JP2020066746A - Steel material for linepipe - Google Patents

Abstract

To provide a steel material for linepipe having excellent low-temperature toughness.SOLUTION: A steel material for linepipe contains, by mass%, C: more than 0.060% to 0.120%, Si: 0.05-0.30%, Mn: 0.50-2.00%, P: 0-0.030%, S: 0-0.0100%, Al: 0.010-0.035%, N: 0.0010-0.0080%, Nb: 0.010-0.080%, Ti: 0.005-0.030%, Ni: 0.001-0.50%, Mo: 0.05-0.30%, O: 0-0.0030%, and the balance being Fe and impurities, and satisfies Formula (1). A ferrite fraction is 60-90%, an effective crystal grain diameter is 15 μm or less, and a coarse crystal grain ratio is 20% or less. When a plane vertical to a rolling direction is RD surface, a rolling surface is ND surface, and a surface vertical to the RD surface and the ND surface is TD surface, in a specific plane with an angle formed by the RD surface and the TD surface of 45°, a degree of integration of a {100} plane is 1.50-2.50.SELECTED DRAWING: Figure 1

Description

  The present invention relates to steel products, and more particularly to steel products for line pipes.

  The pipeline laid on the seabed is composed of a plurality of line pipes. There is a reeling method as a method of laying a line pipe on the seabed. In the reeling method, a long line pipe in which a plurality of ERW steel pipes for line pipes are connected is preliminarily manufactured on land and wound on a spool of a reel barge ship. The wound line pipe is laid on the seabed while being unwound from the spool at sea.

  In the reeling method described above, tensile stress and compression stress due to bending and unbending act on the line pipe. In this case, if the deformability of the electric resistance welded steel pipe for a line pipe is low, local buckling may occur. Since local buckling can be a base point of breakage of the line pipe, it is preferable to suppress occurrence of local buckling. Therefore, a high deformability is required for the electric resistance welded steel pipe for a line pipe used for a seabed pipeline.

  If the difference between the yield strength and the tensile strength of the electric resistance welded steel pipe for line pipe is large, the deformability is high. Therefore, the electric resistance welded steel pipe for a line pipe used for a seabed pipeline is required to have a low yield ratio YR which is a ratio of the yield strength YS to the tensile strength TS.

  The pipeline laid on the seabed is further subjected to pressure from the high pressure fluid passing through it. The pipeline is further subjected to repeated strain due to waves and seawater pressure from the outside. Therefore, not only a low yield ratio YR but also high strength and high low temperature toughness are required for the electric resistance welded steel pipe for a line pipe used for a seabed pipeline.

  International Publication No. 2012/002481 (Patent Document 1) proposes a manufacturing method for enhancing the low temperature toughness of a hot rolled steel sheet for line pipes.

  The hot-rolled steel sheet for line pipes disclosed in Patent Document 1 is, in mass%, C = 0.02 to 0.08%, Si = 0.05 to 0.5%, Mn = 1 to 2%, Nb. = 0.03 to 0.12%, Ti = 0.005 to 0.05%, and the balance is Fe and inevitable impurity elements. In the microstructure at a depth of 1/2 of the plate thickness from the surface of the steel sheet, the proeutectoid ferrite fraction is 3% or more and 20% or less, and the other is low-temperature transformation phase and pearlite of 1% or less. The average grain size is 1 μm or more and 2.5 μm or less, the area average grain size is 3 μm or more and 9 μm or less, the standard deviation of the area average grain size is 0.8 μm or more and 2.3 μm or less, and At a depth of 1/2 thickness, the reflected X-ray intensity ratio {211} / {111} in the {211} direction and the {111} direction with respect to the plane parallel to the steel plate surface is 1.1 or more. Patent Document 1 describes that this steel sheet for line pipes has excellent strength and low temperature toughness by controlling the pro-eutectoid ferrite fraction in the central portion of the thickness, the average grain size, and the texture. Has been done.

International Publication No. 2012/002481

  However, the hot-rolled steel sheet disclosed in Patent Document 1 has a high heating temperature before the rolling step, and the austenite grains may become coarse. In this case, the crystal grains are coarsened and the low temperature toughness may be reduced. Further, Patent Document 1 is for studying suppression of the occurrence of separation, which is peeling parallel to the plate surface, and is not for cracking in the rolling direction.

  An object of the present invention is to provide a steel material for a line pipe having a low yield ratio and excellent low temperature toughness.

The steel material for line pipes according to the present embodiment is, in mass%, C: more than 0.060 to 0.120%, Si: 0.05 to 0.30%, Mn: 0.50 to 2.00%, P: 0-0.030%, S: 0-0.0100%, Al: 0.010-0.035%, N: 0.0010-0.0080%, Nb: 0.010-0.080%, Ti : 0.005 to 0.030%, Ni: 0.001 to 0.50%, Mo: 0.05 to 0.30%, O: 0 to 0.0030%, Ca: 0 to 0.0050%, V: 0 to 0.100%, Cr: 0 to 0.30%, Cu: 0 to 0.30%, Mg: 0 to 0.0050%, and a rare earth element: 0 to 0.0100% The balance consists of Fe and impurities, and has a chemical composition that satisfies the formula (1). In the structure in the central portion of the thickness, the ferrite fraction is 60 to 90%, the effective crystal grain size is 15 μm or less, and the coarse crystal grain ratio, which is the area ratio of the crystal grains having the crystal grain size of 20 μm or more, is 20%. It is the following. When the surface perpendicular to the rolling direction is the RD surface, the rolling surface is the ND surface, and the surface perpendicular to the RD surface and the ND surface is the TD surface, the angle with the RD surface is 45 ° and In the specific plane whose angle is 45 °, the degree of integration of the {100} plane is 1.50 to 2.50.
0.350 ≦ C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 3 + Nb / 3 ≦ 0.400 (1)
Here, the content (mass%) of the corresponding element is substituted into each element symbol of the formula (1).

  The steel material for a line pipe is, for example, a hot rolled steel plate for a line pipe or an electric resistance welded steel pipe for a line pipe.

  In the present embodiment, the effective crystal grain size refers to a position where the orientation difference between adjacent measurement points exceeds 15 ° in the EBSP-OIM (trademark) (Electron Back Scatter Diffraction Pattern-Orientation Image Microscopy) method. , The area average grain size calculated from the grain size and the surface area obtained as crystal grains in the region surrounded by the grain boundaries.

  The steel product for a line pipe according to the present invention has a low yield ratio and excellent low temperature toughness.

FIG. 1 shows that in a steel product for a line pipe, when the surface perpendicular to the rolling direction is the RD surface, the rolling surface is the ND surface, and the surface perpendicular to the RD surface and the ND surface is the TD surface, the angle formed by the RD surface is 45 degrees. FIG. 3 is a perspective view showing a surface (hereinafter, also referred to as a specific surface) which is at an angle of 45 ° with the TD surface. Note that ND indicates the thickness direction. FIG. 2 is a diagram showing the relationship between the accumulation degree of the {100} plane ({100} accumulation degree) in the specific plane and the ductile fracture surface rate (%) when the DWTT test is performed at −30 ° C. FIG. 3 is a schematic diagram of a continuous cooling transformation curve of a steel material for a line pipe according to this embodiment. FIG. 4 is a flow chart showing a manufacturing process of the steel material for a line pipe according to the present embodiment. FIG. 5 is a plan view of the tensile test piece used for the tensile test. In addition, the numerical value in a figure shows a dimension (unit: mm). FIG. 6 is a front view and a side view of the DWTT test piece used in the DWTT test. In addition, the numerical values and t in the figure indicate dimensions (unit: mm).

  The present inventors investigated and examined the yield ratio YR and the low temperature toughness of the steel material for line pipes, and obtained the following findings.

  In order to reduce the yield ratio YR of the steel for line pipes, the carbon (C) content should be increased. If the C content is increased, the hardness of the steel can be increased and the deformation can be promoted uniformly. Therefore, YR of the steel material for line pipes is reduced.

  However, if the C content is increased, the low temperature toughness of the steel material for line pipes is reduced. Then, the present inventors next investigated the low temperature toughness of the steel material for line pipes with increased C content, and obtained the following findings.

  Since the line pipe is filled with a gas or liquid such as natural gas or crude oil at a high pressure, and the maximum stress acts in the circumferential direction of the steel pipe, the crack easily propagates parallel to the steel pipe axis. In ERW steel pipes for line pipes, the pipe axis direction is usually the rolling direction (hereinafter, also referred to as RD direction) during hot rolling of the hot-rolled steel sheet for line pipes, and therefore cracks easily propagate in the RD direction. Is.

  When the steel material for a line pipe is an electric resistance welded steel pipe for a line pipe, when manufacturing the electric resistance welded steel pipe for a line pipe, the plate width direction of the hot rolled steel sheet for a line pipe (hereinafter, also referred to as TD direction) is The ends of the steel material for line pipes are welded to each other so as to be in the circumferential direction of the sewing steel pipe. That is, in the electric resistance welded steel pipe for a line pipe, the pipe axis direction is usually the RD direction. As described above, in the electric resistance welded steel pipe for a line pipe, the crack easily propagates in the RD direction, that is, the pipe axis direction. If crack propagation occurs in the pipe axis direction, the crack area of the line pipe becomes large.

  Therefore, the present inventors have studied a method of suppressing the propagation of cracks in the RD direction corresponding to the pipe axis direction of the electric resistance welded steel pipe in order to improve the low temperature toughness of the steel material for line pipes.

  The present inventors first focused on the texture of steel.

  Carbon steels such as steels for line pipes show a body-centered cubic structure, and in the texture of steel, the {100} plane is a cleavage plane and is {100} compared to {110} planes or {111} planes. The plane is a crystal plane that is easily peeled off when a crack propagates parallel to the {100} plane.

  Therefore, the present inventors considered that cracks are likely to propagate when the normal line of the {100} plane is perpendicular to the RD direction. Therefore, the present inventors considered that the propagation of cracks can be suppressed by inclining the normal line of the {100} plane by 45 ° from the direction perpendicular to the RD direction. This is because the normal line of the {100} plane can be tilted most from the direction perpendicular to the RD direction. More specifically, it is as follows.

  FIG. 1 is a schematic diagram of a minute region in the central portion of the thickness of a steel material for line pipes. When the steel material for line pipes is a steel plate, the illustrated minute region is in the central part of the plate thickness. When the steel material for line pipes is a steel pipe, the plate thickness center part corresponds to the wall thickness center part. That is, the illustrated minute region is in the central portion of the wall thickness. In FIG. 1, a surface perpendicular to the rolling direction is an RD surface, a rolling surface is an ND surface, and a surface perpendicular to the RD surface and the ND surface is a TD surface. A surface having an angle of 45 ° with the RD surface and having an angle of 45 ° with the TD surface is defined as a specific surface. The ND direction indicates the thickness direction (plate thickness direction or wall thickness direction).

  The circumferential direction of the steel pipe is curved, unlike the width direction of the steel plate. However, in the above-mentioned minute area, the circumferential direction is substantially a straight line, and substantially coincides with the straight line TD direction. Therefore, the specific surface shown in FIG. 1 is similarly shown in the electric resistance welded steel pipe for a line pipe.

  Referring to FIG. 1, when the surface perpendicular to the rolling direction is the RD surface, the rolling surface is the ND surface, and the surface perpendicular to the RD surface and the ND surface is the TD surface, the angle formed with the RD surface is 45 °. In addition, the inventors of the present application have considered that if {100} planes are accumulated on a specific plane that forms an angle of 45 ° with the TD plane, peeling on the plane where cracks are most likely to propagate is suppressed. .

  Based on the above idea, the inventors of the present invention have accumulated the {100} plane in a specific plane (hereinafter, referred to as {100} integration degree) and the ductile fracture surface rate when a DWTT test is performed at -30 ° C. We investigated the relationship with (%).

  FIG. 2 is a diagram showing the relationship between the {100} integration degree and the ductile fracture surface ratio (%) when the DWTT test is performed at −30 ° C. FIG. 2 was obtained by the following method.

  ERW steel pipes for line pipes were manufactured using the slabs having the composition of steel A shown in Table 1 in the examples described later.

Specifically, the slab was heated in a heating furnace to a temperature of 1060 to 1200 ° C. Rough rolling was performed on the heated slab. The temperature T ₀ on the delivery side of the final stand of rough rolling was 900 to 985 ° C., and the time t ₀ immediately after the completion of rough rolling to the start of finish rolling was 50 to 230 seconds. After rough rolling, finish rolling was performed at 760 to 800 ° C to manufacture a steel sheet. ROT (runout table) cooling was performed on the steel sheet after finish rolling.

  The hot rolled steel sheet for line pipes was manufactured by the above manufacturing process. The obtained hot-rolled steel sheet for line pipe was wound to obtain a hot-rolled steel sheet for line pipe in the form of hot coil.

  The hot-rolled steel sheet for a line pipe thus obtained was used to produce a pipe by the above-described method to produce an electric resistance welded steel pipe for a line pipe having an outer diameter of 304.8 to 660.4 mm and a wall thickness of 12 to 25 mm.

  Based on the method described below, the {100} integration degree was measured using an EBSD (Electron Back Scatter Diffraction Patterns) method. The measurement conditions in the EBSD method were magnification: 400 times, visual field area: 200 μm × 500 μm, and measurement step: 0.3 μm.

  A low temperature toughness test was conducted on the obtained ERW steel pipe for line pipe. More specifically, a DWTT test was carried out at −30 ° C. in accordance with the API standard 5L3 to determine the ductile fracture surface ratio.

  Referring to FIG. 2, the ductile fracture surface ratio increases as the {100} integration degree increases. When the {100} integration degree exceeds 1.50, the ductile fracture surface ratio becomes 85% or more. When the {100} integration degree exceeds 1.50, the ductile fracture surface ratio does not change so much even if the {100} integration degree becomes higher. That is, the ductile fracture surface ratio with respect to the {100} integration degree has an inflection point near the {100} integration degree of 1.50. As described above, the present invention shows that an electric resistance welded steel pipe for a line pipe having a ductile fracture surface ratio of 85% or more and an excellent low temperature toughness is obtained with an inflection point near the {100} integration degree of 1.50 as a boundary. First discovered.

  The present inventors further investigated and examined the low temperature toughness of the steel material for line pipes, and obtained the following findings.

  (A) Usually, the strength (or hardness) and toughness of steel are in conflict with each other. Ferrite has a soft structure. Therefore, if the structure of the steel is mainly ferrite, the low temperature toughness of the steel is enhanced.

  If the ferrite grains are fine, the low temperature toughness of the steel is further enhanced. In order to make the ferrite grains fine, the heating temperature during rolling is set to 1200 ° C. or lower to suppress the coarsening of the crystal grains. Further, cooling is controlled so that a large number of nucleation sites are generated in the unrecrystallized structure after rolling and a large number of new ferrite grains are generated. In this case, the final ferrite grains become fine, and as a result, the low temperature toughness of the steel increases.

  On the other hand, if the structure of the steel is mainly bainite, laths (elongated structures) are generated in the crystal grains that inherit the old austenite grains as they are, but their orientations are aligned in each block, and each block is substantially uniform. It becomes one crystal grain. Therefore, the size of crystal grains in bainite is determined by the size of prior austenite grains. Therefore, the crystal grains are likely to be coarsened, and as a result, the low temperature toughness of the steel is likely to decrease. Therefore, in the steel material for line pipes of the present embodiment, the structure is mainly ferrite.

  (B) C, Si, Mn, Ni, Cr, Mo, V, and Nb are all S curves (ferrite region, pearlite) of a continuous cooling transformation diagram (Continuous Cooling Transformation Diagram: CCT diagram) of steel for line pipes. Region and bainite region).

  It is defined as F1 = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 3 + Nb / 3. If F1 is too low, the S-curve of the CCT diagram shifts too far to the left. In this case, the low temperature toughness of steel decreases. The reason for this is as follows.

  The magnitude of the driving force at the time of transformation from austenite to ferrite (driving force of phase transformation) correlates with the steel material temperature. When the steel material temperature is high, the driving force for phase transformation is small. Therefore, ferrite transformation nuclei are not easily generated. Furthermore, since the steel material temperature is high, the growth of ferrite grains is fast. As a result, the ferrite grains become coarse. On the other hand, when the steel material temperature is low, the driving force for phase transformation is large. Therefore, ferrite transformation nuclei are likely to be generated. Further, since the steel material temperature is low, the growth of ferrite grains is slow. As a result, the ferrite grains become finer.

  When the S curve of the CCT diagram shifts too much to the left, it enters the ferrite region with the steel material temperature high. Therefore, as described above, the ferrite grains become coarse and the effective crystal grain size becomes large. Furthermore, since a mixed grain structure is likely to occur, the coarse crystal grain ratio increases. In this case, the low temperature toughness of steel decreases. If F1 is too low, the hardenability further deteriorates and sufficient strength cannot be obtained.

  On the other hand, if F1 is too high, the S curve will shift too far to the right. In this case, the cooling curve is less likely to be applied to the ferrite nose. As a result, the amount of hard structures such as bainite and martensite produced increases, and the ferrite fraction in the structures decreases. As a result, the low temperature toughness of steel decreases.

  FIG. 3 is an example of a continuous cooling transformation diagram (Continuous Cooling Transformation Diagram: CCT diagram) of the steel material for a line pipe according to the present embodiment. In FIG. 3, F is a ferrite nose, P is a pearlite nose, and B is a bainite nose. When F1 is 0.350 to 0.400, the S curve (ferrite, pearlite, bainite) of each phase is arranged at an appropriate position in the CCT diagram. In this case, the cooling can be performed mainly through the ferrite region as shown by the cooling curve C1 in FIG. Therefore, a structure mainly composed of ferrite can be generated, and high strength and low temperature toughness can be obtained.

The steel material for a line pipe according to the present embodiment completed based on the above findings is, in mass%, C: more than 0.060 to 0.120%, Si: 0.05 to 0.30%, Mn: 0.50. .About.2.00%, P: 0 to 0.030%, S: 0 to 0.0100%, Al: 0.010 to 0.035%, N: 0.0010 to 0.0080%, Nb: 0. 010 to 0.080%, Ti: 0.005 to 0.030%, Ni: 0.001 to 0.50%, Mo: 0.05 to 0.30%, O: 0 to 0.0030%, Ca : 0 to 0.0050%, V: 0 to 0.100%, Cr: 0 to 0.30%, Cu: 0 to 0.30%, Mg: 0 to 0.0050%, and rare earth element: 0 .About.0.0100%, the balance consisting of Fe and impurities, and having a chemical composition satisfying the formula (1). In the structure in the central portion of the thickness, the ferrite fraction is 60 to 90%, the effective crystal grain size is 15 μm or less, and the coarse crystal grain ratio, which is the area ratio of the crystal grains having the crystal grain size of 20 μm or more, is 20%. It is the following. When the surface perpendicular to the rolling direction is the RD surface, the rolling surface is the ND surface, and the surface perpendicular to the RD surface and the ND surface is the TD surface, the angle with the RD surface is 45 ° and In the specific plane whose angle is 45 °, the degree of integration of the {100} plane is 1.50 to 2.50.
0.350 ≦ C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 3 + Nb / 3 ≦ 0.400 (1)
Here, the content (mass%) of the corresponding element is substituted into each element symbol of the formula (1).

  The above chemical composition may contain Ca: more than 0 and 0.0050%. The chemical composition is one or more selected from the group consisting of V: more than 0 to 0.100%, Cr: more than 0 to 0.30%, and Cu: more than 0 to 0.30%. May be included. The above chemical composition may contain one or more selected from the group consisting of Mg: more than 0 to 0.0050% and rare earth element: more than 0 to 0.0100%.

  The steel material for line pipes is, for example, a hot-rolled steel sheet for line pipes and an electric resistance welded steel pipe for line pipes.

  The above-mentioned electric resistance welded steel pipe for line pipe has a yield strength in the pipe axis direction of 450 to 540 MPa, a tensile strength in the pipe axis direction of 510 to 625 MPa, and a yield ratio which is a ratio of the yield strength to the tensile strength of 0.93. It is the following.

  The electric resistance welded steel pipe for a line pipe may have a wall thickness of 12 to 25 mm and an outer diameter of 304.8 to 660.4 mm.

  The steel material for line pipes of the present embodiment is, for example, a hot-rolled steel sheet for line pipes and an electric resistance welded steel pipe for line pipes. Here, the hot-rolled steel sheet for a line pipe is a hot-rolled steel sheet used for manufacturing an electric resistance welded steel pipe for a line pipe, and corresponds to a material for the electric resistance welded steel pipe for a line pipe.

  In addition, in this specification, "%" regarding an element means the mass%, unless there is particular notice.

[Chemical composition]
The steel material for line pipes of the present embodiment is a hot-rolled steel sheet for line pipes or an electric resistance welded steel pipe for line pipes. The chemical composition of the steel material for line pipes contains the following elements.

C: more than 0.060 to 0.120%
Carbon (C) increases the strength of steel. If the C content is increased, the yield ratio YR can be reduced. As a result, the deformability of steel increases. If the C content is too low, these effects cannot be obtained. On the other hand, if the C content is too high, carbides are formed and the low temperature toughness and ductility of the steel deteriorate. If the C content is too high, the weldability is further reduced. Therefore, the C content is more than 0.060 to 0.120%. The preferable lower limit of the C content is 0.070%, and more preferably 0.080%. The preferable upper limit of the C content is 0.110%, more preferably 0.100%.

Si: 0.05 to 0.30%
Silicon (Si) deoxidizes steel. If the Si content is too low, this effect cannot be obtained. On the other hand, if the Si content is too high, the low temperature toughness of the steel decreases. Therefore, the Si content is 0.05 to 0.30%. The preferable lower limit of the Si content is 0.10%, more preferably 0.15%. The upper limit of the Si content is preferably 0.25%, more preferably 0.21%.

Mn: 0.50 to 2.00%
Manganese (Mn) enhances the hardenability of steel and enhances the strength of steel. If the Mn content is too low, this effect cannot be obtained. On the other hand, if the Mn content is too high, the strength of the steel becomes too high and the low temperature toughness of the steel decreases. Therefore, the Mn content is 0.50 to 2.00%. The preferable lower limit of the Mn content is 0.80%, more preferably 1.00%. The preferable upper limit of the Mn content is 1.80%, more preferably 1.50%.

P: 0 to 0.030%
Phosphorus (P) is an impurity. P reduces the low temperature toughness of steel. Therefore, the P content is preferably as low as possible. Specifically, the P content is 0 to 0.030%. The preferable upper limit of the P content is 0.020%, more preferably 0.015%. On the other hand, the P content may be 0%. However, from the viewpoint of reducing the dephosphorization cost, the P content may be more than 0%, may be 0.001% or more, and may be 0.005% or more.

S: 0 to 0.0100%
Sulfur (S) is an impurity. S combines with Mn to form Mn-based sulfide. Therefore, the low temperature toughness of steel falls. Therefore, the S content is preferably as low as possible. Specifically, the S content is 0 to 0.0100%. The preferable upper limit of the S content is 0.0080%, and more preferably 0.0050%. On the other hand, the S content may be 0%. However, from the viewpoint of reducing the desulfurization cost, the S content may be more than 0%, may be 0.0001% or more, may be 0.0010% or more, and may be 0.0020% or more. It may be.

Al: 0.010 to 0.035%
Aluminum (Al) deoxidizes steel. If the Al content is too low, this effect cannot be obtained. On the other hand, if the Al content is too high, the Al nitrides become coarse and the low temperature toughness of the steel decreases. Therefore, the Al content is 0.010 to 0.035%. The preferable lower limit of the Al content is 0.015%, more preferably 0.020%. The preferable upper limit of the Al content is 0.030%. In the present specification, the Al content means the total Al content in steel.

N: 0.0010 to 0.0080%
Nitrogen (N) forms a nitride and suppresses coarsening of austenite grains during the heating process. In this case, the austenite grains become finer in the rolling step, and the crystal grains after transformation become finer. As a result, the low temperature toughness of steel increases. N further enhances the strength of steel by solid solution strengthening. If the N content is too low, these effects cannot be obtained. On the other hand, if the N content is too high, the carbonitrides are coarsened and the low temperature toughness of the steel is reduced. Therefore, the N content is 0.0010 to 0.0080%. The preferable lower limit of the N content is 0.0020%, and more preferably 0.0025%. The preferable upper limit of the N content is 0.0060%, and more preferably 0.0050%.

Nb: 0.010 to 0.080%
Niobium (Nb) combines with C and N in steel to form fine Nb carbonitrides. The Nb carbonitride suppresses the coarsening of crystal grains and reduces the effective crystal grain size. Therefore, the low temperature toughness of steel is improved. Furthermore, fine Nb carbonitrides enhance the strength of the steel by dispersion strengthening. If the Nb content is too low, these effects cannot be obtained. On the other hand, if the Nb content is too high, the Nb carbonitride becomes coarse and the low temperature toughness of the steel decreases. Therefore, the Nb content is 0.010 to 0.080%. The preferable lower limit of the Nb content is 0.015%. The preferable upper limit of the Nb content is 0.040%, more preferably 0.030%.

Ti: 0.005-0.030%
Titanium (Ti) combines with N in the steel to form TiN, and suppresses the deterioration of the low temperature toughness of the steel due to the solid solution N. Further, fine TiN is dispersed and precipitated, thereby suppressing coarsening of crystal grains. This enhances the low temperature toughness of the steel. If the Ti content is too low, these effects cannot be obtained. On the other hand, if the Ti content is too high, TiN becomes coarse or coarse TiC is generated. In this case, the low temperature toughness of steel decreases. Therefore, the Ti content is 0.005 to 0.030%. The preferable lower limit of the Ti content is 0.007%, more preferably 0.010%. The preferable upper limit of the Ti content is 0.020%, more preferably 0.017%.

Ni: 0.001 to 0.50%
Nickel (Ni) enhances the hardenability of steel and enhances the strength of steel. If the Ni content is too low, this effect cannot be obtained. On the other hand, if the Ni content is too high, this effect is saturated. Therefore, the Ni content is 0.001 to 0.50%. The preferable lower limit of the Ni content is 0.05%, more preferably 0.07%. The preferable upper limit of the Ni content is 0.15%.

Mo: 0.05-0.30%
Molybdenum (Mo) enhances the hardenability of steel and enhances the strength of steel. Mo further refines the austenite grains and enhances the low temperature toughness of the steel. If the Mo content is too low, these effects cannot be obtained. On the other hand, if the Mo content is too high, the on-site weldability of steel deteriorates. Therefore, the Mo content is 0.05 to 0.30%. The preferable lower limit of the Mo content is 0.15%. The preferable upper limit of the Mo content is 0.19%, more preferably 0.18%.

O: 0 to 0.0030%
Oxygen (O) is an impurity. O forms an oxide and reduces the resistance to hydrogen-induced cracking (HIC resistance) of steel. O further reduces the low temperature toughness of the steel. Therefore, the O content is preferably as low as possible. Specifically, the O content is 0 to 0.0030%. The preferable upper limit of the O content is 0.0025%. On the other hand, the O content may be 0%. However, from the viewpoint of reducing the deoxidation cost, the O content may be more than 0%, may be 0.0001% or more, may be 0.0010% or more, and may be 0.0015% or more. Or may be 0.0020% or more.

  The balance of the chemical composition of the steel product for a line pipe according to the present embodiment is Fe and impurities. Here, the impurities are those that are mixed from ore as a raw material, scrap, or the manufacturing environment when industrially manufacturing the steel product for a line pipe, and have an adverse effect on the steel product for a line pipe of the present embodiment. Means an acceptable value within the range of not giving.

[About arbitrary elements]
The chemical composition of the above-mentioned steel material for line pipes may further contain Ca instead of part of Fe.

Ca: 0 to 0.0050%,
Calcium (Ca) is an optional element and may not be contained. When included, Ca controls the morphology of MnS and spheroidizes. In this case, the low temperature toughness of steel increases. However, if the Ca content is too high, coarse oxide inclusions are formed. Therefore, the Ca content is 0 to 0.0050%. The Ca content may be 0%. The preferable lower limit of the Ca content is more than 0%, more preferably 0.0001%, further preferably 0.0010%, further preferably 0.0015%. The preferable upper limit of the Ca content is 0.0045%.

  The chemical composition of the steel material for a line pipe described above may further contain, in place of a part of Fe, one or more selected from the group consisting of V, Cr and Cu. These elements increase the strength of the steel.

V: 0 to 0.100%
Vanadium (V) is an optional element and may not be contained. When contained, V combines with C and N in the steel in the winding process to form fine carbonitrides, and enhances the strength of the steel. The fine V-carbonitride further suppresses the coarsening of the crystal grains and enhances the low temperature toughness of the steel. However, if the V content is too high, the V carbonitrides become coarse and the low temperature toughness of the steel decreases. Therefore, the V content is 0 to 0.100%. The V content may be 0%. The preferable lower limit of the V content is more than 0%, more preferably 0.001%, and further preferably 0.002%. The preferable upper limit of the V content is 0.080%, more preferably 0.070%.

Cr: 0 to 0.30%
Chromium (Cr) is an optional element and may not be contained. When contained, Cr enhances the hardenability of steel and the strength of steel. However, if the Cr content is too high, the hardenability becomes too high and the low temperature toughness of the steel decreases. Therefore, the Cr content is 0 to 0.30%. The Cr content may be 0%. The preferable lower limit of the Cr content is more than 0%, and more preferably 0.01%. The preferable upper limit of the Cr content is 0.20%, more preferably 0.10%, and further preferably 0.05%.

Cu: 0 to 0.30%
Copper (Cu) is an optional element and may not be contained. When contained, Cu enhances the hardenability of steel and enhances the strength of steel. However, if the Cu content is too high, the hardenability becomes too high and the low temperature toughness of the steel decreases. Therefore, the Cu content is 0 to 0.30%. The Cu content may be 0%. The preferable lower limit of the Cu content is more than 0%, more preferably 0.01%, further preferably 0.05%, further preferably 0.10%. The preferable upper limit of the Cu content is 0.25%, and more preferably 0.20%.

  The chemical composition of the steel material for a line pipe described above may further contain one or more selected from the group consisting of Mg and a rare earth element, instead of part of Fe. These elements function as a deoxidizing agent and a desulfurizing agent.

Mg: 0 to 0.0050%
Magnesium (Mg) is an arbitrary element and may not be contained. When contained, Mg functions as a deoxidizing agent and a desulfurizing agent. Further, fine oxides are generated, which also contributes to the improvement of the toughness of the heat affected zone (HAZ). However, if the Mg content is too high, the oxide tends to aggregate or coarsen. As a result, the HIC resistance may be reduced, or the toughness of the base material portion or HAZ may be reduced. Therefore, the Mg content is 0 to 0.0050%. The Mg content may be 0%. The preferable lower limit of the Mg content is more than 0%, more preferably 0.0001%, and further preferably 0.0010%. The preferable upper limit of the Mg content is 0.0030%.

Rare earth element: 0-0.0100%
The rare earth element (REM) is an arbitrary element and may not be contained. When included, REM functions as a deoxidizer and desulfurizer. However, if the REM content is too high, coarse oxides are produced. As a result, the HIC resistance may be reduced, or the toughness of the base material portion or HAZ may be reduced. Therefore, the REM content is 0 to 0.0100%. The REM content may be 0%. The preferable lower limit of the REM content is more than 0%, more preferably 0.0001%, further preferably 0.0010%. The preferable upper limit of the REM content is 0.0070%, and more preferably 0.0050%.

  Here, REM means yttrium (Y) having an atomic number of 39, lanthanum (La) having an atomic number of 57, which is a lanthanoid to lutetium (Lu) having an atomic number of 71, and actinium having an atomic number of 89, which is an actinoid. (Ac) to one or more elements selected from the group consisting of No. 103 to Laurentium (Lr).

[About Formula (1)]
The chemical composition further satisfies equation (1).
0.350 ≦ C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 3 + Nb / 3 ≦ 0.400 (1)
Here, the content (mass%) of the corresponding element is substituted into each element symbol of the formula (1). When the element corresponding to the element symbol in formula (1) is not contained, “0” is substituted for the corresponding element symbol in formula (1).

  As described above, in the chemical composition of the present embodiment, the C, Si, Mn, Ni, Cr, Mo, V, and Nb contents enhance the hardenability of steel.

  It is defined as F1 = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 3 + Nb / 3. If F1 is too low, the S-curve of the CCT diagram shifts too far to the left. In this case, the low temperature toughness of steel decreases. The reason for this is as follows.

  The magnitude of the driving force at the time of transformation from austenite to ferrite (driving force of phase transformation) correlates with the steel material temperature. When the steel material temperature is high, the driving force for phase transformation is small. Therefore, ferrite transformation nuclei are not easily generated. However, since the steel material temperature is high, the growth of ferrite grains is fast. As a result, the ferrite grains become coarse. On the other hand, when the steel material temperature is low, the driving force for phase transformation is large. Therefore, ferrite transformation nuclei are likely to be generated. However, since the steel material temperature is low, the growth of ferrite grains is slow. As a result, the ferrite grains become finer.

  When the S curve of the CCT diagram shifts too much to the left, it enters the ferrite region with the steel material temperature high. Therefore, as described above, the ferrite grains become coarse and the effective crystal grain size becomes large. Furthermore, since a mixed grain structure is likely to occur, the coarse crystal grain ratio increases. In this case, the low temperature toughness of steel decreases. If F1 is too low, the hardenability further deteriorates and sufficient strength cannot be obtained.

  On the other hand, if F1 is too high, the S curve of the CCT diagram shifts to the right (long side). In this case, a hard structure such as bainite or martensite is likely to be formed, and the ferrite fraction in the structure is reduced. As a result, the low temperature toughness of steel decreases. If F1 is too high, the hardenability of the steel becomes too high and the low temperature toughness of the steel decreases.

  When F1 is 0.350 to 0.400, it is easy to keep the temperature of the steel in the ferrite region when the cooling curve C1 shown in FIG. 3 is performed. Therefore, the ferrite fraction in the central portion of the thickness of the steel material can be 60 to 90%, and the low temperature toughness of the steel can be enhanced.

[Ferrite fraction]
The microstructure in the central portion of the thickness of the steel product for a line pipe according to the present embodiment is composed of ferrite, bainitic ferrite, and pearlite, and the balance is precipitates and / or inclusions. Here, the thickness center part means a range of ± 20% t in the plate thickness direction or the plate thickness direction from the plate thickness center or the plate thickness center (that is, from the surface to the plate when the plate thickness or the wall thickness is tmm. 40-60% t in the thickness direction or the thickness direction).

  As described above, if the ferrite fraction of the microstructure in the central portion of the steel is 60% or more and the ferrite grains are fine, the low temperature toughness of the steel increases. In the present specification, the ferrite fraction means the area ratio of ferrite.

  When the ferrite fraction is less than 60%, the effective crystal grain size and / or the coarse crystal grain ratio becomes too large. As a result, the low temperature toughness decreases.

  On the other hand, in the chemical composition of the present embodiment containing C, a metal structure having a ferrite fraction of 90% or less is likely to be formed.

  Therefore, in the structure in the central portion of the thickness of the steel product for a line pipe according to this embodiment, the ferrite fraction is 60 to 90%. The preferable lower limit of the ferrite fraction is 65%, more preferably 70%, and further preferably 75%.

  The ferrite fraction means a ferrite area ratio and is measured by the following method. A sample is taken from the center of the thickness of the steel material for line pipes. When the line pipe steel material is an electric resistance welded steel pipe for a line pipe, a sample is taken from the center portion of the wall thickness at a position deviated from the electric resistance welded portion by 90 ° in the pipe circumferential direction.

  The collected sample is polished with a colloidal silica abrasive for 30 to 60 minutes. The polished sample is analyzed using EBSP-OIM (trademark) to determine the ferrite fraction. The visual field range is 200 μm (rolling direction) × 500 μm (thickness direction) with the center of the wall thickness being the center. The observation magnification is 400 times, and the measurement step is 0.3 μm.

  Specifically, the ferrite fraction is determined by the KAM (Kernel Average Misorientation) method provided in EBSP-OIM (trademark).

  In the KAM method, any one regular hexagonal pixel in the measurement data is set as the center pixel. First approximation using 6 pixels adjacent to this central pixel (7 pixels in total), or 2nd approximation using 12 pixels further outside of these 6 pixels (19 pixels in total) , Or a third approximation (37 pixels in total) using 18 pixels further outside of these 12 pixels, the orientation difference between each pixel is obtained. The calculated orientation difference is averaged, and the obtained average value is set as the value of the pixel at the center. Do this for the whole pixel. In the present embodiment, a display in which the orientation difference between adjacent pixels is 5 ° or less is displayed by the third approximation. In the present embodiment, the area fraction of pixels calculated to have an orientation difference third approximation of 1 ° or less with respect to the entire area of the visual field range is defined as a ferrite fraction. If the orientation difference exceeds the third approximation of 1 °, a structure other than ferrite such as bainite is used.

[About effective grain size]
In the steel product for line pipes of the present embodiment, the effective crystal grain size at the central portion of the thickness of the steel product for line pipes is 15 μm or less. If the effective crystal grain size is too large, the low temperature toughness of the steel decreases. In this embodiment, since the above-mentioned effective crystal grain size is 15 μm or less, excellent low temperature toughness is obtained. The upper limit of the effective crystal grain size is preferably 13 μm, more preferably 10 μm.

  The effective crystal grain size is measured using EBSP-OIM (trademark). Specifically, the sample is sampled and polished in the same manner as the measurement of the ferrite fraction. The polished sample is analyzed using EBSP-OIM ™. More specifically, in the azimuth measurement for each fixed measurement step, the position where the azimuth difference between the adjacent measurement points exceeds 15 ° is set as the grain boundary. 15 ° is the threshold value of the high-angle grain boundary and is generally recognized as a crystal grain boundary. The grain size and the surface area of the crystal grain are obtained by defining the region surrounded by the grain boundaries as the crystal grain. The area average particle size is determined from the obtained particle size and surface area. In the present specification, the obtained area average grain size is defined as the effective crystal grain size. The visual field range is set to 200 μm (rolling direction) × 500 μm (thickness direction) centering on the thickness center portion. The observation magnification is 400 times, and the measurement step is 0.3 μm.

[About coarse crystal grain ratio]
In the measurement using the above-mentioned EBSP-OIM (trademark), the area ratio of the crystal grains having a crystal grain size of 20 μm or more in the central portion of the thickness of the steel material for line pipes is defined as the “coarse crystal grain ratio”. When the crystal grains are coarse, the low temperature toughness of steel decreases. When the coarse crystal grain ratio is 20% or less, excellent low temperature toughness is obtained. The upper limit of the coarse crystal grain ratio is preferably 18%, more preferably 15%. The lower the coarse crystal grain ratio, the more preferable.

The coarse crystal grain ratio is measured using EBSP-OIM (trademark). A sample is taken and polished in the same manner as the measurement of the ferrite fraction. The polished sample is analyzed using EBSP-OIM ™. The visual field range is 200 μm (rolling direction) × 500 μm (thickness direction) with the center of the wall thickness being the center. The observation magnification is 400 times, and the measurement step is 0.3 μm. It can be determined by substituting in the equation (2), where N is the area of the measurement target observed in the EBSP-OIM (trademark) measurement and n is the area of the coarse crystal grains.
Coarse crystal grain ratio (%) = (n / N) × 100 (2)

[Aggregation degree of {100} plane on specific plane: 1.50 to 2.50]
In the steel product for a line pipe according to the present embodiment, when the surface perpendicular to the rolling direction is the RD surface, the rolling surface is the ND surface, and the surface perpendicular to the RD surface and the ND surface is the TD surface, the angle formed by the RD surface is 45. In the specific plane whose angle is 45 ° and the angle with the TD plane is 45 °, the integration degree of the {100} plane is 1.50 to 2.50. This suppresses peeling at the surface where cracks are most likely to propagate. As a result, the propagation of cracks can be suppressed and the low temperature toughness of steel is enhanced.

  When the {100} integration degree is less than 1.50, the low temperature toughness decreases. The upper limit of the {100} integration degree is not particularly limited, but 2.50 is sufficient at the DWTT guaranteed temperature of −30 ° C. Therefore, the {100} integration degree is 1.50 to 2.50. The preferable lower limit of the {100} integration degree is 1.60, more preferably 1.70, further preferably 1.80, and further preferably 2.00. The preferable upper limit of the {100} integration degree is 2.40.

[Method of measuring {100} integration]
The {100} integration degree is measured using EBSP-OIM (trademark). Specifically, a sample is taken from the center of the thickness of the steel material for line pipes. When the line pipe steel material is an electric resistance welded steel pipe for a line pipe, a sample is taken from the center portion of the wall thickness at a position deviated from the electric resistance welded portion by 90 ° in the pipe circumferential direction.

  The collected sample is polished with a colloidal silica abrasive for 30 to 60 minutes. The polished sample is analyzed using the EBSP-OIM ™ EBSD method. The measurement conditions in the EBSD method are: magnification: 400 times, visual field area: 200 μm × 500 μm, measurement step: 0.3 μm. By the EBSD measurement, the spherical harmonic method is used to obtain the {100} integration degree by the texture analysis of the inverse pole figure with respect to the direction perpendicular to the specific surface.

  Microscopically, the circumferential direction of the electric resistance welded steel pipe for line pipe coincides with the TD direction of the hot-rolled steel sheet for line pipe. Therefore, the specific surface shown in FIG. 1 is similarly shown in the electric resistance welded steel pipe for a line pipe. Therefore, regardless of whether the steel product for line pipe is the electric resistance welded steel pipe for line pipe or the hot rolled steel plate for line pipe, the {100} integration degree is measured in the same manner.

  By carrying out the manufacturing process described later, in the structure of the central portion of the thickness, the ferrite fraction can be 60 to 90% or more, the effective crystal grain size can be 15 μm or less, and the coarse crystal grain ratio can be 20% or less. . Furthermore, the {100} integration degree can be set to 1.50 to 2.50. As a result, the low temperature toughness can be enhanced by setting the DWTT guaranteed temperature to -30 ° C or lower.

[Yield strength YS in the pipe axis direction]
When the steel material for line pipes of the present embodiment is an electric resistance welded steel pipe for line pipes, the yield strength YS in the pipe axis direction is preferably 450 to 540 MPa. When the yield strength YS is 450 MPa or more, the strength required for the electric resistance welded steel pipe for line pipes can be more easily satisfied. When the yield strength YS is 540 MPa or less, it is advantageous in suppressing bending deformation or buckling when laying a pipeline formed using an electric resistance welded steel pipe for a line pipe. The more preferable lower limit of the yield strength YS is 460 MPa, more preferably 480 MPa. The more preferable upper limit of the yield strength YS is 530 MPa, more preferably 520 MPa.

[Tensile strength TS in the tube axis direction]
When the steel material for line pipes of the present embodiment is an electric resistance welded steel pipe for line pipes, the tensile strength TS in the pipe axis direction is preferably 510 to 625 MPa. When the tensile strength TS is 510 MPa or more, the strength required for the electric resistance welded steel pipe for a line pipe can be more easily satisfied. When the tensile strength TS is 625 MPa or less, it is advantageous in suppressing bending deformation or buckling when laying a pipeline formed using an electric resistance welded steel pipe for a line pipe. The more preferable lower limit of the tensile strength TS is 530 MPa, more preferably 540 MPa, and further preferably 545 MPa. The more preferable upper limit of the tensile strength TS is 620 MPa, more preferably 600 MPa.

  The yield strength YS and the tensile strength TS can be measured by the following methods. A tensile test piece of full thickness is sampled from a position displaced by 90 ° in the circumferential direction from the electric resistance welded portion of the electric resistance welded steel pipe for a line pipe. The longitudinal direction of the tensile test piece is parallel to the axial direction of the ERW steel pipe for line pipe. The transverse cross section of the tensile test piece (the cross section parallel to the width direction and the thickness direction of the tensile test piece) has an arc shape. The length of the parallel part of the tensile test piece is 50.8 mm, and the width of the parallel part is 38.1 mm. In the present embodiment, the tensile test is performed at room temperature using the above tensile test piece in accordance with the API standard 5CT. Room temperature is, for example, 24 ° C. The yield strength YS and the tensile strength TS are obtained based on the results of the tensile test.

[Yield ratio YR in the tube axis direction]
When the steel product for line pipes of the present embodiment is an electric resistance welded steel pipe for line pipes, the yield ratio YR, which is the ratio of the yield strength YS to the tensile strength TS, is preferably 0.93 or less. If the yield ratio YR is 0.93 or less, the deformability of the electric resistance welded steel pipe for a line pipe is increased. If the deformability is high, even if tensile stress and compressive stress due to bending and unbending act, local buckling and breakage of the line pipe based on the local buckling can be suppressed.

[Production method]
An example of the method for manufacturing the above-mentioned steel material for line pipes will be described. FIG. 4 is a flowchart showing an example of a steel pipe manufacturing process for a line pipe.

  With reference to FIG. 4, in the present manufacturing method, a molten steel satisfying the above-described chemical composition is used to manufacture a slab that is a material (material preparing step: S0). The manufactured slab is heated in a heating furnace (heating step: S1). The heated slab is rolled by a rough rolling mill and a finish rolling mill to manufacture a steel sheet (rolling process: S2). In the rolling step (S2), rough rolling is performed on the slab to produce a rough rolled plate (rough rolling step: S21). Finish rolling is performed on the rough rolled plate by a finish rolling machine to manufacture a steel sheet (finish rolling step: S22). The manufactured steel plate is cooled by ROT (runout table) (ROT cooling step: S3). In the ROT cooling step (S3), first, the steel sheet is strongly cooled by the water cooling device (strong cooling step S31). After the strong cooling, the steel sheet is gradually cooled (gradual cooling step: S32). The steel sheet after ROT cooling is wound up (winding step: S4). The hot rolled steel sheet for line pipes is manufactured by the above manufacturing process.

  Further, a hot-rolled steel sheet for line pipe is formed and welded to produce a pipe, and an electric resistance welded steel pipe for a line pipe is produced (pipe making step: S5). Hereinafter, each step will be described in detail.

[Material preparation step (S0)]
A material having the above chemical composition is prepared. Specifically, molten steel having the above chemical composition is manufactured. A material (slab) is manufactured using molten steel. A slab (slab) may be manufactured by a continuous casting method. An ingot may be manufactured using molten steel, and the ingot may be slab-rolled to manufacture a material (slab).

[Heating step (S1)]
In the heating step (S1), the manufactured slab is heated in a heating furnace. The heating temperature of the slab in the heating furnace is preferably 1060 to 1200 ° C. When the heating temperature is 1060 ° C. or higher, precipitation strengthening after rolling can be obtained and appropriate strength can be obtained. When the heating temperature is 1200 ° C. or lower, coarsening of crystal grains (austenite grains) can be suppressed. If the heating temperature is 1200 ° C. or lower, the temperature T ₀ on the final stand exit side of the rough rolling in the next step can be further maintained appropriately. Therefore, the heating temperature is 1060 to 1200 ° C. The lower limit of the preferable heating temperature is 1100 ° C. The preferable upper limit of the heating temperature is 1160 ° C.

[Rolling process (S2)]
In the rolling step (S2), the slab heated in the heating step (S1) is hot-rolled using a rough rolling mill and a finish rolling mill to form a steel plate. The rolling step (S2) includes a rough rolling step (S21) and a finish rolling step (S22). Both the rough rolling mill and the finish rolling mill include a plurality of rolling stands arranged in a line, and each rolling stand includes a pair of rolls.

[Rough rolling step (S21)]
In the rough rolling step (S21), rough rolling is performed on the prepared slab to manufacture a rough rolled plate.

  As the rough hot rolling machine, for example, a multi-stage hot rolling machine having a plurality of stands is used. For example, a method of performing reciprocal or unidirectional rolling by a two-stage or four-stage hot rolling machine having 1 to 3 stands can be mentioned.

  The total rolling reduction of rough rolling is not particularly limited as long as the effects of this embodiment can be obtained, but it is preferably 60 to 75%.

  The time from immediately after the completion of rough rolling to the start of finish rolling in the next process is controlled according to the temperature at the exit side of the final stand of rough rolling. The term “immediately after completion of rough rolling” means within 5 m from the exit side of the final stand of rough rolling.

  It is preferable that the time from immediately after the completion of rough rolling to the start of finish rolling is short. If the time from immediately after the completion of rough rolling to the start of finish rolling is short, recrystallization becomes difficult in the steel material after rough rolling and before finish rolling. In this case, finish rolling can be performed while maintaining the shape of the crystal grains flattened by rough rolling. As a result, it becomes easier to accumulate.

  The time from immediately after the completion of rough rolling to the start of finish rolling can be changed according to the temperature on the delivery side of the final stand of rough rolling. More specifically, if the temperature on the delivery side of the final stand of rough rolling is low, the shape of flattened crystal grains can be easily maintained. In this case, the time from immediately after the rough rolling to the start of the finish rolling can be lengthened. If the temperature at the exit side of the final stand of rough rolling is high, it becomes difficult to maintain the flattened crystal grain shape. In this case, it is necessary to shorten the time from immediately after the end of rough rolling to the start of finish rolling.

The temperature T ₀ (° C.) on the delivery side of the final stand of rough rolling and the time t ₀ (s) from immediately after the completion of rough rolling to the start of finish rolling satisfy the following formula (3).
t ₀ (s) ≦ −3.7T ₀ +3686 (3)
It is defined as F2 = −3.7T ₀ +3686. When the heating temperature is within the above range and t ₀ (s) is F2 or less, recrystallization is not performed and the shape of the crystal grains flattened by rough rolling is easily maintained. As a result, the {100} plane is more likely to accumulate on the specific plane. On the other hand, if t ₀ (s) exceeds F2, recrystallization occurs, and the shape of the crystal grains flattened by rough rolling cannot be maintained. As a result, the {100} plane is less likely to accumulate on the specific plane.

[Finishing rolling step (S22)]
In the finish rolling step, the obtained rough rolled plate is subjected to finish rolling with a finish rolling machine to manufacture a steel sheet.

  In the finish rolling process, tandem rolling may be performed using a tandem rolling mill including a plurality of rolling stands arranged in line (each rolling stand has a pair of work rolls), and a plurality of passes may be performed. A plurality of passes may be carried out by carrying out reverse rolling using a Sendzimir rolling machine having a pair of work rolls.

  In the finish rolling process, the surface temperature of the steel sheet on the exit side of the final stand of the finish rolling mill is defined as the finish rolling temperature (° C). The finish rolling temperature (° C.) is preferably low. The low temperature is specifically 800 ° C. or lower. When the finish rolling temperature is 800 ° C. or lower, the rolling texture and its transformation texture develop. Thereby, the {100} integration degree can be increased.

However, the finish rolling temperature (° C.) is the Ar ₃ transformation temperature or higher. When the finish rolling temperature is the Ar ₃ transformation temperature or higher, the rolling resistance of the steel sheet can be reduced and the productivity is increased. If the finish rolling temperature is the Ar ₃ transformation temperature or higher, it is possible to prevent the steel sheet from being rolled in the two-phase region of ferrite and austenite. In this case, it is possible to suppress the microstructure of the steel sheet from forming a layered structure and enhance the mechanical properties. Therefore, the finish rolling temperature is preferably not lower than the Ar ₃ transformation temperature. In the steel material for a line pipe of the present embodiment having the above chemical composition, the Ar ₃ transformation temperature is 700 to 750 ° C.

  The total rolling reduction in finish rolling is preferably 60 to 80%. In this case, the degree of {100} integration is further increased.

From the above, the time t ₀ (s) from immediately after the completion of rough rolling to the start of finish rolling is F2 or less, and if the finish rolling temperature is low, the {100} integration degree is 1.50 or more.

  The plate thickness of the steel sheet after finish rolling is 12 to 25 mm. By using the manufacturing method of the present embodiment, excellent toughness can be obtained even when the plate thickness is 12 mm or more.

[ROT cooling step (S3)]
In the ROT (runout table) cooling step (S3), the steel sheet manufactured in the rolling step (S2) is cooled. The ROT cooling step (S3) preferably includes a strong cooling step (S31) and a slow cooling step (S32). As a result, the ferrite fraction increases in the microstructure in the center of the thickness of the steel for line pipes, and the low temperature toughness of the steel increases. Hereinafter, this point will be described in detail.

  FIG. 3 is an example of a CCT diagram of the steel material for a line pipe according to the present embodiment. In FIG. 3, F is a ferrite nose, P is a pearlite nose, and B is a bainite nose.

  As shown in FIG. 3, the ferrite nose is higher than the pearlite nose and the bainite nose. A broken line C2 in FIG. 3 shows a cooling curve (cooling curve C2) by the conventional cooling process. The cooling curve C2 may pass through the pearlite nose. In the conventional cooling method, all of ferrite nose, pearlite nose and / or bainite nose are passed at a uniform speed in the cooling process. Therefore, a large amount of pearlite and / or bainite is generated in the structure, and the ferrite fraction in the structure is reduced.

  Therefore, in the present embodiment, for example, cooling is performed along the cooling curve of the broken line C1 (cooling curve C1). Specifically, in the initial stage of cooling, strong cooling is performed up to the vicinity of the ferrite nose (S31). The rapid cooling of the steel by vigorous cooling results in a large number of strains in the steel resulting in a large number of nucleation sites in the unrecrystallized structure. After strong cooling, slow cooling is performed (S32). At this time, the temperature of the steel is kept within the ferrite region in FIG. As a result, fine ferrite is generated from a large number of nucleation sites generated during strong cooling. As a result, the ferrite fraction in the structure is increased and the crystal grains are refined. Therefore, the low temperature toughness of steel increases. The cooling curve C1 may pass through the pearlite nose.

[Strong cooling step (S31)]
First, the steel sheet is strongly cooled. Strong cooling is, for example, water cooling by a water cooling device. The surface temperature of the steel sheet immediately before water cooling is not particularly limited, but it is preferably the Ar ₃ transformation temperature or higher. If the surface temperature of the steel sheet immediately before water cooling is equal to or higher than the Ar ₃ transformation temperature, it is possible to prevent the decrease in strength due to the coarsening of crystal grains due to grain growth.

  The cooling rate in the strong cooling step (S31) is V1 (° C / s). V1 is calculated by heat conduction. V1 is preferably 5 ° C./s or more in the central portion of the plate thickness. When the cooling rate V1 is less than 5 ° C./s, the degree of supercooling due to cooling is insufficient, and it is not possible to obtain a sufficient ferrite nucleation site. In this case, since the amount of ferrite grains produced is small, the ferrite grains become coarse and the low temperature toughness of the steel deteriorates. Therefore, the cooling rate V1 is 5 ° C./s or more. The preferable lower limit of the cooling rate V1 is 7 ° C./s, and more preferably 8 ° C./s.

  In the strong cooling step (S31), the steel sheet is cooled until the surface temperature of the steel sheet reaches 580 to 680 ° C. In other words, the strong cooling stop temperature T1 is 580 to 680 ° C. If the strong cooling stop temperature T1 is too low, the steel plate temperature passes through the ferrite region and reaches the pearlite region and / or the bainite region in the CCT diagram. In this case, the ferrite fraction decreases and the low temperature toughness of the steel decreases. On the other hand, if the strong cooling stop temperature T1 is too high, the precipitation of Nb, which strengthens the proeutectoid ferrite, becomes overaged, and the strength of the steel decreases. When the strong cooling stop temperature T1 is set to 580 to 680 ° C., the ferrite fraction can be made 60% or more by gradually cooling in the slow cooling step (S4) of the subsequent step, and the low temperature toughness of the steel is enhanced. The strong cooling stop temperature T1 is preferably 600 to 670 ° C, more preferably 610 to 670 ° C.

[Slow cooling step (S32)]
Gradual cooling is performed on the steel plate that has been strongly cooled in the strong cooling step (S31).

  The cooling rate in the slow cooling step (S32) is V2 (° C / s). The cooling rate V2 is preferably 2.0 to 4.0 ° C./s in the central part of the plate thickness. If the cooling speed V2 is too slow, the gradual cooling stop temperature T2 and the winding temperature T3 will be too high in the subsequent steps. In this case, the crystal grains become coarse and the low temperature toughness of the steel decreases. If the cooling rate V2 is too fast, the steel sheet temperature passes through the ferrite region and reaches the pearlite region and / or the bainite region in the CCT diagram. In this case, the ferrite fraction decreases and the low temperature toughness of the steel decreases. Therefore, the cooling rate V2 is 2.0 to 4.0 ° C./s.

  In the slow cooling step (S32), the steel sheet is cooled until the surface temperature of the steel sheet reaches 500 to 670 ° C. In other words, the slow cooling stop temperature T2 is 500 to 670 ° C. If the gradual cooling stop temperature T2 is too low, the steel sheet temperature passes through the ferrite region and reaches the pearlite region and / or the bainite region in the CCT diagram. In this case, the ferrite fraction decreases and the low temperature toughness of the steel decreases. If the gradual cooling stop temperature T2 is too high, the strength of the steel decreases. Therefore, the slow cooling stop temperature T2 is 500 to 670 ° C. The lower limit of the slow cooling stop temperature T2 is preferably 580 ° C, more preferably 590 ° C. The preferable upper limit of the slow cooling stop temperature T2 is 650 ° C, more preferably 635 ° C, and further preferably 620 ° C.

[Winding step (S4)]
In the winding step (S4), the steel sheet cooled in the ROT cooling step (S3) is wound into a coil-shaped hot rolled steel sheet for a line pipe.

  The coil-shaped hot-rolled steel sheet for line pipes is air-cooled after the slow cooling step, and then wound. The surface temperature (hereinafter, referred to as a winding temperature) T3 of the steel sheet during winding is 500 to 650 ° C. If the winding temperature T3 is too low, the coarse crystal grain ratio becomes high, and the low temperature toughness of the steel decreases. On the other hand, if the winding temperature T3 is too high, the crystal grains become coarse and the low temperature toughness of the steel decreases. Therefore, the winding temperature T3 is 500 to 650 ° C. Preferred T3 is 510 to 600 ° C, and more preferably 520 to 560 ° C.

  The hot-rolled steel sheet for line pipes of the present embodiment is manufactured by the above manufacturing process.

[Pipe making step (S5)]
An electric resistance welded steel pipe for a line pipe is manufactured by a known method while rewinding a coil-shaped hot-rolled steel sheet for a line pipe. Specifically, the hot-rolled steel sheet for line pipe is bent into a tubular shape (open pipe) by a continuous forming roll. Subsequently, the seam portion of the open pipe, that is, both end surfaces in the longitudinal direction of the hot-rolled steel sheet for line pipe are welded by the electric resistance welding method. The welded portion of the electric resistance welded steel pipe is subjected to heat treatment once or a plurality of times by induction heating or the like. Through the above steps, the electric resistance welded steel pipe for a line pipe is manufactured.

  In the steel products for line pipes (hot-rolled steel plates for line pipes and electric resistance welded steel pipes for line pipes) manufactured by the above manufacturing process, the ferrite fraction is 60 to 90% or more, effective crystal grains in the structure in the central portion of the thickness. The diameter can be 15 μm or less, and the coarse crystal grain ratio can be 20% or less. Furthermore, the {100} integration degree can be set to 1.50 to 2.50. As a result, the low temperature toughness can be enhanced by setting the DWTT guaranteed temperature to -30 ° C or lower. Further, it is possible to obtain a yield strength YS of 450 to 540 MPa and a tensile strength TS of 510 to 625 MPa in the axial direction of the electric resistance welded steel pipe for a line pipe.

  Slabs were manufactured by continuously casting molten steels of Steel A to Steel N shown in Table 1.

  Using a plurality of slabs of steel A to steel N, ERW steel pipes for line pipes of test numbers 1 to 22 shown in Table 2 were manufactured.

Specifically, the slab was heated to the heating temperature shown in Table 2 in a heating furnace. Rough rolling was performed on the heated slab. Table 2 shows the temperature T ₀ (° C.) on the outlet side of the final stand of the rough rolling, the time t ₀ (seconds) from the end of the rough rolling to the start of the finish rolling, and F2. After rough rolling, finish rolling was performed at the finish rolling temperature shown in Table 2 to manufacture a steel sheet. The rolling reduction in the non-recrystallization temperature range was 60 to 80% in all test numbers. The finish rolling temperature was higher than the Ar ₃ transformation temperature except for the test number 19.

  ROT cooling was performed on the steel sheet after finish rolling. The time from the end of finish rolling to the start of strong cooling was set to 20 seconds or less. In the ROT cooling step, except for Test No. 20, strong cooling was performed at a cooling rate V1 of 5 ° C./s or more until a strong cooling stop temperature T1 of 580 to 680 ° C. was reached. Then, it was gradually cooled at a cooling rate V2 of 2.0 to 4.0 ° C./s until a slow cooling stop temperature T2 of 500 to 670 ° C. (provided that T1> T2 was satisfied).

  A steel plate was manufactured by the above manufacturing process. The obtained steel sheet was wound at a winding temperature T3 of 500 to 650 ° C. (provided that T2> T3 was satisfied) to obtain a hot rolled steel sheet for a line pipe in the form of a hot coil.

  The hot-rolled steel sheet for a line pipe thus obtained was used to produce a pipe by the above-mentioned method to produce an electric resistance welded steel pipe for a line pipe having an outer diameter of 304.8 to 660.4 mm and a wall thickness of 12 mm or more.

[Test method]
[Strength test]
Tensile test pieces were taken from the ERW steel pipe for line pipe of each test number. Specifically, when viewing the ERW steel pipe for a line pipe in the axial direction, the total thickness from the position 90 ° from the welded portion of the ERW steel pipe for a line pipe (the position deviated from the ERW steel pipe in the circumferential direction by 90 °) is measured. A tensile test piece in the tube axis direction was taken. The cross section of the tensile test piece was arcuate, and the longitudinal direction of the tensile test piece was parallel to the longitudinal direction of the steel pipe. The size of the tensile test piece was as shown in FIG. 5, the length of the parallel portion was 50.8 mm, and the width of the parallel portion was 38.1 mm. The numerical values in FIG. 5 indicate the dimensions (unit: mm) of the corresponding parts of the test piece. Using a tensile test piece, a tensile test was carried out at room temperature according to the API standard 5CT. Based on the test results, the yield strength YS (MPa) and the tensile strength TS (MPa) of the electric resistance welded steel pipe for line pipe were obtained. The yield ratio YR was calculated based on the obtained yield strength YS and tensile strength TS.

[Microstructure]
With respect to the electric resistance welded steel pipe for a line pipe, the ferrite fraction, effective crystal grain size, and coarse crystal grain fraction in the central portion of the thickness were measured using EBSP-OIM (trademark) based on the above-mentioned method. The measurement conditions of EBSP-OIM (trademark) in the effective crystal grain size measurement were: magnification: 400 times, visual field area: 200 μm × 500 μm, measurement step: 0.3 μm.

  As analysis software in EBSP-OIM (trademark), "TSL OIM Analysis 7 (trademark)" manufactured by TSL Solutions Inc. was used.

  Further, in the measurement of the ferrite fraction, the type of the remaining part (that is, the structure other than ferrite) in the metal structure of the central portion of the thickness of the base material part was also confirmed.

[{100} degree of integration]
With respect to the electric resistance welded steel pipe for a line pipe, the {100} integration degree was measured using EBSP-OIM (trademark) based on the method described above. The measurement conditions in EBSP-OIM (trademark) were: magnification: 400 times, visual field area: 200 μm × 500 μm, measurement step: 0.3 μm.

[Low temperature toughness test]
DWTT test pieces were taken from the ERW steel pipe for line pipe of each test number. The sampling position was 90 ° from the same weld as the tensile test piece. An arc-shaped member sampled in the pipe circumferential direction from the sampling position was developed into a flat plate shape, and a notch was formed at a 90 ° position. The size of the DWTT test piece was as shown in FIG. The numerical values in FIG. 6 indicate the dimensions (unit: mm) of the corresponding parts of the test piece. t indicates the wall thickness (unit is mm). The longitudinal direction of the DWTT test piece corresponded to the circumferential direction of the ERW steel pipe for line pipe. A DWTT test was performed on the DWTT test piece in accordance with the regulations of ASTM E436. The minimum temperature (DWTT guaranteed temperature) at which the ductile fracture surface ratio reached 85% was determined. When the DWTT guaranteed temperature was -30 ° C or lower, the low temperature toughness was evaluated as high.

[Test results]
Table 3 shows the test results. In Table 3, the notation "P, B" means at least one of pearlite and bainite.

  With reference to Tables 1 to 3, the chemical compositions of the steels of Test No. 1 to Test No. 13 were appropriate and satisfied the formula (1). Furthermore, the manufacturing conditions for all test numbers were appropriate. Therefore, the ferrite fraction of Test No. 1 to Test No. 13 was 60 to 90%, the effective crystal grain size was 15 μm or less, and the coarse crystal grain ratio was 20% or less. Furthermore, the {100} integration degree was 1.50 to 2.50. As a result, the DWTT guaranteed temperature was -30 ° C or lower, and excellent low temperature toughness was exhibited. Further, the yield strength YS in the axial direction of the electric resistance welded steel pipe for line pipe was 450 to 540 MPa, and the tensile strength TS was 510 to 625 MPa. The yield ratio YR was 0.93 or less in all cases.

  On the other hand, in test number 14, although the manufacturing conditions were appropriate, F1 was less than the lower limit of formula (1). Therefore, the crystal grains became coarse, and the effective crystal grain size exceeded 15 μm. Furthermore, the {100} integration degree was less than 1.50. Therefore, the DWTT guaranteed temperature was higher than -30 ° C, and the low temperature toughness was low. Further, in the test number 14, the yield ratio YR was too high.

  In test number 15, although the manufacturing conditions were appropriate, F1 exceeded the upper limit of formula (1). Therefore, the ferrite fraction was less than 60%, and a bainite-based structure was formed. Since the structure was mainly composed of bainite, the {100} accumulation degree was 1.50 or more, but the effective crystal grain size exceeded 15 μm and the coarse crystal grain ratio also exceeded 20%. Therefore, the DWTT guaranteed temperature was higher than -30 ° C, and the low temperature toughness was low. Further, the yield strength YS and the yield ratio YR of the electric resistance welded steel pipe for line pipe were too high.

In test number 16, the heating temperature exceeded 1200 ° C. Therefore, the temperature T ₀ on the delivery side of the final stand of rough rolling becomes too high, and the time t ₀ immediately after the completion of rough rolling to the start of finish rolling exceeds F2. Therefore, the crystal grains became coarse, the effective crystal grain size exceeded 15 μm, and the coarse crystal grain ratio also exceeded 20%. Furthermore, the {100} integration degree was less than 1.50. As a result, the DWTT guaranteed temperature was higher than -30 ° C, and the low temperature toughness was low.

  In test number 17, the heating temperature was less than 1060 ° C. Therefore, the crystal grains became coarse, the effective crystal grain size exceeded 15 μm, and the coarse crystal grain ratio also exceeded 20%. Furthermore, the {100} integration degree was less than 1.50. As a result, the DWTT guaranteed temperature was higher than -30 ° C, and the low temperature toughness was low.

  In test number 18, the finish rolling temperature was too high. Therefore, the {100} integration degree was less than 1.50. As a result, the DWTT guaranteed temperature was higher than -30 ° C, and the low temperature toughness was low.

In test number 19, the finish rolling temperature was lower than the Ar ₃ transformation temperature, which was too low. Therefore, the coarse crystal grain ratio exceeded 20%. Furthermore, the {100} integration degree was less than 1.50. As a result, the DWTT guaranteed temperature was higher than -30 ° C, and the low temperature toughness was low. Further, the yield strength YS and the yield ratio YR of the electric resistance welded steel pipe for line pipe were too high.

  In test number 20, V1 was less than 5 ° C / s. Therefore, the crystal grains became coarse, the effective crystal grain size exceeded 15 μm, and the coarse crystal grain ratio also exceeded 20%. Furthermore, the {100} integration degree was less than 1.50. As a result, the DWTT guaranteed temperature was higher than -30 ° C, and the low temperature toughness was low.

In test number 21, the time t ₀ immediately after the end of rough rolling until the start of finish rolling exceeded F2. Therefore, the {100} integration degree was less than 1.50. As a result, the DWTT guaranteed temperature was higher than -30 ° C, and the low temperature toughness was low.

  In test number 22, the C content was low. Therefore, the yield ratio YR was high.

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

Claims (8)

In mass%,
C: more than 0.060 to 0.120%,
Si: 0.05-0.30%,
Mn: 0.50 to 2.00%,
P: 0 to 0.030%,
S: 0 to 0.0100%,
Al: 0.010 to 0.035%,
N: 0.0010 to 0.0080%,
Nb: 0.010 to 0.080%,
Ti: 0.005-0.030%,
Ni: 0.001 to 0.50%,
Mo: 0.05-0.30%,
O: 0 to 0.0030%,
Ca: 0 to 0.0050%,
V: 0 to 0.100%,
Cr: 0 to 0.30%,
Cu: 0 to 0.30%,
Mg: 0 to 0.0050%, and
Rare earth element: contains 0 to 0.0100%, the balance is Fe and impurities, and has a chemical composition satisfying the formula (1),
In the structure of the central part of the thickness, the ferrite fraction is 60 to 90%, the effective crystal grain size is 15 μm or less, and the coarse crystal grain ratio, which is the area ratio of the crystal grains having the crystal grain size of 20 μm or more, is 20%. Is
When a surface perpendicular to the rolling direction is an RD surface, a rolling surface is an ND surface, and a surface perpendicular to the RD surface and the ND surface is a TD surface, an angle formed by the RD surface is 45 °, and A steel material for a line pipe, in which a specific surface having an angle of 45 ° with a TD surface has a degree of integration of {100} surfaces of 1.50 to 2.50.
0.350 ≦ C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 3 + Nb / 3 ≦ 0.400 (1)
Here, the content (mass%) of the corresponding element is substituted into each element symbol of the formula (1). The steel material for a line pipe according to claim 1,
The chemical composition is% by mass,
A steel material for a line pipe containing Ca: more than 0 to 0.0050%. The steel material for a line pipe according to claim 1 or 2,
The chemical composition is% by mass,
V: more than 0 to 0.100%,
Cr: more than 0 to 0.30%, and
Cu: Steel material for line pipes containing one kind or two or more kinds selected from the group consisting of more than 0 and 0.30%. The steel material for a line pipe according to any one of claims 1 to 3,
The chemical composition is% by mass,
Mg: more than 0 to 0.0050%, and
Rare earth element: steel material for line pipes containing at least one selected from the group consisting of more than 0 and 0.0100%. The steel material for a line pipe according to any one of claims 1 to 4,
The steel product for line pipe is a steel product for line pipe, which is a hot-rolled steel plate for line pipe. The steel material for a line pipe according to any one of claims 1 to 4,
The steel material for line pipes is a steel material for line pipes, which is an electric resistance welded steel pipe for line pipes. The electric resistance welded steel pipe for a line pipe according to claim 6,
For line pipes, the yield strength in the pipe axis direction is 450 to 540 MPa, the tensile strength in the pipe axis direction is 510 to 625 MPa, and the yield ratio, which is the ratio of the yield strength to the tensile strength, is 0.93 or less. ERW steel pipe. An electric resistance welded steel pipe for a line pipe according to claim 7,
ERW steel pipe for line pipe having a wall thickness of 12 to 25 mm and an outer diameter of 304.8 to 660.4 mm.

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