RU2637202C2 - Sheet steel for a thick-strengthen high-strengthening pipe threading with excellent resistance to acid environment, resistance to smoke and low-temperature viscosity and also a main pipe - Google Patents

Abstract

FIELD: metallurgical engineering.

SUBSTANCE: steel contains, wt %: C: 0.04 to 0.08, Mn: 1.2 to 2.0, Nb: 0.005 to 0.05, Ti: 0.005 to 0.03, Ca: 0.0005 to 0.0050, N: 0.001 to 0.008, Si: 0.5 or less, Al: 0.05 or less, P: 0.03 or less, S: 0.005 or less, O: 0.005 or less, Fe and unavoidable impurities - The rest. The microstructure of a part of the outer zone of the sheet located at a distance from 0.9 mm to 1.1 mm from the surface in the thickness direction of the sheet contains deformed ferrite: 5% or more and S_fe1% or less, a mixture of martensite and austenite: 8% or less, the remaining polygonal ferrite and/or bainite, where S_fe1% is determined by the expression: S_fe1=0.6552×T_H-4.7826, where T_H: thickness of thick steel. The microstructure of the central part of the sheet, located at a distance of 1 mm from the centre of the sheet thickness towards the surface of the plate steel, contains a deformed ferrite: 5% or less, a mixture of martensite and austenite: 5% or less, the remaining needle ferrite and/or bainite. Said parts of the outer zone and the central part have an effective grain size, the average value of which, measured by electron diffraction backscattering, is 20 mcm or less.

EFFECT: high resistance to acidic medium, resistance to crushing and low-temperature viscosity.

7 cl, 2 dwg, 5 tbl

Description

FIELD OF THE INVENTION

[0001] The present invention relates to plate steel for a thick-walled high-strength trunk pipe that has excellent acid resistance, shear resistance and low temperature viscosity, in particular, to a steel plate for a thick-walled high-strength trunk pipe, which has excellent acid resistance, resistance crushing and low temperature viscosity, which is therefore optimal for the main pipe for transport ki natural gas or crude oil, and also relates to a main pipe, having excellent resistance to acidic environments, collapse resistance and low temperature toughness.

BACKGROUND

[0002] In recent years, the importance of pipelines as a way of transporting crude oil or natural gas over long distances has grown steadily. Long-distance pipe design approaches for transportation over long distances are mainly based on the standards of the American Petroleum Institute (API). In the past, a main pipe has been developed with excellent tensile strength and low temperature viscosity in order to prevent rupture when applying internal pressure. In order to increase the efficiency of transporting crude oil or natural gas, stronger and thicker trunk pipes are needed. In addition, when laying main pipes in the Arctic regions, in particular, low temperature viscosity is required. However, in most cases, the higher the strength and the greater the thickness, the more difficult it becomes to guarantee the viscosity of the steel material.

[0003] In order to reduce changes in the hardness of the plate material in the direction of the sheet thickness and improve the low temperature viscosity, PLT 1 offers a rolling method in the temperature region where the microstructure becomes biphasic and consists of austenite and ferrite (biphasic region). According to this method, it is possible to convert the microstructure of the plate material into a fine-grained needle-shaped ferrite structure to which island martensite is mixed.

[0004] In addition, recently, the characteristics required of the main pipe have become more diverse. In addition to strength and low temperature stiffness, crushing resistance was sometimes required, allowing the pipe not to crumple under external pressure, or resistance to an acidic environment, allowing the pipe not to crack in an acidic environment that contains hydrogen sulfide, etc. In particular, when laying deepwater sections of the pipeline, it became necessary to simultaneously achieve the opposite characteristics of crushing resistance and low temperature stiffness. However, due to the increased thickness of the main pipe, the simultaneous achievement of shear resistance and low temperature stiffness becomes extremely difficult.

LIST OF REFERENCES

PATENT LITERATURE

[0005] PLT 1: Japanese Patent Publication No. 8-041536A

PLT 2: Japanese Patent Publication No. 2010-084170A

PLT 3: Japanese Patent Publication No. 2010-084171A

PLT 4: Japanese Patent Publication No. 2011-132599A

PLT 5: Japanese Patent Publication No. 2011-163455A

SUMMARY OF THE INVENTION

TECHNICAL PROBLEM

[0006] As noted above, in recent years, the characteristics required of a main pipe for transporting natural gas or crude oil, which is laid at the bottom of the ocean at great depths, have become more complex. Greater thickness, higher strength, low temperature viscosity, resistance to acidic conditions and, in addition, crushing resistance were required.

[0007] In the case of the aforementioned PLT 1, improvement in acid resistance and collapse resistance are not considered at all. In particular, island martensite becomes the starting points for fracture and poses a problem of decreasing fracture toughness.

[0008] In order to solve this problem, a method has been proposed to suppress the formation of solid martensite and suppress the difference in hardness between ferrite and bainite, as well as a method of using fine-grained bainite to suppress the Bausinger effect (see, for example, PLT 2-4).

[0009] In recent years, the characteristics required of the main pipe have become more diverse. Among them, in particular, the characteristics required of the main pipe, which is laid on the ocean floor at great depths, have become more complex. In particular, in addition to the greater thickness, yield strength (YS), tensile strength (TS) and low temperature stiffness (shear area when testing DWTT at -10 ° C), acid resistance and additionally crushing resistance were also required (0 , 2% stress of plastic flow during compression in a circular direction after aging at a temperature of 200 ° C). However, in the prior art (see, for example, PLT 2-5, etc.), it was extremely difficult to simultaneously achieve all of these characteristics.

[0010] The invention, which is disclosed in PLT 2, is devoted to improving crack propagation resistance and low temperature stiffness, but does not consider how to improve acid resistance and crushing resistance. In addition, the invention, which is disclosed in PLT 3, considers low temperature viscosity and shear resistance, but does not consider how to improve resistance to an acidic environment. In addition, the invention disclosed in PLT 4 attempts to achieve a balance of compressive strength and low temperature stiffness, as well as high compressive strength and resistance to acidic conditions, but does not consider the above collapse resistance (0.2% compressive stress in a circular direction after aging at 200 ° C).

[0011] In PLT 5, it was found that in the case of a steel pipe for a main pipe with a sheet thickness of 25 mm or more and up to the American Petroleum Institute (API) X80 standard (tensile strength of 620 MPa or more), make the central part of the sheet thickness a fine-grained structure bainitis is an extremely difficult task. In order to solve such a technical problem, PLT 5 proposes a manufacturing process that lowers the carbon content, turns the microstructure into a microstructure with a low conversion temperature, which is formed mainly from bainite, and based on this steel material, the viscosity of which is improved, adds molybdenum to hardenability and reduces the addition of aluminum in order to use bainite in grains.

[0012] The invention disclosed in PLT 5 improves the hardenability of the base material and makes the effective HAZ grain size smaller due to the fact that the steel plate as a whole consists of a uniform microstructure formed mainly from bainite. The invention, which is disclosed in PLT 5, is aimed at improving the low-temperature stiffness of the weld zone and does not consider how to improve resistance to acidic conditions and resistance to collapse.

[0013] Furthermore, in the central portion of the sheet thickness, rolling due to controlled rolling and cooling rate due to controlled cooling become insufficient. Therefore, even when hardenability improves, as the thickness of the sheet increases, it becomes difficult to make plate steel a generally uniform microstructure.

[0014] Furthermore, in the past, plate steel for a main pipe often had a sheet thickness of 20 mm or less. If the strength corresponded to class X65, etc. according to the API standard, it was possible to easily guarantee various characteristics, such as resistance to acidic conditions, low temperature viscosity and resistance to collapse. This was because during hot rolling, the degree of compression was sufficiently guaranteed, and the effective grain size became smaller and, in addition, the difference in cooling rate between the surface layers and the central part of the thickness was small due to accelerated cooling, so that the microstructure became uniform . In this regard, if the sheet thickness is 25 mm or more, in particular 30 mm or more, it becomes difficult to simultaneously satisfy all the requirements for resistance to acidic conditions, low temperature stiffness and crushing resistance.

[0015] In particular, providing crush resistance and providing low temperature stiffness are opposed things. In the prior art, no material has been developed that can achieve both shear resistance and low temperature stiffness.

[0016] The present invention, in view of this situation, aims at providing a thick-walled high-strength main pipe that would be optimal as a material for the main pipe for transporting natural gas or crude oil and would have a good balance of resistance to acidic conditions, crushing resistance and low-temperature stiffness, as well as plate steel for thick-walled high-strength main pipe.

SOLUTION

[0017] The inventors of the present invention participated in intensive studies focusing on the microstructure and crystalline grain size in a steel plate for a main pipe, so as to obtain a steel plate for a thick-walled high-strength main pipe, which would have excellent acid resistance, shear resistance and low temperature viscosity. As a result, they found that in a thick-walled main pipe (also called a "thick-walled steel pipe") compositions, microstructures, production processes, etc. to achieve (1) both strength and resistance to acidic conditions, (2) both strength and shear resistance of thick-walled steel pipes, and (3) simultaneously strength and low-temperature stiffness of thick-walled steel pipes can be described as follows:

[0018] (1) Achieving both strength and resistance to acidic conditions

In order to increase the strength of the main pipe without impairing the resistance to the acidic environment, it is effective to turn the microstructure of the main material of the main pipe, i.e. plate steel, into a homogeneous structure consisting of acicular ferrite or bainite. In addition, in order to improve the resistance to acidic conditions, it is necessary to suppress the hardening of part of the central segregation. Next, a mechanism causing cracking that occurs in an acidic environment will be explained. Cracking in an acidic environment, in particular hydrogen-induced cracking (HIC), occurs precisely due to hydrogen, which collects around elongated inclusions based on MnS and other defects in steel present in the central segregation section of plate steel. Thus, in an acidic environment, hydrogen that penetrates steel gathers around these defects and forms gas pockets. When the pressure exceeds the value of the steel fracture toughness (KIC), cracking occurs. In addition, if part of the central steel segregation surrounding the inclusions, etc., is hardened, cracking is easily propagated. Therefore, in the main pipe, which is used in an acidic environment, it is important to suppress the formation of elongated MnS and the formation of solid phases in the central segregation part. In particular, it is effective to stop accelerated cooling at a slightly elevated temperature, for example, to stop accelerated cooling after hot rolling so that the temperature of a portion of the central segregation of the steel becomes 400 ° C or more. It should be noted that the “central segregation part” is a part in the central part of the plate thickness of the plate, where manganese and other components are concentrated due to segregation during solidification during casting.

[0019] (2) Achieving both strength and collapse resistance of a thick-walled steel pipe

In the case of a thick-walled steel pipe, in order to simultaneously guarantee strength and collapse resistance, it is effective to add molybdenum, etc. to increase hardenability and use accelerated cooling after hot rolling in order to cause the formation of martensite or bainite with their high dislocation densities and to promote deformation aging. In particular, if you control the stop temperature of accelerated cooling so that it becomes slightly lower, for example, so that the surface temperature of the plate becomes 400 ° C or less, martensite is formed, and strain aging can be facilitated during coating and firing of thick-walled steel pipes (processing by heating and holding the pipe at a temperature of about 200 ° C during coating).

[0020] (3) Achieving both the strength and low temperature stiffness of a thick-walled steel pipe

In the case of a thick-walled steel pipe, compared to a thin-walled steel pipe, the previous austenitic grains (austenitic grains before conversion due to accelerated cooling) become larger and the low temperature viscosity decreases. In addition, compared with the structure of bainite alone, the effective grain size of the structure of acicular ferrite alone is smaller. Even in this case, it is impossible to say that low temperature viscosity is sufficient. Therefore, in order to guarantee low temperature viscosity, the formation of polygonal ferrite is effective. However, polygonal ferrite causes a drop in strength, therefore, in order to guarantee strength, it is effective to turn the structure into a composite of bainite or needle ferrite.

[0021] As explained above, it has been found that it is difficult to simultaneously satisfy the above requirements (1) to (3) in order to simultaneously provide resistance to an acidic environment, low temperature viscosity, and crushing resistance. For example, martensite is effective for crushing resistance (2), while martensite is harmful for resistance to acidic environment (1) and low temperature hardness (3). In addition, for low temperature hardness (3), polygonal ferrite is effective, but the resistance to acidic environment (1) decreases, since the formation of polygonal ferrite causes the structure to become uneven. In addition, polygonal ferrite, which has a low dislocation density, causes a drop in shear resistance. Therefore, the authors of the present invention have studied the method of using thick-walled, that is, the use of hot rolling and subsequent accelerated cooling in order to control the structure by using the temperature difference between the surfaces and the central part due to the thickness of the sheet. In addition, they took into account the fact that in the central part of the sheet thickness it is extremely important to provide resistance to an acidic environment, while in the surface layers it is extremely important to provide resistance to collapse. In addition, in order to guarantee low temperature viscosity, they studied the possibility of reducing the effective grain size.

[0022] Firstly, it was found that in order to guarantee resistance to acidic conditions, strength and low temperature viscosity in the central part of the thickness, it is effective to suppress the formation of deformed ferrite and a mixture of martensite and austenite (hereinafter referred to as “MA”) in order to delay solidification and create a homogeneous structure consisting of one or both of acicular ferrite and bainite. Here, in the central part of the thickness, manganese is concentrated due to segregation. Hardenability is high and ferrite formation is suppressed. However, in order to guarantee low temperature viscosity, the formation of ferrite is effective. It is necessary to create a microstructure so that the amount of ferrite increases in the direction of the surface layers. On the other hand, if soft polygonal ferrite is formed in order to guarantee low temperature viscosity, the yield strength of the surface layers during compression in the circular direction will drop, and as a result, the crushing resistance will drop. In order to solve this problem, the inventors of the present invention have put forward the idea of causing the formation of deformed ferrite in the surface layers and increasing the density of ferrite dislocations in order to promote strain aging and improve crushing resistance. Therefore, they found that the structure of the surface layers should be such that the deformed ferrite should be formed with an area fraction of 5% or more so as to satisfy the requirement for shear resistance, and the MA formation should be suppressed and the remainder should consist of one or both of polygonal ferrite and bainite in order to guarantee low temperature viscosity.

[0023] If the amount of deformed ferrite is large, shear strength increases, but low temperature viscosity deteriorates. In order to guarantee low temperature viscosity, it is necessary to some extent control the amount of deformed ferrite. Thus, it is necessary to appropriately distribute the parts supporting the crushing strength and the parts supporting the low temperature viscosity in accordance with the thickness of the sheet. Thus, the smaller the sheet thickness, the smaller the allowable amount of deformed ferrite in part of the surface layer, while the larger the sheet thickness, the greater the allowable amount of deformed ferrite in part of the surface layer. Therefore, the authors of the present invention investigated the relationship between the allowable amount of deformed ferrite and sheet thickness, and found the optimal ratio. The present invention was made on the basis of these discoveries and has as its essence the following:

[0024] [1] Sheet steel for a thick-walled high-strength main pipe, which has excellent resistance to acidic conditions, shear resistance and low temperature viscosity, which is:

plate steel with a thickness of 25 mm to 45 mm, containing:

C: from 0.04 wt.% To 0.08 wt.%;

Mn: 1.2 wt.% To 2.0 wt.%;

Nb: from 0.005 wt.% To 0.05 wt.%;

Ti: from 0.005 wt.% To 0.03 wt.%;

Ca: from 0.0005 wt.% To 0.0050 wt.%, And

N: from 0.001 wt.% To 0.008 wt.%, With limitations:

Si: 0.5 wt.% Or less

Al: 0.05 wt.% Or less

P: 0.03 wt.% Or less

S: 0.005 wt.% Or less

O: 0.005 wt.% Or less, and

having a residue consisting of iron and inevitable impurities, in which

the microstructure of the part of the surface layer located from the plate steel surface downward in the direction of sheet thickness from 0.9 mm to 1.1 mm, is limited in percentage of the area as follows:

deformed ferrite: 5% or more and S _fe1 %, determined by the following formula 1a or less, and

a mixture of martensite and austenite: 8% or less, and

has a residue consisting of one or both of polygonal ferrite and bainite, and

the microstructure of the part located from the center of the sheet thickness towards the front and rear sides of the steel plate within 1 mm, constituting the central part of the thickness, is limited in percentage of the area as follows:

deformed ferrite: 5% or less

a mixture of martensite and austenite: 5% or less, and

has a residue consisting of one or both of acicular ferrite and bainite, and

part of the surface layer and the central part of the thickness have an average value of the effective grain size, measured by electron backscattering diffraction, of 20 mm or less,

S _fe1 = 0.6552 × T _H -4.7826 formula 1a,

where T _H : thickness of plate steel for a thick-walled high-strength main pipe.

[2] Plate steel for a thick-walled high-strength main pipe having excellent acid resistance, shear resistance and low temperature viscosity in accordance with [1], further comprising one or more of the following:

Cu: 0.50 wt.% Or less

Ni: 0.50 wt.% Or less

Cr: 0.50 wt.% Or less

Mo: 0.50 wt.% Or less

W: 0.50 wt.% Or less

V: 0.10 wt.% Or less

Zr: 0.050 wt.% Or less

Ta: 0.050 wt.% Or less

B: 0.0020 wt.% Or less

Mg: 0.010 wt.% Or less

REM: 0.0050 wt.% Or less

Y: 0.0050 wt.% Or less

Hf: 0.0050 wt.% Or less, and

Re: 0.0050 wt.% Or less.

[3] Plate steel for a thick-walled high-strength trunk pipe having excellent acid resistance, shear resistance and low temperature viscosity in accordance with [1] or [2], in which the aluminum content is 0.005 wt.% Or less.

[4] Plate steel for a thick-walled high-strength main pipe, which has excellent acid resistance, shear resistance and low temperature viscosity in accordance with any one of [1] to [3], in which the tensile strength is from 500 to 700 MPa.

[5] Plate steel for a thick-walled high-strength main pipe, having excellent acid resistance, shear resistance and low temperature viscosity in accordance with any one of [1] to [3], wherein the yield strength after pipe formation is 440 MPa or more, the tensile strength is from 500 to 700 MPa, and the stress of plastic flow by 0.2% when compressed in a circular direction after aging at a temperature of 200 ° C is 450 MPa or more.

[6] A thick-walled high-strength main pipe made by molding plate steel for a thick-walled high-strength main pipe with excellent acid resistance, shear resistance and low temperature viscosity in accordance with any one of [1] to [4], into a pipe followed by an arc by welding adjacent ends, having a yield strength of 440 MPa or more, a tensile strength of 500 to 700 MPa and a plastic flow stress of 0.2% when compressed in a circular direction after aging I at a temperature of 200 ° C equal to 450 MPa or more.

Advantages of the Invention

[0025] According to the present invention, it is possible to provide plate steel for a thick-walled high-strength main pipe having excellent acid resistance, shear resistance and low temperature viscosity, which is therefore optimal as a material for the main pipe for transporting natural gas or crude oil. In particular, it is possible to provide plate steel for a thick-walled high-strength main pipe that has excellent acid resistance, shear resistance and low temperature viscosity, which has a thickness of 25 to 45 mm and which, after being formed into the pipe, has a YS of 440 MPa or more, TS from 500 to 700 MPa, shear area when testing DWTT at -10 ° C, equal to 85% or more, and compressive strength in the circular direction after aging at 200 ° C (0.2% plastic flow stress) 450 MPa or more. Contribution to industry is outstanding.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] FIG. 1 is an optical micrograph of a cross section of a surface portion of a plate of a steel plate for a thick-walled high-strength main pipe of the present invention.

FIG. 2 is a graph that prescribes an upper limit and a lower limit on the percentage of area of deformed ferrite in a portion of a surface layer of steel plate for the thick-walled high-strength main pipe of the present invention.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0027] Next, plate steel for the thick-walled high-strength main pipe of the present invention having excellent acid resistance, shear resistance, and low temperature viscosity (hereinafter also simply referred to as "steel plate for the main pipe" or "plate steel") and method of its manufacture. Next, reasons for limiting components in the steel plate for the thick-walled high-strength main pipe of the present embodiment (main material of the main pipe) will be explained. It should be noted that the characters% mean wt.%, Unless otherwise indicated.

[0028] C: Carbon is an element that improves the strength of plate steel. In this embodiment, 0.04 wt.% Carbon or more should be added. Preferably, carbon is added in an amount of 0.05 wt.% Or more, more preferably 0.055 wt.% Or more. On the other hand, if more than 0.08 wt.% Carbon is added, the low temperature viscosity drops, so that the upper limit of the amount of carbon is set to 0.08 wt.%. Preferably, the upper limit of the amount of carbon is set to 0.07 mass%, more preferably the upper limit is set to 0.065 mass%.

[0029] Mn: Manganese is an element that helps to improve the strength and stiffness of plate steel. In this embodiment, in order to ensure the strength of the steel plate, 1.2 wt.% Or more of manganese is added. Preferably, manganese is added in an amount of 1.4 wt.% Or more, more preferably 1.5 wt.% Or more. On the other hand, if the manganese is added in an excessive amount, the part of the sheet located in the middle of the thickness acquires increased hardness, and the resistance to acidic conditions deteriorates, so that the upper limit of the amount of manganese is set to 2.0 wt.% Or less. Preferably, the upper limit of the amount of manganese is set to 1.8 wt.% Or less, more preferably 1.7 wt.% Or less.

[0030] Nb: Niobium is an element that forms carbides and nitrides and helps to improve strength. In addition, it suppresses recrystallization and helps to reduce grain during hot rolling. For this reason, the lower limit of the amount of niobium is set equal to 0.005 wt.% Or more. Preferably, the lower limit of the amount of niobium is set to 0.010 wt.% Or more, more preferably 0.015 wt.% Or more. On the other hand, if niobium is added in an excessive amount, the strength increases excessively, and the low temperature viscosity deteriorates, so that the upper limit of the amount of niobium is set to 0.05 wt.% Or less. Preferably, the upper limit of the amount of niobium is set to 0.04 wt.% Or less, more preferably 0.03 wt.% Or less.

[0031] Ti: Titanium is an element that forms nitrides and affects the reduction of grain microstructure. The lower limit of the amount of titanium is set equal to 0.005 wt.% Or more in order to make the effective grain size smaller. Preferably, the lower limit of the amount of titanium is set to 0.008 wt.% Or more, more preferably 0.01 wt.% Or more. On the other hand, if titanium is added in an excessive amount, coarse TiN grains are formed and the low temperature viscosity deteriorates, so that the upper limit of the amount of titanium is set to 0.03 wt.% Or less. Preferably, the upper limit of the amount of titanium is set to 0.02 wt.% Or less, more preferably 0.015 wt.%.

[0032] Ca: Calcium is an element that controls the form of sulfides and improves resistance to acidic conditions. In this embodiment, in order to promote the formation of CaS and suppress the formation of MnS elongated in the rolling direction and provide resistance to acidic conditions, the lower limit of the amount of calcium is set to 0.0005 wt.% Or more. Preferably, the lower limit of the amount of calcium is set to 0.0010 wt.%, More preferably 0.0015 wt.%. On the other hand, if calcium is added in an excessive amount, coarse oxides are formed and the low temperature viscosity deteriorates, so that the upper limit of the amount of calcium is set equal to 0.0050 wt.%. Preferably, the upper limit of the amount of calcium is set to 0.0040 wt.% Or less, more preferably 0.0030 wt.% Or less.

[0033] N: In this embodiment, nitrides are used to make the microstructure finer, so that the nitrogen content is set to 0.001 wt.% Or more. Preferably, the amount of nitrogen is set to 0.002 wt.% Or more, more preferably 0.003 wt.% Or more. On the other hand, if nitrogen is contained in an excessive amount, coarse nitrides are formed and the low temperature viscosity deteriorates, so that the upper limit of the amount of nitrogen is set to 0.008 wt.%. Preferably, the upper limit of the amount of nitrogen is set to 0.007 wt.% Or less, more preferably 0.006 wt.% Or less.

[0034] Silicon and aluminum are deoxidizing elements. If they are added for deoxidation, it is sufficient to use any one of them, but both of these elements can also be used. It should be noted that if silicon and aluminum are added in an excessive amount, they degrade the performance of plate steel, so in this embodiment, the upper limits of the silicon and aluminum content are set as follows:

[0035] Si: If silicon is added in an excessive amount, solid MA is formed, in particular in the heat affected zone (HAZ), and the viscosity of the roll weld zone of the steel pipe falls, so that the upper limit of the amount of silicon is set to 0.5 wt.% or less. Preferably, the amount of silicon is set to 0.3 wt.% Or less, more preferably 0.25 wt.% Or less. It should be noted that, as explained above, silicon is an element that is used for deoxidation, and is also an element that helps to increase strength, so that it is preferable that the lower limit of the amount of silicon is 0.05 wt.% Or more, more preferably 0 10 wt.% Or more.

[0036] Al: As explained above, aluminum is a useful deoxidizing element. Preferably, the lower limit of the amount of aluminum is set to 0.001 wt.% Or more, more preferably 0.003 wt.% Or more. However, if aluminum is added excessively, coarse oxides are formed and the low temperature viscosity drops, so that the upper limit of the amount of aluminum is set to 0.05 wt.% Or less. Preferably, the upper limit of the amount of aluminum is set to 0.04 wt.% Or less, more preferably 0.03 wt.% Or less. In addition, by limiting the amount of aluminum to 0.005 wt.% Or less, the viscosity of the heat affected zone can be improved.

[0037] P, S, and O (oxygen) are contained as unavoidable impurities. If they are contained in excessive quantities, this harms the characteristics of plate steel, so in this embodiment, the upper limits of the content of phosphorus, sulfur and oxygen are set as follows:

[0038] P: Phosphorus is an element that causes the brittleness of steel. If the steel contains more than 0.03 wt.% Phosphorus, the low temperature viscosity of the steel is deteriorated, so that the upper limit of the phosphorus content is set to 0.03 wt.% Or less. Preferably, the upper limit of the amount of phosphorus is set to 0.02 mass% or less, more preferably 0.01 mass% or less.

[0039] S: Sulfur is an element that forms MnS and other sulfides. If the steel contains more than 0.005 wt.% Sulfur, the low temperature viscosity and resistance to acidic conditions fall, so that the upper limit of the sulfur content is set to 0.005 wt.% Or less. Preferably, the amount of sulfur is set to 0.003 wt.% Or less, more preferably 0.002 wt.%.

[0040] O: If oxygen is contained in an amount of more than 0.005 wt.%, Coarse-grained oxides are formed and the low temperature viscosity of the steel falls, so that the upper limit of the oxygen content is set to 0.005 wt.% Or less. Preferably, the upper limit of the amount of oxygen is set to 0.003 wt.% Or less, more preferably 0.002 wt.% Or less.

[0041] Furthermore, in the present invention, one or more of Cu, Ni, Cr, Mo, W, V, Zr, Ta and B can be added as elements that improve strength or low temperature viscosity.

[0042] Cu: Copper is an element that is effective for increasing strength without reducing low temperature hardness. Preferably, 0.01 mass% or more of copper is added, more preferably 0.1 mass% or more. On the other hand, copper is an element that facilitates the occurrence of cracking during heating of the steel slab or during roller welding of the steel pipe, so that the amount of copper is preferably set to 0.50 wt.% Or less. More preferably, the amount of copper is set to 0.35 wt.% Or less, and even more preferably 0.2 wt.% Or less.

[0043] Ni: Nickel is an element that is effective for improving low temperature hardness and strength. Preferably, 0.01 mass% or more of nickel is added, more preferably 0.1 mass% or more. Nickel, on the other hand, is an expensive element. From an economic point of view, the amount of nickel is preferably set to 0.50 wt.% Or less. More preferably, the amount of nickel is set to 0.35 mass% or less, and even more preferably 0.2 mass% or less.

[0044] Cr: Chromium is an element that improves the strength of steel due to dispersion hardening. Preferably, 0.01 mass% or more of chromium is added, more preferably 0.1 mass% or more. On the other hand, if chromium is added in an excessive amount, sometimes an increase in strength causes the low temperature viscosity to drop, so that the upper limit of the amount of chromium is preferably set to 0.50 wt.% Or less. More preferably, the amount of chromium is set to 0.35 wt.% Or less, and even more preferably 0.2 wt.% Or less.

[0045] Mo: Molybdenum is an element that improves hardenability and which forms carbonitrides to improve strength. Preferably, 0.01 mass% or more of molybdenum is added, more preferably 0.05 mass% or more. On the other hand, if molybdenum is added in an excessive amount, sometimes an increase in strength causes the low temperature viscosity to drop, so that the upper limit of the amount of molybdenum is preferably set to 0.50 wt.% Or less. More preferably, the amount of molybdenum is set to 0.2 wt.% Or less, and even more preferably 0.15 wt.% Or less.

[0046] W: Tungsten, like molybdenum, is an element that improves hardenability and which forms carbonitrides that improve strength. Preferably, 0.0001 mass% or more of tungsten is added, more preferably 0.01 mass% or more, and even more preferably 0.05 mass% or more. On the other hand, if tungsten is added in an excessive amount, sometimes an increase in strength causes a drop in low-temperature hardness, so that the upper limit of the amount of tungsten is preferably set to 0.50 wt.% Or less. More preferably, the amount of tungsten is set to 0.2 wt.% Or less, and even more preferably 0.15 wt.% Or less.

[0047] V: Vanadium is an element that forms carbides and nitrides and contributes to the improvement of strength. Preferably, 0.001 wt.% Or more vanadium is added, more preferably 0.005 wt.% Or more. On the other hand, if more than 0.10 wt.% Vanadium is added, sometimes this causes a drop in low temperature hardness, so that the amount of vanadium is preferably set to 0.10 wt.% Or less. More preferably, the amount of vanadium is set to 0.05 wt.% Or less, and even more preferably 0.03 wt.% Or less.

[0048] Zr and Ta: Zirconium and tantalum, like vanadium, are elements that form carbides or nitrides and contribute to improved strength. Zirconium and tantalum are preferably added in an amount of 0.0001 mass% or more, more preferably 0.0005 mass% or more, and even more preferably 0.001 mass% or more. On the other hand, if more than 0.050 wt.% Zirconium or tantalum is added, the low temperature viscosity sometimes drops, so that the upper limits of the amount of zirconium and the amount of tantalum are preferably set to 0.050 wt.% Or less. More preferably, this amount is 0.030 wt.% Or less.

[0049] B: Boron is an element that can cause an improvement in hardenability when added in small quantities. In order to increase strength, preferably 0.0001 mass% or more of boron is added. Preferably, 0.0003 wt.% Or more boron is added. On the other hand, if boron is added in an excessive amount, boron precipitates are sometimes formed, and the low temperature viscosity sometimes deteriorates, so that the amount of boron is preferably set to 0.0020 wt.% Or less. More preferably, the amount of boron is set equal to 0.0010 wt.% Or less.

[0050] In addition, in the present invention, one or more elements of Mg, REM, Y, Hf and Re can be added to control the shape of inclusions, such as sulfides and oxides, and to improve low temperature hardness and resistance to acidic conditions.

[0051] Mg: Magnesium is an element that contributes to the improvement of resistance to acidic conditions or low temperature hardness by controlling the shape of sulfides or the formation of fine-grained oxides. Preferably, 0.0001 mass% or more of magnesium is added, more preferably 0.0005 mass% or more, and even more preferably 0.001 mass% or more. On the other hand, if more than 0.010% by weight of magnesium is added, coarse-grained oxides are sometimes easily formed, and the viscosity of the heat affected zone deteriorates, so that the amount of magnesium is preferably set to be 0.010% or less. More preferably, the amount of magnesium is set to 0.005 wt.% Or less, and even more preferably 0.003 wt.% Or less.

[0052] REM, Y, Hf and Re: REM, Y, Hf and Re form sulfides and, in particular, inhibit the formation of MnS elongated in the rolling direction, contributing to an improvement in resistance to acidic conditions. REM, Y, Hf, and Re are all preferably added in an amount of 0.0001 mass% or more, more preferably 0.0005 mass% or more, and even more preferably 0.0010 mass% or more. On the other hand, if REM, Y, Hf or Re is added in an amount of more than 0.0050 wt.%, The oxides increase, which sometimes harms stiffness, so that the upper limit is preferably set to 0.0050 wt.% Or less. More preferably, this amount is set to 0.0030 wt.% Or less.

[0053] In addition, in this embodiment, the residue, in addition to the above elements, essentially consists of iron. Inevitable impurities and other elements that do not harm the action or effect of the present invention can also be added in trace amounts. "Inevitable impurities" means components that are contained in the raw material or which are part of the manufacturing process, and refer to components that are not deliberately incorporated into steel.

[0054] In particular, Si, Al, P, S, O, N, Sb, Sn, Co, As, Pb, Bi, and H may be mentioned. Of these, P, S, O, and N, as explained above. should be controlled at the following levels: Si: 0.5 wt.% or less, Al: 0.05 wt.% or less, P: 0.03 wt.% or less, S: 0.005 wt.% or less, O : 0.005 wt.% Or less, and N: 0.008 wt.% Or less.

[0055] As for the other elements, typically Sb, Sn, Co and As may be contained in an amount of 0.1 wt.% Or less, phosphorus and bismuth can be contained in an amount of 0.005 wt.% Or less, and hydrogen can be contained in an amount of 0 , 0005 wt.% Or less, as inevitable impurities. However, if they are present in the usual ranges, they are not required to be particularly controlled.

[0056] In addition, the optionally added elements Cu, Ni, Cr, Mo, W, V, Zr, Ta, B, Mg, REM, Y, Hf and Re in plate steel for a thick-walled high-strength main pipe in accordance with the present invention can contained as unavoidable impurities, even if they are not consciously turned on. However, these elements do not adversely affect the present invention, even if the number of added elements falls below the lower limit, as long as the number of added elements does not exceed the upper limit of the content if they are deliberately included, as explained above, so that this does not cause problems .

[0057] In addition, in the present invention, in order to provide hardenability to increase strength and low temperature stiffness, the Ceq carbon equivalent, which is calculated in accordance with the following (formula 2) by the contents of C, Mn, Ni, Cu, Cr, Mo and V (wt.%) Is preferably from 0.30 to 0.50. The lower limit of Ceq for increasing strength is more preferably 0.32 or more, and even more preferably 0.35 or more. In addition, the upper limit of Ceq for increasing low temperature hardness is more preferably 0.45 or less, and even more preferably 0.43 or less.

[0058] Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 (formula 2)

[0059] Also, in order to provide low temperature toughness of plate steel and heat affected zones, the cracking susceptibility parameter Pcm, which is calculated according to the following (formula 3) from the contents of C, Si, Mn, Cu, Cr, Ni, Mo and V (wt.%) Is preferably from 0.10 to 0.20. The lower limit of Pcm increases strength, so that more preferably it is 0.12 or more, and even more preferably 0.14 or more. In addition, the upper limit of Pcm increases the low temperature viscosity, so that more preferably it is 0.19 or less, and even more preferably 0.18 or less.

[0060] Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 (formula 3)

[0061] It should be noted that if the selectively contained elements Ni, Cu, Cr, Mo, and V are not intentionally added, they are calculated as 0 in the above (formula 2) and (formula 3).

[0062] Next, the microstructure of the steel plate of the present invention will be explained. The plate steel of the present invention has a sheet thickness of 25 mm or more, more preferably 30 mm or more, and is suitable as plate steel for a thick-walled main pipe (25 mm to 45 mm). In addition, the steel plate of the present invention uses the hot rolling temperature difference or the difference in the cooling rate of accelerated cooling in the surface layers and the middle part of the thickness in order to control the structure, and differs in microstructure in the surface layers and the middle part of the thickness. It should be noted that in the present invention, part of the surface layer of plate steel is a portion of 0.9 mm to 1.1 mm from the surface of plate steel in the thickness direction (i.e., an area within 0.1 mm in the directions to the front and rear surfaces, center which is in the position of 1 mm in the thickness direction from the plate steel surface), while the central part of the plate steel is an area within 1 mm in the directions to the front and rear surfaces from the center of the sheet thickness.

[0063] In part of the surface layer, in order to increase crush resistance, 5% or more of the area of the deformed ferrite is formed. "Deformed ferrite" is a ferrite that extends during hot rolling in the rolling direction. Compared to polygonal ferrite, which is formed upon cooling after rolling, its dislocation density is higher. It is effective for improving crush resistance. An optical micrograph of a cross section of a portion of the surface layer of the steel plate of the present invention is shown in FIG. 1. In addition, the dark gray parts are deformed ferrite. Such a part is shown by an arrow. The portion of the surface layer that is shown in FIG. 1, contains 9.3% deformed ferrite.

[0064] In addition, if there is a lot of deformed ferrite, the crushing strength increases, but the low temperature viscosity deteriorates. Therefore, the inventors of the present invention have found that it is possible to suppress deformed ferrite in the central part in order to increase the low temperature viscosity. As the thickness of the steel plate becomes larger, the temperature difference between the surface layers and the center of the thickness becomes larger. For this reason, as the thickness of the steel plate becomes larger, the amount of deformed ferrite that can be formed in the central part of the sheet thickness becomes smaller, while the amount of deformed ferrite that can be formed in part of the surface layer, getting bigger. Therefore, the authors of the present invention investigated the ratio of the thickness of plate steel and the amount of deformed ferrite in part of the surface layer and found the optimal range.

[0065] FIG. 2 shows the relationship between the thickness of plate steel with a thickness of 25 mm to 45 mm and the upper limit S _fe1 percent of the area of the deformed ferrite in part of the surface layer.

[0066] From FIG. 2, it was found that in order to obtain shear resistance and low temperature viscosity that are optimal for the main pipe for transporting natural gas and crude oil, the percentage of the area of deformed ferrite in the part of the surface layer of plate steel should be equal to or greater than the following lower limit value and equal to or less than the next upper limit value.

The value of the lower limit of the percentage of the area of deformed ferrite in the part of the surface layer of plate steel: 5%.

The value of the upper limit of the percentage of the area of deformed ferrite in part of the surface layer of plate steel: S _fe1 = 0.6552 × T _H -4.7826 formula 1a

(where T _H : plate thickness for thick-walled high-strength main pipe).

[0067] Furthermore, if the percentage of the area of the deformed ferrite exceeds S _fe1 %, the surface layers _harden and the low temperature viscosity deteriorates, so that the percentage of the area of the deformed ferrite is set to S _fe1 % or less. In addition, it is preferable that the upper limit of the percentage of the area of the deformed ferrite in part of the surface layer of plate steel satisfy the following formula 1b.

More preferred upper limit value: S _fe2 = 0.8 × T _H -15 · formula 1b

[0068] As shown in the above formula 1a and formula 1b, the percentage of the area of the deformed ferrite to obtain resistance to acidic environment, shear resistance and low temperature stiffness, optimal for the material of the main pipe for transporting natural gas or crude oil, depends on the thickness of the sheet. The temperature difference during hot rolling between the surface layers and the central part of the thickness and the difference in cooling rates during accelerated cooling strongly depend on the thickness of the sheet, so that the percentage of the area of deformed ferrite should depend on the thickness of the sheet.

[0069] In the part of the surface layer, in order to increase shear resistance, it is preferable to form an MA having a high dislocation density with an area percentage of 0.1% or more. However, MA forms the starting points of failure, and if it is formed in excessive amounts, this degrades the low temperature viscosity. For this reason, MA in part of the surface layer is limited to a percentage of the area of 8% or less. Preferably, the percentage of the area MA in a portion of the surface layer is set to 5% or less, more preferably 3% or less.

[0070] In part of the surface layer, the residue, in addition to the aforementioned deformed ferrite and MA, is a microstructure composed of one or both of polygonal ferrite and bainite. Polygonal ferrite is effective for improving low temperature hardness. It is easily formed in part of the surface layer and gradually decreases towards the central part of the thickness. Bainite is effective for improving strength. Unlike polygonal ferrite, its amount is small in part of the surface layer and gradually increases towards the central part of the thickness. This is because in the central part of the thickness, in comparison with the surface layers, the temperature of rolling during hot rolling and the temperature of the beginning of accelerated cooling become higher.

[0071] In the central part of the thickness, in order to guarantee low temperature viscosity and resistance to acidic conditions, it is necessary to suppress the formation of deformed ferrite. The percent area of deformed ferrite is limited to 5% or less. The percent area of the deformed ferrite is preferably set to 3% or less, more preferably 0%. In the central part of the thickness, it is preferable to suppress the formation of MA, which acts as the starting points of failure and suppresses the hardening of the central part of the thickness. In order to guarantee low temperature viscosity, the percent area MA is limited to 5% or less. Preferably, the percentage of area MA in the central portion of the thickness is set to 4% or less, more preferably 2% or less.

[0072] In the central portion of the thickness, the remainder, other than deformed ferrite and MA, is a microstructure composed of one or both of acicular ferrite and bainite. Polygonal ferrite is effective for improving low-temperature stiffness, but impairs resistance to acidic conditions, so that in the central part of the thickness, the microstructure is preferably a homogeneous microstructure composed of one or both of acicular ferrite and bainite.

[0073] Here, the microstructures of the aforementioned part of the surface layer and the central part of the thickness can be observed with an optical microscope. In particular, the percent area of deformed ferrite and MA can be found by analyzing the image of structures in optical micrographs. It should be noted that for MA, reflective etching is performed, and the percentage of non-color structures is found by image analysis. In addition, the polygonal ferrite that is formed during accelerated cooling is granular. Deformed ferrite is elongated in the rolling direction. In addition, deformed ferrite has a high dislocation density because it is more durable than polygonal ferrite. Therefore, deformed ferrite and polygonal ferrite can be differentiated with respect to the longitudinal axis and the short axis (aspect ratio) or in hardness. Needle ferrite and bainite are pinnate structures and can be differentiated using deformed ferrite and polygonal ferrite.

[0074] In order to guarantee the low temperature viscosity of plate steel, it is effective to increase the boundaries of crystalline grains that provide resistance to crack propagation, that is, to reduce the size of crystalline grains. In the present invention, the size of the region surrounded by high angle grain boundaries with an orientation difference of 15 ° or more, i.e., the effective grain size is reduced in order to improve low temperature viscosity. By reducing the average effective grain size of part of the surface layer and the central part of the thickness, which are measured by electron backscattering diffraction (EBSD), to 20 μm or less, it is possible to guarantee low temperature viscosity. The smaller the effective grain size, the more stable is the resulting high viscosity. Preferably, this value is 10 μm or less.

[0075] It should be noted that the low temperature viscosity of plate steel is estimated by measuring the effective grain size in the central part of the thickness and calculating the average value. In addition, electron backscattering diffraction is used as a means for measuring the effective grain size of various microstructures. The effective grain size is defined as the equivalent circle diameter found by analyzing the structure in the longitudinal direction of the plate steel after rolling by electron backscattering diffraction. It should be noted that in the part of the surface layer this size can be made smaller by using deformed ferrite or polygonal ferrite, but in the central part of the thickness, the formation of deformed ferrite or polygonal ferrite is suppressed, so that the previous austenitic grains can be made smaller by hot rolling .

[0076] Next, the characteristics of the steel plate of the present invention will be explained. When increasing the pressure of the transported crude oil or natural gas to improve the transportation efficiency of pipelines, the main pipe should have increased strength and increased thickness to prevent pipe rupture due to internal pressure. From these points of view, in order to avoid rupture of the main pipe due to internal pressure, the steel plate used for the manufacture of the main pipe preferably has a thickness of 25 mm or more. In addition, this plate steel preferably has a tensile strength of 500 MPa or more. In addition, this plate steel after the formation of the pipe, that is, a part of the steel pipe different from the weld zone and the HAZ zone, for example, a part of the steel pipe from the weld part to positions from 90 ° to 180 ° (positions from “3 hours” to “ 6 hours ”from the seam part) also preferably has a yield strength of 440 MPa or more, and a tensile strength of 500-700 MPa or more. It should be noted that in order to avoid tearing, the thickness of the steel plate is more preferably 30 mm or more, and even more preferably 35 mm or more.

[0077] When laying a pipeline in the Arctic regions, the low temperature viscosity of the main pipe is also required. Low temperature toughness can be evaluated using an impact tensile test (DWT test or simply DWTT). In the present invention, the shear area of a DWTT at a temperature of −10 ° C. of a steel plate prior to pipe formation is preferably 85% or more. In addition, with increased thickness and higher strength of the main pipe, it is difficult to ensure low temperature stiffness, so that the thickness of the steel plate is preferably set to 45 mm or less, and the tensile strength of the steel plate is preferably set to 700 MPa or less. In the production of a steel pipe by cold stamping, the strength of the steel plate after the formation of the pipe tends to become higher than the strength of the steel plate before the formation of the pipe, but the tensile strength of the steel pipe after its formation is also preferably set to 700 MPa or less.

[0078] When laying a pipeline on the ocean floor, it is necessary to resist the main pipe to external pressure (collapse resistance). Collapse resistance is evaluated using a compression test using test specimens taken from a steel pipe, since there is a deformation effect created during cold stamping of steel plate into a steel pipe. To prevent collapse of the main pipe by external pressure, the compressive strength in the circular direction after aging at a temperature of 200 ° C (0.2% plastic flow stress) is preferably 450 MPa or more.

[0079] Next will be explained a method of manufacturing a plate steel of the present invention.

[0080] Plate steel in accordance with the present invention gives structures that differ in the surface layers and in the central part of the thickness by performing one or more hot rolling passes in the temperature range where the microstructure of the surface layers becomes biphasic, consisting of ferrite and austenite (biphasic region), and additionally performing accelerated cooling after hot rolling by cooling with water or other means under conditions whereby the surface temperature th steel plate becomes 400 ° C or less, and after stopping of cooling, heat is recovered. If plate steel has a large thickness, the temperature of the surface layers during hot rolling drops from the temperature in the central part of the thickness. In the central part of the thickness, the formation of ferrite is suppressed in comparison with the surface layers. In addition, the stop temperature of accelerated cooling becomes higher in the central part of the thickness than on surfaces. If you set the conditions for accelerated cooling so that the surface temperature is recovered after accelerated cooling, the temperature of the central part of the plate after stopping the accelerated cooling can be made equal to 400 ° C or more, hardening of the central part of the thickness can be suppressed, and resistance to acidic conditions can be provided .

[0081] Furthermore, in order to provide low temperature hardness, the average effective grain size of the surface layers and the central portion of the thickness should be 20 μm or less. In the surface layer, due to the formation of deformed ferrite and polygonal ferrite, the effective grain size becomes smaller. On the other hand, in the central part of the thickness, the formation of deformed ferrite and polygonal ferrite is suppressed, so that the previous austenitic grains should become smaller. By reducing the average value of the effective grain size, which is measured in the surface layers, and the effective grain size, which is measured in the Central part of the thickness, the effective grain size of the sheet thickness as a whole becomes smaller, and low-temperature viscosity can be ensured. For this reason, during hot rolling, the compression ratio in the recrystallization region must be set to 2.0 or more, and the compression ratio in the non-recrystallization region must be set to 3.0 or more.

[0082] As explained above, by appropriately controlling the conditions of hot rolling and subsequent accelerated cooling, it is possible to obtain not only the strength and low temperature viscosity of plate steel, but also satisfactory composite characteristics of resistance to acidic environment and shear resistance after pipe formation.

[0083] Next, a manufacturing process of plate steel in accordance with the present invention will be explained. First, the steel containing the above components is melted during the steelmaking process, and then cast to produce a steel slab. Casting can be carried out in the usual way, but from the point of view of productivity, continuous casting is preferred. Then, the resulting steel slab is heated, hot rolled, and cooled by means of accelerated cooling in order to produce plate steel. It should be noted that in this embodiment, heating the steel slab, which is performed for hot rolling, is also referred to as “reheating”, and the heating temperature of the steel slab at this time is also called “reheating temperature”.

[0084] The reheating temperature of the hot rolling is set to 1000 ° C or more in order to dissolve carbides, nitrides, etc. that form in the steel slab. In addition, by setting the reheating temperature to 1000 ° C or more, hot rolling in the recrystallization region that is higher than 900 ° C (recrystallization rolling) becomes possible, and the steel structure can be made thinner. It should be noted that the upper limit of the reheat temperature is not prescribed, but in order to suppress the increase in the effective grain size, the reheat temperature is preferably set to 1250 ° C or less. In addition, the reheat temperature is more preferably set to 1200 ° C in order to guarantee a low temperature viscosity, more preferably 1150 ° C or less.

[0085] Hot rolling in accordance with this embodiment consists of a rolling process in a recrystallization region that is above 900 ° C, rolling in a non-recrystallization region that is in a temperature region of 900 ° C or lower, and rolling in a temperature region where the temperature on the surface of the steel plate becomes the temperature leading to a two-phase system of austenite and ferrite (two-phase region), in this order. It should be noted that hot rolling can be started immediately after being removed from the reheating heating furnace, so that the initial hot rolling temperature is not particularly prescribed.

[0086] In order to reduce the effective grain size of the central portion of the plate thickness, it is necessary to set the compression ratio in the recrystallization region that is above 900 ° C to 2.0 or more and accelerate recrystallization. Here, the reduction ratio in the recrystallization region is the ratio of the thickness of the steel slab to the thickness of the sheet at a temperature of 900 ° C.

[0087] Then, hot rolling is performed in the non-recrystallized region, which is in the temperature region of 900 ° C or lower (rolling in the non-recrystallized region). In order to reduce the effective grain size of part of the surface layer of plate steel after accelerated cooling after hot rolling, it is necessary to set the degree of compression during rolling in the area without recrystallization equal to 3.0 or more, and accelerate the conversion using accelerated cooling. More preferably, the rolling reduction ratio in the non-recrystallized region is set to 4.0 or more. It should be noted that in the present invention, the reduction ratio during rolling in the non-recrystallized region is the ratio of the sheet thickness at 900 ° C to the thickness of the sheet after completion of rolling in the non-recrystallized region.

[0088] In hot rolling, rolling (two-phase rolling) is performed in that region (two-phase region) of the surface temperatures of plate steel in which a two-phase austenite and ferrite system is formed. In two-phase rolling, the surface temperature of the plate becomes the initial temperature of the ferrite transformation Ar ₃ or less, but during the period from the beginning to the end of two-phase rolling, the temperature of the central part of the thickness of the plate is maintained so that it is higher than the temperature of the surfaces of the plate, and higher than Ar ₃ . Such a temperature distribution can be realized, for example, by performing accelerated cooling for a short period of time and lowering the temperature only in the surface layers. With this two-phase rolling, the number of passes is set to 1 or more, and the compression ratio is set in the range from 0.1 to 40%. As a result of two-phase rolling, the initial temperature of the accelerated cooling performed later also falls into the two-phase region, so that the strengthening of the central part of the thickness can be suppressed and the low-temperature viscosity can be improved. In addition, the “degree of reduction” is the amount of reduction in plate steel due to rolling, that is, the value obtained by dividing the difference between the thickness of plate steel before rolling and the thickness of plate steel after rolling by the thickness of plate steel before rolling and can be expressed as a percentage (%) etc. In addition, in the parts between the surface layers and the central part of the thickness, the formation of polygonal ferrite is facilitated. This helps to improve low temperature stiffness. In addition, the value of Ar ₃ can be calculated from the contents of C, Si, Mn, Ni, Cr, Cu and Mo (wt.%).

[0089] Ar₃= 905-305C + 33Si-92 (Mn + Ni / 2 + Cr / 2 + Cu / 2 + Mo / 2)

[0090] In the above formula, C, Si, Mn, Ni, Cr, Cu and Mo mean the content of the corresponding elements (wt.%). In addition, Ni, Cu, Cr, and Mo are elements that are selectively added in the present invention. If they are not deliberately added, their content in the formula is indicated as "0".

[0091] The lower limit of the reduction ratio in two-phase rolling is set to 0.1% or more in order to cause the formation of deformed ferrite elongated in the rolling direction. Preferably, the compression ratio of the two-phase rolling is set to 1% or more, more preferably 2% or more. On the other hand, the upper limit of the compression ratio in two-phase rolling is set to 40% or less, since it is difficult to guarantee the compression ratio at low temperature, where the deformation resistance becomes higher. Preferably, the two-phase rolling reduction ratio is set to 30% or less, more preferably 20% or less, and even more preferably less than 10%.

[0092] The final temperature of the two-phase rolling, that is, the final hot rolling temperature, is set to 700 ° C or more as the temperature of the steel plate surfaces, so that the deformed ferrite does not form in an excessive amount. If the final temperature of hot rolling becomes less than 700 ° C, ferrite transformation occurs in the central part of the thickness and, due to deformed ferrite, low temperature viscosity and resistance to acidic conditions sometimes fall. In addition, if the final temperature of the hot rolling drops, the formation of ferrite sometimes causes the concentration of carbon near austenite and facilitates the formation of MA. On the other hand, when the final temperature of the hot rolling is too high, if the final temperature of the accelerated cooling decreases, the central part of the thickness sometimes hardens and the low temperature viscosity drops.

[0093] Then, after the hot rolling is completed, accelerated cooling immediately begins. However, after hot rolling, air cooling is allowed while the steel is transferred from the output side of the rolling mill to the accelerated cooling device. The final temperature of accelerated cooling is set equal to the temperature inside the temperature range from 200 to 400 ° C on the surfaces of plate steel. If accelerated cooling is stopped at a surface temperature of plate steel exceeding 400 ° C, polygonal ferrite is formed in the central part of the thickness, and the resistance to an acidic environment decreases. On the other hand, if accelerated cooling is performed until the surface temperature of the steel plate becomes less than 200 ° C, the central part of the thickness hardens and the low temperature viscosity drops. After accelerated cooling, air cooling is performed in this state. If accelerated cooling is stopped at a surface temperature of the steel plate in the range of 200 to 400 ° C., then the temperature of the surface layers of the steel plate is restored during air cooling. Therefore, the temperature of the central portion of the thickness reaches 400 ° C or more, the hardness drops, and the low temperature viscosity and resistance to acidic conditions can be improved.

[0094] The above manufacturing process can be used to produce plate steel for a high strength main pipe in accordance with the present invention. In addition, when using plate steel for a high-strength main pipe in accordance with the present invention, it is possible to produce a steel pipe for a thick-walled high-strength main pipe having excellent acid resistance, shear resistance and low temperature viscosity as a material. It should be noted that in the production of the steel pipe, it is preferable to use the UOE process of forming steel plate for the high strength main pipe in accordance with the present invention by C-pressing, U-pressing and O-pressing. Alternatively, the JCOE process may be used for the manufacture of a steel pipe using plate steel for a high strength main pipe in accordance with the present invention. A thick-walled high-strength main pipe in accordance with the present invention is produced by molding plate steel for a high-strength main pipe in accordance with the present invention in the form of a pipe followed by arc welding of adjacent ends. For arc welding from the point of view of the stiffness of the weld metal and productivity, arc welding under a flux layer is preferably used. It should be noted that the shear resistance of a thick-walled high-strength main pipe in accordance with the present invention can be estimated by taking circular compression test specimens from steel pipes made using the above methods.

EXAMPLES

[0095] Next, examples of the present invention will be explained, but the present invention is not limited to the conditions that are used in the following examples.

[0096] Steels having the chemical compositions shown in Table 1-1, Table 1-2, Table 2-1, and Table 2-2 were smelted and cast in order to obtain steel slabs. The column "Slab thickness" in Table 3-1 and Table 3-2 shows the thickness of the obtained steel slabs (mm). The steel slabs were reheated and hot rolled in the recrystallization region, which is above 900 ° C. In addition, the “Heating Temperature” column in Table 3-1 and Table 3-2 shows the reheat temperature, and the “Transfer Thickness” column in Table 3-1 and Table 3-2 shows the sheet thickness at 900 ° C after hot rolling in the area of recrystallization and before hot rolling in the area without recrystallization, which is characterized by a temperature of 900 ° C or less. In addition, the “Compression ratio in the recrystallization region” column in Table 3-1 and Table 3-2 shows the ratio of slab thickness to transfer thickness.

[0097] Then, the steel plate having a transfer thickness was hot rolled in an area without recrystallization, which is characterized by a temperature of 900 ° C or less. The column “Thickness of the sheet” in Table 3-1 and Table 3-2 shows the thickness of the sheet after hot rolling in the area without recrystallization and before the two-phase rolling explained later, while the column “The degree of reduction in the area without recrystallization” in Table 3-1 and Table 3-2 shows the value obtained by dividing the transfer thickness by the thickness of the sheet after completion of rolling in the area without recrystallization.

[0098] After hot rolling in the non-recrystallizing region, before the accelerated cooling, the hot rolling finishing process was performed. The surface temperature of the plate at the end of the hot rolling finish process is indicated by the column “Final Finish Rolling Temperature (° C)” in Table 3-1 and Table 3-2. In addition, the number of rolling operations performed during the final hot rolling process, that is, the number of passes, is shown in the column “Number of α + γ rolling passes” in Table 3-1 and Table 3-2, while the reduction ratio plate steel in the finishing process of hot rolling is shown in the column "α + γ degree of reduction (%)" in Table 3-1 and Table 3-2.

[0099] After the hot rolling finish process, immediately after transporting the steel plate to the cooling zone, accelerated water cooling was performed. The initial temperature and the final temperature of accelerated cooling, which was carried out during the manufacturing process of plate steels No. 1-46, are shown in the columns "Initial water cooling temperature (° C)" and "Final water cooling temperature (° C)" in Table 3-1 and Table 3-2. The following manufacturing process was used to produce plate steels No. 1-46.

[0100] Samples for testing were taken from part of the surface layer and the central part of the thickness of the obtained plate steels No. 1-46. These samples were examined to determine their structure using an optical microscope in order to determine the percentage of area of deformed ferrite and the percentage of area MA and confirm the structure of the residue. The residue structure in all plate steels No. 1-46 was a microstructure consisting of one or both of polygonal ferrite and bainite in part of the surface layer, and a microstructure consisting of one or both of acicular ferrite and bainite in the central part of the thickness. It should be noted that the percentage of MA area was measured using a specimen subjected to reflective etching. In addition, average values of effective grain sizes in the surface layers and the central part of the thickness were found using EBSD.

[0101] Strength Measurement of Plate Steel

In addition, two full thickness test specimens in accordance with the American Petroleum Institute API 5L standard (hereinafter referred to simply as “API 5L”) having length directions corresponding to the plate width direction were taken from the central part of the sheet width of each plate obtained No. 1-46. Full thickness test specimens were subjected to tensile tests based on API 2000 at room temperature in order to find yield strengths and tensile strengths. Maximum tensile stress tests were used as a basis for determining tensile strengths.

[0102] Measurement of shear area when testing DWTT plate steel

In addition, a full thickness sample for DWT testing, having a length direction corresponding to the width direction of the plate, was taken from the central part of the sheet width of each of the obtained plate steels No. 1-46. The DWT test was also performed based on the API 2000 standard at -10 ° C in order to measure the shear area of the DWTT.

[0103] Measurement of the strength of the steel pipe and compression test

The resulting plate steels No. 1-46 were used to form pipes using the UOE process, and were welded on the internal and external surfaces by applying heat, shown in Table 5-1 and Table 5-2, using submerged arc welding in order to produce steel pipes with an outer diameter of 30 to 36 inches (plate steel numbers and steel pipe numbers correspond to each other). Then, test samples were taken from steel pipes and were measured to determine strength, and were also subjected to compression tests. The test specimens were cut from the positions of the steel pipes “3 o'clock”, where the zones of the roller weld were defined as “0 hour”, so that the longitudinal directions of the samples for tensile testing corresponded to the longitudinal directions of the steel pipes. The strength of steel pipes was measured based on ASTM E9-09 so as to measure the yield strengths and tensile strengths in the longitudinal directions of the main pipes. Here, the yield strength underloaded by 0.5% was defined as the yield strength. The compression test specimens that were used to test the compression of the steel pipe were obtained by taking parts with a diameter of 22 mm and a length of 66 mm below 3 mm from the inner surfaces of the steel pipes at the 6 o’clock positions of the steel pipes, where the areas of the roller weld were defined as "0 hour". The compression test was conducted based on ASTM E9-09. The compressive strength after aging was found at 200 ° C for 10 minutes (0.2% plastic flow stress).

[0104] Test for hydrogen cracking of steel pipes

In addition, provided that the zone of the roller weld of the steel pipe was defined as “0 hour”, samples for analysis of hydrogen cracking with a width of 20 mm and a length of 100 mm were taken from the positions “3 hours” and “6 hours” of the steel pipe. Samples for the hydrogen cracking test were taken so that the central parts of the thickness of the steel pipes became test positions. The hydrogen cracking test was based on the TM0284 NACE (National Association of Corrosion Engineers) standard and was performed using Solution B as the test solution. For the evaluation, the crack length ratio (CLR) was used.

[0105] The characteristics of plate steel are shown in Table 4-1 and in Table 4-2, while the characteristics of steel pipes are shown in Tables 5-1 and 5-2. Plate steels No. 1-28 correspond to examples of the present invention. As can be clearly seen from Tables 4-1 and 4-2 and from Tables 5-1 and 5-2, steel pipes that were manufactured using these plate steels have yield strengths of 440 MPa or more, and tensile strengths in the range from 500 to 700 MPa. In addition, as shown in Tables 4-1 and 4-2, plate steels had tensile strengths of 500 MPa or more, and had shear areas when testing DWTT at −10 ° C. equal to 85% or more. In addition, as shown in Tables 5-1 and 5-2, steel pipes made by molding these plate steels into a pipe shape, followed by butt welding, showed good results — CLR 10% or less after the hydrogen cracking test and test results compressive 450 MPa or more after strain aging at 200 ° C.

[0106] On the other hand, Steels No. 29-46 are comparative examples. Steels No. 29-40 have a content of chemical components outside the range of the present invention, while Steels No. 41-46 have microstructures outside the range of the present invention and have at least one poorer characteristic of strength, low temperature stiffness, shear resistance, and acid resistance Wednesday. Steel No. 29 has a small amount of carbon and reduced strength and resistance to collapse. On the other hand, Steel No. 30 has a large amount of carbon, Steel No. 31 has a large amount of silicon, and Steel No. 32 has a large amount of manganese. In each comparative example, the tensile strength increases excessively and the low temperature viscosity drops. In addition, the temperature of the Ar ₃ point _of Steel No. 30 is less than 700 ° C, and the plate steel No. 30 is not rolled in a two-phase region in the present invention. However, the amount of carbon contained is large, so that carbon is concentrated near austenite in the central part of the thickness of Steel No. 30, the formation of MA is facilitated, and the resistance to the acidic environment decreases. In addition, Steel No. 32 has a large amount of manganese - 3%, so that the resistance to acidic conditions decreases. Steels No. 33, 34 and 40 have a high content of impurities (phosphorus, sulfur and oxygen), and the low-temperature viscosity decreases. Steels No. 35-39 are examples that have a high content of elements that contribute to the formation of carbides, nitrides, oxides, and sulfides, and which have reduced low temperature viscosity due to precipitates and inclusions. Steels No. 41 and 42 are examples that, respectively, have an insufficient compression ratio in the recrystallization region, as well as a compression ratio in the non-recrystallization region, as a result of which they have a large effective grain size, and therefore the low temperature viscosity decreases. Steel No. 43 has a final hot rolling temperature of 700 ° C or more, but a low Ar ₃ point and does not roll in a two-phase region in the present invention, so that deformed ferrite does not form in the surface layer, the central part of the thickness hardens, and the low temperature viscosity drops . Steel No. 44 is an example in which the final accelerated cooling temperature is high, deformed ferrite and MA are formed in an excessive amount in the central part of the thickness, and the strength decreases. In addition, accelerated cooling is stopped when the surface temperature of the steel plate exceeds 400 ° C, so that polygonal ferrite is formed in the central part of the thickness and the resistance to acidic conditions decreases. Steels Nos. 45 and 46 are examples in which the final rolling temperatures are low, deformed ferrite and MA are formed in excessive amounts in the part of the surface layer and in the central part of the thickness, and the low temperature viscosity and resistance to acidic conditions drop.

[0107]

[0108]

[0109]

Table 2-1 Steel sheet No. Chemical composition (wt.%) Ni Cu Cr Mo V B W Zr Ta Mg Rem Y Re Hf Ceq Pcm Ar ₃ one 0.340 0.156 742 2 0.15 0.10 0.10 0.06 0,0008 0.405 0.170 710 3 0.20 0.10 0.04 0,0008 0.385 0.162 719 four 0.20 0.0051 0.363 0.152 734 5 0.30 0.20 0,050 0.0032 0.360 0.163 725 6 0.15 0.0012 0.363 0.143 717 7 0.10 0.20 0.02 0.0038 0.353 0.145 721 8 0.30 0.415 0.165 714 9 0.30 0.423 0.162 708 10 0.0018 0.310 0.135 753 eleven 0.30 0.20 0.06 0.0042 0.403 0.170 720 12 0.40 0.50 0.0034 0.450 0.171 705 13 0.20 0.40 0.02 0.001 0.456 0.189 706 fourteen 0.35 0.30 0.0033 0.409 0.168 714 fifteen 0.30 0.405 0.164 721 16 0.20 0,0007 0.338 0.147 739 17 0.10 0.10 0,0008 0.408 0.180 723 eighteen 0.30 0.10 0.0029 0.001 0.373 0.158 724 19 0.40 0.30 0,0006 0.443 0.184 701 twenty 0.20 0.50 0.0025 0.375 0.162 707 21 0.001 0.333 0.156 748 22 0.40 0.30 0.10 0.419 0.170 719

23 0.30 0.10 0,357 0.150 732 24 0.30 0.25 0.06 0.427 0.174 709 25 0.28 0.12 0.378 0.156 722 26 0.35 0.20 0.408 0.170 712 27 0.33 0.11 0.391 0.163 715 28 0.29 0.12 0.388 0.165 718

[0110]

Table 2-2 Steel sheet No. Chemical composition (wt.%) Ni Cu Cr Mo V B W Zr Ta Mg Rem Y Re Hf Ceq Pcm Ar ₃ 29th 0.30 0.05 0.349 0,111 743 thirty 0.20 0.20 0.10 0.20 0.528 0.293 690 31 0.40 0.40 0.30 0.30 0.471 0.240 740 32 0.0012 0.560 0.210 611 33 0.382 0.165 717 34 0.30 0,0005 0.415 0.164 712 35 0.327 0.153 749 36 0.30 0.08 0.419 0.170 718 37 0.30 0.386 0.162 729 38 0.40 0.369 0.167 751 39 0.335 0.148 742 40 0.316 0.149 759 41 0.13 0.00 0.363 0.145 719 42 0.00 0,0007 0.387 0.160 708 43 0.50 0.50 0.10 0.00 0.492 0.184 685 44 0.10 0.20 0.10 0,03 0.366 0.154 726 45 0.20 0.40 0.15 0.00 0.389 0.172 714 46 0.50 0.40 0.04 0.436 0.171 709

[0111]

Table 3-1 Steel sheet No. Slab thickness
(mm) Transfer Thickness
(mm) Sheet thickness
(mm) Heating temperature
(° C) The degree of compression in the field of recrystallization The degree of compression in the area without recrystallization Final temperature
finish rolling
(° C) Number of α + γ rolling passes α + γ compression ratio (%) Initial water cooling temperature (° C) Final water cooling temperature (° C) one 240 105 34 1100 2,3 3,1 740 one 3 736 350 2 240 120 thirty 1150 2.0 4.0 709 one 5 705 400 3 240 109 35 1150 2.2 3,1 715 one 2 711 300 four 240 109 35 1200 2.2 3,1 730 one 5 726 300 5 240 121 39 1100 2.0 3,1 720 one 6 716 375 6 240 118 38 1150 2.0 3,1 710 one 2 706 380 7 240 123 35 1200 2.0 3,5 720 one four 716 350 8 240 112 36 1150 2.2 3,1 710 one 5 706 380 9 240 118 38 1200 2.0 3,1 705 one 7 701 300 10 240 118 38 1100 2.0 3,1 750 one 2 746 370 eleven 240 109 35 1150 2.2 3,1 715 one 6 711 320 12 240 112 32 1200 2.1 3,5 704 one 3 700 330 13 240 115 37 1100 2.1 3,1 705 one 7 701 370 fourteen 240 112 32 1150 2.1 3,5 710 one four 706 320 fifteen 240 111 thirty 1200 2.2 3,7 720 one 7 716 300 16 240 112 36 1100 2.2 3,1 735 one 2 731 325 17 240 121 39 1150 2.0 3,1 720 one 5 716 350 eighteen 240 121 39 1100 2.0 3,1 720 one 7 716 400 19 240 112 36 1200 2.2 3,1 700 one 3 696 320

twenty 240 114 thirty 1150 2.1 3.8 705 one 6 701 380 21 240 112 33 1100 2.1 3.4 740 one 7 736 320 22 240 115 37 1150 2.1 3,1 715 one 8 711 370 23 240 one hundred 25 1180 2.0 4.0 730 one one 726 390 24 240 135 45 1100 2.0 3.0 705 one 10 701 320 25 240 one hundred 25 1180 2.0 4.0 730 one 10 715 350 26 240 115 37 1100 2.1 3,1 705 one 10 707 320 27 240 135 45 1100 2.0 3.0 705 2 7 705 300 28 240 135 45 1100 2.0 3.0 705 2 10 700 300

[0112]

Table 3-2 Steel sheet No. Slab thickness
(mm) Transfer Thickness
(mm) Sheet thickness
(mm) Heating temperature
(° C) The degree of compression in the field of recrystallization The degree of compression in the area without recrystallization Final temperature
finish rolling
(° C) Number of α + γ rolling passes α + γ compression ratio (%) Initial water cooling temperature (° C) Final water cooling temperature (° C) 29th 240 109 35 1100 2.2 3,1 710 one 3 710 380 thirty 240 112 32 1200 2.1 3,5 700 one 2 700 370 31 240 112 34 1150 2.1 3.3 700 one four 700 350 32 240 121 39 1100 2.0 3,1 600 one 5 600 320 33 240 118 38 1200 2.0 3,1 700 one 2 700 400 34 240 109 35 1100 2.2 3,1 700 one 7 700 400 35 240 116 33 1160 2.1 3,5 720 one 5 720 370 36 240 115 37 1150 2.1 3,1 700 one 8 700 400 37 240 121 39 1150 2.0 3,1 700 one 3 700 350 38 240 115 32 1130 2.1 3.6 750 one 5 750 350 39 240 118 32 1150 2.0 3,7 720 one 7 720 320 40 240 109 35 1100 2.2 3,1 730 one four 730 330 41 240 152 38 1100 1,6 4.0 700 one 7 700 370 42 240 78 39 1150 3,1 2.0 700 one 5 700 350 43 240 112 36 1150 2.2 3,1 750 0 0 750 400 44 240 105 34 1150 2,3 3,1 700 one 7 700 550 45 240 115 37 1150 2.1 3,1 650 5 6 650 380 46 240 112 36 1100 2.2 3,1 660 3 7 660 400

[0113]

Table 4-1 Steel pipe No. The composition of the parts of the surface layer (%) The composition of the Central part of the sheet thickness (%) Effective size
crystal grains (μm) The tensile strength of sheet steel (MPa) Ductility during DWTT testing (%) The proportion of deformed ferrite MA The proportion of deformed ferrite MA one 6 four <5 2 3 540 90 2 6 four <5 2 four 586 85 3 7 5 <5 3 5 558 85 four 7 5 <5 3 3 525 88 5 6 four <5 2 four 567 92 6 7 5 <5 3 5 596 85 7 9 7 <5 5 four 502 88 8 7 5 <5 3 5 571 98 9 8 6 <5 four 3 551 96 10 6 four <5 2 5 534 91 eleven 7 5 <5 3 6 530 one hundred 12 6 four <5 2 four 584 98 13 6 four <5 2 5 632 97 fourteen 8 6 <5 four 3 580 89

fifteen 9 7 <5 5 four 567 91 16 10 5 <5 3 5 508 90 17 6 four <5 2 6 619 95 eighteen 8 6 <5 four four 548 96 19 9 7 <5 5 3 633 98 twenty 6 four <5 2 four 560 91 21 6 four <5 2 3 541 one hundred 22 8 6 <5 four four 568 one hundred 23 5 3 0 0.1 3 516 one hundred 24 9 7 <5 2 four 598 88 25 eleven four <5 3 3 508 85 26 16 four <5 3 3 530 90 27 21 four <5 3 3 600 one hundred 28 23 four <5 3 3 524 85

[0114]

Table 4-2 Steel pipe No. The composition of the parts of the surface layer (%) The composition of the Central part of the sheet thickness (%) Effective size
crystal grains (μm) The tensile strength of sheet steel (MPa) Ductility during DWTT testing (%) The proportion of deformed ferrite MA The proportion of deformed ferrite MA 29th 9 6 <5 3 5 375 one hundred thirty 7 13 <5 10 four 998 thirty 31 9 6 <5 3 7 837 fifty 32 10 7 <5 four 6 730 75 33 7 four <5 one 3 568 twenty 34 6 3 <5 0 four 560 29th 35 8 5 <5 2 9 522 65 36 5 3 <5 0 7 585 57 37 7 four <5 one 3 557 80 38 9 6 <5 3 5 577 75 39 7 four <5 one 8 509 69 40 6 3 <5 0 9 518 67 41 9 5 <5 2 23 505 57 42 9 5 <5 2 35 551 67 43 0 0 0 0 6 640 fifty 44 6 3 6 9 four 461 85 45 40 fourteen fifteen eleven 5 580 45 46 35 13 10 10 6 585 56

[0115]

Table 5-1 Steel pipe No. Sheet thickness (mm) Outside Diameter (inch) Yield strength of steel pipe (MPa) Tensile strength of steel pipe (MPa) Heat input (kJ / mm) Compression test after aging at a temperature of 200 ° C (0.2% plastic stress) (MPa) Hydrogen Crack Test (CLR (%)) one 34 32 464 545 5 514 0 2 thirty 32 476 595 5.5 561 0 3 35 32 453 566 6 534 0 four 35 32 444 531 7.5 501 0 5 39 32 457 572 6 539 0 6 38 32 449 502 9.5 473 0 7 35 32 466 509 5 480 0 8 36 32 462 578 8.5 545 0 9 38 32 453 566 5 534 0 10 38 32 460 541 7.5 510 0 eleven 35 32 476 596 8.5 562 0 12 32 32 477 597 7 563 0 13 37 32 528 660 4,5 623 0 fourteen 32 32 470 587 6 554 0 fifteen thirty 32 460 575 7 639 0 16 36 32 448 513 6.5 483 0 17 39 32 504 631 7 595 0 eighteen 39 32 443 554 8.5 522 0 19 36 32 514 642 6 606 0 twenty thirty thirty 455 568 8.5 580 0 21 33 32 465 547 7 516 0 22 37 32 476 596 8 562 0 23 25 thirty 440 525 3,5 495 0 24 45 36 487 608 10.5 574 0 25 25 32 451 511 3.6 490 0 26 37 32 478 544 5 525 0 27 45 36 475 612 9.5 550 0 28 45 36 470 533 10 520 0

[0116]

Table 5-2 Steel pipe No. Sheet thickness (mm) Outside Diameter (inch) Yield strength of steel pipe (MPa) Tensile strength of steel pipe (MPa) Heat input (kJ / mm) Compression test after aging at a temperature of 200 ° C (0.2% plastic stress) (MPa) Hydrogen Crack Test (CLR (%)) 29th 35 32 309 387 7 365 0 thirty 32 32 819 1024 6 965 40 31 34 32 672 841 fifteen 793 35 32 39 32 589 736 8.5 628 25 33 38 32 462 577 9 544 45 34 35 32 458 573 7.5 540 35 35 33 32 428 536 twenty 505 34 36 37 32 475 594 6.5 560 45 37 39 32 454 567 6.5 535 45 38 32 32 466 583 7 428 56 39 32 32 415 519 8 456 67 40 35 32 417 522 9.5 492 56 41 38 32 405 506 7 477 0 42 39 32 447 559 7 593 0 43 36 32 515 644 8 607 0 44 34 32 374 468 6.5 508 25 45 37 32 482 602 7 568 45 46 36 32 478 598 8 564 56

Claims (38)

1. Plate steel for a thick-walled high-strength main pipe having a thickness of 25 mm to 45 mm and containing the following elements, in wt.%: C: 0.04 to 0.08 Mn: 1.2 to 2.0 Nb: 0.005 to 0.05 Ti: 0.005 to 0.03 Ca: 0.0005 to 0.0050 N: 0.001 to 0.008 Si: 0.5 or less Al: 0.05 or less P: 0.03 or less S: 0.005 or less O: 0.005 or less Fe and unavoidable impurities - the rest, while the microstructure of the part of the surface layer of the sheet located at distances from 0.9 mm to 1.1 mm from the surface in the direction of the sheet thickness contains deformed ferrite: 5% or more and S _fe1 % or less, a mixture of martensite and austenite: 8% or less , the rest is polygonal ferrite and / or bainite, and S _fe1 % is determined by the expression: S _fe1 = 0.6552 × T _H -4.7826, where T _H : thickness of plate steel; the microstructure of the central part of the sheet, located at a distance of 1 mm from the center of the sheet thickness towards the surface of the plate, contains deformed ferrite: 5% or less, a mixture of martensite and austenite: 5% or less, the rest is needle ferrite and / or bainite, moreover said portion of the surface layer and said central portion have an effective grain size whose average value, measured by electron backscattering diffraction, is 20 μm or less. 2. Plate steel according to claim 1, additionally containing at least one element of, in wt.%: Cu: 0.50 or less Ni: 0.50 or less Cr: 0.50 or less Mo: 0.50 or less W: 0.50 or less V: 0.10 or less Zr: 0.050 or less Ta: 0.050 or less B: 0.0020 or less Mg: 0.010 or less REM: 0.0050 or less Y: 0.0050 or less Hf: 0.0050 or less and Re: 0.0050 or less. 3. The plate steel according to claim 1, in which the aluminum content is 0.005 wt.% Or less. 4. Plate steel according to claim 2, in which the aluminum content is 0.005 wt.% Or less. 5. Plate steel according to any one of paragraphs. 1-4, which has a tensile strength of 500 to 700 MPa. 6. Plate steel according to any one of paragraphs. 1-4, which has a yield strength after the formation of the pipe 440 MPa or more, tensile strength from 500 to 700 MPa, and 0.2% stress of plastic flow in compression in the circular direction after aging at a temperature of 200 ° C of 450 MPa or more . 7. Thick-walled high-strength main pipe made by molding into a pipe a steel plate according to any one of paragraphs. 1-5, followed by arc welding of adjacent ends, having a yield strength of 440 MPa or more, a tensile strength of 500 to 700 MPa and 0.2% stress of plastic flow in compression in the circular direction after aging at a temperature of 200 ° C, comprising 450 MPa or more.

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