JP7448804B2 - ERW steel pipes for line pipes and hot rolled steel plates for line pipes - Google Patents

Description

The present invention relates to an electric resistance welded steel pipe for line pipes and a hot rolled steel plate for line pipes.

Pipelines laid on the ocean floor carry high-pressure fluid through them. The pipeline is further subjected to repeated strain from waves and seawater pressure. Therefore, steel pipes used for submarine pipelines are required to have high strength and high low-temperature toughness.

A pipeline is composed of multiple line pipes. Electric resistance welded steel pipes (hereinafter referred to as electric resistance welded steel pipes) are sometimes used as steel pipes for line pipes. High strength can be obtained by increasing the wall thickness of ERW steel pipes. However, as the wall 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 at which a ductile fracture ratio of 85% or more occurs in the DWTT. The lower the DWTT guaranteed temperature, the higher the low temperature toughness. In recent years, electric resistance welded steel pipes for line pipes are required to have excellent low-temperature toughness.

Patent Document 1 discloses that in the structure at the center of the thickness, the average crystal grain size is 15 μm or less, the coarse grain ratio is 20% or less, the ferrite fraction is 65% or more, and the hard phase fraction is 10 to 20%. Discloses a steel material for line pipes in which the size of the hard phase is 6.0 μm or less. With such a structure, a steel material for line pipes having excellent low-temperature toughness and sufficient strength can be obtained.

On the other hand, microstructure control utilizing the accumulation of strain during finish rolling is known as a means of refining crystal grains.

Patent Document 2 relates to a high-strength steel plate for automobiles, which is finish rolled so that the rolling load of each of the final three rolling stands is 80% or more of the rolling load of the previous rolling stand, and the steel plate is continuously subjected to high-strength steel plate. By applying a load, dynamic recrystallization of austenite occurs in the steel sheet, making the austenite crystal grains finer and introducing a high dislocation density to the austenite grain boundaries. During subsequent forced cooling, nuclei are removed from the austenite grain boundaries. This disclosure discloses increasing the generation frequency of ferrite to increase the generation of fine ferrite grains.

Japanese Patent Application Publication No. 2018-104757 International Publication No. 2019/88104

ERW steel pipes for line pipes used in pipelines that transport crude oil and natural gas over long distances are increasingly being used in deep sea areas, and achieving both thick wall thickness and low-temperature toughness has become a technical issue. In recent years, there has been a demand for electric resistance welded steel pipes for line pipes with a wall thickness of 12 mm or more and a guaranteed DWTT temperature of -40°C or less at the pipe stage. However, as described above, as the wall thickness increases, brittle fracture tends to occur and low temperature toughness decreases, so when increasing the wall thickness, it is necessary to further improve the low temperature toughness.

In view of the above circumstances, it is an object of the present invention to provide a steel material for line pipes that has a thick wall of, for example, 12 mm or more and has excellent low-temperature toughness with a guaranteed DWTT temperature of -40° C. or less.

It is known that grain refinement in the final structure is effective in increasing thickness and toughness. It is especially important to refine the grains at the center of the plate thickness.

Control of austenite before transformation is important for grain refinement in the final structure. Specifically, it is important to accumulate strain in austenite in order to increase the grain boundary area of austenite, introduce deformed bodies, and increase dislocation density. As mentioned above, for hot-rolled steel sheets for automobiles and other applications, many studies are being conducted on structure control and product development that utilize the accumulation of strain during finish rolling.

However, thick hot-rolled steel sheets are thicker than, for example, hot-rolled steel sheets for automobiles, and the amount of rolling reduction in a finish rolling mill is restricted, so strain is difficult to accumulate during finish rolling.

The present inventors have conducted intensive studies on a method of accumulating strain in a thick hot-rolled steel sheet during finish rolling to refine crystal grains. As a result, by controlling the temperature to create a large temperature difference between the center of the plate thickness and the surface layer in the process before finishing rolling, it is possible to accumulate large distortions at the center of the plate thickness due to thermal strain during the finish rolling process. It was found that the crystal grains at the center of the plate thickness can be made finer than before. The present invention has been achieved through further studies based on the above findings, and the gist thereof is as follows.

[1] An electric resistance welded steel pipe for a line pipe including a base metal part and an electric resistance welded part, wherein the chemical composition of the base metal part is, in mass %, C: 0.0030 to 0.120%, Si: 0.05-0.30%, Mn: 0.50-2.00%, P: 0.030% or less, S: 0.0100% or less, Al: 0.010-0.035%, N: 0 .0010 to 0.0080%, Nb: 0.010 to 0.080%, Ti: 0.005 to 0.030%, Ni: 0.01 to 0.50%, Mo: 0.05 to 0.20 %, O: 0.0050% or less, V: 0-0.10%, Ca: 0-0.0050%, Cr: 0-0.30%, Cu: 0-0.30%, Mg: 0- 0.0050%, REM: 0 to 0.0100%, and the remainder: Fe and impurities, and F1 defined by the following formula (1) is 0.30 to 0.38, and the meat of the base material part is In the metal structure of the central part of the thickness, the ferrite fraction is 80 to 95% in terms of area ratio, the remainder is pearlite and/or bainite, and the average crystal grain size is 5.0 μm or less, and An electric resistance welded steel pipe for a line pipe, wherein the average crystal grain size in the metallographic structure of the surface layer portion is 5.0 μm or less, and the difference between the average crystal grain size between the surface layer portion and the wall thickness center portion is 2.0 μm or less.
F1 = C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/3
+Nb/3... Formula (1)
[In formula (1), C, Si, Mn, Ni, Cr, Mo, V, and Nb each represent the content (mass%) of each element. ]

[2] The chemical composition of the base material is, in mass %, V: more than 0% and 0.10% or less, Ca: more than 0% and 0.0030% or less, Cr: more than 0% and 0.30% or less, Cu : more than 0% but less than 0.30%,
The electric resistance welded steel pipe for a line pipe according to [1] above, containing one or more selected from the group consisting of Mg: more than 0% and 0.0050% or less, and REM: more than 0% and 0.0100% or less.

[3] The electric resistance welded steel pipe for a line pipe according to [1] or [2] above, which has a yield strength in the pipe axial direction of 450 to 540 MPa and a tensile strength in the pipe axial direction of 510 to 625 MPa.

[4] The electric resistance welded steel pipe for a line pipe according to any one of [1] to [3] above, which has a wall thickness of 12 to 25 mm and an outer diameter of 304.8 to 660.4 mm.

[5] A hot-rolled steel plate used for manufacturing the electric resistance welded steel pipe for line pipes according to any one of [1] to [4] above, which has a chemical composition in mass% of C: 0.0030 to 0.120. %, Si: 0.05-0.30%, Mn: 0.50-2.00%, P: 0.030% or less, S: 0.0100% or less, Al: 0.010-0.035 %, N: 0.0010 to 0.0080%, Nb: 0.010 to 0.080%, Ti: 0.005 to 0.030%, Ni: 0.01 to 0.50%, Mo: 0. 05-0.20%, V: 0-0.10%, O: 0.0050% or less, Ca: 0-0.0050%, Cr: 0-0.30%, Cu: 0-0.30% , Mg: 0 to 0.0050%, REM: 0.0100%, and the remainder: Fe and impurities, and F1 defined by the following formula (1) is 0.30 to 0.38, In the metal structure at the center of the wall thickness of the steel sheet, the ferrite fraction is 80 to 95% in terms of area ratio, the remainder is pearlite or bainite, and the average crystal grain size is 5.0 μm or less, and the hot rolled steel sheet A hot-rolled steel sheet for line pipes, wherein the average crystal grain size is 5.0 μm or less in the metallographic structure of the surface layer, and the difference in the average grain size between the surface layer and the thickness center portion is 2.0 μm or less. .
F1 = C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/3
+Nb/3... Formula (1)
[In formula (1), C, Si, Mn, Ni, Cr, Mo, V, and Nb each represent the content (mass%) of each element. ]

According to the present invention, it is possible to obtain an ERW steel pipe for a line pipe having a thick wall, excellent low-temperature toughness, and sufficient strength, and a hot rolled steel plate for a line pipe used for manufacturing the ERW steel pipe for a line pipe. Can be done.

FIG. 1 is a plan view of a tensile test piece used in the tensile test. FIG. 2 is a diagram showing the sampling positions of DWTT test pieces used in the DWTT test. FIG. 3 is a front view and a side view of the DWTT test piece used in the DWTT test.

Hereinafter, the electric resistance welded steel pipe for line pipes and the hot rolled steel plate for line pipes (hereinafter, both will be collectively referred to as "steel materials for line pipes") of the present embodiment will be described in detail. "%" with respect to elements means mass % unless otherwise specified.

[Chemical composition]
The chemical composition of the steel material for line pipes of this embodiment contains the following elements.

C:0.0030~0.120%
C is an element that increases the strength of steel. In order to obtain this effect, the C content is set to 0.0030% or more. If the C content is too high, the low-temperature toughness and ductility of the steel may be reduced, and further the weldability may be reduced. Therefore, the C content is set to 0.120% or less. The lower limit of the C content is preferably 0.040%, more preferably 0.050%. A preferable upper limit of the C content is 0.100%, more preferably 0.080%.

Si:0.05~0.30%
Si is an element that functions as a deoxidizer for steel. Furthermore, generation of coarse oxides in the base metal and welded area of the ERW steel pipe is suppressed, and the toughness of the base metal and welded area is improved. In order to obtain these effects, the Si content is set to 0.05% or more. If the Si content is too high, the low temperature toughness of the steel will decrease. Therefore, the Si content is set to 0.30% or less. The preferable lower limit of the Si content is 0.07%, more preferably 0.10%. A preferable upper limit of the Si content is 0.20%, more preferably 0.19%.

Mn: 0.50-2.00%
Mn is an element that improves the hardenability of steel and increases the strength of steel. In order to obtain this effect, the Mn content is set to 0.50% or more. If the Mn content is too high, the strength of the steel may become too high and the low temperature toughness of the steel may decrease. Therefore, the Mn content is set to 2.00% or less. The lower limit of the Mn content is preferably 0.70%, more preferably 1.00%. A preferable upper limit of the Mn content is 1.80%, more preferably 1.50%.

P: 0.030% or less P is an impurity. P reduces the low temperature toughness of steel. Therefore, the P content is set to 0.030% or less. A preferable upper limit of the P content is 0.015%, more preferably 0.01%. The P content is preferably as low as possible, and may be zero.

S: 0.0100% or less S is an impurity. S combines with Mn to form Mn-based sulfides, which deteriorates the low-temperature toughness and SSC resistance of steel. Therefore, the S content is set to 0.0100% or less. A preferable upper limit of the S content is 0.0010%, more preferably 0.0005%. The S content is preferably as low as possible, and may be 0.

Al: 0.010-0.035%
Al is an element that functions as a deoxidizer for steel. Furthermore, generation of coarse oxides in the base metal and welded area of the ERW steel pipe is suppressed, and the toughness of the base metal and welded area is improved. In order to obtain this effect, the Al content is set to 0.010% or more. If the Al content is too high, Al nitrides will become coarse and the low-temperature toughness of the steel will decrease. Therefore, the Al content is set to 0.035% or less. A preferable lower limit of the Al content is 0.012%, more preferably 0.015%. A preferable upper limit of the Al content is 0.020%, more preferably 0.016%. In this specification, Al content means the total Al content in steel.

N: 0.0010-0.0080%
N is an element that forms nitrides and suppresses coarsening of austenite grains during the heating process. Specifically, austenite grains become finer in the rolling process, and crystal grains after transformation become finer. As a result, the low temperature toughness of the steel increases. N further increases the strength of steel through solid solution strengthening. In order to obtain these effects, the N content is set to 0.0010% or more. If the N content is too high, it may coarsen carbonitrides and reduce the low-temperature toughness of the steel. Therefore, the N content is set to 0.0080% or less. A preferable lower limit of the N content is 0.0020%, more preferably 0.0030%. A preferable upper limit of the N content is 0.0060%, more preferably 0.0050%.

Nb: 0.010-0.080%
Nb is an element that combines with C and N in steel to form fine Nb carbonitrides. By forming Nb carbonitride, coarsening of crystal grains is suppressed and the average crystal grain size becomes small. Therefore, it increases the low temperature toughness of steel. Furthermore, fine Nb carbonitrides increase the strength of steel through dispersion strengthening. In order to obtain these effects, the Nb content is set to 0.010% or more. If the Nb content is too high, Nb carbonitrides may become coarse and the low-temperature toughness of the steel may decrease. Therefore, the Nb content is set to 0.080% or less. A preferable lower limit of the Nb content is 0.012%, more preferably 0.020%. A preferable upper limit of the Nb content is 0.070%, more preferably 0.060%.

Ti: 0.005-0.030%
Ti is an element that combines with N in steel to form TiN and suppresses a decrease in low-temperature toughness of steel due to solid solution N. Furthermore, by dispersing and precipitating fine TiN, coarsening of crystal grains is suppressed, thereby increasing the low-temperature toughness of the steel. In order to obtain these effects, the Ti content is set to 0.005% or more. If the Ti content is too high, TiN may become coarse or coarse TiC may be formed, which may reduce the low-temperature toughness of the steel. Therefore, the Ti content is set to 0.00.030% or less. The lower limit of the Ti content is preferably 0.008%, more preferably 0.010%. A preferable upper limit of the Ti content is 0.020%, more preferably 0.015%.

Ni: 0.01~0.50%
Ni is an element that improves the hardenability of steel and increases the strength of steel. In order to obtain this effect, the Ni content is set to 0.01% or more. If the Ni content is too high, this effect will be saturated. Therefore, the Ni content is set to 0.50% or less. The preferable lower limit of the Ni content is 0.05%, more preferably 0.08%. A preferable upper limit of the Ni content is 0.20%, more preferably 0.15%.

Mo: 0.05-0.20%
Mo is an element that improves the hardenability of steel and increases the strength of steel. Mo further refines the austenite grains and increases the low-temperature toughness of the steel. In order to obtain these effects, the Mo content is set to 0.05% or more. If the Mo content is too high, the field weldability of the steel will decrease. Therefore, the Mo content is set to 0.20% or less. The lower limit of the Mo content is preferably 0.10%, more preferably 0.15%. A preferable upper limit of the Mo content is 0.19%, more preferably 0.18%.

O: 0.0050% or less O is an impurity. O forms oxides that reduce the hydrogen-induced cracking resistance of steel. O further reduces the low temperature toughness of the steel. Therefore, the O content is set to 0.0050% or less. A preferable upper limit of the O content is 0.0030%, more preferably 0.0025%. The O content is preferably as low as possible, and may be zero.

V: 0-0.10%
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 carbonitrides further suppress coarsening of crystal grains and improve the low-temperature toughness of the steel. Although these effects can be obtained even with a trace amount of V content, in order to reliably obtain the effects, it is preferable that the V content is 0.01% or more. If the V content is too high, V carbonitrides may become coarse and the low-temperature toughness of the steel may deteriorate. Therefore, the V content is set to 0.10% or less. A more preferable lower limit of the V content is 0.02%, and even more preferably 0.05%. A preferable upper limit of the V content is 0.08%, more preferably 0.07%.

Ca: 0-0.0030%
Ca is an optional element and may not be included. When contained, Ca controls the morphology of MnS to make it spheroidal, and the low-temperature toughness of the steel is enhanced. Although this effect can be obtained even with a trace amount of Ca content, in order to reliably obtain the effect, it is preferable that the Ca content is 0.0001% or more. If the Ca content is too high, coarse oxide inclusions are formed. As a result, the oxide becomes a starting point for fracture, and the low-temperature toughness of the steel decreases. Therefore, the Ca content is 0.0030% or less. A more preferable lower limit of the Ca content is 0.0002%, and even more preferably 0.0005%. A preferable upper limit of the Ca content is 0.0025%, more preferably 0.0020%.

Cr: 0-0.30%
Cr is an optional element and may not be included. When contained, Cr improves the hardenability of the steel and increases the strength of the steel. This effect can be obtained even with a trace amount of Cr, but in order to reliably obtain the effect, it is preferable that the Cr content is 0.01% or more. 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 set to 0.30% or less. The lower limit of the Cr content is preferably 0.05%, more preferably 0.06%. A preferable upper limit of the Cr content is 0.25%, more preferably 0.2%.

Cu: 0-0.30%
Cu is an optional element and may not be included. When contained, Cu increases the hardenability of the steel and increases the strength of the steel. This effect can be obtained even with a trace amount of Cu, but in order to reliably obtain the effect, it is preferable that the Cu content is 0.02% or more. If the Cu content is too high, the hardenability becomes too high and the toughness decreases. Therefore, the Cu content is set to 0.30% or less. The preferable lower limit of the Cu content is 0.05%, more preferably 0.07%. A preferable upper limit of the Cu content is 0.25%, more preferably 0.20%.

Mg: 0-0.0050%
Mg is an arbitrary element and may not be included. When contained, Mg functions as a deoxidizing agent and a desulfurizing agent. Furthermore, fine oxides are generated, which contributes to improving the toughness of the HAZ. These effects can be obtained even with a trace amount of Mg, but in order to reliably obtain the effects, it is preferable that the Mg content is 0.0001% or more. If the Mg content is too high, the oxides tend to aggregate or become coarse, which may result in a decrease in HIC resistance or a decrease in the toughness of the base material or HAZ. Therefore, the Mg content is set to 0.0050% or less. The lower limit of the Mg content is preferably 0.0005%, more preferably 0.0010%. A preferable upper limit of the Mg content is 0.040%, more preferably 0.0030%.

REM: 0-0.0100%,
REM is an arbitrary element and may not be included. Here, "REM" is a rare earth element, i.e., from the group consisting of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Refers to at least one selected element. When REM is included, REM functions as a deoxidizing agent and a desulfurizing agent. Although this effect can be obtained even with a trace amount of REM, in order to reliably obtain the effect, it is preferable that the REM content is 0.0001% or more. If the REM content is too high, coarse oxides are produced, which may result in a decrease in HIC resistance or a decrease in the toughness of the base material or HAZ. Therefore, the REM content is set to 0.0100% or less. A preferable lower limit of the REM content is 0.0005%, more preferably 0.0010%. A preferable upper limit of the REM content is 0.070%, more preferably 0.0050%.

The remainder of the chemical composition of the steel material for line pipe according to this embodiment consists of Fe and impurities. Here, impurities are those that are mixed in from ores as raw materials, scrap, or the manufacturing environment when steel materials for line pipes are industrially manufactured, and have an adverse effect on the steel materials for line pipes of this embodiment. It means that it is permissible within the range that does not give.

[About formula (1)]
The above chemical composition further satisfies formula (1).

F1 = C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/3
+Nb/3... Formula (1)

Here, the content (mass %) of the corresponding element is substituted for each element symbol in formula (1). Furthermore, if the element corresponding to the element symbol in formula (1) is not contained, "0" is substituted for the corresponding element symbol in formula (1).

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

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 nucleation sites are sufficiently generated. As a result, the ferrite crystal grains become coarser and the average crystal grain size becomes larger. 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 will further deteriorate and sufficient strength will not be obtained.

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 likely to be formed, and the ferrite fraction in the structure is reduced. As a result, the low temperature toughness of the steel decreases. If F1 is too high, the hardenability will further increase and the strength of the steel will become too high.

If F1 is 0.30 to 0.38, it is easy to maintain the temperature of the steel in the ferrite region, the ferrite fraction in the center of the thickness of the steel material can be 80% or more, and the low-temperature toughness of the steel is improved. be able to.

[About ferrite fraction]
The structure of the central part of the wall thickness of the steel material for line pipes according to the present embodiment is composed of 80 to 95% ferrite in terms of area ratio, and the remainder is pearlite and/or bainite. Bainite also includes bainitic ferrite that does not contain cementite within the grains or at the grain boundaries. The grain boundaries of bainitic ferrite may contain MA (Martensite-Austenite constituent). Here, the thickness center refers to the range of ±10%t in the plate thickness direction or wall thickness direction from the plate thickness center or wall thickness center (in other words, from the surface to the plate (in the thickness direction or the range of 40 to 60% t).

As described above, if the ferrite fraction of the structure at the center of the thickness of the steel is 80% or more in terms of area ratio, the crystal grains become finer, and as a result, the low-temperature toughness of the steel increases. A preferable lower limit of the ferrite fraction is 83%, more preferably 85%. On the other hand, if the ferrite fraction exceeds 95% in terms of area ratio, desired strength cannot be obtained.

The ferrite fraction is measured by the following method.

A sample is taken from the center of the thickness of the linepipe steel material. The collected sample is polished with colloidal silica polishing agent for 30 to 60 minutes. The polished sample is analyzed using EBSP-OIM (trademark) (Electron Back Scatter Diffraction Pattern-Orientation Image Microscopy) to determine the ferrite fraction.

Specifically, the ferrite fraction is determined using the KAM (Kernel Average Misorientation) method included in EBSP-OIM.

The KAM method uses a certain regular hexagonal pixel (center pixel) in the measurement data and the first approximation using six pixels adjacent to this pixel (total 7 pixels), or the first approximation using these six pixels. Furthermore, for the second approximation (total 19 pixels) that also uses the outer 12 pixels, or the third approximation (total 37 pixels) that also uses the outer 18 pixels of these 12 pixels, the distance between each pixel is The orientation differences are averaged, and the resulting average value is taken as the value of the central pixel. This operation is performed for the entire pixel.

This calculation is performed so as not to cross grain boundaries, and a map is created that expresses the orientation change within the grain. That is, this map represents the strain distribution based on local orientation changes within the grain. In this embodiment, pixels with an orientation difference of 5° or less between adjacent pixels are displayed using the third approximation. In this embodiment, the planarity fraction of a pixel calculated to have an orientation difference of 1° or less in the third approximation is defined as the ferrite fraction. If the misorientation exceeds 1° in the third approximation, it is considered to be a structure other than ferrite such as bainite.

[About average grain size]
Furthermore, in the steel material for line pipes of this embodiment, the average crystal grain size at the center of the wall thickness of the steel material for line pipes is 5.0 μm or less. If the average grain size is too large, the low temperature toughness of the steel will decrease. In this embodiment, since the above-mentioned average grain size is 5.0 μm or less, excellent low-temperature toughness can be obtained. A preferable upper limit of the average crystal grain size is 4.8 μm, more preferably 4.5 μm.

The average grain size is measured using the EBSP-OIM method. A sample is collected and polished in the same way as for measuring the ferrite fraction. The polished sample is analyzed using EBSP-OIM. Specifically, a grain boundary is defined as a position where the difference in orientation between adjacent measurement points exceeds 15° when the orientation is measured at each fixed measurement step. 15° is a threshold value for large-angle grain boundaries, and is generally recognized as a grain boundary. The grain size and surface area of the grain are determined by assuming that the region surrounded by the grain boundary is a crystal grain. The area average particle size is determined from the obtained particle size and surface area. In this embodiment, the obtained area average grain size is taken as the average crystal grain size.

Furthermore, in the steel material for line pipes of this embodiment, the average crystal grain size in the surface layer portion of the steel material for line pipes is 5.0 μm or less. Here, the surface layer portion means a range of 5 to 10% t and a range of 90 to 95% t from the surface in the plate thickness direction or wall thickness direction, where the plate thickness or wall thickness is tmm. If the average grain size is too large, the low temperature toughness of the steel will decrease. In this embodiment, since the average grain size is 5.0 μm or less even in the surface layer of the steel material for line pipes, excellent low-temperature toughness can be obtained. A preferable upper limit of the average crystal grain size is 4.8 μm, more preferably 4.5 μm.

Furthermore, in the steel material for line pipes of this embodiment, the difference in average grain size between the surface layer portion and the thickness center portion is 2.0 μm or less. As a result, uniform low-temperature toughness can be obtained in the surface layer portion and the thickness center portion.

By carrying out the manufacturing process described below, in the metal structure at the center of the wall thickness of the steel material, in terms of area ratio, the ferrite fraction is 80 to 95%, the remainder is pearlite and/or bainite, and the average grain size is 5.0 μm. In the metallographic structure of the surface layer of the steel material, the average grain size can be 5.0 μm or less, and the difference in average grain size between the surface layer and the thickness center portion can be 2.0 μm or less.

As a result, the DWTT guaranteed temperature can be set to −40° C. or lower, and low-temperature toughness can be improved. Furthermore, in the electric resistance welded steel pipe, it is possible to obtain a yield strength in the pipe axial direction of 450 to 540 MPa and a tensile strength in the pipe axial direction of 535 to 760 MPa.

[Production method]
An example of a method for manufacturing the above-mentioned steel material for line pipes will be explained.

[Material preparation process]
First, molten steel having the above-mentioned chemical composition is produced, and a slab is produced using the molten steel. For example, slabs can be manufactured by continuous casting.

[Heating process]
In the heating step, the manufactured slab is heated in a heating furnace. The heating temperature of the slab in the heating furnace is preferably 1060 to 1200°C. If the heating temperature is too high, the crystal grains (austenite grains) will become coarse and the low-temperature toughness will decrease. If the heating temperature is too low, grain refinement during rolling and precipitation strengthening after rolling will not be achieved, resulting in a decrease in strength.

[Hot rolling process]
The hot rolling process is divided into a rough rolling process and a finish rolling process. The slab heated in the heating step is hot rolled using a rough rolling mill and a finishing rolling mill to form a hot rolled steel plate. Both the roughing mill and the finishing mill are equipped with a plurality of rolling stands arranged in a row, and each rolling stand is equipped with a pair of rolls.

In the rough rolling step, the slab is rough rolled at a final rolling temperature of 900 to 1000° C. and a rolling reduction of 60% or more. The rolling reduction ratio in rough rolling is determined by (slab thickness - thickness before finish rolling)/slab thickness x 100 (%).

In this embodiment, the temperature at the center of the plate thickness at the entry side of the finishing rolling machine is set at 770 to 850°C, and by cooling the surface layer of the steel plate at the entry side of the finishing rolling machine, the temperature difference between the center of the plate thickness and 2 mm below the surface layer is reduced to 80°C. The above shall apply. The temperature of the steel plate can be determined, for example, by performing heat conduction analysis from the thickness and length of the steel plate, physical property values such as specific heat, amount of cooling water, and conveyance speed. The surface layer can be cooled, for example, by injecting cooling water onto the steel plate. As a result, since strain is concentrated at the center of the plate thickness, crystal grains can be made fine.

On the exit side of the finishing rolling mill, the surface temperature of the steel plate increases due to recuperation. The surface temperature at the exit side of the finishing rolling machine is preferably set at 720 to 760° C. to prevent productivity from decreasing due to increased rolling resistance of the steel plate.

[ROT cooling process]
In the ROT (runout table) cooling process, the steel plate manufactured in the hot rolling process is cooled. Specifically, the steel plate after finish rolling is cooled by water cooling using a water cooling device, for example. Although the surface temperature of the steel plate immediately before water cooling is not particularly limited, it is preferably equal to or higher than the Ar ₃ transformation point. If the surface temperature of the steel sheet immediately before water cooling is equal to or higher than the Ar ₃ transformation point, it is possible to prevent a decrease in strength due to grain growth and coarsening of crystal grains.

In the cooling process, it is preferable to set the cooling rate to 5 to 20°C/s at the center of the plate thickness, and to strongly cool the plate to 550 to 750°C. If the cooling rate is low, the introduction of strain due to cooling is insufficient, so that sufficient ferrite nucleation sites cannot be obtained. In this case, since the amount of ferrite grains produced decreases, the ferrite grains may become coarse and the low-temperature toughness of the steel may deteriorate. When the cooling rate is high, the structure becomes mainly bainite, which may reduce the low-temperature toughness of the steel.

[Winding process]
In the winding process, the steel plate cooled in the ROT cooling process is wound up to form a coiled hot-rolled steel plate for line pipes.

The surface temperature of the steel sheet (hereinafter referred to as "winding temperature") during winding of a coiled hot rolled steel sheet for line pipes is preferably 450 to 650°C. If the winding temperature is too low, coarse crystal grains may increase and low-temperature toughness may decrease. If the coiling temperature is too high, the crystal grains may become coarse and the low-temperature toughness of the steel may decrease.

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

The electric resistance welded steel pipe for line pipes of this embodiment is manufactured in the following pipe manufacturing process using the above-described hot rolled steel plate for line pipes.

[Pipe manufacturing process]
An electric resistance welded steel pipe for a line pipe is manufactured by a well-known method while unwinding a coiled hot rolled steel plate for a line pipe. Specifically, a hot-rolled steel plate for a line pipe is bent into a cylindrical shape (open pipe) using continuous forming rolls. Subsequently, the joint portion of the open pipe, that is, both end surfaces in the longitudinal direction of the hot-rolled steel plate for a line pipe are welded by electric resistance welding. Through the above steps, an electric resistance welded steel pipe for a line pipe is manufactured.

In the steel materials for line pipes (hot-rolled steel sheets and ERW steel pipes) manufactured by the above manufacturing process, the metal structure in the center of the wall thickness has a ferrite fraction of 80 to 95% in terms of area ratio, and the remainder is pearlite and/or Or bainite, with an average crystal grain size of 5.0 μm or less. Further, in the metal structure of the surface layer of the base material, the average crystal grain size is 5.0 μm or less, and the difference in average crystal grain size between the surface layer and the thickness center portion is 2.0 μm or less. As a result, the DWTT guaranteed temperature can be set to -40°C or lower, and low-temperature toughness can be improved. Furthermore, a yield stress of 450-540 MPa and a tensile strength of 510-625 MPa can be obtained.

The wall thickness and outer diameter of the electric resistance welded steel pipe for line pipes of this embodiment are not particularly limited. As an example, it is suitable as an electric resistance welded steel pipe for a thick line pipe with a wall thickness of 12 to 25 mm and an outer diameter of 304.8 to 660.4 mm (12 to 26 inches).

Molten steels of Steel A to Steel J shown in Table 1 were continuously cast to produce slabs.

Test numbers 1 to 24 shown in Table 2 were conducted using multiple slabs of steel A to O with wall thicknesses of 12 to 25 mm and outer diameters of 304.8 to 660.4 mm (12 to 26 inches). Manufactured electric resistance welded steel pipes for line pipes.

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 then finished rolling was performed using a finishing rolling mill. At this time, the surface of the steel plate was cooled on the entry side of the finishing mill to create a temperature difference between the center of the plate thickness and the surface layer.

ROT cooling was performed on the steel plate after finish rolling. In ROT cooling, cooling was performed at the cooling rate shown in Table 2 to the cooling stop temperature shown in Table 2.

After producing a steel plate through the above production process, it was coiled at the coiling temperature shown in Table 2 to produce a hot-rolled steel plate for line pipes. Further, the hot-rolled steel plate for line pipes was used to manufacture a pipe by the above-described method to produce an electric resistance welded steel pipe for line pipes.

The following tests were conducted on the obtained hot-rolled steel sheets for line pipes and electric resistance welded steel pipes for line pipes.

[Microstructure]
Based on the method described above, the average crystal grain size at the center of the wall thickness and the surface layer, and the ferrite fraction at the center of the wall thickness were measured using EBSP-OIM. The measurement conditions for EBSP-OIM in measuring the average crystal grain size were: magnification: 400 times, visual field area: 200 μm x 500 μm, and measurement step: 0.3 μm.

[distortion]
The strain at the center of the plate thickness was calculated by FEM analysis. MSC's Mark was used as the analysis software, the number of divisions (elements) was 12, and the mesh size was 2.5 mm x 2.5 mm. Moreover, the deformation resistance was determined from the following formula (a).

σ=6310.6ε ^0.407 ×ε′ ^0.115 ×exp(-2.62×10 ^-3 T
-0.669ε) ...(a)

Here, σ is stress, ε is strain, and ε′ is strain rate. Further, ε and ε' have values determined by the following formulas (b) and (c), where t is the plate thickness after rolling and t0 is the plate thickness before rolling.

ε=1.15ln(t/t0)...(b)
ε'=50s ^-1 ...(c)

Note that the strain at the center of the plate thickness is accumulated to promote refinement of crystal grains and uniformity in the thickness direction, and to improve low-temperature toughness, and the value of strain itself is particularly limited. isn't it. From this example, it was confirmed that crystal grains could be refined when a strain of 2.8 or more was accumulated at the center of the plate thickness.

[Strength test]
Tensile test pieces were taken from hot-rolled steel sheets for line pipes and electric resistance welded steel pipes for line pipes of each test number. Hot-rolled steel plates for line pipes were sampled so that the center of the test piece was 1/2 in the thickness direction and the axis of the test piece was perpendicular to the rolling direction. For the tensile test piece of the electric resistance welded steel pipe for a line pipe, a full thickness tensile test piece was taken from a position 90° from the welded part of the electric resistance welded steel pipe for a line pipe when viewed in the axial direction. The cross section of the tensile test piece was arcuate, and the longitudinal direction of the tensile test piece was parallel to the longitudinal direction of the steel pipe. The size of the tensile test piece was as shown in FIG. 1, the length of the parallel part was 50.8 mm, and the width of the parallel part was 38.1 mm. The numerical values in FIG. 1 indicate the dimensions (unit: mm) of the corresponding portions of the test piece. Using a tensile test piece, a tensile test was conducted 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 electric resistance welded steel pipe for line pipes were determined.

[Low temperature toughness test]
DWTT test pieces were taken from the electric resistance welded steel pipes for line pipes of each test number. The sampling position was 90° from the weld, similar to the tensile test piece, and a notch was formed at the 90° position (Figure 2). The size of the DWTT specimen was as shown in FIG. The numerical values in FIG. 3 indicate the dimensions (unit: mm) of the corresponding portions of the test piece. t indicates wall thickness (unit: mm). The longitudinal direction of the DWTT test piece corresponded to the circumferential direction of the electric resistance welded steel pipe for line pipe. Three DWTT specimens were tested at each temperature in accordance with ASTM E 436, and the lowest temperature at which the average value of the ductile fracture ratio of the three specimens was 85% or more was defined as the DWTT guaranteed temperature.

[Test results]
Table 3 shows the test results.

Referring to Tables 1 to 3, the chemical compositions of the steels in test numbers 1 to 13 were appropriate and satisfied formula (1). Furthermore, the manufacturing conditions for all test numbers were appropriate. Therefore, in test numbers 1 to 13, the ferrite fraction is 80 to 95% in the metal structure at the center of the wall thickness of the base material, the remainder is pearlite or bainite, and the average crystal grain size is 5 μm or less. In the metal structure of the surface layer of the base material, the average crystal grain size was 5 μm or less, and the difference in average crystal grain size between the surface layer and the thickness center portion was 2 μm or less.

Furthermore, the yield strength YS in the tube axis direction of the electric resistance welded steel pipes for line pipes was all 450 to 540 MPa, and the tensile strength TS was all 510 to 625 MPa.

On the other hand, in test numbers 14 and 15, although the manufacturing conditions were appropriate, F1 was less than the lower limit of formula (1). Therefore, the crystal grains became coarse.

In test numbers 16 and 17, although the manufacturing conditions were appropriate, F1 exceeded the upper limit of formula (1). Therefore, the ferrite fraction was less than 75%, resulting in a bainite-based structure. The crystal grain size became coarse due to the bainite-based structure. Furthermore, the yield strength YS of the electric resistance welded steel pipe for line pipe exceeded 540 MPa, and the tensile strength TS exceeded 625 MPa, which were too high.

In test number 18, the heating temperature exceeded 1200°C. Therefore, the γ grain size became coarse, and the crystal grain size of the final structure became coarse.

In test number 19, the heating temperature was less than 1060°C. Therefore, in the heating process, Nb became undissolved, and the amount of fine Nb precipitates during winding that contributed to the strength decreased, resulting in a decrease in strength.

In test number 20, the final rough rolling temperature was higher than 1000°C. Therefore, the γ grain size before finish rolling became coarse, and the crystal grain size of the final structure became coarse.

In test number 21, the rough rolling reduction was less than 60%. Therefore, the γ grain size before finish rolling became coarse, and the crystal grain size of the final structure became coarse.

In test number 22, the temperature at the center of the plate thickness on the entry side of finish rolling exceeded 850°C. Therefore, the strain at the center of the plate thickness accumulated during finish rolling became smaller than 2.8, and the crystal grains became coarse.

In test number 23, the temperature at the center of the plate thickness on the entry side of finish rolling was less than 770°C. Therefore, finish rolling was carried out in the two-phase region (γ + α), so processed ferrite was generated, and the yield strength YS of the ERW steel pipe for line pipe exceeded 540 MPa, and the tensile strength TS exceeded 625 MPa, which were too high.

In test number 24, the temperature difference between the center of the plate thickness and 2 mm below the surface layer during finish rolling was less than 80°C. Therefore, in finish rolling, the strain accumulated at the center of the plate thickness became smaller than 2.8, and the crystal grains became coarse.

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

1 ERW welded part 2 DWTT test piece 3 Notch

Claims (5)

An electric resistance welded steel pipe for a line pipe including a base metal part and an electric resistance welded part,
The chemical composition of the base material part is in mass%,
C: 0.0030-0.120%,
Si: 0.05-0.30%,
Mn: 0.50-2.00%,
P: 0.030% or less,
S: 0.0100% or less,
Al: 0.010-0.035%,
N: 0.0010 to 0.0080%,
Nb: 0.010-0.080%,
Ti: 0.005-0.030%,
Ni: 0.01-0.50%,
Mo: 0.05-0.20%,
O: 0.0050% or less,
V: 0 to 0.10%,
Ca: 0-0.0050%,
Cr: 0 to 0.30%,
Cu: 0 to 0.30%,
Mg: 0 to 0.0050%,
REM: 0 to 0.0100%, and the balance: consisting of Fe and impurities,
F1 defined by the following formula (1) is 0.30 to 0.38,
In the metal structure of the central part of the wall thickness of the base material, the area ratio of ferrite is 80 to 95%, the remainder is pearlite and/or bainite, and the average crystal grain size is 5.0 μm or less. ,
In the metal structure of the surface layer part of the base material part, the average crystal grain size is 5.0 μm or less,
An electric resistance welded steel pipe for a line pipe, wherein the difference in the average crystal grain size between the surface layer portion and the wall thickness center portion is 2.0 μm or less.
F1 = C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/3
+Nb/3... Formula (1)
[In formula (1), C, Si, Mn, Ni, Cr, Mo, V, and Nb each represent the content (mass%) of each element. ] The chemical composition of the base material part is in mass%,
V: more than 0% and less than 0.10%,
Ca: more than 0% and less than 0.0030%,
Cr: more than 0% but not more than 0.30%,
Cu: more than 0% and less than 0.30%,
The electric resistance welded steel pipe for a line pipe according to claim 1, containing one or more selected from the group consisting of Mg: more than 0% and not more than 0.0050%, and REM: more than 0% and not more than 0.0100%. The electric resistance welded steel pipe for a line pipe according to claim 1 or 2, wherein the yield strength in the pipe axial direction is 450 to 540 MPa, and the tensile strength in the pipe axial direction is 510 to 625 MPa. The electric resistance welded steel pipe for a line pipe according to any one of claims 1 to 3, having a wall thickness of 12 to 25 mm and an outer diameter of 304.8 to 660.4 mm. A hot-rolled steel plate used for manufacturing the electric resistance welded steel pipe for line pipes according to any one of claims 1 to 4,
The chemical composition is in mass%,
C: 0.0030 to less than 0.120%,
Si: 0.05-0.30%,
Mn: 0.50-2.00%,
P: 0.030% or less,
S: 0.0100% or less,
Al: 0.010-0.035%,
N: 0.0010 to 0.0080%,
Nb: 0.010-0.080%,
Ti: 0.005-0.030%,
Ni: 0.01-0.50%,
Mo: 0.05-0.20%,
V: 0 to 0.10%,
O: 0.0050% or less,
Ca: 0-0.0050%,
Cr: 0 to 0.30%,
Cu: 0 to 0.30%,
Mg: 0 to 0.0050%,
REM: 0.0100%, and the remainder: Fe and impurities,
F1 defined by the following formula (1) is 0.30 to 0.38,
In the metal structure of the central part of the wall thickness of the hot-rolled steel sheet, in area ratio, the ferrite fraction is 80 to 95%, the remainder is pearlite or bainite, and the average grain size is 5.0 μm or less,
In the metal structure of the surface layer of the hot rolled steel sheet, the average crystal grain size is 5.0 μm or less,
A hot rolled steel sheet for a line pipe, wherein the difference in the average grain size between the surface layer portion and the thickness center portion is 2.0 μm or less.
F1 = C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/3
+Nb/3... Formula (1)
[In formula (1), C, Si, Mn, Ni, Cr, Mo, V, and Nb each represent the content (mass%) of each element. ]

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