High-strength hot-rolled steel sheet and manufacturing method thereof, and high-strength electric resistance welded steel pipe and manufacturing method thereof

Abstract

This invention provides a high-strength hot-rolled steel plate and its manufacturing method, as well as a high-strength resistance-welded steel pipe and its manufacturing method. In the high-strength hot-rolled steel plate of this invention, the steel microstructure in the center of the plate thickness contains bainite and ferrite at a specific volume fraction, with an average grain diameter of less than 9.0 μm and a dislocation density of 1.0 × 10⁻⁶. 14 m ‑2 ~1.0×10 15 m ‑2 The steel microstructure at a depth of 0.1 mm from the plate surface contains bainite and ferrite at a specific volume fraction, with an average grain diameter of less than 9.0 μm and a dislocation density of 5.0 × 10⁻⁶. 14 m ‑2 ~1.0×10 15 m ‑2 The maximum small-angle grain boundary density is 1.4 × 10⁻⁶. 6 m ‑1 The following high-strength hot-rolled steel plates have a thickness of 15mm or more.

Description

TECHNICAL FIELD

[0001] The present application relates to a high-strength hot-rolled steel sheet suitable for use as a base material for a line pipe or the like, and a method for manufacturing the same. Also, the present application relates to a high-strength electric resistance welded steel pipe suitable for use in a line pipe or the like, and a method for manufacturing the same. BACKGROUND

[0002] For a steel pipe for a line pipe for long-distance transportation of crude oil, natural gas, or the like, high strength is required in order to improve transportation efficiency by internal fluid pressurization.

[0003] In addition, the inner surface of a steel pipe for a line pipe is required to have high resistance to sulfide stress corrosion cracking (SSC) because it is in contact with a fluid having high corrosiveness including hydrogen sulfide.

[0004] Generally, if the strength of a steel material is increased, the resistance to SSC decreases. In particular, in a steel pipe for a line pipe, it is important to reduce the hardness (strength) of the inner surface of the steel pipe in contact with a fluid in order to ensure the resistance to SSC.

[0005] In the manufacture of a base sheet for a high-strength steel pipe for a line pipe, a TMCP (Thermo-Mechanical Control Process) technique combining controlled rolling and accelerated cooling is applied.

[0006] In this TMCP technique, it is important to increase the cooling speed at the time of accelerated cooling, but the cooling speed of the surface of the steel sheet becomes higher than that of the inside of the steel sheet, and thus the hardness of the surface of the steel sheet becomes excessively high when the thickness of the steel sheet is large. Therefore, a steel sheet manufactured by a conventional TMCP technique is difficult to apply to a line pipe from the viewpoint of the resistance to SSC.

[0007] In order to address the above-described problems, a steel sheet or a steel pipe in which the hardness of the surface is controlled is proposed in, for example, Patent Documents 1 to 3.

[0008] PRIOR ART DOCUMENTS

[0009] PATENT DOCUMENTS

[0010] Patent Document 1: Japanese Patent Application Laid-Open No. 2020-63500

[0011] Patent Document 2: Japanese Patent Application Laid-Open No. 2020-12168

[0012] Patent Document 3: Japanese Patent Application Laid-Open No. 2017-179482 SUMMARY

[0013] However, even if the hardness of the surface of the steel plate or the steel pipe is controlled as in Patent Documents 1 to 3 described above, a region of locally high stress occurs in a part of the crystal grains, in the vicinity of the grain boundaries, and becomes a starting point of SSC. Therefore, there is a case where sufficient SSC resistance cannot be obtained.

[0014] The "region of high stress" described above refers to a part where the dislocation density is locally high. This is a very small region, and therefore it is difficult to evaluate in a hardness test such as a Vickers test, where the region is averaged with the surrounding region of low stress.

[0015] The present application has been made in view of the above-described circumstances, and aims to provide a high-strength hot-rolled steel plate suitable for use as a base material of a high-strength electric resistance welded steel pipe having excellent SSC resistance, a manufacturing method thereof, and a high-strength electric resistance welded steel pipe having excellent SSC resistance, and a manufacturing method thereof.

[0016] Note that the "high strength" in the present application refers to a yield strength of 400 MPa or more in the base material portion of the hot-rolled steel plate and the electric resistance welded steel pipe in the tensile test described later.

[0017] Further, the "excellent SSC resistance" in the present application refers to that, in the 4-point bending corrosion test described later, the base material portion of the hot-rolled steel plate and the electric resistance welded steel pipe does not crack, and the depth of the pitting corrosion that occurs is less than 250 μm, and the maximum value of (depth / width) of the pitting corrosion is less than 3.0.

[0018] Each of the tests described above can be performed using the method described in the Examples described later.

[0019] In the part where the dislocation density is locally high, there are many low-angle grain boundaries. This is because, when there are many dislocations, the dislocations arrange with each other to form a stable structure, and a low-angle grain boundary is formed. However, even if the dislocations form a stable structure, the stress positions caused by the dislocations remain, and therefore the part where there are many low-angle grain boundaries, i.e., the part where the density of low-angle grain boundaries is high, becomes a high stress.

[0020] Therefore, in order to improve the SSC resistance of the steel plate, it is necessary to prevent the part where the density of low-angle grain boundaries is locally high from occurring on the surface of the steel plate.

[0021] The present inventors and others have conducted repeated and intensive studies, and as a result, have obtained the following insight. Even in a thick-walled material having a plate thickness of 15 mm or more, by providing two stages of accelerated cooling of the hot-rolled steel plate, and appropriately controlling the temperature of the surface of the steel plate and the inside of the steel plate, the cooling speed in the cooling process, and the time between the cooling processes, it was found that the part where the density of low-angle grain boundaries is locally high does not easily occur on the surface of the steel plate, and the SSC resistance is improved. Further, it was found that the electric resistance welded steel pipe formed using this steel plate as a base material also has improved SSC resistance due to the same effect.

[0022] The present application is completed based on the above situation, and is constituted by the following gist.

[0023] [1] A high-strength hot-rolled steel sheet, in a steel structure in the center of the sheet thickness,

[0024] a volume fraction of the bainite is 50% or more,

[0025] a volume fraction of the ferrite and the bainite in total is 95% or more,

[0026] the remaining portion contains one or two or more kinds selected from the group consisting of pearlite, martensite, and austenite, and an average grain diameter is 9.0 μm or less,

[0027] a dislocation density is 1.0 x 10 14 m -2 to 1.0 x 10 15 m -2 ;

[0028] in a steel structure at a position of 0.1 mm in the depth direction from the sheet surface,

[0029] a volume fraction of the bainite is 70% or more,

[0030] a volume fraction of the ferrite and the bainite in total is 95% or more,

[0031] the remaining portion contains one or two or more kinds selected from the group consisting of pearlite, martensite, and austenite, and an average grain diameter is 9.0 μm or less,

[0032] a dislocation density is 5.0 x 10 14 m -2 to 1.0 x 10 15 m -2 or more,

[0033] a maximum angle grain boundary density is 1.4 x 10 6 m -1 or more,

[0034] the above high-strength hot-rolled steel sheet has a sheet thickness of 15 mm or more.

[0035] [2] The high-strength hot-rolled steel sheet according to [1], wherein a component composition contains, in mass%, C: 0.020% to 0.15%, Si: 1.0% or less, Mn: 0.30% to 2.0%, P: 0.050% or less, S: 0.020% or less, Al: 0.005% to 0.10%, N: 0.010% or less, Nb: 0.15% or less, V: 0.15% or less, and Ti: 0.15% or less,

[0036] and contains one or two or more selected from the group consisting of Cr: 1.0% or less, Mo: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, Ca: 0.010% or less, and B: 0.010% or less,

[0037] The remainder consists of Fe and inevitable impurities.

[0038] [3] A method for manufacturing a high-strength hot-rolled steel sheet, the method for manufacturing a high-strength hot-rolled steel sheet according to [1] or [2], wherein, after a hot-rolling process of performing hot-rolling on a steel material having the above-described composition is performed, a first cooling process and a second cooling process are performed, and then a process of coiling into a coil is performed,

[0039] In the hot-rolling process,

[0040] after heating to a heating temperature of 1100°C to 1300°C,

[0041] a hot-rolling process of a finish-rolling start temperature of 800°C to 950°C and a finish-rolling end temperature of 750°C to 850°C and a total reduction rate of finish-rolling of 60% or more is performed,

[0042] Next, in the first cooling process,

[0043] a cooling process of an average cooling rate at the center of the sheet thickness of 10°C / s to 60°C / s, a cooling stop temperature of 550°C to 650°C, and a cooling stop temperature of the sheet surface of 250°C to 450°C is performed.

[0044] The time from the end of the first cooling process to the start of the second cooling process is 5 s to 20 s,

[0045] Next, in the second cooling process,

[0046] a cooling process of an average cooling rate at the center of the sheet thickness of 5°C / s to 30°C / s, a cooling stop temperature of 450°C to 600°C, and a cooling stop temperature of the sheet surface of 150°C to 350°C is performed.

[0047] [4] A high-strength electric resistance welded steel pipe having a base material portion and an electric resistance welded portion, wherein, in the steel structure at the center of the wall thickness of the base material portion, the volume fraction of bainite is 50% or more, and the volume fraction of the total of ferrite and bainite is 95% or more,

[0048] the remainder contains one or two or more selected from the group consisting of pearlite, martensite, and austenite, and the average grain diameter is 9.0 μm or less,

[0049] the dislocation density is 2.0 x 1014 / m2 or more 14 m -2~ 1.0 x 10 15 m -2 below;

[0050] The volume fraction of bainite in the steel structure at a position 0.1 mm in the depth direction from the inner surface of the base material portion is 70% or more,

[0051] The volume fraction of ferrite and bainite is 95% or more,

[0052] The remaining portion contains one or two or more selected from the group consisting of pearlite, martensite, and austenite, and the average grain diameter is 9.0 μm or less,

[0053] The dislocation density is 6.0 x 10 14 m -2 ~ 1.0 x 10 15 m -2 ,

[0054] The maximum angle grain boundary density is 1.5 x 10 6 m -1 or less,

[0055] The wall thickness of the base material portion is 15 mm or more.

[0056] [5] The high-strength resistance-welded steel pipe according to [4], wherein the component composition of the base material portion contains, in mass%, C: 0.020% to 0.15%, Si: 1.0% or less, Mn: 0.30% to 2.0%, P: 0.050% or less, S: 0.020% or less, Al: 0.005% to 0.10%, N: 0.010% or less, Nb: 0.15% or less, V: 0.15% or less, and Ti: 0.15% or less,

[0057] and contains one or two or more selected from the group consisting of Cr: 1.0% or less, Mo: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, Ca: 0.010% or less, and B: 0.010% or less,

[0058] and the remaining portion consists of Fe and inevitable impurities.

[0059] [6] A method for manufacturing a high-strength resistance-welded steel pipe, by cold roll forming [1] or

[0060] [2] The high-strength hot-rolled steel sheet is formed into a cylindrical shape, and the circumferential direction end portions of the cylindrical shape are butted to be resistance-welded,

[0061] The upset amount at the time of the resistance welding is 20% to 100% of the sheet thickness of the high-strength hot-rolled steel sheet,

[0062] In the sizing process after the electric resistance welding, the diameter is reduced by 0.5 to 4.0% of the circumference of the steel pipe.

[0063] Effects of the Invention

[0064] According to the present application, it is possible to provide a high-strength electric resistance welded steel pipe having excellent SSC resistance even if the thickness of the steel plate is 15 mm or more, a high-strength hot-rolled steel plate serving as a base material thereof, and a method for manufacturing the same. BRIEF DESCRIPTION OF DRAWINGS

[0065] Figure 1 is a schematic view showing a pipe circumferential direction cross section (a cross section perpendicular to the pipe axial direction) including the periphery of the welded portion of the electric resistance welded steel pipe. DETAILED DESCRIPTION

[0066] Hereinafter, the high-strength hot-rolled steel plate and the high-strength electric resistance welded steel pipe of the present application and the method for manufacturing the same will be described. It should be noted that the present application is not limited to the following embodiments. In the present application, the composition and the steel structure of the base material portion deviated by 90° from the electric resistance welded portion in the pipe circumferential direction cross section are defined when the electric resistance welded portion is defined as 0°. Here, the composition and the steel structure at the position deviated by 90° from the electric resistance welded portion are the same as those at the position deviated by 180° from the electric resistance welded portion, for example.

[0067] First, the reason for defining the steel structure of the high-strength hot-rolled steel plate and the high-strength electric resistance welded steel pipe of the present application will be described.

[0068] In the steel structure of the central portion of the plate thickness of the high-strength hot-rolled steel plate of the present application and the central portion of the wall thickness of the base material portion of the high-strength electric resistance welded steel pipe of the present application, the volume fraction of bainite is 50% or more, the volume fraction of the total of ferrite and bainite is 95% or more, and the remaining portion is composed of one or two or more selected from the group consisting of pearlite, martensite, and austenite.

[0069] In the steel structure of the position 0.1 mm in the depth direction from the surface of the high-strength hot-rolled steel plate of the present application and the position 0.1 mm in the depth direction from the inner surface (the surface on the inside of the pipe) of the base material portion of the high-strength electric resistance welded steel pipe of the present application, the volume fraction of bainite is 70% or more, the volume fraction of the total of ferrite and bainite is 95% or more, and the remaining portion is composed of one or two or more selected from the group consisting of pearlite, martensite, and austenite.

[0070] In the following description, the high-strength hot-rolled steel plate will be sometimes referred to simply as "hot-rolled steel plate", and the high-strength electric resistance welded steel pipe will be sometimes referred to simply as "electric resistance welded steel pipe".

[0071] Here, the ferrite is a soft structure. In addition, the bainite is a structure that is harder than the ferrite, and is softer than the pearlite, the martensite, and the austenite.

[0072] [Volume fraction of bainite]

[0073] When the volume fraction of bainite at the center of the sheet thickness of the hot-rolled steel sheet and the wall thickness of the electric resistance welded steel pipe, or the volume fraction of bainite at a position 0.1 mm in depth from the sheet surface of the hot-rolled steel sheet (hereinafter, also referred to as "position 0.1 mm in depth") and a position 0.1 mm in depth from the inner surface of the pipe of the electric resistance welded steel pipe is less than 50% or 70%, the area fraction of the soft ferrite becomes high, and as a result, the target yield strength in the present application cannot be obtained. Therefore, the volume fraction of bainite at the center of the sheet thickness of the hot-rolled steel sheet and the wall thickness of the electric resistance welded steel pipe is set to 50% or more with respect to the entire steel structure at the same position. The volume fraction of bainite at the center of the sheet thickness of the hot-rolled steel sheet and the wall thickness of the electric resistance welded steel pipe is preferably 60% or more, and further preferably 70% or more. The volume fraction of bainite at the position 0.1 mm in depth from the sheet surface of the hot-rolled steel sheet and the position 0.1 mm in depth from the inner surface of the pipe of the electric resistance welded steel pipe is 70% or more with respect to the entire steel structure at the same position. The volume fraction of bainite at the position 0.1 mm in depth from the sheet surface of the hot-rolled steel sheet and the position 0.1 mm in depth from the inner surface of the pipe of the electric resistance welded steel pipe is preferably 75% or more, and further preferably 80% or more.

[0074] It should be noted that the upper limit of the volume fraction of bainite at the center of the sheet thickness of the hot-rolled steel sheet and the wall thickness of the electric resistance welded steel pipe, and at the position 0.1 mm in depth from the sheet surface of the hot-rolled steel sheet and the position 0.1 mm in depth from the inner surface of the pipe of the electric resistance welded steel pipe is not particularly specified. From the viewpoint of ductility, the volume fraction of bainite at the center of the sheet thickness of the hot-rolled steel sheet and the wall thickness of the electric resistance welded steel pipe is preferably set to 95% or less. In addition, from the viewpoint of SSC resistance, the volume fraction of bainite at the position 0.1 mm in depth from the sheet surface of the hot-rolled steel sheet and the position 0.1 mm in depth from the inner surface of the pipe of the electric resistance welded steel pipe is preferably as high as possible. From the viewpoint of ductility, the volume fraction of bainite at the position 0.1 mm in depth is preferably 99% or less.

[0075] [Volume fraction of ferrite and bainite]

[0076] In the case where the hard microstructure is mixed with ferrite and bainite, the advantage of an increase in ductility is obtained. On the other hand, the interface is likely to become a starting point of SSC by stress concentration due to a difference in hardness, and the SSC resistance is reduced. In addition, the toughness is also reduced. Therefore, the volume fraction of ferrite and bainite at the center of the sheet thickness of the hot-rolled steel sheet and the wall thickness of the electric resistance-welded steel pipe, and at a position 0.1 mm in depth from the surface of the hot-rolled steel sheet and a position 0.1 mm in depth from the inner surface of the electric resistance-welded steel pipe, respectively, is 95% or more of the entire steel microstructure at the same position. The volume fraction of ferrite and bainite is preferably 97% or more, and more preferably 98% or more.

[0077] It should be noted that the upper limit of the volume fraction of ferrite and bainite at the center of the sheet thickness of the hot-rolled steel sheet and the wall thickness of the electric resistance-welded steel pipe, and at a position 0.1 mm in depth from the surface of the hot-rolled steel sheet and a position 0.1 mm in depth from the inner surface of the electric resistance-welded steel pipe is not particularly specified. From the viewpoint of ductility, the volume fraction of ferrite and bainite at the center of the sheet thickness of the hot-rolled steel sheet and the wall thickness of the electric resistance-welded steel pipe is preferably 99% or less. In addition, from the viewpoint of SSC resistance, the volume fraction of ferrite and bainite at a position 0.1 mm in depth from the surface of the hot-rolled steel sheet and a position 0.1 mm in depth from the inner surface of the electric resistance-welded steel pipe is preferably as high as possible. The volume fraction of ferrite and bainite at a position 0.1 mm in depth is preferably set to 99% or less from the viewpoint of ductility.

[0078] In the present application, the volume fraction of ferrite at the center of the sheet thickness of the hot-rolled steel sheet and the wall thickness of the electric resistance-welded steel pipe, and at a position 0.1 mm in depth from the surface of the hot-rolled steel sheet and a position 0.1 mm in depth from the inner surface of the electric resistance-welded steel pipe, respectively, is preferably set to 3% or more of the entire steel microstructure at the same position. In addition, the volume fraction of ferrite at the center of the sheet thickness of the hot-rolled steel sheet and the wall thickness of the electric resistance-welded steel pipe is preferably set to 50% or less. The volume fraction of ferrite at a position 0.1 mm in depth from the surface of the hot-rolled steel sheet and a position 0.1 mm in depth from the inner surface of the electric resistance-welded steel pipe is preferably set to 30% or less. Thereby, the effects of improving ductility and SSC resistance can be further effectively obtained.

[0079] [Remaining portion: 1 or 2 or more selected from the group consisting of pearlite, martensite, and austenite]

[0080] The remaining portions of the center of the plate thickness of the hot-rolled steel plate and the center of the wall thickness of the electric resistance welded steel pipe, and the positions at a depth of 0.1 mm from the plate surface of the hot-rolled steel plate and the positions at a depth of 0.1 mm from the inner surface of the electric resistance welded steel pipe have one or two or more selected from the group consisting of pearlite, martensite, and austenite. When the total volume fraction of these respective structures exceeds 5%, the volume fraction of the hard structure becomes high, the dislocation density and / or the density of the largest angle grain boundary become high, as a result of which the SSC resistance decreases. Therefore, the total volume fraction of these respective structures is set to 5% or less, more preferably 3% or less, with respect to the entire steel structure at the same position.

[0081] The above-described various structures of the austenite set the deformation band in the austenite grain boundary or the austenite grain as a nucleation site. In hot rolling, by increasing the reduction amount at a low temperature at which recrystallization of the austenite is not easily generated, a large number of dislocations can be introduced into the austenite to refine the austenite, and a large number of deformation bands can be introduced into the grain. Thereby, the area of the nucleation site increases and the nucleation frequency becomes high, and the steel structure can be refined.

[0082] In the present application, in the range of ±1.0 mm in the plate thickness direction (depth direction) or the wall thickness direction (depth direction) centered on the center of the plate thickness of the hot-rolled steel plate and the center of the wall thickness of the electric resistance welded steel pipe, even if the above-described steel structure is present, the above-described effects can be similarly obtained. Therefore, in the present application, the "steel structure at the center of the plate thickness (or the wall thickness)" means that the above-described steel structure is present in any one of the range of ±1.0 mm in the plate thickness direction (or the wall thickness direction) centered on the center of the plate thickness (or the wall thickness). In addition, in the range of ±0.06 mm in the plate thickness direction (or the wall thickness direction) centered on the position at a depth of 0.1 mm from the plate surface of the hot-rolled steel plate and the position at a depth of 0.1 mm from the inner surface of the electric resistance welded steel pipe, even if the above-described steel structure is present, the above-described effects can be similarly obtained. Therefore, in the present application, the "steel structure at the position at a depth of 0.1 mm from the plate surface (or the inner surface)" means that the above-described steel structure is present in any one of the range of ±0.06 mm in the plate thickness direction (or the wall thickness direction) centered on the position at a depth of 0.1 mm from the plate surface (or the inner surface).

[0083] Here, the observation of the steel structure can be performed using the method described in the Examples described later.

[0084] First, a test piece for microstructure observation is taken and polished in a manner that a cross section parallel to both the rolling direction and the plate thickness direction of the hot-rolled steel sheet and the plate thickness center portion, and a cross section parallel to both the pipe axis direction and the wall thickness direction of the electric resistance welded steel pipe and the wall thickness center portion are taken, and then, is prepared by nitric acid alcohol etching. The microstructure in the plate thickness (or wall thickness) center portion is observed and photographed using an optical microscope (magnification: 1000 times) or a scanning electron microscope (SEM, magnification: 1000 times). Next, the area ratio of bainite and the remaining portion (ferrite, pearlite, martensite, austenite) is calculated based on the obtained optical microscope image and SEM image. The area ratio of each microstructure is observed in 5 or more fields of view, and is calculated as an average value of the values obtained in each field of view. Note that, in the present application, the area ratio obtained by microstructure observation is used as the volume ratio of each microstructure.

[0085] Ferrite is a product based on diffusion phase transformation, and exhibits a microstructure in which dislocation density is low and recovery is almost complete. Polygonal ferrite and quasi-polygonal ferrite are also included therein.

[0086] Bainite is a complex microstructure of lath-shaped ferrite and cementite with high dislocation density.

[0087] Pearlite is a eutectoid structure of iron and iron carbide (ferrite + cementite), and is a lamellar structure in which linear ferrite and cementite are alternately arranged.

[0088] Martensite is a lath-shaped low-temperature phase transformation microstructure with very high dislocation density. In the SEM image, it exhibits a bright contrast compared to ferrite and bainite.

[0089] Note that, in the optical microscope image and the SEM image, it is difficult to distinguish between martensite and austenite. Therefore, the area ratio of the microstructure observed as martensite or austenite is measured from the obtained SEM image, the volume ratio of austenite is subtracted from the measured value by a method described later, and the obtained value is used as the volume ratio of martensite.

[0090] Austenite is an fcc phase, and the volume ratio of austenite is measured using a test piece prepared by the same method as the test piece for dislocation density measurement, by X-ray diffraction. The volume ratio of austenite is calculated from the integrated intensity of the (200), (220), (311) planes of fcc iron and the (200), (211) planes of bcc iron.

[0091] Further, the steel microstructure of the hot-rolled steel sheet described above is a microstructure in which the average grain diameter in the plate thickness center is 9.0 μm or less, and the dislocation density is 1.0 x 10 14 m -2 ~ 1.0 x 10 15 m -2Further, the steel structure of the hot-rolled steel sheet has an average grain diameter of 9.0 μm or less at a position 0.1 mm in depth from the surface of the sheet, a dislocation density of 5.0 x 10 14 m -2 - 1.0 x 10 15 m -2 , and a maximum low-angle grain boundary density of 1.4 x 10 6 m -1 or less.

[0092] Further, the steel structure of the hot-rolled steel sheet has an average grain diameter of 9.0 μm or less at a position 0.1 mm in depth from the surface of the sheet, a dislocation density of 5.0 x 10 14 m -2 - 1.0 x 10 15 m -2 Further, the steel structure of the hot-rolled steel sheet has an average grain diameter of 9.0 μm or less at a position 0.1 mm in depth from the surface of the sheet, a dislocation density of 5.0 x 10 14 m -2 - 1.0 x 10 15 m -2 Further, the steel structure of the hot-rolled steel sheet has an average grain diameter of 9.0 μm or less at a position 0.1 mm in depth from the surface of the sheet, a dislocation density of 5.0 x 10 6 m -1 or less.

[0093] Here, in the present application, the "average grain diameter" means an average value of the equivalent circle diameters of grains when regions surrounded by boundaries having an orientation difference of 15° or more between adjacent crystals are set as the grains. Further, the "equivalent circle diameter (crystal grain)" means a diameter of a circle equivalent to the area of a grain that is the object.

[0094] In the present application, the "low-angle grain boundary density" means the total length of grain boundaries having an orientation difference of 2° or more and less than 15° per unit area of a certain section. Further, the "maximum low-angle grain boundary density" means a maximum value that can be obtained for the low-angle grain boundary density measured in an arbitrary field of view of 10 μm x 10 μm.

[0095] In a portion where the dislocation density is high, dislocations are arranged with each other to obtain a stable structure, and form low-angle grain boundaries. However, even if the dislocations obtain a stable structure, a stress site due to the dislocations remains, and thus a portion where many low-angle grain boundaries exist, i.e., a portion where the low-angle grain boundary density is high, locally becomes a high stress, and easily becomes a starting point of SSC. The local high stress portion is, for example, an interface of a soft phase in contact with a hard phase, an inclusion, and is a very small region, and thus it is difficult to evaluate in a usual Vickers hardness test or a measurement of dislocation density using X-ray diffraction. The local high stress portion can be evaluated by measuring the maximum low-angle grain boundary density using the SEM / EBSD method described later.

[0096] [average grain diameter]

[0097] In a case where the average grain diameter of the grains at the center of the sheet thickness of the hot-rolled steel sheet and at a position 0.1 mm in depth from the sheet surface, and at the center of the wall thickness of the electric resistance welded steel pipe and at a position 0.1 mm in depth from the inner surface of the pipe exceeds 9.0 μm, the steel structure is not fine enough, and thus the target yield strength in the present application is not obtained. In addition, the toughness is also reduced. Therefore, the average grain diameter of the grains at the center of the sheet thickness of the hot-rolled steel sheet and at a position 0.1 mm in depth from the sheet surface, and at the center of the wall thickness of the electric resistance welded steel pipe and at a position 0.1 mm in depth from the inner surface of the pipe is 9.0 μm or less. This average grain diameter of the grains is preferably 7.0 μm or less, and more preferably 6.5 μm or less. Note that, as the average grain diameter decreases, the dislocation density increases, and the resistance to SSC decreases, and thus the average grain diameter is preferably 3.0 μm or more, and more preferably 4.0 μm or more.

[0098] [dislocation density]

[0099] In a case where the dislocation density at the center of the sheet thickness of the hot-rolled steel sheet is less than 1.0 x 10 14 m -2 and the dislocation density at the center of the wall thickness of the electric resistance welded steel pipe is less than 2.0 x 10 14 m -2 , the dislocation strengthening becomes insufficient, and thus the target yield strength in the present application is not obtained. Therefore, the dislocation density at the center of the sheet thickness of the hot-rolled steel sheet is set to 1.0 x 10 14 m -2 or more. The dislocation density at the center of the sheet thickness of the hot-rolled steel sheet is preferably 2.0 x 10 14 m -2 or more, and more preferably 3.0 x 10 14 m -2 or more. The dislocation density at the center of the wall thickness of the electric resistance welded steel pipe is set to 2.0 x 10 14 m -2 or more. The dislocation density at the center of the wall thickness of the electric resistance welded steel pipe is preferably 2.5 x 10 14 m -2 or more, and more preferably 4.0 x 10 14 m -2 or more.

[0100] On the other hand, in a case where the dislocation density at the center of the sheet thickness of the hot-rolled steel sheet and at the center of the wall thickness of the electric resistance welded steel pipe respectively exceeds 1.0 x 10 15 m -2 , the dislocation density and the maximum angle grain boundary density at the sheet surface and the inner surface of the pipe become high, and the resistance to SSC decreases. In addition, the toughness is also reduced. Therefore, the dislocation density at the center of the sheet thickness of the hot-rolled steel sheet and at the center of the wall thickness of the electric resistance welded steel pipe is respectively set to 1.0 x 10 15 m-2 The dislocation density at the center of the plate thickness of the hot-rolled steel plate and the center of the wall thickness of the electric resistance welded steel pipe is preferably 9.6 x 10 14 m -2 More preferably, it is 9.0 x 10 14 m -2 Further preferably, it is 8.5 x 10 14 m -2 The above.

[0101] In the case where the dislocation density at a position 0.1 mm in depth from the plate surface of the hot-rolled steel plate is less than 5.0 x 10 14 m -2 and the dislocation density at a position 0.1 mm in depth from the inner surface of the electric resistance welded steel pipe is less than 6.0 x 10 14 m -2 , the dislocation strengthening is insufficient, and thus the target yield strength in the present application is not obtained. Therefore, the dislocation density at a position 0.1 mm in depth from the plate surface of the hot-rolled steel plate is set to 5.0 x 10 14 m -2 The above. The dislocation density at a position 0.1 mm in depth from the plate surface of the hot-rolled steel plate is preferably 5.5 x 10 14 m -2 The above. The dislocation density at a position 0.1 mm in depth from the inner surface of the electric resistance welded steel pipe is set to 6.0 x 10 14 m -2 The above. The dislocation density at a position 0.1 mm in depth from the inner surface of the electric resistance welded steel pipe is preferably 6.5 x 10 14 m -2 The above.

[0102] On the other hand, in the case where the dislocation density at a position 0.1 mm in depth from the plate surface of the hot-rolled steel plate and at a position 0.1 mm in depth from the inner surface of the electric resistance welded steel pipe exceeds 1.0 x 10 15 m -2 , the maximum angle grain boundary density of the plate surface and the inner surface becomes high, and the SSC resistance is reduced. In addition, the toughness is also reduced. Therefore, the dislocation density at a position 0.1 mm in depth from the plate surface of the hot-rolled steel plate and at a position 0.1 mm in depth from the inner surface of the electric resistance welded steel pipe is set to 1.0 x 10 15 m -2 The above. The dislocation density at a position 0.1 mm in depth from the plate surface of the hot-rolled steel plate and at a position 0.1 mm in depth from the inner surface of the electric resistance welded steel pipe is preferably 9.0 x 10 14 m -2 More preferably, it is 8.8 x 10 14 m -2 The above.

[0103] [Maximum minimum angle grain boundary density]

[0104] The maximum small-angle grain boundary density at a depth of 0.1 mm from the surface of the hot-rolled steel sheet exceeds 1.4 × 10⁻⁶. 6 m -1 The maximum small-angle grain boundary density at a depth of 0.1 mm from the inner surface of the resistance-welded steel pipe exceeds 1.5 × 10⁻⁶. 6 m -1 In such cases, the localized stress on the plate surface and the inner surface of the tube is high, thus reducing the resistance to SSC (Special Stress Compactness). Therefore, the maximum small-angle grain boundary density at a depth of 0.1 mm from the surface of the hot-rolled steel plate is set to 1.4 × 10⁻⁶. 6 m -1 The following is a preferred maximum small-angle grain boundary density at a depth of 0.1 mm from the surface of the hot-rolled steel sheet: 1.3 × 10⁻⁶. 6 m -1 The maximum small-angle grain boundary density at a depth of 0.1 mm from the inner surface of the resistance-welded steel pipe is 1.5 × 10⁻⁶. 6 m -1 The following is a preferred maximum small-angle grain boundary density at a depth of 0.1 mm from the inner surface of the resistance-welded steel pipe: 1.4 × 10⁻⁶. 6 m -1 the following.

[0105] It should be noted that there is no specific lower limit for the maximum small-angle grain boundary density mentioned above. If pearlite, martensite, or austenite is present, the maximum small-angle grain boundary density increases. It is difficult to set these total volume fractions to 0%, therefore, the maximum small-angle grain boundary density at a depth of 0.1 mm from the surface of the hot-rolled steel sheet is preferably set to 0.080 × 10⁻⁶. 6 m -1 The above. The maximum small-angle grain boundary density at a depth of 0.1 mm from the inner surface of the resistance-welded steel pipe is preferably set to 0.10 × 10⁻⁶. 6 m -1 above.

[0106] Here, as detailed in the embodiments described later, the determination of the average grain diameter, dislocation density, and maximum small-angle grain boundary density of the steel structure can be performed according to the methods described later.

[0107] The average grain diameter was measured as follows. Mirror-polished sections parallel to both the rolling direction and the sheet thickness direction of the hot-rolled steel sheet and parallel to both the pipe axis direction and the wall thickness direction of the electric resistance-welded steel pipe were used to calculate histograms (horizontal axis: grain diameter, vertical axis: proportion of each grain diameter) of grain diameter distribution at the center of the sheet thickness and at a position 0.1 mm deep from the sheet surface of the hot-rolled steel sheet and at the center of the wall thickness and at a position 0.1 mm deep from the inner surface of the electric resistance-welded steel pipe, respectively, using the SEM / EBSD method, and the arithmetic mean of the grain diameters was calculated. The measurement conditions were an acceleration voltage of 15 kV, a measurement area of 100 μm x 100 μm, and a measurement step (measurement resolution) of 0.5 μm, and the measurement values of five fields or more were averaged. Note that in the analysis of the grain diameters, grain diameters less than 2.0 μm were excluded from the analysis targets as measurement noise.

[0108] The dislocation density at the center of the sheet thickness of the hot-rolled steel sheet and at the center of the wall thickness of the electric resistance-welded steel pipe was calculated as follows. After mirror-polishing sections parallel to both the rolling direction and the sheet thickness direction of the hot-rolled steel sheet and parallel to both the pipe axis direction and the wall thickness direction of the electric resistance-welded steel pipe, the surface layer was removed by electrolytic polishing of the polished surface to 100 μm, and test pieces were prepared so that the diffraction plane was the center of the sheet thickness (or wall thickness). X-ray diffraction was performed using the prepared test pieces, and the results were used to calculate the dislocation density using the modified Williamson-Hall method and the modified Warren-Averbach method (Reference Documents 1 and 2). The Burgers vector b, which is the interatomic distance of the slip direction <111> of bcc iron, can be calculated using 0.248 x 10 -9 m.

[0109] [Reference Document 1] T. Ungar and A. Borbely: Appl. Phys. Lett., 69 (1996), 3173.

[0110] [Reference Document 2] M. Kumagai, M. Imafuku, S. Ohya: ISIJ International, 54 (2014), 206.

[0111] The dislocation density at a position 0.1 mm deep from the surface of the hot-rolled steel sheet and at a position 0.1 mm deep from the inner surface of the electric resistance-welded steel pipe was calculated as follows. After mirror-polishing the sheet surface of the hot-rolled steel sheet and the inner surface of the electric resistance-welded steel pipe, the surface layer was removed by electrolytic polishing of the polished surface to 50 μm, and X-ray diffraction was performed in the same manner as for the center of the sheet thickness (or wall thickness) described above, and the dislocation density was calculated.

[0112] The maximum low angle grain boundary density is obtained by mirror polishing cross sections parallel to both the rolling direction and the plate thickness direction of the hot-rolled steel sheet, and cross sections parallel to both the pipe axis direction and the wall thickness direction of the electric resistance welded steel pipe, and using the SEM / EBSD method. At a position 0.1 mm in depth from the surface of the hot-rolled steel sheet and a position 0.1 mm in depth from the inner surface of the electric resistance welded steel pipe, 20 or more fields of view are measured each with a measurement range of 10 μm x 10 μm. For each field of view, the total length of grain boundaries having an orientation difference of 2° or more and less than 15° is calculated, and the low angle grain boundary density of each field of view is obtained. In the present application, the maximum value of the low angle grain boundary densities obtained at the measurement positions is taken as the maximum low angle grain boundary density.

[0113] Next, the preferable range of the composition of the high-strength electric resistance welded steel pipe of the present application and the high-strength hot-rolled steel sheet serving as a base material thereof, and the reasons for the limitations thereof are described from the viewpoint of ensuring the above-described characteristics and steel structure, etc. In the present specification, " % " indicating the composition of the steel is mass % unless otherwise specified.

[0114] C: 0.020% to 0.15%

[0115] C is an element that increases the strength of the steel through solid solution strengthening. In order to ensure the target strength of the present application, it is preferable to contain 0.020% or more of C. However, if the C content exceeds 0.15%, the hardenability becomes high, and hard phases such as pearlite, martensite, and austenite are excessively generated, so the C content is preferably 0.15% or less. The C content is more preferably 0.025% or more, and more preferably 0.12% or less. The C content is more preferably 0.030% or more, and more preferably 0.10% or less.

[0116] Si: 1.0% or less

[0117] Si is an element that increases the strength of the steel through solid solution strengthening. In order to obtain such an effect, it is preferable to contain 0.02% or more of Si. However, if the Si content exceeds 1.0%, the ductility and toughness decrease. Therefore, the Si content is preferably set to 1.0% or less. The Si content is more preferably 0.05% or more, and more preferably 0.70% or less. The Si content is more preferably 0.10% or more, and more preferably 0.50% or less.

[0118] Mn: 0.30% to 2.0%

[0119] Mn is an element that increases the strength of the steel through solid solution strengthening. In addition, Mn is an element that contributes to microstructure refinement by lowering the transformation start temperature. In order to ensure the target strength and steel microstructure of the present application, it is preferable to contain 0.30% or more of Mn. However, when the Mn content exceeds 2.0%, the hardenability becomes high, and hard phases such as pearlite, martensite, and austenite are excessively generated, so the Mn content is preferably set to 2.0% or less. The Mn content is more preferably 0.40% or more, and more preferably 1.9% or less. The Mn content is more preferably 0.50% or more, and more preferably 1.8% or less.

[0120] P: 0.050% or less

[0121] P is a cause of inhomogeneity of the billet due to segregation at grain boundaries, so as an unavoidable impurity, it is preferable to reduce it as much as possible, and the P content is preferably in the range of 0.050% or less. The P content is more preferably 0.040% or less, and more preferably 0.030% or less. It should be noted that the lower limit of P is not particularly specified, but excessive reduction leads to an increase in the cost of refining, so P is preferably 0.001% or more.

[0122] S: 0.020% or less

[0123] S is generally present as MnS in steel, and MnS is drawn very thin in the hot rolling process, which has a negative effect on ductility and toughness. Therefore, it is preferable to reduce S as much as possible in the present application, and the S content is preferably set to 0.020% or less. The S content is more preferably 0.010% or less, and further preferably 0.0050% or less. It should be noted that the lower limit of S is not particularly specified, but excessive reduction leads to an increase in the cost of molten steel, so S is preferably set to 0.0001% or more.

[0124] Al: 0.005% to 0.10%

[0125] Al is an element that functions as a strong deoxidizer. In order to obtain such an effect, it is preferable to contain 0.005% or more of Al. However, when the Al content exceeds 0.10%, the weldability deteriorates, and the number of alumina inclusions increases, and the surface properties deteriorate. Also, the toughness decreases. Therefore, the Al content is preferably set to 0.005% to 0.10%. The Al content is more preferably 0.010% or more, and more preferably 0.080% or less. The Al content is more preferably 0.015% or more, and more preferably 0.070% or less.

[0126] N: 0.010% or less

[0127] N is an unavoidable impurity, an element having an effect of firmly fixing the movement of dislocations to reduce ductility and toughness. In the present application, N is preferably reduced as much as possible as an impurity, but the content of N can be allowed to be 0.010%. Therefore, the content of N is set to 0.010% or less. The content of N is preferably 0.0080% or less. Excessive reduction leads to an increase in manufacturing cost, and therefore the content of N is preferably 0.0010% or more.

[0128] Nb: 0.15% or less

[0129] Nb is an element that contributes to an increase in strength of steel by forming fine carbides and nitrides in the steel. Also, Nb is an element that contributes to refinement of the structure by suppressing coarsening of austenite in hot rolling. In order to obtain the above effects, it is preferable to contain 0.002% or more of Nb. However, if the content of Nb exceeds 0.15%, ductility and toughness are reduced. Therefore, the content of Nb is preferably set to 0.15% or less. The content of Nb is more preferably 0.005% or more, and more preferably 0.13% or less. The content of Nb is more preferably 0.010% or more, and more preferably 0.10% or less.

[0130] V: 0.15% or less

[0131] V is an element that contributes to an increase in strength of steel by forming fine carbides and nitrides in the steel. In order to obtain the above effects, it is preferable to contain 0.002% or more of V. However, if the content of V exceeds 0.15%, ductility and toughness are reduced. Therefore, the content of V is preferably set to 0.15% or less. The content of V is more preferably 0.005% or more, and more preferably 0.13% or less. The content of V is more preferably 0.010% or more, and more preferably 0.10% or less. The content of V is more preferably 0.090% or less.

[0132] Ti: 0.15% or less

[0133] Ti is an element that contributes to an increase in strength of steel by forming fine carbides and nitrides in the steel, and also an element that contributes to reduction of solid-solution N in steel because of high affinity with N. In order to obtain the above effects, it is preferable to contain 0.002% or more of Ti. However, if the content of Ti exceeds 0.15%, ductility and toughness are reduced. Therefore, the content of Ti is preferably set to 0.15% or less. The content of Ti is more preferably 0.005% or more, and more preferably 0.13% or less. The content of Ti is more preferably 0.010% or more, and more preferably 0.10% or less. The content of Ti is more preferably 0.070% or less.

[0134] In addition to the above-mentioned components, the following elements can be contained. Note that each of the following elements (Cr, Mo, Cu, Ni, Ca, and B) can be contained as needed, and thus each of these components can be 0%.

[0135] one or two or more selected from Cr: 1.0% or less, Mo: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, Ca: 0.010% or less, and B: 0.010% or less

[0136] Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.0% or less, Mo: 1.0% or less

[0137] Cu, Ni, Cr, and Mo are elements that improve the hardenability of steel and increase the strength of steel, and can be contained as needed. In order to obtain the above-mentioned effects, in the case where Cu, Ni, Cr, and Mo are contained, it is preferable to set Cu: 0.01% or more, Ni: 0.01% or more, Cr: 0.01% or more, and Mo: 0.01% or more, respectively. On the other hand, excessive Cu, Ni, Cr, and Mo can cause excessive generation of hard phases such as pearlite, martensite, and austenite. Therefore, in the case where Cu, Ni, Cr, and Mo are contained, it is preferable to set Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.0% or less, and Mo: 1.0% or less, respectively. Therefore, in the case where Cu, Ni, Cr, and Mo are contained, it is preferable to set Cu: 0.01% to 1.0%, Ni: 0.01% to 1.0%, Cr: 0.01% to 1.0%, and Mo: 0.01% to 1.0%, respectively. More preferably, Cu: 0.05% or more, Cu: 0.70% or less, Ni: 0.05% or more, Ni: 0.70% or less, Cr: 0.05% or more, Cr: 0.70% or less, Mo: 0.05% or more, and Mo: 0.70% or less. Further preferably, Cu: 0.10% or more, Cu: 0.50% or less, Ni: 0.10% or more, Ni: 0.50% or less, Cr: 0.10% or more, Cr: 0.50% or less, Mo: 0.10% or more, and Mo: 0.50% or less.

[0138] Ca: 0.010% or less

[0139] Ca is an element that contributes to improvement in toughness of the steel by spheroidizing sulfides such as MnS that are thinly stretched in the hot rolling process, and can be contained as needed. In order to obtain the above-mentioned effect, therefore, in the case of containing Ca, it is preferable to contain 0.0005% or more of Ca. However, if the Ca content exceeds 0.010%, Ca oxide clusters are formed in the steel, and the toughness deteriorates. Therefore, in the case of containing Ca, the Ca content is preferably set to 0.010% or less. The Ca content is more preferably 0.0008% or more, and more preferably 0.008% or less. The Ca content is more preferably 0.0010% or more, and more preferably 0.0060% or less.

[0140] B: 0.010% or less

[0141] B is an element that contributes to microstructure refinement by lowering the phase transformation start temperature, and can be contained as needed. Since the above-mentioned effect is obtained, therefore, in the case of containing B, it is preferable to contain 0.0003% or more of B. However, in the case where the B content exceeds 0.010%, the ductility and toughness deteriorate. Therefore, in the case of containing B, the B content is preferably set to 0.010% or less. The B content is more preferably 0.0005% or more, and more preferably 0.0030% or less. The B content is more preferably 0.0008% or more, and more preferably 0.0020% or less.

[0142] The remainder is Fe and inevitable impurities. Among these, as inevitable impurities, O (oxygen) can be allowed to be contained at 0.0050% or less within a range that does not impair the effect of the present application.

[0143] The above-mentioned components are the basic component composition of the base material portion of the high-strength hot-rolled steel sheet and the high-strength electric resistance welded steel pipe of the present application. The target characteristics of the present application are obtained by this basic component composition.

[0144] In the present application, in order to further reduce the hardenability, the carbon equivalent (Ceq) represented by formula (1) is preferably 0.45% or less.

[0145] Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15... (1)

[0146] Here, C, Mn, Cr, Mo, V, Cu, and Ni in formula (1) are the contents (mass%) of each element, and elements not contained are set to have a content of zero.

[0147] In the case where the carbon equivalent exceeds 0.45%, the hardenability becomes high, and hard pearlite, martensite, and austenite are excessively generated. The carbon equivalent is preferably 0.45% or less, more preferably 0.30% or less, and further preferably 0.28% or less. The lower limit of the carbon equivalent is not particularly limited. From the viewpoint of increasing the fraction of bainite, the carbon equivalent is preferably set to 0.20% or more. The carbon equivalent is more preferably 0.22% or more.

[0148] Next, a manufacturing method of the high-strength hot-rolled steel sheet and the high-strength electric resistance welded steel pipe according to one embodiment of the present application will be described.

[0149] The high-strength hot-rolled steel sheet according to the present application is, for example, manufactured by heating a steel material having the above-described composition to a heating temperature of 1100°C to 1300°C, and then performing hot-rolling in which the finish temperature of rough rolling is 900°C to 1100°C, the start temperature of finish rolling is 800°C to 950°C, the finish temperature of finish rolling is 750°C to 850°C, and the total reduction rate in finish rolling is 60% or more (hot-rolling step). Subsequently, in a first cooling step, cooling is performed in which the average cooling rate at the center of the sheet thickness is 10°C / s to 60°C / s, the cooling stop temperature is 550°C to 650°C or less, and the cooling stop temperature at the surface of the sheet is 250°C to 450°C. The time from the end of the first cooling step to the start of a subsequent second cooling step is set to 5 s to 20 s. Subsequently, in the second cooling step, cooling is performed in which the average cooling rate at the center of the sheet thickness is 5°C / s to 30°C / s, the cooling stop temperature is 450°C to 600°C or less, and the cooling stop temperature at the surface of the sheet is 150°C to 350°C. Then, the hot-rolled steel sheet can be manufactured by being coiled into a coil shape.

[0150] In addition, the high-strength electric resistance welded steel pipe according to the present application can be manufactured by forming the manufactured high-strength hot-rolled steel sheet into a cylindrical shape by using a cold roll, resistance welding the both ends of the cylindrical shape in the circumferential direction, and manufacturing an electric resistance welded steel pipe.

[0151] Note that, in the following description of the manufacturing method, "°C" relating to temperature is shown as being set to the surface temperature of the steel material or the steel sheet (hot-rolled sheet) unless otherwise specified. These surface temperatures can be measured using a radiation thermometer or the like. In addition, the temperature at the center of the sheet thickness of the steel sheet can be found by calculating the temperature distribution in the cross section of the steel sheet by heat transfer analysis and correcting the result by the surface temperature of the steel sheet. In addition, "hot-rolled steel sheet" includes a hot-rolled sheet and a hot-rolled steel strip.

[0152] First, the manufacturing method of the hot-rolled steel sheet will be described.

[0153] In the present application, the method of melting the steel material (billet) is not particularly limited. For example, any of a converter, an electric furnace, a vacuum melting furnace, or the like can be used. The casting method is also not particularly limited. For example, the steel material can be manufactured to a desired size by a continuous casting method or the like. Note that even if a casting- block rolling method is used instead of the continuous casting method, there is no problem. The molten steel can be further subjected to secondary refining such as ladle refining.

[0154] Next, the obtained steel material (billet) is heated to a heating temperature of 1100°C to 1300°C, and then the heated steel material is subjected to hot rolling to produce a hot-rolled sheet (hot-rolling step), and then the hot-rolled sheet is subjected to cooling (first cooling step and second cooling step), and then the cooled hot-rolled sheet is coiled into a coil shape (coiling step) to produce a hot-rolled steel sheet.

[0155] Heating temperature: 1100°C to 1300°C

[0156] When the heating temperature is less than 1100°C, the deformation resistance of the rolled material becomes large, and it is difficult to perform rolling. On the other hand, when the heating temperature exceeds 1300°C, the austenite grains become coarse, and fine austenite grains cannot be obtained in the subsequent rolling (rough rolling, finish rolling), and it is difficult to ensure the target average grain diameter of the present application. Therefore, the heating temperature in the hot-rolling step is set to 1100°C to 1300°C. The heating temperature is more preferably 1120°C to 1280°C.

[0157] Note that in the present application, in addition to the conventional method in which the steel material (slab) is manufactured, temporarily cooled to room temperature, and then reheated, an energy-saving process in which the slab is not cooled to room temperature, but directly charged into a heating furnace in a warm state, or immediately subjected to rolling after a slight holding can also be applied without any problem.

[0158] Rough rolling end temperature: 900°C to 1100°C

[0159] When the rough rolling end temperature is less than 900°C, in the subsequent finish rolling, the steel sheet surface temperature becomes below the ferrite transformation start temperature, and a large amount of processing ferrite is generated, and the dislocation density and the maximum angle grain boundary density increase. As a result, it is difficult to ensure the target dislocation density and the target maximum angle grain boundary density of the present application. On the other hand, if the rough rolling end temperature exceeds 1100°C, the reduction in the austenite unrecrystallization temperature region is insufficient, and fine austenite grains cannot be obtained. As a result, it is difficult to ensure the target average grain diameter of the present application, and the yield strength decreases. Therefore, the rough rolling end temperature is set to 900°C to 1100°C. The rough rolling end temperature is more preferably 920°C or higher, and more preferably 1050°C or lower.

[0160] Finish rolling start temperature: 800°C to 950°C

[0161] In the case where the finish rolling start temperature is less than 800°C, the steel sheet surface temperature becomes below the ferrite transformation start temperature in the finish rolling, a large amount of processing ferrite is generated, and the dislocation density and the maximum angle grain boundary density increase. As a result, it is difficult to secure the target dislocation density and the maximum angle grain boundary density of the present application. On the other hand, if the finish rolling start temperature exceeds 950°C, the austenite is coarsened, and sufficient deformation bands are not introduced in the austenite, so it is difficult to obtain the target average grain diameter of the present application, and the yield strength decreases. Therefore, the finish rolling start temperature is set to 800°C to 950°C. The finish rolling start temperature is more preferably 820°C or higher, and more preferably 930°C or lower.

[0162] Finish rolling end temperature: 750°C to 850°C

[0163] In the case where the finish rolling end temperature is less than 750°C, the steel sheet surface temperature becomes below the ferrite transformation start temperature in the finish rolling, a large amount of processing ferrite is generated, and the dislocation density and / or the maximum angle grain boundary density increase. As a result, it is difficult to secure the target dislocation density and the maximum angle grain boundary density of the present application. On the other hand, if the finish rolling end temperature exceeds 850°C, the reduction in the austenite unrecrystallization temperature region is insufficient, and fine austenite grains cannot be obtained. As a result, it is difficult to secure the target average grain diameter of the present application, and the yield strength decreases. Therefore, the finish rolling end temperature is set to 750°C to 850°C. The finish rolling end temperature is more preferably 770°C or higher, and more preferably 830°C or lower.

[0164] Total reduction in finish rolling: 60% or more

[0165] In the present application, in the hot rolling step, the subgrains in the austenite are refined, the ferrite, the bainite, and the balance of the structure generated in the subsequent cooling step and the coiling step are refined, and a steel structure having the target yield strength of the present application is obtained. In order to refine the subgrains in the austenite in the hot rolling step, it is necessary to increase the reduction in the austenite unrecrystallization temperature region and introduce sufficient processing strain. In order to achieve this, in the present application, the total reduction in the finish rolling is set to 60% or more.

[0166] In the case where the total reduction in the finish rolling is less than 60%, sufficient processing strain cannot be introduced in the hot rolling step, so a steel structure having the target average grain diameter of the present application cannot be obtained. The total reduction in the finish rolling is more preferably 65% or more. The upper limit of this total reduction is not particularly specified. If this total reduction exceeds 80%, the effect of the increase in toughness with respect to the increase in the reduction becomes small, and only the equipment load increases. Therefore, the total reduction in the finish rolling is preferably 80% or less. This total reduction is more preferably 75% or less.

[0167] The total reduction in the finish rolling described above refers to the total of the reductions in the respective rolling passes in the finish rolling.

[0168] In the present application, the upper limit of the final plate thickness is not particularly specified, but from the viewpoint of securing the required reduction and steel plate temperature management, the final plate thickness (the plate thickness of the steel plate after finish rolling) is preferably 15 mm to 40 mm.

[0169] After the hot rolling step, a two-stage cooling step is performed on the hot-rolled plate.

[0170] As described above, the accelerated cooling of the cooling step is set to two stages, and the temperature of the steel plate surface and the steel plate interior, the cooling speed, and the time between the respective cooling steps in the cooling step are appropriately controlled. Thereby, a portion with a high density of local low-angle grain boundaries is less likely to occur on the steel plate surface, and thus is particularly important in the present application.

[0171] In the first cooling step, the hot-rolled plate is subjected to cooling with an average cooling speed at the plate thickness center: 10°C / s to 60°C / s, a cooling stop temperature: 550°C to 650°C, and a cooling stop temperature of the plate surface: 250°C to 450°C.

[0172] The average cooling speed at the plate thickness center in the first cooling step: 10°C / s to 60°C / s

[0173] When the average cooling speed from the start of the first cooling step to the temperature region of the cooling stop temperature of the first cooling step described later is less than 10°C / s, measured from the plate thickness center temperature of the hot-rolled plate, the ferrite fraction increases, and thus a steel structure with the target bainite fraction of the present application cannot be obtained. In addition, since the nucleation frequency of ferrite or bainite decreases, they become coarse, and thus a steel structure with the target average grain diameter of the present application cannot be obtained. On the other hand, if the average cooling speed exceeds 60°C / s, measured from the plate thickness center temperature of the hot-rolled plate, a large amount of martensite is generated on the steel plate surface, the maximum low-angle grain boundary density increases, and as a result, the SSC resistance decreases. The average cooling speed at the plate thickness center is preferably 15°C / s or more, and more preferably 18°C / s or more. The average cooling speed at the plate thickness center is preferably 55°C / s or less, and more preferably 50°C / s or less.

[0174] Note that in the present application, it is preferable to start the first cooling step immediately after the finish rolling is completed, from the viewpoint of suppressing the generation of ferrite on the steel plate surface before the first cooling step.

[0175] The cooling stop temperature at the plate thickness center in the first cooling step: 550°C to 650°C

[0176] When the cooling stop temperature at the center of the sheet thickness of the hot-rolled sheet is less than 550°C, the cooling stop temperature at the surface of the steel sheet becomes low, a large amount of martensite is generated at the surface of the steel sheet, the density of the largest angle grain boundary increases, and as a result, the SSC resistance decreases. On the other hand, when the cooling stop temperature at the center of the sheet thickness of the hot-rolled sheet exceeds 650°C, the cooling stop temperature at the surface of the steel sheet becomes high, the ferrite fraction at the center of the sheet thickness increases, and thus a steel structure having the target bainite fraction of the present application cannot be obtained. In addition, the nucleation frequency of ferrite or bainite decreases, and they are coarsened, and thus a structure having the target average grain diameter of the present application cannot be obtained. The cooling stop temperature at the center of the sheet thickness is preferably 560°C or higher, and more preferably 580°C or higher. The cooling stop temperature at the center of the sheet thickness is preferably 630°C or lower, and more preferably 620°C or lower.

[0177] Cooling stop temperature at the surface of the sheet in the first cooling step: 250°C to 450°C

[0178] When the cooling stop temperature at the surface of the hot-rolled sheet is less than 250°C, a large amount of martensite is generated at the surface of the steel sheet, the density of the largest angle grain boundary increases, and as a result, the SSC resistance decreases. On the other hand, when the cooling stop temperature at the surface of the hot-rolled sheet exceeds 450°C, the cooling stop temperature at the center of the sheet thickness becomes high, the ferrite fraction at the center of the sheet thickness increases, and thus a structure having the target bainite fraction of the present application cannot be obtained. In addition, at the center of the sheet thickness, the nucleation frequency of ferrite or bainite decreases, and they are coarsened, and thus a structure having the target average grain diameter of the present application cannot be obtained. The cooling stop temperature at the surface of the sheet is preferably 280°C or higher, and more preferably 290°C or higher. The cooling stop temperature at the surface of the sheet is preferably 420°C or lower, and more preferably 410°C or lower.

[0179] Note that, in the present application, the average cooling rate is set to a value (cooling rate) obtained from ((the center temperature of the sheet thickness of the hot-rolled sheet before cooling - the center temperature of the sheet thickness of the hot-rolled sheet after cooling) / cooling time) unless otherwise specified. The cooling method can include water cooling by spraying water from a nozzle, cooling by spraying a cooling gas, and the like. In the present application, it is preferable to perform the cooling operation (process) on both surfaces of the hot-rolled sheet so that both surfaces of the hot-rolled sheet are cooled under the same conditions.

[0180] After the first cooling step is completed, the hot-rolled sheet is allowed to stand for 5 s to 20 s, and then the second cooling step is performed. In the second cooling step, the hot-rolled sheet is subjected to cooling at an average cooling rate at the center of the sheet thickness of 5°C / s to 30°C / s, a cooling stop temperature of 450°C to 600°C, and a cooling stop temperature at the surface of the sheet of 150°C to 350°C.

[0181] Time from completion of the first cooling step to start of the second cooling step: 5 s to 20 s

[0182] The stand time is set during the period from the end of the first cooling step to the start of the second cooling step, thereby tempering the ferrite or bainite generated in the first cooling step, and reducing the dislocation density.

[0183] If the time from the end of the first cooling step to the start of the second cooling step is less than 5 s, the tempering of the ferrite or bainite is insufficient, the dislocation density at the plate surface increases, the maximum angle grain boundary density increases, and as a result, the SSC resistance decreases. If the time from the end of the first cooling step to the start of the second cooling step exceeds 20 s, the ferrite or bainite at the center of the plate thickness coarsens, and therefore the yield strength decreases. The time from the end of the first cooling step to the start of the second cooling step is preferably 10 s or more, and preferably 18 s or less.

[0184] As a method of setting the stand time during the period from the end of the first cooling step to the start of the second cooling step, for example, the necessary stand time can be ensured by slowing the conveyance speed of the hot-rolled plate in the equipment configured continuously with the first cooling device and the second cooling device.

[0185] Average cooling speed at the center of the plate thickness in the second cooling step: 5°C / s to 30°C / s

[0186] If the average cooling speed from the start of the second cooling step to the temperature range of the cooling stop temperature of the second cooling step described later is less than 5°C / s, measured from the center temperature of the plate thickness of the hot-rolled plate, the ferrite or bainite coarsens, and therefore a structure having the target average grain diameter of the present application cannot be obtained. On the other hand, if the average cooling speed exceeds 30°C / s, measured from the center temperature of the plate thickness of the hot-rolled plate, a large amount of martensite is generated at the surface of the steel plate, and the maximum angle grain boundary density increases. As a result, the SSC resistance decreases. The average cooling speed at the center of the plate thickness is preferably 8°C / s or more, and more preferably 9°C / s or more. The average cooling speed at the center of the plate thickness is preferably 25°C / s or less, and more preferably 15°C / s or less.

[0187] Cooling stop temperature at the center of the plate thickness in the second cooling step: 450°C to 600°C

[0188] When the cooling stop temperature is less than 450°C measured at the center of the thickness of the hot-rolled plate, the cooling stop temperature at the surface of the steel plate becomes low, a large amount of martensite is generated at the surface of the steel plate, the density of the high-angle grain boundaries increases the most, and as a result, the SSC resistance decreases. On the other hand, when the cooling stop temperature exceeds 600°C measured at the center of the thickness of the hot-rolled plate, the cooling stop temperature at the surface of the steel plate becomes high, ferrite or bainite is coarsened, and thus a structure having the target average grain diameter of the present application cannot be obtained. The cooling stop temperature at the center of the thickness is preferably 480°C or higher, more preferably 490°C or higher. The cooling stop temperature at the center of the thickness is preferably 570°C or lower, more preferably 560°C or lower.

[0189] Cooling stop temperature at the surface of the steel plate in the second cooling step: 150°C to 350°C

[0190] When the cooling stop temperature is less than 150°C measured at the surface of the hot-rolled plate, a large amount of martensite is generated at the surface of the steel plate, the density of the high-angle grain boundaries increases the most, and as a result, the SSC resistance decreases. On the other hand, when the cooling stop temperature exceeds 350°C measured at the surface of the hot-rolled plate, ferrite or bainite is coarsened at the center of the thickness, and thus a structure having the target average grain diameter of the present application cannot be obtained. The cooling stop temperature at the surface is preferably 180°C or higher, more preferably 200°C or higher. The cooling stop temperature at the surface is preferably 320°C or lower, more preferably 300°C or lower.

[0191] After the second cooling step, a coiling step of coiling the hot-rolled plate and then allowing the coiled plate to stand is performed.

[0192] In the coiling step, from the viewpoint of the structure of the steel plate, it is preferable to coil at a coiling temperature of 400°C to 600°C measured at the center of the thickness. When the coiling temperature is less than 400°C, a large amount of martensite is generated at the surface of the steel plate, the density of the high-angle grain boundaries increases the most, and as a result, the SSC resistance decreases. If the coiling temperature exceeds 600°C, ferrite or bainite is coarsened, and thus a structure having the target average grain diameter of the present application cannot be obtained. The coiling temperature is more preferably 430°C or higher, more preferably 580°C or lower.

[0193] Next, a method for manufacturing an electric resistance welded steel pipe will be described.

[0194] After the above-described coiling process, a pipe-making process is performed on the obtained hot-rolled steel sheet. In the pipe-making process, the hot-rolled steel sheet is formed into a cylindrical open pipe (round steel pipe) by cold roll forming, the circumferential direction both ends (butted portions) of the cylindrical open pipe are butted and melted by high-frequency resistance heating, and pressure welding is performed by upset forging using an extrusion roll, resistance welding is performed, and a resistance welded steel pipe is manufactured. The resistance welded steel pipe thus manufactured has a base material portion and a resistance welded portion. Then, a sizing process is performed on the resistance welded steel pipe. In the sizing process, the resistance welded steel pipe is reduced in diameter using rolls arranged above and below and to the left and right of the resistance welded steel pipe, the outer diameter and the circularity are adjusted to desired values.

[0195] The upset amount at the time of resistance welding (resistance welding process) is 20% or more of the thickness of the hot-rolled steel sheet, so that inclusions such as oxides and nitrides that cause a decrease in toughness can be discharged together with the molten steel. In the case where the upset amount exceeds 100% of the thickness, the load on the extrusion roll becomes large. In addition, since the processing strain of the resistance welded steel pipe increases, the dislocation density of the inner surface of the pipe increases, the density of the largest angle grain boundary increases, and as a result, the SSC resistance decreases. Therefore, the upset amount is set to 20% to 100% of the thickness. The upset amount is preferably 40% or more and preferably 80% or less.

[0196] The above-described upset amount can be calculated by ((circumference of the open pipe before resistance welding) - (circumference of the resistance welded steel pipe after resistance welding)) / (thickness) x 100 (%).

[0197] The sizing process after resistance welding is performed in order to improve the outer diameter accuracy and the circularity. In order to improve the outer diameter accuracy and the circularity, the steel pipe is reduced in diameter so that the circumference of the steel pipe is reduced by 0.5% or more in total. In the case where the circumference of the steel pipe is reduced by 4.0% or more in total, the amount of bending in the pipe axis direction when passing through the rolls becomes large, the residual stress increases, the dislocation density of the inner surface of the pipe increases, the density of the largest angle grain boundary increases, and as a result, the SSC resistance decreases. Therefore, the steel pipe is reduced in diameter so that the circumference of the steel pipe is reduced by 0.5% to 4.0% in total. The circumference of the steel pipe is preferably 1.0% or more and preferably 3.0% or less.

[0198] Note that, in the sizing process after resistance welding, in order to minimize the amount of bending in the pipe axis direction when passing through the rolls and suppress the generation of residual stress in the pipe axis direction, it is preferable to perform multi-stage reduction in diameter in multiple stands. The reduction in diameter in each stand is preferably performed so that the circumference of the pipe is reduced by 1.0% or less.

[0199] Here, whether a steel pipe is a resistance-welded steel pipe can be determined by cutting the pipe perpendicular to its axis, grinding and corroding the cut surface containing the weld (resistance weld), and then observing it under an optical microscope. Specifically, if the width of the molten and solidified portion of the weld (resistance weld) in the circumferential direction is 1.0 μm to 1000 μm over the entire thickness of the pipe, then it is a resistance-welded steel pipe.

[0200] The aforementioned corrosive liquid can be appropriately selected based on the steel composition and the type of steel pipe.

[0201] Figure 1 The diagram schematically shows a portion of the aforementioned cross-section after corrosion (near the welded section of the resistance-welded steel pipe). The molten solidification section is shown as... Figure 1 The area shown is the molten solidified portion 3, which has a different microstructure and contrast from the base material portion 1 and the heat-affected portion 2. For example, the molten solidified portion of resistance welded steel pipes made of carbon steel and low alloy steel can be identified as a white area observed under an optical microscope in the aforementioned cross-section etched with nitric acid and alcohol. Furthermore, the molten solidified portion of UOE steel pipes made of carbon steel and low alloy steel can be identified under an optical microscope in the aforementioned cross-section etched with nitric acid and alcohol as an area containing a honeycomb-like or dendritic solidified structure.

[0202] The high-strength hot-rolled steel plate and high-strength resistance-welded steel pipe of the present invention are manufactured according to the manufacturing method described above. The high-strength hot-rolled steel plate of the present invention has a wall thickness of 15 mm or more, and the base material of the high-strength resistance-welded steel pipe of the present invention has a wall thickness of 15 mm or more, exhibiting excellent resistance to SSC (Supersonic Sludge Crushing). Furthermore, it also possesses high yield strength.

[0203] Example

[0204] The present invention will now be described in further detail with reference to embodiments. It should be noted that the present invention is not limited to the following embodiments.

[0205] Molten steel with the composition shown in Table 1 is melted to produce slabs (steel billets). The obtained slabs are then subjected to hot rolling, first cooling, and second cooling processes, as well as winding processes, under the conditions shown in Table 2, to produce hot-rolled steel sheets with the final sheet thickness (mm) shown in Table 2.

[0206] After the winding process, the obtained hot-rolled steel sheet is formed into a cylindrical open tube (round steel pipe) using cold rolling rolls. The butt joint of the open tube is then resistance welded to produce a steel pipe billet (pipe manufacturing process). Then, the steel pipe billet is passed through rollers arranged on the top, bottom, left, and right sides of the steel pipe billet to reduce its diameter (sizing process), resulting in resistance-welded steel pipes with the outer diameter (mm) and wall thickness (mm) shown in Table 4.

[0207] From the obtained hot-rolled steel sheet and electric resistance welded steel pipe, various test pieces were taken, and measurement of average grain diameter, measurement of dislocation density, measurement of maximum angle grain boundary density, observation of structure, tensile test, 4-point bending corrosion test were carried out using the following methods. Here, the various test pieces were taken from the center in the width direction in the hot-rolled steel sheet, and from the base material portion which deviates by 90° from the electric resistance welded portion in the pipe circumferential direction when the electric resistance welded portion is set to 0° in the electric resistance welded steel pipe.

[0208] [Measurement of average grain diameter]

[0209] The test pieces for measurement were taken from the hot-rolled steel sheet and the electric resistance welded steel pipe in such a manner that the cross section parallel to both the rolling direction and the sheet thickness direction of the hot-rolled steel sheet, and the cross section parallel to both the pipe axis direction and the wall thickness direction of the electric resistance welded steel pipe were measured, and mirror polished to be produced. The average grain diameter was measured using the SEM / EBSD method. The average grain diameter was measured by taking the orientation difference between adjacent grains, taking the boundary where the orientation difference was 15° or more as a grain boundary. The arithmetic mean of the grain diameter (equivalent circle diameter) of the obtained grain boundaries was taken as the average grain diameter. The acceleration voltage of the measurement conditions was 15 kV, the measurement area was 100 μm x 100 μm, and the measurement step was 0.5 μm.

[0210] Note that, for the grain diameter analysis, grains having a grain diameter of less than 2.0 μm were excluded from the analysis target as measurement noise, and the area ratio was made equal to the volume ratio.

[0211] In addition, the measurement positions were the center of the sheet thickness of the hot-rolled steel sheet and a position 0.1 mm deep from the sheet surface, and the center of the wall thickness of the electric resistance welded steel pipe and a position 0.1 mm deep from the inner surface of the pipe, and the histogram (horizontal axis: grain diameter, vertical axis: graph of the existence ratio at each grain diameter) of the grain diameter distribution was calculated at each position, and the average grain diameter was calculated as the arithmetic mean of the average grain diameter.

[0212] [Measurement of dislocation density]

[0213] The dislocation density of the center of the sheet thickness of the hot-rolled steel sheet and the center of the wall thickness of the electric resistance welded steel pipe was measured as follows. The test pieces for dislocation density were produced by mirror polishing the cross section parallel to both the rolling direction and the sheet thickness direction of the hot-rolled steel sheet, and the cross section parallel to both the pipe axis direction and the wall thickness direction of the electric resistance welded steel pipe, and electrolytically polishing the polished surface by 100 μm to remove the surface processing layer, so that the diffraction surface became the center portion of the sheet thickness (or the wall thickness). The dislocation density of the center of the sheet thickness of the hot-rolled steel sheet and the center of the wall thickness of the electric resistance welded steel pipe was measured using the test pieces by X-ray diffraction, and the result was used to calculate the dislocation density using the modified Williamson-Hall method and the modified Warren-Averbach method (refer to references 1 and 2).

[0214] The dislocation density at a position 0.1 mm deep from the plate surface of the hot-rolled steel plate and at a position 0.1 mm deep from the inner surface of the electric resistance welded steel pipe was measured as follows. The test piece for dislocation density was prepared by taking a cross section parallel to both the rolling direction and the plate thickness direction of the hot-rolled steel plate and a cross section parallel to both the pipe axis direction and the wall thickness direction of the electric resistance welded steel pipe, mirror-polishing, and electrolytically polishing the mirror-polished surface by 50 μm to remove the surface processing layer, with the diffraction surface being located at a position 0.1 mm deep from the plate surface and the inner surface. The dislocation density was measured by X-ray diffraction in the same manner as in the case of the center of the plate thickness (or the wall thickness), and the result was obtained.

[0215] 〔Measurement of maximum low angle grain boundary density〕

[0216] The test piece for measurement was prepared by taking a cross section parallel to both the rolling direction and the plate thickness direction of the hot-rolled steel plate and a cross section parallel to both the pipe axis direction and the wall thickness direction of the electric resistance welded steel pipe, respectively, from the hot-rolled steel plate and the electric resistance welded steel pipe, and mirror-polishing. The maximum low angle grain boundary density was obtained using the SEM / EBSD method.

[0217] At a position 0.1 mm deep from the surface of the hot-rolled steel plate and at a position 0.1 mm deep from the inner surface of the electric resistance welded steel pipe, the measurement range was set to 10 μm x 10 μm, and 20 or more fields of view were measured, respectively. For each field of view, the total length of the grain boundary having an orientation difference of 2° or more and less than 15° was calculated, and the low angle grain boundary density of each field of view was obtained. Here, the maximum value of the low angle grain boundary density obtained at each measurement position was set as the maximum low angle grain boundary density.

[0218] 〔Microstructure observation〕

[0219] The test piece for microstructure observation was prepared by taking a cross section parallel to both the rolling direction and the plate thickness direction of the hot-rolled steel plate and a cross section parallel to both the pipe axis direction and the wall thickness direction of the electric resistance welded steel pipe, respectively, from the hot-rolled steel plate and the electric resistance welded steel pipe, mirror-polishing, and etching with nitric acid alcohol. The microstructure of the center of the plate thickness and the position 0.1 mm deep from the surface of the hot-rolled steel plate and the center of the wall thickness and the position 0.1 mm deep from the inner surface of the electric resistance welded steel pipe was observed and photographed using an optical microscope (magnification: 1000x) or a scanning electron microscope (SEM, magnification: 1000x). From the obtained optical microscope images and SEM images, the area ratio of bainite and the remaining portion (ferrite, pearlite, martensite, austenite) was obtained. The area ratio of each microstructure was observed at 5 or more fields of view, and was calculated as the average of the values obtained in each field of view. Here, the area ratio obtained by microstructure observation was set as the volume ratio of each microstructure.

[0220] Here, ferrite is a product of diffusion phase transformation, and exhibits a structure with low dislocation density and almost recovered structure. It contains polygonal ferrite and simulated polygonal ferrite.

[0221] Bainite is a complex structure of lath-shaped ferrite and cementite with high dislocation density.

[0222] Pearlite is a eutectoid structure of iron and iron carbide (ferrite + cementite), and exhibits a lamellar structure in which linear ferrite and cementite are alternately arranged.

[0223] Martensite is a lath-shaped low-temperature phase transformation structure with very high dislocation density. In the SEM image, it exhibits a bright contrast with ferrite and bainite.

[0224] Note that it is difficult to distinguish between martensite and austenite in the optical microscope image and the SEM image. Therefore, the area ratio of the structure observed as martensite or austenite is determined from the obtained SEM image, and the volume fraction of austenite is subtracted from the value determined by the method described later to obtain the volume fraction of martensite.

[0225] The volume fraction of austenite is measured by X-ray diffraction. Test pieces for measurement of the central portion of the sheet thickness of the hot-rolled steel sheet and the central portion of the wall thickness of the electric resistance welded steel pipe are prepared by grinding so that the diffraction plane becomes the central portion of the sheet thickness of the hot-rolled steel sheet and the central portion of the wall thickness of the electric resistance welded steel pipe, respectively, and then removing the surface processing layer by chemical polishing. In addition, test pieces for measurement of a position 0.1 mm deep from the surface of the hot-rolled steel sheet and a position 0.1 mm deep from the inner surface of the electric resistance welded steel pipe are prepared by mirror polishing so that the diffraction plane becomes the surface of the hot-rolled steel sheet and the inner surface of the electric resistance welded steel pipe, respectively, and then removing the surface processing layer by chemical polishing of the polished surface. The volume fraction of austenite is determined from the integrated intensities of the (200), (220), (311) planes of fcc iron and the (200), (211) planes of bcc iron using Mo Kα rays.

[0226] 〔Tensile Test〕

[0227] Test pieces are JIS No. 5 tensile test pieces taken in the hot-rolled steel sheet so that the tensile direction is parallel to the rolling direction, and in the electric resistance welded steel pipe so that the tensile direction is parallel to the pipe axis direction. The tensile test is performed in accordance with the provisions of JIS Z 2241, and the yield strength (MPa) is measured. Here, the yield strength is the flow stress at a nominal strain of 0.5%.

[0228] 〔4-Point Bend Corrosion Test〕

[0229] A 4-point bending corrosion test piece of 5 mm thickness x 15 mm width x 115 mm length was taken from a hot-rolled steel sheet and an electric resistance welded steel pipe. In the hot-rolled steel sheet, it was taken in a manner that the width direction of the corrosion test piece is perpendicular to the rolling direction and the thickness direction of the hot-rolled steel sheet and the length direction of the corrosion test piece is parallel to the rolling direction of the hot-rolled steel sheet. In the electric resistance welded steel pipe, it was taken in a manner that the width direction of the corrosion test piece is parallel to the pipe circumferential direction of the electric resistance welded steel pipe and the length direction of the corrosion test piece is parallel to the pipe axial direction of the electric resistance welded steel pipe.

[0230] The bending outer side, that is, the corrosion surface was taken in a state that the surface layer is left as it is. A 4-point bending corrosion test was carried out in accordance with the EFC 16 standard on the corrosion surface of the taken test piece, under a tensile stress of 90% of the yield strength obtained from the tensile test described above, using a NACE standard TM0177 Solution A solution, at a hydrogen sulfide partial pressure of 1 bar. After the test piece was immersed in the solution for 720 hours, it was confirmed whether or not cracking had occurred. In addition, at the 1 / 3 position and the 2 / 3 position in the width direction of the test piece after the test, test pieces for observation were taken in a manner that the cross section of the observation surface is parallel to the thickness direction and the length direction. The test pieces for observation obtained were subjected to mirror polishing and optical microscope observation, the depth and the width of all pitting occurring in the portion under the tensile stress were measured, and the maximum depth of the pitting and the maximum value of (depth / width) of the pitting were calculated.

[0231] The results obtained are shown in Tables 3 and 4.

[0232]

[0233]

[0234]

[0235]

[0236] In Tables 3 and 4, the hot-rolled steel sheets of Nos. 1, 4, 7, 11, 14, 18, 20 to 24, 26, 28, 31 and the electric resistance welded steel pipes of Nos. 1, 4, 7, 11, 14, 18, 20, 22, 24, 26, 28, 31 are examples of the present application. The hot-rolled steel sheets of Nos. 2, 3, 5, 6, 8 to 10, 12, 13, 15 to 17, 19, 25, 27, 29, 30, 32, 33 and the electric resistance welded steel pipes of Nos. 2, 3, 5, 6, 8 to 10, 12, 13, 15 to 17, 19, 21, 23, 25, 27, 29, 30, 32, 33 are comparative examples.

[0237] In the steel structure in the center of the plate thickness of any one of the hot-rolled steel plates of the present example, the volume fraction of bainite is 50% or more, the volume fraction of the sum of ferrite and bainite is 95% or more, the remaining portion contains one or two or more selected from the group consisting of pearlite, martensite, and austenite, the average grain diameter is 9.0 μm or less, the dislocation density is 1.0 x 10 14 m -2 ~ 1.0 x 10 15 m -2 In the steel structure in the position 0.1 mm from the plate surface depth, the volume fraction of bainite is 70% or more, the volume fraction of the sum of ferrite and bainite is 95% or more, the remaining portion contains one or two or more selected from the group consisting of pearlite, martensite, and austenite, the average grain diameter is 9.0 μm or less, the dislocation density is 5.0 x 10 14 m -2 ~ 1.0 x 10 15 m -2 , and the maximum angle grain boundary density is 1.4 x 10 6 m -1 The plate thickness of the hot-rolled steel plate is 15 mm or more.

[0238] In the steel structure in the center of the wall thickness of the base material portion of any one of the electric resistance welded steel pipes of the present example, the volume fraction of bainite is 50% or more, the volume fraction of the sum of ferrite and bainite is 95% or more, the remaining portion contains one or two or more selected from the group consisting of pearlite, martensite, and austenite, the average grain diameter is 9.0 μm or less, the dislocation density is 2.0 x 10 14 m -2 ~ 1.0 x 10 15 m -2 In the steel structure in the position 0.1 mm from the inner surface of the base material portion of the pipe, the volume fraction of bainite is 70% or more, the volume fraction of the sum of ferrite and bainite is 95% or more, the remaining portion contains one or two or more selected from the group consisting of pearlite, martensite, and austenite, the average grain diameter is 9.0 μm or less, the dislocation density is 6.0 x 10 14 m -2 ~ 1.0 x 10 15 m -2 , and the maximum angle grain boundary density is 1.5 x 10 6 m -1 The wall thickness is 15 mm or more.

[0239] Further, the yield strength in the tensile test of any one of the hot-rolled steel plates and the electric resistance welded steel pipes of the present example is 400 MPa or more, no cracking occurs in the 4-point bending corrosion test, the depth of the pitting corrosion that occurs is less than 250 μm, and the (depth / width) is less than 3.0.

[0240] On the other hand, the hot-rolled steel sheet and the electric resistance welded steel pipe of Comparative Example No. 2 have a high average cooling rate at the center of the sheet thickness in the first cooling step, and thus a large amount of martensite is generated on the surface of the steel sheet, and the maximum angle grain boundary density increases. As a result, the desired SSC resistance cannot be obtained.

[0241] The hot-rolled steel sheet and the electric resistance welded steel pipe of Comparative Example No. 3 have a low average cooling rate at the center of the sheet thickness in the first cooling step, and thus the ferrite fraction increases on the surface of the steel sheet and at the center of the sheet thickness, and a structure having the target bainite fraction of the present application cannot be obtained. In addition, the ferrite and the bainite are coarsened at the center of the sheet thickness, and a structure having the target average grain diameter of the present application cannot be obtained. As a result, the desired yield strength cannot be obtained.

[0242] The hot-rolled steel sheet and the electric resistance welded steel pipe of Comparative Example No. 5 have a high cooling stop temperature at the center of the sheet thickness in the first cooling step, and thus the cooling stop temperature of the surface of the sheet also becomes high, and the ferrite fraction increases on the surface of the steel sheet and at the center of the sheet thickness, and a structure having the target bainite fraction of the present application cannot be obtained. In addition, the ferrite and the bainite are coarsened at the center of the sheet thickness, and a structure having the target average grain diameter of the present application cannot be obtained. As a result, the desired yield strength cannot be obtained.

[0243] The hot-rolled steel sheet and the electric resistance welded steel pipe of Comparative Example No. 6 have a low cooling stop temperature at the center of the sheet thickness in the first cooling step, and thus the cooling stop temperature of the surface of the sheet also becomes low, and a large amount of martensite is generated on the surface of the steel sheet, and the maximum angle grain boundary density increases. As a result, the desired SSC resistance cannot be obtained.

[0244] The hot-rolled steel sheet and the electric resistance welded steel pipe of Comparative Example No. 8 have a low cooling stop temperature at the surface of the sheet in the first cooling step, and thus a large amount of martensite is generated on the surface of the steel sheet, and the maximum angle grain boundary density increases. As a result, the desired SSC resistance cannot be obtained.

[0245] The hot-rolled steel sheet and the electric resistance welded steel pipe of Comparative Example No. 9 have a long time from the end of the first cooling step to the start of the second cooling step, and thus the ferrite or the bainite is coarsened at the center of the sheet thickness. As a result, the desired yield strength cannot be obtained.

[0246] The hot-rolled steel sheet and the electric resistance welded steel pipe of Comparative Example No. 10 have a short time from the end of the first cooling step to the start of the second cooling step, and thus the dislocation density of the surface of the sheet increases, and the maximum angle grain boundary density increases. As a result, the desired SSC resistance cannot be obtained.

[0247] The hot-rolled steel sheet and the electric resistance welded steel pipe of Comparative Example No. 12 could not obtain the desired SSC resistance because a large amount of martensite was generated on the surface of the steel sheet and the maximum angle grain boundary density increased due to the high average cooling rate at the center of the sheet thickness in the second cooling process.

[0248] The hot-rolled steel sheet and the electric resistance welded steel pipe of Comparative Example No. 13 could not obtain the desired yield strength because ferrite and bainite were coarsened at the center of the sheet thickness and a structure having the target average grain diameter of the present application could not be obtained due to the low average cooling rate at the center of the sheet thickness in the second cooling process.

[0249] The hot-rolled steel sheet and the electric resistance welded steel pipe of Comparative Example No. 15 could not obtain the desired yield strength because ferrite and bainite were coarsened at the center of the sheet thickness due to the high cooling stop temperature at the center of the sheet thickness in the second cooling process. Therefore, a structure having the target average grain diameter of the present application could not be obtained.

[0250] The hot-rolled steel sheet and the electric resistance welded steel pipe of Comparative Example No. 16 could not obtain the desired SSC resistance because a large amount of martensite was generated on the surface of the steel sheet and the maximum angle grain boundary density increased due to the low cooling stop temperature at the center of the sheet thickness in the second cooling process.

[0251] The hot-rolled steel sheet and the electric resistance welded steel pipe of Comparative Example No. 17 could not obtain the desired SSC resistance because a large amount of martensite was generated on the surface of the steel sheet and the maximum angle grain boundary density increased due to the low average cooling rate at the surface of the sheet in the second cooling process.

[0252] The hot-rolled steel sheet and the electric resistance welded steel pipe of Comparative Example No. 19 could not obtain the desired SSC resistance because a large amount of martensite was generated on the surface of the steel sheet and the maximum angle grain boundary density increased due to the high average cooling rate at the center of the sheet thickness in the first and second cooling processes.

[0253] The electric resistance welded steel pipe of Comparative Example No. 21 could not obtain the desired SSC resistance because the dislocation density and the maximum angle grain boundary density at the inner surface of the pipe increased due to the large upsetting amount in the electric resistance welding process.

[0254] The electric resistance welded steel pipe of Comparative Example No. 23 could not obtain the desired SSC resistance because the dislocation density and the maximum angle grain boundary density at the inner surface of the pipe increased due to the high reduction ratio in the sizing process.

[0255] The hot-rolled steel sheet and the electric resistance-welded steel pipe of Comparative Example No. 25 cannot obtain the microstructure having the target bainite fraction of the present application because the average cooling rate at the center of the sheet thickness of the first cooling step and the second cooling step is low. In addition, in the center of the sheet thickness, ferrite and bainite are coarsened, and the steel microstructure having the target average grain diameter of the present application cannot be obtained. As a result, the desired yield strength cannot be obtained.

[0256] The hot-rolled steel sheet and the electric resistance-welded steel pipe of Comparative Example No. 27 cannot obtain the steel microstructure having the target average grain diameter of the present application because the heating temperature of the hot-rolling step is high. As a result, the desired yield strength cannot be obtained.

[0257] The hot-rolled steel sheet and the electric resistance-welded steel pipe of Comparative Example No. 29 cannot obtain the desired SSC resistance because the dislocation density of the sheet surface increases and the maximum angle grain boundary density increases due to the low finish rolling end temperature in the hot-rolling step.

[0258] The hot-rolled steel sheet and the electric resistance-welded steel pipe of Comparative Example No. 30 cannot obtain the desired SSC resistance because the maximum angle grain boundary density of the sheet surface increases due to the low finish rolling start temperature in the hot-rolling step.

[0259] The hot-rolled steel sheet and the electric resistance-welded steel pipe of Comparative Example No. 32 cannot obtain the desired SSC resistance because the maximum angle grain boundary density of the sheet surface increases due to the low finish rolling end temperature in the hot-rolling step.

[0260] The hot-rolled steel sheet and the electric resistance-welded steel pipe of Comparative Example No. 33 cannot obtain the steel microstructure having the target average grain diameter of the present application because the total reduction rate of the finish rolling in the hot-rolling step is low. As a result, the desired yield strength cannot be obtained.

[0261] Explanation of Symbols

[0262] 1: Base material portion

[0263] 2: Weld heat-affected portion

[0264] 3: Fusion solidification portion

Claims

1. A high-strength hot-rolled steel sheet, in a steel structure in the center of the sheet thickness, the volume fraction of bainite is 50% or more, the volume fraction of ferrite and bainite in total is 95% or more, the remainder contains one or two or more kinds selected from the group consisting of pearlite, martensite, and austenite, the average grain diameter is 9.0 μm or less, Dislocation density is 1.0 x 10 14 m -2 ~ 1.0 x 10 15 m -2 ; in a steel structure at a position 0.1 mm in the depth direction from the sheet surface, the volume fraction of bainite is 70% or more, the volume fraction of ferrite and bainite in total is 95% or more, the remainder contains one or two or more kinds selected from the group consisting of pearlite, martensite, and austenite, the average grain diameter is 9.0 μm or less, Dislocation density is 5.0 x 10 14 m -2 ~ 1.0 x 10 15 m -2 , The maximum angle grain boundary density is 1.4 x 10 6 m -1 The following; the high-strength hot-rolled steel sheet contains, in mass%, C: 0.020% to 0.15%, Si: 1.0% or less, Mn: 0.30% to 2.0%, P: 0.050% or less, S: 0.020% or less, Al: 0.005% to 0.10%, N: 0.010% or less, Nb: 0.15% or less, V: 0.15% or less, and Ti: 0.15% or less, further contains one or two or more kinds selected from the group consisting of Cr: 1.0% or less, Mo: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, Ca: 0.010% or less, and B: 0.010% or less, the remainder consists of Fe and inevitable impurities; the high-strength hot-rolled steel sheet has a sheet thickness of 15 mm or more, the high-strength hot-rolled steel sheet has a yield strength of 400 MPa or more.

2. A method of producing a high-strength hot-rolled steel sheet, the method of producing a high-strength hot-rolled steel sheet according to claim 1, when a steel material having the composition is subjected to a hot-rolling step, then subjected to a first cooling step and a second cooling step, and then subjected to a step of coiling into a coil shape, in the hot-rolling step, after heating to a heating temperature of 1100°C to 1300°C, hot-rolling is performed with a finish of rough rolling temperature of 900°C to 1100°C, a start of finish rolling temperature of 800°C to 950°C, a finish of finish rolling temperature of 750°C to 850°C, and a total reduction rate in finish rolling of 60% or more, then, in the first cooling step, cooling is performed with an average cooling rate at the center of the sheet thickness of 10°C / s to 60°C / s, a cooling stop temperature of 550°C to 650°C, and a cooling stop temperature at the sheet surface of 250°C to 450°C, the time from the end of the first cooling step to the start of the second cooling step is 5 s to 20 s, then, in the second cooling step, cooling is performed with an average cooling rate at the center of the sheet thickness of 5°C / s to 30°C / s, a cooling stop temperature of 450°C to 600°C, and a cooling stop temperature at the sheet surface of 150°C to 350°C.

3. A high-strength electric resistance welded steel pipe having a base material portion and an electric resistance welded portion, in a steel structure in the center of the wall thickness of the base material portion, the volume fraction of bainite is 50% or more, the volume fraction of ferrite and bainite in total is 95% or more, the remainder contains one or two or more kinds selected from the group consisting of pearlite, martensite, and austenite, an average grain diameter of 9.0 μm or less, Dislocation density is 2.0 x 10 14 m -2 ~ 1.0 x 10 15 m -2 ; in the steel structure at a position 0.1 mm in the depth direction from the inner surface of the pipe of the base material portion, a volume fraction of bainite of 70% or more, a volume fraction of ferrite and bainite in total of 95% or more, the remaining portion containing one or two or more selected from the group consisting of pearlite, martensite, and austenite, an average grain diameter of 9.0 μm or less, Dislocation density is 6.0 x 10 14 m -2 ~ 1.0 x 10 15 m -2 , The maximum low angle grain boundary density is 1.5 x 10 6 m -1 The following; the base material portion having a composition consisting of, by mass%, C: 0.020% to 0.15%, Si: 1.0% or less, Mn: 0.30% to 2.0%, P: 0.050% or less, S: 0.020% or less, Al: 0.005% to 0.10%, N: 0.010% or less, Nb: 0.15% or less, V: 0.15% or less, and Ti: 0.15% or less, and one or two or more selected from the group consisting of Cr: 1.0% or less, Mo: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, Ca: 0.010% or less, and B: 0.010% or less, the remaining portion consisting of Fe and inevitable impurities; the base material portion having a wall thickness of 15 mm or more, the base material portion having a yield strength of 400 MPa or more.

4. A method for manufacturing a high-strength electric resistance welded steel pipe, by cold roll forming a high-strength hot-rolled steel sheet according to claim 1 into a cylindrical shape, butt joining both ends in the circumferential direction of the cylindrical shape, and performing electric resistance welding, the upset amount at the time of electric resistance welding being 20% to 100% of the sheet thickness of the high-strength hot-rolled steel sheet, in a sizing process after the electric resistance welding, reducing the diameter to reduce the circumference of the steel pipe by a ratio of 0.5% to 4.0%.

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