JP5565420B2 - UOE steel pipe for line pipe - Google Patents

Description

  The present invention relates to a UOE steel pipe, and more particularly to a UOE steel pipe for a line pipe.

Crude oil and natural gas produced in recent years contain wet hydrogen sulfide (H ₂ S). In pipelines that transport excavated crude oil and natural gas, the occurrence of hydrogen induced cracking (hereinafter referred to as HIC) due to hydrogen sulfide is a problem. Therefore, the UOE steel pipe used for the pipeline is required to have excellent HIC resistance.

  Furthermore, recent pipelines are required to improve transportation efficiency. If the operating pressure is increased, the transportation efficiency is improved, but in order to increase the operating pressure, it is necessary to increase the strength of the UOE steel pipe constituting the pipeline. Specifically, a UOE steel pipe having a high strength of API standard X80 grade or higher (tensile strength of 625 MPa or higher) is required.

  Japanese Patent Laid-Open No. 5-271766 proposes a steel plate for pipelines having high strength and excellent HIC resistance. In this document, HIC is considered to be caused by center segregation, and the contents of Mn, P, and S that cause center segregation are reduced. Furthermore, in order to increase the strength, Cr and Mo are contained. This describes that high strength and excellent HIC resistance can be obtained.

JP-A-5-271766

  However, Japanese Patent Application Laid-Open No. 5-271766 is a proposal related to a steel sheet, not a UOE steel pipe, and only mentions improvement of center segregation as a measure for improving HIC resistance. Therefore, even if the center segregation is improved, HIC may occur due to other factors.

  An object of the present invention is to provide a UOE steel pipe for line pipes having excellent HIC resistance even with high strength.

  The UOE steel pipe for line pipes according to the present embodiment is in mass%, C: 0.01 to 0.1%, Si: 0.01 to 0.5%, Mn: 1.0 to 1.7%, P: 0.015% or less, S: 0.002% or less, Cr: 0.01 to 0.45%, Mo: 0.03 to 0.5%, Al: 0.005 to 0.05%, Ca: 0 .0005 to 0.0050%, N: 0.001 to 0.005%, and Ti: 0.005 to 0.03%, with the balance being Fe and impurities and having a bainite single phase structure The hardness of the inner and outer surface layers of the UOE steel pipe is 200 to 255 at HV10, and the hardness of the thickness center part of the UOE steel pipe is 200 to 248 at HV10, and has a tensile strength of 625 MPa or more.

  In this case, the UOE steel pipe for line pipes has excellent HIC resistance even if it has high strength.

  The UOE steel pipe for line pipe according to this embodiment is further replaced with a part of Fe: Cu: 0.5% or less, Ni: 0.5% or less, Nb: 0.05% or less, and V: 0.00. You may contain 1 or more types selected from the group which consists of 10% or less.

FIG. 1 is a distribution diagram of hardness in the radial direction (thickness direction) in the cross section of the UOE steel pipe. FIG. 2 is a cross-sectional view of a UOE steel pipe. FIG. 3 is a diagram showing the relationship between the hardness of the inner and outer surface layers of the steel sheet for UOE steel pipe and the difference value Δ between the hardness of the inner and outer surface layers of the UOE steel pipe and t / D (wall thickness / outer diameter) of the UOE steel pipe. is there.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Hereinafter, “%” of the element content means mass%.

[Outline of UOE steel pipe according to this embodiment]
The present inventors investigated and examined the HIC resistance of high strength UOE steel pipes for line pipes. As a result, the following knowledge was obtained.

  (A) A steel sheet in which center segregation is suppressed has excellent HIC resistance even if it has high strength. However, when a UOE steel pipe is manufactured by performing a pipe manufacturing process such as a C press, a U press, and an O press using such a steel plate, HIC may occur.

  (B) The cause of occurrence of the HIC is an increase in the hardness of the steel sheet in the pipe making process. FIG. 1 is a distribution diagram of hardness in the radial direction (thickness direction) in the cross section of the UOE steel pipe. FIG. 1 was obtained by the following method. Two steel plates having substantially constant hardness in the plate thickness direction and different hardnesses were prepared. The chemical composition of these steel sheets was within the range of the chemical composition of this embodiment described later, and the tensile strength was 625 MPa or more. Two UOE steel pipes (Mark I and Mark II) were manufactured by performing C press, U press and O press on each steel plate. In the cross section of each manufactured UOE steel pipe (cross section perpendicular to the axial direction of the UOE steel pipe), the top of the cross section was assumed to be the origin (0 degree). JIS Z2244 (2009) at a pitch of 0.5 mm in the radial direction (thickness direction) from the outer surface to the inner surface at 90 degrees clockwise position (90 degrees position) and 180 degrees position (180 degrees position). The Vickers hardness test based on the above was carried out. The test force was 10 kgf = 98.07N. Based on the hardness (HV10) obtained, FIG. 1 was completed.

  In FIG. 1, Δ marks are test results of UOE steel pipes with mark I, and ◇ marks are test results of mark II. Referring to FIG. 1, in any UOE steel pipe, the hardness at the central portion in the radial direction (thickness direction) of the steel pipe was substantially the same as the hardness of the steel plate before pipe making. However, the hardness increased from the center of thickness toward the inner and outer surfaces. In the mark I, the maximum hardness of the inner surface layer including the inner surface and the outer surface layer including the outer surface exceeded 255 HV10.

  As a result of performing the HIC test described later on the UOE steel pipes of Mark I and Mark II, HIC was generated in Mark I, and no HIC was generated in Mark II. From FIG. 1 and the HIC test results, the following matters are estimated.

(C) The generation of HIC is first related to the hydrogen concentration (C0) in steel and the allowable hydrogen concentration (Cth) until the steel emits HIC. The inner and outer surface layers have a lower permissible hydrogen concentration Cth than other portions other than the inner and outer surface layers and the thickness center portion. This is because the inner and outer surface layers contain floating inclusions (for example, oxide inclusions such as Al ₂ O ₃ and CaO) generated during continuous casting. This is because Cth (referred to as allowable hydrogen concentration) is lowered.

  (D) On the other hand, the allowable hydrogen concentration Cth at the center of the thickness is lower than the allowable hydrogen concentration Cth of the inner and outer surface layers. This is because carbonitrides are generated and grown by center segregation at the thickness center, and this carbonitride has higher HIC cracking susceptibility than oxide inclusions. Furthermore, since hydrogen tends to aggregate in the central thickness portion due to center segregation, the hydrogen concentration C0 in the central thickness portion is higher than in other portions other than the central thickness portion. That is, from the viewpoint of the hydrogen concentrations C0 and Cth, the HIC cracking susceptibility is highest at the thickness center portion and then at the inner and outer surface layer portions.

  (E) By the way, the hardness of the inner and outer surface layers of the UOE steel pipe is increased by the pipe making process even if there is no variation in the thickness in the thickness direction of the steel sheet as the base material. During pipe production, tensile strain is applied to the outer surface layer (surface layer including the outer surface), and compressive strain is applied to the inner surface layer (surface layer including the inner surface). On the other hand, compared to the inner and outer surface layers, strain is not so much applied to the central wall portion. Therefore, the hardness of the inner and outer surface layers is remarkably increased as compared with the hardness of the thickness center portion. This increase in hardness increases HIC cracking susceptibility.

  (F) From the above, HIC generation in UOE steel pipes takes into account the synergistic effect of increased HIC cracking susceptibility due to changes in hardness due to pipemaking as well as cracking susceptibility related to hydrogen concentration and allowable hydrogen concentration in steel. There is a need to. That is, it is necessary not only to limit the hardness of the thickness center portion having high susceptibility to cracking from the viewpoint of hydrogen concentration, but also to limit the hardness of the inner and outer surface layers that rise during pipe making.

  More specifically, if the upper limit of the hardness at the center of the wall thickness is 248 HV10 and the upper limit of the hardness of the inner and outer surface layers is 255 HV10, the generation of HIC is suppressed even in UOE steel pipes having a strength of API standard X80 grade or higher. The

  Based on the above knowledge, the UOE steel pipe for line pipes according to the present embodiment has been completed. Hereinafter, the details of the UOE steel pipe for line pipe according to the present embodiment will be described.

[Chemical composition]
The UOE steel pipe for line pipes according to this embodiment has the following chemical composition. As above-mentioned,% regarding an element means the mass%.

C: 0.01 to 0.1%
Carbon (C) increases the strength of the steel. However, if the C content is too high, the chemical composition of the steel is included in the peritectic region. Therefore, it becomes difficult to manufacture steel by continuous casting. Therefore, the C content is 0.01 to 0.1%. The lower limit of the preferable C content is higher than 0.01%, more preferably 0.03%, and further preferably 0.04%. The upper limit of the preferable C content is less than 0.1%, more preferably 0.07%, and still more preferably 0.06%.

Si: 0.01 to 0.5%
Silicon (Si) deoxidizes steel. However, if the Si content is too high, the toughness decreases. Therefore, the Si content is 0.01 to 0.5%. The minimum of preferable Si content is higher than 0.01%, More preferably, it is 0.03%, More preferably, it is 0.05%. The upper limit of the preferable Si content is less than 0.5%, more preferably 0.40%, and still more preferably 0.35%.

Mn: 1.0 to 1.7%
Manganese (Mn) increases the strength of the steel. However, if the Mn content is too high, the Mn concentration in the positive segregation zone at the center of the thickness becomes high, and HIC tends to occur in a wet H ₂ S environment. Therefore, the Mn content is 1.0 to 1.7%. The minimum of preferable Mn content is higher than 1.0%, More preferably, it is 1.1%, More preferably, it is 1.2%. The upper limit of the preferable Mn content is less than 1.7%, more preferably 1.6%, and still more preferably 1.5%.

P: 0.015% or less Phosphorus (P) is an impurity. P, like Mn, is easily segregated positively at the center of the thickness and hardens the positive segregation part. Curing of the positive segregation part causes HIC. Therefore, it is preferable that the P content is small. The P content is 0.015% or less. The preferable P content is less than 0.015%, and more preferably 0.010% or less.

S: 0.002% or less Sulfur (S) is an impurity. S forms MnS. MnS is a starting point for generating HIC. Accordingly, the S content is preferably as low as possible. The S content is 0.002% or less. The preferable S content is less than 0.002%, and more preferably 0.001% or less.

Cr: 0.01 to 0.45%
Chromium (Cr) increases the strength and toughness of the steel. However, if the Cr content is too high, weldability is lowered and weld cracks are likely to occur. Therefore, the Cr content is 0.01 to 0.45%. The minimum of preferable Cr content is higher than 0.01%, More preferably, it is 0.05%, More preferably, it is 0.1%. The upper limit of the preferable Cr content is less than 0.45%, more preferably 0.35%, and still more preferably 0.3%.

Mo: 0.03-0.5%
Molybdenum (Mo) increases the hardenability of the steel and increases the strength of the steel. Furthermore, since micro segregation of Mo hardly occurs, generation of HIC due to center segregation is suppressed. However, since Mo is expensive, if Mo is contained excessively, the manufacturing cost increases. Therefore, the Mo content is 0.03 to 0.5%. The minimum of preferable Mo content exceeds 0.03%, More preferably, it is 0.05%, More preferably, it is 0.1%. The upper limit of the preferable Mo content is less than 0.5%, more preferably 0.4%, and still more preferably 0.3%.

Al: 0.005 to 0.05%
Aluminum (Al) deoxidizes steel. However, if the Al content is too high, the cleanliness and toughness of the steel decrease. Therefore, the Al content is 0.005 to 0.05%. The minimum of preferable Al content is higher than 0.005%, More preferably, it is 0.01%, More preferably, it is 0.015%. The upper limit of the preferable Al content is less than 0.05%, more preferably 0.045%, and further preferably 0.04%.

  The Al content in the present embodiment is sol. It means the content of Al (acid-soluble Al).

Ca: 0.0005% to 0.0050%
Calcium (Ca) suppresses the generation of HIC by controlling the form of MnS, which is the starting point of HIC, to be spherical. Ca further forms CaS and suppresses the generation of MnS. On the other hand, if Ca is contained excessively, the effect is saturated and the manufacturing cost increases. Therefore, the Ca content is 0.0005% to 0.0050%. The minimum of preferable Ca content exceeds 0.0005%, More preferably, it is 0.001%, More preferably, it is 0.0015%. The upper limit of the preferable Ca content is less than 0.0050%, more preferably 0.0040%, and still more preferably 0.0030%.

Ti: 0.005% to 0.03%
Titanium (Ti) combines with nitrogen (N) to form TiN. TiN suppresses the coarsening of austenite grains during slab heating and HAZ (welding heat affected zone), and improves the low temperature toughness of the base material and HAZ. However, if the Ti content is too high, TiN is excessively generated and the toughness is lowered. Therefore, the Ti content is 0.005 to 0.03%. The minimum of preferable Ti content exceeds 0.005%, More preferably, it is 0.008%, More preferably, it is 0.01%. The upper limit of the preferable Ti content is less than 0.03%, more preferably 0.025%, and still more preferably 0.02%.

N: 0.001 to 0.005%
Nitrogen (N) combines with the above-mentioned Ti to form TiN and enhances the low temperature toughness of the base material and HAZ. However, if the N content is too high, TiN is excessively formed and the toughness is lowered. Therefore, the N content is 0.001 to 0.005%. The minimum of preferable N content is higher than 0.001%, More preferably, it is 0.0015%, More preferably, it is 0.002%. The upper limit of the preferable N content is less than 0.005%, more preferably 0.0045%, and still more preferably 0.004%.

  The balance of the chemical composition of the UOE steel pipe according to this embodiment consists of Fe and impurities. The impurities here refer to ores and scraps used as raw materials for steel, or elements mixed in from the environment of the manufacturing process.

  The UOE steel pipe according to the present embodiment may further contain one or more selected from the group consisting of Cu, Ni, Nb and V in place of part of Fe. All of these elements are selective elements and increase the strength of the steel.

Cu: 0.5% or less Copper (Cu) is a selective element. Cu increases the hardenability of the steel and increases the strength of the steel. If Cu is contained even a little, the above effect can be obtained. However, if the Cu content is too high, the hot workability of the steel decreases. Further, the surface of the slab is easily broken during continuous casting. Therefore, the Cu content is 0.5% or less. The minimum of preferable Cu content is 0.05%, More preferably, it is higher than 0.05%, More preferably, it is 0.1%. The upper limit of the preferable Cu content is less than 0.5%, more preferably 0.4%, and still more preferably 0.3%.

Ni: 0.5% or less Nickel (Ni) is a selective element. Ni increases the strength of the steel by solid solution strengthening. Ni further increases the toughness of the steel. If Ni is contained even a little, the above effect can be obtained. However, if the Ni content is too high, the weldability of the steel decreases. Therefore, the Ni content is 0.5% or less. The lower limit of the preferred Ni content is 0.05%, more preferably higher than 0.05%, and still more preferably 0.1%. The upper limit of the preferable Ni content is less than 0.5%, more preferably 0.4%, and still more preferably 0.3%.

Nb: 0.05% or less Niobium (Nb) is a selective element. Nb forms carbides in the steel. Nb carbide refines the steel to increase the strength and toughness of the steel. If Nb is contained even a little, the above effect can be obtained. However, if the Nb content is too high, the toughness of the welded portion decreases. Therefore, the Nb content is 0.05% or less. The minimum of preferable Nb content is 0.005%, More preferably, it is higher than 0.005%, More preferably, it is 0.01%. The upper limit of the preferable Nb content is less than 0.05%, more preferably 0.045%, and further preferably 0.04%.

V: 0.10% or less Vanadium (V) is dissolved in steel, or forms carbonitrides in the steel to increase the strength of the steel. If V is contained even a little, the above effect can be obtained. However, if there is too much V content, V carbonitride will coarsen in HAZ and HAZ toughness will fall. Therefore, the V content is 0.10% or less. The minimum of preferable V content is 0.005%, More preferably, it is higher than 0.005%, More preferably, it is 0.01%. The upper limit of the preferable V content is less than 0.10%, more preferably 0.08%, and still more preferably 0.07%.

[Organization]
The structure of the UOE steel pipe according to the present embodiment is a bainite single phase. As used herein, the term "bainite single phase", containing granular Rabe initiative tick ferrite, bainitic ferrite. The structure of the UOE steel pipe may include island martensite (MA), martensite and pearlite, but the total content of MA, martensite and pearlite is 2% or less in terms of area ratio. Hereinafter, MA, martensite, and pearlite are referred to as “hardened structure”.

The tissue is specified by, for example, the following microstructure observation test method. In the cross section of the UOE steel pipe, a portion having a depth of 1/4 of the wall thickness from the outer surface is etched with nital or the like. Observe any 10 to 30 fields (each field 8 to 24 mm ² ) in the etched thickness ¼ part. A 200 × optical microscope is used for observation. By etching, bainite, MA, martensite, and pearlite can be recognized. Therefore, the area ratio of bainite in each field of view and the area ratio of other structures (hardened structures) can be obtained. What averaged the area ratio of the hardening structure | tissue of each visual field is defined as the area ratio (%) of hardening structure | tissue.

  The structure of the UOE steel pipe may further contain inclusions and precipitates. The bainite single phase does not contain polygonal ferrite.

  As described above, the structure of the UOE steel pipe of this embodiment is a bainite single phase, and the area ratio of hardened structures such as MA, martensite, and pearlite is low. Therefore, UOE steel pipe is excellent in HIC resistance even if it has high strength.

[Hardness and tensile strength]
In the UOE steel pipe according to this embodiment, the hardness of the inner and outer surface layers is 200 to 255 at HV10, and the hardness at the center of the wall thickness is 200 to 248 at HV10. The condition that the upper limit of the hardness of the thickness center portion is limited to be lower than the upper limit of the hardness of the inner and outer surface layer portions was found as a result of the inventors' trial and error.

  The hardness of the inner and outer surface layers is measured by the following method. FIG. 2 is a cross-sectional view of the UOE steel pipe (that is, a cross-sectional view cut in the radial direction of the UOE steel pipe). Referring to FIG. 2, UOE steel pipe 100 is divided into 20 equal parts in the thickness t direction. A range from the outer surface 10 of the UOE steel pipe 100 to t / 5 in the direction of the inner surface 20 is defined as an outer surface layer OL. A range from the inner surface 20 to t / 5 in the direction of the outer surface 10 is defined as an inner surface layer IL. A range of t / 10 in the thickness t direction around the center position Pc of the thickness t is defined as the thickness center portion CN.

  The top of the cross section of the UOE steel pipe is assumed to be the origin (0 degree). The outer surface layer OL, the inner surface layer IL, and the thickness center portion CN at 90 degrees clockwise (90 degrees position) and 180 degrees (180 degrees position) are specified. The outer surface layer OL, the inner surface layer IL, and the thickness center portion are all base materials and not welded portions.

  In the specified outer surface layer OL and inner surface layer IL, a Vickers hardness test in accordance with JIS Z2244 (2009) is performed at a pitch of 0.5 mm in the radial direction (thickness t direction). The test force is 10 kgf = 98.07N. When the test force is 10 kgf, the depression formed by the Vickers indenter extends over a plurality of crystal grains.

  Among the plurality of values obtained by the Vickers hardness test in the inner and outer surface layers OL and IL, the maximum value and the minimum value are specified, and these are defined as the hardness (maximum value and minimum value) of the inner and outer surface layers OL. .

  Similarly, in the thickness center portion, the maximum value and the minimum value among the plurality of values obtained by the Vickers hardness test are specified, and these are determined as the hardness (maximum value and minimum value) of the thickness center portion CN. ).

  The maximum value of the hardness of the inner and outer surface layers OL and IL defined by the above method is 255 HV10 or less. And the minimum value of the hardness of inner and outer surface layers OL and IL is 200HV10 or more. Similarly, the maximum value of the hardness at the thickness center is 248 HV10 or less. And the minimum value of the hardness of thickness center part is 200HV10 or more. In short, the hardness of the inner and outer surface layers OL and IL is 200 to 255 HV10, and the hardness at the center of the thickness is 200 to 248 HV10.

  In other words, the upper limit of the allowable hardness is made different between the inner and outer surface layers and the thickness center portion. The reason for this is that the generation of HIC in UOE steel pipes must consider not only the hydrogen concentration in steel and the crack sensitivity associated with the allowable hydrogen concentration, but also the synergistic effect of increased HIC crack sensitivity due to hardness changes due to processing. This is because it is necessary not only to limit the hardness of the center of the wall with high cracking sensitivity from the viewpoint of hydrogen concentration, but also to limit the hardness of the inner and outer surface layers that rise during pipe making.

  In order for the tensile strength TS of the UOE steel pipe to be 625 MPa or more, the hardness of the inner and outer surface layers OL and IL and the hardness of the thickness center portion must be 200 HV10 or more.

[Production method]
An example of the manufacturing method of the UOE steel pipe by this Embodiment is demonstrated. A slab is manufactured by a continuous casting method using molten steel that satisfies the chemical composition described above (continuous casting process). The manufactured slab is rolled to manufacture a steel sheet for UOE steel pipe (rolling process). The manufactured steel plate is piped to produce a UOE steel pipe (pipe making process). Hereinafter, each process will be described in detail.

[Continuous casting process]
Steel is refined to produce molten steel that satisfies the above chemical composition. A slab is manufactured by the continuous casting method using the manufactured molten steel.

  Preferably, the slab is crushed near the final solidification position during continuous casting. Furthermore, it is preferable to select an appropriate water cooling condition and casting speed so as not to cause significant solidification nonuniformity in the casting width direction and casting length direction. In this case, segregation of Mn, P, S, etc. in the thickness center is suppressed. Therefore, the allowable hydrogen concentration Cth at the thickness center can be increased.

[Rolling process]
In the rolling process, the slab manufactured in the continuous casting process is heated in a heating furnace (heating process). The heated slab is rolled into a steel plate with a rolling mill (processing step). The rolled steel sheet is immediately cooled (cooling process). After cooling, annealing is performed as necessary (annealing step). If the rolling process is performed based on the heating process, the processing process, the cooling process, and the annealing process described below, the UOE steel pipe has the above-described structure, hardness, and tensile strength.

[Heating process]
The heating temperature of the slab in the heating furnace is 1000 to 1250 ° C. If the heating temperature is too high, the austenite grains become coarse, so that the crystal grains cannot be refined. Therefore, the HIC resistance is lowered. On the other hand, if the heating temperature is too low, Nb carbonitride produced in the slab during continuous casting cannot be dissolved, and the HIC resistance is lowered. Furthermore, it becomes difficult to obtain crystal grain refinement during rolling and precipitation strengthening after rolling. By setting the heating temperature to 1000 to 1250 ° C., coarsening of austenite grains can be suppressed, and Nb can be dissolved.

Preferably, the heating temperature T (° C.) satisfies the following formula (1).
6770 / (2.26-log (Nb × C)) − 73> T ≧ 6770 / (2.26-log (Nb × C)) − 273 (1)
Here, the content (mass%) of each element is substituted for each element symbol in the formula (1).

  In this case, Nb in steel tends to be dissolved.

  Soaking (soaking) may be performed on the slab. In this case, segregation at the thickness center portion is suppressed. A preferable soaking temperature is 1000 to 1300 ° C., and a soaking time is 20 to 50 hours. In this case, segregation of C, Mn, P, S, etc. is suppressed.

[Processing process]
The surface temperature of the material during rolling is preferably _Ar3 or higher. The _Ar3 point is defined by the following equation (2).
_{A r3 = 910-310 × C-80} × Mn-20 × Cu-15 × Cr-55 × Ni-80 × Mo + 0.35- (t0-8) (2)
Here, the content (mass%) of each element is substituted for each element symbol in the formula (2). Further, the thickness (mm) of the steel sheet after completion of the final rolling is substituted for the symbol t0.

The rolling rate (%) is not particularly limited. Generally, the rolling rate is 50% or more. The rolling rate is defined by the following formula (3).
Rolling ratio = {cross-sectional area of slab (thickness × width)} / {cross-sectional area of steel plate after final rolling (thickness × width)} × 100 (3)

[Cooling process]
After the rolling is completed, the steel sheet is cooled immediately. That is, accelerated cooling is performed. The material surface temperature at the start of cooling is preferably _Ar3 or higher. If the material surface temperature is less than the _Ar3 point, ferrite is generated and carbon discharge is started. Since the hardened structure is easily formed by the carbon discharged from the ferrite phase, the HIC resistance is lowered.

  The cooling rate after rolling is preferably 10 to 40 ° C./sec. Preferably, it is water cooled. If the cooling rate is too slow, diffusion of C and P is promoted. The diffused elements segregate at the grain boundaries or inclusions. On the other hand, if the cooling rate is too high, the steel structure becomes hard to be a bainite single phase because excessive baking occurs and a hardened structure is generated.

  A preferable cooling stop temperature is 350 to 600 ° C. That is, when the surface temperature of the steel sheet reaches 350 to 600 ° C., cooling at the cooling rate is stopped. When cooling immediately after rolling without cooling at the above cooling rate, diffusion of C and P is promoted.

  After cooling at the cooling rate is stopped at the cooling stop temperature, it is preferably left to cool. This is because the toughness is improved by the annealing effect when allowed to cool and the generation of HIC can be suppressed.

[Tensile strength and annealing process of steel sheet after cooling]
It adjusts so that the tensile strength TS0 (MPa) of the steel plate after cooling may satisfy | fill following Formula (4).
TS0 ≦ −1.11 × (t / D × 100) ² + 4.32 × (t / D × 100) +693 (4)
Here, t is the wall thickness (mm) of the UOE steel pipe to be manufactured, and D is the diameter (mm) of the UOE steel pipe to be manufactured.

  FIG. 3 shows a difference value Δ between the hardness of the inner and outer surface layers of the steel sheet and the hardness of the inner and outer surface layers of the UOE steel pipe, and t / D of the UOE steel pipe when the UOE steel pipe is manufactured using the steel plate having the above-described chemical composition. It is a figure which shows the relationship with (%). FIG. 3 was obtained by the following method.

A plurality of steel plates having the chemical composition described above were manufactured by the above manufacturing method. A plurality of UOE steel pipes having different t / D were manufactured by a pipe manufacturing process described later using the manufactured steel sheet. The pipe expansion rate during pipe making was 0.7 to 1.2%. In the steel plate before pipe making and the UOE steel pipe, the maximum value of the hardness of the inner and outer surface layer portions was determined based on the above-mentioned Vickers hardness test. And based on the following formula (5), a difference value Δ (HV10) in hardness was obtained.
Δ = maximum value of hardness of inner and outer surface layers of UOE steel pipe−maximum value of hardness of inner and outer surface layers of steel plate (5)
FIG. 3 was created by using the obtained difference value Δ.

  Referring to FIG. 3, the difference value Δ increases as t / D increases. More specifically, as t / D increases, the hardness of the inner and outer surface layers increases remarkably in a quadratic function, as shown by the curve C0 in FIG. Here, hardness and tensile strength are almost directly proportional. Specifically, as the hardness increases, the tensile strength also increases in a linear function. An expression obtained based on the above knowledge is Expression (4).

  If the chemical composition and microstructure defined in the present invention are satisfied, and the tensile strength TS0 of the steel sheet satisfies the formula (4), the hardness of the inner and outer surface layers of the UOE steel pipe after pipe forming satisfies 200 to 255HV10, And the hardness of thickness center part satisfy | fills 200-248HV10.

When the tensile strength TS0 of the steel plate after the cooling process does not satisfy the formula (4), annealing is performed at less than the _Ac1 point so that the tensile strength TS0 of the steel plate satisfies the formula (4). A preferred annealing temperature is 350 to 600 ° C., and a preferred annealing time is 30 to 90 minutes. In addition, if the tensile strength TS0 of the steel plate after cooling satisfies the formula (4), the annealing process may not be performed.

[Pipe making process]
The manufactured steel sheet is formed into a open pipe by C press, U press, O press or the like. Subsequently, a welded steel pipe is formed on both end faces of the open pipe in the longitudinal direction by a welding method such as a submerged arc welding method. Pipe expansion is performed on the manufactured welded steel pipe to manufacture a UOE steel pipe.

  Slabs were produced by continuously casting molten steels of steel types A to G shown in Table 1.

  The chemical compositions of steel types A to D, F and G in Table 1 were within the range of the chemical composition of the UOE steel pipe of the present invention. On the other hand, the Mn content, P content and S content of the steel type E exceeded the upper limits of the Mn content, P content and S content of the present invention.

  Using a plurality of slabs of steel types A to G, UOE steel pipes with marks 1 to 18 shown in Table 2 were manufactured.

  The slab of each mark was heated in a heating furnace. The heating temperature (° C.) was as shown in Table 2. The slab after heating was rolled to produce a steel plate. Each steel plate was water cooled immediately after rolling. The cooling rate by water cooling was in the range of 10 to 40 ° C./sec. The cooling start temperature (° C.) of the steel plate of each mark was as shown in Table 2. Furthermore, water cooling was stopped at the cooling stop temperature (° C.) shown in Table 2. The steel plate after the water cooling stop was allowed to cool.

Tensile test pieces were collected from a part of the steel sheet after being allowed to cool, and subjected to a tensile test at room temperature (25 ° C.) to obtain a tensile strength TS0 (MPa). Furthermore, F1 value was calculated | required based on following Formula (6).
F1 = −1.11 × (t / D × 100) ² + 4.32 × (t / D × 100) +693 (6)
In short, the F1 value is the right side of Equation (4).

  In the marks 8 to 10 and 13 to 15, the yield strength TS0 of the steel plate after cooling exceeded the F1 value. Therefore, annealing was performed on the steel plates marked 8-11. The annealing temperature was in the range of 350-600 ° C., and the annealing time was 30-90 minutes. The tensile strength TS0 (MPa) and F1 value of the marks 8 to 10 in Table 2 are values after annealing. Annealing was not performed on the steel plates of marks other than marks 8 to 10 and 17.

  In addition, in the mark 8 and the mark 13, manufacturing conditions and steel types other than the presence or absence of annealing were the same. Similarly, marks 9 and 14 and marks 10 and 15 have the same manufacturing conditions and steel types other than the presence or absence of annealing. As described above, the marks 13 to 15 were not annealed.

  After manufacturing a steel plate by the above process, the steel plate of each mark was piped by the above-mentioned method, and the UOE steel pipe was manufactured. The wall thickness t (mm) and outer diameter D (mm) of the manufactured UOE steel pipe were measured. And the tensile test piece was extract | collected from the UOE steel pipe, the tensile test was implemented in normal temperature (25 degreeC), and the yield strength YS (MPa) and tensile strength TS (MPa) were calculated | required.

[Microstructure observation]
Based on the microstructural observation test method described above, the structure of each mark was observed. When the area ratio of the hardened structure was 2% or less, the mark structure was judged to be a bainite single phase. Table 2 shows the structure observation results. “B” in the “Structure” column of Table 2 means that the structure of the corresponding mark is a bainite single phase.

[Vickers hardness test]
Furthermore, based on the above-mentioned Vickers hardness test method, the maximum and minimum values (HV10) of the hardness of the inner and outer surface layers of the UOE steel pipe of each mark, and the maximum and minimum values (HV10) of the hardness at the center of the wall thickness. )

[HIC test]
Test pieces (thickness 10 mm, width 20 mm, length 100 mm) were collected from the UOE steel pipe of each mark. An HIC test was performed using the collected test pieces. The HIC test was subjected to the NACE test specified by NACE TM-02-84, and whether or not HIC occurred in each test piece after the test was judged by an ultrasonic flaw detection method.

[Test results]
Table 2 shows the test results. In the “min” column of “inside / outside surface layer” of “Vickers hardness” in Table 2, the minimum value (HV10) of the hardness of the inside and outside surface layers of the UOE steel pipe of the corresponding mark is described. In the “max” column, the maximum hardness value (HV10) of the inner and outer surface layers of the UOE steel pipe is described. Similarly, in the “min” column of “wall thickness center” of “Vickers hardness”, the minimum hardness value (HV10) of the wall thickness center of UOE steel pipe is described, and in “max” column Describes the maximum value of the hardness at the center of the wall thickness.

  Furthermore, “no crack” in the “HIC test” column in Table 2 means that no HIC was observed in the UOE steel pipe of the corresponding mark. “With crack” means that HIC was observed.

  Referring to Table 2, the chemical composition of marks 1-12 was within the scope of the present invention. Further, the tensile strength was 625 MPa or more, and the strength was API standard X80 grade or more. Furthermore, the maximum value and the minimum value of the hardness of the inner and outer surface layers of the UOE steel pipe were both within the range of the present invention (200 to 255). The maximum value and the minimum value of the hardness at the center of the wall thickness were also within the range of the present invention (200 to 248). Therefore, HIC did not occur in the UOE steel pipes with marks 1 to 12.

  On the other hand, in the marks 13 to 15, the chemical composition was within the range of the present invention, but the tensile strength TS0 of the steel sheet was higher than F1. Therefore, the maximum value of the hardness of the inner and outer surface layers of the UOE steel pipe exceeded the upper limit (255) of the hardness of the inner and outer surface layers of the present invention. Therefore, HIC was observed in the HIC test.

  The Mn content, P content and S content of the marks 16 and 17 all exceeded the upper limit of the present invention. Therefore, the maximum value of the hardness of the thickness center portion of the UOE steel pipe exceeded the upper limit (248) of the hardness of the thickness center portion of the present invention. Therefore, HIC was observed. Since Mn, P, and S are excessively segregated in the thickness center portion, it is considered that the hardness of the thickness center portion is excessively increased.

  In the mark 18, although the chemical composition was within the range of the present invention, an appropriate reduction was not applied to the slab in the continuous casting, and the control for suppressing the center segregation was not sufficient. Therefore, the maximum value of the hardness at the central portion of the UOE steel pipe exceeded the upper limit (248) of the present invention, and HIC was observed.

  While the embodiments of the present invention have been described above, the above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment without departing from the spirit thereof.

Claims (2)

UOE steel pipe for line pipe,
% By mass
C: 0.01 to 0.1%
Si: 0.01 to 0.5%,
Mn: 1.0-1.7%,
P: 0.015% or less,
S: 0.002% or less,
Cr: 0.01 to 0.45%,
Mo: 0.03-0.5%,
Al: 0.005 to 0.05%,
Ca: 0.0005 to 0.0050%,
N: 0.001 to 0.005% and
Ti: 0.005 to 0.03% is contained, the balance consists of Fe and impurities,
It has a bainite single phase structure,
An outer surface layer in the range from the outer surface of the UOE steel pipe having a wall thickness t to t / 5 in the inner surface direction and having a hardness of 200 to 255 in HV10;
An inner surface layer in the range from the inner surface to the outer surface direction of the UOE steel pipe up to t / 5 and having a hardness of 200 to 255 in HV10;
A center of thickness of the UOE steel pipe with a thickness t in the direction of the thickness t in the direction of the thickness t and having a hardness of 200 to 248 at HV10.
A UOE steel pipe for line pipes having a tensile strength of 625 MPa or more. The UOE steel pipe according to claim 1, further comprising:
Instead of part of Fe,
Cu: 0.5% or less,
Ni: 0.5% or less,
Nb: 0.05% or less, and
V: A UOE steel pipe containing one or more selected from the group consisting of 0.10% or less.

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