Carbon fiber and method for producing carbon fiber

Abstract

To provide a carbon fiber having higher tensile strength.SOLUTION: A carbon fiber satisfies the formula (1): 29-(0.034×TM)≤(O / C+N / C)×TM≤40-(0.016×TM), and has a tensile strength of 6,020 MPa or more, wherein TM is a tensile elastic modulus [GPa] of the carbon fiber, O / C is a surface oxygen concentration measured by X-ray photoelectron spectroscopy, and N / C is a surface nitrogen concentration measured by X-ray photoelectron spectroscopy.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a high tensile strength carbon fiber and a method for producing the same. [Background technology]

[0002] Carbon fibers are produced by calcining precursor fibers made from polyacrylonitrile-based fibers, rayon-based fibers, cellulose-based fibers, pitch-based fibers, etc. For example, when carbon fibers are produced using precursor fibers made from polyacrylonitrile-based fibers, a flame-proofing process is performed in which the precursor fibers are heated in an oxygen-containing atmosphere (in a flame-proofing furnace), and a carbonization process is performed in which the fibers that have undergone the flame-proofing process (hereinafter referred to as "flame-proofed fibers") are heated in an inert atmosphere (carbonization furnace). The heating is performed by passing (running) the fibers through the flame-proofing furnace and the carbonization furnace. Heating in the carbonization step is performed using, for example, an electric heater. That is, the atmosphere inside the furnace is heated by the electric heater, and the flame-resistant fiber passes through the heated furnace, thereby indirectly heating the flame-resistant fiber. Carbon fibers having various properties such as tensile properties and compressive properties have been provided by adjusting the temperature conditions in the carbonization process and the fiber drawing conditions (for example, Patent Documents 1 and 2). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 10-25627 [Patent Document 2] Japanese Patent Application Laid-Open No. 2002-54031 Summary of the Invention [Problem to be solved by the invention]

[0004] However, as described in Patent Documents 1 and 2, carbon fibers having high tensile strength properties have not been obtained even when the temperature conditions and drawing conditions in the carbonization step are adjusted.

[0005] In view of the above-mentioned problems, an object of the present invention is to provide a carbon fiber having higher tensile strength and a method for producing the carbon fiber. [Means for solving the problem]

[0006] In order to achieve the above object, a carbon fiber according to one embodiment of the present invention satisfies the relationship of the following formula (1) and has a tensile strength of 6,020 MPa or more. 29-(0.034×TM)≦(O / C+N / C)×TM≦40-(0.016×TM) ··· (1) where "TM" is the tensile modulus of the carbon fiber [GPa], "O / C" is the surface oxygen concentration measured by X-ray photoelectron spectroscopy, and "N / C" is the surface nitrogen concentration measured by X-ray photoelectron spectroscopy. In order to achieve the above object, a carbon fiber according to one aspect of the present invention satisfies the relationship of the following formula (1) and also satisfies any one of the following formulas (2) to (4): 29-(0.034×TM)≦(O / C+N / C)×TM≦40-(0.016×TM) ··· (1) If 228≦TM≦248, 0.050 <O / C<0.127 0.034<N / C<0.036 ··· (2) If 290 ≤ TM ≤ 322, 0.039 <O / C<0.086 0.026<N / C<0.036 ··· (3) If 328 ≤ TM ≤ 374, 0.030 <O / C<0.077 0.020<N / C<0.032 ··· (4) where "TM" is the tensile modulus of the carbon fiber [GPa], "O / C" is the surface oxygen concentration measured by X-ray photoelectron spectroscopy, and "N / C" is the surface nitrogen concentration measured by X-ray photoelectron spectroscopy. In order to achieve the above object, a carbon fiber according to one aspect of the present invention satisfies the relationship of the following formula (1) and also satisfies any one of the following formulas (5) to (7): 29-(0.034×TM)≦(O / C+N / C)×TM≦40-(0.016×TM) ··· (1) If 228≦TM≦248, then 5,020≦TS≦5,580 (5) If 290≦TM≦322, then 6,020≦TS≦6,620 (6) If 328≦TM≦374, then 6,720≦TS≦7,400 (7) where "TM" is the tensile modulus of the carbon fiber [GPa], "TS" is the tensile strength of the carbon fiber [MPa], "O / C" is the surface oxygen concentration measured by X-ray photoelectron spectroscopy, and "N / C" is the surface nitrogen concentration measured by X-ray photoelectron spectroscopy. In order to achieve the above object, a method for producing a carbon fiber according to one aspect of the present invention is a method for producing a carbon fiber in which energy is applied after a carbonization step, The method for producing carbon fibers, wherein the energy is applied so as to satisfy the relationship of the following formula (1) and also to satisfy any one of the following formulas (2) to (4): 29-(0.034×TM)≦(O / C+N / C)×TM≦40-(0.016×TM) ··· (1) If 228≦TM≦248, 0.050 <O / C<0.127 0.034<N / C<0.036 ··· (2) If 290 ≤ TM ≤ 322, 0.039 <O / C<0.086 0.026<N / C<0.036 ··· (3) If 328 ≤ TM ≤ 374, 0.030 <O / C<0.077 0.020<N / C<0.032 ··· (4) where "TM" is the tensile modulus of the carbon fiber [GPa], "O / C" is the surface oxygen concentration measured by X-ray photoelectron spectroscopy, and "N / C" is the surface nitrogen concentration measured by X-ray photoelectron spectroscopy. [Effects of the Invention]

[0007] The carbon fiber according to one aspect of the present invention has high tensile strength. The carbon fiber manufacturing method according to one aspect of the present invention can produce carbon fibers with high tensile strength. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a schematic diagram showing a carbon fiber manufacturing process. [Figure 2] FIG. 1 is a schematic diagram of a surface treatment device. [Figure 3] FIG. 1 is a diagram showing the relationship between strength parameters and tensile strength of fibers. [Figure 4] FIG. 10 is a schematic view of another type of surface treatment device. [Figure 5] FIG. 10 is a schematic view of another type of surface treatment device. [Figure 6] FIG. 10 is a schematic view of another type of surface treatment device. [Figure 7] FIG. 10 is a schematic view of another type of surface treatment device. [Figure 8] FIG. 10 is a schematic view of another type of surface treatment device. DETAILED DESCRIPTION OF THE INVENTION

[0009] <<Summary>> The inventors have been investigating surface modification of carbonized fibers using plasma, which improves the adhesive strength between the carbon fibers and the matrix resin when they are used in composite materials. The adhesive strength between carbon fiber and matrix resin is affected by the amount of functional groups formed on the surface of the carbon fiber. The adhesive strength differs depending on the type of matrix resin, but can be determined by the surface oxygen concentration (O / C), which indicates the ratio of oxygen atoms to carbon atoms on the carbon fiber surface. The inventors further conducted research with a view to surface modification, focusing on the surface nitrogen concentration (N / C) which indicates the abundance ratio of nitrogen atoms to carbon atoms on the surface of carbon fibers. As a result, it was found that the tensile strength of carbon fiber is high when the relationship defined by the surface oxygen concentration (O / C), surface nitrogen concentration (N / C) and tensile modulus of carbon fiber is within a certain range.

[0010] A carbon fiber according to one embodiment of the present invention satisfies the relationship of the following formula (1). 29-(0.034×TM)≦(O / C+N / C)×TM≦40-(0.016×TM) (1) where "TM" is the tensile modulus of the carbon fiber [GPa], "O / C" is the surface oxygen concentration measured by X-ray photoelectron spectroscopy, and "N / C" is the surface nitrogen concentration measured by X-ray photoelectron spectroscopy. The carbon fiber according to this embodiment has a tensile modulus of 240 GPa or more, which allows the tensile strength of the carbon fiber to be increased to a level equal to or greater than that of so-called general-purpose carbon fibers. The carbon fiber according to this embodiment has a tensile strength of 5,000 MPa or more, which makes it possible to obtain high-strength carbon fibers.

[0011] A method for producing carbon fibers according to one aspect of the present invention is a method for producing carbon fibers in which energy is applied after the carbonization step, and the energy is applied so as to satisfy the relationship of the following formula (1). 29-(0.034×TM)≦(O / C+N / C)×TM≦40-(0.016×TM) (1) where "TM" is the tensile modulus of the carbon fiber [GPa], "O / C" is the surface oxygen concentration measured by X-ray photoelectron spectroscopy, and "N / C" is the surface nitrogen concentration measured by X-ray photoelectron spectroscopy. In the method for producing carbon fibers according to this embodiment, the energy is plasma, which allows for easy surface treatment and increased tensile strength.

[0012] <Embodiment> A surface treatment method using the surface treatment device of one embodiment and a method for producing carbon fibers will be described. Here, acrylonitrile-based fibers, which are an example of precursor fibers, are used.

[0013] 1. Carbon fiber manufacturing process FIG. 1 is a schematic diagram showing the carbon fiber manufacturing process.

[0014] Carbon fiber is manufactured using precursor fibers. A single precursor is a bundle of multiple filaments, for example, 12,000 filaments. In some cases, it is called a precursor fiber bundle or a carbon fiber bundle. Precursor 1a can be obtained by spinning a spinning solution containing 90% or more by weight of acrylonitrile by wet spinning or dry wet spinning, followed by washing with water, drying, and stretching. The monomers used for copolymerization include alkyl acrylate, alkyl methacrylate, acrylic acid, acrylamide, itaconic acid, and maleic acid. Usually, the speed at which the precursor 1a is produced differs from the speed at which the precursor 1a is carbonized to produce carbon fibers, so the produced precursor 1a is temporarily stored in a carton or wound around a bobbin.

[0015] 1, the precursor 1a is drawn off, for example, from a bobbin 30 and travels downstream, undergoing various processes along the way and being wound onto a bobbin 39 as carbon fiber. As shown in Figure 1, carbon fiber is manufactured through a flame-retardant process that renders precursor 1a flame-retardant; a carbonization process that carbonizes flame-retardant fiber 1b while stretching it; a surface treatment process that improves the surface of carbonized fiber 1d; a sizing process that applies resin to surface-improved fiber 1e (also referred to as "surface-treated fiber"); and a drying process that dries resin-applied fiber 1f. The dried fiber 1g is wound around a bobbin 39 as carbon fiber 1g. Note that carbon fiber 1g is a fiber that has been subjected to at least a surface treatment on carbonized fiber 1d, and is distinguished from carbonized fiber 1d that has undergone the carbonization process but not the surface treatment process. Here, the process of making the precursor 1a flame-resistant is called the flame-resistant treatment, the process of carbonizing the flame-resistant fiber 1b is called the carbonization treatment, the process of improving the surface of the carbonized fiber 1d is called the surface treatment, the process of attaching resin to the surface-treated fiber 1e is called the sizing treatment, and the process of drying the fiber 1f with the resin attached is called the drying treatment. The treatments and steps are explained below.

[0016] (1) Flameproofing process (flameproofing treatment) The flame-proofing step is carried out using a flame-proofing furnace 3 set to an oxidizing atmosphere of 200 to 350°C. Specifically, the flame-proofing is carried out by passing the precursor 1a through the flame-proofing furnace 3 in an air atmosphere once or multiple times. The oxidizing atmosphere may contain oxygen, nitrogen dioxide, etc. During the flame-proofing process, the precursor 1a is stretched at a predetermined tension in accordance with the carbon fiber 1g to be produced. The stretching ratio during the flame-proofing process is, for example, within a range of 0.7 to 1.3. The precursor 1a is stretched using a plurality of rollers. For example, the stretching is performed using two rollers 5 and 7 on the entrance side of the flame-proofing furnace 3 and three rollers 9, 11, and 13 on the exit side.

[0017] (2) Carbonization process (carbonization treatment) The carbonization step is a step in which the flame-resistant fiber 1b is heated to cause a pyrolysis reaction and carbonize the fiber, and is carried out in an inert atmosphere at a maximum temperature of 300 to 1,800°C. Carbonization is carried out by passing the flame-resistant fiber 1 b through a first carbonization furnace 15 , and then passing the fiber 1 c that has passed through the first carbonization furnace 15 through a second carbonization furnace 17 . Here, the carbonization performed in the first carbonization furnace 15 will be referred to as the "first carbonization" or "first carbonization process," and similarly, the carbonization performed in the second carbonization furnace 17 will be referred to as the "second carbonization" or "second carbonization process."

[0018] The first carbonization is carried out at a maximum temperature of, for example, 300 to 800° C., and the second carbonization is carried out at a maximum temperature of, for example, 500 to 1,800° C. Heating in the carbonization step is carried out using, for example, an electric heater, microwaves, plasma, etc.

[0019] The first carbonization furnace 15 and the second carbonization furnace 17 are provided independently of each other, and an adjusting means for adjusting the tension of the fibers can be provided between the carbonization furnaces 15 and 17. A roller 19 is provided on the entrance side of the first carbonization furnace 15, a roller 21 is provided between the first carbonization furnace 15 and the second carbonization furnace 17, and a roller 23 is provided on the exit side of the second carbonization furnace 17. Specifically, it is preferable to apply a tension of 50 to 200 [g / dtex] in the first carbonization step, and 200 to 1,000 [g / dtex] in the second carbonization step. By applying a tension within this range, a carbon fiber 1g with higher strength can be obtained. The density of the carbonized fiber 1d is 1.5 to 1.9 [g / cm 3 ] and 1.75 to 1.82 [g / cm 3 The diameter of the fiber 1d is preferably 4 to 11 μm.

[0020] (3) Surface treatment process (surface treatment) The surface treatment step is performed by passing the carbonized fiber 1d through a surface treatment device 25. A roller 26 is provided on the outlet side of the surface treatment device 25. When the carbon fiber 1g is used to make a composite material, the surface treatment improves the affinity and adhesion between the carbon fiber 1g and a matrix resin. The surface treatment is generally performed by oxidizing the surface of the carbonized fiber 1d. Atmospheric pressure plasma is used for the surface treatment. The surface treatment device 25 will be described later.

[0021] (4) Sizing process (sizing treatment) The sizing step is performed, for example, by passing the surface-treated fibers 1e through a resin liquid 29. The resin liquid 29 is stored in a resin bath 27. The sizing step improves the convergence of the surface-treated fibers 1e. During the sizing process, the surface-treated fibers 1e pass through the resin liquid 29 while changing their running direction due to a plurality of rollers 31, 33, etc., arranged inside or around the resin bath 27. The resin liquid 29 may be, for example, a liquid or emulsion of epoxy resin, urethane resin, phenol resin, vinyl ester resin, unsaturated polyester resin, or the like dissolved in a solvent.

[0022] (5) Drying process (drying treatment) The drying step is performed by passing the fiber 1f through a drying furnace 35. The dried fiber 1g is wound onto a bobbin 39 via a roller 37 on the downstream side of the drying furnace 35 (winding step).

[0023] 2.Surface treatment equipment 2, the surface treatment device 25 has a cylindrical tube portion 251 extending in the running direction of the carbonized fiber 1d, and an irradiation portion 255 provided in the middle part of the longitudinal direction of the tube portion 251 and irradiating with atmospheric pressure plasma. The carbonized fiber 1d runs inside the tube portion 251.

[0024] (1)Cylinder part The cylindrical portion 251 has a cross-sectional shape that may be, for example, a circular, elliptical, or oval non-cornered shape, or a polygonal shape such as a rectangular, square, or hexagonal shape. The total length of the tubular portion 251 in the running direction is preferably 100 to 10,000 times, and more preferably 200 to 2,000 times, the dimension in the running direction of the irradiation portion 255. The total length of the tubular portion 251 is preferably 20 to 2,000 times, and more preferably 40 to 400 times, the dimension (diameter) of the tubular portion 251 in the plasma irradiation direction. The total length of the tubular portion 251 in the running direction is preferably 100 to 10,000 mm, and more preferably 200 to 2,000 mm. The cylindrical portion 251 here has a through-hole 253 in the center (middle portion) in the longitudinal direction thereof, and atmospheric pressure plasma is irradiated from an irradiation portion 255 to the inside of the cylindrical portion 251 using the through-hole 253 . The cylindrical portion 251 has a cross section of 5 to 4 × 10 with respect to the total cross section area of all the carbonized fibers 1d. 4 It has twice the internal space.

[0025] The through-hole 253 has a slit shape, a circular shape, an elliptical shape, or a polygonal shape such as a pentagon. The slit shape may extend in a direction intersecting the running direction of the carbonized fiber 1d or in a direction parallel to the running direction. The slit shape here extends in a direction perpendicular to the running direction. One through-hole 253 is formed here. The through-holes 253 are preferably provided in such a manner that the carbonized fibers 1d can be seen through the through-holes 253 when viewed from the plasma generation site.

[0026] (2) Irradiation unit The irradiation unit 255 utilizes, for example, a dielectric barrier discharge. The irradiation unit 255 includes a pair of electrodes with a dielectric disposed on at least one of the electrodes, and a nozzle that supplies a mixed gas of nitrogen and oxygen around the pair of electrodes, and generates plasma by causing discharge between the pair of electrodes under atmospheric pressure in the mixed gas supplied from the nozzle. The pair of electrodes may be, for example, flat counter electrodes (parallel plate electrodes). The counter electrodes are arranged, for example, so as to extend in a direction intersecting the running direction of the carbonized fibers 1d or in a direction parallel to the running direction. Here, the counter electrodes are arranged in a direction perpendicular to the running direction, corresponding to the slit-shaped through-holes 253. The generated plasma is irradiated into the interior of the cylindrical portion 251 through the through-hole (irradiation port) 253 by gas from the nozzle. As a result, the carbonized fibers 1d traveling inside the cylindrical portion 251 are irradiated with atmospheric pressure plasma.

[0027] 3. Surface treatment energy The plasma energy used in the surface treatment (surface treatment energy) varies depending on the treatment temperature, tension, etc. of the carbonization step, but is given so as to satisfy the intensity parameters expressed by the following equation (1). 29-(0.034×TM)≦(O / C+N / C)×TM≦40-(0.016×TM) (1) where: O / C: Surface oxygen concentration measured by XPS N / C: surface nitrogen concentration measured by XPS TM: Tensile modulus of carbon fiber (GPa) measured in accordance with JIS R 7606 The value of "(O / C+N / C)×TM" in formula (1) is defined as the "strength parameter." The surface treatment energy tends to increase as the treatment temperature in the carbonization step increases. When the supply gas is a mixed gas of nitrogen and oxygen, the surface oxygen concentration tends to increase as the surface treatment energy increases.

[0028] The surface treatment energy is preferably within a range of 5 to 100 (MJ / kg) when the maximum temperature in the carbonization step is 1,200 to 1,400°C. When the maximum temperature in the carbonization step is 1,200 to 1,400°C, the surface treatment energy is preferably set so that the surface oxygen concentration is within a range of 0.050 to 0.127. When the maximum temperature in the carbonization step is 1,200 to 1,400°C, the surface treatment energy is preferably set so that the surface nitrogen concentration is within a range of 0.034 to 0.036. By applying such surface treatment energy to the carbonized fiber 1d (after the carbonization process), carbon fibers with high tensile strength can be obtained, and the adhesion between the carbon fibers and the matrix resin when made into a composite material can be improved.

[0029] The surface treatment energy is preferably within a range of 20 to 350 (MJ / kg) when the maximum temperature in the carbonization step is 1,400 to 1,600°C. When the maximum temperature in the carbonization step is 1,400 to 1,600°C, the surface treatment energy is preferably set so that the surface oxygen concentration is within a range of 0.039 to 0.086. When the maximum temperature in the carbonization step is 1,400 to 1,600°C, the surface treatment energy is preferably set so that the surface nitrogen concentration is within a range of 0.026 to 0.036. By applying such surface treatment energy to the carbonized fibers 1d, carbon fibers with high tensile strength can be obtained, and the adhesion between the carbon fibers and the matrix resin when made into a composite material can be improved.

[0030] The surface treatment energy is preferably within a range of 45 to 680 (MJ / kg) when the maximum temperature in the carbonization step is 1,600 to 2,000°C. When the maximum temperature in the carbonization step is 1,600 to 2,000°C, the surface treatment energy is preferably set so that the surface oxygen concentration is within a range of 0.030 to 0.077. When the maximum temperature in the carbonization step is 1,600 to 2,000°C, the surface treatment energy is preferably set so that the surface nitrogen concentration is within a range of 0.020 to 0.032. By applying such surface treatment energy to the carbonized fibers 1d, carbon fibers with high tensile strength can be obtained, and the adhesion between the carbon fibers and the matrix resin when made into a composite material can be improved. [Example]

[0031] The present invention will be explained in more detail below by showing examples and comparative examples, but the present invention is not limited to the following examples as long as it does not depart from the gist of the invention. Here, a surface treatment step for performing a surface treatment on the carbonized fiber 1d will be described. First, prior to the Examples and Comparative Examples, the following materials were prepared.

[0032] <Carbonized fiber 1d> In the examples, three types of carbonized fibers 1d were used. Fiber 1d is one of three types produced by subjecting 24,000 precursor fibers 1a to a flame-proofing process in an oxidizing atmosphere at 200 to 350°C, followed by a first carbonization process in an inert atmosphere at a temperature of 300 to 800°C, and a second carbonization process at a maximum temperature of 1,300°C, 1,400°C, or 1,800°C. In the case of fiber 1d, where the maximum temperature in the second carbonization step was 1,300°C, the density of the carbonized fiber 1d was 1.80 g / cm 3 ], the diameter of fiber 1d is 6.9 [μm], the surface oxygen concentration is 0.047, and the surface nitrogen concentration is 0.035 (see Comparative Example 1 in Table 1). For fiber 1d in which the maximum temperature in the second carbonization step is 1,400 [°C], the density of the carbonized fiber 1d is 1.76 [g / cm3 ], the diameter of fiber 1d is 6.4 [μm], the surface oxygen concentration is 0.037, and the surface nitrogen concentration is 0.018 (see Comparative Example 3 in Table 2). For fiber 1d in which the maximum temperature in the second carbonization step is 1,800 [°C], the density of the carbonized fiber 1d is 1.81 [g / cm 3 ], the diameter of fiber 1d is 5.0 [μm], the surface oxygen concentration is 0.029, and the surface nitrogen concentration is 0.013 (see Comparative Example 5 in Table 3).

[0033] <Surface oxygen concentration (O / C)> The surface oxygen concentration (O / C) of the surface-treated fiber 1e after the surface treatment can be determined by XPS (ESCA) according to the following procedure. An X-ray photoelectron spectrometer, ESCA JPS-9000MX, manufactured by JEOL Ltd., was used for the measurement. The fibers were cut and spread out on a stainless steel sample support, and then the photoelectron escape angle was set to 90 degrees. MgKα was used as the X-ray source, and the sample chamber was heated to 1×10 -6 The vacuum was maintained at 0.05 Pa. To correct the peak caused by charging during measurement, first C 1s The binding energy value BE of the main peak of O is set to 284.6 eV. 1s The peak area was determined by drawing a straight baseline in the range of 527 to 540 eV. 1s The peak area was determined by drawing a straight baseline in the range of 281 to 297 eV. 1s O relative to peak 1s The sensitivity correction factor for the peak was 2.6865. The surface oxygen concentration (O / C) of the carbon fiber surface was calculated by the above O 1s Peak area and C 1s It was calculated from the ratio of peak areas.

[0034] <Surface nitrogen concentration (N / C)> The surface nitrogen concentration (N / C) of the surface-treated fiber 1e after the surface treatment can be determined by XPS (ESCA) according to the following procedure. An X-ray photoelectron spectrometer, ESCA JPS-9000MX, manufactured by JEOL Ltd., was used for the measurement. The fibers were cut and spread out on a stainless steel sample support, and then the photoelectron escape angle was set to 90 degrees. MgKα was used as the X-ray source, and the sample chamber was heated to 1×10 -6 The vacuum was maintained at 0.05 Pa. To correct the peak caused by charging during measurement, first C 1s The binding energy value BE of the main peak of N is set to 284.6 eV. 1s The peak area was determined by drawing a straight baseline in the range of 395 to 405 eV. 1s The peak area was determined by drawing a straight baseline in the range of 281 to 297 eV. 1s N relative to peak 1s The sensitivity correction coefficient for the peak was 0.97507. The surface nitrogen concentration (N / C) of the carbon fiber surface was calculated by the above N 1s Peak area and C 1s It was calculated from the ratio of peak areas.

[0035] <Surface treatment> The surface treatment device 25 has a tubular section 251 and an irradiation section 255. One remote atmospheric pressure plasma surface treatment device is used as the irradiation section 255, and the irradiation section 255 has an upstream tubular section of 600 mm on the upstream side and a downstream tubular section of 300 mm on the downstream side. The cross-sectional shape of these tubular sections 251 is rectangular, and the ratio of the cross-sectional area of the internal space of the tubular section 251 to the total cross-sectional area of the input fibers is 40. The surface treatment energy can be adjusted by adjusting the speed (supply speed) of the carbonized fiber 1d traveling inside the surface treatment device 25 (tubular portion 251) or by adjusting the power (surface treatment device output) applied to the pair of electrodes. <Surface treatment energy> The surface treatment energy is obtained by dividing the energy applied to the fibers 1d by the amount of the fibers 1d treated per unit time, and can be calculated by the following formula (2). E T=P T ×W F ×L T / (A N ×LD×V F ×1,000,000) (2) where: E T : Surface treatment energy (MJ / kg) P T : Total output of surface treatment equipment (W) W F : Width of carbon fiber bundle (m) L T : Length of irradiation area (m) A N : Total cross-sectional area (m 2 ) LD: Unit mass of carbon fiber bundle (kg / m) V F : Carbon fiber bundle feeding speed (m / s) is.

[0036] <Characteristics of carbon fiber> The tensile modulus TM is measured in accordance with JIS R 7606. The tensile strength TS is measured in accordance with JIS R 7606.

[0037] Example 1 Fiber 1d carbonized at 1,300°C was used. Fiber 1d had a single fiber fineness of 0.67 dtex and a single fiber count of 24,000. The tensile strength of fiber 1d was 4,520 MPa and the tensile modulus of elasticity was 226 GPa (see Comparative Example 1). The density of fiber 1d was 1.80 g / cm 3 It was. The carbonized fiber 1d was surface treated using plasma in a mixed gas containing oxygen and nitrogen. The nitrogen ratio in the mixed gas was 99.3% and the oxygen ratio was 0.7%. The surface treatment energy was 7 MJ / kg. After the surface treatment, the surface oxygen concentration of fiber 1e was 0.060, the surface nitrogen concentration was 0.036, the tensile modulus of fiber 1e was 228 GPa, the tensile strength was 5,020 MPa, and the strength parameter at this time was 21.9. By carrying out the above surface treatment, the tensile strength of the fiber increased by 1.11 times compared to that of Comparative Example 1, which was not subjected to the surface treatment. These results are shown in Table 1.

[0038] Example 2 The carbonized fiber 1d described in Example 1 was subjected to plasma surface treatment in the same mixed gas as in Example 1 with a surface treatment energy of 15 MJ / kg. After the surface treatment, the surface oxygen concentration of fiber 1e was 0.091, the surface nitrogen concentration was 0.034, the tensile modulus of fiber 1e was 230 GPa, the tensile strength was 5,580 MPa, and the strength parameter at this time was 28.8. The surface treatment increased the tensile strength of the fiber by 1.23 times. The results are shown in Table 1.

[0039] Example 3 The carbonized fiber 1d described in Example 1 was subjected to plasma surface treatment in the same mixed gas as in Example 1 with a surface treatment energy of 30 MJ / kg. After the surface treatment, the surface oxygen concentration of fiber 1e was 0.099 and the surface nitrogen concentration was 0.035. The tensile modulus of fiber 1e was 248 GPa and the tensile strength was 5,270 MPa. The strength parameter at this time was 33.2. By performing this surface treatment, the tensile strength of the fiber increased by 1.17 times. These results are shown in Table 1.

[0040] Example 4 The carbonized fiber 1d described in Example 1 was subjected to plasma surface treatment in the same mixed gas as in Example 1 with a surface treatment energy of 60 MJ / kg. After the surface treatment, the surface oxygen concentration of fiber 1e was 0.114, and the surface nitrogen concentration was 0.036. The tensile modulus of fiber 1e was 232 GPa, and the tensile strength was 5,180 MPa. The strength parameter at this time was 34.8. By performing this surface treatment, the tensile strength of the fiber increased by 1.15 times. These results are shown in Table 1.

[0041] Example 5 Fiber 1d carbonized at 1,400°C was used. Fiber 1d had a single fiber fineness of 0.57 dtex and 12,000 single fibers. The tensile strength of fiber 1d was 5,410 MPa and the tensile modulus of elasticity was 276 GPa (see Comparative Example 3). The density of fiber 1d was 1.76 g / cm 3 It was. The carbonized fiber 1d was subjected to plasma surface treatment in the same mixed gas as in Example 1 with a surface treatment energy of 20 MJ / kg. After the surface treatment, the surface oxygen concentration of fiber 1e was 0.041, and the surface nitrogen concentration was 0.026. The tensile modulus of fiber 1e was 290 GPa, and the tensile strength was 6,020 MPa. The strength parameter at this time was 19.4. By performing this surface treatment, the tensile strength of the fiber increased by 1.11 times. These results are shown in Table 2. Example 6 The carbonized fiber 1d described in Example 5 was subjected to plasma surface treatment in the same mixed gas as in Example 5 with a surface treatment energy of 40 MJ / kg. After the surface treatment, the surface oxygen concentration of fiber 1e was 0.048, and the surface nitrogen concentration was 0.035. The tensile modulus of fiber 1e was 322 GPa, and the tensile strength was 6,620 MPa. At this time, the strength parameter was 26.7. By performing this surface treatment, the tensile strength of the fiber increased by 1.22 times. These results are shown in Table 2.

[0042] Example 7 The carbonized fiber 1d described in Example 5 was subjected to plasma surface treatment in the same mixed gas as in Example 5 with a surface treatment energy of 120 MJ / kg. After the surface treatment, the surface oxygen concentration of fiber 1e was 0.062, and the surface nitrogen concentration was 0.036. The tensile modulus of fiber 1e was 318 GPa, and the tensile strength was 6,480 MPa. The strength parameter at this time was 31.2. By performing this surface treatment, the tensile strength of the fiber increased by 1.20 times. These results are shown in Table 2.

[0043] Example 8 The carbonized fiber 1d described in Example 5 was subjected to plasma surface treatment in the same mixed gas as in Example 5 with a surface treatment energy of 220 MJ / kg. After the surface treatment, the surface oxygen concentration of fiber 1e was 0.080, and the surface nitrogen concentration was 0.034. The tensile modulus of fiber 1e was 305 GPa, and the tensile strength was 6,150 MPa. The strength parameter at this time was 34.8. By performing this surface treatment, the tensile strength of the fiber increased by 1.14 times. These results are shown in Table 2.

[0044] Example 9 Fiber 1d carbonized at 1,800°C was used. Fiber 1d had a single fiber fineness of 0.35 dtex and 12,000 single fibers. The tensile strength of fiber 1d was 6,040 MPa and the tensile modulus of elasticity was 280 GPa (see Comparative Example 5). The density of fiber 1d was 1.81 g / cm 3 It was. The carbonized fiber 1d was subjected to plasma surface treatment in the same mixed gas as in Example 1 with a surface treatment energy of 45 MJ / kg. After the surface treatment, the surface oxygen concentration of fiber 1e was 0.032, and the surface nitrogen concentration was 0.021. The tensile modulus of fiber 1e was 342 GPa, and the tensile strength was 6,720 MPa. The strength parameter at this time was 18.1. By performing this surface treatment, the tensile strength of the fiber increased by 1.11 times. These results are shown in Table 3.

[0045] Example 10 The carbonized fiber 1d described in Example 9 was subjected to plasma surface treatment in the same mixed gas as in Example 9 with a surface treatment energy of 90 MJ / kg. After the surface treatment, the surface oxygen concentration of fiber 1e was 0.035, and the surface nitrogen concentration was 0.032. The tensile modulus of fiber 1e was 374 GPa, and the tensile strength was 7,400 MPa. The strength parameter at this time was 25.1. By performing this surface treatment, the tensile strength of the fiber increased by 1.23 times. These results are shown in Table 3.

[0046] Example 11 The carbonized fiber 1d described in Example 9 was subjected to plasma surface treatment in the same mixed gas as in Example 9 with a surface treatment energy of 250 MJ / kg. After the surface treatment, the surface oxygen concentration of fiber 1e was 0.055, and the surface nitrogen concentration was 0.024. The tensile modulus of fiber 1e was 364 GPa, and the tensile strength was 7,390 MPa. The strength parameter at this time was 28.8. By performing this surface treatment, the tensile strength of the fiber increased by 1.22 times. These results are shown in Table 3.

[0047] Example 12 The carbonized fiber 1d described in Example 9 was subjected to plasma surface treatment in the same mixed gas as in Example 9 with a surface treatment energy of 450 MJ / kg. After the surface treatment, the surface oxygen concentration of fiber 1e was 0.075, and the surface nitrogen concentration was 0.025. The tensile modulus of fiber 1e was 328 GPa, and the tensile strength was 6,920 MPa. The strength parameter at this time was 32.8. By performing this surface treatment, the tensile strength of the fiber increased by 1.15 times. These results are shown in Table 3.

[0048] Comparative Example 1 The carbonized fiber 1d described in Example 1 was not subjected to plasma surface treatment. In other words, it was the same as the carbonized fiber 1d described in Example 1. The surface oxygen concentration of this fiber (1e) was 0.047, and the surface nitrogen concentration was 0.035. The tensile modulus of fiber (1e) was 226 GPa, and the tensile strength was 4,520 MPa. The strength parameter at this time was 18.5. These results are also shown in Table 1.

[0049] Comparative Example 2 The carbonized fiber 1d described in Example 1 was subjected to plasma surface treatment in the same mixed gas as in Example 1 with a surface treatment energy of 120 MJ / kg. After the surface treatment, the surface oxygen concentration of fiber 1e was 0.140, and the surface nitrogen concentration was 0.034. The tensile modulus of fiber 1e was 226 GPa, and the tensile strength was 4,770 MPa. The strength parameter at this time was 39.3. By performing this surface treatment, the tensile strength of the fiber increased by 1.06 times. These results are also shown in Table 1.

[0050] Comparative Example 3 The carbonized fiber 1d described in Example 5 was not subjected to plasma surface treatment. In other words, it was the same as the carbonized fiber 1d described in Example 5. The surface oxygen concentration of this fiber (1e) was 0.037, and the surface nitrogen concentration was 0.018. The tensile modulus of fiber (1e) was 276 GPa, and the tensile strength was 5,410 MPa. At this time, the strength parameter was 15.2. These results are also shown in Table 2.

[0051] Comparative Example 4 The carbonized fiber 1d described in Example 5 was subjected to plasma surface treatment in the same mixed gas as in Example 5 with a surface treatment energy of 400 MJ / kg. After the surface treatment, the surface oxygen concentration of fiber 1e was 0.092, and the surface nitrogen concentration was 0.035. The tensile modulus of fiber 1e was 296 GPa, and the tensile strength was 5,740 MPa. The strength parameter at this time was 37.6. By performing this surface treatment, the tensile strength of the fiber increased by 1.06 times. These results are also shown in Table 2.

[0052] Comparative Example 5 The carbonized fiber 1d described in Example 9 was not subjected to plasma surface treatment. In other words, it was the same as the carbonized fiber 1d described in Example 9. The surface oxygen concentration of this fiber (1e) was 0.029, and the surface nitrogen concentration was 0.013. The tensile modulus of fiber (1e) was 280 GPa, and the tensile strength was 6,040 MPa. The strength parameter at this time was 11.8. These results are also shown in Table 3.

[0053] Comparative Example 6 The carbonized fiber 1d described in Example 9 was subjected to plasma surface treatment in the same mixed gas as in Example 9 with a surface treatment energy of 900 MJ / kg. After the surface treatment, the surface oxygen concentration of fiber 1e was 0.080, and the surface nitrogen concentration was 0.030. The tensile modulus of fiber 1e was 321 GPa, and the tensile strength was 6,320 MPa. The strength parameter at this time was 35.3. By performing this surface treatment, the tensile strength of the fiber increased by 1.05 times. These results are also shown in Table 3.

[0054] [Table 1]

[0055] [Table 2]

[0056] [Table 3]

[0057] Here, in order to distinguish between carbon fibers that have undergone plasma treatment as a surface treatment on carbonized fiber 1d and carbon fibers that have not undergone plasma treatment, the carbon fibers that have undergone plasma treatment will be conveniently designated as "1e" and the carbon fibers that have not undergone plasma treatment will be conveniently designated as "1d". From the results of the Examples and Comparative Examples in Tables 1 to 3, the relationship between the strength parameters and the tensile strength is shown in FIG. As shown in Figure 3, when fiber 1d carbonized at 1,300°C is used, carbon fiber 1e with a strength parameter in the range of 21 to 37 has a tensile strength 1.1 times or more higher than that of carbon fiber 1d that has not been subjected to plasma treatment. Similarly, when fiber 1d carbonized at 1,400°C is used, carbon fiber 1e having a strength parameter in the range of 18 to 36 has a tensile strength 1.1 times or more higher than that of carbon fiber 1d that has not been subjected to plasma treatment. Similarly, when fiber 1d carbonized at 1,800°C is used, carbon fiber 1e having a strength parameter in the range of 16 to 35 has a tensile strength 1.1 times or more that of carbon fiber 1d that has not been subjected to plasma treatment. As described above, when the strength parameters are within a predetermined range corresponding to the carbonization temperature, or when plasma treatment is performed so that the strength parameters are within a predetermined range corresponding to the carbonization temperature, it can be seen that the tensile strength is improved compared to carbon fiber 1d that has not been subjected to plasma treatment.

[0058] As shown in FIG. 3, it can be seen that the strength parameter at which the tensile strength increases by 1.1 times shifts to the left (the side where the strength parameter decreases) as the temperature in the carbonization process increases. On the other hand, the tensile modulus (GPa) of fiber 1d, whose maximum temperature in the second carbonization step was 1,300°C, was 226 to 248, the tensile modulus (GPa) of fiber 1d, whose maximum temperature was 1,400°C, was 276 to 322, and the tensile modulus (GPa) of fiber 1d, whose maximum temperature was 1,800°C, was 280 to 374.It can be seen that the tensile modulus of carbon fiber tends to increase as the maximum temperature in the carbonization step increases (see Table 1). This shows that as the tensile modulus of elasticity of the carbon fiber increases, the strength parameter at which the tensile strength is 1.1 times or more shifts to the left (the side where the strength parameter decreases). Therefore, the range of strength parameters in which the tensile strength is 1.1 times or more can be determined by the tensile modulus of the carbon fiber to be produced. Specifically, if the strength parameter of the carbon fiber is within the range defined by formula (1), the tensile strength of the carbon fiber 1e will be 1.1 times or more that of the carbon fiber 1d that has not been subjected to the plasma treatment of the present invention. Conversely, if the strength parameter of a produced carbon fiber is within the range defined by formula (1), it can be assumed that the carbon fiber was produced by the production method of the present invention. In FIG. 3, "x" indicates a plot of the calculation results using the tensile modulus TM of Examples 3, 6, and 10, which had the highest tensile modulus at each maximum carbonization temperature, in the following formula (3), and "*" indicates a plot of the calculation results using the tensile modulus TM of Examples 1, 5, and 12, which had the lowest tensile modulus at each maximum carbonization temperature, in the following formula (4). Lower limit: 29 - (0.034 x TM) (3) Upper limit: 40 - (0.016 x TM) (4)

[0059] <<Modifications>> Although the present invention has been described above based on the embodiment, it is not limited to the embodiment. For example, any of the modified examples described below may be appropriately combined with any of the embodiment, or multiple modified examples may be appropriately combined.

[0060] (1) In the embodiment, a method for manufacturing carbon fibers having 12,000 filaments has been described, but the present invention can also be applied to surface treatment methods for carbonized fibers and methods for manufacturing carbon fibers using precursor fibers having other numbers of filaments, such as 3,000, 6,000, or 24,000. (2) In the embodiment, a method for producing carbon fiber including a carbonization step has been described, but, for example, a graphitization treatment may be further performed before the surface treatment step. That is, in the embodiment, a method for producing carbon fiber having a tensile modulus of 240 to 360 GPa has been mainly described, but the surface treatment step can also be used for carbonized fiber for high-performance carbon fiber such as high-modulus, medium-modulus, and high-strength carbon fiber. Naturally, it can also be used for the method for producing high-performance carbon fiber.

[0061] (3) In the surface treatment of the embodiment, plasma is irradiated. However, for example, energy may be imparted by exposing the carbonized fiber to a plasma atmosphere. Furthermore, although the irradiation unit 255 uses parallel plate electrodes, it may also use, for example, coaxial cylindrical electrodes, etc. Furthermore, although the irradiation unit 255 uses dielectric barrier discharge to generate plasma, it may also use other means, for example, corona discharge, atmospheric pressure glow discharge, etc. (4) In the embodiment, the irradiation unit 255 blows plasma generated outside the tubular portion 251 into the interior through the through-hole 253 as shown in FIG. 2. However, for example, a pair of electrodes may be placed inside the tubular portion, and the carbonized fibers may run between the pair of electrodes.

[0062] (5) The surface treatment device of the embodiment is an example, and may be any of the following surface treatment devices. (5-1) This will be explained using FIG. In the embodiment, the surface treatment device 25 has the cylindrical portion 251, but the surface treatment device may not have a cylindrical portion. 4(a), the surface treatment device 25a includes a plate portion 251a disposed opposite the traveling fibers 1d, and an irradiation portion 255a disposed at the center of the traveling direction of the plate portion 251a on the opposite side of the fibers 1d. The plate portion 251a has a through-hole 253a at the middle of the traveling direction of the fibers 1d, and atmospheric pressure plasma is irradiated from the through-hole 253a toward the traveling fibers 1d. 4(b), the surface treatment device 25b includes a pair of plate sections 251b arranged opposite the traveling fibers 1d, and an irradiation section 255b arranged at the center of the traveling direction of one of the plate sections 251b on the opposite side of the fibers 1d. One of the plate sections 251b has a through-hole 253b at the middle of the traveling direction of the fibers 1d, and atmospheric pressure plasma is irradiated from the through-hole 253b toward the traveling fibers 1d. 4(c), the surface treatment device 25c includes a pair of plate sections 251c arranged opposite the traveling fiber 1d, and two irradiation sections 255c arranged at the center of the traveling direction of each plate section 251c and on the opposite side of the fiber 1d. Both plate sections 251b have through holes 253c at the middle of the traveling direction of the fiber 1d, and atmospheric pressure plasma is irradiated from the through holes 253c toward the traveling fiber 1d. Here, a plurality of fibers 1d run parallel to one another, and the through holes 253a to 253c are provided so as to extend in the parallel running direction (the direction perpendicular to the paper surface). The number of the through holes 253a to 253c in the plate portions 251a to 251c may be one or more.

[0063] (5-2) This will be explained using FIG. 4 irradiates atmospheric pressure plasma at one point in the running direction of the fibers 1d, but may irradiate atmospheric pressure plasma at multiple points along the running direction of the fibers 1d. Here, a case where atmospheric pressure plasma is irradiated at two points in the running direction will be described. As shown in FIG. 5(a), the surface treatment device 25d has two irradiation units 250d each including a plate portion 251d and an irradiation portion 255d, spaced apart in the running direction of the fibers 1d. 5(b), the surface treatment device 25e has two irradiation units 250e, each equipped with a pair of plate portions 251e and an irradiation portion 255e, spaced apart in the running direction of the fibers 1d. Note that the pair of plate portions 251e may be replaced with a cylindrical portion. The two irradiation units 250d and 250e are arranged on both sides of the fiber 1d in a direction perpendicular to the running direction, thereby reducing unevenness in the irradiation of the fiber 1d with atmospheric pressure plasma.

[0064] (5-3) This will be explained using FIG. Although the surface treatment device 25 of the embodiment and the surface treatment devices 25a to 25c shown in Fig. 4 have through holes 253, 253a to 253c extending in the juxtaposition direction of the plurality of fibers 1d (the direction perpendicular to the paper surface in Fig. 4), the through holes may also be extended in the running direction of the fibers 1d. Here, a case will be described in which the through holes have a shape extending in both the juxtaposition direction and the running direction of the plurality of fibers 1d. 6(a), the surface treatment device 25f includes a plate portion 251f disposed opposite the traveling fibers 1d, and an irradiation portion 255f disposed on the opposite side of the plate portion 251d from the fibers 1d. The plate portion 251f has through holes 253f formed along the traveling direction of the fibers 1d, and atmospheric pressure plasma is irradiated from the through holes 253f toward the traveling fibers 1d. 6(b), the surface treatment device 25g includes a pair of plate sections 251g arranged opposite the traveling fibers 1d, and an irradiation section 255g arranged on one of the plate sections 251g opposite the fibers 1d. One of the plate sections 251g has through holes 253g formed along the traveling direction of the fibers 1d, and atmospheric pressure plasma is irradiated from the through holes 253g toward the traveling fibers 1d. 6(c), the surface treatment device 25h includes a pair of plate sections 251h arranged opposite the traveling fiber 1d, and two irradiation sections 255h arranged on the opposite side of each plate section 251h from the fiber 1d. Both plate sections 251h have through holes 253h formed along the traveling direction of the fiber 1d, and atmospheric pressure plasma is irradiated from the through holes 253h toward the traveling fiber 1d. Here, multiple fibers 1d run parallel to each other, and the through-holes 253f-253h are rectangular or square so as to extend in the parallel running direction. The number of through-holes 253f-253h may be one or multiple along the running direction. Also, a cylindrical portion may be used instead of the pair of plate portions 251g, 251h.

[0065] (5-4) This will be explained using FIG. 6 irradiates atmospheric pressure plasma at one point in the running direction of the fiber 1d, but atmospheric pressure plasma may be irradiated at multiple points along the running direction of the fiber 1d. Here, a case where atmospheric pressure plasma is irradiated at two points in the running direction will be described. As shown in FIG. 7(a), the surface treatment device 25j has two irradiation units 250j each including a plate portion 251j and an irradiation portion 255j, spaced apart in the running direction of the fibers 1d. 7(b), the surface treatment device 25k has two irradiation units 250k, each equipped with a pair of plate portions 251k and an irradiation portion 255k, spaced apart in the running direction of the fibers 1d. Note that the pair of plate portions 251k may be replaced with a cylindrical portion. The two irradiation units 250j and 250k are arranged on both sides in a direction perpendicular to the running direction of the fiber 1d, thereby reducing unevenness in the irradiation of the atmospheric pressure plasma onto the fiber 1d.

[0066] (5-5) This will be explained using FIG. The surface treatment device 25 of the embodiment, the surface treatment device 25e shown in Fig. 5(b) and the surface treatment device 25k shown in Fig. 7(b), irradiate atmospheric pressure plasma at two locations in the running direction of the fiber 1d, but multiple surface treatment devices may be connected to irradiate atmospheric pressure plasma at multiple locations along the running direction of the fiber 1d. Here, a case where atmospheric pressure plasma is irradiated at two locations in the running direction will be described. As shown in Fig. 8(a), the surface treatment device 25m is configured such that two irradiation units 250m each including a pair of plate portions 251m and an irradiation portion 255m are connected in the running direction of the fibers 1d (more precisely, the plate portions 251m of each irradiation unit 250m are connected to each other) and are integrally arranged. In other words, the surface treatment device 25m is configured with a pair of plate portions 251m and a plurality (two) of irradiation portions 255m arranged at intervals in the running direction of the fibers 1d relative to the pair of plate portions 251m. Note that a cylindrical portion may be used instead of the pair of plate portions 251m. As shown in Fig. 8(b), the surface treatment device 25n is configured such that two irradiation units 250n each including a pair of plate portions 251n and an irradiation portion 255n are connected together in the running direction of the fibers 1d (more precisely, the plate portions 251m of each irradiation unit 250n are connected together) and are integrally arranged. In other words, the surface treatment device 25n is configured with a pair of plate portions 251n and a plurality (two) of irradiation portions 255n arranged at intervals in the running direction of the fibers 1d relative to the pair of plate portions 251n. Note that a cylindrical portion may be used instead of the pair of plate portions 251n. The two irradiation units 250m, 250n are arranged alternately on both sides in a direction perpendicular to the running direction of the fiber 1d. This reduces unevenness in the irradiation of the atmospheric pressure plasma onto the fiber 1d, and by arranging them in a connected manner, the generated plasma can be effectively used for surface treatment. The surface treatment devices 25a to 25n shown in Figures 4 to 8 have plate sections 251a to 251n that are longer in the running direction of the fibers 1d than the irradiation sections 255a to 255n, but may have plate sections (cylindrical sections) that are shorter in the running direction of the fibers 1d than the irradiation sections 255a to 255n, or may not have plate sections or cylindrical sections. [Explanation of symbols]

[0067] 1. Fiber 1a precursor 1b Flame-retardant fiber 1d Carbonized fiber 25 Surface Treatment Equipment 251 Cylinder part 255 Irradiation unit

Claims

1. A carbon fiber that satisfies the relationship of the following formula (1) and also satisfies any one of the following formulas (2) to (4): 29-(0.034×TM)≦(O / C+N / C)×TM≦40-(0.016×TM)... (1) If 228≦TM≦248, 0.050<O / C<0.127 0.034<N / C<0.036... (2) When 290≦TM≦305, 0.041<O / C<0.080 0.026<N / C<0.034... (3) If 342≦TM≦374, 0.032<O / C<0.055 0.021<N / C<0.032... (4) Here, "TM" is the tensile modulus of the carbon fiber [GPa], "O / C" is the surface oxygen concentration measured by X-ray photoelectron spectroscopy, and "N / C" is the surface nitrogen concentration measured by X-ray photoelectron spectroscopy.

2. A carbon fiber that satisfies the relationship of the following formula (1) and also satisfies any one of the following formulas (5) to (7). 29-(0.034×TM)≦(O / C+N / C)×TM≦40-(0.016×TM)... (1) If 228≦TM≦248, then 5,020≦TS≦5,580 (5) If 290≦TM≦305, then 6,020≦TS≦6,150 (6) If 342≦TM≦374, then 6,720≦TS≦7,400 (7) Here, "TM" is the tensile modulus of the carbon fiber [GPa], "TS" is the tensile strength of the carbon fiber [MPa], "O / C" is the surface oxygen concentration measured by X-ray photoelectron spectroscopy, and "N / C" is the surface nitrogen concentration measured by X-ray photoelectron spectroscopy.

3. A method for producing carbon fibers, which applies energy that increases the tensile strength by 1.1 times or more after a carbonization step, The method for producing carbon fibers, wherein the energy is applied so as to satisfy the relationship of the following formula (1) and any one of the following formulas (2) to (4): 29-(0.034×TM)≦(O / C+N / C)×TM≦40-(0.016×TM)... (1) If 228≦TM≦248, 0.050<O / C<0.127 0.034<N / C<0.036... (2) If 290≦TM≦322, 0.039<O / C<0.086 0.026<N / C<0.036... (3) If 328≦TM≦374, 0.030<O / C<0.077 0.020<N / C<0.032... (4) Here, "TM" is the tensile modulus of the carbon fiber [GPa], "O / C" is the surface oxygen concentration measured by X-ray photoelectron spectroscopy, and "N / C" is the surface nitrogen concentration measured by X-ray photoelectron spectroscopy.

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