JP7335492B2 - Steel plates and steel pipes for line pipes - Google Patents

Description

TECHNICAL FIELD The present invention relates to a steel plate for line pipes and a steel pipe for line pipes using the same.

BACKGROUND ART In recent years, the mining conditions for oil and gas wells such as crude oil and natural gas (hereinafter collectively referred to as "oil wells") have become severe. The mining environment of an oil well contains CO ₂ , H ₂ S, Cl ^{2 -} , etc. in its atmosphere as the mining depth increases, and the crude oil and natural gas to be mined also contain a large amount of H ₂ S. become.

Therefore, the requirements for the performance of line pipes for transporting these are becoming stricter, and high resistance to sulfide stress cracking (hereinafter also referred to as "SSC resistance") and hydrogen-induced cracking resistance (hereinafter referred to as "HIC resistance") The demand for steel for line pipes having "strength") is increasing.

Steel used in an H ₂ S environment needs to have a low maximum hardness from the viewpoint of improving SSC resistance. Therefore, in steels that require resistance to sulfides, the improvement of techniques for suppressing hardness has become an important issue.

For example, Patent Literature 1 discloses a method for producing high-strength steel with excellent SSC resistance and a tensile strength of 60 kgf/mm ² class. Further, Patent Document 2 discloses a thick steel plate having a tensile strength of 570 to 720 N/mm ² and a small difference in hardness between the welded heat affected zone and the base material, and a method for manufacturing the same. Furthermore, Patent Document 3 discloses a method for manufacturing a high-strength steel sheet for sour line pipes of X60 class or higher, which can reduce surface hardness while preventing a decrease in strength and deterioration of DWTT properties. It is

JP-A-2-8322 Japanese Patent Application Laid-Open No. 2001-73071 Japanese Patent Application Laid-Open No. 2002-327212

Edited by the Iron and Steel Institute of Japan Basic Research Group Bainite Research Group, "Bainite Photographs of Steel 1", The Iron and Steel Institute of Japan, June 1992

According to Patent Documents 1 to 3, it is possible to reduce the hardness of the steel sheet surface by performing tempering after quenching. However, in these documents, a Vickers hardness test is performed with a test force of 98 N (10 kgf) in evaluating hardness. A high test force results in a large measurement area. That is, the average hardness of the metal structure included in a wide area is measured.

However, if there is a tissue with high hardness, even locally, there is a risk that SSC will occur starting there. Therefore, it is necessary to conduct a Vickers hardness test with a lower test force and control the maximum hardness based on the measurement results to be low.

An object of the present invention is to solve the above problems and to provide a steel plate and a steel pipe for line pipes having excellent SSC resistance.

The present invention has been made to solve the above problems, and the gist thereof is the following steel plate and steel pipe for line pipe.

(1) chemical composition, in mass %,
C: 0.02 to 0.08%,
Si: 0.01 to 0.50%,
Mn: 0.5-1.7%,
Nb: 0.001 to 0.100%,
N: 0.0010 to 0.0060%,
Ca: 0.0001 to 0.0050%,
P: 0.03% or less,
S: 0.0025% or less,
Ti: 0.005 to 0.030%,
Al: 0.01-0.04%,
O: 0.004% or less,
Mo: 0-2.0%,
Cr: 0 to 2.0%,
Cu: 0-2.0%,
Ni: 0 to 2.0%,
W: 0 to 1.0%,
V: 0 to 0.20%,
Zr: 0 to 0.050%,
Ta: 0 to 0.050%,
B: 0 to 0.0020%,
REM: 0-0.01%,
Mg: 0-0.01%,
Hf: 0-0.005%,
Re: 0 to 0.005%,
balance: Fe and impurities,
satisfying the following formula (i),
Ceq represented by the following formula (ii) is 0.30 to 0.50,
The metal structure at the center of the sheet thickness contains one or two selected from acicular ferrite and bainite, and the remainder is one or more selected from ferrite, MA phase, and pseudo-pearlite,
The metal structure in the surface layer contains one or two selected from acicular ferrite and bainite, and the balance is tempered martensite containing cementite having a grain size of 100 nm or more, MA phase, and pseudo pearlite 1 A steel plate for line pipes, having a surface layer hardness of 250HV or higher and a maximum hardness of 0.1 or less in the surface layer.
0.05≦Mo+Cr+Cu+Ni≦2.0 (i)
Ceq=C+Mn/6+(Ni+Cu)/15+(Cr+Mo+V)/5 (ii)
However, each element symbol in the formula represents the content (% by mass) of each element contained in the steel, and is zero when not contained.

(2) the chemical composition, in mass %,
W: 0.01 to 1.0%,
V: 0.01 to 0.20%,
Zr: 0.0001 to 0.050%,
Ta: 0.0001 to 0.050%, and
B: 0.0001 to 0.0020%,
containing one or more selected from
The steel plate for line pipes according to (1) above.

(3) the chemical composition, in mass %,
REM: 0.0001 to 0.01%,
Mg: 0.0001-0.01%,
Hf: 0.0001 to 0.005%, and Re: 0.0001 to 0.005%,
containing one or more selected from
The steel plate for line pipes according to (1) or (2) above.

(4) Using the steel plate according to any one of (1) to (3) above,
Steel pipe for line pipes.

In the present invention, "surface layer" means a region from the surface of a steel plate or steel pipe to a depth of 1.0 mm. Also, "HV0.1" means a "hardness symbol" when a Vickers hardness test is performed with a test force of 0.98 N (0.1 kgf) (see JIS Z 2244:2009).

According to the present invention, since the maximum hardness of the surface layer can be suppressed to 250HV0.1 or less, it is possible to obtain a steel plate and a steel pipe having excellent SSC resistance. Therefore, the steel plate and steel pipe according to the present invention can be suitably used as line pipes for transporting crude oil and natural gas containing a large amount of _H2S .

Each requirement of the present invention will be described in detail below.

1. Chemical composition The reasons for limiting each element in the steel plate and steel pipe according to the present invention are as follows. In addition, "%" about content in the following description means "mass %."

C: 0.02-0.08%
C is an element that improves the strength of steel. If the C content is less than 0.02%, a sufficient strength improvement effect cannot be obtained. On the other hand, when the C content exceeds 0.08%, the hardness of the surface layer increases and SSC is likely to occur. Therefore, the C content should be 0.02 to 0.08%. The C content is preferably 0.03% or more. In addition, in order to ensure SSC resistance and suppress deterioration of weldability and toughness, the C content is preferably 0.06% or less.

Si: 0.01-0.50%
Si is an element added for deoxidation. If the Si content is less than 0.01%, a sufficient deoxidizing effect cannot be obtained, and the manufacturing cost increases significantly. On the other hand, when the Si content exceeds 0.50%, the toughness of the weld zone is lowered. Therefore, the Si content should be 0.01 to 0.50%. The Si content is preferably 0.05% or more, more preferably 0.10% or more. Also, the Si content is preferably 0.40% or less, more preferably 0.30% or less.

Mn: 0.5-1.7%
Mn is an element that improves strength and toughness. If the Mn content is less than 0.5%, the effect of addition cannot be sufficiently obtained. On the other hand, when the Mn content exceeds 1.7%, the HIC resistance deteriorates. Therefore, the Mn content should be 0.5 to 1.7%. The Mn content is preferably 1.0% or more, more preferably 1.2% or more. Also, the Mn content is preferably 1.6% or less, more preferably 1.5% or less.

Nb: 0.001-0.100%
Nb is an element that forms carbides and nitrides and contributes to strength improvement. Further, since the addition of Nb expands the non-recrystallization temperature range to a high temperature range, if the Nb content is less than 0.001%, the effect of addition cannot be sufficiently obtained. On the other hand, when the Nb content exceeds 0.100%, coarse carbides and nitrides are formed, resulting in deterioration of HIC resistance and toughness. Therefore, the Nb content should be 0.001 to 0.100%. The Nb content is preferably 0.005% or more, more preferably 0.010% or more. Also, the Nb content is preferably 0.080% or less, more preferably 0.060% or less.

N: 0.0010 to 0.0060%
N is an element that forms a nitride with Nb and contributes to refinement of austenite grain size during heating. If the N content is less than 0.0010%, the effect of addition cannot be sufficiently obtained. On the other hand, when the N content exceeds 0.0060%, coarse carbonitrides are formed, resulting in deterioration of HIC resistance and toughness. Therefore, the N content should be 0.0010 to 0.0060%. The N content is preferably 0.0020% or more. Also, the N content is preferably 0.0050% or less.

Ca: 0.0001-0.0050%
Ca is an element that forms CaS, suppresses the formation of MnS extending in the rolling direction, and contributes to the improvement of HIC resistance. If the Ca content is less than 0.0001%, the effect of addition cannot be sufficiently obtained. On the other hand, when the Ca content exceeds 0.0050%, oxides accumulate and the HIC resistance deteriorates. Therefore, the Ca content should be 0.0001 to 0.0050%. The Ca content is preferably 0.0005% or more, more preferably 0.0010% or more. Also, the Ca content is preferably 0.0045% or less, more preferably 0.0040% or less.

P: 0.03% or less P is an element remaining as an unavoidable impurity. If the P content exceeds 0.03%, the SSC resistance and HIC resistance are lowered, and the toughness of the weld zone is lowered. Therefore, the P content should be 0.03% or less. The P content is preferably 0.015% or less, more preferably 0.01% or less. Note that an excessive reduction in the P content causes a significant increase in manufacturing costs, so 0.001% is the substantial lower limit.

S: 0.0025% or less S is an element that remains as an unavoidable impurity and forms MnS that stretches in the rolling direction during hot rolling, thereby impairing HIC resistance. If the S content exceeds 0.0025%, the HIC resistance is remarkably lowered. Therefore, the S content should be 0.0025% or less. The S content is preferably 0.0015% or less, more preferably 0.0010% or less. Note that an excessive reduction in the S content causes a significant increase in manufacturing costs, so 0.0001% is the substantial lower limit.

Ti: 0.005-0.030%
Ti is an element that forms nitrides and contributes to refinement of crystal grains. If the Ti content is less than 0.005%, the effect of addition cannot be sufficiently obtained. On the other hand, when the Ti content exceeds 0.030%, not only is the toughness lowered, but also coarse nitrides are formed, resulting in lowered HIC resistance. Therefore, the Ti content should be 0.005 to 0.030%. The Ti content is preferably 0.008% or more and preferably 0.015% or less.

Al: 0.01-0.04%
Al is an element added for deoxidation. If the Al content is less than 0.01%, the effect of addition cannot be sufficiently obtained. On the other hand, when the Al content exceeds 0.04%, Al oxides are accumulated to form clusters, and the HIC resistance is lowered. Therefore, the Al content should be 0.01 to 0.04%. The Al content is preferably 0.015% or more and preferably 0.035% or less.

O: 0.004% or less O is an element that inevitably remains after deoxidization, and the smaller the amount, the better. If the O content exceeds 0.004%, oxides are formed and the toughness and HIC resistance are lowered. Therefore, the O content should be 0.004% or less. The O content is preferably 0.003% or less. Note that an excessive reduction in the O content causes a significant increase in manufacturing costs, so 0.001% is the substantial lower limit.

Mo: 0-2.0%
Cr: 0-2.0%
Cu: 0-2.0%
Ni: 0-2.0%
0.05≦Mo+Cr+Cu+Ni≦2.0 (i)
However, each element symbol in the formula represents the content (% by mass) of each element contained in the steel, and is zero when not contained.

Mo, Cr, Cu and Ni are elements that contribute to the improvement of hardenability. In order to adjust Ceq, which is an index of hardenability described later, the total content of these elements is set to 0.05% or more. On the other hand, if the total content exceeds 2.0%, the hardness increases and the SSC resistance decreases. Therefore, the total content of Mo, Cr, Cu and Ni should be 0.05-2.0%. The total content of these elements is preferably 0.07% or more, more preferably 0.1% or more. Also, the total content is preferably 1.0% or less, more preferably 0.9% or less.

W: 0-1.0%
Since W is an element effective in improving strength, it may be contained as necessary. However, if the W content exceeds 1.0%, the toughness may be lowered. Therefore, the W content should be 1.0% or less. The W content is preferably 0.5% or less, more preferably 0.3% or less, and more preferably 0.2% or less. In order to obtain the above effects, the W content is preferably 0.01% or more, more preferably 0.05% or more.

V: 0-0.20%
V is an element that forms carbides and nitrides and contributes to the improvement of strength, so it may be contained as necessary. However, when the V content exceeds 0.20%, the toughness decreases. Therefore, the V content should be 0.20% or less. The V content is preferably 0.10% or less, more preferably 0.08% or less. In order to obtain the above effects, the V content is preferably 0.01% or more, more preferably 0.03% or more.

Zr: 0-0.050%
Zr, like V, forms carbides and nitrides and is an element that contributes to the improvement of strength, so it may be contained as necessary. However, when the Zr content exceeds 0.050%, the toughness may be lowered. Therefore, the Zr content should be 0.050% or less. The Zr content is preferably 0.020% or less, more preferably 0.010% or less. In order to obtain the above effects, the Zr content is preferably 0.0001% or more, more preferably 0.0005% or more.

Ta: 0-0.050%
Ta, like V, forms carbides and nitrides and is an element that contributes to the improvement of strength, so it may be contained as necessary. However, if the Ta content exceeds 0.050%, the toughness may be lowered. Therefore, the Ta content should be 0.050% or less. The Ta content is preferably 0.020% or less, more preferably 0.010% or less. In order to obtain the above effects, the Ta content is preferably 0.0001% or more, more preferably 0.0005% or more.

B: 0 to 0.0020%
B is an element that segregates at the grain boundaries of steel and significantly contributes to the improvement of hardenability, so it may be contained as necessary. However, when the B content exceeds 0.0020%, the toughness may deteriorate. Therefore, the B content should be 0.0020% or less. The B content is preferably 0.0015% or less, more preferably 0.0012% or less. In order to obtain the above effects, the B content is preferably 0.0001% or more, more preferably 0.0005% or more.

REM: 0-0.01%
REM is an element that controls the morphology of sulfide-based inclusions and contributes to the improvement of SSC resistance, HIC resistance and toughness, so it may be contained as necessary. However, when the REM content exceeds 0.01%, oxides are formed, which not only reduces the cleanliness of the steel, but also reduces the HIC resistance and toughness. Therefore, the REM content should be 0.01% or less. Preferably, the REM content is 0.006% or less. In order to obtain the above effects, the REM content is preferably 0.0001% or more, more preferably 0.0010% or more.

Here, REM refers to a total of 17 elements of Sc, Y and lanthanoids, and the content of REM means the total content of these elements.

Mg: 0-0.01%
Mg is an element that forms fine oxides, suppresses coarsening of crystal grains, and contributes to improvement of toughness, so it may be contained as necessary. However, when the Mg content exceeds 0.01%, the oxide aggregates and coarsens, reducing the HIC resistance and toughness. Therefore, the Mg content should be 0.01% or less. The Mg content is preferably 0.005% or less. In order to obtain the above effects, the Mg content is preferably 0.0001% or more, more preferably 0.0010% or more.

Hf: 0-0.005%
Like Ca, Hf is an element that forms sulfides, suppresses the formation of MnS elongated in the rolling direction, and contributes to the improvement of HIC resistance, so it may be contained as necessary. However, if the Hf content exceeds 0.005%, the oxides increase and aggregate and coarsen, impairing the HIC resistance. Therefore, the Hf content should be 0.005% or less. The Hf content is preferably 0.004% or less, more preferably 0.003% or less. In order to obtain the above effects, the Hf content is preferably 0.0001% or more, more preferably 0.0005% or more.

Re: 0 to 0.005%
Like Ca, Re is an element that forms sulfides, suppresses the formation of MnS elongated in the rolling direction, and contributes to the improvement of HIC resistance, so it may be contained as necessary. However, if the Re content exceeds 0.005%, oxides increase and aggregate and coarsen, impairing HIC resistance. Therefore, the Re content should be 0.005% or less. The Re content is preferably 0.004% or less, more preferably 0.003% or less. In order to obtain the above effects, the Re content is preferably 0.0001% or more, more preferably 0.0005% or more.

In the chemical composition of the steel plate and steel pipe 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 is manufactured industrially. means something

Ceq: 0.30-0.50
Ceq is a value that serves as an index of hardenability and is represented by the following formula (ii). If the Ceq is less than 0.30, the required strength cannot be obtained. On the other hand, when the Ceq exceeds 0.50, the surface hardness increases and the SSC resistance decreases. Therefore, Ceq is set to 0.30 to 0.50. Ceq is preferably 0.33 or more and preferably 0.45 or less.
Ceq=C+Mn/6+(Ni+Cu)/15+(Cr+Mo+V)/5 (ii)
However, each element symbol in the formula represents the content (% by mass) of each element contained in the steel, and is zero when not contained.

2. Metal structure In the steel plate and steel pipe according to the present invention, the metal structure at the center of the plate thickness contains one or two selected from acicular ferrite and bainite, and the balance is ferrite, MA phase, and pseudo pearlite ( or “pseudo pearlite”), the metal structure in the surface layer contains one or two selected from acicular ferrite and bainite, and the balance has a grain size of 100 nm or more. It is one or more selected from tempered martensite containing cementite, MA phase, and pseudo-pearlite (or pseudo-pearlite). The reason is as follows.

When martensite is contained in the metal structure inside the steel, the strength of the steel increases excessively, making it difficult to keep the surface hardness low. Therefore, the formation of martensite is prevented by adjusting the chemical composition of the steel, particularly setting the value of Ceq to an appropriate range, and performing controlled cooling after hot rolling as described later.

Therefore, considering the balance between strength and surface hardness, the metal structure at the center of the sheet thickness is made mainly of acicular ferrite and/or bainite. The total area ratio of acicular ferrite and bainite is preferably 50% or more. In the metallographic structure at the center of the plate thickness, the remainder is one or more selected from ferrite, MA phase, and pseudo-pearlite. When the metal structure inside the steel is mainly ferrite, it is difficult to obtain the necessary strength. Therefore, the area ratio of ferrite is preferably 50% or less.

Since the cooling rate of the surface layer is relatively higher than that of the inside of the steel, martensite may partially form in the surface layer during the cooling process after hot rolling. Since martensite is a hard structure, it reduces the SSC resistance. Therefore, the metallographic structure of the surface layer also contains one or two selected from acicular ferrite and bainite, as in the central part of the plate thickness. The total area ratio of acicular ferrite and bainite is preferably 50% or more.

In order to reduce the maximum hardness of the surface layer, it is desirable to make the hardness of the surface layer as uniform as possible. The inclusion of acicular ferrite in the surface layer is preferable because it has the effect of making the hardness uniform.

Even if martensite is generated in the surface layer during the cooling process, it is possible to reduce the hardness by performing a tempering treatment under predetermined conditions after cooling to change the martensite to tempered martensite. In particular, by precipitating cementite having a grain size of 100 nm or more in the tempered martensite, it is possible to sufficiently reduce the hardness. Therefore, in the metallographic structure of the surface layer, the balance is made to be at least one selected from tempered martensite containing cementite having a grain size of 100 nm or more, MA phase, and pseudo-pearlite.

Here, in the present invention, "acicular ferrite", as defined in Non-Patent Document 1, refers to a bainitic ferrite mixed structure containing pseudo-polygonal ferrite. Also, the MA phase (Martensite-Austenite constituent) means a composite of martensite and austenite. In the present invention, the term "MA phase" also includes a metal structure in which the MA phase generated after rolling and accelerated cooling is further subjected to heat treatment such as tempering and contains fine carbides in the structure. Depending on its size, shape, or chemical composition, the MA phase may change its morphology differently even if it receives the same heat history, and the morphology changes to the MA phase containing fine carbides and the pseudo-pearlite described later. there is something

When the MA phase is sufficiently tempered, it decomposes into ferrite and cementite. At this time, a metallographic structure is generated in which cementite is divided into a dotted line in ferrite. In the present invention, “pseudo-pseudo-pearlite” includes not only the metal structure in which the pearlite layered structure defined in Non-Patent Document 1 is divided, but also the metal structure generated by tempering the MA phase.

3. Mechanical properties Maximum hardness of surface layer: 250HV0.1 or less As described above, in order to improve SSC resistance, it is necessary to keep the maximum hardness of steel low. In addition, if there is a tissue with high hardness even locally, SSC may occur starting there. The hardness is evaluated by Therefore, the maximum hardness of the surface layer is set to 250HV0.1 or less. The maximum hardness of the surface layer is preferably 240HV0.1 or less, and the lower the better.

In the present invention, the hardness is measured at intervals of 0.1 mm from a depth position of 0.1 mm to a depth position of 1.0 mm from the surface, and the maximum value is defined as the maximum hardness of the surface layer.

Tensile strength: 460 MPa or more In the steel plate and steel pipe of the present invention, there is no particular limit on the tensile strength, but as a line pipe used in an _H2S environment, materials of grades X52 to X70 are generally used. often used. In order to satisfy the requirement, the tensile strength is preferably 460 MPa or more, more preferably 500 MPa or more.

4. Plate Thickness The plate thickness (wall thickness) of the steel plate and steel pipe of the present invention is not particularly limited. However, the plate thickness (wall thickness) is preferably 16.0 mm or more, more preferably 19.0 mm or more, from the viewpoint of improving the transportation efficiency of the fluid passing through the line pipe. On the other hand, the hardness of the surface layer increases due to work hardening during steel pipe forming, and the surface layer hardness generally increases as the wall thickness increases. Therefore, the plate thickness (wall thickness) is preferably 30.0 mm or less, more preferably 25.0 mm or less.

5. Manufacturing Method The steel material according to the present invention can be manufactured, for example, by the following method, but is not limited to this method.

After the steel having the chemical composition described above is melted in a furnace, a slab is produced by casting. After that, the slab is heated and hot rolled. The conditions in the hot rolling step are not particularly limited, but for example, it is preferable to set the heating temperature before hot rolling to 1000 to 1300°C and the hot rolling finish temperature to 800 to 1000°C.

After hot rolling, the steel is cooled to a temperature below the Bs point within the range where the average cooling rate inside the steel is 5°C/s or more and less than V _{C -90} °C/s. Here, the value of V _C-90 is calculated by the following formula (iii) when the B content contained in the steel is 5 ppm or more, and when the B content is less than 5 ppm, the following formula (iv) Calculated by
log V _C-90 =2.94-0.75β (iii)
β=2.7C+0.4Si+Mn+0.45Ni+0.8Cr+2Mo
logV _C-90 =3.69-0.75β' (iv)
β′=2.7C+0.4Si+Mn+0.45Ni+0.8Cr+Mo
However, each element symbol in the formula represents the content (% by mass) of each element contained in the steel, and is zero when not contained.

Also, the Bs point (° C.) is represented by the following formula (v) and means the temperature at which acicular ferrite and bainite are formed.
Bs=830-270C-90Mn-37Ni-70Cr-83Mo (v)
However, each element symbol in the formula represents the content (% by mass) of each element contained in the steel, and is zero when not contained.

If the average cooling rate is less than 5° C./s, the ferrite-based structure is formed and the required strength cannot be obtained. In order to obtain a structure mainly composed of acicular ferrite and/or bainite, the steel must be cooled to a temperature below the Bs point at a cooling rate of 5° C./s or more in order to suppress the formation of ferrite as much as possible. On the other hand, when the average cooling rate is V _{C -90} ° C./s or more, martensite is formed inside the steel, making it difficult to keep the maximum hardness of the surface layer low.

Furthermore, the surface layer cooling rate is limited to less than V _{C -90} ° C./s from the start of accelerated cooling to the Ms point. In conventional quenched and tempered materials, the surface layer cooling rate is significantly higher than the average cooling rate, but when the surface layer cooling rate is V _{C -90} ° C./s or more, 90% or more of martensite is formed on the steel plate surface, Even if tempering treatment is performed later, the growth of cementite in the tempered martensite is insufficient, making it difficult to reduce the maximum hardness of the surface layer.

Note that the Ms point can be calculated by the following formula (vi).
Ms=545-330C+2Al+7Co-14Cr-13Cu-23Mn-5Mo-4Nb-13Ni-7Si+3Ti+4V (vi)
However, each element symbol in the formula represents the content (% by mass) of each element contained in the steel, and is zero when not contained.

Further, when cooling, it is desirable to perform accelerated cooling intermittently to repeat cooling and reheating. Accelerated cooling of a steel plate causes the surface temperature to be cooled to a lower temperature than the internal temperature. Here, the surface temperature can be double-heated by heat conduction with the internal temperature when the accelerated cooling is temporarily stopped. For example, even if the surface temperature drops to 400°C or less by accelerated cooling, if the internal temperature is 700°C or more when cooling is stopped, it can be reheated to a temperature of 550°C or more by giving an appropriate recovery time. can.

Therefore, when the steel is reheated, a higher self-tempering effect can be obtained than when normal accelerated cooling is performed, which greatly contributes to the decrease in surface hardness. It is preferable to perform the reheating, for example, twice or more. Further, if the maximum temperature after reheating is set to a temperature equal to or higher than the accelerated cooling stop temperature (surface temperature after reheating after stopping final accelerated cooling), a greater self-tempering effect can be obtained.

Then, after the accelerated cooling stop temperature is lowered to the Ms point or lower, tempering is performed at a temperature exceeding 450° C. and Ac ₁ point or lower. By setting the tempering temperature to 450° C. or higher, the cementite contained in the tempered martensite in the metallographic structure of the surface layer grows. This makes it possible to reduce the maximum hardness of the surface layer. Tempering may be performed immediately after accelerated cooling, or may be performed after air-cooling to room temperature after accelerated cooling.

There are no particular restrictions on the tempering time, but by setting the accelerated cooling stop temperature to 400°C or higher, a self-tempering effect can be obtained during air cooling after accelerated cooling. It can be tempered in a shorter time than normal tempering treatment. Therefore, the tempering time can be, for example, 10 to 30 minutes.

Further, the steel pipe according to the present invention can be manufactured by subjecting the above steel plate to UO pipe making, for example. In the UO pipe manufacturing method, for example, a rolled steel plate (raw material) from which scales have been removed from the edge portion is subjected to C-pressing to form a C-shape, followed by U-pressing to form into a U-shape, and further O-pressing. After that, the joints, which are the ends, are butted together, tack welding, inner surface welding, and outer surface welding are performed, and if necessary, the pipe is expanded.

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 having the chemical composition shown in Table 1 was melted and continuously cast into a steel slab. The thickness at this time was 300 mm for steel types J to N, and 240 mm for other steel types A to I and OS. As shown in Table 2, the obtained steel slab was heated to a temperature range of 1000 to 1250°C, hot rolled in a recrystallization temperature range exceeding 900°C, and then rolled to a non-recrystallization temperature of 800 to 900°C. hot rolling was performed in the area.

After hot rolling, water cooling was started from an accelerated cooling start temperature of 750 to 850°C, and stopped at a temperature of 400 to 550°C. As shown in Table 2, accelerated cooling was performed by multistage intermittent cooling and normal accelerated cooling. In addition, accelerated cooling, which is rapid cooling after conventional hot rolling, was also performed. After accelerated cooling, tempering treatment was performed under the conditions shown in Table 2.

A test piece for metallographic observation, a tensile test piece, a test piece for hardness measurement, and an SSC test piece were taken from the tempered steel sheet and subjected to the respective tests.

A cross-section of each steel plate was cut out to obtain a test piece for metallographic observation, wet-polished to a mirror finish, and then subjected to nital corrosion to reveal a microstructure. Then, the structure was observed using an optical microscope at a magnification of 100 to 500 times. For the structure containing tempered martensite, after regrinding, a thin film sample was cut out, and the presence or absence of cementite of 100 nm or more was confirmed with a transmission electron microscope (TEM) at a magnification of 30,000 to 50,000 times.

The tensile test was performed according to JIS Z 2241 using a round bar-shaped tensile test piece. From the result, the tensile strength (MPa) was obtained.

Next, the maximum hardness was measured using the test piece for hardness measurement. Specifically, after cutting out a cross section of each steel plate, the hardness was measured at intervals of 0.1 mm from a depth position of 0.1 mm to a depth position of 1.0 mm from the surface. The maximum value was taken as the maximum hardness of the surface layer.

Also, the SSC test was conducted in accordance with NACE TM0316. Then, those in which SSC did not occur were judged to be acceptable (○), and those in which SSC occurred were judged to be unacceptable (×).

These results are summarized in Table 3.

As can be seen from Table 3, test no. 1, 2, 8, 9 and 13 to 25 had a maximum surface layer hardness of 250HV0.1 or less, and no cracks were observed in the SSC test. Moreover, the tensile strength was also 500 MPa or more.

For them, test no. 3-7, 10-12 and 26-29 are comparative examples that do not meet any of the provisions of the invention.

Test no. In 3 to 5, the surface layer cooling rate was V _{C -90} ° C./s or more. Therefore, the cementite in the martensite was not sufficiently coarsened in the subsequent short-time tempering treatment, resulting in tempered martensite in which only fine cementite of less than 100 nm was dispersed. As a result, the maximum hardness of the surface layer could not be reduced to 250HV0.1 or less.

Test no. In 6, 7, 11 and 12, since the tempering temperature was low, the cementite in the martensite was not sufficiently coarsened, resulting in tempered martensite in which only fine cementite of less than 100 nm was dispersed. As a result, the maximum hardness of the surface layer could not be reduced to 250HV0.1 or less.

Test no. In No. 10, after completion of hot rolling, the steel was rapidly cooled to room temperature and was not reheated. As a result, the self-tempering effect was not obtained, and the cementite in martensite was not sufficiently coarsened in the subsequent short-time tempering treatment, resulting in tempered martensite in which only fine cementite of less than 100 nm was dispersed. As a result, the maximum hardness of the surface layer could not be reduced to 250HV0.1 or less.

Test no. In No. 26, the maximum hardness of the surface layer exceeded 250HV0.1 because the C content was higher than the specified range. Also, test no. In No. 27, sufficient strength was not obtained because the Ceq value was lower than the specified range.

Furthermore, test no. In Test No. 28, the value of Ceq is higher than the specified range. In No. 29, the total content of Mo, Cr, Cu and Ni was higher than the specified range, so the cooling rate of the surface layer could not be made less than V _{C -90} ° C./s. Test no. In No. 29, tempered martensite was observed because martensitic transformation also occurred at the central portion of the plate thickness. Therefore, in these examples, the cementite in the martensite was not sufficiently coarsened in the subsequent short-time tempering treatment, resulting in tempered martensite in which only fine cementite of less than 100 nm was dispersed. As a result, the maximum hardness of the surface layer could not be reduced to 250HV0.1 or less.

Using the steel plates of Examples 2, 9, 14, 18, 22 and 23 of the present invention, pipes were made by the UO pipe making method to obtain steel pipes 1 to 6 shown in Table 4.

A metal structure observation test piece, a tensile test piece, a hardness measurement test piece and an SSC test piece were taken from the obtained steel pipe by the methods described above, and subjected to the respective tests.

These results are summarized in Table 4.

As can be seen from Table 4, Steel Pipes 1 and 2, which satisfy all the requirements of the present invention, had a maximum surface layer hardness of 250HV0.1 or less, and no cracks were observed in the SSC test. Moreover, the tensile strength was also 500 MPa or more.

According to the present invention, since the maximum hardness of the surface layer can be suppressed to 250HV0.1 or less, it is possible to obtain a steel plate and a steel pipe having excellent SSC resistance. Therefore, the steel plate and steel pipe according to the present invention can be suitably used as line pipes for transporting crude oil and natural gas containing a large amount of _H2S .

Claims (4)

The chemical composition, in mass %,
C: 0.02 to 0.08%,
Si: 0.01 to 0.50%,
Mn: 0.5-1.7%,
Nb: 0.001 to 0.100%,
N: 0.0010 to 0.0060%,
Ca: 0.0001 to 0.0050%,
P: 0.03% or less,
S: 0.0025% or less,
Ti: 0.005 to 0.030%,
Al: 0.01-0.04%,
O: 0.004% or less,
Mo: 0-2.0%,
Cr: 0 to 2.0%,
Cu: 0-2.0%,
Ni: 0 to 2.0%,
W: 0 to 1.0%,
V: 0 to 0.20%,
Zr: 0 to 0.050%,
Ta: 0 to 0.050%,
B: 0 to 0.0020%,
REM: 0-0.01%,
Mg: 0-0.01%,
Hf: 0-0.005%,
Re: 0 to 0.005%,
balance: Fe and impurities,
satisfying the following formula (i),
Ceq represented by the following formula (ii) is 0.33 to 0.50,
The metal structure at the center of the sheet thickness contains one or two selected from acicular ferrite and bainite, and the remainder is one or more selected from ferrite, MA phase, and pseudo-pearlite,
The metal structure in the surface layer contains one or two selected from acicular ferrite and bainite , pseudo-pearlite, and the balance is tempered martensite containing cementite having a grain size of 100 nm or more , and from the MA phase. One or more selected, the maximum hardness of the surface layer is 250HV0.1 or less,
A steel plate for line pipes, having a tensile strength of 460 to 560 MPa .
0.05≦Mo+Cr+Cu+Ni≦2.0 (i)
Ceq=C+Mn/6+(Ni+Cu)/15+(Cr+Mo+V)/5 (ii)
However, each element symbol in the formula represents the content (% by mass) of each element contained in the steel, and is zero when not contained. The chemical composition, in mass %,
W: 0.01 to 1.0%,
V: 0.01 to 0.20%,
Zr: 0.0001 to 0.050%,
Ta: 0.0001 to 0.050%, and
B: 0.0001 to 0.0020%,
containing one or more selected from
The steel plate for line pipes according to claim 1. The chemical composition, in mass %,
REM: 0.0001 to 0.01%,
Mg: 0.0001-0.01%,
Hf: 0.0001 to 0.005%, and Re: 0.0001 to 0.005%,
containing one or more selected from
The steel plate for line pipes according to claim 1 or 2. Using the steel plate according to any one of claims 1 to 3,
Steel pipe for line pipes.