JP2018104757A - Steel material for linepipe and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel material for linepipe having excellent low temperature toughness and sufficient strength.SOLUTION: A steel material for linepipe has a chemical composition containing, by mass%, C:0.01 to less than 0.06%, Si:0.05 to 0.2%, Mn:0.5 to 2%, P:0.03% or less, S:0.01% or less, Al:0.01 to 0.035%, N:0.001 to 0.008%, Nb:0.01 to 0.25%, Ti:0.005 to 0.03%, Ni:0.001 to 0.2%, Mo:0.1 to 0.2%, V:0.001 to 0.01%, O:0.003% and the balance Fe with impurities, and satisfying the formula (1). Average crystal particle diameter is 15 μm or less and coarse crystal particle percentage is 20% or less. Ferrite fraction is 65% or more, hard phase fraction is 10 to 20% and size of a hard phase is 6.0 μm or less. 0.30≤C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/3+Nb/3≤0.35 (1).SELECTED DRAWING: Figure 1

Description

  The present invention relates to a steel material and a manufacturing method thereof, and more particularly to a steel material for a line pipe and a manufacturing method thereof.

  Pipelines laid on the seabed allow high-pressure fluid to pass through. Pipelines are also subject to repeated wave distortions and seawater pressure. Therefore, high strength and high low temperature toughness are required for steel pipes used in submarine pipelines.

  The pipeline is composed of a plurality of line pipes. An electric resistance welded steel pipe (hereinafter referred to as an electric resistance steel pipe) may be used as a steel pipe for a line pipe. High strength can be obtained by increasing the thickness of the ERW pipe. However, as the thickness increases, brittle fracture tends to occur and low temperature toughness decreases. As an index of low temperature toughness, there is a DWTT (Drop Weight Tear Test) guaranteed temperature. The DWTT guaranteed temperature means a temperature having a ductile fracture surface ratio of 85% or more in DWTT. It means that low temperature toughness is so high that DWTT guarantee temperature is low. In recent years, excellent low temperature toughness is required for ERW steel pipes for line pipes.

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

  The hot-rolled steel sheet for line pipe disclosed in Patent Document 1 is C = 0.02 to 0.08%, Si = 0.05 to 0.5%, Mn = 1 to 2%, Nb in mass%. = 0.03 to 0.12%, Ti = 0.005 to 0.05%, with the balance being Fe and inevitable impurity elements. In the microstructure at a depth of 1/2 the plate thickness from the steel sheet surface, the pro-eutectoid ferrite fraction is 3% or more and 20% or less, the other is the low temperature transformation phase and 1% or less pearlite, The number average crystal 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, and the standard deviation of the area average grain size is 0.8 μm or more and 2.3 μm or less. The reflection X-ray intensity ratio {211} / {111} in the {211} direction and the {111} direction with respect to a plane parallel to the steel plate surface at a depth of ½ of the thickness is 1.1 or more. This linepipe steel sheet is described as being able to obtain excellent strength and low temperature toughness by controlling the pro-eutectoid ferrite fraction, the average particle diameter, and the texture in the center of the thickness.

International Publication No. 2012/002481

  However, the hot-rolled steel sheet disclosed in Patent Document 1 has a high heating temperature before the rolling process, and austenite grains may become coarse. In this case, the crystal grains become coarse, and the low temperature toughness can be lowered.

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

The steel material for line pipes according to the present embodiment is mass%, C: 0.01 to less than 0.06%, Si: 0.05 to 0.2%, Mn: 0.5 to 2%, P: 0.00. 03% or less, S: 0.01% or less, Al: 0.01 to 0.035%, N: 0.001 to 0.008%, Nb: 0.01 to 0.25%, Ti: 0.005 -0.03%, Ni: 0.001-0.2%, Mo: 0.1-0.2%, V: 0.001-0.01%, O: 0.003% or less, Ca: 0 -0.0030%, Cr: 0-0.3%, Cu: 0-0.3%, and B: 0-0.003%, with the balance consisting of Fe and impurities, the formula (1) It has a chemical composition satisfying In the structure at the center of the thickness, the average crystal grain size is 15 μm or less, and the coarse crystal grain ratio, which is the area ratio of crystal grains having a crystal grain diameter of 20 μm or more, is 20% or less. In the structure at the center of the thickness, the ferrite fraction is 65% or more and the hard phase fraction is 10 to 20%. The size of the hard phase is 6.0 μm or less.
0.30 ≦ C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 3 + Nb / 3 ≦ 0.35 (1)
Here, the content (mass%) of the corresponding element is substituted for 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.

  The steel material for line pipes according to the present invention has excellent low temperature toughness and sufficient strength.

FIG. 1 is a schematic diagram of a continuous cooling transformation curve of a steel product for a line pipe according to the present embodiment. FIG. 2 is a flowchart showing the manufacturing process of the steel product for line pipes according to the present embodiment. FIG. 3 is a plan view of a tensile test piece used in the tensile test. FIG. 4 is a front view and a side view of a DWTT test piece used in the DWTT test.

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

  (A) If the ferrite fraction of the steel structure is 65% or more, fine crystal grains can be obtained, and the low temperature toughness of the steel increases. The heating temperature at the time of rolling is set to 1150 ° C. or less to suppress crystal grain coarsening. Furthermore, a large number of nucleation sites are generated in the unrecrystallized structure after rolling, and cooling is controlled so that 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 steel structure is mainly bainite, laths (elongated structures) are formed in the crystal grains that have inherited the prior austenite grains as they are, but their orientations are aligned for each block, and each block is substantially identical. 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 be reduced. Therefore, in the steel for line pipes of this embodiment, the structure is mainly composed of ferrite.

  However, bainite, perlite, island martensite and MA (Martensite-austentite constituent) increase strength. Therefore, in the steel material for line pipes of this embodiment, the steel structure includes a hard phase containing one or more selected from the group consisting of pearlite, bainite, island martensite, and MA. In this case, the strength of the steel is increased. Furthermore, if the crystal grains of the hard phase are made fine, the low temperature toughness is further increased.

  (B) As an example of the cooling control method, strong cooling is first performed in the cooling process in the ROT (runout table) after rolling, and then slow cooling is performed. Thereby, in the structure of the thickness center part of the steel material for line pipes, the ferrite fraction (the area ratio occupied by ferrite in the structure) increases, and the low temperature toughness of the steel increases. Further, strong cooling is performed following the slow cooling. Thereby, in the structure | tissue of the thickness center part of steel materials, the fine hard phase containing 1 type (s) or 2 or more types selected from the group which consists of pearlite, a bainite, island-like martensite, and MA is obtained. As a result, the strength of the steel is increased and the low temperature toughness is further increased. Hereinafter, this point will be described in detail.

  FIG. 1 is a continuous cooling transformation diagram (CCT diagram: Continuous Cooling Transformation Diagram) of a steel product for a line pipe according to the present embodiment. In FIG. 1, F indicates a ferrite nose, P indicates a pearlite nose, and B indicates a bainite nose.

  As shown in FIG. 1, the ferrite nose exists at a higher position than the pearlite nose and the bainite nose. A broken line C1 in FIG. 1 shows a cooling curve by a conventional cooling process. In the conventional cooling method, the ferrite nose, pearlite nose, and bainite nose are all passed in the cooling process. Therefore, pearlite and bainite are generated in the structure, and the ferrite fraction in the structure is reduced.

  Therefore, in this embodiment, cooling is performed along a cooling curve indicated by a broken line C2. Specifically, in the initial stage of cooling, strong cooling is performed to the vicinity of the ferrite nose (S31). When the steel is rapidly cooled by strong cooling, the degree of supercooling increases, resulting in numerous nucleation sites in the unrecrystallized structure. After strong cooling, slow cooling is performed (S32). At this time, the temperature of the steel is maintained in 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.

  After the slow cooling, further strong cooling is performed (S33). At this time, the temperature of the steel passes through the ferrite region and the pearlite region and reaches the bainite region. Thereby, fine pearlite, bainite, island martensite and MA are generated. Therefore, the strength of the steel is increased and the low temperature toughness is further increased.

  (C) C, Si, Mn, Ni, Cr, Mo, V, and Nb all affect the S curve (ferrite region, pearlite region, and bainite region) of the CCT diagram.

  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 will shift too far to the left. In this case, in the cooling process after rolling, the steel material temperature enters the ferrite region before sufficient nucleation sites are generated. As a result, the ferrite grains become coarse and the average crystal grain size increases. Furthermore, since it tends to be a mixed grain structure, the coarse crystal grain ratio increases. In this case, the low temperature toughness of the steel decreases. If F1 is too low, the hardenability is further lowered and sufficient strength cannot be obtained.

  On the other hand, if F1 is too high, the S curve will shift 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 structure produced increases and the ferrite fraction in the structure decreases. As a result, the low temperature toughness of the steel decreases.

  If F1 is 0.30 to 0.35, the S curve (ferrite, pearlite, bainite) of each phase is arranged at an appropriate position in the CCT diagram. In this case, as shown by a cooling curve C2 in FIG. 1, cooling can be performed mainly through the ferrite region. Therefore, a ferrite-based structure can be generated, and high strength and low temperature toughness can be obtained.

The steel product for line pipes of the present embodiment completed based on the above knowledge is mass%, C: 0.01 to less than 0.06%, Si: 0.05 to 0.2%, Mn: 0.5 -2%, P: 0.03% or less, S: 0.01% or less, Al: 0.01-0.035%, N: 0.001-0.008%, Nb: 0.01-0. 25%, Ti: 0.005-0.03%, Ni: 0.001-0.2%, Mo: 0.1-0.2%, V: 0.001-0.01%, O: 0 0.003% or less, Ca: 0 to 0.0030%, Cr: 0 to 0.3%, Cu: 0 to 0.3%, and B: 0 to 0.003%, with the balance being Fe and It consists of impurities and has a chemical composition that satisfies formula (1). In the structure at the center of the thickness, the average crystal grain size is 15 μm or less, and the coarse crystal grain ratio, which is the area ratio of crystal grains having a crystal grain diameter of 20 μm or more, is 20% or less. In the structure at the center of the thickness, the ferrite fraction is 65% or more and the hard phase fraction is 10 to 20%. In ferrite, the size of the hard phase is 6.0 μm or less.
0.30 ≦ C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 3 + Nb / 3 ≦ 0.35 (1)
Here, the content (mass%) of the corresponding element is substituted for each element symbol of the formula (1).

  The chemical composition may contain Ca: 0.0002 to 0.0030%.

  The chemical composition is one or more selected from the group consisting of Cr: 0.05 to 0.3%, Cu: 0.05 to 0.3%, and B: 0.0002 to 0.003%. It may contain.

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

  The manufacturing method of the steel material for line pipes of this embodiment performs the process which heats the raw material which has the above-mentioned chemical composition at 1060-1150 degreeC, and performs rough rolling and finish rolling with respect to the heated raw material, and is set as a steel plate. And a step of cooling the steel plate after the finish rolling. In the cooling step, a temperature range of 870 to 750 ° C. is strongly cooled at a cooling rate of 10 to 50 ° C./s at the center of the plate thickness, and a temperature range of 750 to 650 ° C. is applied to the strongly cooled steel plate. The step of gradually cooling the sheet thickness center at a cooling rate of 2 to 5 ° C./s, and the cooling range of 650 to 500 ° C. with respect to the gradually cooled steel sheet the center of sheet thickness being 5 to 10 ° C./s. And a strong cooling process. The method further includes a step of winding the cooled steel plate at 500 to 580 ° C.

  The manufacturing method of the ERW steel pipe for line pipes by this embodiment is equipped with the process of shape | molding and welding the above-mentioned hot-rolled steel plate, and producing a pipe.

  Hereinafter, the steel for line pipes of this embodiment will be described in detail. “%” Regarding an element means mass% unless otherwise specified.

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

C: 0.01 to less than 0.06% Carbon (C) increases the strength of steel. If the C content is too low, this effect cannot be obtained. On the other hand, if the C content is too high, carbides are generated and the low temperature toughness and ductility of the steel are reduced. If the C content is too high, the weldability further decreases. Therefore, the C content is 0.01 to less than 0.06%. The minimum with preferable C content is 0.015%, More preferably, it is 0.02%. The upper limit with preferable C content is 0.05%, More preferably, it is 0.048%.

Si: 0.05-0.2%
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 0.2%. The minimum with preferable Si content is 0.07%, More preferably, it is 0.1%. The upper limit with preferable Si content is 0.195%, More preferably, it is 0.19%.

Mn: 0.5-2%
Manganese (Mn) increases the hardenability of the steel and increases the strength of the 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 0.5-2%. The minimum with preferable Mn content is 0.7%, More preferably, it is 1.0%. The upper limit with preferable Mn content is 1.8%, More preferably, it is 1.5%.

P: 0.03% or less Phosphorus (P) is an impurity. P decreases the low temperature toughness of the steel. Therefore, the P content is 0.03% or less. The upper limit with preferable P content is 0.015%, More preferably, it is 0.01%. The P content is preferably as low as possible.

S: 0.01% or less Sulfur (S) is an impurity. S combines with Mn to form a Mn-based sulfide. Therefore, the low temperature toughness and SSC resistance of the steel are reduced. Therefore, the S content is 0.01% or less. The upper limit with preferable S content is 0.001%, More preferably, it is 0.0005%. The S content is preferably as low as possible.

Al: 0.01-0.035%
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 nitride becomes coarse and the low temperature toughness of the steel decreases. Therefore, the Al content is 0.01 to 0.035%. The minimum with preferable Al content is 0.012%, More preferably, it is 0.015%. The upper limit with preferable Al content is 0.017%, More preferably, it is 0.016%. In the present specification, the Al content means the total Al content in steel.

N: 0.001 to 0.008%
Nitrogen (N) forms nitrides and suppresses coarsening of austenite grains during the heating process. In this case, the austenite grains become finer in the rolling process, and the crystal grains after transformation become finer. As a result, the low temperature toughness of the steel is increased. N further increases the strength of the 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 carbonitride is coarsened and the low temperature toughness of the steel is reduced. Therefore, the N content is 0.001 to 0.008%. The minimum with preferable N content is 0.002%, More preferably, it is 0.003%. The upper limit with preferable N content is 0.006%, More preferably, it is 0.005%.

Nb: 0.01 to 0.25%
Niobium (Nb) combines with C and N in the steel to form fine Nb carbonitride. Nb carbonitride suppresses the coarsening of crystal grains and reduces the average crystal grain size. Therefore, the low temperature toughness of the steel is increased. Furthermore, fine Nb carbonitride increases 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 0.01 to 0.25%. The minimum with preferable Nb content is 0.012%, More preferably, it is 0.02%. The upper limit with preferable Nb content is 0.08%, More preferably, it is 0.07%.

Ti: 0.005 to 0.03%
Titanium (Ti) combines with N in the steel to form TiN, and suppresses a decrease in low temperature toughness of the steel due to the solid solution N. Furthermore, coarse TiN is suppressed by the fine TiN being dispersed and precipitated. This increases 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 the steel decreases. Therefore, the Ti content is 0.005 to 0.03%. The minimum with preferable Ti content is 0.008%, More preferably, it is 0.01%. The upper limit with preferable Ti content is 0.02%, More preferably, it is 0.015%.

Ni: 0.001 to 0.2%
Nickel (Ni) increases the hardenability of the steel and increases the strength of the 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 0.2%. The minimum with preferable Ni content is 0.05%, More preferably, it is 0.08%. The upper limit with preferable Ni content is 0.15%, More preferably, it is 0.012%.

Mo: 0.1-0.2%
Molybdenum (Mo) increases the hardenability of the steel and increases the strength of the steel. Mo further refines 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 the steel decreases. Therefore, the Mo content is 0.1 to 0.2%. The minimum with preferable Mo content is 0.12%, More preferably, it is 0.15%. The upper limit with preferable Mo content is less than 0.2%, More preferably, it is 0.19%.

V: 0.001 to 0.01%
Vanadium (V) combines with C and N in the steel in the winding process to form fine carbonitrides and increases the strength of the steel. The fine V carbonitride further suppresses the coarsening of crystal grains and increases the low temperature toughness of the steel. If the V content is too low, these effects cannot be obtained. On the other hand, if V content is too high, V carbonitride will coarsen and the low temperature toughness of steel will fall. Therefore, the V content is 0.001 to 0.01%. The minimum with preferable V content is 0.002%, More preferably, it is 0.005%. The upper limit with preferable V content is 0.008%, More preferably, it is 0.007%.

O: 0.003% or less Oxygen (O) is an impurity. O forms an oxide and reduces the resistance to hydrogen-induced cracking of steel. O further reduces the low temperature toughness of the steel. Therefore, the O content is 0.003% or less. The upper limit with preferable O content is 0.0025%, More preferably, it is 0.002%. The O content is preferably as low as possible.

  The balance of the chemical composition of the steel for line pipe according to the present embodiment is composed of Fe and impurities. Here, the impurities are mixed from the ore as a raw material, scrap, or the manufacturing environment when industrially producing the steel for line pipes, and have an adverse effect on the steel for line pipes of this embodiment. It means that it is allowed in the range that does not give.

[Arbitrary elements]
The chemical composition of the above-described steel for pipes may further contain Ca instead of part of Fe.

Ca: 0 to 0.0030%
Calcium (Ca) is an optional element and may not be contained. When contained, Ca spheroidizes by controlling the form of MnS. In this case, the low temperature toughness of the steel increases. However, if the Ca content is too high, coarse oxide inclusions are formed. Therefore, the Ca content is 0 to 0.0030%. The minimum with preferable Ca content is 0.0002%, More preferably, it is 0.0005%. The upper limit with preferable Ca content is 0.0025%, More preferably, it is 0.0020%.

  The chemical composition of the above-described steel for pipes may further include one or more selected from the group consisting of Cr, Cu, and B instead of a part of Fe. All of these elements are optional elements and increase the strength of the steel.

Cr: 0 to 0.3%
Chromium (Cr) is an optional element and may not be contained. When contained, Cr increases the hardenability of the steel and increases the strength of the 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 0.3%. The minimum with preferable Cr content is 0.05%, More preferably, it is 0.06%. The upper limit with preferable Cr content is 0.25%, More preferably, it is 0.2%.

Cu: 0 to 0.3%
Copper (Cu) is an optional element and may not be contained. When contained, Cu increases the hardenability of the steel and increases the strength of the steel. However, if the Cu content is too high, the hardenability becomes too high and the toughness decreases. Therefore, the Cu content is 0 to 0.3%. The minimum with preferable Cu content is 0.05%, More preferably, it is 0.07%. The upper limit with preferable Cu content is 0.25%, More preferably, it is 0.20%.

B: 0 to 0.003%
Boron (B) is an optional element and may not be contained. When contained, B increases the hardenability and increases the strength of the steel. However, if the B content is too high, coarse nitrides are produced and the low temperature toughness of the steel is reduced. Therefore, the B content is 0 to 0.003%. The minimum with preferable B content is 0.0002%, More preferably, it is 0.00025%. The upper limit with preferable B content is less than 0.0030%, More preferably, it is 0.0028%.

[About Formula (1)]
The chemical composition further satisfies formula (1).
0.30 ≦ C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 3 + Nb / 3 ≦ 0.35 (1)
Here, the content (mass%) of the corresponding element is substituted for each element symbol of the formula (1). When an element corresponding to the element symbol in formula (1) is not contained, “0” is assigned to the corresponding element symbol in formula (1).

  As described above, in the chemical composition of the present embodiment, the contents of C, Si, Mn, Ni, Cr, Mo, V, and Nb increase the hardenability of the 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 (ferrite region, pearlite region, bainite region) of the CCT diagram shifts to the left (short time side). In this case, in the CCT diagram, the steel material temperature enters the ferrite region before the nucleation site is sufficiently generated. As a result, the ferrite crystal grains become coarse and the average crystal grain size increases. Furthermore, mixed grains are likely to occur, and the coarse crystal grain ratio increases. As a result, the low temperature toughness of the steel decreases. If F1 is too low, the hardenability is further lowered 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 time side). In this case, a hard structure is easily generated, and the ferrite fraction in the structure is lowered. As a result, the low temperature toughness of the steel decreases. If F1 is too high, the hardenability is further increased and the strength of the steel becomes too high.

  If F1 is 0.30 to 0.35, when the cooling curve C2 shown in FIG. 1 is performed, the temperature of the steel is easily held in the ferrite region. Therefore, the ferrite fraction in the central part of the thickness of the steel material can be made 65% or more, and the low temperature toughness of the steel can be improved.

[Ferrite fraction]
The structure of the center portion of the thickness of the steel for line pipe according to the present embodiment is composed of ferrite and a hard phase, and the remaining portion is composed of one or more selected from the group consisting of bainitic ferrite and Widmanstatten ferrite. Here, the thickness central portion is a range of ± 20% t from the plate thickness center or the wall thickness direction to the plate thickness direction or the wall thickness direction when the plate thickness or the wall thickness is tmm (that is, from the surface to the plate). It means 30 to 70% t in the thickness direction or the thickness direction).

  As described above, if the ferrite fraction of the structure in the central portion of the steel thickness is 65% or more, the crystal grains are refined, and as a result, the low temperature toughness of the steel is increased. The minimum with a preferable ferrite fraction is 70%, More preferably, it is 75%.

  The ferrite fraction means the ferrite area ratio and is measured by the following method. A sample is taken from the center of the thickness of the steel for line pipe. The collected sample is polished with a colloidal silica abrasive for 30 to 60 minutes. The polished sample is analyzed using EBSP-OIM ™ (Electron Back Scatter Diffraction Pattern-Orientation Image Microscope) to determine the ferrite fraction.

  Specifically, the ferrite fraction is obtained by the KAM (Kernel Average Misoration) method equipped in the EBSP-OIM.

  In the KAM method, a regular hexagonal pixel (center pixel) in the measurement data and a first approximation (total 7 pixels) using 6 pixels adjacent to this pixel, or the 6 pixels In addition, the second approximation (total 19 pixels) using the outer 12 pixels, or the third approximation (total 37 pixels) using the 18 pixels outside the 12 pixels, between each pixel. Are averaged, and the obtained average value is used as the value of the center pixel. This operation is performed on the entire pixel.

  This calculation is performed so as not to cross the grain boundary, and a map expressing the orientation change in the grain is created. That is, this map represents a strain distribution based on local orientation changes in the grains. In the present embodiment, an image having an azimuth difference of 5 ° or less between adjacent pixels is displayed by the third approximation. In the present embodiment, the surface area fraction of a pixel calculated to be an orientation difference third approximation of 1 ° or less is defined as the ferrite fraction. A structure exceeding the third approximate difference of 1 ° is assumed to be a structure other than ferrite such as bainite.

[Average crystal grain size]
In the steel product for line pipes of the present embodiment, the average crystal grain size at the central portion of the thickness of the steel product for line pipes is further 15 μm or less. If the average crystal grain size is too large, the low temperature toughness of the steel will decrease. In this embodiment, since the above-mentioned average crystal grain size is 15 μm or less, excellent low temperature toughness is obtained. A preferable upper limit of the average crystal grain size is 13 μm, and more preferably 10 μm.

  The average crystal grain size is measured using the EBSP-OIM method. Samples are collected and polished in the same manner as the ferrite fraction measurement. The polished sample is analyzed using EBSP-OIM. Specifically, in the azimuth measurement for each constant measurement step, a position where the azimuth difference between adjacent measurement points exceeds 15 ° is defined as a grain boundary. 15 ° is a threshold value for a large tilt grain boundary and is generally recognized as a crystal grain boundary. Using the region surrounded by the grain boundary as crystal grains, the grain size and the surface area of the crystal grains are determined. The area average particle size is determined from the obtained particle size and surface area. In the present specification, the obtained area average particle size is defined as the average crystal particle size.

[About coarse grain ratio]
In the above-described EBSP-OIM measurement, the area ratio of crystal grains having a crystal grain size of 20 μm or more at the center of the thickness of the steel product for line pipes is defined as “coarse crystal grain ratio”. When the crystal grains are coarse, the low temperature toughness of the steel decreases. If the coarse crystal grain ratio is 20% or less, excellent low temperature toughness can be obtained. A preferable upper limit of the coarse crystal grain ratio is 18%, and more preferably 15%. The lower the coarse crystal grain ratio, the better.

The coarse crystal grain ratio can be measured, for example, by the following method. A specimen for EBSP-OIM measurement is collected from the steel for line pipe, and the above-described EBSP-OIM measurement is performed. It can be determined by substituting N into the area of the measurement object observed in the EBSP-OIM measurement and n as the area of the coarse crystal grain, and substituting it into equation (2).
Coarse grain ratio (%) = (n / N) × 100 (2)

[Hard phase fraction]
As described above, the hard phase fraction is 10 to 20%. The hard phase contains one or more selected from the group consisting of pearlite, bainite, island martensite and MA. In this case, the strength of the steel is increased. A preferable upper limit of the hard phase fraction is 15%.

  A hard phase fraction means the area ratio in the whole hard phase. The hard phase fraction can be measured, for example, by the following method. A sample is taken from the center of the thickness of the steel for line pipe. The L section of the collected sample is corroded with a nital etchant. The corroded sample is observed using a scanning electron microscope (SEM) at a magnification of 1000 to 3000 times to determine the hard phase fraction. Specifically, image processing is performed on an SEM with an arbitrary 50 visual fields in the L cross section, and the area ratio of the hard phase in the entire visual field area is calculated.

[About the size of the hard phase]
In the steel material for line pipes of this embodiment, the size of the hard phase at the center of the thickness of the steel material for line pipes is 6.0 μm or less. If the size of the hard phase is too large, the low temperature toughness of the steel will decrease. In this embodiment, since the size of the hard phase is 6.0 μm or less, excellent low temperature toughness is obtained. A preferable lower limit of the size of the hard phase is 2.5 μm, and a preferable upper limit of the size of the hard phase is 5.5 μm.

  The size of the hard phase is determined from the area of the hard phase when an arbitrary 50 visual field is observed with the SEM in the L cross section for which the above-mentioned hard phase fraction is determined. Specifically, the equivalent circle diameter is calculated from the hard phase area, and the 50 average equivalent circle diameters obtained are taken as the size (μm) of the hard phase.

  By carrying out the manufacturing process described later, the average crystal grain size can be 15 μm or less and the coarse crystal grain ratio can be 20% or less in the structure at the center of the thickness. Furthermore, the ferrite fraction can be 65% or more and the hard phase fraction can be 10 to 20% or more. Furthermore, the size of the hard phase can be 6.0 μm or less. As a result, the low temperature toughness can be improved by setting the DWTT guaranteed temperature to −35 ° C. or lower. Furthermore, a yield stress of 435 to 570 MPa and a tensile strength of 535 to 760 MPa can be obtained. Furthermore, the yield ratio YR can be 0.93 or less.

[Production method]
An example of the manufacturing method of the above-mentioned line pipe steel will be described. FIG. 2 is a flowchart showing an example of a method for manufacturing a steel product for line pipes.

  With reference to FIG. 2, in this manufacturing method, the slab which is a raw material is manufactured using the molten steel which satisfy | fills the chemical composition mentioned above (raw material preparation process: S0). The manufactured slab is heated in a heating furnace (heating step: S1). The heated slab is rolled with a roughing mill and a finish rolling mill to produce a steel plate (rolling step: S2). The manufactured steel plate is cooled by an ROT (runout table) (ROT cooling step: S3). In the ROT cooling step (S3), first, the steel sheet is strongly cooled with a water cooling device (strong cooling step S31). After the strong cooling, the steel sheet is gradually cooled (slow cooling process: S32). After the slow cooling, the steel sheet is further strongly cooled (strong cooling step: S33). The steel plate after ROT cooling is wound up (winding step: S4). Through the above manufacturing process, a hot-rolled steel sheet for line pipe is manufactured.

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

[Material preparation process (S0)]
A material having the above chemical composition is prepared. Specifically, molten steel having the above-described chemical composition is manufactured. A raw material (slab) is manufactured using molten steel. A slab may be produced by a continuous casting method. An ingot may be manufactured using molten steel, and the material (slab) may be manufactured by performing ingot rolling on the ingot.

[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 1060 to 1150 ° C. If the heating temperature is too high, the crystal grains (austenite grains) become coarse and the low temperature toughness decreases. On the other hand, if the heating temperature is too low, refinement of crystal grains during rolling and precipitation strengthening after rolling cannot be obtained, and the strength decreases. Accordingly, the heating temperature is 1060 to 1150 ° C.

[Rolling step (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 obtain a steel plate. Both the rough rolling mill and the finish rolling mill include a plurality of rolling stands arranged in a row, and each rolling stand includes a roll pair.

In the rolling process, the surface temperature of the steel plate 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 not less than the Ar ₃ transformation temperature. If the finish rolling temperature is less than Ar ₃ transformation temperature, rolled resistance of the steel sheet and productivity is lowered increases. Furthermore, the steel sheet is rolled in a two-phase region of ferrite and austenite. In this case, the microstructure of the steel sheet forms a layered structure, and the mechanical properties deteriorate. Accordingly, the finish rolling temperature is equal to or higher than the Ar ₃ transformation temperature. In the steel material for line pipes of the present embodiment having the above-described chemical composition, the Ar ₃ transformation temperature is 750 to 850 ° C. The rolling reduction in the austenite non-recrystallization temperature region is preferably 60 to 80%. In this case, the unrecrystallized structure is refined.

  If the manufacturing method of this embodiment is used, excellent toughness can be obtained even if the plate thickness is 12 mm or more.

[ROT cooling step (S3)]
In the ROT (run-out table) cooling step (S3), the steel plate manufactured in the rolling step (S2) is cooled. The ROT cooling step (S3) includes a strong cooling step (S31), a slow cooling step (S32), and a strong cooling step (S33).

[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 plate immediately before water cooling is not particularly limited, but is preferably not less than the Ar ₃ transformation point. If the surface temperature of the steel plate immediately before water cooling is not less than the Ar ₃ transformation point, it is possible to prevent a decrease in strength due to grain growth and crystal grains becoming coarse.

  The cooling rate in the strong cooling step (S31) is set to V1 (° C./s). V1 is 10 to 50 ° C./s at the center of the plate thickness. When the cooling rate V1 is less than 10 ° C./s, the introduction of strain due to cooling is insufficient, so that sufficient ferrite nucleation sites cannot be obtained. In this case, since the generation amount of ferrite grains is reduced, the ferrite grains are coarsened and the low temperature toughness of the steel is lowered. When the cooling rate V1 exceeds 50 ° C./s, the steel sheet temperature does not pass through the ferrite region and reaches the bainite region in the CCT diagram. In this case, the structure is mainly bainite, and the low temperature toughness of the steel is reduced. Therefore, the cooling rate V1 is 10 ° C./s or more. A preferable lower limit of the cooling rate V1 is 12 ° C./s. A preferable upper limit of the cooling rate V1 is 45 ° C./s.

  In the strong cooling step (S31), the steel sheet is cooled in a temperature range where the surface temperature of the steel sheet is 870 to 750 ° C. In other words, the strong cooling stop temperature T1 is 750 ° C. If the strong cooling stop temperature T1 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. On the other hand, if the strong cooling stop temperature T1 is too high, the precipitation of Nb strengthening the pro-eutectoid ferrite becomes over-aged, and the strength of the steel decreases. If the strong cooling stop temperature T1 is set to 750 ° C., the ferrite fraction can be made 65% or more by gradually cooling in the subsequent slow cooling step (S4), and the low temperature toughness of the steel is increased.

[Slow cooling step (S32)]
Slow 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 set to V2 (° C./s). The cooling rate V2 is 2 to 5 ° C./s at the center of the plate thickness. If the cooling rate V2 is too slow, productivity 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 to 5 ° C./s. A preferable lower limit of the cooling rate V2 is 2.5 ° C./s. A preferable upper limit of the cooling rate V2 is 4.5 ° C./s.

  In the slow cooling step (S32), the steel sheet is cooled in a temperature range of 750 to 650 ° C. In other words, the slow cooling stop temperature T2 is 650 ° C. If the slow cooling stop temperature T2 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. If the slow cooling stop temperature T2 is too high, the strength of the steel decreases. Therefore, the slow cooling stop temperature T2 is 650 ° C.

[Strong cooling step (S33)]
Strong cooling is implemented with respect to the steel plate gradually cooled by the slow cooling process (S32).

  The cooling rate in the strong cooling step (S33) is set to V3 (° C./s). The cooling rate V3 is 5 to 10 ° C./s at the center of the plate thickness. If the cooling rate V3 is too slow, a fine hard phase cannot be obtained. As a result, the strength and low temperature toughness of the steel are reduced. If the cooling rate V3 is too fast, the steel plate temperature does not sufficiently pass through the ferrite region and reaches the bainite region in the CCT diagram. In this case, the ferrite fraction is reduced and a steel structure mainly composed of bainite is obtained, and the low temperature toughness of the steel is reduced. Therefore, the cooling rate V3 is 5 to 10 ° C./s. A preferable lower limit of the cooling rate V3 is 5.5 ° C./s. A preferable upper limit of the cooling rate V3 is 9.5 ° C./s.

  In the strong cooling step (S33), the steel sheet is cooled in a temperature range where the surface temperature of the steel sheet is 650 to 500 ° C.

[Winding process (S4)]
In the winding process (S4), the steel sheet cooled in the ROT cooling process (S3) is wound to obtain a coiled hot-rolled steel sheet for a line pipe.

  The surface temperature (hereinafter referred to as the coiling temperature) T4 of the steel sheet at the time of winding the coiled hot-rolled steel sheet for line pipe is 500 to 580 ° C. If the coiling temperature T4 is too low, the coarse crystal grain ratio increases and the low-temperature toughness decreases. On the other hand, if winding temperature T4 is too high, a crystal grain will coarsen and the low temperature toughness of steel will fall. Accordingly, the winding temperature T4 is 500 to 580 ° C. Preferable T4 is 510-570 degreeC, More preferably, it is 520-560 degreeC.

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

  The electric-welded steel pipe for line pipes of this embodiment is manufactured by the following pipe manufacturing process (S5) using the above-mentioned hot-rolled steel sheet for line pipes, for example.

[Pipe making process (S5)]
While rewinding the coiled hot-rolled steel sheet for a line pipe, an electric-welded steel pipe for a line pipe is manufactured by a well-known method. Specifically, the hot-rolled steel sheet for line pipes is formed into a tubular shape (open pipe) by bending with a continuous forming roll. Subsequently, the seam portion of the open pipe, that is, both end faces in the longitudinal direction of the hot-rolled steel sheet for line pipe are welded by the electric resistance welding method. Through the above process, the electric pipe for line pipe is manufactured.

  In the steel for line pipes (hot rolled steel sheet and ERW steel pipe) manufactured by the above manufacturing process, the average crystal grain size is 15 μm or less, the coarse crystal grain ratio is 20% or less, and the ferrite content in the structure at the center of the thickness. The ratio is 65% or more, the hard phase fraction is 10 to 20%, and the size of the hard phase is 6.0 μm or less. As a result, the DWTT guaranteed temperature can be set to −35 ° C. or lower, and the low temperature toughness can be enhanced. Furthermore, a yield stress of 450 to 570 MPa and a tensile strength of 535 to 760 MPa can be obtained.

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

  Using a plurality of slabs of Steel A to Steel J, ERW steel pipes for line pipes of Test No. 1 to Test No. 27 shown in Table 2 were manufactured.

Specifically, each test number slab was heated in a heating furnace. The heating temperature (° C.) was as shown in Table 2. The heated slab was rolled using a roughing mill and allowed to cool to 920 ° C. Thereafter, finish rolling was performed with a finish rolling mill. The rolling reduction in the non-recrystallization temperature range was 60 to 80% for all test numbers. The finish rolling temperature was Ar ₃ or higher.

  ROT cooling was performed on the steel sheet after finish rolling. In ROT cooling, cooling was performed under the cooling conditions shown in Table 2 (strong cooling rate V1 (° C./second), slow cooling rate V2 (° C./second), and strong cooling rate V3 (° C./second)). The strong cooling stop temperature T1 (° C.) and the slow cooling stop temperature T2 (° C.) were 750 ° C. and 650 ° C., respectively, in all the steel plates.

  After manufacturing the steel sheet by the above manufacturing process, winding was performed at a winding temperature T4 shown in Table 2 to manufacture a hot-rolled steel sheet for line pipe. Furthermore, pipes were manufactured by the above-described method using a hot-rolled steel sheet for line pipes, and an ERW steel pipe for line pipes having an outer diameter of 389 mm or more and a wall thickness of 12 mm or more was manufactured.

[Test method]
[Micro structure]
Based on the above-mentioned method, the average crystal grain size, the coarse crystal grain ratio, the ferrite fraction, the hard phase fraction, and the size of the hard phase were measured as described above using EBSP-OIM. The measurement conditions of EBSP-OIM in the average crystal grain size measurement were as follows: magnification: 400 times, visual field area: 200 μm × 500 μm, measurement step: 0.3 μm. The measurement conditions of the SEM in the measurement of the hard phase fraction and the size of the hard phase were: magnification: 1000 to 3000 times, and visual field area 45 μm × 60 μm. For the hard phase, a micro Vickers hardness test was also performed as described above. The test conditions were a test temperature of room temperature (25 ° C.) and a test force of 25 gf. The micro Vickers hardness was 200 Hv or higher in all test numbers.

[Strength test]
Tensile specimens were collected from ERW steel pipes for line pipes of each test number. Specifically, a tensile test piece of full thickness was taken from a position 90 ° (position facing away from the welded portion) of the welded portion of the linepipe ERW steel tube viewed in the axial direction. . The cross section of the tensile specimen was arcuate, and the longitudinal direction of the tensile specimen was parallel to the longitudinal direction of the steel pipe. The size of the tensile test piece was as shown in FIG. 3, the length of the parallel part was 50.8 mm, and the width of the parallel part was 38.1 mm. The numerical value in FIG. 3 shows the dimension (a unit is mm) of the corresponding site | part of a test piece. Using a tensile test piece, a tensile test was performed at room temperature in accordance with the API standard 5CT. Based on the test results, the yield strength YS (MPa) and tensile strength TS (MPa) of the ERW steel pipe for line pipes were determined.

[Low temperature toughness test]
DWTT specimens were taken from ERW steel pipes for line pipes of each test number. The sampling position was the same as the tensile test piece (position facing away from the weld). The size of the DWTT test piece was as shown in FIG. The arc-shaped member sampled from the sampling position was developed into a flat plate shape, and a notch was processed at a 180 ° position. The numerical value in FIG. 4 shows the dimension (a unit is mm) of the corresponding site | part of a test piece. t indicates the thickness (unit: 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 piece was subjected to a DWTT test in accordance with ASTM E 436, and the lowest temperature (DWTT guaranteed temperature) at which the ductile fracture surface ratio was 85% was determined. When the DWTT guaranteed temperature was −35 ° C. or lower, it was evaluated that the low temperature toughness was high.

[Test results]
Table 3 shows the test results.

  Referring 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 production conditions of any test number were appropriate. Therefore, the average crystal grain size of Test No. 1 to Test No. 13 was 15 μm or less, and the coarse crystal grain ratio was 20% or less. Furthermore, the ferrite fraction was 65% or more, and the hard phase fraction was 10 to 20%. It contained one or more selected from the group consisting of hard phase pearlite, bainite, island martensite and MA, and the size of the hard phase was 6.0 μm or less. As a result, the DWTT guaranteed temperature was −35 ° C. or lower, and excellent low temperature toughness was exhibited. Further, the yield strength YS was 435 to 570 MPa, and the tensile strength TS was 535 to 760 MPa.

  On the other hand, in the test number 14, the heating temperature was less than 1060 ° C, V2 exceeded 5 ° C / s, and V3 was less than 5 ° C / s. Therefore, the average crystal grain size exceeded 15 μm, and the coarse crystal grain ratio also exceeded 20%. As a result, the yield strength YS was less than 435 MPa, and the tensile strength TS was also less than 535 MPa. Furthermore, DWTT guarantee temperature was higher than -35 degreeC, and low temperature toughness was low.

  In test number 15, the heating temperature exceeded 1150 ° C, V2 exceeded 5 ° C / s, and V3 was less than 5 ° C / s. Therefore, the average crystal grain size exceeded 15 μm, and the coarse crystal grain ratio also exceeded 20%. Further, the ferrite fraction was less than 65%. As a result, the DWTT guaranteed temperature was higher than −35 ° C. and the low temperature toughness was low.

  In test number 16, since V1, V2, and V3 were too low, the winding temperature T4 exceeded 580 ° C. Therefore, the average crystal grain size exceeded 15 μm and the coarse crystal grain ratio exceeded 20%. Furthermore, the size of the hard phase exceeded 6.0 μm. As a result, the yield strength YS was less than 435 MPa, and the tensile strength TS was also less than 535 MPa. Furthermore, DWTT guarantee temperature was higher than -35 degreeC, and low temperature toughness was low.

  In test number 17, V1 exceeded 50 ° C./s, V3 was less than 5 ° C./s, and winding temperature T4 was less than 500 ° C. Therefore, the average crystal grain size exceeded 15 μm and the coarse crystal grain ratio exceeded 20%. Further, the ferrite fraction was less than 65%, the hard phase fraction exceeded 20%, and the hard phase size exceeded 6.0 μm. As a result, the DWTT guaranteed temperature was higher than −35 ° C. and the low temperature toughness was low.

  In test number 18, since V2 exceeded 5 ° C / s, T4 was less than 500 ° C. Therefore, the hard phase fraction exceeded 20%. The DWTT guaranteed temperature was higher than -35 ° C, and the low temperature toughness was low.

  In test number 19, V1 exceeded 50 ° C./s and the winding temperature T4 was less than 500 ° C. Therefore, the average crystal grain size exceeded 15 μm and the coarse crystal grain ratio exceeded 20%. Further, the ferrite fraction was less than 65%, the hard phase fraction exceeded 20%, and the hard phase size exceeded 6.0 μm. As a result, the DWTT guaranteed temperature was higher than −35 ° C. and the low temperature toughness was low.

  In test number 20, the heating temperature was too high and V1, V2, and V3 were too low, so the winding temperature T4 exceeded 580 ° C. Therefore, the average crystal grain size exceeded 15 μm and the coarse crystal grain ratio exceeded 20%. Furthermore, the size of the hard phase also exceeded 6.0 μm. As a result, the DWTT guaranteed temperature was higher than −35 ° C. and the low temperature toughness was low.

  In test number 21, since the heating temperature was too low and V1, V2, and V3 were too low, the winding temperature T4 exceeded 580 ° C. Therefore, the average crystal grain size exceeded 15 μm and the coarse crystal grain ratio exceeded 20%. Furthermore, the size of the hard phase also exceeded 6.0 μm. As a result, the DWTT guaranteed temperature was higher than −35 ° C. and the low temperature toughness was low.

  In test number 22, the heating temperature was less than 1060 ° C. Therefore, the average crystal grain size exceeded 15 μm, and the coarse crystal grain ratio also exceeded 20%. As a result, the yield strength YS was less than 435 MPa, and the tensile strength TS was also less than 535 MPa. Furthermore, DWTT guarantee temperature was higher than -35 degreeC, and low temperature toughness was low.

  In test number 23, the heating temperature exceeded 1150 ° C. Therefore, the average crystal grain size exceeded 15 μm, and the coarse crystal grain ratio also exceeded 20%. As a result, the DWTT guaranteed temperature was higher than −35 ° C. and the low temperature toughness was low.

  In test number 24, although manufacturing conditions were appropriate, F1 exceeded the upper limit of formula (1). Therefore, the average crystal grain exceeded 15 μm. Furthermore, the ferrite fraction was less than 65%, the hard phase fraction exceeded 20%, and the size of the hard phase exceeded 6.0 μm. Therefore, YS exceeded 570 MPa. Furthermore, DWTT guarantee temperature was higher than -35 degreeC, and low temperature toughness was low.

  In test number 25, although manufacturing conditions were appropriate, F1 was less than the lower limit of formula (1). Therefore, the average crystal grain exceeded 15 μm and the coarse crystal grain ratio exceeded 20%. Furthermore, the hard phase fraction was less than 10%, and the size of the hard phase exceeded 6.0 μm. Therefore, the DWTT guaranteed temperature was higher than −35 ° C. and the low temperature toughness was low.

  In test number 26, V2 and V3 were too low, so the winding temperature T4 exceeded 580 ° C. Therefore, the fraction of the hard phase was also less than 10%. As a result, the yield strength YS was less than 435 MPa, and the tensile strength TS was also less than 535 MPa. Moreover, DWTT guarantee temperature was higher than -35 degreeC, and low temperature toughness was low.

  In test number 27, V3 was too low. Therefore, the size of the hard phase exceeded 6 μm. As a result, the DWTT guaranteed temperature was higher than −35 ° C. and the low temperature toughness was low.

  The embodiment of the present invention has been described above. However, the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately changing the above-described embodiment without departing from the spirit thereof.

Claims (6)

% By mass
C: 0.01 to less than 0.06%,
Si: 0.05-0.2%
Mn: 0.5-2%
P: 0.03% or less,
S: 0.01% or less,
Al: 0.01-0.035%,
N: 0.001 to 0.008%,
Nb: 0.01 to 0.25%,
Ti: 0.005 to 0.03%,
Ni: 0.001 to 0.2%,
Mo: 0.1-0.2%,
V: 0.001 to 0.01%,
O: 0.003% or less,
Ca: 0 to 0.0030%,
Cr: 0 to 0.3%,
Cu: 0 to 0.3%, and
B: 0 to 0.003% is contained, the balance is Fe and impurities, and has a chemical composition satisfying the formula (1),
In the structure of the central portion of the thickness, the average crystal grain size is 15 μm or less, the coarse crystal grain ratio, which is the area ratio of crystal grains having a crystal grain diameter of 20 μm or more, is 20% or less, and the ferrite fraction is 65% or more. And the hard phase fraction is 10-20%,
The steel material for line pipes whose hard phase size is 6.0 μm or less.
0.30 ≦ C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 3 + Nb / 3 ≦ 0.35 (1)
Here, the content (mass%) of the corresponding element is substituted for each element symbol of the formula (1). It is a steel material for line pipes according to claim 1,
The chemical composition is mass%,
Ca: Steel material for line pipes containing 0.0002 to 0.0030%. It is a steel material for line pipes according to claim 1 or 2,
The chemical composition is mass%,
Cr: 0.05 to 0.3%,
Cu: 0.05-0.3% and
B: Steel material for line pipes containing one or more selected from the group consisting of 0.0002 to 0.003%. It is a steel material for line pipes according to claims 1-3,
The steel material for line pipes is a steel material for line pipes, which is a hot-rolled steel plate for line pipes. It is a steel material for line pipes according to claims 1-3,
The steel material for line pipes is a steel material for line pipes, which is an electric resistance steel pipe for line pipes. Heating the material having the chemical composition according to any one of claims 1 to 3 at 1060 to 1150 ° C;
A step of performing rough rolling and finish rolling on the heated material to form a steel plate;
A step of strongly cooling a temperature range of 870 to 750 ° C. at a cooling rate of 10 to 50 ° C./s at the thickness center with respect to the steel plate after the finish rolling,
A step of gradually cooling the strongly cooled steel plate at a temperature range of 750 to 650 ° C. at a cooling rate of 2 to 5 ° C./s at the center of the plate thickness;
A step of strongly cooling the steel sheet that has been slowly cooled at a cooling rate of 5 to 10 ° C./s at a thickness center of a temperature range of 650 to 500 ° C .;
And a step of winding the strongly cooled steel plate at 500 to 580 ° C.

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