JP5842473B2 - High strength welded steel pipe with high uniform elongation characteristics and excellent weld toughness, and method for producing the same - Google Patents

Description

  The present invention is for high-strength line pipes of API standard X80 (tensile strength 700 MPa) or higher, the ratio of 1.5% proof stress to 0.5% proof stress of the base material is 1.10 or higher, and the tensile strength and uniform elongation. The present invention relates to a high-strength welded steel pipe that combines excellent deformation performance satisfying a product of 7500 MPa ·% or more and excellent weld toughness in a low temperature range up to −20 ° C.

  In recent years, line pipes used for the transportation of natural gas and crude oil have been strengthened year by year in order to improve transportation efficiency by increasing pressure and to improve local welding construction efficiency by reducing wall thickness. As a result of ground deformation in Japan, there has been a demand for high deformability that hardly causes buckling even if large deformation occurs in the line pipe.

  The lower the yield ratio, the greater the deformability of the steel material. Therefore, a low yield ratio steel has been developed in which the microstructure of the steel material is a two-phase structure in which a soft ferrite phase and a hard phase such as hard bainite and martensite are appropriately dispersed. Has been.

  For example, Patent Document 1 relates to a method of manufacturing a low yield ratio, low carbon, low alloy, high strength steel, which describes a steel having a structure in which a hard phase is appropriately dispersed in a soft phase, and as a manufacturing method thereof, quenching (Q) and tempering ( In the middle of T), a heat treatment method is disclosed in which quenching (Q ′) from a two-phase region of ferrite and austenite is performed.

  Patent Document 2 relates to a method of manufacturing a high-strength low yield ratio steel pipe for construction, and based on the same concept, in the manufacturing process of a welded steel pipe, it is heated in a two-phase region after performing cold bending and welding at the seam. A method of manufacturing a low yield ratio steel pipe that is post-cooled is disclosed. Patent Document 3 discloses a steel for high-strength line pipes having a low yield ratio, and it is disclosed that a low yield ratio is achieved by a structure in which a processed ferrite that is a soft phase and a hard phase of bainite or martensite are mixed. Yes. Patent Document 4 relates to a method for producing a high-strength and high-toughness steel sheet, and discloses that a low yield ratio is achieved when hard island martensite is dispersed in bainite.

  When the pipe pipe is further deformed after the line pipe undergoes large deformation and buckling occurs, local strain concentration occurs in the pipe, and when the ductile fracture occurrence limit strain is reached, ductile fracture occurs. Since the critical strain at which ductile fracture occurs is considered to correlate with the uniform elongation of the steel material, steels having a high uniform elongation with a low yield ratio have been developed.

  Patent Document 5 relates to a low-yield-ratio high-strength and high-toughness steel sheet and a method for producing the same, and the metal structure is a three-phase structure of ferrite, bainite, and island martensite, and the volume ratio of 3-15% island martensite and volume. A steel sheet containing a retained austenite with a fraction of 2% or more and a uniform elongation in the longitudinal direction of the steel sheet of 12% or more and a method for producing the same are disclosed. Patent Document 6 describes a low yield ratio, high strength, high toughness steel pipe using a low yield ratio, high strength, high toughness steel pipe according to Patent Document 5, and a method for producing the same.

  Patent Document 7 relates to a steel pipe having a high strength of API standard X100 or more (yield strength of about 690 MPa or more and tensile strength of about 760 MPa or more) and excellent HAZ toughness and deformability. In order to prevent the occurrence of ductile cracks (with a strain of about 3% applied), the uniform elongation of the base metal contains 5 to 50% ferrite in an area fraction of 20 μm or less in particle size. It is described that the uniform elongation is ensured by containing 1 to 15% of island martensite.

JP-A-55-97425 Japanese Patent Laid-Open No. 07-150247 Japanese Patent Laid-Open No. 08-209291 JP 2006-265577 A JP 2008-248328 A JP 2008-248330 A JP 2003-293078 A

  As described above, steel pipes having high strength of API standard X100 or higher (yield strength of about 690 MPa or higher, tensile strength of about 760 MPa or higher) and excellent HAZ toughness and deformability have been developed. The effect of high toughness on deformability is not clear enough, and further studies are needed to achieve further improvements in these properties that are expected in the near future. Since the uniform elongation decreases as the tensile strength of the steel increases, it is considered extremely difficult to ensure a uniform elongation of 7% or more when, for example, an increase in strength exceeding 1000 MPa is performed.

  Moreover, when heat-processing after welding a steel pipe in order to obtain a two-phase structure, the low temperature toughness of a weld metal part may be impaired.

  Therefore, in order to help solve the above problems, the present invention is subject to buckling when a steel pipe using high-strength steel of API standard X80 or higher is subjected to bending deformation accompanying ground deformation such as an earthquake. A welded steel pipe having a high uniform elongation and excellent low-temperature toughness in the welded part, and a method for producing the same, in order to prevent the occurrence of ductile fracture from the buckled part even if the pipe buckles The purpose is to provide.

When the buried pipe is subjected to bending deformation due to ground fluctuation, the present inventors aim to suppress the occurrence of buckling inside the bend to the amount of compressive strain at the position to 1%, with the goal of After extensive research on the characteristics, the following findings were obtained.
1 The occurrence of buckling depends on the curing gradient before and after 1% strain of the stress-strain curve, and is suppressed as the curing gradient increases.

According to the actual pipe bending test results, the yield ratio between the 1.5% proof stress value and the 0.5% proof stress value in the stress-strain curve of the pipe used (ratio of the 1.5% proof stress value to the 0.5% proof stress value) is When it is 1.10 or more, buckling does not occur.
2 In such a high yield strength ratio, the form of island-like martensite (also referred to as Martensite-Austenite constituents, hereinafter abbreviated as MA) effective in increasing the strength even if the fraction is low, and the dispersion state in the first phase The uniform elongation is greatly affected.

  Ideal for preventing buckling inside the bend without altering the microstructure of the weld metal necessary to obtain low-temperature toughness by controlling the heat treatment during forming into a steel pipe shape by cold working and the manufacturing method of the raw steel plate It is possible to obtain a microstructure of the base material portion having a suitable MA shape and dispersion state.

The present invention has been made by further investigation based on the obtained knowledge. The composition of the base material part is mass%,
C: 0.05-0.08%
Si: 0.1% or less,
Mn: 1.8-4.0%,
Al: 0.003 to 0.08%,
Nb: 0.01-0.03%,
Ti: 0.005 to 0.025%,
Further Cu: 0.1 to 1.0%,
Ni: 0.1 to 2.0%,
Mo: 0.1 to 1.0%,
Cr: 0.1 to 1.0%,
B: 0.0010 to 0.0030%,
1 type or 2 types or more, and the balance Fe and unavoidable impurities,
The component composition of the weld metal part is mass%,
C: 0.06 to 0.10%,
Si: 0.2 to 0.5%
Mn: 1.6-2.0%,
Al: 0.03% or less,
Nb: 0.005 to 0.025%,
Ti: 0.015-0.040%,
O: 0.015-0.04%,
N: 0.01% or less,
In addition, Cu: 0.1-0.3%,
Ni: 0.1 to 3.5%
Mo: 0.05-1.5%,
Cr: 0.1 to 1.5%
V: 0.025 to 0.1%,
B: 0.0005 to 0.003%,
1 type or 2 types or more, and the balance Fe and unavoidable impurities,
The base material part is an island-shaped martensite having an average aspect ratio of 2.0 or less in which the first phase is bainite and the second phase is dispersed in an area ratio of 5 to 20% in the first phase. 90% or more of martensite has a microstructure present in the prior austenite grain boundaries,
The weld metal part has a microstructure in which the area ratio of acicular ferrite and bainite is 80% or more and the area ratio of island martensite is 5% or less,
High strength welded steel pipe with high uniform elongation and excellent weld toughness.
2. The component composition of the base material part is further mass%,
Ca: 0.0005 to 0.01%,
REM: 0.0005 to 0.02%,
Zr: 0.0005 to 0.01%,
Mg: 0.0005 to 0.01%,
A high-strength welded steel pipe having high uniform elongation as described in 1 and having excellent weld toughness, characterized by containing one or more of the following.
3. % By mass
C: 0.05-0.08%
Si: 0.1% or less,
Mn: 1.8-4.0%,
Al: 0.003 to 0.08%,
Nb: 0.01-0.03%,
Ti: 0.005 to 0.025%,
Further Cu: 0.1 to 1.0%,
Ni: 0.1 to 2.0%,
Mo: 0.1 to 1.0%,
Cr: 0.1 to 1.0%,
B: 0.0010 to 0.0030%,
A steel slab containing one or more of the above and comprising the balance Fe and inevitable impurities is re-heated to Ac ₃ or higher, then hot-rolled at a rolling end temperature Ar ₃ or higher, and a cooling rate of 10 to 80 ° C./s The steel sheet obtained by accelerating cooling to a temperature range of less than 400 ° C. and then air-cooled is formed into a cylindrical shape by cold forming, and then rapidly heated to Ac ₁ or more and Ac ₃ or less, followed by air cooling or water cooling. A method for producing a high-strength welded steel pipe having high uniform elongation and excellent weld toughness, characterized in that after cooling to room temperature, the ends are welded and finally expanded.
4). The component composition of the billet is further mass%,
Ca: 0.0005 to 0.01%,
REM: 0.0005 to 0.02%,
Zr: 0.0005 to 0.01%,
Mg: 0.0005 to 0.01%,
A method for producing a high-strength welded steel pipe having high uniform elongation as described in 3 and having excellent weld toughness, characterized by containing one or more of the following.

  According to the present invention, a high-strength steel pipe of API standard X80 class or higher that is unlikely to cause pipe buckling and subsequent ductile fracture due to ground deformation such as an earthquake, and has low-temperature toughness of a welded portion up to −20 ° C. It can be provided and is extremely useful in industry.

In the present invention, the component composition and the microstructure of the base metal part and the weld metal part are respectively defined.
[Base Material Component Composition] In the following description,% is mass%.
C: 0.05% or more, 0.08% or less C needs to be contained in an amount of 0.05% or more in order to secure the MA area ratio necessary for obtaining high strength of X80 grade or more. On the other hand, if added over 0.08%, in addition to the second phase MA, the formation of cementite leads to a decrease in ductility, so the upper limit is made 0.08%.

Si: 0.1% or less Since Si promotes the formation of MA in the weld heat affected zone and causes a reduction in weld toughness, the upper limit is made 0.1% in order to suppress the formation of MA in the weld.

Mn: 1.8-4.0%
Mn acts as a hardenability improving element, and is an element necessary for suppressing ferrite transformation during accelerated cooling of steel and making the first phase bainite. Furthermore, the addition of a large amount has the effect of reducing the amount of C that can be dissolved in the ferrite phase and increases the C concentration in the untransformed austenite region, thereby increasing the amount of MA produced. In order to make the first phase of the base metal part bainite and the area ratio of MA of the second phase to be 5% or more as described later, it is necessary to contain 1.8% or more. On the other hand, if the addition exceeds 4.0%, the microstructure becomes martensite mainly and the ductility is greatly reduced, so the upper limit is made 4.0%.

Al: 0.003 to 0.08%
Al acts as a deoxidizing element. In order to obtain a sufficient deoxidation effect by simultaneous addition with Si, a content of 0.003% or more is necessary. On the other hand, if added over 0.08%, the cleanliness of the steel is lowered and the uniform elongation is reduced, so the upper limit is made 0.08%.

Nb: 0.01-0.03%
Nb expands the austenite non-recrystallized region during hot rolling, and also acts as an element for improving the hardenability of steel. Further, since the C concentration in the untransformed austenite region is increased similarly to Mn, the amount of MA produced is increased. As will be described later, in order to make the area ratio of MA 5% or more in the microstructure, it is necessary to add 0.01% or more. On the other hand, if added over 0.03%, the toughness of the weld is reduced, so the upper limit is made 0.03%.

Ti: 0.005-0.025%
Ti forms nitrides and has an adverse effect on toughness. It is effective in reducing the amount of dissolved N in steel. In addition, the precipitated TiN suppresses the coarsening of austenite grains due to the pinning effect, and the microstructure is coarse. Control. In order to obtain such an effect, 0.005% or more is contained. On the other hand, if added over 0.025%, TiC is formed, and ductility is lowered by precipitation hardening, so the upper limit is made 0.025%.

In the present invention, one or more of Cu, Ni, Mo, Cr, B, in the present invention, to achieve a high strength exceeding the API standard X80 class as the strength of the base material portion, to improve the strength of the weld heat affected zone, Contains one or more of Cu, Ni, Mo, Cr, B.

Cu: 0.1 to 1.0%
When Cu is contained in an amount of 0.1% or more, it acts as a hardenability improving element and can replace a large amount of Mn addition. However, if the content exceeds 1.0%, Cu is precipitated, and the ratio of 1.5% proof stress to 0.5% proof stress is reduced by precipitation hardening. %.

Ni: 0.1 to 2.0%
Ni is a useful element in the present invention because it acts as a hardenability improving element and does not cause toughness deterioration even when added. This effect is exhibited by containing 0.1% or more, but even if it contains more than 2.0%, the effect of improving the hardenability is saturated. And

Mo: 0.1 to 1.0%
Mo can be contained in order to improve the strength of the base material or the weld heat affected zone. By containing 0.1% or more, it acts as a hardenability improving element and can be used as a substitute for adding a large amount of Mn. However, since it is an expensive element and the increase in strength is saturated even if it is contained in excess of 1.0%, the upper limit is made 1.0%.

Cr: 0.1 to 1.0%
Cr can be contained in order to improve the strength of the base material or the weld heat affected zone. By containing 0.1% or more, it acts as a hardenability improving element and can be used as a substitute for adding a large amount of Mn. However, if the content exceeds 1.0%, the HAZ toughness is remarkably deteriorated. If contained, the upper limit is made 1.0%.

B: 0.0010 to 0.0030%
B can be contained in order to improve the strength of the base material or the weld heat affected zone. B segregates at the austenite grain boundaries and suppresses ferrite transformation, and allows the parent phase (sometimes referred to as the first phase) to be a bainite-based structure and to increase the tensile strength to 800 MPa or more. The ferrite transformation inhibitory effect is exhibited by containing 0.0010% or more, but the effect is saturated even if contained over 0.0030%, so when it is contained, the upper limit is made 0.0030% .

  The above is the basic component composition of the steel base material of the present invention. When the uniform elongation characteristics are further improved, one or more of Ca, REM, Zr, and Mg can be contained. Ca, REM, Zr, and Mg control the form of MnS, which is a non-metallic inclusion in steel, or form oxides or nitrides to improve cleanliness and improve uniform elongation.

Ca: 0.0005 to 0.01%
Ca is an element effective for controlling the form of sulfide in steel, and when contained in an amount of 0.0005% or more, the production of MnS harmful to ductility is suppressed. On the other hand, when the content exceeds 0.01%, a CaO-CaS cluster is formed, and ductility is deteriorated. Therefore, when it is contained, the upper limit is preferably made 0.01%.

REM: 0.0005 to 0.02%
REM is an effective element for controlling the form of sulfide in steel, and when it is contained in an amount of 0.0005% or more, it suppresses the generation of MnS harmful to ductility. On the other hand, since it is an expensive element and the effect is saturated even if it contains more than 0.02%, when it is contained, the upper limit is preferably made 0.02%.

Zr: 0.0005 to 0.01%
Zr forms carbonitrides in steel and brings about a pinning effect that suppresses coarsening of austenite grains. In order to obtain a sufficient pinning effect, it is preferable to contain 0.0005% or more, but if it exceeds 0.01%, the cleanliness in the steel is remarkably lowered, which leads to a reduction in ductility. When it contains, it is preferable that an upper limit shall be 0.01%.

Mg: 0.0005 to 0.01%
Mg has the effect of refining oxides in the steelmaking process and is effective in suppressing coarse oxides that cause ductility reduction. In order to obtain a sufficient oxide refining effect, it is preferable to contain 0.0005% or more, but even if contained over 0.01%, the effect is saturated. The upper limit is preferably 0.01%.

  In the steel pipe base material part according to the present invention, the components other than the above are Fe and inevitable impurities. In the present invention, P and S are unavoidable impurities, and excessive P content segregates at the time of casting to lower the ductility of the steel, and excessive S content promotes the generation of MnS harmful to ductility. Therefore, in any case, it is preferable to reduce as much as possible in consideration of economic efficiency, and the P amount is preferably 0.01% or less and the S amount is preferably 0.003% or less.

[Welding metal component composition] In the following description, "%" means "mass%".
C: 0.06% to 0.10%
In the weld metal, C is required to be 0.06% or more in order to prevent hot cracking of the weld metal. On the other hand, if it exceeds 0.10%, the microstructure of the weld metal is mainly martensite and the toughness is remarkably lowered, so the upper limit is made 0.10%.

Si: 0.2 to 0.5%
Si acts as a deoxidizing element in the weld metal and is an element necessary for controlling the amount of oxygen in the weld metal. If the Si content in the weld metal is less than 0.2%, deoxidation is insufficient and the amount of oxygen in the weld metal increases, resulting in a decrease in toughness. On the other hand, if it exceeds 0.5%, the production of MA harmful to weld metal toughness becomes remarkable, so the upper limit is made 0.5%.

Mn: 1.6-2.0%
Mn also acts as a hardenability improving element in the weld metal. In order to make the tensile strength of the weld metal equal to or higher than the base metal part (700 MPa or more), it is necessary to have a mixed structure of acicular ferrite and bainite, and 1.6% or more of Mn is required. On the other hand, if the content exceeds 2.0%, the microstructure of the weld metal is mainly martensite, and the toughness is significantly reduced, so the upper limit is made 2.0%.

Al: 0.03% or less Al is present in the weld metal as an unavoidable impurity due to dilution from the base metal part, but when it exceeds 0.03%, the formation of TiO described later is inhibited, and the acicular ferrite of the weld metal Therefore, the upper limit is made 0.03%.

Nb: 0.005 to 0.025%
Nb needs to be contained in an amount of 0.005% or more in order to make B exist as a solid solution B at the austenite grain boundary by forming a nitride prior to B with respect to solid solution N in the weld metal. It is. On the other hand, if it exceeds 0.025%, carbides are formed, the weld metal is precipitated and hardened, and the toughness is reduced, so the upper limit is made 0.025%.

Ti: 0.015-0.040%
Ti reacts with oxygen in the weld metal to form TiO and functions as an acicular ferrite transformation nucleus from within the weld metal austenite grains. In order to obtain a fine acicular ferrite structure, it is necessary to generate a large number of TiO, and Ti is required to be 0.015% or more. On the other hand, if it exceeds 0.040%, TiO in the weld metal is aggregated and coarsened to reduce the Charpy impact value, so the upper limit is made 0.040%.

O: 0.015-0.04%
O reacts with the above-mentioned Ti to form TiO and functions as an acicular ferrite transformation nucleus from within the weld metal austenite grains. In order to obtain a fine acicular ferrite structure, it is necessary to produce a large number of TiO, and O is required to be at least 0.015%. On the other hand, if it exceeds 0.04%, TiO in the weld metal is aggregated and coarsened to reduce the Charpy impact value, so the upper limit is made 0.04%.

N: 0.01% or less N in the weld metal exists as an unavoidable impurity. However, when it exceeds 0.01%, the upper limit is set to 0.01% because it dissolves and significantly deteriorates the weld metal toughness. And

In the present invention, one or more of Cu, Ni, Mo, Cr, V, and B are used. In the present invention, one or more of Cu, Ni, Mo, Cr, V, and B are used in order to increase the strength of the weld metal. Added.

Cu: 0.1 to 0.3%
When Cu is contained in an amount of 0.1% or more, it acts as a hardenability improving element and can be substituted for Mn addition. However, if it exceeds 0.3%, Cu liquefaction cracks cause marked weld defects, so when it is contained, the upper limit is made 0.3%.

Ni: 0.1 to 3.5%
Ni acts as a hardenability improving element and does not cause toughness deterioration even when added, so it is a useful element in weld metals. This effect is exhibited when the content is 0.1% or more. However, if it exceeds 3.5%, the hot cracking susceptibility of the weld metal increases and causes weld defects. 5%.

Mo: 0.05-1.5%
By containing 0.05% or more of Mo, it acts as a hardenability improving element and can be used as an alternative to the addition of Mn. However, if added over 1.5%, the microstructure of the weld metal becomes mainly martensite and the toughness is remarkably reduced. Therefore, when it is contained, the upper limit is made 1.5%.

Cr: 0.1 to 1.5%
When Cr is also contained in an amount of 0.1% or more, it acts as a hardenability improving element and can be used as an alternative to Mn addition. However, since it is an expensive element and the effect of increasing the strength is saturated even if the content exceeds 1.5%, the upper limit is made 1.5%.

V: 0.025 to 0.1%
By containing V in an amount of 0.025% or more, carbides are formed even in the weld metal, and the precipitation strengthening contributes to an increase in the strength of the weld metal. Therefore, V can be substituted for the addition of Mn. However, if the content exceeds 0.1%, precipitation strengthening remarkably impairs the weld metal toughness, so the upper limit is made 0.1%.

B: 0.0005 to 0.003%
B has the effect of suppressing the formation of polygonal ferrite from the austenite grain boundaries of the weld metal and making the microstructure of the weld metal a main structure of acicular ferrite. Although the effect of suppressing the formation of polygonal ferrite from the grain boundary is exhibited by inclusion of 0.0005% or more, the effect is saturated even if it exceeds 0.003%, so when it is contained, the upper limit is set 0.003%.

In the steel pipe weld metal according to the present invention, the components other than the above are Fe and inevitable impurities. In the present invention, P and S are unavoidable impurities, and excessive P content segregates at the grain boundaries of the weld metal part to lower ductility and toughness. Excessive S content is a harmful component of MnS. In order to promote the generation, it is preferable to reduce the amount as much as possible in consideration of economic efficiency. The P amount is preferably 0.01% or less, and the S amount is preferably 0.003% or less.
[Base material microstructure]
In the present invention, the microstructure of the steel pipe base material portion is a structure having a bainite-based parent phase and a second phase dispersed and present in the parent phase. The second phase is island martensite (may be referred to as MA) having an average aspect ratio of 2.0 or less, the area ratio is 5 to 20%, and more than 90% is in the prior austenite grain boundaries. It prescribes to the existing organization.

  In order to increase the ratio of the 1.5% proof stress to the 0.5% proof stress (hereinafter also referred to as the proof stress ratio), which is an index of the bending buckling limit strain of the steel pipe, Disperse at a rate of 5-20%.

  When the area ratio is less than 5%, the sufficient yield strength ratio is not obtained. On the other hand, when the area ratio exceeds 20%, even if the aspect ratio of MA described later satisfies the specified condition, the uniform elongation decrease becomes remarkable. To an upper limit of 20%.

  The MA area ratio is about 1000 to 3000 times magnification, and the cross-sectional SEM (scanning electron microscope) photographs of the steel are photographed in four or more fields of view. Calculate by adding the minutes and dividing by the total visual field area.

  The aspect ratio of MA is 2.0 or less. As the shape of the MA grain becomes longer, microscopic breakage tends to occur from the interface between the MA and the first phase, the bainite phase, and as a result, the uniform elongation decreases.

  The aspect ratio of MA is a cross-sectional SEM photograph of steel taken at a magnification of about 1000 to 3000 times, and more than 4 fields of view. For each field of view, the major axis and minor axis of each MA grain are measured by image analysis. After obtaining (= major axis / minor axis), an average value is calculated, and an average value in the entire visual field is further obtained.

  Further, when MA having an aspect ratio of 2.0 or less is present between laths of bainite, although a high yield strength ratio is obtained, the decrease in uniform elongation when the strength is increased is significant. It must be present at the crystal grain interface. Specifically, the MA existing on the prior austenite grain boundary before the bainite transformation needs to be 90% or more of the total MA.

  The fraction of MA present on the prior austenite grain boundaries is the number of particles present on the grain boundaries by confirming the positional relationship with the grain boundaries in the SEM photograph of all MA particles measured by the area ratio measurement described above. And dividing by the total number of MA particles. The prior austenite grain boundaries can be revealed by, for example, picric acid corrosion.

  In the present invention, as a microstructure other than the bainite which is the main phase of the matrix and the island-like martensite which is the second phase, other metal structures having an area ratio of 5% or less, such as ferrite, pearlite and cementite, are used. Or you may contain 2 or more types.

  However, if there is residual austenite as a microstructure other than bainite and island-like martensite, it can be expected to have an effect of improving elongation due to processing-induced transformation. In order to cause a decrease, the area ratio is preferably less than 2%, and more preferably less than 1%.

[Welding metal microstructure]
In the present invention, the microstructure of the steel pipe weld metal is a mixed structure of acicular ferrite and bainite, the sum of the respective area ratios being 80% or more, and the area ratio of island martensite being 5% or less. .

  In order to obtain a high strength of 700 MPa or more in the weld metal, it is necessary to mix the bainite structure with the fine acicular ferrite structure that is transformed with a large number of TiO as nuclei. As the amount of martensite, pearlite, cementite, and the like produced by retransformation afterwards increases, the toughness significantly decreases.

Therefore, the sum of the area ratios of acicular ferrite and bainite in the weld metal needs to be 80% or more, and preferably 90% or more. Further, in the weld metal, hard MA significantly deteriorates toughness even in a small amount, so the area ratio is set to 5% or less, preferably 2% or less.
Hereinafter, a suitable manufacturing method of the steel having the above component composition and the above microstructure will be described.

  The steel temperature condition defined in the present invention refers to the average temperature in the thickness direction of the steel slab or the steel plate.

Steel slab heating temperature: Ac ₃ or higher In order to build the shape and microstructure by hot rolling, the steel slab is heated to a temperature of Ac ₃ or higher for the purpose of austenitizing the steel slab. If it is less than Ac ₃ , untransformed ferrite or the like remains and adversely affects the subsequent microstructure control. In order to completely austenite, the temperature is preferably 1000 ° C. or higher.

On the other hand, the upper limit of the billet heating temperature is preferably 1200 ° C. or less from the viewpoint of base material toughness. In the case of direct feed rolling, hot rolling is started at a temperature of Ac ₃ or higher without reheating. Incidentally, Ac ₃ temperature can be obtained simply by substituting the alloy element content of the steel in the following formula (1).
_{Ac 3 = 961.6-311.9C + 49.5Si-36.4Mn} + 12.7Al-51Cu-29Ni-8.7Cr + 13.5Mo + 308.1Nb-140V + 318.9Ti + 611.2B (1)
In the formula, M (%) indicates the content (mass%) of the element M, and when the element M is not added, it is calculated as 0%.

Hot rolling end temperature: Ar ₃ or higher When the hot rolling end temperature (temperature of the final rolling pass) is lowered to Ar ₃ or lower, so-called processed ferrite is formed in which the ferrite that has undergone transformation during rolling is processed. . As the amount of processed ferrite increases, the yield strength increases and it becomes difficult to achieve the yield ratio necessary for improving the bending buckling strain of the steel pipe. Therefore, the temperature of the final rolling pass is set to Ar ₃ or higher. Preferably, it is set to 800 ° C. or higher. Incidentally, Ar ₃ temperature can be obtained simply by substituting the alloy element addition of the steel in the following formula (2).
Ar ₃ = 910-273C-74Mn-56Ni-16Cr-9Mo-5Cu (2)
In the formula, M (%) indicates the content (mass%) of the element M, and when the element M is not added, it is calculated as 0%.

After the hot rolling, accelerated cooling is performed to make the microstructure first phase bainite. In accelerated cooling, in order to suppress the formation of ferrite, the cooling start temperature is preferably Ar ₃ or higher.

Cooling rate: 10-80 ° C / s
The cooling rate of the accelerated cooling is set to 10 ° C./s or more in order to avoid the formation of ferrite during the accelerated cooling. On the other hand, when the cooling rate exceeds 80 ° C./s, part of martensite transformation occurs and the ductility is remarkably lowered. Therefore, the upper limit is set to 80 ° C./s.

Cooling stop temperature: less than 400 ° C. The cooling stop temperature for accelerated cooling is set to less than 400 ° C. in order to complete the bainite transformation. When the cooling stop temperature is 400 ° C. or higher and the bainite transformation is not completed, untransformed austenite remaining in the bainite becomes MA through the subsequent reheating treatment. Since it exists in the inside, uniform elongation falls.

  The steel pipe which concerns on this invention shape | molds the steel plate manufactured by the above-mentioned manufacturing method into a cylinder shape by the cold working method. There are UOE method, roll bend method and the like as the forming method, and any of them may be used. After the steel plate is formed into a cylindrical shape, reheating treatment is performed.

Reheating temperature: Ac ₁ or more and Ac ₃ or less The bainite which is the first phase of the microstructure is heated to reversely transform a part thereof into austenite. Reverse-transformed austenite is transformed from the triple point of the grain boundary and is transformed in a diffusive manner, so that further transformation-generated MA is prevented from being generated between bainite laths in the subsequent cooling process, and the aspect ratio is small. Further, it is possible to generate MA having a shape with little uniform elongation deterioration.

When the reheating temperature is less than Ac ₁ , since it does not reversely transform to austenite, MA as a hard second phase cannot be generated. On the other hand, when the reheating temperature exceeds Ac ₃ , the entire surface is reversely transformed into austenite, and it is extremely difficult to obtain a state in which the hard phase is dispersed at a predetermined area ratio. A two-phase region of _{1 to} Ac ₃ is used. Incidentally, Ac ₁ temperature can be obtained simply by substituting the alloy element addition of the steel in the following equation (3).
_{Ac 1 = 751-26.6C + 17.6Si-11.6Mn} -169Al-23Cu-23Ni + 24.1Cr + 22.5Mo + 233Nb-39.7V-5.7Ti-895B (3)
In the formula, M (%) indicates the content (mass%) of the element M, and when the element M is not added, it is calculated as 0%.

  The heating rate at the time of reheating is preferably 2 ° C./s or more in order to suppress the generation of reverse transformed austenite generated during heating from between the laths of bainite. After reheating, air cooling or water cooling is performed.

  In order to increase the hardness of MA which is a hard phase and to obtain a higher yield strength ratio stably, it is preferable to water-cool to 200 ° C. or less at a cooling rate of 10 ° C./s or more.

After reheating and cooling in the two-phase region, the end (usually, the end in the rolling direction of the steel sheet) is welded (the end welding may be referred to as seam welding). When the above-described reheating is performed after seam welding, the weld metal is retransformed to produce martensite, pearlite, cementite, or MA, and the sum of the area ratios of acicular ferrite and bainite targeted by the present invention is 80. % Microstructure cannot be obtained. In particular, in the case of a weld metal, a large amount of MA is generated when heated in a temperature range of a two-phase region of Ac ₁ or more and Ac ₃ or less, and does not satisfy the target value of 5% or less of the MA area ratio. For the above reasons, in the present invention, seam welding is always performed after reheating and cooling of the base metal part in the two-phase region so that the microstructure of the weld metal becomes a structure mainly composed of acicular ferrite.
Seam welding is generally single layer welding on the inner and outer surfaces by submerged arc welding, but may be one layer welding by laser or laser arc hybrid welding. After seam welding, pipe expansion is performed to improve the roundness of the pipe. As for the tube expansion condition, it is preferable that the tube expansion rate is 0.6% or more and 2.0% or less from the viewpoint of securing the shape.

  Using steels having chemical compositions A to J shown in Table 1, steel slab heating, rolling and cooling under the conditions shown in Table 2 were performed. 1-14 were produced. Although not shown in Table 1, the contents of P and S, which are inevitable impurities, were both P amount: 0.01% or less and S amount: 0.003% or less.

  From these steel plates, in a direction perpendicular to the rolling direction, a flat plate tensile test piece having a parallel part width of 150 mm and a parallel part length of 600 mm was collected, and after applying a tensile strain corresponding to U-O press forming of a UOE steel pipe, Heating and cooling were performed.

  Next, the inner and outer surface single layer submerged arc welding for simulating the seam welding of the steel pipe was performed using the sample after the heat treatment. Table 3 shows heat treatment conditions and welding heat input conditions.

  Finally, a tensile strain corresponding to UOE steel pipe expansion forming was applied to a test body including a submerged arc weld. As a comparison, after applying tensile strain to the flat plate tensile test piece, submerged arc welding was performed first, followed by heating and cooling, and finally a test specimen that applied tensile strain again was prepared.

  A sample for microstructural observation is collected from the center of the base metal part of the specimen including the submerged arc weld, and the two-stage etching method is used after mirror-polishing the plate thickness section parallel to the rolling longitudinal direction of the original steel sheet. MA appeared.

  After that, using a scanning electron microscope (SEM), a five-view microstructure photograph was randomly taken at a magnification of 2000 times, and the area ratio of MA, the average aspect ratio, and the proportion of MA belonging to ferrite grain boundaries in the photograph were determined. Measured and calculated by image analysis.

  Furthermore, after the same sample was mirror-polished again, it was etched with a 3% nitric acid alcohol etchant and observed with an optical microscope at a magnification of 400 times to confirm the type of the first phase of the microstructure.

  Next, a No. 14A tensile test piece was sampled from the center of the base material part of the same specimen in accordance with JIS Z2201, and a tensile test was performed. The tensile test measured yield strength, tensile strength, and uniform elongation according to JIZ Z2241.

  Similarly, a 2 mm V-notch Charpy impact test piece was sampled from the center of the base material part of the test body according to JIS Z2202, and a Charpy impact test was performed. The Charpy test measured ductile-brittle fracture surface transition temperature (vTrs) according to JIS Z2242.

  Next, a chemical component analysis sample was collected from the weld metal of the submerged arc weld, and the chemical component of the weld metal was measured. Table 4 shows the measurement results.

  A sample for observing the microstructure of the weld metal was collected from the submerged arc weld, and the plate thickness section was mirror-polished, and then MA was revealed using a two-step etching method. Thereafter, a five-view microstructure photograph was randomly taken at a magnification of 2000 using a scanning electron microscope (SEM), and the area ratio of MA in the photograph was measured and calculated by image analysis.

  Furthermore, after the same sample was mirror-polished again, it was etched with a 3% nitric acid alcohol etchant and observed with an optical microscope at a magnification of 400 times to confirm the type of microstructure. Further, a 2 mm V notch Charpy impact test piece was sampled from the submerged arc welded part according to JIS Z2202, and a Charpy impact test of the weld metal was performed. The Charpy test was performed according to JIS Z2242, and the absorbed energy (average value of 3 bars) at a test temperature of −20 ° C. was measured.

  Results of microstructure analysis and machine of the base metal part and weld metal part of the specimen subjected to cold working + simulated heat treatment + inner / outer surface 1 layer submerged arc welding + pipe expansion equivalent cold working to simulate UOE steel pipe manufacturing process Table 5 summarizes the results of the investigation of the physical properties.

  No. 1-1 to 6 are examples in which the base material chemical composition, the weld metal chemical composition, the base metal microstructure, and the weld metal microstructure are within the scope of the present invention. The ratio of the above 1.5% proof stress and 0.5% proof stress, and the product of tensile strength and uniform elongation exceeds 7500 MPa ·%. Furthermore, the weld metal Charpy absorbed energy at the test temperature of −20 ° C. (average of three) Value) exceeded 100 J, and exhibited excellent low temperature toughness.

  On the other hand, comparative example No. which heated / cooled after submerged arc welding was performed after cold work distortion equivalent to a U-O press was given. 1-2, the total area ratio of acicular ferrite and bainite and the MA area ratio are both out of the scope of the present invention due to the heat effect of heating and cooling of the microstructure of the weld metal, and the Charpy absorbed energy of the weld metal is significantly reduced. did.

  In Comparative Example 1-3 in which the reheating temperature after applying cold working strain equivalent to the U-O press was below the lower limit of the present invention, austenitization did not occur during reheating, so the area ratio of MA was 5 which is the lower limit. As a result, the tensile strength was low and the ratio of 1.5% proof stress to 0.5% proof strength did not reach the target of the present invention.

  In Comparative Examples 1-4 in which the reheating temperature exceeded the upper limit of the present invention, MA was reversely transformed into austenite entirely by reheating, and then bainite transformation was performed during air cooling, and MA having a large aspect ratio was generated between bainite laths. . As a result, the product of tensile strength and uniform elongation was less than 7500 MPa ·%.

  In Comparative Example 7 in which the steel slab heating temperature during hot rolling was below the lower limit of the present invention, the steel slab was not completely austenitic, and the remaining ferrite phase remained to the end even after rolling, forming, and heating history. As a result, the MA fraction on the prior austenite grain boundaries was below the range of the present invention due to the influence of MA formation after reheating, and as a result, the product of tensile strength and uniform elongation was below 7500 MPa ·%.

  Similarly, in Comparative Example 8 in which the rolling end temperature during hot rolling was lower than the lower limit of the present invention, proeutectoid ferrite was generated during rolling, so that the tensile strength was reduced, and the MA on the prior austenite grain boundaries after reheating. Since the fraction was below the range of the present invention, the uniform elongation was reduced and the product of tensile strength and uniform elongation was below 7500.

  Further, in Comparative Example 9 in which the cooling rate of accelerated cooling after hot rolling falls below the lower limit of the present invention, the first phase microstructure of the base material part becomes ferrite, and the target API X80 grade tensile strength is Could not be achieved.

In Comparative Example 10 in which the cooling stop temperature of the accelerated cooling exceeded the upper limit of the present invention, MA already existed between the laths of the bainite that had undergone transformation, and this MA remained after the reheating treatment, so that the average aspect ratio of MA was range results around top of the present invention, the product of the tensile strength and the uniform elongation drops below ·% 7500MPa.

Comparative Example No. in which the amount of C in the base material part exceeded the upper limit of the present invention. 11, the first phase is Ri martensite der, as a result the uniform elongation is lowered, the product of the tensile strength and the uniform elongation drops below ·% 7500MPa. Comparative Example No. in which the amount of Si in the base material part exceeded the upper limit of the present invention. 12, the amount of Si in the weld metal also exceeds the upper limit, and as a result of the increase in the MA area ratio in the base metal part and the weld metal, the product of tensile strength and uniform elongation in the base metal part is less than 7500 MPa ·%, Charpy absorbed energy decreased in the weld metal.

  Comparative Example No. in which the amount of Mn in the base material part exceeded the upper limit of the present invention. In No. 13, the microstructure of the base material part was mainly martensite, the uniform elongation was low, and the product of tensile strength and uniform elongation was less than 7500 MPa ·%. Comparative Example No. in which the Nb content of the steel exceeded the upper limit of the present invention. No. 14, since the MA area ratio of the base material part was out of the range of the present invention, the uniform elongation was low, and the product of tensile strength and uniform elongation was less than 7500 MPa ·%.

  Comparative Examples 2-a, 2-b, 2-c, 2-d, 2-e, 2-f, 2-g, and 2-h, in which the chemical components of the weld metal are outside the scope of the present invention, are all mother Although the material part satisfies the target mechanical properties, the weld metal microstructure is outside the scope of the present invention, or as a result of excess TiO or precipitation hardening of Nb or V, the weld metal Charpy absorbed energy is reduced. did.

Claims (4)

The composition of the base material part is mass%,
C: 0.05-0.08%
Si: 0.1% or less,
Mn: 1.8-4.0%,
Al: 0.003 to 0.08%,
Nb: 0.01-0.03%,
Ti: 0.005 to 0.025%,
Further Cu: 0.1 to 1.0%,
Ni: 0.1 to 2.0%,
Mo: 0.1 to 1.0%,
Cr: 0.1 to 1.0%,
B: 0.0010 to 0.0030%,
1 type or 2 types or more, and the balance Fe and unavoidable impurities,
The component composition of the weld metal part is mass%,
C: 0.06 to 0.10%,
Si: 0.2 to 0.5%
Mn: 1.6-2.0%,
Al: 0.03% or less,
Nb: 0.005 to 0.025%,
Ti: 0.015-0.040%,
O: 0.015-0.04%,
N: 0.01% or less,
In addition, Cu: 0.1-0.3%,
Ni: 0.1 to 3.5%
Mo: 0.05-1.5%,
Cr: 0.1 to 1.5%
V: 0.025 to 0.1%,
B: 0.0005 to 0.003%,
1 type or 2 types or more, and the balance Fe and unavoidable impurities,
The base material part is an island-shaped martensite having an average aspect ratio of 2.0 or less in which the first phase is bainite and the second phase is dispersed in an area ratio of 5 to 20% in the first phase. 90% or more of martensite has a microstructure present in the prior austenite grain boundaries,
The weld metal part has a microstructure in which the area ratio of acicular ferrite and bainite is 80% or more and the area ratio of island martensite is 5% or less. High strength welded steel pipe with excellent weld toughness. The component composition of the base material part is further mass%,
Ca: 0.0005 to 0.01%,
REM: 0.0005 to 0.02%,
Zr: 0.0005 to 0.01%,
Mg: 0.0005 to 0.01%,
A high-strength welded steel pipe having high uniform elongation according to claim 1 and excellent in weld zone toughness. % By mass
C: 0.05-0.08%
Si: 0.1% or less,
Mn: 1.8-4.0%,
Al: 0.003 to 0.08%,
Nb: 0.01-0.03%,
Ti: 0.005 to 0.025%,
Further Cu: 0.1 to 1.0%,
Ni: 0.1 to 2.0%,
Mo: 0.1 to 1.0%,
Cr: 0.1 to 1.0%,
B: 0.0010 to 0.0030%,
A steel slab containing one or more of the above and comprising the balance Fe and inevitable impurities is re-heated to Ac ₃ or higher, then hot-rolled at a rolling end temperature Ar ₃ or higher, and a cooling rate of 10 to 80 ° C./s The steel sheet obtained by accelerating cooling to a temperature range of less than 400 ° C. and then air-cooled is formed into a cylindrical shape by cold forming, and then rapidly heated to Ac ₁ or more and Ac ₃ or less, followed by air cooling or water cooling. After cooling to room temperature, the end part is welded, and finally the pipe is expanded. The base material part is composed of bainite in the first phase and 5 to 20% in area ratio in the second phase in the first phase. The island-shaped martensite having a dispersed average aspect ratio of 2.0 or less has a microstructure in which 90% or more of the island-shaped martensite is present in the prior austenite grain boundaries, and the weld metal part is composed of acicular ferrite and The area ratio of bainite is 80 %, And an area ratio of island martensite is 5% or less, a method for producing a high strength welded steel pipe having high uniform elongation and excellent weld toughness. The component composition of the billet is further mass%,
Ca: 0.0005 to 0.01%,
REM: 0.0005 to 0.02%,
Zr: 0.0005 to 0.01%,
The high-strength welded steel pipe having high uniform elongation and excellent weld toughness according to claim 3, characterized by containing one or more of Mg: 0.0005 to 0.01%. Production method.