JP7614484B2 - Electric resistance welded steel pipes for line pipes and hot rolled steel sheets for line pipes - Google Patents

Description

The present invention relates to electric resistance welded steel pipes for line pipes and hot rolled steel sheets for line pipes.

Pipelines laid on the seabed carry high-pressure fluids. They are also subjected to repeated strain from waves and seawater pressure. Therefore, steel pipes used in undersea pipelines must have high strength and high low-temperature toughness.

A pipeline is made up of multiple line pipes. Electric resistance welded steel pipes (hereafter referred to as ERW steel pipes) are sometimes used as steel pipes for line pipes. High strength can be achieved by increasing the wall thickness of the ERW steel pipe. However, as the wall thickness increases, brittle fracture becomes more likely and low-temperature toughness decreases.

The DWTT (Drop Weight Tear Test) guaranteed temperature is an index of low temperature toughness. The DWTT guaranteed temperature refers to the temperature at which the DWTT has a ductile fracture rate of 85% or more. 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 a steel material for line pipes in which, in the structure at the center of the thickness, the average crystal grain size is 15 μm or less, the coarse crystal grain ratio is 20% or less, the ferrite fraction is 65% or more, the hard phase fraction is 10-20%, and the size of the hard phase is 6.0 μm or less. By creating such a structure, a steel material for line pipes having excellent low-temperature toughness and sufficient strength can be obtained.

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

Patent Document 2, which relates to high-strength steel sheets for automobiles, discloses that by performing finish rolling 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 continuously applying a high load to the steel sheet, dynamic recrystallization of austenite occurs in the steel sheet, the austenite crystal grains become finer, and a high dislocation density is introduced at the austenite grain boundaries, which increases the frequency of ferrite nucleation from the austenite grain boundaries during subsequent forced cooling, thereby increasing the generation of fine ferrite grains.

JP 2018-104757 A International Publication No. 2019/88104

Electric resistance welded steel pipes for line pipes used in pipelines that transport crude oil and natural gas over long distances are increasingly being used in deep seas, and achieving both thick wall thickness and low-temperature toughness has become a technical challenge. 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 at the pipe stage of -40°C or less. However, as mentioned above, as the wall thickness increases, brittle fracture becomes more likely and low-temperature toughness decreases, so if the wall thickness is increased, low-temperature toughness must be further improved.

In view of the above circumstances, the present invention aims to provide a steel material for line pipes that has a thickness of, for example, 12 mm or more, excellent low-temperature toughness with a DWTT guaranteed temperature of -40°C or less, and sufficient strength.

It is known that refining the crystal grains in the final structure is effective in increasing the thickness and toughness of the plate. Refining the crystal grains in the center of the plate thickness is particularly important.

Controlling the austenite before transformation is important for refining the crystal grains in the final structure. Specifically, it is important to accumulate strain in the austenite in order to increase the grain boundary area of austenite, introduce deformation bodies, and increase dislocation density. As mentioned above, for hot-rolled steel sheets for automobiles and other uses, much research is being done on structure control and product development that utilizes the accumulation of strain during finish rolling.

However, thick hot-rolled steel sheets are thicker than hot-rolled steel sheets for automobiles, for example, and the amount of reduction in the finishing rolling machine is limited, so distortion is less likely to accumulate during finishing rolling.

The inventors have conducted extensive research into a method for accumulating strain in thick hot-rolled steel plate during finish rolling to refine the crystal grains. As a result, they discovered that by controlling the temperature difference between the center of the plate thickness and the surface layer in the process prior to the finish rolling, it is possible to accumulate large strain in the center of the plate thickness due to thermal strain in the finish rolling process, and to refine the crystal grains in the center of the plate thickness more than ever before. The present invention was made based on the above findings and through further research, and the gist of the invention is as follows.

[1] An electric resistance welded steel pipe for line pipe including a base metal portion and an electric resistance welded portion, wherein the chemical composition of the base metal portion is, in mass%, C: 0.0030 to 0.120%, Si: 0.05 to 0.30%, Mn: 0.50 to 2.00%, P: 0.030% or less, S: 0.0100% or less, Al: 0.010 to 0.035%, N: 0.0010 to 0.0080%, Nb: 0.010 to 0.080%, Ti: 0.005 to 0.030%, Ni: 0.01 to 0.50%, Mo: 0.10 to 0.25%, O: 0.0050% or less, V: 0 to 0.10%, Ca: 0 to 0.0050%, Cr: 0-0.30%, Cu: 0-0.30%, Mg: 0-0.0050%, REM: 0-0.0100%, and the balance: Fe and impurities, wherein F1 defined by the following formula (1) is 0.30-0.42, and in a metal structure at a center portion of a wall thickness of the base material, the ferrite fraction is 40-80%, in terms of area ratio, the balance is bainite and the average crystal grain size is 5.0 μm or less, and in a metal structure at a surface layer portion of the base material, the average crystal grain size is 5.0 μm or less, and a 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.]

[2] The chemical composition of the base material is, in mass%, V: more than 0% and not more than 0.10%, Ca: more than 0% and not more than 0.0050%, Cr: more than 0% and not more than 0.30%, Cu: more than 0% and not more than 0.30%,
The electric resistance welded steel pipe for line pipe according to the above [1], 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] An electric resistance welded steel pipe for line pipe according to [1] or [2] above, in which the yield strength in the axial direction is 555 to 705 MPa and the tensile strength in the axial direction is 625 to 825 MPa.

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

[5] A hot-rolled steel sheet used for manufacturing an electric resistance welded steel pipe for line pipe according to any one of [1] to [4], comprising a chemical composition, in mass%, of C: 0.0030 to 0.120 %, Si: 0.05 to 0.30%, Mn: 0.50 to 2.00%, P: 0.030% or less, S: 0.0100% or less, Al: 0.010 to 0.035%, N: 0.0010 to 0.0080%, Nb: 0.010 to 0.080%, Ti: 0.00 5 to 0.030%, Ni: 0.01 to 0.50%, Mo: 0.10 to 0.25%, V: 0 to 0.01%, O: 0.0050% or less, Ca: 0 to 0.0050%, Cr: 0 to 0.30%, Cu: 0 to 0.30%, Mg: 0 to 0.0050%, R A hot-rolled steel sheet for line pipes, comprising 0.0100% EM and the balance being Fe and impurities, wherein F1 defined by the following formula (1) is 0.30 to 0.42, wherein in a metal structure of a central portion of a thickness of the hot-rolled steel sheet, the ferrite fraction is 40 to 80% in terms of area ratio, the balance being bainite, and the average crystal grain size is 5.0 μm or less, and wherein in a metal structure of a surface layer portion of the hot-rolled steel sheet, the average crystal grain size is 5.0 μm or less, and a difference in the average crystal grain size between the surface layer portion and the central portion of the thickness 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 a thick-walled electric resistance welded steel pipe for line pipe having excellent low-temperature toughness and sufficient strength, and a hot-rolled steel sheet for line pipe used to manufacture the electric resistance welded steel pipe for line pipe.

FIG. 1 is a plan view of a tensile test piece used in the tensile test. FIG. 2 is a diagram showing the collection positions of the 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.

The electric resistance welded steel pipe for line pipe and the hot rolled steel sheet for line pipe (hereinafter, both are collectively referred to as "line pipe steel material") of this embodiment will be described in detail below. "%" for elements means mass % unless otherwise specified.

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

C:0.0030~0.120%
C is an element that increases the strength of steel. 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 decrease, and further Therefore, the C content is set to 0.120% or less. The preferred lower limit of the C content is 0.040%, and more preferably 0.050%. The upper limit of the amount is preferably 0.100%, and more preferably 0.080%.

Si:0.05~0.30%
Silicon is an element that functions as a deoxidizer for steel. It also suppresses the generation of coarse oxides in the base material and welded parts of electric resistance welded steel pipes, improving the toughness of the base material and welded parts. 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 decreases. Therefore, the Si content is set to 0.30% or less. The lower limit of the Si content is preferably 0.07%, and more preferably 0.10%. The upper limit of the Si content is preferably 0.20%, and more preferably 0.19%. .

Mn: 0.50-2.00%
Mn is an element that improves the hardenability and strength of steel. 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 decreases. If the Mn content is too high, 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%, and more preferably 1.00%. The upper limit of the Mn content is preferably 1.80%, and 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. The upper limit of the P content is preferably 0.015%, and more preferably 0.01%. The P content is preferably as low as possible, and may be 0.

S: 0.0100% or less S is an impurity. S combines with Mn to form Mn-based sulfides, which reduces the low-temperature toughness and SSC resistance of the steel. Therefore, the S content is set to 0.0100% or less. The preferred upper limit of the S content is 0.0010%, and 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, and further suppresses the formation of coarse oxides in the base metal and welded parts of electric resistance welded steel pipes, thereby improving the toughness of the base metal and welded parts. In order to obtain this effect, the Al content is set to 0.010% or more. If the Al content is too high, the Al nitrides become coarse and the low-temperature toughness of the steel decreases. Therefore, the Al content is set to 0.010% or more. The Al content is preferably 0.035% or less. The lower limit of the Al content is preferably 0.012%, and more preferably 0.015%. The upper limit of the Al content is preferably 0.020%, and more preferably 0. In this specification, the Al content means the total Al content in the steel.

N: 0.0010-0.0080%
N is an element that forms nitrides and suppresses the coarsening of austenite grains during the heating process. Specifically, the austenite grains become fine in the rolling process, and the crystal grains after transformation become fine. As a result, the low temperature toughness of the steel is improved. N also increases the strength of the steel through solid solution strengthening. To obtain these effects, the N content is set to 0.0010% or more. If the N content is too high, If N is present, the carbonitrides may become coarse, and the low-temperature toughness of the steel may decrease. Therefore, the N content is set to 0.0080% or less. The preferable lower limit of the N content is 0.0020%, and further The upper limit of the N content is preferably 0.0030%. The upper limit of the N content is preferably 0.0060%, and 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. The formation of Nb carbonitrides suppresses the coarsening of crystal grains and reduces the average crystal grain size. Therefore, the low-temperature toughness of the steel is improved. Furthermore, the fine Nb carbonitrides increase the strength of the steel through dispersion strengthening. To obtain these effects, the Nb content is set to 0.010% or more. If the Nb content is too high, Nb carbonitrides become coarse, and the low-temperature toughness of the steel may decrease. Therefore, the Nb content is set to 0.080% or less. The preferred lower limit of the Nb content is The upper limit of the Nb content is preferably 0.012%, and more preferably 0.020%. The upper limit of the Nb content is preferably 0.070%, and more preferably 0.060%.

Ti: 0.005-0.030%
Ti is an element that combines with N in steel to form TiN, and suppresses the deterioration of low-temperature toughness of steel caused by dissolved N. Furthermore, the dispersion and precipitation of fine TiN prevents the coarsening of crystal grains. The Ti content is set to 0.005% or more in order to obtain these effects. If the Ti content is too high, TiN will become coarse and the coarse grains will become hard. The Ti content is therefore set to 0.030 % or less. The preferred lower limit of the Ti content is 0.008%, and more preferably 0.01%. The upper limit of the Ti content is preferably 0.020%, and more preferably 0.015%.

Ni: 0.01~0.50%
Ni is an element that improves the hardenability of steel and increases its strength. To obtain this effect, the Ni content is set to 0.01% or more. If the Ni content is too high, this effect saturates. Therefore, the Ni content is set to 0.50% or less. The preferred lower limit of the Ni content is 0.05%, and more preferably 0.08%. The preferred upper limit of the Ni content is 0. 20%, and more preferably 0.15%.

Mo: 0.10~0.25%
Mo is an element that improves the hardenability and strength of steel. Mo also refines austenite grains and improves the low-temperature toughness of steel. To obtain these effects, the Mo content is set to 0. The Mo content is set to 10% or more. If the Mo content is too high, the field weldability of the steel is reduced. Therefore, the Mo content is set to 0.25% or less. The preferred lower limit of the Mo content is 0.12%. The upper limit of the Mo content is preferably 0.23%, and more preferably 0.20%.

O: 0.0050% or less O is an impurity. O forms oxides and reduces the hydrogen induced cracking resistance of steel. O also reduces the low temperature toughness of steel. Therefore, the O content is set to 0.0050% or less. A preferred upper limit of the O content is 0.0030%, and more preferably 0.0025%. The O content is preferably as low as possible, and may be 0.

V: 0 to 0.10%
V is an optional element and may not be contained. When V is contained, V combines with C and N in the steel during the coiling process to form fine carbonitrides, thereby increasing the strength of the steel. The fine V carbonitrides further suppress the coarsening of crystal grains and increase the low-temperature toughness of the steel. Although these effects can be obtained even with a small amount of V, 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, the V carbonitrides may coarsen, and the low-temperature toughness of the steel may decrease. Therefore, the V content is 0.10% or less. A more preferable lower limit of the V content is 0.02%, and more preferably 0.05%. A preferable upper limit of the V content is 0.08%, and more preferably 0.07%.

Ca: 0-0.0030%
Ca is an optional element and does not have to be contained. When contained, Ca controls the morphology of MnS to make it spheroidal, thereby improving the low-temperature toughness of the steel. This effect can be obtained even with a small amount of Ca. However, 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-based inclusions are formed. As a result, the oxidation The Ca content is more preferably 0.0030% or less. The lower limit of the Ca content is more preferably 0.0002%, and even more preferably 0. The upper limit of the Ca content is preferably 0.0025%, and more preferably 0.0020%.

Cr: 0-0.30%
Cr is an optional element and does not have to be contained. When contained, Cr improves the hardenability of steel and increases the strength of steel. This effect can be obtained even with a small amount of Cr, but it is necessary to ensure the effect. In order to obtain this, the Cr content is preferably 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 preferably 0.01% or more. The Cr content is preferably 0.05% or less, and more preferably 0.06% or less. The Cr content is preferably 0.25% or less, and more preferably 0.30% or less. The lower limit of the Cr content is preferably 0.05%, and more preferably 0.06% or less. The upper limit of the Cr content is preferably 0.25%, and more preferably 0.30% or less. .2%.

Cu: 0-0.30%
Cu is an optional element and does not have to be contained. When contained, Cu improves the hardenability of the steel and increases the strength of the steel. This effect can be obtained even with a small amount of Cu, but it is necessary to ensure that the effect is obtained. In order to achieve this, it is preferable that the Cu content be 0.02% or more. If the Cu content is too high, the hardenability becomes too high and the toughness decreases. Therefore, the Cu content should be 0.30% or less. The lower limit of the Cu content is preferably 0.05%, and more preferably 0.07%. The upper limit of the Cu content is preferably 0.25%, and more preferably 0.20%. be.

Mg: 0-0.0050%
Mg is an optional element and may not be contained. When contained, Mg functions as a deoxidizer and a desulfurizer. It also produces fine oxides and improves the toughness of the HAZ. These effects can be obtained even with a small amount of Mg, but to ensure the effects, it is preferable to set the Mg content to 0.0001% or more. If the Mg content is too high, oxides As a result, there is a risk of 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. Mg Content The lower limit of the Mg content is preferably 0.0005%, and more preferably 0.0010%. The upper limit of the Mg content is preferably 0.040%, and more preferably 0.0030%.

REM: 0-0.0100%,
REM is an optional element and may not be contained. Here, "REM" refers to rare earth elements, i.e., Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, It refers to at least one element selected from the group consisting of Dy, Ho, Er, Tm, Yb, and Lu. When REM is contained, the REM functions as a deoxidizer and a desulfurizer. This effect Although the effect can be obtained even with a small amount of REM, in order to reliably obtain the effect, it is preferable to set the REM content to 0.0001% or more. If the REM content is too high, coarse oxides are generated, and as a result, , there is a risk of causing 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. The preferable lower limit of the REM content is 0.0005%. and more preferably 0.0010%. The upper limit of the REM content is preferably 0.070%, and more preferably 0.0050%.

The remainder of the chemical composition of the line pipe steel material according to this embodiment is composed of Fe and impurities. Here, impurities refer to substances that are mixed in from raw materials such as ore, scrap, or the manufacturing environment when industrially manufacturing line pipe steel material, and are acceptable within a range that does not adversely affect the line pipe steel material of this embodiment.

[Regarding 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 an element corresponding to an 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 increase 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 (shorter time side). In this case, the steel temperature enters the ferrite region on the CCT diagram before nucleation sites are sufficiently formed. As a result, the ferrite grains become coarse and the average grain size becomes large. Furthermore, mixed grains are more likely to occur and the coarse grain ratio increases. As a result, the low temperature toughness of the steel decreases. If F1 is too low, the hardenability also decreases and sufficient strength cannot be obtained.

If F1 is too high, the S curve of the CCT diagram shifts to the right (longer time side). In this case, hard structures are more likely to form and the ferrite fraction in the structure decreases. As a result, the low-temperature toughness of the steel decreases. If F1 is too high, the hardenability also increases and the strength of the steel becomes too high.

If F1 is between 0.30 and 0.42, the temperature of the steel is easily maintained in the ferrite region, the ferrite fraction at the center of the steel thickness can be increased to 80% or more, and the low-temperature toughness of the steel can be improved. The upper limit of the F1 value is preferably 0.40, and more preferably 0.38.

[Ferrite fraction]
The structure of the center of the thickness of the steel material for line pipe according to this embodiment contains 40 to 80% ferrite. The remainder is bainite. Bainite also includes bainitic ferrite that does not contain cementite in the grains or at the grain boundaries. The grain boundaries of bainitic ferrite may contain MA (Martensite-Austenite constituent). Here, the center of the thickness means a range of ±10%t from the center of the thickness in the thickness direction or in the thickness direction (i.e., a range of 40 to 60%t from the surface in the thickness direction or in the thickness direction) when the plate thickness or wall thickness is t mm.

As mentioned above, if the ferrite fraction in the structure at the center of the steel thickness is 40-80% by area, the crystal grains become finer, and as a result, the low-temperature toughness of the steel is improved. The preferred lower limit of the ferrite fraction is 42%, and more preferably 45%. The preferred upper limit is 78%, and more preferably 75%.

The ferrite fraction is measured by the following method:

A sample is taken from the center of the thickness of the line pipe steel material. The sample is polished with colloidal silica abrasive 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 calculated using the KAM (Kernel Average Misorientation) method equipped on the EBSP-OIM.

In the KAM method, the orientation differences between each pixel of a regular hexagon in the measurement data (the central pixel) are averaged for a first approximation using the six pixels adjacent to this pixel (total of seven pixels), or a second approximation using the 12 pixels further out than these six pixels (total of 19 pixels), or a third approximation using the 18 pixels further out than these 12 pixels (total of 37 pixels), and the average obtained is used 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 represents the orientation change within the grain. In other words, this map represents the distribution of strain based on local orientation changes within the grain. In this embodiment, pixels with an orientation difference between adjacent pixels of 5° or less by a third approximation are displayed. In this embodiment, the planar fraction of pixels calculated to have an orientation difference of 1° or less by the third approximation is defined as the ferrite fraction. Those with an orientation difference of more than 1° by the third approximation are considered to be bainite (1) (including bainitic ferrite).

[Average crystal grain size]
In the line pipe steel material of this embodiment, the average grain size at the center of the wall thickness of the line pipe steel material is 5.0 μm or less. If the average grain size is too large, the low-temperature toughness of the steel decreases. In this embodiment, since the above-mentioned average grain size is 5.0 μm or less, excellent low-temperature toughness is obtained. The preferred upper limit of the average grain size is 4.8 μm, and more preferably 4.5 μm.

The average grain size is measured using the EBSP-OIM method. Samples are collected and polished in the same manner as for measuring the ferrite fraction. The polished samples are analyzed using EBSP-OIM. Specifically, the position where the orientation difference between adjacent measurement points exceeds 15° in orientation measurements at each fixed measurement step is considered to be a grain boundary. 15° is the threshold value for high-angle grain boundaries and is generally recognized as a grain boundary. The area surrounded by the grain boundary is considered to be a grain, and the grain size and surface area of the grain are calculated. The area-average grain size is calculated from the obtained grain size and surface area. In this embodiment, the obtained area-average grain size is considered to be the average grain size.

In the line pipe steel material of this embodiment, the average crystal grain size in the surface layer of the line pipe steel material is 5.0 μm or less. Here, the surface layer 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 t mm. If the average crystal grain size is too large, the low-temperature toughness of the steel decreases. In this embodiment, since the average crystal grain size in the surface layer of the line pipe steel material is 5.0 μm or less, excellent low-temperature toughness is obtained. The preferred upper limit of the average crystal grain size is 4.8 μm, and more preferably 4.5 μm.

In addition, in the line pipe steel of this embodiment, the difference in average crystal grain size between the surface layer and the center of the wall thickness is 2.0 μm or less. This makes it possible to obtain uniform low-temperature toughness in the surface layer and the center of the wall thickness.

By carrying out the manufacturing process described below, the metal structure in the center of the thickness of the steel material can be made to have an area ratio of 40 to 80% ferrite and the remainder bainite, with an average crystal grain size of 5.0 μm or less, and the metal structure in the surface layer of the steel material can be made to have an average crystal grain size of 5.0 μm or less, with the difference in average crystal grain size between the surface layer and the center of the thickness being 2.0 μm or less.

As a result, low temperature toughness can be improved by setting the DWTT guaranteed temperature at -40° C. or lower. Furthermore, in the electric resistance welded steel pipe, a yield strength in the axial direction of 550 to 705 MPa and a tensile strength in the axial direction of 625 to 825 MPa can be obtained.

[Production method]
An example of the manufacturing method of the above-mentioned steel material for line pipe will be described.

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

[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) become coarse and the low-temperature toughness decreases. If the heating temperature is too low, the crystal grains are not refined during rolling and precipitation strengthening after rolling is not 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 process is hot rolled using a rough rolling mill and a finish rolling mill to produce a hot rolled steel sheet. Both the rough rolling mill and the finish rolling mill are equipped with multiple rolling stands arranged in a row, and each rolling stand is equipped with a pair of rolls.

In the rough rolling process, the slab is subjected to rough rolling with a final rolling temperature of 900 to 1000°C and a rolling reduction of 60% or more. The rolling reduction of rough rolling is calculated 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 mill is set to 770 to 850°C, and the surface layer of the steel plate is cooled at the entry side of the finishing rolling mill, so that the temperature difference between the center of the plate thickness and 2 mm below the surface layer is set to 80°C or more. The temperature of the steel plate can be determined, for example, by performing a heat conduction analysis from the thickness and length of the steel plate, physical properties such as specific heat, the amount of cooling water, and the conveying speed. The surface layer can be cooled, for example, by spraying cooling water onto the steel plate. This concentrates strain at the center of the plate thickness, making it possible to make the crystal grains fine.

At the exit of the finishing rolling mill, the surface temperature of the steel plate rises due to recuperation. It is preferable that the surface temperature at the exit of the finishing rolling mill be 720 to 760°C so that the rolling resistance of the steel plate does not increase and productivity does not decrease.

[ROT cooling process]
In the ROT (run-out table) cooling process, the steel sheet produced in the hot rolling process is cooled. Specifically, the steel sheet after the finish rolling is strongly cooled, for example, by water cooling using a water cooling device. The surface temperature of the steel sheet immediately before water cooling is not particularly limited, but is preferably equal to or higher than the _Ar3 transformation point. If the surface temperature of the steel sheet immediately before water cooling is equal to or higher than the _Ar3 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 at the center of the plate thickness to 30-100°C/s, and to strongly cool the plate to room temperature to 450°C. If the cooling rate is too low, the ferrite fraction at the center of the plate thickness will be high, making it impossible to obtain the desired strength, and the introduction of strain by cooling will be insufficient, resulting in a large average crystal grain size and a decrease in the low-temperature toughness of the steel. If the cooling rate is too high, the ferrite fraction will decrease, and large amounts of not only bainite but also martensite will be formed, which may decrease the low-temperature toughness of the steel.

If the stop temperature of the strong cooling exceeds 450°C, the ferrite and bainite may coarsen and the low-temperature toughness of the steel may decrease. The preferred stop temperature to achieve both high strength and low-temperature toughness is 350°C or less, and more preferably 200°C or less.

[Winding process]
In the coiling process, the steel sheet cooled in the ROT cooling process is coiled to produce a hot-rolled steel sheet for line pipe in a coil shape.

The surface temperature of the steel sheet when coiling the hot-rolled steel sheet for coiled line pipe (hereinafter referred to as the "coiling temperature") is preferably room temperature to 450°C. If the coiling temperature is too high, the crystal grains may become coarse and the low-temperature toughness of the steel may decrease. The coiling temperature is preferably 350°C or less, and more preferably 200°C or less.

The above manufacturing process produces the hot-rolled steel sheet for line pipe of this embodiment.

The electric resistance welded steel pipe for line pipe of this embodiment is manufactured by the following pipe making process using the above-mentioned hot rolled steel sheet for line pipe.

[Pipe manufacturing process]
While unwinding the coiled hot-rolled steel sheet for line pipe, an electric resistance welded steel pipe for line pipe is manufactured by a known method. Specifically, the hot-rolled steel sheet for line pipe is bent by a continuous forming roll into a cylindrical shape (open pipe). Then, the joints of the open pipe, i.e., both longitudinal end faces of the hot-rolled steel sheet for line pipe, are welded by an electric resistance welding method. Through the above steps, an electric resistance welded steel pipe for line pipe is manufactured.

In the line pipe steel (hot-rolled steel sheet and electric resistance welded steel pipe) manufactured by the above manufacturing process, the metal structure in the center of the wall thickness has an area ratio of 40 to 80% ferrite and the remainder bainite, with an average crystal grain size of 5.0 μm or less. In addition, 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 center of the wall thickness is 2.0 μm or less. As a result, the DWTT guaranteed temperature can be set to -40°C or less, and low-temperature toughness can be improved. Furthermore, a yield stress of 550 to 705 MPa and a tensile strength of 625 to 825 MPa can be obtained.

The wall thickness and outer diameter of the electric resistance welded steel pipe for line pipe of this embodiment are not particularly limited. As an example, it is suitable as an electric resistance welded steel pipe for thick-walled 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).

The molten steels A to J shown in Table 1 were continuously cast to produce slabs.

Using multiple slabs of steels A to O, electric resistance welded steel pipes for line pipes were manufactured with wall thicknesses ranging from 12 to 25 mm and outer diameters ranging from 304.8 to 660.4 mm (12 to 26 inches), with test numbers 1 to 23 shown in Table 2.

Specifically, the slabs of each test number were heated in a heating furnace. The heating temperatures (°C) were as shown in Table 2. The heated slabs were rolled using a rough rolling mill, and then finish-rolled using a finishing mill. During this process, the steel plate surface was cooled at the entry side of the finishing mill, creating a temperature difference between the center of the plate thickness and the surface layer.

After finish rolling, the steel plate was subjected to ROT cooling. In ROT cooling, the plate was cooled to the cooling stop temperature shown in Table 2 at the cooling rate shown in Table 2.

After producing the steel plate by the above manufacturing process, coiling was performed at the coiling temperature shown in Table 2 to produce hot-rolled steel plate for line pipe. Furthermore, the hot-rolled steel plate for line pipe was used to produce pipe by the above-mentioned method to produce electric resistance welded steel pipe for line pipe.

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

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

[distortion]
The strain at the center of the plate thickness was calculated by FEM analysis. Mark by MSC 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. The deformation resistance was calculated 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. In addition, ε and ε' are values that can be calculated 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)

The strain at the center of the plate thickness is accumulated to refine the crystal grains and improve low-temperature toughness, and the value of the strain itself is not particularly limited. This example confirmed that the crystal grains can be refined when a strain of 2.8 or more is accumulated at the center of the plate thickness.

[Strength test]
Tensile test pieces were taken from the hot-rolled steel sheet for line pipe and the electric resistance welded steel pipe for line pipe of each test number. The hot-rolled steel sheet for line pipe was taken so that the center of the test piece was 1/2 in the plate thickness direction and the axis of the test piece was perpendicular to the rolling direction. The tensile test piece of the electric resistance welded steel pipe for line pipe was taken from a position 90° from the welded part of the electric resistance welded steel pipe for line pipe when looking at the electric resistance welded steel pipe for line pipe in the axial direction. The cross section of the tensile test piece was arc-shaped, 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 is 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 part of the test piece. Using the tensile test piece, a tensile test was performed at room temperature in accordance with the 5CT provisions of the API standard. Based on the test results, the yield strength YS (MPa) and tensile strength TS (MPa) of the electric resistance welded steel pipe for line pipe were determined.

[Low temperature toughness test]
DWTT test pieces were taken from the electric resistance welded steel pipe for line pipe of each test number. The taking position was 90° from the welded part as in the case of the tensile test pieces, and a notch was machined at the 90° position (Fig. 2). The size of the DWTT test pieces was as shown in Fig. 3. The numbers in Fig. 3 indicate the dimensions (unit: mm) of the corresponding parts of the test pieces. t indicates the wall thickness (unit: mm). The longitudinal direction of the DWTT test pieces corresponded to the circumferential direction of the electric resistance welded steel pipe for line pipe. Three DWTT test pieces were tested at each temperature in accordance with the provisions of ASTM E 436, and the lowest temperature at which the average value of the ductile fracture surface ratio of the three pieces was 85% or more was defined as the DWTT guaranteed temperature.

[Test Results]
The test results are shown in Table 3.

With reference to Tables 1 to 3, the chemical compositions of the steels of test numbers 1 to 13 were appropriate and satisfied formula (1). Furthermore, the manufacturing conditions of all test numbers were appropriate. Therefore, in test numbers 1 to 13, the metal structure of the base material at the center of the wall thickness was a structure consisting of 40 to 80% ferrite and bainite, the average grain size was 5 μm or less, the metal structure of the base material and the surface layer was 5 μm or less, and the difference in the average grain size between the surface layer and the center of the wall thickness was 2 μm or less. As a result, excellent low-temperature toughness was exhibited. Furthermore, the yield strength YS in the pipe axis direction of the electric resistance welded steel pipes for line pipes was 550 to 705 MPa in all cases, and the tensile strength TS was 625 to 825 MPa in all cases.

On the other hand, in test numbers 14 and 15, although the manufacturing conditions were appropriate, F1 was below the lower limit of formula (1). As a result, 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). As a result, the ferrite fraction was less than 40%, and the yield strength YS of the electric resistance welded steel pipe for line pipe exceeded 705 MPa and the tensile strength TS exceeded 825 MPa, which were too high.

In test number 18, the heating temperature exceeded 1200°C. This caused the gamma grain size to become coarse, resulting in a coarse crystal grain size in the final structure.

In test number 19, the heating temperature was less than 1060°C. As a result, Nb did not dissolve during the heating process, and the amount of fine Nb precipitates during coiling that contribute to strength was reduced, resulting in a low strength.

In test number 20, the final rough rolling temperature was higher than 1000°C. As a result, the gamma grain size before finish rolling became coarse, and the crystal grain size in the final structure became coarse.

In test number 21, the rough rolling reduction was less than 60%. As a result, the gamma grain size before finish rolling became coarse, and the crystal grain size in the final structure became coarse.

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

In test number 23, the temperature at the center of the plate thickness on the finish rolling entry side was less than 770°C. As a result, the finish rolling was performed in the two-phase region (γ + α), which resulted in the formation of processed ferrite, and the yield strength YS of the electric resistance welded steel pipe for line pipe exceeded 705 MPa, and the tensile strength TS exceeded 825 MPa, both of which were too high.

In test number 24, the temperature difference between the center of thickness and the surface layer during finish rolling was less than 80°C. As a result, the strain accumulated in the center of thickness during finish rolling was less than 2.8, causing the crystal grains to become coarse.

In test number 25, in the ROT cooling and coiling process after finish rolling, the cooling stop temperature was higher than 450°C, the cooling rate was less than 30°C/s, and the coiling temperature was higher than 450°C. As a result, the ferrite fraction exceeded 80%, the structure was mainly ferrite, and the strength was reduced.

The above describes an embodiment of the present invention. However, the above-described embodiment is merely an example for implementing the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-described embodiment can be modified as appropriate within the scope of the spirit of the present invention.

1 Electric resistance welded part 2 DWTT test piece 3 Notch

Claims (9)

An electric resistance welded steel pipe for line pipe including a base material portion and an electric resistance welded portion,
The chemical composition of the base material 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: 0.10-0.25%,
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: Fe and impurities;
F1 defined by the following formula (1) is 0.30 to 0.42,
In the metal structure at the center of the thickness of the base material, the ferrite fraction is 40 to 80% in terms of area ratio, the remainder is bainite, and the average crystal grain size is 5.0 μm or less,
In the metal structure of the surface layer of the base material, the average crystal grain size is 5.0 μm or less,
The electric resistance welded steel pipe for line pipe use, wherein the difference in the average crystal grain size between the surface layer portion and the central portion of the wall thickness 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 electric resistance welded steel pipe for line pipe according to claim 1, wherein the F1 is 0.30 to 0.40. The electric resistance welded steel pipe for line pipe according to claim 1 or 2, wherein the F1 is 0.30 to 0.38. The chemical composition of the base material is, in mass%,
V: more than 0% and less than 0.10%,
Ca: more than 0% and less than 0.0050%,
Cr: more than 0% but not more than 0.30%,
Cu: more than 0% and 0.30% or less,
The electric resistance welded steel pipe for line pipe according to any one of claims 1 to 3, 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. 5. The electric resistance welded steel pipe for line pipe according to claim 1, wherein the yield strength in the axial direction is 550 to 705 MPa, and the tensile strength in the axial direction is 625 to 825 MPa. The electric resistance welded steel pipe for line pipe according to any one of claims 1 to 5 , having a wall thickness of 12 to 25 mm and an outside diameter of 304.8 to 660.4 mm. A hot-rolled steel sheet used in the manufacture of an electric resistance welded steel pipe for line pipe according to any one of claims 1 to 6 ,
The chemical composition, in mass%, is
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: 0.10-0.25%,
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 balance: Fe and impurities;
F1 defined by the following formula (1) is 0.30 to 0.42,
In the metal structure of the center of the thickness of the hot-rolled steel plate, the ferrite fraction is 40 to 80%, the remainder is bainite, and the average crystal 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,
The difference in average grain size between the surface layer portion and the central portion of the wall thickness 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 hot-rolled steel sheet for line pipes according to claim 7, wherein the F1 is 0.30 to 0.40. The hot-rolled steel sheet for line pipes according to claim 7 or 8, wherein the F1 is 0.30 to 0.38.

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