Steel pipe and its manufacturing method

Abstract

Provided are: a steel pipe stably having excellent SSCC resistance; and a method for manufacturing the same. This steel pipe has a component composition containing, in mass%, 0.020-0.080% of C, 0.50-1.80% of Mn, 0.01-0.50% of Mo, 0.0010-0.0080% of N, 0.01-0.50% of Si, 0.015% or less of P, 0.0015% or less of S, 0.010-0.080% of Al, and 0.0005-0.0050% of Ca, the remaining portion being Fe and unavoidable impurities. The structure at a position 0.25 mm outward in the pipe radial direction from the inner circumferential surface of the steel pipe is a bainite structure. The area proportion of bainite having a plane direction in which the crystal plane direction / {110 / } is oriented within 15° in the bainite structure is 30.0% or less.

Description

[Technical Field]

[0001] The present invention relates to a steel pipe suitable for use as a line pipe for transporting crude oil or natural gas, particularly a steel pipe having excellent resistance to sulfide stress corrosion cracking (SSCC resistance) and suitable for use in sour environments containing hydrogen sulfide. The present invention also relates to a method for manufacturing the steel pipe. [Background technology]

[0002] In general, steel pipes are produced by forming steel plates produced by a plate mill or a hot rolling mill into steel pipes by UOE forming, press bending, roll forming, or the like.

[0003] Steel pipes for pipelines (line pipes) used to transport oil and natural gas are required to have various strengths, toughness, and weldability depending on the environment they are used in. Furthermore, line pipes used in sour environments containing hydrogen sulfide (H2S) from oil and natural gas require properties that can suppress hydrogen induced cracking (HIC) and sulfide stress corrosion cracking (SSCC), known as sour resistance.

[0004] HIC occurs when hydrogen ions from corrosion reactions are adsorbed onto the steel surface, penetrate into the steel as atomic hydrogen, and then diffuse and accumulate around non-metallic inclusions such as MnS and hard second phase structures in the steel, becoming molecular hydrogen, which then causes cracks due to the internal pressure. HIC is a problem in linepipes, which have a relatively low level of strength compared to oil country tubular goods, and many countermeasures have been disclosed.

[0005] On the other hand, SSCC is known to occur in the high-hardness region of welds in oil country tubular goods and linepipe, and has generally not been considered a major problem in linepipe with relatively low hardness. However, it has been found that in linepipe with relatively high hardness, SSCC may occur due to localized corrosion in the base metal. Therefore, it is considered important to suppress localized corrosion in the base metal and improve SSCC resistance in linepipe used in sour environments.

[0006] Typically, the so-called Thermo-Mechanical Control Process (TMCP), which combines controlled rolling and controlled cooling, is used to manufacture high-strength steel plates for line pipes. Increasing the cooling rate during controlled cooling is an effective way to increase the strength of steel plates using this TMCP technology. However, when controlled cooling is performed at a high cooling rate, the surface layer of the steel plate is rapidly cooled, resulting in a higher hardness in the surface layer compared to the interior of the steel plate. Furthermore, in the case of steel pipes, work hardening occurs when the steel plate is formed into a tubular shape, which increases the hardness of the surface layer of the steel pipe, resulting in a decrease in SSCC resistance.

[0007] In order to solve the above problem, for example, Patent Document 1 discloses a method for determining the dislocation density of a steel sheet at a depth of 0.25 mm below the surface of the steel sheet, where the dislocation density is 1.0 × 10 14 ~7.0×10 14 (m -2 ) and controlling the variation in Vickers hardness, thereby improving SSCC resistance, a high-strength steel plate for sour-resistant line pipes with a tensile strength of 520 MPa or more is disclosed.

[0008] Furthermore, Patent Document 2 discloses a high-strength steel plate for sour service line pipes having a tensile strength of 520 MPa or more, in which the steel structure 0.25 mm below the surface of the steel plate is a bainite structure, and the area ratio of crystal grains in the bainite having a KAM (Kernel Average Misorientation) value of 0.4 or more is set to 50% or less, thereby improving SSCC resistance. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] International Publication No. 2020 / 067209 [Patent Document 2] International Publication No. 2021 / 020220 Summary of the Invention [Problem to be solved by the invention]

[0010] Steel pipes manufactured using steel plates with controlled dislocation density and hardness variations, as described in Patent Document 1, have excellent SSCC resistance. Although the steel pipes thus obtained have sufficient SSCC resistance, there is room for improvement in that these properties cannot be stably obtained.

[0011] Furthermore, a steel pipe manufactured using a steel plate in which the KAM value is controlled in the bainite structure 0.25 mm below the surface of the steel plate, as described in Patent Document 2, has excellent SSCC resistance. Although the steel pipe thus obtained has sufficient SSCC resistance, there is room for improvement in that the property cannot be stably obtained.

[0012] The present invention has been made in view of the above circumstances, and aims to provide a high-strength steel pipe that stably exhibits excellent SSCC resistance and a method for manufacturing the same. Here, "high strength" means a tensile strength of 520 MPa or more. [Means for solving the problem]

[0013] In order to solve the above problems, the present inventors have conducted extensive research into the microstructure of the inner surface of a steel pipe before and after evaluation of SSCC resistance, and into the manufacturing method of the steel pipe, and have obtained the following findings. [1] SSCC begins as localized corrosion. Furthermore, localized corrosion is significantly affected by crystal plane orientation. Therefore, it is important to control the crystal plane orientation of the microstructure of the extreme surface layer of the inner surface of steel pipes, which are exposed to sour environments, in order to suppress localized corrosion, which is the initiation point of SSCC. [2] In order to consistently achieve excellent SSCC resistance, it is important to control the area fraction of bainite with a crystal plane orientation of {110} within 15° within the structure of the extreme surface layer inside the steel pipe, specifically the bainite structure 0.25 mm below the steel plate surface, to 30% or less. [3] In order to achieve the above-mentioned microstructure, it is important to appropriately control the total reduction rate during rolling when manufacturing steel plates for steel pipes and the cooling rate after rolling.

[0014] The present invention was made based on the above findings and further investigations, and the gist of the present invention is as follows. 1. By mass%, C: 0.020~0.080%, Mn: 0.50 to 1.80% Mo: 0.01 to 0.50%, N: 0.0010~0.0080%, Si: 0.01 to 0.50%, P: 0.015% or less, S: 0.0015% or less, Al: 0.010~0.080% and Ca: 0.0005 to 0.0050% and the balance being Fe and unavoidable impurities, A steel pipe in which the structure at a position 0.25 mm radially outward from the inner peripheral surface of the steel pipe is a bainite structure, and the area ratio of bainite in the bainite structure, having a crystal plane orientation of {110} within 15°, is 30.0% or less.

[0015] 2. The component composition is further expressed as follows in mass %: Cu: 0.30% or less, Ni: 0.10% or less, Cr: 0.50% or less, Nb: 0.1% or less, V: 0.1% or less, Ti: 0.1% or less, Zr: 0.02% or less, Mg: 0.02% or less and REM: 0.02% or less 2. The steel pipe according to 1 above, containing one or more selected from the group consisting of:

[0016] 3. A method for producing a steel pipe, comprising heating a steel material having the chemical composition described in 1 or 2 above to a temperature of 1000°C or higher and 1300°C or lower, and then hot rolling the material under conditions where the total reduction A in the austenite region is 95% or lower to obtain a hot-rolled steel plate, and cooling the hot-rolled steel plate to obtain a steel plate, wherein the average cooling rate B in the temperature range of 750°C to 550°C at the steel plate temperature 0.25 mm below the steel plate surface is 20 to 50°C / s, within a temperature range where the cooling start temperature is (Ar3-10°C) or higher at the steel plate surface and the cooling stop temperature is 550°C or lower at the steel plate temperature 0.25 mm below the steel plate surface, and F, as defined by the following formula (1) regarding the total reduction A and the average cooling rate B, satisfies 0 or higher and 30.00 or lower, and the cooled steel plate is used as a steel pipe. F=3.35A+0.03B-286.27 (1) [Effects of the Invention]

[0017] According to the present invention, excellent SSCC resistance can be stably achieved in steel pipes. Furthermore, according to the method for manufacturing a steel pipe of the present invention, steel pipes having excellent SSCC resistance can be stably manufactured. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 2 is a diagram illustrating the crystal plane orientation in a bainite structure. [Figure 2] FIG. 1 is a diagram showing a method for collecting SSCC test specimens in steel pipes. DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, embodiments of the present invention will be described in detail. Note that the following description shows examples of preferred embodiments of the present invention, and the present invention is not limited thereto.

[0020] [Steel pipe] A steel pipe according to one embodiment of the present invention is manufactured using a steel plate having a specific chemical composition and mechanical properties. The steel pipe is characterized in that, after the steel plate is formed into a steel pipe, the innermost surface layer of the steel pipe has a specific microstructure. The reasons for limiting the chemical composition, mechanical properties, and microstructure are explained below. Regarding the chemical composition, since the steel pipe is manufactured using a steel plate, the chemical composition of the steel pipe is the same as that of the steel plate used.

[0021] [Component composition] First, the appropriate range of the chemical composition of the steel pipe, i.e., the steel plate, and the reasons for limiting it will be explained. In the following explanation, "%" as a unit of content represents "% by mass" unless otherwise specified.

[0022] In the chemical composition of the steel pipe according to the present invention, it is important to specify the C content, Mn content, Mo content and N content in order to obtain the strength and SSCC resistance required for the steel pipe.

[0023] C: 0.020 to 0.080% C is an element that contributes to improving the strength of steel sheet. If the C content is less than 0.020%, sufficient strength cannot be ensured, so the C content is set to 0.020% or more. Preferably, the C content is set to 0.025% or more. On the other hand, if the C content exceeds 0.080%, the hardness of the surface layer and central segregation increases during accelerated cooling, resulting in deterioration of SSCC resistance and HIC resistance. In addition, the toughness of the steel sheet also deteriorates. For this reason, the C content is set to 0.080% or less. Preferably, the C content is set to 0.070% or less.

[0024] Mn: 0.50 to 1.80% Mn is an element that contributes to improving the strength and toughness of steel plate, and if the Mn content is less than 0.50%, the above-mentioned effects are not fully exhibited. Therefore, the Mn content is set to 0.50% or more. Preferably, the Mn content is set to 0.80% or more. On the other hand, if the Mn content exceeds 1.80%, the hardness of the surface layer and central segregation increases during accelerated cooling, resulting in deterioration of SSCC resistance and HIC resistance. Weldability also deteriorates. Therefore, the Mn content is set to 1.80% or less. Preferably, the Mn content is set to 1.70% or less.

[0025] Mo: 0.01 to 0.50% Mo is an element that contributes to improving the strength and toughness of steel plate and improving SSCC resistance. If the Mo content is less than 0.01%, the above-mentioned effects are not fully exhibited. Therefore, the Mo content is set to 0.01% or more. Preferably, the Mo content is set to 0.10% or more. On the other hand, if the Mo content is too high, the hardenability becomes excessive, which increases hardness and deteriorates SSCC resistance. In addition, weldability also deteriorates. Therefore, the Mo content is set to 0.50% or less. Preferably, the Mo content is set to 0.40% or less.

[0026] N: 0.0010~0.0080% N is an element that contributes to improving the strength of steel sheet, and therefore, N is contained in an amount of 0.0010% or more. Preferably, the N content is 0.0015% or more. On the other hand, if the N content exceeds 0.0080%, the hardness of the surface layer and central segregation increases during accelerated cooling, resulting in a deterioration in SSCC resistance and HIC resistance. In addition, the toughness of the steel sheet also deteriorates. For this reason, the N content is set to 0.0080% or less. Preferably, the N content is set to 0.0070% or less.

[0027] In the steel pipe according to the present invention, it is important to control the contents of C, Mn, Mo, and N. Furthermore, since it is desirable for steel pipes used in sour environments to ensure HIC resistance and toughness, the composition is such that the following components are added to the above components.

[0028] Si: 0.01 to 0.50% Si is added for deoxidation, but if the Si content is less than 0.01%, the deoxidation effect is insufficient. Therefore, the Si content is set to 0.01% or more. Preferably, the Si content is set to 0.05% or more. On the other hand, if the Si content exceeds 0.50%, the toughness and weldability of the steel plate deteriorate. Therefore, the Si content is set to 0.50% or less. Preferably, the Si content is set to 0.45% or less.

[0029] P:0.015% or less P is an inevitable impurity element that deteriorates weldability and increases the hardness of the center segregation, thereby deteriorating HIC resistance. This tendency becomes more pronounced when the P content exceeds 0.015%, so the P content is set to 0.015% or less. Preferably, the P content is set to 0.008% or less. Meanwhile, since it is desirable to reduce P as much as possible to obtain the aforementioned effects, the lower limit of the P content is not particularly limited and may be 0%. However, excessive reduction leads to increased refining costs, so from the viewpoint of industrial production, the P content is preferably set to 0.001% or more.

[0030] S: 0.0015% or less S is an inevitable impurity element that forms MnS inclusions in steel sheets and deteriorates HIC resistance, so a small amount is preferable, but an S content of up to 0.0015% is acceptable. Preferably, the S content is 0.0010% or less. On the other hand, since it is desirable to reduce S as much as possible to obtain the above-mentioned effects, the lower limit of the S content is not particularly limited and may be 0%. However, excessive reduction leads to increased refining costs, so from the viewpoint of industrial production, the S content is preferably 0.0002% or more.

[0031] Al: 0.010 to 0.080% Al is added as a deoxidizer, but if the Al content is less than 0.010%, the addition effect is ineffective. Therefore, the Al content is set to 0.010% or more, preferably 0.015% or more. On the other hand, if the Al content exceeds 0.080%, the cleanliness of the steel decreases and the toughness of the steel plate deteriorates, so the Al content is set to 0.080% or less. Preferably, the Al content is set to 0.070% or less.

[0032] Ca: 0.0005 to 0.0050% Ca is an element effective in improving HIC resistance by controlling the morphology of sulfide-based inclusions, but if the Ca content is less than 0.0005%, the effect of adding it is insufficient. Therefore, the Ca content is set to 0.0005% or more, preferably 0.0008% or more. On the other hand, if the Ca content exceeds 0.0050%, not only does the above-mentioned effect saturate, but the cleanliness of the steel sheet also decreases, resulting in a deterioration in HIC resistance. Therefore, the Ca content is set to 0.0050% or less, preferably 0.0045% or less.

[0033] A steel pipe according to one embodiment of the present invention has a composition containing the above elements, with the balance being Fe and unavoidable impurities.

[0034] In addition, the chemical composition of the steel pipe in another embodiment of the present invention may optionally further contain one or more elements selected from the group consisting of Cu, Ni, Cr, Nb, V, Ti, Zr, Mg and REM, for the purpose of further improving the properties of the steel pipe.

[0035] Cu:0.30% or less Cu is an element effective in improving the toughness and increasing the strength of steel sheets. When Cu is added, the Cu content is preferably 0.05% or more to obtain this effect. However, if the Cu content exceeds 0.30%, microcracks known as Fischer cracks are likely to occur in an environment with a hydrogen sulfide partial pressure of less than 1 bar. Therefore, when Cu is added, the upper limit is set to 0.30%, and preferably 0.20% or less.

[0036] Ni: 0.10% or less Ni is an element that is effective in improving the toughness and increasing the strength of steel sheets. When Ni is added, the Ni content is preferably 0.01% or more to obtain this effect. However, if the Ni content exceeds 0.10%, microcracks called Fischer cracks are likely to occur in an environment with a hydrogen sulfide partial pressure of less than 1 bar. Therefore, when Ni is added, the upper limit is set to 0.10%, and preferably 0.02% or less.

[0037] Cr:0.50% or less Like Mn, Cr is an effective element for obtaining sufficient strength in steel sheets even with low C content. When adding Cr, the Cr content is preferably 0.05% or more to obtain the above effect. However, if the Cr content exceeds 0.50%, the hardenability becomes excessive, which increases hardness and deteriorates SSCC resistance. Weldability also deteriorates. For this reason, the upper limit of Cr addition is set to 0.50%.

[0038] Nb: 0.1% or less Nb is an element that can be added optionally to increase the strength and toughness of steel sheets. When Nb is added, the Nb content is preferably 0.005% or more to obtain the above-mentioned effect. However, if the Nb content exceeds 0.1%, the toughness of the welded joint deteriorates. Therefore, when Nb is added, the upper limit is set to 0.1%.

[0039] V: 0.1% or less Like Nb, V is an element that can be added optionally to increase the strength and toughness of steel sheets. When V is added, the V content is preferably 0.005% or more to obtain the above-mentioned effect. However, if the V content exceeds 0.1%, the toughness of the welded joint deteriorates. Therefore, when V is added, the upper limit is set to 0.1%.

[0040] Ti: 0.1% or less Like Nb and V, Ti is an element that can be added optionally to increase the strength and toughness of steel sheets. When Ti is added, the Ti content is preferably 0.005% or more to obtain the above-mentioned effects. However, if the Ti content exceeds 0.1%, the toughness of the welded joint deteriorates. Therefore, when Ti is added, the upper limit is set to 0.1%.

[0041] Zr: 0.02% or less Zr is an element that can be added as needed to improve the toughness of steel sheets by refining crystal grains and to improve crack resistance by controlling the properties of inclusions. When Zr is added, the Zr content is preferably 0.0005% or more to obtain these effects. However, when the Zr content exceeds 0.02%, the effects saturate. Therefore, when Zr is added, the upper limit is set to 0.02%.

[0042] Mg: 0.02% or less Like Zr, Mg is an element that can be added as needed to improve the toughness of steel sheets by refining crystal grains and to improve crack resistance by controlling the properties of inclusions. When Mg is added, the Mg content is preferably 0.0005% or more to obtain the above effects. However, if the Mg content exceeds 0.02%, the above effects become saturated. Therefore, the upper limit of Mg addition is set to 0.02%.

[0043] REM: 0.02% or less Like Zr and Mg, REM is an element that can be added optionally to improve the toughness of steel sheets by refining crystal grains and to improve crack resistance by controlling the properties of inclusions. When REM is added, the REM content is preferably 0.0005% or more to obtain the above effects. However, when the REM content exceeds 0.02%, the above effects saturate. Therefore, when REM is added, the upper limit is set to 0.02%.

[0044] Although the present disclosure discloses a technique for improving the SSCC resistance of steel pipes, it is preferable that sour gas resistance performance also satisfies HIC resistance as well as SSCC resistance. To improve HIC resistance, it is preferable that the CP value calculated by the following formula (2) in the chemical composition of the steel pipe be 1.00 or less. 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] ···(2) However, [%X] in the above formula (2) represents the content (mass%) of the X element, and is set to 0 if the element is not contained.

[0045] The CP value is a formula devised to estimate the material quality of the center segregation region from the content of each alloying element. The higher the CP value defined by formula (2), the higher the concentration of elements in the center segregation region, and the higher the hardness of the center segregation region. Therefore, by setting the CP value calculated by formula (2) to 1.00 or less, it is possible to suppress the occurrence of cracks in the HIC test. Furthermore, since the lower the CP value, the lower the hardness of the center segregation region, so if even higher HIC resistance is required, the upper limit can be set to 0.95.

[0046] The balance other than the above elements consists of Fe and unavoidable impurities. However, other trace elements may be included as long as they do not impair the effects of the present invention. For example, O is an element that is inevitably contained in steel sheet, but its content is permissible in the present invention as long as it is 0.0050% or less, preferably 0.0040% or less.

[0047] [Microstructure] Next, the microstructure of the steel pipe of the present invention will be described. In one embodiment of the present invention, the microstructure at a position 0.25 mm radially outward from the inner peripheral surface of the steel pipe (hereinafter also referred to as 0.25 mm below the inner surface of the steel pipe) is a bainite structure mainly composed of bainite. Here, "mainly composed of bainite" means that the area ratio of bainite is 95% or more.

[0048] Bainite structure In order to improve SSCC resistance by maintaining a constant maximum hardness 0.25 mm below the surface of the inner steel pipe, the microstructure 0.25 mm below the surface of the inner steel pipe must be a bainite structure. In particular, if hard phases such as martensite or martensite islands (MA) form in the surface layer from the surface of the inner steel pipe to a depth of 0.25 mm, the hardness of the surface layer on the inner steel pipe increases, increasing the hardness variation in the surface layer on the inner steel pipe and hindering material uniformity. Therefore, the microstructure of the surface layer on the inner steel pipe must be a bainite structure.

[0049] Here, the bainite structure includes structures known as lath bainite or granular bainite, which contribute to transformation strengthening and transform during or after accelerated cooling. The presence of heterogeneous structures, such as ferrite, martensite, pearlite, island martensite, and retained austenite, in the bainite structure can reduce the strength and toughness of the steel pipe, increase its surface hardness, and other problems. Therefore, the smaller the area fraction of structures other than bainite, the better. However, if the area fraction of structures other than bainite is sufficiently low, their effects can be ignored, and a certain amount is acceptable. Specifically, the total area fraction of structures other than bainite (ferrite, martensite, pearlite, island martensite, retained austenite, etc.) is preferably less than 5%.

[0050] Furthermore, it is essential that the area ratio of bainite having a crystal plane orientation of {110} within 15° in the bainite structure be 30% or less. The reason for controlling the microstructure in the surface layer region of the inner steel pipe is that the inside of the steel pipe is in a sour environment and comes into contact with hydrogen sulfide. The reason for controlling the microstructure 0.25 mm below the inner steel pipe surface is that the crack depth at which the occurrence of SSCC is judged is at this 0.25 mm position.

[0051] Area ratio of bainite with crystal plane orientation {110} oriented within 15°: 30.0% or less The present inventors discovered that in sour environments, the {110} crystal plane orientation of the bainite structure serves as the preferential dissolution plane and corrodes (localized corrosion). Furthermore, detailed studies were conducted, focusing on the crystal plane orientation of the bainite structure 0.25 mm below the surface of the inner surface of a steel pipe. Specifically, when the bainite structure is an aggregate of bainite with different crystal plane orientations on either side of a high-angle grain boundary with a misorientation of 15° or more, the inventors discovered that excellent SSCC resistance can be consistently achieved by limiting the area fraction of bainite with a crystal plane orientation within 15° of the {110} crystal plane orientation to 30.0% or less.

[0052] Here, for ease of understanding, "bainite having a crystal plane orientation in which the {110} crystal plane orientation is oriented within 15°" will be explained with reference to Figure 1, assuming that the inner peripheral surface (curved surface) of a steel pipe is flat. That is, the above-mentioned bainite means bainite having a plane orientation in which the axis l perpendicular to the {110} plane is oriented within 15° with respect to the axis L perpendicular to the plate surface, as shown in Figure 1. Note that, since the inner peripheral surface of a steel pipe is actually a curved surface, the above-mentioned axis L can be considered to be the normal to the above-mentioned inner peripheral surface.

[0053] It is believed that keeping the area fraction of bainite with the specific plane orientation at 30% or less will suppress the onset of localized corrosion, which is believed to be the starting point for SSCC. Specifically, "bainite with a plane orientation in which the {110} crystal plane orientation is oriented within 15°" affects SSCC resistance, and by controlling the area fraction of this bainite to 30% or less, excellent SSCC resistance can be consistently achieved, as shown in the examples described below. On the other hand, it has been found that conventional techniques that control the bainite area fraction by the proportion of crystal grains with a KAM value of 0.4 or more, or by controlling the dislocation density and hardness, sometimes fail to control the bainite area fraction to 30% or less, and in such cases, the desired SSCC resistance is not achieved.

[0054] [Tensile strength of steel pipe] The tensile strength (TS) of the steel pipe according to the present invention is not particularly limited as long as the steel pipe can be manufactured. However, since SSCC resistance is less likely to be a problem in steel pipes with low tensile strength, the steel pipe according to the present invention preferably has a tensile strength of 520 MPa or more. In particular, steel pipes with a high tensile strength of 520 MPa or more can be suitably used for applications such as line pipes.

[0055] [Steel pipe manufacturing method] Next, a method for manufacturing a steel pipe according to one embodiment of the present invention will be described. The steel pipe of the present invention can be produced by heating a steel material having the above-mentioned chemical composition, hot-rolling the material to form a hot-rolled steel sheet, subjecting the hot-rolled steel sheet to controlled cooling under predetermined conditions to form a steel sheet, and forming the steel sheet into a steel pipe.

[0056] Steel material The steel material may be in any form. The steel material may be, for example, a steel slab. The method for producing the steel material is not particularly limited, but the steel material may be produced, for example, by melting molten steel having the above-mentioned composition by a conventional method and casting it. The melting may be carried out by any method, such as a converter, an electric furnace, or an induction furnace. From the viewpoint of productivity, the casting is preferably carried out by a continuous casting method, but may also be carried out by an ingot casting method.

[0057] ·Heating temperature: 1000℃ or more and 1300℃ or less The steel material is heated prior to hot rolling. The heating may be carried out after the steel material obtained by a method such as casting has been cooled, or the obtained steel material may be directly subjected to the heating without being cooled.

[0058] If the heating temperature of the steel material is less than 1000°C, the solid solution of carbides is insufficient and the strength required for the steel plate cannot be obtained. Therefore, the heating temperature is set to 1000°C or higher. On the other hand, if the heating temperature exceeds 1300°C, excessive energy is required, resulting in a decrease in productivity. Furthermore, the toughness of the steel plate also deteriorates. Therefore, the heating temperature is set to 1300°C or lower. Note that the heating temperature is the temperature inside the heating furnace, and the steel material is heated to the heating temperature all the way to the center.

[0059] Total reduction in austenite region: 95% or less Next, the heated steel material is hot-rolled to obtain a hot-rolled steel sheet. To produce a steel pipe satisfying the requirements of the present invention, the total reduction in the austenite region (above the Ar3 transformation point) during hot rolling is set to 95% or less. If the total reduction exceeds 95%, austenite with a {111} crystal plane orientation develops on the surface of the sheet due to shear deformation, resulting in the development of a bainite texture with a {110} crystal plane orientation after hot rolling and cooling. As a result, it becomes impossible to keep the area fraction of bainite with a {110} crystal plane orientation within 15° in the bainite structure to 30% or less, resulting in a deterioration in the SSCC resistance of the steel pipe. For this reason, the total reduction is set to 95% or less. On the other hand, to ensure toughness, it is desirable to set the total reduction in the austenite region to 60% or more. Furthermore, from the viewpoint of improving strength and HIC resistance, hot rolling is preferably performed in the austenite region.

[0060] Rolling end temperature To obtain high toughness as a steel pipe, the lower the hot rolling end temperature, the better, but on the other hand, the rolling efficiency decreases. Therefore, the rolling end temperature needs to be set taking into consideration the toughness and rolling efficiency required for the steel pipe. From the viewpoint of improving the strength and HIC resistance of the steel pipe, it is preferable that the rolling end temperature be set to the Ar3 transformation point or higher. The rolling end temperature is the surface temperature of the steel material, and the surface temperature can be measured using a radiation thermometer or the like.

[0061] Here, the Ar3 transformation point means the ferrite transformation start temperature during cooling, and is calculated according to the following formula (3). Ar3(℃)=910-273[%C]-74[%Mn]-56[%Ni]-16[%Cr]-9[%Mo]-5[%Cu] …(3) However, [%X] in the above formula (3) represents the content (mass%) of the X element, and is set to 0 if the element is not contained.

[0062] ·cooling Next, the hot-rolled steel sheet after hot rolling is cooled in a temperature range from a cooling start temperature to a cooling stop temperature, which will be described later. Cooling start temperature: Steel plate surface temperature (Ar3-10℃) or higher If the cooling start temperature is low, the amount of ferrite generated before cooling will be large. In particular, if the temperature drop from the Ar3 transformation point exceeds 10°C, ferrite will be generated in a volume fraction of more than 5%, resulting in a significant decrease in the strength of the steel pipe and a deterioration in HIC resistance. Therefore, the cooling start temperature is set to (Ar3 - 10°C) or higher. The cooling start temperature is set to be equal to or lower than the rolling end temperature. The cooling start temperature is the surface temperature of the hot-rolled steel sheet and can be measured using a radiation thermometer, etc.

[0063] Cooling rate: Average cooling rate of 50°C / s or less from 750°C to 550°C at a temperature 0.25 mm below the steel plate surface In order to increase the strength of steel sheets, i.e., steel pipes, while reducing hardness variations within the steel sheet and improving the uniformity of material properties, it is important to control the cooling rate of the steel sheet surface. In particular, to obtain the desired microstructure 0.25 mm below the steel sheet surface, it is necessary to control the average cooling rate from 750°C to 550°C at the steel sheet temperature 0.25 mm below the surface to 50°C / s or less.

[0064] By making the average cooling rate as slow as possible, it is possible to create bainite with a crystal plane orientation where the {110} crystal plane orientation is not within 15°. Furthermore, as the average cooling rate is slowed, the maximum hardness can be reduced, thereby improving SSCC resistance. If the average cooling rate exceeds 50°C / s, the area fraction of bainite with a crystal plane orientation where the {110} crystal plane orientation is within 15° increases, resulting in a maximum HV0.5 hardness of 230 at 0.25 mm below the steel sheet surface. Furthermore, the transformation texture develops, increasing the area fraction of bainite with a crystal plane orientation where the {110} crystal plane orientation is within 15°, resulting in a deterioration of SSCC resistance after the steel sheet is formed into a pipe. Therefore, the average cooling rate is set to 50°C / s or less, preferably 30°C / s or less. Although there is no particular lower limit to the average cooling rate, if the average cooling rate is too low, ferrite and pearlite are generated, resulting in a steel sheet, i.e., a steel pipe, lacking in strength. From the viewpoint of preventing this lack of strength, the average cooling rate is preferably 20°C / s or more.

[0065] F(=3.35A+0.03B―286.27):30.00 or less As described above, in order to achieve an area fraction of 30.0% or less of bainite with a crystal plane orientation of {110} within 15° at 0.25 mm below the steel sheet surface, the total reduction in the austenite range and the average cooling rate from 750°C to 550°C at the steel sheet temperature 0.25 mm below the steel sheet surface are important. The inventors conducted further studies and found that simply controlling the total reduction and the average cooling rate within a predetermined range is not sufficient. The inventors discovered that, when the total reduction is A (%), the average cooling rate is B (°C / s), and the area fraction of bainite with a crystal plane orientation of {110} within 15° at F (%), F can be expressed by the following formula (1): If F exceeds 30.00, the area fraction of bainite with a crystal plane orientation of {110} within 15° at the sheet surface exceeds 30.0%, resulting in degraded SSCC resistance. On the other hand, the lower limit of F may be 0 or more. F=3.35A+0.03B―286.27···(1)

[0066] Cooling stop temperature: Steel plate temperature 0.25 mm below the surface of the steel plate is 550°C or less After hot rolling, the hot-rolled steel sheet is cooled by controlled cooling from the cooling start temperature to the cooling stop temperature. Here, by setting the cooling stop temperature to 550°C or less, which is the temperature range of bainite transformation, it is possible to generate a bainite phase. If the cooling stop temperature exceeds 550°C, the bainite transformation becomes incomplete, and sufficient strength cannot be obtained. Therefore, the cooling stop temperature is set to 550°C or less. On the other hand, if the cooling stop temperature is less than 250°C, the dislocation density increases, which significantly increases the hardness of the surface layer when the steel is made into a pipe, and there is a risk of deterioration in SSCC resistance. Therefore, it is preferable that the cooling stop temperature be 250°C or more.

[0067] Furthermore, when cooling to a steel sheet temperature of 550°C or less at a point 0.25 mm below the steel sheet surface, if the cooling rate is slow, the cooling will not occur in a stable nucleate boiling state, which may result in variations in hardness at the very surface layer of the steel sheet, a higher maximum Vickers hardness value, and a deterioration in SSCC resistance. Therefore, the average cooling rate from 550°C at a steel sheet temperature 0.25 mm below the steel sheet surface to the cooling stop temperature is preferably 150°C / s or more. A more preferred average cooling rate is 170°C / s or more. There is no particular upper limit to the average cooling rate, but due to equipment constraints, it is preferably 250°C / s or less.

[0068] Although the steel sheet temperature at 0.25 mm below the surface cannot be physically measured directly, it can be determined in real time from the results of calculating the temperature distribution within the sheet thickness cross section by differential calculation using, for example, a process computer based on the surface temperature of the hot-rolled steel sheet at the start of cooling measured with a radiation thermometer and the surface temperature of the hot-rolled steel sheet at the target time of cooling stop. The temperature at 0.25 mm below the steel sheet surface in the above temperature distribution is referred to as "the steel sheet temperature at 0.25 mm below the steel sheet surface" in this specification.

[0069] ·Steel pipe Next, the cooled steel plate is made into a steel pipe. Methods for making the steel plate into a steel pipe include, for example, a manufacturing method in which the steel plate is formed into a tubular shape by press bending, roll forming, UOE forming, etc., and then the butt joint is welded. By the above-mentioned manufacturing method, it is possible to manufacture steel pipes with excellent material uniformity within the steel plate, which are suitable for transporting crude oil and natural gas, such as UOE steel pipes, electric resistance welded steel pipes, spiral steel pipes, etc.

[0070] For example, UOE steel pipes are manufactured by groove-forming the ends of steel plates, forming them into a steel pipe shape using a C press, a U press, or an O press, then seam-welding the butt joints by internal and external welding, and, if necessary, undergoing a pipe expansion process. Any welding method may be used as long as it provides sufficient joint strength and joint toughness, but submerged arc welding is preferred from the viewpoints of excellent weld quality and manufacturing efficiency. [Example]

[0071] The functions and effects of the present invention will be described below using examples, but the present invention is not limited to the following examples. Steel slabs were produced as raw steel materials using a continuous casting method, using steel with the chemical composition shown in Table 1. The resulting steel slabs were heated to the heating temperatures shown in Table 2 and then hot-rolled at the total reduction in the austenite region shown in Table 2 to produce hot-rolled steel sheets with the thicknesses shown in Table 2. The resulting hot-rolled steel sheets were then subjected to controlled cooling using a water-cooled controlled cooling device under the conditions shown in Table 2 to produce steel sheets. The ends of the resulting steel sheets were then beveled and formed into steel pipe shapes using a C press, U press, and O press. The inner and outer butt joints were then seam-welded by submerged arc welding, and the steel pipes were then produced through a pipe expansion process. Actual pipe products were subjected to an outer coating, and an aging heat treatment was performed at 250°C for 1 hour to simulate the outer coating process.

[0072] [Table 1]

[0073] (Measurement of crystal plane orientation) To evaluate the microstructure of the steel pipe manufactured using the above-described manufacturing method, electron backscatter diffraction (EBSD) measurements were performed. Specifically, a test specimen was taken from the inside of the steel pipe, and after descaling, the inner surface was mirror-polished and electropolished so that the measurement surface was located 0.25 mm below the descaled surface. Next, EBSD measurements were performed on the measurement surface using a scanning electron microscope at an accelerating voltage of 20 kV, a measurement area of 300 × 200 μm, and a step size of 0.2 μm. From the obtained measurement results, EBSD analysis software was used to calculate the area fraction of low-index planes {100} oriented within 15°, the area fraction of low-index planes {110} oriented within 15°, the area fraction of low-index planes {111} oriented within 15°, and the area fraction of high-index planes other than those mentioned above. From these calculation results, the area fraction of crystal planes oriented within 15° of the {110} crystal plane orientation was calculated. The results are shown in Table 2.

[0074] As shown in Table 2, the steel pipes of the invention examples and comparative examples, except for Nos. 41 and 42, had a microstructure mainly composed of bainite. Here, "mainly composed of bainite" means that 95% or more of the steel pipe was bainite. On the other hand, Nos. 41 and 42 had a microstructure mainly composed of bainite.

[0075] (SSCC resistance evaluation) The SSCC resistance of steel pipes manufactured using the above-described manufacturing method was evaluated. As shown in Figure 2, 5 × 25 × 125 mm SSCC test specimens were taken from the inner surface of the steel pipe. The inner surface of the steel pipe, the test surface, was left with black scale to preserve the outermost layer. The SSCC test specimens were subjected to a stress of 90% of the actual yield strength of each steel pipe. A four-point bending SSCC test was performed using NACE Standard TM0177 Solution B at a hydrogen sulfide partial pressure of 0.15 bar and a carbon dioxide partial pressure of 0.75 bar. After 720 hours of immersion, the SSCC resistance was judged to be "good" if no cracks were observed, and "poor" if cracks were observed. Here, "cracks" refers to cracks of 0.25 mm or more in the longitudinal center of the specimen after the four-point bending test. The SSCC resistance evaluation results are shown in Table 2.

[0076] (Measurement of tensile strength) The full-thickness test pieces in the circumferential direction of the steel pipes were used as tensile test pieces, and tensile tests were carried out in accordance with ASTM A370 to measure the tensile strength. The tensile strengths of the obtained steel pipes are also shown in Table 2.

[0077] [Table 2]

[0078] As shown in Table 2, all of the invention examples whose component compositions and manufacturing conditions satisfied the appropriate ranges of the present invention had high strength and good SSCC resistance.

[0079] In contrast, all of the comparative examples, whose chemical compositions and / or manufacturing conditions were outside the ranges of the present invention, exhibited poor tensile strength or SSCC resistance. Specifically, Nos. 3, 4, 11, 12, 21, 22, 25, and 28 had chemical compositions outside the ranges of the present invention, resulting in poor tensile strength or SSCC resistance. Furthermore, No. 36, while its chemical composition was within the range, had a total reduction in the austenite region exceeding 95% and an F value exceeding 30.00, resulting in an area fraction of bainite with a crystal plane orientation of {110} within 15° exceeding 30.0%, thereby resulting in poor SSCC resistance. No. 43, while its chemical composition was within the range, had an average cooling rate of 50°C / s or higher and an F value exceeding 30.00, resulting in an area fraction of bainite with a crystal plane orientation of {110} within 15° exceeding 30.0%, thereby resulting in poor SSCC resistance. In addition, in No. 41, the cooling start temperature was below (Ar3-10°C) at the steel plate surface temperature, resulting in a structure of less than 95% bainite, which is not primarily bainite, and therefore the tensile strength was low.In No. 42, the cooling stop temperature was above 550°C at the steel plate temperature 0.25 mm below the steel plate surface, resulting in a structure of less than 95% bainite, which is not primarily bainite, and therefore the tensile strength was low.

[0080] As can be seen from the results shown in Table 2, the steel pipes of the present invention were excellent in both the tensile strength of the steel plate and the SSCC resistance of the steel pipes. On the other hand, the steel pipes of the comparative examples, which were outside the range of the present invention, had low tensile strength or poor SSCC resistance.

Claims

1. In mass%, C: 0.020-0.080%, Mn: 0.50 to 1.80%, Mo: 0.03-0.30%, N: 0.0010-0.0080%, Si: 0.01 to 0.50%, P: 0.015% or less, S: 0.0015% or less, Al: 0.010 to 0.080% and Ca: 0.0005-0.0050% and the balance being Fe and inevitable impurities, A steel pipe having excellent resistance to sulfide stress corrosion cracking, wherein the structure at a position 0.25 mm radially outward from the inner peripheral surface of the steel pipe is a bainite structure, and the area ratio of bainite in the bainite structure having a crystal plane orientation in which the {110} crystal plane orientation is within 15° is 30.0% or less.

2. The composition of the components is further expressed as follows in mass %: Cu: 0.30% or less, Ni: 0.10% or less, Cr: 0.50% or less, Nb: 0.1% or less, V: 0.1% or less, Ti: 0.1% or less, Zr: 0.02% or less, Mg: 0.02% or less; and REM: 0.02% or less 2. The steel pipe having excellent resistance to sulfide stress corrosion cracking according to claim 1, comprising one or more selected from the group consisting of:

3. A steel material having the component composition according to claim 1 or 2 is heated to a temperature of 1000°C or higher and 1300°C or lower, and then hot-rolled under the condition that the total reduction ratio A in the austenite region is 95% or lower to obtain a hot-rolled steel sheet, and the hot-rolled steel sheet is subjected to a cooling start temperature: a steel sheet surface temperature (Ar ３ a cooling stop temperature: within a temperature range of 550°C or less, the steel plate temperature at 0.25 mm below the surface of the steel plate is 750°C to 550°C, the average cooling rate B is 50°C / s or less, and F, defined by the following formula (1) regarding the total rolling reduction A and the average cooling rate B, satisfies 0 or more and 30.00 or less, and a steel plate is cooled to obtain a steel plate, and the cooled steel plate is used as a steel pipe, wherein a structure at a position 0.25 mm outward in the pipe radial direction from the inner peripheral surface is a bainite structure, and in the bainite structure, an area ratio of bainite having a crystal plane orientation in which a crystal plane orientation of {110} is within 15° is 30.0% or less. F=3.35A+0.03B-286.27...(1)

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