JP3937998B2 - Manufacturing method of steel pipe with excellent buckling resistance - Google Patents

Description

【００６５】
【表２】

【００６６】
【発明の効果】
本発明によれば、耐座屈性能に優れ、大地震の際の安全性に優れたＡＰＩＸ５２グレード以下の鋼管が得られ、産業上極めて有用である。
【図面の簡単な説明】
【図１】応力ー歪線図を示し、（ａ）は鋼板、（ｂ）は（ａ）に示す鋼板を用いた鋼管について求めたものを示す。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method of manufacturing a steel pipe used for gas pipelines, water pipes, sewage pipes, and the like, and in particular, steel pipes having API standard X52 grade or less and exhibiting excellent buckling resistance against compressive loads caused by earthquakes and the like. The present invention relates to a manufacturing method of things.
[0002]
[Prior art]
Steel pipes made of carbon steel or low alloy steel have excellent economic efficiency and weldability, and can be manufactured in large quantities and stably. Therefore, UOE steel pipes are used for fluid transport steel pipes such as gas pipelines and water pipes. Widely used as spiral steel pipes, ERW steel pipes and press bend steel pipes.
[0003]
These steel pipes must be seismic-resistant to ensure safety in the event of a large earthquake, lower the yield ratio (ratio of yield stress and tensile strength), and reduce the earthquake energy by plastic deformation. Absorbing low yield ratio steel pipes have been proposed (see, for example, Patent Documents 1 to 3).
[0004]
However, in such a low yield ratio steel pipe, in the case of a steel pipe having a large outer diameter / thickness ratio, the fracture due to cracking due to local buckling caused by repeated tensile and compressive stress in the axial direction of the steel pipe due to a large earthquake. It was not always possible to prevent it sufficiently.
[0005]
In addition, gas pipes for transporting fluids such as gas have been studied for ductile fracture and brittle fracture due to internal pressure acting in the circumferential direction, but studies on axial external force are based on bending stress to prevent damage during laying. At present, the degree of damage is not sufficiently studied as described below.
[0006]
(1) The stress-strain curve obtained by a tensile test in the axial direction of the steel pipe is made flat with no yield shelf, and the buckling resistance of the steel pipe is improved (for example, see Patent Document 4).
[0007]
{Circle around (2)} After hot rolling of the steel sheet, by accelerated cooling from a specific temperature range, the stress-strain curve of the steel pipe is controlled to improve the buckling resistance (see, for example, Patent Documents 5 to 7).
[0008]
All steel pipes produced by these methods have a stress-strain curve without yield shelves and exhibit excellent buckling resistance against axial compression, but after rolling to make the stress-strain curve a desired shape. The steel pipe strength is accelerated to API standard X65 or higher, and a steel pipe for low pressure line pipe of API standard X52 or lower cannot be manufactured.
[0009]
[Patent Document 1]
JP-A-3-173719
[Patent Document 2]
Japanese Patent Laid-Open No. 5-117746
[Patent Document 3]
JP-A-6-264144
[Patent Document 4]
Japanese Patent Laid-Open No. 9-196243 [0013]
[Patent Document 5]
JP-A-9-176734 [0014]
[Patent Document 6]
Japanese Patent Laid-Open No. 9-202922
[Patent Document 7]
JP-A-11-279700 [0016]
[Problems to be solved by the invention]
However, even in low-pressure line pipes using steel pipes of API standard X52 or lower, improvement in buckling resistance is required to improve safety during large-scale earthquakes, while ensuring the safety of structures. Considering the situation where the upper and lower limit strengths are increasing in the material strength standard, the low strength steel pipe of API standard X52 or less is resistant to accelerated cooling, which does not cause accelerated cooling that causes the strength to exceed the upper limit of strength. It is desired to improve the buckling characteristics.
[0017]
An object of this invention is to provide the manufacturing method of the low strength steel pipe below API standard X52 excellent in the buckling resistance property which is hard to raise | generate local buckling by axial loads, such as a big earthquake.
[0018]
[Means for Solving the Problems]
In order to achieve the above-mentioned goal, the present inventors diligently examined the influence of the stress-strain curve of the steel sheet and the steel composition on the buckling resistance of the steel pipe, and obtained the following knowledge.
[0019]
(1) After hot rolling, a steel sheet that is air-cooled has solid solution carbon fixed on the dislocations during the air-cooling process, a clear yield shelf appears in the stress-strain curve, and the yield elongation becomes longer.
[0020]
(2) When a steel sheet showing such a stress-strain curve is cold-formed to form a steel pipe, the yield-shelf is observed in the stress-strain curve obtained in the tube axis direction of the steel pipe. Inferior. Moreover, even if the yield shelf disappears due to cold forming, the work hardening ability is inferior, and local buckling tends to occur with respect to the compressive load in the tube axis direction.
[0021]
(3) If the rolling temperature is too low, processing strain at the time of rolling remains and the work hardening ability of the steel sheet decreases, so the work hardening ability of the steel pipe is low and the buckling resistance performance is poor.
[0022]
(4) Therefore, in order to obtain a steel tube with excellent buckling resistance, the stress-strain curve of the steel sheet should be low in yield elongation and high in work hardening ability.
[0023]
(5) And, in the state of air cooling after hot rolling, a steel sheet having a small yield elongation as described above and excellent in work hardening performance is a Mo addition system in which the steel composition is appropriately adjusted, It is obtained by making the rolling end temperature of hot rolling appropriate.
[0024]
(6) Furthermore, the steel pipe using such a steel plate is excellent in buckling resistance, but when it is made into a steel pipe by cold working, the buckling resistance is further improved dramatically if the pipe is appropriately expanded. .
[0025]
FIG. 1 shows a schematic diagram of a stress-strain curve in the tube axis direction (b) of a steel plate (a) obtained by air cooling after hot rolling and a steel pipe manufactured by cold forming using the steel plate.
[0026]
In FIG. 1A, A shows a case where the yield elongation is long, B shows a case where the yield elongation is small but the work hardening ability is low, and when a steel pipe having a stress-strain curve is produced, In the stress-strain curve in the axial direction, even if the yield elongation remains as in A or the yield shelf disappears as in B, the work hardening ability is still low and the buckling resistance performance as a steel pipe is inferior.
[0027]
On the other hand, when a steel pipe is formed by cold forming under appropriate conditions using a steel plate having a low yield elongation and high work hardening ability, such as C, the stress-strain curve, the stress in the tube axis direction of the steel pipe The strain curve disappears from the yield shelf as in C, and the work hardening ability is high, and a steel pipe with extremely high buckling resistance can be obtained.
[0028]
The present invention has been made by further study based on the above knowledge. The manufacturing method of the steel pipe excellent in the buckling-proof performance by the cold working characterized by including the following processes.
(A) In mass%, C: 0.05 to 0.12%, Si: 0.01 to 0.50%, Mn: 0.5 to 2.0%, Mo: 0.05 to 0.50% , Al: 0.01 to 0.08%, a step of preparing a steel slab comprising the balance Fe and inevitable impurities (b) Rolling end temperature: (Ar3-10) to (Ar3 + 80) After hot rolling at a temperature of 0 ° C., air cooling And making a steel plate.
(C) The process of making the said steel plate into a steel pipe by cold forming.
(D) Tube expansion rate: The final tube forming step for expanding the tube by 0.5 to 1.5%.
[0029]
2. In addition to the composition of the steel slab, Cu: 0.5% or less, Ni: 0.5%, Cr: 0.5% or less, Nb: 0.05% or less, V: 0.1% or less, Ti: 0 The manufacturing method of the steel pipe excellent in the buckling-proof performance by cold work of 1 characterized by containing 0.05% or less of 1 type, or 2 or more types.
[0030]
3. The component composition of the steel slab further comprises one or more of Nb: 0.05% or less, V: 0.1% or less, Ti: 0.05% or less, or 1 or 2 A method of manufacturing steel pipes with excellent buckling resistance by cold working.
[0031]
4). After the hot rolling, the yield elongation of the stress-strain curve of the steel sheet in the air-cooled state is 2.0% or less, and the buckling resistance performance by cold working according to any one of 1 to 3, Excellent steel pipe manufacturing method.
[0032]
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, the component composition and production conditions are defined.
1. Ingredient composition C
C is added in order to secure the strength and toughness of the steel and to improve the buckling resistance performance by the combined addition with 0.05 to 0.5% of Mo. If it is less than 0.05%, the strength is insufficient. On the other hand, if it exceeds 0.12%, the toughness and weldability are deteriorated, and after hot rolling, the yield elongation of the air-cooled steel sheet becomes large, which adversely affects the buckling resistance. Therefore, the content is made 0.05 to 0.12%.
[0033]
Si
Si is added to deoxidize and reduce the strength of ferrite in the steel sheet and improve work hardening ability. If it is less than 0.01%, such curing cannot be obtained. On the other hand, if it exceeds 0.5%, the HAZ toughness and weldability are impaired.
[0034]
Mn
Mn is added to ensure strength toughness. If it is less than 0.5%, strength cannot be obtained. On the other hand, if it exceeds 2.0%, the toughness decreases, so 0.5 to 2.0%.
[0035]
Mo
Mo is an important element in the present invention, ensuring strength, and adding a C content of 0.05 to 0.12% to provide a yield shelf in the stress-strain curve of a steel sheet that has been air-cooled after hot rolling. Add to reduce the effect. If it is less than 0.05%, such an effect cannot be obtained. On the other hand, if it exceeds 0.5%, the microstructure becomes an ash ferrite structure, the strength becomes excessive, and the weldability is impaired. 05 to 0.5%.
[0036]
Al
Al is added for deoxidation. If it is less than 0.01%, such an effect cannot be obtained. If it exceeds 0.08%, the cleanliness is lowered and the toughness is deteriorated, so 0.01 to 0.08%.
[0037]
In the present invention, the basic component composition has been described above, but in order to further improve strength toughness and HAZ toughness, one or more of Cu, Ni, Cr, Nb, V, and Ti can be contained.
[0038]
Cu
Cu is effective in improving toughness and increasing strength, and after hot rolling, reduces the yield elongation of an air-cooled steel sheet in the same manner as Mo. However, if added in excess, weldability deteriorates, so it is added. In this case, the upper limit is 0.5%.
[0039]
Ni
Ni is effective in improving toughness and increasing strength. Like Mo and Cu, Ni reduces the yield elongation of steel sheets in the air-cooled state after hot rolling, but if added in excess, the weldability deteriorates and the product cost increases. When adding, the upper limit is 0.5%.
[0040]
Cr
Cr is effective for improving toughness and increasing strength, and if added excessively, weldability deteriorates. Therefore, when added, the upper limit is 0.5%.
[0041]
Nb
Nb suppresses grain growth during slab heating and rolling, improves the toughness by forming a fine structure, and precipitates fine carbides in cooling after rolling to improve the strength. If it exceeds 0.05%, the toughness of the weld heat-affected zone will be deteriorated.
[0042]
V
V precipitates as fine carbides in the cooling process after hot rolling, and improves the strength. If it exceeds 0.1%, the toughness of the weld heat-affected zone will be deteriorated.
[0043]
Ti
Ti suppresses grain growth at the time of slab heating with TiN, improves the toughness by forming a fine structure, and also improves the toughness of the weld heat affected zone. If it exceeds 0.05%, the toughness of the weld heat-affected zone is deteriorated.
[0044]
2. Production conditions Hot rolling rolling end temperature In order to obtain the desired stress-strain curve of the steel pipe, the rolling end temperature of the steel sheet is a very important manufacturing condition, so that it is close to the ferrite transformation start temperature (Ar3 temperature). Stipulate.
[0045]
When the rolling end temperature is less than Ar3-10 ° C., a large work strain is added to the ferrite generated during rolling, and the work hardening ability of the steel sheet deteriorates. Therefore, the work hardening ability of the steel pipe is similarly lowered, and the buckling resistance is improved. to degrade.
[0046]
On the other hand, when Ar3 + 80 ° C is exceeded, the ferrite produced in the cooling process becomes coarse, the yield elongation of the steel sheet increases remarkably, and even in a steel pipe by cold forming, a yield shelf remains in the stress-strain curve of the steel pipe, Since the buckling resistance is lowered, the rolling end temperature is (Ar3-10) to (Ar3 + 80) ° C.
[0047]
In the present invention, the ferrite transformation start temperature (Ar3 temperature) was determined from Ar3 (° C.) = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo (where each element is in mass%).
[0048]
In addition, the slab heating temperature during hot rolling is preferably 950 to 1200 ° C. from the viewpoint of toughness and rolling efficiency.
[0049]
Cooling after hot rolling Cooling after hot rolling is air cooling. In the case of air cooling, the steel pipe strength is within 575 MPa of API standard X52 tensile strength, but in water cooling, the steel has a large amount of bainite structure, and the strength of the steel pipe exceeds the tensile strength of 575 MPa of API standard X52. Cooling is air cooling.
[0050]
Steel Pipe Forming Process In the present invention, the steel sheet is made into a steel pipe by cold forming. By cold forming, strain is introduced and the yield elongation of the stress-strain curve of the steel pipe is reduced.
[0051]
If the steel pipe is formed by cold, any method such as UOE method, bending roll method and the like may be used, and a specific method for manufacturing the steel pipe is not specified. However, it is specified that the pipe expansion rate is 0.5 to 1.5% in the final pipe forming process.
[0052]
By expanding the pipe, the steel pipe is compressed in the axial direction, the yield strength is lowered by the Bauschinger effect, and the yield elongation disappears, so that the work hardening of the steel pipe is improved and the buckling resistance is remarkably improved.
[0053]
When the tube expansion ratio is 0.5% or less, the effect cannot be obtained. On the other hand, when the tube expansion ratio exceeds 1.5%, the yield strength increases due to work hardening by tube expansion in the pipe circumferential direction, and the yield ratio increases. Since the seam weld is easily broken, the content is set to 0.5 to 1.5%.
[0054]
In addition, when hot-rolled steel sheet is made into a steel pipe by cold forming after hot rolling under the above-described conditions, a steel pipe excellent in buckling resistance can be obtained, but the yield elongation in the stress-strain curve is particularly 2.0. % Or less is preferred.
[0055]
When the yield elongation in the stress-strain curve is 2.0% or less, the yield elongation easily disappears by cold working by pipe forming and pipe expansion in the final pipe forming process, so that the buckling resistance performance of the steel pipe is improved. The yield elongation is JISG0202, which is the amount of strain from the upper yield point to the work hardening start point.
[0056]
【Example】
Steel having the component composition shown in Table 1 was subjected to hot rolling under various conditions to obtain a test steel plate having a thickness of 10.5 mm. In the hot rolling, 65% rolling was performed at a slab heating temperature of 1100 ° C. and a non-recrystallization temperature range, the rolling end temperature was changed, and after the end of rolling, air cooling was performed to room temperature.
[0057]
About the obtained steel plate, the full thickness test piece was extract | collected at right angles to the rolling direction, and the yield elongation was calculated | required from the stress-strain curve.
[0058]
Next, these steel sheets were made into UOE steel pipes having an outer diameter of 610 mm with various tube expansion rates. Yield strength, tensile strength, and yield ratio were determined by a full thickness tensile test taken from the direction of the pipe axis of the steel pipe, and were used as the tensile characteristics of the steel pipe.
[0059]
In the axial compression test, pressure-resistant plates were attached to both ends of a steel pipe having a length of 1,800 mm, axial compression was performed with a large compression test apparatus, and the amount of strain at the point where the compression load was maximized was defined as critical buckling strain.
[0060]
Table 2 shows the tensile properties and the critical buckling strain of the steel pipe together with the steel sheet production conditions. In Table 2, No. Nos. 1 to 7 are examples of the present invention. The yield elongation of the steel sheets is small at 2.0% or less and the tube expansion rate is within 0.5 to 1.5%, so that the critical buckling strain is 0.72 to 1. 02 and excellent buckling resistance.
[0061]
On the other hand, no. Nos. 8 to 13 are comparative examples, and either the component composition or the production conditions are out of the scope of the present invention, and the buckling resistance performance is inferior. No. No. 8 has a low rolling end temperature and is inferior in work hardening performance, and is inferior in buckling resistance even though the yield elongation of the steel sheet is small.
[0062]
No. No. 9 has a high rolling end temperature, a large yield elongation of the steel sheet, and is inferior in buckling resistance. No. No. 10, the rolling end temperature is within the range of the present invention, but the tube expansion rate at the time of pipe making is low and the critical buckling strain is inferior.
[0063]
No. No. 11 has a small amount of Mo. Since No. 13 has too much C content, the yield elongation of the steel sheet is large and the buckling resistance is inferior. No. No. 12, the amount of Mo is large in the range of the present invention, and the yield strength exceeds the upper limit (531 MPa) of API-X52 grade (PSL2 standard).
[0064]
[Table 1]

[0065]
[Table 2]

[0066]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, the steel pipe below the APIX52 grade excellent in buckling-proof performance and excellent in the safety | security at the time of a big earthquake is obtained, and it is very useful industrially.
[Brief description of the drawings]
FIG. 1 shows a stress-strain diagram, wherein (a) shows a steel plate, and (b) shows a steel pipe using the steel plate shown in (a).

Claims (4)

The manufacturing method of the steel pipe excellent in the buckling-proof performance by the cold working characterized by including the following processes.
1. In mass%, C: 0.05 to 0.12%, Si: 0.01 to 0.50%, Mn: 0.5 to 2.0%, Mo: 0.05 to 0.50%, Al: 1. Preparing a steel slab comprising 0.01 to 0.08%, the balance Fe and inevitable impurities Rolling end temperature: Step of hot-rolling at (Ar3-10) to (Ar3 + 80) ° C. and then air-cooling to form a steel plate.
3. A step of forming the steel plate into a steel pipe by cold forming.
4). Tube expansion rate: The final tube forming step for expanding the tube by 0.5 to 1.5%. The component composition of the steel slab further comprises one or more of Cu: 0.5% or less, Ni: 0.5%, Cr: 0.5% or less. A method of manufacturing steel pipes with excellent buckling resistance by hot working. The component composition of the steel slab further comprises one or more of Nb: 0.05% or less, V: 0.1% or less, and Ti: 0.05% or less. The manufacturing method of the steel pipe excellent in the buckling-proof performance by the cold work of description. The yield resistance of the stress-strain curve of the steel sheet as it is air-cooled after hot rolling is 2.0% or less, the seating resistance by cold working according to any one of claims 1 to 3. A method for manufacturing steel pipes with excellent bending performance.

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