JPH06271927A - Production of high carbon steel sheet excellent in formability - Google Patents

Abstract

PURPOSE:To produce a high carbon steel sheet excellent in formability. CONSTITUTION:A hot rolled steel sheet contg., by weight, 0.20 to 0.70% C, 0.05 to 1.00% Si, 0.05 to 0.50% Mn, 0.01 to 1.00% sol.Al, 0.002 to 0.010% N and 0.003 to 0.0050% B, and the balance Fe with impurities (<=0.020% O and <=0,010% S) is thereafter cooled at least to 650 to 780 deg.C at >=20 deg.C/h cooling rate and is subjected to box annealing (secondary annealing) at 650 to 780 deg.C to graphitize >=50 area % of cementite in the steel. It is advantageous to execute cooling to an ordinary temp. because the annealing conditions (soaking temp. and soaking time) for graphitizing >=50 area % of cementite can widely be set. Furthermore, it is effective for promoting the graphitization to incorporate one or more kinds among 0.05 to 2.00% Ni, 0.05 to 2.00% Cu and 0.001 to 0.01% Ca into the steel stock.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a steel having a high carbon content, but has a strength-elongation characteristic comparable to that of a mild steel sheet by graphitizing cementite in ferrite, and forming into a complicated shape. The present invention relates to a method of manufacturing a steel sheet which is capable of exhibiting high strength by heat treatment such as quenching and tempering after forming.

[0002]

2. Description of the Related Art High carbon steel sheets for automobile parts, gears and other mechanical parts, which are soft at the time of molding and require high strength and hardness at the time of use, are shipped with spheroidized cementite. It is often used after being heat-treated after being formed into a desired shape.

However, even if box annealing is performed to spheroidize cementite and further coarsen it, the strength of the material depends on the volume fraction of cementite, and the amount of C specified in the present invention (0.20 to 0.70% by weight). In the steel containing, the tensile strength (TS) is expressed by the following formula (1) when Si content is 0.2% by weight and Mn content is 0.5% by weight and the influence of other components is ignored.

TS (N / mm ² ) = 325 + 375 × C (%) (1) Therefore, when the structure of steel is composed of ferrite and cementite, it contains 0.20 to 0.70% by weight of C. With steel, it was impossible to achieve a tensile strength of 400 N / mm ² or less. To reduce this tensile strength and increase the elongation, the C
It was necessary to reduce the volume fraction of cementite while maintaining the content within a predetermined range.

In order to reduce the volume fraction of cementite while maintaining the C content of steel within a predetermined range, it is necessary to graphitize cementite. However, it is very difficult to graphitize by annealing a steel having a hypoeutectoid composition having a C content of 0.2 to 0.7% by weight defined in the present invention after hot rolling.

As a means for accelerating the graphitization of cementite, the applicant of the present invention has disclosed in JP-A-60-52551 and JP-A-60-53551.
In Japanese Laid-Open Patent Publication No. 317629, a method of hot rolling followed by cold rolling followed by box annealing to graphitize was proposed, but the manufacturing process was complicated. Also, Japanese Patent Laid-Open No. 64-2
As shown in Japanese Patent No. 5946, graphitization is promoted by increasing the Si content in steel, but the addition of Si causes solid solution hardening of steel, and thus the advantage of softening due to graphitization is offset. Will end up.

[0007] After that, the present applicant proposed a high carbon thin steel sheet having good formability and a method for producing the same in Japanese Patent Application Laid-Open No. 4-202744. In this steel sheet, in particular, the P content is limited to 0.020% by weight or less, which is lower than the usual allowable upper limit value (JIS: 0.030% by weight), and the Si content is more than 0.20% by weight and 1.20% by weight. Steel with optimized B and N contents,
Hot rolling at a rolling finishing temperature of 600 to 900 ℃, 400 to 650 ℃
Is a steel sheet manufactured by cooling to a temperature range of 5 to 40 ° C / sec at a cooling rate, winding, and soaking in the temperature range of 600 ° C to Ac ₁ transformation point after winding. Similar BN (boron nitride) is used as a nucleus to promote precipitation and miniaturization of graphite. This steel sheet has excellent workability, and after being processed into a prescribed shape, it can be made into a product with high strength and excellent wear resistance by heat treatment such as quenching and tempering. There was a difficulty that it required.

[0008]

SUMMARY OF THE INVENTION The present invention is a steel having a high carbon content, but is soft at the time of molding by graphitizing the cementite in the ferrite (specifically, the elongation is 40% or more. ), A method of manufacturing a steel sheet that can be formed into a complicated shape and that can exhibit high strength by heat treatment after forming, especially, the graphitization of cementite is efficiently performed in a relatively short time. The object was to provide a method of obtaining the same.

[0009]

In order to solve the above problems, the present inventor has studied a process for promoting the precipitation of graphite and obtained the following findings. That is, also in the method of the present invention, graphite is precipitated by using BN as a nucleus similarly to the method of the invention of the above-mentioned JP-A-4-202744, but in the austenitizing temperature range of 780 to 900 ° C. after hot rolling. If soaking is performed to precipitate fine BN (primary annealing) and then soaking (second annealing) in the temperature range of 650 to 780 ° C, which is advantageous for the precipitation and growth of graphite, the graphite can be treated in a relatively short time. Can be deposited. After performing the primary annealing, the cooling rate until the start of the secondary annealing is 20 ° C./h or more in order to reduce the pearlite lamellar spacing and accelerate the decomposition of pearlite in the secondary annealing. At cooling rates below this,
Due to the coarsening of the pearlite lamella, the annealing time required for graphitization in the secondary annealing becomes extremely long.

The present invention was made on the basis of the above findings, and the gist thereof is the following methods (1) to (3) for producing a high carbon steel sheet. In addition, "%" regarding a component content means "weight%."

(1) C: 0.20 to 0.70%, Si: 0.05 to 1.00
%, Mn: 0.05 to 0.50%, sol.Al: 0.01 to 1.00%, N:
A hot-rolled steel sheet containing 0.002 to 0.010% and B: 0.0003 to 0.0050%, the balance of which is Fe and unavoidable impurities, and P as impurities is 0.020% or less and S is 0.010% or less at 780 to 900 ° C. After holding for 0.1 to 10 hours at 20 ℃
Good formability characterized by cooling to at least 650 to 780 ° C at a cooling rate of / h or more and box annealing at 650 to 780 ° C for 4 hours or more to graphitize 50 area% or more of cementite in steel. Method for producing high carbon steel sheet.

(2) In addition to the components described in (1) above, Ni:
0.05-2.00%, Cu: 0.05-2.00% and Ca: 0.001--0.
It contains one or more of 01% and the balance is Fe and unavoidable impurities. P as an impurity is 0.020% or less and S is
Hot rolled steel sheet with 0.010% or less
After holding for 10 hours, at least at a cooling rate of 20 ° C / h or more
A method for producing a high carbon steel sheet having good formability, which comprises cooling to 650 to 780 ° C., and box annealing at 650 to 780 ° C. for 4 hours or more to graphitize 50 area% or more of cementite in the steel.

(3) Hot-rolled steel sheet at 0.1 to 10 at 780 to 900 ° C
After holding for a time, it is cooled to room temperature at a cooling rate of 20 ° C / h or more, and subsequently box annealed at 650 to 780 ° C for 4 hours or more to graphitize 50 area% or more of cementite in steel. The method for producing a high carbon steel sheet having good formability according to (1) or (2) above.

[0014]

The functions specified in the method of the present invention will be described in detail below.

First, the reasons for limiting the chemical composition of the raw material steel used in the method of the present invention will be explained.

C: 0.20 to 0.70% Generally, the lower the C content, the greater the elongation and the better the workability. However, in order to improve the strength and wear resistance by applying a heat treatment after the forming process, a certain amount of C
Is required. In the method of the present invention, the hardness after quenching
The lower limit was set to 0.20% in order to achieve Hv300 or higher. On the other hand, if the C content exceeds 0.70%, cracks originating from graphite occur during molding, and the formability deteriorates. Therefore, the upper limit of the C content was set to 0.70%.

Si: 0.05 to 1.00% As described above, Si is an effective component for graphitizing cementite. To use this effect to shorten the annealing time
It is necessary to contain 0.05% or more. However, the content is 1.
If it exceeds 00%, solid solution hardening of ferrite becomes remarkable, so the upper limit was made 1.00%.

Mn: 0.05 to 0.50% Mn stabilizes cementite, suppresses decomposition of cementite during annealing and soaking, and significantly inhibits precipitation of graphite. Therefore, the upper limit of the Mn content is set to 0.50%. However, Mn has the effect of enhancing the hardenability of the material, forming MnS by combining with S in the steel to reduce the amount of solute S, and improving the toughness, so addition to some extent is necessary. For this reason, the Mn content is set to 0.05 to 0.50%.

Sol.Al: 0.01 to 1.00% Al (aluminum) acts as a deoxidizing agent and also has an effect of promoting precipitation of graphite during annealing. Because of this, sol.
The lower limit of the Al content is 0.01%. However, when excess Al is present, AlN is preferentially formed over BN, and the effect of BN described above cannot be expected. Therefore, the upper limit of the content is set to 1.00%.

N: 0.002 to 0.010% N is unavoidably contained in steel, but it is an essential and important element for producing BN. N of 0.002% or more is necessary to produce a sufficient amount of BN for the above purpose. On the other hand, when the N content exceeds 0.010%, the solid solution N in the steel increases and the elongation of the steel deteriorates, and the precipitates such as AlN increase and the toughness of the steel plate after heat treatment deteriorates. Since it may cause harmful effects, the upper limit of its content is
It was set to 0.010%.

B: 0.0003 to 0.0050% B forms BN and acts as a nucleus during graphitization of cementite during annealing. For that purpose, it is necessary to contain 0.0003% or more. On the other hand, if the content of Fe exceeds 0.0050%, Fe ₂ B or Fe ₂₃ (C
B) Since a B compound such as ₆ is formed to deteriorate the toughness of the steel sheet, the upper limit of its content is set to 0.0050%.

One of the raw material steels used in the method of the present invention is a steel which, in addition to the above-mentioned components, has the balance of Fe and inevitable impurities. Among the impurities, P and S in particular must be suppressed within the following range.

P: 0.020% or less P segregates at the interface between cementite and ferrite to suppress the migration of C, and significantly inhibits the precipitation of graphite in the voids in the steel. Therefore, it is desirable to make P as low as possible, and the allowable upper limit is set to 0.020%.

S: 0.010% or less It is desirable that the content of S is low. S in steel combines with Mn to form MnS, especially in high carbon steel sheets.
This is because the presence of MnS affects the toughness of the product. After setting the upper limit on the Mn content as described above, the S content is also 0.0
It is necessary to keep it below 10%.

In the method of the present invention, in addition to the steel having the above components, Ni: 0.05-2.00%, Cu: 0.05-2.00% and Ca:
Steels containing one or more of 0.001 to 0.01% can be used. The proper range of action and content of these components is as follows.

Ni: 0.05 to 2.00% Ni is an element that promotes graphitization together with Si, but the effect of solid solution hardening ferrite is not as strong as Si, and is an element effective for softening steel. For that purpose, it is necessary to contain 0.05% or more. On the other hand, excessive addition causes solid solution hardening of ferrite and increase in material cost, so the upper limit of its content is made 2.00%.

Cu: 0.05 to 2.00% Cu is an element which increases the hardenability of steel without inhibiting graphitization and hardly causes solid solution hardening. When it is expected to improve hardenability, it is necessary to contain 0.05% or more. On the other hand, if it is added excessively, Cu that was in solid solution during the cooling during box annealing is aged and precipitates at 0.05 μm or less. , Which forms ultrafine ε Cu, increases the strength and deteriorates the formability, so the upper limit of its content is 2.00%.

Ca: 0.001 to 0.01% Ca is an element that has the effect of promoting graphitization in hot-rolled sheet annealing. In the material steel used in the method of the present invention, when Al is added to promote graphitization, AlN is generated and the formation of BN, which is a feature of the method of the present invention, is suppressed, so the addition of Al must be reduced. I don't get. Therefore, it is desirable to add Ca instead of Al for the purpose of promoting graphitization. In addition, since Ca also has an effect of fixing S that inhibits graphitization and removing its harm, addition of Mn (which is also a graphitization inhibiting element) for fixing S when Ca is added Can be reduced. When expecting such effects, the Ca content must be 0.001% or more. On the other hand, excessive addition causes an increase in cost and
Since Ca-based oxides and sulfides increase, the upper limit of their content is 0.01%.

Next, a method of manufacturing the high carbon steel sheet of the present invention using the above-mentioned raw material steel will be described.

In this manufacturing method, a hot-rolled steel sheet having the above chemical composition is subjected to soaking (primary annealing), cooling,
It is processed in a box annealing (secondary annealing) step to form a structure in which 50% by area or more of cementite in steel is graphitized. The various conditions of the treatment in these steps are as described below. The hot rolling may be carried out under ordinary conditions for producing a hot rolled steel sheet of medium to high carbon steel, for example, heating at 1250 ° C, finish rolling at 850 ° C, and winding at 620 ° C.

Soaking (Primary Annealing) After hot rolling the steel having the above chemical composition, 780 to 90
Soaking is performed at 0 ° C for 0.1-10 hours.

The amount of BN, which becomes a precipitation nucleus of graphite during box annealing (secondary annealing), is insufficient if hot-rolled. To secure a sufficient precipitation amount, after hot rolling, BN
It is effective to carry out soaking in the precipitation temperature range. Therefore, in the method of the present invention, the purpose is to accelerate the precipitation of BN.
It was decided to soak for 0.1 hour or more at a temperature of ℃ or more. On the other hand, if the soaking temperature exceeds 900 ° C, the precipitation amount of BN will decrease, and if the soaking temperature exceeds 10 hours, BN will change to more stable AlN.
It was decided to perform the treatment at 900 ° C. for 0.1 to 10 hours. B
In order to finely disperse N, it is desirable that the soaking time be within 2 hours.

Cooling After primary annealing, at least 650-at a cooling rate of 20 ° C./h or more.
Cool to 780 ° C (the inventions of (1) and (2) above). The cooling rate of 20 ° C./h or more is to reduce the pearlite lamellar spacing in order to promote decomposition of pearlite in box annealing (secondary annealing). Cooling rate is 20 ℃
If it is less than / h, the pearlite lamella becomes coarse and the annealing time required for graphitization in the secondary annealing remarkably increases.

Cooling is performed to at least 650-780 ° C. Cooling to room temperature (invention of (3) above) allows widening of annealing conditions (soaking temperature, soaking time) for graphitizing 50 area% or more of cementite, as shown in Examples described later. However, in the case of cooling to 650 to 780 ° C, that is, to the secondary annealing temperature, or to a temperature range between the secondary annealing temperature and room temperature, there is no need to reheat after the primary annealing, or Since the temperature range of heating is small, there are great advantages in operation.

Box Annealing (Secondary Annealing) After cooling, box annealing is performed at 650 to 780 ° C. for 4 hours or more to graphitize 50 area% or more of cementite in steel.

In order to precipitate graphite in steel, it is desirable to anneal in the ferrite temperature range.

Further, although some precipitation is observed even in the austenite temperature range, the precipitation amount decreases as the temperature rises. Therefore, in the present invention, the lower limit of the annealing temperature for graphitization after precipitation of BN is 6
It was set to 50 ° C. The upper limit of the annealing temperature was set to 780 ° C in order to secure the amount of graphite precipitation. Graphite precipitates by soaking for 4 hours or more, but in order to graphitize 50 area% or more of cementite, it is desirable to soak for 12 hours or more in this temperature range.

The hot-rolled steel sheet produced by the method of the present invention is soft, has excellent formability, and can form martensite in the surface layer or all layers of the product by heat treatment such as quenching and tempering after forming. It has abrasion resistance, is lightweight and has excellent durability. If this hot-rolled steel sheet is used as a material for manufacturing automobile parts and thick-walled gear products that involve blanking and pressing, large-scale equipment such as soft nitriding treatment that was conventionally used can be used. The required treatment is not necessary, and the product as described above can be manufactured by simple induction hardening.

[0039]

Example 1 A hot rolled coil was manufactured by changing the annealing conditions for three types of steels having the adjusted C content shown in Table 1, and the graphitized state and mechanical properties were investigated. Hot rolling is 1250 ℃
After soaking for 1 hour, rolling is started at 1200 ℃,
Finished at 850 ° C. The plate thickness was 5 mm. The cooling rate after finishing was 20 ° C / sec, and the film was wound at 550 ° C.

I. Effects of primary annealing conditions Box annealing in which the above hot-rolled coil is subjected to soaking (primary annealing) for 2 minutes to 10 hours at a predetermined temperature in the temperature range of 800 to 900 ° C, and then to 680 ° C for 24 hours (Secondary annealing)
The obtained steel sheet was examined for graphitization state and mechanical properties. The graphitized state was expressed by the graphitized area ratio defined by the following equation (1). Regarding mechanical properties, a tensile test was performed using JIS No. 5 test pieces (sheet thickness 2.0 mm) cut out from the annealed steel sheet, and the yield strength (YP), tensile strength (TS) and elongation were measured.
(EL) was measured.

[0041]

[Equation 1]

The survey results are shown in FIGS. 1 to 6
Is a diagram showing the relationship between the primary annealing conditions and the graphitized area ratio,
1 and 2 show the results for steel type A, FIGS. 3 and 4 for steel type B, and FIGS. 5 and 6 for steel type C. In addition, Fig. 1, Fig. 3 and Fig. 5 show that after soaking (primary annealing) under the specified conditions, they were cooled to room temperature at a cooling rate of 100 ° C / h and again annealed at 680 ° C for 24 hours (secondary annealing). (Annealing) (separate annealing), FIG. 2, FIG. 4 and FIG.
After soaking (primary annealing) under specified conditions, cooling to 680 ° C at a cooling rate of 100 ° C / h, followed by box annealing (secondary annealing) at this temperature for 24 hours (continuous annealing) ).
It should be noted that in FIGS. 1 to 6, the numbers shown in the figures represent the graphitized area ratio when the primary annealing is performed at the soaking temperature and time indicated at that position. Figure 7 shows the yield strength (YP) and tensile strength (T
It is a figure which shows the average value of S) and elongation (EL).

It can be seen from FIGS. 1 to 6 that the range of primary annealing conditions in which a high graphitization area ratio can be obtained expands as the C content increases. In addition, after the primary annealing, it is cooled to room temperature and is heated again (secondary annealing) in a separate annealing (FIGS. 1 and 3).
And FIG. 5) has a wider range of heating conditions for graphitization than continuous annealing (FIGS. 2, 4, and 6) in which primary annealing is continuously annealed to secondary annealing.

As is clear from the results shown in FIG. 7, the tensile strength and the yield strength are decreased and the elongation is increased with the increase of the graphitized area ratio, which is the target for the steel sheet obtained by the method of the present invention. In order to obtain elongation of 40% or more, it is necessary to set the graphitization area ratio to 50% or more.

From these results, the primary annealing condition for securing the graphitized area ratio of 50% or more is 780 to 900 ° C. × 0.1
It is understood that the content may be appropriately determined depending on the C content in the range of up to 10 hours.

II. Effect of secondary annealing conditions After performing the primary annealing of soaking the hot-rolled coil at 830 ℃ for 30 minutes, cool it to room temperature at 100 ℃ / h and then at 100 ℃ / h.
When the secondary annealing is performed by heating to a specified temperature in the temperature range of 625 to 775 ℃ and soaking at that temperature for 10 to 40 hours (separate type annealing), and the primary anneal that is also soaking at 830 ℃ for 30 minutes. After that, the graphitized area ratio when the secondary annealing is performed by cooling to 100 ° C / h to a specified temperature in the temperature range of 625 to 775 ° C and soaking at that temperature for 10 to 40 hours (continuous annealing). Was investigated. Further, a tensile test was conducted in the same manner as described above to examine the mechanical properties.

The survey results are shown in FIGS. 8 to 13
Is a diagram showing the relationship between the secondary annealing conditions and the graphitized area ratio,
8 and 9 show the results for steel type A, FIGS. 10 and 11 for steel type B, and FIGS. 12 to 13 for steel type C. Further, FIGS. 8, 10 and 12 show the case where the separation type annealing is performed, and FIGS. 9, 11 and 13 show the case where the continuous type annealing is also performed. The numbers in FIGS. 8 to 13 represent the graphitized area ratio when the secondary annealing is performed at the soaking temperature and time indicated at that position. Figure 14 shows the yield strength of steel sheets showing these graphitized area ratios.
It is a figure which shows the average value of (YP), tensile strength (TS), and elongation (EL).

From FIGS. 8 to 13, the range of the secondary annealing conditions in which a high graphitized area ratio can be obtained expands as the C content increases, as in the case of changing the primary annealing conditions. Recognize. Separate annealing (Figs. 8, 10 and 12)
In this case, the range of heating conditions for graphitization is wider than in continuous annealing (FIGS. 9, 11, and 13).

Further, as is clear from the results shown in FIG.
As the graphitized area ratio increases, the tensile strength and yield strength decrease, and the elongation increases. To secure an elongation of 40% or more, it is necessary to set the graphitized area ratio to 50% or more. . To do so, set the soaking temperature during secondary annealing to 650-
It is understood that the set temperature and the soaking time may be appropriately determined depending on the C content within the range of 780 ° C.

Table 2 shows the yield strength (YP) and the tensile strength (T) obtained by the tensile test for the steel sheets having the graphitized area ratio of 50% after the annealing treatment for the steel types A, B and C.
S), elongation (EL) and r value, and yield strength, tensile strength and hardness after quenching and tempering of those steel sheets (all are average values measured twice). Quenching and tempering was performed by soaking the steel sheet in an Ar atmosphere at 880 ° C. for 20 minutes, quenching with oil at 50 ° C., and then tempering at 200 ° C. for 60 minutes.

From these results, all of the steels have a low tensile strength (TS) after annealing, an elongation (EL) of 40% or more and are soft, but have high strength and hardness after quenching and tempering. You can see that

[0052]

Example 2 A steel having the chemical composition shown in Table 3 melted in a vacuum melting furnace was subjected to soaking at 1250 ° C. for 1 hour, and then 12
Hot rolling was started at 00 ° C, and a hot rolled coil with a plate thickness of 5 mm was finished at 850 ° C. The cooling rate after finishing is 20 ℃ / sec, and
It was wound up at 50 ° C. Then, primary annealing and secondary annealing were performed under the conditions shown in Table 4, the graphitized area ratio and mechanical properties of the obtained steel sheet were examined, and the hardness after quenching and tempering was measured. The graphitization area ratio and the mechanical properties were measured in the same manner as in Example 1. In the quenching and tempering treatment, after soaking at 880 ° C for 20 minutes in Ar atmosphere, it was quenched with oil at 50 ° C and tempered at 200 ° C for 30 minutes.

The results of the investigation are shown in Table 4. from this result,
The steel produced by the method of the present invention was soft and had a large elongation after annealing despite having a high C content, and exhibited a high hardness after quenching and tempering.

[0054]

[Table 1]

[0055]

[Table 2]

[0056]

[Table 3 (1)]

[0057]

[Table 3 (2)]

[0058]

[Table 4 (1)]

[0059]

[Table 4 (2)]

[0060]

EFFECTS OF THE INVENTION The present invention provides a method for producing a high carbon steel sheet having good formability by graphitizing 50 area% or more of cementite. The steel sheet according to the method of the present invention is soft, has a large elongation and r-value, and has a high hardness obtained by being subjected to quenching and tempering treatment, and thus is suitable as a material for automobile parts and the like that requires deep drawability and high hardness. Is.

[Brief description of drawings]

FIG. 1 is a diagram showing the effect of primary annealing conditions on the graphitization area ratio of cementite in a steel having a C content of 0.24% (separate annealing in which primary annealing is followed by cooling to room temperature and then secondary annealing).

FIG. 2 is a graph showing the influence of primary annealing conditions on the graphitization area ratio of cementite in a steel having a C content of 0.24% (a continuous type in which secondary annealing is performed after cooling to secondary annealing temperature after primary annealing). Annealing).

FIG. 3 is a diagram showing the influence of primary annealing conditions on the graphitization area ratio of cementite in a steel having a C content of 0.46% (separate annealing).

FIG. 4 is a diagram showing the effect of primary annealing conditions on the graphitization area ratio of cementite in a steel having a C content of 0.46% (continuous type conditions).

FIG. 5 is a diagram showing the effect of primary annealing conditions on the graphitization area ratio of cementite in a steel having a C content of 0.63% (separate annealing).

FIG. 6 is a diagram showing the effect of primary annealing conditions on the graphitization area ratio of cementite in a steel having a C content of 0.63% (continuous type conditions).

FIG. 7 is a diagram showing a relationship between a graphitization area ratio and mechanical properties when primary annealing conditions are changed.

FIG. 8 is a diagram showing the effect of secondary annealing conditions on the graphitization area ratio of cementite in a steel having a C content of 0.24% (separate annealing).

FIG. 9 is a graph showing the effect of secondary annealing conditions on the graphitization area ratio of cementite in a steel having a C content of 0.24% (continuous type condition).

FIG. 10 is a diagram showing the effect of secondary annealing conditions on the graphitization area ratio of cementite in a steel having a C content of 0.46% (separate annealing).

FIG. 11 is a diagram showing the effect of secondary annealing conditions on the graphitization area ratio of cementite in a steel with a C content of 0.46% (continuous condition).

FIG. 12 is a diagram showing the effect of secondary annealing conditions on the graphitization area ratio of cementite in a steel having a C content of 0.63% (separate annealing).

FIG. 13 is a diagram showing the effect of secondary annealing conditions on the graphitization area ratio of cementite in a steel having a C content of 0.63% (continuous condition).

FIG. 14 is a diagram showing a relationship between a graphitization area ratio and mechanical properties when the secondary annealing condition is changed.

Claims (3)

[Claims] 1. By weight%, C: 0.20 to 0.70%, Si: 0.05 to
1.00%, Mn: 0.05 to 0.50%, sol.Al: 0.01 to 1.00%,
N: 0.002-0.010%, B: 0.0003-0.0050%, the balance consisting of Fe and unavoidable impurities, P as an impurity is 0.020% or less, S is 0.010% or less hot rolled steel sheet 780- After holding at 900 ℃ for 0.1-10 hours,
At least 650 to 780 ℃ at a cooling rate of ℃ / h or more, and box annealed at 650 to 780 ℃ for 4 hours or more to graphitize 50 area% or more of cementite in steel. A good method for producing a high carbon steel sheet. 2. By weight%, C: 0.20 to 0.70%, Si: 0.05 to
1.00%, Mn: 0.05 to 0.50%, sol.Al: 0.01 to 1.00%,
N: 0.002-0.010%, B: 0.0003-0.0050%, Ni: 0.05-2.00%, Cu: 0.05-2.00% and Ca: 0.0
A hot-rolled steel sheet containing at least one of 01 to 0.01%, the balance consisting of Fe and unavoidable impurities, with P as impurities of 0.020% or less and S of 0.010% or less at 780 to 900 ° C.
At 0.1 to 10 hours, cool it to at least 650 to 780 ° C at a cooling rate of 20 ° C / h or more, and anneal at 650 to 780 ° C for 4 hours or more to anneal 50% area% or more of cementite in steel into graphite. A method for producing a high-carbon steel sheet having good formability, characterized by: 3. The hot rolled steel sheet is held at 780 to 900 ° C. for 0.1 to 10 hours and then cooled to room temperature at a cooling rate of 20 ° C./h or more,
A method for producing a high carbon steel sheet having good formability according to claim 1 or 2, characterized in that box area annealing is continued at 650 to 780 ° C for 4 hours or more to graphitize 50 area% or more of cementite in the steel. .