KR20210118961A - High strength steel plate for sour-resistant line pipe, 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 not only in HIC resistance but also in SSCC resistance under stricter corrosive environments. The high-strength steel sheet for sour-resistant line pipe of the present invention, in 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% and Ca: 0.0005 to 0.005%, the balance has a component composition consisting of Fe and unavoidable impurities, and the steel structure at 0.5 mm below the surface of the steel sheet has a dislocation density of 1.0×10 ¹⁴ to It has a ^{bainite structure of 7.0×10 14} (m ⁻² ), and the variation in Vickers hardness at 0.5 mm below the surface of the steel sheet is 30 HV or less at 3σ when the standard deviation is σ, and has a tensile strength of 520 MPa or more. characterized.

Description

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

The present invention relates to a high-strength steel sheet for a sour-resistant line pipe having excellent material uniformity in the steel sheet, suitable for use in line pipes in the fields of construction, offshore structures, shipbuilding, civil engineering, and construction industrial machinery, and a method for manufacturing the same is about Further, the present invention relates to a high-strength steel pipe using the above-mentioned high-strength steel sheet for a sour-resistant line pipe.

In general, a line pipe is manufactured by forming a steel sheet manufactured by a plate mill or a hot roll 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 resistance. So-called sour resistance, such as cracking resistance (Sulfide Stress Corrosion Cracking (SSCC) resistance), is required. Among them, in HIC, hydrogen ions due to corrosion reaction adsorb to the surface of steel materials, penetrate into the steel as atomic hydrogen, and diffuse/disperse around non-metallic inclusions such as MnS in steel and hard second-phase structure. It accumulates and becomes molecular hydrogen, which causes cracks by its internal pressure, which has become 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, with respect to SSCC, it is generally known that it occurs in high-strength seamless steel pipes for oil wells or in the high-hardness region of welded parts, and has not been much of a problem in line pipes with relatively low hardness. However, in recent years, the mining environment for crude oil or natural gas is increasingly strict, and it has been reported that SSCC occurs even in the base metal part of line pipes in environments with high hydrogen sulfide partial pressure or low pH. The importance of controlling the hardness of the inner surface layer portion and improving the SSCC resistance in a more severe corrosive environment is pointed out.

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

In order to solve the above problem, for example, in Patent Documents 1 and 2, after rolling, before the surface layer part completes the bainite transformation, controlled cooling at a high cooling rate to recuperate the surface is performed. Disclosed is a method for manufacturing a steel sheet having a small material difference in directions. Further, Patent Documents 3 and 4 disclose a method for manufacturing a steel sheet for line pipe in which the surface layer portion is reduced in hardness by heating the surface of the steel sheet after accelerated cooling from the inside to a high temperature using a high-frequency induction heating device.

On the other hand, when there is a non-uniformity in the thickness of the scale on the surface of the steel sheet, the cooling rate of the steel sheet below it also varies during cooling, so that local variations in the cooling stop temperature within the steel sheet become a problem. As a result, the deviation of the steel sheet material in the sheet width direction occurs due to the non-uniformity of the scale thickness. On the other hand, Patent Documents 5 and 6 disclose a method of improving the shape of a steel sheet by reducing the cooling non-uniformity due to the scale thickness non-uniformity by performing descaling immediately before cooling.

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

However, according to the examination of the present inventors, it became clear that there exists room for improvement in the viewpoint of SSCC resistance in a more severe corrosive environment in the high strength steel plate obtained by the manufacturing method described in the said patent documents 1-6. As the reason, the following are considered.

In the manufacturing method described in Patent Documents 1 and 2, if the transformation behavior is different depending on the components of the steel sheet, the effect of sufficient material homogenization by recuperating 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 multiphase structure such as a ferrite-bainite two-phase structure, in the micro-Vickers test under reduction, the indenter presses a certain structure, Depending on the test, there is a large deviation in the hardness value.

In the manufacturing method described in patent documents 3 and 4, since the cooling rate of the surface layer part in accelerated cooling is large, only heating the surface layer of a steel plate may not be able to fully reduce the hardness of a surface layer part.

On the other hand, in the method described in Patent Documents 5 and 6, by descaling, a reduction in surface quality defects due to indentation damage of scale during hot calibration or a reduction in the variation in the cooling stop temperature of a steel sheet to improve the shape of the steel sheet, However, no consideration is given to the cooling conditions for obtaining a uniform material. This is because, when there is a variation in the cooling rate on the surface of the steel sheet, the hardness of the steel sheet varies. That is, if the cooling rate is slow, "film boiling" in which a film of air bubbles occurs between the surface of the steel sheet and the cooling water when the steel sheet surface cools, and "film boiling" in which air bubbles are separated from the surface by the cooling water before forming a film. "Nucleate boiling" occurs at the same time, and a deviation occurs in the cooling rate of the surface of the steel sheet. As a result, a deviation occurs in the hardness of the surface of the steel sheet. This point is not taken into consideration by the technique described in patent documents 5 and 6.

Therefore, in view of the above problems, an object of the present invention is to provide a high-strength steel sheet for sour line pipe that is excellent not only in HIC resistance but also in SSCC resistance under a stricter corrosive environment, along with an advantageous manufacturing method thereof. Another object of the present invention is to propose a high-strength steel pipe using the high-strength steel sheet for the sour-resistant line pipe.

The present inventors repeated many experiments and examinations about the component composition, microstructure, and manufacturing conditions of steel materials, in order to ensure SSCC resistance in a stricter corrosive environment. As a result, in order to further improve the SSCC resistance of the high-strength steel pipe, it is not sufficient to simply suppress the surface hardness as previously recognized. By setting the bainite structure to a dislocation density of 1.0×10 ^{14 to} 7.0×10 ¹⁴ (m ⁻² ), the increase in hardness in the coating process after pipe making can be suppressed, and as a result, the SSCC resistance of the steel pipe is improved. recognized that Moreover, in order to implement|achieve such a steel structure, it was necessary to strictly control the cooling rate in 0.5 mm below the surface of a steel plate, and it succeeded in discovering the condition. The present invention has been made based on this recognition.

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

[1] In 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%, and Ca : Contains 0.0005 to 0.005%, the balance has a component composition consisting of Fe and unavoidable impurities,

The steel structure at 0.5 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 in Vickers hardness at 0.5 mm below the surface of the steel sheet 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 sheet for sour line pipe, characterized in that.

[2] The component composition further contains, in mass%, one or two or more selected from the group consisting of Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, and Mo: 0.50% or less , The high-strength steel sheet for a sour-resistant line pipe according to the above [1].

[3] The above component composition is further, in mass%, Nb: 0.005 to 0.1%, V: 0.005 to 0.1%, Ti: 0.005 to 0.1%, Zr: 0.0005 to 0.02%, Mg: 0.0005 to 0.02%, and , REM: The high-strength steel sheet for sour line pipe according to the above [1] or [2], containing one or two or more selected from the group consisting of 0.0005 to 0.02%.

[4] In 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% and Ca : A steel piece containing 0.0005 to 0.005% and the balance having a component composition of Fe and unavoidable impurities is heated to a temperature of 1000 to 1300° C., and then hot rolled to obtain a steel sheet,

After that, for the steel plate,

The surface temperature of the steel sheet at the start of cooling: (Ar ₃ -10°C) or more,

Average cooling rate from 750°C to 550°C at the steel sheet temperature at 0.5 mm below the steel sheet surface: 80°C/s or less;

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

Average cooling rate from the steel plate temperature in 0.5 mm below the steel plate surface to the temperature at the time of cooling stop from 550 degreeC: 150 degreeC/s or more, and;

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

A method of manufacturing a high-strength steel sheet for an anti-sour line pipe, characterized in that controlled cooling is performed under the conditions of

[5] The component composition further contains, in mass%, one or two or more selected from the group consisting of Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, and Mo: 0.50% or less , The method for producing a high-strength steel sheet for a sour-resistant line pipe according to the above [4].

[6] The above component composition is further, in mass%, Nb: 0.005 to 0.1%, V: 0.005 to 0.1%, Ti: 0.005 to 0.1%, Zr: 0.0005 to 0.02%, Mg: 0.0005 to 0.02%, and , REM: 0.0005 to 0.02%, the method for producing a high-strength steel sheet for a sour-resistant line pipe according to the above [4] or [5], containing one or two or more selected from the group consisting of 0.0005 to 0.02%.

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

The high-strength steel sheet for a sour-resistant line pipe of the present invention and a high-strength steel pipe using the high-strength steel sheet for a sour-resistant line pipe of the present invention are excellent not only in HIC resistance but also in SSCC resistance under stricter corrosive environments. In addition, according to the method for manufacturing a high-strength steel sheet for a sour-resistant line pipe of the present invention, it is possible to manufacture a high-strength steel sheet for a sour-resistant line pipe that is excellent not only in HIC resistance but also in SSCC resistance under stricter corrosive environments.

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.

(Form for implementing the invention)

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

[Ingredient composition]

First, the component composition of the high-strength steel sheet according to the present disclosure and the reason for limitation thereof will be described. In the following description, all units expressed by % are mass %.

C: 0.02 to 0.08%

Although C effectively contributes to the improvement of strength, when the content is less than 0.02%, sufficient strength cannot be ensured. On the other hand, when C exceeds 0.08%, the hardness of the surface layer portion and the central segregation portion increases during accelerated cooling, so SSCC resistance and HIC resistance deteriorate. 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%

Although Si is added for deoxidation, when the content is less than 0.01%, the deoxidation effect is insufficient, while when it exceeds 0.50%, toughness and weldability are deteriorated, so the amount of Si is limited to 0.01 to 0.50%.

Mn: 0.50 to 1.80%

Mn effectively contributes to the improvement of strength and toughness, but when the content is less than 0.50%, the effect of its addition is insufficient. On the other hand, when it exceeds 1.80%, the hardness of the surface layer part and the central segregation part increases during accelerated cooling, so 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 while deteriorating weldability, HIC resistance deteriorates by raising the hardness of a center segregation part. When it exceeds 0.015 %, since the tendency becomes remarkable, the upper limit is prescribed|regulated as 0.015 %. Preferably it is 0.008 % or less. Although content is so good that it is low, it is set as 0.001 % or more from a viewpoint of refining cost.

S: 0.0002 to 0.0015%

Since S is an unavoidable impurity element and becomes MnS inclusion in steel and deteriorates HIC resistance, it is preferable that it is small, but up to 0.0015% is permissible. Although content is so good that it is low, it is set as 0.0002 % or more from a viewpoint of refining cost.

Al: 0.01 to 0.08%

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

Ca: 0.0005 to 0.005%

Ca is an element effective for improving the HIC resistance by controlling the shape of sulfide inclusions, but when it is less than 0.0005%, the effect of its addition is not sufficient. On the other hand, when it exceeds 0.005%, not only the effect is saturated, but since HIC resistance deteriorates by the fall of the cleanliness of steel, the amount of Ca is limited to the range of 0.0005 to 0.005%.

As mentioned above, although the basic components of the present disclosure have been described, the component compositions of the present disclosure include one or two or more selected from Cu, Ni, Cr and Mo for further improvement of the strength and toughness of the steel sheet, It can contain arbitrarily in the following ranges.

Cu: 0.50% or less

Cu is an element effective for improving toughness and increasing strength, and in order to obtain this effect, it is preferable to contain 0.05% or more, but if the content is too large, weldability deteriorates. do it with

Ni: 0.50% 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 0.05% or more. , when Ni is added, the upper limit is 0.50%.

Cr: 0.50% or less

Like Mn, Cr is an effective element for obtaining sufficient strength even at low C, and in order to obtain this effect, it is preferable to contain 0.05% or more. 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.

Mo: 0.50% or less

Mo is an element effective for improving toughness and increasing strength, and in order to obtain this effect, it is preferable to contain 0.05% or more, but if the content is too large, hardenability becomes excessive. SSCC resistance deteriorates. Moreover, weldability also deteriorates. For this reason, when adding Mo, let 0.50 % be an upper limit.

The component composition of this indication may further contain the 1 type(s) or 2 or more types selected from Nb, V, and Ti or more arbitrarily in the following ranges.

Nb: 0.005-0.1%, V: 0.005-0.1%, Ti: 0.005-0.1%, Zr: 0.0005-0.02%, Mg: 0.0005-0.02%, and REM: 0.0005-0.02% 1 type or 2 types selected from more

Nb, V, and Ti are all elements that can be optionally added in order to increase the strength and toughness of the steel sheet. If the content of each element is less than 0.005%, the effect of the addition is insufficient, whereas if the content exceeds 0.1%, the toughness of the welded portion deteriorates. Zr, Mg, and REM are elements that can be arbitrarily added in order to increase toughness through grain refinement or to increase crack resistance through control of inclusion properties. In all of these elements, if the content is less than 0.0005%, the effect of the addition is insufficient, while if the content exceeds 0.02%, the effect is saturated.

Although the present disclosure discloses a technique for improving SSCC resistance of high-strength steel pipe using high-strength steel sheet for sour-resistant line pipe, needless to say as sour-resistance performance, it is necessary to simultaneously satisfy HIC resistance, For example, it is preferable that the CP value calculated|required by following formula (1) shall be 1.00 or less. In addition, what is necessary is just to substitute 0 for the element not to be 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 an equation devised to estimate the material of the central segregation portion from the content of each alloying element, and the higher the CP value in Equation (1), the higher the component concentration of the central segregation portion, and the central segregation portion hardness increases. Therefore, by making the CP value calculated|required in said formula (1) into 1.00 or less, it becomes possible to suppress the crack generation in an HIC test. Moreover, since the hardness of a center segregation part becomes low as CP value is low, when higher HIC resistance is requested|required, what is necessary is just to set the upper limit to 0.95.

In addition, the remainder other than the above-mentioned elements consists of Fe and an unavoidable impurity. However, as long as the effects of the present invention are not impaired, the inclusion of other trace elements is not prevented. For example, although N is an element unavoidably contained in steel, if the content is 0.007 % or less, Preferably it is 0.006 % or less, it is permissible in this invention.

[Structure of steel plate]

Next, the steel structure of the high-strength steel sheet for a 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, in the surface layer portion, when a hard phase such as martensite or island martensite (MA) is generated, the surface layer hardness increases, the variation in hardness within the steel sheet increases, and the material uniformity is impaired. In order to suppress an increase in surface hardness, a bainite structure is used for the steel structure of the surface layer portion. Here, it is assumed that the bainite structure includes a structure called bainitic ferrite or granular ferrite that transforms during or after accelerated cooling that contributes 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 in toughness, increase in surface hardness, etc. occur, The smaller the tissue fraction, the better. However, when the volume fraction of structures other than the bainite phase is sufficiently low, their influence is negligible, so a certain 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% by volume fraction, it is acceptable because there is no significant effect. do.

Further, the bainite structure also has various forms depending on the cooling rate, but in the present disclosure, the structure of the pole surface layer portion of the steel sheet, specifically, the steel structure of 0.5 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 x10 ¹⁴ (m ^-2). Since the dislocation density decreases in the coating process after pipe making, if the dislocation density of 0.5 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. can Conversely, when the dislocation density of 0.5 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 making, and the hardness increases significantly due to age hardening, which deteriorates the SSCC resistance make it In order to obtain good SSCC resistance after pipe making, the preferable range of dislocation density is 6.0×10 ¹⁴ (m ⁻² ) or less. On the other hand, when the dislocation density of 0.5 mm below the surface ^{of the steel sheet is less than 1.0×10 14} (m ⁻² ), the strength as a steel sheet cannot be maintained. In order to ensure 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, if the dislocation density in the steel structure 0.5 mm below the surface of the steel sheet is in the above range, the pole surface layer portion having a depth of 0.5 mm from the surface of the steel sheet also has the same dislocation density. The effect of improving SSCC resistance is acquired.

Further, when the dislocation density at 0.5 mm below the surface of the steel sheet is 7.0×10 ¹⁴ (m ⁻² ) or less, HV0.1 at 0.5 mm below the surface of the steel sheet becomes 230 or less. From the viewpoint of securing the SSCC resistance of the steel pipe, it is important to suppress the surface layer hardness of the steel sheet, but by setting the HV0.1 at 0.5 mm below the surface of the steel sheet to 230 or less, 0.5 below the surface after the coating process after pipe making HV0.1 in mm can be suppressed to 260 or less, and SSCC resistance can be ensured.

In addition, in the high-strength steel sheet of the present disclosure, it is also important that the variation in Vickers hardness at 0.5 mm below the steel sheet surface is 30 HV or less at 3σ when the standard deviation is σ. When 3σ when measuring Vickers hardness at 0.5 mm below the surface of the steel sheet is more than 30 HV, the hardness variation in the polar surface layer of the steel sheet, that is, the presence of a local high hardness site in the polar surface layer, This is because deterioration of SSCC resistance caused by In addition, when calculating|requiring standard deviation (sigma), it is preferable to measure Vickers hardness at 100 points or more.

Since the high-strength steel sheet of the present disclosure is a steel sheet for steel pipe having a strength equal to or higher than X60 grade of API 5L, it shall 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 manufacturing method of this indication, after heating the steel piece which has the said component composition, it hot-rolls to make a steel plate, and then performs controlled cooling under predetermined conditions with respect to the said steel plate.

[Slab heating temperature]

Slab heating temperature: 1000~1300℃

If the slab heating temperature is less than 1000°C, the solid solution of the carbide is insufficient and the required strength cannot be obtained. On the other hand, if the slab heating temperature exceeds 1300°C, toughness deteriorates. Therefore, the slab heating temperature is 1000 to 1300°C. In addition, it is assumed that this temperature is the furnace temperature of a heating furnace, and a slab is heated at this temperature to the center part.

[Rolling end temperature]

In the hot rolling process, in order to obtain high base material toughness, the lower the rolling end temperature is, the better. On the other hand, since the rolling efficiency decreases, the rolling end temperature at the steel sheet surface temperature is determined in consideration of the required base material toughness and rolling efficiency. need to be set. From the viewpoint of improving strength and HIC resistance, it is preferable that the _{rolling end temperature be equal to or higher than the Ar 3 transformation point 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 from the components of steel, for example, by the following formula. Moreover, in order to obtain high base metal toughness, it is preferable that the reduction ratio in the temperature range of 950 degrees C or less corresponding to the austenite non-recrystallization temperature range be 60% or more. In addition, the surface temperature of a steel plate can be measured with a radiation thermometer etc.

Ar ₃ (°C)=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 at the surface temperature of the steel sheet

When the surface temperature of the steel sheet at the start of cooling is low, the amount of ferrite produced before controlled cooling is large. In particular, when _{the temperature drop from the Ar 3} transformation point exceeds 10° C., more than 5% of ferrite by volume fraction is generated, resulting in a decrease in strength Since the HIC resistance deteriorates as it increases, the surface temperature of the steel sheet at the start of cooling is set to (Ar ₃ -10°C) or higher.

[Cooling rate of controlled cooling]

It is important to control the cooling rate of the surface layer portion and the average cooling rate in the steel sheet in order to reduce the variation in hardness in the steel sheet while increasing the strength, and to improve the material uniformity. In particular, in order to make the dislocation density and 3σ at 0.5 mm below the surface of the steel sheet within the above-mentioned ranges, it is necessary to control the cooling rate at 0.5 mm below the surface of the steel sheet.

Average cooling rate from 750 degreeC to 550 degreeC at the steel plate temperature in 0.5 mm below the steel plate surface: 80 degreeC/s or less

When the average cooling rate from 750°C to 550°C at the steel sheet temperature at 0.5 mm below the surface of the steel sheet exceeds 80°C/s, the dislocation density at 0.5 mm below the surface of the steel sheet exceeds 7.0×10 ¹⁴ (m ⁻² ) becomes As a result, HV0.1 at 0.5 mm below the surface of the steel sheet exceeds 230, and after the coating process after pipe making, HV0.1 at 0.5 mm below the surface exceeds 260, and the SSCC resistance of the steel pipe deteriorates. Therefore, the said average cooling rate shall be 80 degreeC/s or less. Preferably it is 50 degrees C/s or less. The lower limit of the average cooling rate is not particularly limited, but if the cooling rate is excessively small, ferrite or pearlite is formed and the strength is insufficient.

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

When the average cooling rate from 750°C to 550°C from the average steel sheet temperature is less than 15°C/s, a bainite structure is not obtained, and a decrease in strength and deterioration of HIC resistance occur. For this reason, the cooling rate in steel plate average temperature shall be 15 degreeC/s or more. From a viewpoint of the dispersion|variation in steel plate intensity|strength and hardness, it is preferable that the cooling rate of the steel plate average shall be 20 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 80 degreeC/s or less so that a low-temperature transformation product may not produce|generate excessively.

Average cooling rate from 550 degreeC to the temperature at the time of cooling stop from the steel plate temperature in 0.5 mm below the steel plate surface: 150 degreeC/s or more

About cooling of 550 degrees C or less from the steel plate temperature in 0.5 mm below the steel plate surface surface, cooling in a stable nuclei boiling state is required, and the raise of a water density is indispensable. When the average cooling rate from the steel sheet temperature at 0.5 mm below the surface of the steel sheet to the temperature at the time of cooling stop from 550°C is less than 150°C/s, cooling in the nuclei boiling state does not occur, and hardness deviation occurs in the polar surface layer of the steel sheet Thus, 3σ at 0.5 mm below the surface of the steel sheet exceeds 30 HV, and as a result, SSCC resistance deteriorates. Therefore, the said average cooling rate shall be 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, 0.5 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 and the target surface temperature at the time of stopping the cooling measured with a radiation thermometer, for example, a process computer The temperature distribution in the plate thickness section can be obtained in real time by differential calculation using The temperature at 0.5 mm below the surface of the steel sheet in the temperature distribution is referred to as "the temperature of the steel sheet at 0.5 mm below the surface of the steel sheet" in the present specification, and the average value of the temperature in the sheet thickness direction in the temperature distribution is taken It is called "steel plate average temperature" in the specification.

[Cooling stop temperature]

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

After completion of rolling, the bainite phase is produced by rapid cooling to 250 to 550° C., which is the temperature range of bainite transformation in controlled cooling. When the cooling stop temperature exceeds 550°C, the bainite transformation is incomplete and sufficient strength cannot be obtained. Further, if the cooling stop temperature is less than 250°C, the hardness of the surface layer portion increases significantly, and the dislocation density at 0.5 mm below the surface of the steel sheet ^{exceeds 7.0×10 14} (m ⁻² ), so that the SSCC resistance deteriorates. In addition, the hardness of the central segregation portion also increases, and the HIC resistance also deteriorates. Then, in order to suppress deterioration of the material uniformity in a steel plate, the cooling stop temperature of controlled cooling shall be 250-550 degreeC from the steel plate average temperature.

[High-strength steel pipe]

The high-strength steel sheet of the present disclosure is formed into a tubular shape by press bend forming, roll forming, UOE forming, etc., and then welded to the butt, so that the material uniformity in the steel sheet suitable for the transportation of crude oil or natural gas is excellent. High-strength steel pipe (UOE steel pipe, electric resistance steel pipe, spiral steel pipe, etc.) can be manufactured.

For example, UOE steel pipe is manufactured by refining the end of a steel sheet, forming a steel pipe shape by C press, U press, and O press, then seam welding the butt portion by inner welding and outer welding, and further expanding the pipe as necessary manufactured through a process. In addition, although any method may be sufficient as a welding method as long as sufficient joint strength and joint toughness are obtained, it is preferable to use submerged arc welding from a viewpoint of the outstanding welding quality and manufacturing efficiency.

Example

Steel (steel grades A to K) having the component composition shown in Table 1 is made into a slab by a continuous casting method, heated to the temperature shown in Table 2, and then hot rolled at the rolling end temperature and reduction ratio 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 using a water-cooled controlled cooling device under the conditions shown 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 0.5 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 performed using the full-thickness test piece in the direction perpendicular to the rolling direction as the tensile test piece, and the tensile strength was measured. A result is shown in Table 2.

[Measurement of Vickers hardness]

About the cross section perpendicular to the rolling direction, based on JIS Z 2244, the Vickers hardness (HV0.1) of 100 points|pieces was measured in the position 0.5 mm below the steel plate surface, and the average value and standard deviation (sigma) were calculated|required. Table 2 shows the average value and the value of 3σ. Here, since the indentation is reduced by measuring HV0.1 instead of HV10, which is usually used instead of HV0.1, hardness information at a position closer to the surface and hardness information more sensitive to microstructure can be obtained. because it is possible

[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 X-ray diffraction measurement was performed at a position 0.5 mm below the surface of the steel sheet. The dislocation density used the method of converting from the distortion calculated|required from the half width (beta) of X-ray diffraction measurement. 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 the Rachinger method. The Williamsson-Hall method shown below is used for extraction of a deformation|transformation. The expansion of the half width is affected by the crystallite size D and the 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 modifying this formula, 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. Incidentally, the diffraction lines used for calculation are (110), (211), and (220). For the conversion of the dislocation density from the 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 in this example, 0.25 nm.

[Evaluation of SSCC resistance]

The SSCC resistance was evaluated using a part of each of these steel sheets and tubing. The tube pipe was manufactured by refining the end of the steel sheet, forming it into a steel pipe shape by C press, U press, and O press, seam welding the butt portion of the inner surface and the outer surface by submerged arc welding, and then performing a pipe expansion process. As shown in FIG. 1, after flattening the coupon cut out from the obtained steel pipe, the SSCC test piece of 5 x 15 x 115 mm was extract|collected from the inner surface of the steel pipe. At this time, the inner surface, which is the surface to be inspected, was left with mill scale 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 collected SSCC test piece, and using NACE standard TM0177 Solution A solution, hydrogen sulfide partial pressure: 1 bar, 4-point bending according to EFC16 standard It performed based on the SSCC test. After 720 hours of immersion, the case where cracks were not recognized was judged as good SSCC resistance, and (circle), and the case where cracks generate|occur|produced was judged as defective, and it was made into *. A result is shown in Table 2.

HIC resistance was investigated by the HIC test of 96-hour immersion using NACE standard TM0177 Solution A solution. In the HIC test, the case where the crack length ratio (CLR) was 15% or less in the HIC test was judged as good, and the case where it exceeded 15% was taken as ×. A result is shown in Table 2.

The target range of the present invention is a high-strength steel sheet for a sour-resistant line pipe, with a tensile strength of 520 MPa or more, a bainite structure in both the 0.5 mm subsurface position and the t/2 position, and HV0.1 at 0.5 mm below the surface. 230 or less, crack length ratio (CLR) of 15% or less in the SSCC test and HIC test in the high-strength steel pipe piped using the steel sheet.

As shown in Table 2, No. 1 - No. 15 are invention examples in which a component composition and manufacturing conditions satisfy|fill the appropriate range of this invention. All, tensile strength as a steel plate: 520 MPa or more, 0.5 mm below the surface position and t/2 position both have a bainite structure, and the HV 0.1 at 0.5 mm below the surface is 230 or less, SSCC resistance and HIC resistance were also good in the high-strength steel pipe.

On the other hand, No.16-No.23 are comparative examples in which a component composition is within the range of this invention, but manufacturing conditions are outside the range of this invention. In No. 16, since the slab heating temperature was low, homogenization of the microstructure and solid solution of the carbide were insufficient, and thus the strength was low. No. 17 had a low cooling start temperature and formed a layered structure in which ferrite was deposited, so it was low in strength and deteriorated in HIC resistance after tube forming. In No. 18, the controlled cooling conditions were outside the range 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 was heated. In No. 19, the cooling stop temperature was low, the dislocation density at 0.5 mm below the surface was high, and HV0.1 exceeded 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, since the average cooling rate at 750 to 550 °C at 0.5 mm below the surface of the steel sheet exceeded 80 °C/s, the dislocation density at 0.5 mm below the surface became high, and HV0.1 It exceeded this 230, and the SSCC resistance after pipe-making was inferior. Moreover, in No. 23, the HIC resistance in the surface layer part also deteriorated. In No.21 and No.22, since the average cooling rate in 550 degreeC or less in 0.5 mm below the steel plate surface does not satisfy 150 degreeC/s, the non-uniform cooling of a steel plate becomes remarkable, and HV0.1 is average Although 230 or less was satisfied, the hardness variation was large, and since the part with high hardness was generated locally, the SSCC resistance after pipe making was inferior. In Nos. 24 to No. 27, since the component composition of the steel sheet was outside the range of the present invention, the dislocation density at 0.5 mm below the surface was high and the HV0.1 exceeded 230, the SSCC resistance after pipe forming was inferior. Moreover, about No.24 - No.27, since the hardness of a center segregation part increased, HIC resistance was also inferior.

According to the present invention, it is possible to supply a high-strength steel sheet for sour line pipe that is excellent not only in HIC resistance but also in SSCC resistance under a stricter corrosive environment. Therefore, the steel pipe (electric resistance resistance steel pipe, spiral steel pipe, UOE steel pipe, etc.) manufactured by cold forming this steel plate can be used suitably for the transportation of crude oil or natural gas containing hydrogen sulfide which requires 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%, and Ca: 0.0005 to It contains 0.005%, and the balance has a component composition consisting of Fe and unavoidable impurities,
The steel structure at 0.5 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 in Vickers hardness at 0.5 mm below the surface of the steel sheet 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 line pipe, characterized in that. According to claim 1,
The component composition further contains, in mass%, one or two or more selected from the group consisting of Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, and Mo: 0.50% or less. High-strength steel sheet for line pipe. 3. The method of claim 1 or 2,
The above component composition is further, in mass%, Nb: 0.005 to 0.1%, V: 0.005 to 0.1%, Ti: 0.005 to 0.1%, Zr: 0.0005 to 0.02%, Mg: 0.0005 to 0.02%, and REM: A high-strength steel sheet for a sour-resistant line pipe containing one or two or more selected from 0.0005 to 0.02%. 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%, and Ca: 0.0005 to A steel piece containing 0.005% and the balance having a component composition of Fe and unavoidable impurities is heated to a temperature of 1000 to 1300° C. and then hot rolled to obtain a steel sheet,
After that, for the steel plate,
The surface temperature of the steel sheet at the start of cooling: (Ar ₃ -10°C) or more,
Average cooling rate from 750°C to 550°C at the steel sheet temperature at 0.5 mm below the steel sheet surface: 80°C/s or less;
Average cooling rate from 750°C to 550°C at the average steel plate temperature: 15°C/s or more;
Average cooling rate from the steel plate temperature in 0.5 mm below the steel plate surface to the temperature at the time of cooling stop from 550 degreeC: 150 degreeC/s or more, and;
Cooling stop temperature at average steel plate temperature: 250 to 550 °C
A method of manufacturing a high-strength steel sheet for a sour-resistant line pipe, characterized in that controlled cooling is performed under the conditions of 5. The method of claim 4,
The component composition further contains, in mass%, one or two or more selected from the group consisting of Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, and Mo: 0.50% or less. Method for manufacturing high-strength steel sheet for line pipe. 6. The method according to claim 4 or 5,
The above component composition is further, in mass%, Nb: 0.005 to 0.1%, V: 0.005 to 0.1%, Ti: 0.005 to 0.1%, Zr: 0.0005 to 0.02%, Mg: 0.0005 to 0.02%, and REM: A method for producing a high-strength steel sheet for a sour-resistant line pipe, comprising one or two or more selected from the group consisting of 0.0005 to 0.02%. A high-strength steel pipe using the high-strength steel sheet for a sour-resistant line pipe according to claim 1 or 2. A high-strength steel pipe using the high-strength steel sheet for a sour-resistant line pipe according to claim 3.

Images