JP2011063840A - Steel sheet having excellent hic resistance and uoe steel pipe - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel sheet in which, in addition to the control of elements known as HIC generation causes heretofore such as Mn, P and S, in particular, HIC generation caused by the carbonitrides of Nb and Ti can be prevented. <P>SOLUTION: The steel sheet having excellent HIC resistance has a composition comprising 0.02 to 0.07% C, 0.05 to 0.5% Si, 1.10 to 1.60% Mn, ≤0.015%P, ≤0.0020% S, 0.010 to 0.050% Nb, 0.005 to 0.025% Ti, 0.005 to 0.6% sol.Al, 0.0005 to 0.0060% Ca and 0.0015 to 0.007% N, and also satisfying inequality (1): 0.1%≤Cu+Ni+Cr+Mo≤1.5%, and the balance Fe with impurities, and in which the concentration of Nb in a region at a distance of a sheet thickness of 5% from the central part in the sheet thickness direction toward the sheet thickness direction is ≤0.060%, and further, the concentration of Ti is ≤0.025%. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a steel plate and a UOE steel pipe. Specifically, the present invention relates to a thick steel plate having excellent HIC resistance of X60 or higher, which is applied to uses such as line pipes, offshore structures, and pressure vessels, and a UOE steel pipe using the thick steel plate.

  In addition to general properties such as strength, toughness and weldability, line pipes used for the transportation of crude oil and natural gas are used in an environment containing hydrogen sulfide, so hydrogen-induced cracking (Hydrogen) Induced Cracking (hereinafter abbreviated as “HIC”) is required as an important characteristic.

  Many studies on HIC have been conducted in the past because oil leakage, destruction, and explosion accidents caused by HIC have actually occurred in line pipes and oil well pipes in the past. According to them, the generation mechanism of HIC is that hydrogen gas generated in the corrosive environment penetrates into the steel material and diffuses, accumulates around MnS and oxide inclusions in the steel material, and molecularizes, leading to cracking. That is. For this reason, the means roughly divided below are disclosed as measures for preventing HIC.

(1) When MnS is present in a steel material, cracks are generated starting from this, and crack sensitivity increases when MnS extends for a long time during rolling. For this reason, Patent Document 1 discloses an invention in which S in steel is reduced to a fine spheroidized CaS or REM sulfide by reducing the S content in steel and adding Ca or REM to the steel. It is disclosed.

(2) In a portion corresponding to the center segregation portion of the slab, a hard structure such as martensite or bainite is formed due to segregation of C, Mn, P, and the like, and this becomes a propagation path of cracks. For this reason, in Patent Document 2, by reducing the concentration of C, Mn, P, etc. in steel, performing soaking treatment to reduce segregation by diffusion, and further increasing the cooling rate after rolling. An invention for preventing the formation of hard tissue is disclosed. Patent Document 3 discloses an invention in which the center segregation itself is eliminated by bulging the slab once at the stage where the continuously cast unsolidified molten steel remains, and then reducing the slab.

(3) In recent years, with the improvement of the required strength specifications of steel materials, it has become insufficient in many cases to perform the single countermeasure of the above item (1) or (2). For this reason, in Patent Documents 4 to 6, Cu or Ni is added to the steel to form a protective film on the surface to suppress the intrusion of hydrogen into the steel. And the like are used together with a heat treatment (TMCP) during rolling.

Japanese Patent Laid-Open No. 54-110119 JP 61-60866 A JP 09-57410 A Japanese Patent Laid-Open No. 6-220577 Japanese Patent Laid-Open No. 9-209037 JP 2003-226922 A

  As a result of intensive studies by the present inventors, in thick steel plates used for line pipes of X60 class (YP70 ksi class) or higher, there is an HIC that cannot be prevented by the measures described in the above items (1) to (3). Newly found. In the present specification, the “thick steel plate” means a steel plate having a plate thickness of 6.0 mm or more and 38.1 mm or less.

  An object of the present invention is to prevent HIC that can be prevented by a known HIC prevention measure in a thick steel plate used for a line pipe or the like of X60 class (YP70 ksi class) or higher. It is to provide a steel plate and a UOE steel pipe having excellent characteristics.

  In the present invention, in the thick steel plate used for the X60 class (YP70 ksi class) or higher line pipe or the like, the HIC occurrence starting point is mainly Nb or Ti other than the causes described in the above items (1) to (3). It is based on the novel knowledge that it is a carbonitride of

  In the present invention, C: 0.02% or more and 0.07% or less (in this specification, “%” relating to composition means “mass%” unless otherwise specified), Si: 0.05% 0.5% or less, Mn: 1.10% or more and 1.60% or less, P: 0.015% or less, S: 0.0020% or less, Nb: 0.010% or more and 0.050% or less, Ti : 0.005% to 0.025%, sol. Al: 0.005% or more and 0.06% or less, Ca: 0.0005% or more and 0.0060% or less, N: 0.0015% or more and 0.007% or less, and formula (1): 0. 1% ≦ Cu + Ni + Cr + Mo ≦ 1.5% is satisfied, V: 0.10% or less as an optional element, preferably 0.01% or more and 0.10% or less, the balance being Fe and impurities, and in the thickness direction The Nb concentration in a region at a distance of 5% of the plate thickness from the center of the plate toward the plate thickness direction is 0.060% or less and the Ti concentration is 0.025% or less. . However, the element symbol in Formula (1) is content (mass%) of each element.

  From another point of view, the present invention is a UOE steel pipe characterized in that the steel plate according to the present invention described above is used as a material.

  According to the present invention, a steel plate having excellent HIC resistance that is used for X60 class (YP70 ksi class) or higher line pipes and the like and does not cause HIC due to coarse carbonitrides of Ti and Nb, and this steel plate It becomes possible to provide a UOE steel pipe made of a material.

It is a graph which shows the investigation result of the segregation degree of Mn, P, S, Nb, and C using a laser ICP apparatus.

Hereinafter, modes for carrying out the present invention will be described.
First, the reason for limiting the composition of the steel sheet according to the present invention will be described.

[C: 0.02% to 0.07%]
In general, C is known as an element having a great influence on the strength of steel. When the C content is less than 0.02%, it is difficult to obtain a predetermined strength for applications such as line pipes. On the other hand, if the C content exceeds 0.07%, as described above, a macro-segregation portion is formed at the thickness center portion of the slab during continuous casting, which causes generation of HIC. Therefore, the C content is limited to 0.02% or more and 0.07% or less. From the same viewpoint, the upper limit of the C content is preferably 0.06%, and more preferably 0.05%. On the other hand, the lower limit is preferably 0.03%, and more preferably 0.04%.

[Si: 0.05% to 0.5%]
Si is generally one of elements effective for reducing the oxygen concentration in steel as a deoxidizing element by containing 0.05% or more in the steel manufacturing process, and also has an effect of strengthening steel. Further, it is an extremely useful element as an element for increasing the strength. However, when the Si content exceeds 0.5%, striped martensite is generated and the HAZ toughness is deteriorated. For this reason, Si content is limited to 0.05% or more and 0.5% or less. From the same viewpoint, the lower limit of the Si content is preferably 0.10%, and more preferably 0.20%.

  On the other hand, since Si has a strong interaction with Ti, it affects the generation of TiN even though it is not a constituent element. Nb carbonitrides are also highly likely to precipitate with TiN as nuclei, which may cause deterioration of HIC resistance. Therefore, the upper limit of the Si content is preferably less than 0.30%.

[Mn: 1.10% to 1.60%]
Mn is an element that generally has a great influence on the strength of a steel material, but if the Mn content is less than 1.10%, it is difficult to obtain sufficient strength. Moreover, when Mn content exceeds 1.60%, it concentrates in a center segregation part as mentioned above, and HIC-proof performance is deteriorated. For this reason, Mn content shall be 1.10% or more and 1.60% or less. From the same viewpoint, the lower limit of the Mn content is preferably 1.20%, more preferably 1.30%. On the other hand, in order to ensure the HIC resistance at the center segregation part, the upper limit of the Mn content is preferably 1.50%, more preferably 1.40%.

[P: 0.015% or less]
P is one of the impurities inevitably present in the steel, and the P content is preferably as low as possible. P is segregated remarkably because the distribution coefficient at the solid-liquid interface at the time of solidification is small, and as described above, it concentrates at the center segregation part and deteriorates the HIC resistance. Therefore, the P content is 0.015% or less. In order to prevent deterioration of the HIC resistance in the central segregation part of P, the P content is preferably less than 0.008%. However, since significant reduction of P is accompanied by a corresponding increase in steelmaking costs, the P content is preferably 0.003% or more.

[S: 0.0020% or less]
S, like P, is one of the impurities inevitably present in steel, and its content is preferably as low as possible. S is not only segregated remarkably because the distribution coefficient at the solid-liquid interface during solidification is small, but also MnS is generated at the segregated portion as described above and becomes the starting point of HIC. For this reason, S content shall be 0.0020% or less. In order to ensure stable HIC resistance under conditions that are more demanding than those of high strength steel and the like, the upper limit of the S content is preferably set to 0.0010%. However, since significant reduction of S is accompanied by a corresponding increase in steelmaking costs, the S content is preferably 0.0003% or more.

[Nb: 0.010% to 0.050%]
Nb is an element that forms carbonitrides in steel to increase the strength of the steel and is effective in improving toughness. In particular, TMCP controls the microstructure of steel sheets by controlling solid solution and precipitation. Therefore, it is contained by 0.010% or more. However, when the Nb content exceeds 0.050%, the structure cannot be controlled without solid solution even during heating, and the increase in the Nb content means an increase in the amount of Nb carbonitride, and the HIC resistance. May cause a decrease in For this reason, Nb content shall be 0.010% or more and 0.050% or less. In order to ensure the HIC resistance stably under the severe requirements of high strength steel or the like, the Nb content is preferably 0.010% or more and 0.040% or less.

[Ti: 0.005% to 0.025%]
Ti is a constituent element of TiN that improves the strength of steel and forms the basis of the present invention. In addition, N in the steel is fixed as TiN, and the amount of precipitation of NbN and AlN is reduced, so the surface of the slab resulting from the dynamic precipitation of NbN and AlN at the austenite grain boundaries during bending and straightening of continuous cast slabs There is also an effect of preventing cracking. In order to obtain such an effect, Ti is contained by 0.005% or more. However, when the Ti content exceeds 0.025%, not only the toughness of the welded portion is lowered, but also the HIC resistance may be lowered due to an increase in the amount of Ti carbonitride. For this reason, Ti content shall be 0.005% or more and 0.025% or less. More preferably, it is 0.008% or more and 0.020% or less.

[Sol. Al: 0.005% or more and 0.06% or less]
Al is one of the elements effective for reducing the oxygen concentration in steel as a deoxidizing element. The effective content for deoxidation is 0.005% or more. sol. When the Al content is less than 0.005%, desulfurization becomes insufficient, the yield of Ca addition deteriorates, and the effect cannot be obtained sufficiently. HIC is caused by segregation of sulfide and S in steel. appear. However, sol. If the Al content exceeds 0.06%, the alumina produced upon deoxidation may cause HIC. For this reason, sol. Al content shall be 0.005% or more and 0.06% or less. From the same viewpoint, sol. The Al content is more preferably 0.005% or more and 0.04% or less.

[Ca: 0.0005% or more and 0.0060% or less]
In HIC-resistant steel, Ca is contained in order to reduce the S concentration, prevent the formation of MnS, and control the form of sulfide. For this reason, Ca contains 0.005% or more. However, even if the content exceeds 0.0060%, the effect is saturated and the manufacturing cost only increases. For this reason, Ca content shall be 0.0005% or more and 0.0060% or less.

[N: 0.0015% or more and 0.007% or less]
N is an element that inevitably penetrates into steel when it is melted in an air atmosphere such as a converter. It is a constituent element of coarse Nb carbonitride that is focused in the present invention. Nb carbonitrides are not directly associated with N preferentially, but are known to precipitate with crystallized TiN as nuclei. In steel materials, it is an element that forms nitrides with Al, Ti, etc. These nitrides have the effect of refining crystal grains as pinning particles in the process of hot working, and thus affect the mechanical properties of steel materials. And affects the microstructure formation. For this reason, N is contained 0.0015% or more. On the other hand, if the N content exceeds 0.007%, as described above, these nitrides dynamically precipitate at the austenite grain boundaries during continuous casting, which causes slab surface cracks. For this reason, N content shall be 0.0015% or more and 0.007% or less.

[0.1% ≦ Cu + Ni + Cr + Mo ≦ 1.5%]
In HIC-resistant steel, the upper limit of the content of C and Mn is determined for the reason of generation of MnS and reduction of C segregation. For this reason, in order to ensure intensity | strength, alloy elements, such as Cu, Ni, Cr, Mo, are contained. In order to surely obtain the effect of improving the strength, it is effective that the total content of these alloy elements is 0.1% or more. However, if the content of these alloy elements is too large, the hardenability is increased, the strength is increased, and a part of the structure is hardened. As a result, the HIC resistance is deteriorated. Therefore, in the present invention, the total content of these alloy elements is 1.5% or less.

The content of each of these alloy elements will also be described.
Cu improves the hardenability of the steel by containing 0.1% or more, but if the Cu content exceeds 0.5%, the hot workability and machinability of the steel material are reduced and continuous casting is performed. Since it is an element that sometimes induces surface cracks called copper cracks, when it contains 0.2% or more of Cu, it is desirable to contain Ni at a concentration of (1/3) or more. For the above reasons, the Cu content is preferably 0.1% or more and 0.5% or less.

  Ni has the effect of improving the toughness as well as improving the strength of the steel by solid solution strengthening. In order to acquire these effects, it contains 0.1% or more, but even if it contains exceeding 1.0%, the effect will reach a peak and rather the weldability deteriorates. For this reason, it is desirable that the Ni content is 0.1% or more and 1.0% or less.

  Cr increases the strength and toughness of steel, and is effective when high strength specifications such as X80 class are required, so 0.05% or more is contained. On the other hand, as can be seen from the fact that the carbon equivalent Ceq (= C + Mn / 6 + (Cr + Mo) / 5 + (Cu + Ni) 15) is significantly increased only by containing a small amount, by containing a small amount significantly contributes to the strength increase, If it exceeds 0.5%, weld cracking occurs. Therefore, the Cr content is desirably 0.05% or more and 0.4% or less.

  Furthermore, Mo improves the hardenability of the steel sheet and contributes to an increase in strength. Further, it is an element that is difficult to segregate microscopically and has an effect of suppressing the generation of HIC due to center segregation. In order to acquire the effect of containing Mo, it contains 0.02% or more. However, Mo is an expensive element and not only leads to an increase in cost, but if it exceeds 0.5%, a hardened phase such as a bainite or martensite phase is generated and the HIC is rather deteriorated. Is 0.5% or less. Therefore, the Mo content is desirably 0.02% or more and 0.5% or less. Since the influence on the HIC resistance is larger than that of the other three elements described above, the Mo content is preferably 0.3% or less.

Next, arbitrary elements will be described.
[V: 0.10% or less]
V is an effective element for increasing the strength of the steel by forming a solid solution and a carbonitride in ferrite in the steel. However, if the V content exceeds 0.10%, the precipitation state in the weld heat affected zone changes and adversely affects toughness. For this reason, when it contains V, it is desirable that the content is 0.10% or less. In addition, in order to show | play the said effect by V reliably, it is desirable that V content is 0.01%.

Other than the above are Fe and impurities.
[Nb concentration: 0.060% or less, Ti concentration: 0.025% or less in a region at a distance of 5% of the plate thickness from the center in the plate thickness direction toward the plate thickness direction]
The present inventors have found that even if MnS, C, Mn, and P, which have been conventionally considered as a main cause of hindering HIC resistance, are reduced, sufficient HIC resistance may not be secured. , Earnestly studied. As a result, even if the countermeasures for the cause of HIC cracking such as conventional MnS and macrosegregation are sufficiently performed, cracks may occur due to the Nb or Ti carbonitride remaining in the steel as a starting point. found. In other words, in addition to the conventional knowledge, it has been found that newly reducing Nb and Ti is effective for improving the HIC resistance.

  The present inventors have found that the presence or absence of occurrence of HIC can be determined from the degree of segregation in the steel sheet with respect to Nb and Ti by investigating the correlation between the degree of segregation of the steel sheet and the HIC test. That is, the amount of Nb and Ti can be reduced by controlling the segregation.

  An HIC test was performed on a test piece collected from the manufactured steel material. As a result, all occurrences of cracks occurred in the central part of the plate thickness, and this indicates that the central segregation part is the cause of the generation of HIC.

  A detailed investigation of the crack occurrence portion was performed on the test piece where the crack occurred. Inclusions that became the starting point of cracking were analyzed by SEM / EDS, but the concentrations may vary, but carbonitrides containing Nb and denoted as Nb (C, N) (10 vol.) It was found to be “Nb carbonitride” hereinafter.

  As described above, segregation control is essential to reduce the amount of Nb and Ti. In order to evaluate the segregation degree level of the current product and investigate the correlation with the presence or absence of HIC, the present inventors investigated the segregation degree of each element in the central segregation part of the rolled thick steel plate.

  The segregation degree of each element was measured by using a laser ICP device (hereinafter abbreviated as “L-ICP device”). The L-ICP device is a kind of emission analysis device, and can measure about 100 points in a 10 mm length measurement, that is, every 100 μm, so that macrosegregation can be sufficiently evaluated. The degree of segregation was defined by defining the average value of the measurement data of each of the 100 elements as the average composition of the steel sheet and dividing the highest point by the average composition.

  Since the laser beam diameter is about 1 mm, all inclusions of 1 mm or less are absorbed in the measurement data. In general, the inclusion is about several μm, and even a large inclusion is about several tens of μm. By this measurement, it is possible to investigate the degree of segregation sufficiently including the inclusion.

  As a result, the criterion for the occurrence of HIC was determined according to the segregation degree of each element, “Nb concentration in a region at a distance of 5% of the plate thickness from the central portion in the plate thickness direction to 0.060% or less, Ti concentration: 0.025% or less ".

  FIG. 1 is a graph showing the investigation results of the segregation degrees of Mn, P, S, Nb, and C using a laser ICP apparatus. The horizontal axis in the graph of FIG. 1 is the measurement position in the thickness direction, and the vertical axis is the measurement result of each element. The level of the vertical axis varies depending on the element, but the average composition and the degree of segregation can be calculated by this measurement method. In addition to the method using this laser ICP apparatus, it is possible to obtain the degree of segregation by comparing the analysis result with the part without segregation, for example, by measuring the element at the target site using chemical analysis or the like. It is.

  As shown in Examples 1 and Comparative Examples 8 and 9 to be described later, “Nb concentration in a region at a distance of 5% of the plate thickness from the center in the plate thickness direction toward the plate thickness direction: 0.060% Hereinafter, the occurrence of HIC can be determined on the basis of “Ti concentration: 0.025% or less”.

  The determination criteria for the presence or absence of HIC occurrence obtained by the present invention exert a particularly remarkable effect on the HIC of high-strength steel that cannot be determined only by the conventionally known determination criteria. It goes without saying that HIC cracking can be effectively prevented by using the criteria for segregation of Nb and Ti according to the present invention together with the conventionally proposed reduction of C, Mn, P and S.

  Since not only the control of the component and the degree of segregation but also other elements can be controlled, the HIC crack can be prevented more effectively, so these other elements will be described briefly. .

  In steelmaking, application of IR (injection refining) and oxide form control by adding Ca are effective as oxide control. In the next process, continuous casting (hereinafter abbreviated as “CC”), when the central part of the slab solidifies, the gradient is set to a degree corresponding to or slightly above the solidification shrinkage of the slab. It is effective to reduce segregation. On the other hand, it is effective to use appropriate water cooling conditions and to select a casting speed so as not to cause significant solidification non-uniformity in the casting width direction and casting length direction.

In the thick plate, it is effective to first perform heating at 1100 ° C. or more and 1200 ° C. or less. Thereby, it is possible to prevent Nb generated in the slab stage from being dissolved and becoming Nb carbonitride. Next, it is effective to adjust the heating temperature and time according to the Nb amount. In rolling, it is effective to perform water cooling at Ar ₃ points or more. The reason for limiting the water cooling stop temperature is that ferrite is generated at less than ₃ points of Ar and carbon discharge starts, so that a hardened structure is formed due to a large amount of carbon in the structure, and the HIC performance deteriorates as described above. Because it does.

  On the other hand, water cooling is effective because it can reduce the diffusion of elements such as C and P as much as possible and prevent C from being combined with Nb. Although the diffusion rate of each element decreases as the temperature decreases, if it is allowed to cool without water cooling after rolling, it may take a longer time to be exposed to a higher temperature, which may promote element diffusion. The diffused elements segregate at the grain boundaries and inclusions.

  The water cooling rate is desirably 5 ° C./sec or more and 30 ° C./sec or less. This is because if the water cooling rate is too slow, diffusion is promoted, and if the water cooling rate is too high, baking is excessively performed, resulting in a hardened structure.

  The UOE steel pipe manufactured using the steel plate according to the present invention as a raw material does not cause HIC caused by coarse carbonitrides of Ti and Nb.

Furthermore, the present invention will be described more specifically with reference to examples.
Various steel types are continuously cast at a casting speed of 0.7 to 0.8 m / min using a vertical bending slab continuous casting machine having a thickness of 300 mm and a width of 2300 mm. In addition to investigating the precipitates, the precipitates were further investigated after rolling, and further a tensile test and an HIC test were conducted to investigate the HIC crack generation part. Table 1 shows the composition of the cast steel type. The balance other than the elements shown in Table 1 is Fe and impurities.

  On the other hand, the slab continuously cast in this way is heated to about 1100 to 1200 ° C., hot-rolled, and made into a thick steel plate having a thickness of 10 to 25 mm under conditions such that the finish rolling temperature is about 750 to 850 ° C. . Immediately after rolling, water cooling was performed at a cooling rate of 5 to 30 ° C./sec, cooling was stopped at about 400 to 550 ° C., and then air cooling was performed.

  While performing the tensile test about this thick steel plate, it used for the NACE test prescribed | regulated by NACE TM-02-84 as a HIC test, and measured the crack area ratio (CAR,%). Moreover, the deposit was also investigated about the center segregation part of the thick steel plate. At the same time, the plate strength was also measured, and the width direction plate strength C-TS (MPa) was 520 MPa or more. The crack area ratio (CAR,%) was 3% or less.

Sample No. in Table 1 Nos. 1 to 5 are examples of the present invention that satisfy all the conditions of the present invention. 6 to 15 are comparative examples that do not satisfy at least one of the conditions of the present invention.
Specimen No. 1 to 6 have a crack area ratio of 3% or less, and C-TS is 538 MPa or more, which satisfies the X60 class (YP70 ksi class).

On the other hand, the test material No. In No. 7, since the Mn content does not satisfy the lower limit of the range of the present invention, the C-TS is unsatisfactory at 512.
Specimen No. In Nos. 8 and 9, since the Nb absolute amount and the Ti absolute amount do not satisfy the upper limit of the range of the present invention, the crack area ratio is unsatisfactory.

Specimen No. No. 10, since the Ca content does not satisfy the lower limit of the range of the present invention, the sulfide morphology cannot be sufficiently controlled, and the crack area ratio is unsatisfactory.
Specimen No. No. 11 causes segregation and inclusions because the P content does not satisfy the upper limit of the range of the present invention, and the cracked area ratio is unsatisfactory.

Specimen No. No. 12 causes the occurrence of segregation and inclusions because the S content does not satisfy the upper limit of the range of the present invention, and the crack area ratio is poor.
Specimen No. No. 13 causes the occurrence of segregation because the C content does not satisfy the upper limit of the range of the present invention, and the crack area ratio is unsatisfactory.

  Specimen No. No. 14 cannot be used as a pipe because the V content does not satisfy the upper limit of the range of the present invention, the crack area ratio is unsatisfactory, and the toughness of the welded portion is adversely affected.

Specimen No. No. 15 causes the occurrence of segregation and inclusions because the Nb content does not satisfy the upper limit of the range of the present invention, and the crack area ratio is unsatisfactory.
Furthermore, the test material No. No. 16 causes segregation and inclusions because the Ti content does not satisfy the upper limit of the range of the present invention, and the crack area ratio is poor.

Claims (3)

In mass%, C: 0.02% to 0.07%, Si: 0.05% to 0.5%, Mn: 1.10% to 1.60%, P: 0.015% or less S: 0.0020% or less, Nb: 0.010% or more and 0.050% or less, Ti: 0.005% or more and 0.025% or less, sol. Al: 0.005% or more and 0.06% or less, Ca: 0.0005% or more and 0.0060% or less, N: 0.0015% or more and 0.007% or less, and satisfies the following formula (1) In addition, the Nb concentration is 0.060% or less and the Ti concentration is 0.025 in a region consisting of the remaining Fe and impurities and at a distance of 5% of the plate thickness from the central portion in the plate thickness direction toward the plate thickness direction. % Or less steel sheet.
0.1% ≦ Cu + Ni + Cr + Mo ≦ 1.5% (1)
However, the symbol in Formula (1) is content (mass%) of each element.   Furthermore, the steel plate described in Claim 1 containing V: 0.10% or less.   A UOE steel pipe made of the steel sheet according to claim 1 or 2.

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