JP7163777B2 - Steel plate for line pipe - Google Patents

Description

TECHNICAL FIELD The present invention relates to a steel plate for line pipes.

Line pipes are used for transporting petroleum, natural gas, etc. When transporting these over long distances, so-called large-diameter line pipes are used. High strength and toughness are required for the steel plate for line pipes, that is, the base steel plate, which is the raw material.

During use, line pipes may be significantly deformed due to ground changes caused by earthquakes, thawing of frozen soil, freezing, and the like. Therefore, it is required to prevent breakage and damage of the line pipe due to such deformation. For this reason, line pipes are designed in consideration of the allowable strain (strain-based design) that allows steel pipes to maintain soundness against presumed strain.

Based on the above design, the base material steel plate is required to have high deformation performance. Deformation performance is performance for preventing local buckling, which is uneven deformation due to axial compressive force after the base material steel plate is processed into a pipe shape and welded. It is considered that the deformation performance is improved when the yield ratio is low and the uniform elongation is high.

In order to improve deformation performance, the base steel sheet is also required to have improved strain aging resistance. Line pipes undergo strain age hardening due to strain accumulated during pipe manufacturing and heating during coating. When strain age hardening occurs, the strength of the steel pipe increases and the deformation performance decreases. Thus, in line pipes, the higher the strength, the more difficult it is to ensure good deformation performance after strain aging.

In response to the above requirements, techniques have been disclosed for controlling the chemical composition and microstructure to improve the strain aging resistance of steel sheets. For example, Patent Documents 1 and 2 disclose steels having a three-phase structure of ferrite, bainite, and island martensite. Further, Patent Document 3 discloses a steel having a structure composed of ferrite and hard phases (martensite and bainite), in which fine residual austenite is formed in the hard phase.

Japanese Patent Application Laid-Open No. 2008-248328 JP 2008-248330 A Japanese Patent Application Laid-Open No. 2003-253331

In order to provide a base steel sheet with a low yield ratio and high uniform elongation, it is effective to make the metallographic structure of the steel sheet contain a soft phase and a hard phase. For example, the steels disclosed in Patent Documents 1 and 2 have structures containing ferrite and 3% or more island martensite. Further, the steel disclosed in Patent Document 3 has a structure composed of ferrite, martensite and/or bainite. As a result, the steels disclosed in Patent Documents 1 to 3 achieve a low yield ratio and high uniform elongation.

On the other hand, if the metal structure of the base steel sheet contains the above-described soft phase and hard phase, the manufacturing process becomes complicated. Specifically, with the steels disclosed in Patent Documents 1 and 2, it is necessary to perform a step of accelerating the rolling of the steel during production, and then reheating after the accelerated cooling. In addition, in the steel disclosed in Patent Document 3, after accelerated cooling of the rolled steel, it is necessary to perform a step combining one or more of heating, holding, and cooling at a cooling rate of 0.5 ° C./s or less. There is

Complicated manufacturing processes are not preferable in consideration of actual manufacturing. This is because the process of reheating the steel plate after the accelerated cooling described above leads to an increase in manufacturing cost and loss of manufacturing time.

Based on the above, it is an object of the present invention to provide a steel plate for line pipes that can be manufactured by a simple process and has the strength and excellent deformation performance necessary for line pipe materials.

The present invention has been made to solve the above problems, and the gist of the present invention is the following steel sheet for line pipes.

(1) The chemical composition of the steel sheet is % by mass,
C: 0.06 to 0.12%,
Si: 0.10 to 0.50%,
Mn: 1.0-1.8%,
P: 0.02% or less,
S: 0.001% or less,
Nb: 0.005 to 0.050%,
Ti: 0.005 to 0.030%,
Al: 0.010 to 0.040%,
N: 0.001 to 0.005%,
Cu: 0-0.50%,
Ni: 0 to 0.50%,
Cr: 0 to 0.50%,
Mo: 0-0.10%,
V: 0 to 0.50%,
B: 0 to 0.01%,
Ca: 0-0.02%,
REM: 0-0.02%,
Mg: 0-0.02%, and the balance: Fe and impurities,
Pcm represented by the following formula (i) is 0.15 to 0.23,
Ceq represented by the following formula (ii) is 0.38 to 0.43,
The metal structure in the surface layer part and the plate thickness center part of the steel sheet is, in terms of area ratio,
30-70% ferrite and 3% or less hard phase,
the balance is bainite,
and an average crystal grain size of ferrite in the surface layer portion and the plate thickness center portion is 5.0 to 15.0 μm,
The aspect ratio of the bainite crystal grains in the surface layer portion and the plate thickness center portion is 1.0 to 5.0,
The difference between the ferrite area ratio in the surface layer portion and the ferrite area ratio in the plate thickness center portion is 0 to 20%,
The difference between the bainite area ratio in the surface layer portion and the bainite area ratio in the plate thickness center portion is 0 to 20%,
the difference between the aspect ratio of the bainite crystal grains in the surface layer portion and the aspect ratio of the bainite crystal grains in the central portion of the plate thickness is 2.0 or less;
The tensile strength is 460 to 760 MPa, the yield strength is 360 to 600 MPa, the yield ratio is 85% or less, the uniform elongation is 9.0% or more, and the product of the tensile strength and the uniform elongation is 4000 (MPa · %) or more,
A steel plate for line pipes having a plate thickness of 15 to 40 mm.
Pcm=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B (i)
Ceq=C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/14 (ii)
However, each element symbol in the above formulas (i) and (ii) represents the content (% by mass) of each element contained in the steel, and is zero when not contained.

(2) the chemical composition, in mass %,
Cu: 0.10-0.50%,
Ni: 0.10 to 0.50%,
Cr: 0.10 to 0.50%,
Mo: 0.05-0.10%, and V: 0.10-0.50%,
The steel plate for line pipes according to (1) above, containing one or more selected from:

(3) the chemical composition, in mass %,
B: 0.0005 to 0.01%,
The steel plate for line pipes according to (1) or (2) above, containing

(4) the chemical composition, in mass %,
Ca: 0.002-0.02%,
REM: 0.003-0.02%, and Mg: 0.003-0.02%,
The steel plate for line pipes according to any one of (1) to (3) above, containing one or more selected from:

ADVANTAGE OF THE INVENTION According to this invention, the steel plate for line pipes which can be manufactured by a simple process and has the strength|strength required for a line pipe raw material, and the excellent deformation performance can be obtained.

The present inventors set a tensile strength of 460 to 760 MPa and a yield strength of 360 to 600 MPa as target strengths. Then, various investigations were carried out in order to provide the steel plate having the above characteristic values and a plate thickness of 15 to 40 mm with deformability. As a result, the following findings (a) to (e) were obtained.

(a) In order to obtain high deformation performance after strain aging, that is, to achieve both a low yield ratio and high uniform elongation, the metal structure is a composite structure in which a structure that exhibits these two characteristics is formed in a composite manner. is preferable. Specifically, a composite structure containing ferrite and bainite is effective.

(b) An increase in ferrite contributes to an improvement in deformation performance, but does not easily contribute to an improvement in strength. Therefore, it is necessary to limit the ferrite area ratio from the balance between strength and deformation performance. As described above, ferrite does not easily contribute to strength improvement, but strength can be increased by making the average grain size of ferrite finer. However, in this increase in strength, the increase in yield strength is more remarkable than the increase in tensile strength, and the yield ratio increases. Therefore, in order to achieve both strength and a low yield ratio, it is effective to limit the range of the average grain size of ferrite.

(c) Increasing bainite improves strength but reduces deformation performance. Therefore, from the balance between strength and deformation performance, it is necessary to limit the area ratio of bainite as well as ferrite.

(d) The line pipe must have a wall thickness within a certain range. Specifically, the steel plate, which is the raw material, must have a thickness of about 15 to 40 mm. As described above, when the plate thickness is relatively large, the surface layer portion of the steel plate and the central portion of the steel plate may have different properties due to the difference in cooling rate and the like. When the properties of the steel plate surface layer portion and the steel plate center portion differ greatly, the uniform elongation tends to decrease due to such a difference in properties. As an example of properties that reduce the uniform elongation, a difference in metallographic structure between the surface layer portion and the thickness center portion of the steel sheet can be considered. In addition, the difference in aspect ratio of the bainite grains between the surface layer and the center of the sheet thickness is considered to affect the uniform elongation. Moreover, when the difference in the aspect ratio of the bainite crystal grains is large, the uniform elongation value is not improved, and the deformation performance is lowered.

(e) Based on the above, it is desirable that the manufacturing process be the following process. Specifically, it is desirable to perform weak cooling after finish rolling to control the ferrite structure. After that, it is desirable to carry out a step of generating bainite by performing intense cooling. As a result, a steel sheet for line pipes having desired strength and excellent deformation performance can be obtained without requiring a complicated process such as reheating the steel sheet.

The present invention has been made based on the above findings. Each requirement of the present invention will be described in detail below.

1. Chemical composition The reasons for limiting each element are as follows. In addition, "%" about content in the following description means "mass %."

C: 0.06-0.12%
C is an element necessary for improving the strength of the steel sheet. In order to stably obtain a tensile strength of 460 to 760 MPa, the C content should be 0.06% or more. The C content is preferably over 0.07%, more preferably over 0.08%. However, when the C content is excessive, the toughness and weldability of the base metal and the toughness of its weld heat affected zone (hereinafter referred to as "HAZ") are degraded. Furthermore, deterioration of strain aging resistance occurs. Also, the tensile strength may increase excessively. Therefore, the C content is 0.12% or less, preferably 0.10% or less.

Si: 0.10-0.50%
Si has an effect of suppressing precipitation of cementite. Due to this effect, good deformation performance, that is, a low yield ratio and a high uniform elongation can be obtained before and after strain aging. Therefore, the Si content is set to 0.10% or more, preferably more than 0.20%. However, when the Si content is excessive, the toughness of the base metal and HAZ is significantly reduced. In addition, the aspect ratio of the bainite grains in the surface layer and the thickness center increases. The result is higher yield strength and yield ratio and lower uniform elongation. Therefore, the Si content is 0.50% or less, preferably 0.40% or less.

Mn: 1.0-1.8%
Since Mn has the effect of improving the strength of the steel sheet, the Mn content is made 1.0% or more. The Mn content is preferably 1.1% or more, more preferably 1.2% or more. However, if the Mn content is excessive, weld cracks are likely to occur. Furthermore, when the Mn content is excessive, the aspect ratio of the bainite crystal grains in the surface layer portion is large, and the difference in the aspect ratio of the bainite crystal grains between the surface layer portion and the thickness center portion also increases. This results in higher yield strength and yield ratio and lower uniform elongation. That is, it becomes difficult to obtain good deformation characteristics. Therefore, the Mn content should be 1.8% or less. The Mn content is preferably 1.7% or less, more preferably 1.6% or less.

P: 0.02% or less P is an element that causes a decrease in toughness. When the content of P increases, and in particular exceeds 0.02%, the toughness tends to decrease significantly. Therefore, the P content is 0.02% or less, preferably 0.015% or less. The lower the P content, the better.

S: 0.001% or less When the S content is excessive, many inclusions harmful to ductility or toughness are generated. In particular, when the S content exceeds 0.001%, a large amount of inclusions is formed, resulting in a marked decrease in ductility and toughness. Therefore, the S content should be 0.001% or less. The S content is preferably as small as possible, preferably 0.0005% or less.

Nb: 0.005-0.050%
Nb has the effect of improving the strength of the steel sheet and increasing the toughness of the steel pipe base material by manufacturing under appropriate rolling conditions. Therefore, the Nb content is 0.005% or more, preferably 0.010% or more. However, excessive Nb content reduces the toughness in the base metal and HAZ. Therefore, the Nb content is 0.050% or less, preferably 0.030% or less.

Ti: 0.005-0.030%
Ti forms precipitates (TiN) together with N, which is harmful to strain aging resistance, and has the effect of stabilizing N atoms. Formation of such TiN not only greatly improves strain aging resistance, but also refines the structure in the base metal and HAZ. As a result, it has the effect of improving the low-temperature toughness of the base metal and HAZ of the high-strength steel.

Here, if the Ti content is less than 0.005%, the above effect cannot be obtained. Therefore, the Ti content is 0.005% or more, preferably 0.010% or more. However, if the Ti content exceeds 0.030%, the toughness of the base metal and HAZ is lowered. Therefore, the Ti content is 0.030% or less, preferably 0.025% or less. Furthermore, it is preferable that the ratio of the content of Ti and N (Ti/N) is 4.0 or more.

Al: 0.010-0.040%
Al has a deoxidizing effect. In addition, by dissolving Al in the mother phase, the sol. It has the effect of improving uniform elongation as Al (acid-soluble Al). Therefore, the Al content is 0.010% or more, preferably 0.015% or more. However, when the Al content becomes excessive, sol. The amount of Al also increases and the toughness in the HAZ decreases. Moreover, the difference in the aspect ratio of the bainite crystal grains between the surface layer portion and the sheet thickness center portion also increases, and the uniform elongation may decrease. Therefore, the Al content is 0.040% or less, preferably 0.030% or less.

N: 0.001 to 0.005%
N has the effect of refining the austenite grains in combination with Ti, thereby greatly improving the low-temperature toughness. Since the above effect cannot be obtained if the N content is less than 0.001%, the N content is made 0.001% or more. The N content is preferably 0.002% or more. However, if the N content exceeds 0.005%, the amount of solute N dissolved in the matrix increases and the low temperature toughness decreases. Moreover, the difference in the aspect ratio of the bainite crystal grains between the surface layer portion and the sheet thickness center portion also increases, and the uniform elongation may decrease. Therefore, the N content is 0.005% or less, preferably 0.004% or less.

The steel plate for line pipe according to the present invention may contain one or more elements selected from Cu, Ni, Cr, Mo and V in addition to the above elements.

Cu: 0-0.50%
Cu has the effect of improving the strength of the steel sheet, so it may be contained as necessary. However, when the Cu content is excessive, the surface properties and toughness of the steel sheet are significantly degraded. Therefore, the Cu content is 0.50% or less, preferably 0.30% or less. On the other hand, in order to obtain the above effects, the Cu content is preferably 0.10% or more, more preferably 0.15% or more.

Ni: 0-0.50%
Ni has the effect of improving the strength and toughness of the steel sheet. Therefore, it may be contained as necessary. However, when the Ni content exceeds 0.50%, the effect corresponding to the cost increase cannot be obtained. Therefore, the Ni content is 0.50% or less, preferably 0.30% or less. On the other hand, in order to reliably obtain the above effect, the Ni content is preferably 0.10% or more, more preferably 0.15% or more.

Cr: 0-0.50%
Cr has the effect of improving the strength of the steel sheet, so it may be contained as necessary. However, when the Cr content exceeds 0.50%, weld cracks tend to occur. Therefore, the Cr content is 0.50% or less, preferably 0.30% or less. On the other hand, in order to reliably obtain the above effects, the Cr content is preferably 0.10% or more, more preferably 0.15% or more.

Mo: 0-0.10%
Mo has the effect of improving the strength of the steel sheet, so it may be contained as necessary. However, if the Mo content is excessive, strain aging increases the yield strength and impairs the deformability. In addition, deterioration in toughness and weld cracking in the HAZ tend to occur. Therefore, the Mo content is 0.10% or less, preferably 0.08% or less. On the other hand, in order to reliably obtain the above effect, the Mo content is preferably 0.05% or more, more preferably 0.06% or more.

V: 0-0.50%
Since V improves the strength of the steel sheet, it may be contained as necessary. However, excessive V content reduces ductility and toughness. Therefore, the V content is 0.50% or less, preferably 0.40% or less. On the other hand, in order to remarkably obtain the above effects, the V content is preferably 0.10% or more, more preferably 0.15% or more.

The steel sheet for line pipe according to the present invention may contain B in addition to the above elements.

B: 0-0.01%
B is an effective element for improving the strength of the steel sheet, so it may be contained as necessary. However, excessive B content may reduce ductility and toughness. Therefore, the B content is 0.01% or less, preferably 0.007% or less. On the other hand, in order to remarkably obtain the above effect, the B content is preferably 0.0005% or more, more preferably 0.0008% or more.

The steel sheet for line pipe according to the present invention may contain one or more elements selected from Ca, REM, and Mg in addition to the above elements.

Ca: 0-0.02%
REM: 0-0.02%
Ca and REM are elements effective in controlling the morphology of sulfides (particularly MnS) and improving low-temperature toughness, so they may be contained as necessary. However, when Ca exceeds 0.02% or when REM exceeds 0.02%, inclusions containing both elements may coarsen and cluster. As a result, the cleanliness of the steel sheet is impaired, and weldability may also be adversely affected. Therefore, the Ca content should be 0.02% or less. Also, the REM content is 0.02% or less. On the other hand, in order to remarkably obtain the above effect, the Ca content is preferably 0.002% or more. Also, the REM content is preferably 0.003% or more.

Note that REM is a generic term for a total of 17 elements including Sc, Y and lanthanoids, and one or more selected from these elements can be contained. The content of REM means the total amount of the above elements.

Mg: 0-0.02%
Mg has the effect of forming finely dispersed oxides, suppressing coarsening of grain size in the HAZ, and improving low-temperature toughness. Therefore, it may be contained as necessary. However, if the Mg content is excessive, coarse oxides may be formed and the toughness may be lowered. Therefore, the Mg content is 0.02% or less, preferably 0.01% or less. On the other hand, in order to reliably obtain the above effects, the Mg content is preferably 0.003% or more.

In the chemical composition of the steel sheet of the present invention, the balance is Fe and impurities. Here, the term "impurities" refers to components mixed in by various factors in raw materials such as ores, scraps, etc., and in the manufacturing process when steel sheets are manufactured industrially. means something

Pcm: 0.15-0.23
Pcm calculated by the following formula (i) is defined in accordance with JIS G 3136:2012 and is known as a composition susceptible to weld cracking. Pcm is set to 0.15 to 0.23. If Pcm is less than 0.15, the strength in the HAZ is greatly reduced. Therefore, Pcm is set to 0.15 or more, preferably 0.16 or more. On the other hand, when Pcm exceeds 0.23, the weldability deteriorates, and cracks are likely to occur when the steel plate is formed into a tubular shape (CUO formed) and welded. Therefore, Pcm is set to 0.23 or less, preferably 0.22 or less.

Pcm=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B (i)
However, each element symbol in the above formula (i) represents the content (% by mass) of each element contained in the steel, and is zero when not contained.

Ceq: 0.38-0.43
Ceq calculated by the following formula (ii) is defined in accordance with JIS G 3136:2012 and is known as carbon equivalent. Ceq is between 0.38 and 0.43. If the Ceq is less than 0.38, the steel may not be quenched sufficiently, resulting in insufficient strength or toughness. Therefore, Ceq is set to 0.38 or more. On the other hand, when the Ceq exceeds 0.43, excessive quenching results in a structure containing excessive bainite or martensite, and the toughness of the base metal is lowered. Therefore, Ceq is 0.43 or less, preferably 0.42 or less.

Ceq=C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/14 (ii)
However, each element symbol in the above formula (ii) represents the content (% by mass) of each element contained in the steel, and is zero when not contained.

2. Metal structure The steel plate for line pipes according to the present invention basically has a structure consisting of ferrite and bainite. phases may be included. With such a metallographic structure, the yield ratio and uniform elongation do not decrease even if aging treatment is performed when manufacturing the line pipe. Therefore, a steel sheet having high strength and excellent deformation performance can be obtained.

2-1. Metallographic structure of the surface layer of the steel sheet The metallographic structure of the surface layer of the steel sheet contains 30 to 70% ferrite, 3% or less of the hard phase, and the balance is bainite, in terms of area ratio.

2-1-1. Area Ratio and Crystal Grain Size of Ferrite In the metal structure of the surface layer of the steel sheet, the area ratio of ferrite is 30 to 70%. Ferrite in the metallographic structure of the surface layer of the steel sheet is effective in achieving a low yield ratio and high uniform elongation. Therefore, in the metallographic structure of the surface layer of the steel sheet, the area ratio of ferrite is 30% or more, preferably 40% or more. However, excessive ferrite reduces the strength of the steel sheet. Therefore, in the metallographic structure of the surface layer of the steel sheet, the area ratio of ferrite is 70% or less, preferably 60% or less.

The average grain size of ferrite in the surface layer is set to 5.0 to 15.0 μm. As described above, increasing the ferrite content lowers the strength of the steel sheet, but the reduction in strength can be suppressed by refining the ferrite structure. However, if the average crystal grain size of the ferrite in the surface layer portion exceeds 15.0 μm, the decrease in strength cannot be suppressed. Therefore, the average crystal grain size of ferrite in the surface layer is set to 15.0 μm or less. On the other hand, the average crystal grain size of the ferrite in the surface layer is set to 5.0 μm or more in view of manufacturing conditions and the like.

2-1-2. Area ratio of hard phase The metal structure of the surface layer of the steel sheet basically has a metal structure composed of ferrite and bainite, but in addition to the above, it may contain a hard phase with an area ratio of 3% or less. . The hard phase referred to here is a structure such as pearlite, cementite, and MA (island martensite). However, if the amount of the hard phase is excessive, it becomes difficult to achieve both strength and deformation performance. Therefore, in the metallographic structure of the surface layer of the steel sheet, the area ratio of the hard phase is set to 3% or less. The area ratio of the hard phase is preferably as low as possible. The area ratio of the hard phase is preferably less than 1%, more preferably 0%.

2-1-3. Bainite Area Ratio and Aspect Ratio In the metallographic structure of the surface layer of the steel sheet, the remainder other than the ferrite and the hard phase is bainite. Increasing the amount of ferrite reduces the strength of the steel sheet. Therefore, a certain amount of bainite is contained in order to ensure strength. Specifically, bainite has an area ratio of about 30 to 70%. In the metallographic structure of the surface layer portion of the steel sheet, the area ratio of bainite is preferably 30% or more, more preferably 40% or more. However, excess bainite may reduce toughness. Therefore, the bainite content in the metallographic structure of the surface layer of the steel sheet is preferably 70% or less, more preferably 60% or less. In the present invention, bainite includes so-called "bainitic ferrite" and "acicular ferrite".

In the steel sheet according to the present invention, the aspect ratio of bainite grains in the metal structure is also important. The aspect ratio is a value obtained by dividing the major axis of a crystal grain by the minor axis, and a specific method for measuring the aspect ratio will be described later. In the steel sheet according to the present invention, the aspect ratio of the bainite crystal grains in the surface layer portion is controlled within the range described below in order to obtain the desired properties.

The aspect ratio of the bainite crystal grains in the surface layer is set to 1.0 to 5.0. The aspect ratio of the bainite crystal grains in the surface layer is usually 1.0 or more. On the other hand, when the aspect ratio of the bainite crystal grains in the surface layer portion exceeds 5.0, the uniform elongation decreases. Therefore, the aspect ratio of the bainite crystal grains in the surface layer is set to 5.0 or less, preferably 4.5 or less.

2-2. Metallographic structure at the thickness center of the steel sheet The metallographic structure at the thickness center of the steel sheet contains 30 to 70% ferrite, 3% or less of the hard phase, and the balance bainite, in terms of area ratio.

2-2-1. Area Ratio and Crystal Grain Size of Ferrite In the metal structure at the center of the plate thickness of the steel sheet, the area ratio of ferrite is 30 to 70%. Ferrite is also effective in achieving a low yield ratio and high uniform elongation in the metallographic structure at the center of the sheet thickness. Therefore, in the metallographic structure at the central portion of the plate thickness, the area ratio of ferrite is 30% or more, preferably 40% or more. However, an excessive amount of ferrite reduces the strength of the steel sheet. Therefore, in the metal structure at the center of the plate thickness, the area ratio of ferrite is 70% or less, preferably 60% or less.

Also, the average crystal grain size of the ferrite at the central portion of the sheet thickness is set to 5.0 to 15.0 μm. As described above, increasing the ferrite content lowers the strength of the steel sheet, but the reduction in strength can be suppressed by refining the ferrite structure. However, if the average grain size of ferrite at the thickness center exceeds 15.0 μm, the reduction in strength cannot be suppressed. For this reason, the average grain size of ferrite at the center of the plate thickness is set to 15.0 μm or less. On the other hand, in consideration of manufacturing conditions, etc., the average grain size of ferrite at the center of the sheet thickness is set to 5.0 μm or more.

2-2-2. Area ratio of hard phase The steel sheet according to the present invention basically has a metal structure composed of ferrite and bainite as the metal structure at the center of the plate thickness, but in addition to the above, the area ratio is 3% or less. may contain a hard phase of The hard phase is as described above. However, if the amount of the hard phase is excessive, it becomes difficult to achieve both strength and deformation performance. For this reason, the area ratio of the hard phase in the metal structure at the center of the sheet thickness is set to 3% or less. The area ratio of the hard phase is preferably as low as possible. The area ratio of the hard phase is preferably less than 1%, more preferably 0%.

2-2-3. Area ratio and aspect ratio of bainite In the metallographic structure at the center of the plate thickness, the rest other than ferrite and hard phase is bainite. Increasing the amount of ferrite reduces the strength of the steel sheet. Therefore, a certain amount of bainite is contained in order to ensure strength. Specifically, bainite has an area ratio of about 30 to 70%. In the metal structure at the center of the plate thickness, the area ratio of bainite is preferably 30% or more, more preferably 40% or more.

However, excess bainite may reduce toughness. Therefore, the bainite content is preferably 70% or less, more preferably 60% or less, in the metallographic structure at the central portion of the sheet thickness. As described above, bainite includes so-called “bainitic ferrite” and “acicular ferrite”.

In the steel sheet according to the present invention, the aspect ratio of bainite grains in the metal structure is also important. In the steel sheet according to the present invention, the aspect ratio of the bainite crystal grains at the center of the sheet thickness is controlled within the range described below in order to obtain the desired properties.

The aspect ratio of the bainite crystal grains at the center of the plate thickness is set to 1.0 to 5.0. The aspect ratio of the bainite crystal grains at the center of the plate thickness is usually 1.0 or more. On the other hand, when the aspect ratio of the bainite crystal grains at the thickness center exceeds 5.0, the uniform elongation decreases. Therefore, the aspect ratio of the bainite crystal grains at the central portion of the plate thickness is set to 5.0 or less, preferably 4.0 or less.

Here, the previous 2-1. (Metal structure of steel sheet surface layer), and 2-2. Observation of (the metal structure at the thickness center of the steel sheet) can be performed by the following method.

The metal structure of the surface layer is a structure at a depth of 1.0 mm in the thickness direction from the surface after corroding the plate thickness cross section parallel to the rolling direction of the steel plate with nital, and 500 times the magnification of the optical microscope. Visual field and tissue observation are performed, and image analysis is also performed at the same time. Similarly, at the position of the plate thickness center, 5 fields of view are observed for the structure, and image analysis is also performed at the same time. General-purpose application software (Nippon Steel & Sumikin Technology Co., Ltd., trade name: particle analysis) was used for image analysis. Based on these results, the area ratio of ferrite, the average grain size of ferrite, the area ratio of bainite, and the aspect ratio of bainite crystal grains are calculated.

In addition, the crystal grain size is based on JIS G 0551:2013, the crystal grain size of ferrite is obtained by a cutting method, and the average value is taken as the average crystal grain size.

In addition, since bainite crystal grains are crystal grains having a major axis in the rolling direction, the aspect ratio of the bainite crystal grains is determined by taking the maximum length of the crystal grains in the rolling direction as the major diameter and the center of the grains elongated in the rolling direction. The aspect ratio (average value) was calculated by dividing the major axis by the minor axis, with the length of the crystal grain in the plate thickness direction in the part being the minor axis.

2-3. Difference in ferrite area ratio When the difference between the ferrite content in the surface layer portion and the ferrite content in the plate thickness center portion is large, the difference in yield strength and uniform elongation in the steel plate increases. As a result, local strain reduces the uniform elongation across the entire cross section of the steel sheet. Therefore, the difference between the ferrite area ratio in the surface layer portion of the steel sheet and the ferrite area ratio in the thickness center portion (hereinafter simply referred to as "difference in ferrite area ratio") is set to 0 to 20%.

In addition, in the steel sheet according to the present invention, the "difference in ferrite area ratio" is the absolute value of the value obtained by subtracting the ferrite area ratio at the center of the plate thickness from the ferrite area ratio at the surface layer of the steel plate. Further, the smaller the difference in the ferrite area ratio, the better, preferably 15% or less.

2-4. Difference in Bainite Area Ratio When there is a large difference between the amount of bainite in the surface layer and the amount of bainite in the center of the sheet thickness, the difference in yield strength and uniform elongation in the steel sheet also increases. As a result, local strain reduces the uniform elongation across the entire cross section of the steel sheet. Therefore, the difference between the bainite area ratio in the surface layer portion of the steel sheet and the bainite area ratio in the thickness center portion (hereinafter simply referred to as "bainite area ratio difference") is set to 0 to 20%.

In the steel sheet according to the present invention, the "difference in bainite area ratio" is the absolute value of the value obtained by subtracting the bainite area ratio in the central portion of the plate thickness from the bainite area ratio in the steel plate surface layer portion. Moreover, the smaller the difference in bainite area ratio, the better, and it is preferably 15% or less.

2-5. Difference in aspect ratio of bainite grains In the steel sheet according to the present invention, the difference between the aspect ratio of the bainite grains in the surface layer portion and the aspect ratio of the bainite grains in the central portion of the plate thickness (hereinafter, simply referred to as the “difference in aspect ratio”) ) is large, the difference in uniform elongation between the surface layer portion and the inside of the steel plate (plate thickness center portion) increases. As a result, local strain reduces the uniform elongation over the entire cross section of the steel sheet. Therefore, the difference between the aspect ratio of the bainite crystal grains in the surface layer portion and the aspect ratio of the bainite crystal grains in the central portion of the sheet thickness is set to 2.0 or less, preferably 1.5 or less.

The smaller the difference in aspect ratio, the better. In the present invention, the "difference in aspect ratio" is a value obtained by subtracting the aspect ratio of the bainite crystal grains in the central portion of the sheet thickness from the aspect ratio of the bainite crystal grains in the surface layer portion of the steel sheet.

2-6. Properties The tensile properties of the steel sheet according to the present invention are as follows. The strength is the tensile strength and yield strength corresponding to X52-65 grades in the American Petroleum Institute standard API 5L (hereinafter simply referred to as "API 5L"). Specifically, the tensile strength (hereinafter also referred to as “TS”) is 460-760 MPa, and the yield strength (hereinafter also referred to as “YS”) is 360-600 MPa. In addition, the yield ratio (hereinafter also referred to as "YR") is 85% or less, the uniform elongation (hereinafter also referred to as "U.El") is 9.0% or more, and the TS (MPa) and U.S. The product (TS×U.El) with El (%) shall be 4000 (MPa·%) or more.

The YR is expressed as a percentage obtained by dividing the yield strength by the tensile strength. Also, TS×U. El is an index for evaluating the balance between strength and uniform elongation. TS x U.S.A. The larger El is, the better the balance between strength and uniform elongation. These evaluation indices can be calculated by performing a tensile test at room temperature using a tensile test piece based on API 5L.

2-7. Thickness The steel sheet according to the present invention has a thickness in the range of 15 to 40 mm.

2-8. Manufacturing method The steel sheet according to the present invention can obtain the effect as long as it has the above-described structure regardless of the manufacturing method. For example, the steel sheet according to the present invention is stabilized by the following manufacturing method. can be obtained. Specifically, it is preferable to first manufacture a steel slab having the chemical composition described above by, for example, a conventional continuous casting method or the like, and then perform the following steps.

2-8-1. Steel Billet Heating Step It is preferable to heat the manufactured steel billet in a temperature range of 1100 to 1250° C. for soaking. The steel slab contains Nb, and by heating the steel slab to solid-solve Nb in the matrix, the effect of Nb can be reliably obtained. Also, heating the billet facilitates hot rolling. For this reason, it is preferable to set the heating temperature of the steel slab to 1100° C. or higher. On the other hand, if the heating temperature of the billet is too high, unnecessary energy costs will occur. For this reason, the heating temperature of the billet is preferably 1250° C. or lower, more preferably 1200° C. or lower.

2-8-2. Rolling Step Subsequently, the soaked steel slab is preferably hot-rolled to a predetermined thickness. The rolling pass schedule is not particularly limited, but the finish rolling temperature is a condition that greatly affects the properties of the steel sheet. For this reason, finish rolling is preferably carried out in the temperature range of 750 to 850°C. In the following description, the surface temperature of the billet during finish rolling is referred to as the finish rolling temperature.

If the finish rolling temperature is less than 750°C, it becomes difficult to ensure strength. Therefore, the finish rolling temperature is preferably 750° C. or higher. On the other hand, if the finish rolling temperature exceeds 850° C., the ferrite crystal grains become coarse, making it difficult to ensure the strength (TS and YS). Therefore, the finish rolling temperature is preferably 850° C. or lower.

2-8-3. First Stage Cooling Step Subsequently, it is preferable to start water cooling the hot-rolled steel sheet from a temperature range of 750 to 850°C. Further, water cooling is preferably performed at a cooling rate of 15 to 30° C./s. Since water cooling is started from the same temperature range as the finish rolling temperature, water cooling should be started immediately after hot rolling.

If the temperature at which water cooling is started (hereinafter referred to as "water cooling start temperature") is less than 750°C, coarse ferrite is formed before water cooling is started, and low-temperature toughness is lowered. Therefore, it is preferable to set the water cooling start temperature to 750° C. or higher. On the other hand, if the water cooling start temperature is higher than 850° C., the austenite crystal grains before the start of water cooling are not sufficiently refined, the metal structure after that is coarsened, and the low temperature toughness is lowered. Therefore, it is preferable to set the water cooling start temperature to 850° C. or lower.

In addition, when the cooling rate is less than 15 ° C./s, the average grain size of ferrite in the center of the plate thickness becomes coarse, and the ferrite area ratio of the surface layer and the center of the plate thickness increases, resulting in strength decreases. Moreover, the difference in the aspect ratio of the bainite crystal grains between the surface layer portion and the sheet thickness center portion also increases. Therefore, the cooling rate is preferably 15° C./s or more. On the other hand, if the cooling rate is more than 30° C./s, the area ratio of ferrite decreases, and the difference in the area ratio of ferrite and the difference in the area ratio of bainite increase, so the difference in the aspect ratio of bainite crystal grains also increases. U.S.A. El may decrease. Therefore, the cooling rate is preferably 30° C./s or less.

Subsequently, it is preferable to stop water cooling in the temperature range of 600 to 700°C. If the temperature at which water cooling is stopped (hereinafter referred to as “water cooling stop temperature”) is less than 600°C, even if the steel plate enters the bainite region and recovers heat during air cooling, it is not sufficiently reheated to the austenite region. Ferrite is not generated sufficiently during cooling with air cooling of , and the low temperature toughness and U.S. El decreases. Although air cooling depends on the size of the steel sheet, temperature, etc., as a guideline, it should be carried out with a maximum of about 300 seconds. Therefore, it is preferable to set the water cooling stop temperature to 600° C. or higher. On the other hand, when the water cooling stop temperature exceeds 700°C, the ferrite grains become coarse. Therefore, it is preferable to set the water cooling stop temperature to 700° C. or lower.

After the cooling step, air cooling is preferably performed in a temperature range above 600° C. for 60 seconds or longer. If the air cooling is less than 60 seconds, the ferrite area ratio, particularly the ferrite area ratio of the surface layer portion, becomes low. Moreover, the difference in the ferrite area ratio and the difference in the bainite area ratio increase, and the difference in the aspect ratio of the bainite crystal grains increases. As a result, the U.S. El decreases. As a method of air cooling, specifically, the steel plate moving in the water cooling device may be taken out of the water cooling device and left to stand for air cooling (radiation cooling). At this time, since there is a temperature difference between the surface and the inside of the steel sheet, the heat inside the steel sheet is released to the surface of the steel sheet, and the surface temperature rises. Therefore, the surface temperature of the steel sheet does not fall below 600°C during air cooling. It should be noted that the first stage cooling process is from the start of water cooling to the end of air cooling.

2-8-4. Second stage cooling process When the surface temperature of the steel plate reaches 600 to 700°C, water cooling is performed again. This water cooling is preferably carried out in a water cooling apparatus, starting water cooling from a temperature range of 600 to 700° C., and performing water cooling at a cooling rate of 20° C./s or more and less than 35° C./s.

If the water-cooling start temperature in the second water-cooling (hereinafter referred to as "secondary water-cooling start temperature") is less than 600°C, the bainite will not harden sufficiently and the required strength will not be obtained. Therefore, it is preferable to set the secondary water cooling start temperature to 600° C. or higher. On the other hand, when the secondary water cooling start temperature exceeds 700°C, ferrite is not sufficiently formed. Therefore, it is preferable to set the secondary water cooling start temperature to 700° C. or lower.

If the cooling rate in the second water cooling (hereinafter referred to as "secondary cooling rate") is less than 20°C/s, the bainite will not harden sufficiently. Therefore, the secondary cooling rate is preferably 20° C./s or higher. On the other hand, when the secondary cooling rate is 35°C/s or more, low temperature toughness and U.S. El decreases. Therefore, the secondary cooling rate is preferably less than 35°C/s.

At this time, the second water cooling is stopped in the temperature range of 400 to 500°C. When the temperature at which the second water cooling is stopped (hereinafter referred to as “secondary water cooling stop temperature”) is less than 400° C., the aspect ratio of the bainite crystal grains in the surface layer portion increases, and the bainite crystal grains The difference in aspect ratio also increases, and the U.S.A. El decreases. Therefore, it is preferable to set the secondary water cooling stop temperature to 400° C. or higher. On the other hand, if the secondary water cooling stop temperature exceeds 500°C, the strength cannot be ensured. Therefore, it is preferable to set the secondary water cooling stop temperature to 500° C. or lower. After the water cooling is stopped, air cooling (allowing to cool) or slow cooling to room temperature is sufficient. The second stage cooling process is from the start of water cooling again to the end of air cooling (air cooling).

Here, each of the above-mentioned temperatures refers to the average temperature at the surface of the billet and steel plate, that is, the surface temperature, and the “cooling rate” is the temperature difference at the surface of the steel plate at the start and stop of cooling. Refers to a value divided by time.

A line pipe can be manufactured by forming a steel plate according to the present invention manufactured under the above manufacturing conditions into a tubular shape, joining the butt portions, and, if necessary, expanding the pipe and applying a coating for corrosion prevention.

EXAMPLES The present invention will be described in more detail with reference to examples below, but the present invention is not limited to these examples.

A steel slab having a chemical composition shown in Table 1 and a thickness of 140 mm was subjected to heating, rolling, cooling, etc. under the conditions shown in Table 2, and then air-cooled to room temperature to obtain a steel plate. Each temperature shown in Table 2 is the surface temperature of the object to be rolled, that is, the steel billet or steel plate measured using a radiation thermometer.

The structure of each steel sheet obtained was observed, and the tensile properties were investigated by a tensile test.

(Observation of structure and measurement of aspect ratio)
In the structure observation, the area ratio of ferrite and bainite is obtained by expressing the metal structure of the plate thickness cross section (L cross section) parallel to the rolling direction with nital, and using an optical microscope at 500 times for the surface layer and the plate thickness center. Five fields of view were observed, image analysis was performed, and the area ratio of each phase and the average crystal grain size of ferrite were calculated. General-purpose application software (Nippon Steel & Sumikin Technology Co., Ltd., trade name: particle analysis) was used for image analysis. The metallographic structure of the surface layer was observed as a structure at a depth of 1.0 mm from the surface in the plate thickness direction. In addition, the crystal grain size is based on JIS G 0551:2013, the crystal grain size of ferrite is obtained by a cutting method, and the average value is taken as the average crystal grain size.

For the aspect ratio of bainite grains, the metallographic structure of the plate thickness cross section (L cross section) parallel to the rolling direction is expressed with nital, and the surface layer and the plate thickness center are observed with an optical microscope at a magnification of 500 and five fields of view. and calculated. At this time, in the measurement, by image analysis, the maximum length of the grain in the rolling direction is obtained as the major diameter, and the length of the grain in the plate thickness direction at the center of the grain elongated in the rolling direction is taken as the minor diameter. was divided by the minor axis to calculate the aspect ratio of each bainite, and the average value was taken as the aspect ratio. The metallographic structure of the surface layer was observed as a structure at a depth of 1.0 mm from the surface in the plate thickness direction.

(Tensile test)
In the tensile test, a round bar tensile test piece with a width of 38.1 mm and a gauge length of 50.8 mm was taken in the direction (C direction) perpendicular to the rolling direction from the center of the plate thickness of the steel plate, and API 5L (45th) was performed at room temperature according to 0.5% yield strength (yield strength), tensile strength, uniform elongation, and YR (0.5% yield strength/tensile strength) were obtained from the results of the tensile test.

The results are summarized in Table 3 below.

As shown in Table 3, steel grade No. 1 satisfies the chemical composition specified in the present invention and is manufactured under the preferred manufacturing conditions specified in the present invention. 1 to 18 satisfied the metal structure specified in the present invention, and these had X52 to 65 grade tensile strength and yield strength (tensile strength 460 to 760 MPa, yield strength 360 to 600 MPa). . In addition, these steel grades have a YR of 85% or less and U.S. El was also a good value of 11.0% or more, and had excellent deformation performance.

On the other hand, steel grade No. 1 that does not satisfy the chemical composition and metal structure specified in the present invention. Nos. 19-29 did not meet the target values for either strength or deformation performance.

In the following, the steel type No. 19 to 29 will be specifically described.
Steel grade No. 1 has a high first-stage cooling rate and deviates from the preferred manufacturing conditions of the present invention. In No. 19, the ferrite area ratio in the surface layer portion decreased, and the difference in the ferrite area ratio and the difference in the bainite area ratio increased. Moreover, the value of the aspect ratio of the bainite crystal grains in the surface layer portion increased, and the difference in the aspect ratio of the bainite crystal grains also increased. As a result, the U.S. El became a low value.

No. 2 in the surface layer portion, which has a low secondary water cooling stop temperature and is outside the preferred manufacturing conditions of the present invention. In No. 20, the aspect ratio of the bainite crystal grains was large, and the difference in the aspect ratio of the bainite crystal grains was also large. As a result, the U.S. El decreased. No. 6, which has a short air-cooling time and deviates from the preferred production conditions of the present invention. In No. 21, in particular, the ferrite area ratio in the surface layer portion was low, the difference in the ferrite area ratio and the bainite area ratio was large, and the difference in the aspect ratio of the bainite crystal grains was also large. For this reason, the U.S. El became a low value.

No. 1, which has a low water cooling stop temperature and is outside the preferred manufacturing conditions of the present invention. In No. 22, the ferrite area ratio was high and sufficient TS could not be obtained. No. 1, which has a slow first-stage cooling rate and deviates from the preferred manufacturing conditions of the present invention. In No. 23, the ferrite area ratio in the surface layer portion and the plate thickness center portion and the average crystal grain size of ferrite in the plate thickness center portion were particularly high. Moreover, the difference in the aspect ratio of the bainite crystal grains between the surface layer portion and the sheet thickness center portion was large, and the TS was lowered.

No. 1, which has a high finish rolling temperature and is out of the preferred manufacturing conditions of the present invention. In No. 24, the average crystal grain size of ferrite at the center of the plate thickness was particularly large, resulting in low TS and YS. Steel type no. In No. 25, the C content was outside the range defined by the present invention, so the difference in the aspect ratio of the bainite crystal grains in the surface layer portion was also large. As a result, the TS is high and the U.S. El became a low value.

Steel type no. In No. 26, the Si content was outside the range specified by the present invention, so the aspect ratio of the bainite crystal grains in the surface layer portion was large, and the difference in the aspect ratio of the bainite crystal grains in the surface layer portion and the center portion of the plate thickness was also large. . Therefore, YS is high and U.S. El became a low value. Steel type no. In No. 27, the Mn content was outside the range defined by the present invention, so the aspect ratio of the bainite crystal grains in the surface layer portion was large, and the difference in the aspect ratio of the bainite crystal grains in the surface layer portion and the center portion of the plate thickness was also large. . As a result, YS and YR are high, and U.S. El became a low value.

No. In No. 28, the Al content was outside the range defined by the present invention. El became a low value. No. In No. 29, the N content was outside the range defined by the present invention, so the difference in the aspect ratio of the bainite crystal grains between the surface layer portion and the plate thickness center portion was large. El became a low value.

Claims (4)

The chemical composition of the steel sheet, in mass%,
C: 0.06 to 0.12%,
Si: 0.10 to 0.50%,
Mn: 1.0-1.8%,
P: 0.02% or less,
S: 0.001% or less,
Nb: 0.005 to 0.050%,
Ti: 0.005 to 0.030%,
Al: 0.010 to 0.040%,
N: 0.001 to 0.005%,
Cu: 0-0.50%,
Ni: 0 to 0.50%,
Cr: 0 to 0.50%,
Mo: 0-0.10%,
V: 0 to 0.50%,
B: 0 to 0.01%,
Ca: 0-0.02%,
REM: 0-0.02%,
Mg: 0-0.02%, and the balance: Fe and impurities,
Pcm represented by the following formula (i) is 0.15 to 0.23,
Ceq represented by the following formula (ii) is 0.38 to 0.43,
The metal structure in the surface layer part and the plate thickness center part of the steel sheet is, in terms of area ratio,
30-70% ferrite and 3% or less hard phase,
the balance is bainite,
and an average crystal grain size of ferrite in the surface layer portion and the plate thickness center portion is 5.0 to 15.0 μm,
The aspect ratio of the bainite crystal grains in the surface layer portion and the plate thickness center portion is 1.0 to 5.0,
The difference between the ferrite area ratio in the surface layer portion and the ferrite area ratio in the plate thickness center portion is 0 to 20%,
The difference between the bainite area ratio in the surface layer portion and the bainite area ratio in the plate thickness center portion is 0 to 20%,
the difference between the aspect ratio of the bainite crystal grains in the surface layer portion and the aspect ratio of the bainite crystal grains in the central portion of the plate thickness is 2.0 or less;
The tensile strength is 460 to 760 MPa, the yield strength is 360 to 600 MPa, the yield ratio is 85% or less, the uniform elongation is 9.0% or more, and the product of the tensile strength and the uniform elongation is 4000 (MPa · %) or more,
A steel plate for line pipes having a plate thickness of 15 to 40 mm.
Pcm=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B (i)
Ceq=C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/14 (ii)
However, each element symbol in the above formulas (i) and (ii) represents the content (% by mass) of each element contained in the steel, and is zero when not contained. The chemical composition, in mass %,
Cu: 0.10-0.50%,
Ni: 0.10 to 0.50%,
Cr: 0.10 to 0.50%,
Mo: 0.05-0.10%, and V: 0.10-0.50%,
The steel plate for line pipe according to claim 1, containing one or more selected from: The chemical composition, in mass %,
B: 0.0005 to 0.01%,
The steel plate for line pipe according to claim 1 or 2, containing The chemical composition, in mass %,
Ca: 0.002-0.02%,
REM: 0.003-0.02%, and Mg: 0.003-0.02%,
The steel plate for line pipe according to any one of claims 1 to 3, containing one or more selected from.