JPH1129823A - Method for softening medium-or high-carbon steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To extremely soften an annealed material of hot rolled plate of medium- or high-carbon steel. SOLUTION: A hot rolled plate of hypo-eutectoid steel containing 0.1 to 0.8 mass% C is cold rolled at a light draft of >15 to 30%. Subsequently, the resultant cold rolled steel sheet is subjected to three-stage annealing. In this three-stage annealing, the first-stage heating consisting of holding at a temp. in the range between (Ac1 -50 deg.C) and <Ac1 for >=0.5hr is performed and then the second-stage heating consisting of holding at a temp. in the range between Ac12 and (Ac1 +100 deg.C) for 0. 5 to 20 hr and the third-stage heating consisting of holding at a temp. in the range between (Ar1 -50 deg.C) and Ar12 for 2 o 20 hr are continuously carried out and also the cooling rate from the second stage holding temp. down to the third-stage holding temp. is regulated to (5 to 30) deg.C/h.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for softening a hot-rolled steel sheet of a medium- and high-carbon hypoeutectoid steel.

[0002]

2. Description of the Related Art A medium / high carbon steel having a C content of about 0.2 to 0.8 mass% in steel can be hardened and strengthened and has a certain workability in an annealed state before quenching. Also have. For this reason, the hot rolled steel sheet is widely used as a raw material for various mechanical parts such as automobile parts and bearing parts. In manufacturing parts, punching and bending are generally performed, and relatively light drawing and stretch flange forming are sometimes performed. When the shape of the part is complicated, two or three parts are often manufactured by welding. However, in recent years, in order to reduce the manufacturing cost of parts, integral molding of parts and simplification of processing steps have been promoted. This means that the material must endure processing with a higher processing rate (= large plastic deformation) as viewed from the material side. In other words, with the advancement of processing technology, higher ductility has been required for the medium and high carbon steel sheets themselves, which are the raw materials.

[0003] In order to impart high ductility to a steel material, it is fundamental to achieve further "softening". It has been conventionally known that spheroidization of carbides is effective for softening medium and high carbon steel materials. For this purpose, for example, the following heat treatment method is employed. Method of retaining a long time just below the A ₁ transformation point. This method is suitable for spheroidization after cold working. A method in which a temperature is maintained between the A ₁ transformation point and the A ₃ transformation point for a certain period of time, and then gradually cooled to just below the A ₁ transformation point. This method is suitable for spheroidization of the pearlite structure, and has the advantage that spheroidization is completed in a short time. Across the A ₁ transformation point, and heating at a temperature just below the A ₁ transformation point, a method of repeatedly performing the heating at temperatures between the A ₁ transformation point and the A ₃ transformation point. This method is suitable for making the carbide particle size uniform and has the advantage that spheroidization is completed in a short time.
Strict temperature control is required, and it is not always suitable for industrialization in terms of cost. In general, it is known that when a steel material subjected to cold working is subjected to heat treatment, the steel material becomes much softer than when heat treatment is performed without adding cold working.

[0004]

However, in order to provide a hot-rolled steel sheet of medium- and high-carbon steels with a high degree of processing such as a high-integral forming process, the above-mentioned heat treatments require sufficient softening. Cannot be achieved. Cold working and above ~
Even when the above heat treatments are combined, a satisfactory level of softening has not yet been reached. Accordingly, the present invention provides a method of softening a hot-rolled steel sheet of a medium or high carbon steel, which has been widely used in the past, so that it can sufficiently be used for high-forming integrated forming while maintaining its hardenability. Aim.

[0005]

SUMMARY OF THE INVENTION The object of the present invention is to provide a hot rolled steel sheet of hypoeutectoid steel containing 0.1 to 0.8 mass% of C: more than 15% to 30%. subjected to rolling between the following soft reduction cold, then, after the heat of the first stage which holds more than 0.5 hours at a temperature range of less than _{Ac 1 -50 ℃ ~Ac 1, Ac} 1 ~Ac 1 + 100 of ° C. The second stage of heating for 0.5 to 20 hours in the temperature range and the third stage of heating for ₁ to 20 hours in the temperature range of Ar ₁ -50 ° C. to Ar ₁ are continuously performed, and This can be achieved by a method for softening a medium / high carbon steel sheet which is subjected to three-step annealing in which the cooling rate from the eye holding temperature to the third holding temperature is 5 to 30 ° C./h. Here, Ac ₁ is the A ₁ transformation point (° C.) of the steel in the heating process, and Ar is
₁ means the A ₁ transformation point (° C.) in the temperature decreasing process.

A second aspect of the present invention provides the hypoeutectoid steel according to the first aspect of the present invention, in particular, containing 0.1 to 0.8% by mass of C and 0.01% by mass or less of S. It is specified for restricted steel.

According to a third aspect of the present invention, the hypoeutectoid steel according to the first aspect of the present invention is particularly preferably C: 0.1 to 0.8%,
i: 0.15 to 0.40%, Mn: 0.3 to 1.0%, P is 0.03% or less, S is 0.01% or less,
T. Al is limited to a content of 0.1% or less, and the balance is Fe
And steel unavoidable impurities. Here, T. Al means the total amount of Al in the steel.

According to a fourth aspect of the present invention, the hypoeutectoid steel according to the first aspect of the present invention is particularly preferably C: 0.1 to 0.8%, S
i: 0.15 to 0.40%, Mn: 0.3 to 1.0%,
Cu: 0 to 0.30% (including no addition), Ni: 0 to 0
It contains 0.25% (including no addition), Cr: 0 to 0.2% (including no addition), P is 0.03% or less, and S is 0.1 to 0.2%.
01% or less; The content of Al is limited to 0.1% or less, and the balance is defined as steel containing Fe and inevitable impurities. Here, the lower limit of Cu, Ni, Cr is 0.
% Means that the element is not added.

According to a fifth aspect of the present invention, the hypoeutectoid steel according to the first aspect of the present invention is particularly preferably C: 0.1 to 0.8%, S
i: 0.15 to 0.40%, Mn: 0.3 to 1.0%,
Cu: 0 to 0.30% (including no addition), Ni: 0 to 0
2.0% (including no addition), Cr: 0 to 1.2% (including no addition), Mo: 0 to 0.3% (including no addition), and 0.03% of P Hereafter, S is 0.01% or less.
The content of Al is limited to 0.1% or less, and the balance is defined as steel containing Fe and inevitable impurities. Here, the lower limit of 0% of Cu, Ni, Cr and Mo means that the element is not added.

According to a sixth aspect of the present invention, in the invention of the first, second, third or fourth aspect, the rolling reduction of the cold rolling under light pressure is particularly preferably 2%.
0% or more and 30% or less, and the hardness of the steel sheet is Hv1.
It is specified to be extremely soft, not more than 30.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION According to the research of the present inventors,
Subjected to "inter soft reduction cold rolling" in Japanese pressure under constant range with respect to hot-rolled steel sheets of a high carbon steel, and then was subjected to "three-step annealing" under specific conditions sandwiching the A ₁ point of the steel, It was possible to achieve extremely softening, which was difficult to achieve conventionally, without impairing the hardenability inherent in medium- and high-carbon steels. Hereinafter, matters for specifying the present invention will be described.

The present invention is directed to a hypoeutectoid steel containing C: 0.1 to 0.8% by mass. C is the most basic alloying element in carbon steel, and the quenching hardness and the amount of carbide greatly vary depending on its content. In the hypoeutectoid steel having a C content of 0.1% by mass or less, sufficient quench hardness cannot be obtained for application to various machine structural parts. On the other hand, if the C content exceeds 0.8% by mass, the toughness after hot rolling is reduced, and the manufacturability and handleability of the steel strip deteriorates, and sufficient ductility cannot be obtained even after annealing. ,
It becomes difficult to apply to parts with a high degree of processing. Therefore,
In the present invention, from the viewpoint of providing a material steel sheet having both appropriate quenching hardness and workability, steel having a C content in the range of 0.1 to 0.8% by mass is targeted.

S is an element that forms MnS-based inclusions. If the amount of the inclusions is large, the local ductility is deteriorated.
The content is preferably reduced to 0.01% by mass or less. Local ductility also depends on the C content in addition to S, and the higher the C content, the worse the local ductility. In order to maintain good local ductility even when the C content increases to nearly 0.8% by mass, it is desirable to further reduce the S content to 0.005% by mass or less.

Since P deteriorates ductility and toughness, its content is desirably 0.03% by mass or less. A
l is added as a deoxidizer for molten steel, but T.I. A
If the amount exceeds 0.1% by mass, the cleanliness of the steel is impaired, and surface flaws are likely to occur, thereby deteriorating the surface quality of the steel sheet. Therefore, T. Al is desirably 0.1% by mass or less.

[0015] The content of Si is desirably 0.15% by mass or more for deoxidizing molten steel. However, Si hardens ferrite by a solid solution strengthening action, and a large amount of Si causes scale flaws on the steel sheet surface. Further, it also causes a decrease in toughness. Therefore, it is desirable that Si be contained in the range of 0.15 to 0.40% by mass. Mn is an additive element that enhances the hardenability of the steel sheet and is also effective for toughening.
In order to obtain sufficient hardenability, the content is preferably 0.3% by mass or more. However, when it is contained in a large amount exceeding 1.0% by mass, the ferrite is hardened, and the workability deteriorates. Therefore, Mn is desirably contained in the range of 0.3 to 1.0% by mass.

In the present invention, if necessary, Cu, Ni,
The present invention can be applied to steel in which various properties are improved by adding elements such as Cr and Mo. Cu improves the releasability of the oxide scale generated during hot rolling, and is therefore effective in improving the surface properties of the steel sheet. However, when the content is 0.3% by mass or more, fine cracks are easily generated on the steel sheet surface due to the embrittlement of the molten metal. Therefore, Cu can be added in a range of 0.3% by mass or less. A preferable range of the Cu content is 0.10 to 0.15% by mass. Ni is an alloy component that improves hardenability and prevents low-temperature brittleness. Further, Ni has an effect of counteracting the adverse effect of molten metal embrittlement, which is a problem due to the addition of Cu. Therefore, particularly when Cu is added in an amount of about 0.2% or more, it is necessary to add about the same amount of Ni as Cu. Extremely effective. However, a large amount of N exceeding 2.0% by mass
When i is included, the press formability and workability before quenching deteriorate even if softening is performed by performing three-step annealing.
Therefore, when Ni is added, the content is set to a range of 2.0% by mass or less.

Cr is an element that improves hardenability and increases temper softening resistance. However, when a large amount of Cr exceeding 1.2% by mass is contained, even if the steel is softened by performing three-step annealing, press formability and workability before quenching deteriorate. Therefore, when Cr is added, the content is set to a range of 1.2% by mass or less. Mo contributes to improvement of hardenability and tempering softening resistance similarly to Cr when added in a small amount. However, when a large amount of Mo exceeding 0.3% by mass is contained, even if the steel is softened by performing three-step annealing, press formability and workability before quenching deteriorate. Therefore, when Mo is added, the content is set to 0.3% by mass or less.

In the present invention, it is characterized in that performing the inter soft reduction cold rolling the hot-rolled steel sheet prior to the three stages annealing sandwiching the _point A. Generally, in steel subjected to cold working before annealing, recrystallization is promoted during annealing due to the introduced work strain,
As a result, a soft material is obtained as compared with a case where no cold working is performed. However, medium- and high-carbon steels such as those targeted in the present invention have high strength, and the degree of softening obtained by combining general cold working and annealing seems to have been performed on ordinary steel. However, it was not enough to provide an integral molding process with a large degree of processing. However, only in the case where the three-step annealing specified in the present invention is performed, the hardness after the annealing can be further remarkably reduced by preliminarily performing the light rolling under the rolling reduction in a specific range. It is.

More specifically, when the rolling reduction of the cold rolling performed before the three-step annealing exceeds 15%, the hardness after annealing sharply decreases, and the conventional annealing method (the above-mentioned method).
The hardness difference from the case of using is also remarkable. That is, when the rolling reduction exceeds 15%, the effect of the three-step annealing becomes remarkable. Then, as the cold rolling reduction increases, the fragmentation of the carbide proceeds, and the hardness after three-step annealing becomes the lowest at a cold rolling reduction of 20 to 30%. However, when the cold rolling ratio exceeds 30%, 3
The metal structure after the step annealing has a so-called mixed grain structure in which the sizes of ferrite crystal grains are not uniform, and the hardness gradually increases. Therefore, in the present invention, the cold rolling performed before the three-step annealing is light rolling under the light rolling having a reduction ratio of more than 15% and 30% or less.

In particular, when the rolling reduction is in the range of 20 to 30%, a very soft steel having a Hv of 130 or less can be obtained although it is a medium / high carbon steel. When the Hv value becomes 130 or less, many of the general integral forming processes performed on a hot-rolled steel sheet of ordinary steel can be applied. In other words, relatively high-level processing, which could only be applied to ordinary steel in the past, can now be applied to medium- and high-carbon steel sheets. It will be improved.

Next, three-step annealing will be described. In the medium and high carbon steels targeted in the present invention, softening cannot be sufficiently achieved simply by promoting recrystallization, and it is important to control the dispersion form of carbide after annealing. In general, when the steel is heated to a temperature of _one point or more of Ac, fine carbides among the carbides form a solid solution in austenite, and then, when cooled to a temperature of _one point or less of Ar, precipitate again as carbides. At this time, if a certain amount of undissolved carbide can remain in the temperature range of _one point or more of Ac, if the cooling rate is reduced, C dissolved in the austenite will not generate pearlite, and the undissolved carbide will not be formed. Nuclei are precipitated, so that the spheroidization rate of the carbide after annealing is increased. Also, in this case, the number of carbides decreases by heating at _one point or more from the point before annealing, and when the cooling rate is slow, no new nucleation occurs during cooling. As a result, the number of carbides after annealing decreases as compared to before annealing. I do. A decrease in the number of carbides means that a metal structure containing carbides having a large particle size can be obtained because the total carbon amount is constant. In particular, when undissolved carbides serving as nuclei remain uniformly in place, the distance between the carbides also increases. When such a metal structure is obtained, the ductility of the steel is improved.

However, the temperature range above the Ac ₁ point is a region where all carbides of the hypoeutectoid steel form a solid solution in equilibrium. For this reason, the number of undissolved carbides usually decreases when heated to a temperature range of _one point or more of Ac, and then C dissolved in austenite in the course of cooling to a temperature of _one point or less of Ar is recycled pearlite having a wide lamella interval. Precipitates as As a result, the spheroidization rate of carbides becomes extremely low, and a steel sheet with high ductility cannot be obtained.

Therefore, the present inventors have conducted various studies, and as a result,
Prior to heating the steel sheet to more than _one point Ac, by performing the process of heating a predetermined time or longer in advance Ac specific temperature range below _one point, even hypoeutectoid steel, non at a temperature range of not lower than Ac ₁ point It has been found that an appropriate amount of dissolved carbide can be left. In addition, by identifying the time maintained at a specific temperature range after cooling to below ₁ point Ar, was also found that it is possible to obtain an optimum carbide dispersed form to softening. Hereinafter, the conditions of the three-step annealing of the present invention will be described.

[First Stage Heating] The purpose of the first stage heating is as follows.
Ac is to maintain the steel sheet at a temperature of less than _one point and to cut pearlite generated by hot rolling to make spheroidized carbide (cementite). Although the separated carbides are relatively fine, the surface area per unit volume of the carbides decreases due to the progress of spheroidization. As a result, when the second stage Ac is heated to _one point or more, the reduction effect of the carbide / austenite interface area is obtained. The solid solution of the carbide can be delayed. In order to promote the fragmentation and spheroidization reaction of hot-rolled pearlite, a temperature as high as possible within a range of less than Ac ₁ point is desirable. At temperatures lower than Ac ₁ -50 ° C., spheroidization does not proceed sufficiently. On the other hand, if the value of Ac is ₁ or more, the purpose cannot be achieved because hot rolled pearlite having a large interface area easily dissolves in austenite. Therefore the heating temperature in the first stage was a temperature range of less than Ac ₁ -50 ° C. to Ac _1. Further, if the holding time in the temperature range is less than 0.5 hour, spheroidization cannot be sufficiently achieved, so the first stage heating holding time is set to 0.5 hour or more. The upper limit of the holding time need not be particularly defined, but is preferably within 8 hours in consideration of industrial practice. After the first-stage heating, the temperature may be raised as it is to perform the second-stage heating, or the temperature may be once cooled to room temperature and then raised again.
You may provide for the heating of a stage. When it is not possible to secure a holding time of 0.5 hour or more by one heating due to the convenience of the equipment or the like, the first-stage heating may be performed in a plurality of times.
In that case, the total holding time within the above temperature range is 0.1 mm.
Allow at least 5 hours.

[Second Stage Heating] The purpose of the second stage heating is as follows.
The steel sheet that has undergone the first stage heating is maintained at a temperature of at least _one point Ac,
The purpose is to dissolve and eliminate fine carbides in an austenitized portion and to leave relatively large spherical carbides undissolved, and, when ferrite is present, to grow the carbide in that portion by Ostwald ripening. In other words, this is a step of determining the number and dispersion state of undissolved carbides to be the nuclei of carbide precipitation by the third stage heating. If the heating temperature is lower than the Ac ₁ point, no austenite is formed. On the other hand, if the temperature exceeds the temperature of Ac ₁ + 100 ° C., even if the carbides are spheroidized by the first-stage heating, many of them are dissolved and disappeared in austenite, and the number of undissolved carbides becomes too small. Or it no longer exists. Then 3
Recycled pearlite is generated in the course of cooling to the stage, and it cannot be sufficiently softened. If the heating holding time is less than 0.5 hour, the solid solution of fine carbides in austenite is insufficient, and if the heating is performed for a long time exceeding 20 hours, the equilibrium state is approached and the number of undissolved carbides is excessively reduced. Therefore, the second stage heating was to hold 0.5 to 20 hours at a temperature range of Ac ₁ ~Ac ₁ + 100 ℃.

[Third Stage Heating] The purpose of the third stage heating is as follows.
The steel sheet which has been heated in the _first and second stages is kept at a temperature of ₁ point or less, and C discharged from austenite due to austenite → ferrite transformation by cooling from the temperature in the second stage is a core of undissolved carbide. And growing these carbides by Ostwald. In other words, the number of carbides is a step in which the number of undissolved carbides left by heating in the second stage is maintained almost as it is, and the spheroidization ratio of the carbides is increased. If the holding temperature is not Ar ₁ point or less, austenite → ferrite transformation does not occur. If the holding temperature is lower than Ar ₁ -50 ° C. or if the holding time is less than 2 hours, Ostwald growth does not sufficiently proceed. However, even if the holding time exceeds 20 hours, the effect is saturated,
There is no industrial advantage. Thus, the third stage heating was keeping 2-20 hours at a temperature range of Ar ₁ -50 ℃ ~Ar _1.

[Cooling rate from the second-stage holding temperature to the third-stage holding temperature] If this cooling speed is high, the degree of supercooling of austenite increases, and it becomes easy to generate recycled pearlite. In order to sufficiently suppress generation of recycled pearlite, the cooling rate needs to be 30 ° C./h or less. On the other hand, even if the cooling rate is slower than 5 ° C./h, the effect of suppressing reproduced pearlite is saturated, and there is no industrial merit. Therefore, the cooling rate was set to 5 to 30 ° C / h.

[0028]

EXAMPLES In mass%, C: 0.35%, Si: 0.22
%, Mn: 0.72%, P: 0.012%, S: 0.0
11%, Cu: 0.1%, Ni: 0.02%, T.P. A
l: A steel containing 0.007% and the balance being Fe and unavoidable impurities was melted to obtain a hot-rolled steel sheet having a thickness of 4 mm. This hot-rolled steel sheet is subjected to cold rolling at various rolling reductions, and is subjected to conventional annealing and the present invention, respectively.
It was subjected to step annealing. The annealing conditions are as follows. [Conventional annealing] 710 ° C x 5hr holding → cooling rate 10 ° C
/ Hr cooling to 620 ° C → air cooling [3-stage annealing] 690 ° C x 4hr hold → 730 ° C x 4h
r holding → cooling to 690 ° C at a cooling rate of 10 ° C / hr → 6
90 ° C x 4hr hold → 620 ° C at 10 ° C / hr cooling rate
Cooling down to air cooling Note that the Ac ₁ point of this steel is 727 ° C and the Ar ₁ point is 738 ° C.

The hardness of the cross section of each steel sheet after annealing was measured. The result is shown in FIG. It can be seen that when the cold rolling reduction before annealing exceeds 15%, the softening of the steel subjected to the three-step annealing of the present invention rapidly accelerates, and the degree of softening is remarkable as compared with the conventional steel subjected to the annealing. Further, according to the three-step annealing of the present invention, the hardness becomes Hv130 or less when the cold rolling reduction before annealing is in the range of 20 to 30%, and a remarkable softening of 10 or more in Hv value is found as compared with the conventional annealing. Was done.
The metal structure of the steel sheet according to the present invention in which these Hv values are 130 or less is a ferrite crystal having a grain size of FGS.No: 8 to 8.5, carbides are sufficiently spherical, and pearlite remains. I was not able to admit. Further, the steel sheets obtained according to the present invention all maintained good hardenability.

[0030]

According to the present invention, the hardness of a hot-rolled steel sheet of medium and high carbon steel can be remarkably reduced as compared with the conventional steel sheet. Now you can. As a result, advanced processing that could only be applied to ordinary steel in the past can now be applied to hot-rolled steel sheets of medium- and high-carbon steels. It is now possible.
In addition, the hardenability after processing the parts is maintained as before. Furthermore, in the present invention, cold rolling under light rolling is applied, so that the steel sheet according to the present invention can be suitably used for applications requiring good surface texture and plate shape. Therefore, the present invention contributes to expanding the use of medium- and high-carbon steels and reducing manufacturing costs.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the rolling reduction of cold rolling performed before annealing and the hardness after annealing.

Claims (6)

[Claims] 1. A C: 0.1 to 0.8 wt% alms between soft reduction cold rolling the hot-rolled steel sheet of 30% or less than 15% of hypoeutectoid steel containing, then, Ac ₁ -50 ° C. after heating the first stage to hold more than 0.5 hours at a temperature range of to Ac less than _1, Ac ₁ to Ac heating in the second stage to hold 0.5 to 20 hours at a temperature range of ₁ + 100 ° C. and Ar ₁ do -50 ° C. to Ar ₁ of continuously heating the third stage for holding 2-20 hours at a temperature range and the cooling rate from the second stage of the holding temperature to the third stage holding temperature A method for softening a medium / high carbon steel sheet which is subjected to three-step annealing at 5 to 30 ° C./h. 2. The hypoeutectoid steel has a C content of 0.1 to 0.8% by mass.
2. The method for softening a medium- and high-carbon steel sheet according to claim 1, wherein S is limited to a content of 0.01% by mass or less. 3. 3. The hypoeutectoid steel has a C content of 0.1 to 0.1% by mass.
8%, Si: 0.15 to 0.40%, Mn: 0.3 to
1.0%, P is 0.03% or less, S is 0.01%
% Or less; 2. The method for softening a medium / high carbon steel sheet according to claim 1, wherein the content of Al is limited to 0.1% or less, and the balance is steel comprising Fe and unavoidable impurities. 4. The hypoeutectoid steel has a C content of 0.1 to 0.1% by mass.
8%, Si: 0.15 to 0.40%, Mn: 0.3 to
1.0%, Cu: 0 to 0.30% (including no addition), N
i: 0 to 0.25% (including no addition), Cr: 0 to 0.
2% (including no additive), P is 0.03% or less,
S is 0.01% or less; 2. The method for softening a medium / high carbon steel sheet according to claim 1, wherein the content of Al is limited to 0.1% or less, and the balance is steel comprising Fe and unavoidable impurities. 5. The hypoeutectoid steel has a C content of 0.1 to 0.1% by mass.
8%, Si: 0.15 to 0.40%, Mn: 0.3 to
1.0%, Cu: 0 to 0.30% (including no addition), N
i: 0 to 2.0% (including no addition), Cr: 0 to 1.2
% (Including no addition), Mo: 0 to 0.3% (including no addition), 0.03% or less of P, 0.01% or less of S, 2. The method for softening a medium / high carbon steel sheet according to claim 1, wherein the content of Al is limited to 0.1% or less, and the balance is steel comprising Fe and unavoidable impurities. 6. The hot-rolled steel sheet of the hypoeutectoid steel according to claim 1, 2, 3 or 4 is subjected to cold rolling under light pressure of 20% or more and 30% or less, and then Ac ₁ -50 ° C. to Ac After performing the first stage of heating for 0.5 hours or more in a temperature range of less than ₁ , A
c ₁ to Ac continuous heating of the third stage for holding 2-20 hours at a temperature range of ₁ + 100 ° C. in a temperature range of the heating and Ar ₁ -50 ℃ ~Ar ₁ of the second stage to hold 0.5 to 20 hours Then do
In addition, the softness of the medium- and high-carbon steel sheets is reduced to a hardness of Hv130 or less by performing three-step annealing at a cooling rate from the second-stage holding temperature to the third-stage holding temperature of 5 to 30 ° C./h. Method.