JPH11279700A - Steel pipe excellent in buckling resistance and method of manufacturing the same - Google Patents

Abstract

(57) [Summary] [Problem] It is difficult to cause local buckling even when the outer diameter / pipe pressure ratio is large against tensile / compressive stress acting in the axial direction at the time of a large earthquake, and gas pipelines, water pipes, steel pipes A steel pipe excellent in buckling resistance and suitable for use in columns, bridge piers, and the like, and a method for manufacturing the same. SOLUTION: In weight%, C: 0.03 to 0.15%;
Si: 0.01 to 1%, Mn: 0.5 to 2%, the balance being Fe and unavoidable impurities, and the metal structure containing 10 to 50% lower bainite by area fraction. A steel pipe with excellent buckling resistance.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel pipe used for gas pipelines, water pipes, steel pipe columns, bridge piers, etc., and more particularly to a steel pipe excellent in buckling resistance during an earthquake and a method of manufacturing the same.

[0002]

2. Description of the Related Art Carbon steel pipes such as UOE steel pipes, spiral steel pipes, electric resistance welded steel pipes, and press-bend steel pipes or low alloy steel pipes can be manufactured in large quantities and in a stable manner. It is widely used as a pipe for transporting fluids such as gas pipelines and water pipes or as a pillar for civil engineering.

However, when a large earthquake occurs, large tensile and compressive forces are repeatedly applied in the longitudinal direction of these steel pipes, causing local buckling in steel pipes having a somewhat large outer diameter / pipe thickness ratio. Cracking due to buckling can lead to fracture. Conventional steel pipes for construction include, for example, JP-A-3-173719, JP-A-5-65535, JP-A-5-117746, JP-A-5-117747, and JP-A-5-15.
No. 6,357, JP-A-6-49540, JP-A-6-49541, JP-A-6-264143, and JP-A-6-264144. A device in which the yield ratio, which is the ratio between stress and tensile strength, is reduced has been proposed.

[0004]

However, JP-A-3-173719, JP-A-5-65535, JP-A-5-117746, and JP-A-5-117.
No. 747, JP-A-5-156357, JP-A-6-49540, JP-A-6-49541, JP-A-6-264143, JP-A-6-264
The techniques disclosed in each of the 144 publications utilize energy absorption due to plastic deformation, and no studies have been made to prevent local buckling due to compression axial force. In addition, in line pipes for transporting fluids such as gas, studies have been made on ductile fracture and brittle fracture due to internal pressure in which a force acts in the circumferential direction.However, with respect to external force in the axial direction, except for bending deformation during laying, Little has been considered.

SUMMARY OF THE INVENTION In view of the foregoing, an object of the present invention is to prevent local buckling from occurring even when the outer diameter / pipe pressure ratio is large against tensile / compressive stress acting in the axial direction during a large earthquake.
An object of the present invention is to provide a steel pipe excellent in buckling resistance and a method for manufacturing the same, which is suitable for use in gas pipelines, water pipes, steel pipe columns, bridge piers, and the like.

[0006]

In order to solve the above problems and achieve the object, the present invention uses the following means.

(1) The steel pipe of the present invention contains C:
0.03 to 0.15%, Si: 0.01 to 1%, and M
n: 0.5 to 2%, the balance being Fe and unavoidable impurities, and the metal structure contains lower bainite in an area fraction of 10 to 50%. Excellent steel pipe.

(2) The steel pipe of the present invention further comprises, as a steel component, 0.05% to 0.5% of Cu and 0.05% of Ni by weight%.
05 to 0.5%, Cr: 0.05 to 0.5%, Mo:
0.05-0.5%, Nb: 0.005-0.1%,
V: 0.005 to 0.1%, and Ti: 0.005 to
The steel pipe having excellent buckling resistance according to the above (1), characterized by containing one or more kinds selected from the group of 0.1%.

(3) The production method according to the present invention provides a steel having the composition described in the above (1) or (2),
A step of heating to 0 ° C. to perform hot rolling, and a step of heating the hot-rolled steel sheet from a temperature range of Ar ₃ to (Ar ₃ −80) ° C. to 500 ° C.
Cooling at a steel sheet average cooling rate of 15 ° C./sec or more to a temperature range of not more than 15 ° C., and then forming the steel pipe by cold forming, wherein the metal structure contains 10 to 50% lower bainite by area fraction. And a method of manufacturing a steel pipe having excellent buckling resistance. However, Ar ₃ (° C.) = 910-3
10xC% -80xMn% -20xCu% -15xCr
% -55xNi% -80xMo%

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have conducted intensive studies on the buckling resistance to the compressive force in the axial direction of a steel pipe in order to solve the above-mentioned problems, and have obtained the following findings.

The buckling characteristics of a steel pipe largely depend on the microstructure of the steel material. By forming the metal structure into a composite structure containing a certain amount of lower bainite, the buckling resistance is greatly improved. Lower bainite undergoes transformation at relatively low temperatures,
Although the structure is extremely high in strength, when such a mixed structure of hard phase and soft ferrite is formed, the stress-strain curve of the steel material becomes a continuous yield type (stress-strain curve without a yield shelf), The n value (work hardening index) in the strain region where bending occurs is increased, and the buckling resistance is greatly improved.

FIG. 1 shows that 0.08C-0.3Si-1.5M
The SEM (scanning electron microscope) photograph of the steel material manufactured by accelerating cooling n-0.2Mo-0.05Nb-0.06V steel at the cooling rate of 30 degreeC / second after hot rolling is shown. The metal structure is a composite structure mainly composed of lower bainite and ferrite, and contains a small amount of martensite. In order to obtain such a structure, as a result of examining the components and the manufacturing method of the steel material, sufficient strength is obtained, and the components are limited to a range that does not impair the weldability, and after hot rolling, specific By performing accelerated cooling in the above temperature range at a cooling rate equal to or higher than a certain rate, a steel material having the above-described metal structure can be obtained.

Based on the above findings, the present inventors have proposed C,
Controlling accelerated cooling conditions after hot rolling of steel with specified amounts of Si and Mn within a certain range so that the metal structure has a composite structure containing a certain amount of lower bainite, and has excellent buckling resistance. A steel pipe and a method for manufacturing the same have been found, and the present invention has been completed.

That is, the present invention provides a steel composition, a metal structure,
By limiting the manufacturing conditions to the following range, local buckling is unlikely to occur even when the outer diameter / pipe pressure ratio is large against tensile and compressive stresses acting in the axial direction during a large earthquake, and gas pipelines and water A steel pipe excellent in buckling resistance and suitable for use in pipes, steel pipe columns, bridge piers, and the like can be provided.

The reasons for adding the components, the reasons for limiting the components, the reasons for limiting the metal structure, and the reasons for limiting the manufacturing conditions of the present invention will be described below. (1) Component composition range and metal structure C: 0.03 to 0.15% C is an element necessary to secure the strength of the steel material and to promote the formation of lower bainite.
If it is less than 10%, bainite transformation is unlikely to occur, and if added in excess of 0.15%, not only will weldability be impaired, but also yield shelves will be formed on the stress-strain curve and buckling resistance will be reduced. ~ 0.15%. Mn: 0.5 to 2% Mn is added to increase the strength of the steel material.
%, The strength is insufficient, and if it exceeds 2%, the toughness and weldability of the base metal and the weld are deteriorated.
Its content is 0.5-2%.

Si: 0.01 to 1% Si is necessary not only to increase the strength of the steel material but also as a deoxidizing agent in the steel making process, but if it is less than 0.01%, its effect is insufficient, and more than 1%. If added, the toughness of the weld is degraded, so the content is 0.01-1%. In the present invention, in order to increase the strength and toughness,
One or more selected from the following selected component groups may be contained.

(Selective component group) Cu: 0.05-0.5%, Ni: 0.05-0.5
%, Cr: 0.05-0.5%, Mo: 0.05-0.
5% Cu, Ni, Cr and Mo are effective in increasing the strength,
If each is less than 0.05%, the effect is not exhibited,
If it exceeds 0.5%, the weldability deteriorates, so the content is 0.05 to 0.5%. Nb: 0.005 to 0.1%, V: 0.005 to 0.1
%, Ti: 0.005 to 0.1% Nb, V, and Ti are effective elements for improving toughness and strength, but if the content is less than 0.005%, the effect can be effectively exhibited. If the content exceeds 0.1%, the toughness of the weld is deteriorated.
0.1%. In addition, P, S contained as impurity elements, and Al or the like added as a deoxidizing agent may be contained, and these elements do not impair the buckling resistance of the steel of the present invention. Absent.

Metal structure: contains lower bainite in an area fraction of 10 to 50%.

If the area fraction of the lower bainite is less than 10%, the influence of the soft phase is so strong that a high n value cannot be obtained and sufficient buckling resistance cannot be obtained. On the other hand, if it exceeds 50%, the effect of forming a composite structure is reduced, and a high n value cannot be obtained. Therefore, the area fraction of the lower bainite is 10 to 50%. The remaining structure must be mainly composed of a soft ferrite structure, but sufficient characteristics can be obtained even if a small amount of martensite or upper bainite is contained.

By adjusting the component composition range and the metallographic structure described above, local buckling hardly occurs even when the outer diameter / pipe pressure ratio is large with respect to the tensile / compressive stress acting in the axial direction during a large earthquake. It is possible to obtain a steel pipe excellent in buckling resistance, which is suitable for use in gas pipelines, water pipes, steel pipe columns, bridge piers and the like.

A steel pipe having such characteristics can be manufactured by the following manufacturing method.

(2) Steel Pipe Manufacturing Process (Manufacturing Method) Steel adjusted to the above component composition range is melted in a converter, and a steel slab obtained by continuous casting is subjected to 1000-1000.
Heat to 1200 ° C. and perform hot rolling. Next, the hot-rolled steel sheet is cooled from a temperature range of Ar ₃ to (Ar ₃ −80) ° C. to a temperature range of 500 ° C. or less at an average cooling rate of the steel sheet of 15 ° C./sec or more, and then formed by cold forming. And

However, Ar ₃ (° C.) = 910-310 × C
% -80 * Mn% -20 * Cu% -15 * Cr% -55
× Ni% −80 × Mo% a. Heating temperature: 1000-1200 ° C. If the heating temperature is lower than 1000 ° C., the amount of solid solution of carbide forming elements such as Nb is small, and the amount of carbonitride precipitated during rolling is reduced, so that sufficient strength cannot be obtained. On the other hand, if the heating temperature exceeds 1200 ° C., not only does the structure become coarse and the toughness deteriorates, but it also causes scale flaws, so the upper limit is 1200 ° C. b. Cooling start temperature: temperature range of Ar ₃ to (Ar ₃ -80) ° C., where Ar ₃ (° C.) = 910-310 × C% -80
× Mn% -20 × Cu% -15 × Cr% -55 × Ni%
-80 × Mo% By setting the cooling start temperature lower than the Ar ₃ transformation temperature, precipitation of ferrite occurs before cooling, and a composite structure of lower bainite and ferrite can be obtained. But,
When the cooling start temperature exceeds Ar ₃ , the volume fraction of the lower bainite becomes too large, and the buckling resistance decreases. If the cooling start temperature is lower than (Ar ₃ −80) ° C., pearlite is generated, so that a yield shelf is generated on the stress-strain curve, and the buckling resistance decreases. Therefore, the cooling start temperature is Ar ₃ to (A
r ₃ −80) ° C.

C. Cooling stop temperature: 500 ° C. or less If the cooling stop temperature is too high, the strength of the lower bainite decreases, a high n value is not obtained, and the buckling resistance decreases, so the cooling stop temperature needs to be 500 ° C. or less. is there.

D. Steel sheet average cooling rate: 15 ° C./sec or more The cooling rate is the steel sheet average cooling rate from the start of cooling to 500 ° C. When the value is less than 15 ° C./sec, not only the lower bainite structure is not obtained, but also Since sufficient strength cannot be obtained, the lower limit of the cooling rate is 15 ° C./sec. The average cooling rate of the steel sheet is defined as ΔT (° C.) where the decrease in the surface temperature of the steel sheet before and after cooling is defined as ΔT (seconds), and
It is determined by T / Δt (° C./second).

In the present invention, a steel pipe is formed from the above-mentioned steel sheet by cold forming. However, as long as the joint has sufficient deformation performance, any method may be used as the joining method. The size of the steel pipe may be any size that can be manufactured. However, in the case of a thin-walled large-diameter steel pipe having a pipe diameter pipe thickness ratio exceeding 60, buckling occurs in a region of elastic deformation, and the desired effect is not obtained. I can't get it. Hereinafter, examples of the present invention will be described to demonstrate the effects of the present invention.

[0027]

EXAMPLES Steels having the components shown in Table 1 (A to K: steels of the present invention,
L, M: comparative steels) under the conditions (Nos. 1 to 1) shown in Table 2.
2: Example of the present invention, No. 13 to 20: Comparative Example) to obtain various steel sheets. Then, a steel pipe having an outer diameter x a pipe thickness = 711.0 x 16.0 (mm) was manufactured by the UOE process. Micro-observation was performed at several places in the center of the steel pipe in the thickness direction, and the area fraction of the lower bainite was determined by image analysis from ten micrographs of the structure. Further, a tensile test piece having a total thickness was sampled from the longitudinal direction of the steel pipe, a nominal stress-nominal strain curve was measured by a tensile test, and an n value in a strain range of 1 to 4% was obtained. The n value was determined by the following method. The nominal strain epsilon, than the nominal stress sigma, true strain e = ln (1 + ε) , determined the true stress s = σ (1 + ε) , true stress range strain of 1-4% - true strain curve s = Ae ⁿ ( A, n: a constant n when approximated by a constant is defined as an n value.

In the buckling test, after a steel plate is welded to both ends of a steel pipe having a length of 1830 mm, a compression test is performed using a large-sized press test device, and a strain (load reduction / total length) at which load reduction starts due to buckling occurs. ) Was evaluated as buckling strain. Table 2 shows the measurement results of the area fraction, the n-value, the tensile strength, and the buckling strain of the lower bainite.

Inventive Example No. 1-12 all have 1 buckling strain
% Or more, and has excellent buckling resistance. On the other hand, in Comparative Example No. Nos. 13 to 20 have components or production conditions outside the range of the present invention, and have a small or large area fraction of the lower bainite, so that the buckling strain is small and the buckling resistance is poor.

[0030]

[Table 1]

[0031]

[Table 2]

[0032]

As described above, according to the present invention,
By specifying the steel composition, metal structure, and manufacturing conditions, it is possible to provide a steel pipe with excellent buckling resistance against a large compressive load received during a large earthquake, and to provide line pipes and water pipes. It can be said that it is suitable for use in piping, steel pipe columns, bridge piers, etc.

[Brief description of the drawings]

FIG. 1 is a scanning electron microscope (SEM) photograph showing a lower bainite + ferrite structure according to an embodiment of the present invention.

Claims (3)

[Claims] C: 0.03 to 0.15% by weight
, Si: 0.01 to 1%, and Mn: 0.5 to 2%, the balance being Fe and unavoidable impurities, and the metal structure containing 10 to 50% by area fraction of lower bainite. A steel pipe with excellent buckling resistance. 2. The steel composition further comprises Cu:
0.05-0.5%, Ni: 0.05-0.5%, C
r: 0.05 to 0.5%, Mo: 0.05 to 0.5%,
Nb: 0.005 to 0.1%, V: 0.005 to 0.1
The steel pipe excellent in buckling resistance according to claim 1, wherein the steel pipe contains one or more kinds selected from the group consisting of 0.005 to 0.1% and Ti: 0.005 to 0.1%. 3. A step of heating a steel having the composition according to claim 1 or 2 to 1000 to 1200 ° C. to perform hot rolling, and applying a hot-rolled steel sheet to Ar ₃ to (Ar ₃ -80). Cooling at a steel sheet average cooling rate of 15 ° C./second or more from a temperature range of 500 ° C. to a temperature range of 500 ° C. or less, and then forming the steel pipe by cold forming. % Of lower bainite, the method for producing a steel pipe having excellent buckling resistance. However, Ar ₃ (° C.) = 910-31
0xC% -80xMn% -20xCu% -15xCr%
-55 x Ni% -80 x Mo%