KR102497360B1 - High strength steel plate for sour-resistant line pipe and method for manufacturing same, and high strength steel pipe using high strength steel plate for sour-resistant line pipe - Google Patents

Abstract

The present invention provides a high-strength steel sheet for sour line pipe that is excellent in not only HIC resistance but also SSCC resistance in a more severe corrosive environment and SSCC resistance in an environment with a low hydrogen sulfide partial pressure of less than 1 bar. The high-strength steel sheet for sour-resistant line pipe of the present invention contains a predetermined amount of C, Si, Mn, P, S, Al, Mo and Ca, and further contains a predetermined amount of at least one selected from Nb and Ti, The balance has a component composition consisting of Fe and unavoidable impurities, and the steel structure at 0.25 mm below the surface of the steel sheet is a bainite structure with a dislocation density of 1.0 × 10 ¹⁴ to 7.0 × 10 ¹⁴ (m ^-2 ). It is characterized in that the variation of the Vickers hardness in 0.25 mm is 30 HV or less at 3σ when the standard deviation is σ, and has a tensile strength of 520 MPa or more.

Description

High-strength steel plate for sour-resistant line pipe, manufacturing method thereof, and high-strength steel pipe using high-strength steel plate for sour-resistant line pipe FOR SOUR-RESISTANT LINE PIPE}

The present invention relates to a high-strength steel plate for sour-resistant line pipe with excellent material uniformity within the steel plate, which is desirable for use in line pipe in the fields of architecture, marine structures, shipbuilding, civil engineering, and construction industrial machinery, and a method for manufacturing the same. it's about Further, the present invention relates to a high-strength steel pipe using the above high-strength steel sheet for sour-resistant line pipe.

Generally, a line pipe is manufactured by forming a steel plate manufactured by a thick plate mill or a hot rolling mill into a steel pipe by UOE forming, press bend forming, roll forming, or the like.

Here, the line pipe used for transporting crude oil or natural gas containing hydrogen sulfide has, in addition to strength, toughness, weldability, etc., hydrogen induced cracking resistance (HIC (Hydrogen Induced Cracking) resistance) and sulfide stress corrosion cracking resistance ( SSCC (Sulfide Stress Corrosion Cracking) resistance), so-called sour resistance is required. Among them, in HIC, hydrogen ions due to a corrosion reaction are adsorbed on the surface of steel materials, penetrate into the steel as atomic hydrogen, diffuse and accumulate around non-metallic inclusions such as MnS in steel or hard second-phase structures, It becomes hydrogen in the phase and cracks are generated by its internal pressure, which is regarded as a problem in line pipes having a relatively low strength level with respect to oil well pipes, and many countermeasure techniques have been disclosed. On the other hand, SSCC is generally known to occur in high-strength seamless steel pipes for oil wells and in the high hardness region of welded parts, and has not been a problem in relatively low-hardness line pipes. However, in recent years, the harshness of crude oil and natural gas extraction environments has increased, and it has been reported that SSCC also occurs in the parent material portion of the line pipe in an environment where the hydrogen sulfide partial pressure is high or the pH is low. The importance of controlling the hardness and improving the SSCC resistance under a more severe corrosive environment has been pointed out. In addition, in an environment with a relatively low partial pressure of hydrogen sulfide, microcracks called Fischer may occur, and SSCC may occur.

Conventionally, when manufacturing high-strength steel sheets for line pipes, a so-called TMCP (Thermo-Mechanical Control Process) technology combining controlled rolling and controlled cooling is applied. In order to increase the strength of steel materials using this TMCP technique, it is effective to increase the cooling rate during controlled cooling. However, when controlled cooling is performed at a high cooling rate, since the surface layer portion of the steel sheet is rapidly cooled, the hardness of the surface layer portion is higher than that of the inside of the steel sheet, causing variations in the hardness distribution in the thickness direction. Therefore, it becomes a problem from the viewpoint of ensuring material uniformity within the steel sheet.

In order to solve the above problem, for example, in Patent Documents 1 and 2, after rolling, before the surface layer portion completes the bainite transformation, controlled cooling at a high cooling rate that reheats the surface is performed, in the plate thickness direction A method for manufacturing a steel sheet having a small difference in material quality is disclosed. Further, Patent Literatures 3 and 4 disclose a method for manufacturing a steel sheet for line pipe in which the hardness of the surface layer portion is reduced by heating the surface of the steel sheet after accelerated cooling to a higher temperature than the inside using a high-frequency induction heating device.

On the other hand, when there is non-uniformity in the scale thickness on the surface of the steel sheet, variations also occur in the cooling rate of the lower steel sheet during cooling, resulting in a problem of local variation in the cooling stop temperature within the steel sheet. As a result, variations in the material of the steel sheet occur in the sheet width direction due to the non-uniformity of the scale thickness. In contrast, Patent Literatures 5 and 6 disclose a method of improving the shape of a steel sheet by reducing unevenness in cooling caused by unevenness in scale thickness by performing descaling immediately before cooling.

Japanese Patent Publication No. 3951428 Japanese Patent Publication No. 3951429 Japanese Unexamined Patent Publication No. 2002-327212 Japanese Patent Publication No. 3711896 Japanese Unexamined Patent Publication No. 9-57327 Japanese Patent Publication No. 3796133

However, according to the examination of the present inventors, it has been found that there is room for improvement in the high-strength steel sheet obtained by the manufacturing method described in Patent Documents 1 to 6 above from the viewpoint of SSCC resistance in a more severe corrosion environment. As for the reason, the following can be considered.

In the manufacturing methods described in Patent Literatures 1 and 2, if the transformation behavior is different depending on the components of the steel sheet, the effect of sufficient material homogenization by reheating may not be obtained. In addition, when the structure in the surface layer of the steel sheet obtained by the manufacturing method described in Patent Documents 1 and 2 is a biphasic structure such as a ferrite bainite two-phase structure, in the low-load micro-Vickers test, an indenter presses a certain structure Depending on the type of test, there is a large variation in hardness values.

In the production methods described in Patent Literatures 3 and 4, since the cooling rate of the surface layer portion in accelerated cooling is high, there are cases where the hardness of the surface layer portion cannot be sufficiently reduced only by heating the surface of the steel sheet.

On the other hand, in the methods described in Patent Literatures 5 and 6, descaling improves the shape of the steel sheet by reducing surface quality defects due to indentation marks of the scale during hot proofing and reducing the variation in the cooling stop temperature of the steel sheet. , no consideration is given to the cooling conditions to obtain a uniform material. This is because variation occurs in the hardness of the steel sheet if there is variation in the cooling rate on the surface of the steel sheet. That is, when the cooling rate is slow, "film boiling" in which a film of bubbles is generated between the surface of the steel sheet and the cooling water when the surface of the steel sheet is cooled, and "nucleate boiling" in which bubbles are separated from the surface by the cooling water before forming a film are simultaneously This causes variations in the cooling rate of the steel sheet surface. As a result, variation occurs in the hardness of the steel sheet surface. In the techniques described in Patent Literatures 5 and 6, this point is not considered.

In addition, in Patent Documents 1 to 6, the conditions for avoiding Fischer-like microcracks in an environment with a relatively low hydrogen sulfide partial pressure are not clear.

Therefore, in view of the above problems, the present invention provides a high-strength steel sheet for sour-resistant line pipe, which is excellent not only in HIC resistance, but also in SSCC resistance in a more severe corrosive environment and SSCC resistance in an environment with a low hydrogen sulfide partial pressure of less than 1 bar. , It aims to provide together with the advantageous manufacturing method. Another object of the present invention is to propose a high-strength steel pipe using the high-strength steel plate for sour-resistant line pipe.

The inventors of the present invention, in order to secure SSCC resistance in a more severe corrosive environment, repeated numerous experiments and examinations on the component composition, microstructure and manufacturing conditions of steel materials. As a result, in order to further improve the SSCC resistance of high-strength steel pipes, it is not sufficient to simply suppress the hardness of the surface layer as in the conventional knowledge, and in particular, the structure of the extreme surface layer portion of the steel sheet, specifically, the steel structure of 0.25 mm below the surface of the steel sheet By making the bainite structure with a dislocation density of 1.0 × 10 ¹⁴ to 7.0 × 10 ¹⁴ (m ^-2 ), the increase in hardness can be suppressed in the coating process after pipe forming, and as a result, the inside of the steel pipe It was found that the SSCC property was improved. In addition, in order to realize such a steel structure, it is necessary to strictly control the cooling rate in 0.25 mm below the surface of the steel plate, and the conditions have been found. In addition, in an environment with a high hydrogen sulfide partial pressure of more than 1 bar, the addition of Mo is effective in suppressing the occurrence of early cracks, and in an environment with a low hydrogen sulfide partial pressure of less than 1 bar, suppressing the addition of Ni is effective in avoiding microcracks such as Fischer. I found something valid. The present invention is made based on this knowledge.

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

[1] In terms of mass%, C: 0.02 to 0.08%, Si: 0.01 to 0.50%, Mn: 0.50 to 1.80%, P: 0.001 to 0.015%, S: 0.0002 to 0.0015%, Al: 0.01 to 0.08%, Mo : 0.01 to 0.50% and Ca: 0.0005 to 0.005%, and further contains at least one selected from Nb: 0.005 to 0.1% and Ti: 0.005 to 0.1%, the remainder being Fe and unavoidable impurities. has an ingredient composition,

The steel structure at 0.25 mm below the surface of the steel sheet is a bainite structure with a dislocation density of 1.0 × 10 ¹⁴ to 7.0 × 10 ¹⁴ (m ^-2 );

Variation of the Vickers hardness in 0.25 mm below the surface of the steel sheet is 30 HV or less at 3σ when the standard deviation is σ,

A high-strength steel sheet for sour-resistant line pipe, characterized in that it has a tensile strength of 520 MPa or more.

[2] The sour-resistant line pipe according to [1] above, wherein the component composition further contains, in mass%, at least one selected from among Cu: 0.50% or less, Ni: 0.10% or less, and Cr: 0.50% or less. high-strength steel sheet.

[3] The above component composition further contains, in mass%, at least one selected from V: 0.005 to 0.1%, Zr: 0.0005 to 0.02%, Mg: 0.0005 to 0.02%, and REM: 0.0005 to 0.02%, The high-strength steel sheet for sour-resistant line pipe according to [1] or [2] above.

[4] In terms of mass%, C: 0.02 to 0.08%, Si: 0.01 to 0.50%, Mn: 0.50 to 1.80%, P: 0.001 to 0.015%, S: 0.0002 to 0.0015%, Al: 0.01 to 0.08%, Mo : 0.01 to 0.50% and Ca: 0.0005 to 0.005%, and further contains at least one selected from Nb: 0.005 to 0.1% and Ti: 0.005 to 0.1%, the remainder being Fe and unavoidable impurity components After heating a steel piece having the composition at a temperature of 1000 to 1300 ° C., hot rolling to obtain a steel sheet,

Then, for the steel plate,

Steel plate surface temperature at the start of cooling: (Ar ₃ - 10 ° C) or more,

Average cooling rate from 750 ° C. to 550 ° C. at the steel plate temperature at 0.25 mm below the steel plate surface: 50 ° C./s or less;

Average cooling rate from 750 ° C. to 550 ° C. at the steel sheet average temperature: 15 ° C./s or more;

Average cooling rate from 550 ° C. to the temperature at which cooling is stopped at the steel plate temperature at 0.25 mm below the steel plate surface: 150 ° C./s or more, and

Cooling stop temperature at steel sheet average temperature: 250 to 550 °C

Method for producing a high-strength steel sheet for sour-resistant line pipe, characterized in that for performing controlled cooling under the conditions of.

[5] The sour-resistant line pipe according to [4] above, wherein the component composition further contains, in mass%, at least one selected from among Cu: 0.50% or less, Ni: 0.10% or less, and Cr: 0.50% or less. Manufacturing method of high-strength steel sheet for use.

[6] The above component composition further contains, in mass%, at least one selected from V: 0.005 to 0.1%, Zr: 0.0005 to 0.02%, Mg: 0.0005 to 0.02%, and REM: 0.0005 to 0.02%, The method for producing a high-strength steel sheet for sour-resistant line pipe according to the above [4] or [5].

[7] A high-strength steel pipe using the high-strength steel plate for sour-resistant line pipe according to any one of [1] to [3] above.

The high-strength steel plate for sour-resistant line pipe of the present invention and the high-strength steel pipe using the high-strength steel plate for sour-resistant line pipe have not only HIC resistance, but also SSCC resistance in a more severe corrosion environment and an environment with a low hydrogen sulfide partial pressure of less than 1 bar. My SSCC is also excellent. In addition, according to the manufacturing method of the high-strength steel sheet for sour line pipe of the present invention, not only the HIC resistance, but also the SSCC resistance in a more severe corrosive environment and the SSCC resistance in an environment with a low hydrogen sulfide partial pressure of less than 1 bar are also excellent. High-strength steel plates for sour line pipes can be manufactured.

BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic diagram explaining the sampling method of the test piece for evaluation of SSCC resistance in an Example.

Hereinafter, the high-strength steel sheet for sour-resistant line pipe of the present disclosure will be described in detail.

[Ingredient Composition]

First, the component composition of the high-strength steel sheet according to the present disclosure and the reason for its limitation will be described. All units represented by % in the following description are mass %.

C: 0.02 to 0.08%

C effectively contributes to the improvement of strength, but if the content is less than 0.02%, sufficient strength cannot be secured. On the other hand, if it exceeds 0.08%, the hardness of the surface layer portion or center segregation portion increases during accelerated cooling. SSCC resistance and HIC resistance are deteriorated. Moreover, toughness also deteriorates. For this reason, the amount of C is limited to the range of 0.02 to 0.08%.

Si: 0.01 to 0.50%

Si is added for deoxidation, but when the content is less than 0.01%, the deoxidation effect is not sufficient, while when the content exceeds 0.50%, toughness and weldability deteriorate, so the amount of Si is limited to the range of 0.01 to 0.50%.

Mn: 0.50 to 1.80%

Mn effectively contributes to improvement of strength and toughness, but when the content is less than 0.50%, the effect of adding Mn is insufficient. SSCC resistance and HIC resistance are deteriorated. Moreover, weldability also deteriorates. For this reason, the amount of Mn is limited to the range of 0.50 to 1.80%.

P: 0.001 to 0.015%

P, as an unavoidable impurity element, deteriorates weldability and HIC resistance by increasing the hardness of the central segregation portion. Since the tendency becomes remarkable when it exceeds 0.015 %, the upper limit is prescribed|regulated as 0.015 %. Preferably it is 0.008% or less. The lower the content, the better, but from the viewpoint of the refining cost, it is set to 0.001% or more.

S: 0.0002 to 0.0015%

S is an unavoidable impurity element, and since it becomes MnS inclusions in steel and deteriorates HIC resistance, a small amount is preferable, but up to 0.0015% is acceptable. The lower the content, the better, but from the viewpoint of refining cost, it is set to 0.0002% or more.

Al: 0.01 to 0.08%

Al is added as a deoxidizer, but if it is less than 0.01%, there is no addition effect. On the other hand, if it exceeds 0.08%, the cleanliness of the steel decreases and the toughness deteriorates, so the amount of Al is limited to the range of 0.01 to 0.08%.

Mo: 0.01 to 0.50%

Mo is an element effective for improving toughness and increasing strength, and is an element effective for improving SSCC resistance regardless of the hydrogen sulfide partial pressure. In order to obtain this effect, it is necessary to contain 0.01% or more, and it is preferable to contain 0.10% or more. On the other hand, if the content is too large, the quenching property becomes excessive, so the dislocation density described later increases and the SSCC resistance deteriorates. Moreover, weldability also deteriorates. For this reason, the amount of Mo is 0.50% or less, preferably 0.40% or less.

Ca: 0.0005 to 0.005%

Ca is an element effective for improving HIC resistance by morphological control of sulfide-based inclusions, but its addition effect is not sufficient if it is less than 0.0005%. On the other hand, when it exceeds 0.005%, not only the effect is saturated, but also the HIC resistance is deteriorated due to a decrease in the cleanliness of the steel, so the amount of Ca is limited to the range of 0.0005 to 0.005%.

At least one selected from Nb: 0.005 to 0.1% and Ti: 0.005 to 0.1%

Both Nb and Ti are effective elements for increasing the strength and toughness of a steel sheet. When the content of each element is less than 0.005%, the effect of addition is insufficient, while when the content exceeds 0.1%, the toughness of the weld zone deteriorates. Therefore, at least one of Nb and Ti is added in the range of 0.005% to 0.1%, respectively.

Although the basic components of the present disclosure have been described above, the component composition of the present disclosure may optionally contain at least one selected from among Cu, Ni, and Cr within the following ranges in order to further improve the strength and toughness of the steel sheet. there is.

Cu: 0.50% or less

Cu is an element effective for improvement of toughness and increase of strength, and in order to obtain this effect, it is desirable to contain 0.05% or more. However, if the content is too large, weldability deteriorates, so when adding Cu, the upper limit is 0.50%. to be

Ni: 0.10% or less

Ni is an element effective for improving toughness and increasing strength, and in order to obtain this effect, it is preferable to contain 0.01% or more. In order to make it easy to produce the microcracks called Fischer, when adding Ni, 0.10 % is made into an upper limit. Preferably, it is 0.02% or less.

Cr: 0.50% or less

Cr, like Mn, is an effective element for obtaining sufficient strength even at low C. To obtain this effect, it is desirable to contain 0.05% or more of Cr. Dislocation density becomes high, and SSCC resistance deteriorates. Moreover, weldability also deteriorates. For this reason, when adding Cr, 0.50% is made into an upper limit.

The component composition of the present disclosure may further optionally contain at least one selected from among V, Zr, Mg, and REM within the following ranges.

At least one selected from V: 0.005 to 0.1%, Zr: 0.0005 to 0.02%, Mg: 0.0005 to 0.02%, and REM: 0.0005 to 0.02%

V is an element that can be added arbitrarily to increase the strength and toughness of the steel sheet. When the content is less than 0.005%, the addition effect is insufficient, and when the content exceeds 0.1%, the toughness of the weld zone deteriorates. Zr, Mg, and REM are elements that can be arbitrarily added to increase toughness through crystal grain refinement or increase crack resistance through control of inclusion properties. All of these elements have insufficient addition effects when the content is less than 0.0005%, while the effect is saturated when the content exceeds 0.02%, so when adding all of these elements, it is preferable to set them as the range of 0.0005 to 0.02%.

The present disclosure discloses a technique for improving the SSCC resistance of a high-strength steel pipe using a high-strength steel plate for sour-resistant line pipe, but as the sour resistance performance, needless to say, it is necessary to simultaneously satisfy the HIC resistance, For example, it is preferable to make the CP value calculated|required by following Formula (1) into 1.00 or less. In addition, what is necessary is just to substitute 0 for the element which is not added.

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] … (One)

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

Here, the CP value is a formula devised to estimate the material of the central segregation part from the content of each alloy element. The hardness of the segregation part rises. Therefore, by setting the CP value obtained in the above equation (1) to 1.00 or less, it becomes possible to suppress crack generation in the HIC test. In addition, since the hardness of the central segregation portion decreases as the CP value decreases, the upper limit may be set to 0.95 when higher HIC resistance is required.

In addition, remainder other than the above elements consists of Fe and unavoidable impurities. However, the inclusion of other trace elements is not hindered as long as the effect of the present invention is not impaired. For example, N is an element unavoidably contained in steel, but if the content is 0.007% or less, preferably 0.006% or less, it is acceptable in the present invention.

[Organization of Steel Plate]

Next, the steel structure of the high-strength steel sheet for sour-resistant line pipe of the present disclosure will be described. In order to achieve high strength with a tensile strength of 520 MPa or more, the steel structure needs to be a bainite structure. In particular, when a hard phase such as martensite or island martensite (MA) is formed in the surface layer portion, the hardness of the surface layer increases, the variation in hardness within the steel sheet increases, and material uniformity is impaired. In order to suppress the increase in surface layer hardness, the steel structure of the surface layer portion is set as a bainite structure. Sites other than the surface layer portion are also bainite structures, and the structure at the center of the plate thickness as representative of the site should just be a bainite structure. Here, the bainite structure includes a structure called bainitic ferrite or granular ferrite that transforms during or after accelerated cooling contributing to transformation strengthening. When heterogeneous structures such as ferrite, martensite, pearlite, island martensite, and retained austenite are mixed in the bainite structure, a decrease in strength, deterioration of toughness, and an increase in surface layer hardness occur, so that the bainite phase The smaller the tissue fraction other than that, the better. However, when the volume fraction of structures other than the bainite phase is sufficiently low, their influence can be ignored, so any amount is acceptable. Specifically, in the present disclosure, if the total of steel structures other than bainite (ferrite, martensite, pearlite, island martensite, retained austenite, etc.) is less than 5% in terms of volume fraction, there is no significant effect, so it is acceptable do.

In addition, the bainite structure also has various forms depending on the cooling rate, but in the present disclosure, the structure of the extreme surface layer portion of the steel sheet, specifically, the steel structure of 0.25 mm below the surface of the steel sheet, has a dislocation density of 1.0 × 10 ¹⁴ to 7.0 It is important to set it as a bainite structure of × 10 ¹⁴ (m ^-2 ). Since the dislocation density decreases in the coating process after pipe preparation, if the dislocation density at 0.25 mm below the surface of the steel sheet is 7.0 × 10 ¹⁴ (m ^-2 ) or less, the increase in hardness due to age hardening can be minimized. Conversely, when the dislocation density of 0.25 mm below the surface of the steel sheet exceeds 7.0 × 10 ¹⁴ (m ^-2 ), the dislocation density does not decrease in the coating process after pipe forming, and the hardness is greatly increased due to age hardening, and SSCC resistance is deteriorated. let it In order to obtain good SSCC resistance after pipe making, the range of dislocation density is preferably 6.0 × 10 ¹⁴ (m ^-2 ) or less. On the other hand, if the dislocation density at 0.25 mm below the surface of the steel sheet is less than 1.0 × 10 ¹⁴ (m ^-2 ), the strength of the steel sheet cannot be maintained. In order to secure the strength of the X65 grade, it is preferable to have a dislocation density of 2.0 × 10 ¹⁴ (m ^-2 ) or more. In addition, in the high-strength steel sheet of the present disclosure, when the dislocation density in the steel structure at 0.25 mm below the surface of the steel sheet is within the above range, the extreme surface layer portion in the range of 0.25 mm in depth from the surface of the steel sheet also has the same dislocation density, and as a result, the above The effect of improving SSCC resistance is obtained.

Further, when the dislocation density at 0.25 mm below the surface of the steel sheet is 7.0 × 10 ¹⁴ (m ^-2 ) or less, HV 0.1 at 0.25 mm below the surface becomes 230 or less. From the viewpoint of ensuring the SSCC resistance of the steel pipe, it is important to suppress the hardness of the surface layer of the steel plate, but by setting the HV 0.1 at 0.25 mm below the surface of the steel plate to 230 or less, the coating heat treatment process for 1 hour at 250 ° C. After roughening, HV 0.1 at 0.25 mm below the surface can be suppressed to 260 or less, and SSCC resistance can be secured.

In addition, in the high-strength steel sheet of the present disclosure, it is also important that the variation in Vickers hardness at 0.25 mm below the surface of the steel sheet is 30 HV or less at 3σ when σ is the standard deviation. When 3σ when measuring the Vickers hardness at 0.25 mm below the surface of the steel sheet is more than 30 HV, the hardness variation in the extreme surface layer of the steel sheet, that is, the local high hardness portion exists in the extreme surface layer, and the portion This is because the deterioration of my SSCC properties from the starting point occurs. In addition, when obtaining the standard deviation σ, it is preferable to measure the Vickers hardness at 100 points or more.

Since the high-strength steel sheet of the present disclosure is a steel sheet for steel pipes having strength equal to or higher than X60 grade of API 5L, it is assumed to have a tensile strength of 520 MPa or higher.

[Manufacturing method]

Hereinafter, a manufacturing method and manufacturing conditions for manufacturing the high-strength steel sheet for the sour-resistant line pipe will be described in detail. In the production method of the present disclosure, after heating a steel piece having the above component composition, hot rolling is performed to obtain a steel sheet, and then the steel sheet is subjected to controlled cooling under predetermined conditions.

[Slab heating temperature]

Slab heating temperature: 1000 ~ 1300 ℃

If the slab heating temperature is less than 1000°C, the required strength cannot be obtained due to insufficient solid solution of carbides, while if it exceeds 1300°C, toughness deteriorates, so the slab heating temperature is set to 1000 to 1300°C. In addition, this temperature is the temperature inside the furnace of a heating furnace, and it is assumed that the slab is heated to this temperature up to the center.

[Rolling end temperature]

In the hot rolling process, in order to obtain high base metal toughness, the lower the rolling end temperature, the better. On the other hand, since the rolling efficiency decreases, the rolling end temperature in the steel sheet surface temperature takes into account the required base metal toughness and rolling efficiency. you need to set it up. From the viewpoint of improving strength and HIC resistance, it is preferable to set the rolling end temperature to the Ar ₃ transformation point or higher at the surface temperature of the steel sheet. Here, the Ar ₃ transformation point means the ferrite transformation start temperature during cooling, and can be obtained, for example, from the components of the steel by the following formula. Further, in order to obtain high base metal toughness, it is preferable to set the reduction ratio to 60% or more in a temperature range of 950°C or less corresponding to the austenite non-recrystallization temperature range. In addition, the surface temperature of the steel sheet can be measured with a radiation thermometer or the like.

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

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

[Cooling Start Temperature of Controlled Cooling]

Cooling start temperature: (Ar ₃ - 10 ℃) or more from the steel plate surface temperature

If the steel sheet surface temperature at the start of cooling is low, the amount of ferrite produced before controlled cooling increases. In particular, if the temperature drop from the Ar ₃ transformation point exceeds 10°C, ferrite with a volume fraction of more than 5% is produced, resulting in a decrease in strength. Since HIC resistance deteriorates with increase, the steel sheet surface temperature at the start of cooling is set to (Ar ₃ - 10°C) or higher. In addition, the steel sheet surface temperature at the start of cooling is equal to or lower than the rolling end temperature.

[Cooling rate of controlled cooling]

It is important to control the cooling rate of the surface layer portion and the average cooling rate within the steel sheet in order to reduce variation in hardness within the steel sheet and improve material uniformity while achieving high strength. In particular, in order to set the dislocation density and 3σ at 0.25 mm below the steel plate surface to the ranges already described, it is necessary to control the cooling rate at 0.25 mm below the steel plate surface.

Average cooling rate from 750 ° C. to 550 ° C. at the steel plate temperature at 0.25 mm below the steel plate surface: 50 ° C./s or less

When the average cooling rate from 750 °C to 550 °C at the steel plate temperature in 0.25 mm below the steel plate surface exceeds 50 °C/s, the dislocation density in 0.25 mm below the steel plate surface exceeds 7.0 × 10 ¹⁴ (m ^-2 ) become As a result, the HV 0.1 of 0.25 mm below the surface of the steel plate exceeds 230, and after the coating process after pipe forming, the HV 0.1 of 0.25 mm below the surface exceeds 260, and the SSCC resistance of the steel pipe deteriorates. Therefore, the said average cooling rate is made into 50 degrees C/s or less. Preferably it is 45 degrees C/s or less, More preferably, it is 40 degrees C/s or less. The lower limit of the average cooling rate is not particularly limited, but if the cooling rate is excessively low, ferrite and pearlite are formed and the strength is insufficient.

Average cooling rate from 750 ° C. to 550 ° C. at the steel plate average temperature: 15 ° C./s or more

If the average cooling rate from 750°C to 550°C at the average temperature of the steel sheet is less than 15°C/s, the bainitic structure is not obtained, and strength and HIC resistance deteriorate. For this reason, the cooling rate at the steel sheet average temperature is set to 15°C/s or more. From the viewpoint of variations in strength and hardness of the steel sheet, the average cooling rate of the steel sheet is preferably 20°C/s or more. The upper limit of the average cooling rate is not particularly limited, but is preferably 80°C/s or less so that low-temperature transformation products are not excessively produced.

Average cooling rate from 550 ° C. to the temperature at which cooling is stopped from the steel plate temperature at 0.25 mm below the steel plate surface: 150 ° C./s or more

For cooling from the steel sheet temperature at 0.25 mm below the steel sheet surface to 550°C or less, cooling in a stable nucleate boiling state is required, and an increase in water density is indispensable. When the average cooling rate from 550 ° C. to the temperature at which cooling is stopped is less than 150 ° C./s from the steel plate temperature at 0.25 mm below the steel plate surface, cooling in the nucleate boiling state does not occur, and hardness deviation occurs at the extreme surface layer of the steel plate. As a result, 3σ at 0.25 mm below the surface of the steel sheet exceeds 30 HV, and as a result, SSCC resistance deteriorates. Therefore, the said average cooling rate is made into 150 degreeC/s or more. Preferably it is 170 degreeC/s or more. The upper limit of the average cooling rate is not particularly limited, but is preferably 250° C./s or less from facility constraints.

In addition, 0.25 mm below the surface of the steel sheet and the average temperature of the steel sheet cannot be directly measured physically, but based on the surface temperature at the start of cooling measured with a radiation thermometer and the surface temperature at the time of target cooling stop, for example, The temperature distribution in the plate thickness section can be obtained in real time by difference calculation using a process computer. The temperature at 0.25 mm below the surface of the steel sheet in the temperature distribution is referred to as "steel sheet temperature at 0.25 mm below the surface of the steel sheet" in this specification, and the average value of the temperature in the sheet thickness direction in the temperature distribution is Let it be "steel plate average temperature" in a specification.

[cooling stop temperature]

Cooling stop temperature: 250 ~ 550 ℃ at the steel plate average temperature

After completion of the rolling, the bainite phase is formed by quenching by controlled cooling to 250 to 550°C, which is the temperature range of bainite transformation. When the cooling stop temperature exceeds 550°C, bainite transformation is incomplete and sufficient strength cannot be obtained. In addition, when the cooling stop temperature is less than 250 ° C., the hardness of the surface layer portion increases significantly, and the dislocation density at 0.25 mm below the surface of the steel sheet exceeds 7.0 × 10 ¹⁴ (m ^-2 ), so SSCC resistance deteriorates. In addition, the hardness of the central segregation portion is also increased, and the HIC resistance is also deteriorated. Therefore, in order to suppress deterioration of material uniformity within the steel sheet, the cooling stop temperature of the controlled cooling is set to 250 to 550°C in terms of the steel sheet average temperature.

[High strength steel pipe]

After the high-strength steel sheet of the present disclosure is formed into a tubular shape by press-bending, roll forming, UOE forming, etc., and then welding the butted portion, high strength for sour-resistant line pipe with excellent material uniformity within the steel sheet suitable for transportation of crude oil or natural gas Steel pipes (UOE steel pipes, electric welded steel pipes, spiral steel pipes, etc.) can be manufactured.

For example, a UOE steel pipe is obtained by processing the end of a steel plate, forming it into a steel pipe shape by C press, U press, or O press, and then seam-welding the butt portion by inner surface welding and outer surface welding, and further as needed. It is manufactured through the expansion process. Further, any welding method may be used as long as sufficient joint strength and joint toughness are obtained, but from the viewpoint of excellent welding quality and manufacturing efficiency, it is preferable to use submerged arc welding.

Example

Steels (steel grades A to M) having the component compositions shown in Table 1 were made into slabs by the continuous casting method, heated to the temperatures shown in Table 2, and then hot-rolled at the rolling end temperatures and reduction ratios shown in Table 2, , it was set as the steel plate of the plate|board thickness shown in Table 2. Thereafter, the steel sheet was subjected to controlled cooling under the conditions shown in Table 2 using a water-cooled controlled cooling device.

[organization specific]

The microstructure of the obtained steel sheet was observed with an optical microscope and a scanning electron microscope. Table 2 shows the structure at a position 0.25 mm below the surface of the steel sheet and the structure at the center of the sheet thickness.

[Measurement of tensile strength]

A tensile test was conducted using a full-thickness test piece in a direction perpendicular to the rolling direction as a tensile test piece, and the tensile strength was measured. A result is shown in Table 2.

[Measurement of Vickers Hardness]

For a cross section perpendicular to the rolling direction, the Vickers hardness (HV 0.1) of 100 points was measured at a position 0.25 mm below the steel sheet surface in accordance with JIS Z 2244, and the average value and standard deviation σ were obtained. The average value and the value of 3σ are shown in Table 2. Here, the measurement was performed at HV 0.1 instead of the commonly used HV 10 because the indentation becomes smaller by measuring at HV 0.1, making it possible to obtain hardness information at a position closer to the surface and hardness information more sensitive to the microstructure. am.

[Dislocation Density]

A sample for X-ray diffraction was taken from a position having an average hardness, the sample surface was polished to remove scale, and an X-ray diffraction measurement was performed at a position 0.25 mm below the surface of the steel plate. The method of converting the dislocation density from the strain obtained from the half-value width β of the X-ray diffraction measurement was used. In the diffraction intensity curve obtained by normal X-ray diffraction, since two Kα1 rays and Kα2 rays having different wavelengths overlap, they are separated by Rachinger's method. For strain extraction, the Williamsson-Hall method shown below is used. The expansion of the half width is affected by the crystallite size D and strain ε, and can be calculated as the sum of both factors by the following equation. β = β1 + β2 = (0.9λ/(D × cosθ)) + 2ε × tanθ. Further, by transforming this expression, βcosθ/λ = 0.9λ/D + 2ε × sinθ/λ. By plotting βcosθ/λ against sinθ/λ, the strain ε is calculated from the slope of the straight line. In addition, the diffraction lines used for calculation are (110), (211), and (220). For the conversion of dislocation density from strain ε, ρ = 14.4ε ² /b ² was used. In addition, θ means a peak angle calculated from the θ-2θ method of X-ray diffraction, and λ means the wavelength of X-rays used in X-ray diffraction. b is a Burgers vector of Fe(α), and was set to 0.25 nm in this Example.

[Evaluation of my SSCC surname]

SSCC resistance was evaluated by preparing a sample of each of these steel sheets. The steel pipe was manufactured by subjecting the end of the steel plate to improvement processing, forming into a steel pipe shape by C press, U press, and O press, seam welding the abutted parts of the inner and outer surfaces by submerged arc welding, and then going through a pipe expansion process. As shown in Fig. 1, after flattening a coupon cut out from the obtained steel pipe, a 5 x 15 x 115 mm SSCC test piece was sampled from the inner surface of the steel pipe. At this time, the inner surface, which is the surface to be inspected, was kept covered with mill scale in order to leave the state of the outermost layer. A stress of 90% of the actual yield strength (0.5%YS) of each steel pipe was applied to the sampled SSCC test piece, and NACE standard TM0177 Solution A was used, hydrogen sulfide partial pressure: 1 bar, 4 points of EFC 16 standard It was carried out based on the bending SSCC test. In addition, using NACE standard TM0177 Solution B solution, hydrogen sulfide partial pressure: 0.1 bar + carbon dioxide partial pressure: 0.9 bar, it was carried out in accordance with the 4-point bending SSCC test of EFC 16 standard. In addition, NACE standard TM0177 Solution A was used, and hydrogen sulfide partial pressure: 2 bar + carbon dioxide partial pressure: 3 bar was also conducted in accordance with the EFC 16 standard 4-point bending SSCC test. After immersion for 720 hours, when no crack was observed, the SSCC resistance was judged to be good, and the case where cracks occurred was judged to be poor and rated as ×. A result is shown in Table 2.

[Evaluation of my HIC gender]

HIC resistance was investigated by the HIC test of immersion for 96 hours using NACE standard TM0177 Solution A solution, hydrogen sulfide partial pressure: 1 bar. Further, NACE standard TM0177 Solution B was used, and the HIC test was conducted by immersion for 96 hours at a partial pressure of hydrogen sulfide: 0.1 bar + partial pressure of carbon dioxide: 0.9 bar. For HIC resistance, a case where the crack length ratio (CLR) was 15% or less in the HIC test was judged to be good, and a case where the crack length ratio (CLR) exceeded 15% was judged as ○, and a case where it exceeded 15% was evaluated as ×. A result is shown in Table 2.

The target range of the present invention is a high-strength steel sheet for sour-resistant line pipe, tensile strength: 520 MPa or more, microstructure at both the subsurface 0.25 mm position and the t/2 position is a bainite structure, and the HV 0.1 at the subsurface 0.25 mm is 230 Hereinafter, in the high-strength steel pipe manufactured using the steel plate, it was assumed that no crack was observed in the SSCC test and that the crack length ratio (CLR) was 15% or less in the HIC test.

As shown in Table 2, No. 1 to No. 15 are invention examples in which the component composition and production conditions satisfy the appropriate range of the present invention. Tensile strength as a steel sheet: 520 MPa or more, the microstructure at both the subsurface 0.25 mm position and the t/2 position is a bainite structure, and the HV 0.1 at the subsurface 0.25 mm is 230 or less, and the steel plate is manufactured using the high strength SSCC resistance and HIC resistance were also good for steel pipes.

On the other hand, No. 16 to No. 23 are comparative examples in which the component composition is within the scope of the present invention, but the production conditions are outside the scope of the present invention. In No. 16, since the slab heating temperature was low, homogenization of the microstructure and solid solution of carbides were insufficient, and the strength was low. No. 17 had a low cooling start temperature and had a layered structure in which ferrite precipitated, so while having low strength, the HIC resistance after pipe making was deteriorated. In No. 18, the controlled cooling conditions were outside the scope of the present invention, and as a microstructure, a bainite structure was not obtained at the center of the plate thickness, and a ferrite + pearlite structure was obtained, so while being low in strength, HIC resistance after pipe forming was excellent. has deteriorated In No. 19, the cooling stop temperature was low, the dislocation density at 0.25 mm below the surface was high, and the HV 0.1 exceeded 230, so the SSCC resistance after pipe making was poor. In addition, since the hardness of the central segregation portion also increased, the HIC resistance also deteriorated. In No.20 and No.23, the average cooling rate from 750 ° C. to 550 ° C. at the steel plate temperature at 0.25 mm below the surface of the steel plate greatly exceeded 50 ° C./s, so the dislocation density at 0.25 mm below the surface increased, the HV 0.1 exceeded 230, and the SSCC resistance after pipe preparation was poor. Also, in No. 23, the HIC resistance in the surface layer portion was also deteriorated. In No.21 and No.22, since the average cooling rate at 550 ° C. or less at 0.25 mm below the steel plate surface does not reach 150 ° C./s, uneven cooling of the steel plate becomes remarkable, and the HV 0.1 is 230 or less on average was satisfied, but the hardness variation was large and a part with high hardness was generated locally, so the SSCC resistance after pipe preparation was poor. In No.24 to No.27, the component composition of the steel sheet was outside the scope of the present invention, the dislocation density at 0.25 mm below the surface was high, and the HV 0.1 exceeded 230, so the SSCC resistance after pipe forming was poor. Moreover, about No.24 - No.27, since the hardness of the central segregation part increased, HIC resistance was also inferior. No. 28 had poor SSCC resistance in an environment with a low hydrogen sulfide partial pressure because the amount of Ni in the steel sheet was excessive. In No. 29, since the steel sheet did not contain Mo, SSCC resistance deteriorated under a very severe corrosive environment of hydrogen sulfide partial pressure of 2 Bar. In No. 30, the average cooling rate from 750 ° C. to 550 ° C. at the steel plate temperature at 0.25 mm below the steel plate surface exceeded 50 ° C./s, so SSCC resistance was excellent under a very severe corrosion environment of hydrogen sulfide partial pressure of 2 Bar. has deteriorated

According to the present invention, a high-strength steel sheet for sour line pipe having excellent SSCC resistance in a more severe corrosive environment and SSCC resistance in an environment with a low hydrogen sulfide partial pressure of less than 1 bar as well as HIC resistance can be supplied. Therefore, steel pipes (electrically welded steel pipes, spiral steel pipes, UOE steel pipes, etc.) manufactured by cold forming this steel plate can be suitably used for transportation of hydrogen sulfide-containing crude oil or natural gas requiring sour resistance.

Claims (8)

In terms of mass%, C: 0.02 to 0.08%, Si: 0.01 to 0.50%, Mn: 0.50 to 1.80%, P: 0.001 to 0.015%, S: 0.0002 to 0.0015%, Al: 0.01 to 0.08%, Mo: 0.01 to 0.01% 0.50% and Ca: 0.0005 to 0.005%, and further contains at least one selected from Nb: 0.005 to 0.1% and Ti: 0.005 to 0.1%, the remainder being Fe and unavoidable impurities. ,
The steel structure at the center of the plate thickness is a bainite structure, and the steel structure at 0.25 mm below the surface of the steel sheet is a bainite structure with a dislocation density of 1.0 × 10 ¹⁴ to 7.0 × 10 ¹⁴ (m ^-2 );
Variation of the Vickers hardness in 0.25 mm below the surface of the steel sheet is 30 HV or less at 3σ when the standard deviation is σ,
A high-strength steel sheet for sour-resistant line pipe, characterized in that it has a tensile strength of 520 MPa or more. According to claim 1,
A high-strength steel sheet for sour-resistant line pipe, wherein the component composition further contains, in terms of mass%, at least one selected from among Cu: 0.50% or less, Ni: 0.10% or less, and Cr: 0.50% or less. According to claim 1 or 2,
Sour-resistant line containing at least one selected from V: 0.005 to 0.1%, Zr: 0.0005 to 0.02%, Mg: 0.0005 to 0.02%, and REM: 0.0005 to 0.02% in terms of mass%. High-strength steel sheet for pipes. In terms of mass%, C: 0.02 to 0.08%, Si: 0.01 to 0.50%, Mn: 0.50 to 1.80%, P: 0.001 to 0.015%, S: 0.0002 to 0.0015%, Al: 0.01 to 0.08%, Mo: 0.01 to 0.01% A steel slab containing 0.50% and Ca: 0.0005 to 0.005%, further containing at least one selected from Nb: 0.005 to 0.1% and Ti: 0.005 to 0.1%, the remainder having a composition of Fe and unavoidable impurities. After heating to a temperature of 1000 to 1300 ° C., hot rolling to obtain a steel sheet,
Then, for the steel plate,
Steel plate surface temperature at the start of cooling: (Ar ₃ - 10 ° C) or more,
Average cooling rate from 750 ° C. to 550 ° C. at the steel plate temperature at 0.25 mm below the steel plate surface: 20 ° C./s or more and 50 ° C./s or less;
Average cooling rate from 750 ° C. to 550 ° C. at the steel plate average temperature: 15 ° C. / s or more and 80 ° C. / s or less,
Average cooling rate from 550 ° C. to the temperature at which cooling is stopped at the steel plate temperature at 0.25 mm below the steel plate surface: 150 ° C./s or more, and
Cooling stop temperature at steel sheet average temperature: 250 to 550 °C
Controlled cooling was performed under the condition of , the steel structure at the center of the plate thickness was a bainite structure, and the steel structure at 0.25 mm below the surface of the steel sheet had a dislocation density of 1.0Х10 ¹⁴ to 7.0 Х 10 ¹⁴ (m ^-2 ) bay. A high-strength steel sheet having a nitrite structure, a variation of Vickers hardness at 0.25 mm below the surface of the steel sheet is 30 HV or less at 3σ when the standard deviation is σ, and a tensile strength of 520 MPa or more Sour resistance characterized by manufacturing Method for manufacturing high-strength steel sheet for line pipe. According to claim 4,
The above component composition further contains, in terms of mass%, at least one selected from Cu: 0.50% or less, Ni: 0.10% or less, and Cr: 0.50% or less. According to claim 4 or 5,
Sour-resistant line containing at least one selected from V: 0.005 to 0.1%, Zr: 0.0005 to 0.02%, Mg: 0.0005 to 0.02%, and REM: 0.0005 to 0.02% in terms of mass%. Method for manufacturing high-strength steel sheet for pipes. A high-strength steel pipe using the high-strength steel plate for sour-resistant line pipe according to claim 1 or 2. A high-strength steel pipe using the high-strength steel plate for sour-resistant line pipe according to claim 3.

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