KR20240149424A - High-strength steel plate for internal sour line pipe and its manufacturing method, and high-strength steel pipe using high-strength steel plate for internal sour line pipe - Google Patents

Abstract

The present invention provides a high-strength steel plate for a sour line pipe having excellent low-temperature toughness as well as HIC resistance and SSCC resistance. The high-strength steel plate for a sour line pipe of the present invention has a component composition containing, in mass%, C: 0.030 to 0.060%, Si: 0.01 to 0.50%, Mn: 0.80 to 1.80%, P: 0.015% or less, S: 0.0015% or less, Al: 0.010 to 0.080%, Cr: 0.05 to 0.50%, Nb: 0.005 to 0.080%, N: 0.0010 to 0.0080%, and Ca: 0.0005 to 0.0050%, with the remainder being Fe and inevitable impurities, and the steel structure at 0.25 mm below the surface of the steel plate is granular bainite and It is composed of tempered island martensite, has a maximum crystal grain size of 80 ㎛ or less at the center of the plate thickness, an average crystal grain size of 20 ㎛ or less, a brittle-ductile transition temperature of -100 ℃ or less in a Charpy impact test, and a tensile strength of 535 MPa or more.

Description

High-strength steel plate for internal sour line pipe and its manufacturing method, and high-strength steel pipe using high-strength steel plate for internal sour line pipe

The present invention relates to a high-strength steel plate for a sour line pipe having excellent material uniformity within the steel plate, which is preferable for use in line pipes used for transporting crude oil or natural gas, and a method for manufacturing the same. In addition, the present invention relates to a high-strength steel pipe using the high-strength steel plate for a sour line pipe as described above.

Generally, line pipes are manufactured by forming steel plates manufactured by a thick plate mill or a hot rolled mill into steel pipes by UOE forming, press bend forming, and roll forming.

Here, line pipes used for transporting crude oil or natural gas containing hydrogen sulfide require, in addition to strength, toughness, and weldability, so-called sour resistance such as hydrogen induced cracking resistance (HIC (Hydrogen Induced Cracking) resistance) and sulfide stress corrosion cracking resistance (SSCC (Sulfide Stress Corrosion Cracking) resistance). Among these, HIC is a phenomenon in which hydrogen ions due to corrosion reaction are adsorbed on the steel surface, penetrate into the steel as atomic hydrogen, diffuse and accumulate around non-metallic inclusions such as MnS in the steel and hard second phase structures, become molecular hydrogen, and cause cracks due to the internal pressure. This HIC has become a problem in line pipes having a relatively low strength level compared to oil well pipes, and many countermeasure technologies have been disclosed. On the other hand, with regard to SSCC, the importance of controlling the hardness of the inner surface layer of the steel pipe to improve SSCC resistance in a more severe corrosive environment has been pointed out. In addition to these sour properties, the mining environment for crude oil and natural gas has become increasingly stringent in recent years, and the demand for excellent low-temperature toughness is increasing.

Normally, in the production of high-strength steel plates for line pipes, the so-called TMCP (Thermo-Mechanical Control Process) technology, which combines controlled rolling and controlled cooling, is applied. In order to secure the sour resistance of the steel plate using this TMCP technology, it is effective to increase the cooling start temperature during controlled cooling and also to slow down the cooling rate. However, when the cooling start temperature is increased, rolling in the non-recrystallization temperature range becomes insufficient, so there is a limit to the refinement of crystal grains that is effective in improving low-temperature toughness, and excellent low-temperature toughness cannot be secured.

In order to solve the above problem, for example, Patent Document 1 proposes a technology for setting the finishing rolling completion temperature to 700°C or higher and the average crystal grain size to 15.0 μm or lower. In addition, Patent Document 2 proposes a technology for suppressing the growth of crystal grains by setting the holding time from rough rolling to the start of finishing rolling to 300 seconds or less.

Japanese Patent Publication No. 2020-12168 Japanese Patent Publication No. 2020-509181

Although the techniques described in Patent Documents 1 and 2 enable improvement of low-temperature toughness, it is difficult to secure sour resistance in an environment with a high partial pressure of hydrogen sulfide, which is a more severe corrosive environment, because the techniques described in both documents contain ferrite in the metal structure.

In particular, excellent low-temperature toughness is required for pipelines used in cold regions. However, in order to prevent the formation of ferrite, it is necessary to start cooling at a temperature higher than the Ar ₃ point, and since the accumulated strain during rolling is small, it is difficult to make the crystal grain size fine, so excellent low-temperature toughness has not been obtained in the past.

Therefore, the present invention, in consideration of the above problems, aims to provide a high-strength steel plate for a sour line pipe having excellent low-temperature toughness as well as HIC resistance and SSCC resistance, together with an advantageous manufacturing method thereof. In addition, the present invention aims to provide a high-strength steel pipe using the high-strength steel plate for a sour line pipe.

The inventors of the present invention have repeatedly conducted numerous experiments and studies on the composition, microstructure, and manufacturing conditions of the steel plate in order to secure not only the sour resistance but also the low-temperature toughness. As a result, they have found that in order to further improve the low-temperature toughness of the high-strength steel plate, it is necessary to make the maximum crystal grain size at the center of the plate thickness 80 ㎛ or less, and the average crystal grain size 20 ㎛ or less. Furthermore, in order to realize such a steel structure, it is necessary to strictly control the hot rolling conditions in the recrystallization temperature range, and they have succeeded in finding out those conditions. The present invention has been made based on these findings.

That is, the gist of the present invention is as follows.

[1] It has a component composition containing, in mass%, C: 0.030 to 0.060%, Si: 0.01 to 0.50%, Mn: 0.80 to 1.80%, P: 0.015% or less, S: 0.0015% or less, Al: 0.010 to 0.080%, Cr: 0.05 to 0.50%, Nb: 0.005 to 0.080%, N: 0.0010 to 0.0080%, and Ca: 0.0005 to 0.0050%, with the remainder being Fe and inevitable impurities.

The steel structure at 0.25 mm below the surface of the steel plate is composed of granular bainite and tempered island martensite.

The maximum crystal grain size at the center of the plate thickness is 80 ㎛ or less, and the average crystal grain size is 20 ㎛ or less.

The brittle-ductile transition temperature in the Charpy impact test is -100 ℃ or less,

Tensile strength of 535 MPa or more

High-strength steel plate for internal sour line pipe characterized by:

[2] A high-strength steel plate for a sour line pipe as described in [1] above, wherein the above component composition further contains at least one selected from the group consisting of Cu: 0.30% or less, Ni: 0.10% or less, and Mo: 0.50% or less in mass%.

[3] A high-strength steel plate for a sour line pipe according to [1] or [2], wherein the above component composition further contains at least one selected from the group consisting of, in mass%, V: 0.005 to 0.1%, Ti: 0.005 to 0.1%, Zr: 0.0005 to 0.02%, Mg: 0.0005 to 0.02%, and REM: 0.0005 to 0.02%.

[4] A steel slab having a composition containing, in mass%, C: 0.030 to 0.060%, Si: 0.01 to 0.50%, Mn: 0.80 to 1.80%, P: 0.015% or less, S: 0.0015% or less, Al: 0.010 to 0.080%, Cr: 0.05 to 0.50%, Nb: 0.005 to 0.080%, N: 0.0010 to 0.0080%, and Ca: 0.0005 to 0.0050%, with the remainder being Fe and inevitable impurities, is heated to a temperature of 1000 to 1250°C,

After that, hot rolling is performed on the above steel sheet to make a steel plate, satisfying the total reduction ratio in the recrystallization temperature range: 35% or more and 55% or less, and the reduction ratio of the final rolling pass in the recrystallization temperature range: 10% or more.

After that, for the above steel plate,

Steel plate surface temperature at the start of cooling: Ar ₃ point or higher,

Average cooling rate from 750 ℃ to 550 ℃ at 0.25 mm below the steel plate surface: 15 to 35 ℃/s

Average cooling rate from 750 ℃ to 550 ℃ at the center of the plate thickness: 15 ℃/s or more,

Cooling stop temperature: 350 to 550 ℃ at the steel plate temperature at 0.25 mm below the steel plate surface and at the center of the plate thickness

Controlled cooling is performed under the conditions of

A method for manufacturing a high-strength steel plate for a sour line pipe, characterized by:

[5] A method for manufacturing a high-strength steel plate for a sour line pipe as described in [4], wherein the above component composition further contains, in mass%, at least one selected from the group consisting of Cu: 0.30% or less, Ni: 0.10% or less, and Mo: 0.50% or less.

[6] A method for manufacturing a high-strength steel plate for a sour line pipe as described in [4] or [5], wherein the above component composition further contains, in mass%, at least one selected from the group consisting of V: 0.005 to 0.1%, Ti: 0.005 to 0.1%, Zr: 0.0005 to 0.02%, Mg: 0.0005 to 0.02%, and REM: 0.0005 to 0.02%.

[7] High-strength steel pipe using high-strength steel plate for internal sour line pipe described in [1] or [2] above.

[8] High-strength steel pipe using the high-strength steel plate for the internal sour line pipe described in [3] above.

The high-strength steel plate for a sour line pipe of the present invention and the high-strength steel pipe using the high-strength steel plate for a sour line pipe have excellent HIC resistance, SSCC resistance, and low-temperature toughness. In addition, according to the method for manufacturing the high-strength steel plate for a sour line pipe of the present invention, it is possible to manufacture a high-strength steel plate for a sour line pipe having excellent HIC resistance, SSCC resistance, and low-temperature toughness.

Figure 1 is a schematic diagram explaining a method for collecting test pieces for evaluating SSCC properties in an embodiment.

Hereinafter, the high-strength steel plate for the internal sour line pipe of the present invention will be described in detail.

[Ingredients]

First, the composition of the high-strength steel plate according to the present invention and the reasons for its limitation will be explained. In the following description, all units expressed as "%" are "mass%" unless otherwise specified.

C: 0.030 ~ 0.060%

C effectively contributes to the improvement of strength, but sufficient strength cannot be secured when the C content is less than 0.030%, so the C content is set to 0.030% or more, and preferably 0.035% or more. On the other hand, when the C content exceeds 0.060%, the low-temperature toughness deteriorates. In addition, since the hardness of the surface portion and the center segregation portion increases during accelerated cooling, the SSCC resistance and HIC resistance deteriorate. Therefore, the C content is set to 0.060% or less, and preferably 0.050% or less.

Si: 0.01 ~ 0.50%

Si is added for deoxidation, but since the deoxidation effect is not sufficient when the Si content is less than 0.01%, the Si content is set to 0.01% or more, and preferably 0.05% or more. On the other hand, when the Si content exceeds 0.50%, the specific thermal stress of the steel increases and the low-temperature toughness deteriorates, so the Si content is set to 0.50% or less, and preferably 0.45% or less.

Mn: 0.80 ~ 1.80%

Mn effectively contributes to the improvement of strength, but its effect is not sufficiently expressed when the Mn content is less than 0.80%. Therefore, the Mn content is set to 0.80% or more, and preferably 1.00% or more. On the other hand, when the Mn content exceeds 1.80%, the hardness of the surface portion and the center segregation portion increases during accelerated cooling, so that the SSCC resistance and HIC resistance deteriorate. In addition, the weldability also deteriorates. Therefore, the Mn content is set to 1.80% or less, and preferably 1.70% or less.

P: 0.015% or less

P is an inevitable impurity element, and it deteriorates low-temperature toughness and increases the hardness of the surface layer and the center segregation, thereby deteriorating SSCC resistance and HIC resistance. Since this tendency becomes remarkable when the P content exceeds 0.015%, the P content is set to 0.015% or less, and preferably 0.008% or less. In addition, the lower the P content, the better, but from the viewpoint of refining cost, the P content is preferably set to 0.001% or more.

S: 0.0015% or less

S is an inevitable impurity element, and since it becomes an MnS inclusion in steel and deteriorates HIC resistance, it is desirable that it be small. From this viewpoint, the S content is set to 0.0015% or less, and preferably 0.0010% or less. In addition, the lower the S content, the better, but from the viewpoint of refining cost, the S content is preferably set to 0.0002% or more.

Al: 0.010 ~ 0.080%

Al is added as a deoxidizer, but its effect is not sufficiently expressed when the Al content is less than 0.010%. Therefore, the Al content is set to 0.010% or more, and preferably 0.015% or more. On the other hand, when the Al content exceeds 0.080%, the specific thermal stress of the steel increases and the low-temperature toughness deteriorates. Therefore, the Al content is set to 0.080% or less, and preferably 0.070% or less.

Cr: 0.05 ~ 0.50%

Cr, like Mn, is an effective element for obtaining sufficient strength even with a low C content, and to obtain this effect, it is necessary that the Cr content be 0.05% or more. However, if the Cr content is too high, the hardness becomes excessive, so that the hardness of the surface portion or the center segregation portion increases during accelerated cooling, and the SSCC resistance and HIC resistance deteriorate. In addition, the weldability also deteriorates. Therefore, the Cr content is set to 0.50% or less, and preferably 0.45% or less.

Nb: 0.005 ~ 0.080%

Nb, when present as a dissolved Nb, expands the non-recrystallization temperature range during controlled rolling and contributes to the improvement of low-temperature toughness. However, when the Nb content is less than 0.005%, the effect is not sufficiently expressed. Therefore, the Nb content is set to 0.005% or more, preferably 0.010% or more. On the other hand, when the Nb content exceeds 0.080%, coarse carbides are crystallized during solidification, which deteriorates the HIC resistance. Therefore, the Nb content is set to 0.080% or less, preferably 0.060% or less.

N: 0.0010 ~ 0.0080%

N effectively contributes to the improvement of strength, but sufficient strength cannot be secured when the N content is less than 0.0010%. Therefore, the N content is set to 0.0010% or more, and preferably 0.0015% or more. On the other hand, when the N content exceeds 0.0080%, the hardness of the surface portion and the center segregation portion increases during accelerated cooling, so that the SSCC resistance and HIC resistance deteriorate. In addition, the low-temperature toughness also deteriorates. Therefore, the N content is set to 0.0080% or less, and preferably 0.0070% or less.

Ca: 0.0005 ∼ 0.0050 ％

Ca is an effective element for improving HIC resistance by controlling the shape of sulfide inclusions, but when the Ca content is less than 0.0005%, the effect of addition is not sufficient. Therefore, the Ca content is set to 0.0005% or more, preferably 0.0008% or more. On the other hand, when the Ca content exceeds 0.0050%, not only the above-described effect is saturated, but also the cleanliness of the steel deteriorates, thereby deteriorating the HIC resistance. Therefore, the Ca content is set to 0.0050% or less, preferably 0.0045% or less.

Above, the basic components of the composition of ingredients in the present invention have been described, but in the present invention, the composition of ingredients may optionally contain at least one selected from the group consisting of Cu, Ni, and Mo within the following range in order to further improve the strength and toughness of the steel plate.

Cu: 0.30% or less

Cu is an effective element for improving low-temperature toughness and increasing strength, and in order to obtain this effect, it is preferable that the Cu content be 0.05% or more. However, when the Cu content exceeds 0.30%, in an environment with a low hydrogen sulfide partial pressure of less than 1 bar, microcracks called Fischer are likely to be generated, so that the SSCC resistance deteriorates. Therefore, when Cu is contained, the Cu content is set to 0.30% or less, and preferably 0.25% or less.

Ni: 0.10% or less

Ni is an effective element for improving low-temperature toughness and increasing strength, and in order to obtain this effect, it is preferable that the Ni content be 0.01% or more. However, when the Ni content exceeds 0.10%, in an environment with a low hydrogen sulfide partial pressure of less than 1 bar, microcracks called Fischer are likely to be generated, so that the SSCC resistance deteriorates. Therefore, when Ni is contained, the Ni content is set to 0.10% or less, and preferably 0.02% or less.

Mo: 0.50% or less

Mo is an effective element for improving low-temperature toughness and increasing strength, and in order to obtain this effect, the Mo content is preferably 0.01% or more, and more preferably 0.10% or more. On the other hand, if the Mo content is too high, microcracks called Fischer are likely to form in an environment with a low hydrogen sulfide partial pressure of less than 1 bar, so that the SSCC resistance deteriorates. In addition, the weldability also deteriorates. Therefore, when Mo is contained, the Mo content is set to 0.50% or less, and preferably 0.40% or less.

The component composition in the present invention may additionally optionally contain at least one selected from the group consisting of V, Ti, Zr, Mg, and REM within the following range.

At least one selected from the group consisting of V: 0.005 to 0.1%, Ti: 0.005 to 0.1%, Zr: 0.0005 to 0.02%, Mg: 0.0005 to 0.02%, and REM: 0.0005 to 0.02%

Both V and Ti are elements that can be optionally included to increase the strength and low-temperature toughness of the steel plate. For each element, if the content is less than 0.005%, the effect is not sufficiently expressed. Therefore, when these elements are included, the content is preferably 0.005% or more. On the other hand, when the content of these elements exceeds 0.1%, the toughness of the weld deteriorates. Therefore, when these elements are included, the content is preferably 0.1% or less.

Zr, Mg and REM are elements that can be optionally included in order to increase low-temperature toughness through grain refinement or to increase crack resistance through control of inclusion properties. For each element, the effect is not sufficiently expressed when the content is less than 0.0005%. Therefore, when these elements are included, the content is preferably 0.0005% or more. On the other hand, when the content of these elements exceeds 0.02%, the effect is saturated, and therefore, when these elements are included, the content is preferably 0.02% or less.

The present invention discloses a technology for improving the low-temperature toughness of a high-strength steel pipe using a high-strength steel plate for a sour line pipe. However, as sour performance, it is necessary to satisfy HIC resistance, needless to say, and for example, it is preferable that the CP value obtained by the following equation (1) be 1.00 or less. In addition, elements that are not contained may be substituted with 0.

CP = 4.46 [%C] + 2.37 [%Mn]/6 + (1.74 [%Cu] + 1.7 [%Ni])/15 + (1.18 [%Cr] + 1.95 [%Mo] + 1.74 [%V] )/5 + 22.36 [%P] ···(1)

However, [％X] represents the content of element X in the steel (mass%).

Here, the CP value is a formula designed to estimate the material of the central segregation part from the content of each alloy element, and the higher the CP value of the formula (1) above, the higher the component concentration of the central segregation part, and the higher the hardness of the central segregation part. Therefore, by setting the CP value obtained in the formula (1) above to 1.00 or less, it becomes possible to improve the HIC resistance. In addition, since the hardness of the central segregation part decreases as the CP value decreases, in cases where higher HIC resistance is required, the upper limit can be set to 0.95. The lower limit of the CP is not particularly limited, but the CP value can be 0.70 or more.

In addition, the remainder other than the above-mentioned elements is composed of Fe and unavoidable impurities. However, as long as the effect of the present invention is not hindered, the inclusion of other trace elements may also be permitted. For example, O is an element inevitably included in steel, but if its content is 0.0050% or less, preferably 0.0040% or less, it is permitted in the present invention.

[Tissue 0.25 mm below the steel plate surface]

Next, the steel structure of the high-strength steel plate for the sour line pipe of the present invention is described. In order to improve the SSCC resistance and to achieve high strength with a tensile strength of 535 MPa or more, the steel structure 0.25 mm below the surface of the steel plate is made into a structure composed of granular bainite and tempered island martensite. If island martensite is mixed in the steel structure, deterioration of low-temperature toughness and deterioration of SSCC resistance occur, but when the cooling stop temperature is sufficiently high, tempered tempered island martensite (TMA) is formed after the cooling stop, so deterioration of low-temperature toughness and SSCC resistance can be prevented. In addition, since granular bainite and tempered island martensite have lower hardness than other bainites or other martensites, they can improve SSCC resistance.

Here, it is preferable that the steel structure of 0.25 mm below the surface of the steel plate in the present invention is mainly composed of granular bainite. Specifically, it is preferable that the granular bainite is 90% or more in area ratio and the tempered island-like martensite is 10% or less in area ratio. On the other hand, since the presence of tempered island-like martensite is also essential, it is preferable that the granular bainite is 99% or less in area ratio and the tempered island-like martensite is 1% or more in area ratio.

In addition, in the high-strength steel plate of the present invention, if the steel structure 0.25 mm below the surface of the steel plate satisfies the above conditions, the extreme surface layer in the range of 0.25 mm in depth from the surface of the steel plate also has an equivalent steel structure, and as a result, the effect of improving the SSCC resistance is obtained.

In addition, in order to obtain the effect of improving the SSCC properties and increasing the strength more sufficiently, it is desirable that the steel structure of the entire steel plate, including the portion other than the surface layer, satisfy the above conditions. Specifically, it is sufficient that the structure at the center of the plate thickness, representing the “portion other than the surface layer,” satisfies the above conditions.

[Maximum grain size and average grain size at the center of the plate thickness]

In the present invention, it is important to suppress the formation of coarse grains. That is, if the maximum grain size or the average grain size is large, the low-temperature toughness deteriorates. In particular, if the maximum grain size at the center of the plate thickness exceeds 80 µm, the coarse grains easily become the starting point of fracture, so the low-temperature toughness deteriorates significantly. Even if the average grain size exceeds 20 µm, the low-temperature toughness deteriorates. Therefore, it is necessary to make the maximum grain size at the center of the plate thickness 80 µm or less, and the average grain size 20 µm or less. Since the maximum grain size and the average grain size at the center of the plate thickness are preferably smaller, the lower limits thereof are not particularly limited, but in the present invention, the maximum grain size at the center of the plate thickness can be 50 µm or more, and the average grain size can be 10 µm or more. In addition, the "crystal grain size" is defined as the grain size of crystals measured in a range of 1 mm × 1 mm at the center of the plate thickness, the maximum value among them is adopted as the "maximum crystal grain size", and the average value is adopted as the "average crystal grain size", and the crystal grain size is defined as the equivalent diameter of a circle. Here, the center of the plate thickness refers to the position at 1/2 of the total thickness.

[Brittle-Ductile Transition Temperature]

The high-strength steel plate of the present invention has a brittle-ductile transition temperature of -100°C or lower in a Charpy impact test. As a result, excellent low-temperature toughness can be secured. The lower limit of the brittle-ductile transition temperature is not particularly limited, but in the present invention, the brittle-ductile transition temperature can be -120°C or higher.

[tensile strength]

The high-strength steel plate of the present invention is a steel plate for steel pipes having a strength of X65 grade or higher of API 5L, and therefore has a tensile strength of 535 MPa or higher. The upper limit of the tensile strength is not particularly limited, but the tensile strength of the high-strength steel plate of the present invention can be 600 MPa or lower.

[Thickness of steel plate]

The high-strength steel plate of the present invention is not particularly limited in thickness, but a plate having a thickness of 12 to 39 mm is suitable for the above-described use.

[Method for manufacturing high-strength steel plate for internal sour line pipe]

Hereinafter, the manufacturing method and manufacturing conditions for manufacturing the high-strength steel plate for the above-described internal sour line pipe will be specifically described. The manufacturing method of the present invention heats a steel sheet (e.g., a slab) having the above-described component composition, then hot-rolls it into a steel sheet, and then performs controlled cooling on the steel sheet under predetermined conditions.

[Heating temperature of steel plate]

Steel plate heating temperature: 1000 ~ 1250 ℃

When the slab heating temperature is less than 1000°C, the solid solution of carbides becomes insufficient and the amount of solid solution strengthening decreases, so that the required strength is not obtained. Therefore, the slab heating temperature is set to 1000°C or higher, and preferably 1030°C or higher. On the other hand, when the slab heating temperature exceeds 1250°C, the crystal grains become extremely coarsened, and the toughness deteriorates. Therefore, the slab heating temperature is set to 1250°C or lower, and preferably 1200°C or lower. In addition, this temperature is the furnace temperature of the heating furnace, and the slab is heated to this temperature up to the center.

[Hot rolling conditions]

Total pressure reduction in the recrystallization temperature range: 35% or more and 55% or less

In order to make the maximum grain size fine, it is necessary to promote recrystallization of the grains by hot rolling in the recrystallization temperature range and suppress the formation of coarse grains. When the total reduction ratio in the recrystallization temperature range is less than 35%, coarse grains remain because recrystallization is insufficient, which deteriorates the low-temperature toughness. Therefore, the total reduction ratio in the recrystallization temperature range is set to 35% or more, and preferably 38% or more. On the other hand, when the total reduction ratio in the recrystallization temperature range exceeds 55%, the coarsening of the maximum grains can be suppressed, but because the reduction in the non-recrystallization temperature range is insufficient, the average grain size cannot be set to 20 µm or less, which deteriorates the low-temperature toughness. Therefore, the total reduction ratio in the recrystallization temperature range is set to 55% or less, and preferably 52% or less. Here, the "recrystallization temperature range" means a temperature range higher than Tnr, which is obtained from the following equation. In addition, the surface temperature of the steel plate can be measured by a radiation thermometer, etc., and converted into the steel plate temperature at each plate thickness position.

Tnr (℃) = 174 × log{[％Nb] × ([％C] + 12/14 [％N])} + 1444

However, [％X] represents the content of element X in the steel (mass %).

Reduction ratio of the final rolling pass in the recrystallization temperature range: 10% or more

In addition to setting the total reduction ratio in the recrystallization temperature range to 35% or more and 55% or less, it is necessary to sufficiently secure the reduction ratio of the final rolling pass in the recrystallization temperature range to sufficiently promote recrystallization, thereby starting rolling in the non-recrystallization temperature range in a state of uniform grains without coarse grains. When the reduction ratio of the final rolling pass in the recrystallization temperature range is less than 10%, recrystallization is insufficient, so that coarse grains grow during the holding time from rough rolling to the start of finish rolling, thereby deteriorating the low-temperature toughness. Therefore, the reduction ratio of the final rolling pass in the recrystallization temperature range is set to 10% or more, preferably 11% or more. The upper limit of the reduction ratio of the final rolling pass in the recrystallization temperature range is not particularly limited, and a higher reduction ratio is preferable, but the reduction ratio may be 20% or less.

Rolling in the non-recrystallization temperature range, which is performed after completion of rolling in the recrystallization temperature range, is effective for grain refinement because more strain is introduced when rolling at a low temperature. Therefore, it is preferable to perform rolling at as low a temperature as possible within a range that can comply with the cooling start temperature of the cooling treatment described below.

In the hot rolling process, in order to obtain high low-temperature toughness, the lower the rolling end temperature, the better. On the other hand, from the viewpoint of securing sour resistance even in an environment with high hydrogen sulfide partial pressure, it is necessary to set the rolling end temperature based on the fact that the cooling start temperature of the controlled cooling described later needs to be higher than the Ar ₃ point in terms of the steel sheet surface temperature. Here, the Ar ₃ point means the ferrite transformation start temperature during cooling, and can be obtained, for example, from the following equation from the steel composition. In addition, the surface temperature of the steel sheet can be measured with a radiation thermometer, etc.

Ar ₃ point (℃) = 910 - 310 [%C] - 80 [%Mn] - 20 [%Cu] - 15 [%Cr] - 55 [%Ni] - 80 [%Mo]

However, [％X] represents the content of element X in the steel (mass %).

[Controlled cooling conditions]

Cooling start temperature: Ar ₃ point or higher at the steel plate surface temperature

When the steel sheet surface temperature at the start of cooling is lower than the Ar ₃ point, ferrite is generated before controlled cooling, and the strength decreases significantly and the sour resistance deteriorates. Therefore, the steel sheet surface temperature at the start of cooling is set to the Ar ₃ point or higher. In addition, the steel sheet surface temperature at the start of cooling is set to the rolling completion temperature or lower. The upper limit of the steel sheet surface temperature at the start of cooling is not particularly limited, but the temperature may be 900°C or lower.

In order to achieve high strength while obtaining excellent SSCC properties, it is necessary to control the cooling rate at 0.25 mm below the steel plate surface and at the center of the plate thickness.

Average cooling rate from 750 ℃ to 550 ℃ at 0.25 mm below the steel plate surface: 15 to 35 ℃/s

It is important to make the average cooling rate from 750°C to 550°C at a steel plate temperature at 0.25 mm below the steel plate surface as slow as possible to form granular bainite. Since the temperature range from 750°C to 550°C is an important temperature range for bainite transformation, it is important to control the cooling rate in this temperature range. That is, if the average cooling rate exceeds 35°C/s, lath bainite is formed, and the SSCC resistance after pipe making deteriorates. Therefore, the average cooling rate is set to 35°C/s or less, preferably 30°C/s or less. On the other hand, if the average cooling rate is less than 15°C/s, ferrite or pearlite is formed, resulting in insufficient strength. Therefore, from the viewpoint of preventing this, the average cooling rate is set to 15°C/s or more.

In addition, for cooling to a steel plate temperature of 550 ℃ or less at a depth of 0.25 mm below the steel plate surface, if the cooling rate is slow, cooling in a stable nucleate boiling state is not achieved, and there is a concern that the structure may deviate at the extreme surface layer of the steel plate. For this reason, the average cooling rate from a steel plate temperature of 550 ℃ at a depth of 0.25 mm below the steel plate surface to the cooling stop temperature is preferably 150 ℃/s or more. Since there is a concern that the structure may deviate, the average cooling rate is preferably 250 ℃/s or less.

Average cooling rate from 750℃ to 550℃ at the center of plate thickness: 15℃/s or more

When the average cooling rate from 750 ℃ to 550 ℃ at the center of the plate thickness is less than 15 ℃/s, ferrite is generated, causing a decrease in strength and a deterioration in HIC resistance. Therefore, the average cooling rate is set to 15 ℃/s or more. From the viewpoint of suppressing variation in low-temperature toughness, the average cooling rate is preferably set to 17 ℃/s or more. The upper limit of the cooling rate is not particularly limited, but in order to prevent lath bainite from being generated, the average cooling rate is preferably set to 35 ℃/s or less. In addition, with respect to cooling from 550 ℃ or less at the center of the plate thickness, the average cooling rate is preferably set to 15 ℃/s or more and 35 ℃/s or less, but from the viewpoint of suppressing variation in low-temperature toughness.

In addition, the steel plate temperature at 0.25 mm below the steel plate surface and at the center of the plate thickness cannot be directly measured physically, but can be calculated by differential calculation using, for example, a process computer based on the surface temperature at the start of cooling and the surface temperature at the end of cooling, which are measured with a radiation thermometer, and the temperature distribution within the plate thickness cross section can be obtained in real time from the result. The temperature at 0.25 mm below the steel plate surface in the temperature distribution is referred to as “steel plate temperature at 0.25 mm below the steel plate surface” in this specification, and the temperature at the center of the plate thickness in the temperature distribution is referred to as “steel plate temperature at the center of the plate thickness” in this specification.

Cooling stop temperature: 350 to 550 ℃, the steel plate temperature at 0.25 mm below the steel plate surface and at the center of the plate thickness.

If the cooling stop temperature exceeds 550°C, the bainite transformation becomes incomplete and sufficient strength is not obtained. Therefore, the cooling stop temperature is set to 550°C or lower. On the other hand, if the cooling stop temperature is less than 350°C, the island martensite is not sufficiently tempered, so that the low-temperature toughness deteriorates. In addition, if the cooling stop temperature is less than 250°C, the SSCC resistance also deteriorates. Therefore, the cooling stop temperature is set to 350°C or higher, and preferably 400°C or higher.

[High strength steel pipe]

By forming the high-strength steel plate of the present invention into a tubular shape by press bend forming, roll forming, UOE forming, etc. and then welding the butt joints, it is possible to manufacture a high-strength steel pipe (UOE steel pipe, electrically welded steel pipe, spiral steel pipe, etc.) for use as a sour line pipe having excellent material uniformity within the steel plate, which is preferable for transporting crude oil or natural gas. In addition, by using the high-strength steel plate of the present invention in a steel pipe, it is possible to manufacture a steel pipe having excellent SSCC resistance even if a high-hardness region exists in the welded portion.

For example, a UOE steel pipe is manufactured by first processing the end of a steel plate, forming it into a pipe shape by a C press, U press, or O press, and then deep-welding the butt joint by internal and external welding, and further performing an expansion process as necessary. In addition, any welding method may be used as long as sufficient joint strength and joint toughness are obtained, but from the viewpoints of excellent weld quality and manufacturing efficiency, submerged arc welding is preferably used. In addition, expansion can also be performed on a steel pipe that has been formed into a tubular shape by press bend forming and then deep-welded at the butt joint.

Example

Steel (grades A to AE) having the component composition shown in Table 1 was made into a slab by a continuous casting method, and heated, hot rolled, and controlled cooled under the conditions shown in Table 2 to obtain a steel plate. Thereafter, the end portion of the steel plate was subjected to improvement processing, and formed into a steel pipe shape by a C press, a U press, and an O press, and then the butt joint was deep-welded from the inner and outer surfaces by submerged arc welding, and through an expansion process, it was made into a steel pipe.

[Calculation of specific and area ratio of the organization and calculation of maximum and average crystal grain size]

According to the above, samples for metal structure observation were collected from the central part of the length of the steel plate and the central part of the width of the plate. After the cross-section perpendicular to the width direction of this sample was mirror-polished and etched with colloidal silica, crystal data were collected by the EBSD (Electron Backscatter Diffraction) method in a 1 mm × 1 mm field of view at positions 0.25 mm below the surface of the steel plate and at the center of the plate thickness (at a position of 1/2 of the total thickness of the plate) (measurement step: 0.8 ㎛). After data collection, OIM-Analysis and image processing software (ImageJ) were used to specify the structure at 0.25 mm below the surface of the steel plate and the structure at the center of the plate thickness, and the area ratio of each phase was calculated, and the maximum crystal grain size and the average crystal grain size at the center of the plate thickness were calculated. In addition, the crystal grain size was defined as the equivalent circle diameter. The results of these measurements are shown in Table 3.

[Derivation of brittle-ductile transition temperature]

Charpy impact test specimens were collected from the 1/2 thickness position so that the longitudinal direction of the specimen coincided with the width direction of the plate, and subjected to a Charpy impact test. The brittle-ductile transition temperature was derived based on the method described in “New mathematical expression of Charpy absorbed energy transition curve of steel and fracture toughness evaluation (Journal of the Japanese Society of Materials and Strength 17 1 - 13, 1982).” The results are shown in Table 3.

[Measurement of tensile strength and yield strength]

Full thickness tensile test specimens were collected so that the length direction of the test specimens coincided with the width direction of the plate, and a tensile test was performed to measure the tensile strength and yield strength. The results are shown in Table 3.

[My SSCC rating]

As shown in Fig. 1, a test piece (coupon) cut from the obtained steel pipe was flattened, and then a 5 × 15 × 115 mm SSCC test piece was collected from the inner surface of the steel pipe. At this time, in addition to a test piece of only the base material not including the weld, a test piece including both the weld and the base material was collected. The inner surface, which is the surface to be inspected, was left with a black skin formed in order to leave the state of the outermost layer. That is, 0.25 mm below the surface of the steel plate is included in the test piece. A stress of 90% of the actual yield strength (0.5%YS) of each steel pipe was applied to the SSCC test pieces collected in this manner, and a 4-point bending SSCC test was performed in accordance with the EFC16 standard using NACE standard TM0177 Solution A solution at a hydrogen sulfide partial pressure of 1 bar.

In addition, a 4-point bending SSCC test was conducted in accordance with the EFC16 standard using the same NACE standard TM0177 Solution B solution at hydrogen sulfide partial pressure: 0.1 bar + carbon dioxide partial pressure: 0.9 bar.

In addition, a 4-point bending SSCC test was performed in accordance with the EFC16 standard using NACE standard TM0177 Solution A solution at hydrogen sulfide partial pressure: 16 bar + carbon dioxide partial pressure: 5 bar.

After immersing the test pieces in the solution for 720 hours, if no cracks were observed in either the test piece of only the base material without the weld or the test piece including both the weld and the base material, the SSCC resistance was judged to be good and given an ○ rating. If cracks occurred in at least one test piece, the SSCC resistance was judged to be poor and given an × rating. The evaluation results are shown in Table 3.

[My HIC rating]

My HIC resistance was investigated by a HIC test using NACE standard TM0177 Solution A solution, at a hydrogen sulfide partial pressure of 1 bar, and 96 hours of immersion. In addition, I investigated by a HIC test using NACE standard TM0177 Solution B solution, at a hydrogen sulfide partial pressure of 0.1 bar + carbon dioxide partial pressure of 0.9 bar, and 96 hours of immersion. When the crack area ratio (CAR) in the HIC test was 5% or less, the HIC resistance was judged to be good and given an ○ rating, and when it exceeded 5%, the HIC resistance was judged to be poor and given an × rating. The evaluation results are shown in Table 3.

The target scope of the present invention is a high-strength steel plate for a sour line pipe, which has a brittle-ductile transition temperature of -100°C or lower, a tensile strength of 535 MPa or higher, a steel structure at a position 0.25 mm below the surface of the steel plate composed of granular bainite and tempered island martensite, a maximum grain size at the center of the plate thickness of 80 μm or lower, an average grain size of 20 μm or lower, no cracks observed in an SSCC test, and a crack area ratio (CAR) of 5% or lower in an HIC test.

As shown in Tables 2 and 3, Nos. 1 to 10 and 39 to 42 are invention examples whose component compositions and manufacturing conditions satisfy the appropriate ranges of the present invention. All of them, as steel plates, had a brittle-ductile transition temperature of -100°C or less, a tensile strength of 535 MPa or more, a steel structure at a depth of 0.25 mm below the steel plate surface composed of granular bainite and tempered island martensite, a maximum grain size at the center of the plate thickness of 80 μm or less, an average grain size of 20 μm or less, and good SSCC resistance and HIC resistance.

In this regard, Nos. 11 to 25, 37 to 38 have a composition of steel plates outside the scope of the present invention. Nos. 11, 14, and 37 had insufficient solid solution strengthening, so that the strength was insufficient. Nos. 12, 15 to 16, 19, and 21 had poor SSCC resistance and HIC resistance because the hardness increased. Nos. 12 and 21 had a large increase in strength at low temperatures, and the low-temperature toughness deteriorated. No. 16 had a low degree of cleanliness, and the low-temperature toughness deteriorated. Nos. 17, 20, 22, and 38 had poor HIC resistance because inclusions or carbides were generated. Nos. 13 and 18 had poor low-temperature toughness due to an increase in the non-thermal stress of the steel. No.23 to 25 showed deterioration in SSCC resistance at 0.1 bar - _H2S + 0.9 bar - _CO2 because local corrosion was promoted.

No. 26 to 36 are comparative examples whose component compositions are within the scope of the present invention, but whose manufacturing conditions are outside the scope of the present invention. No. 26 had low strength because the slab heating temperature was low, so that carbide solid solution was insufficient. No. 27 had coarsened grains and the low-temperature toughness deteriorated because the slab heating temperature was high. No. 28 had insufficient total reduction ratio in the recrystallization temperature range, so that coarse grains remained, and the low-temperature toughness deteriorated. No. 29 had an excessive total reduction ratio in the recrystallization temperature range, so that the average grain size was large, and the low-temperature toughness deteriorated. No. 30 had insufficient reduction ratio in the final pass in the recrystallization temperature range, so that coarse grains remained, and the low-temperature toughness deteriorated. No. 31 had low strength and deteriorated SSCC resistance at high pressure because the cooling start temperature was low and some ferrite was formed in the surface layer. No. 32 had low strength and poor HIC resistance and high-pressure SSCC resistance because the average cooling rate was low and some ferrite was formed up to the center of the plate thickness. No. 33 had poor SSCC resistance at high pressure because the average cooling rate was high and some lath bainite was formed on the surface. No. 34 had poor low-temperature toughness because the cooling-stop temperature was low and the island martensite was not sufficiently tempered. No. 35 had poor high-pressure SSCC resistance in addition to low-temperature toughness because the cooling-stop temperature was even lower. No. 36 had low strength and poor HIC resistance and high-pressure SSCC resistance because the cooling-stop temperature was high and some ferrite was formed up to the center of the plate thickness.

Industrial applicability

According to the present invention, it is possible to provide a high-strength steel plate and a high-strength steel pipe for a sour line pipe having excellent low-temperature toughness as well as HIC resistance and SSCC resistance.

Claims (8)

A composition having a mass% composition of C: 0.030 to 0.060%, Si: 0.01 to 0.50%, Mn: 0.80 to 1.80%, P: 0.015% or less, S: 0.0015% or less, Al: 0.010 to 0.080%, Cr: 0.05 to 0.50%, Nb: 0.005 to 0.080%, N: 0.0010 to 0.0080%, and Ca: 0.0005 to 0.0050%, with the remainder being Fe and inevitable impurities.
The steel structure at 0.25 mm below the surface of the steel plate is composed of granular bainite and tempered island martensite.
The maximum crystal grain size at the center of the plate thickness is 80 ㎛ or less, and the average crystal grain size is 20 ㎛ or less.
The brittle-ductile transition temperature in the Charpy impact test is -100 ℃ or less,
High-strength steel plate for internal sour line pipe characterized by a tensile strength of 535 MPa or more. In the first paragraph,
A high-strength steel plate for a sour line pipe, wherein the above component composition further contains at least one selected from the group consisting of Cu: 0.30% or less, Ni: 0.10% or less, and Mo: 0.50% or less in mass%. In claim 1 or 2,
A high-strength steel plate for a sour line pipe, wherein the above component composition further contains, in mass%, at least one selected from the group consisting of V: 0.005 to 0.1%, Ti: 0.005 to 0.1%, Zr: 0.0005 to 0.02%, Mg: 0.0005 to 0.02%, and REM: 0.0005 to 0.02%. A steel slab having a composition containing, in mass%, C: 0.030 to 0.060%, Si: 0.01 to 0.50%, Mn: 0.80 to 1.80%, P: 0.015% or less, S: 0.0015% or less, Al: 0.010 to 0.080%, Cr: 0.05 to 0.50%, Nb: 0.005 to 0.080%, N: 0.0010 to 0.0080%, and Ca: 0.0005 to 0.0050%, with the remainder being Fe and unavoidable impurities, is heated to a temperature of 1000 to 1250°C,
After that, hot rolling is performed on the above steel sheet to make a steel plate, satisfying the total reduction ratio in the recrystallization temperature range: 35% or more and 55% or less, and the reduction ratio of the final rolling pass in the recrystallization temperature range: 10% or more.
After that, for the above steel plate,
Steel plate surface temperature at the start of cooling: Ar ₃ point or higher,
Average cooling rate from 750 ℃ to 550 ℃ at 0.25 mm below the steel plate surface: 15 to 35 ℃/s
Average cooling rate from 750 ℃ to 550 ℃ at the center of plate thickness: 15 ℃/s or more,
Cooling stop temperature: 350 to 550 ℃ at the steel plate temperature at 0.25 mm below the steel plate surface and at the center of the plate thickness
A method for manufacturing high-strength steel plates for internal sour line pipes, characterized in that controlled cooling is performed under the conditions of . In paragraph 4,
A method for manufacturing a high-strength steel plate for a sour line pipe, wherein the above component composition further contains, in mass%, at least one selected from the group consisting of Cu: 0.30% or less, Ni: 0.10% or less, and Mo: 0.50% or less. In clause 4 or 5,
A method for manufacturing a high-strength steel plate for a sour line pipe, wherein the above component composition further contains, in mass%, at least one selected from the group consisting of V: 0.005 to 0.1%, Ti: 0.005 to 0.1%, Zr: 0.0005 to 0.02%, Mg: 0.0005 to 0.02%, and REM: 0.0005 to 0.02%. High-strength steel pipe using high-strength steel plate for internal sour line pipe as described in Article 1 or 2. High-strength steel pipe using high-strength steel plate for internal sour line pipe described in Article 3.

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