JPH07188746A - Production of thick-walled steel tube having high toughness and low yield ratio - Google Patents

Abstract

PURPOSE:To produce the steel tube by subjecting a steel, in which composition and the parameter representing the tendency of island martensite formation at the time of heating in two phase region are respectively specified, to hot rolling and to reheating up to specific temp., starting and finishing bending in respectively specified temp. regions, and then applying cooling at specific velocity. CONSTITUTION:A steel, having a composition consisting of, by weight, 0.03-0.20% C, 0.01-0.50% Si, 0.5-2.0% Mn, 0.005-0.10% sol.Al, and the balance Fe, is used. In this steel, the value of the parameter PMA representing the tendency of island martensite formation at the time of heating in two phase region and computed from the equation, is regulated to <=120. This steel is hot-rolled, and the resulting steel plate is reheated up to a temp. in the two phase temp. region between Ac1 and Ac3. Bending is started at a temp. in the region not lower than Ar, and finished at a temp. in the region not lower than Ar1. Then, cooling is done at a cooling velocity noC lower than air cooling velocity. Moreover, one or more kinds among respectively prescribed amounts of the following (a), (b), and (c) can be further incorporated into the above steel: (a) one or more elements among Nb, V, and Ti; (b) one or more elements among Cu, Ni, Cr, and Mo; and (c) B and/or Ca.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a thick-walled steel pipe having a high toughness and a low yield ratio and having a wall thickness of about 40 to 150 mm, which is used for structural members such as high-rise buildings and marine structures or deep-sea submarine pipelines. Regarding

[0002]

2. Description of the Related Art High-strength, high toughness, low yield ratio, high weldability, and other performance are required for thick-walled steel pipes used for structural members such as high-rise buildings and offshore structures, or deep-sea submarine pipelines. . For this reason, relatively thin-walled steel pipes are formed by cold bending using low-constant and high-strength steel sheets that make full use of the so-called TMCP technology such as controlled rolling and controlled cooling. Since deterioration occurs, it is recovered by heat treatment such as stress relief annealing if necessary.

On the other hand, in the case of a thick-walled steel pipe, cold working is impossible from the viewpoint of bending workability of a press machine or the like,
Warm or hot working is adopted. Here, when hot working is performed, the strengthening mechanism by TMCP disappears, so a higher component normalized steel is adopted. Therefore,
Degradation of toughness and weldability cannot be avoided.

Regarding warm working, JP-A-62-54
No. 018 gazette, a steel sheet that has been subjected to controlled rolling or controlled rolling and accelerated cooling is reheated to an Ac ₁ temperature of 750 to 400 ° C. or lower, and immediately or after cooling, processed in a temperature range of 750 to 250 ° C. Therefore, it is proposed that excellent material properties such as toughness can be obtained. Here, the reheating temperature during processing is set to 750 ° C. or lower because the effect of controlled rolling or accelerated cooling is maintained at high heating temperature of Ac ₁ or less and a high quality material is obtained, whereas austenite + ferrite of Ac ₁ or more is obtained. This is because the structure changes when heated to the two-phase region, the effects of controlled rolling and heating and cooling disappear, and the prescribed strength and toughness cannot be obtained.

On the other hand, as a method for lowering the yield ratio, a steel pipe is heated to a temperature range of Ac ₃ -250 ° C. to Ac ₃ -20 ° C. and water-cooled (JP-A-3-87318).
Or heated to above Ac ₃ and air-cooled to Ac ₃ -250 ℃
~ Ac ₃ Water cooling from a temperature range of −20 ° C.
No. 87317) or a method of tempering after these treatments by cold working (Japanese Patent Laid-Open No. 3-21901).
No. 7, JP-A-3-219018), and further, heating a steel pipe to Ac ₃ -200 ° C. or higher to obtain Ac ₃ -200.
℃ to start the strain applied in the above, Ac ₃ -200 ℃ ~Ac ₃ -
Although a method of finishing the strain application in the temperature range of 20 ° C, tempering after water cooling (Japanese Patent Laid-Open No. 4-321) and the like have been proposed, all of them are based on the premise that the steel pipe is subjected to a special treatment. Therefore, the economical efficiency and the productivity are significantly impaired.

[0006]

As described above, in cold working, stress relieving annealing is required after forming because workability deteriorates ductility and toughness, or extra thick steel pipes cannot be produced due to restriction of forming ability. Difficult, TMC in hot working
It is expected that the effect of P cannot be expected and it becomes a high component system, resulting in poor toughness and weldability. On the other hand, in warm forming under heating to an Ac ₁ temperature or less, taking into consideration the temperature drop during processing, it tends to be difficult to form an extremely thick steel pipe due to an increase in deformation resistance, and the yield ratio increases with a decrease in temperature. It has a problem that a low yield ratio cannot be obtained. In addition, heat treatment of the steel pipe to reduce the yield ratio, or treatment such as heat treatment after cold working impairs the economic efficiency, and the prescribed strength / toughness / yield ratio cannot be obtained when manufacturing the steel pipe. A method of giving is desired.

The present invention has been made in view of the above circumstances, and is to manufacture a thick-walled steel pipe having high toughness, a low yield ratio, high strength and excellent weldability with high productivity and economical efficiency. It is an object of the present invention to provide a method for producing a thick-walled steel pipe which has high toughness and a low yield ratio.

[0008]

Means for Solving the Problems The inventors of the present invention have variously studied to obtain a method for producing a thick-walled steel pipe that does not require a large bending facility, does not cause deterioration in toughness, and does not significantly increase the yield ratio. As a result, the following findings were obtained.

That is, by restricting the composition of components to a specific range, even if it is heated to the temperature of the two-phase region of ferrite + austenite and molded from that temperature region, the formation of island martensite is suppressed and deterioration of toughness is caused. Steel sheet is manufactured by hot rolling using this steel, heated to a two-phase region, and subjected to bending for pipe making from this temperature region, and processing at a temperature above the transformation end temperature. It has been found that by completing the process, a high-toughness low-yield ratio steel pipe can be obtained without causing the problem of deterioration of toughness that has been a problem in the conventional two-phase region heating.

It has been known that heating to the two-phase region lowers the deformation resistance and that a low yield ratio can be obtained, but at the same time, it is accompanied by a remarkable deterioration in toughness, and this temperature is characteristic. Although no pipe forming work was performed in the two-phase region, the toughness in the two-phase region heating was found by finding a steel with a composition that does not easily cause deterioration of the toughness when heated and processed in the two-phase region as described above. This is the point of solving the problem of deterioration.

The present invention has been completed on the basis of such findings, and firstly, in% by weight, C: 0.03 to.
0.20%, Si: 0.01 to 0.50%, Mn: 0.
5% to 2.0%, solAl: 0.005 to 0.10%
Containing the balance consisting of Fe and unavoidable impurities, and parameters representative of the (1) shown island martensite tendency in the two-phase region heating which is calculated by the formula below: P values of _{the MA} P _MA ≦ 120 Steel to be a steel plate by hot rolling, A
c ₁ or more and reheated to Ac ₃ following the two-phase region temperature range, it starts bending from Ar ₁ above temperature range, Ar ₁
The present invention provides a method for producing a thick-walled steel pipe having high toughness and a low yield ratio, which is characterized by terminating processing in the above temperature range and then cooling at a cooling rate of air cooling or higher.

Secondly, in% by weight, C: 0.03 to 0.2.
0%, Si: 0.01 to 0.50%, Mn: 0.5% to
2.0%, solAl: 0.005 to 0.10%, Nb: 0.005 to 0.05%, V: 0.0
1 to 0.10%, Ti: 0.005 to 0.10%, containing 1 or 2 or more kinds, consisting of the balance Fe and unavoidable impurities, and calculated by the formula (1) shown below. parameters representing the island martensite tendency in the two-phase region heating: steels value of P _MA is P _MA ≦ 120, and the steel sheet by hot rolling, Ac ₁ or more and Ac ₃ following the two-phase region temperature reheated to the range, to start the bending from Ar ₁ above temperature range, to exit the processing by Ar ₁ or more temperature range, the yield ratio in the subsequent high toughness, characterized by cooling with air or a cooling rate The present invention provides a method for manufacturing a thick-walled steel pipe having a low value.

Third, C: 0.03 to 0.2 by weight%.
0%, Si: 0.01 to 0.50%, Mn: 0.5% to
2.0%, solAl: 0.005 to 0.10%, further Cu: 1.5% or less, Ni: 1.0% or less,
Cr: 1.0% or less, Mo: 0.5% or less, one or more kinds are contained, the balance is Fe and inevitable impurities, and two-phase calculated by the formula (1) shown below. Parameters indicating the tendency of island martensite formation during zone heating:
Steels value of P _MA is P _MA ≦ 120, and the steel sheet by hot rolling, then reheated to and Ac ₃ following the two-phase region temperature range at Ac ₁ or more, the bending from Ar ₁ or more temperature range The present invention provides a method for producing a thick-walled steel pipe having high toughness and a low yield ratio, which is characterized by starting, finishing processing in a temperature range of Ar ₁ or higher, and then cooling at a cooling rate of air cooling or higher.

Fourth, C: 0.03 to 0.2 by weight%.
0%, Si: 0.01 to 0.50%, Mn: 0.5% to
2.0%, solAl: 0.005 to 0.10%, Nb: 0.005 to 0.05%, V: 0.0
1 to 0.10%, Ti: 0.005 to 0.10%, one or more kinds, and Cu: 1.5% or less, Ni:
1.0% or less, Cr: 1.0% or less, Mo: 0.5% or less, one or more types are contained, the balance is Fe and inevitable impurities, and the formula (1) is shown below. parameters representing the island martensite tendency in the two-phase region heating in calculated: the value of P _MA is a steel which is a P _MA ≦ 120, and the steel sheet by hot rolling, and Ac ₃ following the Ac ₁ or more reheated to a two-phase region temperature range, it starts bending from Ar ₁ above temperature range, to exit the processing by Ar ₁ or more temperature range,
The present invention provides a method for producing a thick-walled steel pipe having high toughness and a low yield ratio, which is characterized by cooling at a cooling rate higher than air cooling.

Fifth, by weight%, C: 0.03 to 0.2
0%, Si: 0.01 to 0.50%, Mn: 0.5% to
2.0%, solAl: 0.005-0.10%, C
a: 0.0005 to 0.0050%, balance Fe
And becomes unavoidable impurities, and represents the island martensite tendency in the two-phase region heating is calculated by the following equation (1) Parameters: Steels value of P _MA is P _MA ≦ 120, heat Steel plate by hot rolling, Ac ₁ or more and Ac
₃ was reheated to the following two-phase region temperature ranges, characterized in that to start the bending from Ar ₁ above temperature range, to exit the processing by Ar ₁ or more temperature range, and then cooled in air over a cooling rate And a method of manufacturing a thick-walled steel pipe having a high toughness and a low yield ratio.

Sixth, in% by weight, C: 0.03 to 0.2
0%, Si: 0.01 to 0.50%, Mn: 0.5% to
2.0%, solAl: 0.005 to 0.10%, Nb: 0.005 to 0.05%, V: 0.0
1 to 0.10%, Ti: 0.005 to 0.10%, one or more kinds, and B: 0.0005 to 0.003.
0%, Ca: 0.0005 to 0.0050%, containing one or two kinds, consisting of the balance Fe and unavoidable impurities, and heating in the two-phase region calculated by the following formula (1) Parameter indicating the tendency of island martensite formation: P
Steels value of _MA is P _MA ≦ 120, and the steel sheet by hot rolling, reheating to Ac ₁ or more and Ac ₃ following the two-phase region temperature range, start the bending from Ar ₁ or more temperature range Then
The present invention provides a method for producing a thick-walled steel pipe having high toughness and a low yield ratio, which is characterized by terminating processing in a temperature range of Ar ₁ or higher and then cooling at a cooling rate of air cooling or higher.

Seventh, in% by weight, C: 0.03 to 0.2.
0%, Si: 0.01 to 0.50%, Mn: 0.5% to
2.0%, solAl: 0.005 to 0.10%, further Cu: 1.5% or less, Ni: 1.0% or less,
One or more of Cr: 1.0% or less and Mo: 0.5% or less, and B: 0.0005 to 0.0030%,
Ca: Island containing one or more of 0.0005 to 0.0050%, the balance Fe and unavoidable impurities, and the two-phase region heating calculated by the following formula (1) Parameters that indicate the tendency to form martensite in the form of:
Steels value of P _MA is P _MA ≦ 120, and the steel sheet by hot rolling, then reheated to and Ac ₃ following the two-phase region temperature range at Ac ₁ or more, the bending from Ar ₁ or more temperature range The present invention provides a method for producing a thick-walled steel pipe having high toughness and a low yield ratio, which is characterized by starting, finishing processing in a temperature range of Ar ₁ or higher, and then cooling at a cooling rate of air cooling or higher.

Eighth, C: 0.03 to 0.2 by weight%
0%, Si: 0.01 to 0.50%, Mn: 0.5% to
2.0%, solAl: 0.005 to 0.10%, Nb: 0.005 to 0.05%, V: 0.0
1 to 0.10%, Ti: 0.005 to 0.10%, one or more kinds, Cu: 1.5% or less, Ni: 1.0
% Or less, Cr: 1.0% or less, Mo: 0.5% or less, one or more, and B: 0.0005 to 0.00
30%, Ca: 1 out of 0.0005 to 0.0050%
Parameter indicating the island-like martensite formation tendency at the time of heating in the two-phase region, which contains one or two kinds, is composed of the balance Fe and unavoidable impurities, and is calculated by the following formula (1):
Steels value of P _MA is P _MA ≦ 120, and the steel sheet by hot rolling, then reheated to and Ac ₃ following the two-phase region temperature range at Ac ₁ or more, the bending from Ar ₁ or more temperature range The present invention provides a method for producing a thick-walled steel pipe having high toughness and a low yield ratio, which is characterized by starting, finishing processing in a temperature range of Ar ₁ or higher, and then cooling at a cooling rate of air cooling or higher.

P _MA = 325 × [C%] + 9 × [Si%] + 40 × [Mn%] + 3 × [solAl%] + 380 × [Nb%] (1) Next, the thickness according to the present invention The method of manufacturing a thick steel pipe will be described in detail by dividing the composition and manufacturing conditions.

[Component composition] In the present invention, C, Si, M
n and solAl as basic components, (a) Nb, V,
One or more of Ti, (b) Cu, Ni, C
One or two or more of r and Mo, and one or two of (c) Ca and B are selected as the selected components, and the target steel is the basic component alone or the basic components and these (a) to (). One or more of c) are to be contained. In addition, when only the above (c) is added to the basic component, Ca
Limited to a single person.

Further, in the present invention, as will be described later, P _MA = 325 × [C%] + 9 × [Si%] + 40 × [M
By setting the parameter represented by [n%] + 3 × [solAl%] + 380 × [Nb%] to 120 or less, the tendency of island martensite formation during heating in the two-phase region can be significantly reduced.

The reasons for limiting the range of each component will be described below. In the following explanation, all percentages are% by weight.
Indicates.

C: To secure the strength of this type of steel inexpensively and effectively, C is required to be 0.03%. However, if it exceeds 0.20%, the weldability such as cold cracking and hot cracking is impaired. Therefore, the C content is 0.03 to 0.2.
Specify in the range of 0%.

Si: Si is added for deoxidation, but if it is less than 0.01%, a sufficient deoxidizing effect cannot be obtained, while if it exceeds 0.50%, toughness and weldability are deteriorated. Therefore, the Si content is specified in the range of 0.01 to 0.50%.

Mn: Mn is added as a basic element of steel effective for improving the strength and toughness of the steel, but if it is less than 0.5%, its effect is small, and if it exceeds 2.0%, the weldability is remarkable. to degrade. Therefore, the Mn content is 0.5 to
Specified within the range of 2.0%.

SolAl: SolAl is added for deoxidation of steel, but if it is less than 0.005%, a sufficient effect cannot be obtained, while if it is 0.10%, the effect is saturated. Therefore, Al is specified in the range of 0.005 to 0.10%.

The reasons for limiting the basic components have been described above. The reasons for limiting the selective components shown in (a) to (c) above will be described below.

(A) Nb, V, Ti: These elements bring about an increase in strength due to precipitation strengthening and an improvement in toughness due to the refinement of crystal grains, but when the addition amount increases, the toughness of the welded portion and the like deteriorate. Nb: 0.005-0.05
%, V: 0.01 to 0.10%, Ti: 0.005 to
It is specified in the range of 0.10%.

(B) Cu, Ni, Cr, Mo: These elements can be strengthened without impairing toughness through solid solution strengthening and structural change due to increase in hardenability, but Cu is 1 from the viewpoint of weldability and economy. 0.5% or less, Ni, Cr 1.0
%, And Mo is specified to be 0.5% or less.

(C) B, Ca: B increases the hardenability of steel and exerts a great effect on the increase in strength, but 0.0
If it is less than 005%, this effect is small, and 0.0030
%, The weldability is impaired, so 0.0005 to 0.
Specified within the range of 0030%. Further, Ca is effective in preventing hydrogen-induced cracking and lamellathea by spheroidizing the form of inclusions, but if less than 0.0005%, the effect cannot be obtained, and if added in excess of 0.0050%. However, the effect is saturated, so the range is defined as 0.0005 to 0.0050%.

P _MA : The effect of alloying elements on the tendency to form island martensite when heating in the two-phase region and starting the molding from that temperature was quantitatively examined, and as shown in FIG. It became clear that it has a good correlation with the parameter P _MA calculated by the formula.

P _MA = 325 × [C%] + 9 × [Si%]
+40 x [Mn%] +3 x [solAl%] +380 x
[Nb%] Further, there is a good correlation between the toughness and the area fraction of island martensite. For example, as shown in FIG. 2, by setting the area fraction of island martensite to 5% or less, the Charpy impact test was performed at 0 ° C. The absorbed energy at 100 J is 100 J or more and good toughness is achieved. Here, in order to reduce the area fraction of island martensite to 5% or less, from FIG. 1, P _MA ≦ 1
It should be 20. Therefore, in the present invention, P _MA ≦ 12
It is defined as 0.

[Manufacturing conditions] In the present invention, after hot rolling, A
c ₁ above Ac ₃ was reheated to below the so-called two-phase region temperature region starts bending for forming tubes from Ar ₁ above temperature range, to exit the processing by Ar ₁ or more temperature range, followed by air cool Cooling under the above cooling conditions is an essential condition.

The heating to the two-phase region temperature is performed in order to easily perform the bending without requiring a large facility capacity and to reduce the yield ratio by the two-phase region heating / forming.
The processing end temperature is set to a two-phase region temperature of Ar ₁ or higher. By finishing the work in this range, a small deformation resistance of about 1/3 and a low yield ratio of 80% or less are expected as compared with the cold work. Further, the cooling after processing may be a cooling rate of air cooling or more, and may be water cooling.
Whether it is air-cooled or water-cooled is selected from the viewpoints of facility capacity, workability, and economic efficiency. The cooling rate of air cooling or more as referred to herein means a rate of 0.05 ° C./sec or more.

For the hot rolling, a general method usually used in this field may be adopted. Further, as the bending process, a general method of bending into a cylindrical shape such as a press bend or a bending roll is adopted, but it is not particularly limited.

[0036]

By using the present invention, the following action does not cause deterioration of toughness even if the material is heated in the two-phase region and molded from that temperature region. That is, C, Si, Mn, solAl, and Nb are extracted as five elements that promote the formation of island martensite by adding a large amount of chemical components, and island martensite of these elements during heating in the two-phase region is extracted. The relative contribution to the generation is obtained and expressed as a parameter P _MA . By setting this value to 120 or less, the area fraction of island martensite can be set to 5% or less. Even if heating is performed, significant deterioration of toughness can be prevented.

[0037]

EXAMPLE Steels having the chemical compositions shown in Table 1 were hot-rolled into steel sheets, then reheated to 800 ° C. and immediately formed into a cylindrical shape by press bend. The forming was completed at 690 ° C. and air-cooled. Water cooling was performed to manufacture a steel pipe. Table 1 shows the mechanical properties of the steel pipe manufactured in this manner. Also, P _MA = 325 ×
[C%] + 9 × [Si%] + 40 × [Mn%] + 3 ×
Value of parameter P _MA calculated by [solAl%] + 380 × [Nb%], plate thickness 40 mm, preheating temperature 10
The results of the oblique y-type weld cracking test conducted at 0 ° C are also shown.

In Table 1, steel numbers 1 to 17 are steels of the present invention, and steel numbers 18 to 22 are comparative steels.

[0039]

[Table 1] As shown in Table 1, in the steels of the present invention, cracks did not occur in the weld crack test, good weldability was exhibited, and all tensile strengths were high values of 500 N / mm ² or more.
In addition, the yield ratios were all low values of 70% or less, and a sufficient effect of the two-phase region thermoforming was confirmed. Furthermore, despite the two-phase region hot forming, the Charpy impact values at 0 ° C all show good toughness of 100 J or more, and high strength /
It was confirmed that high toughness / weldability and low yield ratio were simultaneously achieved.

On the other hand, in the case of the comparative steels, both high strength and low yield ratio have been achieved, but there are problems in weldability and toughness, and in particular regarding toughness, the Charpy impact value at 0 ° C. is 21 J or less, which is extremely low. It's getting low. This is due to the coarse island-like martensite formed during the two-phase region hot forming, suggesting the importance of limiting _PMA to 120 or less.

[0041]

As described above, according to the present invention,
Economically manufacture thick-walled steel pipes with a wall thickness of 40 to 150 mm while not requiring large facility capacity and having properties such as high strength, high weldability, and low yield ratio without causing deterioration of toughness. It has become possible.

[Brief description of drawings]

FIG. 1 is a diagram showing a correlation between a parameter P _MA and an area fraction of island martensite.

FIG. 2 is a diagram showing a relationship between an area fraction of martensite and absorbed energy at 0 ° C. in a Charpy impact test.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Takayuki Honda Inventor Marunouchi 1-2-2 Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Junichiro Kawasaki Inventor 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Nippon Steel Tube Co., Ltd.

Claims (8)

[Claims] 1. C: 0.03 to 0.20% by weight,
Si: 0.01 to 0.50%, Mn: 0.5% to 2.0
%, SolAl: 0.005 to 0.10%, the balance Fe and unavoidable impurities, and the island-like martensite formation tendency at the time of heating in the two-phase region calculated by the formula (1) shown below. represents parameters: the value of P _MA is P _MA ≦ 12
0 become steel, and the steel sheet by hot rolling, reheating to Ac ₁ or more and Ac ₃ following the two-phase region temperature range, starts bending from Ar ₁ above temperature range, Ar ₁ or more temperature A method for producing a thick-walled steel pipe having high toughness and a low yield ratio, which comprises finishing processing in a zone and then cooling at a cooling rate higher than air cooling. P _MA = 325 × [C%] + 9 × [Si%] + 40 × [Mn%] + 3 × [solAl%] + 380 × [Nb%] (1) 2. C: 0.03 to 0.20% by weight,
Si: 0.01 to 0.50%, Mn: 0.5% to 2.0
%, SolAl: 0.005-0.10%, Nb: 0.005-0.05%, V: 0.01-
0.10%, Ti: 0.005 to 0.10%, containing one or more kinds, consisting of the balance Fe and inevitable impurities, and a two-phase calculated by the following formula (1) parameters representing the island martensite propensity to generate time-frequency heating: steels value of P _MA is P _MA ≦ 120, and the steel sheet by hot rolling, the Ac ₁ or more and Ac ₃ in the following two-phase region temperature range reheating starts bending from Ar ₁ above temperature range, to exit the processing by Ar ₁ or more temperature range, low yield ratio at a high toughness, characterized by subsequently cooling air cooling or a cooling rate Method for manufacturing thick steel pipe. P _MA = 325 × [C%] + 9 × [Si%] + 40 × [Mn%] + 3 × [solAl%] + 380 × [Nb%] (1) 3. C: 0.03 to 0.20% by weight,
Si: 0.01 to 0.50%, Mn: 0.5% to 2.0
%, SolAl: 0.005 to 0.10%, Cu: 1.5% or less, Ni: 1.0% or less, Cr:
1.0% or less, Mo: 0.5% or less, one or more kinds are contained, and the balance is Fe and unavoidable impurities.
And parameters representative of the island martensite tendency in the two-phase region heating is calculated by the following equation (1): Steels value of P _MA is P _MA ≦ 120, and the steel sheet by hot rolling, Ac Reheat to a two-phase temperature range of ₁ or more and Ac ₃ or less, and start bending from the temperature range of Ar ₁ or more.
A method for producing a thick-walled steel pipe having high toughness and a low yield ratio, characterized by finishing processing in a temperature range of r ₁ or higher and then cooling at a cooling rate of air cooling or higher. P _MA = 325 × [C%] + 9 × [Si%] + 40 × [Mn%] + 3 × [solAl%] + 380 × [Nb%] (1) 4. C: 0.03 to 0.20% by weight,
Si: 0.01 to 0.50%, Mn: 0.5% to 2.0
%, SolAl: 0.005-0.10%, Nb: 0.005-0.05%, V: 0.01-
0.10%, Ti: 0.005 to 0.10%, one or more kinds, and Cu: 1.5% or less, Ni: 1.0
% Or less, Cr: 1.0% or less, Mo: 0.5% or less, one or more of them are contained, the balance is Fe and unavoidable impurities, and calculated by the following formula (1). parameters representing the island martensite tendency in the two-phase region heating that: the steel value of P _MA is P _MA ≦ 120, and the steel sheet by hot rolling, and Ac ₃ following two-phase region at Ac ₁ or more reheated to a temperature range, starts bending from Ar ₁ above temperature range, to exit the processing by Ar ₁ or more temperature range, then the yield in high toughness, characterized by cooling with air or a cooling rate Method for manufacturing thick-walled steel pipe with a low ratio. P _MA = 325 × [C%] + 9 × [Si%] + 40 × [Mn%] + 3 × [solAl%] + 380 × [Nb%] (1) 5. C: 0.03 to 0.20% by weight,
Si: 0.01 to 0.50%, Mn: 0.5% to 2.0
%, SolAl: 0.005 to 0.10%, Ca: 0.
A parameter indicating the tendency of island-like martensite formation during heating in the two-phase region, which is composed of the balance Fe and unavoidable impurities and contains 0005 to 0.0050%, and is calculated by the following formula (1): P _MA A steel having a value of P _MA ≦ 120 is hot-rolled into a steel sheet, reheated to a temperature range of two phases of Ac ₁ or more and Ac ₃ or less, and bending is started from a temperature range of Ar ₁ or more, Finish processing in the temperature range above Ar ₁ ,
Then, a method for producing a thick-walled steel pipe having high toughness and a low yield ratio, characterized by cooling at a cooling rate higher than air cooling. P _MA = 325 × [C%] + 9 × [Si%] + 40 × [Mn%] + 3 × [solAl%] + 380 × [Nb%] (1) 6. C: 0.03 to 0.20% by weight,
Si: 0.01 to 0.50%, Mn: 0.5% to 2.0
%, SolAl: 0.005-0.10%, Nb: 0.005-0.05%, V: 0.01-
0.10%, Ti: 0.005 to 0.10%, one or more kinds, and B: 0.0005 to 0.0030.
%, Ca: 0.0005 to 0.0050% of 1 type or 2 types, the balance Fe and unavoidable impurities, and island martens during heating in the two-phase region calculated by the formula (1). Parameter indicating the site generation tendency: The value of P _MA is P
Steel with _MA ≤ 120 is hot-rolled into a steel plate, and Ac
Reheat to a two-phase temperature range of ₁ or more and Ac ₃ or less,
A thick-walled steel pipe with high toughness and a low yield ratio characterized by starting bending in a temperature range of Ar ₁ or higher, finishing the bending in a temperature range of Ar ₁ or higher, and then cooling at a cooling rate of air cooling or higher. Production method. P _MA = 325 × [C%] + 9 × [Si%] + 40 × [Mn%] + 3 × [solAl%] + 380 × [Nb%] (1) 7. C: 0.03 to 0.20% by weight,
Si: 0.01 to 0.50%, Mn: 0.5% to 2.0
%, SolAl: 0.005 to 0.10%, Cu: 1.5% or less, Ni: 1.0% or less, Cr:
1.0% or less, one or more of Mo: 0.5% or less, and B: 0.0005 to 0.0030%, Ca:
One or more of 0.0005 to 0.0050% is contained, the balance is Fe and unavoidable impurities, and the island-like martensite formation tendency at the time of heating in the two-phase region is calculated by the formula (1). parameters representing the: value of P _MA is P _MA ≦ 12
0 become steel, and the steel sheet by hot rolling, reheating to Ac ₁ or more and Ac ₃ following the two-phase region temperature range, starts bending from Ar ₁ above temperature range, Ar ₁ or more temperature A method for producing a thick-walled steel pipe having high toughness and a low yield ratio, which comprises finishing processing in a zone and then cooling at a cooling rate higher than air cooling. P _MA = 325 × [C%] + 9 × [Si%] + 40 × [Mn%] + 3 × [solAl%] + 380 × [Nb%] (1) 8. C: 0.03 to 0.20% by weight,
Si: 0.01 to 0.50%, Mn: 0.5% to 2.0
%, SolAl: 0.005-0.10%, Nb: 0.005-0.05%, V: 0.01-
0.10%, Ti: 0.005 to 0.10%, one or more, Cu: 1.5% or less, Ni: 1.0% or less, Cr: 1.0% or less, Mo: 1 out of 0.5% or less
Or two or more, and B: 0.0005 to 0.0030
%, Ca: 0.0005 to 0.0050% of 1 type or 2 types, the balance Fe and unavoidable impurities, and island martens during heating in the two-phase region calculated by the formula (1). Parameter indicating the site generation tendency: The value of P _MA is P
Steel with _MA ≤ 120 is hot-rolled into a steel plate, and Ac
Reheat to a two-phase temperature range of ₁ or more and Ac ₃ or less,
A thick-walled steel pipe with high toughness and a low yield ratio characterized by starting bending in a temperature range of Ar ₁ or higher, finishing the bending in a temperature range of Ar ₁ or higher, and then cooling at a cooling rate of air cooling or higher. Production method. P _MA = 325 × [C%] + 9 × [Si%] + 40 × [Mn%] + 3 × [solAl%] + 380 × [Nb%] (1)