JP2009024233A - High carbon steel sheet excellent in hardenability, fatigue property and toughness, and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high carbon steel sheet which is excellent in blanking workability and hardenability thereafter, in which the amount of undissolved carbide and the grain size distribution thereof can easily be controlled by quenching/tempering treatment, and which can produce a component improved in fatigue properties in addition to toughness. <P>SOLUTION: An annealed steel sheet has a constitution containing, by mass, 0.50 to 0.70% C, ≤0.5% Si, 1.0 to 2.0% Mn, ≤0.02% P, ≤0.02% S and 0.001 to 0.10% Al, and further containing one kind or two kinds or more selected from 0.05 to 0.50% V, 0.02 to 0.20% Ti and 0.01 to 0.50% Nb, and having a structure where the spheroidizing ratio of carbides is ≥95%, and further, carbides with the maximum particle diameters of ≤2.5 μm are dispersed. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a high carbon steel sheet exhibiting hardenability suitable for industrial machinery and automobile drive system machine parts, and excellent fatigue characteristics and toughness after heat treatment, and a method for producing the same.

  Steel materials used for industrial machinery and automotive driveline machine parts are required to exhibit excellent hardenability after being processed into parts, and to exhibit excellent fatigue characteristics and toughness after being heat treated. . In particular, parts such as bearings, gears, and knitted needles have been used in recent years by adjusting the cross-sectional hardness after quenching and tempering to a high hardness of 650 to 750 HV in order to obtain high fatigue strength. However, the present situation is that toughness, fatigue strength, and variation occur.

Generally, regarding the variation in fatigue strength, it is said that the influence of the properties of the punched surface, inclusions in steel, surface flaws, etc. is great. For this reason, efforts are being made to reduce inclusions in steel in the steelmaking process. Further, for example, fine blanking is performed so that the punched surface is not damaged, or barrel polishing is performed when there is a surface scratch. Although such measures have certainly been improved, the problem of reduced fatigue strength and variation has not been solved.
Furthermore, if a slight difference occurs in the heat treatment conditions in the same treatment batch during the heat treatment, variations in toughness and fatigue characteristics occur. For this reason, also in terms of materials, high heat treatment stability, toughness, and improvement in fatigue strength are required.

In order to bring out the characteristics of both high toughness and high fatigue strength, in Patent Document 1, the starting point of fracture after heat treatment is reduced by forming a material structure in which a relatively large size of spherical carbide is dispersed in equiaxed ferrite. To improve toughness. In Patent Document 2, toughness is improved by optimizing the amount of carbon during solution treatment and reducing the size of undissolved carbides.
JP 2003-147485 A JP 2006-63384 A

By the way, the techniques introduced in the above Patent Documents 1 and 2 attempt to improve toughness by limiting the size and amount of undissolved carbides. However, nothing is mentioned about the improvement of fatigue characteristics.
In general, industrial machine parts and automobile drive system machine parts are used by exhibiting required characteristics by quenching and subsequent heat treatment after a steel plate is punched into a product shape.
As described above, the steel sheet before punching is excellent in workability and hardenability so as not to cause scratches on the punched surface. And it is necessary to improve fatigue characteristics in addition to toughness by heat treatment after quenching.

  The present invention has been devised to solve such problems, and is excellent in punching workability and subsequent hardenability, and can easily control the amount and particle size distribution of undissolved carbides by quenching and tempering treatment. An object of the present invention is to provide a high-carbon steel sheet capable of producing a part that is capable of producing a fatigue property in addition to toughness.

The high carbon steel sheet having excellent hardenability, fatigue characteristics, and toughness according to the present invention achieves the object, so that C: 0.50 to 0.70 mass%, Si: 0.5 mass% or less, Mn: 1. 0 to 2.0% by mass, P: 0.02% by mass or less, S: 0.02% by mass or less, Al: 0.001 to 0.10% by mass, and V: 0.05 to 0.50 Component composition including one or more of mass%, Ti: 0.02 to 0.20 mass%, Nb: 0.01 to 0.50 mass%, the balance being Fe and inevitable impurities, and carbide It has a structure in which carbides having a spheroidization ratio of 95% or more and a maximum particle size of 2.5 μm or less are dispersed.
The steel sheet of the present invention further has a component composition including one or more of Cr: 0.6% by mass or less, Mo: 0.5% by mass or less, and B: 0.0002 to 0.01% by mass. You can also.

Such a high carbon steel sheet has C: 0.50 to 0.70 mass%, Si: 0.5 mass% or less, Mn: 1.0 to 2.0 mass%, P: 0.02 mass% or less, S: 0.02% by mass or less, Al: 0.001 to 0.10% by mass, V: 0.05 to 0.50% by mass, Ti: 0.02 to 0.20% by mass, Nb: One or more of 0.01 to 0.50% by mass, further Cr: 0.6% by mass or less, Mo: 0.5% by mass or less, B: 0.0002 to 0.01 as necessary Steel having a composition containing one or more of mass%, the balance being Fe and inevitable impurities, finishing temperature: 830-900 ° C., average cooling rate: 30-45 ° C./s, coiling temperature : After hot rolling under conditions of 500 to 680 ° C. and pickling the obtained hot-rolled steel sheet, 650 to (Ac1 + 20) ° C. It is produced by performing annealing for holding the degrees zone 10h or more.
After hot-rolled pickled plate annealing, a step of performing cold rolling of 20% or more and then annealing for 10 hours or more in a temperature range of 650 to (Ac1 + 20) ° C. may be added once or twice.

According to the present invention, by adjusting the component composition and structure of the high-carbon steel sheet, particularly the spheroidization rate and the maximum particle size of the carbide, a high-carbon steel that can be easily punched and has excellent hardenability can be obtained.
For this reason, when the high carbon steel sheet according to the present invention is punched into a desired shape as a raw material, the punched product is quenched at an appropriate temperature and subjected to normal heat treatment to improve fatigue characteristics in addition to high toughness. Drive system mechanical parts and the like are obtained.
Therefore, according to the present invention, a simple punching process can be employed, and a highly reliable drive system mechanical component having desired characteristics can be manufactured at low cost with high productivity by ordinary quenching and subsequent heat treatment.

In general, regarding the toughness of high carbon steel, it is not always clear which metal structure form can improve toughness during heat treatment. In the conventional knowledge about the microstructure and toughness of the metal structure, when heat treatment of eutectoid steel, hypereutectoid steel, etc., heat treatment is performed in which about 0.6% by mass of carbon is dissolved and the remainder is left as undissolved carbide. ing. This is because if the carbon content exceeds 0.6% by mass, the parent structure becomes a lenticular martensite structure with low toughness.
Although it is said that undissolved carbide suppresses the coarsening of austenite crystal grains during quenching, the influence of the form and particle size of undissolved carbide on toughness has not necessarily been clarified. Therefore, the stability of the impact characteristics obtained after quenching and tempering is low, and it is difficult to obtain a drive system mechanical component with high reliability.

Moreover, when high carbon steel is hardened, a hard steel material having a martensite structure is obtained, but the toughness tends to decrease with hardening. On the other hand, hard high carbon steel tends to improve toughness when the prior austenite grains are refined.
Therefore, the present inventors conducted extensive investigations on the influence of the form and particle size of undissolved carbides on toughness and fatigue characteristics.

As a result, in the metal structure after quenching and tempering, it is possible to suppress the formation of lenticular martensite by reducing the amount of dissolved carbon, to reduce large undissolved carbide that becomes the starting point of fracture, and to further refine the fineness of the prior austenite crystal grains It has been found that the addition of an element effective for crystallization improves fatigue characteristics in a desired shape product in addition to high toughness.
Moreover, in order to reduce a large undissolved carbide, the knowledge of the suitable manufacturing conditions which control the metal structure form of the raw material before quenching and tempering was also obtained.

Specifically, in order to facilitate the reduction of undissolved carbide after normal quenching and tempering, it is effective to make the spheroidization of the metal structure carbide of the raw material before heat treatment uniform and to reduce the particle size of the carbide. . By reducing the particle size of the carbide, it is possible to easily form a solution during the subsequent heat treatment. Further, the solution temperature can be made as low as possible.
When the solution temperature is lowered, a large amount of undissolved carbide remains, but the carbide is easily dissolved by reducing the Cr content. Furthermore, the addition of V, Ti, Nb or the like forms fine carbonitrides that are unlikely to start internal destruction, and utilizes the pinning effect of these fine carbonitrides to refine the prior austenite grains.

In addition to improving toughness by reducing the size of prior austenite grains, it is possible to improve toughness and fatigue characteristics by reducing the amount of undissolved carbides and reducing the number of crack sources due to refinement. Although refinement of prior austenite grains causes a decrease in hardenability, the hardenability is secured and improved by containing a large amount of Mn that improves hardenability.
In order to improve the spheroidization rate of the material and reduce the carbonitride grain size, the cooling rate and the coiling temperature after finish rolling in hot rolling may be controlled.

Based on the above results, a driveline machine that uses high-carbon steel sheet as a raw material, can be quenched from a relatively low temperature after being formed into a desired shape by punching, etc., and exhibits excellent toughness and fatigue characteristics after tempering Obtaining knowledge that it is effective to obtain a component having a component composition as specified in the claims, having a high spheroidizing ratio, and having dispersed a carbide having a small particle size. It was.
Details will be described below.

First, the component composition of steel constituting the steel sheet of the present invention will be described.
C: 0.50 to 0.70 mass%
In the quenched and tempered material, in order to stably obtain a hardness of 650 HV, a C amount of at least 0.50 mass% or more is required. However, when an excessive amount of C exceeding 0.70% by mass is contained, lenticular martensite is generated, carbides are precipitated at the prior austenite crystal grain boundaries, and the toughness is lowered. Therefore, the C content is in the range of 0.50 to 0.70 mass%.

Si: 0.5% by mass or less Added as a deoxidizing element. It enhances hardenability and is effective as a solid solution strengthening element for ferrite. However, it is an element that causes internal oxides immediately below the surface during hot rolling, annealing, and heat treatment, and also causes a decrease in toughness and fatigue properties. For this reason, the upper limit of Si content was 0.5 mass%. In addition, since deoxidation is supplemented with other elements, such as Mn and Al, Si may not be added.

Mn: 1.0 to 2.0% by mass
Mn is an element necessary for ensuring hardenability. In the present invention, in order to reduce the prior austenite grain size after quenching and tempering, the hardenability becomes insufficient unless it is less than 1.0% by mass. However, when excess Mn is added so that it exceeds 2.0 mass%, toughness will fall. Manufacturing costs also increase.

P and S: 0.02% by mass or less are components that adversely affect toughness. For this reason, it is preferable to reduce the content as much as possible. However, in this component system, it is possible to suppress adverse effects caused by P and S by setting the upper limit to 0.02 mass%.

Al: 0.001 to 0.10% by mass
It is an effective alloying element as a deoxidizing material for steel. In order to exert the deoxidizing action, it is necessary to add at least 0.001% by mass. However, since an excessive content tends to cause surface defects of the steel sheet, the upper limit of the Al content is set to 0.10% by mass.

V: 0.05-0.50 mass%
It forms carbonitrides in steel, improves strength and toughness, and improves crack propagation resistance by the action of refining prior austenite crystal grains. Such actions and effects are manifested by containing 0.05 mass% or more of V. However, the inclusion of a large amount exceeding 0.50% by mass only saturates the effect of improving the strength, toughness and crack propagation resistance, and causes an increase in manufacturing cost due to expensive elements. Therefore, the V content is in the range of 0.05 to 0.50 mass%.

Ti: 0.02 to 0.20 mass%
It has the effect of refining austenite crystal grains during heat treatment and increasing crack propagation resistance. Moreover, since N in steel is fixed, it is also effective in securing an effective amount of added B. Such an effect of Ti becomes remarkable at 0.02% by mass or more. However, the content exceeding 0.20% by mass causes coarse precipitates to be formed, resulting in deterioration of fatigue characteristics.

Nb: 0.01 to 0.50 mass%
It forms carbonitrides, suppresses the coarsening of austenite grains, and exhibits the effect of improving toughness. Such an action of Nb becomes remarkable at 0.01% by weight or more. However, if excessively adding over 0.50 mass%, coarse precipitates that cause deterioration of fatigue characteristics are formed, so the upper limit was made 0.50 mass%.

Cr: 0.60 mass% or less It has the effect of improving hardenability, strength, and wear resistance. These effects are sufficiently exhibited when the Cr content is 0.10% by mass or more. However, the upper limit is set to 0.60% by mass because there is a detrimental effect on solution of undissolved carbide during heat treatment. Since the hardenability of the present invention is ensured by Mn, it is possible to add no Cr.

Mo: 0.50% by mass or less Mo is an alloy element effective for improving the hardenability of steel. However, since it is an expensive element and its production cost increases when added in a large amount, the upper limit of Mo is set to 0.50% by weight. Since the hardenability of the present invention is ensured by Mn, Mo may be added.

B: 0.0002 to 0.01% by mass
It is an alloy component added as necessary, and has the effect of enhancing hardenability and suppressing grain boundary fracture. The effect of addition of B becomes significant when the B content is 0.0002% by mass or more. However, when the B content exceeds 0.01% by mass, the toughness deteriorates.

In order to obtain a drive system machine part by punching or the like using an annealed steel plate with a carbide spheroidization ratio of 95% or more and a maximum grain size of carbide of 2.5 μm or less , the formability to the drive system machine part, It is required to exhibit desired mechanical properties when used as a machine part.
A carbide having a high spheroidization rate is less likely to be a starting point for microvoid formation during punching than a carbide having insufficient spheroidization, and local ductility is improved. Therefore, formation of a fracture surface on the punched surface can be suppressed. In order to exhibit sufficient local ductility and to obtain a good punched surface property, it is preferable to increase the spheroidization rate. Further, after forming into a part shape, a heat treatment of quenching and tempering is performed to express desired mechanical properties. When focusing on toughness and fatigue characteristics among mechanical properties, the presence of undissolved carbides as the starting point of fracture or cracking is an important position. Details will be given in the examples, but in the structure after quenching and tempering, in order to reduce the undissolved carbide as much as possible, and to reduce the prior austenite grain size to suppress intergranular cracking, It is necessary that the area ratio of spheroidized carbide in an annealed steel sheet as a material for manufacturing drive train mechanical parts is 95% or more and the maximum particle size of carbide is 2.5 μm or less. By refining to such a structure, it is excellent in workability and hardenability, and the normal quenching and tempering treatment after forming into drive system machine parts reduces the undissolved carbide after treatment and makes it tough , Fatigue characteristics can be improved.

Manufacturing conditions In hot rolling, the finishing temperature is limited to 830-900 ° C. When the temperature exceeds 900 ° C., a decarburized layer is formed, and the fatigue characteristics of the heat-treated product are lowered. At a finishing temperature of less than 830 ° C., deformation resistance increases and the rolling mill load becomes too large.
The average cooling rate from finishing to winding is limited to 30 to 45 ° C./s. When the average cooling rate exceeds 45 ° C./s, the coiling temperature becomes low, the lamella spacing of the pearlite becomes small and hard, and it is difficult to pass the plate in a subsequent process such as continuous pickling. At an average cooling rate of less than 30 ° C./s, the plate shape is poor, and the plate passing time becomes long, resulting in a decrease in productivity.
The coiling temperature is limited to 500 to 680 ° C. in order to obtain a uniform pearlite structure. At a coiling temperature exceeding 680 ° C., a decarburized layer and grain boundary oxidation are formed on the surface of the steel sheet. When the coiling temperature is less than 500 ° C., the lamella spacing of the pearlite is small and hard, making it difficult to pass through the post-process.

The drive system mechanical parts are usually used after being annealed and tempered after being formed into a desired shape by stamping or the like. For this reason, annealing is indispensable for softening in order to suppress deterioration of the life of a processing jig such as a mold. Heating is performed for 10 hours or more in a temperature range of 650 ° C to Ac1 + 20 ° C. If it is less than 650 ° C. or less than 10 hours, softening is insufficient. When annealing is performed at a temperature exceeding Ac1 + 20 ° C., carbides are grown abnormally.
In order to increase the spheroidizing rate of the spherical carbide of the annealed sheet, to uniformly disperse it, to equalize the sheet thickness, and to obtain a good sheet shape, the rolled sheet is cold-rolled with a cold rolling rate of 20% or more. The annealing under the same conditions as described above may be repeated once or twice.

As described above, the drive system mechanical parts are usually used after being annealed and tempered after being formed into a desired shape by stamping or the like.
The annealed steel sheet provided by the present invention is also used after being formed into a desired part shape by a normal forming process and then subjected to a normal quenching and tempering treatment.
By the way, since high carbon steel is used with high hardness, the size of the average grain size of the prior austenite after the heat treatment greatly affects the toughness. The annealed steel sheet provided in the present invention is preferably heat-treated by setting the heating for quenching after being formed into a part shape to a temperature that is not too high. For example, when a quenching and tempering treatment is performed by heating to a temperature exceeding 860 ° C., the average grain size of the prior austenite increases to exceed 15 μm, and intergranular cracking is likely to occur. In order to improve toughness and fatigue properties, it is preferable to suppress the quenching temperature to 860 ° C. or less and to set the average grain size of the prior austenite to 15 μm or less. More preferably, it is 10 μm or less.

The tempering treatment after quenching may be performed under normal conditions. Since it is sufficiently solutionized by heating before quenching, the amount of undissolved carbide is extremely small and small. Usually, when there are many undissolved carbides, the toughness and fatigue characteristics are reduced due to an increase in crack generation sources. Moreover, even if there is little undissolved carbide, if undissolved carbide having a large particle size remains, it becomes a crack generation source and reduces toughness and fatigue characteristics. The inventors of the present invention, if the undissolved carbide after quenching and tempering treatment is 1.5% or less in area ratio and the dispersion state of particles having a particle size of 1.5 μm or more is 3 or less per 3000 μm ^{2 of the} observation area, We believe that toughness and fatigue properties can be reduced.
Although details are left to the examples, in fact, when the annealed steel sheet of the present invention is used, the undissolved carbide area ratio is 1.5% or less and the undissolved carbide having a particle size of 1.5 μm or more is 3 per observation area of 3000 μm ^2. Can be less than

Steels having the chemical components shown in Table 1 were melted, hot-rolled under the conditions shown in Tables 2 and 3, and further annealed or processed under the conditions shown in Tables 2 and 3. The plate thickness of the hot-rolled plate was adjusted so that the plate thickness after cold rolling and annealing was 2.0 mm. The area ratio of the spheroidized carbide, the maximum particle size of the spheroidized carbide, and the hardness of the steel plate after the final annealing were measured. The results are shown in Table 4. Since the annealed steel sheet is softened, punching and the like can be easily performed.
Then, heat treatment is performed under the quenching and tempering conditions shown in Table 5, and the hardness, martensite area ratio, prior austenite grain size, area ratio of undissolved carbide, number of undissolved carbide of 1.5 μm or more, impact value, fatigue limit investigated.
The spherical carbide and undissolved carbide were investigated with a laser microscope having an image processing function after etching with a 5% picric alcohol solution.

The prior austenite grain size was measured according to the steel austenite grain size test method specified in JIS G 0551, and the average grain size of the prior austenite was determined by the straight line segment method.
The impact value was determined by a Charpy impact test of JIS Z 2242 using a 2 mm U notch test piece. The impact value was 25 J / cm ² or more.
The fatigue limit was implemented according to the plane bending fatigue test method of a metal flat plate of JIS Z 2275. The test piece used the No. 1 test piece. The specimens were subjected to barrel polishing and chemical polishing after quenching and tempering because the fatigue limit varied due to surface flaws and variations in the finish of the processed surface. A fatigue limit of 1000 N / mm ² or more was considered good.
Tables 6 and 7 show the structure and properties after quenching and tempering.

As can be seen in Tables 6 and 7, the sample No. having the requirements specified in claim 1 is used. 2, 3, 4, 6, 10, 12, 15, 16, 17, 18, 19, 20, 23, 24, 25, 26, 27, 29, 31 have an impact value of 25 J / cm ² or more, fatigue limit : High toughness and fatigue characteristics of 1000 N / mm ² or more.
On the other hand, no. No. 1 has a low fatigue limit because the hardness after quenching and tempering cannot be obtained. No. with high Cr content. No. 5 has an impact value and a fatigue limit of less than the lower limit values because the area ratio of undissolved carbide is high and large. Although it is steel of the present invention, the cooling rate during hot rolling is slow and the coiling temperature is high. In No. 7, cementite is formed thick, the spheroidized carbide obtained after cold rolling annealing becomes large, and a large amount of undissolved carbide remains during subsequent quenching and tempering. For this reason, the impact value and fatigue limit of the quenched and tempered material are less than the lower limit.

  Even if the component composition satisfies the provisions of the present invention, the temperature of annealing condition II is high. In No. 8, since regenerated pearlite was generated, a large amount of undissolved carbide remained, and the impact value and fatigue limit of the quenched and tempered material were below the lower limit. Although the component composition satisfies the provisions of the present invention, the cold rolling ratio after annealing condition I is low. In No. 9, the remaining rod-like carbide was not sufficiently divided, and a large amount of large spheroidized carbide was formed under the annealing condition II, so that a large amount of undissolved carbide remained and the impact value and fatigue limit were below the lower limit. Similarly, no. In No. 11, since the temperature of the annealing condition I is high, many coarse rod-like carbides are formed, so that a lot of undissolved carbides remain and the impact value is high, but the fatigue limit does not reach the lower limit.

  No. Nos. 13, 14, 21, and 22 satisfy the provisions of the present claim, but because the heating temperature at the time of quenching is high, the prior austenite grain size increases and the impact value does not reach the lower limit. No. In hot rolling, since the cooling rate was high and the coiling temperature was low in hot rolling, the steel plate became hard, and subsequent plate passing was impossible, and evaluation after the hot rolling step was stopped. No. No. 30 was hardened because the annealing temperature was low and the die life at the time of punching was deteriorated, so the evaluation after quenching and tempering was stopped. Furthermore, no. In No. 32, since the amount of C is not specified, a large amount of undissolved carbide remains, and the impact value and fatigue limit do not reach the lower limit. No. In No. 33, V, Ti, and Nb are not added, so the prior austenite is large and the impact value and fatigue limit are below the lower limit. No. No. 34 has a large amount of prior austenite grain size due to a small amount of V added, and the impact value and fatigue limit do not reach the lower limit.

No. In No. 35, the amount of Mn is less than the lower limit, and because the quenching failure is recognized, the fatigue limit does not reach the lower limit. No. In No. 36, since the Si amount is high, grain boundary oxidation occurs in the surface layer of the steel sheet, and the fatigue limit does not reach the lower limit. No. No. 37 has a high amount of C and Cr, so that a large amount of undissolved carbide remains, and the impact value is high, but the fatigue limit does not reach the lower limit.
As can be seen from the above, the high carbon annealed steel sheet having the component composition defined in claim 1 and the dispersion form of carbide is excellent in formability because of its low hardness (see hardness in Table 4) and quenching. It is understood that driveline mechanical parts with excellent fatigue characteristics and toughness can be manufactured at low cost because they have excellent properties (see the martensite area ratios in Tables 6 and 7), and there are few undissolved carbides remaining after tempering and they are small. The

Claims (4)

  C: 0.50 to 0.70 mass%, Si: 0.5 mass% or less, Mn: 1.0 to 2.0 mass%, P: 0.02 mass% or less, S: 0.02 mass% or less , Al: 0.001 to 0.10% by mass, V: 0.05 to 0.50% by mass, Ti: 0.02 to 0.20% by mass, Nb: 0.01 to 0.50% by mass % Of the composition, the balance of which is Fe and inevitable impurities, and the carbide having a spheroidization rate of 95% or more and a maximum particle size of 2.5 μm or less was dispersed. A high carbon steel sheet with excellent hardenability, fatigue characteristics and toughness characterized by having a structure.   Furthermore, it has the component composition containing 1 type (s) or 2 or more types of Cr: 0.6 mass% or less, Mo: 0.5 mass% or less, B: 0.0002-0.01 mass%. High carbon steel plate with excellent hardenability, fatigue properties and toughness.   Hot rolling under conditions of finishing temperature: 830 to 900 ° C., average cooling rate: 30 to 45 ° C./s, coiling temperature: 500 to 680 ° C. is applied to the steel having the composition described in claim 1 or 2. Of the high carbon steel sheet excellent in hardenability, fatigue characteristics, and toughness, characterized in that after annealing, the hot rolled steel sheet obtained is pickled and then annealed in a temperature range of 650 to (Ac1 + 20) ° C. for 10 hours or more. Production method.   The hot-rolled pickled plate annealing is followed by adding 20% or more of cold rolling and then annealing for 10 hours or more in a temperature range of 650 to (Ac1 + 20) ° C. once or twice. Of high carbon steel sheet with excellent hardenability, fatigue properties and toughness.