Hot-rolled steel sheet and its manufacturing method

Abstract

To provide a hot rolled steel sheet as a raw material of an electric resistance welded pipe that is suitable for a steel pipe for a machine structure to be used for components of automobile, construction machinery and industrial machinery, and has high strength, and excellent workability and plasticity.SOLUTION: A hot rolled steel sheet has a predetermined component composition, and a steel structure at the sheet thickness center having a volume fraction of bainite of 90% or more, and the balance containing one kind or two or more kinds among ferrite, perlite, martensite and austenite. In the steel structure at the sheet thickness center, a volume fraction of crystal grains having an average crystal grain size of 10.0 μm or less, and a grain diameter of 40.0 μm or more is 40% or less, and a number density of Ti based inclusions having a major axis of 5.0 μm or more is 20 / mm2 or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an electric resistance welded steel pipe suitable for use as a steel pipe for machine structures used in parts of automobiles, construction machinery, and industrial machinery, a hot rolled steel sheet serving as the raw material for the electric resistance welded steel pipe, and a method for manufacturing the electric resistance welded steel pipe and the hot rolled steel sheet. [Background technology]

[0002] In recent years, there has been a demand for higher strength components in automobiles, construction machinery, and industrial machinery in order to improve fuel efficiency and reduce the size of equipment. Furthermore, the steel pipes for machine structures used in these applications are required to have high workability, since they are subjected to processes such as bending, hydraulic bulging, and pipe end flaring depending on the application. Furthermore, since the above-mentioned automobiles, construction machinery, and industrial machinery are sometimes used in cold regions, they also require high low-temperature toughness.

[0003] However, when the strength of a member is increased, the workability and toughness of the base material and the electric resistance welded portion are reduced, making it impossible to use the member for its intended purpose. Furthermore, since electric resistance welded steel pipes lose their workability and toughness due to work hardening during pipe making, the hot-rolled steel sheets used as raw materials are required to have properties that take into account the loss of workability and toughness during pipe making.

[0004] In order to solve the above problems, for example, Patent Document 1 proposes a steel sheet in which bainite accounts for 90 area % or more, the average circular equivalent diameter d of regions surrounded by high-angle grain boundaries with a misorientation of 15° or more is 4 μm or less, and further contains 3 to 10 area % of island martensite having an average circular equivalent diameter of 0.5 to 3 μm and a Vickers hardness Hv of 700 or more.

[0005] In addition, in Patent Document 2, the total area ratio of the martensite phase and the lower bainite structure is 85% or more, the average grain size is 20 μm or less, and the area ratio of grains having an aspect ratio of 0.30 or less is 50% or less, and the {100} <011> ~{211} <011> A hot-rolled steel sheet has been proposed in which the average value of the X-ray random intensity ratio of the orientation group is 6.0 or less and the maximum value is 8.0 or less.

[0006] Furthermore, Patent Document 3 proposes an ad-rolled electric resistance welded steel pipe for torsion beams, in which the area fraction of bainite is 80% or more, the average particle size of the bainite packet grains is 10 μm or less, the average aspect ratio of the packet grains is 2.0 or less, and the tensile strength in the axial direction of the pipe is 750 to 1000 MPa. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-057105 [Patent Document 2] Japanese Patent Application Publication No. 2017-057472 [Patent Document 3] International Publication No. 2019 / 220577 Summary of the Invention [Problem to be solved by the invention]

[0008] However, the steel plates and electric resistance welded steel pipes described in Patent Documents 1 to 3 have improved strength, toughness, workability, fatigue properties, and the like by controlling the average crystal grain size of the main structures, bainite and martensite. However, no consideration has been given to the proportion of coarse grains, inclusions, or workability of electric resistance welds, and there is room for further improvement in toughness and workability.

[0009] The present invention has been made in consideration of the above circumstances, and is applicable to automobiles, construction machines, industrial machines, etc. The present invention aims to provide an electric resistance welded steel pipe having high strength and excellent workability and toughness, suitable for use as a steel pipe for machine structures in products, a hot rolled steel sheet that is the raw material for the electric resistance welded steel pipe, and a method for manufacturing the same.

[0010] In the present invention, "high strength" refers to a hot-rolled steel sheet having a rolling direction yield stress of 650 MPa or more and a tensile strength of 750 MPa or more. Preferably, the hot-rolled steel sheet has a yield stress of 700 MPa or more and a tensile strength of 800 MPa or more. On the other hand, in the present invention, "high strength" means that, in the case of an electric resistance welded steel pipe, the yield stress in the pipe axis direction is 650 MPa or more, the tensile strength is 750 MPa or more, and the Vickers hardness at the center of the wall thickness of the electric resistance weld is 200 HV or more. In the case of an electric resistance welded steel pipe, the yield stress is preferably 700 MPa or more, the tensile strength is 800 MPa or more, and the Vickers hardness is 220 HV or more.

[0011] In the present invention, "excellent workability" means that, in the case of a hot-rolled steel sheet, the yield ratio in the rolling direction (=yield stress / tensile strength × 100) is 90.0% or less and the total elongation in the rolling direction is 15% or more. In the case of a hot-rolled steel sheet, the yield ratio in the rolling direction is preferably 88.0% or less. In addition, in the case of a hot-rolled steel sheet, the total elongation in the rolling direction is preferably 18% or more. On the other hand, "excellent workability" as used herein means that, in the case of an electric resistance welded steel pipe, the yield ratio (= yield stress / tensile strength × 100) in the axial direction is 96.0% or less, the total elongation in the axial direction is 15% or more, and the flattening value of the electric resistance welds is 0.80 or less. In the case of an electric resistance welded steel pipe, the yield ratio in the axial direction is preferably 94.0% or less. In the case of an electric resistance welded steel pipe, the total elongation in the axial direction is preferably 17% or more, and the flattening value of the electric resistance welds is preferably 0.70 or less.

[0012] In the present invention, "excellent toughness" means that, in the case of a hot-rolled steel sheet, the Charpy impact value at -60°C is 100 J / cm 2In the case of a hot-rolled steel sheet, the Charpy impact value at -60°C is preferably 140 J / cm or more. 2 That's all. On the other hand, "excellent toughness" in the present invention means that, in the case of electric resistance welded steel pipes, the Charpy impact value at -20°C is 50 J / cm 2 In the electric resistance welded steel pipe, the Charpy impact value at -20°C is preferably 60 J / cm or more. 2 That's all. [Means for solving the problem]

[0013] The present inventors have conducted extensive research into the above-mentioned problems and have found that in steels mainly composed of bainite, if some of the crystal grains are coarse, the toughness and workability decrease.

[0014] They also found that the presence of a large amount of coarse Ti-based inclusions reduces toughness. If the microstructure is coarse on average, the proportion of high-angle grain boundaries that act as barriers to brittle fracture decreases, thereby reducing the toughness of the steel plate. Also, even if the average grain size of the steel microstructure is small, if a certain proportion of coarse grains are present, these grains will become the starting points for brittle fracture, and the toughness of the steel plate will also decrease.

[0015] In other words, coarse grains have lower strength than the surrounding areas and strain is concentrated therein. Therefore, if a certain proportion of coarse grains is present in the steel structure, the steel becomes prone to premature fracture, the total elongation decreases, and the workability deteriorates. Therefore, by suppressing the generation of bainite that exists as coarse grains, it is possible to improve toughness and workability.

[0016] In addition, Ti-based inclusions have a polygonal shape, and stress tends to concentrate near the corners. Such stress concentrations tend to become the starting points for brittle fracture, and therefore, the presence of many coarse Ti-based inclusions reduces toughness. Therefore, by suppressing the formation of such Ti-based inclusions, toughness can be improved.

[0017] The present invention was completed based on these findings and further investigations. That is, the gist of the present invention is as follows. 1. A hot-rolled steel sheet, In mass%, C: 0.020% or more and 0.200% or less, Si: 0.02% or more and 1.00% or less, Mn: 0.10% or more and 2.50% or less, P: 0.050% or less, S: 0.0200% or less, Al: 0.005% or more and 0.100% or less, N: 0.0100% or less and Ti: 0.030% or more and 0.200% or less and the balance being Fe and unavoidable impurities, The steel has a steel structure at the center of the plate thickness, in which the volume fraction of bainite is 90% or more and the remainder includes one or more of ferrite, pearlite, martensite, and austenite, Furthermore, the steel structure at the center of the plate thickness has an average crystal grain size of 10.0 μm or less, a volume fraction of crystal grains with a grain size of 40.0 μm or more is 40% or less, and the number density of Ti-based inclusions with a major diameter of 5.0 μm or more is 20 pieces / mm 2 Below is hot rolled steel sheet.

[0018] 2. The component composition further includes, in mass%, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.50% or less Nb: 0.050% or less, V: 0.050% or less, Ca: 0.0050% or less and B: 0.0050% or less 2. The hot-rolled steel sheet according to 1 above, comprising one or more of the following:

[0019] 3. The method for producing a hot-rolled steel sheet according to 1 or 2 above, A casting process in which molten steel is cast from the solidification point of the molten steel to 1200°C at an average cooling rate of 0.30°C / s or more to produce a steel material; a hot rolling process in which the steel material is heated to a heating temperature in the range of 1150°C to 1300°C, and then hot-rolled at a rough rolling end temperature of 950°C to 1180°C, a finish rolling end temperature of 850°C to 1000°C, and a total reduction in finish rolling of 50% to 80%; After the hot rolling process, a cooling process is performed in which cooling is performed at an average cooling rate of 10°C / s or more and 60°C / s or less at the center of thickness, and a cooling stop temperature of 400°C or more and 580°C or less at the center of thickness; a winding step of winding the sheet at a temperature of 400°C or higher and 580°C or lower after the cooling step; A method for manufacturing a hot-rolled steel sheet, comprising:

[0020] 4. An electric resistance welded steel pipe having a base metal portion, a welded portion, and a weld heat affected zone, The base material portion is, in mass%, C: 0.020% or more and 0.200% or less, Si: 0.02% or more and 1.00% or less, Mn: 0.10% or more and 2.50% or less, P: 0.050% or less, S: 0.0200% or less, Al: 0.005% or more and 0.100% or less, N: 0.0100% or less and Ti: 0.030% or more and 0.200% or less and the balance being Fe and unavoidable impurities, the base material portion has a steel structure at the center of the wall thickness, in which the volume fraction of bainite is 90% or more and the remainder includes one or more of ferrite, pearlite, martensite, and austenite; Furthermore, the steel structure at the center of the thickness of the base material portion has an average crystal grain size of 10.0 μm or less, a volume fraction of crystal grains with a grain size of 40.0 μm or more is 40% or less, and the number density of Ti-based inclusions with a major diameter of 5.0 μm or more is 20 pieces / mm 2 is as follows: the weld has a steel structure at the center of the wall thickness in which the volume fraction of ferrite is 10% or more, the total volume fraction of ferrite and bainite is 60% or more and 98% or less, and the remainder contains one or more of pearlite, martensite, and austenite; Furthermore, in the electric resistance welded steel pipe, the steel structure at the center of the wall thickness of the welded portion has an average crystal grain size of 15.0 μm or less.

[0021] 5. The composition of the base material portion is further, in mass%, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.50% or less Nb: 0.050% or less, V: 0.050% or less, Ca: 0.0050% or less and B: 0.0050% or less 5. The electric resistance welded steel pipe according to 4 above, which contains one or more of the following:

[0022] 6. A method for producing an electric resistance welded steel pipe according to 4 or 5, a pipe-making process in which the hot-rolled steel sheet is formed into a cylindrical shape by cold rolling, and both circumferential ends of the cylindrical shape are butted together and electric resistance welded; After the pipe-making process, a heat treatment process is performed in which the electric resistance welded portion is heated to 850°C or more and 1050°C or less, air-cooled for 5 seconds or more and 50 seconds or less, and then water-cooled to 200°C or less. a sizing step of adjusting the outer diameter of the pipe using a sizing roll after the weld heat treatment step; A method for producing an electric resistance welded steel pipe, comprising: [Effects of the Invention]

[0023] According to the present invention, it is possible to provide an electric resistance welded steel pipe that has high strength and excellent workability and toughness and is suitable for steel pipes for machine structures used in parts of automobiles, construction machinery, and industrial machinery, a hot-rolled steel sheet that is the raw material for the electric resistance welded steel pipe, and a method for manufacturing the same. [Brief explanation of the drawings]

[0024] [Figure 1] 1 is a schematic diagram of a circumferential cross section (a cross section perpendicular to the axial direction) of an electric resistance welded portion of an electric resistance welded steel pipe. DETAILED DESCRIPTION OF THE INVENTION

[0025] The hot-rolled steel sheet and the electric resistance welded steel pipe using the hot-rolled steel sheet as a base material according to the present invention, as well as the manufacturing methods thereof, will be described below. Note that the present invention is not limited to the following embodiments.

[0026] Here, in the present invention, when the center of the pipe circumference at the point where electric resistance welding has been performed (also referred to as the electric resistance welded portion in the present invention) in a cross section perpendicular to the pipe axis of an electric resistance welded steel pipe is defined as 0°, the chemical composition and steel structure of the base material portion 90° away from such point in the pipe circumference direction are defined as the chemical composition and steel structure of the base material portion of the electric resistance welded steel pipe in the present invention. Although the position 90° away from the electric resistance weld is specified, the same chemical composition and steel structure are also found at a position 180° away from the electric resistance weld, for example.

[0027] First, the reasons for limiting the steel structure of the base material of the hot-rolled steel sheet and electric resistance welded steel pipe of the present invention will be explained. In the steel structure of the hot-rolled steel sheet and electric resistance welded steel pipe of the present invention, the steel structure at the center of thickness of the hot-rolled steel sheet and the center of thickness of the base material of the electric resistance welded steel pipe has a volume fraction of bainite of 90% or more, with the remainder containing one or more of ferrite, pearlite, martensite, and austenite. Furthermore, this steel structure has an average crystal grain size of 10.0 μm or less, and the volume fraction of crystal grains with a grain size of 40.0 μm or more is 40% or less. Furthermore, this steel structure has a number density of Ti-based inclusions with a major axis of 5.0 μm or more of 20 pieces / mm 2 The following is the result. In the present invention, it is essential that the above-mentioned steel structure be defined at least at the center of thickness of the hot-rolled steel sheet and the center of thickness of the base material of the electric resistance welded steel pipe. This is because these are the final solidification positions in the casting process, where the cooling rate is the slowest and inclusions become the coarsest. Also, this is because these are the cooling positions in the cooling process of hot rolling, where the cooling rate is the slowest and crystal grains become the coarsest.

[0028] Here, ferrite is a soft structure, and bainite is a structure that is harder than ferrite and softer than pearlite, martensite, and austenite.

[0029] [Bainite volume fraction] If the volume fraction of bainite at the center of thickness of a hot-rolled steel plate and at the center of thickness of a base material of an electric resistance welded steel pipe is less than 90%, the desired yield stress or tensile strength cannot be obtained. Alternatively, the desired toughness cannot be obtained. Therefore, the volume fraction of bainite at the center of thickness of a hot-rolled steel plate and at the center of thickness of a base material of an electric resistance welded steel pipe is set to 90% or more. The volume fraction of bainite at the center of thickness of a hot-rolled steel plate and at the center of thickness of a base material of an electric resistance welded steel pipe is preferably 93% or more, and more preferably 96% or more. The upper limit of the volume fraction of bainite at the center of thickness of a hot-rolled steel plate and at the center of thickness of the base material of an electric resistance welded steel pipe is not particularly specified, but from the viewpoint of ductility, it is preferably 99% or less. In the present invention, the volume fraction of each structure such as bainite is a value that occupies the entire steel structure in the same field of view at the target position.

[0030] [Remainder: one or more of austenite, ferrite, pearlite, and martensite] The center of thickness of a hot-rolled steel plate and the remainder of the base material of an electric resistance welded steel pipe contain one or more of austenite, ferrite, pearlite, and martensite. Each of these structures has a hardness difference from bainite, and stress concentrates at the interface with bainite. If the total volume fraction of these structures exceeds 10%, the area of ​​the interface with bainite increases, making fracture more likely to occur due to stress concentration, and therefore toughness decreases. Therefore, the total volume fraction of these structures is set to 10% or less, preferably 7% or less, and more preferably 4% or less. Note that, because it is difficult to completely suppress the formation of these structures during the cooling process in hot rolling, a lower limit of approximately 1% is allowed for their total volume fraction.

[0031] Among the above steel structures, the various structures except for austenite have austenite grain boundaries or deformation bands within austenite grains as nucleation sites. That is, by increasing the reduction rate at low temperatures where austenite recrystallization is difficult to occur during hot rolling, 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 within the grains. As a result, in the present invention, the area of ​​nucleation sites increases, the frequency of nucleation increases, and the steel structure can be refined.

[0032] In the present invention, the steel structure can be observed by the method described below or in the examples below. That is, first, test pieces for microstructure observation are taken so that the observation surface is a cross section parallel to both the rolling direction and the thickness direction of a hot-rolled steel sheet and at the center of the thickness, and a cross section parallel to both the axial direction and the thickness direction of an electric resistance welded steel pipe and at the center of the thickness.The observation surface is then polished and then subjected to nital etching. For the structural observation, an optical microscope (magnification: 1000 times) or a scanning electron microscope (SEM, magnification: 1000 times) is used to observe and photograph the structure at the center of the plate thickness (or wall thickness). Next, the area ratios of bainite and the remainder (ferrite, pearlite, martensite, and austenite) are determined from the obtained optical microscope and SEM images. The area ratios of each structure are determined by observing five or more fields in the center of the plate (or wall) thickness, and the average value obtained from each field is used. In the present invention, the area ratio obtained by observing the structure is taken as the volume ratio of each structure. In the present invention, the center of the plate thickness (or wall thickness) is defined as the center position in the plate thickness direction (the position at 1 / 2 of the plate thickness) or the center position in the wall thickness direction (the position at 1 / 2 of the wall thickness). However, the effect of the present invention can be similarly obtained as long as at least the above-mentioned steel structure exists within a range of ±0.20 mm in the plate thickness (or wall thickness) direction from the center of the plate thickness (or wall thickness). Therefore, in the present invention, the "steel structure at the center of the plate thickness (or wall thickness)" refers to a predetermined area (0.10 mm) that exists within a range of ±0.20 mm in the plate thickness (or wall thickness) direction from the center of the plate thickness (or wall thickness). 2 The term "above is preferable") refers to a steel structure in which the above is preferable.

[0033] Ferrite is a product of diffusion transformation and exhibits a nearly recovered structure with low dislocation density. This includes polygonal ferrite and pseudo-polygonal ferrite. Bainite is a complex phase structure of lath-shaped ferrite and cementite with a high dislocation density. Pearlite is a eutectoid structure of iron and iron carbide (ferrite + cementite), and exhibits a lamellar structure in which linear ferrite and cementite are arranged alternately. Martensite is a lath-like structure transformed at low temperatures with an extremely high dislocation density. In SEM images, it exhibits brighter contrast than ferrite and bainite.

[0034] It is difficult to distinguish martensite from austenite in the optical microscope image and SEM image. Therefore, the total area fraction of the structure observed as martensite or austenite in the obtained SEM image is calculated, and the volume fraction of austenite measured by the following method is subtracted from this total value to obtain the volume fraction of martensite. That is, by taking advantage of the fact that austenite is an fcc phase, the volume fraction of austenite is determined by X-ray diffraction. The volume fraction of austenite can be calculated from the integrated intensities of the (200), (220), and (311) planes of fcc iron and the (200) and (211) planes of bcc iron obtained by X-ray diffraction.

[0035] Furthermore, the steel structure at the center of thickness of the hot-rolled steel sheet and the base material of the electric resistance welded steel pipe has an average crystal grain size of 10.0 μm or less, and the volume fraction of crystal grains with a grain size of 40.0 μm or more is 40% or less. Furthermore, the steel structure has a number density of Ti-based inclusions with a major diameter of 5.0 μm or more of 20 pieces / mm 2 The following applies.

[0036] In the present invention, the term "average crystal grain size" refers to the average value of the equivalent circle diameter of crystal grains when the crystal grains are regions surrounded by boundaries where the misorientation of adjacent crystals is 15° or more. Furthermore, the "circle equivalent diameter (crystal grain size)" refers to the diameter of a circle having the same area as the target crystal grain. Furthermore, "Ti-based inclusions" refer to inclusions with a Ti proportion of 50% or more by mass. However, since there is a large measurement error for elements with atomic numbers 1 to 10 (H to Ne in the periodic table) and they may be dirt adhering to the surface, the Ti proportion refers to the proportion of elements with atomic numbers 11 and above (Na and above in the periodic table).

[0037] [Average grain size] If the average grain size of the grains at the center of thickness of the hot-rolled steel sheet and the base material of the electric resistance welded steel pipe exceeds 10.0 μm, the steel structure will not be sufficiently fine, and the yield stress or tensile strength targeted in the present invention will not be obtained. Furthermore, toughness will also decrease. Therefore, the average grain size of the grains at the center of thickness of the hot-rolled steel sheet and the base material of the electric resistance welded steel pipe should be 10.0 μm or less. The average grain size of the grains is preferably 8.0 μm or less. Note that if the average grain size is too small, the yield ratio will tend to increase, so the average grain size is preferably 3.0 μm or more.

[0038] [Grain size: Volume fraction of crystal grains 40.0 μm or larger] If the volume fraction of crystal grains with a grain size of 40.0 μm or more at the center of thickness of a hot-rolled steel sheet and the center of thickness of a base material of an electric resistance welded steel pipe exceeds 40%, the proportion of coarse crystal grains will be high, resulting in a decrease in toughness and / or workability. Therefore, the volume fraction of crystal grains with a grain size of 40.0 μm or more at the center of thickness of a hot-rolled steel sheet and the center of thickness of a base material of an electric resistance welded steel pipe should be 40% or less. The volume fraction of crystal grains with a grain size of 40.0 μm or more at the center of thickness of a hot-rolled steel sheet and the center of thickness of a base material of an electric resistance welded steel pipe is preferably 30% or less.

[0039] [Major diameter: Number density of Ti-based inclusions 5.0 μm or larger] The number density of Ti-based inclusions with a major diameter of 5.0 μm or more at the center of thickness of hot-rolled steel sheets and the base material of electric resistance welded steel pipes is 20 pieces / mm 2 In the case of 0.015 or more, the toughness is reduced due to the large number of coarse polygonal inclusions. Therefore, the number density of Ti-based inclusions with a major diameter of 5.0 μm or more at the center of thickness of the hot-rolled steel plate and the base material of the electric resistance welded steel pipe is 20 pieces / mm 2 The number density of Ti-based inclusions with a major diameter of 5.0 μm or more at the center of thickness of a hot-rolled steel plate and at the center of thickness of a base material of an electric resistance welded steel pipe is preferably 10 pieces / mm 2 The following is the result.

[0040] As will be described in detail in Examples below, the average crystal grain size of the steel structure, the volume fraction of crystal grains with a grain size of 40.0 μm or more, and the number density of Ti-based inclusions with a major axis of 5.0 μm or more can be measured by the following methods.

[0041] To measure the average grain size, a cross section parallel to both the rolling direction and the thickness direction of the hot-rolled steel sheet, and a cross section parallel to both the pipe axis direction and the thickness direction of the base material of the electric resistance welded steel pipe are mirror-polished, and a histogram of the grain size distribution (horizontal axis: grain size, vertical axis: proportion of each grain size) at the center of the thickness of the hot-rolled steel sheet and the center of the thickness of the base material of the electric resistance welded steel pipe is calculated using the SEM / EBSD method, and the arithmetic mean of the grain size is determined. The measurement conditions were: acceleration voltage: 15 kV, measurement area: 300 μm × 300 μm, measurement step size (measurement resolution): 0.5 μm, and the measured values ​​of five or more fields of view were averaged. Note that in the analysis of crystal grain size, crystal grains with a grain size of less than 2.0 μm were excluded from the analysis as measurement noise.

[0042] Grain size: The volume fraction of crystal grains of 40.0 μm or more is calculated from the histogram of the grain size distribution above. The grain size can be determined by calculating the total area ratio of crystal grains having a grain size of 40.0 μm or more, and regarding the total area ratio as the volume ratio.

[0043] The number density of Ti-based inclusions with a major axis of 5.0 μm or more is determined by mirror-polishing cross sections parallel to both the rolling direction and the plate thickness direction of hot-rolled steel sheets, and cross sections parallel to both the pipe axis direction and the wall thickness direction of the base material of electric resistance welded steel pipes, and combining this with SEM measurement of the major axis of the inclusions and elemental analysis using the SEM / EDS method in the same field of view. The number density is measured at the center of the plate thickness of hot-rolled steel plate and the center of the wall thickness of the base material of electric resistance welded steel pipe, in a measurement area of ​​4 mm x 10 mm, and is calculated by averaging measured values ​​from five or more fields of view.

[0044] The steel structure at the center of the wall thickness of the electric resistance welded steel pipe of the present invention has a ferrite volume fraction of 10% or more, a total volume fraction of ferrite and bainite of 60% to 98%, and the remainder containing one or more of pearlite, martensite, and austenite, and further has an average crystal grain size of 15.0 μm or less.

[0045] [Ferrite volume fraction] If the volume fraction of ferrite at the center of the wall thickness of an electric resistance welded part of an electric resistance welded steel pipe is less than 10%, the ductility of the electric resistance welded part decreases, and the flattening value of the electric resistance welded part targeted in the present invention cannot be obtained. Therefore, the volume fraction of ferrite at the center of the wall thickness of an electric resistance welded part of an electric resistance welded steel pipe is set to 10% or more. The volume fraction of ferrite at the center of the wall thickness of an electric resistance welded steel pipe is preferably 20% or more, and more preferably 30% or more. There is no particular upper limit to the volume fraction of ferrite, but from an industrial perspective, it is preferably about 90%.

[0046] [Total volume fraction of ferrite and bainite] If the total volume fraction of ferrite and bainite at the center of the wall thickness of an electric resistance weld of an electric resistance welded steel pipe is less than 60%, the ductility of the electric resistance weld is reduced, and the flattening value of the electric resistance weld targeted in the present invention cannot be obtained. Therefore, the total volume fraction of ferrite and bainite at the center of the wall thickness of an electric resistance weld of an electric resistance welded steel pipe is set to 60% or more. On the other hand, if the total volume fraction of ferrite and bainite at the center of the wall thickness of an electric resistance weld of an electric resistance welded steel pipe is more than 98%, the strength of the electric resistance weld is reduced, and the desired Vickers hardness cannot be obtained. Furthermore, strain is concentrated in the electric resistance weld during a flattening test, and the desired flattening value of the electric resistance weld cannot be obtained. Therefore, the total volume fraction of ferrite and bainite at the center of the wall thickness of an electric resistance welded part of an electric resistance welded steel pipe is set to 60% or more and 98% or less. The total volume fraction of ferrite and bainite at the center of the wall thickness of an electric resistance welded part of an electric resistance welded steel pipe is preferably 70% or more, and more preferably 80% or more. On the other hand, the total volume fraction of ferrite and bainite at the center of the wall thickness of an electric resistance welded part of an electric resistance welded steel pipe is preferably 96% or less, and more preferably 94% or less.

[0047] [Remainder: one or more of pearlite, martensite, and austenite] The remainder at the center of the wall thickness of an electric resistance weld in an electric resistance welded steel pipe contains one or more of pearlite, martensite, and austenite. This is because, to ensure the strength of the electric resistance weld, the remainder must be a structure harder than ferrite and bainite. If the total volume fraction of these structures exceeds 40%, the desired flattening value of the electric resistance weld cannot be obtained. Therefore, the total volume fraction of these structures is set to 40% or less, preferably 30% or less, and more preferably 20% or less.

[0048] [Average grain size is 15.0 μm or less] When the average grain size of the grains at the center of the wall thickness of the electric resistance welded steel pipe exceeds 15.0 μm, the strength of the electric resistance welded part decreases and the desired Vickers hardness cannot be obtained. In some cases, strain is concentrated in the electric resistance weld, making it impossible to obtain the desired flattening value of the electric resistance weld. Therefore, the average grain size of the grains at the center of the wall thickness of the electric resistance weld of the electric resistance welded steel pipe is set to 15.0 μm or less. The average grain size of the grains is preferably 10.0 μm or less. If the average grain size is too small, the ductility tends to decrease, and the flattening value tends to increase. Therefore, the average grain size is preferably 3.0 μm or more.

[0049] [Component composition] Next, from the viewpoint of ensuring the above-mentioned properties and steel structure, the ranges of the chemical composition of the electric resistance welded steel pipe of the present invention and the hot-rolled steel sheet that is the raw material for it, and the reasons for limiting these ranges, will be explained. In this specification, unless otherwise specified, "%" indicating the chemical composition of steel (including steel sheet and steel pipe) is % by mass.

[0050] C: 0.020% or more and 0.200% or less C is an element that increases the strength of steel through solid solution strengthening. In order to ensure the desired strength in the present invention, 0.020% or more of C is contained. On the other hand, if the C content exceeds 0.200%, hardenability increases and hard pearlite, martensite, and austenite are excessively formed, so the C content is set to 0.200% or less. The C content is preferably 0.030% or more and preferably 0.180% or less. Furthermore, the C content is more preferably 0.040% or more and more preferably 0.170% or less.

[0051] Si: 0.02% or more and 1.00% or less Silicon is an element that increases the strength of steel through solid solution strengthening. To achieve this effect, the steel should contain 0.02% or more of silicon. On the other hand, if the silicon content exceeds 1.00%, the ductility and toughness decrease. Furthermore, the melting point of silicon-containing oxides increases, making oxides more likely to remain in the electric resistance weld, thereby decreasing the ductility of the electric resistance weld. For this reason, the silicon content is set to 1.00% or less. The silicon content is preferably 0.05% or more and preferably 0.90% or less. The silicon content is more preferably 0.10% or more and more preferably 0.80% or less.

[0052] Mn: 0.10% or more and 2.50% or less Mn is an element that increases the strength of steel through solid solution strengthening. Furthermore, Mn contributes to microstructural refinement by lowering the transformation start temperature. In order to ensure the strength and steel microstructure targeted in the present invention, 0.10% or more of Mn is contained. On the other hand, if the Mn content exceeds 2.50%, the hardenability increases and hard pearlite, martensite, and austenite are excessively formed. Therefore, the Mn content is set to 2.50% or less. The Mn content is preferably 0.40% or more and preferably 2.30% or less. Furthermore, the Mn content is more preferably 0.50% or more and more preferably 2.00% or less.

[0053] P:0.050% or less Since P segregates at grain boundaries and causes inhomogeneity in the material, it is necessary to reduce it as much as possible, and the P content is set to a range of 0.050% or less. The P content is preferably 0.040% or less, and more preferably 0.030% or less. Although there is no particular lower limit for P, excessive reduction leads to an increase in smelting costs, so P is preferably set to 0.001% or more.

[0054] S: 0.0200% or less S is usually present in steel as MnS, but MnS is thinly drawn in the hot rolling process and has a negative effect on ductility and toughness. For this reason, in the present invention, it is necessary to reduce S as much as possible, and the S content is set to 0.0200% or less. The S content is preferably 0.0100% or less, and more preferably 0.0050% or less. In particular, the lower limit of S is Although not specified, an excessive reduction in S content leads to a rise in smelting costs, so S content is preferably 0.0001% or more.

[0055] Al: 0.005% or more and 0.100% or less Al is an element that acts as a powerful deoxidizer. To achieve this effect, it is necessary to contain 0.005% or more of Al. On the other hand, if the Al content exceeds 0.100%, the weldability deteriorates, and the amount of alumina-based inclusions increases, deteriorating the surface properties. Furthermore, the toughness also decreases. For this reason, the Al content is set to 0.100% or less. The Al content is preferably 0.010% or more and preferably 0.080% or less. Furthermore, the Al content is more preferably 0.015% or more and more preferably 0.070% or less.

[0056] N: 0.0100% or less N is an unavoidably contained impurity, and is an element that has the effect of reducing ductility and toughness by firmly fixing dislocation motion. In the present invention, it is desirable to reduce N as an impurity as much as possible, but an N content of up to 0.0100% is acceptable. Therefore, the N content is set to 0.0100% or less. The N content is preferably 0.0080% or less. There is no particular lower limit for the N content, but since excessive reduction leads to an increase in refining costs, it is preferable that N be 0.0010% or more.

[0057] Ti: 0.030% or more and 0.200% or less Ti is an element that contributes to improving the strength of steel by forming fine carbides and nitrides in the steel. It also contributes to reducing the amount of dissolved N in the steel due to its high affinity with N. To achieve the above-mentioned effects, a Ti content of 0.030% or more is necessary. On the other hand, if the Ti content exceeds 0.200%, coarse Ti-based inclusions are formed during the casting process, making it difficult to achieve the number density of Ti-based inclusions with a major axis of 5.0 μm or more, which is the objective of the present invention, and resulting in a decrease in toughness. Furthermore, ductility also decreases. Therefore, the Ti content is set to 0.200% or less. The Ti content is preferably 0.040% or more and preferably 0.180% or less. The Ti content is more preferably 0.050% or more and more preferably 0.150% or less.

[0058] The above composition may further include the following elements: One or more of Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.50% or less, Nb: 0.050% or less, V: 0.050% or less, Ca: 0.0050% or less, and B: 0.0050% or less

[0059] Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.50% or less Cu, Ni, Cr, and Mo are elements that improve the hardenability and strength of steel and can be added as needed. To achieve the above-mentioned effects, when Cu, Ni, Cr, and Mo are included, the respective amounts of Cu: 0.01% or more, Ni: 0.01% or more, Cr: 0.01% or more, and Mo: 0.01% or more are desirable. On the other hand, excessive inclusion of Cu, Ni, Cr, and Mo may lead to excessive formation of hard pearlite, martensite, and austenite. Furthermore, defects may occur more easily in electric resistance welds, resulting in an increase (deterioration) in the flattening value. Therefore, when Cu, Ni, Cr, and Mo are contained, it is preferable that the Cu content be 0.50% or less, Ni content be 0.50% or less, Cr content be 0.50% or less, and Mo content be 0.50% or less, respectively.

[0060] That is, when Cu, Ni, Cr, and Mo are contained, Cu: 0.01% or more and It is preferable that the content of Ni is 0.01% or more and 0.50% or less, that the content of Cr is 0.01% or more and 0.50% or less, and that the content of Mo is 0.01% or more and 0.50% or less. More preferably, Cu is 0.05% or more and Cu is 0.40% or less, Ni is 0.05% or more and Ni is 0.40% or less, Cr is 0.05% or more and Cr is 0.40% or less, and Mo is 0.05% or more and Mo is 0.40% or less. Still more preferably, Cu is 0.10% or more and Cu is 0.30% or less, Ni is 0.10% or more and Ni is 0.30% or less, Cr is 0.10% or more and Cr is 0.30% or less, and Mo is 0.10% or more and Mo is 0.30% or less.

[0061] Nb: 0.050% or less Nb is an element that contributes to improving the strength of steel by forming fine carbides and nitrides in the steel, and also contributes to refining the structure by suppressing the coarsening of austenite during hot rolling. To achieve the above effects, it is desirable to contain 0.002% or more of Nb. On the other hand, if the Nb content exceeds 0.050%, the ductility and toughness decrease. Therefore, the Nb content is preferably 0.050% or less. The Nb content is more preferably 0.005% or more and more preferably 0.040% or less. Furthermore, the Nb content is even more preferably 0.010% or more and even more preferably 0.030% or less.

[0062] V:0.050% or less V is an element that contributes to improving the strength of steel by forming fine carbides and nitrides in the steel. To achieve the above-mentioned effects, it is desirable for the V content to be 0.002% or more. On the other hand, if the V content exceeds 0.050%, ductility and toughness decrease. For this reason, the V content is preferably 0.050% or less. The V content is more preferably 0.005% or more, and more preferably 0.040% or less. Furthermore, the V content is even more preferably 0.010% or more, and even more preferably 0.030% or less.

[0063] Ca:0.0050% or less Ca is an element that contributes to improving the toughness of steel by spheroidizing sulfides such as MnS that are thinly drawn in the hot rolling process, and can be contained as needed. To achieve the above-mentioned effects, if Ca is contained, it is desirable that the Ca content be 0.0005% or more. On the other hand, if the Ca content exceeds 0.0050%, Ca oxide clusters are formed in the steel, deteriorating toughness. Therefore, if Ca is contained, it is preferable that the Ca content be 0.0050% or less. The Ca content is more preferably 0.0008% or more and more preferably 0.0045% or less. Furthermore, the Ca content is even more preferably 0.0010% or more and even more preferably 0.0040% or less.

[0064] B: 0.0050% or less B is an element that contributes to refining the structure by lowering the transformation start temperature, and can be included as needed. In order to obtain the above-mentioned effects, if B is included, it is desirable to include 0.0003% or more of B. On the other hand, if the B content exceeds 0.0050%, ductility and toughness deteriorate. Therefore, if B is included, the B content is preferably 0.0050% or less. The B content is more preferably 0.0005% or more, and more preferably 0.0040% or less. Furthermore, the B content is even more preferably 0.0008% or more, and even more preferably 0.0030% or less.

[0065] In the above composition, the balance is Fe and unavoidable impurities, but as an unavoidable impurity, it is permissible to contain 0.0050% or less of O (oxygen) as long as the effects of the present invention are not impaired.

[0066] The above-mentioned components are the composition of the base material of the hot-rolled steel sheet and electric resistance welded steel pipe of the present invention. By satisfying this composition, the properties targeted by the present invention can be obtained. Furthermore, the composition of the hot-rolled steel sheet substantially corresponds to the composition of the raw steel and molten steel used in its production.

[0067] In the present invention, it is further preferable to set the Mn / Si value to 2.0 or more and 100.0 or less in order to reduce the flattening value of the electric resistance welded steel pipe. If the Mn / Si ratio is less than 2.0 or more than 100.0, the melting point of the Si-Mn oxides will be high, and they will not melt during electric resistance welding and will remain as inclusions in the electric resistance weld, which may result in an increase (deterioration) in the flattening value. The Mn / Si ratio is more preferably 2.1 or more and more preferably 90.0 or less, and even more preferably 2.2 or more and even more preferably 80.0 or less.

[0068] [Methods of manufacturing hot-rolled steel sheets and electric resistance welded steel pipes] Next, a method for producing a hot-rolled steel sheet and an electric resistance welded steel pipe according to an embodiment of the present invention will be described. The hot-rolled steel sheet of the present invention can be produced by casting molten steel having the above-described chemical composition at an average cooling rate of 0.30°C / s or more from the solidification point of the molten steel to 1200°C to form a steel material (casting process), heating the steel material to a heating temperature of 1150°C to 1300°C, and then hot-rolling the steel material at a rough rolling end temperature of 950°C to 1180°C, a finish rolling end temperature of 850°C to 1000°C, and a total reduction in finish rolling of 50% to 80% (hot rolling process), cooling the steel material at an average cooling rate of 10°C / s to 60°C / s at the center of the thickness and a cooling end temperature of 400°C to 580°C (cooling process), and then winding the steel material into a coil at 400°C to 580°C (winding process).

[0069] Furthermore, the electric-resistance welded steel pipe of the present invention can be produced by forming the hot-rolled steel sheet produced as described above into a cylindrical shape by cold roll forming, butting both circumferential ends of the cylindrical shape together and electric-resistance welding the resulting pipe (pipe-making process), then heating the area where the electric-resistance welded steel pipe has been formed (electric-resistance welded portion) to 850°C or higher and 1050°C or lower, further air-cooling for 5 seconds or higher and 50 seconds or lower, and then water-cooling to 200°C or lower (weld portion heat treatment process), and then adjusting the outer diameter of the pipe using a sizing roll (sizing process).

[0070] In the following description of the manufacturing method, the temperature indicated in "°C" refers to the surface temperature of the steel material or steel plate (hot-rolled plate) unless otherwise specified. These surface temperatures can be measured using a radiation thermometer or the like. The temperature at the center of the steel plate thickness can be determined by calculating the temperature distribution in the cross section of the steel plate using heat transfer analysis and correcting the result by the surface temperature of the steel plate. Furthermore, "hot-rolled steel plate" includes both hot-rolled plate and hot-rolled steel strip.

[0071] First, the method for producing a hot-rolled steel sheet according to the present invention will be described. In the present invention, there is no particular limitation on the method for producing the steel material (steel slab). For example, any of the known methods for producing the steel material (steel slab), such as a converter, an electric furnace, or a vacuum melting furnace, is suitable. There is also no particular limitation on the casting method. For example, a steel material of the desired dimensions can be produced by a known casting method, such as a continuous casting method. However, there is no problem if an ingot-blooming rolling method is used instead of the continuous casting method. The molten steel may further be subjected to secondary refining, such as ladle refining.

[0072] [Average cooling rate from solidification point to 1200°C during casting process: 0.30°C / s or more] In the casting process, if the average cooling rate from the solidification point of the molten steel after melting to 1200°C is less than 0.30°C / s, the Ti-based inclusions will become coarse, making it difficult to ensure the number density of Ti-based inclusions with a major diameter of 5.0 μm or more that is the target of the present invention, and toughness will decrease. For this reason, the average cooling rate from the solidification point to 1200°C is set to 0.30°C / s or more. The average cooling rate is preferably 0.35°C / s or more, and more preferably 0.40°C / s or more. There is no particular upper limit to the average cooling rate. If the average cooling rate exceeds 100°C / s, the effect of improving toughness with increasing cooling rate decreases, and the equipment load only increases. For this reason, the average cooling rate from the solidification point to 1200°C is preferably 100°C / s or less. The average cooling rate is more preferably 80°C / s or less. The freezing point can be calculated by the following formula (1). Freezing point (℃)=1539-(70[C]+8[Si]+5[Mn]+30[P]+25[S]+5[Cu]+4[Ni]+1.5[Cr])...(1) In formula (1), the element symbols represent the content (mass%) of each element in the steel. Elements that are not contained are considered to be 0%.

[0073] In the present invention, the steel material (steel slab) obtained through the casting is heated (reheated) to a heating temperature in the range of 1150°C or more and 1300°C or less, and then the heated steel material is hot rolled (hot rolling process) to form a hot-rolled sheet, which is then cooled (cooling process), and the cooled hot-rolled sheet is wound into a coil (winding process) to form a hot-rolled steel sheet.

[0074] [Heating temperature in the hot rolling process: 1150℃ to 1300℃] In the hot rolling process, if the heating temperature (reheating temperature) is less than 1150°C, the deformation resistance of the rolled material increases, making rolling difficult. On the other hand, if the heating temperature exceeds 1300°C, the austenite grains become coarse, making it difficult to obtain fine austenite grains in the subsequent rolling (rough rolling, finish rolling), and it becomes difficult to ensure the average crystal grain size targeted in the present invention. Therefore, the heating temperature in the hot rolling process is set to 1150°C or higher and 1300°C or lower. The heating temperature is preferably 1170°C or higher and preferably 1280°C or lower.

[0075] In addition to the conventional method of producing a steel slab, cooling it to room temperature and then reheating it, the present invention can also easily apply energy-saving direct rolling processes in which the slab is not cooled to room temperature, but is instead charged into a heating furnace as a hot slab, or is immediately rolled after a short period of heat retention.

[0076] [Rough rolling end temperature: 950℃ to 1180℃] If the rough rolling end temperature in the hot rolling process is less than 950°C, the surface temperature of the steel sheet during the subsequent finish rolling will be equal to or lower than the ferrite transformation start temperature, resulting in the formation of a large amount of ferrite, making it difficult to ensure the volume fraction of bainite targeted in the present invention, and resulting in a decrease in yield stress or tensile strength. On the other hand, if the rough rolling end temperature exceeds 1180°C, the austenite grains will coarsen, making it difficult to ensure the average grain size targeted in the present invention, and resulting in a decrease in toughness. Therefore, the rough rolling end temperature in the hot rolling step is set to 950° C. or higher and 1180° C. or lower. The rough rolling end temperature is more preferably 970° C. or higher, and more preferably 1150° C. or lower.

[0077] [Finishing rolling temperature: 850℃ to 1000℃] If the finish rolling temperature in the hot rolling process is less than 850°C, the surface temperature of the steel sheet during finish rolling will be equal to or lower than the ferrite transformation start temperature, resulting in the formation of a large amount of ferrite, making it difficult to ensure the bainite volume fraction targeted in the present invention, and resulting in a decrease in yield stress or tensile strength. On the other hand, if the finish rolling temperature exceeds 1000°C, the austenite grains will coarsen, making it difficult to ensure the average grain size targeted in the present invention, and resulting in a decrease in toughness. For this reason, the finish rolling temperature in the hot rolling step is set to 850° C. or higher and 1000° C. or lower. The finish rolling temperature is more preferably 870° C. or higher and more preferably 980° C. or lower.

[0078] [Total reduction in finish rolling: 50% to 80%] In the present invention, by refining the subgrains in the austenite in the hot rolling process, the bainite and remaining structures formed in the subsequent cooling and coiling processes are refined, thereby obtaining a steel structure having the desired yield strength. Therefore, to refine the subgrains in the austenite in the hot rolling process, it is necessary to increase the reduction in the austenite non-recrystallization temperature range and introduce sufficient working strain. To achieve this objective, in the present invention, the total reduction in the finish rolling is set to 50% or more. If the total reduction in the finish rolling is less than 50%, sufficient working strain cannot be introduced in the hot rolling process. As a result, a steel structure having the desired average grain size and volume fraction of grains with a grain size of 40.0 μm or more, which are the objectives of the present invention, cannot be obtained. The total reduction in the finish rolling is preferably 55% or more. On the other hand, if the total reduction exceeds 80%, ferrite is likely to form, making it difficult to ensure the bainite volume fraction, which is the objective of the present invention, and the yield stress or tensile strength decreases. Therefore, the total reduction in the finish rolling is set to 80% or less. The total reduction is preferably 75% or less. The total reduction in the finish rolling mentioned above refers to the sum of the reductions in each rolling pass in the finish rolling. In addition, in the present invention, the upper and lower limits of the finished plate thickness are not particularly specified, but from the viewpoint of ensuring the necessary rolling reduction rate and controlling the steel plate temperature, it is preferable that the finished plate thickness (the plate thickness of the steel plate after finish rolling) be in the range of 2 mm or more and 25 mm or less.

[0079] [Average cooling rate at the center of the plate thickness during the cooling process: 10°C / s or more and 60°C / s or less] After the hot rolling step, a cooling step is carried out to cool the hot-rolled sheet. If the average cooling rate in this cooling step is less than 10°C / s, the ferrite volume fraction of the steel structure increases, making it impossible to obtain a steel structure with the bainite volume fraction targeted in the present invention. Furthermore, the frequency of bainite nucleation decreases, and the nuclei become coarse, making it impossible to obtain a steel structure with the average grain size targeted in the present invention. On the other hand, if the average cooling rate exceeds 60°C / s, a large amount of martensite is formed, resulting in reduced workability and toughness. Therefore, the average cooling rate at the center of the thickness of the hot-rolled sheet is set to a range of 10°C / s to 60°C / s. The average cooling rate is preferably 15°C / s or more and preferably 55°C / s or less. In the present invention, from the viewpoint of suppressing the formation of ferrite on the surface of the steel sheet before the cooling step, it is preferable to start the cooling step immediately after the end of finish rolling in the hot rolling step.

[0080] [Cooling stop temperature at the center of the plate thickness during the cooling process: 400°C to 580°C] If the cooling stop temperature at the center of the thickness of the hot-rolled sheet in the cooling process is less than 400°C, a large amount of martensite will be generated, resulting in reduced workability and toughness. On the other hand, if the cooling stop temperature exceeds 580°C, the ferrite volume fraction will increase, making it impossible to obtain a steel structure with the bainite volume fraction targeted in the present invention. Furthermore, the frequency of bainite nucleation will decrease, causing coarsening, making it impossible to obtain a structure with the average grain size targeted in the present invention. Therefore, the cooling stop temperature is set to a range of 400°C to 580°C. The cooling stop temperature is preferably 420°C or higher and preferably 550°C or lower. In the present invention, the average cooling rate is a value calculated by ((temperature at the center of thickness of the hot-rolled sheet before cooling−temperature at the center of thickness of the hot-rolled sheet after cooling) / cooling time).

[0081] The cooling method in the cooling step of the present invention may be water cooling such as spraying water from a nozzle, or cooling by spraying cooling gas, etc. In the present invention, it is preferable to perform the cooling operation (treatment) on both sides of the hot-rolled sheet so that both sides of the hot-rolled sheet are cooled under the same conditions.

[0082] [Temperature at the center of the plate thickness during the coiling process (coiling temperature): 400°C to 580°C] After the cooling step, a coiling step is performed in which the hot-rolled sheet is coiled. After the coiling step, the sheet can be allowed to cool naturally. If the coiling temperature is less than 400°C, a large amount of martensite is generated, resulting in reduced workability and toughness. On the other hand, if the coiling temperature exceeds 580°C, the ferrite volume fraction increases, making it impossible to obtain a steel structure having the bainite volume fraction targeted in the present invention. In addition, the frequency of bainite nucleation decreases, causing these nuclei to coarsen, making it impossible to obtain a structure having the average crystal grain size targeted in the present invention. Therefore, the coiling temperature is set to a range of 400°C to 580°C. The coiling temperature is preferably 420°C or higher and preferably 550°C or lower.

[0083] Next, a method for manufacturing an electric resistance welded steel pipe will be described. The hot-rolled steel sheet obtained by the coiling process (the hot-rolled steel sheet of the present invention) is subjected to a pipe-making process in which the hot-rolled steel sheet is formed into a cylindrical open pipe (round steel pipe) by cold rolling, and both circumferential ends (butt joints) of the cylindrical open pipe are butted together and melted by high-frequency electric resistance heating, while being pressure-welded by upsetting with a squeeze roll and then electric resistance welded to form an electric resistance welded steel pipe.

[0084] The electric resistance welded steel pipe manufactured in this manner has a base material portion which is the above-mentioned hot-rolled steel plate, an electric resistance welded portion (also called a bond portion or simply a welded portion) which is the portion where electric resistance welding is performed, and a weld heat-affected portion which is generated in the base material by the heat generated during such electric resistance welding. After the pipe-making process, the electric resistance welded steel pipe is subjected to a weld heat treatment process. The electric resistance weld after the pipe-making process is heated and then rapidly cooled in the pipe-making process, so it has lower ductility than the base material. Therefore, in this weld heat treatment process, the electric resistance weld is heated and then appropriately cooled, so that the strength, workability, and toughness of the electric resistance weld are all satisfied.

[0085] Here, the upset amount during electric resistance welding (electric resistance welding process) is preferably 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 during welding. However, if the upset amount exceeds 100% of the thickness, the load on the squeeze roll increases. Therefore, the upset amount is preferably 20% or more and 100% or less of the thickness. The upset amount is more preferably 40% or more, and more preferably 80% or less. The upset amount in the present invention can be calculated as follows: ((circumferential length of open pipe immediately before electric resistance welding) - (circumferential length of electric resistance welded steel pipe immediately after electric resistance welding)) / (plate thickness) x 100 (%).

[0086] [Heating temperature in the heat treatment process for welded joints: 850°C to 1050°C] If the heating temperature in the weld heat treatment process is less than 850°C, the electric resistance weld will not be completely austenitized, and the rapidly cooled structure formed in the pipe-making process will remain, resulting in reduced ductility. As a result, the desired flattening value of the electric resistance weld will not be obtained. On the other hand, if the heating temperature exceeds 1050°C, the austenite grains will coarsen, the number of nucleation sites for ferrite and bainite will decrease, and the desired volume fraction of ferrite and the total volume fraction of ferrite and bainite at the center of the wall thickness of the electric resistance weld will not be obtained. Consequently, the desired flattening value of the electric resistance weld will not be obtained. Alternatively, the desired average grain size will not be obtained.

[0087] [Air cooling time during heat treatment of welded joints: 5 seconds or more and 50 seconds or less] Air cooling after heating to the above heating temperature produces ferrite and bainite. If the air cooling time is less than 5 seconds, the desired ferrite volume fraction and the total volume fraction of ferrite and bainite cannot be obtained at the center of the thickness of the electric resistance weld, and the flattening value of the electric resistance weld targeted in the present invention cannot be obtained. On the other hand, if the air cooling time exceeds 50 seconds, the average grain size targeted in the present invention cannot be obtained at the center of the thickness of the electric resistance weld. Or, during air cooling, Since the temperature is below the Ms point, the ferrite volume fraction and the total volume fraction of ferrite and bainite targeted in the present invention cannot be obtained, and the desired flattening value of the electric resistance welded portion cannot be obtained.

[0088] [Temperature after water cooling in the weld heat treatment process: 200°C or less] The ferrite transformation and bainite transformation are completed by water cooling after the air cooling. If the temperature after water cooling exceeds 200°C, the ferrite and bainite will coarsen, and a structure having the average grain size targeted in the present invention will not be obtained.

[0089] After the weld heat treatment process, the pipe is subjected to a sizing process in which the outer diameter of the pipe is adjusted using sizing rolls. In this sizing process, the electric resistance welded steel pipe is reduced in diameter using rolls arranged above, below, left and right of the electric resistance welded steel pipe, thereby adjusting the outer diameter and roundness to desired values.

[0090] This sizing process is carried out to improve the outer diameter accuracy and roundness. To improve the outer diameter accuracy and roundness, it is preferable to reduce the diameter of the steel pipe so that the steel pipe circumferential length is reduced by a total of 0.5% or more. On the other hand, if the steel pipe circumferential length is reduced by a total of more than 4.0%, the amount of bending in the axial direction of the pipe when passing through the rolls increases, and the residual stress of the steel pipe, the dislocation density on the inner surface of the pipe, and the maximum low-angle grain boundary density all increase. As a result, the SSC resistance of the steel pipe decreases. For this reason, it is preferable to reduce the diameter of the steel pipe so that the steel pipe circumferential length is reduced by a ratio of 0.5% to 4.0%. The reduction ratio of the steel pipe circumferential length is more preferably 1.0% or more, and more preferably 3.0% or less. In this sizing process, it is preferable to reduce the diameter in multiple stages using multiple stands in order to minimize the amount of bending in the axial direction of the tube when passing through the rolls and suppress the generation of residual stress in the axial direction of the tube. Furthermore, it is preferable that the reduction in the diameter in each stand is carried out so that the tube circumference is reduced by 1.0% or less.

[0091] Here, whether a steel pipe is an electric resistance welded steel pipe or not can be determined by cutting the electric resistance welded steel pipe perpendicular to the pipe axis direction, polishing and corroding the cut surface including the welded part (electric resistance welded part), and observing it under an optical microscope. Specifically, if the width in the pipe circumferential direction of the bond part of the welded part (electric resistance welded part) is 1.0 μm or more and 1000 μm or less over the entire pipe thickness, it is an electric resistance welded steel pipe.

[0092] The etching solution used for the post-polishing etching may be selected appropriately depending on the steel components and the type of steel pipe. FIG. 1 shows a schematic representation of a portion of the cross section (near the weld of the electric resistance welded steel pipe) after corrosion. As shown in FIG. 1, the bond zone (weld zone) can be visually recognized as a region (bond zone 3) with a different structural morphology and contrast from the base metal zone 1 and the weld heat-affected zone 2. For example, the bond zone 3 of electric resistance welded steel pipes made of carbon steel and low alloy steel can be identified as a region that appears white under an optical microscope in the cross section corroded with nital. Furthermore, the bond zone 3 of UOE steel pipes made of carbon steel and low alloy steel can be identified as a region containing a cellular or dendritic solidification structure under an optical microscope in the cross section corroded with nital.

[0093] Regarding the manufacturing conditions for the steel plate and steel pipe not described above, all of them can be made in accordance with conventional methods. [Example]

[0094] The present invention will be further described below based on examples, but the present invention is not limited to the following examples.

[0095] (Hot-rolled steel sheet manufacturing) Molten steel having the chemical composition shown in Table 1 was produced.

[0096] [Table 1]

[0097] The molten steel was subjected to a casting process under the conditions shown in Table 2 to produce a slab (steel material). The slab was then subjected to a hot rolling process, a cooling process, and a coiling process under the conditions shown in Table 2 to produce a hot-rolled steel sheet.

[0098] [Table 2]

[0099] Various test specimens were taken from the hot-rolled steel sheet, and microstructure observation, measurement of average crystal grain size, measurement of the volume fraction of crystal grains with a grain size of 40.0 μm or more, measurement of the number density of Ti-based inclusions with a major axis of 5.0 μm or more, tensile tests, and Charpy impact tests were performed using the methods described below. Various test specimens were taken from the center in the width direction of the hot-rolled steel sheet. The results are shown in Table 3.

[0100] [Table 3]

[0101] (Manufacture of electric resistance welded steel pipes) The hot-rolled steel sheet thus obtained was then formed into a cylindrical open pipe (round steel pipe) by cold rolling, and the butt joints of the open pipe were electric resistance welded to form a steel pipe material (pipe-making process). The weld heat treatment process was then carried out under the conditions shown in Table 4. The steel pipe material after the weld heat treatment process was reduced in diameter using rolls arranged above, below, left and right of the steel pipe material (sizing process), and electric resistance welded steel pipes with the outer diameter D (mm) and wall thickness t (mm) shown in Table 4 were obtained.

[0102] [Table 4]

[0103] Various test pieces were taken from the electric resistance welded steel pipe and subjected to the following methods: microstructural observation, measurement of average grain size, measurement of the volume fraction of grains with a grain size of 40.0 μm or more, measurement of the number density of Ti-based inclusions with a major axis of 5.0 μm or more, tensile testing, Charpy impact testing, Vickers hardness testing, and flattening testing. The test pieces were taken from the electric resistance weld of the electric resistance welded steel pipe and from the base material, which was 90° circumferentially away from the electric resistance weld, assuming that the electric resistance weld was at 0°. The results are shown in Table 5.

[0104] [Table 5]

[0105] The specific methods for various observations, measurements, and tests are as follows:

[0106] [Structural observation] Test specimens for microstructural observation were prepared by taking specimens from the hot-rolled steel plate and the electric-resistance welded steel pipe so that the observation surface was a cross section parallel to both the rolling direction and the thickness direction of the hot-rolled steel plate for the hot-rolled steel plate, a cross section parallel to both the axial direction and the thickness direction of the electric-resistance welded steel pipe for the base metal portion of the electric-resistance welded steel pipe, and a cross section parallel to both the circumferential direction and the thickness direction of the electric-resistance welded portion of the electric-resistance welded steel pipe, and then mirror-polishing them.

[0107] The observation surface of the test specimen was subjected to nital etching, and then microstructural observation was performed using an optical microscope (magnification: 1000x) or a scanning electron microscope (SEM, magnification: 1000x) at the center of the plate thickness for the hot-rolled steel plate, and at the center of the wall thickness for the base metal portion of the electric resistance welded steel pipe and the electric resistance welded portion of the electric resistance welded steel pipe. From the optical microscope images and SEM images obtained in this manner, the area fractions of bainite, ferrite, and the remainder (pearlite, martensite, and austenite) were determined. The area fractions of each structure were observed in five fields of view, and the values ​​obtained in each field of view were averaged to obtain the average value. In this example, the area fractions obtained by microstructural observation were used as the volume fractions of each structure. Here, ferrite is a product of diffusion transformation, and exhibits a nearly recovered structure with low dislocation density. This includes polygonal ferrite and pseudo-polygonal ferrite.

[0108] Bainite is a complex phase structure of lath-shaped ferrite and cementite with a high dislocation density. Pearlite is a eutectoid structure of iron and iron carbide (ferrite + cementite), and exhibits a lamellar structure in which linear ferrite and cementite are arranged alternately.

[0109] Martensite is a lath-like structure transformed at low temperatures with an extremely high dislocation density. In SEM images, it exhibits brighter contrast than ferrite and bainite.

[0110] Since it is difficult to distinguish between martensite and austenite in optical microscope images and SEM images, the area fraction of the structure observed as martensite or austenite in the obtained SEM images was measured, and the volume fraction of austenite measured by the following method was subtracted from this to determine the volume fraction of martensite. The austenite volume fraction was measured by X-ray diffraction. The test specimens for the center-thickness measurements of the hot-rolled steel sheet and the base metal of the ERW steel pipe were ground so that the diffraction plane was at the center-thickness of the hot-rolled steel sheet and the base metal of the ERW steel pipe, respectively, and then chemically polished to remove the surface layer. Mo Kα radiation was used for the measurements. The integrated intensities of the (200), (220), and (311) planes of fcc iron and the (200) and (211) planes of bcc iron were determined. The normalized integrated intensities were calculated by dividing each value by the theoretical intensity, and the normalized integrated intensity was calculated as the ratio of the austenite normalized integrated intensity, assuming that the normalized integrated intensity is proportional to the volume fraction of each phase. The austenite volume fraction was calculated by the ratio of the normalized integrated intensity of austenite.

[0111] [Measurement of average crystal grain size] To measure the average crystal grain size, first, a histogram of the grain size distribution (a graph with the horizontal axis representing the grain size and the vertical axis representing the proportion of each grain size) was calculated using the SEM / EBSD method, and the arithmetic mean of the grain size was calculated. Specifically, the crystal grain size was determined by determining the misorientation between adjacent crystal grains, and the boundary with a misorientation of 15° or more was considered to be a crystal grain (grain boundary), measuring the circle-equivalent diameter of the crystal grains, and the average circle-equivalent diameter was taken as the average crystal grain size. In this case, the circle-equivalent diameter was the diameter of a circle with the same area as the target crystal grain.

[0112] [Grain size: Measurement of volume fraction of crystal grains 40.0 μm or larger] The volume fraction of crystal grains having a grain size of 40.0 μm or more was determined as the total area fraction of crystal grains having a grain size of 40.0 μm or more from the histogram of the crystal grain size distribution. The measurement conditions were an acceleration voltage of 15 kV, a measurement area of ​​500 μm × 500 μm, and a measurement step size of 0.5 μm. Note that in the crystal grain size analysis, crystal grains with a grain size of 2.0 μm or less were excluded from the analysis as measurement noise, and the obtained area fraction was considered to be equal to the volume fraction.

[0113] [Measurement of the number density of Ti-based inclusions with major diameter of 5.0 μm or more] The number density of Ti-based inclusions with a major axis of 5.0 μm or more was measured by combining SEM measurement of the major axis of the inclusions and SEM / EDS elemental analysis in the same field of view in cross sections parallel to both the rolling direction and the thickness direction of the hot-rolled steel sheet, and in cross sections parallel to both the axial direction and the thickness direction of the base material of the electric resistance welded steel pipe. The measurement positions were the center of the thickness of the hot-rolled steel sheet and the center of the thickness of the base material of the electric resistance welded steel pipe, the measurement area was 4 mm x 10 mm, and the measured values ​​for five fields of view were averaged.

[0114] [Tensile test] For hot-rolled steel sheets, JIS No. 5 tensile test specimens were taken so that the tensile direction was parallel to the rolling direction. On the other hand, for electric resistance welded steel pipes, JIS No. 11 tensile test specimens were taken so that the tensile direction was parallel to the pipe axis. Tensile tests were conducted in accordance with the provisions of JIS Z 2241. Yield stress YS (MPa), tensile strength TS (MPa), and total elongation EL (%) were measured, and the yield ratio YR (%), defined as (YS / TS) x 100, was calculated.

[0115] [Charpy impact test] For hot-rolled steel plates, Charpy impact tests were conducted at -60°C in accordance with JIS Z 2242, with V-notch specimens taken from the center of the plate thickness so that the longitudinal direction of the specimen was parallel to the rolling direction, and the absorbed energy was determined. On the other hand, for electric resistance welded steel pipes, V-notch specimens were taken from the center of the wall thickness of the base material so that the longitudinal direction of the specimen was parallel to the pipe axis, and the tests were conducted at -20°C in accordance with JIS Z 2242, with the absorbed energy determined. The impact value was determined by dividing the absorbed energy by the cross-sectional area of ​​the fracture surface before the test. The number of test pieces for each of the above Charpy impact tests was three, and the average of the obtained impact values ​​was used as the Charpy impact value (vE -60 ) and the Charpy impact value (vE -20 ) was decided.

[0116] [Vickers hardness test] Vickers hardness tests were conducted in accordance with the provisions of JIS Z 2244, using test pieces taken from the electric resistance weld of the electric resistance welded steel pipe so that the measurement surface was parallel to both the circumferential direction and the wall thickness direction of the pipe. The load was 0.1 kgf, and the measurement position was the center of the wall thickness of the weld.

[0117] [Flattening test] The flattening test was carried out in accordance with the method described in JIS G 3441. An annular test piece 100 mm long in the axial direction was taken from the obtained electric resistance welded steel pipe, and the outer surface of the pipe at the weld was polished to a metallic luster. The test piece was compressed at a rate of 10 mm / min, and was stopped when cracks occurred. The height H of the test piece at that point was measured. The flattening value was calculated using the formula H / D, where D is the outer diameter of the steel pipe.

[0118] In Table 3, hot-rolled steel sheets Nos. 1 to 5 and Nos. 14 and 15 are examples of the present invention, and hot-rolled steel sheets Nos. 6 to 13 are comparative examples. In Table 5, Nos. 1 to 5 are examples of the present invention, and Nos. 6 to 15 are comparative examples. Here is an example.

[0119] In the hot-rolled steel sheets of the present invention shown in Table 3, the steel structure at the center of the sheet thickness has a volume fraction of bainite of 90% or more, and the remainder contains one or more of ferrite, pearlite, martensite, and austenite, an average crystal grain size of 10.0 μm or less, a volume fraction of crystal grains with a grain size of 40.0 μm or more of 40% or less, and a number density of Ti-based inclusions with a major axis of 5.0 μm or more of 20 pieces / mm 2 It was as follows.

[0120] On the other hand, in the electric resistance welded steel pipes of the present invention shown in Table 5, the steel structure at the center of the wall thickness of the base material portion has a volume fraction of bainite of 90% or more, with the remainder containing one or more of ferrite, pearlite, martensite and austenite, an average crystal grain size of 10.0 μm or less, a volume fraction of crystal grains with a grain size of 40.0 μm or more of 40% or less, and a number density of Ti-based inclusions with a major diameter of 5.0 μm or more of 20 pieces / mm 2The steel structure at the center of the thickness of the electric resistance weld had a ferrite volume fraction of 10% or more, a total volume fraction of ferrite and bainite of 60% to 98%, and the remainder included one or more of pearlite, martensite, and austenite, and the average crystal grain size was 15.0 μm or less.

[0121] The hot-rolled steel sheets of the examples of the present invention shown in Table 3 all have a yield stress YS of 650 MPa or more, a tensile strength TS of 750 MPa or more, a yield ratio YR of 90.0% or less, a total elongation EL of 15% or more, and a Charpy impact value at -60°C of 100 J / cm 2 That was all.

[0122] On the other hand, the electric resistance welded steel pipes of the present invention shown in Table 5 have a base metal portion with a yield stress YS of 650 MPa or more, a tensile strength TS of 750 MPa or more, a yield ratio YR of 96.0% or less, a total elongation EL of 15% or more, and a Charpy impact strength of 50 J / cm at -20°C. 2 The Vickers hardness at the center of the thickness of the electric resistance welded portion was 200 HV or more, and the flattening value was 0.80 or less.

[0123] In contrast, the comparative example No. 6 hot-rolled steel plate and electric resistance welded steel pipe had a high C content, so the volume fraction of bainite was below the range of the present invention, and the Charpy impact value did not reach the desired value.

[0124] The hot-rolled steel sheet and electric resistance welded steel pipe of Comparative Example No. 7 had a high Si content, and therefore the total elongation, Charpy impact value and flattening value of the electric resistance welded steel pipe did not reach the desired values.

[0125] The comparative hot-rolled steel plate and electric resistance welded steel pipe of No. 8 had a high Mn content, so the volume fraction of bainite was below the range of the present invention, and the Charpy impact value did not reach the desired value.

[0126] The comparative example No. 9 hot-rolled steel plate and electric resistance welded steel pipe had a low Ti content, and therefore the yield stress or tensile strength did not reach the desired value.

[0127] The comparative example No. 10 hot-rolled steel plate and electric resistance welded steel pipe had a high Ti content, so the number density of Ti-based inclusions with a major diameter of 5.0 μm or more exceeded the range of the present invention, and the Charpy impact value did not reach the desired value.

[0128] The comparative example No. 11 hot-rolled steel plate and electric resistance welded steel pipe had a low average cooling rate in the casting process, so the number density of Ti-based inclusions with a major axis of 5.0 μm or more exceeded the range of the present invention, and the Charpy impact value did not reach the desired value.

[0129] The hot-rolled steel plate and electric-resistance welded steel pipe of Comparative Example No. 12 had high reheating temperatures, rough rolling end temperatures, and finish rolling end temperatures in the hot rolling process, so the average crystal grain size exceeded the range of the present invention, and the yield stress and / or tensile strength, and Charpy impact value did not reach the desired values.

[0130] The hot-rolled steel plate and electric-resistance welded steel pipe of Comparative Example No. 13 had a low total reduction ratio in finish rolling, so the volume fraction of crystal grains with a grain size of 40.0 μm or more exceeded the range of the present invention, and the Charpy impact value did not reach the desired value.

[0131] In the comparative example, electric resistance welded steel pipe No. 14, the air cooling time in the heat treatment process of the weld was long, so the average crystal grain size of the electric resistance weld exceeded the range of the present invention, and the Vickers hardness and flattening value did not reach the desired values.

[0132] In the comparative example, electric welded steel pipe No. 15, the heating temperature in the weld heat treatment process was high, so the volume fraction of ferrite and the total volume fraction of ferrite and bainite were below the range of the present invention, and the flattening value did not reach the desired value. [Explanation of symbols]

[0133] 1 Base metal part 2. Weld heat affected zone 3 Bond Section

Claims

1. A hot-rolled steel sheet, In mass%, C: 0.020% or more and 0.200% or less, Si: 0.02% or more and 1.00% or less, Mn: 0.10% or more and 2.50% or less, P: 0.050% or less, S: 0.0200% or less, Al: 0.005% or more and 0.100% or less, N: 0.0100% or less and Ti: 0.030% or more and 0.200% or less and the balance being Fe and unavoidable impurities, The steel has a steel structure at the center of the plate thickness, in which the volume fraction of bainite is 90% or more and the remainder contains one or more of ferrite, pearlite, martensite, and austenite, Furthermore, the steel structure at the center of the plate thickness has an average crystal grain size of 10.0 μm or less, a volume fraction of crystal grains with a grain size of 40.0 μm or more is 40% or less, and the number density of Ti-based inclusions with a major diameter of 5.0 μm or more is 20 pieces / mm ２ Below is hot rolled steel sheet.

2. The component composition further includes, in mass %, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.50% or less, Nb: 0.050% or less, V: 0.050% or less, Ca: 0.0050% or less and B: 0.0050% or less The hot-rolled steel sheet according to claim 1, comprising one or more of the following:

3. The method for producing a hot-rolled steel sheet according to claim 1 or 2, a casting process in which molten steel is cast at an average cooling rate of 0.30°C / s or more from the solidification point of the molten steel to 1200°C to produce a steel material; a hot rolling process in which the steel material is heated to a heating temperature in the range of 1150°C to 1300°C, and then hot-rolled at a rough rolling end temperature of 950°C to 1180°C, a finish rolling end temperature of 850°C to 1000°C, and a total reduction in finish rolling of 50% to 80%; After the hot rolling step, a cooling step is performed in which cooling is performed at an average cooling rate of 10°C / s or more and 60°C / s or less at the center of the thickness, and a cooling stop temperature of 400°C or more and 580°C or less at the center of the thickness; a winding step of winding the sheet at a temperature of 400°C or higher and 580°C or lower after the cooling step; A method for producing a hot-rolled steel sheet, comprising:

Images