WO2009119570A1 - Uoe steel tube for linepipes and process for production thereof - Google Patents

Abstract

Provided is a high-strength UOE steel tube having excellent seismic performance. A high-strength UOE steel tube for linepipes which has a steel composition containing C: 0.03 to 0.10%, Si: 0.05 to 0.50%, Mn: 1.50 to 2.2%, P: 0.025% or less, S: 0.002% or less, Cu: 1.0% or less, Cr: 1.0% or less, Ni: 2.0% or less, Mo: 1.0% or less, Nb: 0.1% or less, V: 0.1% or less, Ti: 0.025% or less, Al: 0.06% or less, N: 0.0050% or less, and Ca: 0.0050% or less at a Ti/N ratio of 4.0 or above with the balance being Fe and impurities and which has either a metal structure consisting of ferrite and bainite or a metal structure consisting of ferrite, bainite and martensite.

Description

UOE steel pipe for line pipe and manufacturing method thereof

This invention relates to the UOE steel pipe for line pipes, and its manufacturing method. Specifically, the present invention is a line pipe of X70 grade or more (pipe axis direction strength of 485 MPa or more) excellent in earthquake resistance, which is designed by a strain-based design and is used in cold regions such as Canada. The present invention relates to a UOE steel pipe for use and a manufacturing method thereof.

In recent years, there has been a great demand for cost reduction for pipelines. For this reason, the UOE steel pipe for pipelines tends to be strengthened. High-strength UOE steel pipes for line pipes have hitherto been stressed (S) stress-based (a) having sufficient strength (TS: highest strength) and (b) being able to withstand internal pressure during use (Stress-Based Design).

In recent years, UOE steel pipes composing line pipes laid in cold regions such as Canada are deformed by tensile stress acting in the axial direction as frozen soil (ground) repeatedly expands and contracts due to seasonal temperature fluctuations. There is concern. In particular, it is absolutely necessary to prevent major accidents from occurring due to the destruction of long-distance pipelines that transport natural gas and oil. For this reason, in order to apply a high-strength UOE steel pipe to a pipeline, the high-strength UOE steel pipe needs to have higher fracture safety than a conventional strength UOE steel pipe. For this reason, the high-strength UOE steel pipe that constitutes a line pipe used particularly in cold regions is excellent not only in strength but also in earthquake resistance evaluated by deformability (for example, yield ratio and uniform elongation) against fracture deformation. Is required.

It is generally known that various factors such as the yield ratio, the size of uniform elongation, and the shape of the stress-strain curve affect the earthquake resistance of steel pipes. Therefore, a high-strength UOE steel pipe for a line pipe used in a cold region is designed based on stress design that emphasizes not only strength but also yield ratio and uniform elongation, for example.

In Patent Document 1 and Patent Document 2, as a steel pipe excellent in earthquake resistance, and in Patent Document 3, as a method of manufacturing a line pipe having high strength, high toughness and excellent earthquake resistance, both UOE steel pipes are used. It is disclosed that having a round-shaped stress-strain curve with no yield point improves seismic resistance. Furthermore, Patent Document 4 discloses that a steel pipe excellent in earthquake resistance is manufactured on a trial basis by cooling after rolling.
JP-A-9-196243 Japanese Patent Laid-Open No. 11-80900 Japanese Patent Laid-Open No. 9-202922 JP 2004-131810 A

In general, all pipeline UOE steel pipes are typically 150 ° C or higher and 250 ° C or lower by high-frequency heating, for example, at the site where they are laid, in order to prevent damage during construction and external corrosion during operation. An external coating is applied with a heat treatment that is held at temperature for about 5 minutes (referred to herein as “pre-coating heat-treatment”).

All of the steel pipes obtained by the conventional techniques disclosed in Patent Documents 1 to 4 described above have a Yield Point type in which the shape of the stress-strain curve after the pre-coating heat treatment is performed has an upper yield point. Therefore, the earthquake resistance is not good.

Usually, the shape of the stress-strain curve of a UOE steel pipe after pipe making is always Round House type because many cold strains are imparted to the UOE steel pipe. Therefore, in order to improve the earthquake resistance of the UOE steel pipe, the stress-strain curve after the pre-coating heat treatment can maintain the Round House type stress-strain curve without changing to the Yield Point type having the upper yield point. At the same time, a low yield ratio can be achieved.

Therefore, the inventors of the present invention, among various factors that affect the earthquake resistance of high-strength UOE steel pipes (UOE steel pipe dimensions, yield ratio, uniform elongation, and stress-strain curve shape, etc.) The present inventors have intensively studied means for maintaining the Round House type in which the shape of the stress-strain curve after the heat treatment does not have an upper yield point.

As a result, the inventors have
(A) Controlling the content ratio (Ti / N) of each of Ti and N in the UOE steel pipe to an appropriate value, and (b) accelerating after rolling is completed in the production stage of the rolled steel sheet as the base material By reducing the free N that fixes dislocations by cooling, and manufacturing the UOE steel pipe by the UOE pipe manufacturing method using this rolled steel sheet as a raw material, the stress of the Round House type shape even after the pre-coating heat treatment is performed. -The inventors have found that high strength UOE steel pipes with strain curves can be produced and have completed the present invention.

In the present invention, C: 0.03% or more and 0.10% or less (unless otherwise specified, “%” in terms of composition means “mass%”), Si: 0.05% or more and 0 50% or less, Mn: 1.50% or more and 2.2% or less, P: 0.025% or less, S: 0.002% or less, Cu: 1.0% or less, Cr: 1.0% or less, Ni: 2.0% or less, Mo: 1.0% or less, Nb: 0.1% or less, V: 0.1% or less, Ti: 0.025% or less, Al: 0.06% or less, N: 0.0050% or less, Ca: 0.0050% or less, and ratio of Ti and N content (Ti / N): 4.0 or more, having a steel composition composed of the balance Fe and impurities. More desirably, the circumferential strength is 550 MPa or more, X70 grade or more (pipe axis direction strength 485MPa or more) UOE steel pipe for line pipe.

From another viewpoint, the present invention provides C: 0.03% to 0.10%, Si: 0.05% to 0.50%, Mn: 1.50% to 2.2%, P : 0.025% or less, S: 0.002% or less, Cu: 1.0% or less, Cr: 1.0% or less, Ni: 2.0% or less, Mo: 1.0% or less, Nb: 0 0.1% or less, V: 0.1% or less, Ti: 0.025% or less, Al: 0.06% or less, N: 0.0050% or less, Ca: 0.0050% or less, and Ti The ratio of Ti and N content (Ti / N): 4.0 or more, and immediately after rolling at a finishing temperature of 700 ° C. or higher and 850 ° C. or lower on a slab having a steel composition consisting of Fe and impurities. A rolled steel sheet obtained by cooling at a cooling rate of 3 ° C./sec to 30 ° C./sec Te, characterized in that the production of UOE steel pipe by UOE steel tube method, preferably the circumferential strength method for manufacturing a UOE steel pipe for X70-grade or line pipe is at least 550 MPa.

In the present invention, the content ratio (Ti / N) of each of Ti and N in the UOE steel pipe is set to an appropriate range, and accelerated cooling is performed after the hot rolling of the rolled steel sheet that is the material of the UOE steel pipe is finished. For this reason, according to the present invention, free N for fixing dislocations can be reduced, and a metal structure composed of ferrite and bainite or a metal structure composed of ferrite, bainite, and martensite can be desirably formed. As a result, according to the present invention, the stress-strain curve of the Round House type shape can be maintained even after the pre-coating heat treatment while reducing the yield ratio of the UOE steel pipe. It is possible to provide a high-strength UOE steel pipe for line pipe.

It is a graph which shows the change of the stress-strain curve before and behind heat processing before a coating. It is a graph which shows the influence of (Ti / N) which exerts on the relationship between the stress and the distortion in the tensile test of the L direction of a test piece. 3 (a) to 3 (g) are graphs showing the shape of the stress-strain curve in the tensile test of Example 1. FIG.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the best mode for carrying out a UOE steel pipe for a line pipe and a method for producing the same according to the present invention will be described in detail with reference to the accompanying drawings.
The reason for limiting the composition of the slab used in the present invention will be described.
(C: 0.03% to 0.10%)
C is an element effective for increasing the strength, and is contained in an amount of 0.03% or more in order to have strength of X70 grade or higher, particularly X100 grade. On the other hand, if the C content exceeds 0.10%, the toughness is significantly lowered, which adversely affects the mechanical properties of the base material and promotes the occurrence of surface scratches on the slab. For this reason, C content shall be 0.03% or more and 0.10% or less.
(Si: 0.05% to 0.50%)
By containing 0.05% or more, Si acts as a deoxidizer and as a component for strengthening steel. However, if the Si content exceeds 0.50%, a lot of striped martensite is generated in the weld heat affected zone, the toughness is extremely deteriorated, and the mechanical properties of the UOE steel pipe are lowered. Therefore, the Si content is limited to 0.05% or more and 0.50% or less. The Si content is preferably determined in consideration of the balance with the plate thickness.
(Mn: 1.50% or more and 2.2% or less)
By containing 1.50% or more, Mn strengthens and strengthens steel. However, if the Mn content exceeds 2.2%, the toughness of the welded portion deteriorates. Therefore, the Mn content is 1.50% or more and 2.2% or less.

(P: 0.025% or less)
P is an impurity contained in steel, and its content is preferably low. However, extreme reduction is accompanied by a corresponding increase in manufacturing costs. Therefore, the P content is 0.025% or less.
(S: 0.002% or less)
If S exceeds 0.002%, the target toughness of the base material cannot be secured. Therefore, the S content is set to 0.002% or less.
(Cu: 1.0% or less)
When Cu is desirably contained in an amount of 0.01% or more, it exerts an effect of strengthening without greatly impairing toughness due to solid solution strengthening and structural change due to the effect of increasing hardenability. However, if the Cu content exceeds 1.0%, Cu checking that is harmful to the surface defects of the slab occurs, which necessitates low-temperature heating of the slab, and increases the restrictions on the manufacturing conditions. Therefore, the Cu content is 1.0% or less.
(Cr: 1.0% or less)
Cr, like Cu and Ni, desirably contains 0.01% or more, thereby exerting an effect of strengthening without greatly impairing toughness due to solid solution strengthening and structural change due to the effect of increasing hardenability. However, if the Cr content exceeds 1.0%, the toughness of the heat-affected zone is lowered. Therefore, the Cr content is 1.0% or less.

(Ni: 2.0% or less)
Ni, like Cu, desirably contains 0.01% or more, thereby exerting an effect of strengthening without greatly impairing toughness due to solid solution strengthening and structural change due to the effect of increasing hardenability. At the same time, it exerts an effect of suppressing deterioration of the toughness of the base material and the heat-affected zone after hot bending. However, if the Ni content exceeds 2.0%, the manufacturing cost increases too much, impairing practicality. Therefore, the Ni content is set to 2.0% or less.
(Mo: 1.0% or less)
When Mo is desirably contained in an amount of 0.01% or more, it is effective for increasing the strength of the base material and the welded portion. However, if the Mo content exceeds 1.0%, the circumferential weldability at the construction site and the toughness of the heat affected zone are deteriorated. Therefore, the Mo content is 1.0% or less.
(Nb: 0.1% or less, V: 0.1% or less)
Nb and Ti both have a great effect on the increase in strength due to precipitation strengthening and hardenability increasing effects, or on the improvement of toughness accompanying crystal grain refinement. However, when the Nb content exceeds 0.1% or the V content exceeds 0.1%, the toughness of the welded portion decreases. Therefore, the Nb content is 0.1% or less, and the V content is 0.1% or less.
(Ti: 0.025% or less)
When Ti is desirably contained in an amount of 0.005% or more, TiN is produced, and the growth of crystal grains in the heat-affected zone is suppressed and the toughness is improved. However, if the Ti content exceeds 0.025%, the amount of dissolved N increases and the toughness of the heat affected zone deteriorates. Therefore, the Ti content is set to 0.025% or less.

(Al: 0.06% or less)
Al, like Si, has an effect as a deoxidizing material, but is preferably contained in an amount of 0.010% or more for complete grain sizing. On the other hand, in order to appropriately control (Al / O) of the weld metal for the purpose of ensuring the low temperature toughness of the welded portion, the Al content is set to 0.06% or less.
(N: 0.0050% or less)
N forms nitrides with V, Ti, etc., and brings about an effect of improving high temperature strength. However, when the N content exceeds 0.0050%, Nb, V, Ti and carbonitride are formed, and the toughness of the base material and the heat-affected zone is lowered. Therefore, the N content is 0.0050% or less. However, when the required level of toughness for the weld heat affected zone is high, the N content is preferably 0.0035% or less.
(Ca: 0.0050% or less)
Ca is effective in controlling the form of inclusions, specifically in spheroidization, and prevents hydrogen-induced cracking and lamellar tear. However, if the Ca content exceeds 0.0050%, the amount of Ca-based inclusions generated becomes excessive, so the Ca content is set to 0.0050% or less. On the other hand, the Ca content is closely related to the S content, and when the S content is 0.0010% or more, it is 0.0005% or more due to the spheroidization of MnS inclusions. Is desirable. In addition, in the steel with extremely high cleanliness having an S content of 0.0010% or less, the lower limit of the Ca content is not particularly required to be set. For these reasons, the Ca content is set to 0.0050% or less.

Other than those described above, Fe and impurities.
(Ti and N content ratio (Ti / N): 4.0 or more)
Taking an X80 grade high strength UOE steel pipe as an example, the relationship between the ratio of the outer diameter D to the wall thickness T (D / T) and the earthquake resistance was examined. As a result, it was confirmed that the buckling strain increased as the ratio (D / T) decreased, and the earthquake resistance improved. For this reason, in order to improve the earthquake resistance of the high-strength UOE steel pipe, it is desirable to first set the ratio (D / T) of the UOE steel pipe to 30 or less. However, the ratio (D / T) may have to be set to more than 30 depending on the transportation pressure, transportation efficiency, and transportation requirement. In the present invention, the characteristics of the material are improved in order to improve and improve the earthquake resistance even in such a case.

Taking an X80 grade UOE steel pipe as an example, the three items of (i) yield ratio YR (YS / TS), (ii) uniform elongation, and (iii) stress-strain curve shape are earthquake resistance (buckling resistance). ) Was investigated.

This study
(A) The yield ratio YR is in the order of 85, 88, 91%, and the bending angles at the time of occurrence of buckling are 13.9 °, 14.5 °, 15.3 °, and the yield ratio YR increases as the value increases. Excellent earthquake resistance.
(B) When the uniform elongation is 5.88%, 7.38%, and 8.88%, the bending angles at the occurrence of buckling are 14.1 °, 14.5 °, and 15.1 °, respectively. The greater the uniform elongation, the better the earthquake resistance. And (C) FIG. 1 is a graph showing changes in the stress-strain curve before and after the pre-coating heat treatment. Case 1 in FIG. 1 is based on the conditions that the finishing temperature is 800 ° C. and the cooling rate after rolling is 20 ° C./sec. 2 shows a stress-strain curve after heat treatment before coating at 200 ° C. for a UOE steel pipe made of the produced rolled steel sheet. Case 2 is a rolled product manufactured under the conditions of a finishing temperature of 800 ° C. and a cooling rate of 20 ° C./sec after rolling. 2 shows a stress-strain curve of a UOE steel pipe made of a steel plate after heat treatment before coating at 270 ° C. FIG.

As shown in the graph of FIG. 1, before case heat treatment before coating, both cases 1 and 2 have Round House type stress-strain curves. However, in case 1 that does not satisfy the conditions defined in the present invention, heat treatment before coating is performed. The shape of the stress-strain curve after the heat treatment before coating in the case 2 that satisfies the conditions specified in the present invention is maintained in the Round House type while the shape of the stress-strain curve after that changes to the Yield Point type. I understand that

In the present invention, among the three items of the yield ratio YR and the uniform elongation and the shape of the stress-strain curve, the stress after the pre-coating heat treatment is performed with particular attention to the shape of the stress-strain curve. -Utilizing the fact that the UOE steel pipe of the Round House type has better earthquake resistance than the UOE steel pipe of the Yield Point type. For this reason, attention was paid to C or N, which is an element that fixes dislocations, and among these, attention was focused on the ratio of Ti and N content (Ti / N).

Then, a confirmation test for changing the ratio (Ti / N) was performed using a test piece, and the change in the shape of the stress-strain curve was examined.
FIG. 2 is a graph showing the influence of (Ti / N) on the relationship between stress and strain in a tensile test in the L direction of a test piece.

As shown in the graph, when (Ti / N) is 2.9, the stress-strain curve has a Yield Point shape, and the values of (Ti / N) are 4.1 and 5.6. As it becomes larger, it becomes a gentle Round House shape without an upper yield point.

As described above, in order to maintain the shape of the stress-strain curve of the UOE steel pipe after the pre-coating heat treatment in the Round House type, it is effective to reduce free N that acts to fix dislocations. . For this purpose, it is effective to set the value of (Ti / N) to 4.0 or more. Furthermore, in order to stably obtain such effects, it is desirable that (Ti / N) is 5.0 or more.

In the present invention, the slab having the steel composition described above is rolled at a cooling rate of 3 ° C./sec to 30 ° C./sec immediately after rolling at a finishing temperature of 700 ° C. or more and 850 ° C. or less according to a conventional method. By performing accelerated cooling, a rolled steel sheet is obtained.

By performing this accelerated cooling, the rolled steel sheet has a metal structure composed of ferrite and bainite or a metal structure composed of ferrite, bainite and martensite.
And in this invention, a UOE steel pipe is manufactured using the rolled steel plate manufactured in this way as a raw material using the well-known UOE pipe manufacturing method. In this UOE pipe manufacturing method, a rolled steel sheet, which is a material, is U-pressed and formed into a U shape, further O-pressed into an O shape and formed into a cylindrical shape, and then a seam at the end. Butt and weld. For those skilled in the art, this is the surrounding pipe making method, and further explanation regarding the UOE pipe making method is omitted.

In the present invention, C: 0.03% to 0.10%, Si: 0.05% to 0.50%, Mn: 1.50% to 2.2%, P: 0.025% or less, S: 0.002% or less, Cu: 1.0% or less, Cr: 1.0% or less, Ni: 2.0% or less, Mo: 1.0% or less, Nb: 0.0. 1% or less, V: 0.1% or less, Ti: 0.025% or less, Al: 0.06% or less, N: 0.0050% or less, Ca: 0.0050% or less, and Ti and Ratio of each N content (Ti / N): 4.0 or more, having a steel composition composed of the balance Fe and impurities, preferably a metal structure composed of ferrite and bainite, or ferrite, bainite and martensite X70 grade or more of the present invention having a metal structure consisting of UOE steel pipe for line pipe is provided.

According to the present invention, the ratio (Ti / N) is limited to an appropriate range, and accelerated cooling is performed after hot rolling of the rolled steel sheet that is the material of the UOE steel pipe, thereby reducing free N and desirably By using a metal structure consisting of ferrite and bainite or a metal structure consisting of ferrite, bainite and martensite, the shape of the stress-strain curve of the UOE steel pipe can be maintained in the Round House type even after heat treatment before coating. As a result, the yield ratio YR of the UOE steel pipe can be suppressed to 90% or less, desirably 85% or less. In this way, it is possible to provide a UOE steel pipe for line pipes of X70 grade or higher that is excellent in earthquake resistance.

Therefore, there is a concern that the UOE steel pipe constituting the laid line pipe may be deformed by the tensile stress acting in the axial direction due to repeated expansion and contraction of frozen soil (ground) due to seasonal temperature fluctuations. As a high-strength UOE steel pipe constituting a line pipe used in cold districts such as Canada, not only the strength but also the earthquake resistance, which is a deformability against fracture deformation, is extremely desirable.

The present invention will be described more specifically with reference to examples.
Rolled steel sheet obtained by rolling a slab having the steel composition shown in Table 1 (Fe and impurities other than those shown in Table 1) at the finishing temperature shown in Table 2, and then immediately cooling at the cooling rate shown in Table 2. A UOE steel pipe having an X80 grade and a wall thickness of 16.2 mm was manufactured by UOE pipe manufacturing method.

The tensile test pieces 1 to 7 cut out from these UOE steel pipes were subjected to a heat treatment simulating a pre-coating heat treatment held at 250 ° C. for 5 minutes, and then subjected to a tensile test.
The chemical compositions and metal structures of the tensile test pieces 1 to 7 are shown in Table 1, and the rolling finishing temperature and cooling rate of the rolled steel sheet as the material, the mechanical properties of the tensile test pieces 1 to 7 (tensile strength TS, yield strength YS). , Yield ratio YR) and toughness (Charpy impact test absorbed energy vE-10 (° C.)) are shown in Table 2. The shape of the stress-strain curve in the tensile test is shown in graphs in FIGS. 3 (a) to 3 (g).

In particular, the shape of the stress-strain curve is classified into two types (I) and (II) as shown below, and is important for evaluating the quality of earthquake resistance.
(I) Round House type:
This is the shape of the stress-strain curve that appears in normal UOE steel pipes that are not heat-treated before coating, and shows excellent earthquake resistance.
(II) Yield Point type:
The shape of the stress-strain curve often appears in the UOE steel pipe after the pre-coating heat treatment, which is inferior to the round type in buckling resistance and unsuitable for use in the strain based design.

From Tables 1 and 2 and FIGS. 3 (a) to 3 (g), the ratio of Ti and N in the UOE steel pipe (Ti / N) is within an appropriate range, and rolling is the material of the UOE steel pipe. Accelerated cooling is performed after hot rolling of the steel sheet is completed. Thereby, while reducing the free N which fixes a dislocation | rearrangement, desirably, it can be set as the metal structure which consists of a ferrite and a bainite, or the metal structure which consists of a ferrite, a bainite, and a martensite, and low yield ratio reduction of a UOE steel pipe As shown in the figure, even after the pre-coating heat treatment, it has a Round House type stress-strain curve. For this reason, it turns out that the UOE steel pipe for line pipes excellent in earthquake resistance can be provided.

Claims (2)

In mass%, C: 0.03% to 0.10%, Si: 0.05% to 0.50%, Mn: 1.50% to 2.2%, P: 0.025% or less , S: 0.002% or less, Cu: 1.0% or less, Cr: 1.0% or less, Ni: 2.0% or less, Mo: 1.0% or less, Nb: 0.1% or less, V : 0.1% or less, Ti: 0.025% or less, Al: 0.06% or less, N: 0.0050% or less, Ca: 0.0050% or less (Ti / N): 4. A UOE steel pipe for line pipes of X70 grade or higher, characterized by having a steel composition consisting of zero or more and the balance Fe and impurities. In mass%, C: 0.03% to 0.10%, Si: 0.05% to 0.50%, Mn: 1.50% to 2.2%, P: 0.025% or less , S: 0.002% or less, Cu: 1.0% or less, Cr: 1.0% or less, Ni: 2.0% or less, Mo: 1.0% or less, Nb: 0.1% or less, V : 0.1% or less, Ti: 0.025% or less, Al: 0.06% or less, N: 0.0050% or less, Ca: 0.0050% or less (Ti / N): 4. A slab having a steel composition consisting of Fe and impurities with a balance of 0 or more is rolled at a finishing temperature of 700 ° C. or more and 850 ° C. or less and then immediately cooled at a cooling rate of 3 ° C./sec or more and 30 ° C./sec or less. To manufacture UOE steel pipe by the UOE pipe manufacturing method using the rolled steel plate The symptom, method for manufacturing UOE steel pipes for X70-grade or line pipe.

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