JP3491148B2 - High strength and high toughness seamless steel pipe for line 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 strength and high toughness seamless steel pipe for line pipes, and more particularly to a high strength and high toughness seamless steel pipe for API-5L X80 line pipes.

[0002]

2. Description of the Related Art Various steel pipes have been developed and put into practical use for pipelines and risers for transporting crude oil and natural gas according to required characteristics. Among them, X80 class strength (yield strength YS: 551 MPa or more, tensile strength TS: 6
A seamless steel pipe having 20 to 827 MPa) has also been developed, but at present, the toughness level is -60 ° C at the fracture surface transition temperature including the weld.

However, the severer environment of oil wells in recent years,
From the viewpoint of saving development costs, materials are required to have higher strength and toughness. A seamless steel pipe having a strength of X80 class is usually adjusted in strength by the following heat treatment method. After manufacturing seamless steel pipe, it is cooled once, reheated and quenched, and then tempered so-called reheating quenching-tempering (RQ-
T) Quenching immediately after manufacturing seamless steel pipe, then tempering, so-called direct quenching-tempering (DQ-T)

[0004]

However, even steels having the same composition have different cooling rates depending on their sizes, and therefore have different strengths after quenching. Therefore, it is necessary to change the tempering conditions. Therefore, it has been a subject to secure appropriate temper softening resistance. Therefore, an object of the present invention is to provide a seamless steel pipe for a line pipe which has high strength of X80 grade and low temperature toughness of a fracture surface transition temperature of −60 ° C. or less, and has appropriate temper softening resistance. To do.

[0005]

[Means for Solving the Problems] In order to achieve the above-mentioned object, the present inventors have earnestly studied and conducted experiments, and have obtained the following important findings. To secure high strength, it is effective to increase the C content, but on the other hand, the low temperature toughness is significantly deteriorated. Further, in order to secure weldability, the lower the amount of C, the more preferable.

Addition of alloying elements such as Mn, Ni and Mo is effective for securing high strength while suppressing the amount of C to a low level. It is known that the addition of a small amount of B is effective for improving the hardenability of a low C steel, but B has a negative effect of adversely affecting the toughness of the welded portion, and its effect is Since it largely depends on the content of precipitate forming elements such as N and Ti, it is difficult to secure stable toughness by adding a trace amount of B.

In order to secure the low temperature toughness and also the hardenability for obtaining the strength of X80 grade, it is necessary to control the amounts of Mo and V within the proper ranges shown in FIG. Furthermore, in order to facilitate strength adjustment by changing tempering conditions,
It is necessary to optimize the temper softening resistance, and for that purpose, it is necessary to control the Nb amount and the V amount within the appropriate range shown in FIG.

The present invention has been completed based on these findings, and the gist of the invention is mass%
C: 0.03 to 0.06%, Si: 0.05 to 0.15%, Mn: 1.6 to 2.0
%, Al: 0.010 to 0.10%, Ni: 0.3 to 0.7%, Mo: 0.10.
~ 0.40%, V: 0.06% or less, Nb: 0.03% or less, Ti: 0.00
3 to 0.020%, N: 0.0010 to 0.0100%, and
Mo + 5V ≧ 0.4%, 2Nb−V ≦ 0%, which is a high-strength, high-toughness seamless steel pipe for a line pipe, the balance of which is Fe and inevitable impurities.

This seamless steel pipe has a strength of X80 grade, a low temperature toughness at a fracture surface transition temperature of −60 ° C. or less, and a difference in yield strength between tempering at 600 ° C. × 30 minutes and tempering at 650 ° C. × 30 minutes. Is 30 ~
Characteristically, it has temper softening resistance of 70 MPa.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the composition (chemical composition) of steel in the present invention as described above will be described below. In addition,% showing a chemical component content (concentration) means mass%. C: 0.03 to 0.06% C is an important element related to the strength of steel, and 0.03% or more is required to improve the hardenability and secure the strength of X80 class, while it exceeds 0.06%. And 0.03 to 0.06% in order to increase the sensitivity to welding cracks.

Si: 0.05 to 0.15% Si is necessary as a deoxidizing agent in steelmaking and for enhancing the strength. If it is less than 0.05%, its effect is poor. On the other hand, if it exceeds 0.15%, the base metal and HAZ are poor. Since it causes deterioration of toughness and deterioration of weldability, it is set to 0.05 to 0.15%. Mn: 1.6 to 2.0% Mn is necessary for enhancing the hardenability and strength, and also has the function of improving the toughness of the base metal and HAZ, but if it is less than 1.6%, it is difficult to obtain these effects. ,on the other hand,
The effect is saturated even if added over 2.0%, so 1.6 ~
2.0%

Al: 0.010 to 0.10% Al acts as a deoxidizing agent in steelmaking, and N
Has the effect of forming AlN by combining with Al to form finer crystal grains and improving toughness. 0.010% for this effect
The above additions are required, but if 0.070% is exceeded, Al ₂ O ₃
There is a concern that the number of system inclusions will increase and the toughness will deteriorate, and that many surface defects will occur. Therefore, Al is 0.010 to 0.10%
And From the perspective of ensuring stable surface quality,
0.010 to 0.050% is preferable.

Ni: 0.3 to 0.7% Ni has the function of improving the toughness of the base material and HAZ. This effect becomes apparent with the addition of 0.3% or more. But,
Even if added in excess of 0.7%, the effect of improving toughness and corrosion resistance saturates, which is disadvantageous because it results in a high cost. Therefore, the Ni content is 0.3 to 0.7%.

Mo: 0.10 to 0.40% Mo is indispensable for improving hardenability and solid solution strengthening, and 0.10% or more is required to obtain the effect,
The addition of more than 0.40% causes deterioration of the toughness of the welded portion, so the content is made 0.10 to 0.40%. V: 0.06% or less V is indispensably added in order to contribute to the optimization of temper softening resistance by precipitating in the matrix as carbonitride, but 0.06%
Exceeding 0.06% deteriorates the toughness of the weld, so 0.06%
Limited to:

Nb: 0.03% or less Nb is essentially added as carbonitride to precipitate in the matrix and contribute to the optimization of temper softening resistance, but 0.03%
If added in excess of 0, the temper softening resistance becomes excessive, so 0.
Limited to less than 03%. Ti: 0.003 to 0.020% Ti forms carbides and refines crystal grains to improve toughness, and precipitates in the matrix to increase strength and contribute to high strength. The effect is exhibited by addition of 0.003% or more, while if added over 0.020%, it becomes difficult to secure hardenability and the toughness deteriorates. For this reason,
Ti is 0.003 to 0.020%. Incidentally, more preferably 0.
It is 010 to 0.018%.

N: 0.0010 to 0.0100% N is 0.00 due to the formation of AlN and carbonitrides of V and Nb.
Although the content of 10% or more is required, the content of more than 0.0100% deteriorates the toughness of the HAZ, so the content is made 0.0010 to 0.0100%. In addition, it is more preferably 0.0030 to 0.0080%. Mo + 5V ≧ 0.4% Even if each component element is within the above limits, if the sum of Mo content and V content is less than 0.4%, hardenability is insufficient and X80 grade strength is secured. Hard to do. Therefore, the amount of Mo and the amount of V must satisfy the relationship of Mo + 5V ≧ 0.4%. (Refer to FIG. 1) 2Nb-V ≦ 0% Even if each component element is within the above-mentioned limited range, if the difference between the double Nb amount and the V amount exceeds 0%, the temper softening resistance becomes excessive, It becomes difficult to adjust the strength simply by changing the tempering conditions regardless of the size. for that reason,
The amount of Nb and the amount of V must satisfy the relationship of 2Nb−V ≦ 0%. (See FIG. 2) P, S, and O are contained as other unavoidable impurities.
From the viewpoint of securing the toughness of the base material, it is desirable to reduce it as much as possible. In addition, P, S, and O are 0.03%, 0.01%, and 0.01, respectively.
Up to% is acceptable.

Next, a preferable manufacturing process of the steel pipe of the present invention will be described. Steel having the above composition is melted in a converter or an electric furnace and solidified by a continuous casting method or an ingot making method to obtain a slab. In the process, ladle refining of molten steel, vacuum degassing, etc. are carried out as necessary. The obtained slab is as it is,
Alternatively, it is further hot rolled to obtain a steel pipe material.

[0018] The steel pipe material is heated to Ac ₃ point or more and hot-rolled by a plug mill method, a mandrel mill method or the like to obtain a seamless steel tube, or a sizer and a stretch reducer are used to produce a desired size while hot. To do. After pipe forming, quenching to obtain the desired strength-toughness balance-
A heat treatment consisting of tempering (Q-T) is performed. Quenching (Q)
Is direct quenching (DQ) that immediately cools from the hot state after pipe making to a temperature below the Ms point (about 200 ° C or less), after cooling to near room temperature after pipe making, and then reheating to the γ region, then below the Ms point. Any of reheat quenching (RQ) for cooling to In order to obtain X80 class strength after Q-T, after quenching from the temperature in the γ region, preferably at a cooling rate of 20 ° C / s or more,
It may be tempered at a temperature appropriately set within the range of less than Ac ₁ point (preferably 550 ° C. or higher). The holding time at the tempering temperature may be appropriately determined and is usually set to about 10 to 120 min.

[0019]

EXAMPLE Steels having the compositions shown in Table 1 were melted in a converter, subjected to vacuum degassing treatment, and solidified by a continuous casting method. These steel pipe materials were made into seamless steel pipes having an outer diameter of 219 mm and a wall thickness of 11.1 mm by a Mannesmann-plug mill type pipe manufacturing facility, and the steel pipes were heat-treated under the conditions shown in Table 2. This heat treatment was performed as follows. After quenching and pipe forming, water quenching directly from the austenite temperature range (D
Q), or ・ Cooling to room temperature and then reheating (950 ℃ × 15 minutes)
Water quenching (RQ) tempering from 900 ℃ · 600 ℃ × 30 minutes, and 650 ℃ × 30 minutes, or · 620 ℃ × 30 minutes Hardness after quenching, tensile properties after tempering (strength: YS, T
S, elongation: El), and 50% fracture surface transition temperature (50% FATT) in the Charpy test were investigated. Also, using a commercially available X80-class welding material, a steel pipe joint was prepared by TIG welding (voltage 15V, current 200A, welding speed 10kJ / min, heat input 18kJ / cm).
The 50% FATT of AZ (1 mm from bond) was investigated. The results are shown in Table 2.

[0020]

[Table 1]

[0021]

[Table 2]

From Table 2, those within the scope of the present invention (Examples of the present invention) exhibited X80 grade strength, excellent low temperature toughness, and appropriate temper softening resistance, while those outside the scope of the present invention. In the sample (Comparative Example), at least one of strength, low temperature toughness, and temper softening resistance was insufficient or excessive.

[0023]

EFFECTS OF THE INVENTION The seamless steel pipe of the present invention has strength of X80 grade, excellent low temperature toughness, and appropriate temper softening resistance, so that the target strength can be easily achieved regardless of size. It is possible to integrate the size components, and it is possible to reduce the cost of steel pipes for line pipes.

[Brief description of drawings]

FIG. 1 is a region diagram showing an appropriate range of Mo amount and V amount.

FIG. 2 is a region diagram showing an appropriate range of Nb amount and V amount.

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-9-41074 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (2)

(57) [Claims] 1. In mass%, C: 0.03 to 0.06%, Si: 0.05
~ 0.15%, Mn: 1.6 to 2.0%, Al: 0.010 to 0.10%, N
i: 0.3 to 0.7%, Mo: 0.10 to 0.40%, V: 0.06% or less, Nb: 0.03% or less, Ti: 0.003 to 0.020%, N: 0.00
Contains 10-0.0100% and Mo + 5V ≧ 0.4%, 2Nb
A high-strength, high-toughness seamless steel pipe for a line pipe, which satisfies the relationship of −V ≦ 0% and consists of the balance Fe and unavoidable impurities. 2. Strength of X80 grade, low temperature toughness of fracture transition temperature -60 ° C. or lower, and tempering at 600 ° C. × 30 minutes and 650 ° C. ×
2. The seamless steel pipe according to claim 1, which has tempering softening resistance such that a difference in yield strength after tempering for 30 minutes is 30 to 70 MPa.