High-strength steel sheet for acid-resistant line pipe, method for producing same, and high-strength steel pipe using high-strength steel sheet for acid-resistant line pipe

Abstract

Provided is a high-strength steel sheet for an acid-resistant line pipe, which has not only excellent HIC resistance and SSCC resistance, but also excellent low-temperature toughness. This high-strength steel sheet for an acid-resistant line pipe has a component composition containing, in mass%, 0.030-0.060% of C, 0.01-0.50% of Si, 0.80-1.80% of Mn, 0.015% or less of P, 0.0015% or less of S, 0.010-0.080% of Al, 0.05-0.50% of Cr, 0.005-0.080% of Nb, 0.0010-0.0080% of N, and 0.0005-0.0050% of Ca, with the remainder being Fe and unavoidable impurities, and the steel structure at 0.25 mm below the surface of the steel sheet is composed of granular bainite and tempered island martensite, the maximum crystal grain size at the center of the sheet thickness is 80 [mu] m or less, the average crystal grain size is 20 [mu] m or less, the ductile-brittle transition temperature in a Charpy impact test is-100 DEG C or less, and the tensile strength is 535 MPa or more.

Description

Technical Field

[0001] The present invention relates to a high-strength steel plate for sour-resistant line pipes having excellent material uniformity in the steel plate, which is suitable for line pipes used in the transportation of crude oil and natural gas, and a method for producing the same. In addition, the present invention relates to a high-strength steel pipe using the high-strength steel plate for sour-resistant line pipes. Background Art

[0002] Generally speaking, a line pipe can be manufactured by forming a steel plate manufactured by a plate rolling mill or a hot rolling mill into a steel pipe by UOE forming, press bending forming, roll forming, or the like.

[0003] Here, in addition to strength, toughness, weldability, etc., the line pipe used in the transportation of crude oil and natural gas containing hydrogen sulfide also requires so-called acid resistance such as resistance to hydrogen induced cracking (HIC (Hydrogen Induced Cracking) resistance) and resistance to sulfide stress corrosion cracking (SSCC (Sulfide Stress Corrosion Cracking) resistance). Among them, HIC refers to the hydrogen ions generated by the corrosion reaction adsorbed on the steel surface, invading the interior of the steel as atomic hydrogen, diffusing and aggregating around non-metallic inclusions such as MnS in the steel or hard second phase structure, becoming molecular hydrogen, and cracking due to its internal pressure. This HIC is considered to be a problem in line pipes with lower strength grades than oil well pipes, and many countermeasure technologies have been disclosed. On the other hand, with regard to SSCC, it is pointed out that it is important to control the hardness of the surface layer of the inner surface of the steel pipe and improve the resistance to SSCC in a more severe corrosion environment. In addition to these acid resistances, in recent years, the mining environment of crude oil and natural gas has become increasingly harsh, and the demand for excellent low-temperature toughness has become higher.

[0004] Generally, when manufacturing high-strength steel plates for line pipes, the so-called TMCP (Thermo-Mechanical Control Process) technology, which combines controlled rolling and controlled cooling, is used. In order to ensure the acid resistance of the steel plate using this TMCP technology, it is effective to increase the cooling start temperature during controlled cooling and reduce the cooling rate. However, when the cooling start temperature is increased, the rolling in the non-recrystallization temperature range becomes insufficient, so the grain refinement that is effective in improving low-temperature toughness is limited, and excellent low-temperature toughness cannot be ensured.

[0005] In order to solve the above problems, for example, Patent Document 1 proposes a technique for setting the finishing temperature of the finishing rolling to 700°C or higher and the average grain size to 15.0 μm or less. In addition, Patent Document 2 proposes a technique for suppressing grain growth by setting the holding time from rough rolling to the start of finishing rolling to within 300 seconds.

[0006] Prior art literature

[0007] Patent Literature

[0008] Patent Document 1: Japanese Patent Application Publication No. 2020-12168

[0009] Patent Document 2: Japanese Patent Application No. 2020-509181 Summary of the invention

[0010] According to the techniques described in Patent Documents 1 and 2, low temperature toughness can be improved. However, since the metal structure in the techniques described in these two documents includes ferrite, it is difficult to ensure acid resistance in an environment with a high hydrogen sulfide partial pressure, which is a more severe corrosive environment.

[0011] In particular, pipelines used in cold regions require excellent low-temperature toughness. However, in order to prevent the formation of ferrite, it is necessary to start cooling at a temperature above the Ar3 point, which reduces the accumulated strain during rolling and makes it difficult to make the crystal grain size fine, so excellent low-temperature toughness has not been obtained in the past.

[0012] Therefore, in view of the above problems, the present invention aims to provide a high-strength steel plate for sour-resistant line pipe having excellent HIC resistance and SSCC resistance and excellent low-temperature toughness, and an advantageous method for producing the same. In addition, the present invention aims to provide a high-strength steel pipe using the high-strength steel plate for sour-resistant line pipe.

[0013] The inventors of the present invention have repeatedly conducted a large number of experiments and studies on the composition, microstructure and manufacturing conditions of steel plates in order to ensure acid resistance and low-temperature toughness. As a result, it was found that in order to further improve the low-temperature toughness of high-strength steel plates, the maximum crystal grain size at the center of the plate thickness needs to be less than 80 μm and the average crystal grain size needs to be less than 20 μm. In addition, in order to achieve such a steel structure, it is necessary to strictly control the hot rolling conditions in the recrystallization temperature range, and the conditions have been successfully found. The present invention is made in view of these findings.

[0014] That is, the gist of the present invention is as follows.

[0015] [1] A high-strength steel plate for sour-resistant line pipe, characterized in that it has the following composition, containing, in mass%, C: 0.030-0.060%, Si: 0.01-0.50%, Mn: 0.80-1.80%, P: 0.015% or less, S: 0.0015% or less, Al: 0.010-0.080%, Cr: 0.05-0.50%, Nb: 0.005-0.080%, N: 0.0010-0.0080% and Ca: 0.0005-0.0050%, with the remainder being Fe and inevitable impurities,

[0016] The steel structure at 0.25 mm below the surface of the steel plate is composed of granular bainite and tempered island martensite.

[0017] The maximum grain size at the center of the plate thickness is less than 80μm, and the average grain size is less than 20μm.

[0018] The ductile-brittle transition temperature in the Charpy impact test is below -100°C.

[0019] The tensile strength is 535 MPa or more.

[0020] [2] The high-strength steel plate for sour-resistant line pipe according to [1] above, wherein the component composition further contains, in mass %, one or more selected from the group consisting of Cu: 0.30% or less, Ni: 0.10% or less, and Mo: 0.50% or less.

[0021] [3] The high-strength steel plate for sour-resistant line pipe according to [1] or [2], wherein the component composition further contains, in mass%, one or more selected from the group consisting of 0.005 to 0.1% V, 0.005 to 0.1% Ti, 0.0005 to 0.02% Zr, 0.0005 to 0.02% Mg, and 0.0005 to 0.02% REM.

[0022] [4] A method for producing a high-strength steel plate for sour-resistant line pipe, characterized in that a steel sheet having a composition comprising, in mass%, 0.030% to 0.060% C, 0.01% to 0.50% Si, 0.80% to 1.80% Mn, 0.015% or less P, 0.0015% or less S, 0.010% to 0.080% Al, 0.05% to 0.50% Cr, 0.005% to 0.080% Nb, 0.0010% to 0.0080% N and 0.0005% to 0.0050% Ca, the remainder being Fe and unavoidable impurities, is heated to a temperature of 1000 to 1250°C,

[0023] Then, the steel sheet is subjected to hot rolling with a total reduction ratio of 35% to 55% in the recrystallization temperature range and a reduction ratio of 10% or more in the final rolling pass in the recrystallization temperature range to produce a steel plate.

[0024] Then, the steel plate is cooled under the following conditions:

[0025] The surface temperature of the steel plate at the beginning of cooling is above the Ar3 point.

[0026] The average cooling rate from 750°C to 550°C using a steel plate thermometer at 0.25mm below the steel plate surface is 15-35°C / s.

[0027] The average cooling rate from 750°C to 550°C at the center of the plate thickness is more than 15°C / s.

[0028] The cooling stop temperature measured by the steel plate thermometer at 0.25 mm below the steel plate surface and at the center of the plate thickness is 350 to 550°C.

[0029] [5] The method for producing a high-strength steel plate for sour-resistant line pipe according to [4] above, wherein the component composition further contains, in terms of mass %, one or more selected from the group consisting of Cu: 0.30% or less, Ni: 0.10% or less, and Mo: 0.50% or less.

[0030] [6] The method for producing a high-strength steel plate for sour-resistant line pipe according to [4] or [5], wherein the component composition further contains, in terms of mass%, one or more selected from the group consisting of V: ​​0.005-0.1%, Ti: 0.005-0.1%, Zr: 0.0005-0.02%, Mg: 0.0005-0.02% and REM: 0.0005-0.02%.

[0031] [7] A high-strength steel pipe using the high-strength steel plate for sour-resistant line pipe according to [1] or [2].

[0032] [8] A high-strength steel pipe using the high-strength steel plate for sour-resistant line pipe according to [3].

[0033] The high-strength steel plate for sour-resistant line pipe of the present invention and the high-strength steel pipe using the same are excellent in not only HIC resistance and SSCC resistance but also low-temperature toughness. In addition, according to the method for producing the high-strength steel plate for sour-resistant line pipe of the present invention, it is possible to produce a high-strength steel plate for sour-resistant line pipe that is excellent in not only HIC resistance and SSCC resistance but also low-temperature toughness. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is a schematic diagram for explaining a method of sampling a test piece for evaluating SSCC resistance in Examples. DETAILED DESCRIPTION

[0035] Hereinafter, the high-strength steel plate for sour-resistant line pipe of the present invention will be described in detail.

[0036] [Ingredients]

[0037] First, the chemical composition of the high-strength steel sheet of the present invention and the reasons for its limitation are described. In the following description, all units represented by "%" are "mass %" unless otherwise specified.

[0038] C: 0.030~0.060%

[0039] C effectively contributes to the improvement of strength, but when the C content is less than 0.030%, sufficient strength cannot be ensured, so the C content is 0.030% or more, preferably 0.035% or more. On the other hand, if the C content exceeds 0.060%, the low-temperature toughness deteriorates. In addition, the hardness of the surface layer and the center segregation part increases during accelerated cooling, so the SSCC resistance and HIC resistance deteriorate. Therefore, the C content is 0.060% or less, preferably 0.050% or less.

[0040] Si: 0.01~0.50%

[0041] Si is added for deoxidation, but when the Si content is less than 0.01%, the deoxidation effect is insufficient, so the Si content is 0.01% or more, preferably 0.05% or more. On the other hand, if the Si content exceeds 0.50%, the non-thermal stress of the steel increases and the low-temperature toughness deteriorates, so the Si content is 0.50% or less, preferably 0.45% or less.

[0042] Mn: 0.80~1.80%

[0043] Mn effectively contributes to the improvement of strength, but when the Mn content is less than 0.80%, its effect cannot be fully manifested. Therefore, the Mn content is 0.80% or more, preferably 1.00% or more. On the other hand, if the Mn content exceeds 1.80%, the hardness of the surface layer and the center segregation part increases during accelerated cooling, so the SSCC resistance and HIC resistance deteriorate. In addition, the weldability also deteriorates. Therefore, the Mn content is 1.80% or less, preferably 1.70% or less.

[0044] P: 0.015% or less

[0045] P is an inevitable impurity element that deteriorates low temperature toughness and increases the hardness of the surface layer and the center segregation part, thereby deteriorating SSCC resistance and HIC resistance. If the P content exceeds 0.015%, this tendency becomes significant, so the P content is 0.015% or less, preferably 0.008% or less. It should be noted that the lower the P content, the better, but from the viewpoint of refining cost, the P content is preferably 0.001% or more.

[0046] S: 0.0015% or less

[0047] S is an inevitable impurity element, and it becomes MnS inclusions in steel to deteriorate HIC resistance, so it is preferably as small as possible. From this point of view, the S content is 0.0015% or less, preferably 0.0010% or less. It should be noted that the lower the S content, the better, but from the viewpoint of refining cost, the S content is preferably 0.0002% or more.

[0048] Al: 0.010~0.080%

[0049] Al is added as a deoxidizer, but when the Al content is less than 0.010%, its effect cannot be fully manifested. Therefore, the Al content is 0.010% or more, preferably 0.015% or more. On the other hand, if the Al content exceeds 0.080%, the non-thermal stress of the steel increases and the low-temperature toughness deteriorates. Therefore, the Al content is 0.080% or less, preferably 0.070% or less.

[0050] Cr: 0.05~0.50%

[0051] Cr is an element that is effective in obtaining sufficient strength even at a low C content, similar to Mn. To obtain this effect, the Cr content needs to be 0.05% or more. However, if the Cr content is too high, the hardenability becomes excessive, so the hardness of the surface layer and the center segregation portion increases during accelerated cooling, and the SSCC resistance and HIC resistance deteriorate. In addition, the weldability also deteriorates. Therefore, the Cr content is 0.50% or less, preferably 0.45% or less.

[0052] Nb: 0.005~0.080%

[0053] If Nb exists in the form of solid solution Nb, the non-recrystallization temperature range during controlled rolling is expanded, which is beneficial to the improvement of low-temperature toughness. However, when the Nb content is less than 0.005%, its effect cannot be fully reflected. Therefore, the Nb content is 0.005% or more, preferably 0.010% or more. On the other hand, if the Nb content exceeds 0.080%, coarse carbides are precipitated during solidification, and the HIC resistance is deteriorated. Therefore, the Nb content is 0.080% or less, preferably 0.060% or less.

[0054] N: 0.0010~0.0080%

[0055] N is effective in improving strength, but when the N content is less than 0.0010%, sufficient strength cannot be ensured. Therefore, the N content is 0.0010% or more, preferably 0.0015% or more. On the other hand, if the N content exceeds 0.0080%, the hardness of the surface layer and the center segregation part increases during accelerated cooling, so the SSCC resistance and HIC resistance deteriorate. In addition, the low temperature toughness also deteriorates. Therefore, the N content is 0.0080% or less, preferably 0.0070% or less.

[0056] Ca: 0.0005~0.0050%

[0057] Ca is an element effective in improving HIC resistance by controlling the morphology of sulfide inclusions, but when the Ca content is less than 0.0005%, the effect of addition is insufficient. Therefore, the Ca content is 0.0005% or more, preferably 0.0008% or more. On the other hand, when the Ca content exceeds 0.0050%, not only the above effect is saturated, but also the cleanliness of the steel decreases, which deteriorates the HIC resistance. Therefore, the Ca content is 0.0050% or less, preferably 0.0045% or less.

[0058] The basic components of the composition in the present invention have been described above. However, in the present invention, in order to further improve the strength and toughness of the steel sheet, the composition may contain any one or more selected from Cu, Ni and Mo within the following ranges.

[0059] Cu: 0.30% or less

[0060] Cu is an element effective in improving low temperature toughness and increasing strength. To obtain this effect, the Cu content is preferably 0.05% or more. However, when the Cu content exceeds 0.30%, fine cracks called fissures (Fisher) are easily generated in an environment with a low hydrogen sulfide partial pressure of less than 1 bar, so the SSCC resistance is deteriorated. Therefore, when Cu is contained, the Cu content is 0.30% or less, preferably 0.25% or less.

[0061] Ni: 0.10% or less

[0062] Ni is an element effective in improving low temperature toughness and increasing strength. To obtain this effect, the Ni content is preferably 0.01% or more. However, when the Ni content exceeds 0.10%, fine cracks called fissures are easily generated in an environment with a low hydrogen sulfide partial pressure of less than 1 bar, so the SSCC resistance deteriorates. Therefore, when Ni is contained, the Ni content is 0.10% or less, preferably 0.02% or less.

[0063] Mo: 0.50% or less

[0064] Mo is an element effective in improving low temperature toughness and increasing strength. To obtain this effect, the Mo content is preferably 0.01% or more, and more preferably 0.10% or more. On the other hand, if the Mo content is too high, fine cracks called fissures are easily generated in an environment with a low hydrogen sulfide partial pressure of less than 1 bar, and the SSCC resistance is deteriorated. In addition, weldability is also deteriorated. Therefore, when Mo is contained, the Mo content is 0.50% or less, preferably 0.40% or less.

[0065] The component composition in the present invention may contain any one or more selected from V, Ti, Zr, Mg and REM within the following range.

[0066] One or more selected from V: 0.005 to 0.1%, Ti: 0.005 to 0.1%, Zr: 0.0005 to 0.02%, Mg: 0.0005 to 0.02%, and REM: 0.0005 to 0.02%

[0067] V and Ti are both elements that can be arbitrarily contained in order to improve the strength and low-temperature toughness of the steel plate. Each element cannot fully demonstrate its effect when the content is less than 0.005%. Therefore, when these elements are contained, the content is preferably 0.005% or more. On the other hand, if the content of these elements exceeds 0.1%, the toughness of the weld deteriorates. Therefore, when these elements are contained, the content is preferably 0.1% or less.

[0068] Zr, Mg and REM are elements that can be arbitrarily contained in order to improve low-temperature toughness by grain refinement or to improve crack resistance by controlling the properties of inclusions. Each element cannot fully demonstrate its effect when the content is less than 0.0005%. Therefore, when these elements are contained, the content is preferably 0.0005% or more. On the other hand, if the content of these elements exceeds 0.02%, the effect is saturated, so when these elements are contained, the content is preferably 0.02% or less.

[0069] The present invention discloses a technique for improving the low temperature toughness of a high-strength steel pipe using a high-strength steel plate for sour-resistant line pipes. However, as sour-resistant performance, HIC resistance must of course be satisfied. For example, the CP value obtained by the following formula (1) is preferably 1.00 or less. It should be noted that elements not contained may be substituted with 0.

[0070] CP=4.46[%C]+2.37[%Mn] / 6+(1.74[%Cu]+1.7[%Ni]) / 15+(1.18[%Cr]+1.95[%Mo]+1.74[%V]) / 5+22.36[%P]···(1)

[0071] Here, [%X] represents the content of element X in steel (mass %).

[0072] Here, the above CP value is a formula designed by estimating the material of the center segregation portion from the content of each alloy element. The higher the CP value of the above formula (1), the higher the component concentration of the center segregation portion, and the higher the hardness of the center segregation portion. Therefore, by making the CP value obtained in the above formula (1) 1.00 or less, the HIC resistance can be improved. In addition, the lower the CP value, the lower the hardness of the center segregation portion, so when higher HIC resistance is required, the upper limit can be 0.95. The lower limit of CP is not particularly limited, and the CP value can be 0.70 or more.

[0073] It should be noted that the remainder other than the above elements is composed of Fe and inevitable impurities. However, as long as the effects of the present invention are not hindered, other trace elements may be contained. For example, O is an element inevitably contained in steel, and as long as its content is 0.0050% or less, preferably 0.0040% or less, it is permitted in the present invention.

[0074] [Structure 0.25mm below the steel plate surface]

[0075] Next, the steel structure of the high-strength steel plate for sour-resistant line pipe of the present invention is described. In order to improve the SSCC resistance and achieve a high strength of 535 MPa or more in tensile strength, the steel structure 0.25 mm below the surface of the steel plate is made of granular bainite and tempered island martensite. If island martensite is mixed in the steel structure, the low-temperature toughness and the SSCC resistance will deteriorate, but if the cooling stop temperature is high enough, it will become tempered island martensite (TMA) after cooling is stopped, so the low-temperature toughness and SSCC resistance can be prevented from deteriorating. In addition, compared with other bainite or other martensite, granular bainite and tempered island martensite have lower hardness and can improve SSCC resistance.

[0076] Here, the steel structure of 0.25 mm below the surface of the steel plate in the present invention is preferably mainly composed of granular bainite. Specifically, it is preferred that the granular bainite is 90% or more in terms of area ratio, and the tempered island martensite is 10% or less in terms of area ratio. On the other hand, since the tempered island martensite must also exist, it is preferred that the granular bainite is 99% or less in terms of area ratio, and the tempered island martensite is 1% or more in terms of area ratio.

[0077] It should be noted that in the high-strength steel plate of the present invention, as long as the steel structure 0.25 mm below the steel plate surface satisfies the above conditions, the extreme surface layer ranging from the steel plate surface to a depth of 0.25 mm also has the same steel structure, and as a result, the above-mentioned effect of improving SSCC resistance can be obtained.

[0078] In order to obtain the effects of improved SSCC resistance and high strength more fully, it is preferred that the entire steel structure of the steel plate including the portion other than the surface portion satisfies the above conditions. Specifically, the structure at the center of the plate thickness representing the "portion other than the surface portion" satisfies the above conditions.

[0079] [Maximum grain size and average grain size at the center of the plate thickness]

[0080] In the present invention, it is important to suppress the formation of coarse grains. That is, if the maximum crystal grain size or the average crystal grain size is large, the low-temperature toughness deteriorates. In particular, if the maximum crystal grain size at the center of the plate thickness exceeds 80 μm, the coarse grains are likely to become the starting point of damage, so the low-temperature toughness is significantly deteriorated. When the average crystal grain size exceeds 20 μm, the low-temperature toughness will also deteriorate. Therefore, the maximum crystal grain size at the center of the plate thickness must be 80 μm or less, and the average crystal grain size must be 20 μm or less. The smaller the maximum crystal grain size and the average crystal grain size at the center of the plate thickness, the better, and there is no special lower limit. In the present invention, the maximum crystal grain size at the center of the plate thickness can be 50 μm or more, and the average crystal grain size can be 10 μm or more. It should be noted that "crystal grain size" refers to the grain size of the grains within the range of 1 mm × 1 mm measured at the center of the plate thickness, and the maximum value thereof is used as the "maximum crystal grain size", and the average value is used as the "average crystal grain size", and the crystal grain size is defined as the equivalent circle diameter. Here, the center of the plate thickness refers to a position that is 1 / 2 of the total thickness.

[0081] [Ductile-brittle transition temperature]

[0082] The high strength steel sheet of the present invention has a ductile-brittle transition temperature of -100°C or less in a Charpy impact test. This ensures excellent low temperature toughness. The lower limit of the ductile-brittle transition temperature is not particularly limited, and in the present invention, the ductile-brittle transition temperature may be -120°C or more.

[0083] [Tensile strength]

[0084] The high-strength steel plate of the present invention is a steel plate for steel pipes having a strength of API 5L X65 grade or higher, and therefore has a tensile strength of 535 MPa or higher. The upper limit of the tensile strength is not particularly limited, and the tensile strength of the high-strength steel plate of the present invention may be 600 MPa or lower.

[0085] [Thickness of steel plate]

[0086] The plate thickness of the high-strength steel plate of the present invention is not particularly limited, but when the plate thickness is 12 to 39 mm, it is suitable for the above-mentioned applications.

[0087] [Method for producing high-strength steel plate for acid-resistant line pipe]

[0088] The manufacturing method and manufacturing conditions for manufacturing the above-mentioned high-strength steel plate for sour-resistant line pipe are specifically described below. The manufacturing method of the present invention is to heat a steel sheet (e.g., slab) having the above-mentioned composition, perform hot rolling to produce a steel plate, and then perform controlled cooling of the steel plate under prescribed conditions.

[0089] 〔Steel sheet heating temperature〕

[0090] Steel sheet heating temperature: 1000~1250℃

[0091] When the steel sheet heating temperature is lower than 1000°C, the solid solution of carbides is insufficient, the amount of solid solution strengthening decreases, and the necessary strength cannot be obtained. Therefore, the steel sheet heating temperature is set to 1000°C or higher, preferably 1030°C or higher. On the other hand, if the steel sheet heating temperature exceeds 1250°C, the grains are extremely coarsened and the toughness deteriorates. Therefore, the steel sheet heating temperature is set to 1250°C or lower, preferably 1200°C or lower. It should be noted that this temperature is the furnace temperature of the heating furnace, and the steel sheet is heated to this temperature until the center.

[0092] [Hot rolling conditions]

[0093] Total reduction ratio in the recrystallization temperature range: 35% to 55%

[0094] In order to make the maximum grains fine, it is necessary to promote the recrystallization of grains and suppress the formation of coarse grains during hot rolling in the recrystallization temperature zone. When the total reduction rate in the recrystallization temperature zone is less than 35%, the recrystallization is insufficient, so coarse grains remain and the low-temperature toughness deteriorates. Therefore, the total reduction rate in the recrystallization temperature zone is set to be 35% or more, preferably 38% or more. On the other hand, if the total reduction rate in the recrystallization temperature zone exceeds 55%, the coarsening of the maximum grains can be suppressed, but the reduction rate in the non-recrystallization temperature zone is insufficient, so the average crystal grain size cannot be made less than 20μm, and the low-temperature toughness deteriorates. Therefore, the total reduction rate in the recrystallization temperature zone is set to be 55% or less, preferably 52% or less. Here, the "recrystallization temperature zone" refers to the temperature zone above Tnr obtained by the following formula. It should be noted that the surface temperature of the steel plate can be measured using a radiation thermometer, etc., and can be converted into the temperature of the steel plate at each plate thickness position.

[0095] Tnr(℃)=174×log{[%Nb]×([%C]+12 / 14[%N])}+1444

[0096] Here, [%X] represents the content of element X in steel (mass %).

[0097] Reduction ratio of the final rolling pass in the recrystallization temperature region: 10% or more

[0098] In addition to making the total reduction rate in the recrystallization temperature zone 35% to 55%, it is also necessary to fully ensure the reduction rate of the final rolling pass in the recrystallization temperature zone to fully promote recrystallization, thereby starting the rolling in the non-recrystallization temperature zone in a state of uniform grains without coarse grains. When the reduction rate of the final rolling pass in the recrystallization temperature zone is less than 10%, the recrystallization is insufficient, so coarse grains grow during the holding time from rough rolling to the start of finish rolling, and the low-temperature toughness deteriorates. Therefore, the reduction rate of the final rolling pass in the recrystallization temperature zone is made to be 10% or more, preferably 11% or more. There is no particular upper limit on the reduction rate of the final rolling pass in the recrystallization temperature zone, the higher the better, and the reduction rate can be 20% or less.

[0099] When rolling in the non-recrystallization temperature region after the rolling in the recrystallization temperature region is completed, a large amount of strain can be introduced under low temperature rolling, which is effective for grain refinement. Therefore, it is preferred to roll at a low temperature as much as possible within the range of the cooling start temperature of the cooling treatment described later.

[0100] In the hot rolling process, in order to obtain high low-temperature toughness, the lower the rolling end temperature, the better. However, from the perspective of ensuring acid resistance even in an environment with high hydrogen sulfide partial pressure, it is necessary to consider setting the rolling end temperature so that the cooling start temperature of the controlled cooling described later needs to be above the Ar3 point with a steel plate surface thermometer. Here, the Ar3 point refers to the ferrite transformation start temperature during cooling, which can be obtained, for example, from the composition of the steel by the following formula. It should be noted that the surface temperature of the steel plate can be measured using a radiation thermometer or the like.

[0101] Ar3 point (℃)=910-310[%C]-80[%Mn]-20[%Cu]-15[%Cr]-

[0102] 55[%Ni]-80[%Mo]

[0103] Here, [%X] represents the content of element X in steel (mass %).

[0104] 〔Controlled cooling conditions〕

[0105] Cooling start temperature: Ar3 point or above based on the steel plate surface thermometer

[0106] When the surface temperature of the steel plate at the start of cooling is lower than the Ar3 point, ferrite is generated before controlled cooling, the strength decreases greatly, and the acid resistance deteriorates. Therefore, the surface temperature of the steel plate at the start of cooling is set to be above the Ar3 point. It should be noted that the surface temperature of the steel plate at the start of cooling is below the rolling end temperature. The upper limit of the surface temperature of the steel plate at the start of cooling is not particularly limited, and the temperature may be below 900°C.

[0107] In order to obtain excellent SSCC resistance and achieve high strength, it is necessary to control the cooling rate 0.25 mm below the surface of the steel plate and in the middle of the plate thickness.

[0108] The average cooling rate from 750°C to 550°C at a temperature of 0.25 mm below the surface of the steel plate is 15 to 35°C / s.

[0109] It is important to slow down the average cooling rate from 750°C to 550°C as measured by the steel plate thermometer at 0.25 mm below the steel plate surface as much as possible to generate granular bainite. The temperature range from 750°C to 550°C is an important temperature range in the bainite transformation, so it is important to control the cooling rate in this temperature range. That is, when the average cooling rate exceeds 35°C / s, lath bainite is generated, and the SSCC resistance after pipe making deteriorates. Therefore, the average cooling rate is 35°C / s or less, preferably 30°C / s or less. On the other hand, when the average cooling rate is lower than 15°C / s, ferrite or pearlite is generated, resulting in insufficient strength, so from the perspective of preventing this phenomenon, the average cooling rate is set to 15°C / s or more.

[0110] It should be noted that cooling with a steel plate temperature of 550°C or less at 0.25 mm below the steel plate surface will not be cooling in a stable nucleate boiling state if the cooling rate is slow, and the structure may deviate in the extreme surface of the steel plate. Therefore, the average cooling rate from 550°C to the cooling stop temperature at a steel plate temperature of 0.25 mm below the steel plate surface is preferably 150°C / s or more. Since the structure may deviate, the average cooling rate is preferably 250°C / s or less.

[0111] Average cooling rate from 750°C to 550°C at the center of the plate thickness: 15°C / s or more

[0112] When the average cooling rate from 750°C to 550°C at the steel plate temperature at the center of the plate thickness is lower than 15°C / s, ferrite will be generated, resulting in reduced strength and deterioration of HIC resistance. Therefore, the average cooling rate is set to 15°C / s or more. From the viewpoint of suppressing the deviation of low-temperature toughness, the average cooling rate is preferably 17°C / s or more. The upper limit of the cooling rate is not particularly limited. In order not to generate lath bainite, the average cooling rate is preferably 35°C / s or less. It should be noted that the cooling to a steel plate temperature of 550°C or less at the center of the plate thickness is not particularly limited. From the viewpoint of suppressing the deviation of low-temperature toughness, the average cooling rate is preferably 15°C / s to 35°C / s.

[0113] It should be noted that the temperature of the steel plate at 0.25 mm below the surface of the steel plate and at the center of the plate thickness cannot be measured directly physically, but the temperature distribution in the plate thickness section can be calculated by differential calculation using, for example, a process computer based on the surface temperature at the start of cooling and the target surface temperature at the stop of cooling measured by a radiation thermometer, and the temperature can be obtained in real time from the result. The temperature at 0.25 mm below the surface of the steel plate in the temperature distribution is defined as the "steel plate temperature at 0.25 mm below the surface of the steel plate" in this specification, and the temperature at the center of the plate thickness in the temperature distribution is defined as the "steel plate temperature at the center of the plate thickness" in this specification.

[0114] Cooling stop temperature: 350~550℃ at the temperature of the steel plate 0.25mm below the surface and at the center of the plate thickness

[0115] If the cooling stop temperature exceeds 550°C, the bainite transformation is incomplete and sufficient strength cannot be obtained. Therefore, the cooling stop temperature is set to 550°C or less. On the other hand, when the cooling stop temperature is lower than 350°C, the island martensite cannot be fully tempered, so the low temperature toughness deteriorates. In addition, when the cooling stop temperature is lower than 250°C, the SSCC resistance also deteriorates. Therefore, the cooling stop temperature is set to 350°C or more, preferably 400°C or more.

[0116] [High-strength steel pipe]

[0117] The high-strength steel plate of the present invention can be formed into a tubular shape by press bending, roll forming, UOE forming, etc., and then the butt joints can be welded to produce a high-strength steel pipe for sour-resistant line pipe (UOE steel pipe, electric-seam steel pipe, spiral steel pipe, etc.) with excellent material uniformity in the steel plate suitable for the transportation of crude oil and natural gas. In addition, by using the high-strength steel plate of the present invention for a steel pipe, a steel pipe with excellent SSCC resistance can be produced even if there is a high hardness region in the weld.

[0118] For example, the UOE steel pipe is manufactured by beveling the end of the steel plate, forming it into a steel pipe shape by C stamping, U stamping, or O stamping, seam welding the butt joint by inner surface welding and outer surface welding, and further through a pipe expansion process as needed. In addition, the welding method can be any method as long as it can obtain sufficient joint strength and joint toughness. From the perspective of excellent welding quality and manufacturing efficiency, submerged arc welding is preferably used. In addition, after being formed into a tubular shape by press bending, the steel pipe formed by seam welding the butt joint can be expanded.

[0119] Example

[0120] Steels (steel types A to AE) having the component compositions shown in Table 1 were formed into slabs by continuous casting, and were subjected to heating, hot rolling, and controlled cooling under the conditions shown in Table 2 to obtain steel plates. Then, the ends of the steel plates were grooved, and after being formed into a steel pipe shape by C-pressing, U-pressing, and O-pressing, the butt joints were seam welded from the inner and outer surfaces by submerged arc welding, and the steel pipes were obtained by a pipe expansion process.

[0121] [Determination of structure and calculation of area ratio as well as calculation of maximum crystal grain size and average crystal grain size]

[0122] A sample for metal structure observation is taken from the center of the length and the center of the width of the steel plate obtained according to the above steps. After the cross section of the sample perpendicular to the width direction of the plate is mirror-polished and etched with colloidal silica, crystal data is collected by EBSD (Electron Backscatter Diffraction) method at 0.25 mm below the surface of the steel plate and the center of the plate thickness (1 / 2 of the total thickness of the plate) in a field of view of 1 mm×1 mm (measurement step: 0.8 μm). After data collection, OIM-Analysis and image processing software (ImageJ) are used to determine the structure at 0.25 mm below the surface of the steel plate and the structure at the center of the plate thickness, calculate the area ratio of each phase, and calculate the maximum crystal grain size and average crystal grain size in the center of the plate thickness. It should be noted that the crystal grain size is defined as the equivalent circle diameter. These measurement results are shown in Table 3.

[0123] [Derivation of ductile-brittle transition temperature]

[0124] A Charpy impact test piece was taken from the half-thickness position so that the long side direction of the test piece coincided with the width direction of the plate, and subjected to the Charpy impact test. The ductile-brittle transition temperature was derived based on the method described in "New mathematical expression of Charpy absorption energy transition curve and fracture toughness evaluation of steel (Journal of the Japan Society of Materials Strength 17 1-13, 1982)". The results are shown in Table 3.

[0125] [Determination of tensile strength and yield strength]

[0126] A total thickness tensile test piece was taken so that the long side direction of the test piece coincided with the plate width direction, and a tensile test was performed to measure the tensile strength and yield strength. The results are shown in Table 3.

[0127] [Evaluation of SSCC resistance]

[0128] like Figure 1As shown, after the test piece (coupon) cut from the obtained steel pipe is flattened, a 5×15×115mm SSCC test piece is taken from the inner surface of the steel pipe. At this time, in addition to the test piece of only the base material without the weld, a test piece containing both the weld and the base material is also taken. The inner surface of the inspected surface is in a state of direct black skin in order to retain the outermost layer. That is, 0.25mm below the surface of the steel plate is included in the test piece. The SSCC test piece thus taken is loaded with a stress of 90% of the actual yield strength (0.5% YS) of each steel pipe, and a four-point bending SSCC test is performed according to the EFC16 standard using the NACE standard TM0177 Solution A solution at a hydrogen sulfide partial pressure of 1 bar.

[0129] In addition, a four-point bending SSCC test was performed in accordance with the EFC16 standard using the NACE standard TM0177 Solution B solution at a hydrogen sulfide partial pressure of 0.1 bar and a carbon dioxide partial pressure of 0.9 bar.

[0130] In addition, a four-point bending SSCC test was performed according to the EFC16 standard using NACE standard TM0177 Solution A at a hydrogen sulfide partial pressure of 16 bar and a carbon dioxide partial pressure of 5 bar.

[0131] After the test pieces were immersed in the solution for 720 hours, in the test pieces of the base material only without the weld part and the test pieces including both the weld part and the base material, the case where no cracks were observed was judged as good SSCC resistance and recorded as ○, and the case where cracks were observed in at least one test piece was judged as poor SSCC resistance and recorded as ×. The evaluation results are shown in Table 3.

[0132] [Evaluation of HIC resistance]

[0133] The HIC resistance was investigated by using NACE standard TM0177 Solution A solution and immersing for 96 hours at a hydrogen sulfide partial pressure of 1 bar. In addition, the HIC test was investigated by using NACE standard TM0177 Solution B solution and immersing for 96 hours at a hydrogen sulfide partial pressure of 0.1 bar + carbon dioxide partial pressure of 0.9 bar. For HIC resistance, the case where the crack area ratio (CAR) in the HIC test is 5% or less is judged as good HIC resistance and recorded as 0, and the case where the crack area ratio (CAR) is greater than 5% is judged as poor HIC resistance and recorded as ×. The evaluation results are shown in Table 3.

[0134] The target range of the present invention is: as a high-strength steel plate for sour-resistant pipeline, the ductile-brittle transition temperature is below -100°C, the tensile strength is above 535 MPa, the steel structure at 0.25 mm below the surface of the steel plate is composed of granular bainite and tempered island martensite, the maximum crystal grain size at the center of the plate thickness is below 80 μm, the average crystal grain size is below 20 μm, no cracks are observed in the SSCC test, and the crack area ratio (CAR) is below 5% in the HIC test.

[0135]

[0136]

[0137]

[0138] As shown in Tables 2 and 3, No. 1 to 10, 39 to 42 are invention examples whose component compositions and manufacturing conditions meet the appropriate range of the present invention. All of them have a ductile-brittle transition temperature of -100°C or less, a tensile strength of 535 MPa or more, a steel structure of granular bainite and tempered island martensite at 0.25 mm below the surface of the steel plate, a maximum crystal grain size of 80 μm or less at the center of the plate thickness, an average crystal grain size of 20 μm or less, and good SSCC resistance and HIC resistance.

[0139] In contrast, the component compositions of the steel plates of No. 11 to 25, 37 to 38 are outside the scope of the present invention. The solid solution strengthening of No. 11, 14, and 37 is insufficient, and the strength is insufficient. No. 12, 15 to 16, 19, and 21 have poor SSCC resistance and HIC resistance due to increased hardness. No. 12 and 21 have deteriorated low-temperature toughness due to a large increase in strength at low temperatures. No. 16 has decreased cleanliness and deteriorated low-temperature toughness. No. 17, 20, 22, and 38 have poor HIC resistance due to the generation of inclusions or carbides. No. 13 and 18 have deteriorated low-temperature toughness due to the increase in non-thermal stress of steel. No. 23 to 25 have deteriorated SSCC resistance under 0.1 bar-H2S + 0.9 bar-CO2 due to the promotion of local corrosion.

[0140] No.26 to No.36 are comparative examples whose component compositions are within the scope of the present invention but whose manufacturing conditions are outside the scope of the present invention. No.26 has insufficient solid solution of carbides due to low slab heating temperature, resulting in low strength. No.27 has coarsened grains and deteriorated low temperature toughness due to high slab heating temperature. No.28 has insufficient total reduction in the recrystallization temperature region, resulting in coarse grains remaining and deteriorating low temperature toughness. No.29 has excessive total reduction in the recrystallization temperature region, resulting in large average grain size and deteriorating low temperature toughness. No.30 has insufficient reduction in the final pass in the recrystallization temperature region, resulting in coarse grains remaining and deteriorating low temperature toughness. No.31 has low strength and deteriorated SSCC resistance under high pressure due to low cooling start temperature, resulting in partial ferrite formation in the surface layer. No.32 has low strength and poor HIC resistance and SSCC resistance under high pressure due to low average cooling rate, resulting in partial ferrite formation up to the center of the plate thickness. In No.33, due to the high average cooling rate, some lath bainite was generated in the surface layer, so the SSCC resistance under high pressure was poor. In No.34, due to the low cooling stop temperature, the island martensite was not fully tempered, so the low temperature toughness was deteriorated. In No.35, due to the further decrease in the cooling stop temperature, both the low temperature toughness and the SSCC resistance under high pressure were deteriorated. In No.36, due to the high cooling stop temperature, some ferrite was generated until the center of the plate thickness, so it had low strength, poor HIC resistance and poor SSCC resistance under high pressure.

[0141] Industrial Applicability

[0142] According to the present invention, a high-strength steel plate for sour-resistant line pipe and a high-strength steel pipe which are excellent in not only HIC resistance and SSCC resistance but also low-temperature toughness can be provided.

Claims

1. A high-strength steel plate for acid-resistant pipeline, characterized in that: The present invention has the following composition, which contains, by mass%, C: 0.030-0.060%, Si: 0.01-0.50%, Mn: 0.80-1.80%, P: 0.015% or less, S: 0.0015% or less, Al: 0.010-0.080%, Cr: 0.05-0.50%, Nb: 0.005-0.080%, N: 0.0010-0.0080% and Ca: 0.0005-0.0050%, and the remainder is composed of Fe and inevitable impurities, The steel structure at 0.25 mm below the surface of the steel plate is composed of granular bainite and tempered island martensite. The maximum grain size at the center of the plate thickness is less than 80μm, and the average grain size is less than 20μm. The ductile-brittle transition temperature in the Charpy impact test is below -100°C. The tensile strength is 535 MPa or more.

2. The high-strength steel plate for acid-resistant line pipe according to claim 1, wherein: The component composition further contains, in mass %, one or more selected from the group consisting of Cu: 0.30% or less, Ni: 0.10% or less, and Mo: 0.50% or less.

3. The high-strength steel plate for sour-resistant line pipe according to claim 1 or 2, wherein: The component composition further contains, in mass%, one or more selected from the group consisting of V: ​​0.005 to 0.1%, Ti: 0.005 to 0.1%, Zr: 0.0005 to 0.02%, Mg: 0.0005 to 0.02%, and REM: 0.0005 to 0.02%.

4. A method for manufacturing a high-strength steel plate for acid-resistant pipeline pipe, characterized in that: A steel sheet having a composition comprising, by mass%, 0.030-0.060% C, 0.01-0.50% Si, 0.80-1.80% Mn, 0.015% or less P, 0.0015% or less S, 0.010-0.080% Al, 0.05-0.50% Cr, 0.005-0.080% Nb, 0.0010-0.0080% N and 0.0005-0.0050% Ca, with the remainder being Fe and unavoidable impurities, is heated to a temperature of 1000-1250° C., Then, the steel sheet is subjected to hot rolling with a total reduction ratio of 35% to 55% in the recrystallization temperature range and a reduction ratio of 10% or more in the final rolling pass in the recrystallization temperature range to produce a steel plate. Then, the steel plate is subjected to controlled cooling under the following conditions: The surface temperature of the steel plate at the beginning of cooling is above the Ar3 point. The average cooling rate from 750°C to 550°C using a steel plate thermometer at 0.25mm below the steel plate surface is 15-35°C / s. The average cooling rate from 750°C to 550°C at the center of the plate thickness is more than 15°C / s. The cooling stop temperature measured by the steel plate thermometer at 0.25 mm below the steel plate surface and at the center of the plate thickness is 350 to 550°C.

5. The method for manufacturing a high-strength steel plate for sour-resistant line pipe according to claim 4, wherein: The component composition further contains, in mass %, one or more selected from the group consisting of Cu: 0.30% or less, Ni: 0.10% or less, and Mo: 0.50% or less.

6. The method for producing a high-strength steel plate for sour-resistant line pipe according to claim 4 or 5, wherein: The component composition further contains, in mass%, one or more selected from the group consisting of V: ​​0.005 to 0.1%, Ti: 0.005 to 0.1%, Zr: 0.0005 to 0.02%, Mg: 0.0005 to 0.02%, and REM: 0.0005 to 0.02%.

7. A high-strength steel pipe using the high-strength steel plate for sour-resistant line pipe according to claim 1 or 2.

8. A high-strength steel pipe using the high-strength steel plate for sour-resistant line pipe according to claim 3.

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