KR20190129957A - High strength steel sheet for internal sour line pipe, manufacturing method thereof and high strength steel pipe using high strength steel sheet for internal sour line pipe - Google Patents

Abstract

The present invention provides a high strength steel sheet for sour line pipes which is excellent in not only HIC resistance but also SSCC resistance in a more severe corrosive environment. The high strength steel sheet for a sour line pipe of the present invention is, 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%, and S: 0.0002 to 0.0015% , Al: 0.01% to 0.08%, and Ca: 0.0005% to 0.005%, the balance has a component composition consisting of Fe and unavoidable impurities, and the steel structure at 0.5 mm under the steel plate surface has a dislocation density of 1.0 × 10 ¹⁴ to It is a bainite structure of 7.0x10 ¹⁴ (m- ² ), and the variation of Vickers hardness in 0.5 mm under the steel plate surface is 30 HV or less at 3σ when the standard deviation is sigma, and has a tensile strength of 520 MPa or more. It features.

Description

High strength steel sheet for internal sour line pipe, manufacturing method thereof and high strength steel pipe using high strength steel sheet for internal sour line pipe

The present invention provides a high-strength steel sheet for sour-resistant line pipe having excellent material uniformity in steel sheet, which is suitable for use in line pipes in the fields of construction, offshore structures, shipbuilding, civil engineering, and construction industries, and a method of manufacturing the same. It is about. Moreover, this invention relates to the high strength steel pipe using the said high strength steel plate for sour line pipes.

Generally, a line pipe is manufactured by shape | molding the steel plate manufactured by the plate mill or hot roll mill into a steel pipe by UOE shaping | molding, press bend shaping | molding, roll forming, etc.

Here, the line pipes used for the transportation of crude oil and natural gas containing hydrogen sulfide include hydrogen organic cracking resistance (HIC (Hydrogen Induced Cracking) resistance) and sulfide stress corrosion in addition to strength, toughness and weldability. So-called sour resistance is required, such as crack resistance (Sulfide Stress Corrosion Cracking). Among them, HIC absorbs hydrogen ions due to the corrosion reaction on the steel surface, invades the steel as atomic hydrogen, and diffuses around non-metallic inclusions such as MnS in the steel and the hard second phase structure. It is accumulated and becomes molecular hydrogen and causes cracking due to the internal pressure, which is a problem in a line pipe having a relatively low strength level relative to an oil well pipe, and many countermeasure techniques have been disclosed. . On the other hand, SSCC is generally known to occur in a high strength seamless steel pipe for oil wells or in a high hardness region of a welded part, and this has not been a problem in a relatively low line pipe. However, in recent years, the mining environment of crude oil and natural gas has become increasingly strict, and it is reported that SSCC occurs also in the base material part of a line pipe in an environment where the hydrogen sulfide partial pressure is high or the pH is low. It is pointed out that controlling the hardness of the inner surface layer portion to improve the SSCC resistance under more stringent corrosion environments.

Usually, at the time of manufacture of the high strength steel plate for line pipes, what is called TMCP (Thermo-Mechanical Control Process) technology which combined control rolling and control cooling is applied. In order to increase the strength of the steel using the TMCP technique, it is effective to increase the cooling rate at the time of controlled cooling. However, when controlled cooling at a high cooling rate, the steel sheet surface layer portion is quenched, so that the hardness of the surface layer portion is higher than that inside the steel sheet, and a variation occurs in the hardness distribution in the plate thickness direction. Therefore, it becomes a problem from the viewpoint of ensuring material uniformity in a steel plate.

In order to solve the said problem, for example, in patent document 1, 2, after rolling, the plate | board thickness by performing the control cooling of the high cooling rate which recuperates the surface before a surface layer part completes a bainite transformation is carried out. The manufacturing method of the steel plate with small material difference of a direction is disclosed. In addition, Patent Documents 3 and 4 disclose a method for producing a steel sheet for line pipe, in which the surface of the steel sheet after accelerated cooling is heated to a high temperature from the inside using a high frequency induction heating apparatus to reduce the hardness of the surface layer portion.

On the other hand, when there is a nonuniformity in the scale thickness of the steel plate surface, a deviation also occurs in the cooling rate of the steel sheet at the bottom thereof during cooling, and a variation in the local cooling stop temperature in the steel sheet becomes a problem. As a result, variation in the steel sheet material occurs in the plate width direction due to non-uniformity of scale thickness. In contrast, Patent Documents 5 and 6 disclose a method of reducing cooling unevenness due to scale thickness unevenness by improving the steel sheet shape by performing descaling immediately before cooling.

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

However, according to the examination of the present inventors, it turned out that there is room for improvement from the viewpoint of SSCC resistance in a more severe corrosion environment in the high strength steel plate obtained by the manufacturing method of the said patent documents 1-6. The reason is as follows.

In the manufacturing methods of patent documents 1 and 2, when transformation behavior differs with the component of a steel plate, sufficient material homogenization effect by reheating may not be acquired. In addition, when the structure in the surface layer of the steel plate obtained by the manufacturing methods of patent documents 1 and 2 is a planar structure like the ferrite-bainite two-phase structure, in the micro-Vickers test which is deteriorating, an indenter will press-in which structure Depending on the test, the variation of the hardness value greatly occurs.

Since the cooling rate of the surface layer part in accelerated cooling is large, the manufacturing method of patent documents 3 and 4 may not fully reduce the hardness of surface layer part only by heating of the steel plate surface.

On the other hand, in the methods described in Patent Literatures 5 and 6, by desqueling, reduction of surface property defects due to indentation damage of scale at the time of hot calibration or reduction of the cooling stop temperature of the steel sheet can be reduced to improve the steel sheet shape. However, no consideration has been given to cooling conditions for obtaining a uniform material. This is because, if a deviation occurs in the cooling rate of the steel sheet surface, a variation occurs in the hardness of the steel sheet. That is, when the cooling rate is slow, when the surface of the steel sheet is cooled, “film boiling” in which 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 are formed. Nucleate boiling "occurs at the same time, causing a deviation in the cooling rate of the steel plate surface. As a result, a deviation occurs in the hardness of the steel plate surface. This point is not considered in the technique of patent documents 5 and 6.

Then, in view of the said subject, an object of this invention is to provide the high strength steel plate for sauer-line pipes which is excellent not only in HIC resistance but also in SSCC resistance in a stricter corrosion environment with its advantageous manufacturing method. Moreover, an object of this invention is to propose the high strength steel pipe using the said high strength steel plate for sour line pipes.

MEANS TO SOLVE THE PROBLEM The present inventors repeated many experiments and examination about the component composition, micro structure, and manufacturing conditions of steel materials, in order to ensure SSCC resistance in a more severe corrosion environment. As a result, in order to further improve the SSCC resistance of the high-strength steel pipe, it is not enough to simply suppress the surface hardness as in the conventional art. In particular, the structure of the pole surface portion of the steel sheet, specifically, the steel structure of 0.5 mm under the surface of the steel sheet By using the bainite structure having a dislocation density of 1.0 × 10 ^{14 to} 7.0 × 10 ¹⁴ (m ⁻² ), an increase in hardness can be suppressed in the coating process after the pipe is produced, and as a result, the SSCC resistance of the steel pipe is improved. Recognized that. Moreover, in order to realize such a steel structure, it is necessary to strictly control the cooling rate in 0.5 mm under the steel plate surface, and succeeded in discovering the conditions. The present invention is based on this recognition.

That is, the summary structure of this invention is as follows.

[1] In mass%, C: 0.02 to 0.08%, Si: 0.01 to 0.50%, Mn: 0.50 to 1.80%, P: 0.001 to 0.015%, S: 0.0002 to 0.0015%, Al: 0.01 to 0.08%, and Ca : 0.0005% to 0.005%, the remainder having a component composition consisting of Fe and unavoidable impurities,

The steel structure under 0.5 mm of the steel plate surface is a bainite structure with a dislocation density of 1.0 × 10 ^{14 to} 7.0 × 10 ¹⁴ (m ⁻² ),

The variation in Vickers hardness at 0.5 mm under the steel plate surface is 30 HV or less at 3σ when the standard deviation is σ,

Having a tensile strength of 520 MPa or more

High strength steel sheet for sour line pipes, characterized in that.

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

[3] The component composition is, in mass%, Nb: 0.005 to 0.1%, V: 0.005 to 0.1%, Ti: 0.005 to 0.1%, Zr: 0.0005 to 0.02%, Mg: 0.0005 to 0.02% and , REM: The high strength steel plate for a sour line pipe as described in said [1] or [2] containing 1 type (s) or 2 or more types chosen from 0.0005 to 0.02%.

[4] In mass%, C: 0.02 to 0.08%, Si: 0.01 to 0.50%, Mn: 0.50 to 1.80%, P: 0.001 to 0.015%, S: 0.0002 to 0.0015%, Al: 0.01 to 0.08%, and Ca : The steel piece containing 0.0005 to 0.005% and remainder having the component composition of Fe and an unavoidable impurity is heated at the temperature of 1000-1300 degreeC, and then hot-rolled to make a steel plate,

Then with respect to the steel sheet,

Steel sheet surface temperature at the start of cooling: (Ar ₃ -10 ℃) above,

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

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

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

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

Controlled cooling is performed under the conditions of the manufacturing method of the high strength steel plate for sour line pipes.

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

[6] The component composition is, in mass%, Nb: 0.005 to 0.1%, V: 0.005 to 0.1%, Ti: 0.005 to 0.1%, Zr: 0.0005 to 0.02%, Mg: 0.0005 to 0.02% and And REM: The manufacturing method of the high strength steel plate for anti-sour line pipes as described in said [4] or [5] containing 1 type (s) or 2 or more types chosen from 0.0005 to 0.02%.

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

The high strength steel pipe using the high strength steel plate for the sour line pipe of the present invention and the high strength steel plate for the sour line pipe is not only excellent in HIC resistance but also excellent in SSCC resistance in a more severe corrosion environment. Moreover, according to the manufacturing method of the high strength steel plate for sauer line pipes of this invention, it is possible to manufacture the high strength steel plate for sauer line pipes which is excellent not only in HIC resistance but also SSCC resistance in a more stringent corrosion environment.

BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic diagram explaining the collection method of the test piece for evaluation of SSCC resistance in an Example.

(Form to carry out invention)

Hereinafter, the high strength steel plate for sour line pipes of this indication is demonstrated concretely.

[Component Composition]

First, the component composition of the high strength steel sheet by this indication and the reason for limitation are demonstrated. In the following description, all the units represented by% are the mass%.

C: 0.02-0.08%

C effectively contributes to the improvement of the strength, but if the content is less than 0.02%, sufficient strength cannot be ensured. On the other hand, if the content exceeds 0.08%, the hardness of the surface layer portion and the central segregation portion increases during accelerated cooling. SSCC resistance and HIC resistance deteriorate. In addition, the toughness deteriorates. For this reason, C amount is limited to 0.02 to 0.08% of range.

Si: 0.01% to 0.50%

Although Si is added for deoxidation, when the content is less than 0.01%, the deoxidation effect is not sufficient. On the other hand, when it exceeds 0.50%, the toughness and weldability are deteriorated, so the amount of Si is limited to 0.01 to 0.50%.

Mn: 0.50 to 1.80%

Mn effectively contributes to the improvement of strength and toughness, but if the content is less than 0.50%, its addition effect is insufficient. On the other hand, if Mn exceeds 1.80%, the hardness of the surface layer portion and the center segregation portion increases during accelerated cooling. SSCC resistance and HIC resistance deteriorate. In addition, weldability deteriorates. For this reason, Mn amount is limited to 0.50 to 1.80% of range.

P: 0.001-0.015%

P is an unavoidable impurity element, deteriorates weldability, and deteriorates HIC resistance by increasing the hardness of the central segregation portion. Since the tendency becomes remarkable when it exceeds 0.015%, an upper limit is prescribed | regulated as 0.015%. Preferably it is 0.008% or less. Although content is so good that it is low, it is made into 0.001% or more from a viewpoint of refining cost.

S: 0.0002 to 0.0015%

Since S is an unavoidable impurity element and becomes a MnS inclusion in steel and degrades HIC resistance, although S is small, up to 0.0015% is acceptable. Although content is so good that it is low, it is made into 0.0002% or more from a viewpoint of refining cost.

Al: 0.01% to 0.08%

Although Al is added as a deoxidizer, when it is less than 0.01%, there is no addition effect, On the other hand, when it exceeds 0.08%, since cleanliness of steel will fall and toughness will deteriorate, Al amount is limited to 0.01 to 0.08%.

Ca: 0.0005 to 0.005%

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

As mentioned above, although the basic component of this indication was demonstrated, the component composition of this indication is 1 type, or 2 or more types chosen from Cu, Ni, Cr, and Mo, for further improvement of the strength and toughness of a steel plate, It can be contained arbitrarily in the following ranges.

Cu: 0.50% or less

Cu is an element effective for improving the toughness and increasing the strength, and in order to obtain this effect, Cu is preferably contained at 0.05% or more. However, if the content is too large, the weldability deteriorates, so the upper limit is 0.50% when Cu is added. It is done.

Ni: 0.50% or less

Ni is an element effective for improving the toughness and increasing the strength, and in order to obtain this effect, Ni is preferably contained at 0.05% or more. However, if the content is too high, it is economically disadvantageous and the toughness of the weld heat affected zone deteriorates. When Ni is added, the upper limit is 0.50%.

Cr: 0.50% or less

Cr is an element that is effective to obtain sufficient strength even at low C, like Mn, and it is preferable to contain 0.05% or more in order to obtain this effect. However, if the content is too large, the quenchability becomes excessive, which will be described later. Dislocation density becomes high and SSCC resistance deteriorates. In addition, weldability deteriorates. For this reason, when adding Cr, 0.50% shall be an upper limit.

Mo: 0.50% or less

Mo is an element effective for improving the toughness and increasing the strength, and in order to obtain this effect, Mo is preferably contained at 0.05% or more. However, when the content is too large, the quenchability becomes excessive, so that the dislocation density described later becomes high, SSCC resistance deteriorates. In addition, weldability deteriorates. For this reason, when Mo is added, 0.50% is made into an upper limit.

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

Nb: 0.005 to 0.1%, V: 0.005 to 0.1%, Ti: 0.005 to 0.1%, Zr: 0.0005 to 0.02%, Mg: 0.0005 to 0.02%, and REM: 0.0005 to 0.02% More than

Nb, V, and Ti are all elements which can be added arbitrarily in order to raise the strength and toughness of a steel plate. In each element, when the content is less than 0.005%, the addition effect is insufficient. On the other hand, when the content exceeds 0.1%, the toughness of the welded section deteriorates. Therefore, when added, it is preferable that the content is in the range of 0.005 to 0.1%. Zr, Mg and REM are elements that can be arbitrarily added to increase toughness through grain refinement or to increase crack resistance through control of inclusion properties. If all of these elements are less than 0.0005% in content, the effect of the addition is insufficient, while if the content is more than 0.02%, the effect is saturated. Therefore, all of these elements are preferably in the range of 0.0005% to 0.02%.

The present disclosure discloses a technique for improving the SSCC resistance of high strength steel pipes using high strength steel sheets for sour line pipes, but needless to satisfy HIC resistance at the same time, not to mention as sour performance. For example, it is preferable to make CP value calculated by following formula (1) into 1.00 or less. In addition, 0 may be substituted for the element which is not added.

CP = 4.66 [% C] + 2.37 [% Mn] / 6 + (1.74 [% Cu] + 1.7 [% Ni]) / 15 + (1.18 [% Cr] + 1.95 [% Mo] + 1.74 [% V]) / 5 + 22.36 [% P]. (One)

However, [% X] shows content (mass%) in the steel of X element.

Here, the CP value is an equation designed to estimate the material of the central segregation unit from the content of each alloying element, and the higher the CP value of the above formula (1), the higher the concentration of components in the central segregation unit, the central segregation unit The hardness of rises. Therefore, by making CP value calculated | required in said Formula (1) into 1.00 or less, it becomes possible to suppress the crack generation in a HIC test. In addition, the lower the CP value, the lower the hardness of the central segregation portion. Therefore, when higher HIC resistance is required, the upper limit thereof may be 0.95.

In addition, remainder other than the above-mentioned element consists of Fe and an unavoidable impurity. However, the content of other trace elements is not prevented unless the effect of the present invention is impaired. For example, although N is an element inevitably contained in steel, in the present invention, the content is acceptable if the content is 0.007% or less, preferably 0.006% or less.

[Organization of Steel Plate]

Next, the steel structure of the high strength steel plate for sour line pipe of this indication is demonstrated. In order to achieve high strength at a tensile strength of 520 MPa or more, the steel structure needs to be bainite structure. In particular, when hard phases, such as martensite and island-like martensite MA, generate | occur | produce the surface layer part, surface layer hardness rises, the variation of the hardness in a steel plate increases, and material uniformity is impaired. In order to suppress the increase in surface hardness, the steel structure of the surface layer portion is made of bainite structure. Herein, the bainite structure is intended to include a structure called bainitic ferrite or granular ferrite which is transformed at the time of accelerated cooling or after accelerated cooling, which contributes to enhanced transformation. When heterostructures such as ferrite, martensite, pearlite, island-like martensite, and retained austenite are mixed in the bainite structure, a decrease in strength, deterioration of toughness, an increase in surface hardness, etc. may occur. The smaller the tissue fraction, the better. However, when the volume fraction of the tissues other than the bainite phase is sufficiently low, since their effects can be ignored, any amount is acceptable. Specifically, in the present disclosure, if the total of steel structures other than bainite (ferrite, martensite, pearlite, island-like martensite, residual austenite, etc.) is less than 5% by volume fraction, it is acceptable because there is no great effect. do.

In addition, there are various forms according to the cooling rate in the bainite structure, but in the present disclosure, the structure of the pole surface layer portion of the steel sheet, specifically, the steel structure 0.5 mm under the surface of the steel sheet, has a dislocation density of 1.0 × 10 ^{14 to} 7.0. It is important to have a bainite structure of 占 10 ¹⁴ (m- ² ). Since the dislocation density decreases in the coating process after the pipe is made, if the dislocation density of 0.5 mm under the steel sheet surface is 7.0 × 10 ¹⁴ (m ⁻² ) or less, the increase in hardness due to age hardening can be minimized. Can be. On the contrary, if the dislocation density of 0.5 mm under the surface of the steel sheet exceeds 7.0 × 10 ¹⁴ (m ⁻² ), the dislocation density does not decrease in the coating process after the pipe is made, and the hardness is greatly increased by aging hardening to deteriorate the SSCC resistance. Let's do it. In order to obtain good SSCC resistance after the tubing, a preferable dislocation density range is 6.0 × 10 ¹⁴ (m ⁻² ) or less. On the other hand, when the dislocation density of 0.5 mm under the steel plate surface is less than 1.0 × 10 ¹⁴ (m ⁻² ), the strength cannot be maintained as the steel plate. In order to secure the strength of the X65 grade, it is preferable to have a dislocation density of 2.0 × 10 ¹⁴ (m ⁻² ) or more. In addition, in the high strength steel plate of this indication, if the dislocation density in the steel structure of 0.5 mm under the steel plate surface is the said range, the pole surface part of the range of 0.5 mm depth from the steel plate surface will also have equivalent dislocation density, As a result, The effect of improving SSCC resistance is obtained.

Moreover, when the dislocation density in 0.5 mm under the steel plate surface shall be 7.0 * 10 <^14> (m <^-2> ) or less, HV0.1 in 0.5 mm under surface will be 230 or less. In order to secure the SSCC resistance of the steel pipe, it is important to suppress the surface hardness of the steel sheet. However, by setting the HV 0.1 at a thickness of 0.5 mm under the surface of the steel sheet to 230 or less, the surface under the surface after the coating process after pipe formation HV0.1 in mm can be suppressed to 260 or less, and SSCC resistance can be ensured.

In addition, in the high strength steel plate of this indication, it is also important that the variation of the Vickers hardness in 0.5 mm under the steel plate surface is 30 HV or less at 3σ when the standard deviation is σ. When 3 σ when the Vickers hardness at 0.5 mm under the surface of the steel sheet is greater than 30 HV, the hardness variation in the pole surface layer of the steel plate, that is, the local high hardness portion is present in the pole surface layer, thus originating the site. This is because the deterioration of the SSCC resistance has occurred. In addition, when obtaining standard deviation (sigma), it is preferable to measure Vickers hardness more than 100 points.

Since the high strength steel plate of this indication is a steel plate for steel pipes which has the intensity | strength more than X60 grade of API 5L, let it have tensile strength of 520 Mpa or more.

[Manufacturing method]

Hereinafter, the manufacturing method and manufacturing conditions for manufacturing the said high strength steel plate for sour line pipes are demonstrated concretely. In the manufacturing method of this indication, after heating the steel piece which has the said component composition, it hot-rolls to make a steel plate, and performs control cooling under predetermined conditions with respect to the said steel plate after that.

[Slab heating temperature]

Slab heating temperature: 1000 to 1300 ° C

If the slab heating temperature is less than 1000 ° C, the solid solution of carbide is insufficient and the required strength is not obtained. On the other hand, if the slab heating temperature exceeds 1300 ° C, the toughness deteriorates, so the slab heating temperature is set to 1000 to 1300 ° C. In addition, this temperature is the furnace temperature of a heating furnace, and a slab is heated to this temperature to the center part.

[Rolling end temperature]

In the hot rolling step, in order to obtain high base material toughness, the lower the rolling end temperature, the better. On the other hand, since the rolling efficiency decreases, the rolling end temperature at the steel plate surface temperature takes into account necessary base material toughness and rolling efficiency. Need to be set. From the viewpoint of improving the strength and HIC resistance, it is preferable that the rolling end temperature, the steel sheet surface temperature above the Ar ₃ transformation point. Here, Ar ₃ transformation point is, means the ferrite transformation starting temperature in the cooling and, for example, can be determined by the following equation from the river component. In addition, in order to obtain high base material toughness, it is preferable to make the reduction ratio in the temperature range below 950 degreeC correspond to austenite uncrystallized temperature range to 60% or more. In addition, the surface temperature of a steel plate can be measured with a radiation thermometer etc.

Ar ₃ (° C.) = 910-310 [% C] -80 [% Mn] -20 [% Cu] -15 [% Cr] -55 [% Ni] -80 [% Mo]

However, [% X] shows content (mass%) in the steel of X element.

[Cooling start temperature of control cooling]

Cooling start temperature: the steel sheet surface temperature of (Ar ₃ -10 ℃) above

When the steel sheet surface temperature at the start of cooling is low, the amount of ferrite generation before the controlled cooling increases, and in particular, when the temperature drop from the Ar ₃ transformation point exceeds 10 ° C, ferrites exceeding 5% are generated in volume fraction, and the strength decreases. since the HIC resistance deteriorates with increased, the steel sheet surface temperature at the start of cooling is to be not less than (Ar ₃ -10 ℃).

[Cooling speed of controlled cooling]

It is important to control the cooling rate of the surface layer portion and the average cooling rate in the steel sheet in order to reduce the variation in hardness in the steel sheet and improve the material uniformity while increasing the strength. In particular, it is necessary to control the cooling rate in 0.5 mm under the steel plate surface in order to make the dislocation density and 3 (sigma) in 0.5 mm under the steel plate surface into the range mentioned above.

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

If the average cooling rate from 750 ° C to 550 ° C exceeds 80 ° C / s at the steel plate temperature at 0.5 mm under the steel plate surface, the dislocation density at 0.5 mm under the steel plate surface exceeds 7.0 × 10 ¹⁴ (m ⁻² ) It becomes. As a result, HV0.1 of 0.5 mm under the surface of the steel sheet exceeds 230, and after the coating process after the tubing, HV 0.1 of 0.5 mm under the surface exceeds 260, which deteriorates the SSCC resistance of the steel pipe. Therefore, the said average cooling rate shall be 80 degrees C / s or less. Preferably it is 50 degrees C / s or less. The lower limit of the average cooling rate is not particularly limited. However, when the cooling rate is excessively small, ferrite and pearlite are formed and the strength is insufficient. Therefore, the lower limit of the average cooling rate is preferably 20 ° C / s or more from the viewpoint of preventing this.

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

If the average cooling rate from 750 ° C to 550 ° C is less than 15 ° C / s at the steel plate average temperature, no bainite structure is obtained and the strength decreases and the HIC resistance deteriorates. For this reason, the cooling rate in steel plate average temperature shall be 15 degreeC / s or more. From the viewpoint of the variation of the steel sheet strength and hardness, the cooling rate of the steel sheet average is preferably set to 20 ° C / s or more. Although the upper limit of the said average cooling rate is not specifically limited, It is preferable to set it as 80 degrees C / s or less so that a low temperature transformation product may not produce excessively.

Average cooling rate from the steel plate temperature at 0.5 mm under the steel plate surface to the temperature at 550 ° C. during cooling stop: 150 ° C./s or more

For cooling at 550 ° C. or lower at the steel sheet temperature at 0.5 mm under the steel sheet surface, cooling in a stable nuclear boiling state is necessary, and an increase in the yield density is indispensable. When the average cooling rate is less than 150 ° C / s from the steel plate temperature at 0.5 mm under the steel plate surface to the temperature at the cooling stop, the cooling in the nucleus boiling state is not performed, and the hardness variation occurs at the pole surface portion of the steel sheet. As a result, 3σ at 0.5 mm under the steel sheet surface exceeds 30 HV, resulting in deterioration of SSCC resistance. Therefore, the said average cooling rate shall be 150 degreeC / s or more. Preferably it is 170 degreeC / s or more. Although the upper limit of the said average cooling rate is not specifically limited, It is preferable to set it as 250 degrees C / s or less from the constraint on an installation.

In addition, 0.5 mm under the steel plate surface and steel plate average temperature cannot be measured directly, but a process computer is based on the surface temperature at the start of cooling measured by the radiation thermometer, and the surface temperature at the time of stop of cooling of a target, for example. By using the difference calculation, the temperature distribution in the plate thickness cross section can be obtained in real time. The temperature in 0.5 mm under the steel plate surface in the said temperature distribution is called "steel plate temperature in 0.5 mm below the steel plate surface" in this specification, and the average value of the temperature of the plate thickness direction in the said temperature distribution was seen. It is called "steel plate average temperature" in a specification.

[Cooling stop temperature]

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

After completion of the rolling, the bainite phase is generated by quenching to 250 to 550 ° C, which is the temperature range of bainite transformation in controlled cooling. If the cooling stop temperature exceeds 550 ° C, bainite transformation is incomplete and sufficient strength cannot be obtained. Moreover, when cooling stop temperature is less than 250 degreeC, hardness rise of a surface layer part becomes remarkable, and since it becomes more than dislocation density 7.0 * 10 <^14> (m- ² ) in 0.5 mm under steel plate surface, SSCC resistance deteriorates. In addition, the hardness of the central segregation portion also increases, and the HIC resistance also deteriorates. Therefore, in order to suppress deterioration of the material uniformity in a steel plate, the cooling stop temperature of control cooling shall be 250-550 degreeC by 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 molding, and the like, and then welded to the butt part, so as to provide excellent uniformity of material in the steel sheet suitable for transportation of crude oil or natural gas. High strength steel pipes (UOE steel pipes, electric wire pipes, spiral steel pipes, etc.) can be manufactured.

For example, a UOE steel pipe improves the edge part of a steel plate, shape | molds it into a steel pipe shape with C press, U press, and O press, seam welds the butt part by inner surface welding and outer surface welding, and further expands as needed. It is manufactured through a process. The welding method may be any method as long as sufficient joint strength and joint toughness are obtained. However, from the viewpoint of excellent welding quality and manufacturing efficiency, it is preferable to use submerged arc welding.

Example

The steels (steel grades A to K), which are the component compositions shown in Table 1, were slab by the continuous casting method, heated to the temperature shown in Table 2, and then hot rolling of the rolling end temperature and reduction ratio shown in Table 2 It was set as the steel plate of the plate thickness shown in Table 2. Then, controlled cooling was performed with respect to the steel plate using the water cooling type control cooling apparatus on the conditions shown in Table 2.

[Specification 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 the position of 0.5 mm under the steel plate surface and the structure at the plate thickness center.

[Measurement of Tensile Strength]

The tensile test was done using the whole thickness test piece of the direction orthogonal to a rolling direction as a tensile test piece, and the tensile strength was measured. The results are shown in Table 2.

[Measurement of Vickers Hardness]

About the cross section perpendicular | vertical to a rolling direction, 100 Vickers hardness (HV0.1) of 100 points was measured in the position of 0.5 mm under the steel plate surface, and the average value and standard deviation (sigma) were calculated | required. The average value and the value of 3σ are shown in Table 2. Here, the measurement of HV0.1 instead of HV10, which is usually used, reduces the indentation by measuring with HV0.1, so that hardness information at a position closer to the surface and hardness information more sensitive to microstructure can be obtained. Because it becomes possible.

[Potential density]

A sample for X-ray diffraction was taken from a position having an average hardness, the sample surface was polished to remove scale, and X-ray diffraction measurement was performed at a position of 0.5 mm under the steel plate surface. The dislocation density used the method of converting from the distortion calculated | required from the half value width (beta) of X-ray-diffraction measurement. In the diffraction intensity curve obtained by normal X-ray diffraction, two of the Kα1 and Kα2 lines having different wavelengths overlap each other, and are separated by the Rachinger method. The Williamsson-Hall method shown below is used for extraction of a deformation | transformation. The expansion of the half-value width is influenced by the size D of the determinant and the deformation 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, this equation is modified to be βcosθ / λ = 0.9λ / D + 2ε × sinθ / λ. By plotting βcosθ / λ with respect to sinθ / λ, the strain ε is calculated from the slope of the straight line. In addition, the diffraction lines used for calculation are set to (110), (211), and (220). In terms of dislocation density from the strain ε it is used the ρ = 14.4ε ² / b ^2. In addition, (theta) means the peak angle computed from the (theta) -2 (theta) method of X-ray diffraction, and (lambda) means the wavelength of the X-ray used by X-ray diffraction. b is the Burgers vector of Fe ((alpha)), and was 0.25 nm in this Example.

[Evaluation of SSCC Resistance]

SSCC resistance was evaluated using a part of each of these steel sheets. The tube was improved by processing the end of the steel sheet, and formed into a steel tube shape by C press, U press, and O press, and then seam welded to the butt parts of the inner and outer surfaces by submerged arc welding, and manufactured through an expansion process. As shown in FIG. 1, after flattening the coupon cut out from the obtained steel pipe, the SSCC test piece of 5x15x115 mm was extract | collected from the steel pipe inner surface. At this time, the inner surface which was a test surface was left with black skin in order to leave the state of the outermost layer. The SSCC test piece collected was loaded with 90% of the stress of the actual yield strength (0.5% YS) of each steel pipe, and the hydrogen sulfide partial pressure was 1 bar using a NACE standard TM0177 Solution A solution. It carried out based on SSCC test. After 720 hours of immersion, it was judged that SSCC resistance was good when the crack was not confirmed, and the case where cracks occurred was judged as defective and was made x. The results are shown in Table 2.

HIC resistance was investigated by 96-hour immersion HIC test using NACE standard TM0177 Solution A solution. The HIC resistance judged that the case where the crack length ratio (CLR) became 15% or less in the HIC test was considered good, and the case of exceeding (circle) and 15% was made into x. The results are shown in Table 2.

The target range of the present invention is a high-strength steel sheet for a sour line pipe, the tensile strength of 520 MPa or more, and the microstructure of both the 0.5 mm position and the t / 2 position under the surface of the bainite structure and the HV 0.1 at 0.5 mm below the surface. It was assumed that cracks were not confirmed by the SSCC test in the high-strength steel pipe formed by using the steel sheet at 230 or less, and the crack length ratio (CLR) was 15% or less in the HIC test.

As shown in Table 2, No.1-No.15 is an example of invention in which a component composition and manufacturing conditions satisfy the appropriate range of this invention. In all, as the steel sheet, the tensile strength: 520 MPa or more, the 0.5 mm position and the t / 2 position under the surface of the microstructure had a bainite structure and HV 0.1 of 0.5 mm or less under the surface of 230 or less. SSCC resistance and HIC resistance were also favorable in the high strength steel pipe.

On the other hand, although No. 16-No. 23 have a component composition in the scope of the present invention, it is a comparative example whose manufacturing conditions are outside the scope of this invention. No. 16 had low strength due to low slab heating temperature due to insufficient homogenization of microstructures and solid solution of carbides. Since No. 17 had a low cooling start temperature and became a layered structure in which ferrite precipitated, it was low in strength and deteriorated the HIC resistance after the tubing. No. 18 is low-strength and has HIC resistance after piping because controlled cooling conditions are outside the scope of the present invention, and since bainite structure is not obtained at the center of the plate thickness as a micro structure, and becomes a ferrite + pearlite structure. Deteriorated. No. 19 had a low cooling stop temperature, had a dislocation density at 0.5 mm under the surface, and HV0.1 exceeded 230, so that the SSCC resistance after the tube was inferior. In addition, since the hardness of the central segregation portion also increased, the HIC resistance also deteriorated. No. 20 and No. 23 have a dislocation density at 0.5 mm under the surface, which is high because the average cooling rate at 750 to 550 ° C. at 0.5 mm under the steel sheet surface exceeds 80 ° C./s. Over 230, the SSCC resistance after piping was inferior. In addition, in No. 23, the HIC resistance at the surface layer portion was also deteriorated. Since No.21 and No.22 have an average cooling rate at 550 ° C. or lower at 0.5 mm below the steel plate surface, they do not satisfy 150 ° C./s, so that non-uniform cooling of the steel sheet becomes remarkable, and HV 0.1 is averaged. Although it satisfies 230 or less, since the hardness variation was large and the hardness was locally generated, the SSCC resistance after piping was inferior. In Nos. 24 to 27, the component composition of the steel sheet was outside the range of the present invention, and the dislocation density at 0.5 mm under the surface became high, and HV0.1 exceeded 230. Thus, the SSCC resistance after the tube was inferior. Moreover, about No.24-No.27, since the hardness of the center segregation part increased, HIC resistance was inferior.

(Industrial availability)

According to the present invention, it is possible to supply a high strength steel sheet for sour line pipes which is excellent not only in HIC resistance but also in SSCC resistance under more stringent corrosion environment. Therefore, the steel pipes manufactured by cold forming this steel plate (sealing steel pipe, spiral steel pipe, UOE steel pipe, etc.) can be used suitably for the transportation of crude oil and natural gas containing hydrogen sulfide which requires sour resistance.

Claims (7)

In mass%, C: 0.02-0.08%, Si: 0.01-0.50%, Mn: 0.50-1.80%, P: 0.001-0.015%, S: 0.0002-0.0015%, Al: 0.01-0.08%, and Ca: 0.0005- 0.005%, and the balance has a component composition of Fe and inevitable impurities,
The steel structure under 0.5 mm of the steel plate surface is a bainite structure with a dislocation density of 1.0 × 10 ^{14 to} 7.0 × 10 ¹⁴ (m ⁻² ),
The variation in Vickers hardness at 0.5 mm under the steel plate surface is 30 HV or less at 3σ when the standard deviation is σ,
Having a tensile strength of 520 MPa or more
High strength steel sheet for sour line pipes, characterized in that. The method of claim 1,
Said component composition further contains 1 type (s) or 2 or more types selected from Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, and Mo: 0.50% or less by mass%. High strength steel sheet for line pipes. The method according to claim 1 or 2,
The component composition is, in mass%, Nb: 0.005 to 0.1%, V: 0.005 to 0.1%, Ti: 0.005 to 0.1%, Zr: 0.0005 to 0.02%, Mg: 0.0005 to 0.02%, and REM: The high strength steel plate for sour-line pipes containing 1 type (s) or 2 or more types chosen from 0.0005 to 0.02%. In mass%, C: 0.02-0.08%, Si: 0.01-0.50%, Mn: 0.50-1.80%, P: 0.001-0.015%, S: 0.0002-0.0015%, Al: 0.01-0.08%, and Ca: 0.0005- After heating the steel piece containing 0.005% and remainder which has a component composition of Fe and an unavoidable impurity at the temperature of 1000-1300 degreeC, it hot-rolls to make a steel plate,
Then with respect to the steel sheet,
Steel sheet surface temperature at the start of cooling: (Ar ₃ -10 ℃) above,
Average cooling rate from 750 degreeC to 550 degreeC at the steel plate temperature in 0.5 mm under steel plate surface: 80 degrees C / s or less,
Average cooling rate from 750 ° C. to 550 ° C. at the steel plate average temperature: 15 ° C./s or more,
Average cooling rate: 150 degreeC / s or more from the steel plate temperature in 0.5 mm of steel plate surfaces to the temperature at the time of cooling stop at 550 degreeC, and
Cooling stop temperature at the steel plate average temperature: 250 to 550 ° C
Controlled cooling is performed under the conditions of the manufacturing method of the high strength steel plate for sour line pipes. The method of claim 4, wherein
Said component composition further contains 1 type (s) or 2 or more types selected from Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, and Mo: 0.50% or less by mass%. Method for producing high strength steel sheet for line pipes. The method according to claim 4 or 5,
The component composition is, in mass%, Nb: 0.005 to 0.1%, V: 0.005 to 0.1%, Ti: 0.005 to 0.1%, Zr: 0.0005 to 0.02%, Mg: 0.0005 to 0.02%, and REM: The manufacturing method of the high strength steel plate for sour line pipes containing 1 type (s) or 2 or more types chosen from 0.0005 to 0.02%. The high strength steel pipe using the high strength steel plate for sour line pipes in any one of Claims 1-3.

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