JP5217556B2 - High strength steel pipe for low temperature excellent in buckling resistance and weld heat affected zone toughness and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength steel pipe having APIX 80 to X100 grade strength, and particularly suitable for steel pipes for transporting natural gas and crude oil used in earthquake zones and frozen land zones where the plate thickness is about 20 mm to 40 mm and ground deformation is severe. It relates to a material excellent in buckling performance and weld heat-affected zone toughness.

  In recent years, welded steel pipes used for the transportation of natural gas and crude oil have been challenged to improve transport efficiency by increasing pressure and to improve the efficiency of local welding work by reducing the wall thickness. Progressing.

  In addition, since the environment in which steel pipes are used is expanding into cold and ground-fluctuating zones, improvement in low-temperature toughness and buckling resistance of welds is also an issue. Development of X100 grade steel pipe is desired.

  In the component design of high-strength steel plates used for X80 and X100 grade steel pipes, B addition is effective in securing strength and toughness, but in the case of steel pipes, weldability such as low temperature cracking susceptibility is also satisfied. In the conventional design of X80 and X100 grade steel pipes, boron with high hardenability in the base steel sheet is used to prevent low temperature cracks at the circumferential welds that connect the steel pipes that are small heat input welds. The component design without adding (B) was the basis (for example, Non-Patent Documents 1 and 2).

However, as the strength of the steel sheet increases, it is also reported that excellent seam weld heat affected zone toughness can be obtained by addition of B depending on the welding heat input of the seam weld (for example, Non-Patent Document 3).
Patent Document 1 also shows that the seam weld heat-affected zone toughness near the melting line is improved by diffusion of B contained in the weld metal into the base metal in the seam weld zone of the steel pipe.

  On the other hand, in the heat-affected zone of the B-added high strength steel, island-like martensite (MA) that is harmful to toughness even when the prior austenite grain size slightly separated from the melting line is as small as 150 μm or less. In a high strength steel, it is difficult to say that the influence of B addition on the toughness of the heat affected zone is well understood.

Even in the component design of thick X80 and X100 grade steel pipes with a tube thickness of over 20mm, excellent low temperature toughness of the weld heat affected zone at seam welds while ensuring strength, toughness, deformation performance and circumferential weldability. In order to ensure, it is necessary to clarify the influence of B addition on the weld heat affected zone structure.
JP 2006-328523 A NKK Technical Report No. 138 (1992), pp 24-31 NKK Technical Review No. 66 (1992) Welding Society Journal No. 50 (1981)

  Therefore, the present invention clarifies the effect of B addition on weldability and weld heat affected zone toughness for the base steel plate used for APIX 80 to X100 grade thick steel pipe, and the tensile strength is 620 MPa or more and 930 MPa or less. A weld bond at −30 ° C. using a base material having a uniform elongation of 5% or more and a ratio of 0.5% yield strength to tensile strength (yield ratio (YR)) of 85% or less. An object of the present invention is to provide a high-strength steel pipe for low temperature having a pipe thickness of 20 mm or more, which has an APIX 80 to X100 class with Charpy absorbed energy of 100 J or more and excellent in buckling resistance and weld heat affected zone toughness.

  In order to develop a low-temperature high-strength steel pipe having a thickness of 20 mm or more and excellent in buckling resistance and weld heat-affected zone toughness, the present inventors have conducted extensive research and obtained the following knowledge.

1. In the heat affected zone (HAZ) of the seam welded portion of the steel pipe, the portion (LBZ: Local Brittle Zone) where the toughness is most reduced is the CGHAZ structure near the bond on the outer surface side, and in the root portion on the inner surface side. The ICCGHAZ structure in which the CGHAZ structure on the inner surface is reheated to a two-phase region (Ac _{1 to} Ac ₃ points), both of which are HAZ coarse-grained regions (regions where the prior austenite particle size near the melting line is 50 μm or more: Coarse- grain HAZ, CGHAZ). The root portion refers to the vicinity of the meeting portion where the inner surface weld metal and the outer surface weld metal cross.

2. And P _CM value of the base material, by adjusting the cooling rate in the temperature range of 500 ° C. from 800 ° C. to transformation gamma-alpha phase in the cooling after welding, regardless of the outer surface side and inner surface side, the microstructure of CGHAZ The toughness is improved by forming a lower bainite structure, or an upper bainite containing a large amount of hard MA, or a lower bainite-based structure in which high-tensile martensite has a certain area fraction or less. In particular, when the lower bainite has a structure in which at least 50% area fraction is secured, the toughness is most improved and the Charpy absorbed energy at −30 ° C. is greatly improved.

3. In order to obtain the above-described microstructure CGHAZ, the addition of boron (B) to the base material is most effective, and the welding heat input is 80 kJ / cm or less (at a cooling rate of 800-500 ° C., 4 ° C./sec or more). If equivalent), the _{P CM} is 0.16 to 0.25 percent of the component composition APIX80~X100 class base metal strength is ensured, a suitable B addition amount range is 5～15Ppm.

  4). When improving buckling resistance: 1. Compression buckling limit strain on the bending compression side at the start of buckling and It is necessary to improve the fracture limit strain on the bending tension side, and it is effective to set the ratio of 0.5% proof stress to the tensile strength (yield ratio) to 85% or less and uniform elongation to 5% or more.

The present invention has been made by further studying the above findings, that is, the present invention
1. The composition of the base material is mass%,
C: 0.03-0.12%,
Si: 0.01 to 0.5%,
Mn: 1.5-3.0%
P: 0.015% or less,
S: 0.003% or less,
Al: 0.01 to 0.08%,
Nb: 0.005 to 0.08%,
Ti: 0.005 to 0.025%,
N: 0.001 to 0.010%,
O: 0.005% or less,
B: 0.0003 to 0.0020%
Further,
Cu: 0.01 to 1%,
Ni: 0.01 to 1%,
Cr: 0.01-1%,
Mo: 0.01 to 1%,
V: 0.01 to 0.1%
Containing one or more of
_{P CM} value calculated by the following formula (1) (unit is%) satisfies the 0.16 ≦ _{P CM} ≦ 0.25, balance being Fe and unavoidable impurities,
In the microstructure, the bainite structure including island-shaped martensite having an area ratio of 2% or more and 15% or less is 95% or more of the entire structure, and the contained island-shaped martensite has an equivalent circle diameter of 3 μm or less, and the tensile property of the base material is 620 MPa. A base material part having a uniform elongation of 5% or more with a tensile strength of 930 MPa or less and a ratio of 0.5% proof stress to tensile strength of 85% or less ;
The composition of the weld metal in seam welding is
C: 0.03-0.10%,
Si: 0.5% or less,
Mn: 1.5-3.0%
P: 0.015% or less,
S: 0.005% or less,
Al: 0.05% or less,
Nb: 0.005 to 0.05%,
Ti: 0.005 to 0.03%,
N: 0.010% or less,
O: 0.015-0.045%,
B: 0.0005 to 0.0050%
Further,
Cu: 0.01 to 1%,
Ni: 0.01-2%,
Cr: 0.01-1%,
Mo: 0.01 to 1%,
V: 0.01 to 0.1%
Containing one or more of
The balance consists of weld metal parts with Fe and inevitable impurities,
The microstructure of the weld heat affected zone where the prior austenite grain size is 50 μm or more in the vicinity of the melting line in the seam weld of the steel pipe is the lower bainite, or the lower bainite with an area ratio of at least 50%, and the upper bainite and / or A high-strength steel pipe for low temperature with excellent buckling resistance and weld heat-affected zone toughness characterized by a mixed structure with martensite.
P _CM (%) = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5 × B (1)
However, each element shows content (mass%).
2. 2. The seam welded portion of the steel pipe welded layer by layer from the inner and outer surfaces in the longitudinal direction of the steel pipe, wherein the weld heat affected zone hardness in the vicinity of the fusion line on the outer surface side satisfies the following formula (2): High strength steel pipe for low temperature with excellent buckling resistance and weld heat affected zone toughness.
250 ≦ HV (98N) ≦ 350 (2)
However, HV (98N): Vickers hardness measured at 10 kgf.
3. The high strength steel pipe for low temperature excellent in buckling resistance and weld heat affected zone toughness according to 1 or 2, wherein the joint strength of the seam welded portion of the steel pipe is 620 MPa or more and 930 MPa or less.
4 . Furthermore, in the chemical composition of the base metal part and / or the weld metal part,
Ca: 0.0005 to 0.01%,
REM: 0.0005 to 0.02%,
Zr: 0.0005 to 0.03%,
Mg: 0.0005 to 0.01%
A high-strength steel pipe for low temperature excellent in buckling resistance and weld heat-affected zone toughness according to any one of 1 to 3 , characterized by containing one or more of the above.
5 . The steel having the base material component described in 1 or 4 is heated to a temperature of 1000 to 1300 ° C., and the cumulative reduction rate at 800 ° C. to 950 ° C. is 10% or more and the cumulative reduction rate at 800 ° C. or less is 75%. % At a rolling finish temperature of 650 ° C. or higher, and accelerated cooling to a temperature of 350 ° C. or higher and lower than 650 ° C. at a cooling rate of 20 ° C./s or more, and then immediately 0.5 ° C./s. A method for producing a steel sheet for high-strength steel pipes for low temperature excellent in buckling resistance and weld heat-affected zone toughness, characterized by reheating to 500 to 700 ° C. above the accelerated cooling stop temperature at the above temperature rising rate .
6 . The steel sheet obtained by the manufacturing method according to 5 is formed into a cylindrical shape, and the welding heat input of each of the inner and outer surfaces when the butt portion is welded layer by layer from the inner and outer surfaces is 80 kJ / cm or less, and the outer surface side and inner surface A method for producing a low-temperature high-strength welded steel pipe excellent in buckling resistance and weld heat-affected zone toughness, characterized in that the heat input balance on the side satisfies the following formula (3):
Inner surface heat input ≦ Outer surface heat input (3)
7 . 7. The method for producing a low-temperature high-strength welded steel pipe according to 6, wherein the pipes are expanded one by one from the inner and outer surfaces in the longitudinal direction of the steel pipe and then expanded at a pipe expansion ratio of 0.4% or more and 2.0% or less.

  According to the present invention, an APIX 80-X100 grade high-strength steel pipe for low temperature with a pipe thickness of 20 mm or more, which is excellent in buckling resistance and weld heat affected zone toughness of seam welded parts, is obtained, which is extremely useful in industry. is there.

  In the present invention, the component composition and tensile properties of the base metal constituting the steel pipe, the component composition of the weld metal in the seam welded portion of the steel pipe, and the prior austenite grain size near the melting line in the longitudinal seam welded portion of the steel pipe are 50 μm or more. Defines the microstructure of the region.

  [Ingredient composition of base material] In the description,% is mass%.

C: 0.03-0.12%
C contributes to an increase in strength by being supersaturated in a low temperature transformation structure. In order to obtain this effect, 0.03% or more of addition is necessary, but if added over 0.12%, the hardness of the circumferential welded portion of the steel pipe is remarkably increased and cold cracking is likely to occur. Therefore, the upper limit is made 0.12%.

Si: 0.01 to 0.5%
Si is an element that acts as a deoxidizer and increases the strength of the steel by solid solution strengthening. However, if less than 0.01%, there is no effect, and if added over 0.5%, the toughness decreases significantly. Therefore, the upper limit is made 0.5%.

Mn: 1.5 to 3.0%
Mn acts as a hardenability improving element. The effect can be obtained by addition of 1.5% or more, but in the continuous casting process, the concentration rises at the center segregation part, and if it exceeds 3.0%, it causes delayed fracture at the center segregation part. Therefore, the upper limit is made 3.0%.

Al: 0.01 to 0.08%
Al acts as a deoxidizing element. A sufficient deoxidation effect can be obtained with addition of 0.01% or more, but if added over 0.08%, the cleanliness in the steel is lowered and the toughness is deteriorated, so the upper limit is 0.08%. And

Nb: 0.005 to 0.08%
Nb has an effect of expanding the austenite non-recrystallized region during hot rolling, and 0.005% or more is added to make the non-recrystallized region 950 ° C. or less. On the other hand, if added over 0.08%, the toughness of HAZ is significantly impaired, so the upper limit is made 0.08%.

Ti: 0.005-0.025%
Ti forms nitrides and is effective in reducing the amount of solute N in the steel. The precipitated TiN suppresses the coarsening of austenite grains by the pinning effect, and contributes to the improvement of the toughness of the base material and HAZ. Addition of 0.005% or more is necessary to obtain the pinning effect, but if added over 0.025%, carbides are formed, and the toughness is significantly deteriorated by precipitation hardening. Is 0.025%.

N: 0.001 to 0.01%
N usually exists as an inevitable impurity in steel, but TiN is formed by addition of Ti. In order to suppress the coarsening of the austenite grains due to the pinning effect by TiN, it is necessary to be present in the steel in an amount of 0.001% or more. Since TiN decomposes in a region heated to a temperature higher than or equal to 0 ° C. and the adverse effect of solute N is significant, the upper limit is made 0.01%.

B: 0.0003 to 0.0020%
B segregates at the austenite grain boundary in the heat affected zone, has the effect of improving hardenability, suppresses the formation of upper bainite containing MA harmful to toughness, and facilitates the formation of lower bainite or martensite.

This effect is significant when added in an amount of 0.0003% or more and 0.0020% or less, and if added over 0.0020%, the toughness decreases due to precipitation of B-based carbides, so the upper limit is 0.0020%. To do. Moreover, since the production | generation of an upper bainite structure will become remarkable when it is less than 0.0003%, a minimum is made into 0.0003%. _{Incidentally,} regardless of _{P CM} value, range maximum effect is obtained is 0.0015% or less, 0.0005% or more.

Cu, Ni, Cr, Mo, V, one or more of Cu, Ni, Cr, Mo, V all act as a hardenability improving element, so for the purpose of increasing the strength, one of these elements, Or two or more of them are added.

Cu: 0.01 to 1%
Cu contributes to the hardenability improvement of steel by adding 0.01% or more. However, since addition of 1% or more causes deterioration of toughness, the upper limit is made 1%, and when Cu is added, 0.01 to 1%.

Ni: 0.01 to 1%
Ni contributes to improving the hardenability of steel by adding 0.01% or more. In particular, even if added in a large amount, it does not cause toughness deterioration, so it is effective for toughening. However, since it is an expensive element, when Ni is added, the upper limit is 1%, and when Ni is added, it is 0. .01 to 1%.

Cr: 0.01 to 1%
Cr also contributes to improving the hardenability of steel by adding 0.01% or more. On the other hand, if added over 1%, the toughness deteriorates, so the upper limit is made 1%, and when adding Cr, the content is made 0.01-1%.

Mo: 0.01 to 1%
Mo also contributes to improving the hardenability of steel by adding 0.01% or more. On the other hand, if added over 1%, the toughness deteriorates, so the upper limit is made 1%, and when Mo is added, it is made 0.01-1%.

V: 0.01 to 0.1%
V forms precipitation strengthening by forming carbonitride, and contributes especially to the softening prevention of a weld heat affected zone. This effect can be obtained by addition of 0.01% or more, but if added over 0.1%, precipitation strengthening remarkably reduces toughness, so the upper limit is made 0.1% and 0 is added when V is added. 0.01 to 0.1%.

O: 0.005% or less, P: 0.015% or less, S: 0.003% or less In the present invention, O, P, and S are inevitable impurities and define the upper limit of the content. O is 0.005% or less in order to suppress the formation of inclusions that are coarse and adversely affect toughness. If the P content is large, the central segregation is remarkable and the base material toughness is deteriorated. If the content of S is large, the amount of MnS produced increases remarkably and the toughness of the base material deteriorates.

P _CM (%): 0.16-0.25
P _CM in weld crack sensitivity index expressed by C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5 × B, each element and content (mass%), the element-free is 0.

In the present invention, for achieving the above 620MPa in joint _strength, the _{P CM} and 0.16% or more and 0.25% or less in terms of the circumferential weldability ensured.

The above is the basic component composition of the base material part of the steel pipe according to the present invention, but when further improving the toughness of the welded part, one or more of Ca, REM, Zr, and Mg are added.
Ca, REM, Zr, Mg
Ca, REM, Zr, and Mg form oxysulfides or carbonitrides in steel, and are mainly added for the purpose of suppressing the austenite grain coarsening in the weld heat affected zone by the pinning effect and improving toughness.

Ca: 0.0005 to 0.01%
In the steelmaking process, when the Ca addition amount is less than 0.0005%, it is difficult to secure CaS due to the deoxidation reaction control, and a toughness improving effect cannot be obtained, so the lower limit of Ca is set to 0.0005%.

  On the other hand, when the amount of Ca added exceeds 0.01%, coarse CaO is likely to be generated, and the toughness including the base material is lowered, and the nozzle of the ladle is blocked and the productivity is hindered. The upper limit is 0.01%, and when added, 0.0005 to 0.01%.

REM: 0.0005 to 0.02%
REM forms an oxysulfide in steel and provides a pinning effect to prevent the weld heat affected zone from becoming coarse by adding 0.0005% or more. However, since it is an expensive element and the effect is saturated even if added over 0.02%, the upper limit is made 0.02%, and when added, it is made 0.0005 to 0.02%.

Zr: 0.0005 to 0.03%
Zr forms carbonitrides in steel and brings about a pinning effect that suppresses the coarsening of austenite grains, particularly in the weld heat affected zone. In order to obtain a sufficient pinning effect, addition of 0.0005% or more is necessary. However, when the addition exceeds 0.03%, the cleanliness in the steel is remarkably lowered and the toughness is lowered. Therefore, the upper limit is made 0.03%, and when added, the content is made 0.0005 to 0.03%.

Mg: 0.0005 to 0.01%
Mg is produced as fine oxides in the steel during the steelmaking process, and in particular, has a pinning effect that suppresses the coarsening of austenite grains in the weld heat affected zone. In order to obtain a sufficient pinning effect, addition of 0.0005% or more is necessary, but if added over 0.01%, the cleanliness in the steel is lowered and the toughness is lowered. The upper limit is 0.01%, and when added, 0.0005 to 0.01%.

  [Component composition of weld metal] In the description,% is mass%.

C: 0.03-0.10%
Also in the weld metal, C is an important element as a steel strengthening element. In particular, in order to achieve overmatching of the joint portion, the weld metal portion also needs to have a tensile strength of 620 MPa or more, and in order to obtain this strength, it is necessary to contain 0.03% or more. On the other hand, if it exceeds 0.10%, hot cracking of the weld metal tends to occur, so the upper limit was made 0.10%.

Si: 0.5% or less Si is useful for deoxidizing the weld metal and ensuring good workability. However, if it exceeds 0.5%, the weld workability is deteriorated. 5%.

Mn: 1.5 to 3.0%
Mn is an important element for increasing the strength of the weld metal. In particular, in order to make the tensile strength 620 MPa or more, it is necessary to contain 1.5% or more. However, if it exceeds 3.0%, the weldability deteriorates, so the upper limit was made 3.0%.

P: 0.015% or less, S: 0.005% or less P and S are segregated at grain boundaries in the weld metal and deteriorate their toughness, so the upper limits were made 0.015% and 0.005%, respectively. .

Al: 0.05% or less Al acts as a deoxidizing element. However, in the weld metal part, deoxidation with Ti has a larger effect of improving toughness, and when there are many inclusions of Al oxide, the weld metal Charpy is increased. Therefore, the upper limit is set to 0.05%.

Nb: 0.005 to 0.05%
Nb is an element effective for increasing the strength of the weld metal. In particular, in order to make the tensile strength 620 MPa or more, it is necessary to contain 0.005% or more. However, if it exceeds 0.05%, the toughness deteriorates, so the upper limit was made 0.05%.

Ti: 0.005 to 0.03%
Ti acts as a deoxidizing element in the weld metal and is effective in reducing oxygen in the weld metal. To obtain this effect, 0.005% or more must be contained, but when it exceeds 0.03%, the excess Ti forms carbides and degrades the toughness of the weld metal. Was 0.03%.

N: 0.010% or less Reduction of the solid solution N in the weld metal also has an effect of improving toughness, and the upper limit is made 0.010% because it is remarkably improved especially by setting it to 0.010% or less.

O: 0.015-0.045%
Reduction of the amount of oxygen in the weld metal has an effect of improving toughness, and the upper limit is made 0.045% because it is remarkably improved especially by setting it to 0.045% or less. On the other hand, if the amount of oxygen in the weld metal is less than 0.015%, the amount of oxide effective for refining the structure of the weld metal decreases, and conversely the toughness of the weld metal deteriorates, so the lower limit was made 0.015%. .

B: 0.0005 to 0.0050%
In a welded pipe for a line pipe with a strength grade of 620 MPa or more and 930 MPa or less, addition of B is effective in order to make the microstructure of the weld metal a fine bainite main structure. Addition of 0005% or more and 0.0050% or less is necessary. A more preferable range is 0.0010% or more and 0.0030% or less.

In the case of adding one or more of Cu, Ni, Cr, Mo, and V, one or more of Cu, Ni, Cr, Mo, and V, Cu: 0.01 to 1.0% or less, Ni: 0 0.01 to 2.0% or less, Cr: 0.01 to 1.0% or less, Mo: 0.01 to 1.0% or less.

  Since Cu, Ni, Cr, and Mo improve the hardenability in the weld metal as well as the base material, 0.01% or more is included for bainite organization. However, as the amount increases, the amount of alloying elements added to the welding wire increases and the wire strength increases significantly. As a result, the wire feedability at the time of submerged arc welding is obstructed. The upper limits were 1.0%, 2.0%, 1.0%, and 1.0%.

V: 0.01 to 0.1%
An appropriate amount of V addition is an effective element because it increases strength without degrading toughness and weldability. However, if it exceeds 0.1%, the toughness of the reheated portion of the weld metal deteriorates significantly, so the upper limit is 0. 0.1%.

  The above is the basic component composition of the weld metal part of the steel pipe according to the present invention. When the toughness of the weld metal part is further improved, one or more of Ca, REM, Zr and Mg are added.

Ca, REM, Zr, Mg
Ca, REM, Zr and Mg are added for the purpose of forming oxysulfides or carbonitrides in steel, suppressing the austenite grain coarsening in the weld metal part by the pinning effect, and improving toughness.

Ca: 0.0005 to 0.01%
When the Ca addition amount is less than 0.0005%, it is difficult to secure CaS due to the deoxidation reaction control, and the toughness improving effect cannot be obtained, so the lower limit of Ca is set to 0.0005%.

  On the other hand, when the Ca addition amount exceeds 0.01%, coarse CaO is likely to be generated and the toughness is lowered. Therefore, the upper limit is 0.01%, and when added, 0.0005 to 0.01%. And

REM: 0.0005 to 0.02%
REM forms an oxysulfide in steel, and adding 0.0005% or more brings about a pinning effect that prevents coarsening of austenite grains in the weld metal part. However, since it is an expensive element and the effect is saturated even if added over 0.02%, the upper limit is made 0.02%, and when added, it is made 0.0005 to 0.02%.

Zr: 0.0005 to 0.03%
Zr forms carbonitride in steel and brings about a pinning effect that suppresses coarsening of austenite grains in the weld metal part. In order to obtain a sufficient pinning effect, addition of 0.0005% or more is necessary, but if added over 0.03%, the cleanliness of the weld metal part is remarkably lowered and the toughness is lowered. Therefore, the upper limit is made 0.03%, and when added, the content is made 0.0005 to 0.03%.

Mg: 0.0005 to 0.01%
Mg is formed as a fine oxide and has a pinning effect that suppresses the coarsening of austenite grains in the weld metal part. In order to obtain a sufficient pinning effect, addition of 0.0005% or more is necessary. However, if it exceeds 0.01%, the cleanliness in the weld metal is lowered and the toughness is lowered. For this reason, the upper limit is set to 0.01%.
05 to 0.01%.

[Microstructure of base material]
In order to obtain a steel pipe having buckling resistance, it is desirable to use an SS curve having a round house type and a high work hardening index (n value) as tensile properties. The yield ratio (0.5% yield strength / tensile strength) is an index equivalent to the n value, and a two-phase structure combining a soft phase and a hard phase is effective to achieve a low yield ratio of 85% or less. It is. Here, tempered bainite having a soft phase and island martensite having a hard phase are utilized.

  By the way, in a thick and high-strength steel sheet exceeding 20 mm, in order to achieve a target ductile fracture surface ratio of 85% or more at −20 ° C. in a toughness evaluation test represented by the DWTT test, it is more than conventional. It is necessary to refine the microstructure.

  In particular, coarse island martensite structures are known to promote the occurrence and propagation of fractures, and in order to secure the target low temperature toughness, the structure size of island martensite and tempered martensite is highly accurate. Control is important.

  The yield ratio correlates with the area ratio of island martensite. If the area ratio is less than 2%, it is difficult to obtain a yield ratio of 85% or less. On the other hand, the ductile fracture surface ratio of toughness DWTT-20 ° C. correlates with the size of island martensite, and when the equivalent circle diameter of the largest island martensite exceeds 3 μm, the ductile fracture surface ratio is 85% or more. Is difficult to achieve.

  The equivalent circle diameter of the largest island martensite correlates with the area ratio of the island martensite. When the area ratio of the island martensite exceeds 15%, how low the unrecrystallized austenite region low temperature side ( Even if the cumulative reduction ratio (800 ° C. or lower) is increased to refine the structure size, the presence of island martensite having an equivalent circle diameter of 3 μm cannot be avoided.

  From the above, the microstructure of the base steel sheet is mainly composed of a bainite structure containing island-shaped martensite having an area ratio of 2% or more and 15% or less, and is defined as an equivalent circle diameter of 3 μm or less in the island-shaped martensite contained.

  Note that “mainly composed of a bainite structure including island-shaped martensite” means that 95% or more of the entire structure is the structure, and that the remainder includes pearlite or martensite. The area ratio of island martensite is identified by observing at least 10 fields of view at random with a scanning electron microscope (magnification 2000 times) at the center of the plate thickness.

[Microstructure of heat affected zone]
With increasing strength of steel pipes, conventional welding heat input tends to form upper bainite containing coarse island martensite as the microstructure of the weld heat affected zone, and low temperature toughness deteriorates. Therefore, it is necessary to suppress the upper bainite containing coarse island martensite to a certain area ratio or less.

  In particular, the lower bainite structure in which fine cementite is precipitated in the lath is known to be excellent in toughness while maintaining high strength, and this structure can be obtained by increasing the hardenability. As a means for improving the hardenability, a method by adding a component such as B or a method by increasing the cooling rate in the γ-α transformation section of the weld heat affected zone by a decrease in welding heat input can be considered.

  On the other hand, in the toughness evaluation test represented by the Charpy test, especially in the weld heat affected zone test, there is a heat affected zone structure heated to various maximum temperatures and a composite structure such as weld metal at the bottom of the notch. Since it is affected not only by the material of each heat-affected zone structure but also by the size of each heat-affected zone structure, variations in toughness are likely to occur.

  For this reason, in order to ensure the stable low temperature toughness, it is necessary to suppress the ratio of the most brittle structure (LBZ) to below a certain fraction. In particular, in order to achieve a cumulative failure probability of 1% or less when the joint HAZ Charpy test is performed 100 times or more at a test temperature of −30 ° C., the welding heat at which the prior austenite grain size is 50 μm or more in the vicinity of the melting line. In the affected area, it is important to suppress the upper bainite structure containing coarse island martensite to 50% or less in area ratio and obtain a lower bainite structure having an area ratio of at least 50%.

[Manufacturing conditions of base steel sheet]
In the present invention, the steel having the above-described component composition is heated to a temperature of 1000 to 1300 ° C., and the cumulative rolling reduction at 800 ° C. to 950 ° C. is 10% or more and the cumulative rolling reduction at 800 ° C. or less is 75%. After hot rolling at a rolling end temperature of 650 ° C. or higher so as to be above, accelerated cooling to a temperature of 350 ° C. or higher and lower than 650 ° C. at a cooling rate of 20 ° C./s or higher, and then immediately 0.5 ° C./s or higher The base steel plate is manufactured by reheating to 500 to 750 ° C., which is equal to or higher than the accelerated cooling stop temperature, at a rate of temperature increase.

  The reason for limiting the manufacturing method of the steel sheet will be described.

Heating temperature: 1000-1300 ° C
In performing hot rolling, the lower limit temperature for complete austenite is 1000 ° C. On the other hand, when the steel slab is heated to a temperature exceeding 1300 ° C., even if TiN pinning is performed, the austenite grain growth is remarkable and the base material toughness deteriorates, so the upper limit was set to 1300 ° C.

Cumulative rolling reduction at 800 ° C. and 950 ° C. or less: 10% or more Rolling on the relatively high temperature side of the austenite non-recrystallized region suppresses mixing such as formation of coarse austenite grains. Since the effect cannot be expected when the cumulative rolling reduction is less than 10%, the cumulative rolling reduction at 800 ° C. to 950 ° C. is set to 10% or more.

Cumulative rolling reduction at 800 ° C. or lower: 75% or more By performing large rolling in this temperature range on the low temperature side of the austenite non-recrystallized region, austenite grains are stretched, and bainite is transformed by accelerated cooling thereafter. The parent phase of the material becomes finer and the toughness is improved.

  In the present invention, in order to achieve a low yield ratio, in order to disperse island martensite in the second phase, it is necessary to promote the refinement of bainite, particularly with a reduction rate of 75% or more, and to prevent a decrease in toughness. Therefore, the cumulative rolling reduction at 800 ° C. or less is set to 75% or more.

Rolling end temperature: 650 ° C or more When the hot rolling end temperature is less than 650 ° C, proeutectoid ferrite is generated from the austenite grain boundary during the air cooling process, which causes a decrease in the strength of the base metal. Therefore, the lower limit temperature was set to 650 ° C.

Accelerated cooling rate: 20 ° C./s or higher To achieve a high strength of 620 MPa or higher, the microstructure must be a bainite-based structure. For this reason, accelerated cooling is performed after hot rolling. When the cooling rate is less than 20 ° C./s, the bainite transformation starts at a relatively high temperature, so that sufficient strength cannot be obtained. Therefore, the cooling rate of accelerated cooling is set to 20 ° C./s or more.

Cooling stop temperature for accelerated cooling: 350 to 650 ° C
This process is an important manufacturing condition in the present invention. In the present invention, C-concentrated untransformed austenite present after reheating is transformed into island martensite during subsequent air cooling.

  That is, it is necessary to stop the cooling in a temperature range where untransformed austenite during the bainite transformation exists. If the cooling stop temperature is less than 350 ° C., the bainite transformation is completed, so that sufficient island martensite cannot be obtained during air cooling, and a low yield ratio of 85% or less cannot be achieved.

  On the other hand, when the temperature exceeds 650 ° C., C is consumed in the pearlite that precipitates during cooling, and no island-like martensite is generated, so the upper limit was set to 650 ° C.

Reheating rate after stopping cooling: When the reheating rate is 0.5 ° C./s or more and less than 0.5 ° C./s, cementite in bainite is coarsened and the toughness of the base material is lowered. .5 ° C / s or more.

Reheating temperature after stopping cooling: 500-750 ° C
By reheating immediately after accelerated cooling, C can be concentrated in untransformed austenite, and island martensite can be generated in the subsequent air cooling process. When the reheating temperature is less than 500 ° C., C concentration to austenite does not occur sufficiently, and the required island-like martensite area ratio cannot be ensured.

  On the other hand, if the reheating temperature exceeds 750 ° C., the bainite transformed by accelerated cooling is austenite again and sufficient strength cannot be obtained, so the reheating temperature is specified to be 750 ° C. or less. There is no need to set the temperature holding time at the reheating temperature.

  Further, in the cooling process after reheating, island-shaped martensite is generated regardless of the cooling rate, and therefore it is preferable that the cooling after reheating is basically air cooling. Here, it is preferable that the reheating after the accelerated cooling is performed by a high-frequency heating device disposed on the same line (in-line) as the accelerated cooling device because it is possible to heat immediately after the accelerated cooling.

  In addition, although it does not specifically limit about the steel making method of steel, From a viewpoint of economical efficiency, it is desirable to cast the steel piece by the steelmaking process by a converter method, and the continuous casting process.

  By the above manufacturing process, the area ratio and particle size of island martensite are controlled, the tensile strength is 620 MPa or more and 930 MPa or less, the uniform elongation is 5% or more, and the ratio of 0.5% proof stress to the tensile strength. However, it is possible to obtain a steel sheet having a high toughness with a ductile fracture surface ratio of 85% or more in a DWTT test at -20 ° C while having a high deformation performance of 85% or less.

[Production conditions for steel pipes]
The high-strength steel pipe for low temperature excellent in buckling resistance and weld heat affected zone toughness according to the present invention is formed into a cylindrical shape with a U-press and an O-press in accordance with a conventional method for forming a base steel plate having the above-described tensile properties. Manufactured by seam welding.

  Seam welding is performed by submerged arc welding on the inner and outer surfaces one after the other after tack welding, and the flux used for submerged arc welding is not particularly limited, and may be a molten type or a fired type. Moreover, preheating before welding or heat treatment after welding is performed as necessary.

Submerged arc welding of the welding heat input (kJ / cm), the heat input 70 kJ / cm in the case _{P CM} of the base material steel plate in the component compositions plate thickness described above in order 20mm~40mm of 0.16 to 0.19% _hereinafter, in the range of heat input 80 kJ / cm when _{P cM} is 0.19 to 0.25%, as the heat affected zone of the microstructure prior austenite grain diameter is more than 50μm in the melt line vicinity, lower It adjusts so that the mixed structure provided with the bainite or the lower bainite of the area ratio at least 50% or more and the upper bainite and / or martensite may be obtained.

  When such a structure is adopted, it is effective in improving the low temperature toughness of LBZ (Local Brittle Zone) where the toughness is most deteriorated in the joint HAZ shown in FIG.

1A shows a Charpy test piece having an outer surface FL notch, and FIG. 1B shows a Charpy test piece having a root-FL notch.
LBZ refers to the CGHAZ structure in the vicinity of the bond on the outer surface side, and refers to the ICCGHAZ structure in which the CGHAZ structure on the inner surface is heated to a two-phase region (Ac1 to Ac3 points) in the root portion on the inner surface side.

  Particularly, if the heat input balance on the outer surface side and the inner surface side satisfies the following equation (3), the γ grain coarsening of the CGHAZ portion on the inner surface side can be suppressed, and FL ( The joint HAZ toughness sampled from the (Fusion line) position can be stably achieved.

“Securing stably” means that the cumulative failure probability is 1% or less when the joint HAZ Charpy test is performed 100 times or more at a test temperature of −30 ° C. or less.
Inner surface heat input ≦ Outer surface heat input (3)
Here, the lower bainite structure refers to the precipitation of carbides mainly composed of cementite in the lath of bainitic ferrite having a lath width of 1 μm or less, and the upper bainite is an island martensite (MA) and / Or it contains cementite. In the case of the above microstructure obtained by seam welding on the outer surface side, the hardness is 250 ≦ HV (98N) ≦ 350.

  After seam welding, pipe expansion is performed at a pipe expansion rate of 0.4% or more and 2.0% or less according to the required roundness. The CGHAZ microstructure of the weld heat-affected zone where the prior austenite grain size is 50 μm or more near the melting line is randomly observed with a scanning electron microscope (magnification 5000 times) at a position of 6 mm from the outer surface. To identify.

  Steels having various chemical compositions shown in Table 1 were melted in a converter and made into a slab of 220 mm thickness by continuous casting, and then steel plates A to I were subjected to hot rolling, accelerated cooling, and reheating conditions shown in Table 2. Produced. The reheating was performed using an induction heating type heating device installed on the same line as the accelerated cooling facility.

  Further, these steel sheets were formed by U press and O press, then subjected to inner seam welding by submerged arc welding and then outer seam welding. Thereafter, the pipe was expanded at a pipe expansion rate of 0.6 to 1.2% to obtain a steel pipe having an outer diameter of 400 to 1626 mm. Table 3 shows the chemical composition of the weld metal parts of the steel pipes 1-16.

  In order to evaluate the joint strength of the obtained steel pipe, a full thickness tensile test piece based on API-5L was taken from the base metal part and the seam welded part, and a tensile test was performed.

  In addition, JIS Z2202 (1980) V-notch Charpy impact test specimens were taken from the two positions of the outer surface FL and Root-FL shown in Fig. 1 from the welded joint of the steel pipe, and the Charpy impact test was performed at a test temperature of -30 ° C. Carried out. The notch position was a position where HAZ and weld metal exist at a ratio of 1: 1.

The microstructure of CGHAZ was observed with a scanning electron microscope (5000 times magnification) at a position 6 mm from the surface of CGHAZ of seam welding on the outer surface side.
Table 4 summarizes the test results of the hardness of CGHAZ and the toughness of CGHAZ (hereinafter referred to as HAZ toughness).

  Further, a V-notch Charpy impact test piece of JIS Z2202 (1980) was collected from the center position of the thickness of the base material portion of the steel pipe, and a Charpy impact test was performed at a test temperature of −40 ° C. Furthermore, a DWTT test piece compliant with API-5L was taken from the steel pipe and tested at a test temperature of −20 ° C. to determine the SA value (Shear Area: ductile fracture surface ratio).

  The tensile strength of the base steel plate is 620 MPa or more and 930 MPa or less, the uniform elongation is 5% or more, the ratio of 0.5% proof stress to the tensile strength is 85% or less, and the test temperature in the base material is −40 ° C. Charpy absorbed energy of 160 J or more, DWTT SA-20 ° C. is 85% or more, and the strength of the seam welded joint of the steel pipe is 620 MPa or more and 930 MPa or less, Charpy absorbed energy at the test temperature of −30 ° C. in CGHAZ is 100 J or more. Within the scope of the present invention.

  Invention Example No. 1 to 5 and 14 show the strength, yield ratio, uniform elongation, toughness of the desired base metal part, and the high HAZ toughness of the seam welded part. In the microstructure of the CGHAZ part, the area ratio is at least 50% or more. A mixed structure having lower bainite and the balance comprising upper bainite and / or martensite was obtained.

  On the other hand, Comparative Example No. 6 to 9 have high welding heat input, and in the microstructure of the joint CGHAZ part, the lower bainite fraction was lower than the lower limit of the present invention, and the upper bainite structure fraction became high. HAZ toughness decreased.

  Comparative Example No. No. 10 has extremely low seam welding heat input, and the fraction of the lower bainite in the microstructure of the joint CGHAZ part is below the lower limit of the present invention, and the fraction of the martensite structure is increased. Toughness decreased.

  Comparative Example No. No. 11 was a B-free system, and the HAB toughness was lowered in both the outer surface side and the inner surface side Root portion because the fraction of the upper bainite structure was increased.

Comparative Example No. _{12, P CM} is below the lower limit of the present invention, in the base metal strength is 620MPa or less, the lower bainite fraction in the microstructure of the joint CGHAZ portion is low, it CGHAZ tissue and upper bainite, outer side, the inner surface side Root The HAZ toughness of both parts decreased.

Comparative Example No. _{13, P CM} value exceeds the upper limit of the present invention, CGHAZ structure becomes martensite, the outer surface side, HAZ toughness on the inner surface side Root portion both decreased.

  Comparative Example No. No. 15 had a high welding heat input on the inner surface side, and the HAZ toughness of the inner surface side root portion was lowered.

  Comparative Example No. No. 16 has a welding heat input of 80 kJ / cm or less on both the inner surface side and the outer surface side, but the welding heat input on the inner surface side is higher than the welding heat input on the outer surface side, and the austenite grain size is large in the microstructure of the root part. Therefore, a coarse upper bainite structure was obtained, and the HAZ toughness on the root side was lowered.

It is a figure explaining the notch position in a welded joint Charpy test, (a) is a Charpy test piece of outer surface FL notch, (b) is a Charpy test piece of Root-FL notch.

Claims (7)

The composition of the base material is mass%,
C: 0.03-0.12%,
Si: 0.01 to 0.5%,
Mn: 1.5-3.0%
P: 0.015% or less,
S: 0.003% or less,
Al: 0.01 to 0.08%,
Nb: 0.005 to 0.08%,
Ti: 0.005 to 0.025%,
N: 0.001 to 0.010%,
O: 0.005% or less,
B: 0.0003 to 0.0020%
Further,
Cu: 0.01 to 1%,
Ni: 0.01 to 1%,
Cr: 0.01-1%,
Mo: 0.01 to 1%,
V: 0.01 to 0.1%
Containing one or more of
_{P CM} value calculated by the following formula (1) (unit is%) satisfies the 0.16 ≦ _{P CM} ≦ 0.25, balance being Fe and unavoidable impurities,
In the microstructure, the bainite structure including island-shaped martensite having an area ratio of 2% or more and 15% or less is 95% or more of the entire structure, and the contained island-shaped martensite has an equivalent circle diameter of 3 μm or less, and the tensile property of the base material is 620 MPa. A base material part having a uniform elongation of 5% or more with a tensile strength of 930 MPa or less and a ratio of 0.5% proof stress to tensile strength of 85% or less ;
The composition of the weld metal in seam welding is
C: 0.03-0.10%,
Si: 0.5% or less,
Mn: 1.5-3.0%
P: 0.015% or less,
S: 0.005% or less,
Al: 0.05% or less,
Nb: 0.005 to 0.05%,
Ti: 0.005 to 0.03%,
N: 0.010% or less,
O: 0.015-0.045%,
B: 0.0005 to 0.0050%
Further,
Cu: 0.01 to 1%,
Ni: 0.01-2%,
Cr: 0.01-1%,
Mo: 0.01 to 1%,
V: 0.01 to 0.1%
Containing one or more of
The balance consists of weld metal parts with Fe and inevitable impurities,
The microstructure of the weld heat affected zone where the prior austenite grain size is 50 μm or more in the vicinity of the melting line in the seam weld of the steel pipe is the lower bainite, or the lower bainite with an area ratio of at least 50%, and the upper bainite and / or A high-strength steel pipe for low temperature with excellent buckling resistance and weld heat-affected zone toughness characterized by a mixed structure with martensite.
P _CM (%) = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5 × B (1)
However, each element shows content (mass%). In the seam welded portion of the steel pipe welded layer by layer from the inner and outer surfaces in the longitudinal direction of the steel pipe, the weld heat affected zone hardness in the vicinity of the fusion line on the outer surface side satisfies the following formula (2). High-strength steel pipe for low temperature with excellent buckling resistance and weld heat-affected zone toughness.
250 ≦ HV (98N) ≦ 350 (2)
However, HV (98N): Vickers hardness measured at 10 kgf.   The high strength steel pipe for low temperature excellent in buckling resistance and weld heat affected zone toughness according to claim 1 or 2, wherein the joint strength of the seam welded part of the steel pipe is 620 MPa or more and 930 MPa or less. Furthermore, in the chemical composition of the base metal part and / or the weld metal part,
Ca: 0.0005 to 0.01%,
REM: 0.0005 to 0.02%,
Zr: 0.0005 to 0.03%,
Mg: 0.0005 to 0.01%
A high-strength steel pipe for low temperature excellent in buckling resistance and weld heat-affected zone toughness according to any one of claims 1 to 3 , characterized by containing at least one of the following. The steel having the base material component according to claim 1 or 4 is heated to a temperature of 1000 to 1300 ° C, and the cumulative rolling reduction at 800 ° C to 950 ° C is 10% or more and 800 ° C or less. Is hot rolled at a rolling finish temperature of 650 ° C. or higher so that the ratio is 75% or higher, then accelerated to a temperature of 350 ° C. or higher and lower than 650 ° C. at a cooling rate of 20 ° C./s or higher, and then immediately 0.5 ° C. A steel sheet for high-strength steel pipes for low temperatures, which is excellent in buckling resistance and weld heat affected zone toughness, characterized by reheating to 500 to 700 ° C. above the accelerated cooling stop temperature at a rate of temperature rise of at least / s Production method. The steel sheet obtained by the manufacturing method according to claim 5 is formed into a cylindrical shape, and the welding heat input of each of the inner and outer surfaces when the butt portion is welded layer by layer from the inner and outer surfaces is 80 kJ / cm or less, and the outer surface side And a method for producing a low-temperature high-strength welded steel pipe excellent in buckling resistance and weld heat-affected zone toughness, characterized in that the heat input balance on the inner surface side satisfies the following formula (3).
Inner surface heat input ≦ Outer surface heat input (3)   The low-temperature high-strength welded steel pipe according to claim 6, wherein the steel pipe is welded one by one from the inner and outer surfaces in the longitudinal direction of the steel pipe and then expanded at a pipe expansion ratio of 0.4% or more and 2.0% or less. Method.

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