JP2001073033A - Production of medium-high carbon steel sheet excellent in local ductility - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a medium-high carbon steel sheet excellent in local ductility. SOLUTION: At the time of subjecting a hot rolled steel sheet contg. 0.10 to 0.60 mass % C to annealing utilizing heating of >=Ac1 point, in the finishing stage of the heating of >=Ac1 point, its metallic structure is made into the one in which the amt. of the α/γ boundaries per γ unit area is >=0.5 μm/μm2, and after that, it is cooled to the temp. of <=Ar1 point at a rate of <=50 deg.C/h. Or, upon the end of the heating in the above annealing, its metallic structure is made into the one in which the number of unmelted carbides per 100 μm2 is >=1 piece, and the amt. of the α/γ boundaries per γ unit area is >=0.3 μm/μm2, and after that, it is cooled to the temp. of <=Ar1 point at a rate of <=50 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 producing a medium and high carbon steel sheet having excellent local ductility.

[0002]

2. Description of the Related Art Steel has a C content of about 0.10 to 0.60% by mass.
The so-called medium- and high-carbon steel sheets can be hardened and strengthened and have some workability in the annealed state before quenching, so they are widely used as materials for automobile parts, various machine parts, and bearing parts. Have been. In manufacturing parts, punching and bending are generally performed, and relatively light drawing and stretch flange forming are sometimes performed. Also, if the part shape is complicated,
Often manufactured by welding two or three parts. These processed parts are processed into various parts through heat treatment.

However, in recent years, in order to reduce the manufacturing cost of parts, integrated molding of parts and simplification of the steps of processing parts 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 workability has been required for the medium and high carbon steel sheets as raw materials. In particular, in recent years, there is an increasing need for a steel sheet material that can withstand not only punching and bending but also advanced processing that requires local ductility such as stretch flange forming (for example, hole expanding).

[0004] Under these circumstances, Japanese Patent Publication No. 61-15930, Japanese Patent Publication No. 5-70685, and Japanese Patent Application Laid-Open No. 4-333527 disclose a method of forming carbides in a steel bar into spheroids by devising a processing method or a heat treatment method. A technique for improving the workability of a steel bar is introduced. However, these methods are all directed to bar steel wire rods, and a method of improving stretch flangeability and hole expandability, which is a problem when the raw material is a plate material, has not been clarified.

Japanese Patent Application Laid-Open No. 8-3687 discloses that C is 0.3
High-carbon steel sheets for processing containing at least 20% by mass, having an area ratio of carbide of not more than 20%, and having at least 30% of carbide having a particle size of 1.5 μm or more are shown. Set the outlet temperature to 750 to 810 ° C, cool at 10 ° C / sec or less, and reduce the difference between the finishing temperature and coil winding temperature to 300 ° C.
The following discloses a method of winding, subjecting to spheroidizing annealing at 720 ° C. for 20 hours, cooling to 100 ° C. at a cooling rate of 26 ° C./Hr, and then air cooling to room temperature. However, although this technique is intended to improve the workability of a steel sheet, a method for improving advanced workability requiring local ductility such as stretch flangeability has not been clarified. In addition, in order to coarsen the carbide particle diameter to 1.5 μm or more, it takes a long time to sufficiently form a solid solution of carbon by heating in a γ temperature range in a quenching process after processing a part. For this reason, it becomes difficult to apply a quenching treatment by short-time heating such as induction hardening.

Further, JP-A-8-120405 discloses that C:
In addition to 0.20 to 0.60%, it contains graphitizing elements such as Si, Al, N, B, Ca, etc., and 10 to 50% of the C content is graphitized, and the steel structure of the cross section is 3 μm or more. A thin steel sheet excellent in workability, which is a ferrite phase in which spheroidized cementite containing a specific amount of graphite particles is dispersed is shown. As its manufacturing method, hot rolling at a finishing temperature of 750 to 900 ° C,
It shows a method of winding at 450 to 650 ° C, annealing at 600 to 720 ° C after cold rolling, and the like. The steel sheet is said to be excellent in hole expandability and secondary workability. However, since it utilizes graphitization of carbon contained, it is necessary to add an element which promotes graphitization, and it cannot be widely applied to general commercially available medium and high carbon steel grades. In addition, the presence of coarse graphite particles of 3 μm or more delays sufficient solid solution of carbon in the heating of the quenching treatment after processing the parts, making it difficult to apply the quenching treatment by heating for a short time.

[0007]

As described above, despite the high needs for medium- and high-carbon steel sheets with particularly improved "stretch flangeability" among workability, general medium- and high-carbon steel types Methods for improving these properties in steel sheet materials have not yet been clarified. In addition, when workability is emphasized, hardenability after working must be sacrificed to some extent at present. Therefore, the present invention can stably improve the stretch flangeability, which has recently been particularly regarded as important, in a general medium- and high-carbon steel grade without adding a special element, and after the component processing. It is an object of the present invention to provide a method for manufacturing a medium- and high-carbon steel sheet material capable of sufficiently securing the hardenability of steel.

[0008]

SUMMARY OF THE INVENTION The object of the present invention is to anneal a steel sheet containing 0.10 to 0.60 mass% of C by using heating at _one point or more of Ac. At the end of heating at _one point or more of Ac, the metal structure has an α / γ interface amount of 0.5 μm / μm ² or more per γ unit area, and then 50 ° C./h or less to a temperature of _one point or less of Ar. This can be achieved by cooling at a rate. When performing this annealing, stepwise heating may be performed during the temperature rise, or stepwise holding may be performed as appropriate at a temperature equal to or lower than the Ar ₁ point during the cooling. Alpha / gamma interface per gamma unit area in _one point or more heating end Ac, for example, by quenching the steel sheet after the completion of the heating for more than _one point Ac, by observing the cross-sectional structure of the steel sheet by scanning electron microscopy You can ask.

The invention according to claim 2 is characterized in that: C: 0.10 to 0.60% by mass
Against hot-rolled steel sheets of steel containing, Ac upon subjected to annealing using a _one point or more heating, undissolved number carbides per 100 [mu] m ² at the end of heating stage above Ac ₁ point is 1 or more,
And the amount of α / γ interface per γ unit area is 0.3 μm / μ
This is a method for producing a medium- and high-carbon steel sheet having excellent local ductility, characterized by forming a metal structure of m ² or more and thereafter cooling it to a temperature of ₁ Ar or less at a rate of 50 ° C./h or less.

Here, the Ac ₁ point means the A ₁ transformation point (° C.) of the steel in the temperature increasing process, and the Ar ₁ point means the A ₁ transformation point (° C.) in the temperature decreasing process. The invention according to claim 3 is C: 0.10-
A hot-rolled steel sheet containing 0.60% by mass is subjected to cold rolling at a rolling reduction of 10% or more and then annealed according to the above-mentioned invention. It is.

According to a fourth aspect of the present invention, there is provided a steel in which S in the subject steel is limited to 0.01% by mass or less.

[0012] The invention of claim 5 likewise provides a C:
0.10 to 0.60%, Si: 0 to 0.40% (including no addition), M
n: 0 to 1.0% (including no addition), P: 0.03% or less, S:
0.01% or less, T.Al: 0.1% or less, the balance being steel composed of Fe and unavoidable impurities. The invention of claim 6 further includes Cr: 0 to 1.6% (including no addition). ), Mo: 0 to 0.3% (including no addition), Cu: 0 to 0.3%
% (Including no addition), Ni: 0 to 2.0% (including no addition)
Is a steel containing The invention of claim 7 further includes Ti: 0.01-0.05%, B: 0.0005-0.0050.
%, N: steel containing 0.01% or less. Here, the lower limit of 0% of Si, Cr, Mo, Cu, Ni means that the element is not added. For example, claim 6
As an example of steel targeted in the above, among these elements, S
i, Cr and Mo alone within the specified range, and other Cu,
Steel to which Ni is not added is exemplified.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have studied in detail means for improving the workability of general medium and high carbon steels. As a result, even when general punching and bending workability is improved, the local ductility is not necessarily improved, and simply improving the local spheroidization of carbides cannot achieve a stable improvement in local ductility. Further, it has been found that the local ductility greatly depends on the dispersion form of carbides in the steel sheet, and specifically, can be improved by further spheroidizing the carbides and increasing the average inter-carbide distance.

There are many unclear points at this time as to why the improvement behavior of the local ductility does not always coincide with the behavior of other workability. The following may be considered. That is, local ductility is a property that is evaluated by the generally hole expansion test, specifically, for example, will expand pushing the hole and push the punch into the hole of diameter d ₀ which previously provided the steel plate thickness to hole edge The hole diameter d when a crack penetrating through is generated is measured, and the hole expansion ratio (d−d ₀ ) / d ₀ at that time is evaluated. Since the hole expansion ratio means the nominal value of the circumferential strain when a crack penetrates the plate thickness at the hole edge, the local ductility is the circumferential direction when the necking or cracking of the hole edge starts to occur This is a characteristic that can be evaluated based on the limit value of strain, which indicates the formability when locally high stress is concentrated. It is considered that such necking and cracking are sensitively caused by very local defects generated during working deformation. In the medium- and high-carbon steel sheets, growth (connection) of microvoids generated from carbide (cementite) as a starting point is cited as a cause of generation of such defects. For this reason, in order to improve the local ductility of the middle and high carbon steel sheets, it is considered important to have a metal structure that minimizes the generation and growth of the microvoids during working deformation. The reason that local ductility behaves differently from other general workability is that microscopic defects that do not affect other workability are sensitive to local ductility. Inferred.

Based on such considerations, if the average inter-carbide distance in the steel sheet can be increased, the connection of microvoids generated from individual carbides can be suppressed, and an improvement in local ductility can be expected. According to the study of the present inventors, it has been confirmed that there is actually a close relationship between the average inter-carbide distance and the local ductility. On the other hand, it was found that increasing the spheroidization rate of individual carbides was also effective in suppressing the formation of microvoids. The present inventors have repeated detailed experiments on a method for achieving such a metal structure, without adding a special element, and within a range that does not inhibit hardenability after processing, and according to the present invention. We have come up with a manufacturing method. Hereinafter, matters for specifying the present invention will be described.

The present invention is directed to a hypoeutectoid steel containing 0.10 to 0.60% by mass of C. 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. C content is 0.10
When the amount of the hypoeutectoid steel is equal to or less than mass%, sufficient quenching hardness cannot be obtained for application to various machine structural parts. On the other hand, if the C content exceeds 0.60% by mass, the toughness after hot rolling is reduced and the productivity and handleability of the steel strip deteriorates, and sufficient ductility cannot be obtained even after annealing. It becomes difficult to apply to parts with high degree. When the C content is close to the eutectoid composition, the α phase does not exist in heating at _one point or more of Ac, so that the annealing specified in the present invention cannot be performed. Therefore, in the present invention, from the viewpoint of providing a material steel sheet having both appropriate quenching hardness and workability, the C content is reduced.
For steel in the range of 0.10 to 0.60 mass%.

S is an element forming MnS-based inclusions. If the amount of the inclusions increases, the local ductility deteriorates. Therefore, it is desirable to reduce the S content in steel as much as possible. In the present invention, the effect of improving the local ductility can be obtained even for general commercial steel in which the S content is not particularly reduced. However, even when the C content is increased to nearly 0.60% by mass, the Elv value and the λ value to be described later are, for example, 45% respectively.
As described above, in order to stably secure a high local ductility of 55% or more, it is desirable to use steel in which the S content is reduced to 0.01% by mass or less.

P deteriorates ductility and toughness.
The content is desirably not more than mass%. Al is added as a deoxidizer for molten steel, but the amount of T.Al in the steel is 0.1%.
If the content exceeds 90% by mass, the cleanliness of the steel is impaired and surface flaws are easily generated on the steel plate. Therefore, the T.Al content is desirably 0.1% by mass or less.

Si is one of the elements having a large effect on local ductility. If Si is added excessively, the ferrite hardens due to the solid solution strengthening action, which causes cracking during molding. Further, when the Si content increases, a scale flaw tends to occur on the surface of the steel sheet during the manufacturing process, which causes a decrease in surface quality. Therefore, when adding Si, 0.40 mass%
The content should be as follows. In applications where workability is particularly important, the Si content is desirably 0.1% by mass or less. Mn is an additive element effective for improving the wear resistance of a steel sheet. If it is contained in a large amount exceeding 1.0% by mass, the ferrite hardens and causes deterioration in workability. Therefore, Mn is 1.0
It is desirable that the content be contained in the range of not more than mass%.

In the present invention, if necessary, Cr, M
Steels with improved properties by adding elements such as o, Cu, Ni, Ti, B, and N can be used. Cr is an element that improves hardenability and increases temper softening resistance. However, a large amount of long annealing and Ac ₁ point or more even annealed using heated before quenching hardly softened press formability and workability of Cr is contained the Ac ₁ point below exceeding 1.6 wt% Deteriorates. Therefore, when Cr is added, the content is set to a range of 1.2% by mass or less. Mo is
The addition of a small amount contributes to the improvement of hardenability and temper softening resistance as in the case of Cr. However, a large amount of M exceeding 0.3% by mass
When o is contained, even if it is subjected to long-time annealing at an Ac ₁ point or less or annealing using heating at an Ac ₁ point or more, it is difficult to soften, and the press formability and workability before quenching deteriorate. Therefore, when Mo is added, the content is set to a range of 0.3% by mass or less.

Cu improves the releasability of the oxide scale generated during hot rolling, and is 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 molten metal. Therefore, Cu can be added in a range of 0.3% by mass or less. N
i is an alloy component that improves hardenability and prevents low-temperature brittleness. Also, Ni has an effect of counteracting the adverse effect of molten metal embrittlement, which is a problem due to the addition of Cu,
In particular, when adding about 0.2% or more of Cu, it is extremely effective to add about the same amount of Ni as the added amount of Cu. However, when a large amount of Ni exceeding 2.0% by mass is contained, Ac ₁
Long annealing and Ac press moldability and workability before quenching hardly softened even annealed using ₁ or more points of heat at the point below comes to deteriorate. Therefore, when adding Ni, the content is set to 2.0% by mass or less.

[0022] Ti is a component added to deoxidize molten steel, but exhibits a denitrifying effect. Further, since N dissolved in the steel sheet is fixed as a nitride, the effective B amount for improving hardenability is increased. Further, it has the effect of forming carbonitrides and preventing crystal grains from becoming coarse during quenching. In order to stably obtain these effects, at least 0.01% by mass or more of Ti
Content is required. However, when a large amount of Ti exceeding 0.05% by mass is contained, it is not only economically disadvantageous but also causes deterioration of local ductility. B significantly improves the hardenability of steel with a very small amount of addition. Further, it has an effect of strengthening the grain boundary by reducing the strain energy of the grain boundary. Further, it is a necessary alloy component in order to stably obtain quenching hardness. The effect of B is
The effect becomes significant when the content is 0.0005% by mass or more. However, even if B is added in an amount exceeding 0.0050% by mass, the effect is saturated, and on the contrary, the toughness is deteriorated. N combines with Ti to form Ti
It is an effective component for forming N and refining crystal grains during quenching. However, when the N content exceeds 0.01% by mass, ductility decreases. Excess N is combined with B and consumes an effective amount of B for improving hardenability. Therefore, in the present invention, the upper limit of the N content is set to 0.01% by mass.

According to the present invention, in order to increase the spheroidization ratio of carbides and to obtain a metal structure having a large average inter-carbide distance in a medium- and high-carbon steel sheet, the steel sheet is subjected to annealing at the end of heating at _one or more points of Ac. There is a great feature in that the amount of α / γ interface (metal structure) is limited. Generally, medium and high carbon steel
_When heated to _one or more temperatures, the carbides dissolve and C forms a solid solution in γ. In the subsequent cooling stage, the α / γ interface and the surface of the undissolved carbide existing in the temperature range of _one or more Ac become nucleation sites, and carbide is deposited. When the α / γ interface amount and undissolved carbide are left to some extent in the temperature range of Ac ₁ point or more, the cooling rate is reduced to cool to ₁ point or less of Ar.
C dissolved in γ does not form as pearlite but precipitates as spherical carbides. As a result, the number of carbides in the final annealed structure is smaller than the number of carbides before annealing. A decrease in the number of carbides means an increase in the average intercarbide distance, and the ductility is improved. On the other hand, in the temperature range of _one or more Ac, α /
If the γ interface amount or undissolved carbide is too small, or if the α / γ interface amount or undissolved carbide is left to some extent and the cooling rate is high, then in the course of cooling to a temperature below Ar ₁ point, γ C dissolved therein precipitates as recycled pearlite having a large lamella spacing. As a result, the spheroidization rate of the carbide becomes extremely low, and the local ductility of the steel sheet does not improve.

Therefore, the present inventors conducted various kinds of annealing using heating at _one or more points of Ac, confirmed the workability thereof, and quenched the steel sheet at the end of heating at _one or more points of Ac into oil from its temperature. The cross-sectional structure was confirmed by scanning electron microscope observation. As a result, α / per unit area of γ
When the γ interface amount is 0.5 μm / μm ² or more, or one or more undissolved carbides remain in 100 μm ² of undissolved carbide, and the α / γ interface amount per γ unit area is 0.3 μm /
It was found that a steel sheet having excellent local ductility can be obtained by reducing the subsequent cooling rate in the case of μm ² or more.

[0025] The Ac supercooling degree of the cooling speed is fast γ from the heating temperature at _one point or more is increased, the reproduction pearlite is easily produced. In order to sufficiently suppress the generation of recycled pearlite, the cooling rate needs to be 50 ° C./h or less.
In order to further improve the spheroidization rate of carbides, 30 ° C /
h or less. On the other hand, if the cooling rate is too slow, it takes a long time for cooling. Therefore, considering the productivity, the cooling rate is desirably 5 ° C./h or more.

In general, in a steel obtained by subjecting a hot-rolled steel sheet to cold working before annealing, recrystallization is promoted during annealing due to the introduced working strain, and as a result, compared to a case where cold working was not performed. To obtain a soft material. In addition, the structure is refined by recrystallization during heating or holding at or below Ac ₁ point, α grains and γ grains are reduced by heating at Ac ₁ or more, and a large amount of α / γ interface tends to remain. . further,
The introduced processing strain promotes the fragmentation and spheroidization of the carbide in the pearlite during heating or holding at or below the Ac ₁ point. In order to obtain the effect of promoting recrystallization and breakage and spheroidization of carbides in pearlite due to processing strain, 10
% Of cold rolling is required. As described above, a metal structure for maintaining high local ductility is obtained.

[0027]

EXAMPLES (Example 1) Table 1 shows the chemical composition of the test steel sheet,
It shows Ac ₁ point, Ar ₁ point and quenching hardness. Ac ₁ point and Ar ₁ point is cooled in a fully γ reduction → 10 ° C. / h and held for 10 minutes at a Atsushi Nobori → 900 ° C. The test steel specimens having a diameter of 5 mm × length 10mm at "10 ° C. / h Heating with a heat pattern
Measure the shrinkage / expansion of the specimen while cooling,
It was determined from the change in the expansion curve. The quenching hardness indicates the hardness when the hot-rolled material is kept at 900 ° C., which is at least _one point of Ac, for 5 minutes and then water-quenched. This quenching hardness is a comparison of the original hardness of a steel material obtained by a general quenching treatment, and does not indicate the hardenability after annealing according to the present invention.

[0028]

[Table 1]

In Table 1, Steel A had a low C content of 0.07% by mass, so that the hardness after quenching was low, and the required hardness as a mechanical part could not be obtained. Steels other than steel A in Table 1 were hot-rolled, then pickled, and each steel sheet was annealed under various conditions. At the end of heating at _one or more Ac points during annealing, it was quenched in water, and the metal structure of the L-section was observed with a scanning electron microscope. In this case, by measuring the alpha / gamma interface length and gamma area in the range of 3000 .mu.m ^2, (alpha
/ Γ interface length) / γ area was determined as α / γ interface amount per γ unit area. When undissolved carbides were present, the number of carbides was measured in the same field of view, and 100 μm ²
The number of undissolved carbides was determined. The steel sheet after annealing was subjected to a tensile test, a notch tensile test, and a hole expansion test with a thickness of 2.3 mm to examine workability.

In the tensile test, a JIS No. 5 tensile test piece was used.
The measurement was performed with the distance between the reference points of the parallel portion being 50 mm. The notch tensile test was performed on a JIS No. 5 tensile test specimen with a 45 ° open angle and a 2 mm deep V at both sides in the width direction at the center in the longitudinal direction of the parallel part.
The test was performed by a tensile test using a notched test piece. The elongation percentage with respect to the gauge length of 5 mm including the V notch was determined after breaking, and the elongation percentage was defined as the notch tensile elongation Elv. The hole expansion test is performed by punching a 10 mm (d ₀ ) hole with a clearance of 20% in the center of a 150 mm square steel plate, and then pushing up the hole with a 50 mmφ ball-head punch. The hole diameter d at the time when a crack penetrating the plate thickness occurs is measured, and the hole expansion ratio λ (%) defined by the following equation
I asked. λ = (d−d ₀ ) / d ₀ × 100 These Elv value and λ value are indexes indicating local ductility, and can quantitatively evaluate stretch flangeability.
Table 2 shows the test results together with the annealing conditions.

[0031]

[Table 2]

The G steel having a C content of more than 0.60% by mass has an annealing condition within the range specified in the present invention, has no α in the cross-sectional structure at _one or more points of Ac, and has a resistance of 100 μm.
The number of carbides in m ² remains as large as 35, but the Elv value is
31% and λ value were only 38%, and the local ductility was poor (No. 12). Even in the case of C steel having a C content within the range specified by the present invention, the Elv value is 30% and the λ value is 42% when the annealing condition is a temperature not higher than the Ac ₁ point outside the range of the present invention.
The local ductility was poor (No. 16). Further, in the C steel having a C content within the range specified in the present invention, the α / γ interface amount per γ unit area in the cross-sectional structure at _one point or more of Ac is 0.5 μm.
No. 13 and No. 14 in which the number of carbides was less than 1 in m / μm ^{2 and} less than 1 in 100 μm ² , both the Elv value and the λ value were low due to the formation of regenerated pearlite. Further, when the α / γ interface amount per γ unit area in the cross-sectional structure at _one or more Ac points is 0.5 μm / μm ² or more, Ac ₁
When the cooling rate from the temperature above the point exceeded 50 ° C./h, both the Elv value and the λ value were low because of the generation of regenerated pearlite (No. 15). In steels B to F having a C content within the range specified in the present invention, when annealed under the conditions specified in the present invention, the Elv value is 38% or more, the λ value is 51% or more,
It showed excellent local ductility. F steel having the same C content and the S content of 0.01% by mass or less as compared with the C steel having the C content within the range specified in the present invention and the S content exceeding 0.01% by mass. Shows even higher Elv and λ values, indicating that it has very excellent local ductility.

Example 2 Using the C steel shown in Table 1, the effect of the cold rolling reduction before annealing on the Elv value and λ value of the annealed material was investigated. Elv after subjecting a hot-rolled steel sheet of C steel to cold rolling at various rolling reductions, and subsequently performing annealing within the range specified in the present invention.
Values and λ values were investigated. The board thickness is 2.3 mm for both,
After annealing under the conditions specified in the present invention, a tensile test, a notch tensile test, and a hole expansion test were performed. Table 3 shows the results of these tests together with the cold rolling reduction and the annealing conditions.

[0034]

[Table 3]

In any of the cold rolling reductions, when annealed under the conditions specified in the present invention, the Elv value is 38% and the λ value is 52%.
It has improved with the above. Among them, in the case of No. 18, No. 19, No. 21, and No. 22 subjected to cold rolling of 10% or more, Elv
The value is 42% and the λ value is 55% or more, which is higher than the case where the annealing of the present invention is performed without performing cold rolling (No. 17, No. 20).
The lv value and the λ value are further improved.

[0036]

As described above, according to the present invention, when performing annealing using heating at _one or more points of Ac on a medium / high carbon steel sheet, α / per γ unit area at the end of heating of annealing is obtained. γ
A steel sheet with improved local ductility can be obtained by controlling the amount of the interface within an appropriate range and then slowly cooling. Moreover, the present invention can be widely applied to general middle and high carbon steel grades, and can improve the local ductility in any of the steel grades, thereby contributing to the expansion of applications of middle and high carbon steel sheets.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI theme coat ゛ (Reference) C21D 9/46 C21D 9/46 S C22C 38/00 301 C22C 38/00 301A 38/54 38/54 F term (Reference) 4K032 AA01 AA02 AA05 AA06 AA11 AA12 AA14 AA16 AA19 AA21 AA23 AA24 AA27 AA29 AA31 AA35 BA01 CD03 CF02 CG01 CH04 4K037 EA01 EA02 EA06 EA07 EA11 EA13 EA15 EA15 EA20 EA17 EA20

Claims (7)

[Claims] 1. A C: 0.10-0.60 with respect to hot-rolled steel sheets of steel containing by mass%, when subjected to annealing using heat above Ac ₁ point, gamma unit area at the end of heating stage above Ac ₁ point A metal structure having an α / γ interface amount of 0.5 μm / μm ² or more per unit area and then cooling to a temperature of ₁ point or less at a rate of 50 ° C./h or less. Manufacturing method of high carbon steel sheet. Wherein C: relative from 0.10 to 0.60% by weight of hot-rolled steel sheet of steel containing, when subjected to annealing using heat above Ac ₁ point, per 100 [mu] m ² at the end of heating stage above Ac ₁ point rate of undissolved number carbides 1 or more, and, alpha / gamma interface per gamma unit area and the metal structure is 0.3 [mu] m / [mu] m ² or more, the following then Ar ₁ point to a temperature below 50 ° C. / h A method for producing a medium- and high-carbon steel sheet having excellent local ductility, characterized by cooling at a low temperature. 3. The hot-rolled steel sheet containing C: 0.10 to 0.60% by mass is subjected to cold rolling at a rolling reduction of 10% or more, followed by annealing according to claim 1 or 2. For manufacturing medium and high carbon steel sheets with excellent local ductility. 4. The hot-rolled steel sheet according to claim 1, wherein S is
A method for producing a medium and high carbon steel sheet having excellent local ductility of 0.01% by mass or less. 5. The hot-rolled steel sheet according to claim 1, wherein, by mass%, C: 0.10 to 0.60%, Si: 0 to 0.40% (including no addition), Mn: 0 to 1.0% (no addition) ), P: 0.03%
Below, S: 0.01% or less, T.Al: 0.1% or less, the balance is F
and a method for producing a medium- and high-carbon steel sheet having excellent local ductility, which is a steel comprising e and unavoidable impurities. 6. The hot-rolled steel sheet according to claim 1, wherein, by mass%, C: 0.10 to 0.60%, Si: 0 to 0.40% (including no addition), Mn: 0 to 1.0% (no addition) ), Cr: 0 ~
1.6% (including no addition), Mo: 0 to 0.3% (including no addition), Cu: 0 to 0.3% (including no addition), Ni: 0 to 2.0
% (Including no addition), P: 0.03% or less, S: 0.01% or less, T.Al: 0.1% or less, with the balance being Fe and unavoidable impurities. Manufacturing method. 7. The hot-rolled steel sheet according to claim 1, wherein, by mass%, C: 0.10 to 0.60%, Si: 0 to 0.40% (including no addition), Mn: 0 to 1.0% (no addition) ), Cr: 0 ~
1.6% (including no addition), Mo: 0 to 0.3% (including no addition), Cu: 0 to 0.3% (including no addition), Ni: 0 to 2.0
% (Including no addition), Ti: 0.01 to 0.05%, B: 0.0005
~ 0.0050%, N: 0.01% or less, P: 0.03% or less, S: 0.
01% or less, T.Al: 0.1% or less, the balance being a steel comprising Fe and unavoidable impurities, a method for producing a medium and high carbon steel sheet having excellent local ductility.