JP7670004B2 - 40kg class non-tempered thick steel plate, welded steel pipe and their manufacturing method - Google Patents

Description

The present invention relates to thick steel plates and welded steel pipes used in pressure vessels, process piping, line pipes, etc. of oil refineries that are in wet hydrogen sulfide corrosive environments, and to methods for manufacturing the same, and in particular to 40 kg class (tensile strength 400 MPa class) thick steel plates and welded steel pipes that have excellent HIC resistance and SR resistance, and to methods for manufacturing the same.

The quality of crude oil is declining year by year, and the hydrogen sulfide concentration is increasing. Therefore, pressure vessels and process piping in oil refining plants are required to have resistance to wet hydrogen sulfide corrosion stress, i.e., excellent hydrogen-induced cracking (HIC) resistance. Technology to ensure HIC resistance in steel has been developed mainly for 50 kg class steels such as API 5L X52MS to X65MS in the line pipe field. For example, in terms of steel composition design, steels with excellent HIC resistance have been developed by reducing the center segregation hardness by using low C, low Mn, and low P, reducing S and adding an optimal amount of Ca to elongate the center segregation, making MnS spheroidal, and suppressing the formation of Ca clusters. In terms of thick steel plate manufacturing methods, it is important to control the microstructure to a uniform one, and steels with excellent HIC resistance have been developed by applying QT (quenching followed by tempering) and accelerated cooling, which make it easy to obtain a bainite single phase structure.

In addition, pressure vessels and process piping generally undergo stress relief annealing (SR (Stress Relieving), and when performed on welds, PWHT (Post Weld Heat Treatment)) after cold working and welding when manufacturing the structure to remove internal stress caused by cold working and welding. SR is a process in which, for example, steel is annealed at 630°C for 180 minutes to remove internal stress. SR resistance refers to the difference in strength before and after the aforementioned SR is performed on a thick steel plate. If SR resistance is high, a strength difference will occur within the thick steel plate when SR is performed locally on the thick steel plate, which may cause deformation, so the smaller the SR resistance, the better (a small strength difference is considered to be excellent SR resistance).

Pressure vessels and process piping often use 40 kg class steels such as ASTM A516-60, 65 and API 5L 5LBMS to X42MS, and there is a demand to develop manufacturing methods that can stably ensure excellent HIC and SR resistance for these strength grades.

To address the above-mentioned issues, the following inventions have been made to date. Patent documents 1 to 4 disclose a method for manufacturing 40-50 kg class steel with excellent HIC resistance by applying a QT process. Patent document 5 discloses a method for manufacturing 40-50 kg class steel with excellent HIC resistance by applying a QT process to fitting members. Patent documents 6 to 7 disclose a method for manufacturing 40-60 kg class steel with excellent HIC resistance by performing TMCP (Thermo-Mechanical Control Process) and a normalizing process consecutively to hot-bent members.

JP 2013-7079 A JP 2013-7080 A JP 2013-23713 A JP 2013-23714 A Japanese Patent Application Publication No. 8-283906 Japanese Patent Application Publication No. 8-283839 Japanese Patent Application Publication No. 8-283840

However, in the methods of Patent Documents 1 to 7, heat treatment such as QT or normalizing is applied to the steel after rolling, so it is assumed that HIC resistance and SR resistance are good, but productivity is poor because heat treatment is applied.

As such, with conventional technology, it was difficult to consistently ensure HIC resistance and SR resistance in the 40 kg strength class, and to manufacture thick steel plates and welded steel pipes with good productivity using a non-tempered process that does not involve QT or normalizing after rolling.

Therefore, an object of the present invention is to provide a 40 kgf/cm2 class non-heat treated thick steel plate having a small difference in strength before and after SR and excellent HIC resistance.
In the present invention, SR refers to heat treatment at, for example, 630°C for 180 minutes to reduce residual stress. In addition, the kilo class in the present invention is a common name for the strength level of steel plate, which specifies the strength range in kgf/ ^mm2 units. In addition, the 40 kilo class refers to thick steel plate or welded steel pipe having a tensile strength of 400 to 520 MPa both before and after SR. In addition, in the present invention, thick steel plate and welded steel pipe manufactured by a non-tempered process that does not involve QT or normalizing are called non-tempered thick steel plate and non-tempered welded steel pipe.

The inventors have conducted extensive research into a method for producing 40 kg class steel that stably ensures HIC resistance and SR resistance without performing a non-tempering process, i.e., QT or normalizing after rolling, and have obtained the following findings.

First, we investigated a method to ensure the HIC resistance of steel using the as-controlled rolling process, which is generally used in the production of 40 kg class steel. However, when produced using the as-controlled rolling process, the microstructure becomes a composite structure consisting of ferrite, pearlite, and bainite, and it was not possible to suppress the occurrence of HIC in the central segregation area.

Therefore, we investigated a method to apply accelerated cooling, which is commonly used in the manufacture of 50 kg class HIC-resistant thick steel plate, to 40 kg class steel.

As a result, it was found that a uniform bainite structure can be obtained and excellent HIC resistance can be obtained by setting the accelerated cooling start temperature at or above Ar3, controlling the accelerated cooling rate within a range in which ferrite does not form during cooling, and setting the cooling stop temperature below the bainite transformation start temperature. It was found that in order to obtain a uniform bainite structure, it is necessary to control the cooling rate during accelerated cooling at Bs (bainite transformation start temperature) to a certain level or higher. On the other hand, the application of accelerated cooling generates hard MA (also called island martensite) between the laths of the bainite structure, which not only deteriorates the HIC resistance before SR, but also causes the MA to decompose into soft cementite when SR is performed, resulting in a decrease in the strength of the thick steel plate. Therefore, in order to reduce the change in strength before and after SR, it is effective to suppress the generation of MA between the bainite laths by setting the accelerated cooling stop temperature at or above the martensite transformation temperature. It was found that this makes it possible to suppress the decrease in strength resulting from the decomposition of MA into carbides by SR.

Furthermore, it was found that in order to adjust the strength to the 40 kg class, it is necessary to control the upper limit of the accelerated cooling rate according to the steel composition, and that, together with the lower limit of the cooling rate controlled to ensure HIC resistance, it is necessary to control it within the range of formula (4).

The present invention has been made based on the above findings and further studies, and the gist of the present invention is as follows.
[1] C: 0.020 to 0.100% by mass,
Si: 0.50% or less,
Mn: 0.50 to 1.50%,
P: 0.020% or less,
S: 0.0020% or less,
Ca: 0.0005-0.0050%,
O: 0.0040% or less;
Furthermore, Cu: 0.30% or less,
Ni: 0.30% or less,
Cr: 0.30% or less,
Mo: 0.05% or less,
Nb: 0.080% or less,
V: 0.080% or less,
Ti: 0.050% or less,
The balance consists of Fe and unavoidable impurities, and
Pcmy defined by formula (1) is 0.100 to 0.140,
PHIC defined by formula (2) is 1.100 or less;
The composition has an ACRM of 0.5 to 5.0 as defined by formula (3),
The microstructure at the 1/8 to 7/8 position in the sheet thickness direction has an area fraction of upper bainite of 95% or more,
A 40 kgf/cm2 class non-tempered thick steel plate having an area fraction of MA of 0.2% or less.
Pcmy=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/7+V/10+5B...Formula (1)
PHIC=4.46C+2.37Mn/6+(1.74Cu+1.70Ni)/15+(1.18Cr+1.95Mo+1.74V)/5+22.36P...Formula (2)
ACRM=(Ca-(1.23O-0.000365))/(1.25S)...Formula (3)
In the formulas (1) to (3), the element symbols indicate the content (mass%) of each element, and when no element is contained, it is set to zero.
[2] A 40 kgf/cm2 class non-heat treated welded steel pipe manufactured by forming the thick steel plate according to [1] into a cylindrical shape and welding the butt joints.
[3] C: 0.020 to 0.100% by mass,
Si: 0.50% or less,
Mn: 0.50 to 1.50%,
P: 0.020% or less,
S: 0.0020% or less,
Ca: 0.0005-0.0050%,
O: 0.0040% or less;
Furthermore, Cu: 0.30% or less,
Ni: 0.30% or less,
Cr: 0.30% or less,
Mo: 0.05% or less,
Nb: 0.080% or less,
V: 0.080% or less,
Ti: 0.050% or less,
The balance consists of Fe and unavoidable impurities, and
Pcmy defined by formula (1) is 0.100 to 0.140,
PHIC defined by formula (2) is 1.100 or less;
A slab having a component composition in which the ACRM defined by the formula (3) is 0.5 to 5.0,
After heating to 1000-1250°C, the plate is hot-rolled to the desired thickness.
A manufacturing method for 40 kgf/cm2 class non-tempered thick steel plate, comprising water cooling from a temperature of Ar3 or higher at a cooling rate CR defined by equation (4) to a cooling stop temperature FCT defined by equation (5), and then air cooling.
Pcmy=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/7+V/10+5B...Formula (1)
PHIC=4.46C+2.37Mn/6+(1.74Cu+1.70Ni)/15+(1.18Cr+1.95Mo+1.74V)/5+22.36P...Formula (2)
ACRM=(Ca-(1.23O-0.000365))/(1.25S)...Formula (3)
25-75(Pcmy+2Nb)≦CR≦190-750(Pcmy+2Nb)...Formula (4)
561-474C-33Mn-17Ni-17Cr-21Mo-1000Nb≦FCT≦830-270C-90Mn-37Ni-70Cr-83Mo-1000Nb...Formula (5)
CR: Cooling rate at the center of thickness when the temperature at the center of thickness reaches Bs (bainite transformation start temperature) (°C/s), FCT: Cooling stop temperature (°C)
In the formulas (1) to (5), the element symbols indicate the content (mass%) of each element, and when no element is contained, it is set to zero.
[4] A method for producing a 40 kgf/cm2 class non-heat treated welded steel pipe, comprising forming the thick steel plate produced by the method described in [3] into a cylindrical shape and welding the butt joint.

According to the present invention, it is possible to provide a 40 kg class non-tempered thick steel plate with a small difference in strength before and after SR and excellent HIC resistance.

The present invention also relates to thick steel plates and welded steel pipes used in pressure vessels and process piping in oil refineries that are subject to a wet hydrogen sulfide corrosive environment, and to a method for manufacturing the same. This allows the manufacture of 40 kgf/cm2 thick steel plates with a small difference in strength before and after SR and excellent HIC resistance without the need for tempering treatment, making it extremely useful in industry.

The reasons for limiting each component of the present invention are explained below.

1. Chemical Composition First, the reasons for specifying the chemical composition of the steel of the present invention will be explained. Note that "%" in the following explanation all means mass%.

C: 0.020-0.100%
C is the most effective element for increasing the strength of steel plates manufactured by accelerated cooling. However, if the C content is less than 0.020%, sufficient strength cannot be ensured, and if it exceeds 0.100%, toughness and HIC resistance are deteriorated. Therefore, the C content is set to be within the range of 0.020 to 0.100%.

Si: 0.50% or less Silicon is added for deoxidation, but if the Si content exceeds 0.50%, the amount of MA (also called island martensite) produced increases, deteriorating HIC resistance and SR resistance. Therefore, the Si content is set to 0.50% or less.

Mn: 0.50-1.50%
Mn is added to improve the strength and toughness of steel, but if the Mn content is less than 0.50%, the effect is insufficient, and the hardenability decreases and bainite does not form, while if the Mn content exceeds 1.50%, the weldability and HIC resistance deteriorate. Therefore, the Mn content is set within the range of 0.50 to 1.50%.

P: 0.020% or less P is an element that is inevitably contained in the composition of steel, and it increases the hardness of the center segregation portion, thereby deteriorating HIC resistance. This tendency becomes more pronounced when the P content exceeds 0.020%. Therefore, the P content is set to 0.020% or less. Preferably, it is set to 0.015% or less.

S: 0.0020% or less S generally becomes MnS-based inclusions in steel, but the addition of Ca controls the shape of the MnS-based inclusions to CaS-based inclusions, thereby suppressing HIC caused by central segregation. However, if the S content is high, the amount of CaS-based inclusions also increases, which can become the starting point of cracks in high-strength materials. This tendency becomes more pronounced when the S content exceeds 0.0020%. Therefore, the S content is set to 0.0020% or less. The S content is preferably 0.0010% or less.

Ca: 0.0005-0.0050%
Ca is an element effective in controlling the morphology of sulfide-based inclusions and improving ductility, but if the Ca content is less than 0.0005%, this effect is lost, and if it exceeds 0.0050%, the effect is saturated and, on the other hand, the toughness is deteriorated due to a decrease in cleanliness. Therefore, the Ca content is set to be within the range of 0.0005 to 0.0050%.

O: 0.0040% or less O is an element that is inevitably contained in steel. If the O content exceeds 0.0040%, MnS in the center segregation cannot be suppressed even if Ca is added, and HIC resistance performance deteriorates, so the upper limit of the O content is set to 0.0040%. The O content is preferably 0.0030% or less.

Pcmy: 0.100-0.140
Pcmy is a modified Mo coefficient of Pcm, which is widely known as an index of weld cold cracking, and has a good correlation with the strength and transformation behavior of low-carbon, low-alloy TMCP steel with a C content of 0.010% or less, such as that of the present invention. Pcmy can be expressed by the following formula (1). If Pcmy is less than 0.100, ferrite will form even if the cooling rate of accelerated cooling is controlled, and HIC resistance cannot be ensured, so the lower limit is set to 0.100. Also, if Pcmy exceeds 0.140, the strength becomes too high and it is not possible to control the strength to the 40 kg class that is the target of the present invention, so the upper limit of Pcmy is set to 0.140.
Pcmy=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/7+V/10+5B...Formula (1)
*Each element symbol indicates the content (mass%) of that element in the steel plate. If the element is not contained, it is entered as zero.
In addition, when the material is not contained, it is considered to be less than the detection limit. Although the amount of B contained in this formula is not specified in the present invention, B less than 0.0001% is regarded as 0 and is considered to be an unavoidable impurity.

PHIC: 1.100 or less PHIC is a formula devised to estimate the material of the central segregation from the content of each alloy element, and the higher the PHIC, the higher the concentration of the central segregation and the higher the hardness of the central segregation. PHIC can be expressed by the following formula (2). In the strength class of the present invention, when PHIC exceeds 1.100, HIC occurs due to hardening of the central segregation, so PHIC is set to 1.100 or less. Note that formula (2) is the sum of the products of the content of each component and the coefficient, so it takes a value of 0 or more.
PHIC=4.46C+2.37Mn/6+(1.74Cu+1.70Ni)/15+(1.18Cr+1.95Mo+1.74V)/5+22.36P...Formula (2)
*Each element symbol indicates the content (mass%) of that element in the steel plate. If the element is not contained, it is entered as zero.

ACRM: 0.5-5.0
Ca has a high affinity with O, and first produces CaO, and the remaining Ca bonds with S to form CaS. ACRM is an index that represents the form of existence of O, S, and Ca in these steels, and can be expressed by the following formula (3). When ACRM is less than 0.5, the amount and size of MnS produced in the center segregation increases, promoting 1/2t HIC. On the other hand, when ACRM exceeds 5.0, the excessively added Ca becomes cluster-like, promoting 1/4t HIC. Therefore, ACRM is set to the range of 0.5 to 5.0. ACRM is preferably set to the range of 1.0 to 4.0.
ACRM=(Ca-(1.23O-0.000365))/(1.25S)...Formula (3)
*Each element symbol indicates the content (mass%) of that element in the steel plate. If the element is not contained, it is entered as zero.

The above are the basic components of the steel plate of the present invention, but in order to obtain the desired strength and toughness, one or more alloy elements selected from the following are added.

Cu: 0.30% or less Cu is an effective element for improving toughness and increasing strength. To obtain this effect, it is preferable to set the Cu content to 0.05% or more. On the other hand, if Cu is contained in an amount exceeding 0.30%, the strength becomes too high and cannot be controlled to the 40 kg class. Therefore, when Cu is contained, it is set to 0.30% or less.

Ni: 0.30% or less Ni is an element that is effective in improving toughness and increasing strength. To obtain this effect, it is preferable to set the Ni content to 0.05% or more. On the other hand, if the Ni content exceeds 0.30%, the strength becomes too high and cannot be controlled to the 40 kg class. Therefore, when Ni is contained, it is set to 0.30% or less.

Cr: 0.30% or less Cr is an element that is effective in increasing strength by improving hardenability. To obtain this effect, it is preferable to set the Cr content to 0.05% or more. On the other hand, if the Cr content exceeds 0.30%, the strength becomes too high and cannot be controlled to the 40 kg class. Therefore, if Cr is contained, it should be 0.30% or less.

Mo: 0.05% or less Mo is an element effective in improving toughness and increasing strength. To obtain this effect, it is preferable to set the Mo content to 0.02% or more. On the other hand, if the Mo content exceeds 0.05%, the strength becomes too high and cannot be controlled to the 40 kg class. Therefore, if Mo is contained, it should be 0.15% or less.

Nb: 0.080% or less Nb is an element that suppresses grain growth during rolling and improves toughness by fine graining. To obtain this effect, the Nb content is preferably 0.006% or more. On the other hand, if the Nb content exceeds 0.080%, the strength becomes too high and cannot be controlled to the 40 kg class. Therefore, when Nb is contained, it is set to 0.080% or less.

V: 0.080% or less V is an element that increases strength without deteriorating toughness. To obtain this effect, it is preferable to set the V content to 0.010% or more. On the other hand, if V is contained in an amount exceeding 0.080%, the strength becomes too high and cannot be controlled to the 40 kg class. Therefore, when V is contained, it is set to 0.080% or less.

Ti: 0.050% or less Ti not only forms TiN to suppress grain growth during slab heating, but also suppresses grain growth in the weld heat affected zone, and improves toughness by refining the grains of the base material and the weld heat affected zone. To achieve this effect, the Ti content is preferably 0.006% or more. However, if the Ti content exceeds 0.050%, the toughness is deteriorated. Therefore, if Ti is contained, it is set to 0.050% or less.

The balance is Fe and unavoidable impurities. The composition of the steel sheet of the present invention includes the basic components and selectively selected components in the amounts described above, with the balance being Fe and unavoidable impurities.

2. Regarding the microstructure, the area fraction of upper bainite (also called bainite) in the microstructure at 1/8 to 7/8 positions in the plate thickness direction is 95% or more. From the viewpoint of ensuring HIC resistance, it is desirable for the structure of the steel plate base material to be a single phase structure, and in order to ensure the desired strength as a 40 kg class steel, a bainite structure is used. The bainite structure fraction of the microstructure at 1/8 to 7/8 positions in the plate thickness direction is desirably 100%, but since HIC resistance is ensured even if 5% or less of ferrite, cementite, MA, etc. are generated, the lower limit is set to 95%. Note that cementite that is generated between bainite laths is considered to be part of bainite, and MA that is generated between bainite laths is included in the area fraction of MA (hereinafter also referred to as MA fraction) described below. The bainite structure fraction is the area fraction when observed with an optical microscope or electron microscope in a plate thickness cross section parallel to the rolling direction.

MA area fraction is 0.2% or less MA (island martensite) refers to a structure with a high carbon content that is generated between laths of bainite, and its structure is a mixture of martensite and austenite. MA is a hard phase, but the hard martensite decomposes by heat treatment and becomes soft cementite. Therefore, steel plates with a large MA area fraction lose strength due to SR. In the case of SR at 630°C or less, which is usually performed with low carbon steel, if the MA area fraction exceeds 0.2%, the strength after SR decreases, so the upper limit is set to 0.2%. The upper limit of the MA area fraction is more preferably 0.1%. The MA area fraction is the area fraction when a plate thickness cross section parallel to the rolling direction is observed with an electron microscope. The measurement position of the MA area fraction is 1/8 to 7/8 in the plate thickness direction, excluding the central segregation part. Specifically, two locations, 1/4 and 3/4 in the sheet thickness direction, are photographed, and the average of the area fractions of MA at these two locations is taken as the area fraction of MA. The MA fraction is more preferably 0%. The area fraction of MA here includes MA generated between bainite laths.

3. Plate Thickness In the present invention, the term "thick steel plate" refers to a steel plate having a thickness of 6 mm or more, in accordance with the usual definition in this technical field. On the other hand, the upper limit of the thickness of the thick steel plate in the present invention is not particularly limited and may be any value.

4. Steel Plate Manufacturing Conditions Next, a method for manufacturing a steel plate according to one embodiment of the present invention will be described.

The method for manufacturing steel plate according to one embodiment of the present invention includes the steps of heating a slab having the above-mentioned composition to 1000-1250°C, hot rolling it to the desired plate thickness, water-cooling it from a temperature of Ar3 or higher at an accelerated cooling rate (also simply referred to as cooling rate) CR defined by the following formula (4) to an accelerated cooling stop temperature (also simply referred to as cooling stop temperature) FCT defined by the following formula (5), and then air-cooling it. By using this manufacturing method, it is possible to manufacture 40 kg class non-tempered thick steel plate with excellent HIC resistance and SR resistance, but the manufacturing method is not limited to the above.

Slab heating temperature: 1000 to 1250°C
The slab heating temperature is set to 1000 to 1250° C. If the slab heating temperature is less than 1000° C., the solid solution of carbides is insufficient and the required strength cannot be obtained, whereas if the temperature exceeds 1250° C., excessive strength is obtained and toughness is deteriorated, so the temperature is set to 1000 to 1250° C. Note that the temperature here is the temperature of the slab immediately after it is removed from the heating furnace, but it may be obtained by heat transfer calculation from information such as the temperature inside the furnace.

Accelerated cooling start temperature: Ar3 or higher In order to suppress the generation of ferrite near the surface of the slab and to obtain a uniform bainite structure that provides excellent HIC resistance, accelerated cooling by water cooling is started at Ar3 or higher. In the present invention, a uniform bainite structure is one that contains 95% or more bainite by area ratio and may contain other structures. The microstructure will be described later. Ar3 can be calculated, for example, by the following formula.
Ar3 (℃) = 910-310C-80Mn-20Cu-55Ni-15Cr-80Mo
*Each element symbol indicates the content (mass%) of that element in the steel plate.

Accelerated cooling rate (°C/s): 25-75 (Pcmy + 2Nb) ≦ CR ≦ 190-750 (Pcmy + 2Nb) ... formula (4)
The cooling rate of accelerated cooling is an important factor in controlling the microstructure and strength. If the cooling rate CR at the center of the plate thickness when the plate thickness temperature reaches Bs exceeds 190-750 (Pcmy + 2Nb) (°C/s), the strength cannot be controlled to 40 kg, so the upper limit is set to 190-750 (Pcmy + 2Nb) (°C/s). On the other hand, if the cooling rate is less than 25-75 (Pcmy + 2Nb) (°C/s), ferrite generation occurs and the bainite single phase structure cannot be created, so the lower limit is set to 25-75 (Pcmy + 2Nb) (°C/s). Note that Bs may be measured by measuring the bainite transformation start temperature of the thick steel plate, or it may be calculated from Bs (°C) = 830-270C-90Mn-37Ni-70Cr-83Mo-1000Nb. Each element symbol represents the content (mass%) of that element in the steel plate, and when the element is not contained, it is set to 0. The temperature and cooling rate at the center of the plate thickness are calculated by heat conduction calculation.

Accelerated cooling stop temperature: 561-474C-33Mn-17Ni-17Cr-21Mo-1000Nb≦FCT≦830-270C-90Mn-37Ni-70Cr-83Mo-1000Nb...Formula (5)
If the accelerated cooling stop temperature FCT exceeds 830-270C-90Mn-37Ni-70Cr-83Mo-1000Nb (°C), ferrite is generated after the accelerated cooling is completed and HIC resistance cannot be ensured, so the upper limit is set to 830-270C-90Mn-37Ni-70Cr-83Mo-1000Nb (°C). On the other hand, if the accelerated cooling stop temperature falls below 561-474C-33Mn-17Ni-17Cr-21Mo-1000Nb (°C), MA is generated between the bainite laths during accelerated cooling, and the strength decreases when SR is performed, so the lower limit is set to 561-474C-33Mn-17Ni-17Cr-21Mo-1000Nb (°C). Each element symbol indicates the content (mass%) of that element in the steel sheet, and when the element is not contained, it is set to zero.

Air cooling after water cooling After the accelerated cooling by water cooling is stopped, air cooling is performed. Air cooling is preferably performed at a rate of 0.01 to 2°C/s. In the present invention, since QT or normalizing is not performed after air cooling, a non-tempered steel plate can be obtained.

5. Welded Steel Pipe Manufacturing Conditions In a welded steel pipe according to one embodiment of the present invention, a thick steel plate having the above-mentioned composition and microstructure is formed into a cylindrical shape, and the butt joint is welded to produce a 40 kgf/cm2 class non-tempered welded steel pipe with excellent HIC resistance and SR resistance. The method of forming into a cylindrical shape is not particularly specified, but for example, the UOE method or the press bending method can be applied. The welding method is also not particularly specified, but for example, submerged arc welding can be applied.

6. Welded Steel Pipe Manufacturing Method In a welded steel pipe manufacturing method according to one embodiment of the present invention, the thick steel plate manufactured by the above manufacturing method is formed into a cylindrical shape, and the butt joint is welded to produce a 40 kgf/cm2 class non-tempered welded steel pipe with excellent HIC resistance and SR resistance. The method of forming into a cylindrical shape is not particularly specified, but for example, the UOE method or the press bending method can be applied. Furthermore, the welding method is not particularly specified, but for example, submerged arc welding can be applied.

Steel with the chemical composition shown in Table 1 was made into a slab by continuous casting, and then the slab was heated under the conditions shown in Table 2, hot rolled, accelerated cooled, and air cooled to produce a non-tempered thick steel plate. Some of the produced steel plates were UO formed (U press forming and O press forming (O press compression rate = 0.3%)), and then the butt joints were seam welded to produce welded steel pipes (hereinafter simply referred to as steel pipes), which were then expanded at an expansion rate of 1.0% to produce UOE steel pipes. The cooling rate of the accelerated cooling after hot rolling was determined by heat conduction calculations.

The microstructure of the steel plate was observed under an optical microscope after mirror polishing of a sample taken from a cross section parallel to the rolling direction, and then Nital etching. The microstructure of the steel pipe was observed under an optical microscope after similarly mirror polishing of a sample taken from a cross section parallel to the rolling direction at a position opposite the weld in the circumferential direction, and then Nital etching. The bainite fraction was determined by taking a magnified photograph of the sample from 1/8 to 7/8 in the thickness direction with an optical microscope, measuring the area fractions of ferrite, clumpy cementite, and clumpy MA, and subtracting the measured fractions (%) from 100 to obtain the bainite fraction (%). The area fraction of MA was evaluated by further electrolytically etching the sample and photographing two locations, 1/4 and 3/4 in the thickness direction, with an electron microscope, measuring the area fraction of MA in the obtained image, and averaging the area fractions of MA at the two locations.

HIC testing was performed in accordance with NACE TM0284. Nine test pieces were cut from the center of the width of each steel plate and from the position opposite the welded part of the steel pipe (meaning the 180° position when the welded part of the steel pipe is considered to be 0°) and used for the test. The average CLR (meaning the crack length ratio of the steel pipe, hereafter also referred to as HIC CLR or simply CLR) was measured, and those with a CLR of 5% or less were deemed to have passed. The CLR was calculated in accordance with NACE TM0284.

Tensile tests were performed on the test pieces before and after SR. SR was performed at 630°C for 180 min on test pieces conforming to ASTM A370. Tensile tests were performed in accordance with ASTM A370, with full thickness tensile tests. The tensile strength (hereinafter also referred to as TS) was deemed to be acceptable if it was 400-520 MPa, which is the strength range of general 40 kg class steel, and if the reduction in strength due to SR was within 30 MPa, it was deemed to be acceptable.

Table 3 shows the microstructure and material test results of the obtained steel plate and steel pipe. The steel plate and welded steel pipe (Examples) manufactured under manufacturing conditions within the specified ranges satisfy the above-mentioned numerical ranges for strength before and after SR (TS), strength reduction due to SR, and area fraction of CLR and MA, whereas the steel plate (Comparative Example) manufactured under manufacturing conditions that do not satisfy the range of the present invention does not satisfy one or more of the above-mentioned numerical ranges.

Claims (4)

C: 0.020 to 0.100% by mass,
Si: 0.50% or less,
Mn: 0.50 to 1.50%,
P: 0.020% or less,
S: 0.0020% or less,
Ca: 0.0005-0.0050%,
O: 0.0040% or less;
Furthermore, Cu: 0.30% or less,
Ni: 0.30% or less,
Cr: 0.30% or less,
Mo: 0.05% or less,
Nb: 0.080% or less,
V: 0.080% or less,
Ti: 0.050% or less,
The balance consists of Fe and unavoidable impurities, and
Pcmy defined by formula (1) is 0.100 to 0.140,
PHIC defined by formula (2) is 1.100 or less;
The composition has an ACRM of 0.5 to 5.0 as defined by formula (3),
The microstructure at the 1/8 to 7/8 position in the sheet thickness direction has an area fraction of upper bainite of 95% or more,
A 40 kgf/cm2 class non-tempered thick steel plate having an area fraction of MA of 0.2% or less.
Pcmy=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/7+V/10+5B...Formula (1)
PHIC=4.46C+2.37Mn/6+(1.74Cu+1.70Ni)/15+(1.18Cr+1.95Mo+1.74V)/5+22.36P...Formula (2)
ACRM=(Ca-(1.23O-0.000365))/(1.25S)...Formula (3)
In the formulas (1) to (3), the element symbols indicate the content (mass%) of each element, and when no element is contained, it is set to zero. A 40 kg class non-tempered welded steel pipe manufactured by forming the thick steel plate according to claim 1 into a cylindrical shape and welding the butt joint. C: 0.020 to 0.100% by mass,
Si: 0.50% or less,
Mn: 0.50 to 1.50%,
P: 0.020% or less,
S: 0.0020% or less,
Ca: 0.0005-0.0050%,
O: 0.0040% or less;
Furthermore, Cu: 0.30% or less,
Ni: 0.30% or less,
Cr: 0.30% or less,
Mo: 0.05% or less,
Nb: 0.080% or less,
V: 0.080% or less,
Ti: 0.050% or less,
The balance consists of Fe and unavoidable impurities, and
Pcmy defined by formula (1) is 0.100 to 0.140,
PHIC defined by formula (2) is 1.100 or less;
A slab having a composition in which the ACRM defined by formula (3) is 0.5 to 5.0 is heated to 1000 to 1250 ° C., and then hot-rolled to a desired plate thickness.
Water-cooling from a temperature of Ar3 or higher at a cooling rate CR defined by formula (4) to a cooling stop temperature FCT defined by formula (5), and then air-cooling.
The microstructure at the 1/8 to 7/8 position in the sheet thickness direction has an area fraction of upper bainite of 95% or more,
A method for manufacturing a 40 kgf/cm2 class non-tempered thick steel plate having an area fraction of MA of 0.2% or less .
Pcmy=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/7+V/10+5B...Formula (1)
PHIC=4.46C+2.37Mn/6+(1.74Cu+1.70Ni)/15+(1.18Cr+1.95Mo+1.74V)/5+22.36P...Formula (2)
ACRM=(Ca-(1.23O-0.000365))/(1.25S)...Formula (3)
25-75(Pcmy+2Nb)≦CR≦190-750(Pcmy+2Nb)...Formula (4)
561-474C-33Mn-17Ni-17Cr-21Mo-1000Nb≦FCT≦
830-270C-90Mn-37Ni-70Cr-83Mo-1000Nb...Formula (5)
CR: Cooling rate at the center of thickness when the temperature at the center of thickness reaches Bs (bainite transformation start temperature) (°C/s), FCT: Cooling stop temperature (°C)
In the formulas (1) to (5), the element symbols indicate the content (mass%) of each element, and when no element is contained, it is set to zero. A method for manufacturing 40 kg class non-tempered welded steel pipes by forming the thick steel plate manufactured by the manufacturing method described in claim 3 into a cylindrical shape and welding the butt joints.