefore, if the ferrite fraction after hot rolling is increased, the degree of accumulation of (100) in the thickness direction cross section in the D direction is reduced, and the propagation of ductile cracks can be suppressed.

Moreover, it examined in detail about the method of manufacturing such a steel plate. When producing a high strength steel having a tensile strength of 800 MPa or more, the microstructure of which is composed of two phases of ferrite and bainite / martensite, the hot rolling finish temperature is set to Ar ₃ point or less, and ferrite is produced when cooled with water. This inventor manufactured the 0.05C-2Mn-Ni-Cu-Cr-Mo-Ti-B type steel plate on various conditions, and investigated the relationship between the finishing temperature of hot rolling, and the area ratio of a ferrite. As a result, as shown in FIG. 3, it was found that there is a correlation between the hot rolling finishing temperature and the area ratio of ferrite.

  Hereinafter, the present invention will be described in detail.

  When the area ratio of the ferrite is less than 1%, bainite martensite having (100) planes accumulated in the thickness direction cross section in the D direction increases, and the high-speed ductile fracture resistance deteriorates. On the other hand, when the area ratio of ferrite exceeds 60%, the strength decreases. Considering the balance between strength and high-speed ductile fracture resistance, the area ratio of ferrite is preferably more than 5% to 20%. The balance is bainite martensite. However, even if the ferrite area ratio is in the range of 1 to 5%, the necessary strength and high-speed ductile fracture resistance can be ensured.

  In the microstructure, ferrite and bainite martensite can be distinguished by structural observation using an optical microscope or scanning electron microscope, and by analyzing the structure photograph taken with an optical microscope or scanning electron microscope, The area ratio of ferrite can be measured.

  The integration of (100) planes in the cross section of the plate thickness in the D direction is set to 3 or less because if it exceeds 3, the high-speed ductile fracture characteristics deteriorate. Note that the accumulation of the (100) plane in the thickness direction cross section in the D direction is based on the measured value of the intensity by X-ray diffraction of a standard sample having a random orientation, so the lower limit is 1.

  Further, if the average grain size of ferrite exceeds 5 μm, the cleaved fracture surface unit becomes large and the propagation energy may decrease, so the average grain size of ferrite is preferably 5 μm or less. The average particle diameter of ferrite can be measured by a cutting method using a structure photograph taken with an optical microscope or a scanning electron microscope.

  Next, the reason for limiting the chemical component of the base material will be described.

  C is an extremely effective element for improving the strength of steel, and preferably contains 0.03% or more of C. However, if the C content is more than 0.10%, the low temperature toughness of the base metal and the weld heat affected zone (referred to as HAZ) may be slightly deteriorated, which may impair on-site weldability. The content is preferably 0.10% or less, and more preferably 0.07% or less.

  Si is an element effective for deoxidation, and it is preferable to contain 0.01% or more. However, if it is added more than 0.6%, the low temperature toughness of HAZ is slightly deteriorated and the on-site weldability may be impaired. For this reason, the upper limit of the Si content is preferably 0.6% or less.

  Mn is an element effective for improving the balance between strength and low temperature toughness of steel, and the lower limit of the Mn content is preferably 1.5% or more. However, if Mn is contained in excess of 2.5%, the hardenability of the steel is increased and the low temperature toughness of the HAZ is deteriorated, and the on-site weldability may be impaired. Therefore, it is preferable that the upper limit of the Mn content is 2.5% or less.

  P and S are impurity elements, and in order to further improve the low temperature toughness of the base material and the HAZ, the upper limits of the P content and the S content are 0.015% or less and 0.003% or less, respectively. It is preferable. Although the lower limit of the P content and the S content is preferably as low as possible, it is not specified, but usually contains 0.001% or more and 0.0001% or more, respectively.

  Ni is an element that improves low-temperature toughness and strength, and in order to obtain the effect, the lower limit of the Ni content is preferably set to 0.1% or more. On the other hand, if the Ni content exceeds 2.0%, weldability may be impaired, so the upper limit of the Ni content is preferably 2.0%.

  Mo is an element that improves the hardenability of steel and forms carbonitride to improve strength. To obtain the effect, Mo content is preferably 0.15% or more. On the other hand, if the Mo content exceeds 0.60%, the strength becomes too high and the low temperature toughness of the HAZ may be impaired, so the upper limit of the Mo content is preferably 0.60%.

  Nb is an element that forms carbides and nitrides and improves the strength of the steel. To obtain this effect, the Nb content is preferably 0.001% or more. On the other hand, if the Nb content is more than 0.10%, the low temperature toughness of the base material and the HAZ may be impaired, so the upper limit of the Nb content is preferably 0.10%.

  Ti is an element that is effective for deoxidation and contributes to the refinement of the crystal grain size by forming a nitride. To obtain the effect, it is preferable to add 0.005% or more. On the other hand, if the Ti content is more than 0.030%, coarse carbides may be produced and the low temperature toughness may be deteriorated. Therefore, the upper limit of the Ti content is preferably 0.030% or less.

  Al is an effective element as a deoxidizing material, but if the Al content exceeds 0.06%, Al-based non-metallic inclusions may increase and inhibit the cleanliness of the steel. The upper limit was made 0.06% or less. Further, since deoxidation can be performed with Ti and / or Si, it is not always necessary to contain Al, and the lower limit may be 0%.

  In the present invention, one or more elements of B, N, V, Cu, Cr, Zr, Ta, Ca, REM, and Mg can be added as elements for improving strength and toughness. .

  B is an element that enhances hardenability and improves the toughness of the weld heat affected zone. This effect becomes prominent with addition of 0.0001% or more, but addition exceeding 0.005% may lead to a decrease in toughness. Therefore, it is preferable that the addition amount of B is in the range of 0.0001 to 0.005%.

  N forms nitrides with Ti, Al, etc., and prevents the austenite grains in the weld heat affected zone from becoming coarse. This effect becomes prominent with addition of 0.0001% or more, but addition exceeding 0.006% may lead to a decrease in toughness. Therefore, it is preferable that the addition amount of N is in the range of 0.0001 to 0.006%.

  V is an element that forms carbides and nitrides in the same manner as Nb and improves the strength of the steel. However, in order to obtain a remarkable effect, V is preferably added in an amount of 0.01% or more. On the other hand, if V is added in excess of 0.10%, the toughness may be lowered, so the upper limit is preferably made 0.10% or less.

  Cu is an element that increases the strength, and is preferably added in an amount of 0.01% or more. On the other hand, if more than 1.0% is added, cracking is likely to occur during heating of the steel slab or during welding, so the upper limit is preferably made 1.0% or less.

  Cr is an element that improves the strength of steel by precipitation strengthening, and the addition of 0.01% or more is effective. On the other hand, if added in a larger amount than 0.8%, the hardenability of the steel is increased and the toughness may be lowered, so the upper limit is preferably made 0.8% or less.

  Zr and Ta are elements that form carbides and nitrides in the same manner as Nb and improve the strength of the steel, and are each preferably added in an amount of 0.0001% or more. On the other hand, if Zr and Ta are added in excess of 0.0050%, toughness may be reduced. Therefore, it is preferable that the upper limit of the addition amount of Zr and Ta is 0.005% or less, respectively.

  Ca and REM suppress the production | generation of the extended | stretched MnS by producing | generating a sulfide, and improve the characteristic in the plate | board thickness direction of steel materials, especially the lamellar resistance. In order to obtain this effect, it is preferable to add 0.0001% or more of Ca and REM, respectively. On the other hand, if Ca and REM are added in excess of 0.01%, Ca and REM oxides increase. Therefore, it is preferable that the upper limit of the addition amount of Ca and REM is 0.01% or less, respectively.

  Mg is an element that generates ultrafine Mg-containing oxides or sulfides such as MgO and MgS, suppresses coarsening of austenite grains, and improves HAZ toughness. In order to acquire this effect, it is preferable to add 0.0001% or more of Mg. On the other hand, if Mg is added in excess of 0.006%, the Mg-containing oxide and sulfide are coarsened, so the upper limit is preferably made 0.006% or less.

  The reason for limiting the components of the weld metal when the steel plate is a steel pipe will be described.

  C is extremely effective for improving the strength of the steel, and in order to obtain the target strength in the martensite structure, the C content is preferably 0.04% or more. On the other hand, if the C content exceeds 0.14%, welding low temperature cracking is likely to occur, and the on-site welded part and seam welding intersect, leading to an increase in the HAZ maximum hardness of the so-called T-cross part. The upper limit is preferably 0.14% or less. A more preferable upper limit of the C content is 0.10% or less.

  In order to prevent the occurrence of blow holes, Si is preferably contained in an amount of 0.05% or more. On the other hand, when the Si content is more than 0.4%, the low temperature toughness may be deteriorated, and particularly when performing inner and outer surface welding or multilayer welding, the low temperature toughness of the reheated portion may be deteriorated. The upper limit is preferably 0.4% or less.

  Mn is an element that improves the balance between strength and low-temperature toughness and forms inclusions that form nuclei for intragranular bainite. In order to obtain this effect, the Mn content is preferably 1.2% or more. On the other hand, if the Mn content is more than 2.2%, segregation is promoted and the low temperature toughness may be deteriorated, making it difficult to produce a welding material. Therefore, the upper limit of the Mn content is 2.2% or less. It is preferable that

  P and S are inevitable impurities, and in order to suppress the low temperature toughness deterioration and reduce the low temperature cracking susceptibility, the smaller the content, the smaller the content of P and S, 0.001% or less, respectively. It is preferable to be 001% or less.

  Ni is an element that enhances hardenability and improves strength and improves low-temperature toughness. In order to obtain this effect, it is preferable to contain 1.3% or more of Ni. On the other hand, if the Ni content is more than 3.2%, hot cracking may occur, so the upper limit of the Ni content is preferably 3.2% or less.

  Cr, Mo, and V are all elements that increase the hardenability and improve the strength. To obtain the effect, Cr + Mo + V is preferably set to 1.0% or more. On the other hand, if Cr + Mo + V is added in a larger amount than 2.5%, low temperature cracking may occur, so the upper limit of the Cr + Mo + V content is preferably 2.5% or less.

  Ti is an element that forms a nitride, oxide, or the like of Ti that serves as a nucleus for formation of intragranular bainite, and preferably contains 0.003% or more. On the other hand, when the Ti content is more than 0.05%, a large amount of Ti carbide is generated and the low temperature toughness may be deteriorated. Therefore, the upper limit of the Ti content is preferably 0.05%.

  Since Al sometimes inhibits the formation of Ti oxides that form nuclei of intragranular bainite, it is preferable that the Al content is low. The upper limit with preferable Al content is 0.02% or less, More preferably, 0.015% or less is good.

  B is an element that enhances the hardenability and improves the low temperature toughness of the weld metal, and preferably contains 0.0003% or more, but if the B content is more than 0.005%, the low temperature toughness is reduced. Since it may deteriorate, the B content is preferably 0.005% or less.

  O is an element that lowers the hardenability and degrades the low temperature toughness of the weld metal. When the amount of O exceeds 0.03%, the low temperature toughness is remarkably deteriorated. On the other hand, if the amount of O is low, cold cracking is likely to occur, and at the same time, the hardness of on-site weldability increases, so the content was made 0.010% or more.

  In addition, the weld metal may contain elements such as Zr, Nb, and Mg that are added to improve the refining and solidification during welding.

  The structure of the weld metal is mainly composed of bainite / martensite and intragranular bainite, and the balance is ferrite and / or retained austenite. In order to set the tensile strength to 800 MPa or more, it is preferable to set the area ratio of bainite / martensite to 50% or more.

  Furthermore, in order to improve the low temperature toughness of the weld metal, it is preferable that the area ratio of intragranular bainite is as large as possible. Bainite martensite and intragranular bainite can be distinguished by observation of the structure with an optical microscope or scanning electron microscope, and the area ratio of bainite martensite and intragranular bainite was photographed with an optical microscope or scanning electron microscope. It can be measured by image analysis using tissue photographs.

Next, the manufacturing method of the steel plate which makes a high-speed ductile fracture characteristic favorable is demonstrated. The steel containing the components shown above is melted in the steel making process, continuously cast, then heated, and hot rolled. The heating temperature of the steel slab is specified at 1100 to 1250 ° C. When the temperature is lower than 1100 ° C., coarse γ grains exist, and the coarse grains remain up to the steel sheet. For this reason, heating temperature was 1100 degreeC or more. On the other hand, when the temperature exceeds 1250 ° C., grain growth occurs, so that coarse grains are likely to be formed, and the low temperature toughness is deteriorated.
Next, recrystallization zone rolling will be described. The rolling temperature of the recrystallization zone rolling was set to 900 ° C. or higher. When the rolling temperature of the recrystallization zone rolling is less than 900 ° C., the austenite cannot be sufficiently recrystallized, and the crystal grains do not become fine. Therefore, the rolling temperature of the recrystallization zone rolling is set to 900 ° C. or higher.

Next, the conditions for non-recrystallization zone rolling will be described.
The rolling conditions in the non-recrystallized region of the present invention are a rolling temperature of 880 ° C. or lower and a cumulative rolling reduction of 60% or higher. If the temperature exceeds 880 ° C., the recrystallization zone rolling is partially performed at the center of the plate thickness, so that the grain size cannot be reduced. In addition, when the cumulative reduction amount is less than 60%, the crystal grain size is not refined, so that it is set to 60% or more.

  On the other hand, in order to increase the area ratio of ferrite, it increases when the end temperature of hot rolling in the non-recrystallized region is lowered. In order to set the area ratio of ferrite to 1 to 60%, it is necessary to set the end temperature of hot rolling in the non-recrystallized region to 800 ° C. or less, and to 800 ° C. or less. A preferred range is 600-780 ° C.

  Furthermore, after completion | finish of hot rolling, the range of 600-350 degreeC is cooled at 0.1-10 degreeC / s. This cooling is preferably water cooling which is easy to control. When the cooling rate is less than 0.1 ° C./s, fine austenite grains grow at the end of the non-recrystallization zone rolling, the average prior austenite grain size exceeds 5 μm, and the low temperature toughness decreases. This cooling rate is preferably 1 ° C./s or more in order to avoid austenite grain growth. If it exceeds 10 ° C./s, the area ratio of ferrite near the surface of the steel sheet may be less than 1%, and needs to be 10 ° C./s or less.

  Furthermore, the steel plate is press-formed into a cylindrical shape, and the ends are submerged arc welded to form a steel pipe.

  Submerged arc welding is a welding with a large dilution of the base metal, and in order to obtain a desired characteristic, that is, a weld metal composition, it is necessary to select a welding material in consideration of the dilution of the base metal. Hereinafter, although the reason for limiting the chemical composition of the welding wire will be described, it is basically a manufacturing method capable of realizing an ultra-high strength line pipe.

  In order to obtain the C content in a range required for the weld metal, C is set to 0.01 to 0.12% in consideration of dilution by the base material component and mixing of C from the atmosphere.

  Si, Mn, Ni, Cr + Mo + V is 0.3% or less in consideration of dilution by the base material component in order to obtain the content of Si, Mn, Ni, Cr + Mo + V in the range required for the weld metal. 1.2-2.4%, 4.0-8.5%, 3.0-5.0%.

  Ti is an element that forms a nitride, oxide, or the like of Ti that forms nuclei for intragranular bainite, and it is preferable to contain 0.005% or more. On the other hand, if the Ti content is more than 0.15%, a large amount of Ti carbide is generated and the low temperature toughness may be deteriorated. Therefore, the upper limit of the Ti content is preferably set to 0.15%.

  Since Al sometimes inhibits the formation of Ti oxides that form nuclei of intragranular bainite, it is preferable that the Al content is low. The upper limit with preferable Al content is 0.02% or less.

  In addition, it is desirable that impurities of P and S are as small as possible, and B can be added to ensure strength. Zr, Nb, Mg, etc. are used for the purpose of deoxidation.

  In addition, welding can be performed with a plurality of electrodes as well as a single electrode. In the case of welding with a plurality of electrodes, it is possible to combine various wires, and it is not necessary for each wire to be in the above-mentioned component range, and the average composition from each wire component and consumption may be in the above-described component range.

  Flux used for submerged arc welding can be broadly classified into fired flux and molten flux. Firing-type fluxes have the advantage that alloy materials can be added and the amount of diffusible hydrogen is low, but they have the disadvantage of being easily pulverized and difficult to use repeatedly. On the other hand, the melt-type flux is in the form of glass powder, has the advantages of high grain strength and is difficult to absorb moisture, and has the disadvantage that diffusible hydrogen is slightly high. When manufacturing the ultra-high strength steel pipe of the present invention, cold cracking is likely to occur. From this point, a fired mold is desirable, but a molten mold that can be recovered and used repeatedly is suitable for mass production and has a low cost. There are low advantages. The cost is high in the baking mold, and the necessity of strict quality control is a problem in the melting mold, but it is within a range that can be handled industrially, and either can be used essentially.

  Next, welding conditions will be described below.

  The initial tack welding performed may be any of MAG arc welding, MIG arc welding, and TIG arc welding. Usually, MAG arc welding. Next, the inner and outer surfaces are preferably submerged arc welding, but may be TIG arc welding, MIG arc welding, or MAG arc welding. The inner and outer surfaces may be welded one by one, but a plurality of passes may be performed.

  When submerged arc welding is performed on the inner and outer surfaces, if the welding speed is less than 1 m / min, seam welding of the line pipe is inefficient, and if it exceeds 3 m / min, the bead shape may become unstable. Therefore, the welding speed of submerged arc welding is preferably in the range of 1 to 3 m / min.

  In addition, when the welding part of tack welding and inner and outer surface welding overlaps, the one where welding heat input is as low as possible is preferable. Also, the welding heat input varies depending on the plate thickness. However, if the heat input is too small, the penetration becomes insufficient, the number of weldings increases, the work efficiency deteriorates, and if the heat input is too large, the heat-affected zone softens. It is large and the toughness of the welded portion is also reduced, so the welding heat input on the inner and outer surfaces is 1.0 to 3.5 kJ / mm.

  After seam welding, roundness is improved by pipe expansion. In order to make a perfect circle, it is necessary to deform to the plastic region. In the case of the ultra-high-strength steel pipe of the present invention, it is preferable that the pipe expansion rate expressed as a percentage obtained by dividing the difference between the circumference after pipe expansion and the circumference before pipe expansion by the circumference before pipe expansion is 0.7% or more. . On the other hand, if the expansion ratio exceeds 2%, the toughness may deteriorate due to plastic deformation of both the base metal and the welded part. Therefore, it is preferable that the tube expansion rate is in the range of 0.7 to 2%.

  Steel made of the chemical components shown in Table 1 was melted and cast into a steel ingot having a thickness of 240 mm. These ingots are heated to 1150 ° C., hot rolled at 900 ° C. or higher to a thickness of 59 to 80 mm in a recrystallization temperature range, and hot rolled in an unrecrystallized zone to a thickness of 14 to 20 mm as it is at 600 to 880 ° C. went. In addition, 900 degreeC or more is a recrystallization temperature range, and 880 degrees C or less is a non-recrystallization temperature range. After hot rolling, the range of 600 to 350 ° C. was water-cooled at an average cooling rate of 1 ° C./s to 10 ° C./s to 350 ° C. or less. The obtained steel plate was press-formed into a cylindrical shape, and after tack welding, the inner and outer surfaces were submerged arc welded with a welding heat input of 2.4 kJ / mm, expanded, and 36 inches (913 mm diameter), The steel pipe was 16 mm thick. Tables 4 and 5 (continued in Table 4) show the manufacturing conditions, characteristics of the base material, test results, and the like at this time.

  Test pieces were collected from the obtained steel pipe base material and subjected to a tensile test in accordance with JIS Z 2241. In addition, a DWTT test piece having a longitudinal direction in the circumferential direction was collected, a press notch was introduced in the thickness direction, a ductile crack was further introduced by three-point bending, and a precrack DWTT test was performed at 0 ° C.

  Steels 1-8 show examples of the present invention. As is clear from Tables 4 and 5 (continuation of Table 4), the pre-crack DWTT energy at 10 ° C. of the base material of each of these steels is 3000 J or more. Moreover, cracks of these steel pipes have stopped in the partial gas burst test. That is, the unstable ductile fracture characteristics are excellent. On the other hand, steel 9-19 shows the comparative example which deviated from this invention method. That is, steels 9 to 13 and 15 do not have a ferrite / martensite two-phase mixed structure depending on whether they deviate from the chemical composition of the base metal or the hot rolling conditions. As a result, since the degree of integration of (100) in the D direction exceeds 3, the pre-crack DWTT energy of the base material is less than 3000 J. Moreover, since these characteristics are poor, the partial gas burst test is also penetrated, and the unstable ductile fracture characteristics are also inferior. Since the base material C deviates from the steel 14, the hardness of the base material is not satisfied. Steels 16, 17, and 19 do not satisfy the weld metal strength or do not satisfy the weld metal toughness because the chemical components or heat input conditions of the weld metal deviate. Steel 18 had many welding defects due to the low heat input of submerged arc welding.

  Tables 2 and 3 (continued in Table 2) show the components of the weld metal and the weld wire of the present invention and the comparative example for reference.

The figure which shows the relationship between a ferrite fraction (%) and pre-rack energy (J). (100) The figure which shows the relationship between an integration degree and pre-rack energy (J).

Claims (13)

  The microstructure is composed of 1-5% ferrite by area and the balance and bainite-martensite, and the degree of integration of (100) in the section rotated 45 ° from the rolling surface with the rolling direction as the axis is 3 or less. Super high strength steel sheet with excellent high speed ductile fracture characteristics.   The microstructure is composed of ferrite with an area ratio of more than 5% to 60% and the balance of bainite / martensite, and the degree of integration of (100) in the section rotated by 45 ° from the rolling surface with the rolling direction as the axis is 3 or less. An ultra-high strength steel sheet with excellent high-speed ductile fracture characteristics.   The ultra-high strength steel sheet excellent in high-speed ductile fracture characteristics according to claim 2, wherein the average grain size of ferrite is 5 µm or less. % By mass
C: 0.03-0.10%,
Si: 0.01 to 0.6%,
Mn: 1.5 to 2.5%
P: 0.015% or less,
S: 0.003% or less,
Ni: 0.1 to 2.0%,
Mo: 0.15-0.60%,
Nb: 0.001 to 0.10%,
Ti: 0.005 to 0.030%,
Al: 0.06% or less, further,
B: 0.0001 to 0.005%,
N: 0.0001 to 0.006%,
V: 0.001 to 0.10%,
Cu: 0.01 to 1.0%,
Cr: 0.01 to 0.8%
Zr: 0.0001 to 0.005%,
Ta: 0.0001 to 0.005%,
Ca: 0.0001 to 0.01%,
REM: 0.0001-0.01%
Mg: 0.0001 to 0.006%
The ultra-high strength steel sheet having excellent high-speed ductile fracture characteristics according to claim 3 or 4, wherein one or more of the above are contained, and the balance consists of iron and inevitable impurities.   It is a manufacturing method of the steel plate in any one of Claims 1-4, Comprising: Steel which consists of a component of Claim 4 is melted, A steel piece is reheated to 1100-1250 degreeC after continuous casting, It is characterized by the above-mentioned. A method for producing an ultra-high strength steel sheet having excellent high-speed ductile fracture characteristics.   The method for producing an ultra-high strength steel sheet having excellent high-speed ductile fracture characteristics according to claim 5, wherein hot rolling is performed in a recrystallization region at 900 ° C or higher after reheating.   The hot rolling with a cumulative reduction amount of 60% or more is performed in a non-recrystallized region of 880 ° C or lower after reheating or subsequent recrystallization region rolling. The manufacturing method of the ultra high strength steel plate excellent in the described high-speed ductile fracture characteristic.   After reheating or subsequent recrystallization zone rolling, non-recrystallization zone rolling is started at 880 ° C. or less, hot rolling is finished in the 600-800 ° C. non-recrystallization zone, The ultra-high-strength steel sheet having excellent high-speed ductile fracture characteristics according to any one of claims 5 to 7, wherein the cooling is performed at an average cooling rate of 600 to 350 ° C at a rate of 0.5 to 10 ° C / s or less. Manufacturing method.   An ultra-high-strength steel pipe excellent in high-speed ductile fracture characteristics, wherein the base material is made of the ultra-high-strength steel sheet according to any one of claims 1 to 4. The composition of the weld metal is mass%,
C: 0.04 to 0.14%,
Si: 0.05-0.4%
Mn: 1.2-2.2%,
P: 0.01% or less,
S: 0.010% or less,
Ni: 1.3-3.2%
Cr + Mo + V: 1.0 to 2.5%,
Ti: 0.003 to 0.050%,
Al: 0.02% or less,
B: 0.005% or less,
O: 0.01 to 0.03%
The ultrahigh strength steel pipe excellent in high-speed ductile fracture characteristics according to claim 9, wherein the balance is made of iron and inevitable impurities.   It is a manufacturing method of the steel pipe of Claim 9 or 10, Comprising: The ultra-high-strength steel plate in any one of Claims 1-4 is shape | molded by the UO process at a tubular shape, and both ends are welding wire-and a baking mold A method for producing an ultra-high strength steel pipe excellent in high-speed ductile fracture characteristics, characterized in that submerged arc welding is performed using a flux or a melt-type flux, followed by pipe expansion.   When the ends are subjected to submerged arc welding using a welding wire and a firing-type flux or a fusion-type flux, welding is performed with a heat input of 1.5 to 3.5 kJ / mm, and then pipe expansion is performed. The manufacturing method of the ultra high strength steel pipe excellent in the high-speed ductile fracture characteristic of Claim 11 characterized by these. % By mass
C: 0.01 to 0.12%,
Si: 0.3% or less,
Mn: 1.2-2.4%
Ni: 4.0 to 8.5%,
Cr + Mo + V: 3.0-5.0%,
Ti: 0.005 to 0.15%,
It is excellent in the high-speed ductile fracture property according to claim 11 or 12, characterized in that submerged arc welding is performed using a welding wire containing Al: 0.02% or less, the balance being iron and inevitable impurities. Manufacturing method of ultra high strength steel pipe.

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