JP2003003233A - High strength steel and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high tensile strength steel (a steel sheet, and a steel pipe including a welded pipe) having unstable fracture resisting characteristics, in which 85% ductile fracture transition temperature SATT 85% in a DWTT test in API standards is <=-30 deg.C, and absorbed energy EDWTT at -30 deg.C is >=5,000 J, and to provide a production method therefor. SOLUTION: The high strengh steel has a composition containing 0.01 to 0.10% C, <=0.30% Si, 1.00 to 2.50% Mn, <=0.010% P, <=0.0008% S, 0.005 to 0.06% Nb, 0.004 to 0.025% Ti, <=0.05% sol. Al, <=0.0040% N and <=0.003% O, and the balance Fe with impurities, and also satisfying the inequality of 20×S+P+5×(N+O)}<=0.045, and has TS(tensile strength) of <=750 MPa.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength steel having a tensile strength of 750 MPa or more, which is suitable for use in line pipes for transporting natural gas and crude oil, various pressure vessels, and the like and is excellent in unstable fracture resistance.

[0002]

2. Description of the Related Art In pipelines for transporting natural gas and crude oil over long distances, there is an increasing need to limit the increase in wall thickness by strengthening the pipe material itself in order to reduce laying costs and transportation costs.

At present, the American Petroleum Institute (API) standardizes X80 (tensile strength of 620 MPa or more) grade steel and puts it into practical use.
Application of high-strength grade steels of 100 (tensile strength of 750 MPa or more) and more than X100 (for example, tensile strength of 900 MPa or more) is also under consideration.

For example, Japanese Unexamined Patent Publication No. 8-199292 and Japanese Unexamined Patent Publication No. 2000-199036 propose a high-strength line pipe of X100 super grade having a high Mn content and a method for producing the same.

In the line pipe, both the strength characteristics and the 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 secure a well-balanced characteristic of both brittle fracture characteristics and unstable ductile fracture characteristics that are ductile unstable fracture characteristics.

Both of the above characteristics cannot be grasped by a small fracture test such as a Charpy impact test, and a brittle fracture resistance characteristic and an unstable ductility which are excellent in a test piece having a full thickness of a steel pipe, for example, a DWTT test specified by API. It is necessary to secure the breakdown resistance characteristics. For high strength line pipes of X100 grade or higher, it is considered necessary to evaluate the unstable fracture characteristics by a full-thickness fracture test.

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 the unstable ductile fracture characteristics in a real pipe are not examined at all. . That is, X100
For high-strength steels of grade or higher, the brittle fracture resistance characteristics and unstable ductile fracture resistance characteristics of actual pipes (both characteristics are collectively referred to as unstable fracture resistance characteristics) are improved to improve safety as structural materials. Little is known about the technology to boost it.

[0008]

SUMMARY OF THE INVENTION In view of the above-mentioned circumstances, an object of the present invention is to provide a tensile strength of 750 MPa or more (preferably 900 MPa or more) and 85% ductility in a DWTT test specified by API standard. Fracture transition temperature (SATT _85% : ° C) is -30 ° C or lower, -30
Energy absorbed at ℃ (vE _DWTT : J) is 5000
High tensile steel with J or more, which has excellent unstable fracture resistance characteristics,
Specifically, it is to provide a steel pipe including a steel plate and a welded pipe and a manufacturing method capable of stably manufacturing these.

[0009]

The present inventor has found the following as a result of repeated experiments and studies in order to achieve the above object.

In the conventional strength grade steel, the difference between the unstable fracture characteristics in the small fracture test and the unstable fracture characteristics of the steel pipe body was relatively small, and the relationship between them was also clear.

On the other hand, X100 (tensile strength 750
For high-strength steels of (MPa or higher) grade or higher, the unstable fracture characteristics of the steel pipe body cannot be grasped by the small-scale fracture test, and the DWTT specified in the API for full-thickness test of steel pipe
It was confirmed that the test needs to be evaluated. That is, the 85% ductile fracture surface transition temperature (S
It was found that the unstable fracture characteristics of the high-strength steel pipe can be grasped by ATT _85% ) and absorbed energy (vE _DWTT ).

In order to obtain excellent unstable fracture resistance characteristics in high-strength steel having a tensile strength of 750 MPa or more without deteriorating the weldability and the toughness of the welded portion during welding, the following means should be taken. I found it good.

(A) The S content of a high-strength steel having a C content of 0.1% by mass or less and a tensile strength of 750 MPa or more of a low C system is 0.0008% by mass or less, preferably 0.0006% by mass. And more preferably 0.0004 mass% or less, for example, even for a steel pipe having a tensile strength of 930 MPa or more, 85% ductile fracture surface transition temperature (SATT _85% ) and absorbed energy (v
E _DWTT ) is improved, brittle fracture resistance characteristics and unstable ductile fracture resistance characteristics in the partial gas burst and full gas burst tests are dramatically improved, and fracture safety as a line pipe is improved.

(B) At the same time as reducing the S content, the contents of P, N and O (oxygen) contained in the steel are calculated by the formula "{20 ×
S + P + 5 × (N + O)} ≦ 0.045 ”, preferably the formula“ {20 × S + P + 5 × (N + O)} ≦ 0.035 ”
When adjusted to a value satisfying the above conditions, the initiation and termination properties of brittle cracks and ductile cracks are improved, and the 85% ductile fracture surface transition temperature (SATT _85% ) and absorbed energy (vE _DWTT ) in the DWTT _test are further improved.

(C) When the total proportions of the martensite phase and the bainite phase in the metal structure of the surface layer portion and the thickness center portion are 95% by volume and 80% by volume, respectively, brittle cracks and ductile cracks are generated. And the stopping characteristics are further improved,
The unstable fracture resistance characteristics of steel pipes become more stable.

The present invention has been completed based on the above findings, and the gist thereof is as follows: (1) and (2) high strength steel, (3) steel plate, and (4) to (5). The method is for producing a steel pipe and (6) high-strength steel.

(1) 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 to 0.06%, Ti: 0.004 to
0.025%, sol. Al: 0.05% or less, N:
0.0040% or less, O: 0.003% or less, Ni:
2.5% or less, Cu: 1.5% or less, Mo: 0.8% or less, Cr: 1.0% or less, V: 0.1% or less, B: 0.
002% or less, Zr: 0.03% or less, Ca: 0.00
It contains 3% or less, and the balance is Fe and impurities.
A high-strength steel made of steel that satisfies the formula and having a tensile strength of 750 MPa or more.

{20 × S + P + 5 × (N + O)} ≦ 0.045 (1) Here, the element symbol in the formula (1) is the content (% by mass) of each element contained in the steel. means.

(2) The total proportion of the martensite phase and the bainite phase in the metal structure of the surface layer portion and the central portion of the wall thickness is
The high-strength steel according to (1) above, which is 95% by volume or more and 80% by volume or more, respectively.

(3) A high-strength steel sheet made of the high-strength steel according to (1) or (2) above.

(4) A high-strength steel pipe made of the high-strength steel described in (1) or (2) above.

(5) The base metal part is a welded steel pipe made of the high-strength steel described in (1) or (2) above, the tensile strength of the weld metal is 700 MPa or more, and the (base metal tensile strength is −
50) A high-strength steel pipe having a MPa or higher and a proportion of the acicular ferrite structure in the weld metal of 10 to 80% by volume.

(6) 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 to 0.06%, Ti: 0.004 to
0.025%, sol. Al: 0.05% or less, N:
0.0040% or less, O: 0.003% or less, Ni:
2.5% or less, Cu: 1.5% or less, Mo: 0.80%
Below, Cr: 1.0% or less, V: 0.1% or less, B:
0.002% or less, Zr: 0.03% or less, Ca: 0.
After heating steel containing 0030% or less and the balance Fe and impurities and satisfying the following formula (1) to 950 to 1200 ° C.,
Manufacture of high-strength steel that is hot-rolled to finish rolling at a finishing temperature of 900 to 600 ° C. and acceleratedly cooled at a cooling rate of 4 ° C./sec or more from a temperature range not lower than 500 ° C. to a temperature of 300 ° C. or less. Method.

{20 × S + P + 5 × (N + O)} ≦ 0.045 (1) Here, the element symbol in the formula (1) is the content (% by mass) of each element contained in the steel. means.

In the present invention described in (1) to (6) above, the elements Ni, Cu, Mo, Cr, V, B, Zr and Ca need not necessarily be positively added and contained. Alternatively, the content may be at the impurity level.

In addition, the surface layer portion in the above (5) means a range from the surface to the 1/10 position of the wall thickness, and the center portion of the wall thickness is from the center of the wall thickness to 1/4 position of the wall thickness. Within the range.

[0027]

BEST MODE FOR CARRYING OUT THE INVENTION The high-strength steel (steel plate,
Steel pipe), its manufacturing method, and the reason for defining the welded steel pipe as described above will be described in detail. Incidentally. In the following, "%" means "mass%" unless otherwise specified.

First, the chemical composition of steel will be described.

C: 0.01 to 0.10% C is contained for the purpose of ensuring strength, but 0.01%
If the content is less than 1, 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 contrary, if the content exceeds 0.10%, not only the toughness of the steel and its welded portion, particularly the weld heat affected zone is deteriorated, but also the unstable fracture resistance characteristic is deteriorated. In addition, the weldability during welding is also reduced. Therefore, the C content is set to 0.01 to 0.10%. The preferred range is 0.02
~ 0.08%, more preferably 0.03 to 0.05
%.

Si: 0.30% or less Si is usually added as a deoxidizing agent, but if its content exceeds 0.30%, not only the toughness of steel and its welded portion is deteriorated, but also the Stable breakdown resistance characteristics also deteriorate. Therefore, the Si content is set to 0.30 or less. A preferred upper limit is 0.15%, and a more preferred upper limit is 0.10%. The lower limit is not particularly specified, but it is desirable that the Si content be 0.02% or more in order to obtain a sufficient deoxidizing effect.

Mn: 1.00 to 2.50% Mn is contained in order to improve hardenability and strength, but a content of less than 1.00% secures a tensile strength of 750 MPa or more. Is difficult. On the contrary, 2.50
If it is contained in excess of%, the toughness of steel and its welded portion will be reduced. Therefore, the Mn content is 1.00 to 2.50.
%. A preferable range is 1.2 to 1.9%, and a more preferable range is 1.2 to 1.7%.

P: 0.010% or less P is an impurity element, which not only lowers the low temperature toughness of the steel and its welds, especially the heat-affected zone of the weld, but also lowers the weldability and further provides an unstable fracture resistance. It also deteriorates the characteristics. Therefore, the lower the P content is, the more preferable it is, but unavoidable mixing is unavoidable, and excessive reduction causes an increase in cost. Therefore, the upper limit is set to 0.010% so as not to cause actual damage. The preferred upper limit is 0.008
%, And a more preferable upper limit is 0.005%. Note that P
The content must satisfy the formula (1) described later.

S: 0.0008% or less S is an impurity element similar to the above P, and not only lowers the low temperature toughness of the steel and its welded part, especially the weld heat affected zone, but also the weldability. Let Further, S, unlike P described above, remarkably deteriorates the unstable fracture resistance characteristics of high-strength steel in a small amount, so that a reduction in its content is an essential element in the present invention. That is, tensile strength 75
It is preferable to make the S content as low as possible in order to impart sufficient unstable fracture resistance characteristics to high-strength steels of 0 MPa or more, and particularly 900 MPa or more, but inevitable mixing is unavoidable and excessive reduction is not possible. Since it causes an increase in cost, it is set to 0.0008% or less as a limit that does not cause actual damage. A preferable upper limit is 0.0006%, and a more preferable upper limit is 0.0004%. The S content will be described later.
It is necessary to satisfy the formula (1).

Nb: 0.005 to 0.06% Nb refines the structure of the steel and significantly improves the toughness of high-strength steel, and also suppresses the occurrence of brittle cracks and ductile cracks and accelerates the stoppage of Nb. Although it is an element that improves the stable fracture resistance characteristic, 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 characteristic is rather deteriorated. Therefore, the Nb content is 0.005 to 0.
It was set to 06%. The preferred range is 0.005-0.03
%, And a more preferable range is 0.005 to 0.02%.

Ti: 0.004 to 0.025% Ti refines the structure of steel and its heat-affected zone,
Although it is an element that improves the low temperature toughness of steel and its heat affected zone, if the content is less than 0.004%, the above effect cannot be obtained. On the other hand, if the content exceeds 0.015%, not only the low-temperature toughness of the steel and its welds, especially the heat-affected zone of the weld, is impaired, but also the weldability is reduced and the unstable fracture resistance property is also reduced. . Therefore, the Ti content is set to 0.004 to 0.025%. The preferred range is 0.
004 to 0.015%, more preferably 0.004
Is about 0.010%.

Sol. Al: 0.05% or less Al is an element that is usually added as a deoxidizer, and the following N contained as an impurity in steel is fixed and stabilized as AlN to improve unstable fracture resistance characteristics. However, the content of sol. 0.05% in Al content
If it exceeds, not only the characteristics of the welded portion are deteriorated, but also the weldability is rather deteriorated. Therefore, the Al content is so
l. The Al content is 0.05% or less. A preferable upper limit is 0.035%, and a more preferable upper limit is 0.025%. The lower limit need not be specified, but in order to obtain the above-mentioned effects sufficiently, sol. Al content 0.000
It is desirable to be 5% or more.

N: 0.0040% or less N is an impurity element, which lowers the toughness of the steel and also reduces the ductile fracture resistance, and is extremely harmful for improving the unstable fracture resistance characteristics. If it exceeds 0.0040%, the desired unstable fracture resistance cannot be ensured.
Therefore, the N content is set to 0.0040% or less. A preferred upper limit is 0.0025%, and a more preferred upper limit is 0.0
Although it is 020%, the lower the N content, the better.
The N content must satisfy the formula (1) described later.

O (oxygen): 0.003% or less O is an impurity element similar to the above-mentioned N, which lowers the toughness of the steel and also reduces the ductile fracture resistance, thus improving the unstable fracture resistance characteristics. It is an extremely harmful element, and if its content exceeds 0.003%, desired unstable fracture resistance characteristics cannot be secured. Therefore, the O content is 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 must satisfy the formula (1) described later.

Relationship between P, S, N and O: The contents of these elements are as follows within the above-mentioned ranges.
The content must satisfy the formula (1). That is,
If the contents of P, S, N and O do not satisfy the formula (1) below, the occurrence of brittle cracks and ductile cracks in the steel, the propagation stopping properties are significantly reduced, and the desired unstable fracture resistance properties are Cannot be secured. The preferable upper limit value calculated on the left side of the following formula (1) is 0.035. In this case, the unstable breakdown resistance characteristic is further improved. These facts are also clear from the examples described later.

{20 × S + P + 5 × (N + O)} ≦ 0.045 (1) where the element symbol in the formula (1) is the content (% by mass) of each element contained in the steel. Means

The above formula (1) is obtained from the results of the experiments conducted by the inventor of the present invention. Based on the results obtained, each element is defined by the unstable fracture resistance characteristics of steel, specifically, the API standard. 85% ductile fracture transition temperature (SA
TT _85% ) and the effect on absorbed energy (vE _DWTT ) are investigated, and this is the formula first determined by multiple analysis.

Experiment content: After subjecting many steels having different chemical compositions to hot rolling at a finishing temperature of 1000 to 800 ° C., 7
300 from 50-600 ℃ at a cooling rate of 18-35 ℃ / sec
Accelerated cooling process to cool below ℃
m steel plate is obtained. Then, a test piece is taken from a direction orthogonal to the rolling direction of the obtained steel sheet, and D specified by API is taken.
A W-T test was performed and the 85% ductile fracture surface transition temperature (SA
TT _85% ) and absorbed energy (vE _DWTT ) are investigated.

The above equation (1) represents the following.
That is, S forms sulfides such as MnS, and a part thereof is extended by rolling to become fine sulfides, which significantly promotes the generation and propagation of brittle cracks and further deteriorates ductility-propagation crack resistance. Therefore, the degree of influence is the largest, as the coefficient is 20. N and O respectively form nitrides and oxides to deteriorate ductile growth crack resistance, and promote the initiation and propagation of brittle cracks and the propagation of ductile growth cracks even in a solid solution state, but the coefficient is 5 As described above, the degree of influence is smaller than that of S. P segregates into micro or macro to facilitate the generation of brittle cracks and ductile cracks, but as the coefficient is 1, its influence is much smaller than that of S, N and O.

The high-strength steel of the present invention is sufficient if it has the chemical composition described above, but if necessary, Ni, Cu,
Any one or more of Cr, Mo, V, B, Ca and Zr may be positively added and contained. In this case, high-strength, corrosion resistance, and unstable fracture resistance properties are further improved without impairing the low-temperature toughness and weldability of the steel and its welds, especially the heat-affected zone of welding, and thicker steel plates and steel pipes, etc. Can be obtained.

Ni: 2.5% or less (desirable lower limit of 0.2% at the time of addition) Ni improves the low temperature toughness and brittle crack propagation stopping performance of the steel to improve the unstable fracture resistance characteristics, as well as welding. It also has the effect of improving the sex. These effects can be obtained even at the impurity amount level, but become remarkable when the content is 0.2% or more. However, even if the content exceeds 2.5%, not only the margin for improving the above-mentioned effect becomes small for the cost increase, but also excessive residual austenite is generated by the quenching-tempering treatment, and the yield strength decreases. It may happen. For this reason,
When positively added and contained, the Ni content is 0.2 to
2.5% is preferable.

Cu: 1.5% or less (desirable lower limit of 0.1%) Cr: 1.0% or less (desirable lower limit of 0.1)
%) Mo: 0.8% or less (desirable lower limit at the time of addition is 0.1
%) V: 0.1% or less (desirable lower limit of addition is 0.005
%) B: 0.003% or less (desirable lower limit at the time of addition is 0.0
(003%) All of these elements have the effect of improving hardenability and strengthening the steel. This effect is
Cu, Cr and Mo can be obtained even at the impurity level.
In 0.1% or more, V is 0.005% or more, and B is 0.0003% or more. But C
If u is contained in an amount of more than 1.5%, the toughness of steel and its welded portion may be impaired, and the hot ductility may be significantly reduced. When Cr, Mo, V, and B are contained in an amount of more than 1.0%, 0.8%, 0.1%, and 0.003%, respectively, the strength increases excessively,
The toughness of steel and its welds may be compromised. For this reason, Cu, Cr, M when positively added and contained
The contents of o, V and B are 0.1 to 1.5, respectively.
%, 0.1-1.0%, 0.1-0.8%, 0.005
˜0.1%, 0.0003 to 0.003% is preferable.

Among the above elements, Cr has the effect of increasing the strength and toughness by the precipitation strengthening action during tempering and Mo has the effect of strengthening the solid solution, and Mo has a combined effect with the essential component Nb. At the same time as promoting the refinement of the structure, an appropriate amount (0.5 to 5% by volume) of retained austenite is dispersed in the steel to improve the unstable fracture resistance characteristics. Further, V also has an effect of stabilizing elements (N, C, O) harmful to the strain aging resistance and improving the strain aging resistance.

Ca: 0.003% or less (desirable lower limit of 0.0005%) Zr: 0.03% or less (desirable lower limit of 0.0
(05%) All of these elements control the morphology of inclusions in the steel, improve the toughness and corrosion resistance of the steel and its welds, and add elements harmful to brittle fracture (N, C, O). It has the effect of stabilizing and improving the unstable fracture resistance characteristics. Although these effects can be obtained even at the impurity amount level, Ca becomes significant at a content of 0.0005% or more and Zr of 0.005% or more. However, when Ca is contained in an amount of more than 0.003% and Zr is more than 0.03%, not only the cleanliness of the steel is lowered and the toughness of the steel and its welded portion is lowered,
The unstable ductile crack fracture resistance properties are also reduced. Therefore, the contents of Ca and Zr when positively added and contained are 0.0005 to 0.003% and 0.005 to 0.005%, respectively.
It is good to set it to 0.03%.

Metal Structure: The high strength steel of the present invention is
Although used as a steel plate or a steel pipe, the metal structure has a total ratio of the martensite phase and the bainite phase in the metal structure of the surface layer portion of 95% by volume or more, and the martensite phase in the metal structure of the central portion of the wall thickness. It is desirable that the total proportion of the bainite phase and the bainite phase is 80% by volume or more, and in this case, the unstable fracture resistance characteristics are further improved.

The high-strength steels (including steel plates and seamless steel pipes) of the present invention detailed above are reheat-quenched after ordinary hot rolling as long as the chemical composition of the steel satisfies the conditions specified in the present invention. It is also possible to manufacture by means of a tempering method, a normal hot rolling method followed by a direct quenching and tempering method, or a normal hot rolling method followed by an accelerated cooling treatment. It is preferable to manufacture by a method of performing accelerated cooling treatment after hot rolling under the following conditions.

Heating temperature: If the heating temperature is lower than 950 ° C., tensile strength of 750 MPa or more may not be secured in some cases. On the other hand, if the heating temperature exceeds 1200 ° C., the elements (N, C, O) harmful to the occurrence of brittle fracture and the termination of ductile fracture become insufficiently stabilized after the subsequent hot rolling, resulting in a desired unstable fracture resistance. In some cases, the characteristics cannot be secured. Therefore, it is desirable that the heating temperature be 950 to 1200 ° C.

Finishing temperature of hot rolling: When the finishing temperature of hot rolling is less than 600 ° C., tensile strength of 750 MPa or more may not be secured in some cases. Further, when the finishing temperature of hot rolling exceeds 900 ° C., the refinement of the structure due to rolling and subsequent accelerated cooling is not sufficient, and the elements (N, C) harmful to the occurrence of brittle fracture and the termination of ductile fracture are stabilized. In some cases, it may not be possible to ensure the desired unstable breakdown resistance characteristics. Therefore, the finishing temperature for hot rolling is 6
It is desirable to set the temperature to 00 to 900 ° C.

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 secured. Therefore, the water cooling start temperature at the time of accelerated cooling is preferably 500 ° C. or higher.

Cooling rate: Cooling rate during accelerated cooling is 4 ° C. /
If it is less than seconds, coarse upper bainite may be mixed in the structure, and good low temperature toughness and unstable fracture resistance characteristics may not be secured. Therefore, the cooling rate during accelerated cooling is 4 ° C.
/ Sec is recommended. The cooling rate may be 4 ° C./second or more, and it is not necessary to specify the upper limit.

Water-cooling stop temperature: When the water-cooling stop temperature at the time of accelerated cooling exceeds 300 ° C., not only a tensile strength of 750 MPa or more cannot be secured, but also an appropriate amount existing in steel (0.5
Retained austenite (about 5% by volume) may decompose and the desired unstable fracture resistance characteristics may not be secured. Therefore, the water cooling stop temperature during accelerated cooling is preferably 300 ° C. or lower.

Next, the welded steel pipe of the present invention will be described.

The welded steel pipe of the present invention may be manufactured by any well-known pipe manufacturing method as long as the steel plate made of the high-strength steel of the present invention is used as a base material. In particular,
Forging welded steel pipe, electric resistance welded steel pipe, laser welded steel pipe, electron beam welded steel pipe, plasma arc welded steel pipe, TIG welded steel pipe, SAW welded steel pipe represented by UOE steel pipe and spiral steel pipe, MAG welded steel pipe, MIG welded steel pipe, etc. You can

However, in the welded steel pipe, the tensile strength of the weld metal is 700 MPa or more, and the tensile strength of the base metal is −5.
0) MPa or more, and the proportion of the acicular ferrite structure in the weld metal must be 10 to 80% by volume.

The reason is as follows. That is,
If the tensile strength of the weld metal is less than 700 MPa, the tensile strength of the welded joint does not exceed 750 MPa, and if the tensile strength of the weld metal is less than (tensile strength of base material −50) MPa. The strain at the time of deformation concentrates on the weld metal and the heat affected zone of the weld, and the desired unstable fracture resistance cannot be secured. Further, 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 secured.

Here, the welded steel pipe satisfying the above conditions is
In the case where the steel pipe 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, a steel plate made of the high-strength steel of the present invention is used and obtained by welding in accordance with a conventional method. To be

In the case of SAW welded steel pipe represented by TIG welded steel pipe, UOE steel pipe and spiral steel pipe, MAG welded steel pipe and MIG welded steel pipe, the steel sheet made of the high-strength steel of the present invention is used as the base material, and the chemical composition is Is the same as the high strength steel of the present invention, and a welding wire having a Pcm value defined by the following formula (2) in the range of 0.20 to 0.32 is used, and the welding site is Ar, He, N. It can be obtained by welding by shielding with a gas such as ₂ , CO ₂ or by using a flux of a firing type or a melting type.

Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B (2) Here, the element symbol in the formula is the content (mass%) of each element contained in the steel. .

[0063]

EXAMPLES Ten kinds of steels having the chemical compositions shown in Table 1 were made into steel plates having a thickness of 20 mm under various conditions shown in Table 2,
The obtained steel plate is used as a base material and has an outer diameter of 1014 mm and a length of 12
m UOE welded steel pipe was manufactured.

At this time, the welding was carried out by using a four-electrode SAW welding machine and performing a one-pass welding on each of the inner and outer surfaces under the condition of the heat input amount of 3 to 4 kJ / mm. Further, as the welding wire, one having a chemical composition similar to that of each steel and having a Pcm value defined by the above-mentioned formula (2) of 0.27 to 0.30 was used. In addition,
The pipe expansion ratio after welding was 0.8 to 1.2%.

With respect to each of the obtained steel pipes, the strength of the base material (yield strength YS (MPa), tensile strength TS (MP
a)), toughness (fracture transition temperature vTs: ° C), volume ratio (vol.%) of martensite (M) + bainite (B) in the metal structure of the surface layer portion and the center portion, contained in the weld metal. Volume ratio of acicular ferrite (AF) (v
ol. %), Tensile strength TS (MPa) of weld metal and toughness (Charpy absorbed energy vE at test temperature −30 ° C.)
While examining _{−30 ° C.} (J), it was subjected to a DWTT test specified by the API standard, and a 85% ductile fracture surface transition temperature (SAT
T _85% : ° C) and absorbed energy (vE _DWTT :
J) was examined, and these results are also shown in Table 2.

The tensile strength and toughness of the weld metal were examined by cutting out a test piece from the weld metal portion.

As is clear from Table 2, in the UOE welded steel pipes of trial numbers 1 to 9 satisfying the conditions specified in the present invention, the tensile strength TS of the base metal is 765 MPa or more, and the tensile strength of the weld metal is all. TS is as high as 822 MPa or more, toughness is good, the fracture surface transition temperature of the base material is -86 ° C or less, and the Charpy absorbed energy of the weld metal is 105 J or more, and D
WTT test 85% ductile fracture transition temperature SATT _85%
Is −41 ° C. or lower, and the DWTT absorbed energy vE _{DWTT−30 °} C. at −30 ° _C. is 6887 J or higher, which are significantly higher than the target −30 ° C. or lower and 5000 J or higher.

On the other hand, the formula (1) defined in the present invention is satisfied, but the S content is 0.00, which is the upper limit value defined in the present invention.
The UOE welded steel pipe of trial No. 10 made of the steel of the substitution H exceeding 08% has good strength and toughness of the base metal and the weld metal, but has a SATT _85% of −18 ° C. and vE.
_{DWTT-30 ° C} is 2775J, which is the target of -3
It is well below 0 ° C and above 5000J.

Although the content of each element is within the range specified in the present invention, the UOE welded steel pipe of trial No. 11 made of the steel of the substitute I which does not satisfy the formula (1) specified in the present invention is Although the strength and toughness of the base metal are good, the toughness of the weld metal is poor because the _acicular ferrite structure is not contained in the weld metal, SATT _85% is -22 ° C, vE _{DWTT-
30 ° C} is 3125J, which is the target of -30 ° C or less,
It is well below 5000J.

Further, the UOE welded steel pipe of trial No. 12, which is made of the steel of the substitute J, which satisfies the formula (1) defined by the present invention, but whose C and N contents exceed the upper limits defined by the present invention, is Although the base metal has good strength and toughness, the tensile strength of the weld metal is 938 MPa, which is the value obtained by subtracting 50 MPa from the tensile strength of the base metal.
Because of the lower value of 902 MPa, WATT _85% was -15 ° C and vE _{DWTT-30 ° C} was 3112J, both of which were significantly lower than the target of -30 ° C or less and 5000J or more.

The above results show that the high-strength steel of the present invention,
Steel plate, seamless steel pipe and UO made of this high strength steel
Needless to say, it means that similar results can be obtained with welded steel pipes other than E-welded steel pipe.

[0072]

[Table 1]

[Table 2]

The high-strength steel, the steel sheet made of this steel, and the steel pipe including the welded pipe of the present invention are excellent in the unstable fracture resistance characteristic despite the high strength. Therefore, for example, if the steel pipe of the present invention is used as a line pipe to construct a pipeline, the safety of the pipeline can be dramatically improved, which greatly contributes to the industry.

Continued front page    F-term (reference) 4K032 AA01 AA02 AA04 AA08 AA11                       AA14 AA15 AA16 AA17 AA19                       AA21 AA22 AA23 AA24 AA26                       AA27 AA29 AA31 AA35 AA36                       AA39 BA01 CA01 CA02 CC02                       CC03 CC04 CD02 CD03

Claims (6)

[Claims] 1. C: 0.01 to 0.10% by mass% and S
i: 0.30% or less, Mn: 1.00 to 2.50%,
P: 0.010% or less, S: 0.0008% or less, N
b: 0.005 to 0.06%, Ti: 0.004 to 0.
025%, sol. Al: 0.05% or less, N: 0.0
040% or less, O: 0.003% or less, Ni: 2.5%
Below, Cu: 1.5% or less, Mo: 0.8% or less, C
r: 1.0% or less, V: 0.1% or less, B: 0.002
%, Zr: 0.03% or less, Ca: 0.003% or less, balance Fe and impurities, and a steel satisfying the following formula (1), and a tensile strength of 750 MPa or more. steel. {20 × S + P + 5 × (N + O)} ≦ 0.045 (1) Here, the element symbol in the formula (1) means the content (mass%) of each element contained in the steel. 2. The high strength according to claim 1, wherein the total proportions of the martensite phase and the bainite phase in the metal structure of the surface layer portion and the central portion of the wall thickness portion are 95% by volume and 80% by volume, respectively. steel. 3. A high-strength steel sheet comprising the high-strength steel according to claim 1. 4. A high-strength steel pipe made of the high-strength steel according to claim 1. 5. A welded pipe made of the high-strength steel according to claim 1, wherein the base metal portion has a tensile strength of 70.
0 MPa or more and (tensile strength of base material −50) MPa
As described above, a high-strength steel pipe in which the proportion of the acicular ferrite structure in the weld metal is 10 to 80% by volume. 6. C: 0.01 to 0.10% by mass% and S
i: 0.30% or less, Mn: 1.00 to 2.50%,
P: 0.010% or less, S: 0.0008% or less, N
b: 0.005 to 0.06%, Ti: 0.004 to 0.
025%, sol. Al: 0.05% or less, N: 0.0
040% or less, O: 0.003% or less, Ni: 2.5%
Below, Cu: 1.5% or less, Mo: 0.80% or less, C
r: 1.0% or less, V: 0.1% or less, B: 0.002
% Or less, Zr: 0.03% or less, Ca: 0.0030%
Includes the following, balance Fe and impurities, and (1)
After heating the steel satisfying the formula to 950 to 1200 ° C., hot rolling is performed to finish the rolling at a finishing temperature of 900 to 600 ° C., and a temperature range of not lower than 500 ° C. to a temperature of 300 ° C. or lower is 4 ° C. / A method for producing high-strength steel in which accelerated cooling is performed at a cooling rate of at least 2 seconds. {20 × S + P + 5 × (N + O)} ≦ 0.045 (1) Here, the element symbol in the formula (1) means the content (mass%) of each element contained in the steel.