JP3166652B2 - Method for producing high carbon thin steel sheet with excellent formability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a high carbon steel strip having excellent workability used for clutch parts, chain parts, office equipment parts, automobile parts, and the like.

[0002]

2. Description of the Related Art A high carbon steel sheet is usually formed into a required shape by pressing, such as cutting, punching, bending, and drawing, and then subjected to a heat treatment such as quenching and tempering to be refined to a required strength. Therefore, it is often applied to the intended use. For this reason, at the stage of a steel sheet product, it is important that it is as soft as possible and has excellent workability. However, since it is premised that the required mechanical properties are obtained by heat treatment such as quenching and tempering, compared to a thin steel sheet applied to a part as it is in a normal press working,
It has a high content and contains many elements for securing hardenability, and it is inevitable that the workability is inferior. But,
Forming into a complex shape by pressing and heat-treating it into a final-use part can be manufactured much more rationally than forming the required shape by cutting or welding. Requirements are increasing. As a means of obtaining the required strength by heat treatment after processing a thin steel sheet, there is a method of forming a low carbon steel sheet having good workability, carburizing and quenching from the surface after forming. This is particularly useful when abrasion resistance of the surface is required. However, since carburizing heat treatment at a high temperature for a long time is required, there are disadvantages such as an increase in heat treatment cost and deformation of a molded product.

[0003] As a means for improving the workability of a high carbon thin steel sheet, spheroidizing annealing of cementite is widely used. The structure of high carbon steel obtained by ordinary hot rolling etc. is a pearlite structure composed of layered cementite and ferrite.However, this cementite is granulated by management of annealing conditions etc., the hardness decreases, Elongation is improved. However, workability, especially elongation, is not always improved depending on the composition and history, and annealing conditions for spheroidization tend to be high in temperature, long in time, and high in processing cost.

[0004] When spheroidizing annealing is performed on a high carbon steel sheet for the purpose of improving workability, there are some more problems. Since spheroidizing annealing requires a long time for soaking, a thin steel plate is coiled and box-annealed. When the thin steel sheet is wound in a coil shape, heat transfer in the direction perpendicular to the direction of the winding axis of the coil is extremely poor. For this reason, in the box annealing method, the heat history of heating and cooling is greatly different between a portion in contact with the furnace atmosphere outside the coil and a portion close to the atmosphere and the vicinity of the center portion of the coil winding layer. As in the case of soft annealing, it is good if the purpose can be achieved if the temperature is sufficiently controlled,
In the spheroidizing annealing, the required temperature range is narrow, and the holding time and the cooling rate also affect the results. Therefore, the variation in the characteristics depending on the coil position increases, and the yield of the portion that passes the target performance deteriorates. This tendency increases as the width and the total weight of the coil increase, and hinders an improvement in productivity. Further, thin steel sheets are required to be beautiful and have good surface properties for plating, painting, etc., and annealing is usually performed in a reducing atmosphere in order to minimize oxidation.
For this reason, decarburization easily occurs due to the atmosphere, and special attention is required for spheroidizing annealing at a high temperature for a long time.

As an example of changing the composition of high carbon steel for the purpose of improving workability, for example, Japanese Patent Application Laid-Open No. 63-100161 discloses a steel strip having a carbon content of 0.2% or more in order to improve cold forgeability. An invention has been proposed in which Mn and Si of steel are reduced, and instead of Cr and B, hardenability is compensated. However,
In the case of strip steel, manufacturing conditions such as rolling are different, heating and cooling rates such as annealing are different, and the deformation of the material during processing is different, so if it is applied to thin steel products as it is, will a significant effect be obtained? It is not clear. In addition, as an example of improving the workability as much as possible with the aim of obtaining high strength by heat treatment, C is set to 0.
Steels that have been reduced to 20 to 0.25, have reduced Si and Mn, and have been added with Cr and B have been introduced (Nissin Steel Engineering Report: Vol. 45 (1981)
Dec), p.30). In this case, although the workability is greatly improved by lowering the C, the carburizing heat treatment is a heat treatment that requires a long time at a high temperature. It is.

One of the present inventors has invented a method for producing a high carbon steel sheet having excellent workability and excellent toughness after heat treatment such as quenching, and is disclosed in JP-A-5-98356 and JP-A-5-98357. It was presented in the gazette. In these cases, the amount of C
It is reduced to 0.4% or less, Mn is increased, and B is added. However, since a large amount of Mn is added, the workability is not expected to be significantly improved, and the heat treatment conditions after working the obtained steel sheet are suggested, but the manufacturing conditions of the steel sheet are not clear. In the case of high carbon steel sheet, its workability is greatly affected by the process until it is made into a steel sheet product, for example, spheroidizing annealing conditions, so simply controlling the component range improved workability. I can't say it. There are several examples of restricting the chemical composition of high carbon thin steel sheets in order to achieve both improvement in workability and securing strength by heat treatment after forming. At present, regarding the improvement of workability of high carbon steel sheets,
It is common to take measures mainly on spheroidizing annealing. However, there are not many examples in which workability is improved by a combination of manufacturing conditions and components other than spheroidizing annealing, and many problems remain regarding not only product performance but also rationalization of a manufacturing method.

[0007]

SUMMARY OF THE INVENTION The present invention is intended to provide a high carbon thin steel sheet which has excellent press workability and has sufficient strength by heat treatment such as quenching and tempering after working and forming. It is intended to provide a method for manufacturing the semiconductor device.

[0008]

As an example of steel having excellent workability and sufficient strength obtained by heat treatment, there is B-added steel used for high-strength bolts in strip steel products.
This steel is required to have good cold forging workability and rolling workability for forming a steel bar into a bolt, and sufficient hardenability to obtain high strength by heat treatment. For this purpose, the workability is improved by lowering the C content to about 0.3%, and the decrease in hardenability due to the reduced C content is compensated for by the addition of B.
Further, the quenching effect is enhanced by using a water-based cooling agent so that sufficient quench hardening is obtained.

For the purpose of improving the workability of the high carbon thin plate and securing sufficient hardness after quenching, various studies are to be made with reference to the B-added steel for bolts in this strip. I made it. In the case of a thin steel plate, since the mass or thickness of a portion cooled by quenching from a high-strength bolt of a bar steel or the like is small, it is advantageous that the cooling rate can be increased without particular consideration of the cooling capacity of the refrigerant.

[0010] Conventionally used high carbon steel sheets which are processed by a press or the like and heat-treated are those having a C content of 0.45% to 0.60%, that is, S45C, unless wear resistance is required such as a cutting tool. Most are carbon steels from to S60C. In parts processing industry to use these steel sheets, it was investigated hardness of as-quenched state of the heat treatment operation after processing, in many cases that is a degree of greater than 50 at H _R C or less. H _R C50 with quenching
Hardness of ~ 53, when sufficient quenching is performed,
It can be easily achieved at a C content of about 0.35%. Nevertheless, adopting steel with a C content of more than 0.45% is
For one thing, it was considered that the effect of increasing the hardenability by increasing the amount of C was expected.

However, when the amount of C increases, the workability of steel deteriorates remarkably. This is because cementite in steel, which increases with the amount of C, is hard and brittle, so that it is likely to be cracked by deformation of the steel and become a starting point of a crack. Spheroidizing annealing is performed to suppress the risk of starting the crack. In other words, the conventional method of using high carbon steel sheets is
Since a stable hardenability can be easily secured in the heat treatment after processing, a steel containing an excessive amount of C is used, and a steel having a high C content is inferior in workability, so that sufficient spheroidizing annealing is required. I thought it was.

Therefore, it is considered that if sufficient hardenability can be ensured by other means, the C content can be reduced and workability can be improved. Therefore, attention was paid to B addition.
B has a significant hardenability improving effect with a small amount compared to other elements, and hardly affects the workability of the annealed material. In addition, the effect of improving the hardenability hardly depends on the added amount, but is strongly influenced by the C content, and is larger as the C amount is smaller. This is extremely convenient for the purpose of improving workability by lowering the amount of C and compensating for a decrease in hardenability due to the reduction of the amount of C. Therefore, further investigations were made on the application conditions when using B-added steel with a reduced C content as a basic condition and using it as a high carbon thin steel sheet.

As a result of confirming the effect of the addition of B when the C content is 0.35% for a plate having a thickness of 3 mm, sufficient hardness can be obtained as-quenched.
It was confirmed that even though the Mn content was much larger than C and the specified Mn amount was reduced to about 1/2, the value was still sufficiently large. When the content of Mn increases, the base material becomes harder. Therefore, reducing the content of Mn is preferable for improving the cold workability. However, as long as the workability is not deteriorated, the hardenability is higher, so that a stable heat treatment can be performed. Therefore, when Mn is reduced, Cr is added to some extent.

When the hardenability is improved by adding B,
The presence of solute N greatly reduces its effect. When Al is added, N can be fixed, but the effect of adding B becomes unstable when the content is insufficient or depending on the heat history. This is because Al may not be able to sufficiently fix N, and a small amount of Ti, which has much stronger bonding force with N than Al, is generally used. Also in this thin steel plate, it was confirmed that a stable effect of improving the hardenability of B can be obtained by adding a small amount of Ti corresponding to the chemical equivalent of N mixed as an impurity.

However, it has been found that after the quenching of the pressed molded product, the austenite grains are coarsened in portions having a small bending radius or portions subjected to high strength processing, such as near the cut surface of the shearing process. The coarsening of austenite grains significantly deteriorates the toughness of the product. There are two possible reasons for this coarsening. One is that strong processing causes more strain energy to accumulate in the part than others,
This is because when the part is recrystallized by heating, the released energy becomes a driving force for crystal grain growth and the grains become coarse.

Another is due to the disappearance of the grain growth inhibiting effect. When Al is added for the purpose of deoxidation, Al remains in the steel and combines with N existing in the steel as an impurity to form Al.
Form N. Al and N dissolved in the steel in the slab heating temperature range are combined in hot rolling and the subsequent process, and are dispersed and precipitated as fine AlN particles in the steel. This fine AlN has the effect of strongly inhibiting the growth of austenite grains. Therefore, steel deoxidized with Al has been known as a fine-grained steel for some time, and it is considered that the coarse graining of the strongly worked portion as described above did not appear if fine AlN precipitates were present. . On the other hand, when Ti is added for the purpose of sufficiently fixing N, Ti has a much stronger bonding force with N than Al, so that Ti is added in the temperature range immediately after solidification.
N is formed. TiN formed at a high temperature becomes a relatively coarse precipitate, and the effect of suppressing austenite crystal grain growth is weak. That is, even if Al is contained, there is no solid solution N to be bonded in the hot rolling temperature range, so that fine AlN is not formed and growth of austenite grains is not suppressed.

Therefore, as a result of conducting various studies for the purpose of suppressing coarsening of austenite grains in the strongly worked portion, it was found that it is effective to contain a relatively large amount of Ti. From the stability and solubility product of the compound, the Ti added to the steel first binds to N as described above,
It is presumed to form iN and then combine with S. On top of that, if there is further excess Ti, it reacts with C to form TiC.
Generate. Although depending on the amount of Ti and the amount of C, TiC may form a solid solution to some extent in the slab heating stage of hot rolling, and may precipitate finely in the subsequent process, and may have an effect of suppressing coarsening of austenite grains. Conceivable. However, the presence of an excessive amount greatly deteriorates the processability, so that sufficient attention must be paid to the added amount.

The high carbon thin steel sheet thus obtained is
Conventional S45C-S
The workability was better than that of a 60C thin steel sheet, and the steel had sufficient hardenability. In order to further improve this good workability, further studies were made on hot rolling and annealing conditions suitable for this steel. As a result, in the hot rolling, the finishing temperature is brought as close as possible to the Ac ₃ point, and in the annealing after winding and pickling, the material is heated to a temperature exceeding the Ac ₁ point, so that it is softer and more stretchable. It was found that an excellent high carbon steel sheet could be obtained. Since these steels have a slightly lower C content, the spheroidization ratio determined by microscopic observation is not necessarily sufficient, but is superior to a sufficiently spheroidized thin steel sheet having a larger carbon content. Has workability.

By rolling to a required thickness in the hot rolling step and performing the above-described annealing, a thin steel sheet having sufficient strength and toughness can be obtained by heat treatment with excellent workability.
However, in order to meet the requirements for thinner steel sheets, steel sheets that require more accurate thickness, or steel sheets that require strict surface properties, cold rolling and annealing are also excellent in workability and after heat treatment. A steel sheet with sufficient strength can be manufactured.

When improving workability by spheroidizing annealing,
In particular, as the coil shape becomes wider and larger by annealing, the difference in spheroidization between coil portions becomes larger, and the variation in characteristics becomes larger. On the other hand, in the steel having the chemical composition according to the method of the present invention, the variation in mechanical properties due to the difference in annealing conditions is small, and even in a large coil, the variation in properties depending on the location is small.

Based on the above examination results, the allowable range of variation of each manufacturing condition factor was clarified, and the present invention was completed. The gist of the present invention is as follows.

(1) By weight%, C: 0.25 to 0.45%, S
i: 0.05% or less, Mn: 0.2 to 0.5%, Cr: 0.05 to 0.50
%, B: 0.0005 to 0.0050%, sol. Al: 0.01 to 0.10%,
N: 0.008% or less, S: 0.015% or less, Ti content
A high carbon steel which is 0.06% or less and satisfies the following formula, and the balance substantially consists of Fe and unavoidable impurities, with finishing temperature in the range of (Ar ₃ points + 20 ° C.) to (Ar ₃ points + 50 ° C.) After hot rolling, winding in a temperature range of 550 to 600 ° C., pickling, and then in an atmosphere containing 95% by volume or more of hydrogen, Ac ₁ point to (A
c ₁ point + 30 ° C) After soaking for 1 to 10 hours in the temperature range,
A method for producing a high carbon steel sheet, comprising annealing at a cooling rate of 20 ° C./h to a temperature lower than (Ar ₁ point-50 ° C.).

Ti (%) − [(48/14) × N (%) + (48/32) × S (%)] ≧ 0.005 (2) C: 0.25 to 0.45% by weight% , Si: 0.05% or less, Mn: 0.2 to 0.5%, Cr: 0.05 to 0.50%, B: 0.00
05 to 0.0050%, sol. Al: 0.01 to 0.10%, N: 0.008%
Hereinafter, S: 0.015% or less, a high carbon steel having a Ti content of 0.06% or less and satisfying the above formula and the balance substantially consisting of Fe and unavoidable impurities, was prepared by setting the finishing temperature to (A
hot rolling a range of r ₃ point + 20 ℃) ~ (Ar ₃ point + 50 ° C.), wound at a temperature range of 550 to 600 ° C., after pickling, Ac in an atmosphere containing hydrogen 95% by volume or more ₁ Point ~ (Ac ₁ point + 30
C) in a temperature range of 1 to 10 hours, and then 3 to 20 ° C / h
Annealing at a cooling rate of less than (Ar ₁ point -50 ° C), and then cold rolling, 650 ° C
A method for producing a high-carbon cold-rolled steel sheet, comprising performing annealing at a temperature range of (Ac ₁ point −10 ° C.) for 2 hours or more.

In any of the manufacturing methods, the effect of the method of the present invention is not impaired even if a light temper rolling is performed after the final annealing for the purpose of correcting the shape or suppressing the stretcher strain.

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION

 (1) Chemical composition of steel "%" in the following chemical composition is all percentages by weight.

The content range of C is set to 0.25 to 0.45%. When the content of C is less than 0.25%, hardness after quenching becomes insufficient. On the other hand, when the C content exceeds 0.45%, the workability is not improved even if sufficient annealing is performed, and the object of the present invention cannot be achieved. In particular, when B is added, the hardenability can be effectively improved as the C content is reduced.

The content of Si is 0.15 to 0.35% in JIS standard G-3311 relating to high carbon steel sheets, and is added for the purpose of deoxidation. However, in the case of the present invention, since deoxidation is mainly performed with Al, it is not particularly necessary to contain it. Rather, Si not only increases the hardness of the annealed steel sheet and deteriorates the workability, but also deteriorates the surface properties as a thin steel sheet product.
As a limit that does not have such an effect, the content
0.05% or less.

The content range of Mn is set to 0.2 to 0.5%. Mn
Is contained to prevent hot brittleness due to S, one of the inevitable impurities, and to ensure hardenability. For that purpose, a content of at least 0.2% or more is required. The hardenability is greatly improved depending on the content, and the cost is low, so it is desirable to add as much as possible. However, it is an element that hardens the steel base, and when the content increases, the workability deteriorates.
Up to 5%.

Cr is a hardenability improving element similar to Mn, but does not harden the substrate as much as Mn. Further, in order to improve the hardenability, it is more effective to combine a plurality of elements than to add a large amount of only one kind of element.
It is contained in the range of 0.50%. If the content of Cr is less than 0.05, the effect of improving hardenability is small, but if the content is large, not only does the ductility deteriorate, but also the surface treatment properties required for thin steel products such as plating and painting. Is degraded, so it should be at most 0.50%.

B can be obtained without impairing the workability of the annealed material.
Hardenability can be greatly improved. The content is
If it is less than 0.0005%, the effect is not sufficient, but if it is more than this, almost constant hardenability improvement effect can be obtained regardless of the amount. However, if it exceeds 0.005%, the steel may be embrittled. Therefore, the range of the content is 0.0005 to 0.0050
%.

Ti is added for the purpose of fixing solid solution N in steel and suppressing growth of austenite crystal grains. The solute N easily bonds with B, and reduces the effect of improving the hardenability of B. Therefore, Ti is added to form TiN, and N in a solid solution state is excluded. In addition, it produces fine TiC and also has an effect of suppressing austenite crystal grain growth during heating. Since Ti in steel has a higher affinity for N and S than C and usually binds to them first, it is necessary to contain a minimum amount of Ti that can generate TiC. Therefore, Ti needs to be contained in an amount that satisfies at least the following equation.

Ti (%) − [(48/14) × N (%) + (48/32) × S (%)] ≧ 0.005 However, if too large, excessive TiC precipitates are generated. Since the workability is deteriorated and the toughness after the heat treatment is significantly deteriorated, its content is limited to at most 0.06%.

Sol.Al (acid-soluble Al) is contained as a result of molten steel deoxidation to maintain the soundness of the slab.
Usually, it has an effect of forming fine precipitates of AlN by combining with N in steel and suppressing coarsening of austenite crystal grains. In the case of the method of the present invention, Al is not formed because N is almost completely fixed by Ti. However, as a result of sufficiently deoxidizing the molten steel and further improving the yield of added Ti and B as much as possible, the content of 0.01% or more is essential. However, if the content increases, the workability is impaired and the surface properties deteriorate, so the content should be 0.10% or less. That is, the content of sol.Al is set to 0.01 to 0.10%.

The elements which are mixed as unavoidable impurities generally impair the workability, so the smaller the better, the better. Among them, N and S generate precipitates which impair workability and increase the required amount of Ti. Therefore, the smaller the content, the better. The content of N is set to 0.008% or less as a limit at which the generated precipitate does not affect the workability and the toughness of the heat-treated product. S may be desirable to contain S to some extent because it has the effect of improving machinability in the case of processed and formed products whose cutting is the main, such as strip steel products, but press forming is mainly used for thin steel sheets. The limit is not more than 0.015%, at which the effect on workability of the steel does not become apparent.

(2) Steel Sheet Manufacturing Conditions The high-carbon thin steel sheet may be hot-rolled to a required thickness, or may be further cold-rolled after hot rolling to finish to a thinner thickness. .

In steel having a chemical composition determined by the method of the present invention,
After hot rolling, by pickling and annealing, a steel sheet with excellent workability can be obtained, but in that case, the workability such as elongation of the steel sheet after annealing depends on the finishing temperature of hot rolling. receive. Therefore, the finishing temperature of hot rolling is set to (Ar ₃
(Point + 20 ° C) to (Ar ₃ points + 50 ° C). The hot rolling is preferably finished at a temperature as low as possible in the austenite region. This is because the higher the finishing temperature, the larger the crystal grains, the coarser the pearlite becomes, and the morphology does not change significantly even after annealing,
Poor workability. Therefore, the upper limit temperature of the finishing is set to Ar ₃ points + 50 ° C. Further, there is a fear that below the transformation temperature is too low, Sonaruto since rolling control Kitaichi becomes difficult to increase the deformation resistance, the temperature of the lower limit Ar ₃
Up to the point + 20 ° C.

The coiling temperature for hot rolling is in the range of 550 to 600 ° C. If it is wound at a temperature exceeding 600 ° C., the pearlite lamellar becomes coarse, the shape does not change much during annealing, and the workability is not improved. Also, at a winding temperature of less than 550 ° C., there is a possibility that a bainite structure may be formed depending on the position of the coil.

Annealing is performed by pickling after hot rolling, and from Ac ₁ point to (A
c ₁ point + 30 ° C) soak for 1 to 10 hours in a temperature range of 3 to 20 ° C /
At a cooling rate of h, the temperature is gradually cooled until the temperature reaches a temperature lower than Ar ₁ point −50 ° C. After that, the temperature is lowered to room temperature by cooling or forced cooling. The annealing for heating to a temperature of _one or more Ac is performed because the elongation is greatly improved. However, if the temperature becomes too high and the temperature exceeds Ac ₁ point + 30 ° C., the solid solution of cementite re-dissolves too much,
After cooling, pearlite appears again and deteriorates elongation. If the soaking time is less than 1 hour, sufficient softening cannot be obtained, and if it exceeds 10 hours, the ductility is not further improved and heating energy is wasted. The rate of cooling after soaking, especially when passing through the Ar ₁ transformation point, should be as slow as possible because if it is too fast, pearlite will be generated and solid solution C will be insufficient to reprecipitate and elongation will deteriorate. Is preferred. However, if it is too slow, the efficiency will only be reduced, so the range of the cooling rate is 3 to 20 ° C./h. Further, in order to sufficiently precipitate solid solution C and improve elongation, the cooling is carried out slowly at this cooling rate until Ar ₁ point-50 ° C.

To meet the demand for a thinner steel sheet, a steel sheet requiring a higher precision sheet thickness, or a steel sheet having strict surface properties, the above-described annealed hot-rolled steel sheet is further added. Cold rolling and annealing can also produce a steel sheet having excellent workability and having sufficient strength after heat treatment. In this case, the hot rolling conditions and the annealing conditions after the hot rolling are as described above, but the slow cooling after the soaking of the annealing may be up to an Ar ₁ point to −20 ° C. this is,
This is because cold rolling is sufficiently possible even if a large amount of solid solution C remains before the cold rolling, and the precipitation of solid solution C further proceeds in the subsequent process such as annealing.

The rolling reduction of the cold rolling is not particularly limited as long as the required thickness and the shape can be sufficiently controlled.
It may be about 30-70%. If necessary, intermediate annealing may be performed during the cold rolling. The intermediate annealing or the final annealing is performed in a temperature range of 650 ° C. to (Ac ₁ point−10 ° C.). This is because at temperatures lower than 650 ° C, softening is insufficient and workability is insufficient, whereas at temperatures exceeding ₁ point Ac, coarse pearlite is easily formed after cold rolling, solute C is increased, and workability is greatly increased. It is because it is deteriorated. Therefore, the upper limit is set to Ac ₁ point-10 ° C. in order to prevent overheating. The soaking time requires at least 2 hours for sufficient softening, but if the holding time is longer than this, the effect of further improvement is small, so it may be appropriately selected.

The inert atmosphere in the furnace at the time of annealing the hot-rolled steel sheet and the cold-rolled steel sheet is a gas containing little nitrogen gas and having a dew point as low as possible and containing 95% by volume or more of hydrogen. The purpose of annealing in a hydrogen atmosphere is that hydrogen gas has a good thermal conductivity, so that when a coil of a thin steel sheet is annealed, a temperature deviation depending on a portion can be greatly reduced. In addition, since a thin steel sheet extremely dislikes oxidation and dirt on the surface, it is preferable to use hydrogen gas also from this point. The reason for avoiding the mixing of nitrogen gas is that, in the case of steel containing Al, if the surface is cleaned and heated to a high temperature in an atmosphere containing hydrogen and nitrogen, nitrogen absorption may occur, and solute Al may be changed to AlN. ,
The higher the annealing temperature, the greater the risk. In particular, in the case of a thin steel sheet, since the ratio of the surface area to the volume is large, almost all of the Al in the steel is converted to AlN, and the elongation may be significantly deteriorated. Also, the reason why the dew point is set low is to suppress decarburization from the surface during annealing.

In the description of the present invention, the transformation temperature is indicated as Ac ₁ or Ac ₃ when the temperature is raised, and Ar ₁ or Ar ₃ when the temperature is lowered. The transformation temperature varies depending on the chemical composition of the steel and the heating or cooling rate. Strictly, it is necessary to measure the steel by transforming it at the same heating or cooling rate. However, in reality, since the heating or cooling speed in the step that affects the effect of the present invention is not large, the difference between the transformation temperatures of heating and cooling may be small and substantially the same. Therefore, it is assumed that Ac ₁ and Ar ₁ are the same as A ₁ , and that Ac ₃ and Ar ₃ are the same as A ₃ , and are estimated from the chemical composition using the following equation. A ₁ (° C.) = 723 + 29.1Si (%) − 10.7
Mn (%) + 16.9Cr (% ) ······ A 3 (℃) = 881-205.7C (%) + 53.1Si (%) - 15.0Mn (%) - 0.7Cr (%) ··· ...

[0043]

【Example】

[Example 1] Steel having the chemical composition shown in Table 1 was melted into a slab, heated to 1200 ° C, and then hot-rolled to a width of 91 mm.
The resulting coil was finished into a hot-rolled thin steel sheet having a thickness of 0 mm and a thickness of 4 mm, and the obtained coil was pickled and box-annealed in a hydrogen atmosphere. Table 2 shows the finishing temperature of hot rolling, winding temperature, soaking temperature of box annealing, time,
The slow cooling rate from the soaking temperature and the slow cooling end temperature are shown respectively. In the furnace at the time of annealing, after air is sufficiently replaced with nitrogen gas, hydrogen gas with a dew point of -50 ° C or less and purity of 99% or more is introduced and further replaced, and a complete hydrogen atmosphere is obtained at 600 ° C or more. I did it.

[0044]

[Table 1]

[0045]

[Table 2]

With respect to the annealed steel sheet, JIS 13 parallel to the rolling direction at a position approximately 25 mm from the center and the edge of the sheet.
A tensile test piece of No. B was collected and measured for strength and elongation.

Table 2 also shows these results.

Steel No. 12 has a chemical composition equivalent to S50C which is usually used, and Test No. 18 in Table 2 shows tensile test values of a steel plate obtained from this steel. Test numbers 1, 3, 4, 6, 8, 9, 11, 12, and 13 that fall within the range defined by the present invention.
By comparing the results, it is clear that the effect of the method of the present invention is obvious, and the elongation is remarkably large and the workability is excellent. This largely depends on the effect of lowering the amount of C. Even if the workability is improved by simply reducing the C content, the hardenability is often insufficient, but the steel sheet according to the method of the present invention has sufficient hardenability as a thin steel sheet. In addition, comparison of test numbers 3 and 4, and 2 and 5, or test numbers 8 and 9 and 7
As can be seen from the comparison with Examples 10 and 10, even if the steel has a chemical composition falling within the range of the present invention, it is not sufficient if the hot rolling conditions or the annealing conditions of the hot-rolled sheet are insufficient. It is clear that a steel sheet with even better elongation can be obtained within the range defined by.

Example 2 Steel numbers 1 to 3 shown in Table 1
Using a slab having a composition of 6, 9, 11, 12 and 14, hot rolling was performed at a finishing temperature of 780 to 890 ° C and a winding temperature of 580 ° C to finish a hot-rolled steel coil having a width of 910 mm and a thickness of 3 mm. The obtained coil was pickled, annealed under different conditions, and then
%, And cold-rolled to a thickness of 1 mm. The cold-rolled steel sheet was annealed at 680 ° C. for 4 hours and then subjected to 0.5% temper rolling. The annealing atmosphere was an atmosphere containing 95% or more of hydrogen in both the hot-rolled steel sheet and the cold-rolled steel sheet.
About the obtained steel sheet,
A tensile test piece of JIS No. 13B was sampled at a position of 25 mm parallel to the rolling direction, and the strength and elongation were measured.

Table 3 shows the annealing conditions applied to the hot-rolled steel sheet and the measurement results of the strength and elongation of the obtained cold-rolled steel sheet.

[0051]

[Table 3]

Next, from these cold-rolled steel sheets, a width of 30 mm,
Cut out a test piece of 100mm length, inner radius r = plate thickness,
After bending at an angle of 180 °, it was heated at 880 ° C for 30 minutes and quenched in oil kept at 80 ° C. The hardness in the as-quenched state and the austenite grain size at the bent portion were investigated. The results are shown in Table 3.

In the same manner as in Example 1, the test number 33 using the steel number 12 was used.
Is a steel plate corresponding to conventional S50C. In comparison with this, when the case where the chemical composition and the manufacturing conditions fall within the ranges defined by the present invention, a steel sheet with excellent elongation is obtained even after cold rolling and annealing. Also, when the C content exceeds 0.3%, the hardness as quenched is higher than H _R C50, and it can be seen that there is no grain growth by heating even under severe processing. Further, when looking at the tensile test values of the central portion and the end portion of the steel sheet, the difference of the present invention example is smaller than that of the test sample No. 33, and the variation due to the position in the coil annealing is greatly reduced.

[0054]

[Effect of the Invention] After processing by press molding and the like,
If the method of the present invention is used for producing a high carbon thin steel sheet subjected to heat treatment such as quenching and tempering, a steel sheet having excellent press workability and having sufficient strength can be easily obtained by heat treatment after forming.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C21D 9/46-9/48 C21D 38/00-38/60

Claims (2)

(57) [Claims] (1) In terms of% by weight, C: 0.25 to 0.45%, Si: 0.
05% or less, Mn: 0.2-0.5%, Cr: 0.05-0.50%,
B: 0.0005 to 0.0050%, sol. Al: 0.01 to 0.10%, N:
0.008% or less, S: 0.015% or less, Ti content is 0.06%
% Or less and the following formula is satisfied, and the balance is substantially Fe
And high-carbon steel consisting of unavoidable impurities is hot-rolled at a finishing temperature of (Ar ₃ points + 20 ° C.) to (Ar ₃ points + 50 ° C.), wound up in a temperature range of 550 to 600 ° C., and acidified. After washing, in an atmosphere containing 95% by volume or more of hydrogen, Ac ₁ point to (Ac ₁
3 to 20 hours after soaking for 1 to 10 hours in the temperature range of
A method for producing a high carbon steel sheet, comprising annealing at a cooling rate of ° C / h to a temperature lower than (Ar ₁ point-50 ° C). Ti (%) − [(48/14) × N (%) + (48/32) × S (%)] ≧ 0.005 ・ ・ ・ 2. C .: 0.25 to 0.45% by weight, Si: 0.
05% or less, Mn: 0.2-0.5%, Cr: 0.05-0.50%,
B: 0.0005 to 0.0050%, sol. Al: 0.01 to 0.10%, N:
0.008% or less, S: 0.015% or less, Ti content is 0.06%
% Or less and the following formula is satisfied, and the balance is substantially Fe
And high-carbon steel consisting of unavoidable impurities is hot-rolled at a finishing temperature of (Ar ₃ points + 20 ° C.) to (Ar ₃ points + 50 ° C.), wound up in a temperature range of 550 to 600 ° C., and acidified. After washing, in an atmosphere containing 95% by volume or more of hydrogen, Ac ₁ point to (Ac ₁
3 to 20 hours after soaking for 1 to 10 hours in the temperature range of
After annealing at a cooling rate of ° C./h to gradually lower the temperature to below (Ar ₁ point-50 ° C.), cold rolling is performed, and 650 ° C.
A method for producing a high-carbon cold-rolled steel sheet, comprising annealing at a temperature in a temperature range of _{1 to} 10 ° C for 2 hours or more. Ti (%) − [(48/14) × N (%) + (48/32) × S (%)] ≧ 0.005 ・ ・ ・