KR102524176B1 - 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 composition consisting of Fe and unavoidable impurities, the steel structure at 0.25 mm below the steel sheet surface is a bainite structure with a dislocation density of 1.0 × 10 ¹⁴ to 7.0 × 10 ¹⁴ (m ^-2 ), and the steel sheet surface The variation of Vickers hardness in the bottom 0.25 mm is 30 HV or less at 3σ when the standard deviation is σ, and the variation of Vickers hardness in the plate thickness direction is 30 HV or less at 3σ when the standard deviation is σ, It is characterized by having 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 sheet for sour-resistant line pipe having excellent material uniformity within the steel sheet and a method for manufacturing the same, which is preferable for use in line pipes in the fields of architecture, offshore structures, shipbuilding, civil engineering, and construction industrial machinery. Further, the present invention relates to a high-strength steel pipe using the high-strength steel plate for sour-resistant line pipe.

In general, 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 not only strength, toughness, weldability, etc., but also hydrogen induced cracking resistance (HIC (Hydrogen Induced Cracking) resistance) and sulfide stress corrosion cracking resistance. So-called sour resistance such as (SSCC (Sulfide Stress Corrosion Cracking) 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 and causes cracks due to its internal pressure, which is a problem in line pipes having a relatively low strength level with respect to oil wells, and many countermeasure technologies have been disclosed. On the other hand, SSCC is generally known to occur in high-strength seamless steel pipes for oil wells and high-hardness regions of welded parts, and has not been a problem in relatively low-hardness line pipes. However, in recent years, the environment for mining crude oil and natural gas has become more and more severe, and it has been reported that SSCC also occurs in the base material portion of the line pipe in an environment where the hydrogen sulfide partial pressure is high or the pH is low, and the inner surface layer portion of the steel pipe The importance of improving SSCC resistance in a more severe corrosive environment by controlling the hardness of the metal 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.

In general, 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, which is a material in the plate thickness direction. A manufacturing method of a steel plate with a small difference 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, if there is unevenness 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 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 study of the present inventors, it has been found that there is room for improvement in the high-strength steel sheet obtained by the production method described in Patent Documents 1 to 6 above from the viewpoint of SSCC resistance in a more severe corrosion environment. The reason for this is considered to be as follows.

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 biphasic structure, in the low-load micro-Vickers test, the indenter Depending on which tissue is indented and tested, the hardness value varies greatly.

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, the hardness of the surface layer portion may not 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 straightening 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 variations occur in the hardness of the steel sheet if the cooling rate on the surface of the steel sheet is irregular. That is, if the cooling rate is slow, when the surface of the steel sheet is cooled, "film boiling" in which a film of bubbles is generated between the surface of the steel sheet and the cooling water, and "nucleate boiling" in which the bubbles are separated from the surface by the cooling water before forming a film occur simultaneously As a result, variations occur 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, in addition to HIC resistance, 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, high-strength steel sheet for sour-resistant line pipe , 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 repeated numerous experiments and studies on the component composition, microstructure, and manufacturing conditions of steel materials in order to ensure SSCC resistance in a more severe corrosive environment. As a result, in order to further improve the SSCC resistance of high-strength steel pipes, it is not sufficient to simply suppress the surface layer hardness as 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 ), it is possible to suppress a rise in hardness in the coating process after pipe making, and as a result, the inside of the steel pipe SSCC properties were found to be 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 sheet, and it has succeeded in finding the condition. 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 balance being Fe and unavoidable impurities Has a component composition consisting of

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 ),

The variation of the 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,

The variation of the Vickers hardness in the plate thickness direction is 30 HV or less at 3σ when the standard deviation is σ,

having a tensile strength of 520 MPa or more

High-strength steel plate for sour-resistant line pipe, characterized in that.

[2] The sour-resistant line 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 for pipes.

[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 the above [1] or [2].

[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 impurities A steel piece having the component composition is heated to a temperature of 1000 to 1300° C., then hot-rolled 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 ℃ to 550 ℃ at average temperature of steel sheet: 15 ℃ / 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 to steel plate average temperature: 250 to 550 ° C

Controlled cooling is performed under the condition of

After that, the steel sheet is reheated under conditions of steel sheet average temperature: greater than the cooling stop temperature and 450 to 600°C.

[5] The sour-resistant line 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. Method for manufacturing high-strength steel sheet for pipes.

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

Although C effectively contributes to the improvement of strength, if the content is less than 0.02%, sufficient strength is not secured, while if it exceeds 0.08%, the hardness of the surface layer portion and center segregation portion increases during accelerated cooling, so SSCC resistance 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 deteriorate. 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 is an unavoidable impurity element, and degrades 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 irrespective of the partial pressure of hydrogen sulfide. 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 the addition effect is not sufficient when 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 welded part deteriorates. Therefore, at least one of Nb and Ti is added in the range of 0.005% to 0.1%, respectively.

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

Cu: 0.50% or less

Cu is an element effective in improving toughness and increasing strength, and in order to obtain this effect, it is preferable 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. In order to obtain this effect, it is preferable to contain 0.05% or more of Cr. Dislocation density increases 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%, whereas 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-resistant performance, needless to say, it is necessary to simultaneously satisfy the HIC resistance, e.g. 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] ... (1)

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

Here, the CP value is an equation devised to estimate the material of the central segregation portion from the content of each alloy element. hardness rises Therefore, by setting the CP value obtained in the above formula (1) to 1.00 or less, it becomes possible to suppress the generation of cracks in the HIC test. Further, 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 and variation in hardness within the steel sheet increases, thereby impairing material uniformity. 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 a bainite structure, and the structure at the center of the plate thickness on behalf of the site only needs to 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-like martensite, and retained austenite are mixed in the bainite structure, a decrease in strength, deterioration of toughness, increase in surface layer hardness, etc. occur. The smaller the tissue fraction other than the bainite phase, 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, as long as 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. do it as

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 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 forming, 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 rise in hardness due to age hardening can be minimized. Conversely, when the dislocation density at 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 greatly increases due to age hardening, thereby improving SSCC resistance. degrade In order to obtain good SSCC resistance after pipe forming, 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 desirable 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, if the dislocation density in the steel structure 0.25 mm below the surface of the steel sheet is in 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 securing 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 of the 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, local high hardness portion exists in the extreme surface layer, and the portion is This is because deterioration of SSCC resistance 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.

In the same way of thinking, in the high-strength steel sheet of the present disclosure, it is also important that the variation of the Vickers hardness in the sheet thickness direction is 30 HV or less at 3σ when σ is the standard deviation.

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

[Manufacturing method]

Hereinafter, the 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 manufacturing method of the present disclosure, after heating a steel piece having the above component composition, hot rolling is performed to obtain a steel sheet, the steel sheet is then subjected to controlled cooling under predetermined conditions, and then the steel sheet is reheated.

[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 furnace temperature of a heating furnace, and it is assumed that a slab is heated to this temperature to the center part.

[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 at the surface temperature of the steel sheet 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 in terms of 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: At least (Ar ₃ - 10 ℃) at the surface temperature of the steel plate

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 less 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 sheet surface to the ranges described above, it is necessary to control the cooling rate at 0.25 mm below the steel sheet 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 at 0.25 mm below the steel plate surface exceeds 50 °C/s, the dislocation density at 0.25 mm below the steel plate surface exceeds 7.0 × 10 ¹⁴ (m ^-2 ) become As a result, the HV 0.1 at 0.25 mm below the surface of the steel plate exceeds 230, and after the coating process after pipe manufacture, the HV 0.1 at 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 average steel plate 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 at the steel plate temperature at 0.25 mm below the steel plate surface: 150 ° C./s or more

For cooling to 550°C or less at a steel sheet temperature at 0.25 mm below the steel sheet surface, 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 at 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. Then, 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. Although the upper limit of the said average cooling rate is not specifically limited, It is preferable to set it as 250 degrees C/s or less from restrictions on facilities.

In addition, 0.25 mm below the surface of the steel sheet and the average temperature of the steel sheet cannot be measured directly 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 process The temperature distribution within the plate thickness section can be obtained in real time by difference calculation using a computer. The temperature at 0.25 mm below the steel sheet surface in the temperature distribution is referred to as "steel sheet temperature at 0.25 mm below the steel sheet surface" in this specification, and the average value of the temperature in the sheet thickness direction in the temperature distribution is It is called "steel plate average temperature" in the specification.

[cooling stop temperature]

Cooling stop temperature: 250 to 550 ° C as average steel plate temperature

After completion of the rolling, the bainite phase is formed by rapidly cooling to 250 to 550°C, which is the temperature range of bainite transformation, by controlled cooling. When the cooling stop temperature exceeds 550°C, bainite transformation is incomplete and sufficient strength is not 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.

[Reheating conditions]

Reheating temperature: Average steel sheet temperature, exceeding the cooling stop temperature, and 450 to 600 ° C.

In the present disclosure, after completion of rolling, the steel sheet is subjected to online reheating after rapidly cooling to 250 to 550°C, which is the temperature range of bainite transformation, by controlled cooling. SSCC resistance can be improved by subjecting the steel sheet to a temperature higher than the cooling stop temperature by the reheating to temper and soften the bainite phase. However, if the reheating temperature is less than 450 ℃, the surface layer softening effect is insufficient, and if the reheating temperature exceeds 600 ℃, the strength and DWTT (Drop Weight Tear Test: Drop Weight Tear Test) properties deteriorate. Let it be 600 degreeC.

In the present disclosure, from the viewpoint of reducing production efficiency and fuel cost required for heat treatment, it is preferable to perform reheating immediately after stopping the controlled cooling. Here, reheating immediately after stopping controlled cooling means reheating within 120 seconds after stopping controlled cooling.

For softening the surface layer, it is preferable to raise the temperature by 50°C or more from the reheating start temperature, that is, the cooling stop temperature, during reheating. In addition, it is preferable to perform cooling after reheating basically by air cooling.

[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 refining 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 abutted portion by inner surface welding and outer surface welding. And, if necessary, it is manufactured through a tube expansion process. Further, the welding method may be any method 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. Immediately thereafter, the steel sheet was reheated using an online induction heating device so that the average temperature of the steel sheet reached the "reheating temperature" in Table 2.

[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 of 0.25 mm below the surface of the steel sheet and the structure at the center of the plate 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 of 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 as the average hardness and hardness deviation at 0.25 mm below the surface of the steel sheet. Similarly, for a cross section perpendicular to the rolling direction, in accordance with JIS Z 2244, from a position 0.25 mm below the surface of the steel sheet to a position 0.25 mm below the surface on the opposite side of the steel sheet at a pitch of 0.5 mm in the plate thickness direction, The Vickers hardness (HV 0.1) was measured and the standard deviation σ was obtained. The value of 3σ is shown in Table 2 as the hardness variation in the plate thickness direction. Further, when the Vickers hardness measurement in the sheet thickness direction is continued at a pitch of 0.5 mm, after the first measurement at a position within 1.25 mm below the steel sheet surface on the side opposite to the measurement start side, the steel sheet surface on the opposite side to the measurement start side The Vickers hardness at the lower 0.25 mm position is measured, and the measurement is finished. Here, the reason why the HV 0.1 was measured instead of the normally used HV 10 is that since the indentation becomes smaller by measuring with the HV 0.1, it becomes possible to obtain hardness information at a position closer to the surface or 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 steel plate surface. 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 spread 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 SSCC resistance]

SSCC resistance was evaluated by roughening using a part of each of these steel plates. The steel pipe was manufactured by subjecting the end of a steel plate to improvement processing, forming into a steel pipe shape by a C press, a U press, or an O press, seam-welding the butted parts of the inner and outer surfaces by submerged arc welding, and 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 collected from the inner surface of the steel pipe. At this time, the inner surface, which is the surface to be inspected, was kept as it was formed with mill scale in order to leave the state of the outermost layer. Stress of 90% of the actual yield strength (0.5% YS) of each steel pipe was applied to the sampled SSCC test piece, NACE standard TM0177 Solution A was used, hydrogen sulfide partial pressure: 1 bar, EFC 16 standard 4-point bending It was implemented based on the SSCC test. In the same way, NACE standard TM0177 Solution B was used, and hydrogen sulfide partial pressure: 0.1 bar + carbon dioxide partial pressure: 0.9 bar, and the test was conducted in accordance with the EFC 16 standard 4-point bending SSCC test. 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, the SSCC resistance was judged as good when no cracks were observed, and the case where cracks occurred was judged as poor and was evaluated as ×. A result is shown in Table 2.

[Evaluation of HIC resistance]

HIC resistance was investigated by the HIC test of immersion for 96 hours using NACE standard TM0177 Solution A solution at hydrogen sulfide partial pressure: 1 bar. Moreover, it investigated by the HIC test of immersion for 96 hours using NACE standard TM0177 Solution B solution, hydrogen sulfide partial pressure: 0.1 bar + carbon dioxide partial pressure: 0.9 bar. For HIC resistance, in the HIC test, a case where the crack length ratio (CLR) was 15% or less 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 0.25 mm and t/2 below the surface, bainite structure, HV 0.1 at 0.25 mm below the surface 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 plate: 520 MPa or more, the microstructure at both the 0.25 mm position and the t/2 position below the surface is a bainite structure, and the HV 0.1 at 0.25 mm below the surface is 230 or less, and the high strength produced using the steel plate The SSCC resistance and HIC resistance of the steel pipe were also good.

On the other hand, No. 16 to No. 23 are comparative examples in which the component composition is within the range of the present invention, but the production conditions are outside the range 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. Since No. 17 had a low cooling start temperature and became a layered structure in which ferrite precipitated, while being low in 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. It has deteriorated. No. 19 had a low cooling stop temperature, a high dislocation density at 0.25 mm below the surface, and an HV 0.1 exceeding 230, so the SSCC resistance after pipe making was inferior. 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 at 750°C to 550°C greatly exceeded 50°C/s at the steel sheet temperature at 0.25 mm below the surface of the steel sheet, so the dislocation density at 0.25 mm below the surface high, HV 0.1 exceeded 230, and SSCC resistance after pipe preparation was inferior. Also, in No. 23, the HIC resistance in the surface layer portion was also deteriorated. In No.21 and No.22, the average cooling rate at 550°C or less at 0.25 mm below the surface of the steel sheet did not reach 150°C/s, so the uneven cooling of the steel sheet became remarkable, and the HV 0.1 was 230 or less on average. was satisfied, but the hardness variation was large and a locally high hardness portion was generated, so the SSCC resistance after pipe preparation was inferior. In No.24 to No.27, the component composition of the steel sheet was outside the range 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 inferior. Also, for No. 24 to No. 27, since the hardness of the center segregation portion increased, the 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 was deteriorated under a very severe corrosive environment of hydrogen sulfide partial pressure of 2 Bar. No. 30 had no effect of softening the surface layer because reheating was not performed, and the SSCC resistance sometimes deteriorated under a very harsh corrosive environment of hydrogen sulfide partial pressure of 2 Bar. In No. 31, the average cooling rate from 750 ° C to 550 ° C exceeded 50 ° C / s at the steel plate temperature at 0.25 mm below the steel plate surface, so SSCC resistance was excellent in a very severe corrosion environment of hydrogen sulfide partial pressure of 2 Bar. There was a case of deterioration.

According to the present invention, it is possible to supply 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. 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 transporting 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 balance being Fe and unavoidable impurities. Have,
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 ),
The variation of the 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,
The variation of the Vickers hardness in the plate thickness direction is 30 HV or less at 3σ when the standard deviation is σ,
having a tensile strength of 520 MPa or more
High-strength steel plate for sour-resistant line pipe, characterized in that. 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,
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%, High-strength steel plate for line pipe. 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 having a component composition of Fe and unavoidable impurities. After heating the steel piece to a temperature of 1000 to 1300 ° C., hot rolling to make a steel plate,
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 average steel plate 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 to steel plate average temperature: 250 to 550 ° C
Controlled cooling is performed under the condition of
Thereafter, the steel sheet is reheated under the condition of steel sheet average temperature: exceeding the above cooling stop temperature and further at 450 to 600°C, and the steel structure at the center of the sheet thickness is a bainite structure and is 0.25 mm below the surface of the steel sheet. The steel structure of is a bainite structure with a dislocation density of 1.0 × 10 ¹⁴ to 7.0 × 10 ¹⁴ (m ^-2 ), and the variation in Vickers hardness in 0.25 mm below the surface of the steel sheet is 3σ when the standard deviation is σ. 30 HV or less, the deviation of the Vickers hardness in the plate thickness direction is 30 HV or less at 3σ when the standard deviation is σ, and a high-strength steel plate having a tensile strength of 520 MPa or more is manufactured for sour-resistant line pipe, characterized in that Manufacturing method of high-strength steel sheet. According to claim 4,
The above 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. Method for producing a high-strength steel sheet for sour-resistant line pipe. According to claim 4 or 5,
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%, Method for manufacturing high-strength steel sheet for line pipe. 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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