JP6825748B2 - High-strength steel sheet for sour-resistant pipe and its manufacturing method, and high-strength steel pipe using high-strength steel sheet for sour-resistant pipe - Google Patents

Description

INDUSTRIAL APPLICABILITY The present invention is a high-strength steel plate for sour line pipes having excellent material uniformity in steel plates, which is suitable for use in line pipes in the fields of construction, marine structures, shipbuilding, civil engineering, and machinery for the construction industry, and production thereof. It's about the method. The present invention also relates to a high-strength steel pipe using the above-mentioned high-strength steel plate for sour-resistant pipes.

Generally, a line pipe is manufactured by forming a steel plate produced 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, line pipes used for transporting crude oil containing hydrogen sulfide and natural gas have strength, toughness, weldability, etc., as well as hydrogen-induced cracking resistance (HIC (Hydrogen Induced Cracking) resistance) and sulfide resistance. So-called sour resistance such as stress corrosion cracking resistance (SSCC (Sulfide Stress Corrosion Cracking) resistance) is required. Among them, in HIC, hydrogen ions due to the corrosion reaction are adsorbed on the surface of the steel material, penetrate into the steel as atomic hydrogen, and diffuse and accumulate around non-metal inclusions such as MnS in the steel and the hard phase 2 structure. As a result, it becomes molecular hydrogen and cracks occur due to its internal pressure, which has been a problem in line pipes with a relatively low strength level with respect to oil well pipes, 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 in the high hardness range of welds, and has not been regarded as a problem for line pipes with relatively low hardness. .. However, in recent years, the mining environment for crude oil and natural gas has become more and more severe, and it has been reported that SSCC occurs even in the base metal part of line pipes in an environment where the partial pressure of hydrogen sulfide is high or the pH is low. It has been pointed out that it is important to control the hardness of the inner surface layer of the steel pipe to improve the SSCC resistance in a more severe corrosive environment. Further, in an environment where the partial pressure of hydrogen sulfide is relatively low, fine cracks called fisher may occur, and SSCC may occur.

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

In order to solve the above problems, for example, in Patent Documents 1 and 2, in the plate thickness direction, for example, after rolling, controlled cooling at a high cooling rate is performed to reheat the surface of the surface layer portion before completing the bainite transformation. A method for manufacturing a steel sheet having a small material difference is disclosed. Further, Patent Documents 3 and 4 disclose a method for manufacturing a steel sheet for a line pipe, which uses a high-frequency induction heating device to heat the surface of the steel sheet after accelerated cooling to a higher temperature than the inside to reduce the hardness of the surface layer portion. Has been done.

On the other hand, if the scale thickness on the surface of the steel sheet is uneven, the cooling rate of the steel sheet below the steel sheet also varies during cooling, and the local cooling stop temperature in the steel sheet becomes a problem. As a result, the steel plate material varies in the plate width direction due to the unevenness of the scale thickness. On the other hand, Patent Documents 5 and 6 disclose a method of reducing the cooling unevenness caused by the scale thickness unevenness and improving the steel sheet shape by performing descaling immediately before cooling.

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

However, according to the study by the present inventors, there is room for improvement in the high-strength steel sheet obtained by the manufacturing methods described in Patent Documents 1 to 6 from the viewpoint of SSCC resistance under a harsher corrosion environment. found. The possible reasons for this are as follows.

In the manufacturing methods described in Patent Documents 1 and 2, if the transformation behavior differs depending on the composition of the steel sheet, the effect of sufficient material homogenization by reheating may not be obtained. Further, when the structure of the surface layer of the steel plate obtained by the production methods described in Patent Documents 1 and 2 is a double-phase structure such as a ferrite-bainite two-phase structure, the indenter has any structure in the low-load micro-Vickers test. There is a large variation in the hardness value depending on whether the test is performed by pushing in.

In the manufacturing methods described in Patent Documents 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 Documents 5 and 6, descaling reduces surface texture defects due to scale indentation defects during hot straightening and reduces variations in the cooling stop temperature of the steel sheet to improve the shape of the steel sheet. However, no consideration has been given to the cooling conditions for obtaining a uniform material. This is because if the cooling rate on the surface of the steel sheet varies, the hardness of the steel sheet varies. That is, when the cooling rate is slow, when the surface of the steel plate is cooled, a film of bubbles is generated between the surface of the steel sheet and the cooling water, and the bubbles are separated from the surface by the cooling water before forming the film. "Nucleate boiling" occurs at the same time, causing variations in the cooling rate of the steel plate surface. As a result, the hardness of the surface of the steel sheet varies. This point is not taken into consideration in the techniques described in Patent Documents 5 and 6.

Further, in Patent Documents 1 to 6, the conditions for avoiding fine cracks such as Fisher in an environment where the partial pressure of hydrogen sulfide is relatively low are not clear.

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

The present inventors have repeated numerous experiments and studies on the 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 hardness as previously found, and in particular, the structure of the polar surface layer of the steel sheet, specifically the surface of the steel sheet. By forming the steel structure of the lower 0.25 mm into a bainite structure with a dislocation density of 1.0 × 10 ^{14 to} 7.0 × 10 ¹⁴ (m- ² ), the increase in hardness can be increased in the coating process after pipe formation. It was found that it can be suppressed and as a result, the SSCC resistance of the steel pipe is improved. Furthermore, in order to realize such a steel structure, it is necessary to strictly control the cooling rate at 0.25 mm below the surface of the steel sheet, and we have succeeded in finding the condition. Further, in an environment where the partial pressure of hydrogen sulfide exceeds 1 bar, the addition of Mo is effective in suppressing the generation of initial cracks, and in an environment where the partial pressure of hydrogen sulfide is less than 1 bar, suppressing the addition of Ni is like Fisher. It was found to be effective in avoiding microcracks. The present invention is based on this finding.

That is, the gist structure 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 It contains one or more selected from Nb: 0.005 to 0.1% and Ti: 0.005 to 0.1%, and has a component composition in which the balance consists of Fe and unavoidable impurities.
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 in 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.
A high-strength steel sheet for sour line pipes, which has a tensile strength of 520 MPa or more.

[2] The above-mentioned component composition further contains at least one selected from Cu: 0.50% or less, Ni: 0.10% or less, and Cr: 0.50% or less in mass%. The high-strength steel plate for sour-resistant pipes according to [1].

[3] Further, the component composition is, in mass%, V: 0.005 to 0.1%, Zr: 0.0005 to 0.02%, Mg: 0.0005 to 0.02% and REM: 0. The high-strength steel sheet for sour line pipe according to the above [1] or [2], which contains at least one selected from 0005 to 0.02%.

[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 1000 to 1300 steel pieces containing one or more selected from Nb: 0.005 to 0.1% and Ti: 0.005 to 0.1%, and the balance having a component composition of Fe and unavoidable impurities. After heating to a temperature of ℃, it is hot-rolled to make a steel sheet.
After that, with respect to the steel sheet
Cooling at the start of the steel sheet surface _{temperature: (Ar} 3 -10 ℃) or higher,
Average cooling rate from 750 ° C to 550 ° C at steel plate temperature 0.25 mm below the surface of the steel plate: 50 ° C / s or less,
Average cooling rate from 750 ° C to 550 ° C at average steel sheet temperature: 15 ° C / s or more,
Average cooling rate: 150 ° C / s or more from 550 ° C to the temperature at which cooling is stopped at a steel plate temperature of 0.25 mm below the surface of the steel sheet, and cooling stop temperature: 250 to 550 ° C at the average temperature of the steel sheet.
A method for manufacturing a high-strength steel sheet for sour-resistant pipes, which comprises performing controlled cooling under the above conditions.

[5] The above-mentioned component composition further contains at least one selected from Cu: 0.50% or less, Ni: 0.10% or less, and Cr: 0.50% or less in mass%. The method for manufacturing a high-strength steel plate for sour-resistant pipes according to [4].

[6] Further, the component composition is, in mass%, V: 0.005 to 0.1%, Zr: 0.0005 to 0.02%, Mg: 0.0005 to 0.02% and REM: 0. The method for producing a high-strength steel sheet for sour line pipes according to the above [4] or [5], which contains at least one selected from 0005 to 0.02%.

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

The high-strength steel pipe for sour line pipes of the present invention and the high-strength steel pipe using the high-strength steel plate for sour line pipes have not only HIC resistance but also SSCC resistance under more severe corrosion environment and less than 1 bar. It also has excellent SSCC resistance in an environment with low hydrogen sulfide partial pressure. Further, according to the method for producing a high-strength steel plate for sour line pipes of the present invention, not only HIC resistance but also SSCC resistance in a harsher corrosion environment and resistance in an environment with a low hydrogen sulfide partial pressure of less than 1 bar are obtained. It is possible to manufacture a high-strength steel plate for sour line pipes having excellent SSCC properties.

It is a schematic diagram explaining the method of collecting the test piece for evaluation of SSCC resistance in an Example.

Hereinafter, the high-strength steel sheet for sour-resistant pipes 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 its limitation will be described. In the following description, all units indicated by% are mass%.

C: 0.02 to 0.08%
C effectively contributes to the improvement of strength, but if the content is less than 0.02%, sufficient strength cannot be secured, while if it exceeds 0.08%, the hardness of the surface layer portion and the central segregation portion is hard during accelerated cooling. The SSCC resistance and the HIC resistance are deteriorated due to the increase in the resistance. In addition, toughness also deteriorates. Therefore, 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 if the content is less than 0.01%, the deoxidizing effect is not sufficient, while if it exceeds 0.50%, the toughness and weldability deteriorate, so the amount of Si is 0. Limited to the range of 01 to 0.50%.

Mn: 0.50 to 1.80%
Mn effectively contributes to the improvement of strength and toughness, but its addition effect is poor when the content is less than 0.50%, while when it exceeds 1.80%, the hardness of the surface layer portion and the central segregation portion during accelerated cooling. As a result, SSCC resistance and HIC resistance deteriorate. Weldability also deteriorates. Therefore, 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, which deteriorates weldability and also deteriorates HIC resistance by increasing the hardness of the central segregated portion. If it exceeds 0.015%, the tendency becomes remarkable, so the upper limit is set to 0.015%. It is preferably 0.008% or less. The lower the content, the better, but from the viewpoint of refining cost, it should be 0.001% or more.

S: 0.0002 to 0.0015%
S is an unavoidable impurity element and is preferably a small amount because it becomes an MnS inclusion in steel and deteriorates HIC resistance, but up to 0.0015% is allowed. The lower the content, the better, but from the viewpoint of refining cost, it should be 0.0002% or more.

Al: 0.01 to 0.08%
Al is added as an antacid, but if it is less than 0.01%, there is no effect of addition, while if it exceeds 0.08%, the cleanliness of the steel is lowered and the toughness is deteriorated, so that the amount of Al is 0. Limited to the range of 01 to 0.08%.

Mo: 0.01-0.50%
Mo is an element effective for improving toughness and increasing strength, and is an element effective for improving SSCC resistance regardless of the partial pressure of hydrogen sulfide. In order to obtain this effect, it is necessary to contain 0.01% or more, and preferably 0.10% or more. On the other hand, if the content is too large, the hardenability becomes excessive, so that the dislocation density described later becomes high and the SSCC resistance deteriorates. Weldability also deteriorates. Therefore, 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 controlling the morphology of sulfide-based inclusions, but its addition effect is not sufficient if it is less than 0.0005%. On the other hand, if it exceeds 0.005%, not only the effect is saturated but also the HIC resistance is deteriorated due to the decrease in the cleanliness of the steel, so the Ca amount is limited to the range of 0.0005 to 0.005%. ..

One or more selected from Nb: 0.005 to 0.1% and Ti: 0.005 to 0.1% Nb and Ti are both effective elements for increasing the strength and toughness of the steel sheet. .. If the content of each element is less than 0.005%, the effect of adding the element is poor, while if it exceeds 0.1%, the toughness of the welded portion deteriorates. Therefore, at least one of Nb and Ti shall be added in the range of 0.005 to 0.1%, respectively.

The basic components of the present disclosure have been described above, but the component composition of the present disclosure includes one or more selected from Cu, Ni and Cr in the following range in order to further improve the strength and toughness of the steel sheet. Can be arbitrarily contained in.

Cu: 0.50% or less Cu is an element effective for improving toughness and increasing strength, and it is preferable to contain 0.05% or more in order to obtain this effect, but if the content is too large, welding is performed. Since the property deteriorates, the upper limit is 0.50% when Cu is added.

Ni: 0.10% or less Ni is an element effective for improving toughness and increasing strength, and it is preferable to contain 0.01% or more in order to obtain this effect, but it exceeds 0.10%. When Ni is added, the upper limit is 0.10% when Ni is added in order to facilitate the formation of fine cracks called Fisher in an environment where the partial pressure of hydrogen sulfide is less than 1 bar. Preferably, it is 0.02% or less.

Cr: 0.50% or less Cr is an element effective for obtaining sufficient strength even at low C like Mn, and it is preferable to contain 0.05% or more in order to obtain this effect. If the amount is too large, the hardenability becomes excessive, so that the dislocation density described later becomes high and the SSCC resistance deteriorates. Weldability also deteriorates. Therefore, when Cr is added, the upper limit is 0.50%.

The component composition of the present disclosure may further optionally contain one or more selected from V, Zr, Mg and REM within the following range.

One selected from V: 0.005-0.1%, Zr: 0.0005-0.02%, Mg: 0.0005-0.02% and REM: 0.0005-0.02% As described above, V is an element that can be arbitrarily added in order to increase the strength and toughness of the steel sheet. If the content is less than 0.005%, the effect of addition is poor, while if it exceeds 0.1%, the toughness of the weld deteriorates. Therefore, when adding, the range should be 0.005 to 0.1%. preferable. Zr, Mg and REM are elements that can be arbitrarily added to increase toughness through grain refinement and to increase crack resistance through control of inclusion properties. When the content of each of these elements is less than 0.0005%, the effect of adding them is poor, while when the content exceeds 0.02%, the effect is saturated. Therefore, when they are added, they are all 0.0005-0. It is preferably in the range of 02%.

This disclosure discloses a technique for improving the SSCC resistance of a high-strength steel pipe using a high-strength steel plate for a sour line pipe, but it goes without saying that the sour resistance is HIC resistance. At the same time, it is necessary to be satisfied, and for example, the CP value obtained by the following equation (1) is preferably 1.00 or less. In addition, 0 may be substituted 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] indicates the content (mass%) of the X element in the 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, and the higher the CP value of the above equation (1), the higher the component concentration of the central segregation portion. Increases, and the hardness of the central segregated portion increases. Therefore, by setting the CP value obtained in the above equation (1) to 1.00 or less, it is possible to suppress the occurrence of cracks in the HIC test. Further, the lower the CP value, the lower the hardness of the central segregated portion. Therefore, when higher HIC resistance is required, the upper limit thereof may be 0.95.

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

[Structure of steel plate]
Next, the steel structure of the high-strength steel sheet for sour-resistant pipes of the present disclosure will be described. The steel structure needs to have a bainite structure in order to increase the tensile strength to 520 MPa or more. In particular, when a hard phase such as martensite or island-shaped martensite (MA) is generated in the surface layer portion, the surface layer hardness increases, the variation in hardness in the steel sheet increases, and the material uniformity is impaired. .. In order to suppress the increase in surface hardness, the steel structure of the surface layer shall be a bainite structure. A portion other than the surface layer portion is also a bainite structure, and the structure at the center of the plate thickness may be a bainite structure on behalf of the portion. Here, the bainite structure includes a structure called bainitic ferrite or granular ferrite that transforms during accelerated cooling or after accelerated cooling, which contributes to strengthening the transformation. When heterogeneous structures such as ferrite, martensite, pearlite, island-like martensite, and retained austenite are mixed in the bainite structure, the strength decreases, the toughness deteriorates, and the surface hardness increases. Therefore, structures other than the bainite phase The smaller the fraction, the better. However, if the volume fraction of the tissue other than the bainite phase is sufficiently low, those effects can be ignored, and a certain amount is acceptable. Specifically, in the present disclosure, if the total volume fraction of steel structures other than bainite (ferrite, martensite, pearlite, island-like martensite, retained austenite, etc.) is less than 5%, there is no significant effect and is acceptable. It shall be done.

Further, the bainite structure also has various forms depending on the cooling rate. In the present disclosure, the structure of the polar surface layer of the steel sheet, specifically, the steel structure 0.25 mm below the surface of the steel sheet is defined as the dislocation density 1. It is important to have a bainite structure of 0 × 10 ^{14 to} 7.0 × 10 ¹⁴ (m- ² ). Since the dislocation density decreases in the coating process after pipe formation, if the dislocation density of 0.25 mm below the surface of the steel sheet is 7.0 × 10 ¹⁴ (m- ² ) or less, the increase in hardness due to age hardening is minimized. It can be suppressed to the limit. On the contrary, when the dislocation density of 0.25 mm below the surface of the steel sheet exceeds 7.0 × 10 ¹⁴ (m- ² ), the dislocation density does not decrease in the coating process after pipe formation, and the hardness increases significantly by age hardening. It deteriorates SSCC resistance. The preferred range of dislocation densities for obtaining good SSCC resistance after pipe formation is 6.0 × 10 ¹⁴ (m- ² ) or less. On the other hand, if the dislocation density of 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 X65 grade, it is preferable to have a dislocation density of 2.0 × 10 ¹⁴ (m- ² ) or more. 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 within the above range, the dislocation density is the same for the polar surface layer portion in the range of 0.25 mm deep from the surface of the steel sheet. As a result, the effect of improving the SSCC resistance can be obtained.

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 HV 0.1 at 0.25 mm below the surface is 230 or less. From the viewpoint of ensuring the SSCC resistance of the steel pipe, it is important to suppress the surface hardness of the steel sheet. However, by setting the HV 0.1 at 0.25 mm below the surface of the steel sheet to 230 or less, 250 after pipe formation. After undergoing a coating heat treatment process at ° C. for 1 hour, HV0.1 at 0.25 mm below the surface can be suppressed to 260 or less, and SSCC resistance can be ensured.

Further, in the high-strength steel sheet of the present disclosure, it is also important that the variation in Vickers hardness at 0.25 mm below the surface of the steel sheet is 30 HV or less at 3σ when the standard deviation is σ. When 3σ when measuring the Vickers hardness at 0.25 mm below the surface of the steel sheet exceeds 30 HV, the hardness variation in the polar surface layer of the steel sheet, that is, the presence of a local high hardness portion on the polar surface layer causes the portion. This is because the SSCC resistance deteriorates starting from. When determining the standard deviation σ, it is preferable to measure the Vickers hardness at 100 points or more.

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

[Production method]
Hereinafter, the manufacturing method and manufacturing conditions for manufacturing the high-strength steel sheet for sour-resistant pipes will be specifically described. In the manufacturing method of the present disclosure, a steel piece having the above-mentioned composition is heated and then hot-rolled to obtain a steel sheet, and then the steel sheet is subjected to controlled cooling under predetermined conditions.

[Slab heating temperature]
Slab heating temperature: 1000-1300 ° C
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., the toughness deteriorates. Therefore, the slab heating temperature is set to 1000 to 1300 ° C. It should be noted that this temperature is the temperature inside the heating furnace, and the slab is heated to this temperature up to the center.

[Rolling end temperature]
In the hot rolling process, in order to obtain high base metal toughness, the lower the rolling end temperature, the better, but on the other hand, the rolling efficiency decreases, so the rolling end temperature at the steel sheet surface temperature is the required base material toughness and rolling. It is necessary to set in consideration of efficiency. From the viewpoint of improving the strength and HIC resistance, it is preferable that the rolling end temperature is the Ar ₃ transformation point or higher at the steel sheet surface temperature. 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 equation. Further, in order to obtain high base metal toughness, it is desirable that the reduction rate in the temperature range of 950 ° C. or lower, which corresponds to the austenite unrecrystallized temperature range, be 60% or more. The surface temperature of the steel sheet can be measured with a radiation thermometer or the like.
Ar ₃ (° C.) = 910-310 [% C] -80 [% Mn] -20 [% Cu] -15 [% Cr] -55 [% Ni] -80 [% Mo]
However, [% X] indicates the content (mass%) of the X element in the steel.

[Cooling start temperature for controlled cooling]
Cooling start temperature: the steel plate surface temperature (Ar 3 _-10 ° C.) or higher cooling starting steel sheet surface temperature is low, the number of ferrite amount before controlled cooling, in particular the amount of temperature drop of from Ar ₃ transformation point is 10 ° C. the generated ferrite exceeding 5% by volume fraction exceeds, the strength decreases to HIC resistance is deteriorated with increases, the steel sheet surface temperature of the cooling at the start and (Ar 3 _-10 ℃) or higher .. The surface temperature of the steel sheet at the start of cooling is equal to or lower than the rolling end temperature.

[Cooling rate of controlled cooling]
It is important to control the cooling rate of the surface layer and the average cooling rate in the steel sheet in order to reduce the variation in hardness in the steel sheet and improve the material uniformity while increasing the strength. In particular, in order to set the dislocation density and 3σ at 0.25 mm below the surface of the steel sheet within the above-mentioned ranges, it is necessary to control the cooling rate at 0.25 mm below the surface of the steel sheet.

Average cooling rate from 750 ° C to 550 ° C at 0.25 mm below the surface of the steel sheet: 50 ° C / s or less Average cooling rate from 750 ° C to 550 ° C at 0.25 mm below the surface of the steel sheet is 50 ° C / If it exceeds s, the dislocation density at 0.25 mm below the surface of the steel sheet exceeds 7.0 × 10 ¹⁴ (m- ² ). As a result, the HV0.1 of 0.25 mm below the surface of the steel sheet exceeds 230, and after the coating process after pipe making, the HV0.1 of 0.25 mm below the surface exceeds 260, and the SSCC resistance of the steel pipe deteriorates. To do. Therefore, the average cooling rate is set to 50 ° C./s or less. It is preferably 45 ° C./s or less, more preferably 40 ° C./s or less. The lower limit of the average cooling rate is not particularly limited, but if the cooling rate becomes excessively small, ferrite and pearlite are generated and the strength becomes insufficient. Therefore, from the viewpoint of preventing this, it is preferably 20 ° C./s or more.

Average cooling rate from 750 ° C to 550 ° C at average steel sheet temperature: 15 ° C / s or more If the average cooling rate from 750 ° C to 550 ° C at average steel sheet temperature is less than 15 ° C / s, bainite structure cannot be obtained and strength Deterioration and deterioration of HIC resistance occur. Therefore, the cooling rate at the average temperature of the steel sheet is set to 15 ° C./s or more. From the viewpoint of variations in steel sheet strength and hardness, 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 it is preferably 80 ° C./s or less so that the low temperature transformation product is not excessively produced.

Average cooling rate from 550 ° C to the temperature at which cooling is stopped at a steel plate temperature of 0.25 mm below the surface of the steel plate: 150 ° C / s or more Stable nuclear boiling for cooling at a steel plate temperature of 0.25 mm below the surface of the steel plate of 550 ° C or less Cooling in the state is required, and an increase in water density is essential. If the average cooling rate is less than 150 ° C / s from 550 ° C to the temperature at which cooling is stopped at a steel plate temperature of 0.25 mm below the surface of the steel sheet, cooling does not occur in the nucleate boiling state, and the hardness varies at the extreme surface layer of the steel sheet. , And 3σ at 0.25 mm below the surface of the steel sheet exceeds 30 HV, resulting in deterioration of SSCC resistance. Therefore, the average cooling rate is set to 150 ° C./s or higher. It is preferably 170 ° C./s or higher. The upper limit of the average cooling rate is not particularly limited, but is preferably 250 ° C./s or less due to equipment restrictions.

Although the 0.25 mm below the surface of the steel plate and the average temperature of the steel plate cannot be physically measured directly, the surface temperature at the start of cooling and the surface temperature at the target cooling stop measured by a radiation thermometer are also included. In addition, for example, the temperature distribution in the plate thickness cross section can be obtained in real time by difference calculation using a process computer. The temperature at 0.25 mm below the surface of the steel sheet in the temperature distribution is defined as "the temperature of the steel sheet at 0.25 mm below the surface of the steel sheet" in the present specification, and the average value of the temperatures in the thickness direction in the temperature distribution is the "steel plate" in the present specification. "Average temperature".

[Cooling stop temperature]
Cooling stop temperature: 250 to 550 ° C at average steel sheet temperature
After the rolling is completed, the bainite phase is formed by quenching to 250 to 550 ° C., which is the temperature range of bainite transformation, by controlled cooling. If the cooling stop temperature exceeds 550 ° C., the bainite transformation is incomplete and sufficient strength cannot be obtained. Further, when the cooling stop temperature is less than 250 ° C., the hardness of the surface layer portion increases remarkably, and the dislocation density exceeds 7.0 × 10 ¹⁴ (m- ² ) at 0.25 mm below the surface of the steel sheet. Therefore, SSCC resistance Deteriorates. In addition, the hardness of the central segregation portion becomes high, and the HIC resistance also deteriorates. Therefore, in order to suppress the deterioration of the material uniformity in the steel sheet, the cooling stop temperature of the control cooling is set to 250 to 550 ° C. as the average temperature of the steel sheet.

[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 the butt portion is welded to provide excellent material uniformity in the steel sheet suitable for transporting crude oil and natural gas. High-strength steel pipes for sour-resistant pipes (UOE steel pipes, welded steel pipes, spiral steel pipes, etc.) can be manufactured.

For example, in a UOE steel pipe, the end portion of a steel sheet is grooved, formed into a steel pipe shape by C press, U press, and O press, and then the butt portion is seam welded by inner surface welding and outer surface welding, and further, if necessary. Manufactured through a pipe expansion process. The welding method may be any method as long as sufficient joint strength and joint toughness can be obtained, but from the viewpoint of excellent welding quality and manufacturing efficiency, submerged arc welding is preferably used.

Steels (steel grades A to M) having the composition shown in Table 1 are made into slabs by a continuous casting method, heated to the temperatures shown in Table 2, and then hot-rolled at the rolling end temperature and rolling reduction ratio shown in Table 2. The steel plate with the thickness shown in Table 2 was used. Then, the steel sheet was controlled-cooled using a water-cooled control cooling device under the conditions shown in Table 2.

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

[Measurement of tensile strength]
A tensile test was performed using a full-thickness test piece in the direction perpendicular to the rolling direction as a tensile test piece, and the tensile strength was measured. The results are shown in Table 2.

[Measurement of Vickers hardness]
For the cross section perpendicular to the rolling direction, 100 points of Vickers hardness (HV0.1) were measured at a position 0.25 mm below the surface of the steel sheet in accordance with JIS Z 2244, and the average value and standard deviation σ were obtained. .. Table 2 shows the average value and the value of 3σ. Here, the measurement with HV0.1 instead of the normally used HV10 is because the indentation becomes smaller by the measurement with HV0.1, so that the hardness information at a position closer to the surface and more sensitive to the microstructure are obtained. This is because it is possible to provide various hardness information.

[Dislocation density]
A sample for X-ray diffraction was taken from a position having average hardness, the surface of the sample was polished to remove scale, and X-ray diffraction measurement was performed at a position 0.25 mm below the surface of the steel sheet. The dislocation density was converted from the strain obtained from the half-value width β of the X-ray diffraction measurement. In the diffraction intensity curve obtained by ordinary X-ray diffraction, since two Kα1 lines and Kα2 lines having different wavelengths overlap, they are separated by the method of Rachinger. The Williamsson-Hall method shown below is used to extract the strain. The spread of the half-value range is affected by the crystallite size D and strain ε, and can be calculated by the following equation as the sum of both factors. β = β1 + β2 = (0.9λ / (D × cosθ)) + 2ε × tanθ. Further modifying this equation, βcosθ / λ = 0.9λ / D + 2ε × sinθ / λ. By plotting βcosθ / λ against sinθ / λ, the strain ε is calculated from the slope of a straight line. The diffraction lines used for the calculation are (110), (211), and (220). For the conversion of the dislocation density from the strain ε, ρ = 14.4 ε ² / b ² was used. Note that θ means the peak angle calculated by 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), which is 0.25 nm in this example.

[Evaluation of SSCC resistance]
The SSCC resistance was evaluated by forming a pipe using a part of each of these steel sheets. Pipe making is manufactured through a pipe expansion process in which the end of the steel sheet is grooved, formed into a steel pipe shape by C press, U press, and O press, and then the butt portion of the inner and outer surfaces is seam welded by submerged arc welding. did. As shown in FIG. 1, after flattening the coupon cut out from the obtained steel pipe, a 5 × 15 × 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 left with black skin in order to retain the state of the outermost layer. A stress of 90% of the actual yield strength (0.5% YS) of each steel pipe is applied to the collected SSCC test piece, and using NACE standard TM0177 Solution A solution, hydrogen sulfide partial pressure: 1 bar, EFC 16 standard. The 4-point bending SSCC test was performed in accordance with the above. Further, using a NACE standard TM0177 Solution B solution, hydrogen sulfide partial pressure: 0.1 bar + carbon dioxide partial pressure: 0.9 bar was carried out in accordance with the EFC16 standard 4-point bending SSCC test. Furthermore, using the NACE standard TM0177 Solution A solution, hydrogen sulfide partial pressure: 2 bar + carbon dioxide partial pressure: 3 bar was also performed in accordance with the 4-point bending SSCC test of the EFC16 standard. When no cracks were observed after 720 hours of immersion, the SSCC resistance was judged to be good, and when cracks occurred, it was judged to be defective and marked as x. The results are shown in Table 2.

[Evaluation of HIC resistance]
The HIC resistance was examined by a 96-hour immersion HIC test at a partial pressure of hydrogen sulfide of 1 bar using a NACE standard TM0177 Solution A solution. In addition, using a NACE standard TM0177 Solution B solution, hydrogen sulfide partial pressure: 0.1 bar + carbon dioxide partial pressure: 0.9 bar was examined by a 96-hour immersion HIC test. The HIC resistance was evaluated as good when the crack length ratio (CLR) was 15% or less in the HIC test, and was evaluated as x when it exceeded 15%. The results are shown in Table 2.

The target range of the present invention is a high-strength steel sheet for sour line pipes with a tensile strength of 520 MPa or more, a bainite structure at both 0.25 mm and t / 2 positions below the surface, and HV0.1 at 0.25 mm below the surface. It was determined that no cracks were observed in the SSCC test and the crack length ratio (CLR) was 15% or less in the HIC test in the high-strength steel pipe formed by using the steel plate.

As shown in Table 2, No. 1-No. Reference numeral 15 denotes an example of the invention in which the component composition and the production conditions satisfy the appropriate range of the present invention. In both cases, the tensile strength of the steel sheet is 520 MPa or more, the microstructure at both the 0.25 mm and t / 2 positions below the surface is a bainite structure, and the HV 0.1 at 0.25 mm below the surface is 230 or less. The high-strength steel pipe produced had good SSCC resistance and HIC resistance.

On the other hand, No. 16-No. 23 is a comparative example 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. No. In No. 16, since the slab heating temperature was low, the homogenization of the microstructure and the solid solution of carbides were insufficient, and the strength was low. No. No. 17 had a low cooling start temperature and had a layered structure in which ferrite was precipitated, so that the strength was low and the HIC resistance after pipe formation was deteriorated. No. 18 had a low strength and HIC resistance after pipe formation because the controlled cooling conditions were outside the scope of the present invention, a bainite structure could not be obtained at the center of the plate thickness as a microstructure, and a ferrite + pearlite structure was formed. The sex has deteriorated. No. In No. 19, the cooling stop temperature was low, the dislocation density at 0.25 mm below the surface was high, and the HV 0.1 exceeded 230, so that the SSCC resistance after pipe formation was inferior. In addition, the hardness of the central segregated portion was also increased, so that the HIC resistance was also deteriorated. In No. 20 and No. 23, the average cooling rate from 750 ° C. to 550 ° C. at the steel sheet temperature 0.25 mm below the surface of the steel sheet greatly exceeded 50 ° C./s, so that the dislocation density at 0.25 mm below the surface was high. As a result, HV0.1 exceeded 230, and SSCC resistance after pipe making was inferior. In addition, No. In No. 23, the HIC resistance at the surface layer was also deteriorated. In No. 21 and No. 22, since the average cooling rate at 550 mm or less below the surface of the steel sheet is less than 150 ° C./s, uneven cooling of the steel sheet becomes remarkable, and HV0.1 is 230 on average. Although the following was satisfied, the SSCC resistance after pipe formation was inferior because the hardness variation was large and a portion having a locally high hardness was generated. In No. 24 to No. 27, the composition of the steel sheet was out of 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 that the SSCC resistance after pipe formation was increased. The sex was inferior. In addition, No. 24 to No. With respect to 27, the hardness of the central segregated portion increased, so that the HIC resistance was also inferior. No. In No. 28, since the amount of Ni in the steel sheet was excessive, the SSCC resistance in an environment where the partial pressure of hydrogen sulfide was low deteriorated. No. In No. 29, since the steel sheet does not contain Mo, the SSCC resistance deteriorated in a very severe corrosion environment of hydrogen sulfide partial pressure of 2 Bar. No. In No. 30, the average cooling rate from 750 ° C. to 550 ° C. exceeded 50 ° C./s at the steel sheet temperature 0.25 mm below the surface of the steel sheet, so that the SSCC resistance deteriorated in a very severe corrosion environment with a partial pressure of hydrogen sulfide of 2 Bar. did.

According to the present invention, a high-strength steel plate for sour line pipes having excellent not only HIC resistance but also SSCC resistance in a harsher corrosion environment and SSCC resistance in an environment with a low partial pressure of hydrogen sulfide of less than 1 bar. Can be supplied. Therefore, steel pipes (electrosewn steel pipes, spiral steel pipes, UOE steel pipes, etc.) manufactured by cold forming this steel sheet can be suitably used for transporting crude oil and natural gas containing hydrogen sulfide, which requires sour resistance. ..

Claims (7)

By 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: It contains 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 Nb: 0. It contains .005 to 0.1% , optionally Ti: 0.005 to 0.1 %, and has a component composition with the balance consisting of Fe and unavoidable impurities.
The microstructure at the center of the plate thickness (t / 2 position) is the bainite structure, and the steel structure at 0.25 mm below the surface of the steel sheet has a dislocation density of 1.0 × 10 ^{14 to} 7.0 × 10 ¹⁴ (m- ^2). ) Is a bainite organization
The variation in 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.
A high-strength steel sheet for sour line pipes, which has a tensile strength of 520 MPa or more. The first aspect of the present invention, wherein the component composition further contains one or more selected from Cu: 0.50% or less, Ni: 0.10% or less, and Cr: 0.50% or less in mass%. The high-strength steel plate for sour line pipes described. The composition of the components is further increased by mass%, V: 0.005-0.1%, Zr: 0.0005-0.02%, Mg: 0.0005-0.02% and REM: 0.0005-. The high-strength steel sheet for sour line pipe according to claim 1 or 2, which contains at least one selected from 0.02%. By 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: It contains 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 Nb: 0. A steel piece containing .005 to 0.1% , optionally Ti: 0.005 to 0.1 %, and the balance having a composition of Fe and unavoidable impurities, at a temperature of 1000 to 1300 ° C. After heating, it is hot-rolled to make a steel sheet.
After that, with respect to the steel sheet
Cooling at the start of the steel sheet surface temperature: (Ar ₃ -10 ℃) or higher,
Average cooling rate from 750 ° C to 550 ° C at steel plate temperature 0.25 mm below the surface of the steel plate: 50 ° C / s or less,
Average cooling rate from 750 ° C to 550 ° C at average steel sheet temperature: 15 ° C / s or more,
Average cooling rate: 150 ° C / s or more from 550 ° C to the temperature at which cooling is stopped at a steel plate temperature of 0.25 mm below the surface of the steel sheet, and cooling stop temperature: 250 to 550 ° C at the average temperature of the steel sheet.
What condition row control cooling in the a microstructure bainite structure at the mid-thickness portion (t / 2 position), the steel structure in the surface of the steel sheet under 0.25mm is, the dislocation density 1.0 × 10 ¹⁴ ~ It has a Bainite structure of 7.0 × 10 ¹⁴ (m- ² ), and the variation in Vickers hardness at 0.25 mm below the surface of the steel sheet is 30 HV or less at 3σ when the standard deviation is σ, and the tensile strength is 520 MPa or more. A method for producing a high-strength steel sheet for a sour-resistant line pipe, which comprises producing a high-strength steel sheet having strength . According to claim 4, the component composition further contains at least one selected from Cu: 0.50% or less, Ni: 0.10% or less, and Cr: 0.50% or less in mass%. The method for manufacturing a high-strength steel plate for sour-resistant line pipes described. The composition of the components is further increased by mass%, V: 0.005-0.1%, Zr: 0.0005-0.02%, Mg: 0.0005-0.02% and REM: 0.0005-. The method for producing a high-strength steel sheet for sour line pipe according to claim 4 or 5, which contains at least one selected from 0.02%. A high-strength steel pipe using the high-strength steel plate for sour-resistant line pipe according to any one of claims 1 to 3.

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