JP2003321730A - High strength steel sheet for line pipe having excellent HIC resistance and method for producing the same - Google Patents

Abstract

(57) [Summary] [Problem] A high-strength steel sheet of API X65 grade or higher, which has excellent HIC resistance to HIC at the central segregation part and HIC generated from the vicinity of the surface or from inclusions. At low cost without adding a large amount of alloying elements. SOLUTION: In mass%, C: 0.02 to 0.08%,
Si: 0.01 to 0.50%, Mn: 0.5 to 1.8
%, P: 0.01% or less, S: 0.002% or less, W:
0.05-1.0%, Ti: 0.005-0.04%,
Al: 0.01 to 0.07%, the balance being substantially composed of Fe, and C / (W + Ti), which is the ratio of the C content in atomic% to the total content of W and Ti, is 0.1%. 5 to 3.0, wherein the metal structure is substantially a two-phase structure of ferrite and bainite, and a precipitate containing Ti and W is dispersed and precipitated in the ferrite phase. Use a high-strength steel sheet with excellent HIC characteristics.

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 sheet having a strength of API standard X65 grade or higher, which is suitable for manufacturing steel pipes. TECHNICAL FIELD The present invention relates to a strength steel plate and a manufacturing method thereof.

[0002]

2. Description of the Related Art Line pipes used for transporting crude oil and natural gas containing hydrogen sulfide have strength, toughness, weldability, hydrogen-induced crack resistance (HIC resistance) and stress corrosion crack resistance (resistance). So-called sour resistance such as SCC) is required. Hydrogen-induced cracking (HIC) of steel is caused by the adsorption of hydrogen ions by the corrosion reaction on the surface of the steel and invasion of the steel as atomic hydrogen into the interior of the steel, surrounding non-metallic inclusions such as MnS in the steel and the hard second phase structure. It is said that it diffuses and accumulates in the, and cracks occur due to the internal pressure. In order to prevent such hydrogen-induced cracking, JP-A-54-110119 discloses that
By adding Ca and Ce in an appropriate amount with respect to the amount of S, the formation of needle-shaped MnS is suppressed, and the formation and propagation of cracks are suppressed by changing the form into finely dispersed spherical inclusions with low stress concentration. , A method for producing steel for line pipes having excellent HIC resistance is disclosed. Also, JP-A-61-60866
JP-A-61-165207 discloses reduction of elements having a high segregation tendency (C, Mn, P, etc.), soaking in a slab heating stage, and accelerated cooling during transformation during cooling. Disclosed is a steel excellent in HIC resistance, which suppresses the formation of a hardened structure such as island martensite that becomes a starting point of cracks in the center segregated portion and martensite and bainite that become crack propagation paths. Further, regarding a high-strength steel sheet of X80 grade having excellent HIC resistance, JP-A-5-9575, JP-A-5-271766, and JP-A-7-173 are disclosed.
No. 536, etc., while controlling the morphology of inclusions by adding Ca at low S, the central segregation is suppressed as low C and low Mn, and the accompanying strength reduction is caused by addition of Cr, Mn, Ni, etc. and accelerated cooling. The method of supplementing is disclosed. But,
All of the above methods for improving the HIC resistance are targeted at the center segregation portion. High-strength steel sheets that exceed X65 grades such as API X80 grade are often manufactured by accelerated cooling or direct quenching, so the steel sheet surface with a fast cooling rate hardens compared to the inside and hydrogen-induced cracking occurs near the surface. . Further, the microstructure of these high strength steel sheets obtained by accelerated cooling is a structure in which bainite or acicular ferrite is relatively highly susceptible to cracking not only in the surface but also in the interior, and measures against HIC in the central segregation part were taken. Even in such cases, it is difficult to eliminate HIC originating from sulfide-based or oxide-based inclusions in high-strength steel of API X80 grade. Therefore, if the HIC resistance of these high-strength steel sheets is a problem, it is necessary to take measures against HIC on the surface of the steel sheet or HIC starting from sulfide-based or oxide-based inclusions. On the other hand, as a high-strength steel excellent in HIC resistance, which does not include block-shaped bainite or martensite whose microstructure is highly susceptible to cracking, Japanese Patent Laid-Open No. 7-216500 discloses a ferrite-bainite two-phase structure, API
A high strength steel material having excellent HIC resistance of X80 grade is disclosed. Further, in JP-A-61-227129 and JP-A-7-70697, SCC (SSCC) resistance and H resistance are improved by using a ferrite single-phase microstructure.
A high-strength steel having improved IC properties and utilizing precipitation strengthening of carbide obtained by adding a large amount of Mo or Ti is disclosed.

[0003]

However, JP-A-7-2
The bainite structure of the high-strength steel described in Japanese Patent No. 16500 is a structure having relatively high cracking susceptibility, though not so much as block-shaped bainite or martensite, and the amount of S and Mn is strictly limited to make Ca treatment essential. Since it is necessary to improve the HIC property, the manufacturing cost is high. Also,
It is difficult to stably obtain a ferrite-bainite two-phase structure by using the rolling / cooling method described in JP-A-7-216500. On the other hand, the ferrite phase described in JP-A-61-227129 and JP-A-7-70697 has a structure with rich ductility and has extremely low cracking susceptibility, so that compared with steels having a bainite structure or an acicular ferrite structure. The HIC resistance is greatly improved. However, since the strength of the ferrite single phase is low, it is disclosed in JP-A-61-227.
The steel described in Japanese Patent No. 129 uses a steel to which a large amount of C and Mo is added, to increase the strength by precipitating a large amount of carbides, and in the steel strip of Japanese Patent Laid-Open No. 7-70697, Ti is used.
The added steel is wound around a steel strip at a specific temperature and the precipitation strengthening of TiC is used to increase the strength. However, JP-A-61
In order to obtain a ferrite structure in which Mo carbides described in JP-A-227129 are dispersed, it is necessary to perform cold working after quenching and tempering and then perform tempering again, which not only increases the manufacturing cost but also increases the Mo carbides. Has a large particle size of about 0.1 micron, and the effect of increasing strength is low.
It is necessary to obtain a predetermined strength by increasing the contents of Mo and Mo and increasing the amount of carbides. In addition, JP-A-7-7
TiC used in the high-strength steel described in Japanese Patent Publication No. 0697
Is finer than Mo carbide and is effective for precipitation strengthening. However, although it is apt to coarsen due to the influence of the temperature during precipitation, no countermeasure is taken against coarsening of precipitate. Therefore, precipitation strengthening is not sufficient and a large amount of T
i addition is required.

Therefore, an object of the present invention is to solve the above problems of the prior art, and to provide a high strength steel plate of API X65 grade or higher, which is for HIC in the central segregation portion and HIC generated near the surface or inclusions. Therefore, it is to provide a high-strength steel sheet for line pipes having excellent HIC resistance at low cost without adding a large amount of alloying elements.

[0005]

The features of the present invention for solving the above problems are as follows.

(1)% by mass, C: 0.02 to 0.0
8%, Si: 0.01 to 0.50%, Mn: 0.5 to
1.8%, P: 0.01% or less, S: 0.002% or less, W: 0.05 to 1.0%, Ti: 0.005 to 0.
C / (W + Ti), which is the ratio of the amount of C in atomic% to the total amount of W and Ti, containing 04% and Al: 0.01 to 0.07% with the balance being substantially Fe. Is 0.5 to 3.0, the metal structure is substantially a two-phase structure of ferrite and bainite, and precipitates containing Ti and W are dispersed and precipitated in the ferrite phase. A high-strength steel sheet for line pipes with excellent HIC resistance.

(2) Further, in mass%, Nb: 0.0
05-0.05% and / or V: 0.005-0.
C / (W + Ti + Nb + V), which is the ratio of the amount of C in atomic% to the total amount of W, Ti, Nb, and V, is 0.5.
.About.3.0, the metal structure is substantially a two-phase structure of ferrite and bainite, and Ti and W are contained in the ferrite phase.
And Hb resistance to HIC according to (1), characterized in that a complex precipitate containing Nb and / or V is dispersed and precipitated.
High strength steel plate for line pipes with excellent characteristics.

(3) C, 0.02 to 0.0% by mass
8%, Si: 0.01 to 0.50%, Mn: 0.5 to
1.8%, P: 0.01% or less, S: 0.002% or less, W: 0.05 to 1.0%, Ti: 0.005 to 0.
04%, containing Al: 0.01 to 0.07%, and mass%
In the total amount with W, the content of Mo is 0.05 to 1.0%, the balance is substantially Fe, and the ratio of the C amount in atomic% and the total amount of W, Mo, and Ti. C / (W + Mo + T)
i) is 0.5 to 3.0, the metal structure is substantially a two-phase structure of ferrite and bainite, and precipitates containing Ti, Mo, and W are dispersed and precipitated in the ferrite phase. A high-strength steel sheet for line pipes, which has excellent HIC resistance.

(4) Furthermore, in mass%, Nb: 0.0
05-0.05% and / or V: 0.005-0.
Contains 10 and C content in atomic% and W, Mo, Ti, N
C / (W + Mo + Ti + Nb, which is the ratio of the total amount of b and V)
+ V) is 0.5 to 3.0, and Ti is contained in the ferrite phase.
A high-strength steel sheet for line pipes having excellent HIC resistance as set forth in (3), characterized in that a complex precipitate containing, and Mo, W, and Nb and / or V is dispersed and precipitated.

(5) Further, in mass%, Cu: 0.5
It is characterized by containing one or more selected from 0% or less, Ni: 0.50% or less, Cr: 0.50% or less, and Ca: 0.0005 to 0.0050% ( A high-strength steel sheet for line pipes having excellent HIC resistance according to any one of 1) to (4).

A steel containing the chemical composition described in any one of (6), (1) to (5) is heated at a heating temperature of 1000-
After hot-rolling under the conditions of 1300 ° C., rolling end temperature: 750 ° C. or higher, cooling rate: 5 ° C./s or higher, 400 to 600 ° C.
Accelerated cooling up to 0.5 ° C immediately after cooling
A method for producing a high-strength steel sheet for line pipes excellent in HIC resistance, which comprises reheating to a temperature of 550 to 700 ° C. at / s or more.

(7), H-resistant, characterized by being manufactured using the steel sheet according to any one of (1) to (5)
High strength steel pipe with excellent IC characteristics.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have diligently studied a microstructure of a steel material and a method for manufacturing a steel sheet in order to improve both HIC resistance and high strength. As a result, in order to achieve both high strength and HIC resistance, it is most effective to make the microstructure a ferrite + bainite two-phase structure having a small strength difference between the ferrite structure and the bainite structure. By performing the manufacturing process of accelerated cooling and subsequent reheating, the ferrite phase, which is a soft phase, is strengthened by the fine precipitates containing Ti, W, etc., and the bainite phase, which is a hard phase, is softened, resulting in a small strength difference. It was found that a ferrite + bainite two-phase structure can be obtained. And C
It was found that the precipitation strengthening by the carbide can be utilized to the maximum extent by optimizing the addition amounts of W and Ti with respect to. In addition, if Nb and / or V are added in combination, it is possible to increase the strength of the ferrite phase by dispersing and depositing a precipitate containing Ti, W, and Nb and / or V. , Nb, V
It was found that the precipitation strengthening by carbide can be utilized to the maximum extent by optimizing the addition amount of.

The present invention has a high-strength steel sheet for line pipes having a two-phase structure of a ferrite phase in which the above-described precipitates containing Ti, W, etc. are dispersed and precipitated, and a bainite phase, and excellent in HIC resistance. The present invention relates to a manufacturing method, and since the steel sheet manufactured in this way does not have an increase in hardness in the surface layer portion like a steel sheet having a bainite or acicular ferrite structure obtained by conventional accelerated cooling or the like, HIC from the surface layer portion is not present. Does not occur. Further, since the two-phase structure of the ferrite phase and the bainite phase having a small strength difference has extremely high resistance to cracking, it becomes possible to suppress HIC from the central portion of the steel sheet and inclusions.

The high strength steel sheet for line pipes of the present invention will be described in detail below. First, the structure of the high-strength steel sheet for line pipes of the present invention will be described.

The metal structure of the steel sheet of the present invention is substantially a ferrite + bainite two-phase structure. Since the ferrite phase is rich in ductility and has low cracking susceptibility, high HIC resistance can be realized. Further, the bainite phase has excellent strength and toughness. The two-phase structure of ferrite and bainite is
Generally, it is a mixed structure of a soft ferrite phase and a hard bainite phase, and steel materials having such a structure tend to accumulate hydrogen at the interface between the ferrite phase and the bainite phase, and at the same time, the interface propagates cracks. HIC resistant as it becomes a route
The characteristics are inferior. However, in the present invention, by adjusting the strengths of the ferrite phase and the bainite phase to reduce the strength difference between the two, it is possible to achieve both high HIC resistance and high strength.
When one or more different metal structures such as martensite and pearlite are mixed in the ferrite + bainite two-phase structure, HIC is likely to occur due to hydrogen accumulation and stress concentration at the different phase interface. The smaller the organizational fraction except the phase, the better. However, if the volume fraction of the structure other than the ferrite phase and the bainite phase is low, the effect can be ignored, so that another metal structure having a total volume fraction of 5% or less, that is, one kind of martensite, pearlite, or the like is used. You may contain 2 or more types. Although the bainite fraction is not particularly specified, it is desirable that the bainite fraction is 10% or more from the viewpoint of ensuring the toughness of the base material and the ferrite fraction is 5% or more from the viewpoint of ensuring the strength.

Next, the precipitates dispersed and precipitated in the ferrite phase in the present invention will be described.

In the steel sheet of the present invention, the ferrite phase is strengthened by the dispersed precipitation of the precipitate containing W and Ti as a base in the ferrite phase, and the strength difference between ferrite and bainite is reduced, which is excellent. HIC resistance characteristics can be obtained. Since this precipitate is extremely fine, it has no effect on the HIC resistance. W and Ti are elements that form carbides in steel, and strengthening steel by precipitation of WC and TiC has been conventionally performed.
In the present invention, by adding W and Ti in combination, and by finely precipitating a composite carbide containing W and Ti in the steel, a greater strength than in the case of precipitation strengthening of WC and / or TiC is obtained. The feature is that an improvement effect can be obtained. This unprecedented large strength improving effect is due to W and T
This is because the composite carbide containing i as a base is stable and has a slow growth rate, and thus extremely fine precipitates having a particle size of less than 10 nm can be obtained.

The composite carbide containing W and Ti as a basic component is composed of W, Ti and C only, and W and Ti
The total amount of C and the amount of C are combined in the vicinity of an atomic ratio of 1: 1 and are very effective in increasing the strength. In the present invention, by adding Nb and / or V in combination,
It was found that the precipitate is a composite carbide containing W, Ti, and Nb and / or V, and similar precipitation strengthening can be obtained.

In the above composite carbide, part of W is M
It was found that it could be replaced with o. When W is contained, a fine composite carbide containing Ti, Mo, and W or a fine composite carbide containing Ti, Mo, W, and Nb and / or V is formed to increase the strength. Greatly contribute to.

In the present invention, the composite carbide mainly composed of W and Ti, which is a precipitate that is dispersed and precipitated in the steel sheet, is obtained by producing a steel sheet using the production method of the present invention for steel having the components described below. , And can be obtained by dispersing in a ferrite phase. When the high-strength steel sheet for line pipes of the present invention contains precipitates other than the composite carbide mainly composed of W and Ti, the effect of increasing the strength by the composite carbide of W and Ti is not impaired, and the HIC resistance property is improved. It should not be deteriorated.

Next, the chemical composition of the high strength steel sheet for line pipes used in the present invention will be described. Unless otherwise specified in the following description, all units shown in% are% by mass.

C: 0.02 to 0.08% C is an element that contributes to precipitation strengthening as a carbide, but 0.02
If it is less than 0.1%, sufficient strength cannot be secured, and if it exceeds 0.08%, toughness and HIC resistance are deteriorated, so the C content is specified to be 0.02 to 0.08%.

Si: 0.01 to 0.50% Si
Is added for deoxidation, but if it is less than 0.01%, the deoxidizing effect is not sufficient, and if it exceeds 0.50%, toughness and weldability are deteriorated, so the Si content is 0.01 to 0.50. Specify as%.

Mn: 0.5 to 1.8% Mn is added for strength and toughness, but if it is less than 0.5%, its effect is not sufficient, and if it exceeds 1.8%, the weldability and HIC resistance deteriorate, so the Mn content is 0.5-0.5%. Specified at 1.8%.

P: 0.01% or less. Since P is an unavoidable impurity element that deteriorates weldability and HIC resistance,
The upper limit of P content is specified to 0.01%.

S: 0.002% or less. In general, S is an MnS inclusion in steel and deteriorates the HIC resistance. However, 0.002%
Since there is no problem if it is below, the upper limit of the S content is 0.00
Specify 2%.

W: 0.05 to 1.0% W is an important element in the present invention, and when contained in an amount of 0.05% or more, it suppresses the pearlite transformation during cooling after hot rolling and forms fine composite precipitates with Ti to increase strength. Make a big contribution. However, if added in excess of 1.0%, a hardened phase such as martensite is formed and the HIC resistance is deteriorated, so the W content is specified to be 0.05 to 1.0%.

Ti: 0.005 to 0.04%. T
i, like W, is an important element in the present invention. 0.
By adding 005% or more, a complex precipitate is formed with W, which greatly contributes to the strength increase. If added in excess of 0.04%, the toughness of the weld heat affected zone will be deteriorated, so the Ti content is specified to be 0.005 to 0.04%.

Al: 0.01 to 0.07%. Al
Is added as a deoxidizer, but if it is less than 0.01%, it has no effect, and if it exceeds 0.07%, the cleanliness of the steel decreases and the HIC resistance deteriorates. ~
Specify to 0.07%.

C / C, which is the ratio of the amount of C to the total amount of W and Ti,
(W + Ti): is 0.5 to 3.0. The increase in strength according to the present invention is due to the precipitates (mainly carbides) containing Ti and W. In order to effectively utilize the precipitation strengthening by this composite precipitate, the amount of C and W and Ti which are carbide forming elements are used.
The relationship with the amount is important, and by adding these elements in an appropriate balance, a thermally stable and extremely fine composite precipitate can be obtained. At this time, when the value of C / (W + Ti) represented by the content of each element in atomic% is less than 0.5 or exceeds 3.0, the amount of one of the elements is excessive, and it depends on the formation of a hardened structure. The value of C / (W + Ti) is defined to be 0.5 to 3.0 because it causes deterioration of HIC resistance and deterioration of toughness. However, each element symbol is the content of each element in atomic%. When the content of mass% is used, the value of (C / 12.01) / (W / 183.84 + Ti / 47.9) is specified to be 0.5 to 3.0.

In the present invention, one or two of Nb and V shown below may be contained for the purpose of further improving the strength of the steel sheet.

Nb: 0.005 to 0.05% N
Although b improves the toughness by making the structure finer, it forms a composite precipitate with Ti and W, and contributes to the strength increase of the ferrite phase. However, if it is less than 0.005%, there is no effect, and if it exceeds 0.05%, the toughness of the weld heat affected zone deteriorates, so the Nb content is specified to be 0.005 to 0.05%.

V: 0.005 to 0.10% V also forms a composite precipitate with Ti and W, like Nb,
Contributes to an increase in the strength of the ferrite phase. But 0.00
If it is less than 5%, there is no effect, and if it exceeds 0.1%, the toughness of the weld heat affected zone deteriorates, so the V content is 0.005 to 0.005.
Specify to 0.1%.

When Nb and / or V is contained, the ratio of the amount of C to the total amount of W, Ti, Nb and V, C,
/ (W + Ti + Nb + V): is set to 0.5 to 3.0.
The strengthening according to the present invention depends on the precipitate containing Ti and W, but when Nb and / or V is contained, it becomes a composite precipitate (mainly carbide) containing them. At this time, C / (W + Ti + Nb +) represented by the content of each element in atomic%
If the value of V) is less than 0.5 or exceeds 3.0, the amount of one of the elements is excessive and the resistance to H due to the formation of a hardened structure.
C / (W + T as it causes deterioration of IC characteristics and toughness.
The value of (i + Nb + V) is defined to be 0.5 to 3.0. However, each element symbol is the content in atomic%. When using the content of mass%, (C / 12.01) / (W / 183.84 + Ti / 4
The value of (7.9 + Nb / 92.91 + V / 50.94) is specified to be 0.5 to 3.0.

In the present invention, Mo may be contained.

0.05 to 1.0% in total with Mo: W
And Mo can replace W in a fine composite precipitate containing W and Ti, so the total amount of W and Mo is specified to be 0.05 to 1.0%. When Mo is contained, the value of C / (W + Mo + Ti + Nb + V) is set to 0.5 to 3.0. When using a content of mass%, (C / 12.01) / (W / 183.
9 + Mo / 95.9 + Nb / 92.91 + V / 50.94 + Ti / 47.9) value of 0.5 to 3.0
And

In the present invention, for the purpose of further improving the strength and HIC resistance of the steel sheet, the following Cu, Ni, Cr, C
You may contain 1 type (s) or 2 or more types of a.

Cu: 0.50% or less. Cu is an element effective for improving the toughness and increasing the strength, but if added in a large amount, the weldability deteriorates, so if added, 0.50%
Is the upper limit.

Ni: 0.50% or less. Ni is an element effective for improving the toughness and increasing the strength, but if added in a large amount, the HIC resistance is deteriorated.
The upper limit is 50%.

Cr: 0.50% or less. Cr is Mn
Similar to the above, it is an element effective for obtaining sufficient strength even with low C, but if added in a large amount, the weldability deteriorates.

Ca: 0.0005 to 0.0050%. Ca is an element effective for improving the HIC resistance by controlling the morphology of sulfide inclusions, but if it is less than 0.0005%, its effect is not sufficient, and if it exceeds 0.0050%, the effect saturates. On the contrary, since the HIC resistance is deteriorated due to a decrease in cleanliness of steel, the Ca content is specified to 0.0005 to 0.0050% when added.

The balance other than the above is substantially Fe.
The fact that the balance consists essentially of Fe means that those containing other trace elements including unavoidable impurities can be included in the scope of the present invention unless the effects of the present invention are lost.

Next, a method for manufacturing the high strength steel sheet for line pipes of the present invention will be described.

The high-strength steel sheet for line pipes of the present invention uses the steel having the above-mentioned composition, and the heating temperature is 1000 to 1
Hot rolling is performed at 300 ° C. and rolling end temperature: 750 ° C. or higher, then cooled to 400 to 600 ° C. at a cooling rate of 5 ° C./s or more, and immediately after cooling, at a temperature rising rate of 0.5 ° C./s or more. 5
By reheating to a temperature of 50 to 700 ° C, W and T
A fine composite carbide mainly composed of i is dispersed and precipitated in a ferrite phase, and a bainite phase is softened to produce a composite structure. Here, the temperature is the average temperature of the steel sheet.
Hereinafter, each manufacturing condition will be described in detail.

Heating temperature: 1000 to 1300 ° C.
If the heating temperature is lower than 1000 ° C, the solid solution of carbide is insufficient to obtain the required strength, and if it exceeds 1300 ° C, the toughness deteriorates.

Rolling end temperature: 750 ° C. or higher. If the rolling end temperature is low, the structure will be extended in the rolling direction and H
Not only the IC characteristics are deteriorated, but also the subsequent ferrite transformation rate is reduced and it is necessary to lengthen the reheating time after rolling, which is not preferable in terms of manufacturing efficiency.
℃ or above.

Immediately after completion of rolling, cooling is performed at a cooling rate of 5 ° C./s or more. If cooling or slow cooling is performed after the rolling is completed, precipitates are precipitated from the high temperature region, and the precipitates easily coarsen and the ferrite phase cannot be strengthened. Therefore, it is an important manufacturing condition in the present invention to perform rapid cooling (accelerated cooling) to a temperature optimum for precipitation strengthening and prevent precipitation from a high temperature range. If the cooling rate is less than 5 ° C./s, the effect of preventing precipitation in the high temperature region is not sufficient and the strength decreases, so the cooling rate after rolling is specified to be 5 ° C./s or more. As a cooling method at this time, any cooling equipment can be used depending on the manufacturing process.

Cooling stop temperature: 400 to 600 ° C.
Accelerated cooling after rolling is 400-
By rapidly cooling to 600 ° C., a bainite phase is generated and the driving force for ferrite transformation during reheating is increased. By increasing the driving force, it is possible to promote the ferrite transformation in the reheating process and complete the ferrite transformation in a short time reheating. Cooling stop temperature is 40
When the temperature is lower than 0 ° C, bainite or martensite single-phase structure is formed, or even if ferrite + bainite two-phase structure is formed, island-like martensite (MA) is generated and HIC resistance is deteriorated. Since the ferrite transformation during reheating is not completed and pearlite is deposited and the HIC resistance is deteriorated, the accelerated cooling stop temperature is specified to be 400 to 600 ° C.

Immediately after the accelerated cooling, reheating is performed to a temperature of 550 to 700 ° C. at a heating rate of 0.5 ° C./s or more. This process is an important manufacturing condition in the present invention. The fine precipitates that contribute to the strengthening of the ferrite phase precipitate simultaneously with the ferrite transformation during reheating. In order to strengthen the ferrite phase by the fine precipitates and soften the bainite phase at the same time, and to obtain a structure with a small strength difference between the ferrite phase and the bainite phase, immediately reheat to a temperature range of 550 to 700 ° C after accelerated cooling. is necessary. If the heating rate during reheating is less than 0.5 ° C / s, it takes a long time to reach the target reheating temperature, which deteriorates the production efficiency and causes pearlite transformation. Cannot be obtained and sufficient strength cannot be obtained. If the reheating temperature is less than 550 ° C, ferrite transformation is not completed, and untransformed austenite transforms to pearlite during subsequent cooling.
The characteristics deteriorate, and if the temperature exceeds 700 ° C, the precipitates become coarse and sufficient strength cannot be obtained. Therefore, the reheating temperature range is set to 550 to 7
Specified at 00 ° C. It is not necessary to set the temperature holding time at the reheating temperature. When the production method of the present invention is used, even if the material is cooled immediately after being reheated, ferrite transformation sufficiently progresses, so that high strength due to fine precipitation can be obtained. In order to surely complete the ferrite transformation, it is possible to keep the temperature for 30 minutes or less, but if the temperature is kept for more than 30 minutes, coarsening of precipitates may occur and the strength may be lowered. The cooling rate after reheating may be set appropriately,
Air cooling is preferable because ferrite transformation proceeds even in the cooling process after reheating. It is also possible to cool at a higher cooling rate than air cooling as long as it does not hinder the ferrite transformation.

FIG. 1 shows an outline of the thermal history when the manufacturing method of the present invention described above is used.

As equipment for reheating to a temperature of 550 to 700 ° C., a heating device can be installed on the downstream side of the cooling equipment for accelerated cooling. As the heating device, it is preferable to use a gas combustion furnace or an induction heating device capable of rapidly heating the steel sheet. The induction heating device is easier to control the temperature than the soaking furnace and the cost is relatively low.
It is particularly preferable because the steel sheet after cooling can be quickly heated. Further, by arranging a plurality of induction heating devices in series continuously, even when the line speed and the type and size of the steel sheet are different, it is only necessary to arbitrarily set the number of induction heating devices to be energized. It is possible to freely control the reheating temperature. Since the cooling rate after reheating may be any rate, it is not necessary to install special equipment downstream of the heating device.

FIG. 2 shows a schematic view of an example of a production line for carrying out the production method of the present invention. As shown in FIG. 2, a hot rolling mill 3, an accelerated cooling device 4, an in-line induction heating device 5, and a hot leveler 6 are arranged in the rolling line 1 from the upstream side to the downstream side. By installing the in-line induction heating device 5 or another heat treatment device on the same line as the hot rolling mill 3 which is a rolling facility and the accelerated cooling device 4 which is a subsequent cooling facility,
Since the reheating treatment can be performed quickly after the completion of rolling and cooling, the steel sheet after rolling and accelerated cooling can be immediately heated to 550 ° C. or higher.

The steel sheet of the present invention produced by the above production method is formed into a steel pipe by press bend forming, roll forming, UOE forming, etc., and is a steel pipe for transporting crude oil or natural gas (electric resistance welded steel pipe, spiral steel pipe, UOE steel pipe) and the like. Steel pipe manufactured using the steel sheet of the present invention,
Since it has high strength and excellent HIC resistance, it is also suitable for transporting crude oil and natural gas containing hydrogen sulfide.

[0055]

EXAMPLES Steels having the chemical composition shown in Table 1 (steel types A to N) were made into slabs by a continuous casting method, and the slabs were made to have plate thicknesses of 18 and 2.
6 mm thick steel plates (No. 1-26) were manufactured.

[0056]

[Table 1]

After the heated slab is rolled by hot rolling, it is immediately cooled using a water-cooled type accelerated cooling equipment,
Reheating was performed using an induction heating furnace or a gas combustion furnace.
The cooling equipment and induction heating furnace were in-line type. Table 2 shows the manufacturing conditions of each steel plate (No. 1-26).

The microstructure of the steel sheet manufactured as described above was observed with an optical microscope and a transmission electron microscope (TEM). The components of the precipitate were analyzed by energy dispersive X-ray spectroscopy (EDX). Further, the tensile properties and HIC resistance properties of each steel sheet were measured. The measurement results are also shown in Table 2.
As for the tensile properties, a tensile test was performed using a full-thickness test piece in the rolling vertical direction as a tensile test piece, and the yield strength and tensile strength were measured.
Then, in consideration of manufacturing variations, the yield strength 480
API X65 with MPa or more and tensile strength of 580 MPa or more
It was evaluated as a high-strength steel sheet of a grade or higher. For HIC resistance, HIC test according to NACE Standard TM-02-84 with a dipping time of 96 hours was performed.
When the C property was judged to be good, it was shown as ◯, and when cracking was shown as x.

[0059]

[Table 2]

In Table 2, No. 1 to No. 1 of the present invention are shown.
All of 13 had a chemical composition and a manufacturing method within the scope of the present invention, were high in tensile strength of 580 MPa or more, and were excellent in HIC resistance. The structure of the steel sheet is substantially a ferrite + bainite two-phase structure, and Ti and W, and for some steel sheets, Nb and / or V, and Mo.
Fine carbide precipitates containing and having a particle size of less than 10 nm were dispersed and precipitated.

In Nos. 14 to 20, the chemical components are within the scope of the present invention, but the manufacturing method is outside the scope of the present invention, so that the structure is not a ferrite + bainite two-phase structure, and Since the carbide was not dispersed and precipitated, the strength was insufficient and cracking occurred in the HIC test. No. 21 to 2
No. 6 has a chemical composition outside the scope of the present invention, so that coarse precipitates are not formed or precipitates containing Ti and W are not dispersed and precipitated, so that sufficient strength cannot be obtained, or a HIC test is performed. A crack occurred.

There was no particular difference in the results when the reheating was performed in the induction heating furnace or the gas combustion furnace.

[0063]

As described above, according to the present invention, the AP
A steel sheet having high strength of IX65 grade or higher and excellent in HIC resistance can be manufactured at low cost without adding a large amount of alloying elements. Therefore, it is possible to manufacture steel pipes such as electric resistance welded steel pipes, spiral steel pipes and UOE steel pipes having excellent characteristics.

[Brief description of drawings]

FIG. 1 is a graph showing an outline of thermal history in the manufacturing method of the present invention.

FIG. 2 is a schematic view showing an example of a production line for carrying out the production method of the present invention.

[Explanation of symbols]

1: rolling line, 2: Steel plate, 3: hot rolling mill, 4: Accelerated cooling device, 5: In-line induction heating device, 6: Hot leveler

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Nobuyuki Ishikawa             1-2-1, Marunouchi, Chiyoda-ku, Tokyo             Main Steel Pipe Co., Ltd. F-term (reference) 4K032 AA01 AA04 AA08 AA11 AA14                       AA16 AA22 AA23 AA27 AA29                       AA31 AA35 AA36 AA37 BA01                       CA02 CA03 CC03 CC04 CD02                       CD03 CF01 CF02

Claims (7)

[Claims] 1. In mass%, C: 0.02 to 0.08%,
Si: 0.01 to 0.50%, Mn: 0.5 to 1.8
%, P: 0.01% or less, S: 0.002% or less, W:
0.05-1.0%, Ti: 0.005-0.04%,
Al: 0.01 to 0.07%, the balance substantially consisting of Fe, and C / (W + Ti), which is the ratio of the amount of C in atomic% to the total amount of W and Ti, is 0. 5 to 3.0, the metal structure is substantially a two-phase structure of ferrite and bainite, and precipitates containing Ti and W are dispersed and precipitated in the ferrite phase. High strength steel plate for line pipes with excellent HIC characteristics. 2. Further, Nb: 0.005 by mass%.
0.05% and / or V: 0.005 to 0.10 is contained, and C / (W + Ti + Nb + V), which is the ratio of the C amount in atomic% and the total amount of W, Ti, Nb, and V, is 0.5. ~ 3.
0, the metal structure is substantially a two-phase structure of ferrite and bainite, and Ti, W and N are contained in the ferrite phase.
The high-strength steel sheet for line pipes having excellent HIC resistance according to claim 1, characterized in that a composite precipitate containing b and / or V is dispersed and precipitated. 3. In mass%, C: 0.02 to 0.08%,
Si: 0.01 to 0.50%, Mn: 0.5 to 1.8
%, P: 0.01% or less, S: 0.002% or less, W:
0.05-1.0%, Ti: 0.005-0.04%,
Al: 0.01-0.07% is contained, the total amount with W in mass% is 0.05-1.0% Mo, and the balance is essentially Fe, and atomic% C amount and W, Mo,
C / (W + Mo + Ti), which is the ratio of the total amount of Ti, is 0.
5 to 3.0, the metal structure is substantially a two-phase structure of ferrite and bainite, and Ti in the ferrite phase,
A high-strength steel sheet for line pipes, which is excellent in HIC resistance, characterized in that precipitates containing Mo and W are dispersed and precipitated. 4. Nb: 0.005 by mass%
0.05% and / or V: 0.005 to 0.10 is contained, and C / (W + Mo + Ti + Nb + V), which is the ratio of the C amount in atomic% and the total amount of W, Mo, Ti, Nb, and V, is 0. 0.5 to 3.0, Ti and Mo in the ferrite phase
A high-strength steel sheet for line pipes having excellent HIC resistance according to claim 3, characterized in that a complex precipitate containing, and W and Nb and / or V is dispersed and precipitated. 5. Further, in mass%, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, C
a: 1 selected from 0.0005 to 0.0050%
A high-strength steel sheet for line pipes having excellent HIC resistance according to any one of claims 1 to 4, characterized in that it contains one or more kinds. 6. A steel containing the chemical component according to claim 1 is heated at a heating temperature of 1000 to 1.
After hot rolling at a temperature of 300 ° C and a rolling end temperature of 750 ° C or more, accelerated cooling is performed at a cooling rate of 5 ° C / s or more to 400 to 600 ° C, and immediately after cooling, a temperature raising rate of 0.5 ° C / s
The method for producing a high-strength steel sheet for a line pipe excellent in HIC resistance, characterized by reheating to a temperature of 550 to 700 ° C as described above. 7. A HI resistant product manufactured by using the steel sheet according to claim 1. Description:
High-strength steel pipe with excellent C characteristics.