JP2009174024A - High-strength steel sheet having excellent pwht resistance, and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength steel sheet exhibiting excellent PWHT (post-weld heat treatment) resistance. <P>SOLUTION: Disclosed is a high-strength steel sheet having a componential composition comprising 0.03 to 0.07% C, 0.01 to 0.5% Si, 1.5 to 2.5% Mn, 0.1 to 0.5% Mo, ≤0.08% Al, 0.005 to 0.035% Ti and 0.005 to 0.07% Nb, and in which Ceq value (mass%)=C+Mn/6+(Cu+Ni)/12+(Cr+Mo+V)/5 and P value (atomic%)=[Mo]+[Ti]+[Nb]+[V] satisfy 9×Ceq value+4×P value≥4.8, and also, 0.6 (atomic%)≤[C]/([Mo]+[Ti]+[Nb]+[V])≤1.7 (atomic%) is satisfied, and essentially composed of a bainitic structure in which the fraction of insular martensite is ≤2%, wherein, (Ti and/or Nb) compound carbides essentially composed of Mo with an equivalent circle diameter of ≤10 nm are dispersed by ≥30 pieces per 1 μm<SP>2</SP>, and the total precipitation amount of the compound carbides is ≥0.03 mass%. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention is a high-strength steel sheet having API X100 grade or more suitable for steel pipes, pressure vessels, etc., particularly PWHT resistance having excellent strength and toughness even after performing post-weld heat treatment (PWHT). It relates to a high-strength steel sheet.

Riser steel pipes used for oil and gas drilling and the like are often welded and joined by forged products (for example, connectors) having a very large amount of alloy elements by circumferential welding. In addition, steel pipes or steel plates used as piping members for power plants and other strength members are often welded to Cr-Mo steel or the like. In such a case, the PWHT treatment (post-weld heat treatment) is usually performed for the purpose of removing residual stress by welding. However, since there is a concern that the heat treatment may cause a decrease in strength or toughness, the PWHT treatment is performed. Steel pipes and steel materials to be used are required to have strength and toughness even after PWHT treatment. In recent years, API has been developed from the viewpoint of improving operational efficiency and reducing material costs due to increased pressure.
The demand for high-strength steel pipes or steel materials of X100 grade or higher is also increasing.

As conventional techniques related to such a high-strength steel pipe excellent in PWHT resistance, there are those disclosed in Patent Documents 1 and 2.
Japanese Patent Laid-Open No. 11-50188 JP 2001-158939 A

  However, in the technique described in Patent Document 1, since the strength reduction due to the PWHT treatment is compensated by precipitation of Cr carbide during PWHT, a large amount of Cr is required, which not only increases the material cost but also improves weldability and Decrease in toughness becomes a problem. On the other hand, the technology described in Patent Document 2 focuses on improving the characteristics of the seam weld metal when manufacturing a steel pipe, and no special consideration is given to the base material, and the strength of the base material by the PWHT treatment is not considered. Since the reduction is unavoidable, it is necessary to increase the strength before PWHT by controlled rolling or accelerated cooling.

  Therefore, an object of the present invention is to solve such problems of the prior art, have a high strength higher than API X100 grade, and exhibit high PWHT resistance without adding a large amount of alloy elements, and a high strength steel plate It is to provide a strength steel pipe. Moreover, the other object of this invention is to provide the manufacturing method which can manufacture stably the high strength steel plate which has such the outstanding characteristic.

The gist of the present invention for solving the above problems is as follows.
[1] By mass%, C: 0.03 to 0.07%, Si: 0.01 to 0.5%, Mn: 1.5 to 2.5%, Mo: 0.1 to 0.5% , Al: 0.08% or less, Ti: 0.005 to 0.035%, Nb: 0.005 to 0.07%, Ceq value represented by the following formula (1) and the following formula (2) P value represented by the following equation (i):
9 × Ceq value + 4 × P value ≧ 4.8 (i)
Ceq value = C + Mn / 6 + (Cu + Ni) / 12 + (Cr + Mo + V) / 5
… (1)
However, the element symbol of the formula (1) indicates mass% of each contained element.
P value = [Mo] + [Ti] + [Nb] + [V] (2)
However, the element symbol of the formula (2) indicates atomic% of each contained element.
And having a component composition satisfying the following formula (ii):
0.6 ≦ [C] / ([Mo] + [Ti] + [Nb] + [V]) ≦ 1.7
... (ii)
However, the element symbol in the formula (ii) indicates atomic% of each contained element.
A composite mainly composed of a bainite structure having an island-like martensite (MA constituent) fraction of 2% or less in a steel sheet microstructure, an equivalent circle diameter of 10 nm or less, mainly composed of Mo, and containing Ti and / or Nb. A high-strength steel sheet having excellent PWHT resistance, wherein 30 or more carbides are dispersed per 1 μm ^{2 and} the total precipitation amount of the composite carbide is 0.03% by mass or more.

[2] In the high-strength steel sheet of the above [1], the composite carbide further includes, in mass%, V: 0.005 to 0.1%, mainly Mo, and Ti and / or Nb. A high-strength steel sheet excellent in PWHT resistance, characterized by containing V.
[3] In the high-strength steel sheet according to [1] or [2], further, by mass, Cu: 0.5% or less, Ni: 0.5% or less, Cr: 0.5% or less, Ca: 0 .PWHT resistance characteristics characterized by containing one or more selected from 0005 to 0.0035%, REM: 0.0005 to 0.01%, B: 0.002% or less Excellent high strength steel plate.
[4] A steel having any one of the above-mentioned component compositions [1] to [3] is hot-rolled at a heating temperature of 1100 to 1300 ° C. and a cumulative reduction ratio of 800 ° C. or less: 70% or more, Cooling start temperature: 700 ° C. or more, cooling rate: 20 ° C./s or more and accelerated cooling to a temperature of less than 300 ° C., and then immediately 550 to 700 at a temperature increase rate of 0.5 ° C./s or more and less than 10 ° C./s. A method for producing a high-strength steel sheet having excellent PWHT resistance characteristics, characterized by reheating to a temperature of ° C.
[5] A steel pipe made of the steel sheet of any one of [1] to [3], wherein the steel sheet is formed into a cylindrical shape in the longitudinal direction, and the butt portion is welded in the longitudinal direction one layer at a time from the inner and outer surfaces of the pipe A high-strength steel pipe obtained by

The high-strength steel sheet of the present invention has high strength equal to or higher than API X100 grade, and can maintain excellent strength and toughness even after PWHT treatment. For this reason, it is a steel plate useful as a material for a steel pipe, a pressure vessel or the like that is subjected to PWHT treatment.
Moreover, the manufacturing method of this invention can manufacture stably the high strength steel plate which has such the outstanding characteristic.

  In order to achieve both improvement in the PWHT resistance and high strength, the present inventors have conducted detailed studies on changes in the microstructure of the steel material due to the PWHT treatment. In general, steel plates for welded steel pipes and steel plates for welded structures are severely limited in chemical composition from the viewpoint of weldability. Therefore, high-strength steel sheets of X65 grade or higher are manufactured by accelerated cooling after hot rolling. Therefore, the microstructure is mainly bainite or a structure containing island-like martensite (MA constituent) in the bainite. When PWHT treatment is performed on steel having such a structure, the island-like martensite structure in the bainite is obtained. However, strength degradation is inevitable due to decomposition by tempering. Further, in order to compensate for the strength reduction due to tempering, there is a method of precipitating Cr carbide or the like during the PWHT treatment, but the carbide is easily coarsened, resulting in a reduction in toughness. Thus, it is clear that there is a limit to securing strength and toughness even after PWHT treatment by transformation strengthening. Therefore, as a result of intensive studies on the microstructure structure capable of obtaining excellent PWHT resistance, the present inventors have obtained the following findings (a) to (c).

(A) The microstructure of the steel may be a microstructure that does not change in shape before and after the PWHT treatment. For that purpose, the island-like martensite decomposed by the PWHT treatment may be suppressed to 2% or less, and the carbon in the steel may be dispersed and precipitated as thermally stable fine carbides.
(B) As a result of examining various precipitates precipitated in steel, composite carbides composed of Ti, Nb and Mo, or further composite carbides containing V are extremely small in size under 10 nm under an appropriate component balance. It turned out that it became a fine deposit and was thermally stable. In particular, if 30 or more such fine composite carbides are dispersed per 1 μm ^{2 in} steel and the total precipitation amount is 0.03% by mass or more, a decrease in strength after PWHT treatment can be suppressed.
(C) In order to precipitate the fine carbide of (b) above, the steel having a specific alloy component is used and cooled at a temperature lower than the bainite transformation end temperature in the process of cooling by accelerated cooling after hot rolling. And rapid reheating may be performed immediately.

The metal structure of the steel subjected to the thermal history as in (c) is a bainite structure with a low dislocation martensite and a high dislocation density immediately after cooling is stopped, but the generation of cementite is suppressed by Mo, and the carbon Since it exists in a supersaturated state, it is preferentially deposited on dislocations as a composite carbide composed of Ti, Nb and Mo, or further a composite carbide containing V by subsequent reheating.
A steel in which a composite carbide composed of Ti, Nb and Mo as described above, or a composite carbide further containing V is dispersed and precipitated, not only provides high strength by precipitation strengthening, but also finely by heating at about 700 ° C. or less. Since the carbide is not decomposed or coarsened, the high strength is maintained even after the PWHT treatment.

Hereinafter, the details of the high-strength steel sheet and the manufacturing method thereof according to the present invention will be described.
[Chemical composition]
First, chemical components of the high-strength steel plate of the present invention will be described. In the following description of the content of each element, all units shown in% are mass%.
・ C: 0.03-0.07%
C is an element that increases the strength of steel, and in order to obtain a desired structure and obtain desired strength and toughness, the content of 0.03% or more is required. On the other hand, if the content exceeds 0.07%, the weldability deteriorates, and cracks are likely to occur, and the base metal toughness and weld heat affected zone toughness (hereinafter referred to as “HAZ toughness”) are reduced. Therefore, the C content is 0.03 to 0.07%, preferably 0.04 to 0.06%.

・ Si: 0.01-0.5%
Si is an element that acts as a deoxidizer and increases the strength of the steel material by solid solution strengthening. However, if it is less than 0.01%, there is no effect, while if it exceeds 0.5%, HAZ toughness is remarkably increased. Deteriorate. For this reason, Si content is 0.01 to 0.5%, preferably 0.05 to 0.2%.
・ Mn: 1.5-2.5%
Mn is an element that has the effect of enhancing the hardenability of steel and improving the strength and toughness, and needs to be contained in an amount of 1.5% or more, but the inclusion exceeding 2.5% deteriorates the weldability. There is a fear. For this reason, Mn content is 1.5 to 2.5%, preferably 1.8 to 2.0%.

-Al: 0.08% or less Al acts as a deoxidizer during steelmaking, and inclusion exceeding 0.08% causes a decrease in toughness. For this reason, the Al content is 0.08% or less, preferably 0.01 to 0.05%.
Mo: 0.1-0.5%
Mo is an important element in the present invention. By containing 0.1% or more, Mo forms a fine composite carbide with Ti, Nb, and V while suppressing pearlite transformation during cooling after hot rolling. , Greatly contribute to strength increase. However, if it exceeds 0.5%, the HAZ toughness is deteriorated. For this reason, Mo content is made into 0.1 to 0.5%.

Ti: 0.005-0.035%
When Ti is added in an amount of 0.005% or more, it forms a composite carbide with Mo and greatly contributes to an increase in strength. However, addition exceeding 0.035% causes deterioration of HAZ toughness and base metal toughness. For this reason, Ti content shall be 0.005-0.035%.
・ Nb: 0.005 to 0.07%
Nb improves toughness by refining the structure, but forms a composite carbide with Mo and contributes to an increase in strength. However, if it is less than 0.005%, there is no effect, while if it exceeds 0.07%, the HAZ toughness deteriorates. For this reason, Nb content shall be 0.005-0.07%.

In the present invention, the chemical components of the steel sheet must further satisfy the conditions of the following formulas (i) and (ii).
・ 9 × Ceq value + 4 × P value ≧ 4.8 (i)
In this formula (i), the Ceq value is represented by the following formula (1), and the P value is represented by the following formula (2). In particular, the Ceq value has a correlation with the intensity before the PWHT process, and is often used as an index of intensity. The P value is an index for precipitation strengthening. From the regression equation obtained from the experiment, when 9 × Ceq value + 4 × P value <4.8, the API after PWHT processing
High strength of X100 grade cannot be obtained. Therefore, 9 × Ceq value + 4 × P value ≧ 4.8.
Ceq value = C + Mn / 6 + (Cu + Ni) / 12 + (Cr + Mo + V) / 5
… (1)
However, the element symbol of the formula (1) indicates mass% of each contained element.
P value = [Mo] + [Ti] + [Nb] + [V] (2)
However, the element symbol of the formula (2) indicates atomic% of each contained element.

In addition, the total amount in atomic% of the above element in the formula (2) is the sum of the number of atoms of Mo, Ti, Nb, V contained in steel, Fe, Mo, Ti, Nb, V, and other alloy elements. Although it can obtain | require by ratio with the total number of atoms of, it can also obtain | require by following (3) Formula using content in the mass% of Mo, Ti, Nb, and V.
(Mo / 95.9 + Nb / 92.91 + V / 50.94 + Ti / 47.9) / (100 / 55.85) × 100 (3)
However, the element symbol of the formula (3) indicates mass% of each contained element.

0.6 ≦ [C] / ([Mo] + [Ti] + [Nb] + [V]) ≦ 1.7
... (ii) However, the element symbol of the formula (ii) indicates the atomic% of each contained element.
The strengthening of the steel sheet of the present invention is due to a composite carbide composed of Ti, Nb and Mo, or a composite carbide further containing V. In order to effectively use the precipitation strengthening by this composite carbide, the relationship between the amount of C and the amount of carbide forming elements Mo, Ti, Nb, and V is important, and these elements are added in an appropriate balance. By doing so, a thermally stable and very fine composite carbide can be obtained. At this time, [C] / ([Mo] + [Ti] + [Nb] +, which is the ratio of the content of C in atomic% and the total content of Mo, Ti, Nb, and V in atomic%) The value of [V]) is 0.6 to 1.7. When the value of [C] / ([Mo] + [Ti] + [Nb] + [V]) is less than 0.6 or exceeds 1.7, the amount of any element is excessive, and the present invention is aimed at. Hardened structures other than the composite carbide are formed excessively, leading to deterioration of PWHT resistance and toughness.
In addition, when using content of the mass%, it is good also considering the following formula (iii) and calculating the value from 0.6 to 1.7.
(C / 12.01) / (Mo / 95.9 + Nb / 92.91 + V / 50.94 + Ti / 47.9) (iii)
However, the element symbol of the formula (iii) indicates mass% of each contained element.

The steel plate of the present invention may contain V in order to further increase the strength. Moreover, in order to further improve the strength and toughness, one or more selected from Cu, Ni, Cr, Ca, REM, and B may be contained.
・ V: 0.005-0.1%
V, like Nb, forms a composite carbide with Mo and contributes to an increase in strength. However, if it is less than 0.005%, there is no effect, while if it exceeds 0.1%, the HAZ toughness deteriorates. For this reason, when adding V, the content shall be 0.005-0.1%.
Cu: 0.5% or less Cu is an element effective for improving toughness and increasing strength, but if added excessively, weldability deteriorates. Therefore, when Cu is added, its content is reduced to 0. .5% or less.

-Ni: 0.5% or less Ni is an element effective for improving toughness and increasing strength. However, when added excessively, the PWHT resistance is lowered. 0.5% or less.
-Cr: 0.5% or less Cr is an element effective for obtaining sufficient strength even at low C, as with Mn. However, if excessively added, weldability deteriorates. The content is 0.5% or less.
・ Ca: 0.0005 to 0.0035%
Ca is an element effective for improving toughness by controlling the form of sulfide inclusions. However, if it is less than 0.0005%, the effect is not sufficient, and if added over 0.0035%, it is effective. Saturates, but rather degrades the toughness due to a reduction in the cleanliness of the steel. For this reason, when adding Ca, the content shall be 0.0005 to 0.0035%.

・ REM: 0.0005 to 0.01%
REM is also an element effective for improving toughness by controlling the form of sulfide inclusions in steel, but if it is less than 0.0005%, its effect is not sufficient, while it is added in excess of 0.01%. However, the effect is saturated, and rather, the toughness is deteriorated due to a decrease in the cleanliness of the steel. For this reason, when adding REM, the content shall be 0.0005 to 0.01%.
-B: 0.002% or less B segregates at the austenite grain boundaries and suppresses ferrite transformation, thereby contributing to the prevention of strength reduction particularly in the heat affected zone of the weld. However, B is added in excess of 0.002%. But the effect is saturated. For this reason, when adding B, the content shall be 0.002% or less.

The balance other than the above consists essentially of Fe. Therefore, as long as the effects of the present invention are not lost, the inclusion of other trace elements including inevitable impurities is not prevented.
In the present invention, V, Cu, Ni, and Cr are selective additive elements. Therefore, when these elements are not added, the value of the element amount in the above formulas (i) and (ii) Becomes “zero”.

[Precipitation form of microstructure and composite carbide]
The steel sheet of the present invention has a microstructure mainly composed of a bainite structure having an island-like martensite (MA) fraction of 2% or less, and further has an equivalent circle diameter of 10 nm or less, 30 or more composite carbides containing Ti and / or Nb as a main component are dispersed per 1 μm ² , and the total precipitation amount of the composite carbide (composite carbide having an equivalent circle diameter of 10 nm or less) is 0.03% by mass or more. It is necessary. Moreover, when the steel plate of this invention contains V, the composite carbide which satisfies said conditions will contain V further.
Here, the equivalent circle diameter of the composite carbide is the diameter of the circle when the area of the composite carbide obtained by image processing is converted into the area of the circle.

If the microstructure form before the PWHT treatment is controlled as described above, it is possible to maintain a tensile strength of 760 MPa or more without decreasing the strength even after the PWHT treatment of about 700 ° C. or less.
Note that the microstructure of the steel sheet and the precipitation form of the composite carbide as described above need to be satisfied regardless of the position in the thickness direction of the steel sheet. For example, for the island-like martensite fraction, Then, the area ratio may be measured and identified by observing at least 10 fields of view randomly with a scanning electron microscope (magnification 2000 times). In addition, the precipitation form of Mo-based composite carbide was observed at random in the center of the plate thickness section with a transmission electron microscope (magnification 30000 times) at least 10 fields of view, and the precipitation form (area of each composite carbide and composite carbide) (Dispersion density) may be identified.

[Production conditions]
Hereinafter, the manufacturing conditions of the high-strength steel sheet of the present invention will be described.
The present invention uses a combination of transformation strengthening due to bainite transformation during accelerated cooling and precipitation strengthening due to fine carbides precipitated during reheating after accelerated cooling, thereby increasing strength without adding a large amount of alloying elements. In addition, even when the PWHT treatment is performed, since the fine carbide is thermally stable, it is maintained as it is during the PWHT treatment, and the strength is maintained even after the PWHT treatment.
In the present invention, for example, after applying a cumulative pressure of a certain level or more in the non-recrystallized austenite region, accelerated cooling is started from the austenite single phase region, cooling is stopped at the bainite transformation end temperature or less, and immediate rapid reheating is performed. This makes it possible to combine and use transformation strengthening and precipitation strengthening most effectively. When the cumulative reduction ratio in the non-recrystallized austenite region (800 ° C. or less) is small, the transition of accumulated dislocations from processed austenite to transformed bainite is not sufficient, and fine dispersion of composite carbide preferentially precipitates on the dislocations. Becomes insufficient and the precipitation strengthening amount decreases, so the cumulative rolling reduction in the non-recrystallized austenite region needs to be 70% or more. In addition, when cooling is started from a two-phase region of Ar ₃ points or less, polygonal ferrite is mixed, and the strength decrease before PWHT treatment is large. Therefore, the cooling start temperature needs to be 700 ° C. or more, which is Ar ₃ points or more. .

Specifically, the high-strength steel sheet of the present invention can be manufactured under the following manufacturing conditions. That is, using steel having the above-described composition, hot rolling is performed at a heating temperature of 1100 to 1300 ° C. and a cumulative rolling reduction at 800 ° C. or less: 70% or more, and thereafter from 700 ° C. or more exceeding Ar ₃ point. Cooling is started at a cooling rate of 20 ° C./s or more, accelerated cooling is performed to a temperature of less than 300 ° C., and immediately thereafter a temperature of 550 to 700 ° C. at a heating rate of 0.5 ° C./s or more and less than 10 ° C./s. Is reheated until the main component is a bainite structure having an island-like martensite (MA) fraction in the microstructure of the steel sheet of 2% or less, the equivalent circle diameter is 10 nm or less, Mo is the main component, and Ti and / or containing Nb, or further dispersed fine composite carbide is 1 [mu] m ² per 30 or more, including a V, and the total deposition amount of the composite carbides is high-strength steel sheet of the present invention which is 0.03 wt% or more It is. Here, each temperature of the manufacturing conditions mentioned above is the average temperature of a steel plate (average temperature of a plate | board thickness direction).

In hot rolling, when the heating temperature is less than 1100 ° C., the dissolution of coarse carbides in the steel slab becomes insufficient, and precipitation strengthening becomes insufficient. On the other hand, if the temperature exceeds 1300 ° C., the initial γ grains become coarse and the toughness deteriorates. On the other hand, when the cumulative rolling reduction at 800 ° C. or lower is less than 70%, as described above, the fine dispersion of the composite carbide becomes insufficient, and the precipitation strengthening amount decreases.
In accelerated cooling after hot rolling, when the cooling start temperature is less than 700 ° C., as described above, polygonal ferrite is mixed and the strength reduction before PWHT treatment is large. On the other hand, when the cooling rate is less than 20 ° C./s, the bainite structure becomes coarse and it is difficult to ensure the strength before the PWHT treatment. In addition, when the cooling stop temperature is 300 ° C. or higher, the bainite matrix in the microstructure becomes soft, the strength before PWHT treatment decreases, and the island-like martensite fraction exceeds 2%, and the strength decreases after PWHT treatment. It is easy to invite.

In reheating after accelerated cooling, if the rate of temperature increase is less than 0.5 ° C./s, fine dispersion of the composite carbide becomes difficult. On the other hand, when the rate of temperature rise is 10 ° C./s or more, the time until precipitation of composite carbide is short, the amount of precipitation of composite carbide before PWHT treatment is less than 30, and the strength before PWHT treatment is reduced. In addition, when the reheating temperature is less than 550 ° C., the precipitation of the composite carbide becomes insufficient. On the other hand, when it exceeds 700 ° C., the strength is greatly reduced.
The high-strength steel pipe of the present invention is a steel pipe made of the above-described high-strength steel plate, and is formed into a cylindrical shape in the longitudinal direction according to a normal pipe making method, and the butt portion is formed from the inside and outside of the pipe It is obtained by welding one layer at a time in the longitudinal direction.

Steels of the chemical composition shown in Table 1 (steel types A to M) were made into slabs by a continuous casting method, and No. 2 shown in Tables 2 and 3 were used. 1 to 20 thick steel plates (thickness 20 mm) were produced.
In the production of this steel plate, after the hot slab is hot-rolled, it is immediately cooled using water-cooled accelerated cooling equipment, and an inline induction furnace or gas combustion installed on the same line as the accelerated cooling equipment. Reheating was performed using a furnace.
Table 2 shows the manufacturing conditions of each steel plate.
About the tensile characteristic of the manufactured steel plate, the tensile test was done by making the full thickness test piece of the same direction as a rolling direction into a tensile test piece, and measured the tensile strength. The strengths targeted in the present invention are a yield strength of 690 MPa or more and a tensile strength of 760 MPa or more. About HAZ toughness, the Charpy test was done using the test piece which added the heat history equivalent to heat input 20-50 kJ / cm with the reproduction | regeneration thermal cycle apparatus. A sample having Charpy absorbed energy at −10 ° C. of 100 J or higher was evaluated as good (◯), and a sample having a Charpy absorbed energy of less than 100 J was determined as poor (×).

Further, in order to investigate the PWHT resistance characteristics, PWHT treatment was performed on each steel plate using a gas atmosphere furnace. The heat treatment conditions at this time were 650 ° C. and 700 ° C. for 2 hours, and then removed from the furnace and cooled to room temperature by air cooling. And while investigating the microstructure form and precipitate form of the steel plate before PWHT processing, the tensile property and HAZ toughness of the steel plate before and after PWHT processing were measured. Table 3 shows the measurement results.
In Table 3, No. which is an example of the present invention. Nos. 1 to 7 have a strength drop of 50 MPa or less even after PWHT treatment at 700 ° C., maintain a high strength of a yield strength of 690 MPa or more and a tensile strength of 760 MPa or more, and have good base metal toughness and HAZ toughness. It was.

On the other hand, the comparative example No. Nos. 8, 9, 13, and 14 have chemical components within the scope of the present invention, but the dispersion amount of the fine composite carbide of 10 nm or less is less than 30 per 1 μm ² , and is a base material before or after PWHT treatment. Strength and base metal toughness deteriorated. No. which is a comparative example. No. 10, the chemical composition is within the scope of the present invention, but the ferrite structure is mixed in the steel plate microstructure before the PWHT treatment, and the island-like martensite (M · A constituent) fraction exceeds 2%. The base material strength before and after the PWHT treatment deteriorated. No. which is a comparative example. 11 and 12, although the chemical composition is within the scope of the present invention, the fraction of island martensite (MA constituent) in the steel sheet microstructure before PWHT treatment exceeds 2%, and the base material after PWHT treatment Strength or base metal toughness before PWHT treatment deteriorated. No. which is a comparative example. Since chemical components 15 to 18 are outside the scope of the present invention, sufficient base material strength / toughness was not obtained, or HAZ toughness was inferior. No. which is a comparative example. In No. 19, the value of the parameter X was outside the range of the present invention, and in particular, the base metal strength after the PWHT treatment at 700 ° C. was deteriorated. Moreover, No. which is a comparative example. No. 20, the value of the parameter Y was outside the range of the present invention, and the base material strength after the PWHT treatment was deteriorated.

Claims (5)

In mass%, C: 0.03-0.07%, Si: 0.01-0.5%, Mn: 1.5-2.5%, Mo: 0.1-0.5%, Al: 0.08% or less, Ti: 0.005 to 0.035%, Nb: 0.005 to 0.07%, Ceq value represented by the following formula (1) and represented by the following formula (2) P value satisfies the following formula (i),
9 × Ceq value + 4 × P value ≧ 4.8 (i)
Ceq value = C + Mn / 6 + (Cu + Ni) / 12 + (Cr + Mo + V) / 5
… (1)
However, the element symbol of the formula (1) indicates mass% of each contained element.
P value = [Mo] + [Ti] + [Nb] + [V] (2)
However, the element symbol of the formula (2) indicates atomic% of each contained element.
And having a component composition satisfying the following formula (ii):
0.6 ≦ [C] / ([Mo] + [Ti] + [Nb] + [V]) ≦ 1.7
... (ii)
However, the element symbol in the formula (ii) indicates atomic% of each contained element.
A composite mainly composed of a bainite structure having an island-like martensite (MA constituent) fraction of 2% or less in a steel sheet microstructure, an equivalent circle diameter of 10 nm or less, mainly composed of Mo, and containing Ti and / or Nb. A high-strength steel sheet having excellent PWHT resistance, wherein 30 or more carbides are dispersed per 1 μm ^{2 and} the total precipitation amount of the composite carbide is 0.03% by mass or more.   Further, the composite carbide containing, in mass%, V: 0.005 to 0.1%, mainly composed of Mo, and containing Ti and / or Nb, further contains V. A high-strength steel sheet having excellent PWHT resistance described.   Furthermore, in mass%, Cu: 0.5% or less, Ni: 0.5% or less, Cr: 0.5% or less, Ca: 0.0005 to 0.0035%, REM: 0.0005 to 0.01 %, B: One type or two or more types selected from 0.002% or less are contained, The high-strength steel sheet having excellent PWHT resistance according to claim 1 or 2.   The steel having the component composition according to any one of claims 1 to 3 is hot-rolled at a heating temperature of 1100 to 1300 ° C and a cumulative reduction ratio of 800 ° C or less: 70% or more, and then a cooling start temperature: 700 ° C. or higher, cooling rate: accelerated cooling to a temperature of 20 ° C./s or higher and lower than 300 ° C., and immediately thereafter, a temperature rising rate of 0.5 ° C./s or higher and lower than 10 ° C./s to a temperature of 550 to 700 ° C. A method for producing a high-strength steel sheet having excellent PWHT resistance, characterized by reheating.   It is a steel pipe which uses the steel plate according to any one of claims 1 to 3 as a raw material, and is obtained by forming the steel plate into a cylindrical shape in the longitudinal direction and welding the butt portion one layer at a time from the inside and outside of the tube. High strength steel pipe characterized by that.