JP3770106B2 - High strength steel and its manufacturing method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-strength steel having a tensile strength of 750 MPa or more and excellent in unstable fracture resistance characteristics, which is suitable for use in line pipes and various pressure vessels for transporting natural gas and crude oil.
[0002]
[Prior art]
In pipelines that transport natural gas and crude oil over long distances, there is a growing need to increase the strength of the pipe material itself and limit the increase in wall thickness, with the aim of reducing laying and transportation costs.
[0003]
Currently, in the American Petroleum Institute (API), X80 (tensile strength of 620 MPa or more) grade steel has been standardized and put into practical use, and even higher strength X100 (tensile strength of 750 MPa or more) and more than X100 (for example, Application of high-strength grade steel having a tensile strength of 900 MPa or more is also being studied.
[0004]
For example, Japanese Patent Application Laid-Open No. 8-199292 and Japanese Patent Application Laid-Open No. 2000-199036 propose a high-strength line pipe of X100 super grade with a high Mn content and a method for manufacturing the same.
[0005]
In line pipes, both strength characteristics and unstable fracture characteristics are important among the required characteristics to be provided as a structural material. In particular, regarding the latter unstable fracture characteristics, it is necessary to ensure a well-balanced characteristic of both brittle fracture characteristics and unstable ductile fracture characteristics which are ductile unstable fracture characteristics.
[0006]
Both of the above characteristics cannot be grasped by a small fracture test such as a Charpy impact test, and have excellent brittle fracture resistance characteristics and unstable ductile fracture resistance characteristics in DWTT tests specified by API, for example, full-thickness steel pipe test pieces. It is necessary to ensure. In the case of a high-strength line pipe of X100 grade or higher, it is considered necessary to evaluate unstable fracture characteristics by a full thickness fracture test.
[0007]
However, in the techniques disclosed in both of the above publications, the fracture characteristics are evaluated only by the Charpy impact test, and the brittle fracture characteristics and unstable ductile fracture characteristics in the actual pipe are not studied at all. In other words, in high-strength steels of X100 grade or higher, brittle fracture resistance characteristics and unstable ductile fracture resistance characteristics (hereinafter, both characteristics are collectively referred to as unstable fracture resistance characteristics) in actual pipes are improved as structural materials. Little is known about technology to improve safety.
[0008]
[Problems to be solved by the invention]
The subject of the present invention is based on the above-mentioned circumstances, and has a tensile strength of 750 MPa or more (preferably 900 MPa or more) and 85% ductile fracture surface transition temperature (SATT) in the DWTT test specified in the API standard._85%  : ° C.) absorbed energy (vE) at −30 ° C. or lower and −30 ° C._DWTT: J) of 5000 J or more, to provide a high-strength steel excellent in unstable fracture resistance characteristics, specifically a steel pipe including a steel plate and a welded pipe, and a production method capable of stably producing them. It is in.
[0009]
[Means for Solving the Problems]
As a result of repeated experiments to achieve the above-mentioned problems, the present inventor has found the following.
[0010]
In the conventional strength grade steel, the difference between the unstable fracture characteristics by the small fracture test and the unstable fracture characteristics of the steel pipe body is relatively small, and the relationship between them is clear.
[0011]
On the other hand, in the case of high strength steel of X100 (tensile strength of 750 MPa or more) grade or higher, the unstable fracture characteristics of the steel pipe body cannot be grasped in the small fracture test, and it is specified in the API which is the full thickness test of the steel pipe. It was confirmed that it was necessary to evaluate in the DWTT test. That is, 85% ductile fracture surface transition temperature (SATT) in the DWTT test._85%  ) And absorbed energy (vE_DWTT) Revealed that the unstable fracture characteristics of high-strength steel pipes can be grasped.
[0012]
And in high-strength steel with a tensile strength of 750 MPa or more, in order to obtain excellent unstable fracture resistance characteristics without impairing the weldability and weld zone toughness at the time of welding, the following measures may be taken. I found out.
[0013]
(a) The S content of a low strength C high strength steel having a C content of 0.1 mass% or less and a tensile strength of 750 MPa or more is 0.0008 mass% or less, preferably 0.0006 mass%, more preferably Is 0.0004 mass% or less, for example, even if the steel pipe has a tensile strength of 930 MPa or more, the 85% ductile fracture surface transition temperature (SATT) in the DWTT test_85%  ) And absorbed energy (vE_DWTT), The brittle fracture resistance characteristics and the unstable ductile fracture resistance characteristics in the partial gas burst and full gas burst tests are dramatically improved, and the fracture safety as a line pipe is improved.
[0014]
  (b) The content of P, N and O (oxygen) contained in the steel simultaneously with the reduction of the S contentThe expressionWhen adjusted to a value satisfying “{20 × S + P + 5 × (N + O)} ≦ 0.035”, the occurrence and stopping characteristics of brittle cracks and ductile cracks are improved, and 85% ductile fracture surface transition temperature (SATT) in the DWTT test._85%) And absorbed energy (vE_DWTT)ForI will go up.
[0015]
(c) When the total proportion of the martensite phase and the bainite phase in the metal structure of the surface layer portion and the thickness center portion is 95% by volume or more and 80% by volume or more, the occurrence and stopping characteristics of brittle cracks and ductile cracks This further improves the unstable fracture resistance characteristics of the steel pipe.
[0016]
  The present invention has been completed based on the above findings, and the gist thereof is as follows.(1)High strength steel,(2)Steel sheet,(3)-(4)Steel pipe, and(5)The manufacturing method of high strength steel.
[0017]
  (1) By mass%, C: 0.01 to 0.10%, Si: 0.30% or less, Mn: 1.00 to 2.50%, P: 0.010% or less, S: 0.0008 %, Nb: 0.005 to 0.06%, Ti: 0.004 to 0.025%, sol. Al:0.015-0.05%,N: 0.0040% or less, O: 0.003% or less, Ni: 0 to 2.5%, Cu: 0 to 00.62%, Mo: 0 to 0.8%, Cr: 0 to 1.0%, V: 0 to 0.1%, B: 0 to 0.002%, Zr: 0 to 0.03%, Ca: 0 It is made of steel that contains ~ 0.003%, the balance is Fe and impurities, and satisfies the following formula (1), and the tensile strength is 750 MPa or moreFurthermore, the total ratio of the martensite phase and the bainite phase in the metal structure of the surface layer portion and the central thickness portion is 95% by volume or more and 80% by volume or more, respectively.High strength steel.
[0018]
  {20 × S + P + 5 × (N + O)} ≦0.035.... (1)
  Here, the element symbol in the formula (1) means the content (% by mass) of each element contained in the steel.
[0020]
  (2)the above(1)A high-strength steel plate made of the described high-strength steel.
[0021]
  (3)the above(1)A high-strength steel pipe made of the described high-strength steel.
[0022]
  (4)The base material is above(1)It is a welded steel pipe made of the described high-strength steel, the tensile strength of the weld metal is 700 MPa or more, and (base material tensile strength −50) MPa or more, and the proportion of the acicular ferrite structure in the weld metal is 10 to 10. High-strength steel pipe that is 80% by volume.
[0023]
  (5)In mass%, C: 0.01 to 0.10%, Si: 0.30% or less, Mn: 1.00 to 2.50%, P: 0.010% or less, S: 0.0008% or less, Nb: 0.005-0.06%, Ti: 0.004-0.025%, sol. Al:0.015-0.05%,N: 0.0040% or less, O: 0.003% or less, Ni: 0 to 2.5%, Cu: 0 to 00.62%, Mo: 0 to 0.8%, Cr: 0 to 1.0%, V: 0 to 0.1%, B: 0 to 0.002%, Zr: 0 to 0.03%, Ca: 0 The steel satisfying the following formula (1) is heated to 950 to 1200 ° C. with the remaining Fe and impurities including -0.003%, followed by hot rolling to finish rolling at a finishing temperature of 900 to 600 ° C. A method for producing high-strength steel that is accelerated and cooled at a cooling rate of 4 ° C./second or higher from a temperature range not lower than 500 ° C. to a temperature of 300 ° C. or lower.
[0024]
  {20 × S + P + 5 × (N + O)} ≦0.035.... (1)
  Here, the element symbol in the formula (1) means the content (% by mass) of each element contained in the steel.
[0025]
  In addition, said (1)-(5)In the present invention described in the above, it is not always necessary to positively add and contain the elements Ni, Cu, Mo, Cr, V, B, Zr and Ca.
[0026]
  Also, above(1)The surface layer portion in the range from the surface to the 1/10 position of the thickness, and the thickness center portion refers to the range from the thickness center to the 1/4 position of the thickness.
[0027]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the reason why the high-strength steel (steel plate, steel pipe), the manufacturing method thereof, and the welded steel pipe of the present invention are defined as described above will be described in detail. Note that. In the following, “%” means “% by mass” unless otherwise specified.
[0028]
First, the chemical composition of steel will be described.
[0029]
C: 0.01 to 0.10%
C is contained for the purpose of securing the strength. However, if the content is less than 0.01%, the hardenability is insufficient and it is difficult to secure a tensile strength of 750 MPa or more, and the toughness is not sufficient. On the other hand, if the content exceeds 0.10%, not only the toughness of the steel and its welded portion, particularly the weld heat affected zone, but also the unstable fracture resistance characteristics are lowered. Moreover, the weldability at the time of welding construction also falls. For this reason, C content was made into 0.01 to 0.10%. A preferable range is 0.02 to 0.08%, and a more preferable range is 0.03 to 0.05%.
[0030]
Si: 0.30% or less
Si is usually added as a deoxidizing agent. However, if its content exceeds 0.30%, not only the toughness of the steel and its welded portion is lowered, but also the unstable fracture resistance characteristics are lowered. For this reason, Si content was 0.30 or less. A preferable upper limit is 0.15%, and a more preferable upper limit is 0.10%. The lower limit is not particularly defined, but the Si content is preferably 0.02% or more in order to obtain a sufficient deoxidation effect.
[0031]
Mn: 1.00-2.50%
Mn is added to improve the hardenability and increase the strength, but if the content is less than 1.00%, it is difficult to ensure a tensile strength of 750 MPa or more. On the other hand, if the content exceeds 2.50%, the toughness of the steel and the welded portion thereof decreases. For this reason, Mn content was made into 1.00-2.50%. A preferable range is 1.2 to 1.9%, and a more preferable range is 1.2 to 1.7%.
[0032]
P: 0.010% or less
P is an impurity element that not only lowers the low temperature toughness of the steel and its welded part, particularly the welded heat affected zone, but also lowers the weldability and further reduces the unstable fracture resistance characteristics. Therefore, the lower the P content, the better. However, inevitable mixing is inevitable, and excessive reduction leads to an increase in cost. Therefore, the upper limit is set to 0.010% as a limit that does not cause actual harm. A preferable upper limit is 0.008%, and a more preferable upper limit is 0.005%. In addition, P content needs to satisfy | fill (1) Formula mentioned later.
[0033]
S: 0.0008% or less
S is an impurity element similar to P described above, and not only lowers the low temperature toughness of the steel and its welds, particularly the weld heat affected zone, but also lowers the weldability. Further, unlike P described above, S is an element in which the content reduction is indispensable in the present invention, because the unstable fracture resistance characteristics of high-strength steel are significantly deteriorated in a small amount. That is, in order to impart sufficient unstable fracture resistance characteristics to high strength steel having a tensile strength of 750 MPa or more, especially 900 MPa or more, it is preferable to reduce the S content as much as possible, but inevitable mixing is inevitable. Since excessive reduction leads to an increase in cost, the limit is set to 0.0008% or less as a limit that does not cause actual harm. A preferable upper limit is 0.0006%, and a more preferable upper limit is 0.0004%. In addition, S content needs to satisfy | fill (1) Formula mentioned later.
[0034]
Nb: 0.005 to 0.06%
Nb is an element that refines the structure of steel and significantly improves the toughness of high-strength steel, and also improves the unstable fracture resistance characteristics by suppressing the occurrence and stopping of brittle cracks and ductile cracks. If the content is less than 0.005%, the above effect cannot be obtained. On the other hand, if the content exceeds 0.06%, not only the weldability is impaired, but also the unstable fracture resistance characteristics are deteriorated. For this reason, Nb content was made into 0.005 to 0.06%. A preferable range is 0.005 to 0.03%, and a more preferable range is 0.005 to 0.02%.
[0035]
Ti: 0.004 to 0.025%
Ti is an element that refines the structure of steel and its weld heat-affected zone and improves the low-temperature toughness of steel and its weld heat-affected zone, but the above effect cannot be obtained with a content of less than 0.004%. . On the other hand, if the content exceeds 0.015%, not only the low temperature toughness of the steel and its welded part, particularly the welded heat-affected zone, is impaired, but also the weldability is lowered, and the unstable fracture resistance characteristics are also lowered. . For this reason, Ti content was made into 0.004-0.025%. A preferable range is 0.004 to 0.015%, and a more preferable range is 0.004 to 0.010%.
[0036]
  sol. Al:0.015-0.05%
  AlIs an element usually added as a deoxidizer,Its content is sol. With Al content of 0.015% or more,The following N contained as impurities in steel is fixed as AlN and stabilized, and has the effect of improving unstable fracture resistance characteristics.The on the other hand,Its content is sol. If the Al content exceeds 0.05%, not only the properties of the welded portion deteriorate, but also the weldability decreases. For this reason, the content of Al is sol. With Al content0.015-0.05%Whendid. A preferable upper limit is 0.035%, and a more preferable upper limit is 0.025%.The
[0037]
N: 0.0040% or less
N is an impurity element, and it is extremely harmful to the improvement of unstable fracture resistance characteristics because it lowers the toughness of the steel and also lowers the ductile fracture resistance. If the content exceeds 0.0040%, the desired content Unstable fracture resistance characteristics cannot be secured. For this reason, N content was made into 0.0040% or less. The preferable upper limit is 0.0025%, and the more preferable upper limit is 0.0020%, but the lower the N content, the better. In addition, N content needs to satisfy | fill (1) Formula mentioned later.
[0038]
O (oxygen): 0.003% or less
O is an impurity element similar to N described above, and is an element that is extremely harmful to the improvement of unstable fracture resistance because it lowers the toughness of the steel and lowers the ductile fracture resistance. If it exceeds 003%, the desired unstable fracture resistance characteristics cannot be secured. Therefore, the O content is set to 0.003% or less. The preferable upper limit is 0.0018%, and the more preferable upper limit is 0.0012%, but the lower the O content, the better. The O content needs to satisfy the formula (1) described later.
[0039]
  Relationship of P, S, N and O:
  The content of these elements needs to satisfy the following formula (1) within the above-described range. That is, when the contents of P, S, N, and O do not satisfy the following formula (1), the occurrence of brittle cracks and ductile cracks in steel, the propagation stop property are remarkably lowered, and the desired unstable fracture resistance The characteristics cannot be securedYes. thisThis is also clear from examples described later.
[0040]
  {20 × S + P + 5 × (N + O)} ≦0.035.... (1)
  Here, the element symbol in the formula (1) means the content (% by mass) of each element contained in the steel.
[0041]
The above formula (1) is the result of the following experiment conducted by the present inventor. From the obtained results, each element is an unstable fracture resistance characteristic of steel, specifically, the DWTT test specified in the API standard. 85% ductile fracture surface transition temperature (SATT)_85%  ) And absorbed energy (vE_DWTT) This is a formula that is determined only after investigating the effect on
[0042]
Experiment contents:
For many steels with different chemical compositions, after hot rolling at a finishing temperature of 1000 to 800 ° C., the plate is subjected to accelerated cooling treatment that is cooled from 750 to 600 ° C. to 300 ° C. or less at a cooling rate of 18 to 35 ° C./second. A steel plate with a thickness of 20 mm is obtained. Next, a test piece was taken from the direction perpendicular to the rolling direction of the obtained steel sheet, and subjected to a DW T test specified by API, and an 85% ductile fracture surface transition temperature (SATT) was obtained._85%  ) And absorbed energy (vE_DWTT)
[0043]
Also, the above equation (1) represents the following. That is, S forms a sulfide such as MnS, and a part thereof is extended by rolling to become a fine sulfide, which significantly promotes the generation and propagation of brittle cracks and further degrades the ductile crack resistance. Therefore, the degree of influence should be the largest such that the coefficient is 20. N and O form nitrides and oxides, respectively, to deteriorate the ductile progress crack resistance, and also promote the generation and propagation of brittle cracks and the propagation of ductile progress cracks in the solid solution state, but the coefficient is 5 As shown, the impact is small compared to S. P segregates microscopically or macroscopically to facilitate the occurrence of brittle cracks and ductile cracks, but has a much smaller influence than S, N, and O so that the coefficient is 1.
[0044]
  High strength steel of the present inventionThe chemical composition ofSaid aboveStuffIf necessary, one or more of Ni, Cu, Cr, Mo, V, B, Ca and Zr may be positively added and contained as necessary. In this case, the high strength, corrosion resistance and unstable fracture resistance characteristics are further improved without impairing the low temperature toughness and weldability of the steel and its welded parts, especially the welded heat affected zone. Can be obtained.
[0045]
  Ni:0-2.5%(The desirable lower limit at the time of addition is 0.2%)
  NiThe addition of is optional. NiIsIf added,In addition to improving low temperature toughness and brittle crack propagation stopping performance to improve unstable fracture resistance characteristics, it also has the effect of improving weldability. These effectsIs 0.2It becomes remarkable at a content of more than%. However, even if the content exceeds 2.5%, not only the cost for improving the effect is reduced for the cost increase, but also excessive retained austenite is generated by quenching-tempering treatment, and the yield strength is reduced. May end up. For this reason,The Ni content was 0 to 2.5%.The Ni content in the case of positive addition is preferably 0.2 to 2.5%.
[0046]
  Cu: 00.62% (The desirable lower limit when added is 0.1%)
  Cr: 0 to 1.0% (desirable lower limit when added is 0.1%)
  Mo: 0 to 0.8% (desirable lower limit at the time of addition is 0.1%)
  V: 0 to 0.1% (desirable lower limit when added is 0.005%)
  B: 0 to 0.003% (desirable lower limit when added is 0.0003%)
  Addition of these elements is optional. Any of these elements, when added, has the effect of improving hardenability and strengthening the steel. This effect becomes significant at a content of 0.1% or more for Cu, Cr and Mo, 0.005% or more for V, and 0.0003% or more for B. However, Cu is0.62If the content exceeds 50%, the toughness of the steel and its welds is impaired, and the hot ductility may be significantly reduced. Further, when Cr, Mo, V and B are contained in amounts exceeding 1.0%, 0.8%, 0.1% and 0.003%, respectively, the strength increases excessively, and steel and The toughness of the weld may be impaired. For this reason, the contents of Cu, Cr, Mo, V and B are set to 0 to 0, respectively.0.62%, 0-1.0%, 0-0.8%, 0-0.1% and 0-0.003%. The contents of Cu, Cr, Mo, V and B when actively added and contained are each 0.1 to0.62%, 0.1 to 1.0%, 0.1 to 0.8%, 0.005 to 0.1%, and 0.0003 to 0.003%.
[0047]
Of the above elements, Cr also has the effect of increasing the strength and toughness by the precipitation strengthening action during tempering treatment, and Mo has the effect of increasing the strength and toughness by the solid solution strengthening action. At the same time, there is an effect of dispersing the appropriate amount (0.5 to 5% by volume) of retained austenite in the steel and improving the unstable fracture resistance characteristics. V also has the effect of stabilizing elements (N, C, O) that are harmful to the strain aging characteristics and improving the strain aging characteristics.
[0048]
  Ca:0-0.003%(The desirable lower limit at the time of addition is 0.0005%)
  Zr:0-0.03%(The desirable lower limit at the time of addition is 0.005%)
  These elementsThe addition of is optional. These elementsIsIf added,Both control the form of inclusions in the steel, improve the toughness and corrosion resistance of the steel and its welds, stabilize the elements (N, C, O) harmful to brittle fracture, and provide unstable fracture resistance characteristics It has the effect | action which improves. These effectsIs CaIn this case, it becomes remarkable when the content is 0.0005% or more and Zr is 0.005% or more. However, if Ca is contained in an amount exceeding 0.003% and Zr exceeds 0.03%, not only the cleanliness of the steel is lowered, but the toughness of the steel and its welded portion is lowered, as well as unstable ductile crack resistance. The characteristics are also degraded. For this reason,The contents of Ca and Zr were 0 to 0.003% and 0 to 0.03%, respectively.The content of Ca and Zr in the case of positive addition is preferably 0.0005 to 0.003% and 0.005 to 0.03%, respectively.
[0049]
  About the metal structure:
  The high-strength steel of the present invention is used as a steel plate or a steel pipe, but the metal structure is 95% by volume or more in the total proportion of the martensite phase and the bainite phase in the metal structure of the surface layer portion, and the thickness center portion. The total proportion of martensite phase and bainite phase in the metal structure ofThein this caseNot goodStable breakdown resistance characteristicsForI will go up.
[0050]
  The high-strength steel of the present invention detailed above (including steel plates and seamless steel pipes) is a chemical composition of steel.And metallographic structureAs long as the conditions specified in the present invention are satisfied, a method of tempering by reheating and quenching after normal hot rolling, a method of quenching and tempering directly after normal hot rolling, and also normal hot rolling Although it is possible to manufacture by an accelerated cooling method later, etc., it is preferable to manufacture by an accelerated cooling process after hot rolling under the following conditions for reliable and stable manufacturing.
[0051]
Heating temperature:
If the heating temperature is less than 950 ° C., a tensile strength of 750 MPa or more may not be ensured. On the other hand, if the heating temperature exceeds 1200 ° C., the element (N, C, O) harmful to the occurrence of brittle fracture and ductile fracture stop after the subsequent hot rolling becomes insufficient, and the desired unstable fracture resistance The characteristics may not be ensured. For this reason, the heating temperature is desirably 950 to 1200 ° C.
[0052]
Hot rolling finishing temperature:
When the finishing temperature of hot rolling is less than 600 ° C., a tensile strength of 750 MPa or more may not be ensured. Moreover, when the finishing temperature of hot rolling exceeds 900 ° C., the structure is not sufficiently refined by rolling and subsequent accelerated cooling, and the elements (N, C) that are harmful to the occurrence of brittle fracture and to stop ductile fracture are stabilized. In some cases, the desired unstable fracture resistance characteristics cannot be ensured. For this reason, it is desirable that the finishing temperature of hot rolling be 600 to 900 ° C.
[0053]
Water cooling start temperature:
If the water cooling start temperature during accelerated cooling is less than 500 ° C., a tensile strength of 750 MPa or more may not be ensured. For this reason, the water cooling start temperature at the time of accelerated cooling is preferably 500 ° C. or higher.
[0054]
Cooling rate:
When the cooling rate during accelerated cooling is less than 4 ° C./second, coarse upper bainite is mixed in the structure, and good low temperature toughness and unstable fracture resistance characteristics may not be ensured. For this reason, the cooling rate during accelerated cooling is preferably 4 ° C./second. The cooling rate may be 4 ° C./second or more, and it is not necessary to specify the upper limit.
[0055]
Water cooling stop temperature:
When the water cooling stop temperature during accelerated cooling exceeds 300 ° C., not only a tensile strength of 750 MPa or more cannot be secured, but also an appropriate amount (0.5 to 5% by volume) of retained austenite present in the steel decomposes and is desired. Unstable fracture resistance characteristics may not be ensured. For this reason, the water cooling stop temperature at the time of accelerated cooling is preferably 300 ° C. or lower.
[0056]
Next, the welded steel pipe of the present invention will be described.
[0057]
The welded steel pipe of the present invention may be manufactured by any known pipe manufacturing method as long as the base material is a steel plate made of the high-strength steel of the present invention. Specifically, forged steel pipes, ERW welded steel pipes, laser welded steel pipes, electron beam welded steel pipes, plasma arc welded steel pipes, TIG welded steel pipes, SAW welded steel pipes represented by UOE steel pipes and spiral steel pipes, MAG welded steel pipes, MIG welding A steel pipe etc. can be mentioned.
[0058]
However, in the welded steel pipe, the tensile strength of the weld metal is 700 MPa or more, (base material tensile strength−50) MPa or more, and the proportion of the acicular ferrite structure in the weld metal is 10 to 80% by volume. There is a need.
[0059]
The reason is as follows. In other words, if the tensile strength of the weld metal is less than 700 MPa, the tensile strength of the weld joint does not exceed 750 MPa, and the tensile strength of the weld metal is less than (base material tensile strength−50) MPa. If it exists, the distortion at the time of a deformation | transformation concentrates on a weld metal and a welding heat affected zone, and it becomes impossible to ensure a desired unstable fracture resistance characteristic. Furthermore, if the proportion of the acicular ferrite structure in the weld metal is less than 10% by volume, the low temperature toughness and deformability of the weld metal are insufficient, and the desired unstable fracture resistance characteristics cannot be ensured. This is because if it exceeds 80% by volume, the desired strength cannot be ensured.
[0060]
Here, the welded steel pipe satisfying the above condition is a forged steel pipe, an electric resistance welded steel pipe, a laser welded steel pipe, an electron beam welded steel pipe, a plasma arc welded steel pipe, and a steel plate made of the high strength steel of the present invention. It is obtained by welding and pipe making according to a conventional method.
[0061]
In the case of SAW welded steel pipe, MAG welded steel pipe, MIG welded steel pipe typified by TIG welded steel pipe, UOE steel pipe and spiral steel pipe, the steel plate made of the high-strength steel of the present invention is used as a base material, and the chemical composition is the present invention. A welding wire having a Pcm value in the range of 0.20 to 0.32 defined by the following formula (2) is used, and the welding site is Ar, He, N₂, CO₂It can be obtained by welding by shielding with a gas such as, or by using a calcined or molten flux.
[0062]
Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B (2)
Here, the element symbol in a formula is content (mass%) of each element contained in steel.
[0063]
【Example】
Ten types of steel having the chemical composition shown in Table 1 are made into steel plates having a thickness of 20 mm under various conditions shown in Table 2, and a UOE welded steel pipe having an outer diameter of 1014 mm and a length of 12 m using the obtained steel plate as a base material. Manufactured.
[0064]
At that time, welding was performed for each pass of the inner and outer surfaces using a four-electrode SAW welder under a heat input of 3 to 4 kJ / mm. As the welding wire, one having a chemical composition almost the same as that of each steel and having a Pcm value of 0.27 to 0.30 defined by the above-described equation (2) was used. The tube expansion rate after welding was set to 0.8 to 1.2%.
[0065]
And about each obtained steel pipe, intensity | strength of a base material part (yield strength YS (MPa), tensile strength TS (MPa)), toughness (fracture surface transition temperature vTs: ° C), metal of surface layer part and center part Volume ratio (vol.%) Of martensite (M) + bainite (B) in the structure, volume ratio (vol.%) Of acicular ferrite (AF) contained in the weld metal, tensile strength TS of the weld metal (MPa) and toughness (Charpy absorbed energy vE at a test temperature of -30 ° C)_{-30 ° C}While examining (J), it was subjected to the DWTT test specified in the API standard, and the 85% ductile fracture surface transition temperature (SATT)._85%  : ° C) and absorbed energy (vE_DWTT: J) and these results are shown together in Table 2.
[0066]
The tensile strength and toughness of the weld metal were examined by cutting a test piece from the weld metal portion.
[0067]
As is apparent from Table 2, in all of the UOE welded steel pipes of trial numbers 1 to 9 that satisfy the conditions specified in the present invention, the tensile strength TS of the base metal is 765 MPa or more, and the tensile strength TS of the weld metal is 822 MPa. The fracture surface transition temperature of the base metal is -86 ° C or lower, the Charpy absorbed energy of the weld metal is 105J or higher, and the toughness is 85% ductile fracture surface transition temperature SATT of the DWTT test._85%  DWTT absorption energy vE at −41 ° C. or lower and −30 ° C._{DWTT-30 ℃}No less than 6887 J, both significantly exceeding the target of −30 ° C. or lower and 5000 J or higher.
[0068]
  In contrast, the formula (1) defined in the present inventionDoes not meetThe UOE welded steel pipe of the trial number 10 made of steel with the mark H exceeding 0.0008% of the upper limit specified in the present invention has good strength and toughness of the base metal and the weld metal, but the SATT_85%-18 ° C, vE_{DWTT-30 ℃}2775J, both of which are well below the target of -30 ° C and below 5000J.
[0069]
In addition, the content of each element is within the range specified in the present invention, but the UOE welded steel pipe of trial number 11 made of steel of symbol I that does not satisfy the formula (1) defined in the present invention is a base material. Although the strength and toughness are good, since the weld metal does not contain an acicular ferrite structure, the weld metal has poor toughness._85%  Is -22 ° C, vE_{DWTT-30 ℃}3125J, both of which are well below the target of -30 ° C and below 5000J.
[0070]
  Further, the formula (1) defined in the present inventionDoes not meetThe UOE welded steel pipe of No. 12 made of steel with the symbol J exceeding the upper limit specified by the present invention in the contents of C and N, although the strength and toughness of the base metal are good, the tensile strength of the weld metal Is 902 MPa which is lower than 938 MPa which is a value obtained by subtracting 50 MPa from the tensile strength of the base material._85%-15 ° C, vE_{DWTT-30 ℃}3112J, both of which are well below the target of -30 ° C or lower and 5000J or higher.
[0071]
The above results mean that the same results can be obtained with the high-strength steel of the present invention, a steel plate made of this high-strength steel, a seamless steel pipe, and a welded steel pipe other than a UOE welded steel pipe. Not too long.
[0072]
[Table 1]

Claims (5)

In mass%, C: 0.01 to 0.10%, Si: 0.30% or less, Mn: 1.00 to 2.50%, P: 0.010% or less, S: 0.0008% or less, Nb: 0.005-0.06%, Ti: 0.004-0.025%, sol. Al: 0.015 to 0.05 %, N: 0.0040% or less, O: 0.003% or less, Ni: 0 to 2.5%, Cu: 0 to 0.62 %, Mo: 0 to 0 0.8%, Cr: 0 to 1.0%, V: 0 to 0.1%, B: 0 to 0.002%, Zr: 0 to 0.03%, Ca: 0 to 0.003% in the balance Fe and impurities, and made of steel which satisfies the following equation (1), a tensile strength of Ri der least 750 MPa, furthermore, martensite and bainite phase occupying the surface layer portion and a thick central portion of the metal structure total proportion of, respectively, 95% by volume or more and 80% by volume or more der Ru high strength steel.
{20 × S + P + 5 × (N + O)} ≦ 0.035 (1)
Here, the element symbol in the formula (1) means the content (% by mass) of each element contained in the steel. A high-strength steel plate made of the high-strength steel according to claim 1 . A high-strength steel pipe made of the high-strength steel according to claim 1 . The base metal part is a welded steel pipe made of the high-strength steel according to claim 1 , wherein the weld metal has a tensile strength of 700 MPa or more, and (base material tensile strength−50) MPa or more, and the weld metal in the weld metal. A high-strength steel pipe in which the proportion of the curl ferrite structure is 10 to 80% by volume. In mass%, C: 0.01 to 0.10%, Si: 0.30% or less, Mn: 1.00 to 2.50%, P: 0.010% or less, S: 0.0008% or less, Nb: 0.005-0.06%, Ti: 0.004-0.025%, sol. Al: 0.015 to 0.05 %, N: 0.0040% or less, O: 0.003% or less, Ni: 0 to 2.5%, Cu: 0 to 0.62 %, Mo: 0 to 0 0.8%, Cr: 0 to 1.0%, V: 0 to 0.1%, B: 0 to 0.002%, Zr: 0 to 0.03%, Ca: 0 to 0.003% Then, the steel satisfying the following formula (1) with the balance Fe and impurities is heated to 950 to 1200 ° C., and then hot-rolled to finish the rolling at a finishing temperature of 900 to 600 ° C. and not lower than 500 ° C. A method for producing high-strength steel that is accelerated and cooled from a temperature range to a temperature of 300 ° C. or lower at a cooling rate of 4 ° C./second or higher.
{20 × S + P + 5 × (N + O)} ≦ 0.035 (1)
Here, the element symbol in the formula (1) means the content (% by mass) of each element contained in the steel.