WO2018181564A1

WO2018181564A1 - High strength steel sheet for sour-resistant line pipe, method for manufacturing same, and high strength steel pipe using high strength steel sheet for sour-resistant line pipe

Info

Publication number: WO2018181564A1
Application number: PCT/JP2018/012956
Authority: WO
Inventors: 周作太田; 横田　智之; 長谷　和邦; 雄太田村
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2017-03-30
Filing date: 2018-03-28
Publication date: 2018-10-04
Also published as: EP3604592A4; EP3604592A1; JP6844691B2; JPWO2018181564A1; KR20210118960A; KR20190129097A; BR112019020236A2; EP3604592B1; CN110475894B; CN110475894A

Abstract

The present invention provides a high strength steel sheet for a sour-resistant line pipe, the high strength steel sheet having excellent HIC resistance and SSCC resistance under extremely severe corrosive environments, and having excellent hardness uniformity in the thickness direction. This high strength steel sheet for a sour-resistant line pipe has a predetermined component composition, and is characterized in that the steel structure 0.5 mm below the surface of the steel sheet is a bainite structure having a dislocation density of 0.5×10¹⁴-7.0×10¹⁴ (m^-2), the difference ΔHV between the average Vickers hardness value 0.5 mm below the surface of the steel sheet and the average Vickers hardness value at the center with respect to the thickness of the steel sheet is 25 HV or less, and the tensile strength of the steel sheet is at least 520 MPa.

Description

High-strength steel sheet for sour line pipes, method for producing the same, and high-strength steel pipe using high-strength steel sheets for sour line pipes

The present invention is suitable for use in line pipes in the fields of architecture, offshore structures, shipbuilding, civil engineering, and construction industrial machines, and is a high-strength steel sheet for sour-resistant pipes with excellent material uniformity in the steel sheet and its manufacture. It is about the method. The present invention also relates to a high-strength steel pipe using the above-described high-strength steel plate for sour line pipes.

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

Here, line pipes used for transporting crude oil and natural gas containing hydrogen sulfide are resistant to hydrogen induced cracking (HIC (Hydrogen Induced Cracking)) and sulfides in addition to strength, toughness and weldability. So-called sour resistance such as stress corrosion cracking resistance (SSCC (Sulfide-Stress-Corrosion-Cracking) resistance) is required. In particular, in HIC, hydrogen ions from the corrosion reaction are adsorbed on the steel surface, penetrate into the steel as atomic hydrogen, and diffuse and accumulate around non-metallic inclusions such as MnS and hard second-phase structures in the steel. As a result, it becomes molecular hydrogen and cracks are caused by the internal pressure thereof, which is a problem in a line pipe having a relatively low strength level with respect to an oil well pipe, and many countermeasure techniques have been disclosed. On the other hand, SSCC is generally known to occur in high-strength seamless steel pipes for oil wells and high hardness regions of welds, and has not been regarded as a problem for line pipes with relatively low hardness. . However, in recent years, it has been reported that the mining environment for crude oil and natural gas has become increasingly severe, and SSCC also occurs in the base material of the line pipe in an environment where the hydrogen sulfide partial pressure is high or the pH is low. The importance of improving the SSCC resistance in a more severe corrosive environment by controlling the hardness of the steel pipe inner surface layer has been pointed out.

Usually, when manufacturing 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 steel 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 plate, and the hardness distribution in the thickness direction varies. Therefore, it becomes a problem from the viewpoint of ensuring the material uniformity in the steel plate.

In order to solve the above problem, for example, in Patent Documents 1 and 2, there is a material difference in the plate thickness direction by interrupting accelerated cooling after rolling, reaccelerating the surface, and then performing accelerated cooling again. A method for manufacturing a small steel sheet is disclosed. Patent Documents 3 and 4 disclose a method for manufacturing a steel plate for a line pipe, which uses a high-frequency induction heating device to heat the steel plate surface after accelerated cooling to a higher temperature from the inside to reduce the hardness of the surface layer portion. Has been.

On the other hand, if there is unevenness in the scale thickness on the surface of the steel plate, the cooling rate of the steel plate underneath will also vary during cooling, causing local variations in the cooling stop temperature within the steel plate. As a result, the unevenness of the scale thickness causes variations in the steel plate material in the plate width direction. On the other hand, Patent Documents 5 and 6 disclose a method for improving the steel plate shape by reducing the uneven cooling due to the uneven thickness of the scale by performing descaling immediately before the cooling.

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

However, according to the study by the present inventors, there is room for improvement in terms of HIC resistance and SSCC resistance in a more severe corrosive environment in the high-strength steel sheets obtained by the production methods described in Patent Documents 1 to 6 above. Turned out to be. The reason is as follows.

In the production methods described in Patent Documents 1 and 2, if the transformation behavior differs depending on the steel plate components, the effect of sufficient material homogenization by recuperation may not be obtained.

In the manufacturing methods described in Patent Documents 3 and 4, since the cooling rate of the surface layer portion in accelerated cooling is large, the hardness of the surface layer portion may not be sufficiently reduced only by heating the steel sheet surface.

On the other hand, the methods described in Patent Documents 5 and 6 improve the steel sheet shape by descaling to reduce surface quality defects due to indentation of the scale during hot correction and to reduce variation in the cooling stop temperature of the steel sheet. However, no consideration is given to the cooling conditions for obtaining a uniform material. That is, in the techniques described in Patent Documents 5 and 6, the cooling rate of the surface layer portion in the accelerated cooling is not considered at all. Therefore, there is a possibility that the hardness of the surface layer portion may not be sufficiently reduced at the cooling rate for securing the tensile properties in the center of the plate thickness, and as a result, the hardness may vary in the plate thickness direction. Concerned.

Therefore, in view of the above problems, the present invention provides a high-strength steel sheet for sour line pipes that is excellent in HIC resistance and SSCC resistance under a more severe corrosive environment and excellent in hardness uniformity in the thickness direction. It is intended to provide with its advantageous manufacturing method. Another object of the present invention is to propose a high-strength steel pipe using the high-strength steel sheet for sour-resistant pipes.

The present inventors repeated numerous experiments and examinations on the component composition, microstructure, and production conditions of the steel material in order to ensure HIC resistance and SSCC resistance under a more severe corrosive environment. As a result, in order to further improve the SSCC resistance of high-strength steel pipes, it is not sufficient to simply suppress the surface layer hardness as in the past. In particular, the structure of the extreme surface layer portion of the steel sheet, specifically the steel sheet surface By making the steel structure of 0.5 mm below into a bainite structure with a dislocation density of 0.5 × 10 ¹⁴ to 7.0 × 10 ¹⁴ (m ⁻² ), an increase in hardness can be reduced in the coating process after pipe forming. It was found that the SSCC resistance of the steel pipe was improved as a result.

Furthermore, in order to realize such a steel structure, both the thermal history at 0.5 mm below the steel sheet surface in the controlled cooling and the thermal history of the steel sheet average are strictly controlled, and then the excess introduced by the controlled cooling. We found that it is important to reduce the dislocations by induction heating. Further, by performing induction heating under a predetermined condition in consideration of the steel plate surface temperature T ₁ at the start of cooling in the controlled cooling and the cooling stop temperature T ₂ at the steel plate average temperature, the variation in hardness in the plate thickness direction is caused. It was found that it can be significantly reduced. The present invention has been made based on this finding.

That is, the gist configuration of the present invention is as follows.
[1] 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: 0.0002 to 0.0015%, Al: 0.01 to 0.08% and Ca: 0.0005 to 0.005%, and the CP value obtained by the following formula (1) is 1 0.000 or less, and the balance has a component composition consisting of Fe and inevitable impurities,
The steel structure at 0.5 mm below the steel sheet surface is a bainite structure having a dislocation density of 0.5 × 10 ¹⁴ to 7.0 × 10 ¹⁴ (m ⁻² ),
The difference ΔHV between the average value of Vickers hardness at 0.5 mm below the steel sheet surface and the average value of Vickers hardness at the center of the steel sheet thickness is 25 HV or less,
A high-strength steel sheet for sour line pipes, characterized by having a tensile strength of 520 MPa or more.
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 element X in steel.

[2] The component composition was further selected by mass% from 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 sour-resistant pipes according to [1] above, containing one or more kinds.

[3] The component composition is further selected from Nb: 0.005 to 0.1%, V: 0.005 to 0.1%, and Ti: 0.005 to 0.1% by mass%. The high-strength steel sheet for sour-resistant pipes according to the above [1] or [2], containing one or more kinds.

[4] 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: 0.0002 to 0.0015%, Al: 0.01 to 0.08% and Ca: 0.0005 to 0.005%, and the CP value obtained by the following formula (1) is 1 A steel slab having a component composition of Fe and unavoidable impurities in the balance is heated to 1000-1300 ° C., and then hot-rolled into a steel plate,
After that,
Steel sheet surface temperature T ₁ at the start of cooling: (Ar ₃ −10 ° C.) or more,
Average cooling rate from 750 ° C. to 550 ° C. at a steel plate temperature at 0.5 mm below the steel plate surface: 100 ° C./s or less,
Average cooling rate from 750 ° C. to 550 ° C. at the steel plate average temperature: 15 ° C./s or more, and cooling stop temperature T ₂ at the steel plate average temperature: 250 to 550 ° C.
Controlled cooling under the conditions of
Thereafter, the steel sheet is reheated by induction heating so that the average temperature of the steel sheet is equal to or higher than the cooling stop temperature T ₂ and the steel sheet surface temperature is a heating temperature T ₃ of 550 to 750 ° C. Manufacturing method of high strength steel plate for sour line pipes.
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 element X in steel.

[5] The component composition is further selected by mass% from 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 sour-resistant pipes according to [4] above, which contains one or more kinds.

[6] The component composition is further selected from Nb: 0.005 to 0.1%, V: 0.005 to 0.1% and Ti: 0.005 to 0.1% by mass%. The manufacturing method of the high strength steel plate for sour-resistant pipes as described in said [4] or [5] containing 1 type (s) or 2 or more types.

[7] The reheating is performed so as to satisfy a condition that a TP defined by the following formula (2) is 0.50 or more, according to any one of the above [4] to [6]. Manufacturing method of high strength steel plate for sour line pipe.
TP = (T ₃ −T ₂ ) × T ₂ / (T ₁ −T ₂ ) ² (2)

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

The high-strength steel plate for sour line pipe and the high-strength steel pipe using the high-strength steel plate for sour line pipe of the present invention are excellent in HIC resistance and SSCC resistance in a more severe corrosion environment, and in the thickness direction. Excellent hardness uniformity. Further, according to the method for producing a high-strength steel sheet for sour line pipes of the present invention, the HIC resistance and SSCC resistance in a more severe corrosive environment are excellent, and the hardness uniformity in the thickness direction is also excellent. A high-strength steel sheet for sour line pipes can be manufactured.

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

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

C: 0.02 to 0.08%
C contributes effectively 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 increases during accelerated cooling. , HIC resistance and SSCC resistance deteriorate. In addition, toughness deteriorates. For this reason, the C content is limited to a 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 deoxidation effect is not sufficient. On the other hand, if it exceeds 0.50%, the toughness and weldability are deteriorated. It is limited to the range of 01 to 0.50%.

Mn: 0.50 to 1.80%
Mn contributes effectively to the improvement of strength and toughness, but if the content is less than 0.50%, the effect of addition is poor, while if it exceeds 1.80%, the hardness of the central segregation part increases during accelerated cooling. Therefore, the HIC resistance is deteriorated. Moreover, weldability also deteriorates. For this reason, the amount of Mn is limited to the range of 0.50 to 1.80%.

P: 0.001 to 0.015%
P is an inevitable impurity element, and deteriorates the weldability and also increases the hardness of the center segregation part to deteriorate the HIC resistance. Since the tendency will become remarkable when it exceeds 0.015%, an upper limit is prescribed | regulated to 0.015%. Preferably it is 0.008% or less. The lower the content, the better, but 0.001% or more from the viewpoint of refining costs.

S: 0.0002 to 0.0015%
S is an unavoidable impurity element, and is preferably MnS inclusion in the steel, so that the HIC resistance is degraded. The lower the content, the better, but 0.0002% or more from the viewpoint of refining costs.

Al: 0.01 to 0.08%
Al is added as a deoxidizer, but 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 is lowered and the toughness is deteriorated. It is limited to the range of 01 to 0.08%.

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

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

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

Ni: 0.50% or less Ni is an element effective for improving toughness and increasing strength. To obtain this effect, it is preferable to contain 0.05% or more, but if the content is too large, it is economical. This is not only disadvantageous, but also the toughness of the weld heat affected zone deteriorates. Therefore, when Ni is added, the upper limit is 0.50%.

Cr: 0.50% or less Cr, like Mn, is an element effective for obtaining sufficient strength even at low C. To obtain this effect, it is preferable to contain 0.05% or more. If the amount is too large, weldability deteriorates, so when Cr is added, the upper limit is 0.50%.

Mo: 0.50% or less Mo is an element effective in improving toughness and increasing strength. To obtain this effect, it is preferable to contain 0.05% or more, but if the content is too large, welding is performed. When the Mo is added, the upper limit is 0.50%.

The component composition of the present disclosure may further contain one or more selected from Nb, V and Ti within the following ranges.

One or more selected from Nb: 0.005 to 0.1%, V: 0.005 to 0.1% and Ti: 0.005 to 0.1% Any of Nb, V and Ti Is an element that can be optionally added to increase the strength and toughness of the steel sheet. When the content of each element is less than 0.005%, the effect of addition is poor. On the other hand, when the content exceeds 0.1%, the toughness of the welded portion deteriorates. It is preferable to be in the range.

This disclosure discloses a technique for improving the SSCC resistance of a high-strength steel pipe using a high-strength steel plate for sour line pipes. Needless to say, the sour-proof performance is not limited to HIC resistance. Since it is necessary to satisfy simultaneously, CP value calculated | required by the following (1) formula shall be 1.00 or less. Note that 0 may be substituted for elements 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 element X in steel.

Here, the CP value is an expression devised for estimating the material of the center segregation part from the content of each alloy element. The higher the CP value of the above formula (1), the higher the component concentration of the center segregation part. Increases and the hardness of the central segregation part increases. Therefore, it is possible to suppress the occurrence of cracks in the HIC test by setting the CP value obtained in the above equation (1) to 1.00 or less. Further, the lower the CP value, the lower the hardness of the center segregation part. Therefore, when higher HIC resistance is required, the upper limit may be set to 0.95.

Note that the balance other than the above-described elements is composed of Fe and inevitable impurities. However, the content of other trace elements is not hindered unless the effects of the present invention are impaired.

[Structure of steel sheet]
Next, the steel structure of the high-strength steel sheet for sour line pipes according to the present disclosure will be described. In order to increase the 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 is increased, the hardness variation in the steel sheet is increased, and the material uniformity is inhibited. . In order to suppress the increase in surface hardness, the steel structure of the surface layer portion is a bainite structure. Here, 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 different types of 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. The smaller the fraction, the better. However, when the volume fraction of the structure other than the bainite phase is sufficiently low, the influence thereof can be ignored, so that a certain amount is acceptable. Specifically, in the present disclosure, if the sum of the steel structures other than bainite (ferrite, martensite, pearlite, island martensite, residual austenite, etc.) is less than 5% in volume fraction, there is no significant influence, so Shall be.

Further, although there are various forms of bainite structure depending on the cooling rate, in the present disclosure, the structure of the extreme surface layer portion of the steel sheet, specifically, the steel structure of 0.5 mm below the steel sheet surface has a dislocation density of 0. It is important to have a bainite structure of 5 × 10 ¹⁴ to 7.0 × 10 ¹⁴ (m ⁻² ). Since the dislocation density decreases in the coating process after pipe forming, if the dislocation density 0.5 mm below the steel sheet surface is 7.0 × 10 ¹⁴ (m ⁻² ) or less, the increase in hardness due to age hardening is minimized. To the limit. Conversely, when the dislocation density of 0.5 mm below the steel sheet surface exceeds 7.0 × 10 ¹⁴ (m ⁻² ), the dislocation density does not decrease in the coating process after pipe forming, and the hardness increases greatly by age hardening. To deteriorate the SSCC resistance. In order to obtain good SSCC resistance after pipe forming, a preferable range of dislocation density is 6.0 × 10 ¹⁴ (m ⁻² ) or less. On the other hand, if the dislocation density 0.5 mm below the steel sheet surface is less than 0.5 × 10 ¹⁴ (m ⁻² ), the strength of the steel sheet cannot be maintained. In order to ensure the strength of the X65 grade, it is preferable to have a dislocation density of 1.0 × 10 ¹⁴ (m ⁻² ) or more. In the high-strength steel sheet of the present disclosure, if the dislocation density in the steel structure 0.5 mm below the steel sheet surface is in the above range, the extreme surface layer part in the range of 0.5 mm depth from the steel sheet surface also has an equivalent dislocation density. As a result, the effect of improving the SSCC resistance can be obtained.

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

Moreover, in the high-strength steel sheet of the present disclosure, in addition to the average value of Vickers hardness at 0.5 mm below the steel sheet surface being 230 HV or less, the material properties at the center of the sheet thickness can be secured while suppressing the hardness of the surface layer. From the viewpoint, it is also important that the difference ΔHV between the average value of Vickers hardness at 0.5 mm below the steel sheet surface and the average value of Vickers hardness at the center of the steel sheet thickness is 25 HV or less. More preferable ΔHV is 20 HV or less.

Since the high-strength steel sheet of the present disclosure is a steel pipe steel sheet having an API 5L X60 grade or higher strength, it has 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, after heating a steel slab having the above composition, it is hot-rolled into a steel plate, and then the steel plate is subjected to controlled cooling under a predetermined condition, and then the steel plate is re-induced by induction heating. Heat.

[Slab heating temperature]
Slab heating temperature: 1000-1300 ° C
If the slab heating temperature is less than 1000 ° C., the required strength cannot be obtained due to insufficient solid solution of the carbide. On the other hand, if the slab heating temperature exceeds 1300 ° C., the toughness deteriorates, so the slab heating temperature is set to 1000 to 1300 ° C. This temperature is the furnace temperature of 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 a high base metal toughness, the lower the rolling end temperature, the better. However, 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 strength and HIC resistance, it is preferable that the rolling end temperature is not less than the Ar ₃ transformation point 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 steel components by the following formula, for example. In order to obtain high base metal toughness, it is desirable that the rolling reduction in a temperature range of 950 ° C. or lower 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 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 element X in steel.

[Cooling start temperature of control cooling]
Steel plate surface temperature T ₁ at the start of cooling: (Ar ₃ −10 ° C.) or more When the steel plate surface temperature at the start of cooling is low, the amount of ferrite produced before controlled cooling increases, and in particular, the temperature drop from the Ar ₃ transformation point When the temperature exceeds 10 ° C., ferrite with a volume fraction exceeding 5% is generated, and the strength drop increases and the HIC resistance deteriorates. Therefore, the steel sheet surface temperature at the start of cooling is (Ar ₃ −10 ° C.) or more. And

[Cooling rate of controlled cooling]
Suppressing the cooling rate on the surface layer (specifically, a depth of 0.5 mm below the surface of the steel sheet) in order to reduce the hardness variation in the steel sheet and improve the material uniformity while increasing the strength However, it is necessary to ensure the cooling rate in the transformation temperature section centered on the plate thickness.

Average cooling rate from 750 ° C. to 550 ° C. at a steel plate temperature at 0.5 mm below the steel plate surface: 100 ° C./s or less Average cooling rate from 750 ° C. to 550 ° C. at a steel plate temperature at 0.5 mm below the steel plate surface is 100 ° C. / If it exceeds s, the dislocation density at 0.5 mm below the steel sheet surface exceeds 7.0 × 10 ¹⁴ (m ⁻² ). As a result, HV0.1 of 0.5 mm below the steel sheet surface exceeds 230, and after passing through the coating process after pipe forming, HV0.1 at 0.5 mm below the surface exceeds 260, and the SSCC resistance of the steel pipe deteriorates. To do. Therefore, the said average cooling rate shall be 100 degrees C / s or less. Preferably it is 80 degrees C / s or less. The lower limit of the average cooling rate is not particularly limited. However, if the cooling rate is excessively small, ferrite and pearlite are generated and the strength becomes insufficient. Therefore, from the viewpoint of preventing this, it is preferably set to 10 ° C./s or more.

Average cooling rate from 750 ° C. to 550 ° C. at the average temperature of the steel plate: 15 ° C./s or more When the average cooling rate from 750 ° C. to 550 ° C. at the average temperature of the steel plate is less than 15 ° C./s, the bainite structure is not obtained and the strength Decrease and deterioration of HIC resistance occur, or the variation in hardness in the thickness direction increases. For this reason, the cooling rate at the steel plate average temperature is set to 15 ° C./s or more. From the viewpoint of variations in steel plate strength and hardness, the average cooling rate of the steel plate is preferably 20 ° C./s or more. The upper limit of the average cooling rate is not particularly limited, but is preferably set to 80 ° C./s or less so that the low temperature transformation product is not excessively generated.

Note that the 0.5 mm below the steel plate surface and the average steel plate temperature cannot be physically measured directly, but the surface temperature at the start of cooling measured with a radiation thermometer and the surface temperature at the target cooling stop are also measured. In addition, for example, the temperature distribution in the cross section of the plate thickness can be obtained in real time by difference calculation using a process computer. The temperature at 0.5 mm below the steel sheet surface in the temperature distribution is defined as “steel temperature at 0.5 mm below the steel sheet surface” in this specification, and the average value of the temperature in the plate thickness direction in the temperature distribution is “ Average temperature ”.

[Cooling stop temperature]
Steel sheet average temperature and cooling stop temperature T ₂ : 250 to 550 ° C
After rolling, the bainite phase is generated by quenching to 250 to 550 ° C., which is the temperature range of bainite transformation, by controlled cooling. When the cooling stop temperature exceeds 550 ° C., the bainite transformation is incomplete and sufficient strength cannot be obtained. In addition, when the cooling stop temperature is less than 250 ° C., martensite and island martensite (MA) are generated, and in particular, the variation in hardness in the thickness direction becomes large. Therefore, in order to suppress deterioration of material uniformity in the steel plate, the cooling stop temperature of the controlled cooling is set to 250 to 550 ° C. as the steel plate average temperature.

[Induction heating conditions]
Induction heating temperature T ₃ : 550 to 750 ° C. at the steel sheet surface temperature
In this embodiment, after the controlled cooling, it is important to temper the high-density dislocations introduced into the bainite by the controlled cooling. Thereby, the dislocation density at 0.5 mm below the steel sheet surface was 7.0 × 10 ¹⁴ (m ⁻² ) or less, and excellent SSCC resistance was obtained, and the average of the Vickers hardness at 0.5 mm below the steel sheet surface was obtained. The difference ΔHV between the value and the average value of Vickers hardness at the center of the steel plate thickness can be 25 HV or less. Here, if the induction heating temperature is lower than 550 ° C., a sufficient tempering effect cannot be obtained, and even if the dislocation density of the surface layer can be 7.0 × 10 ¹⁴ (m ⁻² ) or less, ΔHV is It cannot be less than 25HV. If the induction heating temperature exceeds 750 ° C., the center of the plate thickness is also tempered, and there is a possibility that a predetermined strength cannot be obtained. Therefore, in order to secure the strength at the center of the plate thickness while suppressing deterioration of material uniformity in the steel plate, the ultimate temperature of the on-line induction heating is set to 550 to 750 ° C. at the steel plate surface temperature. In the present embodiment, it is important that only the surface layer portion is tempered without tempering the inside of the steel sheet as much as possible in order to suppress a decrease in strength, and therefore, an online induction heating device is used for heating.

Regarding reheating conditions, it is preferable that TP defined by the following formula (2) satisfies 0.50 or more and 1.50 or less. More preferably, it is 0.60 or more and 1.00 or less.
TP = (T ₃ −T ₂ ) × T ₂ / (T ₁ −T ₂ ) ² (2)
TP is a relational expression of tempering with respect to the degree of supercooling of the controlled cooling, and when this satisfies 0.50 or more, the dislocation of the surface layer portion introduced by the accelerated cooling is sufficiently recovered and excessive in the center of the plate thickness. Since tempering is not imitated, it is possible to remarkably suppress variation in hardness in the thickness direction. Specifically, ΔHV can be set to 20 or less.

[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, ERW steel pipes, spiral steel pipes, etc.) can be manufactured.

For example, in UOE steel pipe, the end of a steel plate is grooved and 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. Manufactured through a tube expansion process. Any welding method may be used as long as sufficient joint strength and joint toughness can be obtained, but it is preferable to use submerged arc welding from the viewpoint of excellent welding quality and manufacturing efficiency.

Steels (steel types A to I) having the composition shown in Table 1 were made into slabs by a continuous casting method, heated to the temperatures shown in Table 2, and then hot rolled at the rolling end temperatures and reduction rates shown in Table 2. Thus, a steel plate having a thickness shown in Table 2 was obtained. Thereafter, controlled cooling was performed on the steel sheet using a water-cooled control cooling device under the conditions shown in Table 2. Immediately thereafter, the steel sheet was reheated by the method shown in “Heating Method” in Table 2 so that the steel sheet surface temperature became the “maximum temperature during reheating” in Table 2.

[Identify organization]
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 plate thickness.

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

[Measurement of Vickers hardness]
With respect to the cross section perpendicular to the rolling direction, 20 points of Vickers hardness (HV0.1) were measured at a position 0.5 mm below the steel sheet surface in accordance with JIS Z 2244, and the average value was obtained. Similarly, 20 points of Vickers hardness (HV0.1) were measured at the center of the plate thickness, and the average value was obtained. And the absolute value (DELTA) HV of the difference of both was calculated | required. Here, instead of the normally used HV10, the measurement was performed with HV0.1, because the indentation is reduced by measuring with HV0.1, so the hardness information at a position closer to the surface and more sensitive to the microstructure This is because it is possible to perform accurate hardness information.

[Dislocation density]
A sample for X-ray diffraction was collected from a position having an average hardness, the sample surface was polished to remove the scale, and X-ray diffraction measurement was performed at a position 0.5 mm below the steel sheet surface. The dislocation density was converted from the strain obtained from the half width β of the X-ray diffraction measurement. In the diffraction intensity curve obtained by normal X-ray diffraction, the Kα1 line and the Kα2 line having different wavelengths overlap each other, so that they are separated by the Rachinger method. The Williamsson-Hall method shown below is used for distortion extraction. The spread of the half width is affected by the size D of the crystallite and the strain ε, and can be calculated by the following equation as the sum of both factors. β = β1 + β2 = (0.9λ / (D × cos θ)) + 2ε × tan θ. Further, this equation is transformed to βcos θ / λ = 0.9λ / D + 2ε × sin θ / λ. By plotting β cos θ / λ against sin θ / λ, the strain ε is calculated from the slope of the 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 = 14.4ε ² / b ² was used. Here, 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), and in this example, it was 0.25 nm.

[Evaluation of SSCC resistance]
SSCC resistance was evaluated by pipe forming using a part of each of these steel plates. Pipe making is performed after the end of the steel plate is grooved and formed into a steel pipe shape by C-press, U-press and O-press, then the butt part of the inner and outer surfaces is seam welded by submerged arc welding, and the tube is expanded. did. As shown in FIG. 1, after flattening a 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 test surface, was left with a black skin to leave the outermost layer. The collected SSCC test piece was loaded with 90% of the actual yield strength (0.5% YS) of each steel pipe, and using NACE TM0177 Solution A solution, hydrogen sulfide partial pressure: 1 bar, EFC16 standard The four-point bending SSCC test was conducted. A case where no crack was observed after immersing for 720 hours was judged as good when the SSCC resistance was good, and a case where a crack occurred was judged as poor and was marked as x. The results are shown in Table 2.

[Evaluation of HIC resistance]
The HIC resistance was evaluated as “Good” when a HIC test was conducted with an immersion time of 96 hours in accordance with NACE Standard TM-02-84. As evaluated. The results are shown in Table 2.

The target range of the present invention is that the tensile strength is 520 MPa or more as a high-strength steel plate for sour line pipes, the microstructure is a bainite structure at 0.5 mm below the surface and the t / 2 position, and HV0.1 is 0.5 mm below the surface. Is 230 or less, the absolute value ΔHV of the difference between the hardness at 0.5 mm below the surface and the hardness at the center of the plate thickness is 25 or less, and no cracks are observed in the SSCC test in a high-strength steel pipe made using the steel plate In addition, no cracks were observed in the HIC test.

As shown in Table 2, No. 1-No. 9 is an invention example in which the component composition and production conditions satisfy the appropriate range of the present invention. In either case, the tensile strength of the steel sheet is 520 MPa or more, the microstructure is a bainite structure at both the 0.5 mm position and the t / 2 position below the surface, the HV0.1 is 0.5 or less and the ΔHV is 25 or less at 0.5 mm below the surface, SSCC resistance and HIC resistance were also good in the high-strength steel pipe made using the steel plate.

On the other hand, No. 10-No. No. 16 is a comparative example in which the component composition is within the scope of the present invention but the production conditions are outside the scope of the present invention. No. In No. 10, the cooling stop temperature was low, so the difference in hardness between the surface layer and the center of the plate thickness was large. In Nos. 11 and 12, the controlled cooling condition was outside the range of the present invention, and the dislocation density was significantly increased in the steel sheet surface layer, so that the surface layer hardness increased and SSCC was generated. No. In No. 13, the average cooling rate of the steel plate was not sufficiently secured, and ferrite was formed at the center of the plate thickness. In No. 14, the heating temperature in the on-line induction heating was not optimal, so that a hardness difference in the plate thickness direction occurred. No. No. 15 is tempered by furnace heating, but has a low strength because the rate of temperature rise is slow and the entire thickness is tempered on average. No. No. 16 is a case in which reheating is not performed, and since the surface layer is not softened by tempering, the dislocation density of the surface layer is high, which causes the occurrence of SSCC. Also, the thickness variation in the thickness direction is large. In No. 17 to No. 20, the component composition of the steel sheet is outside the scope of the present invention, and the HIC resistance is deteriorated.

According to the present invention, it is possible to supply a high-strength steel sheet for sour line pipes that is excellent in HIC resistance and SSCC resistance in a more severe corrosive environment and excellent in hardness uniformity in the thickness direction. . Therefore, a steel pipe (such as an electric resistance steel pipe, a spiral steel pipe, or a UOE steel pipe) manufactured by cold forming this steel sheet can be suitably used for transporting crude oil or natural gas containing hydrogen sulfide that requires sour resistance. .

Claims

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%, and Ca: 0.0005 to 0.005%, and the CP value obtained by the following formula (1) is 1.00 or less And the remainder has a component composition consisting of Fe and inevitable impurities,
The steel structure at 0.5 mm below the steel sheet surface is a bainite structure having a dislocation density of 0.5 × 10 14 to 7.0 × 10 14 (m −2 ),
The difference ΔHV between the average value of Vickers hardness at 0.5 mm below the steel sheet surface and the average value of Vickers hardness at the center of the steel sheet thickness is 25 HV or less,
A high-strength steel sheet for sour line pipes, characterized by having a tensile strength of 520 MPa or more.
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 element X in steel.
In addition, the component composition may be one by mass selected from 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 sour-resistant pipes according to claim 1, containing two or more kinds.
The component composition is further selected by mass% from Nb: 0.005 to 0.1%, V: 0.005 to 0.1%, and Ti: 0.005 to 0.1%. Or the high-strength steel plate for sour-resistant pipes of Claim 1 or 2 containing 2 or more types.
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%, and Ca: 0.0005 to 0.005%, and the CP value obtained by the following formula (1) is 1.00 or less And the balance of the steel slab having the composition of Fe and inevitable impurities is heated to a temperature of 1000 to 1300 ° C. and then hot-rolled into a steel sheet,
After that,
Steel sheet surface temperature T 1 at the start of cooling: (Ar 3 −10 ° C.) or more,
Average cooling rate from 750 ° C. to 550 ° C. at a steel plate temperature at 0.5 mm below the steel plate surface: 100 ° C./s or less,
Average cooling rate from 750 ° C. to 550 ° C. at the steel plate average temperature: 15 ° C./s or more, and cooling stop temperature T 2 at the steel plate average temperature: 250 to 550 ° C.
Controlled cooling under the conditions of
Thereafter, the steel sheet is reheated by induction heating so that the average temperature of the steel sheet is equal to or higher than the cooling stop temperature T 2 and the steel sheet surface temperature is a heating temperature T 3 of 550 to 750 ° C. Manufacturing method of high strength steel plate for sour line pipes.
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 element X in steel.
In addition, the component composition may be one by mass selected from Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, and Mo: 0.50% or less. The manufacturing method of the high strength steel plate for sour-resistant pipes of Claim 4 containing 2 or more types.
The component composition is further selected by mass% from Nb: 0.005 to 0.1%, V: 0.005 to 0.1%, and Ti: 0.005 to 0.1%. Or the manufacturing method of the high strength steel plate for sour-proof pipes of Claim 4 or 5 containing 2 or more types.
The high strength for a sour line pipe according to any one of claims 4 to 6, wherein the reheating is performed so as to satisfy a condition that TP defined by the following formula (2) is 0.50 or more. A method of manufacturing a steel sheet.
TP = (T 3 −T 2 ) × T 2 / (T 1 −T 2 ) 2 (2)
A high-strength steel pipe using the high-strength steel sheet for sour-resistant pipes according to any one of claims 1 to 3.