WO2008081956A1 - High carbon hot-rolled steel sheet and method for production thereof - Google Patents

Abstract

Disclosed is a high carbon hot-rolled steel sheet showing excellent flexural properties after being stretched. Also disclosed is a method for producing the high carbon hot-rolled steel sheet. The method for producing the high carbon hot-rolled steel sheet comprises the steps of: hot-rolling a steel comprising the following components (by mass): C: 0.2-0.7%, Si: 2% or less, Mn: 2% or less, P: 0.03% or less, S: 0.03% or less, Sol.Al: 0.01% or less and N: 0.01% or less at a finishing temperature which is equal to or higher than a temperature lower by 20°C than the Ar3 transformation point to form a hot-rolled steel sheet; cooling the hot-rolled steel sheet to a temperature of 650°C or below at a cooling rate of not less than 60°C/sec and less than 120°C/sec; winding-up the cooled hot-rolled steel sheet at a winding temperature of 600°C or lower; and annealing the wound hot-rolled steel sheet at an annealing temperature of equal to or higher than 640°C and equal to or lower than the Ac1 transformation temperature.

Description

 Description High carbon heat and its ^^ method

 The present invention relates to a high carbon heat 3¾, particularly a high carbon heat excellent in bending after processing, and the Mit method. Background sickle

 High carbon steel used for tools or automobile parts (gears, missions), etc. is processed into various complex shapes, and therefore excellent workability is required by users. On the other hand, in recent years, the demand for component manufacturing costs has increased, and B & ^ processing methods have been changed to save processing steps. For example, as described in Non-Patent Document 1, as a processing rod for automatic parts using high charcoal ^ ¾, it is possible to increase the thickness and to open a fiber processing chamber that has drastically increased ¾ ^. Has been put into practical use. At the same time, high-carbon steel is required to be able to be processed without any problems even if it combines multiple processing modes such as pulling, drawing, stretching, bending, and hole expansion. In particular, since bending often occurs after bending after applying traction!], Bending 14 after excellent traction!] Is desired.

To date, several techniques have been studied to improve the processability of high carbon. For example, in Patent Document 1, after hot rolling a high-carbon coal of a predetermined chemical composition, descaling, heating in a hydrogen atmosphere above 95 volume m ^ specified by the chemical composition ^ A method for producing a soft, high-coal zone with excellent uniformity and workability by cooling at a cooling rate of 1 ° C or less at a temperature of 100 ° C / hr or less is being tested. Further, Patent Document 2, was cooled 10～100D, until? Of ί ^ ίΡ ^ in the 20~500Ό was rolled at finishing above (A _Cl transformation point + 30), after l~10 ¾¾¾f, 500 ~ reheated in warm舰of (^ in transformation point + 30)卷取Ri., 650 as needed (a _Cl transformation point + 30) in good workability by soaking 1 hour or more A method of producing high carbon flakes is being tested. Further, in Patent Document 3, a steel containing 0.2 to 0.7% by mass of C is hot-rolled at a finishing temperature (Ar ₃ transformation point−20 ° C.) or higher, and then cooled to a BS degree of 120 ° C./second. And i ^ i ff stop at 650 ° C or below

As a result, a method for producing high carbon heat® with excellent stretch flangeability has been proposed.

 Non-Patent Document 1: Journal of the JSTP, 44, 2003, p.409-413

 Patent Document 1: JP-A-9-157758

 Patent Document 2: JP-A-5-9588

 Patent Document 3: Japanese Patent Laid-Open No. 2003-13145 Disclosure of Invention

 However, the high carbon hot-spring of β¾ in these submissive techniques is superior in properties when processed in a single processing mode such as tension or 7¾, but bends after tensile processing. Combining multiple processing modes; ^ had problems such as cracking.

 An object of the present invention is to provide a high carbon heat exciter having excellent bending properties after tensile processing and an i * "method thereof.

As a result of studying the bending characteristics after high carbon heat ^^, the inventors have determined the amount of Sol. A1 in the steel, the cooling conditions after hot rolling, the cutting temperature, and the annealing temperature. The ability to control properly S has been found to be extremely important ^ and the ferrite grain size obtained by the later measurement method should be 5.0 / zm or less, and the ferrite grains with a force aspect ratio of 0 or more It was found that by controlling the area ratio below 1 TO, it was possible to obtain excellent bending characteristics after pulling. The present invention has been made on the basis of the above findings, and has a mass of 34, C: 0.2-0.71 Si: 2% or less, ¾fo: 2% or less, P: 0.03% or less, S: 0.0 , Sol. A1: A process of hot rolling steel with a content of 0.01% or less and N: 0.01% or less to a finish of (Ar ₃ transformation point-20 ° C) or more to form a hot plate The process of cooling the heat plate to 60 ° C / second or more and 120 ° C / second to 650 ° C or less and the heat after the tilt itself and卷取Ru step by preparative, heat ^^ after Ri preparative IiP5, to «a step of纖by a _Cl transformation point of纖above 64TfC, the manufacturing i * law high carbon hot ^^ with.

 In the method of the present invention, in the step of ΙίίΙΚ, heat is glued to 600 ° C or less with a paste of 80 Ό / sec or more and less than 120 ° C / sec. It ’s better to pick it up below 550 ° C.

 The present invention also relates to a high-carbon steel sheet that is formed into a cocoon and has a mass of CO. 2

~ 0.7%, Si: 2% ¾, ¾fci: 2 ¾TF, P: 0.03% ¾, S: 0.0 «, Sol. A1: 0.01% ¾, N: 0.01% Has a thread! ^ To舍有the following ferrite grain size is not less less 5 · 0 _M m, the force ^ one aspect ratio 4.0 or more of ferrite grains area ratio is under 15% ¾, high carbon Heat ^^ Here, ferrite is the average value of the particle diameters obtained by approximating the ferrite grains to circles by image WW, and the aspect ratio is still obtained by elliptically approximating the ferrite grains by image ^. It is the average value of (ellipse long axis) / (ellipse short axis). Specifically, after polishing a rough «1 $ cross section in the rolling direction of the steel plate, and corroding 1/4 of the heel with nital liquid (glass ^ ethanol), ¾ S-type electron microscope ^^ Observe the Miku mouth knives at double magnification, using Media Cybrnetics image ^? ^ Software "Image Pro Plus ver. 0" (Bl) The aspect ratio of the grains was determined. Furthermore, the area ratio of 40 or more aspect ratio is obtained for each ferrite grain, and this is divided by the whole field of view to obtain the area ratio of each field of view. Of ferrite grains. '

 The area ratio of ferrite grains having a self-spect ratio of 40 or more is more preferable. In addition, in the present invention, in addition to the above-described steel thread reel, at least one selected from B, Cr, Ni, o, Cu, Ti, b, W, V, and Zr in the following content ranges: It is also possible to include seeds.

 B: 0.005 mass y. Cr: 3.5 mass% or less, Ni: 3.5 mass% or less, Mo: 0.7 mass% or less, Cu: 0.1 quality *% or less, Ti: 0.1 mass% or less, b: 0.1 mass% or less, W, V, Zr: 0.1 mass in total.

 According to the present invention, it is possible to produce a high carbon heat flux that is excellent in bending properties even after being subjected to processing such as tensile processing. Brief Description of Drawings

 FIG. 1 is a diagram showing the relationship between the area ratio of ferrite grains having an aspect ratio of 40 or more and the bending 14 after pulling. Best Mode for Invention

 In the following, the high-carbon coal «which is the present invention and its method will be described in detail. Unless otherwise specified, “%”, which is a unit of component content, means “quality *%”.

Steel castle C amount: C forms carbides! It is an important element that gives after ¾Λ. If the amount of C is less than 0.2%, a sufficient bow daughter as a part will not be obtained later. On the other hand, if the amount of C exceeds 0.7%, even if the ferrite grain size is 5.0 μπι or less and the area ratio of ferrite grains having a force-spect ratio of 0 or more is less than 19½¾, sufficient Bending properties after [working!] Can not be obtained. In addition, the hardness after hot rolling becomes remarkably high and the steel sheet becomes brittle, so that handling is inconvenient, and a bow as a structural iffl part after j½A is also provided. Therefore, the amount of C is specified as 0.2 to 0.7%. In addition,! To place more emphasis on the hardness of ^ A l ^, it is preferable that the C content exceeds O. S%, and that the C content is 0.5% ¾ or less in order to improve the workability.

 Si amount: Since Si tends to graphitize carbides and inhibit j½A property, the amount is specified to be 2¾ or less, preferably 0.5% or less.

 Mn content: When Mn is contained in ii¾, the ductility tends to be lowered. Therefore, the amount is specified to be 2% or less, preferably 1% or less.

 P content: When P is contained in ■, ductility such as stretch flangeability is reduced, and cracks are generated and become fragile. Therefore, the content is specified to be 0.0 «lower, preferably 0.02% ¾ lower.

 S i: When S is included in ■, as with P, ductility such as stretch flangeability is reduced, and cracks are generated L ^ t, so its content is 0.0 «lower, preferably 0. Specified. .

 Sol. M amount: SoL Al is the most important element in the present invention. That is, when the amount of Sol. Al exceeds 0.01%, when nitrogen is used as a relatively non-oxidizing atmosphere with relatively high efficiency, A1N is formed on the surface layer when heat is generated in a nitrogen atmosphere. The inventors of the present invention have found that the bending force 14 after working 11 hours is markedly reduced by the surface hardening of the steel sheet and significantly decreases. Therefore, the amount of Sol. A1 is specified to be 0.01% or less.

 N amount: When N is contained in »j, the ductility is lowered, so the amount is 0 · 01% ¾ lower, preferably 0. lower.

 Here, reducing each of the above elements to a predetermined amount or less, for example, less than 0.0001%, incurs an increase in cost.

 Fe and unavoidable impurities, but for example, B, Cr, Ni, Mo, Cu, Ti, Nb within the range usually added for the purpose of improving i½A b properties and softening resistance. , W,

Even if at least one element such as V or Zr is added, the effect of the present invention is not impaired. Specifically, these elements are: Β: 0.005% or less, Cr: 3.5% or less, Ni: 3.5% or less, Mo: 0.7% or less, 01: 0. 1 lower, Ti: 0. 1% lower, Nb: ai lower, W, V, Zr: 0.1 total lower. For the above purposes, Β: 0 · 0005% or more, Cr: 0.05% or more, Ni: 0.05% or more, Mo: 0.05% ¾, Cu: 0.01% ¾, Ti: 0.01 or more Nb: 0.01, and W, V, Zr: It is preferable to contain 0.01 ¾ in total. Further, even if an element such as Sn or Pb is used as an impurity as in the case of itiii, the effect of the present invention is not affected.

Cow

If the finish of the hot rolling is less than (Ar ₃ transformation point-20 ° C), it will be partially rolled in the ferrite region, and the following ferrite will exceed 5.0 m. to degrade. Therefore, the hot rolling finish should be (Ar ₃ transformation point-20 ° C) or higher. The Ar ₃ transformation point can be calculated from the following equation (1), but actually measured may be used.

Ar ₃ transformation point ^ lO ^ OSX E ^ + YX SiJ-SOX Bln] · · · (1)

Here, M represents the content of the element M 00. Depending on the elements contained, correction terms may be introduced. For example, to contain Cr, Mo, Ni force S, -ll X [Cr], +31.5 X [Mo], -15 A correction term such as 2 X [Ni] may be added to the right-hand side of equation (1).

Cooling conditions after hot rolling: In the present invention, the amount of Sol. A1 is low, and although the grain growth is hardly inhibited by the pinning of A1N, ferrite grains are transformed. This is because the strain imparted to the austenite grains during rolling is accumulated by cooling after the hot rolling, and the strain accumulated in the subsequent stage contributes as the core of the ferrite grains. It is estimated to be. If the cooling ^ after hot rolling is 60 ° C / second, the strain applied to the austenite grains during rolling is less likely to accumulate. Then, the ferrite grains grow and stick. As a result, the ferrite grain size exceeds 5.0 μιη, and the bending habit after the pulling is deteriorated. On the other hand, when the temperature is 120 ° C / sec or more, the area ratio of ferrite grains with a force aspect ratio of 40 or more exceeds 15%. Similarly, the bend 14 after tensile processing deteriorates. This is because when the cooling ¾g is 120 ° C / sec or more, the strain applied to the austenite grains becomes excessive after the rolling, so that the equiaxed ferrite grains grow after that. Is estimated to be difficult. Based on the above, cooling after hot rolling should be 60 ° C / 涉 or more and 120 ° C / second or less. The upper limit of ffil ^ is preferably 115 ° C / sec. If the end point of such ¾ ^, ie, the cooling stop is higher than 650 ° C, the strain accumulated in the austenite will be released during the cooling until the heat is wound. As a result, the ferrite grain size after 超 え exceeds 5.0 / zm, and the bending properties after traction!] Deteriorate. Therefore, the cooling temperature is 650 ° C or less, preferably 600 ° C or less. In addition, since there is a problem in the measurement accuracy, it is preferable that the cooling rate is 500 ° C. or higher.

 After reaching the stop, there is no need to stipulate, and natural cooling may be used, or forced cooling may be encouraged by reducing the power. It is preferable to forcibly cool to ¾g to suppress recuperation from the average H4.

 卷取 ： Cooling after heat is removed, but if the tempering exceeds 600 ° C, the strain force made austenite at the time of hot rolling is released. Elite particle size exceeds 5.0μ πι, and bending difficulty after traction DIE deteriorates. Therefore, the tapping shall be 600 ° C or less. In order to obtain a sufficient t & t self-cooling effect, it is preferable to set the temperature at a lower temperature than when the self-cooling is stopped. Note that, since the shape of the heat ^^ deteriorates, the scraping is preferably 200 ° C or higher, more preferably 350 ° C or higher. When the ratio of ferrite grains with an aspect ratio of 40 or more is reduced to 1TO, the bending characteristics are further improved. To achieve this, ί ^ ί «is 80 ° C / sec. It is necessary to keep the temperature below 600 ° C and force winding up to 550 ° C.

 Descaling: After removal of the hot cake, the scale is usually descaled before the next. The scale removal means is not particularly limited, but it is preferable to use the usual method. '

Heat ^^ ^: The heat wrapping board after being removed by scissors or the like is subjected to a force S as spheroidization in order to spheroidize the carbide. At that time, if 纖 is less than 60 ° C, the ferrite core length becomes insufficient, the area ratio of ferrite grains with an aspect ratio of 40 or more exceeds 15%, and the bending characteristics after drawing are deteriorated. To do. On the other hand,纖¾ is austenitization partially proceeds exceeds A _Cl transformation point, since pearlite is formed during cooling, bending after pulling働卩E ^ deteriorates. Therefore, S ^ of the hot plate is set to 60 ° C or more and _ACl transformation point or less. In order to obtain better stretch flangeability, it is preferable that the thickness of the steel sheet is 680 ° C or higher. The Ac / transformation point can be calculated from the following equation (2), but actually measured values can also be used. Ac! Transformation point = 754 83-32. 25 X [C] +23. 32 X [Si]-17. 76 X [Mn] '· · · (2)

Here, [M] represents the content (mass) of the element M. Depending on the contained elements, correction terms may be introduced. For example, ^^ containing Cr, Mo, and V 17. Add 3 X [Cr], +4 51 X [Mo], +15. 62 X [V] and the corresponding correction term to the right side of Equation (2).

 Heat time is 8 ~ 80 hours ¾g force S preferred. The obtained carbide in the soot was measured at about 1/4 position of the chemical, average aspect ratio of about 5.0 or less.

 In order to melt the high coal slag of the present invention, both the converter and the electric power can be shelfd. In addition, the high coal mine that has been difficult in this way is made into a slab by rolling ingots for one minute or continuous. Slabs are usually heated and then hot rolled. In the case of a slab made of steel, it may be applied as it is or as it is heat-retained for the purpose of suppressing the temperature drop and rolled for the first time. In addition, when the slab is heated and hot-rolled, the slab temperature should be 1280 ° C or less to avoid deterioration of the surface state due to the scale. The hot rolling can be performed by omitting the rolling and only rolling. In order to ensure the finish, it is also possible to heat the plate with a heating means such as a sheet bar heater during hot rolling. In addition, the coil can be kept warm by means such as a slow cooling cover after winding, in order to promote shaping or to reduce hardness. ¾J? Of the heat wrapping plate is not particularly limited as long as the production of the present invention can be maintained, but a heat of 1.0 to 10.πππ is particularly preferable.

 The heat ^^ fiber can be done either in a box or, ^. After dredging, temper rolling is performed as necessary. Since this temper rolling does not affect the i¾AtL property, there are no particular restrictions on the conditions.

 The heat ^ produced by the method of the upper bowl invention is a heat summary that has been subjected to heat denaturation, and as described above, the average aspect ratio is about 5.0 or less. This is a hot plate with carbides.

Further, the thermal steel plate of the present application has a ferrite grain size of 5.0 zm or less. Ferrite grain size has an effect on bending after rolling, and when the ferrite grain size exceeds 5.0 / zm, many Tsuruta carbides are ejected into the ferrite grains, In Loe, fine voids generated at the interface between the carbide and the parent phase (ferrite) are connected in the bending process and cracks occur. By setting the ferrite grain size to 5.0 z ra or less, the fine carbides in the ferrite grains are reduced, and the Hengda void generated in the! Since it becomes difficult to be connected in the bending after the bending, it is possible to suppress cracking. Furthermore, the area ratio power of ferrite grains with an aspect ratio of 4.0 or more is 5% ¾ under the heat of ^. Like the ferrite grain size, the ferrite grain shape affects the bending characteristics after the erosion, and if the ferrite grain has an aspect ratio of 4.0 or more, the aspect ratio is 40 or more. At the grain boundary between the ferrite grains of less than 40 and the equiaxed ferrite grains, [1] «severe cracking occurs and becomes ^J H". Such an aspect ratio of 40 or more If the area ratio of ferritic grains exceeds 15%, cracks will occur in the bending process starting from a crack at the pulling S¾D.The area ratio of ferrite grains with such an aspect ratio of 40 or more By making it 15% or less, it is possible to suppress cracking by bending after the pulling opening, more preferably, the area ratio of ferrite grains having an aspect ratio of 40 or more is 10% ¾ or less. It is.

Example 1

 Steels A to E and Z having the chemical composition shown in Table 1 were heated to 1250 ° C, hot-rolled under the conditions shown in Table 2, and m. 1 ~ 20 of ¾ 5. 011111 was produced. Note that was performed in a nitriding atmosphere.

 Here,, o · 1 to 10 are examples of the present invention, and Νο. 11 to 20 are comparative examples. And the aspect ratio and area ratio of ferrite port and ferrite grain were measured by the following methods. In addition, the following method was applied to the bending after 11 hours of working.

 Ferrite grain size, ferrite grain aspect ratio and area ratio: Here, the ferrite grain size is an average value of the grain size obtained by approximating the ferrite grain to a circle by image interpretation. The pect ratio is the average value of (ellipse major axis) / (ellipse minor axis) obtained by ellipsoidal approximation of ferrite grains from the image ^^. Specifically, after polishing the TO cross section in the rolling direction of the steel sheet and corroding 1/4 of W with nital liquid (glass ^ ethanol), «type electron microscope« 9 magnification 1500

The image of the micro yarn is observed at double magnification, and the image of the ferrite particle size and the ferrite particle size are determined by image folding using the “Image Pro PI us ver.4 0” (ΊΜ) software of Media Cybernetics. The spectrum ratio was determined. Furthermore, an area ratio of 40 or more in aspect ratio is obtained for each ferrite grain, and this is divided by the total area of the field of view to obtain an area ratio for each field of view. -0 or more ferrite grain area ratio.

 Also, grind the TO cross section in the rolling direction of the copper plate and place 1/4 of

After being corroded with (picric acid; ethanol) Observe the above picture ^? ^ Software determined the aspect ratio (longest diameter) of carbides. Then, the aspect ratios obtained for each carbide were averaged (number average) to obtain the average aspect ratio, and the results were obtained.

 Bending characteristics after tensile processing: JIS No. 5 strips with a parallel width of 3 Onm taken from the direction perpendicular to the rolling direction were used, and tension was applied in accordance with JIS Z 2241. After applying a pre-strain of 15%, bending was performed by a press bending method in accordance with JIS Z 2248. Pong ^^ D in ~ m experiment is set to lmn, and it is performed 3 times, and the crack is not generated in all three times. The cracking force S generated was designated as X. In addition, ○ was an invention example.

 The results are shown in Table 3. Examples No. 1 to 10, which are examples of the present invention, have a ferrite grain size of 5.0 / zm or less, and the area ratio of ferrite grains having a force and a aspect ratio of 40 or more is 15%. Below: Tsutsu, excellent in bending 14 after working 11e. In the examples of the present invention, it was also confirmed that the average aspect ratio of the carbide was 5.0 or less, and the carbide was spheroidized.

Figure 1 shows the percentage of ferrite grains with an aspect ratio of 40 or more in ferrite with a ferrite of 5 or less. When the ferrite grain size is set to 5.0 / xm or less and the area ratio of ferrite grains having a force-spect ratio of 4_0 or more is reduced by 15% ¾ It can be seen that excellent bending characteristics after pulling can be obtained.

Table 1 (mass%)

Table 2

Table 3

Difficult example 2

 F copper (C: 0.31%, Si: 0.18%,: 0.68%, P: 0.012 S: 0.00331 Sol. A1: 0.005%, ^ 0.0040%. Transformation point: 785 ° C, transformation point '737 ° 0,

G copper (C: 0.23%, Si: 0.18%, Μη · 0.7β¾, P: 0.016 ^ 5: 0.0040 Sol.A1: 0.008%, N: 0.00281 Cr: 1.2 ₃ Transformation point: 785 ° C, Transformation point: 759),

 Steel H (C: 0.32¾, Si: 1.2%, ifa: 1.5%, P: 0.025 S: 0.010%, Sol. Α1: 0.006%, N: 0.00701 3 Transformation point: 80 ° C, Transformation point: 746 ° C ),

Copper I (C: 0.35%, Si: 0.20%, Mn: 0.68 P: 0.012%, S: 0.00381 Sol: A1: 0.005% N: 0.00331 Mo: 0.17%, Cr: 0.98%, ₃ transformation points: 773 ° C , Change: 754 ° 0, and

 The steel E shown in Table 1 was made into a slab and then heated to 1230 ° C. Hot rolling and heat fibering were performed under the conditions shown in Table 4, and 赚 o.21-37 in 娜 .5 was applied. Note that the fiber was made in a nitriding atmosphere (0¾ atmosphere ^). For the obtained heat ^, the aspect ratio and area ratio of ferrite and ferrite grains were measured by the same method as i, and the bending characteristics after the pulling were examined. In addition, as in Example 1, the effect of carbide masculinization was checked.

The _three transformation points and transformation points of F copper to I steel were obtained from the above formula (1) or (2). For G steel and I copper containing Cr or Mo, the above corrections were made. It is obtained using the term.

The results are shown in Table 5. It can be seen that in steel plates 21 to 27 in which the cooling conditions are constant, No. 22-26, whose control speed is within the range of the present invention, has remarkably superior bending characteristics after tensile processing. From Νο.23 to 26, the area ratio of ferrite grains with an aspect ratio of 40 or more can be reduced by 10% ¾. Also, ^^ ¾¾¾¾ was investigated as constant, 1¾-28 to 33 are within the scope of the present invention for both ^^ stop a¾ and winding, o. I understand that. In the case of ¾1¾.33 where the stop is 600 or less and the recovery is 550 ° or less, the ratio of ferrite grains having an aspect ratio of 40 or more can be reduced by 1 «. In all of the examples of the present invention, the average aspect ratio of the carbides is 5.0 or less, and it is shown that the carbides are shaped. E ~ I copper whose extinction is within the scope of the present invention is excellent in bending properties after tensile processing, including G and I steels, which include iron and silicon other than the ingredients. Show.

Table 4

Table 5

Claims

: Ft range

1.Quality, C: 0.2 ~ 0.7 Si: 2¾¾, Μη: 2% ¾, P: 0.03, S: 0.03% ¾TF, Sol.Al:0.01%¾, N: 0.01 % Of steel containing less than or equal to (Ar ₃ transformation point-20 ° C)

Cooling at 60 ° C / s to 120 ° C / s and cooling to 650 ° C or less,

 A process of winding the hot water after IS cooling at a temperature of 600 ° C or less;

The process of annealing the heat-treated steel plate after mi is annealed at a temperature not lower than 60 ° C and not higher than the _ACl transformation point.

2. In the process of heat treatment, is the heating plate at a cooling rate of 80 ° C / second or more and 120 ° C / second or less and 600 ° C or less? However, in the process of taking force, take it at a temperature below 550 ° C.

Claim 1 A process for producing | 5¾ high carbon heat.

3. Steel destruction was selected from B, Cr, Ni, Mo, Cu, Ti, Nb, W, V, and Zr in the following content ranges. A method for producing Stt's high charcoal according to claim 1 or 2, comprising at least one species;

Quality: Β ... 0.005% or less, Cr: 3.5% or less, Ni: 3.5¾ or less, Mo: 0.7% or less, Cu: 0.1 or less, Ti: 0.1% or less, Nb: 0.1% or less, W, V, Zr: 0.1 or less in total.

4. High-temperature hot-rolled steel sheet, which is the heat state

 4%, C: 0.2-0.7%, Si: 2% ¾, Μη: ¾¾, P: 0.03% ¾, S: 0.0 «, Sol.A 1: 0.01% or less, N: 0. Having a composition containing no more than 01%,

 Ferrite, 5.0 / zm or less, and the area ratio of ferrite grains with a force-spect ratio of 40 or more is 15 times lower.

High carbon steel sheet;

Here, the ferrite grain size is a grain obtained by approximating a ferrite grain to a circle by drawing. The aspect ratio is the average value of (ellipse major axis) / (ellipse minor axis) obtained by approximating the ferrite grains to an ellipse by image i? F. '

5. The area ratio of ferrite grains with an aspect ratio of 40 or more is low.

High carbon heat according to claim 4 ^

6. In addition to the above, at least one selected from among B, Cr, Ni, Mo, Cu, Ti, Nb, W, V, and Zr with the following content ranges is added to the above. contains,

 The high-carbon hot plate according to claim 4 or 5 according to claim 4, wherein the material is 4%, B: 0.005% or less, Cr: 3.5% or less, Ni: 3.5% or less, Mo: 0.7 or less, Cu: 0.1% or less,

 Ti: 0.1% or less, Nb: 0.1% or less, W, V, Zr: 0.1% or less in total.