JP2018168441A - High strength steel sheet for sour linepipe resistance, manufacturing method therefor and high strength steel pipe using high strength steel sheet for sour linepipe resistance - Google Patents

Abstract

To provide a high strength steel sheet for sour linepipe resistance excellent in HIC resistance and SSCC resistance under severer corrosive environment.SOLUTION: The high strength steel sheet for sour linepipe resistance has a component composition containing C:0.02 to 0.08%, Si:0.01 to 0.5%, Mn:0.5 to 1.8%, 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%, having CP value calculated by a prescribed formula of 1.00 or less and the balance Fe with inevitable impurities, in which a steel structure at 0.5 mm below a steel sheet surface is a ferrite structure with dislocation density of 0.5×10to 7.0×10(m).SELECTED DRAWING: None

Description

  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 resistance) and sulfide resistance in addition to strength, toughness, weldability, etc. So-called sour resistance such as stress corrosion cracking (SSCC) 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 a high-strength steel sheet for a line pipe, a so-called TMCP (Thermo-Mechanical Control Process) technique in which controlled rolling and controlled cooling are combined 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, when the scale thickness on the steel sheet surface is uneven, the cooling rate of the steel sheet below is also varied during cooling, which causes a problem of local variation in the cooling stop temperature within the steel sheet. 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, the high-strength steel sheets obtained by the production methods described in Patent Documents 1 to 6 have room for improvement in terms of HIC resistance and SSCC resistance in a more severe corrosive environment. Turned out to be. The reason is as follows.

  In the manufacturing methods described in Patent Documents 1 and 2, if the transformation behavior varies depending on the components of the steel sheet, a sufficient material homogenizing effect by recuperation may not be obtained. Moreover, when the structure in the surface layer of the steel sheet obtained by the manufacturing method described in Patent Documents 1 and 2 is a multiphase structure such as a ferrite-bainite two-phase structure, in the low load micro Vickers test, the indenter has any structure. The hardness value varies greatly depending on whether or not the test is performed.

  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. This is because if the cooling rate of the steel sheet surface varies, the hardness of the steel sheet varies. That is, when the cooling rate is slow, when the steel sheet surface cools, a film of bubbles is generated between the steel sheet surface and the cooling water, and the film is separated from the surface by the cooling water before the bubbles form the film. "Nucleate boiling" occurs simultaneously, and the cooling rate of the steel sheet surface varies. As a result, the hardness of the steel sheet surface varies. This is not considered in the techniques described in Patent Documents 5 and 6.

  Therefore, in view of the above problems, the present invention aims to provide a high-strength steel sheet for sour line pipes having excellent HIC resistance and SSCC resistance under a more severe corrosive environment, together 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 in addition to the HIC resistance of high-strength steel pipes, it is not sufficient to simply suppress the surface layer hardness as in the past, particularly the structure of the extreme surface layer portion of the steel sheet. Specifically, the steel structure 0.5 mm below the surface of the steel sheet is made a ferrite structure with a dislocation density of 0.5 × 10 ^{14 to} 7.0 × 10 ¹⁴ (m ⁻² ), so that the coating process after pipe forming It was found that the allowance for increasing the hardness of the steel pipe can be suppressed, and as a result, the SSCC resistance of the steel pipe is improved. And although the pole surface layer part is a structure mainly composed of ferrite, the structure inside the steel sheet is bainite, so that the strength of the steel sheet can be secured while suppressing the surface layer hardness.

  Furthermore, in order to realize such a steel structure, it is necessary to perform descaling under predetermined conditions immediately before the controlled cooling, and in the subsequent controlled cooling, it is necessary to strictly control the cooling rate at 0.5 mm below the steel sheet surface. , Succeeded in finding out the conditions. The present invention has been made based on this finding. Note that conventional descaling techniques such as Patent Documents 5 and 6 are merely intended to reduce subsequent cooling unevenness by reducing the scale. In the present invention, by performing descaling under a predetermined condition and performing controlled cooling under a predetermined condition, ferrite having a dislocation density suppressed as described above is formed in the extreme surface layer.

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 ferrite structure with a dislocation density of 0.5 × 10 ^{14 to} 7.0 × 10 ¹⁴ (m ⁻² ),
A high-strength steel plate for sour line pipes, characterized in that the steel structure in the center of the plate thickness is a bainite structure.
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 by mass% from Nb: 0.005 to 0.1%, V: 0.005 to 0.1%, and Ti: 0.005 to 0.1%. 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 After the steel slab having a composition of Fe and unavoidable impurities is heated to a temperature of 1000 to 1300 ° C., it is hot-rolled into a steel plate,
Thereafter, descaling is performed with respect to the steel plate at a steel plate surface temperature of (Ar ₃ + 20 ° C.) or less, and the collision pressure of the jet flow on the steel plate surface is 1 MPa or more,
After that,
Steel plate surface temperature at the start of cooling: (Ar ₃ −100 ° C.) or more and (Ar ₃ −10 ° C.) 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: 80 ° 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 at the steel plate average temperature: 250 to 550 ° C.
A method for producing a high-strength steel sheet for sour line pipes, wherein controlled cooling is performed under the conditions of:
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 by mass% from Nb: 0.005 to 0.1%, V: 0.005 to 0.1%, and Ti: 0.005 to 0.1%. 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] A high-strength steel pipe using the high-strength steel sheet for sour-resistant pipes according to any one of [1] to [3].

  The high-strength steel sheet for sour line pipes and the high-strength steel pipe using the high-strength steel sheet for sour line pipes of the present invention are excellent in HIC resistance and SSCC resistance in a more severe corrosive environment. Further, according to the method for producing a high-strength steel sheet for sour line pipes of the present invention, it is possible to produce a high-strength steel sheet for sour line pipes excellent in HIC resistance and SSCC resistance in a more severe corrosive environment. it can.

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 of the present disclosure will be specifically described.

[Ingredient composition]
First, the component composition of the high-strength steel sheet according to the present disclosure and the reason for limitation 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. In addition, the HIC resistance and the SSCC resistance deteriorate. In addition, toughness deteriorates. For this reason, C amount is limited to 0.02 to 0.08% of range.

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, which degrades weldability and deteriorates the resistance to HIC by increasing the hardness of the central segregation part. 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 inclusions in the steel, so that it is preferable to be small because it deteriorates the HIC resistance. However, up to 0.0015% is allowed. 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, when 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 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 optionally contain one or more selected from Nb, V and Ti within the following range.

Nb: 0.005 to 0.1%, V: 0.005 to 0.1% and Ti: One or more selected from 0.005 to 0.1% Nb, V and Ti are either Is an element that can be optionally added to increase the strength and toughness of the steel sheet. For each element, if the content is less than 0.005%, the effect of addition is poor. On the other hand, if it 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 sheet for sour line pipes. 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.

  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.

[Steel structure]
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 achieve a high strength with a tensile strength of 520 MPa or more, the steel structure at the center of the plate thickness needs to be a bainite structure. On the other hand, in order to suppress the increase in surface layer hardness, the steel structure having a surface layer portion of 0.5 mm is made a ferrite structure. If the surface ferrite layer is too thick, the hardness immediately below the ferrite phase may increase due to C diffusion associated with ferrite transformation, so the ferrite layer is preferably 1.0 mm or less. When different structures such as bainite, martensite, pearlite, island martensite, and retained austenite are mixed in the ferrite structure, variation in surface hardness increases by evaluating individual layers during microhardness evaluation. The smaller the fraction of the structure other than the ferrite phase, the better. However, when the volume fraction of the structure other than the ferrite 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 steel structures other than ferrite (bainite, martensite, pearlite, island martensite, retained austenite, etc.) is less than 5% in volume fraction, there is no significant effect. Shall be acceptable.

In the present disclosure, the structure of the extreme surface layer portion of the steel sheet, specifically, the steel structure 0.5 mm below the surface of the steel sheet is converted into a dislocation density of 0.5 × 10 ^{14 to} 7.0 × 10 ¹⁴ (m ⁻² ). It is important to have a ferrite structure. 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 formation, 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 2.0 × 10 ¹⁴ (m ⁻² ) or more. In the high-strength steel sheet of the present disclosure, if the dislocation density in the steel structure 0.5 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.

If the dislocation density at 0.5 mm below the steel sheet surface is 7.0 × 10 ¹⁴ (m ⁻² ) or less, 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.

  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 the steel slab having the above composition, it is hot-rolled into a steel sheet, and then descaling is performed on the steel sheet under a predetermined condition, and then the predetermined condition is applied to the steel sheet. Control cooling below.

[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 because the solid solution of the carbide is insufficient. On the other hand, if it exceeds 1300 ° C., the toughness deteriorates, so the slab heating temperature is 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 anti-HIC performance, it is preferable that the rolling end temperature is equal to or higher than the Ar ₃ transformation point at the steel sheet surface temperature. Here, the Ar ₃ transformation point means a 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.

[Descaling]
After the hot rolling is completed, descaling is performed on the steel sheet immediately before the controlled cooling. This is one of the important steps for forming ferrite uniformly on the surface layer portion.

Steel plate surface temperature at the start of descaling: (Ar ₃ + 20 ° C.) or less When the steel plate surface temperature at the start of descaling exceeds (Ar ₃ + 20 ° C.), ferrite is not formed to a position 0.5 mm below the steel plate surface, This is because the surface hardness increases and the SSCC resistance deteriorates. The lower limit of the steel sheet surface temperature is not particularly limited, it is preferable from the viewpoint of avoiding excessive formation of ferrite layers and the (Ar ₃ -30 ℃) or higher.

Collision pressure of jet flow on steel plate surface: 1 MPa or more When impact pressure of jet flow on steel plate surface is less than 1 MPa, descaling may be insufficient and unevenness of scale may occur. Since it becomes difficult to form a phase, the pressure is set to 1 MPa or more. The upper limit of the collision pressure is not particularly limited, but is preferably 10 MPa or less from the viewpoint of equipment capacity.

[Cooling start temperature of control cooling]
Steel plate surface temperature at the start of cooling: (Ar ₃ −100 ° C.) or more and (Ar ₃ −10 ° C.) or less The steel plate surface temperature at the start of cooling is particularly important. There is a need. When the temperature drop from the Ar ₃ transformation point is less than 10 ° C., the surface layer structure becomes a ferrite-bainite two-phase structure, and the dislocation density at 0.5 mm below the steel sheet surface is 7.0 × 10 ¹⁴ (m ⁻² ), The resistance to SSCC deteriorates. For this reason, the steel plate surface temperature at the start of cooling was set to (Ar ₃ −10 ° C.) or less. Preferably not more than (Ar ₃ -30 ℃). When the temperature drop from the Ar ₃ transformation point exceeds 100 ° C., the ferrite transformation progresses in the entire thickness of the plate, and a ferrite-bainite two-phase structure is formed at the center of the thickness, resulting in an increase in strength reduction and resistance to HIC. Characteristics deteriorate. Therefore, the steel sheet surface temperature of the cooling at the start and (Ar ₃ -100 ℃) or higher. More preferably, it is (Ar ₃ -80 ° C.) or higher.

[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 of 0.5 mm below the steel plate surface: 80 ° C./s or less Average cooling rate from 750 ° C. to 550 ° C. at a steel plate temperature of 0.5 mm below the steel plate surface is 80 ° C. / When s is exceeded, the dislocation density in the 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 80 degrees C / s or less. Preferably it is 60 degrees C / s or less. The lower limit of the average cooling rate is not particularly limited, but is preferably 10 ° C./s or more from the viewpoint of avoiding excessive formation of the ferrite layer.

Average cooling rate from 750 ° C. to 550 ° C. at the steel plate average temperature: 15 ° C./s or more When the average cooling rate from 750 ° C. to 550 ° C. at the steel plate average temperature is less than 15 ° C./s, a bainite structure is formed at the center of the plate thickness. Without being obtained, the strength is lowered, the HIC resistance is deteriorated, or the variation in hardness is increased. 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 from the viewpoint of avoiding hardness variation due to formation of a hard phase such as martensite.

  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 “steel plate in this specification” Average temperature ”.

[Cooling stop temperature]
Steel plate average temperature and cooling stop temperature: 250-550 ° C
After the ferrite transformation of the surface layer is completed, the bainite phase is generated by quenching to 250 to 550 ° C. which is the temperature range of the bainite transformation by controlled cooling. When 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 martensite (MA) are generated, and particularly the hardness of the surface layer portion is remarkably increased, and the variation in hardness in the thickness direction is increased. 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.

[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.

  The steel (steel types A to I) having the composition shown in Table 1 is made into a slab by a continuous casting method, heated to the temperature shown in Table 2, and then hot-rolled at the rolling finish temperature and the rolling reduction shown in Table 2. Thus, a steel plate having a thickness shown in Table 2 was obtained. Thereafter, the steel sheet was descaled so that the collision pressure of the jet flow on the steel sheet surface was 2.0 MPa at the steel sheet surface temperature shown in Table 2. Thereafter, the steel sheet was controlled to be cooled using a water-cooled control cooling device under the conditions shown in Table 2.

[Identification of 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, 50 Vickers hardness (HV0.1) 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. 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 Williamson-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 by performing an HIC test with an immersion time of 96 hours in accordance with NACE Standard TM-02-84. If no crack was observed, the HIC resistance was judged good. 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 at the 0.5 mm position below the surface is the ferrite phase, the microstructure at the t / 2 position is the bainite structure, and below the surface. It was decided that no crack was observed in the SSCC test and no crack was found in the HIC test in a high-strength steel pipe made using the steel plate with an HV0.1 of 0.5 or less at 0.5 mm.

  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 plate is 520 MPa or more, the microstructure at the 0.5 mm position below the surface is the ferrite phase, the microstructure at the t / 2 position is the bainite structure, and the HV0.1 at 0.5 mm below the surface is 230 or less. The SSCC resistance and the HIC resistance were good in the high-strength steel pipe made using the steel plate.

  In contrast, no. 10-No. 19 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, since descaling was not used, the surface layer structure was not limited to the ferrite phase but was inferior in SSCC resistance. No. No. 11, the descaling temperature, the cooling start temperature, and the cooling rate are out of the range of the present invention, and the dislocation density and hardness of the surface layer are increased, leading to deterioration of SSCC resistance. No. In No. 12, the descaling temperature and the cooling start temperature are outside the range of the present invention, and the surface layer structure is bainite. Therefore, the dislocation density and hardness of the surface layer are increased, and the SSCC resistance is deteriorated. In No. 13, the cooling start temperature was low, ferrite was formed over the entire plate, the strength was insufficient, and the HIC resistance was deteriorated. No. No. 14 had a high descaling start temperature, and ferrite was not sufficiently formed on the surface layer. Therefore, the surface layer hardness and dislocation density increased, and SSCC was generated. No. No. 15, although the descaling condition was within the range, the cooling start temperature was high, and the formation of surface ferrite was not sufficient, which caused the occurrence of SSCC. No. In No. 16, the cooling rate at 0.5 mm below the surface of the steel plate was remarkably fast, so martensite was formed in the surface layer portion, and as a result, the hardness of the surface layer portion increased and the SSCC resistance also deteriorated. No. 17 is No. Since the cooling rate is not as fast as 16, the surface layer structure is ferrite, but since the cooling rate is outside the range of the present invention, the hardness of the surface layer is increased and the SSCC resistance is deteriorated. No. In No. 18 and No. 19, the controlled cooling conditions were outside the scope of the present invention, and a bainite structure was not obtained at the center of the plate thickness as a microstructure, and the strength was low. In No. 20 to No. 23, the component composition of the steel sheet was outside the range of the present invention, and HIC cracking occurred.

  ADVANTAGE OF THE INVENTION According to this invention, the high strength steel plate for sour line pipes excellent in HIC resistance and SSCC resistance in a severer corrosive environment can be supplied. 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 (7)

In mass%, C: 0.02 to 0.08%, Si: 0.01 to 0.50%, Mn: 0.50 to 1.80%, P: 0.001 to 0.015%, S: 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 ferrite structure with a dislocation density of 0.5 × 10 ^{14 to} 7.0 × 10 ¹⁴ (m ⁻² ),
A high-strength steel plate for sour line pipes, characterized in that the steel structure in the center of the plate thickness is a bainite structure.
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. In mass%, C: 0.02 to 0.08%, Si: 0.01 to 0.50%, Mn: 0.50 to 1.80%, P: 0.001 to 0.015%, S: 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 after the steel slab having the composition of Fe and inevitable impurities as the balance is heated to a temperature of 1000 to 1300 ° C., it is hot-rolled into a steel plate,
Thereafter, descaling is performed with respect to the steel plate at a steel plate surface temperature of (Ar ₃ + 20 ° C.) or less, and the collision pressure of the jet flow on the steel plate surface is 1 MPa or more,
After that,
Steel plate surface temperature at the start of cooling: (Ar ₃ −100 ° C.) or more and (Ar ₃ −10 ° C.) 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: 80 ° 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 at the steel plate average temperature: 250 to 550 ° C.
A method for producing a high-strength steel sheet for sour line pipes, wherein controlled cooling is performed under the conditions of:
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.   A high-strength steel pipe using the high-strength steel plate for sour-resistant pipes according to any one of claims 1 to 3.

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