JP6798565B2 - High-strength steel sheets for sour-resistant pipes and high-strength steel pipes using them - Google Patents

Description

The present invention relates to a high-strength steel plate for sour-resistant pipes, which is used in the field of sour-resistant pipes and has excellent material uniformity in steel sheets, particularly HIC characteristics, and high-strength steel pipes using the same. is there.

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

Here, line pipes used for transporting crude oil containing hydrogen sulfide and natural gas have strength, toughness, weldability, etc., as well as hydrogen-induced cracking resistance (HIC (Hydrogen Induced Cracking) resistance) and sulfide resistance. So-called sour resistance such as stress corrosion cracking resistance (SSCC (Sulfide Stress Corrosion Cracking) resistance) is required. Among them, in HIC, hydrogen ions due to the corrosion reaction are adsorbed on the surface of the steel material, penetrate into the steel as atomic hydrogen, and diffuse and accumulate around non-metal inclusions such as MnS in the steel and the hard phase 2 structure. As a result, it becomes molecular hydrogen, and cracks are caused by its internal pressure.

Several methods have been proposed to prevent such HIC. Patent Documents 1 and 2 describe that for high-strength steel sheets, while controlling the morphology of sulfide-based inclusions by adding low S and Ca, central segregation is suppressed by lowering C-Mn, and the strength is reduced accordingly. Has been proposed as a method of supplementing with the addition of Cr, Mo, Ni and the like and accelerated cooling.

On the other hand, from the viewpoint of increasing the size of steel structures and reducing costs, there is an increasing demand for steel sheets having higher strength and toughness. For the purpose of improving the characteristics of steel sheets, reducing alloying elements, and omitting heat treatment, high-strength steel sheets are usually manufactured by applying the so-called TMCP (Thermo-Mechanical Control Process) technology, which combines controlled rolling and controlled cooling.

In order to increase the strength of steel materials using TMCP technology, it is effective to increase the cooling rate during controlled cooling. However, when controlled cooling is performed at a high cooling rate, the surface layer portion of the steel sheet is rapidly cooled, so that the hardness of the surface layer portion is higher than that inside the steel sheet, and the hardness distribution in the plate thickness direction varies. Therefore, there is a problem from the viewpoint of ensuring the material uniformity in the steel sheet.

In order to solve the above problems, for example, in Patent Documents 3 and 4, a line in which the surface of the steel sheet after accelerated cooling is heated to a higher temperature than the inside to reduce the hardness of the surface layer portion by using a high frequency induction heating device. A method for manufacturing a steel plate for a pipe is disclosed.

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

Japanese Unexamined Patent Publication No. 5-271766 Japanese Unexamined Patent Publication No. 7-173536 JP-A-2002-327212 Japanese Patent No. 3711896 Japanese Unexamined Patent Publication No. 9-57327 Japanese Patent No. 3796133

However, although the techniques described in Patent Documents 1 to 4 are all targeted at the central segregation portion, the uniformity of HIC resistance characteristics in the plate width direction is not considered. There is a problem that the variation in the central segregation in the plate width direction at the slab stage affects the variation in the HIC resistance characteristics of the rolled steel sheet in the plate width direction.

Further, according to the study by the present inventors, there is room for improvement even in the high-strength steel plate obtained by the manufacturing methods described in Patent Documents 5 and 6 from the viewpoint of uniformity of HIC resistance characteristics in the plate width direction. found. The possible reasons for this are as follows. That is, in the methods described in Patent Documents 5 and 6, descaling reduces surface texture defects due to scale indentation defects during hot straightening and reduces variations in the cooling stop temperature of the steel sheet to improve the shape of the steel sheet. However, no consideration has been given to the cooling conditions for obtaining a uniform material.

As described above, conventionally, when a combination of low-cost components and controlled cooling at a high cooling rate, it has not been possible to manufacture a high-strength steel sheet having material uniformity in the steel sheet and HIC resistance characteristics.

Therefore, in view of the above problems, the present invention provides a high-strength steel plate for sour-resistant pipes which is excellent in HIC-resistant characteristics and suppresses variations in HIC-resistant characteristics in the plate width direction, and a high-strength steel pipe using the same. With the goal.

In order to solve the above problems, the present inventors have prevented HIC from being generated from the central segregation portion in a high-strength steel sheet having strength of API standard X65 grade, and suppressed variation in HIC resistance characteristics in the plate width direction. In order to improve the material uniformity in the steel sheet, the composition of the steel material, the microstructure, and the manufacturing method were intensively studied. As a result, by adopting specific conditions for the secondary cooling of the slab and performing controlled cooling after hot rolling under the specific conditions, in the plate width direction of the steel sheet. The present invention was completed based on the finding that the variation in central segregation can be suppressed.

That is, the gist structure of the present invention is as follows.
[1] In terms of mass%, C: 0.02 to 0.08%, Si: 0.01 to 0.50%, Mn: 0.50 to 1.80%, P: 0.001 to 0.015%. , S: 0.0002 to 0.0015%, Al: 0.01 to 0.08% and Ca: 0.0005 to 0.005%, and the balance is composed of Fe and unavoidable impurities. And
In the cross section perpendicular to the rolling direction of the steel sheet, the number of Mn-enriched spots with a major axis length exceeding 1.5 mm, which is similar to an elliptical shape, is in the measurement area of ± 5 mm in the sheet thickness direction from the center of the sheet thickness in the plate width direction. 3 or less per 100 mm of length
The HIC resistance is 10% or less in CAR at the W / 4, W / 2, and 3W / 4 positions from one end of the steel plate in the plate width direction, where W is the plate width.
The variation in HIC resistance characteristics in the plate width direction is 5% or less at 3σ when the standard deviation of CAR is σ.
A high-strength steel sheet for sour line pipes, which has a tensile strength of 520 MPa or more.

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

[3] The component composition further comprises, in mass%, 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 line pipe according to the above [1] or [2], which contains one or more selected types.

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

The high-strength steel sheet for sour line pipes of the present invention has excellent HIC resistance characteristics, and variations in HIC resistance characteristics in the plate width direction are suppressed. Therefore, the high-strength steel pipe of the present invention using this high-strength steel plate is excellent in HIC resistance, and variations in HIC resistance in the circumferential direction of the pipe are suppressed.

It is a schematic diagram of the cross section of the steel plate C explaining the position of the EMPA analysis area in an Example. It is a schematic diagram of the cross section of the steel plate C explaining the position of the sampling part of the HIC test piece in an Example. It is a figure explaining the secondary cooling method of a slab in continuous casting for manufacturing a high-strength steel plate of this embodiment, (A) is the cooling at the time of injecting cooling water from one two-fluid spray nozzle. It is a schematic diagram which shows the water injection range and the water amount density distribution, (B) is the cooling water injection range when cooling water is injected from two two-fluid spray nozzles, the water amount density distribution, and the lap allowance of the injection range. It is a schematic diagram which shows.

Hereinafter, the high-strength steel sheet for sour-resistant pipes of the present disclosure will be specifically described.

[Ingredient composition]
First, the component composition of the high-strength steel sheet according to the present disclosure and the reason for its limitation will be described. In the following description, all units indicated by% are mass%.

C: 0.02 to 0.08%
C effectively contributes to the improvement of strength, but if the content is less than 0.02%, sufficient strength cannot be secured, while if it exceeds 0.08%, the hardness of the surface layer portion increases during accelerated cooling. Therefore, the HIC resistance property deteriorates. In addition, toughness also deteriorates. Therefore, the amount of C is limited to the range of 0.02 to 0.08%.

Si: 0.01 to 0.50%
Si is added for deoxidation, but if the content is less than 0.01%, the deoxidizing effect is not sufficient, while if it exceeds 0.50%, the toughness and weldability deteriorate, so the amount of Si is 0. Limited to the range of 01 to 0.50%.

Mn: 0.50 to 1.80%
Mn effectively contributes to the improvement of strength and toughness, but when the content is less than 0.50%, the effect of adding Mn is poor, while when it exceeds 1.80%, the hardness of the surface layer portion increases during accelerated cooling. , HIC resistance deteriorates. Weldability also deteriorates. Therefore, the amount of Mn is limited to the range of 0.50 to 1.80%.

P: 0.001 to 0.015%
P is an unavoidable impurity element, which deteriorates weldability and also deteriorates HIC resistance by increasing the hardness of the central segregated portion. If it exceeds 0.015%, the tendency becomes remarkable, so the upper limit is set to 0.015%. It is preferably 0.008% or less. The lower the content, the better, but from the viewpoint of refining cost, it should be 0.001% or more.

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

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

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

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

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

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

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

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

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

One or more selected from Nb: 0.005 to 0.1%, V: 0.005 to 0.1%, and Ti: 0.005 to 0.1% Nb, V and Ti Both are elements that can be optionally added to increase the strength and toughness of the steel sheet. If the content of each element is less than 0.005%, the effect of adding it is poor, while if it exceeds 0.1%, the toughness of the weld deteriorates. Therefore, when it is added, it is 0.005 to 0.1%. It is preferably in the range of.

The balance other than the above elements consists of Fe and unavoidable impurities. However, the inclusion of other trace elements is not hindered as long as the effects of the present invention are not impaired.

[Mn enrichment spot]
In the high-strength steel plate for sour line pipes of the present disclosure, in a cross section perpendicular to the rolling direction (plate length direction) of the steel plate, a length close to an elliptical shape is formed in a measurement region of ± 5 mm in the plate thickness direction from the center of the plate thickness. It is important that the number of Mn enriched spots having a shaft length of more than 1.5 mm is 3 or less per 100 mm of length in the plate width direction.

In the present specification, the “Mn-enriched spot” means a portion where the Mn concentration is higher than the addition amount (Mn content in the steel sheet) due to segregation. Specifically, the Mn content in the steel sheet is 1. When it is .50% or less, it is specified as a portion where the Mn concentration is 1.60% or more, and when the Mn content in the steel sheet is more than 1.50% and 1.80% or less, the Mn concentration is in the steel sheet. It is identified as a site that is 0.10% or more higher than the Mn content of.

According to the study by the present inventors, among the Mn-enriched spots specified as described above, HIC cracks are likely to occur from the Mn-enriched spots having a major axis length of more than 1.5 mm, and the length It was found that HIC cracking occurs when the number of Mn-enriched spots having a shaft length of more than 1.5 mm exceeds 3 per 100 mm of length in the plate width direction. Therefore, in the present disclosure, the number of Mn enriched spots having a major axis length exceeding 1.5 mm is set to 3 or less per 100 mm in the plate width direction.

In the present disclosure, "the number of Mn-enriched spots having a major axis length exceeding 1.5 mm per 100 mm in the plate width direction" shall be measured by the following method. First, a sample for analysis is cut out from the steel sheet, and the sample is adjusted by polishing. At this time, the surface of the sample is set to have a cross section (C cross section) perpendicular to the plate length direction of the steel plate. Then, as shown in FIG. 1, in this C cross section, where W is the plate width, the W / 4 position, the W / 2 position, and the 3W / 4 position (hereinafter,, from one end in the plate width direction of the steel plate). Each of the three points that are the center of the thickness of the steel sheet (t / 2 position; t is the plate thickness) at the "W / 4 position", "W / 2 position", and "3W / 4 position"). The Mn concentration is mapped by an electron probe microanalyzer (EMPA) for three regions of ± 5 mm (thickness 10 mm) in the plate thickness direction and ± 200 mm (width 400 mm) in the plate width direction. Depending on the plate width of the steel plate, the above three regions may overlap to form one region. Mapping is performed using an electron probe having an accelerating voltage of 25 kV and a diameter of 0.15 mm. In this EPMA analysis region (thickness 10 mm × width 400 mm), the number of Mn-enriched spots having a major axis length exceeding 1.5 mm is counted and converted into the number per 100 mm length in the plate width direction.

The steel structure of the high-strength steel plate for sour-resistant pipes disclosed in the present disclosure preferably has a bainite structure in order to increase the tensile strength to 520 MPa or more. Here, the bainite structure includes a structure called bainitic ferrite or granular ferrite that is transformed during or after accelerated cooling, which contributes to strengthening the transformation. When heterogeneous structures such as ferrite, martensite, pearlite, island-like martensite, and retained austenite are mixed in the bainite structure, the strength decreases, the toughness deteriorates, and the surface hardness increases. Therefore, structures other than the bainite phase The smaller the fraction, the better. However, if the volume fraction of the tissue other than the bainite phase is sufficiently low, those effects can be ignored, and a certain amount is acceptable. Specifically, in the present disclosure, if the total volume fraction of steel structures other than bainite (ferrite, martensite, pearlite, island-like martensite, retained austenite, etc.) is less than 5%, there is no significant effect and is acceptable. It shall be done.

[Uniformity of HIC resistance characteristics in the plate width direction]
In the high-strength steel plate for sour line pipe of the present disclosure, the HIC resistance property is 10% or less in CAR at the W / 4 position, W / 2 position, and 3W / 4 position, and the plate width direction It is important that the variation in HIC resistance characteristics is 5% or less at 3σ when the standard deviation of CAR is σ. This means that the HIC resistance is excellent and the variation in the HIC resistance in the plate width direction is suppressed. The HIC resistance characteristics of the W / 4 position, the W / 2 position, and the 3W / 4 position are preferably 5% or less in CAR.

In the present disclosure, "HIC resistance characteristics at W / 4 position, W / 2 position, and 3 W / 4 position" shall be evaluated by the following method. As shown in FIG. 2, in the C cross section of the steel sheet, the thickness is 20 mm × width centered on the center of the plate thickness (3 points in total) at the W / 4, W / 2 position, and 3 W / 4 position in the plate width direction. Take a test piece with a size of 20 mm. Three samples are taken from each of the three test pieces thus obtained, and a total of nine samples are subjected to a HIC (hydrogen-induced cracking) characteristic investigation. In this survey, based on TM0284 defined by NACE, the survey is performed in a Method A environment, and the crack generation area ratio (CAR) is determined as a hydrogen-induced crack determination criterion. In the high-strength steel sheet for sour line pipes of the present disclosure, all nine CARs thus obtained are 10% or less, preferably 5% or less.

Further, in the present disclosure, "variation in HIC resistance characteristics in the plate width direction" shall be evaluated as 3σ when the standard deviations of the above nine CARs are calculated as σ.

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

[Production method]
Hereinafter, the manufacturing method and manufacturing conditions for manufacturing the high-strength steel sheet for sour-resistant pipes will be specifically described. In the manufacturing method of the present disclosure, steel having the above composition is continuously cast into a slab, which is heated and then hot-rolled to form a steel sheet, and then controlled cooling is performed on the steel sheet. .. At this time, by performing the secondary cooling in continuous casting under specific conditions and performing slab heating and controlled cooling under specific conditions, the HIC resistance is excellent and the variation in HIC resistance in the plate width direction is suppressed. High-strength steel sheets for sour-resistant pipes can be manufactured.

[Secondary cooling method for slabs during continuous casting]
As shown in FIGS. 3A and 3B, cooling water is sprayed in a mist form from a plurality of two-fluid spray nozzles 10A and 10B arranged at predetermined intervals in the width direction of the slab 20, and the slab 20 is formed. It is a method of cooling while feeding in the longitudinal direction, and the position where the ratio of the two-fluid spray nozzle 10 to the water amount density directly under the two-fluid spray nozzle 10 is 50% is the cooling water in the width direction of the slab 20. The wrapping allowance of the cooling water injection range injected from the adjacent two-fluid spray nozzles 10A and 10B is 1.6S or more and 2.4S or less, using the one having a distance S (mm) from both ends of the injection range. A secondary cooling method for slabs is used, which is characterized in that it is within the range.

FIG. 3 schematically shows the injection range and the water density distribution of the cooling water jetted from the two-fluid spray nozzle, and FIG. 3 (A) shows the amount of water with respect to the water density immediately below the two-fluid spray nozzle 10. The distance S from both ends of the injection range in which the density ratio is 50% is shown, and FIG. 3B shows the lap allowance of the injection range of the cooling water injected from the two two-fluid spray nozzles 10A and 10B. It is shown.

The distance S from both ends of the injection range of the cooling water injected from the two-fluid spray nozzle 10 can be obtained by the following method. First, the water density distribution in the width direction of the slab of cooling water sprayed from the two-fluid spray nozzle 10 is measured. For the water amount density distribution, the bifluid spray nozzle 10 is arranged above the measuring basin group divided into a large number in the width direction of the slab 1, and the cooling water injected from the bifluid spray nozzle 10 is weighed for each measuring basin. Can be measured by.

The reason for setting the lap allowance to be in the range of 1.6S or more and 2.4S or less is as follows. That is, when a plurality of two-fluid spray nozzles are arranged to secondarily cool the slab, the slab is arranged so that the water amount density of the cooling water sprayed from each two-fluid spray nozzle is uniform over the width direction of the slab. Even so, since the collision pressure is low at both ends of the cooling water injection range, the cooling ability of the slab becomes low, and the slab cannot be uniformly cooled over the width direction. However, if the wrap allowance is in the range of 1.6S or more and 2.4S or less, the slab is uniformly cooled over the width direction in consideration of the collision pressure distribution in addition to the water density distribution in the width direction of the slab. can do. That is, according to this method, the slab can be cooled without reducing the cooling capacity in the region where the injection range of the cooling water injected from the adjacent two-fluid spray nozzles 10A and 10B wraps, and the slab can be cooled. The surface temperature deviation in the width direction can be reduced, and cooling can be performed almost uniformly. This makes it possible to manufacture a slab with little variation in central segregation in the width direction.

In addition, in FIG. 3B, an example using two two-fluid spray nozzles 10A and 10B was described, but even when three or more two-fluid spray nozzles are arranged to secondarily cool the slab. The wrapping allowance of the cooling water injection range may be set as described above for adjacent two or more two-fluid spray nozzles.

Further, as the two-fluid spray nozzle, for example, a mist nozzle provided with a cooling water and air supply pipe, a mixing pipe and a nozzle tip can be used, but the two-fluid spray nozzle is not limited thereto.

[Slab heating temperature]
Slab heating temperature: 1000-1300 ° C
If the slab heating temperature is less than 1000 ° C., the solid solution of the carbide is insufficient and the required strength cannot be obtained. On the other hand, if the slab heating temperature exceeds 1300 ° C., the toughness deteriorates. Therefore, the slab heating temperature is set to 1000 to 1300 ° C. It should be noted that this temperature is the temperature inside the heating furnace, and the slab is heated to this temperature up to the center.

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

[Cooling start temperature for controlled cooling]
Cooling start temperature: the steel plate surface temperature (Ar ₃ -10 ° C.) or higher cooling starting steel sheet surface temperature is low, the number of ferrite amount before controlled cooling, in particular the amount of temperature drop of from Ar ₃ transformation point is 10 ° C. to produce the ferrite exceeds 5% by volume fraction exceeds, for deteriorated HIC resistance with reduction in strength is increased, the steel sheet surface temperature of the cooling at the start and (Ar ₃ -10 ℃) or higher ..

[Cooling rate of controlled cooling]
Average cooling rate from 750 ° C to 550 ° C at average steel sheet temperature: 15 ° C / s or more If the average cooling rate from 750 ° C to 550 ° C at average steel sheet temperature is less than 15 ° C / s, bainite structure cannot be obtained and strength Deterioration and deterioration of HIC resistance characteristics occur. Therefore, the cooling rate at the average temperature of the steel sheet is set to 15 ° C./s or more. From the viewpoint of variations in steel sheet strength and hardness, the average cooling rate of the steel sheet is preferably 20 ° C./s or more. The upper limit of the average cooling rate is not particularly limited, but it is preferably 80 ° C./s or less so that the low temperature transformation product is not excessively produced.

[Cooling stop temperature]
Cooling stop temperature: 250 to 550 ° C at average steel sheet temperature
After the rolling is completed, the bainite phase is formed by quenching to 250 to 550 ° C., which is the temperature range of bainite transformation, by controlled cooling. If the cooling stop temperature exceeds 550 ° C., the bainite transformation is incomplete and sufficient strength cannot be obtained. Further, when the cooling stop temperature is less than 250 ° C., the hardness of the surface layer portion increases remarkably. Preferably, it is 350 to 500 ° C.

Although the average temperature of the steel plate cannot be physically measured directly, for example, a process computer can be used based on the surface temperature at the start of cooling and the surface temperature at the target cooling stop measured by a radiation thermometer. The temperature distribution in the plate thickness cross section can be obtained in real time by using the difference calculation. The average value of the temperature in the plate thickness direction in the temperature distribution is referred to as the "average temperature of the steel plate" in the present specification.

[High-strength steel pipe]
The high-strength steel sheet of the present disclosure is formed into a tubular shape by press bend forming, roll forming, UOE forming, etc., and then the butt portion is welded to provide excellent material uniformity in the steel sheet suitable for transporting crude oil and natural gas. High-strength steel pipes for sour-resistant pipes (UOE steel pipes, welded steel pipes, spiral steel pipes, etc.) can be manufactured.

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

The steels (steel grades A to M) having the composition shown in Table 1 were prepared into slabs having a slab width of 1600 mm by a continuous casting method. In the secondary cooling, the lap allowance in the injection range of the cooling water sprayed in a mist shape from the three two-fluid spray nozzles arranged at predetermined intervals in the width direction was used as the value shown in Table 2 for the secondary cooling. The distance S from both ends of the cooling water injection range in the width direction of the slab 20 to the position where the ratio to the water density immediately below the two-fluid spray nozzle is 50% was fixed at 70 mm.

The slab thus obtained was heated to the temperature shown in Table 2 and then hot-rolled at the rolling end temperature and the rolling reduction ratio shown in Table 2 to obtain a steel sheet having a plate thickness shown in Table 2. Then, the steel sheet was controlled-cooled using a water-cooled control cooling device under the conditions shown in Table 2.

[Organization identification]
The microstructure of the obtained steel sheet was observed with an optical microscope and a scanning electron microscope. Table 3 shows the structure of the steel sheet at the center of the thickness (t / 2 position).

[Evaluation of tensile properties]
A total thickness test piece (API5L standard) in the direction perpendicular to rolling is taken from the steel sheet obtained at each level, and a tensile test is performed using this as a tensile test piece to measure the yield strength (0.5% proof stress) and tensile strength. did. The target range is a yield strength of 450 MPa or more and a tensile strength of 520 MPa or more. The results are shown in Table 3.

[Evaluation of variation in HIC resistance characteristics in the plate width direction]
Three samples were taken from each of the W / 4, W / 2, and 3W / 4 positions by the method described above, and the CAR was measured. The maximum value among the nine measured values thus obtained is shown in the column of "HIC resistance characteristics" in Table 3. Table 3 also shows 3σ when the standard deviations of the nine CARs are calculated as σ. The maximum value is 10% or less, and 3σ is 5% or less.

[Measurement of Mn concentrated spot]
The number of Mn-enriched spots having a major axis length of more than 1.5 mm per 100 mm in the plate width direction was measured by the method described above. The target range is 3 or less. The results are shown in Table 3.

[DWTT test]
DWTT test pieces conforming to API-5L are collected from the steel sheets obtained at each level and tested at a test temperature of 0 to -80 ° C, and the SA value (Shear Area: ductile fracture surface ratio) becomes 85%. The transition temperature was calculated. The target range for the transition temperature is -50 ° C or lower. The results are shown in Table 3.

No. 1 to 13 are examples of the invention, the component composition is within the range of the present invention, and the manufacturing method is within the range of suitable conditions for obtaining the steel sheet of the present invention. In each case, the yield strength was 450 MPa or more, the tensile strength was 520 MPa or more, the 85% SATT in the DWTT test was −50 ° C. or less, the variation in the HIC resistance characteristics in the plate width direction was small, and all the characteristics were good.

On the other hand, No. 14 to 22 are comparative examples, and the composition of the components is within the range of the present invention, but the production method is outside the range of suitable conditions for obtaining the steel sheet of the present invention. No. In No. 14, the slab heating temperature was low, the homogenization of the microstructure and the solid solution of carbides were insufficient, and the strength was low.
No. In No. 15, the cooling start temperature was low and ferrite was deposited too much, so that the strength was low and the HIC resistance was inferior.
No. 16 and No. No. 18 had low strength and was inferior in HIC resistance characteristics because pearlite was excessively precipitated in the central portion of the plate thickness as a microstructure when the control cooling conditions were out of the preferable range.
No. In No. 17, the cooling stop temperature was low and a hard phase of martensite or island-shaped martensite (MA) was formed, so that the DWTT characteristics and the HIC resistance characteristics were inferior.
No. 19-No. In each of No. 22, the secondary cooling conditions in the slab stage were outside the range of suitable conditions, the Mn concentration in the central segregated portion was large, the HIC resistance characteristics in the steel plate plate width direction varied widely, and the HIC characteristics were inferior.
No. 23-No. In No. 27, the component composition was out of the range of the present invention, the Mn concentration in the central segregated portion was large, the HIC characteristics in the steel plate plate width direction varied widely, and the HIC characteristics were inferior.

The high-strength steel sheet for sour line pipes of the present invention has excellent HIC resistance characteristics, and variations in HIC resistance characteristics in the plate width direction are suppressed. Therefore, steel pipes (electrosewn steel pipes, spiral steel pipes, UOE steel pipes, etc.) produced by cold forming this steel sheet can be suitably used for transporting crude oil and natural gas containing hydrogen sulfide, which requires sour resistance. ..

10, 10A, 10B two-fluid spray nozzle 20 slab

Claims (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: It contains 0.0002 to 0.0015%, Al: 0.01 to 0.08% and Ca: 0.0005 to 0.005%, and has a component composition in which the balance consists of Fe and unavoidable impurities.
In the cross section perpendicular to the rolling direction of the steel plate, where W is the plate width, from one end in the plate width direction of the steel plate, at the center of the thickness of the steel plate at the W / 4, W / 2, and 3W / 4 positions. ± 5 mm in the thickness direction around the respective certain three points, Oite the measurement region of ± 200 mm in the sheet width direction, the number of Mn concentrated spot beyond major axis 1.5mm approximates an elliptical shape, 3 or less per 100 mm of length in the plate width direction,
In the cross section perpendicular to the rolling direction of the steel plate, where W is the plate width, from one end in the plate width direction of the steel plate, at the center of the thickness of the steel plate at the W / 4, W / 2, and 3W / 4 positions . A test piece having a thickness of 20 mm and a width of 20 mm was collected around each of the three points, and three samples were taken from each of the three test pieces thus obtained to make a total of nine samples. On the other hand, when the HIC characteristic test was performed, the HIC resistance characteristics of all the samples were 10% or less in CAR.
The variation in HIC resistance characteristics in the plate width direction is 5% or less at 3σ when the standard deviation of the CARs of the nine samples is σ.
A high-strength steel sheet for sour line pipes, which has a tensile strength of 520 MPa or more. The component composition is further selected from Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, and Mo: 0.50% or less in mass%. The high-strength steel plate for sour-resistant pipes according to claim 1, which contains two or more types. The component composition was further selected from Nb: 0.005 to 0.1%, V: 0.005 to 0.1%, and Ti: 0.005 to 0.1% in mass%. The high-strength steel plate for sour line pipe according to claim 1 or 2, which contains seeds or two or more kinds. A high-strength steel pipe using the high-strength steel plate for sour-resistant line pipe according to any one of claims 1 to 3.

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