JP2689864B2 - High toughness steel pipe - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-toughness steel pipe having high strength and excellent fracture toughness, which is suitable for offshore platforms used for the development and production of oil and natural gas in deep water.

[0002]

2. Description of the Related Art In recent years, crude oil wells and natural gas gas wells have tended to become deeper and deeper, and as the development of oil and natural gas in deep seas has progressed, it has become deeper and deeper on offshore platforms. Although various types have been adopted, the tension leg platform is the most promising in terms of shortening the installation period, reducing the amount of steel used, and high operational safety. The steel material used for this tension leg platform is required to have higher strength and fatigue strength in seawater than conventional steel materials for offshore platforms, and at the same time, to have fracture toughness values and weldability equivalent to those of the conventional steel materials. Furthermore, due to the increase of hydrogen sulfide in oil and gas, some steel materials have sulfide stress corrosion cracking resistance (SSC resistance
C) Sex is required.

Conventionally, in order to obtain excellent fracture toughness, 9
Although high Ni steel materials such as% Ni steel have been used, the recent demands for steel material performance have become higher and the fracture toughness has become lower temperature, and the conventional technology cannot meet the demand. As a steel pipe satisfying both high strength and high fracture toughness, C: 0.1 to 0.5%, Si: 0.1 to 0.3%,
Mn: 0.2 to 0.8%, Cr: 1.0 to 4.0% included, Al: 0.005 to 0.1%, P and S
Both are reduced to 0.005% or less, and N: 0.004% or less, and Mo: 0.2 to 1.0%, Nb:
When hot rolling is performed on a seamless steel pipe material having a composition containing 0.01 to 0.1% together with the total amount of Zr and / or Ti of 0.005 to 0.1%, and then temper rolling. During the tempering process following quenching, 620 ° C or higher A _c1
Manufacturing method of high-strength seamless steel pipe, which is maintained for a predetermined time according to the wall thickness t of the pipe in a temperature range below the point (Japanese Patent Publication No. 2-25969).
Gazette), C: 0.15 to 0.40%, Si: 0.05
~ 0.50%, Mn: 0.50-2.00%, Cr:
1.00 to 3.00%, Mo: 0.10 to 1.00%,
Al: 0.005-0.050%, Nb: 0.005-
High toughness seamlessness containing 0.050%, the balance consisting of Fe and inevitable impurities, P in the impurities is 0.025% or less, S is 0.015% or less, and O is 0.0010% or less. A steel pipe (Japanese Patent Laid-Open No. 5-51699) and the like are known.

[0004]

[Problems to be Solved by the Invention] Japanese Patent Publication No. 2-259
The steel pipe obtained by the method disclosed in Japanese Patent Publication No. 69 has high strength and excellent toughness only by the tempering treatment following quenching, but the fracture toughness Kee at low temperatures is not taken into consideration. There is a problem that the desired fracture toughness Kee cannot be obtained by only one quenching and tempering treatment. Further, the steel pipe disclosed in Japanese Unexamined Patent Publication No. 5-51699 has high strength and excellent toughness as hot-rolled, but does not consider fracture toughness Kee at low temperatures.

The object of the present invention is to provide a yield point of 550 N / mm.
² or more, and a tensile strength of 620N / mm ² or more high strength, -2
It is to provide a high toughness steel pipe having a fracture toughness Kee at 0 ° C. of 180 N / mm ² √m or more.

[0006]

Means for Solving the Problems The present inventors have conducted various tests and studies to achieve the above object. As a result, a steel pipe having excellent fracture toughness can be obtained by containing an appropriate amount of Ni and suppressing the oxygen content as low as possible.
Further, it was clarified that a steel pipe having excellent fracture toughness can be obtained by subjecting the steel pipe to repeated heat treatments of normalizing-quenching-quenching-tempering, and arrived at the present invention.

That is, according to the present invention, C: 0.02 to 0.
06%, Si: 0.05 to 0.35%, Mn: 1.00
~ 1.50%, Cu: 1.00 to 1.50%, Ni:
1.50 to 2.50%, Cr: 0.40 to 0.80%,
Mo: 0.40 to 0.60%, Al: 0.005 to 0.
050%, Nb: 0.020 to 0.060%, P: 0.
015% or less, S: 0.010% or less, O: 0.001
A high toughness steel pipe characterized by being manufactured by using a steel containing 0% or less and the balance being Fe and inevitable impurities, and then repeatedly heat-treating normalizing-quenching-quenching-tempering.

[0008]

The function and effect of each component element contained in the steel constituting the steel pipe of the present invention and the reason for limiting the content thereof will be described below. C is an element that enhances the strength of steel,
If it exceeds 0.06%, the toughness is adversely affected. on the other hand,
If it is less than 0.02%, sufficient strength cannot be obtained.
It was set to 02 to 0.06%. Si is an essential element as a deoxidizing agent during steel making, but if it is less than 0.05%, a sufficient deoxidizing effect cannot be obtained, and if it exceeds 0.35%, the effect is saturated. It was set to 0.35%. Mn is an element that enhances the strength of steel, but if it is less than 1.00%, sufficient strength cannot be obtained, and if it exceeds 1.50%, the strength becomes too high and the toughness is adversely affected. 1.50
%.

Cu is an element that enhances the strength of steel.
If less than 1.00%, a sufficient effect cannot be obtained, and 1.50%
If it exceeds, the effect will be saturated, so 1.00 to 1.50%
And Cr is an element that increases the strength of steel, but 0.4
If it is less than 0%, a sufficient effect cannot be obtained, and if it exceeds 0.80%, the effect is saturated, so the content was made 0.40 to 0.80%. Mo is an element that enhances the strength of steel, but 0.40%
If it is less than 0.1%, a sufficient effect cannot be obtained, and if it exceeds 0.60%, the effect is saturated, so 0.40 to 0.60% is set. N
i is an element that enhances the strength and toughness of steel, but 1.50%
If it is less than 2.50%, a sufficient effect cannot be obtained, and if it exceeds 2.50%, the effect is saturated and it is not economical.
It was set to 2.50%.

Al is an element used for deoxidizing steel,
When it is less than 0.005%, a sufficient deoxidizing effect cannot be obtained, and
If it exceeds 050%, the effect will be saturated, so 0.005-
0.050%. Nb has the effect of improving the strength and toughness of steel by containing a trace amount, but 0.02
If it is less than 0%, a sufficient effect cannot be obtained, and if it exceeds 0.060%, the effect is saturated, so 0.020 to 0.060%.
And P reduces the hot workability of steel, and
Since it is desirable to reduce as much as possible in order to reduce the toughness, the content is set to 0.015% or less. S, like P, lowers the hot workability of steel and it is desirable to reduce it as much as possible in order to lower the toughness, so it was made 0.010% or less. O greatly affects the toughness of high strength materials having a yield point of 550 to 650 N / mm ² . In order to improve the fracture toughness Kee, it is necessary to reduce the content as much as possible, and it is 0.00 in consideration of use in a cold region of about -20 ° C.
It was set to 10% or less.

In order to manufacture the steel pipe of the present invention, the steel containing each of the above chemical components is hot-rolled into a seamless steel pipe by an ordinary seamless steel pipe manufacturing method such as the Mannesmann mandrel mill method. Then normalizing-tempering-tempering-
Repeated heat treatment of tempering is applied. The repeated heat treatments of normalizing-tempering-tempering-tempering are performed to refine the crystal grains of the steel. Note that the normalizing is also aimed at the uniform solid solution of carbides, and the heating is performed at a relatively high temperature as described later. Further, tempering is performed to adjust the strength to a predetermined value. By performing the above heat treatment, a steel pipe having excellent fracture toughness at a low temperature in which the crystal grains of the steel are refined can be obtained.

[0012]

EXAMPLES Steels having the chemical composition shown in Table 1 were used, and hot rolling was carried out by an ordinary Mannesmann mandrel mill to obtain an outer diameter of 32.
A seamless steel pipe with a thickness of 3.9 mm and a thickness of 60 mm was produced and
The heat treatment shown in FIG. Then, the heat-treated seamless steel pipe was subjected to a tensile test (yield point and tensile strength) and a fracture toughness test (Kee measurement, according to ASTM E992). Table 2 shows the results. Fracture toughness test is AST
According to ME992, the Kee value was calculated | required by carrying out the tensile fracture | rupture in the test piece which carried out the fatigue crack in the notch tip in the atmosphere of -20 degreeC. In Table 2, N in the heat treatment column indicates normalization at 980 ° C, Q indicates quenching at 920 ° C, and T indicates tempering at 640 ° C.

[0013]

[Table 1]

[0014]

[Table 2]

As shown in Table 2, the steel pipe of the present invention has a high strength of which the yield point exceeds 550 N / mm ² , and
Even at −20 ° C., it shows an excellent fracture toughness Kee of 180 N / mm ² √m or more. On the other hand, the steel pipes of the comparative examples have high strength with a yield point exceeding 550 N / mm ² , but the fracture toughness Kee at -20 ° C is 165 N / m.
The fracture toughness is greatly reduced to m ² √m or less.

[0016]

As described above, the steel pipe of the present invention is
A steel pipe that exhibits high strength and excellent fracture toughness, and is suitable as a steel material for offshore platforms used for the development and production of oil and natural gas in deep water, or as a member for other mechanical structures.

Claims (1)

(57) [Claims] 1. C: 0.02-0.06%, Si: 0.
05-0.35%, Mn: 1.00-1.50%, C
u: 1.00 to 1.50%, Ni: 1.50 to 2.50
%, Cr: 0.40 to 0.80%, Mo: 0.40
0.60%, Al: 0.005-0.050%, Nb:
0.020 to 0.060%, P: 0.015% or less,
A steel containing S: 0.010% or less, O: 0.0010% or less, and the balance of Fe and unavoidable impurities was used to make a pipe, and then the heat treatment was repeated: normalizing-quenching-quenching-tempering. High toughness steel pipe characterized by the above.