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

Abstract

To provide a 40 kg class non-thermal thick steel plate having small strength difference between before and after SR, and excellent HIC resistance.SOLUTION: A 40 kg class non-thermal thick steel plate characterized in that a component composition contains, in mass%, C: 0.020 to 0.100% and others, Pcmy is 0.100 to 0.140, PHIC is 1.100 or less, ACRM is 0.5 to 5.0, each of these is specified by formulas (1) to (3), 95% or more of a microstructure in the 1 / 8 to 7 / 8 position in a plate thickness direction is upper bainite, and an area fraction of MA is 0.2% or less. Pcmy=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 7+V / 10+5B Formula (1) PHIC=4.46 C+2.37 Mn / 6+(1.74 Cu+1.70 Ni) / 15+(1.18 Cr+1.95 Mo+1.74 V ) / 5+22.36 P. Formula (2) ACRM=(Ca-(1.23O-0.000365)) / (1.25S) Formula (3).SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to thick steel plates and welded steel pipes used for pressure vessels, process piping, line pipes, etc. of oil refineries that are located in wet hydrogen sulfide corrosive environments, and to methods for manufacturing the same, and in particular to 40 kgf / cm2 class (tensile strength 400 MPa class) thick steel plates and welded steel pipes having excellent HIC resistance and SR resistance, and methods for manufacturing the same. [Background technology]

[0002] The quality of crude oil is declining year by year, and the hydrogen sulfide concentration is increasing. Therefore, the pressure vessels and process piping of oil refineries are required to have resistance to wet hydrogen sulfide corrosion stress, i.e., excellent hydrogen-induced cracking (HIC) resistance. Technologies to ensure HIC resistance in steels have been developed mainly for 50 kg class steels such as API 5L X52MS-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 the manufacturing method of thick steel plates, 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 are easy to obtain a bainite single phase structure.

[0003] Furthermore, pressure vessels and process piping generally undergo stress relief annealing (called SR (Stress Relieving), and when performed on welds it is also called PWHT (Post Weld Heat Treatment)) after cold working and welding when manufacturing the structure, to remove internal stress caused by the cold working and welding. SR is a process in which steel is annealed for 180 minutes at 630°C to remove internal stress. SR resistance refers to the difference in strength before and after SR as described above 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 plate, which can cause deformation, so the smaller the SR resistance, the better (a small difference in strength is considered to be excellent SR resistance).

[0004] Pressure vessels and process piping often use 40 kg class steels such as ASTM A516-60, 65 and API 5L 5LBMS~X42MS, and there is a demand for the development of manufacturing methods that can stably ensure excellent HIC and SR resistance for these strength grades.

[0005] In response to the above-mentioned problems, the following inventions have been made. 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 continuously performing a TMCP (Thermo-Mechanical Control Process) and a normalizing process to hot-bent members. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2013-7079 A [Patent Document 2] JP 2013-7080 A [Patent Document 3] JP 2013-23713 A [Patent Document 4] JP 2013-23714 A [Patent Document 5] Japanese Patent Application Publication No. 8-283906 [Patent Document 6] Japanese Patent Application Publication No. 8-283839 [Patent Document 7] Japanese Patent Application Publication No. 8-283840 Summary of the Invention [Problem to be solved by the invention]

[0007] However, in the methods of Patent Documents 1 to 7, since heat treatment such as QT or normalizing is applied to the steel after rolling, it is presumed that the HIC resistance and SR resistance are good, but the productivity is poor because heat treatment is applied.

[0008] As described above, with conventional technology, it was difficult to stably ensure HIC resistance and SR resistance in the 40 kg strength class, and at the same time, to produce thick steel plates and welded steel pipes productively using a non-tempered process that does not perform QT ​​or normalizing after rolling.

[0009] 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 630°C for 180 minutes to reduce residual stress. In addition, the kilogram class in the present invention refers to kgf / mm 2 It is a common name for the strength level of steel plates that specifies the strength range in units. Also, 40 kg class refers to thick steel plates or welded steel pipes with tensile strength of 400 to 520 MPa before and after SR. In the present invention, thick steel plates and welded steel pipes manufactured by the non-tempered process without QT or normalizing are called non-tempered thick steel plates and non-tempered welded steel pipes. [Means for solving the problem]

[0010] The present inventors have conducted extensive research into a method for producing a 40 kgf / cm2 class steel that stably ensures HIC resistance and SR resistance without performing a non-thermal refining process, i.e., without performing QT or normalizing after rolling, and have obtained the following findings.

[0011] First, we investigated ways to ensure the HIC resistance of steel using the as-controlled rolling process, which is generally applied in the production of 40 kg class steel. However, when manufactured using the as-controlled rolling process, the microstructure becomes a composite structure consisting of ferrite, pearlite, and bainite, and the occurrence of HIC in the central segregation area could not be suppressed.

[0012] Therefore, we investigated a method of applying accelerated cooling, which is commonly used in the manufacture of 50 kgf / cm2 class HIC-resistant thick steel plate, to 40 kgf / cm2 class steel.

[0013] As a result, it was found that a uniform bainite structure can be obtained by setting the start temperature of accelerated cooling at Ar3 or higher, controlling the cooling rate of accelerated cooling to a range in which ferrite does not form during cooling, and setting the cooling stop temperature below the bainite transformation start temperature, and that excellent HIC resistance performance can be obtained. 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 (the 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 reduces the strength of the thick steel plate when SR is performed by decomposing the MA into soft cementite. Therefore, in order to reduce the change in strength before and after SR, it is effective to suppress the formation of MA between the bainite laths by setting the accelerated cooling stop temperature above the martensite transformation temperature. It was found that this can suppress the strength reduction caused by the decomposition of MA into carbides due to SR.

[0014] 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 equation (4).

[0015] 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] By weight C: 0.020~0.100%, Si: 0.50% or less, Mn: 0.50-1.50%, P: 0.020% or less, S: 0.0020% or less, Ca: 0.0005 to 0.0050%, O: Contains 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, Contains one or more selected from 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 the formula (3), The microstructure at the 1 / 8 to 7 / 8 position in the sheet thickness direction has an upper bainite area fraction of 95% or more, A 40kgf / cm2 class non-tempered thick steel plate with 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)...Equation (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-tempered welded steel pipe manufactured by forming the thick steel plate described in [1] into a cylindrical shape and welding the butt joint. [3] By weight C: 0.020~0.100%, Si: 0.50% or less, Mn: 0.50-1.50%, P: 0.020% or less, S: 0.0020% or less, Ca: 0.0005 to 0.0050%, O: Contains 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, Contains one or more selected from 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 the formula (3) is 0.5 to 5.0, After heating to 1000-1250℃, the plate is hot-rolled to the desired thickness. A manufacturing method for 40 kgf / cm2 class non-tempered thick steel plate in which the plate is water-cooled 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-cooled. 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)...Equation (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) (℃ / s), FCT: Cooling stop temperature (℃) 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 manufacturing method for 40 kgf / cm2 class non-tempered welded steel pipes, in which the thick steel plate manufactured by the manufacturing method described in [3] is formed into a cylindrical shape and the butt joint is welded. Effect of the Invention

[0016] According to the present invention, it is possible 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.

[0017] Furthermore, the present invention relates to thick steel plates and welded steel pipes used for pressure vessels and process piping in oil refineries, which are located in wet hydrogen sulfide corrosive environments, and a manufacturing method thereof. This makes it possible to manufacture 40 kgf / mm2 thick steel plates with a small difference in strength before and after SR and excellent HIC resistance without performing thermal refining treatment, which is extremely useful industrially. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] The reasons for limiting each of the constituent elements of the present invention will be explained below.

[0019] 1. 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 %.

[0020] 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%.

[0021] Si:0.50% or less Silicon is added for deoxidation, but if the silicon content exceeds 0.50%, the amount of MA (also called island martensite) formed increases, deteriorating HIC performance and SR resistance. Therefore, the silicon content is set to 0.50% or less.

[0022] Mn: 0.50 to 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 it exceeds 1.50%, the weldability and HIC resistance deteriorate. Therefore, the Mn content is set to the range of 0.50 to 1.50%.

[0023] P:0.020% or less P is an element that is inevitably included in the composition of steel, and it increases the hardness of the center segregation area, 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, and preferably 0.015% or less.

[0024] S: 0.0020% or less In steel, S generally becomes MnS-based inclusions, but the addition of Ca controls the morphology from MnS to CaS-based inclusions, thereby suppressing HIC caused by center 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.

[0025] Ca: 0.0005~0.0050% Ca is an effective element for controlling the morphology of sulfide-based inclusions and improving ductility, but if the Ca content is less than 0.0005%, the effect is ineffective, 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 the range of 0.0005 to 0.0050%.

[0026] O: 0.0040% or less O is an element that is inevitably contained in steel. If the O content exceeds 0.0040%, the addition of Ca cannot suppress MnS in the center segregation, and the 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.

[0027] 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 below 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 considered to be 0 and is regarded as an unavoidable impurity.

[0028] 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; 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.

[0029] ACRM: 0.5 to 5.0 Ca has a high affinity with O, and first produces CaO, and the remaining Ca combines with S to form CaS. ACRM is an index that indicates the form 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 a range of 0.5 to 5.0. ACRM is preferably set to a range of 1.0 to 4.0. ACRM=(Ca-(1.23O-0.000365)) / (1.25S)...Equation (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.

[0030] 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 contained.

[0031] Cu: 0.30% or less Cu is an element that is effective in improving toughness and increasing strength. To achieve this effect, it is preferable to set the Cu content to 0.05% or more. On the other hand, if the Cu content exceeds 0.30%, the strength becomes too high and cannot be controlled to the 40 kg class. Therefore, if Cu is contained, it should be 0.30% or less.

[0032] Ni: 0.30% or less Ni is an element that is effective in improving toughness and increasing strength. To achieve 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, if Ni is contained, it should be 0.30% or less.

[0033] Cr:0.30% or less Cr is an element that is effective in increasing strength by improving hardenability. To achieve 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.

[0034] Mo: 0.05% or less Mo is an element that is effective in improving toughness and increasing strength. To achieve 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.

[0035] Nb: 0.080% or less Nb is an element that suppresses grain growth during rolling and improves toughness by refining grains. 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, if Nb is contained, it should be 0.080% or less.

[0036] V:0.080% or less V is an element that increases strength without deteriorating toughness. To obtain this effect, it is preferable that the V content is 0.010% or more. On the other hand, if the V content exceeds 0.080%, the strength becomes too high and cannot be controlled to the 40 kg class. Therefore, if V is contained, it should be 0.080% or less.

[0037] Ti: 0.050% or less Ti not only forms TiN and suppresses grain growth during slab heating, but also suppresses grain growth in the heat-affected zone of welding, and improves toughness by fine-graining the base material and the heat-affected zone of welding. To achieve this effect, the Ti content is preferably 0.006% or more. However, if the Ti content exceeds 0.050%, toughness is deteriorated. Therefore, if Ti is contained, it should be 0.050% or less.

[0038] 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.

[0039] 2. Microstructure The microstructure at the 1 / 8 to 7 / 8 position in the plate thickness direction has an upper bainite (also called bainite) area fraction of 95% or more. From the viewpoint of ensuring HIC resistance, the structure of the steel plate base material is preferably a single phase structure, and is made a bainite structure to ensure the desired strength as a 40 kg class steel. The bainite structure fraction in the microstructure at the 1 / 8 to 7 / 8 position in the plate thickness direction is preferably 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 forms between bainite laths is considered to be part of bainite, and MA that forms 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.

[0040] The area fraction of MA is 0.2% or less MA (island martensite) refers to a structure with a high carbon content that occurs between laths of bainite, and its structure is a mixture of martensite and austenite. MA is a hard phase, but hard martensite decomposes during heat treatment to become soft cementite. Therefore, steel sheets with a large area fraction of MA 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 area fraction of MA exceeds 0.2%, strength after SR decreases, so the upper limit is set to 0.2%. The upper limit of the area fraction of MA is more preferably 0.1%. The area fraction of MA 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 area fraction of MA is the position 1 / 8 to 7 / 8 in the plate thickness direction, excluding the central segregation part. Specifically, two positions, 1 / 4 and 3 / 4 in the plate thickness direction, are photographed, and the average of the area fractions of MA at these two positions 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.

[0041] 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.

[0042] 4.Thick steel plate manufacturing conditions Next, a method for manufacturing a steel plate according to one embodiment of the present invention will be described.

[0043] A method for producing a steel plate according to one embodiment of the present invention includes the steps of heating a slab having the above-mentioned composition to 1000 to 1250°C, hot rolling it to a 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 production method, a 40 kg class non-tempered thick steel plate excellent in HIC resistance and SR resistance can be produced, but the production method is not limited to the above.

[0044] Slab heating temperature: 1000~1250℃ 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., the strength becomes excessive and the toughness deteriorates, 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.

[0045] 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.

[0046] Accelerated cooling rate (℃ / s): 25-75(Pcmy+2Nb)≦CR≦190-750(Pcmy+2Nb)···Equation (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 thickness when the temperature at the center of thickness reaches Bs exceeds 190-750 (Pcmy+2Nb) (℃ / s), the strength cannot be controlled to 40 kg, so the upper limit is set to 190-750 (Pcmy+2Nb) (℃ / s). On the other hand, if the cooling rate is less than 25-75 (Pcmy+2Nb) (℃ / s), ferrite will be generated and it will not be possible to create a bainite single phase structure, so the lower limit is set to 25-75 (Pcmy+2Nb) (℃ / s). Note that Bs can be measured by measuring the bainite transformation start temperature of the thick steel plate, but it can also be calculated from Bs (℃) = 830-270C-90Mn-37Ni-70Cr-83Mo-1000Nb. Each element symbol indicates the content (mass%) of that element in the steel plate, and if the element is not contained, it is set to zero. The temperature and cooling rate at the center of the plate thickness are calculated by heat conduction calculation.

[0047] 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 (℃), ferrite will be generated after the accelerated cooling is completed and HIC resistance will not be ensured, so the upper limit is set to 830-270C-90Mn-37Ni-70Cr-83Mo-1000Nb (℃). On the other hand, if the accelerated cooling stop temperature is below 561-474C-33Mn-17Ni-17Cr-21Mo-1000Nb (℃), MA will be generated between the bainite laths during accelerated cooling, and the strength will decrease when SR is performed, so the lower limit is set to 561-474C-33Mn-17Ni-17Cr-21Mo-1000Nb (℃). Each element symbol represents the content (mass%) of that element in the steel plate, and if it is not contained, it is set to zero.

[0048] Air cooling after water cooling After the accelerated cooling by water cooling is stopped, air cooling is performed. The 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 thick steel plate can be obtained.

[0049] 5. Manufacturing conditions for welded steel pipes In the 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.

[0050] 6. Welded steel pipe manufacturing method In a method for producing a welded steel pipe according to one embodiment of the present invention, the thick steel plate produced by the above-mentioned method is formed into a cylindrical shape and the butt joint is welded to produce a 40 kgf / cm2 class non-heat treated welded steel pipe with excellent HIC resistance and SR resistance. The method for 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. EXAMPLES

[0051] 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 calculated by heat conduction calculation.

[0052] [Table 1]

[0053] [Table 2]

[0054] The microstructure of the steel plate was observed by mirror polishing a sample taken from a cross section of the plate thickness parallel to the rolling direction, followed by nital etching, and then by optical microscopy. The microstructure of the steel pipe was observed by mirror polishing a sample taken from a cross section of the plate thickness parallel to the rolling direction at a position opposite to the weld in the circumferential direction, followed by nital etching, and then by optical microscopy. The bainite fraction was determined by taking a magnified photograph of the plate thickness direction from 1 / 8 to 7 / 8 using 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 above sample, photographing two locations, 1 / 4 and 3 / 4 in the plate thickness direction, using an electron microscope, measuring the area fraction of MA in the obtained image, and averaging the area fractions of MA at the two locations.

[0055] The HIC test was conducted 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 0°). The average CLR (meaning the crack length ratio of the steel pipe, hereinafter 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.

[0056] Tensile tests were performed on the test pieces before and after SR. SR was performed at 630℃ for 180min 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 400-520MPa, which is the strength range of general 40kg class steel, and if the reduction in strength due to SR was within 30MPa, it was deemed to be acceptable.

[0057] The microstructures and material test results of the obtained steel plates and steel pipes are shown in Table 3. The steel plates and welded steel pipes (Examples) manufactured under manufacturing conditions within the specified ranges had strength before and after SR (TS), strength reduction due to SR, and area fractions of CLR and MA that satisfied the above-mentioned numerical ranges, whereas the steel plates (Comparative Examples) manufactured under manufacturing conditions that did not satisfy the ranges of the present invention did not satisfy one or more of the above-mentioned numerical ranges.

[0058] [Table 3]

Claims

1. By mass% C: 0.020-0.100%, 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 claim 1 into a cylindrical shape and welding the butt joints.

3. By mass% C: 0.020-0.100%, 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.

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 according to claim 3 into a cylindrical shape and welding the butt joints.

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