JP6844691B2 - High-strength steel sheets for sour-resistant pipes and their manufacturing methods, and high-strength steel pipes using high-strength steel sheets for sour-resistant pipes - Google Patents

Description

INDUSTRIAL APPLICABILITY The present invention relates to a high-strength steel plate for sour line pipes having excellent material uniformity in a steel plate, 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, the line pipe used for transporting crude oil containing hydrogen sulfide and natural gas has 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 second phase 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, it has been reported that the mining environment for crude oil and natural gas has become more and more severe, and 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.

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 steel materials 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, there is a difference in material in the plate thickness direction due to interruption of accelerated cooling after rolling, reheating of the surface, and then acceleration cooling again. A method for manufacturing a small steel sheet 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 of 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 variation in the cooling stop temperature in the steel sheet becomes a problem. As a result, the material of the steel sheet varies in the 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 plate 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 studies by the present inventors, the high-strength steel sheets obtained by the manufacturing methods described in Patent Documents 1 to 6 have room for improvement in terms of HIC resistance and SSCC resistance under a harsher corrosion environment. It turned out that there is. 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.

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. 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, the hardness of the surface layer portion may not be sufficiently reduced at the cooling rate for ensuring the tensile property at the center of the plate thickness, and as a result, the hardness may vary in the plate thickness direction. I am concerned.

Therefore, in view of the above problems, the present invention provides a high-strength steel plate for sour line pipes, which is excellent in HIC resistance and SSCC resistance in a harsher corrosion environment and also has excellent hardness uniformity in the plate thickness direction. It is intended to be provided 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 HIC resistance and SSCC resistance in a more severe corrosion 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 lower 0.5 mm steel structure into ^{a bainite structure with a dislocation density of 0.5 × 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 characteristics of the steel pipe are improved.

Furthermore, in order to realize such a steel structure, both the thermal history at 0.5 mm below the surface of the steel sheet and the thermal history of the average of the steel sheet in controlled cooling are strictly controlled, and then the excess introduced by controlled cooling is introduced. It was found that it is important to reduce the number of dislocations by induction heating. _{Further, by performing the induction heating under predetermined conditions in consideration of the steel sheet surface temperature T 1} at the start of cooling in the controlled cooling _{and the cooling stop temperature T 2} at the average temperature of the steel sheet, the hardness varies in the plate thickness direction. It was found that it can be significantly reduced. 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% and Ca: 0.0005 to 0.005%, and the CP value obtained by the following formula (1) is 1. It has a component composition of .00 or less, and the balance is composed of Fe and unavoidable impurities.
The steel structure 0.5 mm below the surface of the steel sheet is a bainite structure with ^{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 surface of the steel plate and the average value of Vickers hardness at the center of the steel plate thickness is 25 HV or less.
A high-strength steel sheet for sour line pipes, which has 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 the X element in the steel.

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

[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% in mass%. The high-strength steel sheet for sour line pipe according to the above [1] or [2], which contains only one type or two or more types.

[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% and Ca: 0.0005 to 0.005%, and the CP value obtained by the following formula (1) is 1. A steel piece having a component composition of Fe and unavoidable impurities, which has a balance of 0.00 or less, is heated to a temperature of 1000 to 1300 ° C. and then hot-rolled to obtain a steel sheet.
After that, with respect to the steel sheet
Cooling at the start of the steel sheet surface temperature _{T 1: (Ar 3 -10 ℃} ) or higher,
Average cooling rate from 750 ° C to 550 ° C at the steel plate temperature 0.5 mm below the surface of the steel plate: 100 ° C / s or less,
Average cooling rate from 750 ° C to 550 ° C at average steel sheet temperature: 15 ° C / s or higher, and cooling stop temperature T ₂ : 250 to 550 ° C at average steel sheet temperature
Control cooling under the conditions of
Then, 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 surface temperature of the steel sheet is the heating temperature T _{3 of 550 to 750 ° C.} A method for manufacturing high-strength steel sheets 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 the X element in the steel.

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

[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% in mass%. The method for producing a high-strength steel sheet for a sour line pipe according to the above [4] or [5], which contains only one type or two or more types.

[7] The resistance according to any one of the above [4] to [6], wherein the reheating is performed so as to satisfy the condition that the TP defined by the following formula (2) is 0.50 or more. A method for manufacturing high-strength steel sheets for sour line pipes.
TP = (T _3- T ₂ ) x T ₂ / (T _1- T ₂ ) ² ... (2)

[8] 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 and the high-strength steel pipe using the high-strength steel plate for sour line pipes of the present invention are excellent in HIC resistance and SSCC resistance in a harsher corrosion environment, and in the thickness direction. It is also excellent in hardness uniformity. Further, according to the method for producing a high-strength steel plate for sour line pipes of the present invention, it is excellent in HIC resistance and SSCC resistance in a harsher corrosion environment, and is also excellent in hardness uniformity in the plate thickness direction. High-strength steel sheets for sour-resistant pipes can be manufactured.

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 increases during accelerated cooling. , HIC resistance and SSCC resistance deteriorate. In addition, the 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 when the content is less than 0.50%, the effect of adding Mn is poor, while when it exceeds 1.80%, the hardness of the central segregated portion increases during accelerated cooling. Therefore, the HIC resistance deteriorates. 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%.

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%. ..

The basic components of the present disclosure have been described above, but the component composition of the present disclosure is 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 arbitrarily contained in the following range.

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: 0.50% or less Ni 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 it is economical if the content is too large. Not only is it disadvantageous, but 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 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 weldability deteriorates. Therefore, when Cr is added, the upper limit is 0.50%.

Mo: 0.50% or less Mo 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 Mo is added.

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

One or more selected from Nb: 0.005 to 0.1%, V: 0.005 to 0.1% and Ti: 0.005 to 0.1% Nb, V and Ti Is also an element that can be optionally added to increase 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. It is preferably in the range of.

The present 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. Since it is necessary to be satisfied at the same time, the CP value obtained by the following equation (1) is set to 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 content of other trace elements is not hindered as long as the action and effect of the present invention are not impaired.

[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. 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. 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, but in the present disclosure, the structure of the polar surface layer of the steel sheet, specifically, the steel structure 0.5 mm below the surface of the steel sheet 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 formation, if the dislocation density 0.5 mm below the surface of the steel sheet is 7.0 × 10 ¹⁴ (m- ² ) or less, the increase in hardness due to age hardening is minimized. It can be suppressed to the limit. On the contrary, when the dislocation density of 0.5 mm below the surface of the steel sheet ^{exceeds 7.0 × 10 14} (m- ² ), the dislocation density does not decrease in the coating process after pipe formation, and the hardness increases significantly by age hardening. It deteriorates SSCC resistance. The preferred dislocation density range 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 0.5 mm below the surface of the steel sheet ^{is less than 0.5 × 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 ^{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 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 0.5 mm 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 0.5 mm below the surface of the steel sheet is 7.0 × 10 ¹⁴ (m- ² ) or less, the HV 0.1 at 0.5 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. After passing through the process, HV0.1 at 0.5 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, in addition to the average value of Vickers hardness 0.5 mm below the surface of the steel sheet being 230 HV or less, the material properties at the center of the sheet thickness are ensured 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 surface of the steel sheet and the average value of Vickers hardness at the center of the steel plate thickness is 25 HV or less. A more preferable ΔHV is 20 HV or less.

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, then hot-rolled to obtain a steel sheet, then the steel sheet is subjected to controlled cooling under predetermined conditions, and then the steel sheet is re-heated by induction heating. Heat.

[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 it is assumed that the slab is heated to this temperature up to the center.

[Rolling end temperature]
In the hot rolling process, in order to obtain high base metal toughness, the lower the rolling end temperature, the better, 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 equal to or higher than the _{Ar 3 transformation point at the surface temperature of the steel sheet.} Here, the Ar ₃ transformation point means the ferrite transformation start temperature during cooling, and can be obtained from the components of steel, for example, by the following 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 at the start of the steel sheet surface temperature T _1: the (Ar 3 _-10 ° C.) or higher cooling starting steel sheet surface temperature is low, the more the ferrite amount before controlled cooling, in particular the amount of temperature drop of from Ar ₃ transformation point There was produced ferrite exceeding 5% by volume fraction exceeds 10 ° C., for HIC resistance deteriorates the strength reduction is increased, the steel sheet surface temperature of the cooling start is (Ar ₃ -10 ℃) or higher And.

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

Average cooling rate from 750 ° C to 550 ° C at a steel plate temperature 0.5 mm below the surface of the steel sheet: 100 ° C / s or less Average cooling rate from 750 ° C to 550 ° C at a steel plate temperature 0.5 mm below the surface of the steel sheet is 100 ° C / If it exceeds s, the dislocation density at 0.5 mm below the surface of the steel sheet exceeds 7.0 × 10 ¹⁴ (m- ² ). As a result, the HV0.1 of 0.5 mm below the surface of the steel sheet exceeds 230, and after the coating process after pipe making, the HV0.1 of 0.5 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 100 ° C./s or less. It is preferably 80 ° 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 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 If 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 cannot be obtained and the strength The decrease, deterioration of HIC resistance, and large variation in hardness in the plate thickness direction may 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.

Although the 0.5 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 0.5 mm below the surface of the steel sheet in the temperature distribution is defined as the “steel plate temperature 0.5 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 at the average temperature of the steel sheet T ₂ : 250 to 550 ° C
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., martensite and island-shaped martensite (MA) are generated, and the variation in hardness in the plate thickness direction becomes particularly large. Therefore, in order to suppress the deterioration of the material uniformity in the steel sheet, the cooling stop temperature of the controlled cooling is set to 250 to 550 ° C. on the average temperature of the steel sheet.

[Induction heating conditions]
Induction heating temperature T ₃ : 550 to 750 ° C at the surface temperature of the steel sheet
In the present embodiment, it is important to temper the high-density dislocations introduced into bainite by controlled cooling after controlled cooling. As a result, the dislocation density at 0.5 mm below the surface of the steel sheet is 7.0 × 10 ¹⁴ (m- ² ) or less, excellent SSCC resistance is obtained, and the average Vickers hardness at 0.5 mm below the surface of the steel sheet is average. 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, when 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 set to 7.0 × 10 ¹⁴ (m- ² ) or less, ΔHV is set. It cannot be 25 HV or less. Further, if the induction heating temperature exceeds 750 ° C., the center of the plate thickness may also be tempered, and a predetermined strength may not be obtained. Therefore, in order to secure the strength at the center of the sheet thickness while suppressing the deterioration of the material uniformity in the steel sheet, the ultimate temperature of the online induction heating is set to 550 to 750 ° C. at the surface temperature of the steel sheet. In the present embodiment, in order to suppress the decrease in strength, it is important to reheat only the surface layer portion without tempering the inside of the steel sheet as much as possible. Therefore, an online induction heating device is used for heating.

Regarding the reheating conditions, it is preferable that the 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 ) x T ₂ / (T _1- T ₂ ) ² ... (2)
TP is a relational expression of tempering with respect to the supercooling degree of controlled cooling, and by satisfying 0.50 or more, TP is excessive at the center of the plate thickness while sufficiently recovering the dislocation of the surface layer portion introduced by accelerated cooling. Since it does not cause tempering, it is possible to remarkably suppress variations in hardness in the plate thickness direction. Specifically, ΔHV can be 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, welded steel pipes, spiral steel pipes, etc.) can be manufactured.

For example, in a UOE steel pipe, the end portion of a steel plate 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 I) 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 subjected to controlled cooling under the conditions shown in Table 2 using a water-cooled controlled cooling device. Immediately after that, the steel sheet was reheated by the method shown in "Heating method" in Table 2 so that the surface temperature of the steel sheet became the "maximum temperature at the time of reheating" 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 0.5 mm below the surface of the steel sheet and the structure at the center of the sheet thickness.

[Measurement of tensile strength]
A tensile test was performed using 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]
With respect to the cross section perpendicular to the rolling direction, Vickers hardness (HV0.1) at 20 points was measured at a position 0.5 mm below the surface of the steel sheet in accordance with JIS Z 2244, and the average value was obtained. Similarly, the Vickers hardness (HV0.1) at 20 points was measured at the center of the plate thickness, and the average value was obtained. Then, the absolute value ΔHV of the difference between the two was obtained. Here, what was measured with HV0.1 instead of the normally used HV10 is that the indentation becomes smaller by measuring 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 0.5 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 each other, they are separated by the Rachinger method. 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. 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]
For the HIC resistance, an HIC test with a immersion time of 96 hours according to NACE Standard TM-02-84 was performed, and if no cracks were observed, it was judged that the HIC resistance was good, and if cracks occurred, it was ×. Evaluated as. 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 0.5 mm and t / 2 positions below the surface, and HV0.1 at 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 the high-strength steel pipe made using the steel plate. , And no cracks were found by the HIC test.

As shown in Table 2, No. 1-No. Reference numeral 9 denotes an example of the invention in which the component composition and the production conditions satisfy the appropriate range of the present invention. In each case, the tensile strength of the steel sheet is 520 MPa or more, the microstructure at both 0.5 mm and t / 2 positions below the surface is a bainite structure, and the HV 0.1 at 0.5 mm below the surface is 230 or less and the ΔHV is 25 or less. The high-strength steel pipe formed using the steel plate also had good SSCC resistance and HIC resistance.

On the other hand, No. 10-No. Reference numeral 16 denotes 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. 10, since the cooling stop temperature was low, the difference in hardness between the surface layer and the center of the plate thickness was large. In Nos. 11 and 12, the dislocation density increased remarkably in the surface layer of the steel sheet when the controlled cooling conditions were outside the range of the present invention, so that the surface hardness increased and SSCC was generated. No. In No. 13, the cooling rate on average of the steel sheet was not sufficiently secured, and ferrite was formed at the center of the sheet thickness, which caused a decrease in strength. In No. 14, since the heating temperature in the online induction heating was not optimal, a difference in hardness in the plate thickness direction was generated. No. No. 15 is tempered by heating in a furnace, but the temperature rise rate is slow and the entire plate thickness is tempered on average, so that the strength is low. No. No. 16 is a case where 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. In addition, there is a large variation in hardness in the plate thickness direction. In No. 17 to No. 20, the 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 plate for sour line pipe, which is excellent in HIC resistance and SSCC resistance in a harsher corrosion environment and also has excellent hardness uniformity in the plate thickness direction. .. 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 (8)

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% and Ca: 0.0005 to 0.005%, and the CP value calculated by the following formula (1) is 1.00 or less. The balance is composed of Fe and unavoidable impurities.
The steel structure 0.5 mm below the surface of the steel sheet is a bainite structure with ^{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 surface of the steel plate and the average value of Vickers hardness at the center of the steel plate thickness is 25 HV or less.
A high-strength steel sheet for sour line pipes, which has 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 the X element in the steel. The component composition is further selected from Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, and Mo: 0.50% or less in mass%. The high-strength steel sheet for sour-resistant pipes according to claim 1, which contains two or more types. 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% in mass%. The high-strength steel sheet for sour line pipe according to claim 1 or 2, which contains two or more kinds. 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% and Ca: 0.0005 to 0.005%, and the CP value calculated by the following formula (1) is 1.00 or less. A steel piece having the composition of Fe and unavoidable impurities as the balance is heated to a temperature of 1000 to 1300 ° C. and then hot-rolled to obtain a steel sheet.
After that, with respect to the steel sheet
Cooling at the start of the steel sheet surface temperature _{T 1: (Ar 3 -10 ℃} ) or higher,
Average cooling rate from 750 ° C to 550 ° C at the steel plate temperature 0.5 mm below the surface of the steel plate: 100 ° C / s or less,
Average cooling rate from 750 ° C to 550 ° C at average steel sheet temperature: 15 ° C / s or higher, and cooling stop temperature T ₂ : 250 to 550 ° C at average steel sheet temperature
Control cooling under the conditions of
Then, by induction heating, there is a steel sheet average temperature is the cooling stop temperature T ₂ above, and the steel sheet surface temperature by reheating the steel plate so that the heating temperature T ₃ of 550 to 750 ° C., the steel sheet surface under The steel structure at 0.5 mm is ^{a bainite structure with a dislocation density of 0.5 × 10 14 to} 7.0 × 10 ¹⁴ (m- ² ), and the average value of Vickers hardness at 0.5 mm below the surface of the steel sheet and the steel sheet plate. A method for producing a high-strength steel sheet for sour line pipes, which comprises producing a high-strength steel sheet having a difference ΔHV from the average value of bainite hardness at the center of thickness of 25 HV or less.
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. The component composition is further selected from Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, and Mo: 0.50% or less in mass%. The method for producing a high-strength steel sheet for sour-resistant pipes according to claim 4, which contains two or more types. 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% in mass%. The method for producing a high-strength steel plate for sour-resistant pipes according to claim 4 or 5, which contains two or more of them. The high strength for sour line pipe according to any one of claims 4 to 6, wherein the reheating is performed so as to satisfy the condition that the TP defined by the following formula (2) is 0.50 or more. Manufacturing method of steel plate.
TP = (T _3- T ₂ ) x T ₂ / (T _1- T ₂ ) ² ... (2) 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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