JP6772823B2 - Steel materials for line pipes and their manufacturing methods - Google Patents

Description

The present invention relates to a steel material and a method for producing the same, and more particularly to a steel material for a line pipe and a method for producing the same.

Pipelines laid on the seabed allow high-pressure fluid to pass through. The pipeline is also subject to repeated strains due to waves and seawater pressure. Therefore, steel pipes used for submarine pipelines are required to have high strength and high low temperature toughness.

The pipeline is composed of a plurality of line pipes. As a steel pipe for a line pipe, an electric resistance welded steel pipe (hereinafter referred to as an electric resistance welded steel pipe) may be used. High strength can be obtained by increasing the wall thickness of the electrosewn steel pipe. However, if the wall thickness is increased, brittle fracture is likely to occur and the low temperature toughness is lowered. 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. The lower the DWTT guaranteed temperature, the higher the low temperature toughness. In recent years, electric resistance sewn steel pipes for line pipes are required to have excellent low temperature toughness.

Thick-walled electrosewn steel pipes for line pipes are also required to have a lower yield ratio (YR). As a method of laying submarine line pipes, the reeling method, which is carried out using a laying vessel equipped with a large spool, is increasingly being adopted. In the reeling method, a connecting pipe is manufactured on land by butt welding (peripheral welding) the pipe ends of electric resistance sewn steel pipes. The manufactured connecting pipe is wound on a large spool of the laying ship and then transported to the laying site. On the ocean of the laying site, lay it on the seabed while unwinding the connecting pipe wound around a large drum.

In the reeling method, the electric resistance steel pipe is once wound on a spool and then rewound at sea. Therefore, tensile stress and compressive stress due to bending and bending back are applied to a part of the electrosewn steel pipe. If the deformability of the power-stitched steel pipe is low, the application of these stresses may cause local buckling of the power-stitched steel pipe or break a part of the power-stitched steel pipe. Therefore, electric resistance sewn steel pipes for line pipes are required to have not only high strength and excellent low temperature toughness but also a low yield ratio.

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

The hot-rolled steel sheet for line pipes disclosed in Patent Document 1 has 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%, and the balance is composed of Fe and unavoidable impurity elements. In the microstructure at a depth of 1/2 of the plate thickness from the surface of the steel plate, the pro-eutectoid ferrite fraction is 3% or more and 20% or less, and the others are the low temperature transformation phase and pearlite of 1% or less, and the entire microstructure The number average crystal grain size is 1 μm or more and 2.5 μm or less, the area average particle size is 3 μm or more and 9 μm or less, the standard deviation of the area average particle size is 0.8 μm or more and 2.3 μm or less, and the plate from the steel plate surface to the plate. At a depth of 1/2 of the thickness, the reflected X-ray intensity ratio {211} / {111} in the {211} direction and the {111} direction with respect to the surface parallel to the steel plate surface is 1.1 or more. It is stated that this steel sheet for line pipes can obtain excellent strength and low temperature toughness by controlling the proeutectoid ferrite fraction at the center of the thickness, the average particle size, and the texture.

International Publication No. 2012/002481

However, in the hot-rolled steel sheet disclosed in Patent Document 1, the heating temperature before the rolling process is high, and the austenite grains may become coarse. In this case, the crystal grains may become coarse and the low temperature toughness may decrease. Further, Patent Document 1 does not disclose measures for local buckling assuming a reeling method. Therefore, local buckling may occur in the electric resistance welded steel pipe manufactured by using this hot-rolled steel sheet.

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

The steel material for line pipes according to this embodiment has C: 0.06 to 0.12%, Si: 0.05 to 0.3%, Mn: 0.5 to 2%, P: 0.03 in mass%. % Or less, S: 0.01% or less, O: 0.003% 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: 0.01 to 0.2%, Mo: 0.01 to 0.2%, Cu: 0.01 to 0.3%, Cr: 0 to 0 It contains 0.3%, V: 0 to 0.01%, B: 0 to 0.003%, and Ca: 0 to 0.0030%, and the balance consists of Fe and impurities. Has a chemical composition that meets. 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 the crystal grains having a crystal grain size 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.
0.35 ≤ C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 3 + Nb / 3 ≤ 0.40 (1)
Here, the content (mass%) of the corresponding element is substituted for each element symbol of the formula (1).

The steel material for line pipes is, for example, a hot-rolled steel plate for line pipes or an electrosewn steel pipe for line pipes.

The steel material for line pipes according to this embodiment has excellent low temperature toughness and strength and a low yield ratio.

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

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

(A) If the C content in the steel is increased, the tensile strength of the steel is increased by work hardening after yielding. As a result, the yield ratio (yield strength / tensile strength) becomes low. Specifically, if the C content is 0.06 to 0.12%, the C direction (the plate width direction, the direction perpendicular to the rolling direction) and the L direction (longitudinal direction) are assumed to satisfy the conditions described below. The yield ratio (direction, rolling direction) is 0.93 or less.

(B) When the ferrite fraction of the structure of the steel is 65% or more, fine crystal grains can be obtained and the low temperature toughness of the steel is enhanced. The heating temperature during rolling is set to 1150 ° C. or lower to suppress coarsening of crystal grains. 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 finer, and as a result, the low temperature toughness of the steel is increased.

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 for each block, and each block is substantially one. It becomes one crystal grain. Therefore, the size of crystal grains in bainite is determined by the size of old austenite grains. Therefore, the crystal grains tend to be coarsened, and as a result, the low temperature toughness of the steel tends to decrease.

However, bainite, pearlite, island martensite and MA (Martensite-austenite constituent) increase strength. Therefore, the steel material for line pipes of the present embodiment includes a hard phase in which the steel structure 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. Further, if the crystal grains of the hard phase are made finer, the low temperature toughness is further enhanced.

(C) As an example of the cooling control method, in the cooling step in the ROT (runout table) after rolling, strong cooling is first performed, and then slow cooling is performed. As a result, the ferrite fraction (the area ratio occupied by ferrite in the structure) increases in the structure at the center of the thickness of the steel material for line pipes, and the low temperature toughness of the steel increases. Further, slow cooling is followed by strong cooling. As a result, a fine hard phase containing one or more selected from the group consisting of pearlite, bainite, island martensite and MA is obtained in the structure of the central part of the thickness of the steel material. As a result, the strength of the steel is increased and the low temperature toughness is further increased. This point will be described in detail below.

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

As shown in FIG. 1, the ferrite nose is located higher than the pearlite nose and the bainite nose. The broken line C1 in FIG. 1 shows a cooling curve by a conventional cooling process. In the conventional cooling method, the ferrite nose, the pearlite nose, and the bainite nose are all passed through in the cooling process. Therefore, pearlite and bainite are formed in the structure, and the ferrite fraction in the structure decreases.

Therefore, in the present embodiment, cooling is performed along the cooling curve of the broken line C2. 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 strong cooling increases the degree of supercooling, 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 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 is increased.

After 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. This produces fine pearlite, bainite, island martensite and MA. Therefore, the strength of the steel is increased, and the low temperature toughness is further increased.

(D) 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 shifts too far to the left. In this case, in the cooling process after rolling, the steel temperature enters the ferrite region before sufficient nucleation sites are formed. Therefore, the ferrite grains become coarse and the average crystal grain size becomes large. Further, since it tends to have a mixed grain structure, the coarse crystal grain ratio becomes large. 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 shifts too much 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 steel decreases.

When F1 is 0.35 to 0.40, the S curves (ferrite, pearlite, bainite) of each phase are arranged at appropriate positions in the CCT diagram. In this case, as shown in the cooling curve C2 in FIG. 1, cooling can be performed mainly through the ferrite region. Therefore, a ferrite-based structure can be formed, and high strength and low temperature toughness can be obtained.

The steel material for line pipes of the present embodiment completed based on the above findings has a mass% of C: 0.06 to 0.12%, Si: 0.05 to 0.3%, Mn: 0.5 to 2%, P: 0.03% or less, S: 0.01% or less, O: 0.003% 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: 0.01 to 0.2%, Mo: 0.01 to 0.2%, Cu: 0.01 to 0 .3%, Cr: 0 to 0.3%, V: 0 to 0.01%, B: 0 to 0.003%, and Ca: 0 to 0.0030%, with the balance being Fe and impurities. It is composed of and has a chemical composition satisfying the 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 the crystal grains having a crystal grain size 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.
0.35 ≤ C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 3 + Nb / 3 ≤ 0.40 (1)
Here, the content (mass%) of the corresponding element is substituted for each element symbol of the formula (1).

The chemical composition is one or 2 selected from the group consisting of Cr: 0.01 to 0.3%, V: 0.001 to 0.01%, and B: 0.0002 to 0.003%. It may contain more than a seed. Further, the chemical composition may contain Ca: 0.0005 to 0.0030%.

The steel material for line pipes is, for example, a hot-rolled steel plate for line pipes or an electrosewn steel pipe for line pipes.

The method for producing a steel material for line pipes of the present embodiment includes a step of heating a material having the above-mentioned chemical composition at 106 to 1150 ° C., and rough rolling and finish rolling of the heated material to obtain a steel sheet. It includes a step and a step of cooling the steel sheet after finish rolling. In the cooling step, the 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 the temperature range of 750 to 650 ° C. is set for the strongly cooled steel plate. The process of slowly cooling the center of the plate thickness at a cooling rate of 2 to 5 ° C / s, and the cooling rate of the center of the plate thickness of 5 to 10 ° C / s in the temperature range of 650 to 500 ° C for the slowly cooled steel plate. It is equipped with a process of strong cooling with. A step of winding the cooled steel sheet at 500 to 580 ° C. is further provided.

The method for manufacturing an electrosewn steel pipe for a line pipe according to the present embodiment includes a step of forming and welding the above-mentioned hot-rolled steel sheet to produce the pipe.

Hereinafter, the steel material for line pipes of the present embodiment will be described in detail. Unless otherwise specified, "%" for an element means mass%.

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

C: 0.06 to 0.12%
Carbon (C) increases the strength of steel. C further increases the tensile strength by work hardening and lowers the yield ratio. If the C content is too low, this effect cannot be obtained. On the other hand, if the C content is too high, carbides will be formed and the low temperature toughness and ductility of the steel will decrease. If the C content is too high, the weldability is further reduced. Therefore, the C content is 0.06 to 0.12%. The lower limit of the C content is preferably 0.08%, more preferably 0.085%. The preferable upper limit of the C content is 0.10%.

Si: 0.05-0.3%
Silicon (Si) deoxidizes steel. Si further enhances the hardenability of steel and enhances the strength of 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 strength of the steel becomes too high and the low temperature toughness of the steel decreases. Therefore, the Si content is 0.05 to 0.3%. The preferred lower limit of the Si content is 0.07%, more preferably 0.1%. The preferred upper limit of the Si content is 0.2%, more preferably 0.195%, still more preferably 0.19%.

Mn: 0.5-2%
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 0.5 to 2%. The preferred lower limit of the Mn content is 0.7%, more preferably 1.0%. The preferred upper limit of the Mn content is 1.8%, more preferably 1.5%.

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

S: 0.01% or less Sulfur (S) is an impurity. S combines with Mn to form Mn-based sulfide. Therefore, the low temperature toughness and sour resistance of steel are lowered. Therefore, the S content is 0.01% or less. The preferred upper limit of the S content is 0.001%, more preferably 0.0005%. It is preferable that the S content is as low as possible.

O: 0.003% or less Oxygen (O) is an impurity. O forms an oxide and reduces the hydrogen-induced cracking resistance of steel. In addition, it reduces low temperature toughness. Therefore, the O content is 0.003% or less. The preferred upper limit of the O content is 0.0025%, more preferably 0.0020%. The O 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 oxide becomes coarse and the low temperature toughness of the steel decreases. Therefore, the Al content is 0.01 to 0.035%. The lower limit of the Al content is preferably 0.017%, more preferably 0.020%. The preferred upper limit of the Al content is 0.030%, more preferably 0.025%. In the present specification, the Al content means the total Al content.

N: 0.001 to 0.008%
Nitrogen (N) forms a nitride and suppresses the coarsening of austenite grains during the heating step. 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 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 lowered. Therefore, the N content is 0.001 to 0.008%. The preferred lower limit of the N content is 0.002%, more preferably 0.003%. The preferred upper limit of the N content is 0.006%, more preferably 0.005%.

Nb: 0.01 to 0.25%
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 average crystal grain size. Therefore, the low temperature toughness of steel is increased. Furthermore, the fine Nb carbonitride enhances 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 0.01-0.25%. The preferred lower limit of the Nb content is 0.012%, more preferably 0.02%. The preferred upper limit of the Nb content is 0.08%, more preferably 0.07%.

Ti: 0.005 to 0.03%
Titanium (Ti) combines with N in steel to form TiN, and suppresses the decrease in low temperature toughness of steel due to solid solution N. Further, fine TiN is dispersed and precipitated to suppress coarsening of crystal grains. 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 preferred lower limit of the Ti content is 0.008%, more preferably 0.01%. The preferred upper limit of the Ti content is 0.02%, more preferably 0.015%.

Ni: 0.01-0.2%
Nickel (Ni) enhances the hardenability of steel and enhances the strength of steel. On the other hand, if the Ni content is too high, this effect will be saturated. Therefore, the Ni content is 0.01-0.2%. The preferred lower limit of the Ni content is 0.05%, more preferably 0.08%. The preferred upper limit of the Ni content is 0.15%, more preferably 0.10%.

Mo: 0.01-0.2%,
Molybdenum (Mo) enhances the hardenability of steel and enhances the strength of steel. Mo further refines the austenite granules and enhances low temperature toughness. On the other hand, if the Mo content is too high, the on-site weldability of the steel will deteriorate. Therefore, the Mo content is 0.01-0.2%. The preferred lower limit of the Mo content is 0.05%, more preferably 0.1%. The preferred upper limit of the Mo content is less than 0.2%.

Cu: 0.01-0.3%
Copper (Cu) enhances the hardenability of steel and enhances the strength of steel. On the other hand, if the Cu content is too high, the hardenability becomes too high and the low temperature toughness decreases. Therefore, the Cu content is 0.01-0.3%. The preferred lower limit of the Cu content is 0.05%, more preferably 0.1%. The preferred upper limit of the Cu content is 0.25%, more preferably 0.20%.

The balance of the chemical composition of the steel material for line pipes according to this embodiment is composed of Fe and impurities. Here, the impurities are those mixed from ore, scrap, manufacturing environment, etc. as raw materials when the steel material for line pipe is industrially manufactured, and adversely affect the steel material for line pipe of the present embodiment. Means what is allowed within the range that does not give.

[About arbitrary elements]
The chemical composition of the above-mentioned steel material for line pipes may further contain one or more selected from the group consisting of Cr, V and B instead of a part of Fe. Both of these elements increase the strength of steel.

Cr: 0-0.3%
Chromium (Cr) is an optional element and may not be contained. When contained, Cr enhances the hardenability of the steel and enhances the strength of the steel. Cr further enhances the work hardening of the steel after yielding to increase the tensile strength and reduce the yield ratio. However, if the Cr content is too high, the hardenability becomes too high and the low temperature toughness decreases. Therefore, the Cr content is 0 to 0.3%. The lower limit of the Cr content is preferably 0.01%, more preferably 0.1%. The preferred upper limit of the Cr content is 0.25%, more preferably 0.2%.

V: 0-0.01%
Vanadium (V) is an optional element and may not be contained. When contained, V combines with C and N in the steel during the winding process to form fine carbonitrides, increasing the strength of the steel. The fine V-carbonitride further suppresses the coarsening of crystal grains and enhances the low temperature toughness of the steel. If the V content is too low, these effects cannot be obtained. However, if the V content is too high, the V carbonitride becomes coarse and the low temperature toughness of the steel decreases. Therefore, the V content is 0 to 0.01%. The preferred lower limit of the V content is 0.001%, more preferably 0.005%. The preferred upper limit of the V content is 0.008%, more preferably 0.005%.

B: 0 to 0.003%
Boron (B) is an optional element and may not be contained. When contained, B enhances hardenability and enhances the strength of steel. However, if the B content is too high, coarse nitrides will be formed and the low temperature toughness of the steel will decrease. Therefore, the B content is 0 to 0.003%. The preferred lower limit of the B content is 0.0002%, more preferably 0.00025%. The preferred upper limit of the B content is 0.0028%.

The chemical composition of the above-mentioned steel material for line pipes may further contain Ca instead of a part of Fe.

Ca: 0-0.0030%,
Calcium (Ca) is an optional element and may not be contained. When contained, Ca controls the morphology of MnS to spheroidize. In this case, the low temperature toughness of the steel is increased. However, if the Ca content is too high, coarse oxide-based inclusions will be formed. Therefore, the Ca content is 0 to 0.0030%. The preferable lower limit of the Ca content is 0.0005%. The preferred upper limit of the Ca content is 0.0025%, more preferably 0.0020%.

[About equation (1)]
The chemical composition further satisfies the formula (1).
0.35 ≤ C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 3 + Nb / 3 ≤ 0.40 (1)
Here, the content (mass%) of the corresponding element is substituted for each element symbol of the formula (1). Further, when the element corresponding to the element symbol in the formula (1) is not contained, "0" is substituted for the corresponding element symbol in the formula (1).

As described above, in the chemical composition of the present embodiment, the C, Si, Mn, Ni, Cr, Mo, V, and Nb contents affect the S curve in 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 (ferrite region, pearlite region, bainite region) of the CCT diagram shifts to the left side (short time side). In this case, in the CCT diagram, the steel temperature enters the ferrite region before the nucleation sites are sufficiently formed. As a result, the ferrite crystal grains become coarse and the average crystal grain size becomes large. Further, mixed grains are likely to occur, and the coarse crystal grain ratio becomes large. As a result, the low temperature toughness of steel decreases. If F1 is too low, the hardenability is further lowered and the strength of the steel is lowered.

On the other hand, if F1 is too high, the S curve of the CCT diagram shifts to the right side (long-time side). In this case, a hard structure is likely to be formed, and the ferrite fraction in the structure is lowered. As a result, the low temperature toughness of steel decreases. If F1 is too high, the hardenability is further high and the strength of the steel is too high.

When F1 is 0.35 to 0.40, the ferrite fraction at the center of the thickness of the steel material can be 65% or more and the hard phase fraction can be 10 to 20%, thereby increasing the low temperature toughness of the steel. Can be done.

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

As described above, when the ferrite fraction of the structure at the center of the thickness of the steel is 65% or more, the crystal grains become finer, and as a result, the low temperature toughness of the steel is enhanced. The preferred lower limit of the ferrite fraction is 70%, more preferably 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 material for line pipes. The collected sample is polished with a colloidal silica abrasive for 30 to 60 minutes. The polished sample is analyzed using EBSP-OIM ™ (Electron Backscatter Diffraction Pattern Microscopy) to determine the ferrite fraction.

Specifically, the ferrite fraction is determined by the KAM (Kernel Average Missionation) method installed in the EBSP-OIM.

In the KAM method, a regular hexagonal pixel (center pixel) in the measurement data and the first approximation (7 pixels in total) using 6 pixels adjacent to this pixel, or these 6 pixels Between each pixel for the second approximation (19 pixels in total) using the 12 pixels outside it, or the third approximation (37 pixels in total) using 18 pixels further outside these 12 pixels. The orientation difference of is averaged, and the obtained average value is taken as the value of the pixel at the center. Do this for the entire pixel.

Perform this calculation so as not to cross the grain boundaries, and create a map that expresses the orientation change within the grain. That is, this map shows the strain distribution based on local orientation changes within the grain. In the present embodiment, the directional difference between adjacent pixels of 5 ° or less is displayed by the third approximation. In the present embodiment, the planar fraction of the pixel calculated to have an orientation difference of 1 ° or less is defined as the ferrite fraction. If the orientation difference exceeds 1 °, a hard phase structure other than ferrite such as bainite shall be used.

[About average grain size]
Further, in the line pipe steel material of the present embodiment, the average crystal grain size at the central portion of the thickness of the line pipe steel material is 15 μm or less. If the average grain size is too large, the low temperature toughness of the steel will decrease. In the present embodiment, since the above-mentioned average particle size is 15 μm or less, excellent low-temperature toughness can be obtained. The preferred upper limit of the average crystal grain size is 13 μm, more preferably 10 μm.

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

[About coarse grain ratio]
In the above-mentioned EBSP-OIM measurement, the area ratio of crystal grains having a crystal grain size of 20 μm or more at the central portion of the thickness of the steel material for line pipe is defined as “coarse crystal grain ratio”. If the grains are coarse, the low temperature toughness of the steel will decrease. When the coarse grain ratio is 20% or less, excellent low temperature toughness can be obtained. The preferred upper limit of the coarse grain ratio is 18%, more preferably 15%. The lower the coarse grain ratio, the more preferable.

The coarse grain ratio can be measured by, for example, the following method. A test piece for EBSP-OIM measurement is collected from the steel material for line pipe, and the above-mentioned EBSP-OIM measurement is performed. It can be obtained by substituting into the equation (2), where N is the area of the measurement target observed in the EBSP-OIM measurement and n is the area of the coarse crystal grains.
Coarse grain ratio (%) = (n / N) x 100 (2)

[About hard phase fraction]
As mentioned 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. The preferred upper limit of the hard phase fraction is 18%.

The hard phase fraction means the area ratio of the entire 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 material for line pipes. The L cross section of the collected sample is corroded with a nital corrosive solution. The corroded sample is observed using a SEM (Scanning Electron Microscope) at a magnification of 1000 to 3000 times, and the hard phase fraction is determined. Specifically, the SEM of an arbitrary 50 visual fields in the L cross section is image-processed, and the area ratio of the hard phase in the total visual field area is calculated.

[About the size of the hard phase]
Further, in the line pipe steel material of the present embodiment, the size of the hard phase at the central portion of the thickness of the line pipe steel material 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 the present embodiment, since the size of the above-mentioned hard phase is 6.0 μm or less, excellent low temperature toughness can be obtained. The preferred lower limit of the hard phase size is 2.5 μm, and the preferred upper limit of the hard phase size is 5.5 μm.

The size of the hard phase is obtained from the area of the hard phase when an arbitrary 50 visual fields are observed by SEM in the L cross section obtained from the above-mentioned hard phase fraction. Specifically, the circle-equivalent diameter is calculated from the hard phase area, and the average circle-equivalent diameter of the obtained 50 pieces is defined as the hard phase size (μm).

By carrying out the manufacturing process described later, the average crystal grain size can be set to 15 μm or less and the coarse crystal grain ratio can be set to 20% or less in the structure at the center of the thickness. Further, the ferrite fraction can be 65% or more, and the hard phase fraction can be 10 to 20% or more. Further, 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. Further, a yield stress of 435 to 570 MPa and a tensile strength of 535 to 760 MPa can be obtained. Further, the yield ratio YR can be 0.93 or less.

[Hardness of hard phase]
The hardness of the hard phase is 200 Hv or more in the Micro Vickers hardness test. The hardness of the hard phase is measured as follows. In the hard phase of the obtained steel material for line pipes, five arbitrary regions are selected. In each selected region, Vickers hardness (Hv) is measured according to JIS Z2244 (2009). The test conditions are that the test temperature is room temperature (25 ° C.) and the test force is 25 gf. The average of the obtained values (5 in total) is defined as the hardness of the hard phase.

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

With reference to FIG. 2, in the present manufacturing method, a slab as a raw material is manufactured using molten steel satisfying the above-mentioned chemical composition (material preparation step: S0). The produced slab is heated in a heating furnace (heating step: S1). The heated slab is rolled by a rough rolling machine and a finishing rolling machine to produce a steel sheet (rolling process: S2). The manufactured steel sheet is cooled by ROT (runout table) (ROT cooling step: S3). In the ROT cooling step (S3), first, the steel sheet is strongly cooled by a water cooling device (strong cooling step S31). After strong cooling, the steel sheet is slowly cooled (slow cooling step: S32). After the slow cooling, the steel sheet is further strongly cooled (strong cooling step: S33). The steel sheet after ROT cooling is wound up (winding process: S4). A hot-rolled steel sheet for line pipes is manufactured by the above manufacturing process.

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

[Material preparation process (S0)]
A material having the above-mentioned chemical composition is prepared. Specifically, a molten steel having the above-mentioned chemical composition is produced. A 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 ingot may be block-rolled to produce a material (slab).

[Heating step (S1)]
In the heating step (S1), the produced slab is heated in a heating furnace. The heating temperature of the slab in the heating furnace is 1060-1150 ° 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, the crystal grains during rolling cannot be refined and the precipitation strengthening after rolling cannot be obtained, and the strength is lowered. Therefore, the heating temperature is 1060-1150 ° 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 sheet. Both the rough rolling mill and the finishing rolling mill are provided with a plurality of rolling stands arranged in a row, and each rolling stand has a roll pair.

In the rolling process, the surface temperature of the steel sheet on the exit side of the final stand of the finish rolling machine is defined as the finish rolling temperature (° C.). The finish rolling temperature (° C.) is equal to or higher than the Ar ₃ transformation point. If the finish rolling temperature is less than the Ar ₃ transformation point, the rolling resistance of the steel sheet increases and the productivity decreases. Further, the steel sheet is rolled in the two-phase region of ferrite and austenite. In this case, the microstructure of the steel sheet creates a layered structure, which deteriorates the mechanical properties. Therefore, the finish rolling temperature is equal to or higher than the Ar ₃ transformation point. In the line pipe steel material of the present embodiment having the above-mentioned chemical composition, the Ar ₃ transformation temperature is 750 to 850 ° C. The reduction rate in the austenite unrecrystallized temperature range is preferably 60 to 80%. In this case, the unrecrystallized structure is miniaturized.

By using the manufacturing method of the present embodiment, excellent toughness can be obtained even if the plate thickness is 12 mm or more.

[ROT cooling step (S3)]
In the ROT (runout table) cooling step (S3), the steel sheet produced 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 sheet immediately before water cooling is not particularly limited, but it is preferably Ar ₃ transformation point or higher. When the surface temperature of the steel sheet immediately before water cooling is Ar ₃ transformation point or higher, it is possible to prevent a decrease in strength due to grain growth and coarsening of crystal grains.

The cooling rate in the strong cooling step (S31) is V1 (° C./s). V1 is 10 to 50 ° C./s at the central portion 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 a sufficient ferrite nucleation site cannot be obtained. In this case, since the amount of ferrite grains produced 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 becomes mainly bainite, and the low temperature toughness of the steel decreases. Therefore, the cooling rate V1 is 10 ° C./s or higher. The preferable lower limit of the cooling rate V1 is 12 ° C./s. The 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 in which 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 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 that reinforces the proeutectoid ferrite becomes overaging, and the strength of the steel decreases. When the strong cooling stop temperature T1 is set to 750 ° C., the ferrite fraction can be increased to 65% or more by slow cooling in the slow cooling step (S4) in the subsequent step, and the low temperature toughness of the steel is enhanced.

[Slow cooling step (S32)]
Slow cooling is performed on the steel sheet 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 2 to 5 ° C./s at the center of the plate thickness. If the cooling rate V2 is too slow, the productivity will decrease. If the cooling rate V2 is too fast, 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. Therefore, the cooling rate V2 is 2 to 5 ° C./s. The preferable lower limit of the cooling rate V2 is 2.5 ° C./s. The 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 in which the surface temperature of the steel sheet is 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 performed on the steel sheet that has been slowly cooled in the slow cooling step (S32).

The cooling rate in the strong cooling step (S33) is V3 (° C./s). The cooling rate V3 is 5 to 10 ° C./s at the central portion 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 decreases, the steel structure becomes mainly bainite, and the low temperature toughness of the steel decreases. Therefore, the cooling rate V3 is 5 to 10 ° C./s. The preferable lower limit of the cooling rate V3 is 5.5 ° C./s. The 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 in which the surface temperature of the steel sheet is 650 to 500 ° C.

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

The surface temperature (hereinafter referred to as the winding temperature) T4 of the steel sheet at the time of winding the hot-rolled steel sheet for a coiled line pipe is 500 to 580 ° C. If the take-up temperature T4 is too low, the coarse grain ratio increases and the low temperature toughness decreases. On the other hand, if the take-up temperature T4 is too high, the crystal grains become coarse and the low temperature toughness of the steel decreases. Therefore, the take-up temperature T4 is 500 to 580 ° C. The preferred T4 is 510-570 ° C, more preferably 520-560 ° C.

By the above manufacturing process, the hot-rolled steel sheet for line pipe of this embodiment is manufactured.

The electric resistance welded steel pipe for a line pipe of the present embodiment is manufactured by, for example, the following pipe making step (S5) using the hot-rolled steel plate for a line pipe described above.

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

In the steel materials for line pipes (hot-rolled steel sheets and electric resistance pipes) manufactured by the above manufacturing process, the average grain size is 15 μm or less, the coarse grain ratio is 20% or less, and the ferrite content is 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 guaranteed DWTT temperature can be set to −35 ° C. or lower, and the low temperature toughness can be improved. Further, a yield stress of 450 to 570 MPa and a tensile strength of 535 to 760 MPa can be obtained. Further, the yield ratio YR is 0.93 or less.

The molten steels of steels A to J shown in Table 1 were continuously cast to produce a slab.

Using a plurality of slabs of steels A to J, electrosewn steel pipes for line pipes of test numbers 1 to 26 shown in Table 2 were manufactured.

Specifically, the slabs of each test number were heated in a heating furnace. The heating temperature (° C.) was as shown in Table 2. The heated slab was rolled using a rough rolling mill and allowed to cool to 920 ° C. After that, finish rolling was carried out with a finish rolling mill. The reduction rate in the unrecrystallized temperature range was 60 to 80% in all test numbers. The finish rolling temperature was Ar ₃ points or more.

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./sec), slow cooling rate V2 (° C./sec), and strong cooling rate V3 (° C./sec)). The strong cooling stop temperature T1 (° C.) and the slow cooling stop temperature T2 (° C.) were 750 ° C. and 650 ° C., respectively, for all the steel sheets.

After the steel sheet was manufactured by the above manufacturing process, the steel sheet was wound at the winding temperature T4 shown in Table 2 to manufacture a hot-rolled steel sheet for line pipes. Further, a hot-rolled steel plate for a line pipe was used to produce a pipe by the above-mentioned method, and an electrosewn steel pipe for a line pipe having an outer diameter of 389 mm or more and a wall thickness of 12 mm or more was manufactured.

[Test method]
[Micro tissue]
Based on the above method, the average grain size, coarse grain ratio, ferrite fraction, hard phase fraction, and hard phase size were measured using EBSP-OIM as in the test method described above. The measurement conditions of EBSP-OIM in the average crystal grain size measurement were magnification: 400 times, visual field area: 200 μm × 500 μm, and measurement step: 0.3 μm. The SEM measurement conditions for measuring the hard phase fraction and the size of the hard phase were a magnification of 1000 to 3000 times and a visual field area of 45 μm × 60 μm. For the hard phase, the 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]
Two tensile test pieces were taken from the electric resistance steel pipe for line pipe of each test number. Specifically, when the electric pipe for line pipe was viewed in the axial direction, two tensile test pieces having a total thickness were collected from a position of 90 ° from the welded portion of the electric pipe for line pipe. The cross sections of the two tensile test pieces are both arcuate, the longitudinal direction of one tensile test piece is the rolling direction (L direction), and the longitudinal direction of the other tensile test piece is the plate width direction (C direction). )Met. The size of the tensile test piece was as shown in FIG. 3, 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. 3 indicate the dimensions (unit: mm) of the corresponding parts of the test piece. Tensile test pieces were used to carry out tensile tests at room temperature in accordance with the API standard 5CT. Based on the test results, the yield strength (MPa) and tensile strength (MPa) of the electrosewn steel pipe for line pipes in the L and C directions were determined. The average yield strength in the L and C directions obtained was defined as the yield strength YS (MPa) of the test number. Similarly, the average of the obtained tensile strengths in the L and C directions was defined as the tensile strength TS (MPa) of the test number.

Further, the yield ratio YR in the L direction and the yield ratio YR in the C direction were determined based on the yield strength and the tensile strength in each of the obtained directions (L direction, C direction).

[Low temperature toughness test]
DWTT test pieces were collected from the electric resistance sewn steel pipes for line pipes of each test number. The sampling position was the same as that of the tensile test piece (position facing the welded part). The size of the DWTT test piece was as shown in FIG. The arc-shaped member collected from the collection position was developed into a flat plate shape, and a notch was machined at the 180 ° position. The numerical values in FIG. 4 indicate the dimensions (unit: mm) of the corresponding parts of the test piece. t indicates the wall thickness (unit: mm). The longitudinal direction of the DWTT test piece corresponded to the circumferential direction of the electric resistance sewn steel pipe for line pipe. The DWTT test piece was subjected to a DWTT test in accordance with the regulations of ASTM E 436, and the minimum 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, the low temperature toughness was evaluated as high.

[Test results]
Table 3 shows the test results.

With reference to Tables 1 to 3, the chemical compositions of the steels of Test Nos. 1 to 13 were appropriate and satisfied formula (1). Furthermore, the manufacturing conditions of all test numbers were appropriate. Therefore, the average crystal grain size of Test Nos. 1 to 13 was 15 μm or less, and the coarse crystal grain ratio was 20% or less. Further, the ferrite fraction was 65% or more, and the hard phase fraction was 10 to 20%. Hard phase It contained one or more selected from the group consisting of pearlite, bainite, island martensite and MA, and the size of the hard phase was 6.0 μm or less. As a result, the guaranteed DWTT temperature was −35 ° C. or lower, showing excellent low temperature toughness. Further, the yield strength YS was 435 to 570 MPa, and the tensile strength TS was 535 to 760 MPa. Further, the yield ratio YR was 0.93 or less in both the C direction and the L direction.

On the other hand, in test number 14, 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, the DWTT guaranteed temperature was higher than −35 ° C., and the low temperature toughness was low.

In test number 15, the heating temperature exceeded 1150 ° C. 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, V1, V2 and V3 were too low and the temperature was gradually cooled, so that 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 yield strength YS was less than 435 MPa, and the tensile strength TS was also less than 535 MPa. The yield ratio YR also exceeded 0.93 in the C and L directions. Furthermore, the DWTT guaranteed temperature was higher than −35 ° C., and the low temperature toughness was low.

In test number 17, V3 was less than 5 ° C./s, so the take-up temperature T4 exceeded 580 ° C. Therefore, the size of the hard phase exceeded 6.0 μm. As a result, the tensile strength TS was less than 535 MPa. The yield ratio YR also exceeded 0.93 in the C and L directions. Furthermore, the DWTT guaranteed temperature was higher than −35 ° C., and the low temperature toughness was low.

In test number 18, the take-up temperature T4 was less than 500 ° C. because V2 exceeded 5 ° C./s. Therefore, the average crystal grain size exceeded 15 μm, and the coarse crystal grain ratio exceeded 20%. Further, the ferrite fraction decreased and the hard phase fraction exceeded 20%. The size of the hard phase also exceeded 6.0 μm. Therefore, the DWTT guaranteed temperature was higher than −35 ° C., and the low temperature toughness was low.

In test number 19, V1 was above 50 ° C./s, V3 was below 5 ° C./s, and the take-up temperature T4 was below 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 size of the hard phase also exceeded 6.0 μm. Therefore, the DWTT guaranteed temperature was higher than −35 ° C., and the low temperature toughness was low.

In test number 20, although the production conditions were appropriate, F1 exceeded the upper limit of the formula (1). Therefore, the average crystal grain exceeded 15 μm. Further, the ferrite fraction was less than 65%, the hard phase fraction was over 20%, and the hard phase size was over 6.0 μm. Therefore, the DWTT guaranteed temperature was higher than −35 ° C., and the low temperature toughness was low. Furthermore, the yield strength YS exceeded 570 MPa. The yield ratio YR also exceeded 0.93 in the L direction.

In test number 21, although the production conditions were appropriate, F1 was less than the lower limit of the formula (1). Therefore, the average crystal grain size exceeded 15 μm, and the coarse crystal grain ratio also exceeded 20%. Further, 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. Further, the tensile strength TS was less than 535 MPa, which was low. The yield ratio YR also exceeded 0.93 in the C and L directions.

In test number 22, V1, V2 and V3 were too low, so that the take-up 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 yield ratio YR exceeded 0.93 in the C and L directions. Furthermore, the DWTT guaranteed temperature was higher than −35 ° C., and the low temperature toughness was low.

In test number 23, the take-up temperature T4 exceeded 580 ° C. because V1, V2 and V3 were too low. 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 yield strength YS was less than 435 MPa, and the tensile strength TS was also less than 535 MPa. The yield ratio YR also exceeded 0.93 in the C and L directions. Furthermore, the DWTT guaranteed temperature was higher than −35 ° C., and the low temperature toughness was low.

In test number 24, the take-up temperature T4 exceeded 580 ° C. because V2 and V3 were too low. Therefore, the hard phase fraction was also 10% or less. As a result, the yield strength YS was less than 435 MPa, and the tensile strength TS was also less than 535 MPa. The yield ratio YR also exceeded 0.93 in the C and L directions. Furthermore, the DWTT guaranteed temperature was higher than −35 ° C., and the low temperature toughness was low.

At test number 25, V2 was too high and V3 was too low. Therefore, the hard phase fraction exceeded 20%. As a result, the DWTT guaranteed temperature was higher than −35 ° C., and the low temperature toughness was low.

At test number 26, V3 was too low. Therefore, the size of the hard phase exceeded 6 μm. As a result, the yield ratio YR also exceeded 0.93 in the C and L directions. Furthermore, the DWTT guaranteed temperature was higher than −35 ° C., and the low temperature toughness was low.

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

Claims (6)

By mass%
C: 0.06 to 0.12%,
Si: 0.05-0.3%,
Mn: 0.5-2%,
P: 0.03% or less,
S: 0.01% or less,
O: 0.003% or less,
Al: 0.01-0.035%,
N: 0.001 to 0.008%,
Nb: 0.01-0.25%,
Ti: 0.005 to 0.03%,
Ni: 0.01-0.2%,
Mo: 0.01-0.2%,
Cu: 0.01-0.3%,
Cr: 0-0.3%,
V: 0-0.01%,
B: 0 to 0.003% and
Ca: contains 0 to 0.0030%, the balance is composed of Fe and impurities, and has a chemical composition satisfying the formula (1).
In the structure at the center of the thickness, the coarse crystal grain ratio, which is the area ratio of crystal grains having an average crystal grain size of 15 μm or less and a crystal grain size of 20 μm or more, is 20% or less, and the ferrite content is 65% or more. And the hard phase fraction is 10 to 20%.
A steel material for line pipes with a hard phase size of 6.0 μm or less.
0.35 ≤ C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 3 + Nb / 3 ≤ 0.40 (1)
Here, the content (mass%) of the corresponding element is substituted for each element symbol of the formula (1). The steel material for line pipes according to claim 1.
The chemical composition is
Cr: 0.01-0.3%,
V: 0.001 to 0.01% and
B: A steel material for line pipes containing one or more selected from the group consisting of 0.0002 to 0.003%. The steel material for line pipes according to claim 1 or 2.
The chemical composition is
A steel material for line pipes containing Ca: 0.0005 to 0.0030%. The steel material for line pipe according to any one of claims 1 to 3.
The steel material for line pipes is a steel material for line pipes, which is a hot-rolled steel plate for line pipes. The steel material for line pipe according to any one of claims 1 to 3.
The steel material for line pipes is a steel material for line pipes, which is an electrosewn steel pipe for line pipes. A step of heating a material having the chemical composition according to any one of claims 1 to 3 at 1060-1150 ° C.
A process of performing rough rolling and finish rolling on the heated material to obtain a steel sheet, and
A step of strongly cooling the steel sheet after finish rolling in a temperature range of 870 to 750 ° C. at a cooling rate of 10 to 50 ° C./s.
A step of slowly cooling the strongly cooled steel sheet in a temperature range of 750 to 650 ° C. at a cooling rate of 2 to 5 ° C./s.
A step of strongly cooling the slowly cooled steel sheet in a temperature range of 650 to 500 ° C. at a cooling rate of 5 to 10 ° C./s.
The method for producing a steel material for a line pipe according to any one of claims 1 to 5 , further comprising a step of winding the strongly cooled steel sheet at 500 to 580 ° C.

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