JP3470632B2 - Steel pipe for line pipe excellent in buckling resistance and method of manufacturing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a gas pipe line.
In the steel pipe to be used for emission or the like, the steel pipe, and a method for producing the same, particularly for line pipe superior in buckling resistance屈特of earthquake.

[0002]

2. Description of the Related Art Carbon steel pipes such as UOE steel pipes, spiral steel pipes, electric resistance welded steel pipes, press bend steel pipes and low alloy steel pipes can be manufactured in large quantities and stably, and therefore, they have excellent economical efficiency and welding workability. Therefore, it is widely used as a pipeline for fluid transportation such as a gas pipeline or a water pipe or a pillar material for civil engineering and construction. However, many of the steel pipes used in these applications have a large outer diameter / pipe thickness ratio, and when a large earthquake occurs, large tensile and compressive forces are repeatedly applied in the longitudinal direction of the steel pipe, resulting in local buckling. In some cases, cracking due to buckling caused rupture.

Despite the need for countermeasures against such local buckling due to the axial force of compression, conventionally proposed seismic resistant steel pipes are Japanese Patent Laid-Open Nos. 3-173719 and 5-1993.
65535, JP-A-5-117746, JP-A-5-117747, and JP-A-5-156357.
JP-A-6-49540, JP-A-6-49
As disclosed in Japanese Patent Application Laid-Open No. 541, Japanese Patent Application Laid-Open No. 6-264143, Japanese Patent Application Laid-Open No. 6-264144, etc., the yield ratio, which is the ratio of the yield stress to the tensile strength, is reduced, and energy absorption by plastic deformation is utilized. It was a low yield ratio steel pipe. In addition, for line pipes for transporting fluids such as gas, ductile fracture and brittle fracture due to internal pressure where force acts in the circumferential direction have been studied, but for external force in the axial direction, except for bending stress during laying. In most cases, it is not taken into consideration.

[0004]

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and is unlikely to cause local buckling with respect to tensile and compressive stress acting in the axial direction which is expected in a large earthquake or the like. Provided are a steel pipe for a line pipe having excellent buckling resistance and a method for manufacturing the same.

[0005]

[Means for Solving the Problems] In order to achieve the above object, the inventors of the present invention conducted extensive studies on buckling resistance characteristics against the compressive force in the axial direction of the steel pipe, and examined the relationship between the tensile characteristics of the steel material and the microstructure. The following findings were obtained.

1. The buckling resistance of steel pipes is that the stress-strain curve in the tensile properties of steel is a continuous yield type (stress-strain curve without yielding shelf), and n value: work hardening index in the strain region of about several% where buckling occurs. If is high, it greatly improves.

2. The metal structure capable of obtaining such tensile properties is a mixed structure composed of a hard island martensite and a relatively soft structure of ferrite or bainite.
Shows an example of such an organization. Fig. 1 shows 0.08C-
0.3Si-1.5Mn-0.2Mo-0.05Nb-
Scanning electron microscope (SEM) of the metallographic structure of 0.06V steel
As a result of observation by, the white observed portion is island martensite. The island-like martensite is formed by transforming a portion where carbon is concentrated into martensite in the process of ferrite transformation or bainite transformation from austenite, and has an extremely high hardness structure.

The present invention has been made based on these findings. A steel pipe used for line pipes that transport fluids.
I, in weight%, C: 0.03~0.15%, Si :
0.01 to 1% Mn: contains 0.5% to 2%, the area fraction contains 3.4 to 15% of the island martensite, before
The average aspect ratio of island martensite is 2-15
Line pipe superior in buckling resistance屈特properties, characterized in that
For steel pipe .

2. % By weight, and further Cu: 0.05-
0.5%, Ni: 0.05 to 0.5%, Cr: 0.05
~ 0.5%, Mo: 0.05-0.5%, Nb: 0.0
05-0.1%, V: 0.005-0.1%, and T
i: One or more selected from the group of 0.005 to 0.1% is contained, and the steel pipe for a line pipe excellent in buckling resistance according to claim 1, characterized in that:

[0010]

3. The steel having the composition described in 1 or 2 is heated to 1000 to 1200 ° C., and the rolling reduction in the non-recrystallization temperature range
There and performing rolling 50% or more heat, the hot rolled steel sheet from the temperature range of Ar ₃ to Ar ₃ -80 ° C., cooled on the steel plate an average cooling rate on 5 ° C. / sec or more <br/> and to stop the cooling at a temperature range of 380 ° C. or less, then, comprises the step of forming a steel tube by cold forming, in an area fraction metal structure containing 3.4 to 15% of the island martensite, wherein Island-shaped martensite flat
A method for producing a steel pipe for a line pipe , which is excellent in buckling resistance, characterized in that an average aspect ratio is 2 to 15 . However,
_{Ar 3 (℃) = 910-310 *} C (%) - 80 * Mn
(%)-20 * Cu (%)-15 * Cr (%)-55 *
Ni (%)-80 * Mo (%)

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The component composition range, metal structure, and manufacturing conditions of the present invention will be described below.

1. Ingredient composition range C: 0.03 to 0.15% C is an element necessary for securing the strength of the steel material and promoting the formation of island martensite. 0.03
If it is less than 0.1%, the amount of island martensite is insufficient,
%, The weldability will deteriorate, so 0.0
3 to 0.15%.

Si: 0.01 to 1% Si enhances the strength and is added as a deoxidizing agent in the steelmaking process. If it is less than 0.01%, the effect is not sufficient, and if it is added in excess of 1%, the toughness of the welded portion deteriorates, so it is made 0.01 to 1%.

Mn: 0.5-2% Mn is added to enhance the strength. If it is less than 0.5%, the strength is insufficient, and if it is added in excess of 2%, the toughness of the base material and the welded portion and the weldability deteriorate, so the content is made 0.5 to 2%.

Cu: 0.05 to 0.5%, Ni: 0.0
5 to 0.5%, Cr: 0.05 to 0.5%, Mo: 0.
05-0.5% Cu, Ni, Cr and Mo are selective elements, and in order to enhance the strength, one or more kinds are added. If it is less than 0.05%, there is no effect, and if it exceeds 0.5%, the weldability deteriorates.

Nb: 0.005 to 0.1%, V: 0.0
05-0.1%, Ti: 0.005-0.1% Nb, V, and Ti are selective elements, and when enhancing toughness and strength, one or more kinds are added. 0.005%
If less than 0.1%, there is no effect, and if over 0.1%, the toughness of the welded portion deteriorates, so 0.005 to 0.1% is set.

In the present invention, the effect can be obtained if the above-specified elements are contained, and P, which is an impurity element, can be obtained.
Even if S, Al added as a deoxidizer, and other elements are contained, they are not damaged at all.

2. Area fraction of metal structure island martensite: 3.4 to 15% When the area fraction of island martensite is less than 3.4% ,
Since the influence of the soft phase is strong and the n value is low, the buckling resistance is deteriorated. If it exceeds 15%, the toughness deteriorates, and thus 3.
4 to 15%. If the remaining structure is lower in strength than island martensite, sufficient buckling resistance can be obtained.

In general, the remainder is a structure containing either or both of ferrite and bainite because island martensite is formed during the ferrite transformation from austenite or the bainite transformation.

Average aspect ratio of island martensite:
2-15 In the present invention, in order to further improve the buckling resistance and obtain sufficiently stable properties, the average aspect ratio of island martensite is specified. If the average aspect ratio is less than 2,
The strain to the soft phase near the island-like martensite / soft phase interface is not sufficient and the plastic deformation of the soft phase is delayed, resulting in a stress-strain curve with a yield shelf and sufficient buckling resistance cannot be obtained. There are cases.

On the other hand, even under the manufacturing conditions in which the average aspect ratio is 15 or more, plastic strain due to rolling remains in the soft phase, and n
Since the value decreases and sufficient buckling resistance may not be obtained, the value is set to 2 to 15. The aspect ratio is indicated by the major axis / minor axis of island martensite, and in the case of the average aspect ratio,
It is obtained by using image processing or the like from the aspect ratio of dozens to hundreds of island martensites.

By adjusting the composition range and the metal structure described above, local buckling is less likely to occur against a tensile / compressive stress acting in the axial direction even when the outer diameter / tube thickness ratio is large in the event of a large earthquake. It is possible to provide a steel pipe excellent in buckling resistance suitable for use in a gas pipeline, a water pipe, a steel pipe column, a bridge pier, and the like.

The steel pipe having such characteristics can be manufactured by the following steps.

3. Manufacturing method Slab heating temperature: 1000 to 1200 ° C. When the heating temperature is less than 1000 ° C., carbonitride forming elements such as Nb do not form a solid solution sufficiently and carbonitrides precipitated during rolling are reduced, and sufficient strength is obtained. Absent. If it exceeds 1200 ° C, the structure becomes coarse, the toughness deteriorates, and scale flaws easily occur, so the temperature is set to 1000 to 1200 ° C.

Rolling ratio in the non-recrystallization temperature region: 50% or more Rolling in the non-recrystallization temperature region is effective for ensuring high strength and toughness and for increasing the aspect ratio of island martensite after accelerated cooling. is there. However, if the rolling reduction in the non-recrystallization temperature range is less than 50%, the effect is insufficient, so the rolling reduction in the non-recrystallization region is set to 50% or more.

Cooling start temperature: Ar _{3 to} Ar ₃ -80 ° C. When the cooling start temperature is higher than Ar ₃ , the precipitation amount of ferrite before cooling is small and the production amount of island martensite is lowered. On the other hand, when the temperature is lower than Ar ₃ -80 ° C, pearlite starts to be generated and island martensite cannot be obtained. Therefore, the cooling start temperature is Ar ₃ to Ar ₃ -80 ° C.
And

Steel sheet average cooling rate: 5 ° C./sec or more The steel sheet average cooling rate is ΔT (° C.), the cooling time is Δt (sec), and the cooling time is ΔT (sec).
Calculated as / Δt (° C / sec). In the present invention, in order to obtain a necessary metallographic structure and sufficient strength, the cooling rate is 5 ° C / se.
c or more.

Cooling stop temperature: 380 ° C. or lower The cooling stop temperature ensures the amount of island martensite formed,
In order to obtain a high n value, the temperature is set to 380 ° C or lower .

In the present invention, the steel plate under the above manufacturing conditions is formed into a steel pipe by cold forming. The joining method is not particularly limited as long as the joint has sufficient deformability. The size of the steel pipe is not particularly limited as long as it can be manufactured. However, in the case of a thin-walled large-diameter steel pipe having a pipe diameter / tube thickness ratio of more than 60, buckling occurs in the elastic deformation region, and the intended effect cannot be obtained.

[0031]

EXAMPLES Steels having the components shown in Table 1 (A to J: steels of the present invention,
(K, L: comparative steel) after being formed into a steel sheet under the conditions shown in Table 2,
A steel tube having an outer diameter (711.0 mm) * tube thickness (16.0 mm) was manufactured by the UOE process.

The area fraction of island martensite was determined by image analysis of 10 microstructure photographs in the central portion of the tube thickness.
For the n value, a full-thickness test piece is sampled from the longitudinal direction of the steel pipe, and the strain range is 1 to 4% from the nominal stress-nominal strain curve by the tensile test.
The value at was calculated.

From the nominal strain ε and the nominal stress σ, the true strain e = ln
(1 + ε), true stress s = σ (1 + ε) is calculated, and the strain is 1 to
The true stress-true strain curve is represented by s = Ae ⁿ (A, n:
The constant n when approximated by (constant) is the n value.

In the buckling test, a steel plate having a length of 1830 mm is welded to both ends of a steel pipe, and then a compression test is carried out by a large-scale press tester to determine the strain (rolling down amount / total length) at which load reduction starts due to buckling. It was evaluated as bending strain.

Table 2 also shows the results of measuring the area fraction, average aspect ratio, n value, tensile strength and buckling strain of island martensite. No. 1, 3, 4, 6-10, 12
The metal structures of the examples are such that the area ratio of island martensite and the average aspect ratio satisfy the requirements of the present invention, and both have buckling strain of 1.0% or more and excellent buckling resistance. Is shown. No. Comparative Examples 14 to 22 do not satisfy the requirements of the present invention regarding island martensite,
Both have poor buckling resistance.

[0036]

[Table 1]

[0037]

[Table 2]

[0038]

According to the present invention, was excellent in buckling resistance屈特against large compressive load applied to, such as when a large earthquake Rainpa
It is possible to manufacture a type for steel pipe, the steel pipe is Gasupai
Suitable for use in line pipes such as planes .

[Brief description of drawings]

FIG. 1 is a scanning electron microscope (SEM) photograph showing the formation of island martensite according to an embodiment of the present invention.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Masamitsu Doi Inventor Masamitsu Marunouchi 1-2-2 Marunouchi, Tokyo Inside Nippon Kokan Co., Ltd. (56) Reference JP-A-8-239732 (JP, A) JP-A-7 -90478 (JP, A) JP-A-9-165621 (JP, A) JP-A-8-225853 (JP, A) JP-A-6-299238 (JP, A) (58) Fields investigated (Int.Cl) . ⁷ , DB name) C22C 38/00-38/60 C21D 8/10

Claims (3)

(57) [Claims] 1. Used in a line pipe for transporting a fluid
Steel pipe having a weight percentage of C: 0.03 to 0.15
%, Si: 0.01 to 1%, Mn: 0.5 to 2%, and an area fraction of 3.4 to 15% of island martensite, and the average aspect of the island martensite. Ratio is
La excellent in buckling resistance屈特properties, characterized in that 2 to 15
Steel pipe for in-pipe . 2. In weight%, further Cu: 0.05-0.
5%, Ni: 0.05 to 0.5%, Cr: 0.05 to
0.5%, Mo: 0.05 to 0.5%, Nb: 0.00
5 to 0.1%, V: 0.005 to 0.1%, and Ti:
1 or 2 selected from the group of 0.005 to 0.1%
The steel pipe for a line pipe excellent in buckling resistance according to claim 1, characterized by containing at least one kind. 3. The composition according to claim 1 or 2.
Steel is heated to 1000-1200 ℃ and rolled in the non-recrystallization temperature range.
The rate of 50% or more for hot rolling, and hot rolling
Steel sheet from the temperature range of Ar ₃ to Ar ₃ -80 ° C. was, 5 ° C. / sec
The steel plate is cooled at the above average cooling rate, and the temperature is 380 ° C or lower.
Cooling is stopped in the zone and then cold-formed into steel pipe
An island with a metal structure having an area fraction of 3.4 to 15%
Containing martensite, the island-shaped martensite
Resistance to an average aspect ratio of 2 to 15
A method for manufacturing a steel pipe for a line pipe having excellent buckling characteristics. However
, Ar ₃ (° C.) = 910−310 * C (%) − 80 *
Mn (%)-20 * Cu (%)-15 * Cr (%)-5
5 * Ni (%)-80 * Mo (%)